A. Westphalen

Invited Article : Vector and Bragg Magneto-optical Kerr effect for the analysis of nanostructured magnetic arrays

Experimental and theoretical aspects of obtaining the magnetic information carried by laser beams diffracted from an array of micro- or nanosized magnetic objects are reviewed. We report on the fundamentals of vector magneto-optic Kerr effect (MOKE), Bragg-MOKE, and second-order effects in the Kerr signal in longitudinal Kerr geometry as well as on an experimental setup used for vector and Bragg-MOKE experiments. The vector and Bragg-MOKE technique in combination with micromagnetic simulation is a reliable tool for measuring the complete magnetization vector and for characterizing the reversal mechanism of lateral magnetic nanostructures. We discuss the Bragg-MOKE effect for three standard domain configurations during the magnetization reversal process and present the expected behavior of the magnetic hysteresis loops.

Quantitative description of the azimuthal dependence of the exchange bias effect

While the principal features of the exchange bias between a ferromagnet and an antiferromagnet are believed to be understood, a quantitative description is still lacking. We show that interface spin disorder is the main reason for the discrepancy of model calculations versus experimental results. Taking into account spin disorder at the interface between the ferromagnet and the antiferromagnet by modifying the well known Meiklejohn and Bean model, an almost perfect agreement can be reached. As an example this is demonstrated for the CoFe/IrMn exchange biased bilayer by analysing the azimuthal dependence of magnetic hysteresis loops from MOKE measurements. Both exchange bias and coercive fields for the complete 360° angular range are reproduced by our model.

Anisotropy of ferromagnetism in Co-implanted rutile

Magnetic anisotropy of cobalt implanted single-crystalline rutile has been studied by means of magneto-optical Kerr effect (MOKE) and superconducting quantum interference device (SQUID) techniques. We observed for the first time strong angular dependence of the remanent magnetization and coercive field in the plane of the implanted surface: twofold anisotropy for the (100)-substrate and fourfold anisotropy for the (001)-substrate samples. The observation opens up new possibilities to tailor magnetic anisotropies of the material. Possible origins of ferromagnetism and anisotropies in dielectric and diamagnetic single-crystalline TiO2 samples after Co-ion implantation are discussed.

Hyper-domains in exchange bias micro-stripe pattern

A combination of experimental techniques, e.g. vector-MOKE magnetometry, Kerr microscopy and polarized neutron reflectometry, was applied to study the field induced evolution of the magnetization distribution over a periodic pattern of alternating exchange bias (EB) stripes. The lateral structure is imprinted into a continuous ferromagnetic/antiferromagnetic EB bilayer via laterally selective exposure to He-ion irradiation in an applied field. This creates an alternating frozen-in interfacial EB field competing with the external field in the course of the re-magnetization. It was found that in a magnetic field applied at an angle with respect to the EB axis parallel to the stripes the re-magnetization process proceeds via a variety of different stages. They include coherent rotation of magnetization towards the EB axis, precipitation of small random (ripple) domains, formation of a stripe-like alternation of the magnetization, and development of a state in which the magnetization forms large hyper-domains comprising a number of stripes. Each of those magnetic states is quantitatively characterized via the comprehensive analysis of data on specular and off-specular polarized neutron reflectivity. The results are discussed within a phenomenological model containing a few parameters, which can readily be controlled by designing systems with a desired configuration of magnetic moments of micro- and nano-elements.

Bragg magneto-optical Kerr effect measurements at Co stripe arrays on Fe(001)

We report on magneto-optical Kerr effect (MOKE) measurements of a patterned spin valve system, consisting of few-micrometers-wide Co stripes on a continuous Fe film separated by a 14-nm-thick Cr spacer layer. The spin valve array was studied by regular longitudinal MOKE in specular geometry as well as in Bragg MOKE geometry, using the diffraction spots from the grating for hysteresis measurements. We have investigated the shape of the hysteresis loops and the Kerr amplitude as a function of the diffraction order and for various grating periods. The hysteresis loops measured at the diffracted spots reveal an amplification of the Kerr signal in the field regime where the magnetization of the Fe and Co layers is antiparallel.

Extended longitudinal vector and Bragg magneto-optic Kerr effect for the determination of the chirality distribution in magnetic vortices

We have measured longitudinal magneto-optic Kerr effect (MOKE) for off-specular beams diffracted by a square array of Permalloy nanodots outside the plane of incidence and found that only the measurements performed outside the plane of incidence are sensitive to the distribution of rotational sense of vortices (vortex chirality) in the dot array. An asymmetry was introduced into the dot shape to ensure a uniform vortex chirality distribution in the dot array and to obtain a clear, well-defined chirality contrast in the MOKE signal. We also demonstrate an alternative method to measure the vortex chirality in dots of asymmetric shape without switching the chirality during magnetization reversal. In addition to the experiment, we have developed a general formalism that can be used to describe MOKE measured for off-specular beams inside as well as outside of the plane of incidence in all three Kerr geometries (polar, longitudinal, and transversal) with an arbitrary polarization state of incident light. Combin...

Magnetization reversal in nanowires with a spiral shape

Magnetic nanowires have been shaped in the form of spirals and arranged in different patterns. A two-dimensional periodic array of Fe spiral structures was fabricated by electron-beam lithography. The spirals had a radius of 2.8 μm, a linewidth of 100 nm, and a thickness of 20 nm. The magnetization reversal was studied by longitudinal vector magneto-optic Kerr effect (MOKE) in specular geometry as well as in Bragg MOKE geometry, using the diffraction spots from the grating for hysteresis measurements. The measurements are compared with the results of micromagnetic simulation, which allows a detailed interpretation of the experimental data. The magnetization reversal is characterized by an onion state in remanence and a coercivity which is different for the inner and outer parts of the spiral structures. In general the inner parts of the spiral are more stable and switch later than the outer ones. The switching of the outer parts depends on the boundary condition.

Magnetization Reversal Studies of Periodic Magnetic Arrays via Scattering Methods

Magnetic patterns with different shapes and aspect ratios provide control over the remanent domain state, the coercivity, and over different types of reversal mechanisms. We discuss magneto-optical, soft x-ray resonant scattering, and neutron scattering methods for evaluating the vector magnetization during reversal and the higher Fourier components of the magnetization distribution. These scattering methods, providing a statistical averaged signal, contrast real space methods, which give information on individual islands.

Magnetization Reversal of Stripe Arrays

Magnetic stripes with different aspect ratios provide control over the remanent domain state, the coercivity, and over different types of reversal mechanisms. We discuss two methods for evaluating the magnetization vector during reversal: vector magneto-optical Kerr effect and polarized neutron reflectivity. Both methods are compared and critically discussed.