Magnetic Multilayers as a Way to Increase the Magnetic Field Responsiveness of Magnetocaloric Materials

RESEARCH ARTICLE Copyright © 2012 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 12, 7432–7436, 2012 Magnetic Multilayers as a Way to Increase the Magnetic Field Responsiveness of Magnetocaloric Materials R. Caballero-Flores1, V. Franco1∗, A. Conde1, L. F. Kiss2, L. Péter2, and I. Bakonyi2 1Dpto. Física de la Materia Condensada, ICMSE-CSIC, Universidad de Sevilla, P.O. Box 1065, 41080 Sevilla, Spain 2Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 49, Hungary The magnetocaloric response of Ni–Cu based multilayers has been studied with the aim of optimizing their magnetic field dependence. In contrast to the behavior of single phase materials, whose peak magnetic entropy change follows a power law with exponents close to 0.75, multilayering leads to exponents of ∼1 for an extended temperature span close to the transition temperature. This demonstrates that nanostructuring can be a good strategy to enhance the magnetic field responsiveness of magnetocaloric materials. Keywords: Magnetocaloric Effect, Magnetic Refrigeration, Nanostructured Materials, Multilayers. 1. INTRODUCTION Solid-state magnetic refrigeration at room temperature based on the magnetocaloric effect (MCE) is a field of active research in recent years.1–3 Its energy-efficient and environment-friendly features are the prominent advantages over conventional gas compression-expansion refrigeration technology that is used nowadays.4 The MCE is the reversible temperature change (Tad) of a magnetic material due to the application of an external magnetic field change (H) under adiabatic conditions. Alternatively, the MCE can be characterized as the reversible magnetic entropy change (SM) experimented by the magnetic material due to the application of an external magnetic field Hat temperature Tunder isothermal conditions: SMT H =Hf H0 0 MT H T    H dH (1) where H =Hf−H0,0is the magnetic permeability of vacuum, and MTH is the magnetization of the material. In this work, it is considered that 0H0=0T. Along with the SMvalues, magnetic refrigerants are also characterized by the refrigerant capacity5(RC) defined as the heat transferred per unit volume of the refrigerant material from the hot (at temperature Thot ∗Author to whom correspondence should be addressed. to the cold (at temperature Tcoldreservoirs in the used thermodynamic cycle, which is associated with the width of the SMT  curve RCT H =Thot Tcold SMTHdT (2) where T =Thot −Tcold. In the literature, it is usual to determine the value of T according to the temperature values corresponding to the full width at half maximum of the SMT  curve (TFWHM), yielding in this case the definition of the refrigerant capacity denoted as RCAREA. In order to make the magnetic refrigeration technology feasible for the domestic market, there is the need to find materials with large SMand RC values in the environment of the working temperature of the refrigerator, and that these magnitudes are achieved with the application of a moderate magnetic field. According to the experimental results, it can be assumed that the field dependence of SMand RC follow power laws of the field67 SMT H =aT HnT H (3) RCAREAH =bH(4) where the exponent n, in general, depends on temperature and field. For single-phase materials, the asymptotic values of nare 1 and 2 when the values of temperature are quite 7432 J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 9 1533-4880/2012/12/7432/005 doi:10.1166/jnn.2012.6521 RESEARCH ARTICLE Caballero-Flores et al. Magnetic Multilayers as a Way to Increase the Magnetic Field Responsiveness of Magnetocaloric Materials far below and above the Curie temperature (TC), respectively. Also for single-phase materials, the minimum value of this exponent at the Curie temperature, nT =TC,is field independent and can be expressed in terms of the critical exponents as n=1−1−/+. It has been observed experimentally that for typical amorphous alloys the exponent nT =TCtakes a value of the order of 0.7 (2/3 for the mean field theory). On the other hand, the exponent that controls the field dependence of the refrigerant capacity RCAREA is related to the critical exponents as7=1+/ + (taking the value 4/3 for the mean field theory). Note that the exponents nand are related via the expression =n+1/ +). For multiphase materials, however, it has been shown that they exhibit higher values of the exponent nthan single-phase materials in the temperature range between the Curie temperatures of the constituent phases.8 The study of MCE using multilayers/superlattices has been recently presented as an alternative method to obtain tunable and improved magnetocaloric materials9with respect to the corresponding polycrystalline or amorphous systems, especially when they present second-order phase transitions.10 The goal of the present work is to study the field dependence of SMand RC in multiphase materials with a multilayer nanostructure, in order to find materials with values of the exponent nhigher than 0.7 in a wide temperature span where the MCE is maximum, i.e., in the environment of their transition temperatures. This could be used to design magnetic refrigerant materials with enhanced field responsiveness for moderate magnetic fields. 2. EXPERIMENTAL DETAILS In this work, three Ni–Cu based multilayer samples, with phases whose nominal Curie temperatures vary between 192 and 449 K,11 have been prepared by a electrodeposition technique using different repetition schemes: sample G1 (): three periods of nine layers in which the current density and the deposition times per layer have been varied; sample G2 (•): six periods of nine layers with the same current density and deposition times per layer than those for G1; and sample G3 (): three periods of nine layers with the same current density and double deposition times than those for G1 and G2. Electrodeposition was performed at 50 C under current control by using an Elektroflex 453 power source. An acetate-stabilized citrate bath was used with the following composition: nickel sulfate (from NiSO4· 6H2O) 0.3 mol/liter, nickel acetate (from Ni(CH3COO)2· 4H2O) 0.05 mol/liter, copper sulfate (from CuSO4·5H2O) 0.025 mol/liter, sodium citrate (from Na3C6H5O7·2H2O) 0.25 mol/liter, and sodium chloride (NaCl) 0.03 mol/liter. This bath was similar to those used for the preparation of Ni–Cu alloys1112 and NiCu/Cu multilayers.13–15 The application of Ni acetate made it unnecessary to adjust the solution pH.16 The concentration of the bath components was optimized to obtain a slowly increasing Ni content between 3 and 25 mA cm−2current density. In this current density range, the Ni content of the d.c. deposits changes between 72 and 82 at.%, which corresponds to the Curie temperature range of 192 to 449 K for Ni–Cu alloys. Due to the 2–3 at.% Co content of the resulting deposit, the Curie temperatures were systematically higher than those of the Ni–Cu alloys with identical Cu content. The field and temperature dependence of MTH of the typically 3 mm wide, 3 mm long and ∼2–5 m thick ribbon-shaped samples has been measured (up to a value of magnetic field 0Hf=15 T applied in the plane of the ribbons and in 10 mT increments, and from 323 K to 773 K in 10 K increments) in a vibrating sample magnetometer. 3. RESULTS AND DISCUSSION The temperature evolution of the extremal values of the derivative with respect to temperature at low field of the experimental specific magnetization data Td/dTHcan be used to determine the distribution of the Curie temperatures (TC) of the existing phases in the samples. Figure 1 indicates that the studied multiphase systems are constituted by different magnetic phases with a broad Curie temperature distribution TC∼150 K. Its determination for the samples G2 and G3 has been obtained from the inflection points of the T curves. It can be seen in Figure 1 that the Curie temperature distribution of the phases present in the sample increases as the total deposition time increases, that for the samples G2 and G3 they vary between ∼375 K and ∼525 K, and that they can be higher than the nominal Curie temperature of the monolayer with the largest TC(449 K). The magnetic entropy change SMcaused by the variation of the magnetic field has been calculated by numerical integration of the Maxwell relationship (Eq. (1)). Figure 2 shows the temperature dependence of the SMT  curves at a maximum magnetic field 0Hf=15 T for the three studied samples G1, G2 and G3. The SMT  curves have a broad peak, characteristic of multiphase systems, which emerges from the overlapping of several secondorder phase transitions occurring in the broad temperature span TC. It must be noted that in the samples studied, the SMT curves have maximum values (Spk M)ata temperature of ∼500 K, and that the table-like shape of the SMT  curves becomes more extended as the total deposition time increases, appearing this table-like shape from ∼450 K and ∼420 K for G2 and G3, respectively. It can be also seen in Figure 2 that the full temperature width at half maximum of the peak TFWHM has a value of 175 K (G1 and G3) and 225 K (G2). The reason why these nanocomposite materials exhibit broad SMT  peaks is J. Nanosci. Nanotechnol. 12, 7432–7436, 2012 7433 RESEARCH ARTICLE Magnetic Multilayers as a Way to Increase the Magnetic Field Responsiveness of Magnetocaloric Materials Caballero-Flores et al. 300 400 500 600 700 800 0 5 10 15 20 300 400 500 600 700 800 –0.10 –0.05 0.00 300 400 500 600 700 800 0 5 10 15 20 300 400 500 600 700 800 –0.10 –0.05 0.00 300 400 500 600 700 800 0 5 10 15 20 300 400 500 600 700 800 –0.10 –0.05 0.00 σ (emu g–1) G2 dσ/dT (emu g–1 K–1) T (K) G3 T (K) ΔTC G1G1 G2 G3 Fig. 1. Temperature dependence of the specific magnetization T and its derivative d/dT Hat low field (0H=10−3T) of the studied multiphase samples G1 (upper panels), G2 (central panels) and G3 (lower panels). A Curie temperature distribution of TC∼150 K can be obtained. the overlapping of the magnetocaloric responses of the existing phases. The field dependence of the different magnetocaloric responses of the samples has been studied in the whole temperature range defined by the maximum values of the magnetic entropy change Spk M, and this field dependence expressed as Spk MH ∝Hnhas been fitted according to Eq. (3). Although, in general, nis field dependent for the whole temperature range in multiphase systems, the small field dependence close to the peak lets us to assume a field independent value on nfor the fit, otherwise the logarithmic derivative nT H =dlnSM/d lnHmust be used. 300 400 500 600 700 800 0.00 0.05 0.10 0.15 |ΔSM| (J K–1 kg–1) T (K) G1 G2 G3 Fig. 2. Temperature dependence of the magnetic entropy change SM corresponding to an applied field oHf=15 T of the samples G1 (), G2 (•) and G3 (), indicating that all of them exhibit a broad temperature span TFWHM. The results as presented in Figure 3 indicate that in all the studied cases the field dependence of SMis nearly linear in the broad temperature span TCof the Curie temperatures of the existing phases. The values of the exponent nhave been indicated in brackets and this result demonstrates that nanostructured systems are good candidates to control the field dependence of the magnetocaloric response of the materials used in magnetic refrigeration at room temperature. The field and temperature evolution of the exponent nin the temperature range TCfor which the 0.0 0.5 1.0 1.5 0.00 0.05 0.10 0.15 |ΔSpk| (J·K–1·kg–1) M H (T) G1 (n=1.01) G2 (n=1.00) G2 (n=1.00) Fitted data Fig. 3. Field dependence of the maximum entropy change in the studied samples. Symbols correspond to the maximum values at a temperature of 500 K (G1), and any temperature in the ranges from 450 K to 500 K (G2) or from 420 K to 500 K (G3). Solid lines indicate the non-linear fit according to Eq. (3), corresponding to experimental data of the maximum magnetic entropy change values. 7434 J. Nanosci. Nanotechnol. 12, 7432–7436,2012 RESEARCH ARTICLE Caballero-Flores et al. Magnetic Multilayers as a Way to Increase the Magnetic Field Responsiveness of Magnetocaloric Materials 300 400 500 600 700 0.5 1.0 1.5 2.0 ΔTC,multiphase n T (K) Fe77Co5.5Ni5.5Zr7B4Cu1 G2 TC,single-phase 0.0 0.5 1.0 G1 0.0 0.5 1.0 G2 n 0.0 0.5 1.0 1.5 0.0 0.5 1.0 G3 H (T) (a) (b) Fig. 4. (a) Temperature dependence of the exponent nat a maximum magnetic field 0Hf=15 T for sample G1 (similar behavior for G2 and G3 has been found). (b) Field dependence of the exponent nin the studied samples at the temperatures of the peaks of the SMT  curves from 0H0=0Tto0Hf=15 T for samples G1 (upper panel), G2 (central panel) and G3 (lower panel). The exponent nis field independent from a value of field 0H∼025 T and takes a value close to 1. SMT  curves present maximum values, as presented in Figure 4, is another indication that the field dependence of SMis nearly linear in the temperature span defined by the peaks of the SMT  curves. Figure 4(a) shows the temperature dependence of the exponent n for sample G2 (similar behavior for the others sample G1 and G3 has been found). A comparison to the experimental data corresponding to the single-phase sample Fe77Co55Ni55Zr7B4Cu1(open symbols) indicates that while the exponent ntakes the abovementioned minimum value for single-phase system at the Curie temperature (vertical arrow and dashed line at the Curie temperature Tcsingle-phase, for multiphase systems the exponent ntakes the asymptotic value 1 in the whole ferromagnetic region (horizontal arrow, which is associated with the distribution of the Curie temperatures TCmultiphase, getting the asymptotic value 2 in the paramagnetic one. This field dependence represented by the exponent n could be used to design materials with enhanced mag- 0.0 0.5 1.0 1.5 0 10 20 30 RCAREA (J kg–1) H (T) G1 (ξ=1.12) G2 (ξ=1.02) G3 (ξ=1.04) Fitted data Fig. 5. Field dependence of the refrigerant capacity RCAREA in the studied samples G1 (), G2 (•) and G3 (). Solid lines indicate the non-linear fit according to Eq. (4) corresponding to experimental data. netocaloric behavior, showing that magnetic multilayers are a way to increase the magnetic field responsiveness of magnetocaloric materials for moderate magnetic fields. On the other hand, Figure 4(b) shows that the exponent ntakes constant values close to 1 for fields above 0H∼025 T, which is the field required for magnetic saturation. This behavior of the exponent nis contrary to that presented usually in the literature that indicates a decreasing dependence of exponent nwhen the magnetic field increases. The refrigerant capacity RCAREA of the studied samples has been obtained according to Eq. (2) and its field dependence has been fitted using Eq. (4) in the magnetic field range between 0H=025 T and 0Hf=15 T, as indicated in Figure 5. The values of the exponent have been indicated in brackets. The results indicate approximately the same dependence on the applied field. 4. CONCLUSIONS Electrodeposition proved to be a flexible technique to produce multilayered materials in which the composition and hence the Curie temperature changes from one layer to another, hence providing an unusual magnetocaloric behavior. The magnetocaloric responses of three Ni–Cu based multilayer samples have been investigated. In all the cases studied, the field dependence of SMis linear in a broad environment of the Curie temperature, demonstrating that nanostructuring is a good strategy to control the way in which magnetocaloric materials respond to the field. Although the studied samples do not present high SMvalues, all of them present a large temperature span TFWHM ∼200 K as a result of the overlapping of the magnetocaloric responses of the existing phases with a broad Curie temperature distribution TC∼150 K. In this wide Curie temperature range, the exponent ncontrolling the J. Nanosci. Nanotechnol. 12, 7432–7436, 2012 7435 RESEARCH ARTICLE Magnetic Multilayers as a Way to Increase the Magnetic Field Responsiveness of Magnetocaloric Materials Caballero-Flores et al. field dependence of the magnetic entropy change exhibits a value close to 1. Acknowledgments: The financial support of the Spanish Ministry of Science and Innovation and EU FEDER (Project MAT 2010-20537), the PAI of the Regional Government of Andalucía (Project FQM-6462), the United States Office of Naval Research (Project N00014-11-1-0311) and of the Hungarian Scientific Research Fund (OTKA grants K68612 and NN-79846) is acknowledged. R. Caballero-Flores acknowledges a research fellowship from the Regional Government of Andalucía. References and Notes 1. K. A. Gschneidner, Jr, V. K. Pecharsky, and A. O. Tsokol, Rep. Prog. Phys. 68, 1479 (2005). 2. E. Brück, J. Phys. D 38, R381 (2005). 3. A. M. Tishin and Y. I. 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