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The magnetization processes in exchange-biased MnPd∕Fe bilayers have been investigated using polarized neutron reflectivity and vibrating sample magnetometry. The measurements show that by breaking the symmetry of the intrinsic cubic fourfold anisotropy of the Fe film the induced unidirectional anisotropy radically changes the magnetization processes. If the exchange bias is large the induced unidirectional anisotropy is able to pull the net magnetization of the sample to the bias direction after saturation along any of the magnetic hard Fe⟨110⟩ directions. However, if the exchange bias is small it will only give rise to a difference in net magnetization along the magnetic easy axes adjacent to the saturation direction. Thus, the net magnetic moment along the bias direction depends on the relative magnitudes of the cubic and the unidirectional anisotropies.

The depolarization of a polarized neutron beam after transmission through a granular magnetic medium is related to the size and orientation of the ferromagnetic domains. Unfortunately, this relationship is simple only for a limited number of cases. In order to investigate the interpretation of neutron depolarization data, calculations for a three-dimensional model system of granular magnetic domains are presented. A comparison is made between the equations generally used to approximate the average domain size and the actual average domain size of the model system. Calculations are performed for several configurations of monodisperse spherical domains, as well as for a polydisperse distribution of nonspherical domains. While there is a good agreement between the actual domain size and that derived from the calculated neutron depolarization at low volume fractions of magnetic domains, at increasing volume fractions there is an increasing discrepancy. This discrepancy is partially due to magnetic correlation...

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.

Exchange bias of a partially oxidized thin film of ferromagnetic Co was studied by magnetization measurements and polarized neutron reflectivity (PNR). The magnetization curve shows strong effects of training with cycling of the magnetic field. Reflectivity measurements with the field parallel to the cooling field showed the onset of spin-dependent diffuse scattering—off the specular reflection—after a training cycle. Such scattering, of the Yoneda type, is due to misaligned Co domains possibly close to the Co/CoO interface. Subjecting the field cooled Co/CoO pair to a field perpendicular to the cooling field causes a rotation of the magnetization. The PNR measurements confirmed earlier susceptibility studies by indicating that the rotation of the magnetization is reversible in fields up to 400 Oe. The rotation of the magnetization of Co is uniform across the film thickness.

The magnetization reversal in exchange biased FeF2/Co films was studied using polarized neutron reflectometry and magneto-optical imaging (MOI). After field cooling in a field H perpendicular to the c axis (the easy axis of FeF2) through the FeF2 Neel temperature, the magnetization curve measured with H parallel to the c axis showed a double loop, one with a positive exchange bias field (HE) and one with a negative HE. This behavior suggests that the antiferromagnet (AFM) is split into two types of domains with HE in opposite directions along the c axis. However, the observation of spin-flip reflectivity in the field region separating the two loops indicates that some rotation of the Co magnetization also occurred during reversal. Concurrently, small inhomogeneous domains with sizes much less than 30μm were observed using MOI. This implies that small AFM domains with opposing HE created during cooling and the resulting frustration of the interface exchange interaction with the ferromagnet are responsible ...

We studied the magnetic properties of a ferritin-core array, deposited on a Si wafer and encapsulated in silica. The array was formed by a simple evaporation-induced self-assembly mechanism of ferritin proteins, and it consists of 6 nm ferritin cores crystallized in a two-dimensional hexagonal lattice with a core-core distance of about 10.5 nm. Magnetization studies indicate that the array undergoes a magnetic transition around 210 K. Our small-angle neutron-scattering studies provide evidence that huge net magnetic moments are stabilized in each core and that long-range magnetic order between the core moments occurs in the array.