Cubic-type Heusler compound ${\mathrm{Mn}}_2\mathrm{FeGa}$ thin film with strain-induced large perpendicular magnetic anisotropy

This study reports the development of ferrimagnetic cubic Heusler compound ${\mathrm{Mn}}_2{\mathrm{Fe}}_x\mathrm{Ga}$ (MFG) thin films with large perpendicular magnetic anisotropy (PMA). By growing MFG on a Cr buffer layer, a cubic ${\mathrm{X}}_{\mathrm{a}}$ phase with a tetragonal distortion $c/a\approx 1.04$ induced by the buffer layer was obtained in the range of $x=1.0–1.3$, leading to large PMA which exceeds $0.75\mathrm{MJ}{\mathrm{m}}^{-3}$ in stoichiometric ${\mathrm{Mn}}_2\mathrm{FeGa}$ $(x=1)$. Synchrotron Mössbauer spectroscopy revealed that these cubic MFG thin films show good chemical ordering close to the ${\mathrm{X}}_{\mathrm{a}}$-ordered state. First-principles calculations demonstrated that, in ${\mathrm{X}}_{\mathrm{a}}$-ordered cubic MFG, the characteristic electronic structure of Fe around the Fermi level causes a large uniaxial magnetocrystalline anisotropy under the tetragonal strain consistent with experiment. The half-metallic-like band structure of cubic MFG was also shown to be preserved under the strain. Cubic ${\mathrm{Mn}}_2\mathrm{FeGa}$ with its small saturation magnetization, large PMA, and possibility of being highly spin polarized make this material an ideal candidate for the development of magnetic random-access memory and other spintronic devices.

Figure 1

(a) Bright field cross-sectional TEM image for the stoichiometric ${\mathrm{Mn}}_2\mathrm{FeGa}$ (30 nm)/Cr (20 nm)/MgO(001) sample. (b) High-resolution TEM image of the MFG/Cr interface. (c) EDS mapping results for the image shown in part (a). RHEED patterns along the [110] and [100] azimuths of the MgO(001) substrate for (d) a 20 nm thick Cr buffer grown on the MgO(001) substrate and (e) a 30 nm thick stoichiometric MFG thin film grown on top of the Cr buffer layer.

Figure 2

(a) The XRD profiles of the MFG/Cr samples with Fe concentrations, $x=0.8$, 1.0, and 1.3 respectively. (b) and (c) XRD profiles of the MFG/Cr sample with $x=1.0$ for the (111) and (110) crystallographic planes, respectively. (d) $\varphi$ scan of the (202) reflection.

Figure 3

(a) In-plane (IP) and out-of-plane (OP) magnetization curves measured at room temperature for the stoichiometric MFG/Cr sample. (b) Magnetic anisotropy constant ($K_{\mathrm{u}}$) and saturation magnetization ($M_s$) as a function of Fe concentration $x$. (c) Magnetic coercivity (${\mu }_0{H}_c$) and squareness of the hysteresis loop ($M_r/M_s$) also a function of $x$.

Figure 4

Mössbauer spectra of MFG/Cr samples with different Fe concentrations ($x$). The sample with $x=0.8$ shows a peak which is a convolution of two magnetic components: the blue component due to the presence of tetragonal MFG and the golden component due to cubic MFG. For the samples with $x=1.0$ and 1.3, only a single component due to cubic MFG was observed. The fitted peak positions are highlighted by the bars above each spectrum.

Figure 5

(a) Total DOS for ${\mathrm{X}}_{\mathrm{a}}$-ordered ${\mathrm{Mn}}_2\mathrm{FeGa}$ without tetragonal strain ($\lambda =0\%$). (b)–(d) Projected LDOSs for this ordered state of the Mn $4a$, Mn $4c$, and Fe $4b$ atoms, respectively. (e) Total DOS for ${\mathrm{X}}_{\mathrm{a}}$-ordered ${\mathrm{Mn}}_2\mathrm{FeGa}$ with $\lambda =4.1\%$ and (f)–(h) LDOS for this case, respectively.

Figure 6

The tetragonal strain $\lambda$ dependence of $K_{\mathrm{u}}$. The inset is the theoretical model of the ${\mathrm{X}}_{\mathrm{a}}$-ordered state of cubic ${\mathrm{Mn}}_2\mathrm{FeGa}$.

Figure 7

The results of the second-order perturbation analysis of the magnetocrystalline anisotropy energy for cubic ${\mathrm{Mn}}_2\mathrm{FeGa}$ with $\lambda =4.1\%$.