Antiferromagnets (AFMs) have attracted increasing attention in state-of-the-art research. Their important role in enhancing the hardness of ferromagnetic electrodes through the exchange-bias effect in microelectronics, has been broadly extended by new perspectives in spintronic applications. AFMs also facilitate current-induced switching of their order parameter owing to the absence of shape anisotropy and action of spin torques through the entire volume. Additional non-trivial spintronic effects originating from a non-vanishing Berry phase might occur in non-collinear AFMs. Non-collinear planar AFMs without mirror symmetry are predicted to exhibit an anomalous Hall, Kerr, magnetic circular dichroism (MCD), and other effects, which were not encountered in AFMs so far.

Scientists from the Max Planck Institute for Chemical Physics of Solids, Dresden, the ILL in Grenoble, and the HLD have studied the new room-temperature tetragonal non-collinear Heusler AFM Pt2MnGa. Low-field magnetization measurements exhibit a non-saturating(almost linear) increase up to 7 T (Figure 1, left), similar to antiferromagnetic or paramagnetic materials. Only a narrow hysteresis at 2 K (inset in Figure 1) indicates a very weak ferromagnetic component at low temperature. To probe the magnetic response in very high fields, we applied magnetic-field pulses of 60 T. The corresponding magnetization data at 257 and 1.5 K (Figure 1, right) increase monotonically and do not saturate. Only a broadened step-like feature is observed at 1.5 K indicating a metamagnetic transition close to 14 T.The detailed magnetic structure was determined by neutrondiffraction experiments. The results suggest that the magnetic order is a spiral in the Mn sublattices. Owing to the inversion symmetry, the left- and right-handed spirals are equally stable. The large energy barrier between left- and right-handed spirals can efficiently be overcome via a precessional reorientation of magnetization, induced by magnetic-field pulses perpendicular to the spiral axis (Figure 2).

This suggests Pt2MnGa as a convenient candidate for a non-volatile magnetic memory based on the helicity vector as a bit of information.

Reference:
Room-temperature tetragonal noncollinear Heusler antiferromagnet Pt2MnGa.
S. Singh, S. W. D’Souza, J. Nayak, E. Suard, L. Chapon, A. Senyshyn, V. Petricek, Y. Skourski, M. Nicklas, C. Felser, and S. Chadov,
Nat. Commun. 7, 12671 (2016).

Figure 1: Magnetization of Pt2MnGa. Field-dependent magnetization hysteresis loops (left) up to 7 T at 2 (blue) and 300 K (red) and (right) up to 60 T at 1.5 K (blue) and 257 K (red). The insets in the left panel show the low-field data in enlarged scale.

Figure 2: Switching of the magnetic helicity. The application of an external magnetic-field pulse Hext (indicated by the green arrow) perpendicular to the spiral wave vector q causes the precessionof local moments (rotating along the dashed green curves), which changes the helicity of a spiral from a left-handed screw (a), through the intermediate cycloidal state (b), finally to the right-handed screw (c).