Heteroepitaxial growth of tetragonal Mn$_{2.7-x}$Fe$_{x}$Ga$_{1.3}$ (0 $\leqslant$ x $\leqslant$ 1.2) Heusler films with perpendicular magnetic anisotropy

This work reports on the structural and magnetic properties of Mn$_{2.7-x}$Fe$_{x}$Ga$_{1.3}$ Heusler films with different Fe content x (0 $\leqslant$ x $\leqslant$ 1.2). The films were deposited heteroepitaxially on MgO single crystal substrates, by magnetron sputtering. Mn$_{2.7-x}$Fe$_{x}$Ga$_{1.3}$ films with the thickness of 35 nm were crystallized in tetragonal D0$_{22}$ structure with (001) preferred orientation. Tunable magnetic properties were achieved by changing the Fe content x. Mn$_{2.7-x}$Fe$_{x}$Ga$_{1.3}$ thins films exhibit high uniaxial anisotropy Ku $\geqslant$ 1.4 MJ/m3, coercivity from 0.95 to 0.3 T and saturation magnetization from 290 to 570 kA/m. The film with Mn$_{1.6}$Fe$_{1.1}$Ga$_{1.3}$ composition shows high Ku of 1.47 MJ/m3 and energy product ${(BH)_{max}}$ of 37 kJ/m3, at room temperature. These findings demonstrate that Mn$_{2.7-x}$Fe$_{x}$Ga$_{1.3}$ films have promising properties for mid-range permanent magnet and spintronic applications.


Introduction
Currently, intensive efforts are made to develop novel permanent magnets in order to reduce or even completely replace their rare earth content (Nd, Sm). [1][2][3] The increasing global demand for permanent magnets has been driven by the development of high-efficiency motors and generators for various clean energy applications, such as wind turbines for power generation, electrical vehicles, and magnetic refrigeration. Therefore, the replacement of rare earth in permanent magnets became necessary, due to the volatility of their prices and the strategic issues associated with them. Moreover, new permanent magnets need to contain environmentally sustainable elements with lower environmental impact than rare earths.
Apart from high-end applications, new magnets with mid-range performance are required as well, where novel hard magnetic materials are predicted to bridge the gap between low-cost hard ferrites and expensive rare-earth-based magnets. 2 To achieve this aim, the new class of permanent magnets need to meet the criteria of high Curie temperature (T c ) and high saturation magnetization (M s ) combined with strong uniaxial magnetic anisotropy (K u ), in order to achieve a large energy product value (BH) max at room temperature (RT). 4 Different approaches have been proposed to develop rare earth free permanent magnets. One of them is to induce magnetocrystalline anisotropy through tetragonal distortion in phases possessing high magnetization, such as Fe x Co 1-x alloy films grown on an appropriate buffer layer, [5][6][7] or doped by a third element [8][9][10][11] . Another promising approach is the exchange-coupled nanocomposites, where one phase with large coercivity H c is combined with a high-magnetization phase M s . [12][13][14][15] The third involves searching and investigating new magnetic compounds exhibiting high magnetocrystalline anisotropy. As an example, it has been reported that the novel non-cubic Zr 2 Co 11 and HfCo 7 alloys show attractive hard magnetic properties with large anisotropy. 16,17 Among new materials, Mn-based magnetic compounds 18,19 have attracted much interest in recent years, due to their high magnetocrystalline anisotropy, such as the tetragonal Mn x Ga, 20,21 MnAl 22 and the hexagonal MnBi. 23,24 Heusler compounds are a remarkable class of materials with tunable multifunctional properties and a huge potential for various applications. 25,26 Tetragonally distorted D0 22 Mnbased Heusler compounds, such as the ferrimagnetic Mn x Ga and Mn x Ge (x = 2-3) show large magnetocrystalline anisotropy and large coercivity, as well as high Curie temperature. 19,[27][28][29][30] However, they suffer from low saturation magnetization, due to the antiparallel coupling of the magnetic moments in the Mn atoms at 4d (Mn-II) and 2b (Mn-I) sites. 27,31 Therefore, the magnetization in the Mn-based tetragonal Heusler compounds has to be enhanced for the potential permanent magnets applications. Towards to this direction, it was reported that the tetragonal phase of the Mn 38.5 Fe 32.5 Ga 29 compound shows a coercivity of 0.33 T and a remanent magnetization of 20 Am 2 kg -1 at RT. 32 Mn 3-x Y x Ga thin films with perpendicular magnetic anisotropy and tunable magnetization may be realized by the substitution of Mn by ferromagnetic elements (Y=Co, Fe). [33][34][35] This tunable magnetic behavior make these materials interesting for spintronic applications as well.
In this work, we present the structural and magnetic properties of tetragonally distorted Mn 2.7-x Fe x Ga 1.3 films with strong perpendicular magnetic anisotropy. For this purpose, we performed systematic X-ray diffraction (XRD), transmission electron microscopy (TEM) and magnetic characterization of films, heteroepitaxially grown on MgO substrates.

Experimental details
Mn 2.7-x Fe x -Ga 1.3 (x = 0-1.2) films with thickness of 35 nm have been deposited on single crystal MgO (100) substrates. For the deposition, a BESTEC UHV magnetron sputtering system has been used with Mn (2"), Fe (2"), and Mn 50 Ga 50 (2") sources in confocal geometry. The target to substrate distance was 17 cm. Prior to the deposition, the chamber was evacuated to a base pressure less than 2 x 10 -8 mbar, while the process gas (Ar 5N) pressure was 3 x 10 -3 mbar. Mn-Fe-Ga films were grown by co-sputtering at 350 °C, and then post-annealed in situ for additional 20 minutes to improve chemical ordering. The power of the different sources was adjusted, in order to reach the desired composition of the Mn-Fe-Ga films. Special attention was paid to keep Ga content equal for consistency, and to investigate the substitution of Mn by Fe. All samples were capped at room temperature with a 2 nm thick Al film to prevent oxidation. Stoichiometry was estimated by energy-dispersive X-ray analysis (EDX) and verified by inductively coupled plasma optical emission spectrometry (ICP-OES). X-ray diffraction (XRD) was collected with a Panalytical Diffractometer X'PERT 3 MRD, by using Cu-Kα 1 radiation (λ=1.5406 Å). The film thicknesses were determined by X-ray reflectivity (XRR) measurements. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were performed by a FEI Tecnai G2 F20 microscope at 200 kV and a FEI TITAN −3 G2 80-300 microscope equipped with a SuperX energy-dispersive X-ray spectroscopy (EDXS) analyzer at 300 keV, respectively. TEM samples were prepared by focused ion beam milling (FIB). A protective Pt layer was deposited before the cross sectioning. Magnetic measurements were carried by out using a Quantum Design (MPMS 3) magnetometer.

Results and discussion
Different XRD measurements, such as θ-2θ, rocking curve, and phi-scans were performed to study the structure, the crystallinity, and the heteroepitaxial relationship between the films and the substrate. XRD patterns of the films with different Fe content x are shown in Fig. 1 (a). All samples have been indexed by assuming a tetragonal D0 22 phase. In the tetragonal D0 22 cell of stoichiometric Mn 3 Ga, Mn, and Ga occupy two and one crystallographic sites, respectively. The resulting space group is I4/mmm with MnII on 4d (0, 1/2, 1/4), MnI on 2b (0, 0, 1/2), and Ga on 2a (0, 0, 0), as depicted in Fig. 2

(b). Fe in Mn 2.7-
x Fe x Ga 1.3 is expected to be preferentially substituted on the 4d site, due to its higher electronegativity, as previously known for Heusler compounds. 36 Table I. Lattice parameter c is deduced from the (002) and (004)   The heteroepitaxial relationship between the film and substrate was further confirmed by the TEM analysis. Fig. 2 (a)  Typical in-plane and out-of-plane magnetization hysteresis loops of the Mn 1.9 Fe 0.8 Ga 1.3 film, measured at 300 K are shown in Fig. 4 (a). The perpendicular magnetic anisotropy is manifested by the easy saturation axis oriented along the film normal. The outof-plane coercive field is 0.78 T and the saturation magnetization is 390 kA/m. The shape of the in-plane magnetization curve reveals a small in-plane moment, which might be originated from a slight atomic disorder. 28 The out-of-plane magnetization hysteresis loops of the Mn 2.7- x Fe x Ga 1.3 series, measured at 300 K are summarized in Fig. 4 (b). All compositions show clearly that the easy magnetization axis is oriented normal to the film plane.   Large coercivity values and the maximum energy product (BH) max 46 and Zr 2 Co 11 thin film assemblies (132 kJ/m 3 ). 16 Here, the moderate squareness ratio (S = 0.8) limits the magnitude of (BH) max , which can be further optimized by an appropriate buffer layer. 35

Conclusions
In summary, we have studied the structural and magnetic properties of Mn 2.7-x Fe x Ga 1. This tunable magnetic behavior constitutes an attractive option for spintronic applications.
The combination of large K u ≥ 1.4 MJ/m 3 and high (BH) max up to 37 kJ/m 3 makes Mn 2.7- x Fe x Ga 1.3 films candidate materials for mid-range permanent magnets.