Mathieu Malfait

Enhanced annealing effect in an oxygen atmosphere on Ga1−xMnxAs

We report on in situ resistivity measurements on Ga1−xMnxAs during post-growth annealing in different atmospheres. A drop in the resistivity is observed when the Ga1−xMnxAs is exposed to oxygen, which indicates that the passivation of Mn interstitials (MnI) at the free surface occurs through oxidation. The presence of oxygen can therefore be an important annealing condition for the optimization of Ga1−xMnxAs thin films, all the more since the oxidation appears to be limited to the sample surface. Annealing in an oxygen-free atmosphere leads to an increase in the resistivity indicating a second annealing mechanism besides the outdiffusion of MnI. According to our magnetization and Hall effect data, this mechanism reduces the amount of magnetically and electrically active Mn atoms.

Direct local observation of magnetic anisotropy in microscopic Ga1−xMnxAs dots

The effect of the patterning of a Ga1−xMnxAs (x=0.08) film into an array of microscopic dots on its magnetic properties is locally studied by means of scanning Hall probe microscopy. The measured stray field patterns indicate the presence of a single domain state with a uniform magnetization within the Ga1−xMnxAs dots. The magnetic anisotropy of the dot array is directly imaged, revealing a [100] easy axis in the as-grown sample and a [11¯0] easy axis orientation after annealing. In contrast to a temperature dependent anisotropy in the plain Ga1−xMnxAs film, no magnetic easy axis reorientation is observed within the experimentally accessible temperature range.

Magnetic anisotropy and magnetization reversal in Ga1−xMnxAs layers studied by polarized neutron reflectometry

The magnetic anisotropy and the in-plane magnetization reversal mechanism of Ga0.945Mn0.055As thin films, grown on GaAs(001), were investigated. Samples A and B, with sample A grown 45 degrees C lower than sample B, were analyzed by magnetization hysteresis measurements and polarized neutron reflectivity (PNR) magnetization reversal experiments. Magnetization hysteresis loops and temperature-dependent magnetization curves accounted for an in-plane uniaxial magnetic anisotropy, with a significant out-of-plane magnetization, for sample A, while an in-plane biaxial magnetic anisotropy with in-plane as easy axes was evidenced for sample B. PNR magnetization reversal experiments showed the occurrence of a spin-flip maximum upon magnetization reversal only for sample B. A mechanism of magnetization reversal proceeding by 180 degrees domain-wall nucleation and propagation is proposed for sample A, while an incoherent rotation mechanism by 90 degrees domains is proposed for sample B. The reversal mechanism is shown to be correlated to the anisotropy of the samples which depends on the T/T-c ratio. (c) 2005 American Institute of Physics.

Magnetotransport properties of a room temperature ferromagnet (Ga, Mn)N

Heavily Mn-doped GaN thin films were fabricated by using solid-state diffusion. The magnetic properties of the samples were determined by Hall effect measurements and by using a superconducting quantum interference device. The results show ferromagnetic behavior even above room temperature, the Curie temperature being 330 K. The samples were of n-type and the estimated electron concentration was /spl ap/1/spl times/10/sup 20/ cm/sup -3/. Due to the spin-disorder scattering, a negative magnetoresistance was observed and it was largest at Curie temperature. The measured sheet resistance versus temperature could also be explained by the spin disorder scattering model for ferromagnetic metals. From the measurement results, the value of the exchange interaction parameter between the electron spins and the magnetic moments of the Mn atoms was estimated to be about 1 eV. The material properties were studied also by using secondary ion mass spectroscopy and X-ray diffraction, which showed some segregation of Ga/sub x/Mn/sub y/.

Pulsed-field studies of the magnetization reversal in molecular nanomagnets

We report experimental studies of crystals of

Magnetotransport in ferromagnetic pn-diode of (Ga,Mn)As and (Ga,Mn)N

{\mathrm{Mn}}_{12}

Sign inversion of the high-field Hall slope in epitaxial La0.5Ca0.5MnO3 thin films

molecular magnetic clusters in pulsed magnetic fields with sweep rates up to

Electrical transport in Mn-doped GaAs pn-diodes

4\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.3em}{0ex}}\mathrm{T}∕\mathrm{s}

Large magnetoresistance in a ferromagnetic GaMnAs/GaAs Zener diode

. The steps in the magnetization curve are observed at fields that are shifted with respect to the resonant field values. The shift systematically increases as the rate of the field sweep goes up. These data are consistent with the theory of the collective dipolar relaxation in molecular magnets.

MBE growth of MgGeAs2 : Mn on GaAs substrate

Here we have demonstrated a large magnetoresistance in tunnelling (Ga,Mn)As/GaAs diodes at low temperatures.The (Ga,Mn)As and (Ga,Mn)N pn-diodes were fabricated by using Molecular Beam Epitaxy (MBE).Firstly, heavily doped n+-GaAs was grown on top of n-type GaAs substrate. After that the magnetic Mn doped GaAs layer with different Mn concentrations was grown by MBE. The growth temperature was only 230 C in order to inhibit formation of secondary phases and segregation of Mn at the surface. Finally, the ohmic contacts were evaporated by an electro-beam evaporator. The structure of the (Ga,Mn)N pn-diode was similar, except the fact that the n-and p-type areas were in the opposite order. In addition, the n-type magnetic layer (Ga,Mn)N was made by solid state diffusion of Mn inside the MBE chamber. The direct magnetization measurements were performed in order to confirm that the (Ga,Mn)As and (Ga,Mn)N layers are ferromagnetic. However, no magnetic field dependence in the diode current was observed in the I-V characteristics of the pn-diodes, which were made of Mn doped GaAs or GaN and which had a lightly doped n-side.