Direct Detection of Pure ac Spin Current by X-Ray Pump-Probe Measurements

Despite recent progress in spin-current research, the detection of spin current has mostly remained indirect. By synchronizing a microwave waveform with synchrotron x-ray pulses, we use the ferromagnetic resonance of the Py (${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$) layer in a $\mathrm{Py}/\mathrm{Cu}/{\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}/\mathrm{Cu}/\mathrm{Co}$ multilayer to pump a pure ac spin current into the ${\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}$ and Co layers, and then directly probe the spin current within the ${\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}$ layer and the spin dynamics of the Co layer by x-ray magnetic circular dichroism. This element-resolved pump-probe measurement unambiguously identifies the ac spin current in the ${\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}$ layer.

Figure 1

Top row: Static XMCD measurements at the Ni, Co, and Mn $L_{3,2}$ edges show that Py and Co are ferromagnetic, and the ${\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}$ is paramagnetic. Bottom row: Element-specific hysteresis loops obtained by monitoring field dependence of the Ni, Co, and Mn $L_3$ XMCD. The Cu layers eliminate magnetic polarization and coupling of the ${\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}$ by the Py and Co layers.

Figure 2

Ac XMCD measurements of the Py precession in $\mathrm{Py}/\mathrm{Cu}/{\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}/\mathrm{Cu}$. (a) The Py magnetic moment precession amplitude exhibits a Lorentzian-shaped FMR peak at $H_{\text{res}}=235\mathrm{Oe}$ with a full-width half-maximum of $\mathrm{\Delta }{H}_{1/2}=64\mathrm{Oe}$. (b) The sinusoidal time dependence of the Ni $L_3$ XMCD signal reveals the precession of the Py magnetic moment. A clear phase shift occurs as the magnetic field crosses the resonance field.

Figure 3

(a) Schematic drawing of the magnetic moment precession in each layer due to the pure spin current pumped by the Py FMR. Note the inverted cone of precession for the Mn moment as described by Eq. (1). (b) Spin precession within the Py, ${\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}$, and Co layers revealed by ac XMCD measurements using LCP (red dots) and RCP (green dots) x rays at the Ni, Mn, and Co edges, respectively. The absence of any oscillations below the Mn $L_3$ edge energy (purple solid dots) confirms the absence of any artifacts in the measurement. (c) The relative magnitude and phase of the Py, ${\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}$, and Co spin precession. The ${\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}$ spin precession is a direct indicator of the ac spin current.

Figure 4

For the $\mathrm{Py}/\mathrm{MgO}/{\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}$ sample, (a) Ni spin precession at the Py resonance field. (b) Absence of Mn XMCD indicates the absence of the Mn spin precession at the Py resonance field. (c) Total power absorption showing a broad ESR peak at $H=1300\mathrm{Oe}$ in addition to the sharp Py FMR peak. The ESR arises from all conduction electrons in the sample. (d) The absence of Mn AC XMCD at $H=1300\mathrm{Oe}$ shows that the ESR does not contribute to the Mn ac XMCD signal.

Figure 5

(a) Py and Co magnetic moment precession at different magnetic fields (dots are experimental data and lines are sinusoidal fits). The amplitude is normalized for clarity. (b) Ni and Co ac XMCD as a function of applied field. At the Py FMR field of $H_{\text{res}}=235\mathrm{Oe}$, the Co amplitude also shows a peak due to spin pumping. (c) Phase of the ac XMCD signals. The Py precession shows the $\pi$-phase change typical of FMR across the resonance field. The phase of the ${\mathrm{Cu}}_{75}{\mathrm{Mn}}_{25}$ is identical to that of Py as indicated by Eq. (1). The Co phase exhibits a characteristic bipolar behavior that is a fingerprint of ac spin-current driven precession. The solid lines in (b) and (c) are calculated results (see Supplemental Material [32]). (d) From the schematic diagram of the ac spin current, RF-field torque ${\overrightarrow{\tau }}_{\mathrm{RF}}$, and the total torque ${\overrightarrow{\tau }}_{\text{tot}}$, in the spin precession plane, it is easy to understand the bipolar phase variation, whereby ${\varphi }_{\mathrm{Co}}<{\varphi }_{\mathrm{Co}}^0$ for $H>H_{\text{res}}$ and ${\varphi }_{\mathrm{Co}}>{\varphi }_{\mathrm{Co}}^0$ for $H<H_{\text{res}}$ (see the main text).