Determination of the magnetocaloric properties using a PPMS
Contents
1. Application Note 1085 200 Determination of the magnetocaloric properties using a PPMS J B Monteiro R D dos Reis F G Gandra University of Campinas Physics Institute Brazil N R Dilley Quantum Design Most studies in the literature related to the magnetocaloric effect MCE make use of specific heat or magnetization measurements which lead to the entropy change through the use of the Maxwell relation A very interesting alternative presented by Plackowski et al using a small Peltier element has proven to be quite efficient to determine both the specific heat and the MCE properties Although there are more elaborate schemes the basic idea is to use the Peltier element as a heat flux sensor instead of a heat pump which allows the determination of the total heat exchanged by the sample with the heat reservoir Here we show a similar setup to be used with the Quantum Design PPMS to determine the MCE of magnetic materials exploring its versatility in terms of temperature and magnetic field control In figure 1 we show the top view of the Peltier element attached to a blank puck P101 Here the Peltier dimensions are 6x8mm and 2 3mm high and this unit has 32 thermocouple pairs Figure 1 left top view of the puck with the Peltier element mounted over a brass plate This plate also holds the heat shield in place right lateral view showing a single crystal glued with silver paint The low temperature epoxy in white was2. Gadolinium rt T 290K L fi i fi i fi i fi L 0 20 40 60 80 time min Gd disc corrected for demag T 290K Our data 4 Porcari et al Bahl and Nielsen pan foo Z Figure 7 The upper panel shows the sensor resistance corresponding to the field change shown in the lower panel The field was changed in 0 5T steps using a 190 Oe s sweep rate Figure 8 Temperature change at T 290K for a Gd disc 3 8mm diameter and 1 9mm in height with m 160mg corrected for demagnetization The literature data are shown for comparison Application Note 1085 200 Rev AO 3 26 2014 Quantum Design www qdusa com Determination of the magnetocaloric properties using a PPMS The whole process can be controlled by the PPMS using a Visual Basic programming a resource provided by the PPMS under Sequence Advanced Macros A simple routine example is presented at the end of the text Obviously the first method full magnetic field sweep is not adiabatic and some heat is lost to the thermal bath leading to a smaller temperature change The second method sequence of small field steps assures a zero entropy change but it might require several field steps and of course is time consuming But the results are quite good using just a simple hardware construction on a Resistivity puck This setup is quite convenient is capable of measuring the temperature change even at
3. 1 Open fila For Input As 1 Do While Not EOF 1 Input 1 titulo Loop Close r0 Val titulo Kill fila campo campo 5000 field incremented by 5 kOe fila E GML Data Gd DeltaT res1 dat PPMS SetField campo 190 0 0 1 WaitFor 0 5 0 Do While Abs actualfield campo gt 5 PPMS GetField actualfield estado Loop PPMS SendPpmsCommand MEASURE 70 replyStr errorStr 0 0 PPMS SequenceMeasure LOG 1 PPMS SequenceMeasure UPL 0 1 E GML Data Gd DeltaT res1 dat PPMS SequenceMeasure UPL 1 PPMS SetField campo 190 0 0 0 set persistent mode WaitFor 0 5 0 Open fila For Input As 1 Do While Not EOF 1 Input 1 titulo Loop Close 8 Application Note 1085 200 Rev AO 3 26 2014 Quantum Design www qdusa com Determination of the magnetocaloric properties using a PPMS r1 Val titulo Kill fila s 0 4874 1 1456e 3 temp DeltaT r1 r0 s temp temp DeltaT Open filaout For Append As 2 Print 2 tempini Print 2 campo Print 2 temp Close If campo lt fieldmax 200 Then PPMS SetTemperature temp 1 000000 1 set new temp 1K min WaitFor 0 5 0 PPMS SetField campo 190 0 0 0 WaitFor 3 timedel 0 GoTo inicio Else PPMS SetField 0 190 0 0 0 t1 10 fieldmax 190 WaitFor 0 t1 0 tempini tempinitsteptemp If tempini lt tempfin Then GoTo newtemp End If End Sub Sub Main Call fn_Sequence2 End Sub NOTE User variables and file names are in RED Quantum Design Application Note 1085 200 Re
4. applied just on the border of the Peltier element plate to mechanically hold it in place The thermal contact is provided by a layer of Apiezon N grease previously applied to the Peltier element lower plate AT Plackowski Y Wang and A Junod Rev Sci Instrum 73 7 2755 2002 S M Kuepferling C Sasso V Basso L Giudici IEEE Trans Magn 43 6 2764 2007 3 For example Custom Thermoelectric 03201 9G30 08RA Quantum Design Application Note 1085 200 Rev AO 1 www qdusa com 3 26 2014 Determination of the magnetocaloric properties using a PPMS The thermal contact between the Peltier element and the brass plate is made by a thin layer of Apieson N grease and a low temperature epoxy in white was applied externally to the lower Peltier plate in order to provide mechanical stability Both surfaces of the Peltier element are metallized to improve the thermal contact with the sample and with the puck and also because it is easier to clean after the sample removal although a non metalized unit will work as well alternatively pre tinned units can be soldered to the brass plate To calibrate the Peltier element we followed the same procedure described in the literature and to read the Peltier voltage with the PPMS resources we used the PPMS bridge configured as a voltmeter The heat flux sensor voltage for the empty system or addenda was recorded during a temperature sweep at 0 5K min rate from 20K up to 310K and a second
5. sweep was made with a m 822mg high purity copper sample used as a standard With this procedure we determined the system heat capacity C T and the Peltier element sensitivity A T with A S K where S is the Seebeck coefficient and K is the thermal conductance of the Peltier element In figure 2 we show the Peltier voltage for a polycrystalline sample of Gd Ge 2Si 3 with m 913mg as function of the temperature and for different magnetic fields Here we discarded the initial data until the temperature sweep rate was stable at 0 5 K min From this data we Vp T ae Csys T for each field considering that the system is not affected by a magnetic field of 2T or less For comparison obtained the heat capacity of the sample using C T purposes we also plotted the specific heat obtained with the relaxation method available with the PPMS for a small piece m 29mg of the same sample fig 2b The evident difference is due to the characteristic of the relaxation technique when measuring a phase transition especially a first order transition with hysteresis Note that QD provides a specific procedure to analyze the specific heat data in these cases and we refer the reader to slope analysis discussion in the Heat Capacity Option Users Manual 1085 150 available at www qdusa com pharos This material is known for presenting the Giant MCE so it turns out to be an interesting example of the Peltier use Taking the heat flow curves
6. 240 245 250 255 260 265 270 275 280 Temperature K Figure 2 a The Peltier voltages obtained sweeping the temperature at 0 5K min at three different field values b the specific heat at different fields the inset shows a comparison with the technique c the calculated entropy change for the Gd Ge2 2Siis sample for a 1T and 2T field change relaxation In figure 3 we show the Peltier voltage and AS obtained for DyCo for a field change of 2T This material also presents a first order transition at T 137K but with no measurable hysteresis In this case the relaxation technique and the heat flux method give quite similar results for cp The Peltier voltage also shows a peak which is the signature of a first order transition which seems to disappear with the field increase 0 0 0 5 Figure 3 The Peltier voltage 1 obtained with a 0 5K min 10 rate and entropy change for S AH 2T for DyCop The inset at E 1 5 the right shows the specific 2 heat at H 0 and H 2T S 20 obtained by the Peltier O eltier gt AS Jikg K f E m 129mg andi By the 5 25 8 senile method TD i g naaar m mg A i F yor 3 0 3 5 4 0 80 100 120 140 160 180 200 Temperature K Quantum Design Application Note 1085 200 Rev AO 3 www gdusa com 3 26 2014 Determination of the magnetocaloric properties using a PPMS The results for a Gd sample are presented in figure 4 Here the Peltier element
7. high fields and is only limited by the mechanical strain of the sample Therefore the PPMS provides a very flexible platform for experiments related to the direct determination of the MCE properties by using simple adaptations of a Peltier heat flux sensor or a specific setup with a temperature sensor to a Resistivity puck Quantum Design Application Note 1085 200 Rev AO 7 www gdusa com 3 26 2014 Determination of the magnetocaloric properties using a PPMS Sub fn_Sequence2 Dim replyStr As String Dim errorStr As String Dim titulo As String Dim R As Variant Dim Rp As Variant Dim actualfield As Double Dim estado As Long tempini 282 initial temperature steptemp 2 temperature increment tempfin 310 final temperature filaout E GML Data Gd DeltaT DeltaTxH6 dat output file name timedel 600 time delay sec between field steps fieldmax 40000 maximum field Oe newtemp PPMS GetField actualfield estado temp tempini campo 0 PPMS SetTemperature temp 1 000000 1 WaitFor 0 5 0 PPMS SetField campo 190 0 0 1 set desired field in drive mode Do While Abs actualfield campo gt 5 PPMS GetField actualfield estado Loop PPMS WaitFor 3 timedel 0 inicio fila E GML Data Gd DeltaT resO dat auxiliary filename temporary PPMS SendPpmsCommand MEASURE 70 replyStr errorStr 0 0 PPMS SequenceMeasure LOG 1 PPMS SequenceMeasure UPL 0 1 E GML Data Gd DeltaT resO dat PPMS SequenceMeasure UPL
8. ignal in detail b right the corresponding temperature change at several fields as function of the temperature This data was obtained for a parallelepiped shaped 3x3x6mm Gd sample corresponding AT for each temperature in 1K interval taken after about 10 min for stabilization The limitation of this experiment is imposed by the maximum field sweep of 190 Oe s which is not fast enough to simulate an adiabatic condition For example with a m 160 Quantum Design Application Note 1085 200 Rev AO 5 www qdusa com 3 26 2014 Determination of the magnetocaloric properties using a PPMS mg Gd sample the system has an estimated time constant around 2 min while it takes 1 75 min for the PPMS to bring the field from zero up to 2 T leading to an underestimated AT However an alternative way is to measure sequentially in steps of 0 5 T which requires 0 43 min each After each field change the PPMS temperature is set to the new temperature of the sample driven by the field and so on until the maximum field is reached With this procedure the entropy change between initial and final states is zero and it is also possible to reach high field values We obtained good results up to 4T but not limited to for a Gd sample where the total temperature change is obtained by summing AT for each field step AT 6K for AHapp 4T as seen in figures 7 and 8 lt 53 4 53 2 53 0 52 8 52 6 52 4 4 3
9. j T V T A T we can obtain the entropy for each field using Sa Yip SG aT Sy Csys T dT S T and then evaluate the entropy change AS S H S H 0 as shown in figure 2c INB thermoelectric InbS1 031 015 3 Following the procedures depicted in QD application note 1076 303 we used the bridge Port 1 and connected the Peltier element to pins 5 3 and 6 4 The command ExecCmd portcmd BR_VCNF 0 2 1 0 00 was added to the sequence configuring current zero at channel 1 e Usually we start the temperature sweep about 10K below the desired initial temperature Otherwise it will be necessary to obtain Cys for each field 8 or simply by AS iTar 2 Application Note 1085 200 Rev AO Quantum Design 3 26 2014 www qdusa com Determination of the magnetocaloric properties using a PPMS Peltier Voltage mV c J mol K 15 H 2 04 2 54 3 04 3 54 a 4 04 T T T T 220 240 260 280 300 320 Temperature K 600 3 3 gt PPMS 600 3 gt pps avg 3 t Potier lt 500 A nee Sot S Har tlg 3 ote a a 800 te 400 a Oe on Rak b Po a i 220 240 260 280 f T K 300 4 200 E 1 il 1 1 1 220 240 260 280 300 320 340 Temperature K AS mJ g K 84 aha A A A a 74 aH 1T a c A AH 2T r 6 4 a A A a 54 a p a 4 4 a ries E e A 1 ae o E 34 ae A 1 e 24 Lt 14 0 T T T T T T T T F
10. tivity puck is held in place by two screws which are also used to fix the two holders for the radiation shield not shown In figure 6 a the temperature sensor resistance was recorded as function of the temperature for a field change of 2T The correspondence between the temperature sensor resistance and temperature can also be obtained from this data In figure 6 b we plot the Y Toloukian and E Buyco Thermophysical properties of matter vol 4 pg 74 plenum 1970 1 Here we used a Lakeshore bare chip cernox 1030 with 100pA excitation fixed with Apiezon or the GE varnish The estimated time constant is of the order of 2 minutes using the same sample of the text 4 Application Note 1085 200 Rev AO Quantum Design 3 26 2014 www gdusa com Determination of the magnetocaloric properties using a PPMS Figure 5 The proposed setup for the temperature change measurement The manganin wires serve just as a support for the sample The thin glass plate under the sample is used to distribute the force caused by the field gradient avoiding deformation of the wires On top of the sample there is a Cernox sensor Two copper supports not shown are also fixed by the screws and hold the radiation shield 53 0 52 8 52 6570 915 920 934 T oc My 400 800 1200 1600 270 280 290 300 310 time min T K Figure 6 a left Resistance of the Cernox sensor recorded at different temperatures during a 2T field sweep The inset shows the s
11. v AO 9 www qdusa com 3 26 2014
12. voltage does not show a peak since this is a second order type transition The specific heat is in agreement with the relaxation results as is the entropy change with the literature data Peltier voltage mV 220 240 260 280 300 320 Temperature K Figure 4 The Peltier voltage specific heat and entropy change for Gd with m 612 7mg In the right inset the line represents the literature data Another interesting aspect on the use of the PPMS to study the MCE is to determine the temperature change that a sample experiment when the magnetic field is changed in an adiabatic process A simple setup using a PPMS Resistivity puck P102 can be used to estimate AT using the PPMS resources In figure 5 we show the setup where four O 0 12mm Manganin wires support a thin glass plate glued with GE7031 varnish The sample is also glued on the glass plate with the varnish and on top of it a temperature sensor is appropriately fixed This configuration was adopted considering that the effective thermal conductance between the sample and the puck should be very small but still enough to allow a reasonable waiting time to change the measurement temperature The glass plate is used to distribute to the wires the strong magnetic forces acting on the sample The temperature sensor uses the channel 2 terminals on the Resistivity puck The reading of the sensor resistance is made in the usual way by using the PPMS user bridge The PC board on the Resis
Download Pdf Manuals
Related Search