Magnetic and thermal studies of doped rare-earth chalcogenide nanostructures in solution and the solid-state

Abstract

The europium chalcogenides (oxide, sulfide, selenide) are intrinsically ferromagnetic semiconductors. Research with these types of materials is motivated by applications in fields such as spintronics, which can take advantage of the coupling of magnetic and electronic properties. By synthesizing these materials on the nanoscale, we can exercise strong control over their size and morphology for potential application in devices. Changes in size and structure also control their properties.

We have been able to synthesize these materials through both solution-based and solid-state techniques. The solution-based route has allowed the synthesis of ligand capped europium sulfide nanoparticles through the decomposition of a europium dithiocarbamate precursor. Particle size control is accomplished through manipulation of temperature, time and ligand to precursor ratio. Our group has previously shown that this manipulation of particle size can tune the magnetic ordering temperature. By introducing changes in capping ligands used, we can influence particle shape, albeit to a limited degree; this was the motivation for developing a hydrothermal/solid-state synthesis of these materials as well.

By utilizing a hydrothermal method, we can synthesize single-crystalline europium hydroxide (Eu(OH)3) nanorods and nanowires. This starting material can be used in a variety of chemical transformation methods to create a wide range of materials. Dehydration of the material yield single-crystalline europium oxide (Eu2O3) which maintains the morphology during the conversion. Eu2O3 can be reacted with hydrogen sulfide to form europium sulfide (EuS), europium metal to form europium monoxide (EuO), ammonium halides to form europium oxyhalides (EuOX), or hydrolyzed back to Eu(OH)3, all while maintaining the wire morphology of the starting material.

In addition, the stoichiometry can be finely controlled in the materials through doping with different elements. By doping the europium chalcogenides with trivalent lanthanides, electrons can be injected into the conduction band. Because of the connection of the magnetic and electronic properties, electron doping can be used to improve the magnetic properties, notably by increasing the magnetic ordering temperature. These materials have a strong magnetic moments, colossal magnetoresistance, and strong magneto-optical Kerr effect, but are limited for application in devices by their low magnetic ordering temperatures, the weakest aspects of these materials.

We also discovered a wide range of alkali-doping in EuS materials. By doping with alkali-metals such as sodium, we have been able to maintain the sodium chloride-type crystal structure of europium sulfide (Eu1-xNaxS), while the observing a contraction of the unit cell. We have shown through XPS and Mössbauer studies that as the sodium content decreases, europium is reduced from the trivalent to divalent state during synthesis. In addition to changes in valence, the magnetic properties are affected by sodium doping as well. Replacing europium ions with sodium destroys the magnetic communication responsible for ferromagnetic order, but does not disrupt the antiferromagnetic communication, which is overwhelmed by the ferromagnetism in undoped europium sulfide.