Spin transfer oscillators emitting microwave in zero applied magnetic field

Using pillar-shaped spin valves with the magnetization of the reference layer being pinned perpendicularly to the easy axis of the free layer, we show that spin-transfer-induced microwave emission can be obtained at exactly zero applied magnetic field and in its vicinity. The frequency tunability (typically 150 MHz/mA), the spectral purity (quality factor up to 280), and the power (up to 5 nV/Hz1/2) of the emission compares well with other spin-transfer oscillators based on spin-valve nanopillars. This ability to get satisfactory microwave emission without needing bulky magnetic field sources may arise from a small nonvanishing field-like term in the current-induced torque. It may be of interest for the design of submicron microwave sources.