Theoretical Study On The Microwave Assisted Switching Of Magnetic Moment For Grains

The minimum switching field threshold and switching speed are key parameters for the magnetic recording in writing. As an isolate grain, microwave assisted switching could enhance minimum switching field threshold and switching speed. The reversing of the grains, however, correlates with each other, because of the interaction between them. This paper presents the dynamic precession of the moments of the grains based on Landau-Lifshitz equation, and the impact of the interaction between the grains on the minimum switching field threshold and the speed of the switching process in the microwave field is investigated. The main jobs and conclusions are as follows.1. When angle θ between the applied field (Hdc) and the anisotropy field (Hk) is small, the impact of the single microwave (hdc) and two microwaves (hdc) with different frequencies on the grain is calculated. It is found that two waves can reduce the switching speed and the minimum switching field. While the frequencies of the waves with same amplitude are smaller than the resonance frequency (fR), comparing to the wave with the same amplitude and the same frequency, the switching speed is relatively enhanced. While no field is applied and the amplitude of the wave equals to0.1Hk. the moment of the grain processes obviously with the microwave as the fac equals to fr.2. The impact of the applying order of wave and magnetic field on the switching process of an isolate grain is investigated as a function of Hdc when fac fixed to fR. It is found that the switching speed is enhanced when magnetic field and wave are applied at the same time. The minimum switching field is reduced when the wave is applied firstly comparing to the case that no wave is applied, but switching speed becomes slow comparing to the case that two field is applied simultaneously. 3. As to two grains with different anisotropic field directions and interaction between them, it is found that the speed of the processional motion is slow and the switching field threshold is not continuous as one wave is applied. For a relatively weak exchange interaction, the threshold is increased, and the strong exchange interaction causes the motions of the moments to be similar to an independent grain. The minimum switching field is reduced with the increase of the angle between the reversal field and the anisotropy field, however the moments of the grains will not switch when the angle is up to a certain value. While the exchange interaction (J) or dipolar interaction (D) is increased, both the minimum switching field and switching time increase. But when J is increased to a certain value, the motions of the grains are similar to that of an isolate grain. As the frequencies of microwaves are in the range from0.7fR to0.8fR, the minimum switching field can be reduced.4. Concerning the impact of the volume of the grain on the moment switching, it is found that the switching is similar with two isolate grains when the volume ratio of the two grains is bigger than0.8. When the ratio is smaller than0.3, the motion behavior of the bigger moment is as to an isolate grain. The small one, however, could not switch at the same condition when two same grains can switch.