Influence of La and Ce additions on the magnetocaloric effect of Fe–B–Cr-based amorphous alloys

Influence of La and Ce additions on the magnetocaloric effect of Fe–B–Cr-based amorphous alloys J. Y. Law,1V. Franco,2and R. V. Ramanujan1,a兲 1School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 2Dpto. Física de la Materia Condensada, ICMSE-CSIC, Universidad de Sevilla, P.O. Box 1065, 41080 Sevilla, Spain 共Received 14 March 2011; accepted 14 April 2011; published online 10 May 2011兲 The magnetic entropy change 共⌬SM兲, temperature of peak ⌬SM共Tpk兲and refrigerant capacity 共RC兲 in Fe共RE兲80B12Cr8共RE=La, Ce, or Gd兲alloys were studied. Increasing La, Ce, and Gd content led to relatively constant, decrease, and increase in Tpk, respectively. Both the phenomenologically constructed universal curve for ⌬SMand field dependence power laws demonstrated that these alloys exhibited similar critical exponents at Curie temperature. With 5% Ce added to Fe80B12Cr8, Tpk could be tuned near room temperature with relatively constant peak ⌬SM.Fe 79B12Cr8La1 exhibited enhanced RC compared to Gd5Si2Ge1.9Fe0.1. The tunable Tpk and enhanced RC are needed in active magnetic regenerators. © 2011 American Institute of Physics.关doi:10.1063/1.3589353兴 Magnetic refrigeration 共MR兲offers a competitive alternative to conventional vapor compression refrigeration systems due to its high energy efficiency and environmental friendliness.1–3It employs the magnetocaloric effect 共MCE兲, which is related to the reversible temperature change in a magnetocaloric material 共MCM兲subjected to varying magnetic field under adiabatic conditions.1The Curie temperature 共TC兲of MCM plays a significant role in magnetocaloric studies because MCE peaks near TC. Gadolinium, a wellknown MCM, exhibits large MCE with a second order magnetic transition 共SOMT兲at TCnear room temperature 共RT兲. However, its high cost, poor corrosion resistance, and restricted availability necessitate the development of MCM for near RT MR. An ideal MCM should be low cost and exhibit good refrigerant capacity 共RC兲and peak magnetic entropy change 共兩⌬SM pk兩兲 with little hysteresis. Iron-based amorphous alloys fulfill such requirements. They offer low magnetic hysteresis, high electrical resistivity, enhanced corrosion resistance, good mechanical properties, and tunable TCby composition variation.4–6In addition, the elements involved are abundant and fabrication costs are reasonable, making them highly competitive. Fe80Cr8B12 amorphous alloys exhibit promising MCE near RT, Cr additions enhance the corrosion resistance of the alloy.7Gd additions to this alloy have been studied previously8with the aim of tuning the TCof the alloy. Fe79B12Cr8Gd1alloy displayed enhanced 兩⌬SM pk兩 共⬃33% larger than Fe80B12Cr8兲and RC values 关⬃29% larger than Gd5Si2Se1.9Fe0.1 共Ref. 9兲兴. These findings suggest that studies of the effect of rare earths 共REs兲additions on the MCE of Fe-based amorphous alloys would be useful. The type of RE additions to transition metals 共TMs兲, such as Fe, influences the net magnetic moment of the alloys significantly: Gd and heavy RE contribute to ferrimagnetism while light RE contribute to ferromagnetism, which generally results in a larger net magnetic moment than the former.10 The magnetism in such alloys depends on the nature of coupling between the RE and TM moments. As the 4f-5delectron exchange interaction at the RE site is ferromagnetic, the antiferromagnetic 5d共RE兲-3d共TM兲electron interactions result in ferromagnetic coupling of light RE to TM moments and vice versa for heavy RE.11 This present work shows that La, Ce, and our previously studied Gd additions to Fe–B–Cr amorphous alloys influence the temperature of ⌬SM pk 共Tpk兲 differently. Alloys with nominal composition Fe80−xB12Cr8Lax 共x=0, 1, 5, 10, and 15兲and Fe80−yB12Cr8Cey共y=2, 5, 10, and 15兲were melt spun into ribbons. The ribbons are denoted by their 共a兲La content as La1, La5, La10, and La15, and 共b兲Ce content as Ce2, Ce5, Ce10, and Ce15. The amorphicity of the ribbons was confirmed by x-ray diffraction. Magnetic properties were measured by Lakeshore 7407 vibrating sample magnetometer for magnetic fields up to 15 kOe. The magnetic entropy change 共⌬SM兲due to the variation in applied magnetic field 共H兲was determined from ⌬SM=兰0 H共 ⳵ M/ ⳵ T兲HdH. RC is calculated in two ways in this study: 共a兲RCFWHM: the product of ⌬SM pk times the full temperature width at half maximum of the peak 共RCFWHM =⌬SM pk⫻ ␦ TFWHM兲, and 共b兲RCAREA: numerical integration of the area under the ⌬SM共T兲curves, using the full temperature width at half maximum of the peak as the integration limits. The compositional dependence of 兩⌬SM pk兩,T pk, and RCFWHM are shown in the main panel of Fig. 1. For both La and Ce series, 兩⌬SM pk兩values remains relatively constant for RE content up to 5 at. % and progressively decreases for higher concentrations. The 兩⌬SM pk兩for Fe80−xRExB12Cr8共RE =La or Ce; x=1–5 at. %兲alloys compare favorably with the base Fe80B12Cr8amorphous alloy.7For higher Gd content, the decrease in 兩⌬SM pk兩for the Gd alloy series was larger than those of the La and Ce series 共Gd content ⱖ3 at. %兲. ⌬SMhas been shown to be correlated with the magnetic moment of the material in other alloy systems.8,12,13 To investigate whether such a relationship holds between 兩⌬SM pk兩 and the magnetic moment of Fe80−xB12Cr8Laxand Fe80−yB12Cr8Ceyamorphous alloys, the temperature dependence of magnetization was measured below TC. The low temperature spontaneous magnetization 共M0兲was obtained by the linear extrapolation of M共T3/2兲plots. The experimental 兩⌬SM pk兩values were plotted as a function of M0in the inset a兲Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: ⫹65 67904342. FAX: ⫹65 67909081. APPLIED PHYSICS LETTERS 98, 192503 共2011兲 0003-6951/2011/98共19兲/192503/3/$30.00 © 2011 American Institute of Physics98, 192503-1 Downloaded 31 Jan 2013 to 161.111.152.13. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions of Fig. 1,a共i兲for both alloy series as well as the previously studied Fe80−zB12Cr8Gdz共z=1–11 at. %兲alloys.8兩⌬SM pk兩increases with increasing M0, evidencing the abovementioned linear correlation between 兩⌬SM pk兩and M0. This also attributes to the compositional dependence of 兩⌬SM pk兩. As La atoms do not exhibit magnetic moment,11 the trend of 兩⌬SM pk兩decreasing with higher La concentration in La alloys reflect dilution of the magnetic coupling between the Fe moments. The M0 values for the Ce alloy series were observed to be lower than the corresponding La alloys in Fig. 1,a共ii兲. The Ce ion in Fe–Ce alloys was reported to exhibit tetravalent valence state unlike other trivalent RE ions.14 Hence, the localized 4felectron of Ce atom can be easily displaced to the 5dconduction band. This results in a further distinct hybridization of 5d states with 3dstates in Fe–Ce alloys, which can lead to lower magnetization in Fe–Ce alloys compared to other amorphous light RE-Fe alloys.15,16 For the Gd alloy series, the large magnetic moment of Gd compared to the other RE investigated in this study led to the highest 兩⌬SM pk兩value 共1 at. % Gd兲. Its reduced 兩⌬SM pk兩for increasing Gd additions is due to the antiparallel coupling between Gd and Fe moments, leading to reduced M0for higher Gd content. The temperature of the peak magnetic entropy change 共Tpk兲can be considered as Tcfor low H.17 Tpk shows little variation as La content increases, consistent with earlier reports in amorphous Fe–La binary alloys.18 The presence of Cr and B in our alloys displaces TCto higher temperatures compared to the literature.18–20 In contrast, Tpk reduces as Ce concentration increases. The Ce alloy series displays lower Tpk values compared to the La alloy series.21 The magnetic ordering temperatures for Gd and Ce alloys were the highest and lowest, respectively.22 Ce was reported to adopt the tetravalent state in Ce–Fe alloys, which can lead to reduction in the Fe–Fe distance, which in turn reduces TC,23 and thus these alloys exhibit lowest TCcompared to other light RE alloys.22 Gd has been reported to display the largest TCamong all RE,22,23 which led de Gennes to correlate this maximum with the maximal value of 4fspin in Gd.24 In addition, Tpk in the La series showed little variation with La content compared to those of Ce and Gd containing alloys. In contrast, Ce alloys show a monotonic decrease in Tpk with increasing Ce content. Unlike the La and Ce series, Tpk was observed to increase with higher Gd additions.25,26 RCFWHM reduced with increasing RE content, with the Gd alloy series displaying the largest decrease compared to the La and Ce alloy systems. For Ce15, the decreasing trend of RC shows a larger negative slope compared to that of the La15 alloy. To study this reduction in RCFWHM, the compositional dependence of full temperature width at half maximum 共 ␦ TFWHM兲of 兩⌬SM pk兩was plotted 关inset of Fig. 1共c兲兴. For RE=15 at. %: ␦ TFWHM increased for the La15 alloy while the Ce15 alloy showed a large reduced ␦ TFWHM among the Ce alloy series. For La alloy series, ␦ TFWHM did not significantly change with La concentration. On the other hand, ␦ TFWHM decreased with increasing Ce concentration. The decrease in RC for the La alloy series could be attributed to the reduction in 兩⌬SM pk兩共RC being proportional to 兩⌬SM pk兩 and ␦ TFWHM兲since 兩⌬SM pk兩generally plays a dominant role in RC maximization for SOMT materials.27 For the Ce alloy series, the reduced 兩⌬SM pk兩and ␦ TFWHM contributes to a much lower RC value, e.g., in the Ce15 alloy. For the Gd series, the large decreasing trend of RC could be attributed to the large reduction in 兩⌬SM pk兩with Gd addition. Literature reports of the properties of MCM are usually published at the maximum available magnetic field in individual laboratories. To compare different literature values, the field dependence studies of ⌬SM pk and RC should be known. The field dependence of magnetic entropy change can be expressed as ⌬SM pk⬀Hn, validated experimentally in several soft magnetic amorphous alloys and some RE-based crystalline MCM.17 The exponent nis field invariant at TCor Tpk.17 The field dependence of RC can also be represented by a power law expression: RCFWHM⬀HN. Exponents n 共and N兲 should have similar values for a given alloy series since they are controlled by the critical exponents of the alloy series. As an example, ⌬SM pk共H兲and RC共H兲for the La5 and Ce5 alloys are presented in the insets of Fig. 2. The good fits shown aid the determination of the exponents n and N. For the La alloys: n=0.79⫾0.01 and N=1.20⫾0.01; and Ce alloys: n=0.77⫾0.03 and N=1.20⫾0.01. These values are close to those obtained for Gd alloys 共n=0.75⫾0.01 and N =1.16⫾0.01兲.8Both RCFWHM and RCAREA have been predicted to scale with field with the same value of exponent,17 in good agreement with our experimental results. When 兩RCAREA兩for the La1 and La5 alloy are extrapolated to H =50 kOe, values of 386 J kg−1 and 356 J kg−1, respectively, are obtained, which are up to ⬃9% higher than the well-known MCM—Gd5Si2Ge1.9Fe0.1.9For Ce2 alloys, its 兩RCAREA兩yield 367 J kg−1 when extrapolated to 50 kOe, FIG. 1. 共Color online兲Compositional dependence of 共a兲兩⌬SM pk兩,共b兲Tpk, and 共c兲RCFWHM for the as-spun ribbons at H=11 kOe. 共1a兲inset: 共i兲spontaneous magnetization dependence of 兩⌬SM pk兩and 共ii兲compositional dependence of M0.共1c兲inset: compositional dependence of ␦ TFWHM. 192503-2 Law, Franco, and Ramanujan Appl. Phys. Lett. 98, 192503 共2011兲 Downloaded 31 Jan 2013 to 161.111.152.13. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions which shows ⬃3% improvement over Gd5Si2Ge1.9Fe0.1.9A phenomenological universal curve for ⌬SMis used to extrapolate magnetocaloric properties to magnetic fields and/or temperatures which are inaccessible in different laboratories. This aids the performance evaluation of different MCM. Its construction requires the normalization of ⌬SM共T兲with respect to their peaks:17 ⌬S⬘=⌬SM/⌬SM pk and rescaling the temperature axis ␪ using two reference temperatures 共Tr兲such that ⌬SM共Tr兲/⌬SM pk=0.6. The rescaled ⌬SM共T兲curves for La and Ce alloys are presented in the main panel of Fig. 2. The normalized ⌬SMcurves collapse nicely onto a single curve for each alloy series, confirming that they exhibit SOMT, that the universal curve is applicable to these alloys and that their critical exponents should be the same. In summary, additions of La and Ce to Fe–B–Cr amorphous alloys led to different influences on Tpk. With increasing RE content, Tpk decreases for Fe80−yB12Cr8Ceybut increases for Fe80−zB12Cr8Gdz. La additions have little effect on Tpk. For x=1 and 5 at. % in Fe80−xB12Cr8Laxand Fe80−xB12Cr8Cexamorphous alloys, Tpk can be tuned near RT with 兩⌬SM pk兩comparable to the base Fe80B12Cr8alloy. Both ⌬SM pk共H兲and RC共H兲agrees with theoretical predictions of a power law dependence, yielding exponents as 0.79 and 1.20, respectively, for the La alloy series and 0.77 and 1.20, respectively, for the Ce alloys. These are in good agreement with the previous results for the Gd alloy series. Fe79B12Cr8La1alloys exhibit enhanced RC compared to the well-known MCM Gd5Si2Ge1.9Fe0.1. This work was supported by the U.S. AOARD 共Grant No. AOARD-08-4018兲, program manager, Dr. R. 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Lett. 98, 192503 共2011兲 Downloaded 31 Jan 2013 to 161.111.152.13. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions