Abstract
The magnetization dynamics in a spin torque oscillator (STO) consisting of two in-plane magnetized free layers is studied by solving the Landau-Lifshitz-Gilbert equation and evaluating the Lyapunov exponent numerically. The phase diagrams of the oscillation frequencies of the magnetizations and magnetoresistance and the maximum Lyapunov exponent are obtained from the numerical simulations. The phase synchronization is found in the low current region, whereas the magnetizations oscillate with different frequencies in the middle current region. On the other hand, positive Lyapunov exponents found in the high current region indicate the existence of chaos in the STO.
Highlights
Spin torque oscillator (STO) is a nonlinear oscillator in nanoscale, and generates an oscillating power having the frequency on the order of gigahertz through giant or tunnel magnetoresistance (MR) effect.1–3 A conventional structure of spin torque oscillator (STO) consists of three ferromagnets, called free, reference, and pinned layers, with a nonmagnetic spacer between the free and reference layers
The phase synchronization is found in the low current region, whereas the magnetizations oscillate with different frequencies in the middle current region
The magnetization dynamics in an STO is detected through the giant or tunnel magnetoresistance (MR) effect in the experiment,23 which depends on m1⋅m2.27,28 On the other hand, the microwave field required in microwave assisted magnetization reversal (MAMR) reflects the oscillations of the magnetizations, m1 and m2
Summary
Spin torque oscillator (STO) is a nonlinear oscillator in nanoscale, and generates an oscillating power having the frequency on the order of gigahertz through giant or tunnel magnetoresistance (MR) effect. A conventional structure of STO consists of three ferromagnets, called free, reference, and pinned layers, with a nonmagnetic spacer between the free and reference layers. The magnetization direction in the reference layer is fixed by the pinned layer, whereas that in the free layer can change its direction by applying magnetic field and/or electric current. Another type of STO has been proposed for a new scheme of magnetic recording, namely microwave assisted magnetization reversal (MAMR).. At the beginning of the study on MAMR, the STO consisted of an inplane magnetized free layer and perpendicularly magnetized reference and pinned layers.. The latest design of the STO for MAMR consists of two in-plane magnetized ferromagnets called field-generation layer and spin-injection layers.. Chaos is found in the high current region, which is identified from positive Lyapunov exponents
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