S. H. Chun

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.

Colossal magnetoresistance is a Griffiths singularity.

It is now widely accepted that the magnetic transition in doped manganites that show large magnetoresistance is a type of percolation effect. This paper demonstrates that the transition should be viewed in the context of the Griffiths phase that arises when disorder suppresses a magnetic transition. This approach explains unusual aspects of susceptibility and heat capacity data from a single crystal of La0.7Ca0.3MnO3.

Coexistence of localized and itinerant carriers near TC in calcium-doped manganites

We explore the possibility that polaronic distortions in the paramagnetic phase of La{sub 0.67}Ca{sub 0.33}MnO{sub 3} manganites persist in the ferromagnetic phase by considering the observed electrical resistivity to arise from coexisting field- and temperature-dependent polaronic and band-electron fractions. We use an effective medium approach to compute the total resistivity of the two-component system, and find that a limit with low percolation threshold explains the data rather well. To test the validity of this model, we apply it to the thermoelectric coefficient. We propose a plausible mean-field model that reproduces the essential features of a microscopic model and provides a comparison with the experimental mixing fraction, as well as the magnetization and magnetic susceptibility. {copyright} {ital 1999} {ital The American Physical Society}

Griffiths singularities and magnetoresistive manganites

The large, so-called colossal magnetoresistivity of doped LaMnO 3 has attracted considerable attention, but is only one unusual feature of the ferromagnetic transition in these compounds. We examine in this paper the progression of magnetic and thermodynamic behavior as the transition temperature is made to vary from 360 K to 218 K by changing the divalent dopant. Single crystals of La 0 . 7 Sr 0 . 3 MnO 3 , as is well known, show modest magnetoresistivity and conventional critical behavior. La 0 . 6 6 Pb 0 . 3 4 MnO 3 , and to an even greater extent, La 0 . 7 Ca 0 . 3 MnO 3 , have unusual magnetic properties extending more than 100 K above the transition. We treat the properties of the latter samples in the context of a Griffiths phase in which the transition temperature is depressed from its maximum value T G by random bond-angle bending.

Saturated ferromagnetism and magnetization deficit in optimally annealed Ga1-xMnxAs epilayers

We examine the Mn concentration dependence of the electronic and magnetic properties of optimally annealed

Spin-polarized tunneling in hybrid metal-semiconductor magnetic tunnel junctions

{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}

Magnetotransport in Manganites and the Role of Quantal Phases: Theory and Experiment

epilayers for

Charge transport in manganites: Hopping conduction, the anomalous Hall effect, and universal scaling

1.35%l~xl~8.3%.

Magnetocaloric effect and temperature coefficient of resistance of La2/3(Ca,Pb)1/3MnO3

The Curie temperature