Independence of the spin current from the Néel vector orientation in antiferromagnet CoO

Spin pumping from ferromagnetic Fe into antiferromagnetic CoO across a Ag spacer layer was studied using ferromagnetic resonance (FMR) in Py/CoO/Ag/Fe/Ag(001). The thin Py film on top of CoO permits an alignment of the CoO Néel vector through field cooling in two otherwise equivalent [110] and $\left[1\overline{1}0\right]$ crystalline axes which are parallel and perpendicular to the Fe magnetization direction, respectively. Fe FMR linewidth is measured as a function of Ag thickness in 10–20-GHz frequency range and in 180–330 K temperature range. We find that there exists an anisotropy in the Fe FMR damping for parallel and perpendicular alignment of the Fe and CoO spins. However, such anisotropic damping exists only at thin Ag thickness where there exists a magnetic interlayer coupling between Fe and CoO, and vanishes at thick Ag thickness where the interlayer coupling becomes negligible but permitting spin-current transmission into CoO. Our result indicates the absence of anisotropic spin current for parallel and perpendicular alignment of the Fe and CoO spin axes.

Figure 1

(a), (b) Schematic drawing of the sample structure and FMR measurement. Here $x$- and $y$ axes are defined as CoO [110] and $\left[1\overline{1}0\right]$ axes, and $z$ axis is the sample normal direction. In the FMR measurement, the external field (H) is applied along the CPW strip line direction ($y$ axis). Thus field cooling ($H_{\mathrm{FC}}$) and Py/CoO coupling create (a) perpendicular and (b) parallel alignment between the frozen CoO spin Néel vector (yellow arrows) and the Fe magnetization (red arrows). Blue arrows indicate the Py magnetization during field cooling. (c) LEED patterns from sample of MgO(3 nm)/Py(2 nm)/CoO(10 nm)/Ag(8 nm)/Fe(4 nm)/Ag(001).

Figure 2

Fe hysteresis loops for (a) $H\parallel H_{\mathrm{FC}}$ and (b) $H\perp H_{\mathrm{FC}}$. The interlayer coupling between Fe and CoO decreases with increasing Ag thickness and vanishes above 4.5-nm Ag thickness. (c) CoO spectra measured with linear polarized x rays for $H_{\mathrm{FC}}\parallel x$. XMLD spectra of (d) $H_{\mathrm{FC}}\parallel x$ and (e) $H_{\mathrm{FC}}\parallel y$ are equal and opposite, confirming that CoO spins are aligned along $y$ and $x$ directions, respectively. (f) The unchanged XMLD (with fixed x-ray polarization, $E\parallel x$ and $E\parallel y$) after rotating a 4000-Oe field from $x$ to $y$ axis proves the frozen CoO spins after field cooling. (g) The $\mathrm{CoO}{R}_{\mathrm{L}3}$ ratio exhibits $\mathrm{co}{\mathrm{s}}^2\phi$ dependence on x-ray polarization angle ($\phi$), showing that CoO spins are aligned perpendicularly to $H_{\mathrm{FC}}$. Solid lines are cosine fits to the data. Inset shows the schematic of XMLD measurement, with $\phi$ defined as the angle between x-ray polarization and $x$ axis. All measurements were performed at 200 K.

Figure 3

Fe FMR spectra measured at 17.5 GHz with $H_{\mathrm{FC}}\parallel x$ and $H_{\mathrm{FC}}\parallel y$ in (a) $d_{\mathrm{Ag}}=2\text{−}\mathrm{nm}$ sample and (b) $d_{\mathrm{Ag}}=8\text{−}\mathrm{nm}$ sample. The shading with arrows indicates the FMR full linewidth (2ΔH). Frequency-dependent FMR linewidth (ΔH) for $H_{\mathrm{FC}}\parallel y$ and $H_{\mathrm{FC}}\parallel x$ from (c) $d_{\mathrm{Ag}}=2\text{−}\mathrm{nm}$ sample and (d) $d_{\mathrm{Ag}}=8\text{−}\mathrm{nm}$ sample. Solid lines are fitting with Eq. (1). The linewidth from a reference sample of Ag(8 nm)/Fe(4 nm) is also shown in (d) to indicate the existence of spin-current pumping into CoO. (e) Coercivity ($H_{\mathrm{C}}$), splitting field ($H_{\mathrm{S}}$), and (f) FMR damping of Fe layer as a function of the Ag spacer layer thickness. All measurements were performed at 200 K.

Figure 4

(a) Fe FMR damping for $H_{\mathrm{FC}}\parallel x$ and $H_{\mathrm{FC}}\parallel y$ as a function of temperature from $d_{\mathrm{Ag}}=2$- and $d_{\mathrm{Ag}}=8\text{−}\mathrm{nm}$ samples. (b) Fe hysteresis loops of $d_{\mathrm{Ag}}=2\mathrm{nm}$ sample for $H\perp H_{\mathrm{FC}}$.