Manual - Vampire - University of York
Contents
1. e reate material interfacial rougnnesg 0 reate interfacial roughness random seed reate interfacial roughness number of seed point reate interfacial roughness typq 0 0 reate interfacial roughness seed radiug reate interfacial roughness seed radius variancg reate interfacial roughness mean heigh reate interfacial roughness maximum heighf reate interfacial roughness height field resolution Bee hte Ge wn oe ewes A een me eee ae a eee nie ee ee ee ee ee Lag Spot ones eee ee ee ee ee Go 2 PIMU O n n 3 n n n 3 a oy oy 3 on D 3 DIT Iz a N o ty a K n mens ens mens mens mn O mens mensions unit cell size y ons unit Cell siZe Z QJ O 31 3 ojo ojo o 3 2 N D Ons SyS Ons SyS Ons SyS Ons SyS ons par a 3 fe 5 5 o ons par QJ O O w 8 ps o T N ons par ons par ons par dimensions particle array offset y GIMPSIZOSM ss ee a e o A a e a E A e spacing e snape SsSMADSAACIOM Vil ee oe ak a o SO a Se ae ee 8 e snape e array 0 dimensions macro cell sizel ation Contro sim integrato S prog sim surtace anisotropy threshold surface anisotropy nearest neighbour rangel time s 3 fo D O R on eA D Q p D 22. 2 2 0 0 0 0 00000 32 32 output magnetisation s lt s e c a o o e 32 output magnetisation length 2 en 32 output mean magnetisation length oa aaa a a 33 output material magnetisation o oa a a 33 output material mean magnetisation length 33 outp UM a gA Bohs g Be Saath Be dt Aes A BS eB Se He ws Se we ee A 33 butput mean total torqud 0 2 000022 e 33 DUTPUECONSTAINEDOR send ee Arpe aene rc da a a a 33 output constralntliheta 2 ooa ee 33 output material mean torqug aoaaa ee 34 A dare e ae git li Bare e ot 34 els DEA Gee cee OY eee a 34 output ononon temperaturg en 34 OUTOULTOLAIZFCNEIGW sew arad RAE Soh ba PaO Se a e 34 putputimean total energY 0 0 2 34 output anisotropy energy o e o 34 output mean anisotropy energy 2 2 0000 34 output cubic anisotropy energy o e o 35 output mean cubic anisotropy energy o o 35 output surface anisotropy energy 0 e e 35 output mean surface anisotropy energy o eee 35 Be ine A ee Sneha Gow ge ut he oe hc oe 35 output mean exchange energy o e o 35 output applied Hield energY o 0 e e 35 imean applied tield energy 35 A aise toto mith Bh ag ak 35 We a eee ee ee A 35 35 35 35 35 outp D AAA AN 35 Contigurat
3. default this is disabled and specifying a demagnetisation factor adds an effective field such that the total field is given by Hio Hext Hint M Ng where M is the magnetisation of the sample and Ng is the demagnetisation factor of the macroscopic sample The components of the demagnetisation factor must sum to 1 In general the demagnetisation factor should be used without the dipolar field as this results in counting the demagnetising effects twice However the possibility of using both is not prevented by the code 30 sim mpi mode geometric decomposition replicated data replicated data staged sim integrator random seed Integer default 12345 Sets a seed for the psuedo random number generator Simulations use a predictable sequence of psuedo random numbers to give repeatable results for the same simulation The seed determines the actual sequence of numbers and is used to give a different realisation of the same simulation which is useful for determining statistical properties of the system sim constraint rotation update sim constraint angle theta float default 0 When a constrained integrator is used in a normal program this variable controls the angle of the magnetisation of the Whole system from the x axis degrees In constrained simulations such as c c anisotropy this has no effect sim constraint angle theta minimum float default 0 sim constraint angle theta maximum sim constraint angle theta increment
4. in simulations for determining the most common magnetic properties of a system D for example Curie temperature hysteresis loops or even a time series Additionally the parameters for these simulations such as applied field maximum temperature temperature increment etc can be set 19 Input and Output Files VAMPIRE requires at least two files to run a simulation the input file and the material file The input file defines all the properties of the simulated system such as the dimensions or particle shape as well as the simulation parameters and program output The material file defines the properties of all the materials used in the simulation and is usually given the mat file extension A sample material file Co mat is included with the code which defines a minimum set of parameters for Co The output of the code includes a main output file which records data such as he magnetisation timesteps temperature etc The format of the output file is fully customisable so that the amount of output data is limited to what is useful In addition o the output file the other main available output are spin configuration files which with post processing allow output of snapshots of the magnetic configurations during he simulation Sample input files Sample input and output files are included in the source code distribution but the files for a simple test simulation which computes the time dependence of the magnetisation of a cubic sys
5. on O i O O lt 3 3 IS 3 IS ep 3 op time step laser pulse temporal protile d 3 o 3 3 3 3 3 3 3 3 3 3 IS 3 sim laser pulse time milaser DUISS DOWEN 6 6 s s cose aeu ewi eto eneen 29 5 dr a da EE 29 NA oh Bae deans Geom 29 Aare tae axe 29 e 30 30 sa eh dieg daa 30 a 30 30 My HE Ri ae ah ce ooh sn SEG he stn Re Maye Hee A A 30 sim applied tield strength o a a ee 30 30 30 30 30 30 Lah fg tne a Se ae ees 30 Sim demagnetisation facton s a oss ioes 0 0 30 S p GE pee ar Be Bh ke A VA Ap We o oe ye Bee ew 31 Be Be ih tee My BO Ae GBS he eh QAR Bo ates ce a 31 hod Se Ae Reo ee He ne Geek TR we wee amp 31 A Be 31 beta cis Geen a eh 31 sim constraint angle theta maximum 2 2 0 020040 31 Sim constraint angle theta incremen 31 Lite drag LE ot Asha Si ah A a eS 31 A ede Bat ae te we us 31 rara ta tas 31 ala he aad dd de os E 31 e a A ate ots 31 A AA ee Be ee Ae E a 31 Dala QU DUN ssc fro oe RAR ee wR ee a e 32 OUTPUTUME STEOS e cH A bd pka BAe eA EA A GS OR mw Bos 32 OUTOUTKEAIETIMNG o so aha Ghia Goda badd boa A eee EE ES 32 OUTDULTEMPCKAtUIE lt aoe ck a ae Be ew 32 output applied tield strength 2 2 en 32 output applied tield unit vectoy 2 aa ee 32 output applied tield alignmeny
6. the folder where Vampire Parallel exe is and click Properties go to Sharing Tab and click Share Then if your name is there OK else choose your name from dropdown menu and click Add In the permission level choose to be Read write next to your name e Every PC Put the files C Program Files x86 MPICH2 bin smpd exe C Program Files x86 MPICHAa bin mpiexec exe and Vampire Parallel exe in the excep tion list of Windows Firewall maybe there already in this list Bring up the windows firewall from the control panel Click on Allow a program or feature through Windows Firewall click Change settings and click Allow another program and Browse Go to find these files in their directories and the Vampire Parallel exe go in the Network then to the running PC and then to the folder Optional Only for the running PC In order to make the log files to join and the program finish as expected you must do that Run regedit and find the key HKEY_CURRENT_USER Software Microsoft Command Processor and click add DWORD with the name DisableUNCCheck and give it value 1 will appear o 0 x 1 Hex Run the code To run VAMPIRE on multiple machine execute the following command mpiexec hosts number_of_hosts PC1 np1 PC2 np2 dir PC1 Vampire PC1 Vampire Vampire Parallel exe number_of_hosts number of PCs you want to use PC1 name of the 15t PC np1 number of cores to use in PC1 PC2 n
7. to a wide range of open source libraries and tools such as openmpi rasmol and povray For the serial version compilation is the same as for linux following a simple make command in the source directory Similarly for the parallel version openmpi needs to be installed via MacPorts and compilation is usually straightforward using make parallel Compiling on Windows In order to compile the code on Windows systems you will need to have Microsoft Visual Studio 2010 or later and open the file Vampire vcxproj with it The project has two versions Debug and Release where you can choose the current one in the drop down menu on the top toolbar The first is for debugging the code and the executable which is created will not be a stand alone executable The Release version is for creating a stand alone executables for the serial version Click Build gt Project to 14 compile the code Finally you can compile a 64 bit version if you choose from the drop down menu in the top toolbar in MS Visual Studio It writes Win32 so when you click it the x64 option will appear 15 3 Running the code To run the code in all version you first need to specify an input file and material file which must reside in the same directory where you run the code Example files are available in the source code distribution or from the Download section of the website http vampire york ac uk download index html Linux Debian Ubuntu and Mac OS X
8. years of continuous development with an aim to make atomistic simulation of magnetic materials routinely available to the non specialist researcher Before now using atomistic models to simulate magnetic systems required in depth and technical knowledge of the underlying theoretical methods computer programming skills and the ability to debug and understand intricate computational problems The code is designed with ease of use in mind and includes an extensive set of input parameters to control the simulations through a plain text input file Subject to future funding it is also hoped to develop graphical user interfaces for Mac OS X and Windows which should make using the code more accessible The VAMPIRE project is still very much under active development and a yearly release schedule in the Autumn is planned to make the latest improvements available for everyone These features are always available during the development stages from the develop branch of the code but with the caveat that they are not always fully reliable Feedback of any bugs or errors to the VAMPIRE developers is always welcome as well as any feature requests or enhancements We hope that as the VAMPIRE project develops it will become a useful tool for the magnetics community for specialists and non specialists alike 1 Background theory While the underlying theory behind the atomistic spin model is well known in the scientific literature in the fo
9. zrespectively as well as the longitudinal susceptibility Xm The data is output in four columns Xx Xy Xz ANA Xm The susceptibility is useful for identifying the critical temperature for a system as well as atomistic parametrization of the micromagnetic Landau Lifshitz Bloch LLB equation output electron temperature Outputs the instantaneous electron temperature as calcu lated from the two temperature model output phonon temperature outputs the instantaneous phonon lattice temperature as calculated from the two temperature model output total energy output mean total energy output anisotropy energy output mean anisotropy energy 34 output cubic anisotropy energy output mean cubic anisotropy energy output surface anisotropy energy output mean surface anisotropy energy output exchange energy output mean exchange energy output applied field energy output mean applied field energy output magnetostatic energy output mean magnetostatic energy output second order uniaxial anisotropy energy output mean second order uniaxial anisotropy energy output mpi timings output gnuplot array format output output rate integer default 1 controls the number of data poin the output file or prin every sim time steps i the updated statistic e g magnetization every time which is s written to ed to screen By default VAMPIRE calculates statistics once ncrement number o However sometimes you
10. In the directory including the input and material files typing vampire will run the code in serial mode For the parallel mode with openmpi mpirun np 2 vampire will run the code in parallel mode on 2 CPUs Increasing the np argument will run on more cores Windows In order to run any Windows version of VAMPIRE you need to have Microsoft Visual C 2008 Redistributable library or newer installed on your PC Usually this is installed by default but if the executable fails to run then download and install it from here http www microsoft com en gb download details aspx id 29 Serial version The serial version can be run by double clicking the executable file where the executable input and material files are in the same directory You may also want to run the code using the command line in case there are error messages In this case you should change to the directory containing the input files and executables using the usual cd commands VAMPIRE can then be run by simply typing vampire Parallel version using MPICH2 1 PC The parallel version of VAMPIRE requires a working installation of MPICH2 which must be installed as follows 16 Set up MPICH2 e Download and install MPICH2 from http Awww mpich org static tarballs 1 4 1p1 mpich2 1 4 1p1 win ia32 msi e Putits bin directory to the path This is how to do that Right Click My Computer and then Properties Click on left the Advanced system se
11. VAMPIRE User Manual VAMPIRE User Manual Software version 4 0 Manual written by Richard F L Evans and Andreas Biternas Copyright O 2014 Department of Physics The University of York Heslington York YO10 5DD All rights Reserved The VAMPIRE software package is principally developed and maintained by Richard F L Evans Code contributors Weijia Fan Phanwadee Chureemart Thomas Ostler Joe Barker Andreas Biternas and Roy Chantrell The entire VAMPIRE package is available under the GNU General Public License You are free to use vampire for personal academic and commercial research and to modify the source code as you wish For details of the licence check the README file in the source code or consult www gnu org copyleft gpl html The VAMPIRE source code is available from www github com richard evans vampire This manual software features tutorials and more information is available from the VAMPIRE webpage at http vampire york ac uk Table of Contents 2 5 3 9 Introducing VAMPIRE 2 e 9 10 A DIT MODE oc koe Rhee RRR SRE RE RTE 10 dh haces Syn ees caer on carne we Gy lados Mara Gea a 11 SPIN Dynamic ss ee Gow ae ks eo KR ah a ew 11 Se es Fee e Oo soe ele te ee 12 13 rotor per eee 13 II e amp oA eee ln be es bk ek he ee Bd es ee a 13 Pad ore ate ee ees ee eae oe 14 OMPMINGFON LINUX x Loco Sh a RR wok Ge te Be RR rsh es De a 14 oiling V OD oe dee See eee dde do ea 14 pi
12. ame of the 27d PC np2 number of cores to use in PC2 WPC 1 Vampire shared directory where all the PCs can find the executable and input files PC 1 Vampire Vampire Parallel exe the executable location 18 4 Getting Started VAMPIRE is a powerful software package capable of simulating many different systems and the determination of parameters such as coercivity Curie points reversal dynamics statistical behaviour and more This chapter contains an overview of the capabilities of VAMPIRE and how to use them Feature Overview The features of VAMPIRE are split into three main categories material parameters structural parameters and simulation parameters Details of these parameters are given in the following chapters but between them they define the parameters for a particular simulation Materials Material parameters essentially define the magnetic properties of a class of atoms including magnetic moments exchange interactions damping constants etc VAM PIRE includes support for up to one hundred defined materials and material param eters control the simulation of multilayers random alloys core shell particles and lithographically defined patterns Structures Structural parameters define properties such as the system size shape particle size or voronoi grain structures In combination with material parameters they essentially define the system to be simulated Simulations VAMPIRE includes a number of built
13. demagnetizing field and output of the magnetic configuration Finer discretisation lead to more accurate results at the cost of significantly longer run times The cell size should always be less than the system size as highly asymmetric cells will leads to significant errors in the demagnetisation field calculation Simulation Control The following commands control the simulation including the program maximum temperatures applied filed strength etc sim integrator exclusive string default Ilg heun Declares the integrator to be used for the simulation Available options are Ilg heun monte carlo Ilg midpoint 26 constrained monte carlo hybrid constrained monte carlo sim program exclusive bool defines the simulation program to be used sim program benchmark program which integrates the system for 10 000 time steps and exits Used primarily for quick performance comparisons for different system architectures processors and during code performance optimisation sim program time series program to perform a single time series typically used for switching calculations ferromagnetic resonance or to find equilibrium magnetic configurations The system is usually simulated with constant temperature and applied field The system is first equilibrated for sim equilibration time steps time steps and is then integrated for sim time steps time steps sim program hysteresis loop program to simulate a dynamic hysteresis lo
14. e the next chapter on running VAMPIRE From version 4 0 onwards a copy of 13 qvoronoi is integrated into VAMPIRE for generating granular structures Compiling from source The best way to get the vampire source code is using git a distributed version control program which enables changes in the code to be tracked Git is readily available on linux git core package on ubuntu and Mac via MacPorts To get vampire from the Github repository checkout your own copy of the repository using git clone git github com richard evans vampire git This way updates to the code can be easily merged with the downloaded version Compiling is generally as easy as running make in Unix platforms Compiling on Linux In order to compile in linux a working set of development tools are needed which on ubuntu includes the packages build essential and g VAMPIRE should compile without issue following a simple make command in the source directory For the parallel version a working installation of openmpi is recommended which must usually include a version of the development tools openmpi bin and openmpi dev packages on ubuntu Compilation is usually straightforward using make parallel Compiling on Mac OSX With OS X compilation from source requires a working installation of Xcode available for free from the Mac App Store In addition command line tools must also be installed A working installation of MacPorts is recommended to provide access
15. eate particle spacing If the system size is insufficient to contain at least a single entire particle of size create particle size then no atoms will be generated and the program will terminate with an error create voronoi film Generates a two dimensional voronoi structure of particles with a mean grain size of create particle size and variance create voronoi size variance as a fraction of the grain size If create voronoi size variance 0 then hexagonal shaped grains are generated The spacing between the grains defined by the initial voronoi seed points is controlled by create particle spacing The pseudo random pattern uses a predefined random seed and so the generated structure will be the same every time A different structure can be generated by setting a new random seed using the create voronoi random seed parameter Depending on the desired edge structure the first row can be shifted using the create voronoi row offset flag which changes the start point of the voronoi pattern The create voronoi rounded grains parameter generates a voronoi structure but then applies a grain rounding algorithm to remove the sharp edges create voronoi size variance float Controls the randomness of the voronoi grain struc ture The voronoi structure is generated using a hexagonal array of seed points appropriately spaced according to the particle size and particle spacing The seed points are then displaced in x and y according to a gaussian di
16. ection and e is the easy axis unit vector Positive values of k4 give a preferred easy axis orientation and negative values give a preferred easy plane orientation of the spin material cubic anisotropy constant float default 0 0 J atom Defines the local cubic magnetocrystalline anisotropy constant at each atomic site The anisotropy energy is given by the expression Ko Ey Sk Sy 82 where Sx y z are the components of the local spin direction and ke is the cubic anisotropy constant Positive values of ke give a preferred easy axis orientation along the 001 directions medium hard along the 110 directions and hard along the 111 directions Negative values give a preferred easy direction along the 111 directions medium ahead along the 110 directions and hard along the 100 directions material uniaxial anisotropy direction float vector default 001 A unit vector e de scribing the magnetic easy axis direction for uniaxial anisotropy The vector is entered in comma delimited form For example material 1 uniaxial anisotropy direction 0 0 1 The unit vector is self normalising and so the direction can be expressed in standard form with length r 1 or in terms of crystallographic directions e g 111 material surface anisotropy constant float default 0 0 J atom Describes the surface anisotropy constant in the N el pair anisotropy model The anisotropy is given by a 39 summation over nearest neighb
17. efault 1 0 defines the phenomenological relaxation rate damping in dynamic simulations using the LLG equation For equilibrium properties the damping should be set to 1 critical damping while for realistic dynamics the damping should be representative of the material Typical values range from 0 005 to 0 1 for most materials 37 material exchange matrix index float default 12 x 107 J link Defines the pairwise exchange energy between atoms of type index and neighbour index The pair wise exchange energy is independent of the coordination number and so the total exchange integral will depend on the number of nearest neighbours for the crystal lattice The exchange energy must be defined between all material pairs in the simulation with positive values representing ferromagnetic coupling and negative values representing anti ferromagnetic coupling For a ferromagnet with nearest neighbour exchange the pairwise exchange energy can be found from the Curie temperature by the meanfield expression 3kg Te io 1 EZ where Jj is the exchange energy kg is the Boltzmann constant To is the Curie temperature z is the coordination number number of nearest neighbours and e is a correction factor to account for spin wave fluctuations in different crystal lattices If a custom unit cell file is used the exchange values defined here are ignored material atomic spin moment float 0 01 ug default 1 72 ug Defines the local effective sp
18. egative first derivative of the complete spin Hamiltonian such that 1 90 A 1 2 ett lig 08 1 2 where sg is the local spin moment The inclusion of the spin moment within the effective field is significant in that the field is then expressed in units of Tesla given a Hamiltonian in Joules The LLG is integrated numerically using the Heun numerical scheme which allows the time evolution of the spin system to be simulated Citations If you use VAMPIRE for your research please cite the following article Atomistic spin model simulations of magnetic nanomaterials R F L Evans W J Fan P Chureemart T A Ostler M O A Ellis and R W Chantrell J Phys Condens Matter 26 103202 2014 If you use the constrained Monte Carlo method in addition please cite Constrained Monte Carlo method and calculation of the temperature dependence of magnetic anisotropy P Asselin R F L Evans J Barker R W Chantrell R Yanes O Chubykalo Fesenko D Hinzke and U Nowak Phys Rev B 82 054415 2010 If you use the temperature rescaling method please cite Quantitative simulation of temperature dependent magnetization dynamics and equilibrium prop erties of elemental ferromagnets R F L Evans U Atxitia and R W Chantrell Phys Rev B 91 144425 2015 12 2 Installation This chapter covers the requirements installation and support for VAMPIRE on different platforms System Requirements VAMPIRE is de
19. etisation length outputs the time averaged normalized magnetiza tion length ml output material magnetisation outputs the instantaneous normalized magnetization for each material in the simulation The data is output in blocks of four columns with one block per material defined in the material file e g 4 my me my 1 s m e ai m mp mz mp Note that obtaining the actual macroscopic magnetization length from this data is not trivial since it is necessary to know how many atoms of each material are in the system This information is contained within the log file giving the fraction of atoms which make up each material However it is usual to also output the total normalized magnetization of the system to give the relative ordering of the entire system output material mean magnetisation length outputs the time averaged normalized mag netization length for each material e g m mo mn output total torque outputs the instantaneous components of the torque on the system T MS x H in three columns Tx Ty Tz units of Joules In equilibrium the total torque will be close to zero but is useful for testing convergence to an equilibrium state for zero temperature simulations output mean total torque Outputs the time average of components of the torque on the system T X2 H S X Hj in three columns Tx Ty Tz In equilibrium the total torque will be close to zero but the average torque is useful for ext
20. float 0 001 360 default 5 Incremental Change of angle of m from z direction in constrained simulations Controls the resolution of sim constraint angle phi sim constraint angle phi minimum sim constraint angle phi maximum sim constraint angle phi increment sim monte carlo algorithm spin flip uniform angle hinzke nowak sim checkpoint flag default false Enables checkpointing of spin configuration at end of simulation sim save checkpoint end sim save checkpoint continuous sim save checkpoint rate 1 sim load checkpoint restart sim load checkpoint continue 31 Data output The following commands control what data is output to the output file The order in which they appear is the order in which they appear in the output file Most options output a single column of data but some output multiple columns particularly vector data or parameters related to materials where one column per material is output output time steps Outputs the number of time steps or Monte Carlo steps completed during the simulation so far output real time Outputs the simulation time in seconds The real time is given by the number of time steps multiplied by sim time step default value is 1 0 x 10715 s The real time has no meaning for Monte Carlo simulations output temperature Outputs the instantaneous system temperature in Kelvin output applied field strength Outputs the strength of the applied field in Tesla For hysteresis simulations the s
21. h as FM AFM or ECC recording media Aa ee se ee ase ses eee eee ree system max z 1 material 2 maximum 2 minimum 2 maximum 1 material 1 minimum 1 Et a ue a system min z 0 Figure 6 1 Schematic diagram showing definition of a multilayer system consisting of two materials The minimum height and maximum height are defined as a fraction of the total z height of the system The heights of the material are applied when the crystal is generated and so in general further geometry changes can also be applied for example cutting a cylinder shape or voronoi granular media while preserving the multilayer structure The code will also print a warning if materials overlap their minimum maximum ranges since such behaviour is usually but not always undesirable material maximum height float 0 1 default 1 0 defines the maximum height of the material as a fraction of the total height z of the system See material minimum height for more details material core shell size float 0 1 default 1 0 defines the radial extent of a material as a fraction of the particle radius This parameter is used to generate core shell nanoparticles consisting of two or more distinct layers The core shell size is compatible with spherical ellipsoidal cylindrical truncated octahedral and cuboid shaped particles In addition when particle arrays are generated all particles are also core shell type This option is also comparable with the minimum max
22. ign of the applied field strength changes along a fixed axis and is represented in the output by a similar change in sign output applied field unit vector outputs a unit vector in three columns Ax hy indicating the direction of the external applied field hz output applied field alignment outputs the dot product of the net magnetization direc tion of the system with the external applied field direction m H output material applied field alignment outputs the dot product of the net magnetiza direction of each material defined in the material file with the external applied fie direction ma AL mo A E r n Al output magnetisation outputs the instantaneous magnetization of the system data is output in four columns mx my mz m giving the unit vector direction he magnetization and normalized length of the magnetization respectively he localized spin moments y are taken into account in the summation 32 The normalized length of the magnetization m gt u Sj gt H is given by the sum o all moments in the system assuming ferromagnetic alignment of all spins Note tha output magnetisation length outputs the instantaneous normalized magnetization leng m gt 0 u4iSi gt 2 H where the saturation value is defined by ferromagnetic ion d The O h alignment of all spins in the system Note that the localized spin moments u are taken into account in the summation output mean magn
23. imal number of neighbours to classify as surface atom sim surface anisotropy nearest neighbour range float default oo Sets the interaction range for the nearest neighbour list used for the surface anisotropy calculation 28 sim time step sim total time steps sim loop time steps sim time steps increment sim equilibration time steps sim simulation cycles sim maximum temperature sim minimum temperature sim equilibration temperature sim temperature sim temperature increment sim cooling time sim laser pulse temporal profile square two temperature double pulse two temperature double pulse square sim laser pulse time sim laser pulse power sim second laser pulse time sim second laser pulse power sim second laser pulse maximum temperature 29 sim second laser pulse delay time sim two temperature heat sink coupling sim two temperature electron heat capacity sim two temperature phonon heat capacity sim two temperature electron phonon coupling sim cooling function exponential gaussian double gaussian linear sim applied field strength sim maximum applied field strength sim equilibration applied field strength sim applied field strength increment sim applied field angle theta sim applied field angle phi sim applied field unit vector sim demagnetisation factor float vector default 000 vector describing the compo nents of the demagnetising factor from a macroscopic sample By
24. imum height 41 core shell size material 2 maximum height 1 particle size core shell size minimum height particle size Figure 6 2 a Schematic diagram showing definition of a nanoparticle with two materials with different radii core shell size is defined as a fraction of the particle radius particle size 2 b Schematic diagram showing side on iew of a cylinder consisting of two materials with different core shell size and different maximum heights Part of the core material is exposed while the other part is covered with the other material options allowing for partially filled or coated nanoparticles material interface roughness float 0 1 default 1 0 defines interfacial roughness in multilayer systems material intermixing index float 0 1 default 1 0 defines intermixing between adjacent materials in multilayer systems The intermixing is defined as a fraction of the total system height and so small values are usually used The intermixing defines the mixing of material index into the host material and can be asymmetric a gt b l b gt a material density float 0 1 default 1 0 defines the fraction of atoms to remove randomly from the material density material continuous flag default off is a keyword which defines materials which ignore granular CSG operations such as particles voronoi media and particle arrays material fill space flag default off is a key
25. in moment for each atomic site Atomic moments can be found from ab initio calculations or derived from low temperature measurements of the saturation mag netisation The atomic spin moments are related to the macroscopic magnetisation by the expression Es Mga Ms gt where ais the lattice constant nis the number of atoms per unit cell and Mg is the saturation magnetisation in units of J T m3 A m Note that unlike micromagnetic simulations atomistic simulations always use zero K values of the spin moments since thermal fluctuations of the magnetisation are provided by the model Small values lt 1p will typically lead to integration problems for the LLG unless sub femtosecond time steps are used material uniaxial anisotropy constant float default 0 0 J atom Defines the local second order single ion magnetocrystalline anisotropy constant at each atomic site The anisotropy energy is given by the expression E ko S ej Where is the local spin direction and e is the easy axis unit vector Positive values 38 of ko give a preferred easy axis orientation and negative values give a preferred easy plane orientation of the spin material second uniaxial anisotropy constant float default 0 0 J atom Defines the local fourth order single ion magnetocrystalline anisotropy constant at each atomic site The anisotropy energy is given by the expression E ky S ej Where is the local spin dir
26. ion outpuy lt ee o e o 36 ERAN A ees ed 36 eee cee E ee oe eh ee ee 36 ONTIG AtOMS MIM ENY ea ea Ra ds AAA 36 onfig atoms MIN Y ww 36 on on on ONTIG ATOMS MAXA e e ee a e p e aa a ha a abe Cae ua ai oni MaCcro sCOlS o 2 acel otk al ee aE e w as aa boas de a eG oa on ontIg IOENUNY SUNMACG ALOIMNS 2 oo ooo A Material File Command Reference ater DO PO 1L fo HO JL PO L L fo o er o DO PO 1 PO of JL MO o wj V er ater a a D T 2 o y o 2 o 10 o o D xample m a Nu m materalg lt a a i Ded ewinwt IN sida clado de a ld as o as ea errada A rr ad E E O E Meath sty E E E E rial reiative gamma ww o a a a E a Inttial spin direction ooa aaa a erial material elemen ee we ee eH erial geometry tile aoaaa a a a a a O ee er ee errr er eee edita nl deh ocd a tereag ieee dos yeas A See ba ee ea EN eB Aah Awd ios ie eps CERRAR ARA enakdenSi A A ee aE GO a SNShOOMINUOUR e kb Ree Bo ee ee eR we EO spa erlal couple to phononic temperature al al emperature rescaling exponenf emperature rescaling Curle lemperalure STAM 2 5 e a A e E 44 Introducing VAMPIRE VAMPIRE is a state of the art atomistic simulator for magnetic nanomaterials This software is the culmination of five
27. ling WINCIOWO ne s ee ee we Bw ee le ee Oe 14 B R O del 16 Running on Linux and MacOS Ml o a 16 R g WINGOWG o oo ba othe oe he Baw APR ds de Ae BS 16 oe SSO SH Bd Sat ASM as Sl 16 Parallel version TPC 2 e 16 Parallel version 2 PCs 0 2 deu maa eE aa a 17 4 O d 19 AAA ETA 19 Input and Output FlleS o 20 5 O PUTTICS o a ls a a Be a oN 20 o Input File Command Reference rea rea reate cylinde S8ate CMPSOIO e co ss a BE A BO e E reate spnere reate truncated octahedroy 2 0 eee ee reate particle feate ParliCle Amrayyl se abet ae bcd ob hth Yo hc O Binge Re EATS VOLOMOIMIM 24 Boe Be Se Be Bl a a Go Med Be eee a Be Gees real voronol siZ varlance e S ooa os a a a a g a e a a a an reate voronol row otfseq ooa aoa a reate voronol random seeq 2 a ee reate voronol rounagea graing d ro E ee reate voronol rounaea grains area e a 2 ee feate partiCle Panilyi s e soe aoa ma ata e abe a a a reate crystal structurej 4 a be wee soea we BR ee ee MEATS SINGIS SSM 05 0 oF i tad Se Sih A aay ie a RE Ee E A ee Be a reale perlodiC DOUNQAMES X e gt s a ee reale perlodIC DOUNQATNES Y gt ee reale perlodiC DOUNQAMES 7 a ww ee reate select material by neigny o a eee reate select material by geometry reate fi reate interfacial roughness 0
28. llowing a very brief overview of the fundamental theory is presented for the benefit of those who do not wish to study the methods in great detail If more information is required then a comprehensive review of the methods implemented in VAMPIRE is available from the project website Atomistic Spin Models Atomistic spin models form the natural limit of two distinct approaches namely micromagnetics and ab initio models of the electronic structure In micromagnetics a material is discretized into small domains where the magnetization is assumed to be fully ordered within If the micromagnetic cell size is reduced to less than 1 nm then the magnetization is no longer a true continuum but a discrete entity considering localized moments on individual atoms Similarly when the electronic properties of the system are considered the quantum mechanical properties can be mapped onto atomic cores in a manner similar to molecular dynamics where the effective properties can often be treated in a classical approximation The advantage of the atomistic model over micromagnetics is that it naturally deals with atomic ordering and variation of local properties seen in real materials such as interfaces defects roughness etc The discrete formulation also allows the simulation of high temperatures above and beyond the Curie temperature where the usual continuum micromagnetic approach breaks down Such effects or often central to current problems in mag
29. may want to plo you want to collect s file which is controlled by this keyword For example if outpu sim time steps increm atistics much more ent 10 then statist once every 10 time steps and the new stat 100 time steps time steps Usual frequently than you y you want to output he default behaviour the time evolution of an average where output to the output e 10 and output ra ics and average values will be updated istics will be written 35 o the output file every Configuration output These options enable the output of spin configuration snapshots config atoms flag enables the output of atomic spin configurations config atoms output rate int 0 default 1000 Determines the rate configuration files are output as a multiple of sim time steps increment config atoms minimum x config atoms minimum y config atoms minimum z config atoms maximum x config atoms maximum y config atoms maximum z config macro cells Enables the output of macro cell spin configurations config macro cells output rate config identify surface atoms Boo default false Flag to enable identification of surface atoms in configuration and xyz file output 36 6 Material File Command Reference The material file defines all the magnetic properties of the materials used in the simulation including exchange anisotropy damping etc Material properties are defined by an index number for each ma
30. mol The contrast is particularly useful in inspecting the generated structures particularly ones with a high degree of complexity material geometry file string default specifies a filename containing a series of connected points in space which is used to cut a specified shape from the material in a process similar to lithography The first line defines the total number of points which must be in the range 3 100 A minimum of three points is required to define a polygon The points are normalised to the sample size and so all points are defined as X y pairs in the range 0 1 with one point per line The last point is automatically connected first so need not be defined twice material alloy host flag default off is a keyword which if specified scans over all 40 other materials to replace the desired fraction of host atoms with alloy atoms This is primarily used to create random alloys of materials with different properties such as FeCo NiFe or disordered ferrimagnets such as GdFeCo material alloy fraction index float 0 1 default 0 0 defines the fractional number of atoms of the host material to be replaced by atoms of material index material minimum height float 0 1 default 0 0 defines the minimum height of the material as a fraction of the total height z of the system By defining different minimum and maximum heights it is easy to define a multilayer system consisting of different materials suc
31. nd ending at sim maximum temperature in steps of sim temperature increment At each temperature the system is first equi 27 librated for sim equilibration steps time steps and then a statistical average is taken over sim loop time steps In general the Monte Carlo integrator is the optimal method for determining the Curie temperature and typically a few thousand steps is sufficient to equilibrate the system To determine the Curie temperature it is best to plot the mean magnetization length at each temperature which can be specified using the output mean magnetisation length keyword Typically the temperature dependent magnetization cen be fitted using the function mT lt zs gt 2 ay 5 1 where Tis the temperature Tc is the Curie temperature and B 0 34 is the critical exponent sim program field cooling sim program temperature pulse sim program cmc anisotropy sim enable dipole fields flag enables calculation of the demagnetising field sim enable fmr field sim enable fast dipole fields Bool default false Enables fast calculation of the demag field by pre calculation of the interaction matrix sim dipole field update rate Integer default 1000 Number of timesteps between re calculation of the demag field Default value is suitable for slow calculations fast dynamics will generally require much faster update rates sim enable surface anisotropy sim surface anisotropy threshold Int default native Determines min
32. netism such as materials for spin electronics heat assisted magnetic recording or ultrafast laser processes Similarly for ab initio calculations mapping onto an effective spin model allows apply the full quantum mechanical deal of the properties to much larger systems and the consideration of dynamic effects on much longer timescales 10 The Spin Hamiltonian The basis of the atomistic spin model is the spin Hamiltonian which describes the fundamental spin dependent interactions at the atomic level neglecting the effects of potential and kinetic energy and electron correlations The spin Hamiltonian is typically defined as H JiSj S Ko Y S Hs X Happ S i ij i describing exchange uniaxial anisotropy and applied field contributions respec ively Important parameters are the Heisenberg exchange Jj the anisotropy constant kp and the atomic spin moment Ug S is a unit vector which describes he orientation of the local spin moment In most magnetic materials the exchange interactions are the dominant contribution usually by two orders of magnitude and gives rise to the atomic ordering of the spin directions For ferromagnetic materials parallel alignment of spins Jj gt 0 while for anti ferromagnetic materials antiparallel alignment of spins Jj lt 0 While the exchange interaction determines the ordering of the spins it is usually isotropic and so there is no preferential orientation of all the spins in the sy
33. op in user defined field range and precision The system temperature is fixed and defined by sim temperature The system is first equilibrated for sim equilibration time steps time steps at sim maximum applied field strength applied field For normal loops sim maximum applied field strength should be a saturating field After equilibration the system is integrated for sim loop time steps at each field point The field increments from sim maximum applied field strength to sim maximum applied field strength in steps of sim applied field increment and data is output after each field step sim program static hysteresis loop program to perform a hysteresis loop in the same way as a normal hysteresis loop but instead of a dynamic loop the equilibrium condition is found by minimisation of the torque on the system For static loops the temperature must be zero otherwise the torque is always finite At each field increment the system is integrated until either the maximum torque for any one spin is less than the tolerance value 1078 T or if sim loop time steps is reached Generally static loops are computationally efficient and so sim loop time steps can be large as many integration steps are only required during switching i e near the coercivity sim program curie temperature Simulates a temperature loop to determine the Curie temperature of the system The temperature of the system is increased stepwise starting at sim minimum temperature a
34. our atoms given by k nn S E gt gt Ss x rj j where ks is the surface anisotropy constant and rj is a unit vector between sites and j Defining this material parameter for any material enables calculation of the surface anisotropy during the simulation at some performance cost so only define this if actually required in the simulation material relative gamma float default 1 defines the gyromagnetic ratio of the material relative to that of the electron Y 1 76 T71s71 Valid values are in the range 0 01 100 0 For most materials y 1 material initial spin direction float vector bool default 001 false determines the initial direction of the spins in the material Value can wither be a unit vector defining a direction in space or a boolean which initialises each spin to a different random direction equivalent to infinite temperature As with other unit vectors a normalised value or crystallographic notation e g 110 may be used material material element string default Fe defines a purely descriptive chemical element for the material which gives visual contrast in a range of interactive atomic structure viewers such as mol rasmol etc In rasmol Fe is a gold colour H is white Li is a deep red O is red B is green and Ag is a medium grey This parameter has no relevance to the simulation at all and only appears when outputting atomic coordinates which can be post processed to be viewable in ras
35. racting effective anisotropies or exchange using constrained Monte Carlo simulations output constraint phi outputs the current angle of constraint from the z axis for constrained simulations using either the Lagrangian Multiplier Method LMM or Constrained Monte Carlo CMC integration methods output constraint theta outputs the current angle of constraint from the x axis for constrained simulations using either the Lagrangian Multiplier Method LMM or Constrained Monte Carlo CMC integration methods 33 output material mean torque Outputs the time average of components of the torque on the each material system tT in blocks of three columns with one block for each material defined in the material file e g Cep rt DL K 72 23 KTA tn TA Computing the torque on each material is particularly useful for determining equilibrium properties of multi component systems with constrained Monte Carlo simulations In certain cases the components of a system different materials can exert equal and opposite torques on each other giving a total system torque of zero The decomposition of the torques for each material allows the determination of internal torques in the system output mean susceptibility outputs the components of the magnetic susceptibility x The magnetic susceptibility is defined by DA 2 2 Xa EF ma Ma where a Xx y z m giving the directional components of the magnetization in x y and
36. rameter create full should be used when importing a complete system such as a complete nanoparticle and where a further definition of the system shape is not required This is the default if no system truncation is defined create cube Cuts a cuboid particle of size lx ly lz create particle size from the defined crystal lattice gt he he create cylinder Cuts a cylindrical particle of diameter create particle size from defined crystal lattice The height of the cylinder extends to the whole extent o system size create system size z in the z direction create ellipsoid Cuts an ellipsoid particle of diameter create particle size with frac tional diameters of dimensions particle shape factor x dimensions particle shape factor y dimensions particle shape factor z from the defined crystal lattice create sphere Cuts a spherical particle of diameter create particle size from the defined crystal lattice create truncated octahedron Cuts a truncated octahedron particle of diameter create particle size from the defined crystal lattice 22 create particle Defines the creation of a single particle at the centre of the defined system If create particle size is greater than the system dimensions then the outer boundary of the particle is truncated by the system dimensions create particle array Defines the creation of a two dimensional array of particles on a square lattice The particles are separated by a distance cr
37. res unit cell create crystal structure string sc fcc bcc default sc Defines the default crystal lattice to be generated create single spin flag Overrides all create options and generates a single isolated spin create periodic boundaries x flag Creates periodic boundaries along the x direction Parallel version is implemented but untested use with caution create periodic boundaries y flag creates periodic boundaries along the y direction Parallel version is implemented but untested use with caution create periodic boundaries z flag creates periodic boundaries along the z direction Parallel version is implemented but untested use with caution create select material by height create select material by geometry create fill core shell particles create interfacial roughness 24 create material interfacial roughness create interfacial roughness random seed create interfacial roughness number of seed points create interfacial roughness type create interfacial roughness seed radius create interfacial roughness seed radius variance create interfacial roughness mean height create interfacial roughness maximum height create interfacial roughness height field resolution System dimensions The commands here determine the dimensions of the generated system dimensions unit cell size float 0 1 default 3 54 Defines the size of
38. signed to be generally portable and compilable on Linux Unix Mac OSX and Windows with a range of different compilers By design the software has a very minimal dependence on external libraries to aid compilation on the widest possible range of platforms without needing to first install and configure a large number of other packages VAMPIRE is designed to be maximally efficient on high performance computing clusters and scalable to thousands of processors and as such is the recommended platform if you have access to appropriate resources Hardware Requirements VAMPIRE has been successfully tested on a wide variety of x86 and power PC processors Memory requirements are generally relatively modest for most systems though larger simulations will require significantly more memory VAMPIRE is generally computationally limited and so the faster the clock speed and number of processor cores the better Binary installation Compiled binaries of the latest release version are available to download from http vampire york ac uk download for Linux Mac OS X and Windows platforms For the Linux and Mac OS X releases a simple installation script install sh installs the binary in opt vampire and appends the directory to your environment path The Windows binary is redis tributable and must simply be in the same directory as the input file however before running the code you need to install the Microsoft Visual C 2008 Redistributable se
39. stem Most magnetic materials are anisotropic that is the spins have a preferred orientation in space which arises at the atomic level due to the local crystal environment henc its full name of magnetocrystalline anisotropy In the model this is most common as the easy axis The strength of the anisotropy is determined by the anisotrop constant in our case kz where positive value prefer alignment along the z axis whi e y uniaxial anisotropy where the spins prefer to lie along a single preferred axis known y e negative values prefer alignment around the x y plane The last term describes the coupling of the spin system to an externally applied field Happ or Zeeman field The applied field is used to reverse the orientation of the spins and can be used in the simulation to calculate hysteresis loops for example Spin Dynamics The spin Hamiltonian describes the energetics of the system but says nothing about the dynamic behaviour For that the Landau Lifshitz Gilbert LLG equation is used to describe the dynamics of atomic spins The LLG is given by 98 ___ Y r aaa x Hoy AS x S x Hb 1 1 11 where S is a unit vector representing the direction of the magnetic spin moment of site i y is the gyromagnetic ratio and Hi is the net magnetic field on each spin The atomistic LLG equation describes the interaction of an atomic spin moment with an effective magnetic field which is obtained from the n
40. stribution of width create voronoi size variance times the particle size The variance must be in the range 0 0 1 0 Typical values for a realistic looking grain structure are less than 0 2 and larger values will generally lead to oblique grain shapes and a large size distribution create voronoi row offset flag default false Offsets the first row of hexagonal points to generate a different pattern e g 2 3 2 grains instead of 3 2 3 grains create voronoi random seed int Sets a different integer random seed for the voronoi seed point generation and thus produces a different random grain structure 23 create voronoi rounded grains flag default false Controls the rounding of voronoi grains to generate more realistic grain shapes The algorithm works by expanding a polygon from the centre of the grain until the total volume bounded by the edges of the grain is some fraction of the total grain area defined by create voronoi rounded grains area This generally leads to the removal of sharp edges create voronoi rounded grains area float 0 0 1 0 default 0 9 Defines the fractional grain area where the expanding polygon is constrained in the range 0 0 1 0 Values less than 1 0 will lead to truncation of the voronoi grain shapes and very small values will generally lead to circular grains A typical value is 0 9 for reasonable voronoi variance create particle centre offset shifts the origin of a particle to the centre of the nea
41. tem are given here Sample vampire input file to perform benchmark calculation for v4 0 dimensions unit cell size 3 54 A dimensions system size x 7 7 nm dimensions system size y 7 7 nm dimensions system size z 7 7 nm material file Co mat 20 H Simulation attributes sim temperature 300 0 sim time steps increment 1000 sim total time steps 10000 sim time step 1 0E 15 sim program benchmark sim integrator 11g heun output real time output temperature output magnetisation output magnetisation length screen time steps screen magnetisation length Co mat material 1 material name Co material 1 damping constant 1 0 material 1 exchange matrix 1 11 2e 21 material 1 atomic spin moment 1 72 muB material 1 uniaxial anisotropy constant 1 0e 24 material 1 material element Ag material 1 minimum height 0 0 material 1 maximum height 1 0 21 5 Input File Command Reference The input file can accept a large number of commands and this chapter gives a comprehensive list of all the options and what they do Commands are in the form category keyword value where value can be optional depending on the keyword System Generation The following commands control generation of the simulated system including dimensions crystal structures etc create full Uses the entire generated system without any truncation or consideration of the create particle size pa
42. terial starting at one Material properties are then defined as follows materialfindex keyword value unit ollowed by a carriage return so that each property is defined on a separate line The defined keywords are listed below The material file is largely free format apart rom the first line which must specify the number of materials for the simulation The material properties can be defined in any order and if omitted the default value will be used When the same property for a particular material is defined in the file the ast definition reading top to bottom will be used Comments can be added to the ile using the character which moves the file parser to the next line material num materials int 1 100 default 1 defines the number of materials to be used in the simulation and must be the first uncommented line in the file If more than n materials are defined then only the first n materials are actually used The maximum number of different materials is currently limited to 100 If using a custom unit cell hen the number of materials in the unit cell cell must match the number of materials here otherwise the code will produce an error material material name string default material n defines an identifying name for the material with a maximum length of xx characters The identifying name is only used in the output files and does not affect the running of the code material damping constant float 0 0 10 0 d
43. the unit cell dimensions unit cell size x Defines the size of the unit cell if asymmetric dimensions unit cell size x Defines the size of the unit cell if asymmetric dimensions unit cell size z Defines the size of the unit cell if asymmetric dimensions system size Defines the size of the symmetric bulk crystal dimensions system size x Defines the total size if the system along the x axis dimensions system size y Defines the total size if the system along the y axis 25 dimensions system size z Defines the total size if the system along the z axis dimensions particle size float Defines the size of particles cut from the bulk crystal dimensions particle spacing Defines the spacing between particles in particle arrays or voronoi media dimensions particle shape factor x float 0 001 1 default 1 0 Modifies the default particle shape to create elongated particles The selected particle shape is modified by changing the effective particle size in the x direction This property scales the as a fraction of the particle size along the x direction dimensions particle shape factor y float 0 001 1 default 1 0 dimensions particle shape factor z float 0 001 1 default 1 0 dimensions particle array offset x 0 10 A Translates the 2 D particle array the chosen distance along the x direction dimensions particle array offset y dimensions double macro cell size determines the macro cell size for calculation of the
44. ttings On the Advanced tab click the Environment Variables at the bottom Inthe System Variables list at the bottom scroll down to find the Path variable Select it and click Edit Go to the end of Variable value and write Mpich2path bin where Mpich2path is the path where you install your Mpich2 The put it if does not exist before So e g what have in my pc is C Program Files x86 MPICHA2 bin e Open a Command Prompt and write smpd install mpiexec remove mpiexec register where give as username the exact username for login in the windows and password your exact password for login windows smpd install Run the code To run the parallel version it is necessary to launch the code with the mpiexec command To do this first open the MSDOS prompt Navigate to the directory containing the binary and execute the following command mpiexec n number_of_processors Vampire Parallel exe replacing number_of_processors with the number of processors cores you want to use in this system e g mpiexec n 4 Vampire Parallel exe for a quad core machine Parallel version using MPICH2 2 PCs For multiple PCs in addition to a working MPICH2 implementation you must also setup the PCs shared folder and firewall as detailed below 17 Configure MPICH2 for multiple PCs e For every PC install MPICH2 as detailed in the last section e Only for the running PC Right click
45. word which defines materials which obey granular CSG operations such as particles voronoi media and particle arrays but in fill the void created This is useful for embedded nanoparticles and recording media with dilute interlayer coupling 42 material couple to phononic temperature flag default off couples the spin system of the material to the phonon temperature instead of the electron temperature in pulsed heating simulations utilising the two temperature model Typically used for rare earth elements material temperature rescaling exponent float 0 10 default 1 0 defines the exponent when rescaled temperature calculations are used The higher the exponent the flatter the magnetisation is at low temperature This parameter must be used with temperature rescaling curie temperature to have any effect material temperature rescaling curie temperature float 0 10 000 default 0 0 defines the Curie temperature of the material to which temperature rescaling is applied Example material files 43 Bibliography 44
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