Reduction in Switching Field for a Granular Perpendicular Medium Using Microwave Assisted Magnetic Recording

Abstract

This paper describes the basic physics of magnetization switching in the presence of a dc bias and an in-plane radio-frequency (RF) field using a micromagnetic model based on the Landau-Lifshitz-Gilbert formulation for the magnetization dynamics. We start with the case of a uniform granular media at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> = 0 K without any distributions in grains or medium properties. The switching behavior of this medium is essentially the same as that of a single grain and can be described by replacing <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">HK</i> of the grain with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> ' <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> = <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">HK</i> - 4pi <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ms</i> of the medium. Significantly smaller values of the dc bias field compared to the Stoner-Wohlfarth field are needed to reverse the medium for the assumed medium properties at low damping constant alpha. This reduction in dc bias field is progressively less at higher values of damping constant. However, increasing RF field magnitude can provide a significant reduction in the dc bias field even for high alpha values. As grain size distributions are added, the coherent precession of the medium grains can be sustained only for very short duration due to varying self-demagnetization of individual grains. Once the coherent precession is lost, a higher dc bias field compared to the case of uniform granular medium is required to completely reverse all the grains in the medium. At finite temperature and with the inclusion of grain size and magnetic materials distributions, the coherent precession is completely lost, thereby requiring even larger dc bias field for complete reversal of the grains. However, this dc bias field is still significantly smaller compared to the dc field required without the presence of RF field. The need for smaller dc bias field may mitigate the writeability requirements for high areal density recording.