T. Gredig

Unidirectional coercivity enhancement in exchange-biased Co/CoO

A unidirectional coercivity enhancement, exhibiting the same behavior as a positive exchange bias, has been discovered in a temperature range below the blocking temperature in Co/CoO bilayers. Below this temperature range, the usual shift of the center of the M–H loops to the negative or antiparallel to the cooling field direction is found. This behavior is observed in both magnetic hysteresis loops and transport properties. The positive exchange bias can be explained by reversible changes in the interfacial pinning by the antiferromagnet causing an asymmetric magnetization reversal and a unidirectional coercivity enhancement along the cooling field direction.

Magnetization reversal in exchange biased Co/CoO probed with anisotropic magnetoresistance

The magnetization reversal in exchange coupled polycrystalline Co/CoO bilayers has been investigated as a function of CoO thickness using anisotropic magnetoresistance as a probe. The anisotropic magnetoresistance (AMR) was measured during the magnetization reversal and it was used to determine the orientation of the magnetization. For thin CoO layers large training effects were present; ergo the first hysteresis loop after field cooling was not the same as the second. The magnitude of the observed training was found to decrease with increasing CoO thickness. In the samples where substantial training was observed, the first magnetization reversal was dominated by nucleation of reversed domains. For the reversal from the antiparallel state back to the parallel direction, the AMR is consistent with a rotation process. In thicker CoO films where the training was less, the asymmetry was drastically reduced. A simple model that couples the antiferromagnetic grains to the ferromagnetic layer simulates qualitati...

Ferromagnetic domain distribution in thin films during magnetization reversal

It is shown that polarized neutron reflectometry can determine in a model-free way not only the mean magnetization of a ferromagnetic thin film at any point of a hysteresis cycle, but also the mean-square dispersion of the magnetization vectors of its lateral domains. This technique is applied to elucidate the mechanism of magnetization reversal of an exchange-biased Co/CoO bilayer. The reversal process above the blocking temperature

Domain formation in exchange biased Co/CoO bilayers

{T}_{b}

Rotation of exchange anisotropy in biased Co/CoO bilayers

is governed by uniaxial domain switching, while below

Magnetic relaxation in exchange-coupled Co/CoO bilayers measured with ac-anisotropic magnetoresistance

{T}_{b}

The use of the anisotropic magnetoresistance to study exchange coupling

the reversal of magnetization for the trained sample takes place with substantial domain rotation.

Role of magnetic aftereffect in coercivity enhancement of Co/CoO bilayers

The magnetic behavior of exchange biased Co/CoO polycrystalline thin films has been investigated using magnetometry and magneto-optical (MO) imaging. For CoO layer thicknesses of about 30 A, these films exhibit a strong training effect below the blocking temperature of 130 K. A sharp initial reversal of the magnetization of the FM Co layer after field-cooling is followed by S-shaped magnetization loops with reduced coercive fields. The MO images show that during the initial magnetization reversal the remagnetization front moves from the edge of the sample into the homogenously magnetized film, leaving behind an irregular pattern of domains of the order of 10 μm. These domains, once generated during the first reversal, do not expand or move on subsequent magnetization loops and can be erased only by heating above the blocking temperature. This suggests that the domains are related to domains in the antiferromagnetic CoO layer.

Determination of exchange anisotropy by ac-AMR and planar Hall effect

Magnetic field induced irreversible changes of the exchange anisotropy in Co/CoO bilayers were investigated. Cobalt films were grown by dc magnetron sputtering and then partially oxidized. They were then field cooled in 7000 Oe to 4.2 K to induce the exchange bias. A variable magnitude magnetic field was applied in the film plane at various angles with respect to the exchange bias direction. The effects of this variable magnetic field on the exchange coupling were studied by the reversible anisotropic magnetoresistance technique. Three qualitatively different behaviors of the exchange anisotropy direction were observed. The particular behavior was determined by the magnitude of the applied magnetic field and the rotation angle. A simple phenomenological model of the exchange anisotropy was developed to explain the experimental results, which allows for explicit quantitative separation of the unidirectional and the rotatable parts of the exchange anisotropy.

Positive Exchange Bias Induced by Temperature

The complex ac-susceptibility of field-cooled Co/CoO bilayers has been measured by perturbing the magnetization with a small (5 Oe) ac field, and detecting the response through anisotropic magnetoresistance using a double modulation technique. Samples of Ta(3 nm)/Co(8 nm)/CoO(3 or 10 nm)/Cu(3 nm) grown by sputter deposition on an amorphous silicon nitride substrate have been studied using a frequency of 100 Hz. The relaxation of the exchange anisotropy in response to the perturbation allows the activation energies and relaxation times of the CoO grains to be investigated. Both the real and imaginary response show prominent thickness-dependent peaks at the blocking temperature, as well as a peak near 80 K which is insensitive to CoO film thickness. The measurements are well-described by a generic linear relaxation model of the anisotropy field, using a bimodal distribution of activation energies for the CoO grains.