Abstract
Using nonlinear system theory and numerical simulations, we map out the static and dynamic phase diagrams in the zero applied field of a spin torque nano device with a tilted polarizer (TP). We find that for sufficiently large currents, even very small tilt angles (β>1°) will lead to steady free layer precession in zero field. Within a rather large range of tilt angles, 1°<β<19°, we find coexisting static states and hysteretic switching between these using only current. In a more narrow window (1°<β<5°) one of the static states turns into a limit cycle (precession). The coexistence of current-driven static and dynamic states in the zero magnetic field is unique to the TP device and leads to large hysteresis in the upper and lower threshold currents for its operation. The nano device with TP can facilitate the generation of large amplitude mode of spin torque signals without the need for cumbersome magnetic field sources and thus should be very important for future telecommunication applications based on spin transfer torque effects.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Spin transfer torque can be used to reverse the magnetic state in so-called spin transfer torque random access memory (STT-RAM), or excite coherent spin waves at GHz frequencies in spin torque oscillators (STOs). In conventional STOs with in-plane magnetizations, one needs to apply a large magnetic field for signal generation, which is a significant complication and limits the miniaturization of STO-based devices. Zero field STO operation has consequently become an intensely researched topic.
Main results. Using an analytical eigenvalue analysis, and complementing it with magnetodynamic simulations, we map out the detailed static and dynamic phase diagram of a novel spin torque device where the magnetization of the fixed layer is tilted at an arbitrary angle out of the film plane. While this device has the combined advantage of zero-field operation and large signal output, we show that it also possesses a surprisingly rich phase diagram with hysteretic switching between coexisting static and dynamic states at zero applied field.
Wider implications. The tilt angle of the polarizer magnetization introduces a new degree of freedom in spin torque devices and may open up a wide range of polarizer angle dependent phenomena in both oscillatory devices and STT-RAM.
Figure. (a) Schematic of a tilted polarizer spin torque nano device. (b) Two coexisting spiral and node type eigenstates. Brown region: node type eigenstates; blue region: spiral type eigenstates. The yellow and green curves represent two examples of the dynamical evolution of the magnetization which finally relaxes in S and N regions, respectively.