Effect of Pt substitution on the magnetocrystalline anisotropy of ${\mathrm{Ni}}_2\text{MnGa}$: A competition between chemistry and elasticity

The magnetocrystalline anisotropy (MAE) of ${\mathrm{Ni}}_{2-x}{\mathrm{Pt}}_x\mathrm{MnGa}(0\le x\le 0.25)$ alloys are investigated using the singular point detection technique and density functional theory. A slight reduction in MAE as compared to that of ${\mathrm{Ni}}_2\mathrm{MnGa}$ is observed due to Pt substitution. The calculated MAE varies almost linearly with the orbital moment anisotropy. A competition between the elastic and the chemical contributions explains the observed trend of the MAE with increasing Pt content. The large MAE in combination with the previously reported increase of the martensitic transition temperature makes these alloys promising candidates for ferromagnetic shape memory applications near room temperature.

Figure 1

Martensitic start ($T_M$), austenitic start ($T_A$), and magnetic transition temperature ($T_C$) of ${\mathrm{Ni}}_{2-x}{\mathrm{Pt}}_x\mathrm{MnGa}$ as a function of chemical composition $x$ as obtained in our measurements. Our previous theoretical results on $T_M$ [18] are also shown for comparison.

Figure 2

Example of singular point detection measurement for ${\mathrm{Ni}}_{1.85}{\mathrm{Pt}}_{0.15}\mathrm{MnGa}$ at 213 K. Upper panel: magnetic field pulse. Lower panel: second and third derivatives of the magnetization showing the flexus in $d^{3}M$/${dH}^3$ in the increasing $H\left(t\right)$ branch and the minimum at $d^{2}M$/${dH}^2$ in the decreasing $H\left(t\right)$ branch.

Figure 3

(a) Anisotropy constant ($K_A$) and anisotropy field ($H_A$) as a function of temperature for polycrystalline ${\mathrm{Ni}}_{2-x}{\mathrm{Pt}}_x\mathrm{MnGa}$ ($0\le x\le 0.25$) alloys. The vertical dashed lines correspond to the temperatures where the martensite to austenite transition starts on heating. Solid lines are guides to the eye used to show the variation. (b) $K_A$ and $H_A$ at 78 K for all measured samples.

Figure 4

Dependence of the MAE and the OMA on the Pt concentration in ${\mathrm{Ni}}_{2-x}{\mathrm{Pt}}_x\mathrm{MnGa}$ as obtained from ab initio calculations. The inset shows the variation of the theoretically calculated MAE and the experimentally determined $K_A$ with increasing Pt concentration. Both quantities in the inset are plotted taking their respective values for ${\mathrm{Ni}}_2\mathrm{MnGa}$ as a reference.

Figure 5

(a) Calculated MAE of nonmodulated ${\mathrm{Ni}}_2\mathrm{MnGa}$ for different $c/a$ ratios. Magenta $▾$ indicate results obtained by us, while $\diamond$ and $\circ$ symbols refer to previous theoretical results published in Refs. [56, 57], respectively. The MAE for the $7M$ martensite phase is marked by $\blacksquare$. (b) Calculated MAE as a function of the Pt concentration in ${\mathrm{Ni}}_{2-x}{\mathrm{Pt}}_x\mathrm{MnGa}$ for nonmodulated tetragonal structures with $c/a\approx 1.25$ ($▴$) and 0.90 ($▴$). $•$ correspond to experimental data. All MAEs are plotted taking the MAE of ${\mathrm{Ni}}_2\mathrm{MnGa}$ as the reference.

Figure 6

Changes in the MAE resulting from (a) the chemical effect and (b) the elastic effect due to different Pt concentrations. Ionic relaxation refers to optimization of internal coordinates.