Coercive Field and Magnetization Deficit in Ga(1-x)Mn(x)As Epilayers

We have studied the field dependence of the magnetization in epilayers of the diluted magnetic semiconductor Ga(1-x)Mn(x)As for 0.0135 < x < 0.083. Measurements of the low temperature magnetization in fields up to 3 T show a significant deficit in the total moment below that expected for full saturation of all the Mn spins. These results suggest that the spin state of the non-ferromagnetic Mn spins is energetically well separated from the ferromagnetism of the bulk of the spins. We have also studied the coercive field (Hc) as a function of temperature and Mn concentration, finding that Hc decreases with increasing Mn concentration as predicted theoretically.Comment: 15 total pages -- 5 text, 1 table, 4 figues. Accepted for publication in MMM 2002 conference proceedings (APL

Spin-polarized Tunneling in Hybrid Metal-Semiconductor Magnetic Tunnel Junctions

We demonstrate efficient spin-polarized tunneling between a ferromagnetic metal and a ferromagnetic semiconductor with highly mismatched conductivities. This is indicated by a large tunneling magnetoresistance (up to 30%) at low temperatures in epitaxial magnetic tunnel junctions composed of a ferromagnetic metal (MnAs) and a ferromagnetic semiconductor (GaMnAs) separated by a nonmagnetic semiconductor (AlAs). Analysis of the current-voltage characteristics yields detailed information about the asymmetric tunnel barrier. The low temperature conductance-voltage characteristics show a zero bias anomaly and a V^1/2 dependence of the conductance, indicating a correlation gap in the density of states of GaMnAs. These experiments suggest that MnAs/AlAs heterostructures offer well characterized tunnel junctions for high efficiency spin injection into GaAs.Comment: 14 pages, submitted to Phys. Rev.

Non-Drude Optical Conductivity of (III,Mn)V Ferromagnetic Semiconductors

We present a numerical model study of the zero-temperature infrared optical properties of (III,Mn)V diluted magnetic semiconductors. Our calculations demonstrate the importance of treating disorder and interaction effects simultaneously in modelling these materials. We find that the conductivity has no clear Drude peak, that it has a broadened inter-band peak near 220 meV, and that oscillator weight is shifted to higher frequencies by stronger disorder. These results are in good qualitative agreement with recent thin film absorption measurements. We use our numerical findings to discuss the use of f-sum rules evaluated by integrating optical absorption data for accurate carrier-density estimates.Comment: 7 pages, 3 figure

Global versus Local Ferromagnetism in a Model for Diluted Magnetic Semiconductors Studied with Monte Carlo Techniques

A model recently introduced for diluted magnetic semiconductors by Berciu and Bhatt (PRL 87, 107203 (2001)) is studied with a Monte Carlo technique, and the results are compared to Hartree-Fock calculations. For doping rates close to the experimentally observed metal-insulator transition, a picture dominated by ferromagnetic droplets formed below a T* scale emerges. The moments of these droplets align as the temperature is lowered below a critical value Tc<T*. Our Monte Carlo investigations provide critical temperatures considerably smaller than Hartree-Fock predictions. Disorder does not seem to enhance ferromagnetism substantially. The inhomogeneous droplet state should be strongly susceptible to changes in doping and external fields.Comment: 4 pages, 4 figure

Clustered States as a New Paradigm of Condensed Matter Physics

We argue that several materials of much current interest in condensed matter physics share common phenomenological aspects that only very recent investigations are unveiling. The list includes colossal magnetoresistance manganites, high temperature superconducting cuprates, diluted magnetic semiconductors, and others. The common aspect is the relevance of intrinsic inhomogeneities in the form of "clustered states", as explained in the text.Comment: elsart3, 6 pages, 5 figures. Invited paper for the Conference Proceedings of the International Conference on Magnetism, Rome, July 200

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 3D 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. * To whom correspondence should be addressed. E-mail: [email protected]

Saturated Ferromagnetism and Magnetization Deficit in Optimally Annealed (Ga,Mn)As Epilayers

We examine the Mn concentration dependence of the electronic and magnetic properties of optimally annealed Ga1-xMnxAs epilayers for 1.35% < x < 8.3%. The Curie temperature (Tc), conductivity, and exchange energy increase with Mn concentration up to x ~ 0.05, but are almost constant for larger x, with Tc ~ 110 K. The ferromagnetic moment per Mn ion decreases monotonically with increasing x, implying that an increasing fraction of the Mn spins do not participate in the ferromagnetism. By contrast, the derived domain wall thickness, an important parameter for device design, remains surprisingly constant.Comment: 8 pages, 4 figures, submitted for Rapid Communication in Phys Rev

Electronic and magnetic properties of GaMnAs: Annealing effects

The effect of short-time and long-time annealing at 250C on the conductivity, hole density, and Curie temperature of GaMnAs single layers and GaMnAs/InGaMnAs heterostructures is studied by in-situ conductivity measurements as well as Raman and SQUID measurements before and after annealing. Whereas the conductivity monotonously increases with increasing annealing time, the hole density and the Curie temperature show a saturation after annealing for 30 minutes. The incorporation of thin InGaMnAs layers drastically enhances the Curie temperature of the GaMnAs layers.Comment: 4 pages, 6 figures, submitted to Physica

Photoemission studies of Ga1−x_{1-x}Mnx_{x}As: Mn-concentration dependent properties

Using angle-resolved photoemission, we have investigated the development of the electronic structure and the Fermi level pinnning in Ga$_{1-x}$Mn$_{x}$As with Mn concentrations in the range 1--6%. We find that the Mn-induced changes in the valence-band spectra depend strongly on the Mn concentration, suggesting that the interaction between the Mn ions is more complex than assumed in earlier studies. The relative position of the Fermi level is also found to be concentration-dependent. In particular we find that for concentrations around 3.5--5% it is located very close to the valence-band maximum, which is in the range where metallic conductivity has been reported in earlier studies. For concentration outside this range, larger as well as smaller, the Fermi level is found to be pinned at about 0.15 eV higher energy.Comment: REVTeX style; 7 pages, 3 figure

Theoretical models of ferromagnetic III-V semiconductors

Recent materials research has advanced the maximum ferromagnetic transition temperature in semiconductors containing magnetic elements toward room temperature. Reaching this goal would make information technology applications of these materials likely. In this article we briefly review the status of work over the past five years which has attempted to achieve a theoretical understanding of these complex magnetic systems. The basic microscopic origins of ferromagnetism in the (III,Mn)V compounds that have the highest transition temperatures appear to be well understood, and efficient computation methods have been developed which are able to model their magnetic, transport, and optical properties. However many questions remain.Comment: 4 pages, 4 figures, review, to be published in Curr. Appl. Phy