Observation of the topological Hall effect and signature of room-temperature antiskyrmions in Mn-Ni-Ga $D_{2d}$ Heusler magnets

Topologically stable nontrivial spin structures, such as skyrmions and antiskyrmions, display a large topological Hall effect owing to their quantized topological charge. Here, we present the finding of a large topological Hall effect beyond room temperature in the tetragonal phase of a Mn-Ni-Ga based ferrimagnetic Heusler shape memory alloy system. The origin of the field induced topological phase, which is also evidenced by the appearance of dips in the ac-susceptibility measurements, is attributed to the presence of magnetic antiskyrmions driven by $D_{2d}$ symmetry of the inverse Heusler tetragonal phase. Detailed micromagnetic simulations assert that the antiskyrmionic phase is stabilized as a result of interplay among inhomogeneous Dzyaloshinskii-Moriya interaction, the Heisenberg exchange, and the magnetic anisotropy energy. The robustness of the present result is demonstrated by stabilizing the antiskyrmion hosting tetragonal phase up to a temperature as high as 550 K by marginally varying the chemical composition, thereby driving us a step closer to the realization of ferrimagnetic antiskyrmion based racetrack memory.

Figure 1

(a) Crystal structure of ${\mathrm{Mn}}_2\mathrm{NiGa}$: MnI, Ga, MnII, and Ni atoms are represented by red, green, blue, and yellow balls, respectively. The $ab$ plane is shown in light green color. (b) Schematic representation of one-dimensional spin propagation along [100] (helix) and [110] (cycloid) in the $ab$ plane. (c) Spin configuration of a ferrimagnetic antiskyrmion in the present system. (d) Field-cooled (FC) and field-heating (FH) temperature dependence of magnetization $\left[M\right(T\left)\right]$ for ${\mathrm{Mn}}_2\mathrm{NiGa}$ and ${\mathrm{Mn}}_{2.1}{\mathrm{NiGa}}_{0.9}$ measured in 0.1 T field. Inset shows the high-temperature $M\left(T\right)$ curves showing the Curie temperature ($T_C$). (e) Field dependent magnetization $\left[M\right(H\left)\right]$ loops measured at 2 K for ${\mathrm{Mn}}_2\mathrm{NiGa}$ and ${\mathrm{Mn}}_{2.1}{\mathrm{NiGa}}_{0.9}$. The inset shows low-field region of the data.

Figure 2

Field dependent Hall resistivity (${\rho }_{xy}$, filled squares) measured at different temperatures for (a)–(c) ${\mathrm{Mn}}_2\mathrm{NiGa}$, (d)–(f) ${\mathrm{Mn}}_{2.1}{\mathrm{NiGa}}_{0.9}$. The inset of (a) shows an expanded view of the ${\rho }_{xy}$ in the second quadrant. The solid lines represent calculated Hall resistivity without topological Hall contribution as described in the text.

Figure 3

Field dependent ac susceptibility measured at different temperatures for (a)–(f) ${\mathrm{Mn}}_2\mathrm{NiGa}$ and (g)–(i) ${\mathrm{Mn}}_{2.1}{\mathrm{NiGa}}_{0.9}$.

Figure 4

Field dependent topological Hall resistivity (${\rho }_{xy}^T$) at different temperatures for (a) ${\mathrm{Mn}}_2\mathrm{NiGa}$ and (b) ${\mathrm{Mn}}_{2.1}{\mathrm{NiGa}}_{0.9}$. The $H\text{−}T$ phase diagram showing field dependence of ${\rho }_{xy}^T$ at different temperatures derived from the topological Hall effect measurements for (c) ${\mathrm{Mn}}_2\mathrm{NiGa}$ and (d) ${\mathrm{Mn}}_{2.1}{\mathrm{NiGa}}_{0.9}$. (e) Magnetic anisotropy $K_{\mathrm{u}}$ vs DMI, (f) $K_{\mathrm{u}}$ vs exchange stiffness constant ($A$), phase diagrams illustrating the stability of antiskyrmion (ASk) phase for different $K_{\mathrm{u}}$, $D$, and $A$ values. Dotted lines represent boundary between different phases. Different colors in the phase diagram represent number density of antiskyrmions ($n$).