Identifying the octupole antiferromagnetic domain orientation in Mn3NiN by scanning anomalous Nernst effect microscopy

Abstract

The intrinsic anomalous Nernst effect in a magnetic material is governed by the Berry curvature at the Fermi energy and can be realized in non-collinear antiferromagnets with vanishing magnetization. Thin films of (001)-oriented Mn3NiN have their chiral antiferromagnetic structure located in the (111) plane facilitating the anomalous Nernst effect unusually in two orthogonal in-plane directions. The sign of each component of the anomalous Nernst effect is determined by the local antiferromagnetic domain state. In this work, a temperature gradient is induced in a 50 nm thick Mn3NiN two micrometer-size Hall cross by a focused scanning laser beam, and the spatial distribution of the anomalous Nernst voltage is used to image and identify the octupole macrodomain arrangement. Although the focused laser beam width may span many individual domains, cooling from room temperature to the antiferromagnetic transition temperature in an in-plane magnetic field prepares the domain state, producing a checkerboard pattern resulting from the convolution of contributions from each domain. These images together with atomistic and micromagnetic simulations suggest an average macrodomain of the order of 1 μm2.

The Impact of Local Strain Fields in Noncollinear Antiferromagnetic Films

“…[ 51,52 ] However, this requires highly specialized techniques, both in terms of the sample preparation and equipment, that is beyond the scope of the present paper. In MNN thin films, our measurements of AHE, [ 11,13,16 ] MOKE, [ 20 ] and ANE, [ 21,23 ] while still large in comparison to the weak magnetic moment, are greatly reduced in comparison to theoretical predictions, which may partially result from mixed tetragonality regions within the films, particularly close to the interface as we have shown here.…”

“…Now considering the AFM structure, the Γ 4g structure has eight possible variants, corresponding to the 8 (111) planes. [ 21 ] Each variant may, under a tetragonal distortion, produce a piezomagnetic moment oriented in one of the eight <112> directions, with the direction of the moment determined by the local Γ 4g variant (Figure 5f). However, for a single Γ 4g variant, moving from c > a to c < a reverses the direction of the net moment (Figure 5g,h).…”

“…Recently there has been great interest in antiferromagnetic materials for their use in energy efficient computing [1][2][3][4] and neuromorphic hardware [5,6] with the potential for terahertz operating frequencies, lower thermal losses, and enhanced on-chip packing density. [7][8][9][10] In particular, noncollinear antiferromagnets (AFMs) such as Mn 3 A (A = Sn, Ge, Pt) and Mn 3 AN (A = Ni, Ga, Sn) have been shown to possess intrinsic anomalous Hall effect (AHE), [11][12][13][14][15][16][17] magneto-optical Kerr effect (MOKE), [18][19][20] anomalous Nernst effect (ANE), [21][22][23] and the tunneling magnetoresistance effect [24] which allows for easy readout of the antiferromagnetic state, an obstacle for many collinear antiferromagnetic families, [25][26][27] although encouraging progress with altermagnets may help overcome this bottleneck. [28][29][30][31] Although it is theoretically understood that the AHE may be observed in fully compensated noncollinear AFM with M = 0, [32][33][34][35] experimentally the observation of the anomalous Hall effect in noncollinear AFM systems is accompanied with a small net moment.…”

Atomic Displacements Enabling the Observation of the Anomalous Hall Effect in a Non‐Collinear Antiferromagnet

“…Our results also show that the formation of Mn displacements is not limited to a certain material or deposition technique. Beyond the AHE and SHE, Mn displacements may, furthermore, be involved in the measurement of other transport effects that require a domain imbalance, such as the anomalous Nernst effect, [43,44] the magneto-optical Kerr effect [20] as well as longitudinal spin current polarization. [11,13] We show that tetragonal distortions in thin films are not a prerequisite to measuring these effects, but that their observation may be enabled, instead, by atomic displacements.…”