Measurements of noise properties of MgO-based magnetic tunnel junctions were performed in the frequency range from 100MHzto18GHz. A pronounced narrow peak at the frequency of around 7GHz with a full width at half-maximum as low as 21MHz has been observed. The peak amplitude and 1∕f noise have a threshold dependence on the dc bias current. The narrow peak is present only at one current polarity for an antiparallel orientation of the magnetic layers adjacent to the barrier, which is consistent with spin-momentum transfer theoretical predictions.

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Low resistance-area product and high spin polarization lead to current-driven precession of the magnetization in CoFeB∕MgO∕CoFeB magnetic tunnel junctions due to spin-transfer torque (STT) effects. Current-driven precession of the magnetization leads to pronounced narrow peaks in the frequency range of 4–7GHz with a full width at half maximum as low as 21MHz. The peak amplitudes have a threshold dependence on the dc bias current. Experimental results show that the STT-driven microwave generation can also occur in MgO-based junctions at maximum resistance state but at opposite current polarity, which corresponds to precession of the magnetization of the reference layer (RL) electrode. This conclusion is supported by the peak frequency dependence on magnetic field. The maximum generated power was 35nW at a peak frequency of about 6GHz. The estimated maximum angle change of the RL in-plane magnetization rotation is 19° and corresponds to a large angle precession.

The high power ferromagnetic resonance (FMR) response, as well as butterfly curves of the spin wave instability threshold microwave field amplitude hcrit versus in-plane static field H profiles, have been measured for Permalloy films with thicknesses of 104, 128, and 270nm at a nominal pumping frequency of 9.37GHz. The hcrit values range from about 1 to 7Oe. Both the resonance saturation response at the FMR field and the subsidiary absorption (SA) response for static fields below the FMR field are similar in appearance to those for bulk ferrites. Butterfly curves over the SA response region, while similar to those for ferrites, exhibit a film thickness dependent band edge cutoff effect not found in bulk ferrites. The SA butterfly curve data were analyzed on the basis of a spin wave instability theory adapted to thin films. The observed shift in the SA band edge cutoff with thickness agrees with calculations based on the thin film dispersion response and the assumption of first order instability processes with critical modes at one half the pumping frequency. The fitted SA spin wave linewidths give values consistent with metallic relaxation processes, but indicate critical modes with wave vectors that always make relatively small 0°–20° angles with the static field, very different from the critical modes for bulk ferrites. Three key conclusions from this work are (1) the nonlinear microwave FMR response in Permalloy films is a threshold effect related to well established spin wave instability processes, (2) the details of the SA response are controlled largely by the thin film spin wave dispersion, and (3) these nonlinear processes occur for very small precession angles.

Unusual metallic behavior in nanostructured cobalt ferrite at superparamagnetic regime J. Appl. Phys. 112, 063926 (2012) Investigation of structural, dielectric, and magnetic properties of hard and soft mixed ferrite composites J. Appl. Phys. 112, 054323 (2012) Magneto-optical study of holmium iron garnet Ho3Fe5O12 Low Temp. Phys. 38, 863 (2012) Growth and ferromagnetic resonance of yttrium iron garnet thin films on metalsThe ferromagnetic resonance ͑FMR͒ linewidth, the field dependent effective linewidth, and the parallel pump spin wave linewidth were measured for spheres and disks prepared from a block of hot isostatic pressed ͑hipped͒ polycrystalline yttrium iron garnet ͑YIG͒. All linewidths as well as static magnetization data indicate close to 100% density. Vibrating sample magnetometer measurements give an average saturation induction 4M s of 1825 G. The FMR half-power linewidths for the spheres at 9.5 GHz were 13 Oe. Linewidths measured over the 9.5-18 GHz frequency range show a small but distinct drop and agree with Schlömann's theory of anisotropy-dominated two-magnon scattering for polycrystalline ferrites. The effective linewidth versus field data at 10 GHz show a region of strong absorption that corresponds to the width of the spin wave manifold for low wave numbers and a high field value of about 2 Oe. Parallel pumping measurements give minimum spin wave linewidths of 1.2 and 0.6 Oe at 9 and 16.7 GHz, respectively. The 16.7 GHz spin wave linewidths correspond to half-frequency spin waves at 8.35 GHz. The extrapolated linewidths at zero wave number are about 0.5 Oe and match the established intrinsic linewidths expected for YIG single crystals at 8 -9 GHz. The spin wave linewidths increase linearly with wave number and are consistent with a transit time scattering process with scattering lengths that are about ten times greater than the average grain size.