Anomalous Hall effect in noncollinear antiferromagnetic Mn3NiN thin films

Abstract

We have studied the anomalous Hall effect (AHE) in strained thin films of the frustrated antiferromagnet Mn 3 NiN. The AHE does not follow the conventional relationships with magnetization or longitudinal conductivity and is enhanced relative to that expected from the magnetization in the antiferromagnetic state below T N = 260 K. This enhancement is consistent with origins from the noncollinear antiferromagnetic structure, as the latter is closely related to that found in Mn 3 Ir and Mn 3 Pt where a large AHE is induced by the Berry curvature. As the Berry-phase-induced AHE should scale with spin-orbit coupling, yet larger AHE may be found in other members of the chemically flexible Mn 3 AN structure.

The Biaxial Strain Dependence of Magnetic Order in Spin Frustrated Mn3NiN Thin Films

“…Thus, due to the out‐of‐plane magnetization of our strained Mn 3 NiN films, we expect ρ xy to include a measurable AHE contribution. Additionally, a further intrinsic contribution to the AHE arises when the films adopt the Γ 4 g noncollinear magnetic structure, due to the Berry curvature associated with its bandstructure . To estimate the normal Hall contribution we use density functional theory (DFT) calculations at zero temperature of the carrier density as 2.4 × 10 21 cm −3 , which gives a normal Hall contriubition R o,th = 26 × 10 −2 µΩ cm T −1 .…”

“…For film #3, where the lattice parameter and transition temperature is roughly the same as the bulk (see Table 1 and Figure S2, Supporting Information), ρ xy ,sat is small above T N and increases upon cooling. This behavior is similar to that found for films on LSAT (#1 and #2), however even for zero strain one expects a significant AHE arising from the Γ 4 g structure …”

“…In both ρ xy ,sat and H C we can identify three clear regimes of behavior: (i) the regime above T N , where films under compressive and tensile strain show large and small AHE, respectively; (ii) the intermediate temperature regime below T N , which is consistent with contributions to the AHE from the Γ 4 g magnetic structure; (iii) a low temperature regime where all films show a decrease in ρ xy ,sat , generally below T / T N < 0.5. This latter feature is most marked in the LSAT 100 nm film (#2, see Figure S1b, Supporting Information); upon cooling below 100 K the coupling of ρ xy to the field weakens and reduces the coercivity.…”

“…While the phase diagram predicted for Mn 3 GaN is plotted with respect to net magnetization, our phase diagram for Mn 3 NiN shares many qualitative similarities: (i) the T N dependence with strain is in the same direction; (ii) different high and low ρ xy ,sat states exist in the opposite strain regimes above T N ; (iii) high ρ xy ,sat states exist at low temperature for both compressive and tensile strain; (iv) the transition at T N becomes second‐order‐like at large compressive strains, in the opposite sense to Mn 3 GaN where large tensile strains result in second‐order behavior . Furthermore, noticeable differences to the Mn 3 GaN magnetization phase diagram are also present: (i) films with small strain show large ρ xy ,sat below T N due to the Berry curvature contribution to the AHE that is allowed despite negligible magnetization; (ii) a region of low ρ xy ,sat exists below T N for small compressive strain. This may exist because compressive strain is predicted to reduce the AHE, however as further compressive strain is applied the net magnetization due to the piezomagnetism may then act as an additional contribution to the AHE.…”

“…Furthermore, noticeable differences to the Mn 3 GaN magnetization phase diagram are also present: (i) films with small strain show large ρ xy ,sat below T N due to the Berry curvature contribution to the AHE that is allowed despite negligible magnetization; (ii) a region of low ρ xy ,sat exists below T N for small compressive strain. This may exist because compressive strain is predicted to reduce the AHE, however as further compressive strain is applied the net magnetization due to the piezomagnetism may then act as an additional contribution to the AHE.…”