Influence of Site Occupancy on the Structure, Microstructure and Magnetic Properties of Ternary and Quasi-ternary Alloys of Ni-Mn-Ga

Influence of site occupancy on the structure, microstructure and magneticproperties of ternary and quasi-ternary alloys of Ni-Mn-GaV. Seshubai, A. Satish Kumar, and M. Ramudu Citation: AIP Conf. Proc. 1461, 74 (2012); doi: 10.1063/1.4736874 View online: http://dx.doi.org/10.1063/1.4736874 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1461&Issue=1 Published by the American Institute of Physics. Related ArticlesFerromagnetism in ZrFe12−xAlx and HfFe12−xAlx (x = 6.0, 6.5, 7.0) J. Appl. Phys. 111, 093918 (2012) The magnetic and magnetocaloric properties of NdFe12−xMox compounds J. Appl. Phys. 111, 07A949 (2012) Structural and magnetic study of SmTAl single crystals (T=Pd and Ni) J. Appl. Phys. 111, 07E146 (2012) Scatter in nonlinear ultrasonic measurements due to crystallographic orientation change induced anisotropy inharmonics generation J. Appl. Phys. 111, 054905 (2012) Structures and magnetic properties of Sm5Fe17 melt-spun ribbon J. Appl. Phys. 111, 07E322 (2012) Additional information on AIP Conf. Proc.Journal Homepage: http://proceedings.aip.org/ Journal Information: http://proceedings.aip.org/about/about_the_proceedings Top downloads: http://proceedings.aip.org/dbt/most_downloaded.jsp?KEY=APCPCS Information for Authors: http://proceedings.aip.org/authors/information_for_authors

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Influence of Site Occupancy on the Structure, Microstructure and Magnetic Properties of

Ternary and Quasi-ternary Alloys of Ni-Mn-Ga

V. Seshubai, A. Satish Kumar and M. Ramudu School of Physics, University of Hyderabad, Hyderabad – 500046, India.

Abstract: A systematic study of the effect of cobalt, selectively substituted for Ni and Mn, in the modulated orthorhombic (7M) Ni50Mn29Ga21 alloy has led to interesting correlations between the resultant structure and microstructure. Substitution of Co for Mn resulted in the stabilization of a non-modulated tetragonal (NM) phase at higher Co content and caused suppression of long-range twin deformation leading to sporadic islands within which twin variants were confined. On the other hand, substitution of Co for Ni does not to alter either the superstructural ordering or the long-range twin deformation. A study of the compositional dependence of saturation magnetization measured at 5 K is shown to throw light on the site preference of cobalt and iron atoms substituted in Mn-rich alloys of quasi-ternary Ni-Mn-Ga-(Fe,Co) system. The study reveals that the dopant atoms occupy the regular Mn site, rather than the vacant Ga site, with ferromagnetic exchange relative to the moments on Ni and Mn sub-lattices. These effects are attributed to have their origin in minimizing the stresses generated by the corresponding atomic volume changes incorporated by doping.

Keywords: Ni-Mn-Ga-(Co,Fe); Structure; Saturation magnetization; Site preference. PACS: 75.20 En, 75.30 Cr, 75.50 Cc.

INTRODUCTION

Ferromagnetic Shape Memory Alloys (FSMAs) are class of smart materials that exhibit multifunctional properties like large ferromagnetic and thermal shape memory effects [1-3], magneto-caloric effect [4] and magneto-resistance [5]. Most studied among them are the Ni-Mn-Ga alloys whose properties are very sensitive to composition and thermo-mechanical treatments. In the austenitic phase, Ni2MnGa compound has L21 atomic order, cubic with space group [6]. The crystal structure of the martensitic phases in the above system has been reported to be tetragonal or monoclinic with five layered modulation (5M) [7], orthorhombic or monoclinic with seven layered modulation (7M) [8] or non-modulated tetragonal (NM) [6, 9]. In addition to having large magneto-crystalline anisotropy, it is essential for a material to exhibit favorable structural ordering and microstructural (twin deformation) properties for rendering it suitable for shape memory applications [1]. Among the martensitic phases, large magnetic-field-induced strain (MFIS) of 6% and 10% respectively have been reported in the 5M and 7M structures at considerably low stresses [1]. The stress levels required to cause considerable strain are very high in

Functional MaterialsAIP Conf. Proc. 1461, 74-86 (2012); doi: 10.1063/1.4736874

© 2012 American Institute of Physics 978-0-7354-1065-7/$30.00

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NM structures [10]. Considerable strain deformation and large recovery ratio have been observed in polycrystalline Ni53.5Mn26.0Ga20.5 alloy that crystallizes in 7M monoclinic structure with magneto-structural transformation, owing to low residual stresses and multimode twinning [11].

The magnetic moment in Ni-Mn-Ga Heusler alloy is mainly localized on the Mn sites, as confirmed by both experiments [12, 13] and theoretical calculations [14]. From magnetization measurements, the total saturation magnetic moment per formula unit (μB/f.u.) of Ni2MnGa is reported [12] to be 4.17. The magnetic moment is localized primarily on the manganese atoms with μMn ~ 3.5 μB, while the moment on nickel atoms is quite small with μNi ~ 0.33 μB. However, in Mn-rich compositions, a decreased net magnetic moment is observed experimentally which is due to the excess Mn atoms occupying vacant Ga site with their moments coupled antiferromagnetically to those on Mn at regular sites [15].

Alloying Ni-Mn-Ga with another element influences both the crystal and electronic structures of the parent compound and affects the stability of austenitic and martensitic states; it can thus influence the physical properties drastically [16, 17]. Many recent studies on quasi-ternary Ni–Mn–Ga–X alloys have been directed at the selective partial substitution of 3d elements like Co and Fe and also introduction of light elements like boron. Cobalt doped Ni-Mn-Ga alloys have shown exciting phenomena such as meta-magnetic transformations [18], magnetic field induced martensitic transformations [19] and large magnetic entropy change across the transformations [20]. Most of the reports on the effect of Co- and Fe- doped either for Ni or for Mn discuss the variations in their martensitic transformation temperature (TM), Curie temperature (TC) and the magnetization [17, 21, 22]. However, there has not been clear understanding on the effect of substitutions on the crystal structure, superstructural ordering and the associated microstructural properties of the alloys of Ni-Mn-Ga system and site preference of magnetic and non-magnetic atoms, from a systematic study.

The crystal structures of the Ni-excess compositions in the series Ni53-

xMn25Ga22Cox with x = 0 to 14 [23], Ni56-xMn25Ga19Cox (x = 0, 2 to 14), Ni56Mn25-

yGa19Coy (y = 4, 8) and Ni56-z/2Mn25-z/2Ga19Coz (z = 4, 6) [24] were all of non-modulated tetragonal symmetry before and after Co-doping. The alloys (Ni50.26Mn27.30Ga22.44)100-xCox (x = 0, 2, 4 and 6), which do not have excess Ni, were also of non-modulated tetragonal symmetry on Co doping [25]. Since modulated crystal structure is one of the essential criteria for achieving high magnetic strain in FSMAs at relatively low magnetic fields, we have chosen to investigate the effect of Co doping in Mn excess Ni50Mn29Ga21 alloy which has 7M orthorhombic structure in the martensitic state, and displays wide spread twin deformation. This composition is known to exhibit large magnetic field induced strain in single crystals [1], which arises from the high magneto-crystalline anisotropy and low twinning stresses, caused by the modulated crystal structure in the martensitic phase [1, 26, 27] and hence is chosen as parent compound in the present study.

In addition to crystal structure, one of the important key factors governing the ferromagnetic shape memory effect is the net magnetization per formula unit, which depends on the relative orientation of the magnetic moments on Mn and the dopant (Fe, Co) atoms. Therefore, it is interesting to investigate the site occupancy and the

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magnetic moment orientation of dopants in Ni-Mn-Ga alloy system relative to Mn moments. This article discusses the outcome of selective partial substitution of Co for Ni, and for Mn, in the compound Ni50Mn29Ga21 through a systematic study of the structural and magnetic properties of a series of alloys.

EXPERIMENTAL PROCEDURES

We have synthesized different series of polycrystalline alloys in the present work. These alloys were prepared by induction melting of appropriate quantities of the constituent elements Ni, Ga and Co of 99.999% purity and Mn of 99.99% purity under argon atmosphere. To compensate for estimated loss of Mn, additional 1.2 wt.% was added during melting. The ingots were homogenized under vacuum in quartz ampoules for 8 days at 830 oC and quenched into ice water. The chemical compositions of the samples are determined using the Energy Dispersive X-ray Analyses (EDAX) facility on a JEOL scanning electron microscope. X-ray diffraction (XRD) studies were carried out using fine powders after annealing them in argon atmosphere for 12 h at 500 oC followed by furnace cooling. X-ray diffractograms at ambient temperature were recorded using a Philips X’pert X-ray diffractometer with Cu Kα radiation. Optical microstructures were recorded in the samples, after being mounted in bakelite and polished to quarter micron level, using Zeiss make inverted metallurgical microscope. A thin circular disk of 3 mm in diameter, was thinned further to electron transparency using a Gatan ion polishing system and was studied using an FEI TECHNAI G2 transmission electron microscope (TEM) operating at 200 kV and selected area electron diffraction patterns were also recorded. The lattice parameters were obtained from the XRD patterns using Philips X’pert plus software. Magnetic isotherms (M-H curves) were recorded in the temperature range 5 K - 380 K using a Vibrating Sample Magnetometer (VSM) on PPMS of Quantum design make.

RESULTS AND DISCUSSION

The EDAX compositions of the two series of alloys under study, with Co substituted for Ni and for Mn in Ni-Mn-Ga, are given in Table 1. The parent alloy Ni50Mn29Ga21 is referred to as C-0 while the alloys of Ni50-xCoxMn29Ga21 series with x = 1.3, 1.9, 2.4, 3.3 and 4.1 will be referred to as CNi-x, and those of Ni50Mn29-yCoyGa21 series with y = 1.8, 2.4, 3.3, 4.0, 4.7 and 6.8 will be referred to as CMn-y, respectively. 3.1. Crystal structure 3.1.1. Ni50-xMn29Ga21Cox (CNi-x) alloys

Figure 1 shows the XRD patterns of the CNi-x series of alloys recorded at room temperature (RT). The pattern of C-0 is indexed to an orthorhombic unit cell with 7-layer modulation (7M) in the b-direction. The lattice parameters computed using the XRD pattern agree well with those reported in literature [26] for this composition. Alloy CNi-1.3 also exhibits 7M orthorhombic structure while for samples with higher x content ( 2.4) the crystal structure transforms gradually to cubic L21 structure at RT. The CNi-2.4, CNi-3.3 and CNi-4.1 alloys are austenitic at RT, and their TM are

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below RT [17]. In CNi-1.9 alloy, the austenitic and martensitic phases are found to coexist at RT since its TM (302 K) is very close to RT and the XRD pattern is indexed accordingly.

FIGURE 1. Powder X-ray diffraction (XRD) patterns from alloys of CNi-x series recorded at RT. In the case of CNi-1.9 alloy, the reflections from austenitic phase are marked with a prefix ‘A’. 3.1.2. Ni50Mn29-yCoyGa21 (CMn-y) alloys

FIGURE 2. Powder X-ray diffraction (XRD) patterns from CMn-y series at RT. The additional reflections from modulated orthorhombic crystal structure are marked with a suffix ‘M’.

The XRD patterns recorded at RT in the CMn-y series of alloys, with cobalt doped for manganese, are shown in Fig. 2. All the alloys of this series are in martensitic state at RT. The CMn-1.8 alloy crystallizes in 7M orthorhombic (7M-O) structure similar to C-0 while the patterns for higher y = 2.4 to 4.7 are more complex. The pattern of CMn-6.8 alloy with y = 6.8, at RT, is fully indexed to non-modulated tetragonal structure (NM), with I4/mmm space group as shown in Fig. 3 (a). A good agreement can be observed between the observed and calculated profiles. The lattice parameters are a = 3.876(1) Å and c = 6.402(7) Å with axial ratio c/a of 1.65 and are comparable to those reported in literature [28] for Ni58.0Mn19.4Ga22.6 alloy that crystallizes in NM.

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A close examination shows that the pattern for CMn-3.3 has similarity to that of CMn-6.8 except for the extra reflections at angles (2θ) of 44.1o, 45o, 64o and 81.3o. The SAED patterns recorded in a thinned CMn-3.3 sample using a TEM, showed coexistence of 7M-O and NM structures [17]. Accordingly, the XRD reflections of this alloy shown in Fig. 2 could be indexed to an admixture of these two phases. By Rietveld analysis, the ratio of the NM phase to the 7M-O phase was calculated to be 80:20. XRD patterns of CMn-2.4, CMn-4.0 and CMn-4.7 were also identified as admixtures of the two phases. The percentage of the non-modulated martensitic phase increases with cobalt substitution for Mn: and it amounts to nearly 40% at x = 2.4 to 80% at x = 3.3 and 90% at x = 4.7. At x = 6.8, the alloy is a fully non-modulated martensite with tetragonal structure. Compositions with higher Co content were explored in the case of the CMn-y series of alloys, unlike in the case of the CNi-x series, so as to confirm the occurrence of the NM tetragonal phase in single phase form at higher Co content. The lattice parameters of CNi-x and CMn-y alloys are given in Table 1. Figure 3 (b) shows the variation of the percentage of NM phase stabilized in the alloys of CMn-y series, as determined from Rietveld analysis. The figure suggests that a critical concentration of 1.6 at. % Co substituted for Mn induces the minimum stresses required for the crystallization of NM tetragonal phase in 7M-O martensitic structure of C-0 alloy.

FIGURE 3. (a) Powder X-ray diffraction pattern from the CMn-6.8 alloy at RT. The circles indicate the experimental data while the line represents the profile obtained from a Rietveld analysis. (b) Variation of the percentage of NM phase stabilized in the alloys of CMn-y series showing that 1.6 at. % Co substituted for Mn induces crystallization of NM phase. 2.2. Microstructure

Figure 4 shows the optical microstructures recorded in polycrystalline specimens in the martensitic phase, after polishing down to 0.25 μm. Figure 4(a) shows the martensitic twin variants of the Co-free composition C-0, with parallel bands-like morphology and crossing of thermo-elastic martensite variants. These are spread over the entire sample indicating long-range deformation. The microstructure of CNi-1.3 shown in Fig. 4 (b) is similar to that of C-0. Figure 4 (c) shows that well defined twins are absent in a few regions in CNi-1.9 at RT, which is close to TM, supporting the

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TABLE 1. The crystal structure at room temperature, lattice parameters of modulated martensite, non-modulated martensite and cubic structure of Co doped Ni-Mn-Ga alloys. 7 M-O, NM-T and C stands for seven layer modulated orthorhombic, non-modulated tetragonal and cubic crystal structure respectively.

Code

Crystal structure

at RT

Lattice parameters at RT for

modulated martensitic phase

non-modulated martensitic phase

austenitic phase

a (Å)

b (Å)

c (Å)

a (Å)

b (Å)

c (Å)

a (Å)

C-0 7M-O 4.208(2) 29.371(1) 5.588(9) ----- ----- ----- ----- CNi-1.3 7M-O 4.192(2) 29.386(7) 5.624(6) ----- ----- ----- ----- CNi-1.9 7M-O+C 4.190(1) 29.429(8) 5.642(2) ----- ----- ----- 5.835(1) CNi-2.4 C ----- ----- ----- ----- ----- ----- 5.834(4) CNi-3.3 C ----- ----- ----- ----- ----- ----- 5.835(3) CNi-4.1 C ----- ----- ----- ----- ----- ----- 5.836(6) CMn-1.8 7M-O 4.195(1) 29.258(6) 5.598(2) ----- ----- ----- ----- CMn-2.4 7M-O+ NM-T 4.190(1) 29.429(8) 5.642(1) 3.905(4) 3.905(4) 6.448(2)

-----

CMn-3.3 7M-O+ NM-T 4.131(9) 28.832(1) 5.793(2) 3.903(7) 3.903(7) 6.445(9) ----- CMn-4.0 7M-O+ NM-T 4.134(2) 28.816(9) 5.791(4) 3.899(2) 3.899(2) 6.428(5) ----- CMn-4.7 7M-O+ NM-T 4.137(2) 28.716(3) 5.793(2) 3.897(5) 3.897(5) 6.416(9)

-----

CMn-6.8 NM-T ----- ----- ----- 3.876(1) 3.876(1) 6.402(7) -----

FIGURE 4. The figure shows optical micrographs from C-0, CNi-x and CMn-y alloys at RT. (a) and (b) show widespread twining in the alloy with 7M crystal structure. In (c), short range twin variants along with regions without twinning are seen in CNi-1.9 alloy which is a mixture the 7M and cubic austenitic phases. In (d) the austenite phase shows no twinning. Figures (e) to (h) show sporadic appearance of small islands within which twin variants occur on a fine scale for the alloys of the CMn-y series.

picture of coexisting austenitic and martensitic phases as deduced from XRD pattern. The microstructures of the alloys CNi-3.3 (Fig. 4(d)) and CNi-4.1 (not shown) did not show any twin variants since they are in cubic austenite phase at RT. In contrast, the microstructures of all the alloys with Co doped for Mn, shown in Figs. 4 (e) to (h), do exhibit twins but only in the form of sporadic islands. Faint twin variants are found within the islands and long-range deformation is conspicuously absent.

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It can be seen that formation and nature of twin variants are strongly correlated to the presence of structural ordering in martensitic state. This is also evident from the XRD measurements performed at 150 K on CNi-3.3 and CMn-3.3 alloys (Fig 5 (a)) and the microstructure recorded in CNi-3.3 alloy (Fig 5 (b)) at 150 K, where both the alloys are in martensitic phase. Figure 5(a) shows that the majority phase of CMn-3.3 alloy is retained to be NM structure to low temperatures. It also shows that the CNi-3.3 alloy which is a cubic austenite at RT is observed to transform to 7M-O in martensitic phase. These results show that the structure and nature of lattice modulation are not affected in the martensitic phase, by substitution of Co for Ni when x varies from 0 to 4.1 in CNi-x series. On the contrary, Co doped for Mn changes the structure (from 7M-O for C-0) to NM for CMn-y alloys, for y > 1.6, in the martensitic phase.

Additionally Fig. 5 (b) shows that widespread twin variants and crossing twins, which were absent at RT in Fig. 4 (d), appear at 150 K and are typical of 7M orthorhombic structure of the parent alloy (Fig. 4(a)) and are very different from the islands seen in CMn-y series with NM structure. These effects indicate that the local stresses generated by doping have a strong bearing on the structural and microstructural modifications. Possible origin of these effects in relation to atomic volume changes is discussed in the latter sections.

FIGURE 5. (a) XRD patterns recorded at 150 K in CNi-3.3 and CMn-3.3 alloys. The pattern from CNi-3.3 is indexed to a 7M orthorhombic unit cell and that from CMn-3.3 to a mixture of NM and 7M unit cells. Suffix ‘M’ stands for reflections from the 7M phase in the pattern of the CMn-3.3 alloy. (b) Widespread twin variants characteristic of 7M orthorhombic structure is observed when the CNi-3.3 alloy is cooled to 150 K. The alloy which is cubic at RT, has a martensitic 7M structure at 150 K as seen from (a).

3.3. Site preference of magnetic and non-magnetic atoms

Of recent, studies are directed towards understanding the role of site occupancy of Ni, Mn and Ga in off-stoichiometric Ni2MnGa alloys [29-31]. From neutron diffraction and magnetic studies, Richard et al. reported [29] that in Ni - rich Ni-Mn-Ga alloys, the excess Ni displaces Mn at regular sites, rather than occupying vacant Ga sites. The moments on Ni atoms at Mn site are ordered ferromagnetically relative to the moments in the Ni and Mn sub-lattices. The displaced Mn and excess Mn atoms, if any, would occupy the vacant Ga sites with their moments coupled antiferromagnetically to the sum of magnetic moments on the Ni and Mn sub-lattices. Chen Jie et al. [30] have supported this view from their calculations for Ni-rich Ni-Mn-Ga alloys based on Density Functional Theory (DFT). They stated that the smaller

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lattice distortion and the lower-energy electronic structure explain as to why the excess Ni occupies Mn sites. In Mn - rich Ni - poor compositions like Ni43.8Mn37.8Ga18.4 alloy Lázpita et al. [31] have proposed that 25 at.% Mn occupy regular Mn site while 6.2 at.% of the excess Mn occupy the vacant Ni site while the remaining 6.6 at.% of the excess Mn occupy the vacant Ga sites. The Mn at Ni site being the nearest to Mn at regular site, these Mn-Mn exhibit antiferromagnetic exchange. This renders the Mn at Ga site to be coupled ferromagnetically relative to Mn at regular site, unlike in Ni excess compositions [31].

To study the site preference of substituted atoms in this system, various alloys of quasi-ternary Ni50Mn25+xGa25-x-yMy (M = Co, Fe) system are examined whose compositions are listed in Table 2. These compositions are selected such that Ni content is nearly 50%. The field dependence of magnetization of the alloys has been measured at 5 K up to a field (H) of 5 T. The measured values of saturation magnetization (Msat) at 5K and the corresponding magnetic moment determined per formula unit (f.u.) (referred to as μexp) in units of μB/f.u. are tabulated in Table 2 for all the alloys. TABLE 2. The chemical composition from EDAX, corresponding code of alloy, electron per atom ratio and the experimentally determined magnetic moment per formula unit at 5 K of Ni-Mn-Ga, Ni-Mn-Ga-Co and Ni-Mn-Ga-Fe alloys. The magnetic moment values were deduced from the magnetization values obtained using the PPMS as discussed in the text.

S.No Alloy composition Code e/a Measured magnetic moment per formula

unit (μB/f.u.) at 5 K

Curie temperature TC (K)

1 Ni50Mn29Ga21 M12 7.650 3.47 374 2 Ni50.4Mn27.6Ga22 M13 7.656 3.53 372

3 Ni51.1Mn28.5Ga20.4 M14 7.717 3.21 369 4 Ni50.5Mn26.5Ga21.6Co1.4 C20 7.679 3.36 373 5 Ni49.8.Mn27.2Ga21.2Co1.8 C21 7.681 3.26 380 6 Ni50.7Mn25Ga21.6Co2.7 C22 7.711 3.2 371 7 Ni50.7Mn25.3Ga20.5Co3.5 C23 7.771 2.96 388 8 Ni51.8Mn24.6Ga20.2Co3.5 C25 7.815 2.95 393 9 Ni50.1Mn26Ga21.5Co2.4 C38 7.691 3.37 382.5

10 Ni50.4Mn25.2Ga20.9Co3.5 C60 7.746 3.16 384 11 Ni50.7Mn23.3Ga21.3Co4.7 C46 7.763 2.82 391 12 Ni50.1Mn25.4Ga20.3Co4.2 C52 7.775 2.97 375 13 Ni50.3Mn27.5Ga20.8Fe1.4 F26 7.692 3.25 374 14 Ni50.4Mn26Ga20.5Fe3.1 F28 7.741 3.10 396 15 Ni50.5Mn25.7Ga21.5Fe2.3 F30 7.678 3.35 389 16 Ni50.3Mn24.7Ga20.4Fe4.6 F47 7.74 3.06 397 17 Ni50.1Mn26.7Ga19.6Fe3.6 F53 7.751 3.10 391

The μexp for Ni50Mn29Ga21 (referred to as M12) is determined to be 3.47 μB/f.u. and is in agreement with the value reported for this composition [32]. We have computed the calculated magnetic moment (μcal) for M12 sample considering a contribution of 3.35 μB and 0.33 μB respectively from the Mn and Ni atoms. Here, the moments on Mn and Ni at regular sites are considered to have ferromagnetic exchange, while those on the excess Mn occupying Ga positions are antiferromagnetically coupled to the Mn and Ni atoms at regular sites [15, 29]. Our measured μexp for M12 agrees well with the μcal of 3.47μB/f.u. The μexp values

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measured at 5 K for the other off-stoichiometric compositions Ni50.7Mn27.7Ga21.6 (M13) and Ni51.1Mn28.5Ga20.4 (M14) also are in good agreement with the corresponding μcal values computed on lines similar to that discussed for M12. These calculations consider that for Mn-rich alloys with a constant (50 at.%) Ni content, the extra Mn atoms occupy the vacant Ga site with their moments coupled antiferromagnetically to those on Mn sub-lattice.

Since the magnetic moment of 1.72 μB present on Co atom [33] is less when compared to 3.35 μB on Mn atom, one expects an increase in magnetization if Co atoms replace the Mn atoms at Ga site, coupled antiferromagnetically. In Fig. 6, we show the temperature dependence of magnetization for a subset of the alloys listed in Table 1, viz. Ni50Mn29Ga21 (M12), Ni50.1Mn26Ga21.5Co2.4 (C38), Ni50.4Mn25.2Ga20.9Co3.5 (C60) and Ni50.7Mn23.3Ga21.3Co4.7 (C46), in which atomic concentration of Ni and Ga are approximately constant and Co concentration increases at the expense of that of Mn [34].

FIGURE 6. Temperature dependence of saturation magnetization (Msat) for alloys Ni50Mn29Ga21 (M12), Ni50.1Mn26Ga21.5Co2.4 (C38), Ni50.4Mn25.2Ga20.9Co3.5 (C60), Ni50.7Mn23.3Ga21.3Co4.7 (C46), in the temperature interval 5 - 400 K. A reduction in Msat can be seen with increasing cobalt concentration in these alloys in which the tomic concentrations of Ni and Ga are approximately constant and Co concentration increases at the expense of Mn concentration.

One can see that the magnetization reduces with an increase in Co content. Thus, there is a serious disagreement between the experimental results and the expected trend. Experimentally observed reduction of saturation magnetization in Fig. 6 with Co replacing Mn atoms, suggests that Co atoms might show a preference to replace the Mn at regular Mn site, rather than those at Ga site. Then one needs to examine the order in the magnetic moments on Co, as to whether they are ferromagnetically or antiferromagnetically coupled with respect to Ni and Mn sub-lattices, consistent with the observed Msat values for all the alloys.

In order to study the site preference and magnetic order of the dopant (Co or Fe) atoms in Ni-Mn-Ga system, four possible configurations [34] are examined with an objective to achieve the best possible agreement between the experimental and the calculated magnetic moments.

In Configuration I, the Co or Fe atoms are assumed to occupy the vacant Ga site and exhibit ferromagnetic exchange with magnetic moments of the Ni and Mn sub-lattices while in Configuration II, the Co or Fe atoms occupy the vacant Ga site but

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with antiferromagnetic exchange coupling. In configuration III, the dopant atoms are assumed to occupy the regular Mn site with ferromagnetic exchange while the excess and displaced Mn atoms occupy the Ga site with antiferromagnetic exchange.

FIGURE 7. The magnetic moments of the Ni and Mn sub-lattices (Mn-I) are parallel to each other (marked by an up-arrows). Co or Fe atoms displace the Mn atoms from the Mn sub-lattice. The displaced Mn and the excess Mn atoms occupy the Ga sub-lattice (Mn-II). The magnetic moments of the displaced Mn and the excess Mn atoms align antiparallel to the Ni and Mn sub-lattice, as marked by down-arrows. In Configuration III, the magnetic moments on Co or Fe, which reside on the regular Mn sites order parallel to the magnetic moments on Ni and Mn sub-lattice, as marked by up-arrows.

In configuration IV, we consider the dopant atoms to occupy the Mn site but with antiferromagnetic exchange and the excess and displaced Mn that occupy the Ga site also to exhibit antiferromagnetic exchange. One should note that the actual moment configuration could differ depending on the actual composition of the alloys under study. The details of the moments in Configurations III are shown in Fig. 7.

The solid lines in Fig. 8 indicate the best fit lines for each of the four configurations showing the variation of magnetic moment μcal, calculated knowing the composition, with e/a for all the alloys under study presented in Table 2. The open circles represent the measured magnetic moment μexp with e/a for all the alloys and evidently show the agreement with the line representing the calculated magnetic moment values computed for Configuration III.

This remarkable agreement supports the view that the Co or Fe atoms substituted for Mn occupy regular Mn sites with ferromagnetic exchange, and that the excess Mn and displaced Mn atoms occupy the Ga sites with antiferromagnetic exchange.

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FIGURE 8. The calculated magnetic moment per formula unit is plotted as solid lines versus the electron/atom ratio of the compounds, for the four assumed configurations I, II, III and IV. The experimental magnetic moment per formula unit for all the alloys, at 5 K (represented by open circles) fall on the calculated line corresponding to Configuration III.

CONCLUSIONS

The effect of cobalt substitution for Ni or Mn in Ni50Mn29Ga21 alloy with 7M orthorhombic structure on the crystal structure and the microstructure is investigated. The results reveal that 1.6 at. % Co substituted for Mn induces crystallization of NM tetragonal phase in 7M-O martensitic structure of C-0 alloy and that the crystal structure / ordering and microstructure are strongly correlated. An examination of the atomic volumes [29, 35] of the elements involved reveals that substitution of Co, whose atomic volume (VCo) is 6.7 cm3/mol, for Ni (with VNi = 6.6 cm3/mol) causes an atomic volume change of only 1.7%. On the other hand, substitution of Co for Mn (VMn = 7.4 cm3/mol) causes 9.5% volume change. This possibly induces a large localized stress when Co is doped for Mn and suppresses the superstructural ordering, while substitution of Co for Ni does not seem to affect 7M-O ordering as well as wide spread twinning. The effect of atomic volume change is also reflected in the microstructures which show suppression of long range twin deformation, associated with absence of lattice modulation in CMn-y series. The correlation of saturation magnetization at low temperatures also supports the picture that the dopants Co and Fe show a preference to occupy the place of Mn with atomic volume closer to them, rather than that of Ga with much larger volume,VGa = 11.8 cm3/mol. It appears that site preferences of Co or Fe are correlated to minimization of the effects associated with lattice distortion in Mn-rich Ni-Mn-Ga alloys and are governed by the atomic volume changes caused by dopants.

ACKNOWLEDGEMENTS

Funding from Aeronautical Development Agency (ADA), Bangalore in the form of a project (Ref. No. ADA/AF/DISMAS/163/2005) is gratefully acknowledged. ASK thanks CSIR (Ref. No. 03 (1065) 106/EMR-II) and MR thanks UGC-RFSMS for

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fellowship. Authors thank Centre for Nanotechnology, University of Hyderabad for PPMS and TEM facilities.

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Documents

Influence of Site Occupancy on the Structure, Microstructure and Magnetic Properties of Ternary and Quasi-ternary Alloys of Ni-Mn-Ga