Journal
PHYSICAL REVIEW B
Volume 79, Issue 1, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevB.79.014436
Keywords
cobalt alloys; coercive force; ferromagnetic materials; iridium alloys; magnetic anisotropy; magnetic domain walls; magnetic hysteresis; magnetic multilayers; magnetisation reversal; manganese alloys; Permalloy; stochastic processes; vortices
Funding
- U.S. Department of Energy Office of Science Laboratory [DE-AC02-06CH11357]
- Spanish CICYT [MAT2007-66302-C02]
- Catalan DGR [2005-SGR-00401]
- Institut Catala de Nanotecnologia
- ICREA Funding Source: Custom
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We investigate the reversal behavior of circularly exchange-biased micron-sized bilayer disks of Permalloy (Py)/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 activation) inherent in single-layer ferromagnetic disks and causes the magnetic behavior to be reproducible over time.
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