Coupling of Ferromagnetic and Antiferromagnetic Spin Dynamics in ${\mathrm{Mn}}_2\mathrm{Au}/\mathrm{NiFe}$ Thin Film Bilayers

We investigate magnetization dynamics of ${\mathrm{Mn}}_2\mathrm{Au}/\mathrm{Py}$ (${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$) thin film bilayers using broadband ferromagnetic resonance (FMR) and Brillouin light scattering spectroscopy. Our bilayers exhibit two resonant modes with zero-field frequencies up to almost 40 GHz, far above the single-layer Py FMR. Our model calculations attribute these modes to the coupling of the Py FMR and the two antiferromagnetic resonance (AFMR) modes of ${\mathrm{Mn}}_2\mathrm{Au}$. The coupling strength is in the order of 1.6 T nm at room temperature for nm-thick Py. Our model reveals the dependence of the hybrid modes on the AFMR frequencies and interfacial coupling as well as the evanescent character of the spin waves that extend across the ${\mathrm{Mn}}_2\mathrm{Au}/\mathrm{Py}$ interface.

Figure 1

(a) Real part of the background corrected VNA signal of a ${\mathrm{Mn}}_2\mathrm{Au}$ (40 nm)/Py (10 nm) sample. The dashed line indicates the expected FMR for a bare Py thin film. (b) Resonance frequencies $f_1$ and $f_2$ vs external magnetic field $H_0$ obtained from fitting data in panel (a). The lines are fits to the modified Kittel equation from our model (see text). (c) Same as (b) but for a ${\mathrm{Mn}}_2\mathrm{Au}$ (40 nm)/Py (5 nm) sample. (d) The zero-field mode frequencies $f_1(H_0=0)$ and $f_2(H_0=0)$ increase with increasing $d_{\mathrm{FM}}^{-1}$. The scatter in the $f_1$ data points is attributed to weak $f_1$ intensity.

Figure 2

(a) Sketch of the sample structure and coupling of modes. The FMR with frequency $f_{\mathrm{FM}}$ couples to the two antiferromagnetic modes in the AFM. This results in hybrid modes with frequencies $f_1$ and $f_2$ with perpendicular wave vector $k_{i,\perp }$. The Néel vector excitations decay within $1/{\kappa }_i$ (see text). (b) The effective stiffness fields resulting from the mode coupling exceed 0.6 T for Py thickness $d_{\mathrm{FM}}\le 5\mathrm{nm}$.

Figure 3

(a) BLS spectrum. The solid lines are Lorentzian fits used to determine $f_2$, the dashed line represents the background signal. (b) The spin-wave dispersion of samples with 5 nm and 30 nm Py obtained by fitting the BLS spectra (points) and fit to Eq. (1) (lines). Spectra are shifted on the $y$ axis by the $f_2(k_{\parallel }=0)$ mode frequency for clarity. (c) The stiffness field $B_2^{\mathrm{eff}}$ extracted from fitting the BLS measurements is in full agreement with that observed in FMR measurements.