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Materials Science and Engineering B 175 (2010) 253–260

Contents lists available at ScienceDirect

Materials Science and Engineering B

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

nhanced soft magnetic properties and magnetocaloric effect in B substitutedmorphous Fe–Zr alloy ribbons

ebabrata Mishraa, Mallikarjuna Gurrama, Anvesh Reddya, A. Perumala,∗, P. Saravananb, A. Srinivasana

Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, IndiaDefence Metallurgical Research Laboratory, Hyderabad 500 058, India

r t i c l e i n f o

rticle history:eceived 5 May 2010eceived in revised form 20 July 2010ccepted 22 July 2010

ACS:8.55.Qr1.60.+a5.60.−d5.30.Gw

a b s t r a c t

Magnetic properties and magnetocaloric effect in amorphous Fe89−xBxZr11 (x = 0–10) alloys prepared bymelt spinning technique have been investigated. The crystallization temperature, Curie temperature(TC), and room temperature saturation magnetization (MS) of the alloys increase almost linearly with Baddition. Coercivity decreases from 13 Oe to 0.015 Oe with the addition of 5 at.% B and then increasesto 0.063 Oe for the alloy with 10 at.% B. Low temperature thermomagnetization curves of Fe89Zr11 alloyshow a bifurcation between field-cooled and zero-field-cooled curves below 35 K. No such bifurcation wasobserved in the alloys containing B. High temperature thermomagnetic curves indicate an amorphous-to-crystalline transition above 800 K corresponding to the precipitation of �-Fe phase. These studies revealthat addition of B not only enhances the crystallization temperature, but also decreases the degree of

eywords:morphous alloysagnetic domain structure

urie temperatureagnetic inhomogeneityagnetocaloric effect

competition between ferromagnetic and antiferromagnetic interactions and there by enhances the softmagnetic nature of the alloys. Magnetic domain structure shows large-sized domains with smooth andrelatively straight domain walls for the Fe84B5Zr11 alloy. The magnetic domain structure analysis is usedto understand the origin of the ultralow coercivity in these alloys. Due to the enhanced soft magneticproperties of these alloys and the possibility of tuning TC and MS by B addition, change in magneticentropy (�SM) was also investigated. �SM showed enhancement from 1.3 J/(kg-K) for the Fe89Zr11 alloy

79B10

to 1.73 J/(kg-K) for the Fe

. Introduction

The study of magnetic properties of Fe-rich amorphous andanocrystalline alloys has received considerable attention in the

ast two decades [1,2]. Amorphous (a-) Fe-rich Fe-M (M = Zr, Hf)ased alloys exhibit unusual properties such as double transitionehavior [3,4], invar effect [5], electrical resistivity anomalies [6],nd magnetoelastic behavior [7]. Hence, these alloys have been aertile ground for the exploration of novel magnetism. On the otherand, Fe-rich a-Fe100−xBx (11 ≤ x ≤ 25) alloys are known to be goodoft magnetic materials [8,9]. The Curie temperature (TC) of theselloys increases up to x = 25 at.% and decreases drastically with fur-her addition of B [8]. The saturation magnetization varies just in thepposite way to the TC variation with B composition [9]. Also, the

hermal stability of the amorphous phase is relatively low, whichs further aggravated with an increase in Fe content.

Substitution of Fe by other transition metal or metalloid ele-ents in a-Fe–Zr alloys brings out drastic changes in their magnetic

∗ Corresponding author. Tel.: +91 361 2582714; fax: +91 361 2582749.E-mail addresses: perumal@iitg.ernet.in, alagarsamy.perumal@gmail.com

A. Perumal).

921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2010.07.038

Zr11 alloy for a field change of 0–18 kOe.© 2010 Elsevier B.V. All rights reserved.

behavior [10,11]. In this regard, Fe–B–Zr based amorphous andnanocrystalline alloys with different B content have been exten-sively studied because of their excellent soft magnetic properties inthe nanocrystalline state [12,13], and complex magnetic exchangeinteraction in the amorphous phase [14,15]. However, it is to benoted that most of the Fe–B–Zr alloys investigated earlier containlower Zr content (≤7 at.%). Recently, Huang et al. [16] reported thatthe crystallization stages of the a-Fe80B8Zr12 alloy under controlledheating were different from those observed for Fe–B–Zr alloys withlow Zr content [17]. This suggests that the crystallization kineticsand magnetic properties of Fe–B–Zr alloys are strongly depen-dent on the relative amount of both Zr and B contents. Therefore,correlating the magnetic properties of the amorphous phase withmagnetic domain structures is important. Also, soft magnetic amor-phous alloys have recently received a lot of attention as refrigerantmaterials due to their low cost, low energy loss, higher electri-cal resistivity and tunable TC [18–21]. Soft magnetic alloy such asFINEMET alloys investigated earlier show a maximum change of

�Sm = 1.1 J/(kg-K) for a field change of 0–15 kOe [18]. �Sm value ofFe–B–Zr based alloy having low Zr concentration (Zr = 6 at.%) hasbeen found out to be 1.4 J/(kg-K) for 0–15 kOe field change [19]. Itis well known that the addition of B decreases the melting tem-perature and diffusivity of the amorphous alloys at certain B/M

2 and Engineering B 175 (2010) 253–260

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2

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ple. The variation of crystallization temperature (on-set Tcrys) withB addition is shown in Fig. 2. It can be observed from the figurethat Tcrys increases with increasing B addition. This indicates thatthe thermal stability of the amorphous phase increases in Fe–B–Zr

54 D. Mishra et al. / Materials Science

tomic ratios, improves the glass forming ability and thermal stabil-ty of the alloys and in general influences their magnetic properties22]. The existing reports on a-Fe–B–Zr alloys are limited to thenderstanding of the crystallization behavior and optimization oficrostructure of a limited number of compositions of these alloys.Therefore, it is necessary to understand the effect of system-

tic B addition on the (i) magnetic exchange interactions, (ii)hermomagnetic properties, (iii) magnetic domain structures, andiv) magnetocaloric effect (MCE) of a-Fe–B–Zr alloys. This workresents systematic investigations carried out on the temperatureependent magnetic properties and room temperature magneticomain structures of melt-spun Fe89−xBxZr11 (0 ≤ x ≤ 10) alloysith high Zr content to understand the influence of the metalloid

B) content on the magnetic properties, phase transitions and MCEf Fe–B–Zr alloys.

. Experimental details

Alloys with nominal compositions of Fe89−xBxZr11 (x = 0, 2.5,, 7.5, and 10) were prepared by melting appropriate amountsf high purity Fe, Zr, and B in an arc melting furnace underrgon atmosphere. Rapidly solidified ribbons of 2–3 mm width and0–30 �m thickness were produced in controlled argon gas atmo-phere using a vacuum melt spinner. The speed of the copperheel of the melt spinner was optimized at 35 m/s. The amor-hous nature of the as-made ribbons was confirmed by powder-ray diffraction (XRD, Rigaku RINT 2000) using Cu K� radiation.he microstructure and magnetic domain structure of metallo-raphically prepared ribbons were characterized by transmissionlectron microscope (TEM, FEI Tecnai G2 F30 Lorentz). The mag-etic domain structure was observed using Lorentz microscopy inresnel mode. The specimen was tilted to 20◦ angle to the verticaln order to induce an in-plane field component. Thermal stabilitynd crystallization behavior of the alloys were determined usingDifferential Scanning Calorimeter (DSC, Perkin-Elmer DSC 7).

SC curves were recorded in the heating cycle under a constanteating rate of 20 K/min. Room temperature magnetic hystere-is (M–H) loops and temperature dependent thermomagnetizationata were obtained using Vibrating Sample Magnetometer (VSM,akeshore 7410) between 20 K and 1100 K, while the low tempera-ure thermomagnetization curves were obtained between 5 K and00 K using superconducting quantum interference device basedagnetometer (SQUID, MPMS XL 7). Temperature dependent mag-

etization measurements were carried out under different externalpplied magnetic fields (Happ). Curie temperature of the samplesas determined from the minimum in the derivative of the ther-omagnetization data measured at an applied field of 100 Oe.

oom temperature coercivity of the samples was measured usingoercimeter (Forster Koerzimat, Model 1.095).

. Results and discussion

.1. Structural and thermal properties

Fig. 1(a) shows the XRD patterns of the a-Fe89−xBxZr110 ≤ x ≤ 10) alloys. All the samples exhibit amorphous structureith two broad humps near 2� = 43◦ and 78◦. High-resolution TEM

HRTEM) micrograph of Fe84B5Zr11 sample depicted in Fig. 1(b)eveals the characteristic amorphous microstructure devoided ofny local lattice fringes. Also, the selected area electron diffrac-

ion (SAED) pattern shows a halo diffraction ring corresponding tohe amorphous structure. Similar features were also observed inll other samples studied. Thermal stability of the Fe–B–Zr alloysas investigated by analyzing the DSC curves recorded in the heat-

ng cycle. The DSC curve of each sample recorded up to 915 K (as

Fig. 1. (a) XRD patterns of a-Fe89−xBxZr11 alloys and (b) HRTEM micrograph andSAED pattern of a-Fe84B5Zr11 alloy.

shown as inset in Fig. 2 for x = 5 and 10) revealed one exothermicpeak corresponding to the crystallization of the amorphous sam-

Fig. 2. Variations of crystallization temperature (on-set Tcrys) with B content ina-Fe89−xBxZr11 alloy ribbons. Inset shows DSC curves for a-Fe89−xBxZr11 alloys withx = 5 and 10 obtained in the heating cycle under a constant heating rate of 20 K/min.

D. Mishra et al. / Materials Science and E

Fobv

awuirtdIaalr(t

3

c(1

temperature. The Fe89Zr11 sample shows almost zero magnetiza-tion at 300 K due to the paramagnetic nature of the sample. Withincreasing temperature, the flat region with almost zero magne-tization can be observed up to 520 K followed by a slow increase

ig. 3. Temperature dependent ZFC (unfilled symbols) and FC (filled symbols) curvesbtained at different applied fields [Happ = 100 Oe (a,c,e,g,i) and 1000 Oe (b,d,f,h,j)]etween 5 K and 300 K for a-Fe89−xBxZr11 alloy ribbons. Inset shows an expandediew of Fig.3(a) at temperatures below 65 K.

lloys due to the enhancement in the energy of glass formationith increasing B addition [23–25]. The observed results can also benderstood on the basis of a thermodynamical model [26]. Accord-

ng to this model, the thermal stability of the amorphous phase iselated to the glass transition temperature (Tg) and the amorphiza-ion stabilization parameter (˛), both of which are compositionependent according to the Redlich-Kister-Muggianu formalism.

t follows that the addition of B in Fe–Zr alloy enhances both Tg

nd ˛ [27]. Also, the observed results in the present investigationgree with the behavior observed upon B addition in other simi-ar Fe based alloy systems [16,28–30]. A comparison of the presentesults on Fe89−xBxZr11 (0 ≤ x ≤ 10) alloys with that of Fe93−xBxZr70 ≤ x ≤ 10) [31] shows that Zr stabilizes the amorphous phase andends to increase Tcrys when Zr content is high [16].

.2. Magnetic properties

Temperature dependent zero-field-cooled (ZFC) and field-ooled (FC) magnetization curves were obtained for Fe89−xBxZr110 ≤ x ≤ 10) at two different applied fields (Happ = 100 Oe and000 Oe) and the same are depicted in Fig. 3. In the temperature

ngineering B 175 (2010) 253–260 255

dependent magnetization measurements, the sample was initiallycooled to 4.2 K under zero-field condition. Magnetization was thenrecorded as the sample was heated to room temperature undera constant applied field (ZFC process). Subsequently, the samplewas cooled under the same constant applied field to 4.2 K andthen the magnetization was recorded as the sample was heated toroom temperature (FC process). The thermomagnetization curvesshow the following features: (i) for the Fe89Zr11 sample, a suddenincrease in magnetization around 260 K corresponding to a transi-tion from paramagnetic to ferromagnetic phase (TC) was observed.On cooling this sample further, one more transition was observedaround 35 K, from where the ZFC and FC curves start to bifurcate(inset of Fig. 3(a)). However, when the magnitude of the appliedfield is increased, the bifurcation point disappeared and at thesame time, the transition observed around 260 K gets broadened(Fig. 3(b)). (ii) For the Fe86.5B2.5Zr11 alloy, the low temperaturetransition was not observed even at 100 Oe and the transition atthe higher temperature region shifts toward room temperature.(iii) TC and room temperature magnetization increase as the B con-tent is increased in the alloys. These results are in good agreementwith the previously reported values obtained in other Fe–Zr basedalloy [32]. It is to be noted that no bifurcation between ZFC and FCwas observed in the present investigations for the alloys with x > 0.However, the bifurcation between ZFC and FC has been reported inFe89B4Zr7 [32] alloy measured at 100 Oe applied field. This can beattributed to the fact that the addition of Zr in Fe-rich Fe–Zr systemenhances the ferromagnetic interaction in the alloys [11], which inturn reduces the spin frustration leading to the shift of the bifur-cation between ZFC and FC to lower temperatures. The higher Zrcontent in the present set of alloys has enhanced the ferromagneticinteraction and suppressed the bifurcation phenomenon.

To determine the TC and to understand the magnetic propertiesat high temperature, thermomagnetic measurements were per-formed on Fe89−xBxZr11 alloys under an applied field of 100 Oewhile heating from room temperature and the data obtained aredepicted in Fig. 4. The heating rates used in the measurementsare 2, 3, and 5 K/min in the temperature ranges of 300–593 K,594–773 K and 774–1150 K, respectively. The inset of Fig. 4 displaysthe thermal derivative of the magnetization data as a function of

Fig. 4. Temperature dependent magnetization curves obtained under the appliedfield of 100 Oe between 300 K and 1100 K in the heating process for a-Fe89−xBxZr11

alloy ribbons.

256 D. Mishra et al. / Materials Science and E

Fcis

irttancpcptm�stdsssatcBTitFc

T

wia

t

ig. 5. Variations of (a) Curie temperature, (b) saturation magnetization and (c)oercivity with B substitution in a-Fe89−xBxZr11 alloy ribbons. The filled symbolsn the figure represent the data obtained in the present work, while the unfilledymbols refer to the data taken from the literature (Refs. [11,15,34]).

n magnetization up to 788 K. This can be attributed either to theelease of the stress accumulated in the amorphous ribbons duringhe quenching process or to the initiation of nucleation of �-Fe crys-allites. However, the annealing studies revealed that the a-Fe–B–Zrlloy ribbons annealed up to 673 K for 1 h are still amorphous inature [33]. Hence, the continuous increase in the magnetizationan be attributed to the release of accumulated stress in the amor-hous ribbons. The marked increase in magnetization around 800 Korresponds to the amorphous-to-crystalline transition with therecipitation of �-Fe phase. On heating the sample beyond 935 K,he magnetization decreases to zero at about 1029 K. This drop in

agnetization is associated with the TC of �-Fe phase. TC of pure-Fe is 1043 K and the lower TC observed in the present studieshows that the precipitates are not pure �-Fe. On the other hand,he room temperature magnetization of the Fe86.5B2.5Zr11 sampleecreases from 5 emu/g to zero upon heating and remains con-tant up to 550 K. On further heating, the magnetization increaseslowly up to 840 K and in contrast to the Fe89Zr11, exhibits two-harp step-like increase around 860 K and 930 K, respectively. Afterttaining a maximum at about 980 K, the magnetization decreaseso zero around 1030 K. These results show that the amorphous-to-rystalline transition shifts to higher temperature with increasingcontent, which is in good agreement with the Tcrys data (Fig. 2).

he high temperature magnetic behavior of all the alloys contain-ng B was found to be similar. TC of Fe89−xBxZr11 alloys obtained inhe present work is shown along with the variations of TC of othere–B–Zr based alloys [11,15] in Fig. 5. The variation of TC with Bontent can be approximately expressed by the linear relation,

C (K) = 266.3 + 10.2xB (1)

here xB is the B concentration on at.%. TC of the alloys investigatedn this work is higher than those reported for other Fe–B–Zr basedlloys.

To understand the evolution of soft magnetic properties at roomemperature, the variations of MS and coercivity (HC) with B content

ngineering B 175 (2010) 253–260

in a-Fe89−xBxZr11 alloys were analyzed. Fig. 5(b) and (c) depict thevariations of MS and HC with B addition. The value of MS increasesto 37 emu/g for the Fe86.5B2.5Zr11 alloy and eventually reaches amaximum of 105 emu/g in Fe79B10Zr11 alloy. HC decreases enor-mously from 13 Oe to 0.015 Oe with the addition of 5 at.% B andincreases slightly to 0.063 Oe for the alloy with x = 10. This could beascribed to the (i) variation in the magnetostriction of a-Fe–B–Zralloys with B addition [34], since HC is directly proportional to themagnetostriction [35] and (ii) local magnetic inhomogeneities (fre-quently referred as clusters [36,37]) in the amorphous alloys withthe addition of B. Slawska-Waniewska et al. [34] reported that themagnetostriction of a-Fe–B–Zr alloys decreases largely with theaddition of up to 8 at.% B and thereafter increases slightly with fur-ther increase in B. According to the micromagnetic theory [35],HC is directly proportional to the product of the magnetostric-tion and square root of the domain wall width. In addition, themagnetic inhomogeneities in the amorphous alloys can result insensible intrinsic pinning of domain walls. Thus, the reductions ofmagnetostriction and magnetic inhomogenities with B addition arebelieved to be responsible for the excellent soft magnetic softnessin a-Fe–B–Zr alloys.

On the other hand, the values of MS increase linearly at a rateof 8.7 emu/g per at.% B. To understand the relative effects of B andZr on Fe–B–Zr alloys, the variations of TC and MS with B contentin the currently investigated alloys are compared with the valuesreported in the literature for similar alloys [15,34]. It is observedthat the rate of increase in TC increases from 10.2 K per at.% B forthe a-Fe89−xBxZr11 sample with 11 at.% Zr to 18 K per at.% B forthe a-(Fe92Zr8)100−xBx sample with 8 at.% Zr content. But, the rateof increase in MS decreases from 8.7 emu/g per at.% B for the a-Fe89−xBxZr11 sample with 11 at.% Zr to 5.5 emu/g per at.% B for thea-Fe93−xBxZr7 sample with 7 at.% Zr content. This suggests that thevariations of TC and MS in Fe-rich Fe–B–Zr alloys mainly dependon the relative amounts of Zr and B on the stabilization of localmagnetic structure. It is worthy to point out that this is the firstreport to show enhanced magnetic properties (ultralow HC andhigh MS) in Fe89−xBxZr11 ribbons in as-made condition without anyheat-treatment [1,38].

To study the effect of local magnetic inhomogeneities ina-Fe–B–Zr alloys, temperature dependent HC and MS measure-ments were carried out from 20 K to temperatures just above TC.Fig. 6 shows the M–H loops of a-Fe89−xBxZr11 (x = 0, 2.5, 5, and 10)alloys recorded at different temperatures. M–H curves of a-Fe89Zr11sample obtained at room temperature exhibit predominant para-magnetic nature. However, the room temperature M–H curves of Bsubstituted a-Fe–B–Zr samples show strong non-linear variation ofmagnetization indicating the dominance of ferromagnetic nature inthese samples. The extracted values of MS(T) of different samplesare shown in Fig. 7(a). It may be noted that the magnetization valuemeasured at 15 kOe has been taken as the saturation magnetization.MS of a-Fe89Zr11 sample increases with decreasing temperature andalmost attains saturation below 50 K. On the other hand, MS val-ues not only increase with decreasing temperature for a particularsample, but also increase with increasing B content in a-Fe–B–Zralloys. A maximum MS value of ∼175 emu/g was observed at 20 Kfor the Fe79B10Zr11 sample. In order to compare the variation ofHC of all the a-Fe–B–Zr samples at different temperatures, the nor-malized coercivity [HC(T)/HC(20 K)] as a function of temperature isplotted in Fig. 7(b). The normalized values of HC for Fe89Zr11 sampledecreases initially with increasing temperature up to 200 K whichis a typical behavior exhibited by soft ferromagnetic materials, and

then shows a rapid increase around 300 K (near the TC of the amor-phous phase), followed by a large decrease above 320 K. HC at TCof the amorphous phase (TC,A) decreases with increasing B con-tent and remains constant for alloys with x > 2.5 at.%. This magnetichardening observed close to TC,A has been reported in a variety of

D. Mishra et al. / Materials Science and Engineering B 175 (2010) 253–260 257

ibbons

natHspfini(atrmia

mfmmetxisatt

Fig. 6. M–H loops of a-Fe89−xBxZr11 (x = 0, 2.5, 5, and 10) alloy r

anocrystalline systems [39–41] with two-phase structure and isttributed to the loss of ferromagnetic exchange coupling betweenhe fine crystallites through the paramagnetic amorphous matrix.owever, XRD and HRTEM studies on the presently investigated

amples (already discussed in Section 3.1) clearly reveal the com-lete amorphous structure without any local lattice fringes and/orne crystallites. This leads to the inference that the observed mag-etic hardening behavior is due to the presence of local magnetic

nhomogeneities and/or finite spin clusters [36]. So, HC increasesowing to the strong ramification of spin clusters just above TC)s the temperature is increased above TC. On further increasinghe temperature, the spin clusters shrink in size and ultimatelyesult in total magnetic disorder in the spin system (or the for-ation of the paramagnetic state). With increased B substitution

n a-Fe–B–Zr alloys, the local magnetic inhomogeneities decreasend the magnetic hardening effect disappears.

Good soft magnetic properties are obtained when (i) both theagnetic domain wall motion and magnetic domain rotation occur

or a small change in the energy of the system and (ii) saturationagnetostriction is very low, resulting in a drastic reduction inagnetic anisotropy [42]. The enhanced soft magnetic properties

xhibited by the Fe89−xBxZr11 samples motivated us to investigateheir magnetic domain structure. Fig. 8 shows the Lorentz images of= 5 and 10 samples. Although smooth domain walls are observed

n both x = 5 and 10 samples, the average domain size in x = 10 ismaller than that of the x = 5 sample. This indicates that the aver-ge effective anisotropy of the samples increases slightly whenhe B content is increased from 5 to 10 at.% [34,35]. It is knownhat average effective anisotropy increases due to magnetoelastic

recorded (while heating from 20 K) at different temperatures.

coupling and short-range stresses due to quasidislocation dipolespresent in amorphous based alloys [43]. Also, large-sized domainsindicate a strong exchange coupling and good magnetic softness[35]. On increasing the applied field, the domain walls moved outrapidly (Fig. 8(b)–(f)) and disappeared completely at the saturationfield. This exercise confirmed that the domains can be displaced atvery low applied field strengths, which is a characteristic of softmagnetic materials.

Attempts to model the composition dependence of TC of softmagnetic alloys on the basis of the average molecular field modelhave led to the expression [8],

TC = J(r)ZS(S + 1)3kB

(2)

where J(r) is the distance dependent inter-atomic exchange inte-gral, Z is the coordination number, S is the atomic spin quantumnumber and kB is Boltzmann constant. However, this model ismostly applicable to materials with either pure ferromagnetic orpure antiferromagnetic state, which is not the case in the currentlyinvestigated samples. The Fe89−xBxZr11 samples show an almostlinear variation of TC with B addition as shown in Fig. 5(a). Thisbehavior is similar to that of a-Fe–Zr based alloys [11,15]. The dou-ble transition behavior observed in Fe89Zr11 below 35 K might bedue to the presence of noncollinear spin structure resulting from

the co-existence of ferromagnetic and antiferromagnetic couplings.This is mainly due to the sensitivity of the Fe–Fe exchange inter-action to atomic distance, which in turn causes fluctuations in themagnitude and can also reverse the sign of the magnetic exchangeintegral. The average coordination number (Z) of a-Fe84Zr16 alloy

258 D. Mishra et al. / Materials Science and E

Ft

hFcaecto

Fi

ig. 7. Variations of (a) saturation magnetization and (b) normalized coercivity withemperature for a-Fe89−xBxZr11 (x = 0, 2.5, 5, and 10) alloy ribbons.

as been found to be 11.6 [44]. Since Z of the ferromagnetic bcce and antiferromagnetic fcc Fe are 8 and 12, respectively, onean presume the existence of antiferromagnetic exchange inter-

ction in Fe84Zr16 alloy. According to the model proposed by Ryant al. [45,46], the compositional dependence of TC in Fe–B–Zr alloysan be attributed to the variation in spin structure and its correla-ion length resulting from the concentration effect on the numberf antiferromagnetic coupling existing in the parent Fe–Zr alloy.

ig. 8. Room temperature magnetic domain structures of a-Fe89−xBxZr11 alloy ribbons witn the figures correspond to a scale of 1 �m.

ngineering B 175 (2010) 253–260

When B is added to Fe–Zr, the values of TC and MS increase linearlywithin the entire concentration range measured. This might be aconsequence of the decrease in the degree of competition betweenferromagnetic and antiferromagnetic interactions with increasingB content, which removes the spin frustration present in the par-ent Fe-Zr alloy and stabilizes the ferromagnetic nature with its spinstructure approaching parallel alignments. Hence, it can be inferredthat the noncollinear spin structure present in the parent Fe–Zr ismodified due to the substitution of B.

The ultra soft magnetic properties in these samples would proveto be advantageous in magnetic refrigeration applications [47].As these samples have very low hysteresis loss and Curie tem-perature close to room temperature these are advantageous forindustrial refrigeration applications. Since the magnetic momentin the present alloys is higher than that of the samples with lowerZr content, it would be interesting to study the MCE of these alloyswith higher Zr content. Fig. 9 shows the isothermal magnetiza-tion curves of a-Fe89−xBxZr11 (x = 5, 10) alloys around TC. The M–Hdata were obtained on the heating cycle as a function of increas-ing magnetic field at each temperature. The temperature intervalbetween the adjacent curves in the figure is 2 K. With increasingtemperature, all the samples show a gradual change in magneti-zation from non-linear to linear variations with the applied field.MCE of a magnetic material was evaluated from the field depen-dent magnetization curves using a numerical approximation to theequation,

�SM =∫ H

0

(∂M

∂T

)H

dH (3)

where the partial derivative is replaced by finite differences andthe integration is performed numerically. The change in the mag-netic entropy (�SM) was calculated from the area under the M–Hcurves at different temperatures using Eq. (3). Fig. 10 shows the

variations of �SM with temperature for a-Fe–B–Zr alloys calcu-lated for a magnetic field change of 18 kOe. The �SM curve forx = 0 sample shows atypical behavior, i.e., a tendency to saturatebelow 205 K and larger width. This behavior can be attributedto the presence of local magnetic inhomogeneities and/or fine

h x = 5 (a–c) and x = 10 (d–f) observed at different applied fields. The horizontal bars

D. Mishra et al. / Materials Science and Engineering B 175 (2010) 253–260 259

F ns cloT

mwdrtdtuiaftss�aot[tatl

Fa

ig. 9. Isothermal magnetization curves of a-Fe89−xBxZr11 (x = 5 and 10) alloy ribbohe temperature interval between the adjacent curves in the figure is 2 K.

agnetic clusters in this sample. With increasing B content, theidth of the �SM curves decreases and the flatness in the �SM dataisappears within the temperature range of investigation. Theseesults lend support to the proposal [48] that any magnetic clus-ers or local magnetic inhomogeneities present in the Fe–Zr alloysiminishes upon B addition and the general observation that B addi-ion stabilizes the amorphous phase in Fe–Zr alloys. In order tonderstand such atypical behavior in detail, one needs to obtain

sothermal magnetization curves over wide range of temperaturesnd analyze the variations in �SM as a function of temperatureor all these samples. The present interest is to merely point outhe possibility of attaining high �SM in these alloys. It can also beeen from Fig. 10 that �SM increases from 1.3 J/(kg-K) for Fe89Zr11ample to 1.73 J/(kg-K) for Fe79Zr11B10 sample. The increase inSM is mainly due to the enhanced ferromagnetic properties of

-Fe–B–Zr alloys with B addition. It is to be noted that �SM valuesbtained in the present samples are relatively higher comparedo those reported for NANOPERM [1.4 J/(kg-K)] [49] and FINEMET

1.1 J/(kg-K)] [18] alloys for 0–15 kOe field change. Taking note ofhe enhancement in ferromagnetic nature of a-Fe–Zr alloy with theddition of Zr [11], one can attribute this observed increase in �SM

o the relative influences of both Zr and B on the stabilization ofocal magnetic structure of a-Fe–B–Zr alloy.

ig. 10. Variations of �SM with temperature (calculated for 18 kOe field change) for-Fe89−xBxZr11 (x = 0, 5, and 10) alloy ribbons close to TC .

[[

[

se to TC measured in heating process and as a function of increasing applied field.

4. Conclusion

Temperature dependent magnetic properties, room tempera-ture magnetic domain structures, and crystallization behavior ofa-Fe89−xBxZr11 alloys have been investigated. Addition of B in Fe–Zralloys improves the thermal stability of the amorphous phase. Adetailed analysis of magnetization data and magnetic parametersshows that the addition of B decreases the antiferromagnetic cou-pling significantly, removes the spin frustration observed in theparent Fe–Zr alloy, and stabilizes the ferromagnetic nature of sam-ples with its spin structure approaching parallel alignments. Theenhancement in magnetic properties strongly depends on the rel-ative amounts of both Zr and B present in Fe–B–Zr alloys. Theamorphous-to-crystalline transition with the precipitation of �-Fephase shifts to high temperature with increasing B. The magneticdomain walls of Fe–B–Zr alloys are relatively smooth and straight,and the large-sized domains indicate lower magnetic anisotropyand hence an enhancement in soft magnetic properties. A mod-erate increase in the magnetocaloric effect in these alloys withhigh B suggests potential application of these alloys in magneticrefrigeration.

Acknowledgements

This work was financially supported by DST, India videproject nos. SR/FTP/PS-32/2004, SR/S2/CMP-19/2006, andSR/S5/NM-108/2006 and DAE-BRNS, India, vide project no.2005/20/34/1/BRNS/376. We thank Prof. K. Hono for his permissionto use TEM.

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