The effect of the thickness and microstructural features on the switching behaviour of granular CoCrPt:SiO2 films with perpendicular magnetic anisotropy was investigated. TEM plane view and cross section analysis indicate that, while the very first layers grow as uniform nanograins in dose contact, the growth proceeds by formation of well-defined CoCrPt columnar islands (7 nm average size) separated by the silicon oxide, the distance among the islands remaining roughly constant along the whole thickness.The relation between such non-uniform microstructure and the magnetization reversal mechanism at room temperature was investigated by performing hysteresis loops at variable angle as well as time dependent measurements by using a vector vibrating sample magnetometer. Numerical micromagnetic simulations of the hysteresis loops have been carried out to support the description of the experimental observations. The results showed a coexistence of coherent and incoherent reversal processes, the former being more and more pronounced with increasing the magnetic layer thickness, consistently with the microstructural investigations. (C) 2013 Elsevier B.V. All rights reserved.

We investigate the effect of Co+ irradiation on the magnetization dynamics of CoCrPt:SiO2 granular media. Increasing irradiation levels reduce the saturation magnetization and effective anisotropy, which decrease the intrinsic magnetization precession frequency. Furthermore, increasing intergranular exchange coupling results in a qualitative change in the behavior of the magnetic material from a collection of individual grains to a homogeneous thin film, as evidenced in both the switching behavior and dynamics. The frequency change cannot be explained by single crystal macrospin modeling, and can only be reproduced by the inclusion of the dipolar effects and anisotropy distribution inherent in a granular medium.

We show switching probability measurements of individual bits of a bit patterned media in the highly inhomogeneous write field of a recording head. The behavior of the switching probability as a function of the applied field pulse width t deviates from a simple Arrhenius—Néel model for the magnetization reversal. The data agree well with an extended model that assumes a normal distribution of energy barriers. We compare the extracted energy barrier distribution to the switching field distribution in a uniform perpendicular magnetic field measured at long time scales.

We demonstrate a 2.5-fold coercivity reduction in FePt based exchange coupled composite bit patterned media (ECC-BPM) by coupling a lower anisotropy Co/Pd–Co/Ni-multilayer system to the top of a high anisotropy FePt L10 film. Furthermore the tight switching field distribution (SFD) of the lower anisotropy system reduces the SFD of the ECC-BPM composite system compared to a single layer FePt film. The relative amount of switching field and SFD reduction in these ECC-BPM arrays agree with corresponding micromagnetic simulations.

We demonstrate a 2.5-fold coercivity reduction in FePt based exchange coupled composite bit patterned media (ECC-BPM) by coupling a lower anisotropy Co/Pd–Co/Ni-multilayer system to the top of a high anisotropy FePt L10 film. Furthermore the tight switching field distribution (SFD) of the lower anisotropy system reduces the SFD of the ECC-BPM composite system compared to a single layer FePt film. The relative amount of switching field and SFD reduction in these ECC-BPM arrays agree with corresponding micromagnetic simulations.

An approach for tailoring the magnetic properties by ion irradiation of granular perpendicular CoCrPt:SiO(2) films grown on silica particles with sizes down to 10 nm was investigated. The as-prepared samples reveal an intriguing scaling dependence of the coercive field and remnant magnetization: both parameters are found to decrease with decreasing particle size. However, Co(+) irradiation at a low fluence of 0.5 x 10(14) cm(-2) already results in an opposite scaling behavior. It is assumed that this modification is due to the enhancement of the intergranular magnetic exchange coupling of the granular CoCrPt:SiO(2) film initiated by Co(+) irradiation resulting in a modified reversal behavior. Further increase of the irradiation fluence beyond 1.6 x 10(14) ions cm(-2) leads to a degradation of the magnetic layer properties, lowering the remnant magnetization and the coercive field in the easy-axis direction. Moreover, the local magnetic properties of the samples were analyzed by magnetic force microscopy revealing magnetic multi-domain cap structures.

Granular CoCrPt–SiO2 films with perpendicular magnetic anisotropy were deposited onto arrays of SiO2 nanoparticles with diameters down to 10 nm. Columnar CoCrPt grains with their c-axis pointing perpendicular to the particle surface were formed, creating a unique hedgehog-like cap structure. This peculiar structure induced by the curvature of the particles substantially modifies the magnetic properties. Underneath the CoCrPt pillars a continuous Co-rich layer was observed, which gives rise to enhanced intergranular exchange coupling resulting in single domain states. The temperature dependence of the coercivity and the angular dependence of the switching field were extracted and correlated with the microstructure.

Arrays of self-assembled polystyrene spheres with various particle sizes have been used as a substrate to study the exchange bias effect along the out-of-plane direction of Pt/Co multilayers capped with IrMn layers. The evolution of the reversal process of the resulting magnetic nanocaps was investigated by magnetic force microscopy ( MFM) and magnetic transmission x-ray microscopy ( M-TXM). Tip-sample interaction-induced irreversible and reversible switching events have been observed during multiple scanning cycles in MFM imaging which are ascribed to the so-called training effect. During M-TXM imaging a drastic change in morphology has been found due to the x-ray exposure, leading to the formation of much larger spherical particles. Interestingly, these merged particles reveal again an exchange coupled single-domain magnetic cap with magnetic behaviour similar to magnetic films deposited directly on spheres of similar size.