K. C. Ku

Highly enhanced Curie temperature in low-temperature annealed [Ga,Mn]As epilayers

We report Curie temperatures up to 150 K in annealed Ga1−xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) suggest that the higher Curie temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material, suggesting that the free surface of Ga1−xMnxAs epilayers may be important in determining their physical properties.

Effects of annealing time on defect-controlled ferromagnetism in Ga1−xMnxAs

We have studied the evolution of the magnetic, electronic, and structural properties of annealed epilayers of Ga1−xMnxAs grown by low temperature molecular beam epitaxy. Annealing at the optimal temperature of 250 °C for less than 2 h significantly enhances the conductivity and ferromagnetism, but continuing the annealing for longer times suppresses both. These data indicate that such annealing induces the defects in Ga1−xMnxAs to evolve through at least two different processes, and they point to a complex interplay between the different defects and ferromagnetism in this material.

Exchange biasing of the ferromagnetic semiconductor Ga1−xMnxAs

We demonstrate the exchange coupling of a ferromagnetic semiconductor (Ga1−xMnxAs) with an overgrown antiferromagnet (MnO). Unlike most conventional exchange biased systems, the blocking temperature of the antiferromagnet (TB=48±2K) and the Curie temperature of the ferromagnet (TC=55.1±0.2K) are comparable. The resulting exchange bias manifests itself as a clear shift in the magnetization hysteresis loop when the bilayer is cooled in the presence of an applied magnetic field and an enhancement of the coercive field.

Capping-induced suppression of annealing effects on Ga1−xMnxAs epilayers

We have studied the effects of capping ferromagnetic Ga1−xMnxAs epilayers with a thin layer of undoped GaAs, and we find that even a few monolayers of GaAs have a significant effect on the ferromagnetic properties. In particular, the presence of a capping layer only 10 monolayers thick completely suppresses the enhancement of the ferromagnetism associated with low temperature annealing. This result, which demonstrates that the surface of a Ga1−xMnxAs epilayer strongly affects the defect structure, has important implications for the incorporation of Ga1−xMnxAs into device heterostructures.

Andreev reflection and pair-breaking effects at the superconductor/magnetic semiconductor interface

Department of Physics, Purdue University, West Lafayette, IN 47907(Dated: February 2, 2008)We investigate the applicability of spin polarization measurements using Andreev reflection ina point contact geometry in heavily doped dilute magnetic semiconductors, such as (Ga,Mn)As.While we observe conventional Andreev reflection in non-magnetic (Ga,Be)As epilayers, our mea-surements indicate that in ferromagnetic (Ga,Mn)As epilayers with comparable hole concentrationthe conductance spectra can only be adequately described by a broadened density of states anda reduced superconducting gap. We suggest that these pair-breaking effects stem from inelasticscattering in the metallic impurity band of (Ga,Mn)As and can be explained by introducing a finitequasiparticle lifetime or a higher effective temperature. For (Ga,Mn)As with 8% Mn concentrationand 140 K Curie temperature we evaluate the spin polarization to be 83 ± 17%.

Coercive field and magnetization deficit in Ga1−xMnxAs epilayers

We have studied the field dependence of the magnetization in epilayers of the diluted magnetic semiconductor Ga1−xMnxAs 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 expected for full saturation of all the Mn spins. These results suggest that the spin state of the nonferromagnetic 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.

Optoelectronic control of spin dynamics at near-terahertz frequencies in magnetically doped quantum wells

We use time-resolved Kerr rotation to demonstrate the optical and electronic tuning of both the electronic and local moment (Mn) spin dynamics in electrically gated parabolic quantum wells derived from II-VI diluted magnetic semiconductors. By changing either the electrical bias or the laser energy, the electron spin precession frequency is varied from 0.1 to 0.8 THz at a magnetic field of 3 T and at a temperature of 5 K. The corresponding range of the electrically-tuned effective electron g-factor is an order of magnitude larger compared with similar nonmagnetic III-V parabolic quantum wells. Additionally, we demonstrate that such structures allow electrical modulation of local moment dynamics in the solid state, which is manifested as changes in the amplitude and lifetime of the Mn spin precession signal under electrical bias. The large variation of electron and Mn-ion spin dynamics is explained by changes in magnitude of the sp−d exchange overlap.

Exchange biasing of the ferromagnetic semiconductor (Ga,Mn)As by MnO (invited)

We provide an overview of progress on the exchange biasing of a ferromagnetic semiconductor (Ga1−xMnxAs) by proximity to an antiferromagnetic oxide layer (MnO). We present a detailed characterization study of the antiferromagnetic layer using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, transmission electron microscopy, and x-ray reflection. In addition, we describe the variation of the exchange and coercive fields with temperature and cooling field for multiple samples.

Growth and characterization of MnAs/ZnSe ferromagnet/semiconductor hybrid heterostructures

We report a detailed study of the molecular beam epitaxy of MnAs/ZnSe heterostructures on GaAs (100) substrates under varying growth conditions. As with the MnAs/GaAs system, these MnAs/ZnSe epilayers show ferromagnetism at room temperature, although with different structural and magnetic characteristics. We study the impact of parameters such as the As/Mn beam equivalent pressure ratio, substrate temperature, templating, and substrate misorientation on the MnAs/ZnSe heterostructures utilizing in situ reflectance high energy electron diffraction, as well as ex situ x-ray diffraction and atomic force microscopy. This structural information is correlated with magnetic properties obtained from magneto-optical Kerr effect and superconducting quantum interference device magnetometry. Finally, we discuss preliminary studies on the properties of MnAs/ZnSe/MnAs trilayers for spin-valve type applications.

Fabrication and Characterization of Modulation-Doped ZnSe/(Zn,Cd)Se (110) Quantum Wells: A New System for Spin Coherence Studies

We describe the growth of modulation-doped ZnSe/(Zn,Cd)Se quantum wells on (110) GaAs substrates. Unlike the well-known protocol for the epitaxy of ZnSe-based quantum structures on (001) GaAs, we find that the fabrication of quantum well structures on (110) GaAs requires significantly different growth conditions and sample architecture. We use magnetotransport measurements to confirm the formation of a two-dimensional electron gas in these samples, and then measure transverse electron spin relaxation times using time-resolved Faraday rotation. In contrast to expectations based upon known spin relaxation mechanisms, we find surprisingly little difference between the spin lifetimes in these (110)-oriented samples in comparison with (100)-oriented control samples.