E. Dan Dahlberg

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

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.

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

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.

Detecting two magnetization components by the magneto‐optical Kerr effect

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.

Micromagnetic Microscopy and Modeling

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...

Improved spatial resolution in magnetic force microscopy

Electron beam deposited spikes for use in magnetic force microscopy have been grown onto atomic force microscope tips and coated with magnetic thin films using thermal evaporation. The resulting magnetically active regions are a close approximation to monopoles or dipoles located on the very end of the spikes. We show that these tips image with increased spatial resolution and less sample perturbation than the standard, commercially available tips.

Training effects and the microscopic magnetic structure of exchange biased Co/CoO bilayers

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.

Domain structures in single crystal magnetite below the Verwey Transition as observed with a low‐temperature magnetic force microscope

The magnetic domain structures on the {110} plane of magnetite (Fe 3 O 4 ) below the Verwey transition (T V =120K) were studied using a Low-Temperature Magnetic Force Microscope (LTMFM). At 298K, domain structures consisted of arrays of 180°, 109° and 71° walls, typical for magnetite with cubic anisotropy. At 77K (below T V ), the cubic style patterns disappeared and transformed into uniaxial patterns consistent with the uniaxial magnetocrystalline symmetry of the low-temperature monoclinic phase of magnetite. We also observed two distinct styles of domain patterns below T V : (1) wide domains separated by straight 180° walls along the in-plane [100] easy axis ; and (2) intricate wavy walls with reverse spike domains characteristic of out-of-plane easy axes. This intimate mixture of domain styles within adjacent areas of the crystal reflects variations in the direction of the magnetic easy axes in different regions produced by c-axis twinning of the crystal below T v . The thermal dependence of planar and wavy-wall patterns show little change from 77K until 110K, where patterns disappear. Upon cooling back to 77K, domain structures are different from the initial 77K states, indicating that renucleation of different domain states occurs by cycling near T V .

Localized micromagnetic perturbation of domain walls in magnetite using a magnetic force microscope

Magnetic force microscope (MFM) profiles of domain walls (DWs) in magnetite were measured using commercially available MFM tips. Opposite polarity profiles of a single DW segment were obtained by magnetizing the MFM tip in opposite directions perpendicular to the sample surface. During a measurement, the field of the tip locally magnetized the DW, resulting in a more attractive tip‐sample interaction. The difference between opposite polarity DW profiles provided a qualitative measurement of the reversible changes in DW structure due to the localized field of the MFM tip.

Oscillatory Exchange Coupling and Positive Magnetoresistance in Epitaxial Oxide Heterostructures

Oscillation in the exchange coupling between ferromagnetic La(2/3)Ba(1/3)MnO3 layers with paramagnetic LaNiO3 spacer layer thickness has been observed in epitaxial heterostructures of the two oxides. This behavior is explained within the RKKY model employing an ab initio calculated band structure of LaNiO3, taking into account strong electron scattering in the spacer. Antiferromagnetically coupled superlattices exhibit a positive current-in-plane magnetoresistance.

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...