The influence of Co addition on the magnetocaloric effect of Nanoperm-type amorphous alloys

The influence of Co addition on the magnetocaloric effect of Nanoperm-type amorphous alloys V. Franco, J. S. Blázquez, and A. Conde Citation: Journal of Applied Physics 100, 064307 (2006); doi: 10.1063/1.2337871 View online: http://dx.doi.org/10.1063/1.2337871 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/100/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The effect of distributed exchange parameters on magnetocaloric refrigeration capacity in amorphous and nanocomposite materials J. Appl. Phys. 111, 07A334 (2012); 10.1063/1.3679456 Influence of Co and Ni addition on the magnetocaloric effect in Fe 88 − 2 x Co x Ni x Zr 7 B 4 Cu 1 soft magnetic amorphous alloys Appl. Phys. Lett. 96, 182506 (2010); 10.1063/1.3427439 Magnetocaloric effect in Fe–Zr–B–M ( M = Mn , Cr, and Co) amorphous systems J. Appl. 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Box 1065, 41080 Sevilla, Spain 共Received 3 March 2006; accepted 21 June 2006; published online 22 September 2006兲 The effect of Co addition on the magnetocaloric effect of amorphous alloys with Nanoperm-type composition has been studied for temperatures above room temperature. Co addition produces an increase in the maximum magnetic entropy change and a shift of its associated temperature to higher temperatures. The maximum refrigerant capacity 共RC兲value obtained in this study is 82 J kg−1 for a maximum applied field H=15 kOe. This value is ⬃30% larger than that of a Mo-containing Finemet-type alloy measured under the same experimental conditions. However, the RC of the alloys, when calculated from temperatures corresponding to the half-maximum entropy change value, deteriorates with the presence of Co in the alloy. The field dependence of the magnetic entropy change has also been analyzed, showing a power dependence for all the magnetic regimes of the samples. This field dependence at the Curie temperature deviates from mean field predictions. © 2006 American Institute of Physics.关DOI: 10.1063/1.2337871兴 INTRODUCTION The magnetocaloric effect 共MCE兲, reported by Warburg in 1881,1consists in the temperature change of a magnetic material upon the application of a magnetic field. When magnetized, the entropy of the spin subsystem is decreased and, under adiabatic conditions, the transfer of energy to the lattice provokes the heating of the material. Conversely, the adiabatic demagnetization of the material causes its cooling, an effect that has been employed for reaching temperatures below 1 K since 1933.2 MCE phenomenon is not new, and there were proposals for employing it at temperatures close to room temperature since 1976.3In 1998 a prototype was efficiently tested for room-temperature magnetic refrigeration.4This long delay in the application of the MCE at high temperatures was not due to conceptual difficulties, but to technological ones, the selection of adequate materials being among them. To display a significant MCE response, a material needs to exhibit an appreciable temperature dependence of magnetization in the application temperature range. Therefore, for cooling at temperatures around 1 K, paramagnetic materials can be used as their magnetic response increases when approaching 0 K. However, their response is negligible at temperatures close to room temperature. In order to obtain materials with a noticeable temperature dependence of magnetization at higher temperatures, ferromagnetic materials can be used. When the Curie temperature of the material is approached, the ferromagnetic-paramagnetic transition causes a peak in the magnetic entropy change associated with the magnetization/demagnetization of the material. As the Curie temperature of Gd is close to room temperature 共294 K兲and its magnetic moment is large, this element was the material of choice for developing the initial roomtemperature prototypes.3,4 The efforts for finding materials which can be employed as high temperature magnetic refrigerants are not futile, because refrigeration based on MCE is energetically more efficient than that of conventional gas compression-expansion refrigerators and more environmental friendly as neither ozone-depleting nor global-warming volatile refrigerants are required. Therefore, currently there is an intensive research in this field,5–7 with two main objectives: the maximization of material properties and the reduction of material cost. In this respect, the peak entropy change 共兩⌬SM pk兩兲 has been maximized with the so-called giant MCE 共Ref. 8兲and giant inverse MCE,9while cost reduction is being investigated by using transition metal based alloys instead of rare earth based materials.10 In particular, there is a growing interest in studying the applicability of soft magnetic amorphous alloys as magnetic refrigerants11–15 due to their reduced magnetic hysteresis 共virtually negligible兲, enhanced electrical resistivity, and tunable Curie temperature. In the case of bulk amorphous alloys,16,17 outstanding mechanical properties are also exhibited. All these characteristics are beneficial for a successful application of the material. The aim of this work is to analyze MCE of soft magnetic amorphous alloys of the Nanoperm family, taking into consideration the influence of Co addition on the refrigerant capacity of the material, and to study the field dependence of the magnetic entropy change. As the magnetic moment and Curie temperature of Fe based amorphous alloys depend on Co addition, it could be expected that it will also have an influence on MCE. EXPERIMENT Amorphous ribbons 共⬃5 mm wide and 20–30 ␮ m thick兲of Fe83Zr6B10Cu1and Fe78Co5Zr6B10Cu1were obtained by melt spinning. The amorphous character of the as-quenched alloys was checked by x-ray diffraction. The field dependence of magnetization was measured in a Lakeshore 7407 vibrating sample magnetometer using a maxia兲Electronic mail: [email protected] JOURNAL OF APPLIED PHYSICS 100, 064307 共2006兲 0021-8979/2006/100共6兲/064307/4/$23.00 © 2006 American Institute of Physics100, 064307-1 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 150.214.182.116 On: Fri, 22 Jan 2016 15:32:26 mum applied field H=15 kOe with field steps of 50 Oe, for constant temperatures in the range of 300–625 K with increments of 10 K. Prior to the measurements, the stress of the samples was relaxed by preannealing them at 700 K. The MCE can be characterized by the magnetic entropy change due to the application of a magnetic field H, which can be evaluated from the processing of the temperature and field dependent magnetization curves using a numerical approximation to the equation ⌬SM= 冕 0 H 冉 ⳵ M ⳵ T 冊 H dH,共1兲 where the partial derivative is replaced by finite differences and the integration is performed numerically. In order to compare the performance of different materials, either the peak entropy change 兩⌬SM pk兩or the refrigerant capacity 共RC兲is used. The refrigeration at low temperatures requires a narrow temperature span of the refrigeration cycle, making 兩⌬SM pk兩the parameter of choice for comparing low temperature materials, while high temperature refrigeration implies a wider temperature range and, consequently, RC is employed for comparison. According to Wood and Potter,18 the RC of a reversible refrigeration cycle operating between Thand Tc共the temperatures of the hot and cold reservoirs, respectively兲is defined as RC=⌬SM⌬T, where ⌬SMis the magnetic entropy change at the hot and cold ends of the cycle and ⌬T=Th−Tc. Moreover, hysteresis losses can be taken into account when evaluating the refrigerant material by subtracting them from the computed RC,19 making the comparison between materials with different coercivities more straightforward. The optimal refrigeration cycle is that which maximizes the RC. RESULTS AND DISCUSSION Temperature dependence of the magnetic entropy change Figure 1 shows the temperature dependence of the magnetic entropy change corresponding to an applied field H =15 kOe for the two studied samples. Co addition produces an enhancement in the temperature at which the maximum entropy change takes place, in agreement with the influence of Co content on the Curie temperature of the amorphous phase 共TC am兲for these alloys.20 It is worth mentioning that, in contrast to this behavior, amorphous alloys with larger metalloid concentration exhibit a monotonous decrease of TC am with increasing Co content.21–23 In the case of the studied alloys, Co addition leads to an increase of the 兩⌬SM pk兩value, unlike the observed behavior for the FeCoSiAlGaPCB alloy series, with larger metalloid content.17 It has to be noted that an estimate of the error in 兩⌬SM pk兩is below 3%, which is below the differences observed for both samples. The explanation of this influence of the metalloid content on TC am and 兩⌬SM pk兩should be ascribed to the change in the local environment of Fe and Co atoms and its effect on the exchange interaction between atoms.23 The 兩⌬SM pk兩value of the Cocontaining Nanoperm-type alloy is ⬃50% larger than that of a Mo-containing Finemet-type alloy, with a comparable TC am, measured under the same experimental conditions,14 while the Co-free Nanoperm-type alloy increases the peak entropy change by ⬃31% with respect to the same Finemet-type alloy. Comparing with the Fe70B5C5Si3Al5Ga2P10 amorphous alloy,17 the Co-containing alloy presents a comparable value of 兩⌬SM pk兩, with the advantage of a peak temperature Tpk, which is 100 K closer to room temperature for the alloy studied in the present work. Field dependence of the magnetic entropy change The magnetic entropy change not only depends on the measuring temperature but also on the value of the maximum applied field. Figure 2 shows the combined field and temperature dependence of 兩⌬SM兩for the two studied alloys. For all temperatures, the magnetic entropy change increases with increasing maximum applied field H. The field dependence can be expressed as FIG. 1. 共Color online兲Temperature dependence of the magnetic entropy change for a maximum applied field of 15 kOe. FIG. 2. 共Color online兲Field and temperature dependences of the magnetic entropy change for Fe83Zr6B10Cu1共bottom兲and Fe78Co5Zr6B10Cu1共top兲. 064307-2 Franco, Blázquez, and Conde J. Appl. Phys. 100, 064307 共2006兲 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 150.214.182.116 On: Fri, 22 Jan 2016 15:32:26 ⌬SM⬀Hn,共2兲 where ndepends on the magnetic state of the sample. For ferromagnets above their Curie temperature, the direct integration of the Curie-Weiss law indicates that n=2.5However, for temperatures below the Curie temperature no analytical expression is available. Moreover, the intrinsic irreversible behavior of ferromagnetic materials has to be taken into account in order to derive a thermodynamical model,24 a consideration which is specially important for materials with non-negligible hysteresis 共which is not the case in the present study兲. Nevertheless, on the basis of a mean field approach, the field dependence of the magnetic entropy change at the Curie temperature has been predicted to correspond to n =2/3.25 In order to determine the field dependence of the experimental 兩⌬SM兩data for the different magnetic regions of the studied samples, a local exponent16 can be calculated as n=dln兩⌬SM兩 dln H.共3兲 Figure 3 shows the temperature dependence of the local exponents for the Co-free alloy for applied fields of 0.5, 1, and 1.5 T. Some general features of the curves, also fulfilled for the Co-containing alloy, are worth mentioning. In the ferromagnetic regime the local exponent is n⬇1. At the Curie temperature of the amorphous alloy, a decrease in nis observed. However, it does not reach 2/3, as predicted by the mean field approach, but decreases only to ⬃0.74. Above TC am the local exponent increases up to n⬇2, as predicted by the Curie-Weiss law. It should be recalled that this law is accurate for low fields and high temperatures, which explains the faster increase of nfor smaller applied fields, as the high temperature condition is achieved at lower temperatures for smaller applied fields. The final increase in the exponent is ascribed to numerical errors, as the magnetic entropy change is negligible at these temperatures. The field dependence of ⌬SMat Tpk for the Co-free alloy is also analyzed in Fig. 4 by representing ⌬SM pk versus field and fitting the curve to a power law 共equivalent results are obtained for the Co-containing alloy兲. The value obtained for the exponent 共⬃0.76兲is comparable to the minimum in Fig. 3. This value of the exponent, higher than the mean field predictions, could be due to local inhomogeneities in the amorphous alloy, causing a distribution of Curie temperatures, although the amorphous character of the alloy can also have an effect. On the other hand, discrepancies with mean field theories in the vicinity of a transition temperature can be expected. Refrigerant capacity As previously mentioned, the comparison between different materials for high temperature magnetic refrigeration should be based on the refrigerant capacity. Figure 5 shows the refrigerant capacity of the studied alloys as a function of the temperature of the cold reservoir, Tc, ranging from room temperature up to Tpk of each alloy. As Tcseparates from Tpk, the refrigerant capacity of the material increases. An optimal refrigeration cycle can be found for the Co-containing alloy in the experimental temperature range, as evidenced by a maximum in RC. The maximum RC values obtained in this study are 77 and 82 J/kg for the Co-free and Co-containing alloys, respectively, indicating a superior performance of the Co-containing alloy for refrigeration cycles with Tcabove room temperature. However, the shape of the curve for the Co-free alloy suggests a maximum RC below room temperature for this alloy. Instead of comparing the maximum obtained RC values, if we compare the RC values of refrigeration cycles with temperatures corresponding to half of 兩⌬SM pk兩共i.e., the temperature span of the cycle corresponds to the width at halfFIG. 3. 共Color online兲Temperature dependence of the local exponent determining the field dependence of the magnetic entropy change of the Fe83Zr6B10Cu1alloy for three different maximum applied fields. The lines are a guide to the eyes. FIG. 4. 共Color online兲Field dependence of the peak entropy change for the Fe83Zr6B10Cu1alloy. The line is the fitting to the data. FIG. 5. 共Color online兲Dependence of the refrigerant capacity on the temperature of the cold end of the refrigeration cycle for the Fe83Zr6B10Cu1and Fe78Co5Zr6B10Cu1alloys. 064307-3 Franco, Blázquez, and Conde J. Appl. Phys. 100, 064307 共2006兲 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 150.214.182.116 On: Fri, 22 Jan 2016 15:32:26 maximum兲, the consideration of the Co-containing alloy as the one with a better performance no longer holds. In this case RC values are 71 and 63 J/kg for the Co-free and Cocontaining alloys, respectively. This is in agreement with the previous suggestion of an optimal cycle for the Co-free alloy with a cold end below room temperature. These values are comparable to recent results on the magnetocaloric effect of Co–Cr containing Nanoperm-type alloys.15 Figure 6 presents the relationship between Thand Tcfor cycles which produce the maximum RC for a given Tc. For the Co-containing alloy, the optimal refrigeration cycle corresponds to Th⬇531 K and Tc⬇325 K, while for the Cofree alloy the maximum obtained RC corresponds to Th ⬇440 K and Tc⬇308 K. The maximum RC value measured in this work favorably compares to the previously mentioned Finemet-type and Fe70B5C5Si3Al5Ga2P10 amorphous alloys, being ⬃30% larger in the present case. CONCLUSIONS The effect of Co addition on the magnetocaloric effect of Fe83Zr6B10Cu1and Fe78Co5Zr6B10Cu1amorphous alloys has been studied. The temperature at which the maximum magnetic entropy change takes place is shifted to higher temperatures with Co addition, simultaneously increasing the value of this peak entropy change. However, the refrigerant capacity of the alloys, when calculated from the half-maximum temperatures, deteriorates with the presence of Co in the alloy. These alloys favorably compare to other amorphous materials considered as candidates for high temperature magnetic refrigeration, such as Finemet-type and FeCoSiAlGaPCB alloys. The refrigerant capacity is increased by ⬃30% for the present case. The field dependence of the magnetic entropy change has also been analyzed, showing a power dependence for all the magnetic regimes of the samples. In the ferromagnetic range the exponent is 1. In the paramagnetic regime, well above the Curie temperature, the power is 2, in agreement with the Curie-Weiss law. However, the field dependence at the Curie temperature deviates from mean field predictions. ACKNOWLEDGMENTS This work was supported by the Spanish Government and EU-FEDER 共Project No. MAT 2004-04618兲and the PAI of Junta de Andalucía. One of the authors 共J.S.B.兲is grateful to Junta de Andalucía for a research contract. 1E. Warburg, Ann. Phys. Chem. 13,141共1881兲. 2W. F. Giauque and G. P. MacDougall, Phys. Rev. 43,768共1933兲. 3G. V. Brown, J. Appl. Phys. 47, 3673 共1976兲. 4C. A. Zimm, A. Jastrab, A. Sternberg, V. K. Pecharsky, K. A. Gschneidner, Jr., M. G. Osborne, and I. E. Anderson, Adv. Cryog. Eng. 43,1759共1998兲. 5A. M. Tishin, in Handbook of Magnetic Materials, edited by K. H. J. 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Phys. 100, 064307 共2006兲 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 150.214.182.116 On: Fri, 22 Jan 2016 15:32:26