Epitaxial Ferromagnetic Thin Films Research Articles

Spintronics: recent progress and tomorrow's challenges

This article reviews the recent advances of epitaxial ferromagnetic thin films and heterostructures as well as devices towards semiconductor-based spin-electronics or often called spintronics. We review the developments of III–V based ferromagnetic thin films and heterostructures grown by molecular beam epitaxy, with focus on the Mn-delta-doped GaAs/p-AlGaAs heterostructures with high ferromagnetic transition temperature, and control of the spin-dependent properties. As a new direction in spintronics research, we present our recent study on a new silicon-based spin device, spin metal-oxide-semiconductor field-effect transistor (spin MOSFET), and novel reconfigurable or reprogrammable logic gates using spin MOSFETs, which are compatible with the current CMOS technology and promising for reconfigurable computing.

Magnetization dynamics and magnetotransport in epitaxial nanostructures

The latest magnetoresistive results reported for electrochemically-defined nanocontacts between Ni wires are still quite controversial (Ninth Joint MMM Intermag Conference). We have used e-beam lithography applied to epitaxial Ni films to fabricate nanobridges in more controlled geometry than that of electrochemical deposition. Our studies are important for the understanding of suitable production procedures for Ni nanocontacts. The idea behind our scheme is that epitaxial ferromagnetic thin films exhibit rich magnetic anisotropy and large coercivities at room temperature, associated with only few defects in the crystal lattice. They also exhibit higher anisotropy than polycrystalline films and domain walls with narrower widths are expected. Thus, we speculate that this type of structure would favor ballistic magnetoresistance and possibly domain-wall trapping provided that there is a small enough constriction as predicted by Bruno [Phys. Rev. Lett. 83, 2425 (1999)].

Epitaxial ferromagnetic thin films and heterostructures of Mn-based metallic and semiconducting compounds on GaAs

We present two approaches to integrate magnetic materials with III–V semiconductors. One is epitaxial ferromagnetic metallic films and heterostructures on GaAs (0 0 1) substrates. Although crystal structure, lattice constant, chemical bonding and other properties are dissimilar, ferromagnetic hexagonal MnAs thin films and MnAs/NiAs ferromagnet/nonmagnet heterostructures (HSs) are grown on GaAs by molecular beam epitaxy (MBE). Multi-stepped magnetic hysteresis are controllably realized in MnAs/NiAs HSs, making this material promising for the application to multi-level nonvolatile recording on semiconductors. The other approach is to prepare a new class of GaAs based magnetic semiconductor, GaMnAs, by low-temperature molecular beam epitaxy (LT-MBE) on GaAs (0 0 1). New III–V based superlattices consisting of ferromagnetic semiconductor GaMnAs and nonmagnetic semiconductor AlAs are also successfully grown. Structural and magnetic properties of these new heterostructures are presented.

Anisotropy and orientational dependence of magnetization reversal processes in epitaxial ferromagnetic thin films.

We have undertaken a detailed study of the macroscopic and microscopic magnetization reversal processes in epitaxial ferromagnetic thin films with varying cubic and uniaxial magnetocrystalline anisotropy strengths. The macroscopic magnetization reversal processes were observed with in-plane magneto-optic Kerr effect (MOKE) vector magnetometry as a function of the relative anisotropy strengths and on the orientation of the applied field with respect to the anisotropy directions. Measurements of the component of magnetization in the plane of the sample and perpendicular to the applied field allow a precise determination of the relative orientation of the hard and easy in-plane anisotropy axes. This can be used to accurately determine the ratio of uniaxial to cubic anisotropy constants, when the ratio is less than one. The ratios obtained from MOKE agree well with those obtained by Brillouin light scattering (BLS). MOKE vector magnetometry reveals loop features that can be associated with either one or two irreversible jumps in the direction of the magnetization, depending sensitively on the anisotropy ratio and the orientation of the applied field. Minimum-energy calculations predict that the reversal process should proceed by a continuous rotation of the magnetization vector with either one or two irreversible jumps between single-domain states, depending on the applied field orientation and the nature of the anisotropy of the film. The calculations provide a good qualitative description of the observed reversal process, although the magnetic microstructure influences the exact values of the switching fields.Sequences of Lorentz microscopy images were made in the vicinity of the two switching fields to reveal the microscopic reversal mechanism. These images provide direct evidence that the reversal proceeds by domain-wall motion between single domain states. In the case where the applied field is oriented away from the easy axis direction, a single domain intermediate state is observed during magnetization reversal, as suggested by the MOKE measurements and predicted by the calculations. In addition, the reversal mechanism with the field applied close to an easy axis reveals that the single jump observed in the MOKE loops actually corresponds to a two-step process on a microscopic scale.

Epitaxial ferromagnetic thin films and superlattices of Mn-based metallic compounds on GaAs

Epitaxial ferromagnetic thin films grown directly on III–V compound semiconductors can lead to the integration of magnetism and high-speed III–V electronics/photonics, offering possibilities of hybrid ferromagnetic-semiconductor devices. Despite stringent material requirements for this purpose, we have successfully grown some of Mn-based metallic compounds sharing common atoms with the underlying III–V using molecular beam epitaxy (MBE). First, it is shown that ferromagnetic MnGa thin films with perpendicular magnetization were obtained on (001) GaAs substrates. The c axis, which is the easy axis for magnetization, of the tetragonal unit cell is found to be desirably aligned perpendicular to the film plane. Second, in order to derive more functionality and to increase the freedom in material design, we have also created a new class of magnetic superlattices (SLs) of metallic compounds, MnGa/NiGa, on GaAs substrates. The MnGa/NiGa SLs show even stronger perpendicular magnetization and evenly-spaced multiple steps in the hysteresis loops in extraordinary Hall effect measurements. Third, ferromagnetic hexagonal MnAs, an As-based compound which is more compatible with existing III–V MBE technology, is shown to be successfully grown on GaAs. It was found that the MBE-grown MnAs/GaAs heterostructures have a unique epitaxial relationship and a mono-atomic template plays a critical role in determining the epitaxial orientation and magnetic properties.

SURFACE AND INTERFACE MAGNETISM BY MÖSSBAUER SPECTROSCOPY

SURFACE AND INTERFACE MAGNETISM BY MÖSSBAUER SPECTROSCOPY