Modeling of intermediate behavior in Co/Pt vertical magnetization multilayers

Abstract

Magneto-optical Kerr effect (MOKE) hysteresis loops were acquired at various radial positions on a 25 mm diameter disk of a (0.3 nm Co/1.2 nm Pt) 15 multilayer. Two distinct types of behavior: unimodal, with two stable states, and bimodal, with four stable states, were observed. At the center of the disk, the loop exhibited bimodal features. At the outer edge, the reversal behavior was significantly altered, and the loop was unimodal. The radial variations in the magnetization are attributed to diffusion and interface effects arising from nonuniform heating in the film deposition process. We model these media to understand the physics of the magnetization processes. The bimodal loops are characterized by columns of CoPt 3 and of Pt threading the multilayers, strongly coupling them, and fully polarizing the Pt layers. Then, this thread is lost due to the different growth conditions at the edge of the disk, altering the polarization of the Pt layers. The overall behavior is modeled by assuming that in the unimodal region, there are a certain number of smooth layers starting at the substrate, and the remaining layers are rougher. In the intermediate region, vertical threads start forming at random points whose length increases as the medium becomes more bimodal. When the medium is fully bimodal, it consists solely of CoPt 3 and Pt threads that pierce the entire medium.