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Intermetallics 18 (2010) 2464e2467

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Intermetallics

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Short communication

The role of boron on the magneto-caloric effect of FeZrB metallic glasses

Pablo Álvarez a, Pedro Gorria a,*, Jorge Sánchez Marcos b, Luis Fernández Barquín c, Jesús A. Blanco a

aDepartamento de Física, Universidad de Oviedo, Calvo Sotelo, s/n, 33007 Oviedo, Asturias, Spainb Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, SpaincDepartamento CITIMAC, F. Ciencias, Universidad de Cantabria, 39005 Santander, Spain

a r t i c l e i n f o

Article history:Received 26 May 2010Received in revised form20 July 2010Accepted 22 July 2010Available online 21 August 2010

Keywords:A. magnetic intermetallicsB. magnetic propertiesG. ambient-temperature usesG. magnetic applications

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

0966-9795/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.intermet.2010.07.018

a b s t r a c t

Fe-rich FeZrB metallic glasses exhibit magneto-caloric effect (MCE) around room temperature. Amor-phous ribbons of two different compositions, Fe91Zr7B2 and Fe88Zr8B4, with respective Curie temperaturevalues of 230 and 285 K have been studied. Although the maximum magnetic entropy change is rela-tively moderate (jDSM jmax w 3 J K�1 kg�1 under an applied magnetic field change from 0 to 50 kOe), theMCE spreads over a broad temperature interval (ΔT w 200 K), giving rise to a large refrigerant capacityloss (RC w 435 J kg�1) without any hysteresis. The Curie temperature can be easily tuned between 200and 350 K by changing the boron content. Therefore, the MCE can be controlled over a wide temperatureinterval, thus making these amorphous alloys promising candidates for magnetic refrigeration near roomtemperature.

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1. Introduction

The search for new materials displaying magneto-caloric effect(MCE), for applications in magnetic refrigeration, is currently anactive research field [1e5]. For applications near room temperature,a material with high or giant magnetic entropy change, jDSM j,inside a narrow temperature interval (as occurring around a first-order phase transition), is not well suited. It is needed a broadjDSM jðTÞ peak because the operational temperature differencebetween the hot and cold reservoirs in the refrigerant engineusually expands more than 50 K. The latter condition is fulfilled bysome ferromagnetic materials, which exhibit a moderate MCE overa wide temperature range associated to the second-order magneticphase transition (SOMPT) [2,6]. Well-known examples of suchmaterials are Gd, rare-earth intermetallic compounds or somemanganites [2,3,7e9]. However, moderate MCE has been recentlyobserved in disordered, nanostructured and amorphous magneticmaterials displaying SOMPT [10e15]. Although the maximum ofjDSM j is relatively low compared to that of other crystallinecompounds, the broad jDSM jðTÞ peak gives rise to large values forthe refrigerant capacity (RC) [6,13,16].

Fe-rich FeZrB metallic glasses offer several advantages asmagnetic refrigerant materials around room temperature: (i) ultra-soft magnetic properties and absence of energy losses due to

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hysteresis effects, (ii) easily modifiable temperature range for theMCE by selecting the appropriate composition, and (iii) high elec-trical resistivity (r z 130 mU cm at 295 K), good mechanicalbehaviour and corrosion resistance [17,18]. In addition, FeZrBmetallic glasses combine relatively high values for the saturationmagnetization (above 1.5 mB/Fe at.) with tunable TC values between200 and 400 K [18,19].

Fe-rich FeZr metallic glasses are ferromagnetic below roomtemperature and possess crystallization temperatures above 750 K[20]. These alloys display striking magnetic behaviours including re-entrant spin-glass, exceptional magneto-volume effects or a reduc-tion of TC with increasing Fe content, due to the strong competitionbetween FeeFemagnetic interactions [21e23]. The addition of boronup to 10%wt. gives rise to a large increase in TC (up to around 400 K),without losing complexity in the magnetic behaviour [18,22,24].Moreover, these alloys have attracted huge attention because afteradequate heat treatments a stable nanocrystalline microstructure isreached, thus displaying ultra-soft magnetic properties [25,26] andallowing its use in a number of commercial applications [27]. In thisletter we show that FeZrB amorphous alloys exhibit large RC valueswith an easy-to-control temperature range of operation between 100and 450 K by small changes in the composition.

2. Materials and experimental details

Fe91Zr7B2 (B2) and Fe88Zr8B4 (B4) amorphous ribbons, with1.5 mm� 20 mm cross sections, were fabricated by the melt-spinningmethod after preparing the master alloys (around 2 g of mass) in an

P. Álvarez et al. / Intermetallics 18 (2010) 2464e2467 2465

arc-melting furnace under Ar atmosphere. The melt spinner equip-ment was operated using a stain steel wheel (50 m/s linear speed)under a controlled Ar atmosphere. The X-ray diffraction patterns cor-responding to either ribbon faces show only broad haloes, thus con-firming a complete amorphous state and absence of crystallinity in theribbons. Isothermal magnetization vs. applied magnetic field, M(H)curves, were measured using an SQUID magnetometer in thetemperature range between 50 and 370 K. At each temperature thedata were collected under constant dc applied magnetic field steps ofH¼ 1 kOe in the magnetic field range between 0 and 50 kOe.

3. Results and discussion

In Fig. 1 the temperature evolution of the M(H) curves for bothB2 and B4 samples are depicted. The values of TC (230 � 5 and285� 5 K for B2 and B4, respectively) have been estimated from thelow-field M(T) curves, as the minimum of the dM/dT vs. T curves.

The MCE as a function of the magnetic field and the temperaturehas been obtained by integrating the adequate Maxwell relation [2]:

DSMðH; TÞ ¼ SMðH; TÞ � SMð0; TÞ ¼ZH

0

�vMvT

�HdH

where SM(H,T) and SM(0,T) are the magnetic entropy under anapplied magnetic field H and in the absence of any magnetic field,

Fig. 1. Temperature and applied magnetic field dependences of the magnetization forboth Fe91Zr7B2 (upper panel, B2) and Fe88Zr8B4 (bottom panel, B4) amorphous ribbons.

respectively, and at a fixed temperature T. Hence, the resultsobtained after applying this procedure to the whole set of M(H)curves provide the temperature and/or magnetic field dependencesof the magnetic entropy, ΔSM(T,H).

Both amorphous samples exhibit ΔSM(T,H) surfaces with broadmaxima around T¼ TC, as it can be observed in the 3D view of Fig. 2,thus indicating that these materials exhibit MCE in a widetemperature range.

The calculated maximum values for jDSM j are 2.8 and3.3 J kg�1 K�1 for B2 and B4 alloys, respectively, under a magneticfield change between 0 and 50 kOe. In order to compare the MCEbetween different materials the refrigerant capacity (RC) is a veryuseful parameter, defined as the amount of heat that can betransferred between the cold and hot reservoirs in an ideal refrig-eration cycle, at temperatures Tcold and Thot respectively [28,29].The Tcold and Thot are commonly taken as those temperatures where

Fig. 2. Temperature and magnetic field dependences of jDSM j for B2 and B4 samples.

Fig. 4. Almost linear dependence of the Curie temperature with the percentage of Featoms in Fe-rich FeZrB alloys. Data are taken from Refs. [31e33].

P. Álvarez et al. / Intermetallics 18 (2010) 2464e24672466

jDSM j is half of its maximum.We have estimated the RC values fromnumerical integration of the area under jDSM jðTÞ curve by usingTcold and Thot as the integration limits. The obtained RC values areclearly higher to those of GdSiGe compounds [30], reaching435 J kg�1 (z3.5 � 0.1 J cm�3), which is ca 90% of that corre-sponding to pure gadolinium (z4 J cm�3).

It must be pointed out that even though the maximum value ofjDSM j for the B2 sample is around 15% lower, due to its smallermagnetization (a reduction of 15% comparedwith that of B4 sampleis observed at 5 K [17]), the smoother decrease of M around TC forB2 compound results in a broader jDSM jðTÞ curve, giving rise toroughly identical RC value for both samples.

In Fig. 3 we show the jDSM jðTÞ curves for applied magnetic fieldchange of ΔH ¼ 20 and 50 kOe, in these FeZrB amorphous alloys ΔTcan reach 200 K under a magnetic field change of ΔH ¼ 50 kOe. IfΔH ¼ 20 kOe, which is a value that can be reached without usingsuperconducting magnets, the RC and ΔT values for the B2(B4)sample are 148(159) J kg�1 and 163(150) K respectively.

Moreover, the compositional dependence of the Curie temper-ature in these alloys follows an almost linear tendency witha negative slope (see Fig. 4 for TC vs. Fe content plot), being nearlyindependent of the relative amount of Zr and B. Hence, thetemperature range for the refrigeration cycle can be tuned between100 and 450 K, taking into account that the value of TC can bestraightforwardly adjusted between 200 and 350 K via smallchanges in the composition of FeZrB metallic glasses (a decrease of1 at. % in the amount of Fe approximately equals to an increase of20 K in the value of TC, see Fig. 4).

Fig. 3. Temperature dependence of jDSM j for ΔH ¼ 20 kOe (blue squares) andΔH ¼ 50 kOe (red circles) for Fe91Zr7B2 and Fe88Zr8B4 samples. The area of the dashedregions corresponds to the values of RC (see text). (For interpretation of references tocolour this figure legend the reader is referred the web version of this article).

4. Conclusions

In summary, FeZrB amorphous alloys exhibit low magneticentropy change maxima (around 3 J kg�1 K�1 for ΔH ¼ 50 kOe), butexpanded over wide temperature ranges, thus giving rise to largeRC values, reaching 435 J kg�1 (3.5 J cm�3) and without anyhysteresis loses. Moreover, the Curie temperature can be easilycontrolled by small compositional changes in the range 200e350 K.Furthermore, the possibility of combination of two or more alloysby selecting the adequate TC values could be a good option fora hypothetical near room temperature magnetic refrigerator oper-ating in multistage mode. The latter could ensure a more or lessconstant value of jDSM j between the hot and cold ends of therefrigeration cycle, which is the optimal condition for an idealEricsson refrigeration cycle. Therefore, these alloys are promisingcandidates for their potential application in magnetic refrigerationat about room temperature.

Acknowledgments

Financial support from FEDER and the Spanish MICINN throughresearch projects NAN2004-09203-C04-03 and MAT2008-06542-C04 is acknowledged. Two of us (PA & JSM) thank FICyTandMICINNfor PhD and “Juan de la Cierva” research contracts, respectively.

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