Ginzburg-Landau theory of spin pumping through an antiferromagnetic layer near the Néel temperature

Abstract

Spin pumping is a microwave-driven means for injecting spins from a ferromagnet into the adjacent target material. The insertion of a thin antiferromagnetic layer between the ferromagnet and the target material is known to enhance the spin pumping signal. Here, in view of describing dynamic fluctuations of the Néel order parameter, we develop Ginzburg-Landau theory of the spin pumping in a ferromagnet/antiferromagnet/heavy metal trilayer in the vicinity of the antiferromagnetic Néel temperature $T_{\rm N}$. When there exists an interfacial exchange interaction between the ferromagnetic spins and the antiferromagnetic Néel order parameter at the ferromagnet/antiferromagnet interface, we find a strongly frequency-dependent enhancement of the pumped spin current that is peaked at $T_{\rm N}$. The present finding offers an explanation for the enhanced spin pumping with strong frequency dependence observed in a Y$_3$Fe$_5$O$_{12}$/CoO/Pt system.

Figures (4)

• Figure 1: Schematic illustration of the system considered in this work. Here, FI, AFI, and M refer to a ferromagnetic insulator, antiferromagnetic insulator, and heavy metal, respectively, and $d_{\rm A}$ is the thickness of the AFI layer. Besides, ${\bm S}$ is the spin in the FI layer, ${\bm m}$ and ${\bm n}$ respectively represent the magnetization vector and Néel vector in the AFI layer, and ${\bm \sigma}$ denotes itinerant spin density in the M layer.
• Figure 2: Temperature dependence of the pumped spin current density $j_{\rm s}^{\rm M}(d_{\rm A})$ for the magnetic coupling case calculated from Eq. (\ref{['eq:jsRES01']}) for several values of $\widetilde{\omega}$. Here, the data is normalized by its value at $T= 1.5 T_{\rm N}$, and $\widetilde{\Gamma}_m= 0.1$, $\widetilde{\Gamma}_n= 0.5$, and $K_0= 0.1$ are used.
• Figure 3: The same as in Fig. \ref{['fig:jm-temp01']} but for the Néel coupling case.
• Figure 4: Angular frequency dependence of the pumped spin current density divided by the spin voltage, $j_{\rm s}^{\rm M}(d_{\rm A})/V_{\rm s}$, calculated from Eq. (\ref{['eq:jsRES01']}) at several different temperatures. The magnetic coupling case [(a), (b), (c)] and the Néel coupling case [(d), (e), (f)] are shown. The data is normalized by its value at $T=1.5T_{\rm N}$, and $\widetilde{\Gamma}_m= 0.1$, $\widetilde{\Gamma}_n= 0.5$, and $K_0= 0.1$ are used.