Two-step Magnetization Reversal Research Articles

Metastable antiferromagnetic domain configurations in exchange biased bilayers

We report on exchange bias relaxation in NiO/FeNi bilayers. FeNi films have been sputtered on top of NiO films, previously grown on quartz substrates using pulsed laser deposition. As-prepared bilayers show enhanced coercivity but no bias. Their hysteresis loops are essentially isotropic within the sample plane. Exchange bias relaxation is observed after the samples have been magnetized once, and let in their remanent state: exchange bias progressively builds up in zero external magnetic field, over a few days timescale. This behavior may be related to the reorientation of antiferromagnetic domains triggered by the exchange coupling with the ferromagnetic domains throughout the interface. Relaxed samples are exchange biased along the magnetization direction, which becomes an easy axis. Along the perpendicular direction, a two-step magnetization reversal is observed. This two-step process is erased when the samples are field-annealed at elevated temperatures, and it is replaced by the rotation process usually observed along the hard axis. These results suggest that two different antiferromagnetic domain structures can be stabilized at room temperature, both giving rise to exchange bias. This may be related to the competition between the field-induced exchange bias direction, and the antiferromagnetic anisotropy easy axis within the NiO layer.

Kerr imaging of a Co/Pt bimodal magneto-optical medium

Spatially resolved magneto-optical (MO) images of a (0.3 nm Co/1.2 nm Pt)15 multilayer film with perpendicular magnetic anisotropy have been obtained by Kerr microscopy. This material exhibits a major hysteresis loop with two-step magnetization reversal. The MO images display four possible “stable” magnetic states distinguished by four different intensities. This behavior is explained by the presence of two different magnetic phases each which has a stable magnetization state either parallel or antiparallel to the applied field, and which may reverse quasi-independently from one another.