1

Magnetocaloric effect in melt spun MnCoGe ribbons

C.F. Sánchez-Valdésa, J.L. Sánchez Llamazares,b,1 H. Flores-Zúñigab, and D. Ríos-Jarab, P. Alvarez-Alonsoc and Pedro Gorriad aInstitut de Ciencia de Materials de Barcelona (C.S.I.C.), Campus U.A.B., 08193 Bellaterra, Spain bDivisión de Materiales Avanzados, Instituto Potosino de Investigación Científica y Tecnológica, Camino a la

presa San José 2055, CP 78216, San Luis Potosí, Mexico. cDepartamento de Electricidad y Electrónica, Universidad del País Vasco (UPV/EHU), 48940 Leioa, Spain dDepartamento de Física, EPI, Universidad de Oviedo, Gijón, Spain.

DOI: 10.1016/j.scriptamat.2013.03.022

Abstract- Single phase MnCoGe ribbons with the NiIn2-type structure were produced by using the

melt spinning technique. We reduced by two orders of magnitude, compared with that of the parent

bulk alloy, the annealing time for stabilizing the lower-symmetry orthorhombic TiNiSi-type crystal

structure. Both phases exhibit second-order magneto-caloric effect, with ∆SMpeak ≈-2.8 (-4.0) J/kg·K

and RC ≈ 238 (281) J/kg for the maximum isothermal magnetic entropy change and refrigerant

capacity, respectively, for a magnetic field change of 5 T.

Keywords: MnCoGe melt spun ribbon; magnetocaloric effect; magnetic entropy change; refrigerant capacity.

1E-mail: jose.sanchez@ipicyt.edu.mx

 

2

The stoichiometric MnCoGe compound exhibits polymorphism, and can adopt either the hexagonal

Ni2In-type (space group P63/mmc) or the orthorhombic TiNiSi-type (space group Pnma) crystal

structure at room temperature [1]. Both phases are ferromagnetically ordered, although the values for

the Curie temperature, TC, and saturation magnetic moment, mS, differ: TC≈275 and 355 K, and mS≈2.8

and 4.1 µB/f.u., for the hexagonal and orthorhombic phases, respectively [2,3]. Both Mn and Co have

magnetic moments, although the main contribution comes from the Mn atoms [2,4].

The orthorhombic variant, usually referred as the low-temperature phase, is obtained after

homogenization achieved by thermal annealing at temperatures in the range 773-1123 K [5-9],

followed by slow or fast cooling to room temperature. However, a serious practical drawback in the

homogenization process is the annealing time, which typically requires several days. On heating, this

phase undergoes a first-order diffusionless structural phase transformation to the hexagonal phase (i.e.,

a martensitic-type transformation) with a large thermal hysteresis of ∼ ΔT= 40 K [3,10]. The

temperature for the beginning of the structural transition Tstr varies between 420 K [5] and 650 K

[3,10]. Although the high-temperature stable phase is hexagonal (for T>Tstr), it has been also reported

that when the alloy is annealed at high temperatures and rapidly quenched in water, this hexagonal

phase can be obtained at room temperature in a metastable state [2,11].

An interesting feature of MnCoGe-based alloys is the strong interplay between structure and

magnetism. Several factors such as Co-vacancies [8,12,13], Mn-deficiency [14], the application of

hydrostatic pressure [10,11,15], substitution of a small amount of Mn or Co by a fourth element

[3,5,8,9,11,16,17], and the introduction of interstitial elements [18-20], may considerably affect the

intrinsic magnetic properties of both structural variants, as well as the value of Tstr.

The magneto-caloric response of MnCoGe-based alloys has drawn considerable attention in the last

few years. Large, and even giant, peak values of the isothermal magnetic entropy change ΔSMpeak have

been reported in pure [6,13] or doped alloys [8,9,15-20] with the orthorhombic crystal structure. In

3 MnCo0.95Ge1.14 alloys a sizable ΔSM

peak of -6.4 Jkg-1K-1 is induced by a rather low magnetic field

change of µoΔH= 1 T [6]. The effect has been explained by the large sharp drop in magnetization close

to the ferromagnetic-to-paramagnetic transition as a consequence of the negative exchange striction of

the lattice around TC (i.e., due to the strong magneto-elastic coupling nature of the magnetic transition).

More recently, a giant magnetic entropy change has been found in interstitially modified MnCoGeBx

[16,19], and MnCoGeCx [19] alloys, or by partial replacement of Mn by Cr in Mn1-xCrxCoGe

[15,16,19], by reducing Tstr below the Curie point of the orthorhombic phase.

Until now, MnCoGe alloys have been produced as bulk samples by means of conventional melting

techniques followed by lengthy thermal annealing. In view of the current interest and great potential of

these compounds as magneto-caloric materials near room temperature, here we report the synthesis of

the equi-atomic MnCoGe compound by rapid solidification using the melt spinning technique, as well

as the characterizations of the crystal structure, the magnetic properties, and the magneto-caloric effect

(MCE) to a maximum magnetic field change up to µoΔHmax = 5 T. To our knowledge, neither the

synthesis of MnCoGe melt spun ribbons nor their magneto-caloric properties, have been reported yet.

Significantly, the use of rapid solidification to produce the orthorhombic variant of this compound

results in a substantial reduction in the annealing time.

Rapidly solidified ribbons (with thicknesses ≈ 30-35 µm) were produced by melt spinning in an Ar

atmosphere at a wheel linear speed of 20 ms-1 from as-cast pellets of nominal composition of MnCoGe

previously obtained by arc melting from highly pure elements (> 99.9%) under an Ar atmosphere as

well. Mn losses during arc melting were carefully compensated by adding the appropriate excess of this

element. Samples were annealed at 923 K for 1 hour to stabilize the orthorhombic structure. Annealing

was followed by water quenching. X-ray powder diffraction (XRD) patterns were obtained with a

Bruker AXS model D8 Advance diffractometer using Cu-Kα radiation. Microstructure and elemental

4 composition were determined using a FEI/Philips XL30 FEG ESEM equipped with an energy

dispersive analysis system (EDS).

Magnetization measurements were performed using a PPMS platform equipped with a vibrating

sample magnetometer module. The magnetic field µoH was applied along the major length of the

ribbon samples (typically, ~ 4 mm) to minimize the demagnetizing field effect. Magnetization versus

temperature, M(T), curves were recorded under a low applied magnetic field of 5 mT with the aim of

accurately determining the value of TC. The magnetic entropy change as a function of the temperature

curves, ΔSM(T), were obtained by numerical integration of the Maxwell relation

( ) ∫ ⎟⎠

⎞⎜⎝

⎛∂∂

=Δ maxB

oB

M dBTMTS from a set of isothermal magnetization curves M(µoH) measured up to

µoHmax=5 T. The refrigerant capacity RC, that measures the thermal efficiency of the material on the

energy transfer from cold to hot reservoirs for an ideal thermodynamic cycle, was estimated using the

following three well-established methods: RC-1 = ΔSMpeak x δTFWHM [20],

∫ ΔΔ=−

cold

hot

T

T BM dTTSRC )]([2 [21],

and, RC-3 by maximizing the product ⎜ΔSM ⎜ x ΔT below the ΔSM(T) curve (usually referred as the

Wood and Potter method) [22,23]. In case of RC-1 and RC-2, Thot and Tcold are the temperatures that

define the temperature interval of the full width at half maximum of the ΔSM(T) curve (i.e., δTFWHM=

Thot - Tcold).

Figs. 1(a) and (b) show the room temperature XRD patterns obtained for as-quenched (aq) and

annealed samples, respectively. Both patterns were refined using the FullProf analytical package based

on the Rietveld method [24], with good reliability factors from the refinements (aq sample: RB=9.3 %,

Rf=9.3 %, and χ2=2.2; annealed sample: RB=3.6 %, Rf =3.3 %, and χ2=2.0).

As-quenched ribbons crystallize as a single-phase hexagonal NiIn2-type structure with lattice

parameters a=4.083(1) Å and c=5.313(1) Å, which roughly agree with those reported for bulk alloys

[3]. The homogeneous distribution of chemical elements and the single-phase character of the ribbons

5 were also confirmed by SEM examinations in the backscattered electron emission mode (i.e., no minor

or secondary phases were detected). Numerous EDS analyses performed on both cross section and

ribbon surfaces for different ribbon flakes confirmed that the average chemical composition of the

samples is close to the nominal one (i.e., 1:1:1) within experimental error (~0.1 % wt.). The XRD

pattern corresponding to the annealed sample [Figs. 1 (b)] reveals the formation of the orthorhombic

TiNiSi-type structure as the major phase (98 % wt.) with lattice parameters a=5.958(2) Å; b=3.822(1)

Å; c =7.059(2) Å (in agreement with Refs. 3 and 4). A residual amount of the hexagonal phase (~2 %

wt.; a=4.060(1) Å; c=5.344(1) Å) is also present in the sample.

The typical microstructures of aq and annealed samples are shown in the inset of Figs. 1(a) and (b),

respectively. The aq ribbons are fully crystalline indicating that the alloy exhibits fast nucleation and

growth kinetics. In both samples, polyhedral micronic grains with an average grain size of ≈5 µm are

observed. Due to the low annealing temperature needed to form the orthorhombic phase, the

microstructure does not show any substantial change. Both samples were found to be quite fragile with

numerous intergranular cracks (that are more clearly observed in the annealed ribbons).

The low-field M(T) curves measured for the hexagonal (open squares), and orthorhombic (closed

circles) phases, together with the respective dM/dT vs T curves (inset), are shown in Figure 1(c). The

TC values inferred from the minima in the dM/dT(T) curves, of 275 K (hexagonal) and 355 K

(orthorhombic), are in good agreement with previously reported data for the bulk MnCoGe compound

[1,2,14]. The small knee at around 275 K in the M(T) curve (indicated in the figure by a vertical arrow)

is attributed to an impurity from the residual hexagonal phase detected by XRD. Moreover, no

noticeable thermal hysteresis is observed between the heating and cooling pathways of the curve (≤ 2

K).

Figs. 2(a) and (b), and (c) and (d) show the M(µoH) curves and the resultant Arrott plots,

respectively, for aq and annealed ribbons (the temperature intervals for which the curves were

6 determined, as well as the ΔT step between them, are given in the graphs). The Arrott plots were used

to check the nature of the ferromagnetic-to-paramagnetic transition of both samples. The Arrott plots

for the hexagonal phase exhibit positive slopes in the whole temperature range, with changes in

curvature around TC (from negative to positive), thus confirming the second-order character of the

transition. Those of the annealed ribbons show a slightly different behaviour. As is observed in Fig.

2(d), between 355 K and 380 K the Arrott plots display a smooth inflection (a dashed rectangle in the

figure highlights this observation), thus suggesting that in the 1:1:1 phase the coupling between

structural changes around TC that results from the lattice distortion of the orthorhombic phase and the

magnetic structure is weak. A very different situation is found in Mn or Co deficient MnCoGe alloys in

which strong magneto-elastic coupling leads to a sharp fall in the magnetization at TC giving rise to a

giant MCE [6,13].

Fig. 3(a) shows the ΔSM(T) curves at 2 and 5 T, along with the │ΔSMpeak│versus (µoH)2/3 curves

(inset). At 5 T, the aq sample shows a moderate absolute peak value of the magnetic entropy change of

2.8 Jkg-1K-1, along with a working temperature span δTFMHW of 85 K. The left inset shows that

│ΔSMpeak│depends linearly on (µoH)2/3 as expected for materials with second-order ferromagnetic

transitions [20,25]. At 5 T (2 T), the annealed sample shows a │ΔSMpeak│ of 4.0 (2.0) Jkg-1K-1, and a

δTFMHW value of 68 K (50 K). In agreement with the Arrott plots, │ΔSMpeak│is nearly proportional to

(µoH)2/3, which, again, is a consequence of the second-order-like nature of the magnetic transition

[20,25]. Also, the set of field-up and field-down magnetization isotherms around TC measured up to

µoHmax=3 T shown in Fig. 3(c) exhibit fully reversible behaviour.

Fig. 3(b) compares the field dependence of both, the refrigerant capacity RC-2 and the characteristic

temperatures, Thot and Tcold, that defines the full-width at half-maximum of the ΔSM(T) curve for both

phases. The increase of RC for the orthorhombic phase is mainly due to the higher entropy change

since both compounds have similar values of δTFWHM. Moreover, the RC-1 value of the samples

7 produced (99 and 281 J kg-1, for a field change of 2 and 5 T, respectively), may be similar to the one

deduced from the ΔSM(T) curve, for chemically modified MnCoGe alloys with a giant MCE

[6,8,9,13,15-20]. For all of them ΔSM(T) is quite sharp and narrow, leading to δTFWHM values of

typically 5-7 K, which limits the practical use of the material as a magnetic refrigerant. An outline of

the magnetocaloric properties at 2 and 5 T for the samples is given in Table I (i.e., the absolute value of

ΔSMpeak, RC-1, RC-2, and RC-3 and the related temperature parameters are listed).

In summary, the hexagonal and orthorhombic variants of the stoichiometric MnCoGe compound

were produced as rapidly solidified polycrystalline ribbons and their magneto-caloric properties

studied. The use of melt spinning technique to produce the orthorhombic variant reduces considerably

the annealing time (at 923 K) with respect to that reported for bulk alloys. The MCE that exhibits both

structural variants is within the expected range for materials with second-order ferromagnetic phase

transitions. The estimated refrigerant capacity (RC-1 = 238 J/kg and 281 J/kg for the hexagonal and

orthorhombic variants, respectively) is nearby that of modified MnCoGe giant MCE alloys, owing to

the larger dTFWHM values and almost negligible thermal hysteresis of MnCoGe ribbons. Therefore,

the possibility of tuning the structural and magnetic transition temperatures through small

compositional changes and/or heat treatments may open up new avenues of scientific research for room

temperature magnetic refrigerants.

Acknowledgements - Authors acknowledge: (a) the support received from CONACYT, Mexico,

under the project number 156932, Laboratorio Nacional de Investigaciones en Nanociencias y

Nanotecnología (LINAN, IPICyT), and Spanish CICyT (project number MAT2011-27573-C04-02);

(b) the technical support received from M.Sc. G.J. Labrada-Delgado and B.A. Rivera-Escoto , and A.G.

Lara-Rodríguez (IIM-UNAM, Mexico DF) is gratefully recognized. C.F.S.V. thanks Spanish CSIC for

the Ph.D. Grant received (JAEPRE-08-00508).

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10 FIGURE CAPTIONS

FIG. 1. (color online) Experimental (red dots) and calculated (violet line) X-ray powder diffraction

patterns using the Rietveld method for as-quenched (a) and annealed (b) MnCoGe alloy ribbons.

Vertical lines indicate the positions of the Bragg reflections for the hexagonal (Ni2In-type) and

orthorhombic (NiTiSi-type) phases. Insets: typical microstructure observed at the free ribbon surface.

(c) M(T) and dM/dT versus T curves (inset) at 5 mT for as-quenched (open squares) and annealed (full

circles) MnCoGe ribbons. The horizontal arrows indicate the heating/cooling regime.

FIG. 2. (color online) Isothermal magnetization curves for as-quenched (a) and annealed (b) MnCoGe

ribbons together with the corresponding Arrott plots [(c) and (d)], respectively.

FIG. 3. (color online) (a) ΔSM(T) curves at µoΔHmax= 2 and 5 T for the hexagonal (open and full

squares) and orthorhombic (open and full circles) variants in MnCoGe alloy ribbons. Inset: ΔSMpeak as a

function of (µoH)2/3. (b) Field dependence of the refrigerant capacity RC-2 (see the text for definition)

for the hexagonal (full squares) and orthorhombic (full circles) MnCoGe phase. Inset: field dependence

of the temperatures Thot and Tcold that define the full-width at half-maximum of the ΔSM(T) curve (i.e

δTFWHM= Thot - Tcold). (c) Isothermal magnetization curves measured in increasing and decreasing

magnetic field for annealed MnCoGe ribbons in the temperature interval close to the magnetic phase

transition of the MnCoGe orthorhombic phase.

TABLE CAPTIONS

TABLE I. Peak magnetic entropy change ΔSMpeak (absolute value), Tcold, Thot, δTFWHM, δTRC-3 and

refrigerant capacity values RC-1, RC-2, and RC-3 for field change values of µoΔHmax= 2 T and 5 T.

11 TABLE I.

Material AS-QUENCHED ANNEALED

µoΔHmax (T) µoΔHmax (T) 2 T 5 T 2 T 5 T |ΔSM

peak| (Jkg-1K-1)

1.5 2.8 2.0 4.0

Tcold (K) 241 225 322 317 Thot (K) 295 310 373 385 δTFWHM (K) 54 85 50 68 RC-1 79 238 99 281 RC-2 60 176 76 218 RC-3 49 135 50 143 Tcold (K)* 179 180 318 305 Tcold (K)* 311 326 374 391 δTRC-3 (K) 132 146 56 86 * associated with RC-3.

12

FIGURE 1

13

FIGURE 2

14

FIGURE 3