Magnetization reversal in circularly exchange-biased ferromagnetic disks

Abstract

We investigate the reversal behavior of circularly exchange-biased micron-sized bilayer disks of Permalloy ͑Py͒/IrMn and CoFe/IrMn. A circular exchange bias is induced by imprinting the vortex configuration of the ferromagnetic layer into the IrMn when the disks are cooled in zero external field through the blocking temperature of IrMn. The resulting circular exchange bias has a profound effect on the reversal behavior of the ferromagnetic magnetization. In Py/IrMn disks the reversal takes place via vortex motion only, and the behavior is controlled by the exchange bias; it is reversible over a range of small fields and the vortex maintains a single chirality throughout reversal, determined by the chirality of the exchange bias. In CoFe/IrMn disks the non-negligible magnetocrystalline anisotropy causes a reversal via both vortices and domain walls resulting in a finite coercivity, and the behavior is controlled by microstructure. We verify that circular exchange bias does not give rise to a hysteresis loop shift. It lowers coercivity with respect to the field-cooled case, and in Py/IrMn disks it even causes completely reversible magnetic behavior. In both Py/IrMn and CoFe/IrMn disks, circular exchange bias removes the randomness ͑i.e., stochastic processes due to thermal activation͒ inherent in singlelayer ferromagnetic disks and causes the magnetic behavior to be reproducible over time.

Simulations of magnetic vortex dynamics in exchange-biased sub-micron-sized disks

“…This configuration is equivalent to what is obtained experimentally by a ZFC process, as was reported in Refs. [16][17][18][19]. As a validation test, we have also done simulations with the exchange bias defined as a fixed circular field, and the results were equivalent to the results obtained in the case of a fixed spin layer.…”

“…The larger observed values of nucleation and annihilation fields for exchange-biased vortex are in agreement with results reported in literature. [17][18][19] It was suggested that the increase in both nucleation and annihilation fields must be equal to the exchange field. 16,21 On the other hand, Sort et al 21 observed that simulation results differ to experimental data by presenting slightly larger annihilation fields, which is in accordance with our results.…”

“…15,16 However, when a submicron-sized disk in vortex configuration in the FM layer is Zero-Field Cooled (ZFC), the vortex configuration is imprinted in the AFM layer, inducing a circular exchange bias. [17][18][19][20] It is also possible to imprint displaced vortices in the AFM layer. For this, the cooling process should occur in an applied field insufficient to saturate the disk.…”

Temperature dependent magnetization reversal of exchange biased magnetic vortices in IrMn/Fe microcaps

“…In this regard, magnetization reversal in exchange biased permalloy (Py)/IrMn disks of 1 lm diameter was investigated. [4][5][6][7][8][9] The magnetization reversal in such structures is found to be dependent on the applied field direction. If the applied in-plane field is along the exchange bias direction, the magnetization reverses via nucleation and annihilation of the vortex.…”

An O(N) and parallel approach to integral problems by a kernel-independent fast multipole method: Application to polarization and magnetization of interacting particles

“…The magnetostatic problem is ubiquitous for any system that involves patterned micronor sub-micron size magnetic bodies, such as read or write heads in magnetic hard disk drives, magnetic nanoparticles 25 , coupled magnetic disks 26,27 , or artificial spin ices 28 . In patterned ferromagnetic systems, there typically arises a competition between the short-range exchange interactions and long-range magnetostatic interactions.…”