Mn2YZ systems where Y is an element with 3d electrons and Z is an element from Group III or IV of the periodic table, by and large, belong to the family of Heusler compounds. These compounds have attracted considerable attention due to their multifunctional properties. Many compounds in this series show magnetic shape memory effect and magnetocaloric effect, useful for magnetomechanical actu- ators [20, 49] and magnetic refrigeration [17]. On the other hand, few compounds are promising for spintronics applications such as STT-MRAM [50], spin-valves [51], magnetic tunnel junction [52] and spin-gapless semiconductor [53].
In general, the Heusler compound X2YZ, crystallizes in the cubic L21 (Space group no. 225; F m¯3m) structure where the X atoms occupy the Wyckoff position 8c(1/4, 1/4, 1/4), the Y and Z atoms occupy the positions 4b(1/2, 1/2, 1/2) and 4a(0, 0, 0) respectively [54]. In X2YZ, X and Y are transition metal elements and Z is the main group element. The crystal structure of cubic L21 is shown in Fig.1.5 (a). Another prototype of Heusler compound, called “inverse” Heusler, is formed in Hg2CuTi (space group no. 216; F¯43m) structure. In the inverse Heusler structure, the atom at 4b site gets interchanged with one of the atoms at 8c site of regular Heusler alloy. Thus, the X atoms of X2YZ compounds occupy the Wyckoff positions 4c(3/4, 3/4, 3/4) and 4b(1/2, 1/2, 1/2) whereas the Y and the Z atoms occupy the 4c(1/4, 1/4, 1/4) and 4a(0, 0, 0) positions respectively [54]. The corresponding crystal structure is shown in Fig.1.5 (b). An alternative arrangement of the atoms
1.5 Mn2YZ systems: Experimental and theoretical background can also be made as the 4a site is symmetric with 4b and 4c with 4d. Depending on the number of valence electrons of the constitute elements, a system crystallizes in the regular or inverse Heusler structure. If the number of valence electrons of the transition metal element Y is larger than that of the transition metal element X, the system crystallizes in the inverse Heusler structure [54–58]. Experimental and theo- retical studies on numerous Heusler compounds established the above rules for site occupancies of X2YZ Heusler compounds with few exceptions. Another structural phase in the Heusler compounds is the tetragonal derivative of the cubic structure which has been widely reported in magnetic shape memory alloys. The tetragonal structure can be generated through a rotation of the cubic structure by 45◦ along [110] direction. The cubic unit cell can also be described in terms of tetragonal cell with c/a ratio of √
2 and the corresponding relations between the cell parameters are ctet=ccub and atet=acub/√
2. Recently, other structural phases such as different modulated structures representing the microstructures of the sample during marten- sitic transformation, cubic Cu3Au-like structure and hexagonal structure have been reported [59–62]. The details of these structural phases have been described later for different compounds.
Heusler alloys received a tremendous attention when magnetic shape memory effect was observed in Ni2MnGa which undergoes a martensitic phase transformation at 202 K and has a Curie temperature of 380 K [63]. Ni2MnGa crystallizes in a highly ordered L21 type crystal structure, where Ni, Mn and Ga atoms occupy the 8c, 4b and 4a Wyckoff positions respectively. However, active research started in late 1996 when a 0.2% of MFIS was observed by Ullako and co-workers [18]. Since then, the Heusler Ni-Mn-Ga system has been explored extensively. The reason was that several modulated martensite phases were observed in this system with the composition ratio of Ni, Mn and Ga near 2:1:1. A 6% strain was obtained in the martensitic phase of Ni-Mn-Ga alloy with a five-layered modulated (10M) structure [19, 21]. A MFIS of 10% was also observed in the same system with a seven-layered modulated structure (14M) [20, 21]. In spite of having large MFIS, a martensitic transformation temperature (Tm) below the room temperature and a relatively low Curie temperature (Tc) were serious hindrances in exploiting the functionalities of Ni2MnGa. This is because of the fact that the Tm being lower than the room temperature makes the commercial realization of the material for shape memory applications difficult. As an alternative, many off-stoichiometric materials were investigated with high content of Ni and low content of Mn or with high content of Mn and low content of Ga with mixed outcomes.
In Mn2YZ series, Mn2NiGa is the well studied compound as it is directly related with Ni2MnGa. It was synthesized during the ongoing process of functionality im- provement of Ni2MnGa by varying the composition of Ni and Mn. Liuet al. were the first to synthesize the Mn2NiGa which crystallized in inverse Heusler structure at high temperatures [64]. As was described earlier, the two Mn atoms occupy the 4a and 4c Wyckoff positions in the Hg2CuTi lattice; we denote them as MnI and MnII respectively throughout the thesis. Ni and Ga occupy the 4b and 4d sites respectively. Mn2NiGa exhibits a martensitic transformation around room temper- ature (Tm=270 K) and has a quite high Curie temperature of 588 K [64]. A MFIS of 4% was also observed in the non-modulated (NM) tetragonal structure [64]. The large values of Tm and Tc compared to the prototype Ni2MnGa make this mate- rial promising for better practical applications. In Mn2NiGa, the Mn atoms are ferrimagnetically coupled resulting in a low saturation magnetization compared to Ni2MnGa [65, 66]. Inspite of low magnetization, the ferrimagnetic coupling be- tween two Mn atoms gives rise to a plethora of interesting physical properties in Mn2NiGa [66]. Singh et al. reported spin-valve-like magnetoresistance in Mn2NiGa at room temperature [51]. Recently, a notable inverse magnetocaloric effect (MCE) was reported in Mn2NiGa [67]. Experimentally, the existence of few modulated martensite phases has been also reported which can further improve the MFIS in Mn2NiGa [68,69].
Another compound in this series, Mn3Ga, a promising material for spintronics applications, has also been studied extensively. Mn3Ga crystallizes in cubic, tetrag- onal and hexagonal structures as reported by different experiments [59, 60, 70,71].
In the tetragonal and hexagonal phases, a high perpendicular magneto-crystalline anisotropy and a large Exchange Bias are observed, which are useful for STT- MRAM [70] and magnetic tunnel junction [60] applications, respectively. These studies on Mn2NiGa and Mn3Ga, motivate the researchers to explore Mn2YZ sys- tems with different Y and Z elements. For example, Mn2FeGa has been synthesized in similar crystallographic phases like Mn3Ga [61, 62,72]. In the tetragonal phase, Mn2FeGa behaves like an exchange spring which is useful for magnetic storage tech- nology [61]. A giant exchange bias has also been reported in both tetragonal and hexagonal phases [61,62]. Another compound, Mn2CoGa, a high spin-polarized ma- terial, has been studied in bulk and thin films and its possible applications have been explored [73, 74]. Recently, Mn2CoAl has been realized as a spin-gapless semicon- ductor [53]. From DFT calculations Mn2NiAl, Mn2NiSn and Mn2NiIn are predicted to be magnetic shape memory materials [75–78]. However, the shape memory effect
1.6 The importances of first-principles electronic structure calculations towards inquiring