W. L. Lim

In1−xMnxSb—a narrow-gap ferromagnetic semiconductor

A narrow-gap ferromagnetic In1−xMnxSb semiconductor alloy was grown by low-temperature molecular-beam epitaxy on CdTe/GaAs hybrid substrates. Ferromagnetic order in In1−xMnxSb was unambiguously established by the observation of clear hysteresis loops both in direct magnetization measurements and in the anomalous Hall effect, with Curie temperatures TC ranging up to 8.5 K. The observed values of TC agree well with the existing models of carrier-induced ferromagnetism.

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 Mnu200a(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 ...

Annealing-dependent magnetic depth profile in Ga 1 − x Mn x As

We have studied the depth-dependent magnetic and structural properties of as-grown and optimally annealed Ga 1 - x Mn x As films using polarized neutron reflectometry. In addition to increasing the total magnetization, the annealing process was observed to produce a significantly more homogeneous distribution of the magnetization. This difference in the films is attributed to the redistribution of Mn at interstitial sites during the annealing process. Also, we have seen evidence of significant magnetization depletion at the surface of both as-grown and annealed films.

Growth and properties of ferromagnetic In1-xMnxSb alloys

Abstract We discuss a new narrow-gap ferromagnetic (FM) semiconductor alloy, In 1− x Mn x Sb, and its growth by low-temperature molecular-beam epitaxy. The magnetic properties were investigated by direct magnetization measurements, electrical transport, magnetic circular dichroism, and the magneto-optical Kerr effect. These data clearly indicate that In 1− x Mn x Sb possesses all the attributes of a system with carrier-mediated FM interactions, including well-defined hysteresis loops, a cusp in the temperature dependence of the resistivity, strong negative magnetoresistance, and a large anomalous Hall effect. The Curie temperatures in samples investigated thus far range up to 8.5 K , which are consistent with a mean-field-theory simulation of the carrier-induced ferromagnetism based on the 8-band effective band-orbital method.

Curie temperature limit in ferromagnetic Ga 1 − x Mn x As

We provide experimental evidence that the upper limit of ∼110 K commonly observed for the Curie temperature T C of Ga 1 - x Mn x As thin films (thickness >50 nm) is caused by Fermi-level-induced hole saturation. Ion channeling, electrical, and magnetization measurements on a series of Ga 1 - x - y Mn x Be y As layers show a dramatic increase of the concentration of Mn interstitials accompanied by a reduction of T C with increasing Be concentration, while the free hole concentration remains relatively constant at ∼5 X 10 2 0 cm - 3 . These results indicate that the concentrations of free holes and ferromagnetically active Mn spins are governed by the position of the Fermi level, which controls the formation energy of compensating interstitial Mn donors.

Correlation of Mn lattice location, free hole concentration, and curie temperature in ferromagnetic GaMnAs

We provide experimental evidence that the electrical and magnetic characteristics of Ga1−xMnxAs for a given x depend primarily on the distribution of Mn atoms over their different possible locations in the crystal lattice. Using combined channeling Rutherford backscattering and particle-induced X-ray emission, we show that optimal postgrowth annealing—which leads to an increase of the Curie temperature TC and is accompanied by an increase of free hole concentration and saturation magnetization—is caused by the reduction in the number of Mn atoms occupying interstitial positions. On the other hand, when Ga1−xMnxAsis additionally doped with Be, we observe that—while the hole concentration remains nearly constant—there occurs a strong decrease of TC together with a dramatic increase in the concentration of Mn interstitials. These results indicate that there is a thermodynamic limit imposed on the maximum Curie temperature in Ga1−xMnxAs.

Optical studies of carrier and phonon dynamics in Ga1−xMnxAs

We present a time-resolved optical study of the dynamics of carriers and phonons in Ga1−xMnxAs layers for a series of Mn and hole concentrations. While band filling is the dominant effect in transient optical absorption in low-temperature-grown GaAs, induced absorption by trapped electrons become important with increasing Mn concentration in Ga1−xMnxAs, as inferred from the sign of the absorption change. We find that carrier trapping and trapped carrier recombination becomes faster as Mn concentration increases in GaMnAs. We also report direct observation on lattice vibrations in Ga1−xMnxAs layers via the reflective electro-optic sampling technique. The data show increasingly fast dephasing of longitudinal-optical phonon oscillations for samples with increasing Mn and hole concentration, which can be understood in term of phonon scattering by the holes.