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We study nanopillar spin-torque oscillators processed from low-resistance-area product MgO-based magnetic tunnel junctions. The influence of spin torque can be seen in quasistatic experiments as a strong astroid distortion, consistent with a pure Slonczewski-type spin torque. At microwave frequencies, the spin-torque results in pronounced magnetization auto-oscillations with a clear threshold behavior, though only evidenced in the antiparallel configuration. Two kinds of auto-oscillations are seen depending on the polarity of the voltage applied to the junction. Free layer oscillations require a large easy axis applied field and electrons flowing from the reference layers to the free layer. Acoustic excitation of the reference synthetic ferrimagnet can be seen, provided the electron flow is reversed, indicating a clear similarity with the behavior of conventional all-metallic spin valves. The analysis of the threshold indicates that the amplitude of the spin torque is neither proportional to the current nor to the voltage and it is not accompanied by any significant fieldlike torque.

We present an experimental and theoretical study of the magnetic field dependence of the mode frequency of thermally excited spin waves in rectangular shaped nanopillars of lateral sizes 60x100, 75x150, and 105x190 nm2, patterned from MgO-based magnetic tunnel junctions. The spin wave frequencies were measured using spectrally resolved electrical noise measurements. In all spectra, several independent quantized spin wave modes have been observed and could be identified as eigenexcitations of the free layer and of the synthetic antiferromagnet of the junction. Using a theoretical approach based on the diagonalization of the dynamical matrix of a system of three coupled, spatially confined magnetic layers, we have modeled the spectra for the smallest pillar and have extracted its material parameters. The magnetization and exchange stiffness constant of the CoFeB free layer are thereby found to be substantially reduced compared to the corresponding thin film values. Moreover, we could infer that the pinning of the magnetization at the lateral boundaries must be weak. Finally, the interlayer dipolar coupling between the free layer and the synthetic antiferromagnet causes mode anticrossings with gap openings up to 2 GHz. At low fields and in the larger pillars, there is clear evidence for strong non-uniformities of the layer magnetizations. In particular, at zero field the lowest mode is not the fundamental mode, but a mode most likely localized near the layer edges., Comment: 16 pages, 4 figures, (re)submitted to PRB

We present a numerical study of oscillation modes in magnetic tunnel junction nanopillars and investigate the frequency and the full width at half maximum of the power spectrum as a function of applied field and applied current. We show that the line width reaches a minimum of 14.5 MHz as the system approaches the threshold current, and increases sharply to 308 MHz as the current is increased beyond the threshold current. The initial line narrowing is due to an increased coherence in the uniform precession mode, while the line broadening above threshold arises from the intrinsic oscillator nonlinearity combined with overlapping contributions from edge modes. We show that these results are in good agreement with recent experiments on MgO-based oscillators.

We have studied the effect of nanopillar patterning on the saturation magnetization of the CoFeB free layer in MgO magnetic tunnel junctions. Before patterning, the free layer magnetization is measured by ferromagnetic resonance and is found to be close to the bulk value, with no detectable interface anisotropy. After patterning, the shape anisotropy and the frequency of the main spin wave mode indicate that the free layer magnetization is substantially reduced. Current dependent measurements indicate that this is not due to Joule heating. Size dependent measurements indicate that the magnetization reduction most likely arises from process damage during the etching step.