S. Miyanishi

Negative magnetoresistance in GaAs with magnetic MnAs nanoclusters

We show a negative magnetoresistance (MR) in GaAs with magnetic MnAs nanoclusters (about 1.5% at 30 K in 1 T). The clusters were formed in a two step process consisting of the molecular beam epitaxy of (Ga,Mn)As layer and the subsequent annealing. The origin of the negative MR is attributed to the presence of the MnAs clusters. The mechanism is considered to be a spin-dependent scattering of carriers by MnAs clusters which decreases when the direction of the magnetization between the clusters aligns with the magnetic field.

EPITAXIAL MNGA/(MN,GA,AS)MNGA TRILAYERS : GROWTH AND MAGNETIC PROPERTIES

Epitaxial ferromagnet/semiconductor/ferromagnet sandwiches with the nominal structure δ Mn60Ga40/GaAs/δ Mn54Ga46 have been grown on GaAs(001) by molecular beam epitaxy, with spacer layer thicknesses ranging from 2 to 19 monolayers (ML) GaAs nominally. A strong antiferromagnetic coupling field μ0Hs=−12 to −39 mT at room temperature is observed for spacer layers of 4–14 ML GaAs nominally, a weaker ferromagnetic coupling of +1 to +7 mT exists outside of this thickness region. Magnetic contamination of the spacer layer and the inclusion of antiferromagnetic Mn2As cannot be ruled out.

Molecular‐beam epitaxy of Mn1+xSb thin films and substrate temperature dependence of their magneto‐optical properties

Ferromagnetic Mn1+xSb thin films have been grown on (001) GaAs substrates by molecular‐beam epitaxy at substrate temperatures (TS) in the range from 50 to 400 °C. The excess Mn composition (x) of the grown films was found to change uniquely depending on TS. The polar magneto‐optical Kerr effect measurement demonstrated a strong TS dependence of Kerr rotation and Kerr ellipticity spectra of Mn1+xSb films from the infrared through ultraviolet wavelength region. A possible origin of the TS dependence of the spectra is discussed in connection with the composition of Mn1+xSb films.

STRUCTURAL AND MAGNETIC PROPERTIES OF EPITAXIAL (0001) MNSB THIN FILMS GROWN ON (111) B GAAS : INFLUENCE OF INTERFACE QUALITY

We have succeeded in growing epitaxial (0001) MnSb/(111)B GaAs films with an atomically flat heterointerface using a GaAs buffer layer. For MnSb films with a thickness of 2–20 A, the in-plane strain in the films arising from a lattice mismatch between MnSb and GaAs is compressive, which was observed by in situ reflection high-energy electron diffraction measurement. On the other hand, a rough interdiffused interface was obtained in samples grown without a GaAs buffer layer. In contrast to the case of samples with flat interfaces, the in-plane strain is tensile, which is attributed to interdiffusion of Sb into GaAs substrates. The magnetization of the MnSb films with the thickness of this range shows a strong dependence on the interface quality. The saturation magnetization and the saturation magnetic field of the samples with the rough interface are smaller than those of the samples with the atomically flat interface.

Influence of GaAs (001) surface termination on the in-plane magnetic anisotropies of MnSb epitaxial films

We have studied the in-plane magnetic anisotropy of epitaxial MnSb (1101) films grown on GaAs (001) by molecular beam epitaxy. The MnSb films were grown on (2×4) and (4×6) reconstructed GaAs surfaces at 250 and 50 °C. At 250 °C, the films showed a strong twofold in-plane magnetic anisotropy independent of the GaAs surface reconstruction. In contrast, at 50 °C, the in-plane anisotropy appeared only on the (2×4) reconstructed surface. The anisotropic crystallographic domain structure of the MnSb films is thought to cause the magnetic anisotropy. The anisotropic domain formation is explained by the different chemisorption of the Mn adatom on the GaAs surface as a function of the termination.

Observation of antiferromagnetic coupling in δ-MnGa/(Mn,Ga,As)/δ-MnGa trilayers

Abstract We present the magnetic properties of δ-MnGa/(Mn,Ga,As)/δ-MnGa trilayers. The spacer layer consists of nominally 2 to 19 monolayers (ML) GaAs, but we have indications for an important incorporation of Mn and the possible formation of antiferromagnetic Mn 2 As. Antiferromagnetic (AFM) coupling is observed for spacer layer thicknesses of 4 to 14 ML GaAs, ferromagnetic (FM) coupling exists outside this region. The largest observed coupling field was −87.0 mT (−870 Oe) at 17 K for a sample with a 12 ML spacer layer, causing a cross-over between both branches of the hysteresis loop and a negative remanence. In one sample (16 ML) the coupling changes from AFM at low temperature to FM at room temperature.

Epitaxial Mn2Sb thin films grown by molecular-beam epitaxy on (001) GaAs and their magnetic and magneto-optical properties

We have succeeded in growing monocrystalline ferrimagnetic Mn2Sb thin films on (001) GaAs by molecular-beam epitaxy. From the reflection high-energy electron and x-ray diffraction patterns, the orientation of Mn2Sb films grown on (001) GaAs substrates was found to be with the c axis normal to the surface. This direction is also the easy magnetization direction as determined by superconducting quantum interference device measurements at room temperature. Magneto-optical spectra of these films showed a much larger magneto-optical Kerr effect than bulk data from the infrared to the blue wavelength region.

Temperature dependence of interlayer coupling in δ MnGa/(Ga,As,Mn)/δ MnGa trilayers

We present the temperature dependence of the interlayer coupling, from 15 to 300 K, in ferromagnet/semiconductor/ferromagnet trilayers with the nominal structure of 10 nm δMn60Ga40/GaAs (n monolayers)/20 nm δMn54Ga46 [n=6–16 monolayers (ML) nominally] grown on (001) GaAs substrates by molecular-beam epitaxy. Since compositional analysis showed a strong diffusion of Mn into the GaAs spacer layer, we represent the trilayer structure as MnGa/(Ga,As,Mn)/MnGa. The magnetic-circular-dichroism loops showed antiferromagnetic coupling between both MnGa layers for the samples with a spacer layer from 6 to 14 ML GaAs nominally in the whole temperature range. The temperature coefficient of the coupling field was found to be positive for 6–8 ML spacers and negative for 10–14 ML spacers. We interpret these facts as the competition between two (or more) coupling mechanisms. For the sample with a 16 ML GaAs spacer layer, the interlayer coupling was antiferromagnetic below 200 K, but ferromagnetic above 200 K.

CORRELATION BETWEEN THE SIGNS OF THE MAGNETORESISTANCE AND OF THE INTERLAYER COUPLING IN MNGA/(MN,GA,AS)/MNGA TRILAYERS

A correlation is found between the sign of the magnetoresistance ΔR=(RP−RAP) and the sign of the interlayer coupling in MnGa/(Mn,Ga,As)/MnGa trilayers (subscript P, AP=parallel, antiparallel magnetic alignment of both MnGa layers). Ferromagnetic coupling results in ΔR 0. This correlation has not been observed before and can not be explained by the usual model for giant magnetoresistance in magnetic multilayers.

Schottky barrier height of MnSb(0001)/GaAs(111)B contacts: Influence of interface structure

The Schottky barrier height (SBH) of MnSb(0001)/n-GaAs(111)B diodes was investigated in terms of current–voltage characteristics for three different GaAs surfaces, GaAs (19×19), GaAs (2×2), and sulfur passivated GaAs. We observed that the SBH and the ideality factor changed significantly depending on the GaAs surface structure prepared before the MnSb growth. The sulfur passivated sample was superior to the others in that it has a lower ideality factor and higher barrier. The SBH fell off linearly with increasing ideality factor n. The SBH of MnSb(0001)/n-GaAs(111)B was estimated to be 0.94 eV by extrapolating the linear relationship to n=1.