Y. Sasaki

Above-room-temperature ferromagnetism in GaSb/Mn digital alloys

Digital alloys of GaSb/Mn have been fabricated by molecular-beam epitaxy. Transmission electron micrographs showed good crystal quality with individual Mn-containing layers well resolved, no evidence of three-dimensional MnSb precipitates was seen in as-grown samples. All samples studied exhibited ferromagnetism with temperature-dependent hysteresis loops in the magnetization accompanied by metallic p-type conductivity with a strong anomalous Hall effect (AHE) up to 400 K (limited by the experimental setup). The anomalous Hall effect shows hysteresis loops at low temperatures and above room temperature very similar to those seen in the magnetization. The strong AHE with hysteresis indicates that the holes interact with the Mn spins above room temperature. All samples are metallic, which is important for spintronics applications.

Effect of additional nonmagnetic acceptor doping on the resistivity peak and the Curie temperature of Ga1-xMnxAs epitaxial layers

We have investigated the effect of additional doping by Be on the properties of Ga1−xMnxAs (x=0.03). For this relatively low value of x, the Curie temperature is observed to increase with increasing Be concentration. We show that the temperature dependence of the resistivity at zero magnetic field, including the resistivity maximum near the Curie temperature, can be successfully described by the magnetoimpurity scattering model proposed by Nagaev [Phys. Rep. 346, 387 (2001)] in both the paramagnetic and the ferromagnetic temperature regions. Quantitative analysis of the data in terms of this model yields the value of the p–d exchange energy |N0β|≈1.6 eV for Ga0.97Mn0.03As.

Effect of Be doping on the properties of GaMnAs ferromagnetic semiconductors

We have studied two series of molecular beam epitaxy grown Ga1−xMnxAs epilayers with several different Be doping levels. Two Mn concentrations x were chosen for this study: 0.03 and 0.05, and these values were maintained constant in each series. These samples were characterized by using SQUID and magnetotransport measurements. A systematic increase of the Curie temperature TC was observed in SQUID measurements on the series of Ga1−xMnxAs with x=0.03. The resistivity measured at zero magnetic field shows a local maximum near the Curie temperature, reflecting the effects of critical scattering near TC. The observed increase of TC in Ga1−xMnxAs for this low range of x can be explained by the increase of the free carrier concentrations in the system arising from Be doping. However, in the series of Ga1−xMnxAs with the higher concentration of Mn (x=0.05), the measurements reveal that the TC systematically decreases with increasing Be doping level. We discuss this effect in terms of a fundamental limitation of ...

Magnetization relaxation in (Ga,Mn)As ferromagnetic semiconductors

We describe a theory of Mn local-moment magnetization relaxation due to p-d kinetic-exchange coupling with the itinerant-spin subsystem in the ferromagnetic semiconductor (Ga,Mn)As alloy. The theoretical Gilbert damping coefficient implied by this mechanism is calculated as a function of Mn-moment density, hole concentration, and quasiparticle lifetime. Comparison with experimental ferromagnetic resonance data suggests that in annealed strongly metallic samples, p-d coupling contributes significantly to the damping rate of the magnetization precession at low temperatures. By combining the theoretical Gilbert coefficient with the values of the magnetic anisotropy energy, we estimate that the typical critical current for spin-transfer magnetization switching in all-semiconductor trilayer devices can be as low as ∼ 10 5 A cm - 2 .

Optical properties of epitaxial ZnMnTe and ZnMgTe films for a wide range of alloy compositions

Zn1−xMnxTe and Zn1−xMgxTe ternary wide-gap semiconductor alloys were grown by molecular beam epitaxy on (100) GaAs substrates over a wide range of compositions (0⩽x⩽0.75 and 0⩽x⩽0.67, respectively). Values of the band gap were measured by photoluminescence at 12 K, and by optical reflectivity at room temperature. The wavelength dependence of the indices of refraction n of these ternary systems was also measured for these alloys at wavelengths below their respective energy gaps. The measurements were performed using a combination of the prism coupler method and reflectivity. Compilation of these results allows us to establish a set of empirical parameters for the two alloy families, that can be used to calculate the index of refraction for an arbitrary alloy composition and arbitrary wavelength. It is interesting that the values of n show a surprisingly linear dependence on the corresponding energy gaps for both these alloy systems.

Determination of free hole concentration in ferromagnetic Ga1−xMnxAs using electrochemical capacitance–voltage profiling

We demonstrate that electrochemical capacitance–voltage profiling can be used to determine the free hole concentration in heavily p-type doped low-temperature-grown GaAs films. This provides a simple and reliable method for measuring the hole concentration in ferromagnetic Ga1−xMnxAs semiconductor alloys. The method overcomes the complications that arise from the anomalous Hall effect term which affects standard transport studies of carrier concentration in conducting ferromagnetic materials. Specifically, we find that the maximum Curie temperature of about 111 K found for our Ga0.91Mn0.09As samples corresponds to a hole concentration of 1021 cm−3.

Anisotropic magnetoresistance in Ga 1 − x Mn x As

We have measured the magnetoresistance in a series of Ga

Enhancement of magnetic coercivity and ferromagnetic transition temperature by proximity effects in the GaMnAs–ZnMnSe multilayer system

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Ferromagnetic resonance in GaMnAs

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Ultrafast dynamics of interfacial electric fields in semiconductor heterostructures monitored by pump-probe second-harmonic generation

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