Influence of Co addition on the magnetocaloric effect of FeCoSiAlGaPCB amorphous alloys

Influence of Co addition on the magnetocaloric effect of FeCoSiAlGaPCB amorphous alloys V. Franco, J. M. Borrego, and A. Conde Dpto. Física de la Materia Condensada, ICMSE-CSIC, Universidad de Sevilla, P.O. Box 1065, 41080 Sevilla, Spain S. Roth Leibniz Institut für Metallische Werkstoffe, IFW Dresden, Postfach 270016, D 01171 Dresden, Germany 共Received 9 January 2006; accepted 22 February 2006; published online 28 March 2006兲 The FeCoSiAlGaPCB alloys can be prepared as bulk amorphous materials, with outstanding mechanical properties and increased electrical resistivity. These features can be beneficial for their application as a magnetic refrigerant. The influence of Co addition on the magnetic entropy change of the alloy has been studied. This compositional modification displaces the temperature of the peak entropy change closer to room temperature, but reduces the refrigerant capacity of the material. For the Co-free alloy, the peak entropy change is increased with respect to a Finemet alloy containing Mo, but its refrigerant capacity is not enhanced. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2188385兴 Magnetic refrigeration is a field of research that has gained increasing attention, as it is considered an alternative to the gas compression-expansion cycle. At temperatures close to room temperature, rare-earth-based materials are among the most relevant ones.1–4 To display a big magnetocaloric response, two requirements have to be fulfilled: the material needs to exhibit a big magnetic moment and, also, a strong temperature dependence of magnetization close to the working temperature. This second condition, related to a magnetic phase transition, can be achieved in two different ways. Either by a first-order phase transition, which produces an abrupt temperature change of the magnetic moment and, therefore, a remarkable peak in the magnetic entropy change 共⌬SM兲at the transition temperature, or by a second order phase transition, which causes a more smeared peak in ⌬SM. However, the remarkable hysteresis that appears in some materials, associated to first order phase transitions, may reduce the actual efficiency of the cooling process.5It has been pointed out, nevertheless, that in order to compare the characteristics of different materials as candidates for magnetic refrigerants, their refrigerant capacity 共RC兲in a reversible cycle, connected to the entropy absorbed by the refrigerant at the cold end of the cycle and its temperature span, should be used.6 The search for low-cost materials for high-temperature magnetic refrigeration is a field of current interest.7–12 Recently it has been shown that some soft-magnetic amorphous alloys are good candidates for this application,11 with a refrigerant capacity that is comparable to that of low-hysteretic Gd-based materials.4It has also been shown that the nanocrystallization of the alloy, although broadening the ⌬SM peak, does not improve the RC of the material. Besides the peak entropy change, RC and material cost, there are other factors that should be taken into account for a material to be efficiently applied,12 such as mechanical properties, corrosion resistance, electrical resistivity, etc. Multicomponent Fe-based bulk amorphous alloys, together with their promising magnetic properties,13,14 present outstanding mechanical properties.13,15 Pieces for application devices, with the final required shape, could be prepared by mold casting. For the Fe–共Al, Ga兲–共P, C, B, Si兲alloy series, cylinders with up to 3 mm in diameter have been obtained.13 Moreover, the high electrical resistivity associated to the noncrystalline character of the alloys is a beneficial feature for their application as refrigerant materials. However, there are few studies concerning the magnetocaloric response of bulk amorphous alloys.16 It has been shown that the Co addition to this family of alloys produces a decrease of its Curie temperature, TC am.14 Therefore, this compositional change could be a way for tuning the ⌬SMpeak temperature. In this work the magnetic entropy change of the FeCoSiAlGaPCB series is characterized, analyzing the influence of Co addition on the refrigerant capacity of the material. Amorphous ribbons, ⬃25 ␮ m thick and ⬃10 mm wide, with nominal composition 共FexCoyBzCu兲Si3Al5Ga2P10 共Table I兲were prepared by single-roller melt spinning. The amorphous character of the as-quenched alloys was checked by x-ray diffraction. Previously to the measurements, samples were stress relaxed by thermal annealing in an Ar atmosphere at 675 K for 30 min. The field dependence of magnetization was measured in a Lakeshore 7407 Vibrating Sample Magnetometer using a maximum applied field H=15 kOe with field steps of 50 Oe, for constant temperatures in the range 300–720 K in increments of 10 K. The magnetic entropy change due to the application of a magnetic field Hhas been calculated from the numerical approximation to the equation TABLE I. Composition of the studied 共FexCoyBzCu兲Si3Al5Ga2P10 alloys. Alloy A B C D E F x70 56 43 29 17 5 y01426405263 z5 6 8 9 10 12 u543210 APPLIED PHYSICS LETTERS 88, 132509 共2006兲 0003-6951/2006/88共13兲/132509/3/$23.00 © 2006 American Institute of Physics88, 132509-1 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: 150.214.230.47 On: Mon, 30 Nov 2015 13:03:39 ⌬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. Figure 1 shows the temperature dependence of the magnetic entropy change corresponding to an applied field H =15 kOe for the different studied samples. There is a remarkable correlation between the ⌬SMpeak temperatures 共Tpk兲of each sample and its corresponding TC am calculated by the “kink point” method 共Fig. 2兲. Although Tpk is shifted to temperatures closer to room temperature as the Co content in the alloy increases, the magnitude of the peak 共兩⌬SM pk兩兲 continuously decreases with increasing Co content 共Fig. 2兲. This compositional dependence of 兩⌬SM pk兩is in agreement with the continuous decrease of the magnetic moment per transition metal atom as Co content increases in this series of alloys.14 The RC of the samples has been calculated by the Wood and Potter method6共⌬SM⌬Tin Fig. 3兲, either using the optimal cycle temperatures or imposing room temperature as the temperature of the cold reservoir 共the cold and hot reservoir temperatures, Tcand Th, are plotted in Fig. 4 in both cases兲. To allow the comparison with published data for other materials, the product of the peak entropy change times the full width at half maximum of the peak 共⌬SM pk ⌬TFWHM in Fig. 3兲, and the area under the ⌬SM共T兲curves using the temperatures at half-maximum of the peak as the integration limits 共Area in Fig. 3兲have also been calculated. For the sample with the lowest Fe content, the RC for the optimal cycle could not be calculated, as the peak is too close to room temperature and Tc共optimal兲is below the available experimental range. As the Co content increases, the RC of the samples decreases. The minor differences in ⌬SM⌬Tfor the optimal cycle and for that with Tcat room temperature evidence that the best performance of these materials is achieved for a cold reservoir close to room temperature. Figure 4 shows the temperatures of the optimal cycles and those of the cycles with Tcat room temperature. While Thdisplays no big differences between both cycles, Tc共optimal兲presents a more scattered character. This is related to the fact that the low-temperature part of the ⌬SM共Fig. 1兲has a smaller temperature dependence and a minor change in the ⌬SMvalue corresponding to the selected temperatures of the cycles implies a relevant change in the associated temperature. As Co content increases, the temperature span of the cycles is reduced. Although the Co addition in the FeCoSiAlGaPCB alloy series can be used to fine tune TC am and, consequently, Tpk,it decreases both 兩⌬SM pk兩and the RC of the material. The Cofree alloy presents a RC comparable to that of a Mo-Finemet alloy with a similar Fe content,11 although 兩⌬SM pk兩and the temperature span are bigger in the present case. FIG. 1. 共Color online兲Temperature dependence of the magnetic entropy change for the different studied alloys. Lines are a guide for the eye. FIG. 2. 共Color online兲Compositional dependence of the Curie temperature of the alloys, the peak entropy change temperature, and the maximum magnetic entropy change. Lines are a guide for the eye. FIG. 3. 共Color online兲Compositional dependence of the refrigerant capacity 共RC兲calculated by three different methods 共for the Wood and Potter method, RC has been calculated for the optimal reversible refrigeration cycle and for cycles departing from room temperature兲. Lines are a guide for the eye. FIG. 4. 共Color online兲Compositional dependence of the temperatures of the cold and hot reservoirs for the optimal cycles 共solid symbols兲and for cycles departing from room temperature 共open symbols兲. Lines are a guide for the eye. 132509-2 Franco et al. Appl. Phys. Lett. 88, 132509 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: 150.214.230.47 On: Mon, 30 Nov 2015 13:03:39 This work was supported by the Spanish Government and EU-FEDER 共Project MAT 2004-04618兲and the PAI of Junta de Andalucía. 1A. M. Tishin, in Handbook of Magnetic Materials Vol. 12, edited by K. H. J. 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