J Supercond Nov Magn (2012) 25:2431–2436DOI 10.1007/s10948-012-1632-z

O R I G I NA L PA P E R

Annealing Influence on the Microstructure and MagneticProperties of Ni–Mn–In Alloys Ribbons

L. González-Legarreta · T. Sánchez · W.O. Rosa · J. García · D. Serantes ·R. Caballero-Flores · V.M. Prida · L. Escoda · J.J. Suñol · V. Koledov · B. Hernando

Received: 30 April 2012 / Accepted: 8 May 2012 / Published online: 26 May 2012© Springer Science+Business Media, LLC 2012

Abstract We report on the structure, microstructure and in-verse magnetocaloric effect associated with the first-ordermartensitic phase transition, in Heusler Ni50.0Mn35.5In14.5

alloy ribbons. We have studied the short-time vacuum an-nealing influence at 1048 K, 1073 K, 1098 K, and 1123 Kin these properties. At room temperature, an increase in thedegree of structural order for ribbons annealed up to 1098 Kwas observed, corresponding to cubic L21 austenite phase.Meanwhile, for the sample annealed at 1123 K a monoclinic10M martensitic phase was detected. A comparison of mag-netic entropy change as a function of the applied field, af-ter using zero-field-cooling thermomagnetic and isothermalmagnetization measurements, has been made for the sampleannealed at 1073 K.

Keywords Martensitic transformation · Melt spinning ·Magnetocaloric effect · Ni–Mn–In

L. González-Legarreta · T. Sánchez · W.O. Rosa · J. García ·D. Serantes · R. Caballero-Flores · V.M. Prida · B. Hernando (�)Departamento de Física, Universidad de Oviedo, Calvo Sotelos/n, 33007 Oviedo, Spaine-mail: grande@uniovi.es

W.O. RosaCentro Brasileiro de Pesquisas Físicas, 22290-180 Urca,Rio de Janeiro, Brazil

L. Escoda · J.J. SuñolUniversidad de Girona, Campus Montilivi (Ed. PII),Lluís Santaló s/n, 17003 Girona, Spain

V. KoledovKotelnikov Institute of Radio Engineering and Electronics, RAS,Moscow 125009, Russia

1 Introduction

Off-stoichiometric Ni–Mn–X (X = In, Sn, Ga) ferromag-netic shape memory alloys have drawn more attention re-cently [1] since the discovery of a magnetic field-inducedreverse martensitic transformation in Ni–Mn–In [2]. In thesealloys martensitic transformation (MT) on cooling convertsthe ferromagnetic L21 structure into a low magnetizationmartensite phase [3], being therefore accompanied by alarge magnetization drop (�M) [4]. The occurrence of suchmagneto-structural transition opens the possibility for mag-netic shape memory effect in addition to other propertiesresulting from the interaction between the lattice and mag-netic structures, such us giant magnetoresistance and inversemagnetocaloric effect (IMCE) [5], which are of great tech-nical interest for practical applications in sensing and mag-netic refrigeration. The IMCE or a positive magnetic entropychange (�SM ) is due to the low magnetization value of themartensite phase (MP) in comparison to that of the parentphase (or austenite AP). It is of scientific interest to under-stand the magneto-structural complex behavior displayed bythese alloys. For evaluating the magnetic entropy change ina magnetic material, as a function of temperature due to anisothermal change of magnetic field, �SM can be estimatedfrom the experimentally obtained isothermal M–H curves(or thermomagnetic measurements) using the Maxwell rela-tion [6, 7]:

�SM(T ,H) =∫ H

0μ0

∂M(T ,H)

∂T|H dH. (1)

In the Ni–Mn-based Heusler alloys, there is a strong inter-play between magnetic and structural properties. It dependson the atomic order as long as the variations on the config-urational ordering of the constituting elements in the crys-tal lattice affect both the MT and the alloy magnetic mo-

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Fig. 1 XRD diffraction patternsof the Ni50.0Mn35.5In14.5as-quenched ribbon (a), andannealed ribbons during 10minutes for: (b) 1048 K,(c) 1073 K, (d) 1098 K, and(e) 1123 K. In (f), a zoom of thediffraction pattern for thesample annealed at 1123 K isdisplayed

ments. This is related to the modification of both the elec-tronic structure and the lattice site occupancy by the mag-netic atoms. The atomic order can be modified either by sub-jecting an alloy to different thermal treatments or by chang-ing the alloy composition. Nevertheless, this last procedureis not useful when studying the effect of atomic order on theMT since it implies the modification of the valence electrondensity, e/a, which greatly affects the MT [4, 8–10]. In thiswork, the structural and magnetic properties undergone byNi50.0Mn35.5In14.5 ribbons have been studied. The effect ofthermal treatments has been analyzed on X-ray diffractionpatterns, microstructure and magnetic entropy changes.

2 Experimental

Off-stoichiometric Ni–Mn–In Heusler alloys were preparedby arc-melting with high pure metals (>99.98 %) in an ar-gon atmosphere. Ingots were melted several times to en-sure a good homogeneity. Moreover, to compensate for Mnlosses during the melting, an excess of a few wt.% Mn wasadded to each ingot. Then, master alloys were inductionmelted in quartz tubes in a melt spinning system and ejectedin argon environment onto the polished surface of a copperwheel rotating at an elevated linear speed of 48 m/s. Rib-bon flakes around 1.5–2.0 mm width were obtained. Someribbons wrapped in a Ta foil and sealed in quartz tubes in anargon atmosphere were annealed during 10 minutes at differ-ent temperatures ranging from 1048 K to 1123 K with a 25 Kstep. The crystal structures were analyzed by X-ray diffrac-tion (XRD) at room temperature (RT) measured with a D8Discover (Brucker), in the range between 20◦ ≤ 2θ ≤ 100◦by employing Cu-Kα radiation (λ = 1.5418 Å), workingat 40 kV and 40 mA. The ribbons composition was de-termined by energy dispersive X-ray spectroscopy (EDX).The samples morphology and microstructure was observedby scanning electron microscopy (SEM, JEOL 6100). Mag-netic measurements were carried out by the isothermal M–H curves and by the temperature dependence of the magne-tization using zero-field-cooling (ZFC), field-cooling (FC)

and field-heating (FH) protocols, in the temperature rangefrom 50 K up to 400 K and applied magnetic field upto 30 kOe using a vibrating sample magnetometer (VSM-VersaLab, QD). Magnetic entropy change (�SM ) and refrig-erant capacity (RC) were obtained from ZFC thermomag-netic measurements in different applied magnetic fields.

3 Results and Discussion

3.1 Structure and Morphology

In order to study the annealing effect on the crystalline struc-ture and the degree of order in annealed ribbons in com-parison with the as-quenched sample, we have collectedthe XRD patterns at RT. Figures 1(a)–1(f) show diffractionpatterns of as-quenched and annealed ribbons at 1048 K,1073 K, 1098 K, and 1123 K, respectively. In the diffrac-tion pattern of the as-quenched sample (see Fig. 1a), it canbe observed a pure austenite phase with a face-centered cu-bic (fcc) structure, according to previous results stating thatthe Martensitic transformation starts at 257 K [11]. More-over, the super-lattice reflection peaks proving the presenceof the second-neighbor order, such as (111) and (311) in-dicating that an ordered L21 is developed with a latticeparameter ac = 0.5987(3) nm. After annealing ribbons at1048 K, 1073 K, and 1098 K (see Figs. 1b, 1c, 1d), an in-creasing in the intensity of super-lattice reflections such as(111), (311), and (331) is detected, as the annealing tem-perature increases, when the number of counts reflectedin the intensity is compared among them and with thecorresponding to the as-quenched ribbon. This indicates ahigher degree of order in annealed samples than in the as-quenched one. Also, an increasing of the lattice parameteris found, being larger for the ribbon annealed at 1098 K(ac = 0.5995(2) nm).

It should be remarked that a highly order has been ob-tained in the ribbons subjected to only 10 min of annealing.This is considerable shorter than in bulk samples, where nor-mally more than 2 h of annealing is required for increasing

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Fig. 2 Micrographs of the Ni50.0Mn35.5In14.5 for as-quenched ribbon(a) and annealed ribbons during 10 minutes at 1048 K (b), 1073 K (c),1098 K (d), and 1123 K (e)

the ordering [12]. However, after annealing at 1123 K for10 min (see Fig. 1e–1f), the structure at RT has changed;the Bragg peaks have been indexed according to the 10M

monoclinic structure, indicating a lower degree of orderthan the other ones, corresponding to the martensite phase,which is textured in the (125) reflection. The calculated lat-tice parameters being a = 0.431(5) nm, b = 0.58(6) nm,c = 2.115(1) nm, and β = 87.91◦.

The room-temperature microstructure of the as-quenchedand annealed ribbons is shown in Figs. 2(a)–2(e). In thesefigures, it can be observed columnar grains, which grow per-pendicular from the ribbon surface in contact with the wheelone during the quenching process, as can be appreciated inthe upper part of the figures. On this surface the annealingeffect reinforces the grain structure.

For the as-quenched ribbon (see Fig. 2a), a grain sizearound 2.2 µm is observed in the columnar structure. How-ever, for annealed ribbons (see Fig. 2b, 2c, 2d, 2e), re-crystallization and a higher order than as-quenched sam-ple are developed, being the columnar structure clearer andgrain sizes increasing when the annealing temperature rises,reaching sizes around 8.7 µm. After an exhaustive studyby EDX microanalysis on different places at the ribbonssurface and transversal section, an averaged compositionof Ni50.0Mn35.5In14.5 for either the as-quenched and/or an-nealed samples was determined. The estimated error in de-termining the concentration of each element is ±0.1 %. Themicroanalysis confirms the chemical homogeneity of the al-loy without influence of the grain shape on composition.

Fig. 3 ZFC, FC and FH temperature dependence of the magnetiza-tion of the Ni50.0Mn35.5In14.5 as-quenched ribbon obtained at differentfields

The valence electron concentration per atom for this alloyis e/a = 7.9.

3.2 Magnetism

Figure 3 displays the ZFC, FC and FH temperature depen-dence of the as-quenched Ni50.0Mn35.5In14.5 sample for dif-ferent applied fields. Before starting the ZFC measurementsthe samples were heated up to 350 K, in AP paramagneticphase, at zero applied magnetic field. Afterwards, they werecooling down to 50 K where the magnetic field was applied.Qualitatively similar behaviors have been obtained for allannealed samples. Some common features can be seen asso-ciated with these samples: (i) All samples undergo two mainphase transitions near the room-temperature range associ-ated with the Martensitic (magneto-structural) transforma-tion, from the low-temperature MP to the high-temperatureAP, and with the magnetic transformation from the low-temperature ferromagnetic AP to the high-temperature para-magnetic AP; (ii) positive values of the change in the mag-netization �M = MAP − MMP are obtained for all samples,where MAP and MMP are the magnetization values of theAP and MP, respectively. These values of �M vary be-tween 35 and 55 emu/g in the studied samples, and takeplace over a temperature interval of width ranging from25 to 50 K, where there is an AP and MP phase coexis-tence.

To characterizing the MCE in these samples, the temper-ature dependence of �SM has been calculated from Eq. (1)after using the ZFC and isothermal M–H protocols. TheZFC protocol has previously been mentioned. In this mode,the follow approximation in Eq. (1) has been considered:the output M only depends on the final (input) applied fieldH , being independent of the magnetization process (i.e., nohysteresis has been considered). Concerning the isothermalM–H protocol, the samples were heated up to 400 K at zeroapplied field and cooled down to the measurement tempera-

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Fig. 4 Temperature dependence of �SM obtained from Eq. (1) of the Ni50.0Mn35.5In14.5 as-quenched ribbon (a) and annealed ribbons during 10minutes at 1048 K (b), 1073 K (c), 1098 K (d), and 1123 K (e)

ture. Then, the field was increased from zero up to 30 kOemeasuring the isothermal magnetization change as a func-tion of the field, and then samples were demagnetized. It hasbeen pointed out how the magnitude of the isothermal mag-netic field-induced entropy change crucially depends on themeasurement protocol [13, 14]. Isothermal M(H) curveswere recorded for each sample in a range from a temperaturebelow the one at the respective MP starting, and up to a tem-perature higher above the Curie temperature for the AP [11].In this way, the temperature measuring range covers both thestructural and magnetic transition for all samples.

Figures 4(a)–4(e) show the �SM(T ) curves for the wholeexperimental magnetic field range of the ZFC protocol, indi-cating the existence of the inverse and direct MCE with re-markable maximum values around 25 J/kg K and −3 J/kg K,respectively. It can be seen that: (i) the maximum valuesof �SM of the as-quenched sample can be enhanced af-ter the annealing process; (ii) there is a pronounced (weak)field evolution of the temperature of the positive (negative)peaks toward lower (higher) temperatures, which can bedue to the thermodynamic equivalence (opposition) of T

and H as driving forces for the Martensitic (ferromagnetic-

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Fig. 5 M–H (lines) and ZFC (solid symbols) dependence of �SM

obtained from Eq. (1) of the Ni50.0Mn35.5In14.5 annealed ribbon at1073 K

to-paramagnetic in AP) transformation. These field evolu-tions of the temperature of the positive peaks toward smallertemperatures are higher for samples annealed at 1048 Kand 1073 K. In order to correlate this undercooling of the�SM(T ) curves to the concomitant overheating in the ZFCmode (associated with the thermal hysteresis), the �SM val-ues obtained in the ZFC (Fig. 4c) and in the M–H proto-cols have been compared (Fig. 5) for the sample annealed at1073 K.

Figure 5 shows that these opposite effects (the overheat-ing in the ZFC and the undercooling due to thermodynamicequivalence of T and H ) can be totally compensate when a30 kOe field is applied. It should be taken in account that thisfield value corresponds to the available limit in our magne-tometer. Further studies are being carried out in order to an-alyze the influence of the thermal- and magnetic-hysteresison the magnetocaloric responses of these samples.

An estimation of the RC has been obtained by the numer-ical integration of the area under the field-dependent �SM

versus T curves (Fig. 4), using as the integration limits thetemperature widths (δT ): 10 K, 20 K, and 30 K correspond-ing to a certain fraction of both positive and negative peaks.Figure 6 indicates for a 20 kOe applied field that, contrary togeneral assumptions, there is a value of the span δT that sat-urates the RC values associated with the magneto-structuraltransition (RCSTR), and that when δT is the same for bothtransition, the structural transition, presents higher RC thanthe magnetic one (RCMAG).

4 Conclusions

After short annealing Ni50.0Mn35.5In14.5 Heusler alloy rib-bons at 1048 K, 1073 K, 1098 K, and 1123 K, the orderdegree and lattice parameter of the L21 fcc structure is in-creased with the annealing temperature except for the sam-ple annealed at 1123 K that develops a monoclinic 10M

phase at RT. All as-quenched and annealed ribbons display

Fig. 6 δT spanning dependence of RC associated with magne-to-structural (RCSTR) and magnetic (RCMAG) phase transition of theNi50.0Mn35.5In14.5 as-quenched and annealed ribbon

columnar grains perpendicular to ribbon surfaces with grainsizes ranging from 2.2 µm to 8.7 µm. Also, the magneticentropy change is enhanced after annealing, and the mag-netic field evolution of temperatures at which the entropychange peaks for both structural and magnetic transition isobserved. For the ribbon annealed at 1073 K, a 30 kOe ap-plied field allows that entropy change peaks at the same tem-perature as when it is measured from the zero-field-coolingM(T ) and isothermal M(H) measurements.

Acknowledgements The authors are thankful to Spanish MICINNfor financial support: MAT2009-13108-C02-01-02 and MAT2010-20798-C05-04. L. González-Legarreta also thanks to MICINN for aFPI grant and J. García to FICYT for a “Severo Ochoa” grant.

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