Direct-Current Induced Dynamics in ${\mathrm{C}\mathrm{o}}_{90}{\mathrm{F}\mathrm{e}}_{10}/{\mathrm{N}\mathrm{i}}_{80}{\mathrm{F}\mathrm{e}}_{20}$ Point Contacts

We have directly measured coherent high-frequency magnetization dynamics in ferromagnetic films induced by a spin-polarized dc current. The precession frequency can be tuned over a range of several gigahertz by varying the applied current. The frequencies of excitation also vary with applied field, resulting in a microwave oscillator that can be tuned from below 5 to above 40 GHz. This novel method of inducing high-frequency dynamics yields oscillations having quality factors from 200 to 800. We compare our results with those from single-domain simulations of current-induced dynamics.

Figure 1

(a) $dV/dI$ vs $I$ with ${\mu }_{0}H=0.1\mathrm{T}$. (b) High frequency spectra taken at several different values of current through the device, corresponding to the symbols in (a). Variation of $f$ with $I$ (inset).

Figure 2

(a) In-plane $f$ vs $H$ dispersion curve along with a fit to Eq. (1). Error bars (FWHM) are smaller than the data points. (b) Frequency spectra for $I=5\mathrm{m}\mathrm{A}$ to 9 mA in 0.5 mA steps with $\mu H=0.06\mathrm{T}$ showing responses at both $f$ and $2f$. (Inset) $f$ to $2f$ amplitude ratios as a function of $I$ for two different fields.

Figure 3

Low-frequency power spectra for two different currents along with a fit to the data at $I=11\mathrm{m}\mathrm{A}$ to a Lorentzian function. The fitted center frequency is $f_0=0\pm 50\mathrm{M}\mathrm{H}\mathrm{z}$. (inset) The corresponding $dV/dI$ curve.

Figure 4

(a) Plot of $f$ vs $I$ with amplitude shown in a linear color scale from 0 (blue) to $0.9\mathrm{n}\mathrm{V}/{\mathrm{H}\mathrm{z}}^{1/2}$ (red), discretization results from measuring spectra in $500\mu \mathrm{A}$ intervals. (b) Out-of-plane $f$ vs $H$ dispersion curve. Data correspond to the highest $f$ at a given $H$. Error bars (FWHM) are smaller than the data points. (inset) Spectral peak at 1.3 T and $I=11\mathrm{m}\mathrm{A}$ along with a fit.