M. J. Wilson

Synthesis and characterization of an n=6 Aurivillius phase incorporating magnetically active manganese, Bi7(Mn,Ti)6O21

Epitaxial films of Bi7Mn3.75Ti2.25O21 were prepared to yield a previously unsynthesized material. The superlattice phase is produced by incorporating the magnetoelectric BiMnO3 into the perovskite substructure of the ferroelectric Bi4Ti3O12, a strategy which is hoped to yield previously undiscovered multiferroic materials. X-ray diffraction and transmission electron microscopy (TEM) confirm synthesis of an epitaxial n=6 Aurivillius phase. Magnetization measurements show ferromagnetic behavior with a Curie point of 55K, but electronic polarization measurements show no remanent polarization. Rutherford backscattering spectrometry indicates a channeling minimum χmin of 22%, consistent with the high density of out-of-phase domain boundaries observed by TEM.

Interlayer and interfacial exchange coupling in ferromagnetic metal/semiconductor heterostructures

We describe a systematic study of the exchange coupling between a magnetically hard metallic ferromagnet (MnAs) and a magnetically soft ferromagnetic semiconductor (GaMnAs) in bilayer and trilayer heterostructures. An exchange spring model of MnAs/GaMnAs bilayers accounts for the variation of the exchange bias field with layer thickness and composition. We also present evidence for hole-mediated interlayer exchange coupling in MnAs/p-GaAs/\GaMnAs trilayers and study the dependence of the exchange bias field on the thickness of the spacer layer.

Spin valve effect in self-exchange biased ferromagnetic metal/semiconductor bilayers

We report magnetization and magetoresistance measurements in hybrid ferromagnetic metal/semiconductor heterostructures comprised of MnAs∕(Ga,Mn)As bilayers. Our measurements show that the (metallic) MnAs and (semiconducting) (Ga,Mn)As layers are exchange coupled, resulting in an exchange biasing of the magnetically softer (Ga,Mn)As layer that weakens with layer thickness. Magnetoresistance measurements in the current-perpendicular-to-the-plane geometry show a spin valve effect in these self-exchange biased bilayers. Similar measurements in MnAs∕p-GaAs∕(Ga,Mn)As trilayers show that the exchange coupling diminishes with spatial separation between the layers.

Measurements of nanoscale domain wall flexing in a ferromagnetic thin film.

We use the high spatial sensitivity of the anomalous Hall effect in the ferromagnetic semiconductor Ga(1-x)Mn(x)As, combined with the magneto-optical Kerr effect, to probe the nanoscale elastic flexing behavior of a single magnetic domain wall in a ferromagnetic thin film. Our technique allows position sensitive characterization of the pinning site density, which we estimate to be ∼10(14) cm(-3). Analysis of single site depinning events and their temperature dependence yields estimates of pinning site forces (10 pN range) as well as the thermal deactivation energy. Our data provide evidence for a much higher intrinsic domain wall mobility for flexing than previously observed in optically probed μm scale measurements.

Ferromagnetic resonance study of MnAs∕(Ga,Mn)As bilayers

We report the investigation of the static and dynamic magnetic properties of type-A MnAs∕Ga0.945Mn0.055As (001) bilayers. Static magnetization measurements show them to be ferromagnetically coupled with an exchange bias field of ∼340Oe. The magnetocrystalline anisotropy constants of the (Ga,Mn)As layer were determined by X-band ferromagnetic resonance (FMR) spectroscopy. The (Ga,Mn)As layers are magnetically inhomogeneous as evidenced by a strong broadening of the (Ga,Mn)As uniform mode linewidth. The MnAs FMR spectra reveal the presence of a small MnAs fraction with a different orientation.

Local and nonlocal magnetic behavior of dilute manganese-doped nickel alloys

We report magnetic behavior of a dilute ferromagnetic alloy Ni1−xMnx, 0<x<25%. Magnetic circular dichroism (MCD) is used to examine the local magnetic moment on each element, and superconducting quantum interference device magnetometry is used to evaluate the magnetization of the alloy as a whole. Both MCD and hysteresis loops show a collapse in moment at x≈15% measured at 100 K. The Mn doping appears to disturb the long-range ordering of the host nickel spins which, in the concentration range studied, is a precursor disordered ferromagnet prior to a spin-glass phase.

Substrate orientation dependence of ferromagnetism in (Ga,Mn)As

We describe a comprehensive study of the properties of both ordered and random alloy Ga1−xMnxAs grown on (001), (311), (201), and (110) GaAs substrates. Magnetization measurements show a systematic variation in the Curie temperature with epitaxial orientation in both types of samples, even for the same average Mn concentrations. Electrical characterization and thermal annealing of the random alloy Ga1−xMnxAs samples suggest that the density of As antisite defects depends on the growth surface, providing an extrinsic origin for the observed dependence of Curie temperature on substrate orientation.

Tunneling Magnetoresistance in Exchange Biased Ferromagnetic Semiconductor Tunnel Junctions

Summary form only given.Here, we demonstrate the observation of TMR in exchange-biased MTJs derived from the ferromagnetic semiconductor (Ga,Mn)As. Although still limited to operation at low temperatures, these devices provide an important step forward in exploring proof-of-concept semiconductor spintronic tunneling devices.