CEMS Faculty

Magnetism and magnetic materials are the central focus of our group’s research. We employ a wide variety of materials such as magnetic nanostructures, thin films, multilayered heterostructures, and bulk polycrystals and single crystals. We have interests in several areas including the interplay between electronic transport and magnetism in novel materials, and the study of interfaces between dissimilar magnetic materials. The intention is to focus on topics that are attractive from the viewpoint of fundamental science but lie in close proximity to important technological applications, particularly in information storage. Each of the projects requires fabrication of magnetic materials/structures, detailed atomic level characterization, and in-depth measurement by numerous techniques, particularly electrical, magnetic and neutron scattering methods. As such the work often requires that collaborations be forged both within the University of Minnesota and with external collaborators. The bulk of the research can be divided into five primary categories:

(i) magnetic phase separation in perovskite cobaltites,

(ii) highly spin polarized ferromagnets,

(iii) spin transport in metals,

(iv) magnetic nanostructure arrays by block copolymer patterning, and,

(v) perovskite oxide heterostructures.

The work on perovskite cobaltites mostly involves the study of a fascinating phenomenon known as magneto-electronic phase separation, where chemically homogeneous materials are found to exhibit intrinsic inhomogeneities in electronic and magnetic properties. These inhomogeneities correlate with some of the most important properties of complex oxides such as high temperature superconductivity and colossal magnetoresistance. In our work on cobaltites we are examining the phenomenology, consequences, and origin of formation of magnetic clusters by combining a wide array of bulk property measurements (e.g. magnetometry, transport, heat capacity, etc.) with powerful local probes such as NMR and small-angle neutron scattering. The ultimate goal is a fundamental understanding of the origin of the phase separation effects in a model system. In our most recent work we have focused on the important effects of local doping fluctuations at the nanoscale, which can explain many of the observed features of the electronic inhomogeneity in these systems

Our group’s effort in the area of highly spin polarized, or “half-metallic”, ferromagnets is focused on the use of transition metal disulfides. We have recently demonstrated that the conduction electron spin polarization can be controllably tuned (up to 85 %) by alloy composition in Co1-xFexS2. In essence we use “band engineering” to control the position of the Fermi level, leading to control over the sign and magnitude of the spin polarization. Following this proof of principle demonstration with bulk polycrystalline materials we have now proceeded to optimize single crystal growth by chemical vapor transport, which increases the spin polarization even further. Current research is focusing on deposition of thin films by ex-situ sulfidation and reactive magnetron sputtering, so that we can utilize this tunable spin polarization system in heterostructured spintronic devices. Spin polarizations up to 90 % have been demonstrated in thin film form. Future work will involve exploiting this tunable highly polarized material in fundamental studies of spin transport in semiconductors (GaAs) and non-magnetic metals.

In a collaborative project with Prof. Paul Crowell's group (Physics, UMN) we are studying spin transport in metallic systems. We are fabricating lateral spin valves for non-local measurement of spin injection efficiency and spin diffusion length. Our fabrication schemes enable deposition of epitaxial metallic spin transport channels with controlled interface transparency, disorder, etc. The ultimate goal of the work is to unravel the critical structure-property relationships for spin injection efficiency and diffusion length in model metal-based systems. At the current time our work is focused on the temperature dependence of the non-local spin valve signal in systems such as Co/Cu and NiFe/Cu, including establishing multiple self-consistent measurement schemes for the spin diffusion length.

In a project aimed at developing materials for potential extremely high density recording media we are using block copolymer thin films as templates for fabrication of large area arrays of magnetic nanostructures. Our prior work focused on the important issue of characterizing and understanding the pattern transfer with these block copolymer lithography techniques. Thus far we have demonstrated the ability to make 35 nm dot arrays by “lift-off” type techniques as well as 25 nm diameter antidot arrays using the more reproducible pattern transfer methods. We have applied these techniques to perpendicular magnetic anisotropy materials as well as pursuing various strategies for achieving improved alignment, order and monodispersity in feature size (e.g. electric field alignment). We have also discovered a method for achieving spontaneous perpendicular alignment of minority phase cylinders, eliminating the need for lengthy annealing steps. Our most recent work involves use of these methods to pattern antidot arrays in complex oxide thin films to study phase separation under dimensional confinement.

Finally, we are also exploring the area of perovskite oxide heterostructures. Perovskites offer unique opportunities for heterostructure fabrication as they enable the assembly of chemically compatible lattice matched interfaces between materials with widely varied electronic and magnetic properties. We are pursuing various concepts using high pressure reactive magnetron sputtering for deposition of epitaxial thin films. Examples include the study of heterostructures containing doped cobaltite components for studies of interfacial magnetic phase separation, the use of SrTiO3 thin films for achieving very high electrostatically induced charge densities in oxide and organic conductors, the investigation of the transport properties of semiconducting SrTiO3, spin injection with manganites, and the study of bipolar magnetic heterostructures. The major part of our recent work has been focused on the interfacial phase separation effect at cobaltite/titanate interfaces, which we have recently verified directly using SANS.

In terms of experimental techniques our work involves bulk fabrication methods such as solid-state reaction and chemical vapor transport single crystal growth, in addition to thin film growth by UHV sputtering and molecular beam epitaxy. Structural characterization methods such as high-resolution x-ray diffraction and reflectivity, scanning probe microscopy, and various forms of spectroscopy and electron microscopy are of vital importance. The measurement techniques we employ include magnetometry, magnetotransport, and, most importantly, various forms of neutron scattering and reflectivity.