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Materials Chemistry and Physics 131 (2011) 18–22

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

aterials science communication

agneto-caloric effect in the pseudo-binary intermetallic YPrFe17 compound

ablo Álvareza, Pedro Gorriaa,∗, José L. Sánchez Llamazaresb, María J. Péreza, Victorino Francoc,arian Reiffersd, Jozef Kovácd, Inés Puente-Orenche, Jesús A. Blancoa

Departamento de Física, Universidad de Oviedo, Calvo Sotelo, s/n, 33007 Oviedo, SpainDivisión de Materiales Avanzados, Instituto Potosino de Investigación Científica y Tecnológica, Camino a la presa San José 2055, CP 78216, San Luis Potosí, MexicoDepartamento de Física de la Materia Condensada, ICMSE-CSIC, Universidad de Sevilla, P.O. Box 1065, 41080 Sevilla, SpainInstitute of Experimental Physics, Watsonova 47, SK-04001 Kosice, SlovakiaInstitute Laue Langevin, 6 rue Jules Horowitz, 38042 Grenoble, France

r t i c l e i n f o

rticle history:eceived 16 March 2011eceived in revised form0 September 2011

a b s t r a c t

We have synthesized the intermetallic YPrFe17 compound by arc-melting. X-ray and neutron powderdiffraction show that the crystal structure is rhombohedral with R3m space group (Th2Zn17-type). Theinvestigated compound exhibits a broad isothermal magnetic entropy change �SM(T) associated with theferro-to-paramagnetic phase transition (TC ≈ 290 K). The |�SM| (≈2.3 J kg−1 K−1) and the relative cooling

ccepted 27 September 2011

eywords:ntermetallic compounds

agnetic materialsrystal structure

power (≈100 J kg−1) have been calculated for applied magnetic field changes up to 1.5 T. A single mastercurve for �SM under different values of the magnetic field change can be obtained by a rescaling of thetemperature axis. The results are compared and discussed in terms of the magneto-caloric effect in theisostructural R2Fe17 (R = Y, Pr and Nd) binary intermetallic alloys.

© 2011 Elsevier B.V. All rights reserved.

hermomagnetic effects

. Introduction

Since the discovery of the magneto-caloric effect (MCE) aroundoom temperature (TC ≈ 295 K) for gadolinium [1] the research inCE has attracted huge interest from both experimental and the-

retical points of view ([2–7] and references therein). The mainotivations are the enhanced performance (efficiency, mechanical

ibration, size, etc.) and reduced environmental impact of refriger-tion systems based on the MCE effect [2,3], because most of thehort-comings of the existing vapour-compression gas technologyould be avoided [2,8]. In this way, several families of compoundsave been investigated during the last two decades with the aimf optimizing both the magnitude and the temperature range forhe MCE [5,9–11]. A needed requisite for a magnetic material inrder to display a large MCE response is to exhibit a large magneti-ation change around the magnetic phase transition temperature.rom the point of view of the implementation of these materialsn magnetic refrigeration systems, another important parameters the relative cooling power (RCP) ([12] and references therein),

hich gives a figure of merit of how much heat could be transferred

etween the hot and cold reservoirs by the magnetic refrigerant

n an ideal thermodynamic cycle. In the case of materials withecond-order phase transition, the peak value of the isothermal

∗ Corresponding author.E-mail address: pgorria@uniovi.es (P. Gorria).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.09.062

magnetic entropy change |�SPeakM | is smaller than that observed in

materials undergoing first-order magnetic phase transitions [3,4,8].Nevertheless the lack of magnetic field hysteresis and the largeroperation temperature range in materials displaying a second-order magnetic phase transition give commonly rise to higher RCPvalues [12–14].

Most of the current prototypes for room temperature mag-netic refrigeration employ rare-earth-based materials (with therare-earth being mainly Gd) [2,8]. However, some Fe-rich R2Fe17(R = rare earth) compounds have shown values of RCP compa-rable with those of Gd-based magnetic materials. In addition,these R2Fe17 alloys are of interest due to lower cost of the maincomponent (Fe), easy fabrication procedures and absence of dis-advantageous hysteresis effects [15–17]. Within the whole R2Fe17series the alloys with R = Y, Pr or Nd have the largest magneticmoment per formula unit, and therefore the higher |�SPeak

M | value,owing to the collinear ferromagnetic order with magnetic order-ing temperatures, TC, around 310 K [18]. These facts make themsuitable to be used in magnetic refrigeration as active magneticregenerative systems operating around room temperature [19].Moreover, the value of TC in this 2:17 type of compounds canbe tuned by mixing two rare earth elements (i.e. in the formR2−xR′

xFe17) [20], or by partial substitution of Fe for other 3d-atom

[21]. The crystal structure of the binary intermetallic R2Fe17 com-pounds can be either of Th2Zn17-type (rhombohedral R3m spacegroup) for light rare earths, or Th2Ni17-type (hexagonal P/63mmc

istry and Physics 131 (2011) 18–22 19

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2

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�

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Fig. 1. Observed (dots) and calculated (solid line) powder diffraction patterns forYPrFe17 alloy collected at T = 300 K. Positions of the Bragg reflections are repre-

P. Álvarez et al. / Materials Chem

pace group) for heavy rare earths [22,23]. In the case of R = Y, Gdnd Tb or in pseudo-binary intermetallic R2−xR′

xFe17 alloys with aixture of two different R atoms, both crystal structures can coex-

st, with the rare-earths sharing the same crystallographic sites24–27].

In the present work we have studied the crystal structure of aew pseudo-binary intermetallic YPrFe17 alloy by means of neutronnd X-ray powder diffraction, together with its magnetic propertiesnd the magneto-caloric effect up to a maximum applied magneticeld change of �0�H = 1.5 T. The experimental results are com-ared with those measured in binary Y2Fe17, Pr2Fe17 and Nd2Fe17.

. Experimental details and data analysis

As-cast ingots with Y2Fe17, Pr2Fe17, Nd2Fe17 and YPrFe17 nom-nal compositions were prepared from 99.99% pure elementsrelative to rare earth content in the case of Pr) by standardrc-melting technique under a controlled Ar atmosphere. The poly-rystalline as-cast pellets were sealed under vacuum in quartzmpoules and further annealed during one week at 1263 K. Afternishing the heat treatment the samples were quenched directly

n water. All the subsequent measurements were carried out onulverised samples. The crystal structure was determined at roomemperature (T = 290 K) by both X-ray (XRD) and neutron powderiffraction (ND). XRD studies were performed in a high-resolution-ray powder diffractometer (Seifert model XRD3000) operating

n Bragg–Brentano geometry. The scans in 2� were performedetween 2 and 160◦ with 0.02◦ steps and counting times of 20 ser point using Cu K� radiation (� = 1.5418 A). The ND pattern wasollected on the high-intensity D1B two-axis powder diffractome-er at the ILL (Grenoble) with a neutron wavelength of � = 2.52 A,h of acquisition time and an angular range of 80◦ in 2� (in steps of.2◦). The full-profile analysis of the diffraction patterns was carriedut with the FullProf suite package [28]. Peak broadening origi-ated from small crystals and/or microstrain effects [29] was notetected.

The low-magnetic field magnetization as a function of temper-ture M(T) curves were recorded in a Faraday susceptometer underheating rate of 2 K min−1. Isothermal magnetization curves, M(H),ere measured with a Lakeshore model 7407 VSM vibrating sam-le magnetometer in the temperature range between 90 and 450 Kith a maximum applied magnetic field of 1.5 T, and in a Quan-

um Design MPMS-5 T magnetometer in the temperature range0–390 K with applied magnetic fields up to 5 T. At each tempera-ure the magnetization was measured for a large number of selectedalues of the applied magnetic field (≈150 for the VSM measure-ents and 50 for the MPMS) with the aim of gaining accuracy in

he estimation of the isothermal magnetic entropy change, |�SM|.he value of |�SM| at each temperature T, due to a change of thepplied magnetic field from H = 0 to H = Hmax, was calculated usinghe Maxwell relation:

SM(T, H) = SM(T, H) − SM(T, O) =∫ H

0

(∂M(T †, H†)

∂T †

)T†=T

dH′

(1)

After applying this procedure to the whole set of M(H) curveshe value of |�SM| for a given applied magnetic field change and atselected temperature is obtained by numerical approximation ofq. (1), where the partial derivative is replaced by finite differencesnd then the integral is calculated by means of numerical meth-ds [10,12]. In addition, the relative cooling power (RCP) has been

alculated using three different criteria (see [12] and referencesherein for details): RCP-1(H) = |�Smax

M |(H) × ıTFWHM(H), whereTFWHM is the full width at half maximum of |�SM|(T) curve; RCP-2 isbtained by numerical integration of the area below |�SM|(T)

sented by vertical bars; the first row corresponds to the rhombohedral Th2Zn17-typephase while the second one is associated with an �-Fe impurity (<3%). Theobserved–calculated difference is depicted at the bottom of each figure.

curve using those temperatures at which |�SM | = |�SPeakM |/2 as the

integration limits; and RCP-3 is the maximum value of the prod-uct |�SM| × (Thot − Tcold) below the |�SM(T)| curve, provided that|�SM|(Thot) = |�SM|(Tcold). The first one (RCP-1), which only takesinto account the peak value and the full width at half maximum ofthe |�SM(T)| curve, is the simplest and the less accurate, althoughit is the most used for comparing different materials. The secondone (RCP-2) takes into account the shape of the |�SM(T)| curve,and gives a measure of the amount of heat that the material couldabsorb if the refrigerant thermodynamic cycle and the |�SM(T)|curve were coincident. However, for applications, the most accu-rate is the third definition (RCP-3), as it considers that most of thethermodynamic cycles operate under adiabatic processes, and alsogives the optimum working temperature range.

3. Results and discussion

In Fig. 1 the room temperature neutron (upper panel) and X-ray(bottom panel) powder diffraction patterns of the YPrFe17 sampleare shown.

Both XRD and neutron diffraction patterns have been refined byusing the Rietveld method in multi-pattern mode (see [28] for fur-ther technical details). The observed intensity peaks can be indexedas the Bragg reflections corresponding to a rhombohedral Th2Zn17-type crystal structure with R3m space group (#164), and if thehexagonal setting is chosen the lattice parameters are: a = 8.540 (1)Å and c = 12.419 (1) Å. No traces of the hexagonal Th2Ni17-type crys-

tal structure have been found, as confirmed by neutron diffractionfrom which information of the whole sample is attained, in contrastwith XRD. Whereas in other pseudo-binary alloys of the 2:17 familya disordered rhombohedral structure has been proposed in order to

20 P. Álvarez et al. / Materials Chemistry and Physics 131 (2011) 18–22

Table 1Crystallographic parameters, cell volume and atomic coordinates of the studiedYPrFe17 (R3m) compound obtained from the both NPD and XRD patterns in multi-pattern fit.

a (Å) 8.540 (1)c (Å) 12.419 (1)c/a 1.454V (Å3) 784.3 (2)Pr/Y (6c)

z 0.348 (3)Fe1 (6c)

z 0.092 (1)Fe3 (18f)

x 0.293 (1)Fe4 (18h)

x 0.169 (2)z 0.489 (1)

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tM

Fd

0

0.5

1

1.5

2

2.5

400350300250200

Pr2Fe

17YPrFe

17Y

2Fe

17Nd

2Fe

17

| ΔSM | (J kg

-1 K-1)

Temperature, T (K)

μ0∆H = 1.5 T

0

0.2

0.4

0.6

0.8

1

1.210.8

Fig. 3. Temperature dependence of the magnetic entropy change in the pseudo-binary YPrFe17 compound around the |�SM| peak for an applied magnetic fieldchange �0�H = 1.5 T. Data for binary R2Fe17 (R = Y, Pr and Nd) are also shown forcomparison. The lines connecting the calculated points are guides for the eyes. Inset:normalized �SM/�SPeak

Mvs. T/TC for the four intermetallic alloys.

RB 5.7�2 (%) 1.5

xplain the strong intensity reduction observed for many reflexions30,31], in the present case there is no such a reduction, even thoughhen compared with the diffraction patterns of the binary Y2Fe17,

r2Fe17 or Nd2Fe17 alloys [16,32]. From the fit of the diffractionatterns it is evident that the 6c site corresponding to the R atoms

s equally shared between Y and Pr with the same atomic coordi-ates. The main crystallographic information is given in Table 1.he values for the cell parameters are, as it could be expected,etween those of the Y2Fe17 and Pr2Fe17 (a = 8.46 A, c = 12.39 And a = 8.585 A; c = 12.464 A, respectively) [16,23,33]. The ratio/a ∼ 1.54 is in good agreement with those reported for the rhom-ohedral crystal structure in these 2:17-type alloys [16,32,33].

In order to estimate the Curie temperature of the sample, theagnetization vs. temperature, M(T), curve under a low appliedagnetic field �0H = 5 mT (not shown) has been measured. There-

fter, the value of TC has been taken as the minimum of the dM/dT vs.curve, which is a commonly adopted criterion [12,27,32]. In thisay, TC = 291 ± 5 K for YPrFe17, which is in between those reported

alues for Pr2Fe17 (TC = 286 ± 2 K [16]), and for Y2Fe17 (TC = 310 ± 4 K18]). Therefore, it seems that by mixing Y and Pr the Curie temper-ture of the resulting pseudo-binary alloy can be tuned betweenhose values of the pure binary compounds. This fact is due to thetrong magneto-volume coupling existing in this family of inter-etallic compounds, hence, the mixing of different R ions give rise

o slight variations in the Fe–Fe next neighbour distances along the-axis, thus modifying the value of TC [16].

Fig. 2 shows a 3D surface plot representing simultaneously the

emperature and magnetic field dependences of the magnetization,(H,T) for YPrFe17 alloy.

ig. 2. 3D surface corresponding to the temperature and applied magnetic fieldependences for the magnetization of the YPrFe17 compound.

The isothermal magnetic entropy change, |�SM| has been calcu-lated from the set of isothermal magnetization vs. applied magneticfield, M(H), curves depicted in Fig. 2 and following the procedureexplained in the previous section. In Fig. 3 the temperature depen-dence of |�SM| for the maximum applied magnetic field change,from 0 to �0Hmax = 1.5 T is shown. In addition, data for Y2Fe17,Pr2Fe17 and Nd2Fe17 binary intermetallic compounds with thesame rhombohedral Th2Zn17-type crystal structure are also shownfor comparison. The maximum value for the magnetic entropychange, |�SPeak

M |, for the YPrFe17 compound is 2.3 J kg−1 K−1, whichis just in between those values for the binary Pr2Fe17 (2.6 J K−1 kg−1)and Y2Fe17 (1.9 J K−1 kg−1) alloys. The latter can be understoodtaking into account that |�SPeak

M | is roughly proportional to themagnetization change of the alloy across the second order ferro-to paramagnetic phase transition. In the case of Pr2Fe17, the mag-netic moments of Pr and Fe sublattices are parallel to each other,hence, the contribution of Pr atoms (≈3 �B/Pr atom [34]) to thenet magnetization of the alloy is additive, while in the case ofY2Fe17, Yttrium atoms do not carry any magnetic moment, andthe net magnetization of the alloys comes exclusively from theFe sublattice. Assuming that: (i) the Fe atoms possess the samevalues for the magnetic moment in Pr2Fe17, YPrFe17 and Y2Fe17alloys; (ii) the M(T) curves show a very similar trend for thethree alloys; and (iii) the Pr3+ ions in YPrFe17 have their mag-netic moments parallel to those of Fe atoms, we could expectthat the substitution of half of the Pr3+ ions by Y ones shouldgive rise to a decrease in the magnetic entropy change, respectto that of Pr2Fe17, down to an approximate value given by:|�SM|(YPrFe17) ≈ [|�SM|(Pr2Fe17) + |�SM|(Y2Fe17)]/2.

We summarize in Table 2 the values for TC, |�SM| (peak value for�0�H = 1.5 T) together with the RCP estimated by using the threecriteria previously defined. Although Y2Fe17 exhibits the lowest|�SM| the RCP-1 is the highest due to a broader |�SM|(T) peak as itcan be observed in the inset of Fig. 3, where |�SM| vs. the reducedtemperature T/TC is plotted.

In Fig. 4 the magnetic field dependence of the RCP for YPrFe17is depicted. From a linear fit of the RCP(H) curves for �0H > 1 T, wehave extrapolated the values for an applied magnetic field changeof 2 T in order to compare with available date for Gd (see Table 2),which is the archetypical magneto-caloric material with second

order magnetic phase transitions.

P. Álvarez et al. / Materials Chemistry and Physics 131 (2011) 18–22 21

Table 2Curie temperature, TC , magnetic entropy change, |�SM|, reference temperatures Tr1

and Tr2 (see text for details) and relative cooling power, RCP, obtained from thethree methods (see text). Extrapolated values of RCP for a magnetic field change�0�H = 2 T are compared with those for Gd taken from Ref. [2].

Alloy YPrFe17 Pr2Fe17 Y2Fe17 Nd2Fe17 Gd

TC (K) 290(5) 286(2) 303(4) 339(2) 291(2)|�SM| (1.5 T) (J kg−1 K−1) 2.3 2.6 1.9 2.5 –Tr1 (K) 269 268 278 314 –Tr2 (K) 312 308 334 349 –RCP-1 (1.5 T) (J kg−1) 98 101 112 85 –RCP-2 (1.5 T) (J kg−1) 75 78 86 64 –RCP-3 (1.5 T) (J kg−1) 51 55 56 57 –

−1

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fwF

Fd

0

0.2

0.4

0.6

0.8

1

840-4-8

YPrFe17

∆SM

/∆S M

Peak

θ

0

0.2

0.4

0.6

0.8

1

3210-1-2-3θ

Fig. 5. Normalized |�SM| vs. reduced temperature for the pseudo-binary YPrFe17

RCP-1 (2 T) (J kg ) 145 156 155 133 200RCP-2 (2 T) (J kg−1) 111 116 121 98 147RCP-3 (2 T) (J kg−1) 77 81 82 81 135

The pseudo-binary YPrFe17 alloy exhibits RCP values compara-le to those of the R2Fe17 compounds with R = Y, Pr or Nd, and bothCP-1 and RCP-2 are ca. 75% of those for pure Gd [2].

Moreover, it has been proposed, based on a phenomenologicalpproach, that |�SM|(T) curves for different magnetic field changesan collapse into a single master curve, after an appropriate nor-alization, in ferromagnetic materials exhibiting a second orderagnetic phase transition [35]. The master curve is obtained in

he following way: firstly, the |�SM|(T) curves are normalized tots maximum value |�SPeak

M | for each value of the applied magneticeld change. Secondly, the temperature axis is rescaled using twoifferent reference temperatures:

= − T − TC

Tr1 − TCT < TC (2)

= T − TC

Tr2 − TCT > TC (3)

here Tr1 and Tr2 are the temperatures at which |�SM | = a �SPeak

M |, with 0 ≤ a ≤ 1. In our case we have chosen a = 0.5 (thisalue makes Tr1 and Tr2 coincident with those temperatures athich |�SM | = |�SPeak

M |/2 [36]). The master curves for the YPrFe17ompounds are shown in Fig. 5, and it can be observed how theSM/�SPeak

M vs. � curves for different magnetic field values almostollapse into a unique one for each compound.

Moreover, it can be shown that if the �SM/�SPeakM vs. � curve

or YPrFe17 compound, corresponding to �0�H = 1.5 T, is comparedith those of the binary R2Fe17 alloys (R = Y, Pr, Nd, see inset in

ig. 5), the curves almost overlap in the range −1 ≤ � ≤ 1. This

0

20

40

60

80

100

1.41.210.80.60.40.20

YPrFe17

RCP-2RCP-1

RCP-3

RC

P (J

/kg)

µ0H (T)

ig. 4. Magnetic field dependence of the relative cooling power (RCP). See text foretails.

compound. The inset shows the comparison with the curves for the binary Y2Fe17,Pr2Fe17 and Nd2Fe17 under a magnetic field change �0�H = 1.5 T.

feature means that for the binary or pseudo-binary R2Fe17 inter-metallic compounds (with R = Pr, Nd, Y and YPr) the MCE is mainlygoverned by the Fe sublattice (as Yttriun is not magnetic, in Y2Fe17only 3d–3d interactions arise), because such collapse of the curvesoccurs in the immediacy of TC (� = 0), which corresponds to thetemperature range of operation of an hypothetical refrigerationcycle (note that � = ±1 corresponds to T = Tr1 or Tr2). Discrepan-cies for |�| > 1, could be due to (i) differences in the R-Fe magneticinteractions far from the magnetic transition temperature (� < −1)favoured by the increase of the rare earth magnetic moment forT < TC, and (ii) the existence of magnetic fluctuations and/or crys-talline electric field for � > 1 (see [7] and references therein). It isworth noting that the width of the |�SM|(T) is different for thefour alloys (see Fig. 3 and Table 2). While the curves for Pr andYPr are rather similar (Tr2 − TC ≈ 20 K), that corresponding to Y2Fe17alloy is broader (Tr2 − TC = 31 K), and the associated with Nd2Fe17 ismuch sharper, so Tr2 is closer to the TC value (Tr2 − TC = 10 K). There-fore, the use of two reference temperatures allows us to properlynormalize the temperature dependence of the magnetic entropychange via a master curve. On the other hand, the use of a mastercurve representation could help in detecting and studying, from amore fundamental viewpoint, different co-existing magnetic phe-nomena [37–39].

4. Summary and conclusions

The magnetic properties and the magneto-caloric effect in thepseudo-binary YPrFe17 alloy have been studied. Room temperatureX-ray and neutron powder diffraction confirm that the compoundcrystallizes into the ordered Th2Zn17-type rhombohedral crystalstructure. The magnetic entropy change has been obtained byisothermal magnetic measurements, showing that the introduc-tion of the non-magnetic Y atoms leads to a shift of the temperaturewhere the maximum of |�SM| is obtained with a small reductionof the peak value. The calculated values for the RCP in YPrFe17 canreach 75% of the pure Gd, Therefore, we could expect that an ade-quate mixture of Pr or Nd with Y in R2Fe17 compounds allows usto tune the Curie temperature around room temperature (between285 and 340 K) with almost similar values for the RCP. Therefore,(YPrNd)2Fe17 compounds could be potential candidates for its use

in magnetic refrigeration, provided that the observed change in themagnetic entropy were accompanied by a moderate adiabatic tem-perature change. Finally, the magnetic entropy change for several

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agnetic fields can be represented using a master curve represen-ation for all these alloys.

cknowledgments

We thank Spanish MICINN and FEDER programme for financialupport through the research project MAT2008-06542-C04-03 andhe Slovak Grant Agency VEGA 2/0007/09. P.A. is grateful to FICyTor Ph.D. contract. The Slovak Research and Development Agencycontract No. VVCE-0058-07), the CLTP as the Centre of ExcellenceAS and P.J. Safárik University, the CEX Nanofluid as the Centre ofxcelence SAS, the 7.FP EU–MICROKELVIN and the SCT’s at the Uni-ersity of Oviedo (XRD measurements) are also acknowledged. Welso thank ILL and Spanish CRG-D1B for allocating neutron beamime.

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