http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

Understanding the correlation between magnetic properties and nanostructure involves collaborative efforts between chemists, physicists, and materials scientists to study both fundamental properties and potential applications. This article introduces a classification of nanostructure morphology according to the mechanism responsible for the magnetic properties. The fundamental magnetic properties of interest and the theoretical frameworks developed to model these properties are summarized. Common chemical and physical techniques for the fabrication of magnetic nanostructures are surveyed, followed by some examples of recent investigations of magnetic systems with structure on the nanometer scale. The article concludes with a brief discussion of some promising experimental techniques in synthesis and measurements.

Magnetic Properties of Nanostructured Materials

Granular materials display a variety of behaviors that are in many ways different from those of other substances. They cannot be easily classified as either solids or liquids. This has prompted the generation of analogies between the physics found in a simple sandpile and that found in complicated microscopic systems, such as flux motion in superconductors or spin glasses. Recently, the unusual behavior of granular systems has led to a number of new theories and to a new era of experimentation on granular systems.

https://science.sciencemag.org/content/sci/255/5051/1523.full.pdf

Physics of the Granular State

Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

TOPICAL REVIEW: Nanomagnetics

Direct ferromagnetic antiferromagnetic exchange biasing energy is determined by small reversible rotations of the magnetization away from the unidirectional easy axis using an externally applied magnetic field. The angle the magnetization makes relative to the easy direction is determined by measuring the anisotropic magnetoresistance. This technique gives energies larger by a factor of 2 than the traditional measurements of the shift in the hysteresis loop (an irreversible process) of the ferromagnetic layer in bilayer samples of Co/CoO. An apparent Co thickness variation of the experimentally determined exchange biasing interface energy indicates the Co magnetization is not uniform but probably spirals through the thickness of the film.

Use of the anisotropic magnetoresistance to measure exchange anisotropy in Co/CoO bilayers

The longitudinal and transverse magneto-optical Kerr effects have been used for understanding the magnetization processes in single crystal Fe/GaAs (100) thin films. By using both of these Kerr effects it is possible to concurrently detect two orthgonal in-plane magnetization components. Presented here are Kerr hysteresis curves for magnetic fields directed along the in-plane 〈100〉 and 〈110〉 directions of the Fe films. For the 〈100〉 direction the magnetization curves are square, while for the 〈110〉 unusual overshoots are present in the Kerr hysteresis curves. In order to understand the origin of these curves for the 〈110〉 direction, simulations were done using the Fresnel reflection coefficients for the in-plane Kerr effects and a coherent-rotation model for the magnetization process. There is good agreement between the simulated Kerr hysteresis curves and the experimental data. The overshoots along with the general analyzer dependence of the hysteresis curves are reproduced in the simulations. However, the magnitude of the reversal and saturation fields of the modeled loops cannot be brought into agreement with the data. For applied fields near the 〈110〉 directions, the analysis suggests that the reversal occurs through the nucleation and/or unpinning of 90\ifmmode^\circ\else\textdegree\fi{} domains at two distinct transition fields followed by coherent rotation.

Magnetization reversal in (100) Fe thin films.

A novel technique was detecting two orthogonal in‐plane magnetization components is presented. This technique utilizes the magneto‐optical Kerr effects to sense the two components. These components of magnetization are parallel and perpendicular to the plane of incidence of the light beam. The ability to sense two components, individually or simultaneously, is a result of the disparity in the longitudinal and transverse Kerr effects. Based on the Frensel reflection coefficients of these two effects, an anlysis is presented describing this dual component sensitivity. The physical conditions are given for simultaneous and individual detection of the two in‐plane magnetization components. To substantiate this analysis, magneto‐optical measurements are made on single‐crystal Fe films. The results are discussed in the context of dual component sensitivity.

Detecting two magnetization components by the magneto‐optical Kerr effect

Magnetotransport measurements provide an ideal probe to determine the various anisotropy energies in epitaxial magnetic films. The extraordinary Hall effect (EHE) can be used to determine the perpendicular or surface anisotropy energy while the anisotropic magnetoresistance (AMR) can be used to investigate the in‐plane anisotropy energies. The advantage of magnetotransport over more tranditional measurement techniques used to determine these anisotropy energies is the ease of the technique, the lack of a need for sophisticated equipment, and the insensitivity of the techniques to the magnetic properties of a semiconducting or insulating substrate. Both the EHE and the AMR have been used to study the magnetic properties of epitaxial iron films grown on GaAs substrates. The results of the EHE and the AMR study and how the various anisotropy energies compare with those determined by the more traditional techniques of ferromagnetic resonance and vibrating sample magnetometry will be discussed.

Magnetotransport: An ideal probe of anisotropy energies in epitaxial films (invited)

With the miniaturization of magnetic technologies, the need to understand magnetization on length scales below a micron is becoming increasingly important. This booming interest in micro magnetics has fueled a renaissance in both micro‐magnetic modeling and measurement techniques. Conversely, the codevelop‐ment of modeling and imaging has made possible recent advances in this critical area of magnetism. On the modeling side, the rapid development of high‐speed computing has had a tremendous impact on micromagnetics simulations. On the measurement side, a number of microscopies have been developed for imaging on a length scale of tens of nanometers. Figure 1 shows an image of rows of bits in a magneto‐optical medium. The bits were both written and imaged using a magnetic force microscope. Results on this length scale provide information that can be used in models and also challenge models’ predictive capabilities. The image on the cover of this issue shows naturally occurring domain patterns in a single‐cr...

E. Dan Dahlberg

Papers

Use of the anisotropic magnetoresistance to measure exchange anisotropy in Co/CoO bilayers

Magnetization reversal in (100) Fe thin films.

Detecting two magnetization components by the magneto‐optical Kerr effect

Magnetotransport: An ideal probe of anisotropy energies in epitaxial films (invited)

Micromagnetic Microscopy and Modeling