Spintronics is an important field of electronics that utilizes the two degrees of freedom of electrons: charge and spin angular momentum. Chiral antiferromagnets are attracting attention as spintronics materials due to their high resonant frequency and minimal stray fields. $D$0₁₉-Mn₃Sn (Fig. 1(a)) is a chiral antiferromagnet with a noncolinear spin structure. Its Mn spins form a spin structure called a cluster magnetic octupole [1]. The magnetic effects on Mn₃Sn are known to be anisotropic due to its anisotropic magnetic structure, which originates from the kagome crystal structure [2]. Utilizing Mn₃Sn as a material for spintronics devices requires Mn₃Sn/ferromagnet heterostructures because these structures produce current-induced magnetic effects. Spin current transport at a metal/metal interface is characterized by the spin mixing conductance $g^{\uparrow\downarrow}_{\mathrm{eff}}$. A previous study [3] evaluated the $g^{\uparrow\downarrow}_{\mathrm{eff}}$ at the Mn₃Sn/NiFe (Ni₈₁Fe₁₉, permalloy) interface using spin pumping. In this study, the anisotropy of the spin mixing conductance has been investigated.

A Mn₃Sn($011\bar{0}$) (20 nm)/MgO spacer (0-3.5 nm)/NiFe (4 nm) multilayer sample was prepared by molecular beam epitaxy method. The Mn₃Sn ($011\bar{0}$ thin film has kagome planes perpendicular to the film plane. Spin mixing conductance is characterized by the spin pumping. Spin current transport at metal/ferromagnet interface is evaluated from the damping constant of the ferromagnet. The damping of the NiFe ferromagnet is characterized using a time-resolved magneto-optical Kerr effect (TRMOKE). Figure 1(b) shows a schematic of the TRMOKE measurements, where the ferromagnetic spin in the NiFe layer, excited by the pump pulse, is detected by the probe pulse via MOKE. An external magnetic field was applied at 45° in the $xy$ and $zx$ planes. The TRMOKE signal is shown in Fig. 2(a). Based on the Landau-Lifshitz-Gilbert equation, $$\frac{d\bm{M}}{dt} = -\gamma\bm{M}\times\bm{H}_{\mathrm{eff}}+\frac{\alpha}{M_{s}}\bm{M}\times\frac{d\bm{M}}{dt}$$ damping constant $\alpha$ was evaluated for each MgO spacer thickness as shown in Fig. 2(b). Here, $\bm{M}$ is magnetization of NiFe and $\bm{H}_{\mathrm{eff}}$ is the effective magnetic field.

We find that the damping was large where the MgO spacer thickness was thin because the nonmagnetic insulator MgO can cut the spin transport at Mn₃Sn/NiFe interface. The obtained damping constant was used to obtain the mixing conductance $g^{\uparrow\downarrow}_{\mathrm{eff}}$, which was found to be $g^{\uparrow\downarrow}_{\mathrm{eff}}$ = 3.6 ± 0.6 ×10¹⁸ m^-2 when the field was applied out of the Mn₃Sn kagome plane and $g^{\uparrow\downarrow}_{\mathrm{eff}}$ = 1.5 ± 0.1 ×10¹⁸ m^-2 when the field was applied in the Mn₃Sn kagome plane.

There are several possible reasons for the anisotropy of $g^{\uparrow\downarrow}_{\mathrm{eff}}$. One possibility is the spin transparency of conduction electrons at the NiFe/Mn₃Sn interface. The anisotropic band structure of Mn₃Sn [1] may cause anisotropy in spin transparency. However, it is difficult to explain the anisotropy in terms of spin absorption efficiency, because the spin diffusion length ($\sim$ 1 nm) of Mn₃Sn [4] is much smaller than the Mn₃Sn thickness. Furthermore, since the damping enhancement anisotropy is greater than twice as large, the anisotropy cannot be solely attributed to the spin current via conduction electrons. We believe the damping enhancement anisotropy is caused by magnetic coupling between NiFe and Mn₃Sn via the exchange spring effect [5].

References

• [1] M.-T. Suzuki et al., Phys. Rev. B 95, 94406 (2017).
• [2] S. Nakatsuji, N. Kiyohara, and T. Higo, Nature 527, 212 (2015).
• [3] J.-Y. Yoon et al., Nature Mater. 22, 1106 (2023)
• [4] P. K. Muduli et al., Phys. Rev. B 99, 184425 (2019)
• [5] H. Kosaki et al., Phys. Rev. B 111, 024418 (2025).

Authors

• H. Kosaki, S. Sakamoto, T. Hatajiri, T. Higo, S.Nakatsuji, and  S.Miwa