FeMnGa and NiMnGa alloys in the light of the diffraction and microscopy techniques

J. Kopeček¹,  K. Onderková¹, D. Král², L. Klimša¹, L. Straka¹ and O. Heczko¹

¹Department of Functional Materials, Academy of Sciences of the Czech Republic, Institute of Physics,

Na Slovance 2, CZ-18221 Prague, Czech Republic;

²Department of Physics of Materials, Institute of Physics of Charles University, Ke Karlovu 5, CZ-12116 Prague 2, Czech Republic

Whereas NiMnGa ferromagnetic shape memory alloys became an icon of the field in the last two decades, FeMnGa alloys are still waiting for exploration. There are several expectations. Heusler type alloy Fe₂MnGa is one of the few compounds exhibiting ferromagnetic shape memory effect, i.e. reversible martensitic and ferromagnetic transformation in response to the magnetic field (beyond conventional response to temperature or mechanical stress) [1-2].  Multi-ferroic behavior in this system is connected with interesting magneto-transport and magneto-optic properties [3-4]. Since couple of presentation is dedicated to NiMnGa system - even from the same site, let switch our attention in this article to “poor relative” FeMnGa. It is known that the magnetic moment strongly depends on Fe content or Fe/Mn ratio, and the ordering varies from ferromagnetic to antiferromagnetic or paramagnetic.

To open an investigation of this system in our laboratory, we prepared a wide set of samples covering a wide surroundings of the stoichiometric Heusler alloy (Fe₂MnGa).  All samples were metallographically prepared and conventionally studied using LOM, SEM and its hyphenated analytical techniques energy-dispersive X-ray spectrometry (EDS) and electron backscatter diffraction (EBSD).  The crystallographic structures were analysed by X-ray diffraction (XRD) to obtain information on the structures present in the samples, because there are doubts about the high-temperature phase structure, i.e., whether it is D0₃ [3] or L1₂ [5], see Fig. 1.

The structural studies were found to be complicated as proper lattice parameters are not known and good polycrystals are not yet available. Thus only the rough estimate of the presented phases will be presented. Peculiarities of structure and microstructure found during investigation will be presented in the lecture, e.g. as in  Fe₄₀Mn₂₅Ga₃₅ alloy in  Fig. 2,  where tetragonal L1₀ a cubic D0₃ structures are presented.

D03	L12
L10	Fe, Mn, Ga

Figure 1. Structures in the system Fe – Mn – Ga.

Apart from structural investigation, the functional properties of the samples were measured: magnetic properties by vibration magnetometer, and magneto-optic properties by ellipsometry.  Saturated magnetization sharply grows, when iron content overcomes approx. 30 at. %, and nearly immediately drops above 50 at. % of iron in alloys with 25 at.% manganese content. We can speculate about transition from ferromagnetic to antiferromagnetic state in these samples. The investigated FeMnGa samples exhibit magneto-optical spectra similar to those of pure Fe or some Fe compounds, suggesting higher contribution of free electrons to their magneto-optical properties compared to NiMnGa. The amplitude of Kerr effect scales with composition similarly as the saturation magnetization. At room temperature the amplitude of magneto-optical effect of ferromagnetic alloys was approximately five times higher than for off-stoichiometric Ni-Mn-Ga allowing observation of magnetic domains by Kerr effect. The striking difference between magneto-optical spectra of Fe-Mn-Ga and Ni-Mn-Ga suggest that the mechanism of martensitic transformation might be different in these systems [6] despite the same Heusler type.

a	b
c	d

Figure 2. Fe₄₀Mn₂₅Ga₃₅ alloy; a) optical microscopy; EBSD orientation map, visible are cubic L1₂ grain, whereas tetragonal L1₀ grains are black as evaluation quality was poor due to the misfit in c/a ratio; c) scale orientation in EBSD map; d) summarized diffraction pattern from large grain material. The summation over rotated and inclined sample was applied, but it was found insufficient for finding proper tetragonal lattice parameters.

This work was supported in part by the MEYS CR FUNBIO CZ.2.16/3.1.00/21568 (SEM purchase), LO1409 and LM2015088 projects (SEM maintenance) and CSF project 14-36566G (AdMat).

1.         Omoti T, et al.  2009  Appl. Phys. Lett. 95 082508

2.         Zhu W, et al.  2009  Appl. Phys. Lett. 95 222512

3.         Kudryavstev Y V, et al.  2012  Acta Mater. 60 4780

4.         Khovaylo V V, et al.  2013  Phys. Rev. B 87 174410

5.         Gasi T, et al.  2013  J. Appl. Phys. 113 17E301

6.         Heczko O  2014  Mater. Sci. Technol. 30 1559