E. D. Dahlberg

Unidirectional coercivity enhancement in exchange-biased Co/CoO

A unidirectional coercivity enhancement, exhibiting the same behavior as a positive exchange bias, has been discovered in a temperature range below the blocking temperature in Co/CoO bilayers. Below this temperature range, the usual shift of the center of the M–H loops to the negative or antiparallel to the cooling field direction is found. This behavior is observed in both magnetic hysteresis loops and transport properties. The positive exchange bias can be explained by reversible changes in the interfacial pinning by the antiferromagnet causing an asymmetric magnetization reversal and a unidirectional coercivity enhancement along the cooling field direction.

Henkel plots in a temperature and time dependent Preisach model

The effect of finite temperature T and observation time t on the Henkel plots of ac and thermally demagnetized systems has been investigated within the framework of a generalized Preisach model, in which it is assumed that thermally activated hopping will occur over all energy barriers W<W* =k/sub B/TIn(t//spl tau//sub 0/), where /spl tau//sub 0/ is a microscopic time, and will systematically drive the Preisach plane towards equilibrium. The Preisach distribution function is assumed to be a factorized product of a Gaussian coercive field distribution, with mean value h~/sub c/ and dispersion /spl sigma//sub c/, and a Gaussian interaction field distribution, with a self-consistent mean-field average h~/sub int/=km and dispersion /spl sigma//sub s/. Increases in temperature or observation time cause a progressive collapse of the hysteresis cycle, as expected, and also enhance demagnetizing-like curvature in Henkel plots, at least for ac demagnetized systems. An exception is a thermally demagnetized system with k=0, which has a linear Henkel plot independent of W*. Varying the effective time for thermal relaxation of the magnetization from branch to branch of the hysteresis cycle can have the effect of imitating mean field interactions of both magnetizing-like and demagnetizing-like sign in systems with k=0, and can even lead to Henkel plots which violate the lower boundary i/sub d/=-i/sub r/.

Structural and magnetic properties of triode-sputtered epitaxial γ′-Fe4N films deposited on SrTiO3 (001) substrates

Highly ordered, single-crystal Fe4N films were prepared on single-crystal SrTiO3 (001) substrates by reactive magnetically enhanced dc triode sputtering. Analytical techniques, including x-ray diffraction, reflectivity, ion channeling, and atomic force microscopy, were used to characterize the structure and morphology of the films. The magnetic properties of the films were measured by magnetic force microscopy and superconducting quantum interference device magnetometry. A technique that utilizes anisotropic magnetoresistance was employed for measurement of the magnetocrystalline anisotropy of the Fe4N films.

Rotation of exchange anisotropy in biased Co/CoO bilayers

Magnetic field induced irreversible changes of the exchange anisotropy in Co/CoO bilayers were investigated. Cobalt films were grown by dc magnetron sputtering and then partially oxidized. They were then field cooled in 7000 Oe to 4.2 K to induce the exchange bias. A variable magnitude magnetic field was applied in the film plane at various angles with respect to the exchange bias direction. The effects of this variable magnetic field on the exchange coupling were studied by the reversible anisotropic magnetoresistance technique. Three qualitatively different behaviors of the exchange anisotropy direction were observed. The particular behavior was determined by the magnitude of the applied magnetic field and the rotation angle. A simple phenomenological model of the exchange anisotropy was developed to explain the experimental results, which allows for explicit quantitative separation of the unidirectional and the rotatable parts of the exchange anisotropy.

Henkel plots in a thermally demagnetized scalar Preisach model

The implications of thermal demagnetization with respect to the observation of interaction effects in Henkel plots are discussed within the framework of a scalar moving Preisach model. The Preisach distribution is assumed to be a product of a Gaussian coercive field distribution and a Gaussian interaction field distribution. Numerical calculations of the magnetizing and demagnetizing remanences show that, by contrast with other demagnetizing procedures, thermal demagnetization yields Henkel plots whose direction of curvature is uniquely related to the sign of the mean interaction field. In particular, a distribution of interaction fields which is symmetric about the origin yields the same linear Wohlfarth relation as a completely noninteracting system, while demagnetizing (magnetizing)-like mean fields always curve the Henkel plot below (above) the Wohlfarth line. The simple linearity of the Henkel plot in the absence of mean field effects was exploited to evaluate the effectiveness of a recent proposal for experimentally suppressing shape demagnetizing effects in perpendicular recording media. >

Moment determination of magnetic force microscope tips by imaging superparamagnetic films

A method to characterize the magnetic moment of a magnetic force microscopy (MFM) tip is presented. The response of the MFM while imaging a superparamagnetic film is compared to a nonmagnetic reference. A simple model is used to compare the experimental results with a calculation of the response of the MFM tip due to the interactions with the magnetization of a superparamagnetic film.

Critical magnetic fields of superconducting thin films of ErRh4B4

Abstract In thin films of the re-entrant ternary superconductor ErRh 4 B 4 the perpendicular critical field H c⊥ has been found to be larger than the parallel critical field H c 11 . These unusual results have been shown to be consistent with a simple model which considers the effect of the magnetization induced in the film by the externally applied field but inconsistent with a more detailed model which invokes multiple pair-breaking mechanisms.

Magnetic force microscopy studies of the domain structure of Co/Pd multilayers in a magnetic field

We have measured the magnetic domain patterns in Co/Pd multilayers of varying thickness using magnetic force microscopy in the presence of an external magnetic field applied perpendicular to the multilayers. We find that the domain patterns evolution is in qualitative agreement with existing theories for single layer thin films. Our results are in reasonable agreement with a theoretical model of domains appropriate to multilayer films.

Interactions and thermal effects in systems of fine particles: a Preisach analysis of CrO/sub 2/ audio tape and magnetoferritin

We present an analysis and comparison of magnetization and remanence data from two particulate systems: commercial CrO/sub 2/ audiotape and a frozen suspension of magnetoferritin particles. The measurements were performed over a range of temperatures, 25/spl deg/C/spl les/T/spl les/120/spl deg/C for CrO/sub 2/ and 5 K/spl les/T/spl les/10 K for magnetoferritin, and over a range of fields sufficient to saturate the remanence at each temperature. The CrO/sub 2/ tape was prepared by both ac demagnetization and by thermal demagnetization from above the Curie temperature, while the magnetoferritin was thermally demagnetized from above its highest blocking temperature. The data were analyzed within the framework of a scalar Preisach model that included thermally activated relaxation and a reversible nonlinearity. In the case of CrO/sub 2/, our fits showed that changes in hysteresis with temperature were due primarily to thermal variations in the spontaneous moment and the anisotropy of the particles, and in their magnetostatic interaction fields, and hence to changes in the free-energy landscape. In magnetoferritin, the temperature dependence of the magnetic response was due almost exclusively to changes in the relative populations of blocked and unblocked particles which relax by thermal activation over a fixed distribution of energy barriers. Our analysis of interaction effects was based on the Wohlfarth-Henkel parametric plot of the principal magnetizing and demagnetizing remanences. These plots showed curvature indicative of demagnetizing-like interactions in both thermally demagnetized magnetoferritin and ac-demagnetized CrO/sub 2/, but essentially no curvature in thermally demagnetized CrO/sub 2/. Our analysis revealed that there were no long-range, mean field interactions in either system, but that thermal demagnetization of CrO/sub 2/ produced a random initial state that was insensitive to fluctuations in the interaction field due to disorder, provided that mean field effects were absent, while thermal demagnetization of magnetoferritin produced a highly asymmetric initial state resembling that induced by ac demagnetization, which was sensitive to these fluctuations.

Sputter deposition of thin films of superconducting Er(RhB)4

Thin films of essentially single‐phase ErRh4B4 have been sputter deposited from an arc‐melted target of ErRh4B4 in an ultrahigh‐vacuum system incorporating a liquid‐nitrogen cryogettering can. Optimum conditions for depositing the films were determined. The films exhibit reentrant superconducting transitions with Tc’s somewhat different from those reported for bulk material.