Dana A. Schwartz

Ferromagnetism in oxide semiconductors

Over the past five years, considerable work has been carried out in the exploration of candidate diluted oxide magnetic semiconductors with high Curie temperatures. Fueled by early experimental results and theoretical predictions, claims of ferromagnetism at and above room temperature in doped oxides have abounded. In general, neither the true nature of these materials nor the physical causes of the magnetism have been adequately determined. It is now apparent that these dilute magnetic systems are deceptively complex. We consider two well-characterizedn-type magnetically doped oxide semiconductors and explore the relationship between donor electrons and ferromagnetism.

Above-room-temperature ferromagnetic Ni2+-doped ZnO thin films prepared from colloidal diluted magnetic semiconductor quantum dots

We report the preparation of spin-coated nickel-doped zinc oxide nanocrystalline thin films using high-quality colloidal diluted magnetic semiconductor (DMS) quantum dots as solution precursors. These films show robust ferromagnetism with Curie temperatures above 350K and 300K saturation moments up to 0.1Bohr magnetons per nickel. These results demonstrate a step toward the use of colloidal zero-dimensional DMS nanocrystals as building blocks for the bottom-up construction of more complex ferromagnetic semiconductor nanostructures.

Magnetic circular dichroism of ferromagnetic Co2+-doped ZnO

Cobalt-doped ZnO (Co2+:ZnO) films were studied by magnetic circular dichroism (MCD) spectroscopy. A broad 300K ferromagnetic MCD signal was observed between 1.4 and 4.0eV after exposure of paramagnetic Co2+:ZnO films to zinc metal vapor, attributed to low-energy photoionization transitions originating from a spin-split donor impurity band in ferromagnetic n-type Co2+:ZnO.

A simple room-temperature preparation of colloidal ZnO quantum dots from homogenous polar aprotic solutions

A simple, versatile, and scalable procedure for synthesizing high-quality colloidal ZnO quantum dots from homogeneous polar aprotic solutions has been developed. TEM and x-ray diffraction data show that the nanocrystals synthesized by this procedure are highly crystalline, pseudo-spherical, and have a narrow size distribution. This procedure allows large quantities of homogeneous ZnO quantum dots to be prepared with relative ease, and is also conducive to the introduction of dopants.