A. J. Shapiro

Effect of 3d, 4d, and 5d transition metal doping on damping in permalloy thin films

The effect of doping on the magnetic damping parameter of Ni80Fe20 is measured for 21 transition metal dopants: Ti, V, Cr, Mn, Co, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, and Au. For most of the dopants, the damping parameter increases linearly with dopant concentration. The strongest effects are observed for the 5d transition metal dopants, with a maximum of 7.7×10−3 per atomic percent osmium.

Refractive index study of AlxGa1−xN films grown on sapphire substrates

A prism coupling method was used to measure the ordinary (no) and extraordinary (ne) refractive indices of AlxGa1−xN films, grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) on sapphire, at several discrete wavelengths from 442 nm to 1064 nm. In addition, spectroscopic transmittance and reflectance, correlated with the prism coupling results, were used to measure no as a continuous function of wavelength between the band gap of each sample (255 nm to 364 nm, depending on Al fraction) and 2500 nm. The Al mole fractions (x), determined by energy dispersive x-ray spectroscopy (EDS), were x=0.144, 0.234, 0.279, 0.363, 0.593, and 0.657 for the HVPE-grown samples, and x=0.000, 0.419, 0.507, 0.618, 0.660, and 0.666 for the MOCVD-grown samples. The maximum standard uncertainty in the EDS-determined value of x was ±0.02. The maximum standard uncertainty in the refractive indices measured by prism coupling was ±0.005 and a one-Sellmeier-term equation was adequate to fit ...

Multistep magnetic switching in single-crystal ( 001 ) Co 2 MnGe films

Multistep magnetic switching with single, double, and triple loops has been observed in single-crystal (001) Co 2 MnGe Heusler alloy films epitaxially grown on Gads (001) substrates due to a combined magnetic anisotropy consisting of cubic magnetocrystalline anisotropy and uniaxial anisotropy. Theoretical calculations of the hysteresis loops using coherent rotation show excellent agreement with the experimental results. Domain patterns and magnetization rotation in the Co 2 MnGe films have been studied using the magneto-optical indicator film (MOIF) imaging technique. Magnetic domains and domain walls, as well as the coherent rotation of magnetization in single-step, double-step, and triple-step switching have been clearly observed. The magnetic switching field and domain wall pattern (90° and 180° domain walls) observed from the MOIF images are in excellent agreement with the results of the hysteresis measurements and the theoretical model.

Spring magnet films

The properties of exchange-spring-coupled bilayer and superlattice films are highlighted for Sm-Co hard magnet (nominally Sm/sub 2/Co/sub 7/) and Fe or Co soft magnet layers. The hexagonal Sm-Co is grown via magnetron sputtering in a- and b-axis epitaxial orientations. In both cases the c-axis, in the film plane, is the easy axis of magnetization. Trends in coercivity with film thickness are established and related to the respective microstructures of the two orientations. The magnetization reversal process for the bilayers is examined by magnetometry and magneto-optical imaging, as well as by simulations that utilize a one-dimensional model to provide the spin configuration for each atomic layer. The Fe magnetization is pinned to that of the Sm-Co at the interface, and reversal proceeds via a progressive twisting of the Fe magnetization. The Fe demagnetization curves are reversible as expected for a spring magnet. Comparison of experiment and simulations indicates that the spring magnet behavior can be understood from the intrinsic properties of the hard and soft layers. Estimates are made of the ultimate gain in performance that can potentially be realized in this system.

Direct experimental study of the exchange spring formation process

The remagnetization of a soft ferromagnetic film exchange coupled with a high-coercivity ferromagnetic film is studied by a magneto-optic imaging technique. If the magnetic field is antiparallel to the macroscopic unidirectional in-plane anisotropy, the soft layer reverses via the formation of exchange springs consisting of subdomains with opposite spin twistings. However, if the field is instead rotated in-plane, remagnetization initially proceeds via formation of a single uniform exchange spring. Then, at a critical angle, the spring incoherently untwists, leading again to subdomains with opposite chirality. These phenomena are attributed to the influence of inhomogeneity in the unidirectional magnetic anisotropy.

Direct experimental study of the magnetization reversal process in epitaxial and polycrystalline films with unidirectional anisotropy

Direct observation of the magnetization reversal of epitaxial NiO/NiFe bilayers grown on (001) MgO and on polycrystalline Si substrates was performed by using the magneto-optical indicator film technique. It was shown that the unidirectional-axis magnetization reversal proceeds by domain nucleation and growth. A new phenomenon, an asymmetry in the activity of the domain nucleation centers, has been revealed. Remagnetization of the bilayer is shown to be governed by defect structures in the antiferromagnetic layer.

Chirality of a forming spin spring and remagnetization features of a bilayer ferromagnetic system

Distribution of a magnetic moment in an exchange-coupled bilayer Fe/SmCo epitaxial structure grown on a (110) MgO substrate is visualized by the magnetooptic indicator film technique. The direction and the magnitude of the effective magnetization in this structure are determined both under external magnetic fields of variable magnitude and direction and after the removal of these fields. It is shown that such a heterostructure is remagnetized by a nonuniform rotation of a magnetic moment both along the thickness of a sample and in its plane. A field antiparallel to the axis of unidirectional anisotropy gives rise to spin springs with opposite chiralities in different regions of the magnetically soft ferromagnetic layer. The contributions of these springs to the net magnetization cancel out, thus decreasing the averaged magnetic moment and the remanent magnetization without their rotation. When the external field deviates from the easy axis, the balance is violated and the sample exhibits a quasi-uniform rotation of the magnetic moment. Asymmetry in the rotation of the magnetic moment is observed under the reversal of the field as well as under repeated remagnetization cycles. It is established that a monochiral spin spring is also formed in a rotating in-plane magnetic field when the magnitude of the field exceeds the critical value. Possible mechanisms of remagnetization in this system are discussed with regard to the original disordered orientation of magnetization of the magnetically soft layer with respect to the easy axis, which is defined by the variance of unidirectional anisotropy axes of this layer on the interface.

In-Plane Field Effects on the Dynamics of Domain Walls in Ultrathin Co Films With Perpendicular Anisotropy

We studied how the application of an in-plane field affects the asymmetries of the domain nucleation activity and domain wall velocity revealed previously in ultrathin Co films with perpendicular anisotropy. It is established that the asymmetries survive and new effects arise under application of an additional field parallel to the film surface. It is found that the mobility of different parts of a circular domain wall vary drastically under application of additional planar field. The domain wall velocity in ultrathin Co films, unlike that in low-damping garnet films, slows down dramatically upon application of a planar field. The domain wall part that is moving perpendicular to the in-plane field direction decelerates most relative to the other parts of the domain wall. Moreover, domain wall parts moving in the same direction as the in-plane field and in the opposite direction have considerably different velocities. This anisotropy of domain wall velocity rotates with the field as the in-plane field direction is varied. A new asymmetry in the domain wall velocity in the presence of both in-plane and perpendicular fields is observed when the perpendicular field is reversed.

Antiferromagnetic spin structure and domains in exchange-coupled multilayers

As revealed by the observation of memory effects and domain imaging, the antiferromagnetic (AFM) spin structure in exchange bias is not static. During reversal, the AFM spins form an exchange spring connected with the ferromagnet (FM). We have observed hybrid domain walls consisting of FM and AFM sections and their evolution using the magnetooptical indicator film technique. The external magnetic field moves only the FM section of the hybrid domain walls, leading to the formation of an exchange spring parallel to the interface. The nucleation and unwinding of the exchange spring occur at different locations, and the propagation depends strongly on the chirality of the FM domain walls.

Magneto-optical indicator film study of the hybrid exchange spring formation and evolution processes

The elementary events of the remagnetization processes in nanocomposite magnetic bilayers were investigated using iron-garnet indicator films with in-plane anisotropy. We have observed hybrid domain walls consisting of both ferromagnetic and antiferromagnetic sections perpendicular to the interface. The external magnetic field shifts only the ferromagnetic part of the domain walls. This leads to the formation of a hybrid exchange spin spring parallel to the interface. The processes of spring nucleation and untwisting occur at different locations. With the field oriented antiparallel to the macroscopic unidirectional anisotropy, remagnetization of the soft ferromagnet layer in the hard/soft nanocomposite starts by the formation of an exchange spring consisting of micrometer-scale sub-domains with opposite direction spin twisting. A rotating magnetic field (smaller than some critical value) creates firstly a single-chiral spin spiral; this spiral then loses stability, incoherently untwists and gradually inverts its chirality with increasing field rotation. Untwisting of the hybrid exchange spring at higher fields leads to the creation of unusual hybrid non-180° domain walls. The initial (ground) state of the bilayer with such noncollinear magnetized domains is not restored after stopping the field rotation and returning it to zero. The revealed phenomena are attributed to the influence of the dispersion in the unidirectional anisotropy induced by magnetization frustration in the interface and bilayer crystal lattice defects.