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We fabricated a spin-torque oscillator (STO) having a nanopillar-shaped magnetic tunnel junction with perpendicularly magnetized FeB free and in-plane magnetized CoFeB reference layers. The perpendicular magnetization of the FeB was stabilized by strong perpendicular magnetic anisotropy induced at both the MgO tunnel barrier/FeB and FeB/MgO cap interfaces. Under a perpendicular field (3 kOe), the STO exhibited a large emission power (0.55 W), a high frequency (6.3 GHz) and a high Q factor (135) simultaneously, all of which are the largest to date among nanopillar-shaped STOs. The bias voltage dependence of the oscillation property was well explained by the macrospin model.

Highly sensitive microwave devices that are operational at room temperature are important for high-speed multiplex telecommunications. Quantum devices such as superconducting bolometers possess high performance but work only at low temperature. On the other hand, semiconductor devices, although enabling high-speed operation at room temperature, have poor signal-to-noise ratios. In this regard, the demonstration of a diode based on spin-torque-induced ferromagnetic resonance between nanomagnets represented a promising development, even though the rectification output was too small for applications (1.4 mV mW(-1)). Here we show that by applying d.c. bias currents to nanomagnets while precisely controlling their magnetization-potential profiles, a much greater radiofrequency detection sensitivity of 12,000 mV mW(-1) is achievable at room temperature, exceeding that of semiconductor diode detectors (3,800 mV mW(-1)). Theoretical analysis reveals essential roles for nonlinear ferromagnetic resonance, which enhances the signal-to-noise ratio even at room temperature as the size of the magnets decreases.

We prepared magnetic tunnel junction films with PtMn/CoFe/Ru/CoFeB/MgO tunnel barrier/FeB free layer/MgO cap layer/Ta multilayers using sputtering and measured magnetic and magnetoresistive properties of the films at room temperature. The magnetization curves of the FeB plane film measured under perpendicular-to-plane magnetic fields showed much smaller saturation fields (Hs) than those expected from the demagnetizing field. Hs decreased from 4 to 0.4 kOe with increasing MgO cap layer thickness. The small Hs is due to the perpendicular magnetic anisotropy (PMA) induced at both MgO barrier–FeB and FeB–MgO cap interfaces. After microfabrication, the small free layer cells having a 1.6 nm thick MgO cap layer showed a magnetization easy axis in the perpendicular-to-plane direction. By inducing PMA from both upper and lower interfaces, we can stabilize the magnetization of the relatively thick (2 nm) FeB free layer in the perpendicular-to-plane direction.

We report on rf current-induced excitation of the ferromagnetic resonance in CoFeB/MgO/CoFeB magnetic tunnel junctions under a perpendicular magnetic field. By choosing an appropriate external field and using an Fe-rich CoFeB free layer, the effective precession of the free layer could be excited. In a measurement of homodyne detection, a large dc output voltage of 180 µV was obtained when an rf signal power of -25 dBm was applied. The sensitivity of this junction, as an rf rectifier, reaches about 170 mV/mW (280 mV/mW after impedance matching correction), which is the same order compared with that of a Schottky diode operated at room temperature.

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