Switching of magnetization by nonlinear resonance studied in single nanoparticles

Abstract

Magnetization reversal in magnetic particles is one of the fundamental issues in magnetic data storage. Technological improvements require the understanding of dynamical magnetization reversal processes at nanosecond time scales. New strategies are needed to overcome current limitations. For example, the problem of thermal stability of the magnetization state (superparamagnetic limit) can be pushed down to smaller particle sizes by increasing the magnetic anisotropy. High fields are then needed to reverse the magnetization, which are difficult to achieve in current devices. Here we propose a new method to overcome this limitation. A constant applied field, well below the switching field, combined with a radio-frequency (RF) field pulse can reverse the magnetization of a nanoparticle. The efficiency of this method is demonstrated on a 20-nm-diameter cobalt particle by using the microSQUID (superconducting quantum interference device) technique. Other applications of this method might be nucleation or depinning of domain walls.

Strategy to reduce minimal magnetization switching field for Stoner particles

“…A similar result was experimentally confirmed in Ref. 10. It will be shown later that a ͑linearly polarized͒ rf-field used in Ref.…”

“…Most studies have assumed the magnetic field to be time independent. However, a very recent experiment 10 has shown that a dramatic reduction of the minimal field is possible by applying a small rf field pulse ͑the decrease in the constant field is much larger than the amplitude of the rf field͒. In this study, it has been shown that a small timedependent magnetic field can affect the magnetization of a Stoner particle such that the magnetization can move upward in its energy landscape against the dissipation effect.…”

“…This problem was first studied by Stoner and Wohlfarth 4 ͑SW͒ who showed that a field h larger than the SW-limit h SW can switch the magnetization from its initial state to the target value through a ringing effect. [5][6][7][8][9] However, recent theoretical and experimental studies [8][9][10][11][12] have shown that the minimal switching field can be smaller than the SW limit. Most studies have assumed the magnetic field to be time independent.…”

Magnetization switching by microwaves synchronized in the vicinity of precession frequency

“…Microwave-assisted magnetization reversal [1][2][3][4][5][6][7][8][9][10] is a fascinating subject in magnetism for practical applications such as high-density recording. An oscillating field generated by microwaves excites a small-amplitude oscillation of the magnetization in the ferromagnet and significantly reduces the switching field to, typically, half of the uniaxial anisotropy field for an optimized microwave frequency.…”

“…In previous works on microwave-assisted magnetization reversal, [1][2][3][4][5][6][7][8][9][10] the microwave source is isolated from the ferromagnet. Recently, however, an alternative system has been investigated both experimentally and numerically 11,12) in which a spin torque oscillator (STO) is used as the microwave source.…”