Miniature susceptometer User Manual - Infoscience
Contents
1. susceptome ters with a primary coil and a pair of balanced secondaries The purpose of the miniature dimensions of the susceptometer is to fit it inside a clamp type pressure cell The general structure of an AC susceptometer is first recalled Those typically use insulated copper wire wound up in a certain number of turns and on several layers forming 3 different coils a primary coil plugged to an AC voltage source creating the uniform external oscillating magnetic field in which bathe a pair of balanced secondaries Induction in the secondaries from an applied field of amplitude Hae will lead to an AC tension at the edge of those Furthermore the two secondaries are connected in an anti series circuit with the goal of cancelling most of the background signal this naturally requires the secondaries to be as identical as possible From this basic situation we can insert a sample in one of the secondaries which modifies the magnetic permeability u of the space filled by the sample This will result in an unbalance of the 2 secondaries directly linked to the magnetic permeability of the sample and thus to its magnetic susceptibility through the relation u fio 1 x A generic example of such a device is shown on Fig 1 thermometer cryostat Figure 1 Scheme of a generic AC susceptometer The building part was mostly achieved during my internship at LQM this summer 2008 It was achieved using a home made set up already in us2. Miniature susceptometer User Manual Cevey Laurent laurent cevey epfi ch December 24 2008 Contents 1 Introduction 1 1 1 P sceptomelers ng eh eS RE ee Ok Ee we bE eS eo oe ea 2 L2 Pea ck Bake Be Roe bee oe ewe Ohew eee ed 2 2 Technical specifications 2 21 Design cocada ii ee we Ee ko hee De oreo e oe Be eS 2 2 2 Geometrical characteristics 2 2 e de rda da bia bk ata 4 2 3 Calibration and background signal so o sc soco ccu ta cece go 4 24 Field configuration 22 2 saors ea eose ee ee 5 3 User instructions 5 31 Preliminary caution on manipulation lt lt s 4 4 54 25848448 ee is 5 mee General Ingtructiong a es sore ca Sae ee Ee Re He Y 5 a0 Usein SQUID ervinieenient dep he Ye OEE Yaw ee Aw REY OR 6 34 Ube im premire cel eco hha bd edwed be Sed is cabana db he a dh 7 3 5 Comment on breaking repairing the MSM 8 1 Introduction This user manual s purpose is to help the user to get familiar with the miniature suscep tometer MSM and to allow them to make use of it in the most conscious manner It also contains instructions and advice on the last tasks to be done before putting the MSM to real use Hopefully this will help and lead to the obtaining of interesting datas and results 1 1 Susceptometers As their name indicates The purpose of susceptometers is to measure the magnetic suscep tibility x of materials AC susceptometers specifically allow a direct measurement of the actual AC susceptibi
3. Such application has not yet been achieved nor proven possible Testings are still to be performed inside the pressure cell to see the influence of the environnement Indeed even though the design is such that most background signal should be cancelled measurements in SQUID environnement have shown serious perturbations 3 5 Comment on breaking repairing the MSM A repairing of the coil has already been performed after the breaking of a wire right at the surface of the coil the side part Silver paint has been used to connect two end of wires after removing the insulating coating as soldering them would most probably have given a dramatic perturbation on the signal the soldering material containing superconducting compounds Stycast has then been used to cover the silver paint and secure the mending It seems that the mending has been successful with a neat connection as comparison of results before and after the repair have shown to be similar Finally the perspective of such a surgeon work should motivate the reader to be very careful not breaking the wire connection References 1 C Pfleiderer et al Rev Sci Instrum 68 3 1997 1532 2 http www stoner leeds ac uk techniques ACsuscept htm consulted the 15 12 08
4. cessary connection Slin S2in or Slout S2ouz has until now been made far from the coils leaving the whole length of the wires available for different future set up of the MSM Technical details on the coils are given in the next section 2 2 Figure 2 shows an early image of the MSM at the construction stage Figure 2 Picture under microscope of the MSM after finishing the first mounting stage winding up the 16 layers of the coils and before pulling the inner teflon rod out The layers are visible on the edge as well as the 6 outgoing wires 2 2 Geometrical characteristics The dimensions and design characteristics of the MSM are now detailed Composing elements 50 wm Cu wire Stycast Epoxy resin GE varnish Layers and turns Ls 12 Ng 570 Lp 10 Np amp 470 and Ls2 4 Ngo amp 160 for the inner secondary the primary and the outer secondary respectively where the last layer of secondary 2 was only partially filled see the calibration part Dimensions Inner diameter d 1 mm Outer diameter D 4 mm Length L 3 mm 2 3 Calibration and background signal A critical part of the coil making is the calibration between the 2 secondaries for minimizing the background signal To this end 2 extra layers with regard to the original 3 and a half layers from the design computations were first wound up on the external secondary After 2 calibration processes the final optimized structure of the coil was set
5. e for making coils for other suscep tometers The supporting structure for building the coil side walls and the inner base rod were made out of Teflon This was the main trick to optimize the compactness of the susceptometer Indeed other susceptometers often include a permanent supporting structure on which the coils are wound up but here the idea was to maximize the available space for the sample while minimizing the inner diameter of the coils thus offering a higher sensitivity This was achieved by mounting the coils on a 1mm Teflon rod while ensuring a firm Stycast base in the first layers and by removing the supporting rod once the suscep tometer finished A concentric arrangement of the coils was chosen the secondaries sandwiching the pri mary in order to perform calibration on the outer secondary coil The coils as well as the whole device are made solid by using Stycast Epoxy resin The advantage of using Stycast is mostly its excellent strengh and adhesive properties but also the long time of curation which allows a working time of over 2h For the last layers eventually after calibration for the last layer only GE varnish has been used to fix the coil leaving possibility for future modifications on the coil The design with 3 coils stacked coils naturally gives 6 wires coming out of the MSM in total those have been kept pretty long L 10 cm but can be even tually shortened to the minimum sufficient length The ne
6. e of the general circuit for measurement with the MSM As for the samples that can be used in the MSM most crystals are suitable A powder sample has proven to be difficult to prepared for measurement in the MSM because of the small dimensions Naturally the crystal sample is to be chosen and prepared in order to fill up the largest space possible inside the MSM This will give the best sensitivity and will make it most likely to get a signal from the sample The ideal sample configuration is thus a 3 mm long crystal rod with diameter 0 9 mm Various methods can be used for cutting a crystal in the right dimension and shape 3 3 Use in SQUID environnement As preliminary testing measurements it was proposed to measure temperature dependance of certain given materials The SQUID Superconducting Quantum Interference Device apart from its own capacity to measure AC magnetic susceptibility has such a temperature controlled environnement with temperatures possibly reaching as low as Tmin 2 15 K The way of using MSM in this environnement is to attach it to a rod designed for entering the SQUID with a set of 8 connections on both ends one wooden end with free connections for soldering wires and the other with a set of wires sticking out The MSM itself is securely set in a piece of straw of common use in SQUID measurements with one end closed and the other end attached to the end of the rod More precisely a system with a hole and a slit o
7. lity 444 whereas other tools such as SQUID magnetometers need computing the gradient of the magnetisation curve M H AC susceptometers basically use 3 coils a primary and a pair of balanced secondaries the details of the structure and work ing principle will be given in the design section 2 1 Susceptometers come in different shapes designs and sizes ranging them from versatile to very specific tools 1 2 Motivations The first purpose of constructing a miniature susceptometer is to allow its use in an ex treme environnement namely under very high pressures P 30kbar A clamp pressure cell achieving those conditions has been designed and build at LQM with an inner diameter of x 5mm Other methods consist in having the whole pressure cell with sample inside a large susceptometer The advantage of the miniature susceptometer is to fit entirely inside the P cell thus potentially dramatically improving the sensitivity of the measurement One of the original goals of the miniature coil is to measure SCBO SirCu2 BO3 2 in the P cell and to observe a possible quantum phase transition At this date this measurement hasn t been achieved yet 2 Technical specifications This part contains the full description of the MSM its design construction dimensions and the characteristics of the magnetic field when plugging the MSM to a voltage source 2 1 Design The design was proposed by Ivica Zivkovic and follows those of miniaturized AC
8. n the side allow both the wires to come out and the straw to slip on the wooden end of the rod while fixing it using Capton tape Fig 4 shows the MSM in such set up ready for measurement in the SQUID Figure 4 MSM in the SQUID set up before putting it in the protecting straw A smaller straw is used for putting away the long wire connection A GGG sample rod is visible on the MSM ready for measurement Once set inside the SQUID environnement temperature control can be performed on the MultiVu SQUID control program The MSM itself is controlled simply by a lock in amplifier giving the excitation voltage and measuring the Data as described in the previous section Up to now no automatized routine has been written for gathering temperature dependance datas with the MSM which means datas have to be gathered manually with controlling the temperature and reading the associate value for Susceptibility Unfortunately comparison between measurements in the SQUID and measurements in a more free environnement namely putting the rod directly in a liquid He dewar through a slitted rubber cap have shown the existence of a strong environnement perturbation in the SQUID Still nice calibration observations are possible while taking into account that higher BG signal but typically measuring a weak paramagnet won t be possible in this environnement 3 4 Use in pressure cell The final purpose of the MSM is to be used in the pressure cell
9. ry caution on manipulation It is necessary to emphasize on the fragility of the device Naturally its very small size and specifically the use of 50 um Cu wire makes its manipulation most delicate The most fragile part is the side of the MSM where the wires stick out of the coils a connection there has already broken once see section 3 5 As basic care measures the user should always be patient while manipulating the coil and ideally shouldn t let the coil be hanging by holding the wires and lifting it up The body part held with stycast and GE varnish can be more safely touched and can be used for holding the coil with tweezers 3 2 General Instructions First we describe the circuit which is to be set up for measuring with the MSM This is mostly shown on Fig 3 A basic but important remark is now in order Generally excitation is supplied by a voltage source but what matters in our enquiry is actually a control of the current flowing through the primary coil Thus a conversion has to be made with the problem that a changing resis tance requires an adapted voltage for getting the same current This is most simply done by inserting a 1 Ohm resistance in the primary coil circuit and setting the excitation voltage in order to get the desired signal at the 1 Ohm resistance boundaries typically chosing a current J 1 mA Lock in R 1Q Ref Sine A TB CH1 CH2 in out O O oO MSM Figure 3 Schem
10. tled with 3 layers and 14 extra turns on the 4th layer carefully wound up in the middle part of the layer The background signal has been measured in liquid Na T 77 K for a value of R Uge T 77K 3 16 uV 4 with applied excitation J 1 mA v 1 kHz and where the imaginary part of the signal was neglectible the phase is 6 90 deg with respect to applied excitation due to the induction derivative relation Ugg d y dt dI dt A later measurement in liquid He T 4 2 K has given a background signal of Ugc T 4 2K 3 604 uV Ong 79 8 with J 1 mA v 990 Hz Here an out of phase dissipative part of the signal exists which is a certain deviation from the ideal case From the calibration computations in particular the approximate signal value of 5 3 uV per turn it seems difficult to lower the BG signal more Naturally the important factor is then the sample signal with BG signal ratio 2 4 Field configuration A computation with a specific program for modelling susceptometers has given a field value at the boundaries of the coils of about 853 Oes A and in the middle of about 1361 Oes A The average over the whole length using a quadratic regression then gives a value of field H 1192 Oes A The main purpose of knowing this value is to try determine the absolute scaling factor giving the relation Umsm V x SI No such relationship could yet be established 3 User instructions 3 1 Prelimina
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