Spin-transfer in nanometre-scale magnetic devices results from the torque on a ferromagnet owing to its interaction with a spin-polarized current and the electrons' spin angular momentum. Experiments have detected either a reversal or high-frequency (GHz) steady-state precession of the magnetization in giant magnetoresistance spin valves and magnetic tunnel junctions with current densities of more than 10(7) A cm(-2). Spin-transfer devices may enable high-density, low-power magnetic random access memory or direct-current-driven nanometre-sized microwave oscillators. Here we show that the magnetization oscillations induced by spin-transfer in two 80-nm-diameter giant-magnetoresistance point contacts in close proximity to each other can phase-lock into a single resonance over a frequency range from approximately <10 to >24 GHz for contact spacings of less than about approximately 200 nm. The output power from these contact pairs with small spacing is approximately twice the total power from more widely spaced (approximately 400 nm and greater) contact pairs that undergo separate resonances, indicating that the closely spaced pairs are phase-locked with zero phase shift. Phase-locking may enable control of large arrays of coupled spin-transfer devices with increased power output for microwave oscillator applications.

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We measured switching of a thin film nanomagnet driven by spin-polarized current in giant magnetoresistance spin valves as small as 50 nm×100 nm. Spin-transfer reversal is observed in both dc current and magnetic field sweeps, with a switching current of ∼5 mA, for example, for a bit with ∼900 Oe switching field in zero current. We studied the dependence of spin-transfer switching on the relative angle φ between the layer magnetizations by using a magnetic field to orient the magnetization of a bulk magnetic layer at an angle to a patterned layer held in place by shape anisotropy. The critical current is a minimum for collinear magnetizations and diverges as 1/|cos φ| as φ increases to 90°, consistent with switching current calculations using the Slonczewski spin-transfer torque model.

We used ferromagnetic artificial pinning centers in superconducting NbTi wires to achieve a large critical current density (J c) in a magnetic field. Four wires were fabricated that contained nanometer-sized arrays of Ni or Fe pins inside micron-sized filaments of Nb 0.36 Ti 0.64 alloy. A ferromagnetic pin volume of only 2% Ni produced J c 's ͑e.g., 2500 A/mm 2 at 5 T, 4.2 K͒ that were comparable to those of commercial wires that have a pin volume of ϳ20% Ti. We conclude that ferromagnetic artificial pins are more effective than nonmagnetic pins for a given volume percent.