(i) Magnetism in the perovskite cobaltites
Our 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 these model systems. In 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. We have also expanded the work to include narrow bandwidth systems such as (Pr,Ca)CoO3, layered Ruddlesden-Popper phases such
as La1-xSr1+xCoO4, and lightly hole-doped LaCoO3.
(ii) Perovskite oxide heterostructures
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 focused on the interfacial phase separation effect at cobaltite/titanate interfaces, which is closely related to the strain-induced O vacancy ordering in these materials.
(iii) Spin transport in 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. Our work reveals the importance of surface spin relaxation, which we are currently quantifying, in addition to implicating interdiffusion at the F/N interface as important. We are carrying out Polarized Neutron Reflectometry (PNR) studies to quantify such effects.
(iv) Highly spin-polarized ferromagnets
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 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. We are now working on exploiting this tunable highly polarized material in fundamental studies of spin transport in semiconductors (GaAs) and non-magnetic metals. Recent progress includes achieving a detailed understanding of intergranular transport in thin polycrystalline films, in addition to developing reproducible methods for deposition on GaAs.
(v) Sulfide-based heterostructures and photovoltaics
Due to their outstanding potential as next generation solar absorber materials, we are also working on sulfides for photovoltaics. In collaborative work with Profs. Eray Aydil (CEMS, UMN) and Steve Campbell (Electrical Engineering, UMN) we are working on Cu2 ZnSnS4, an excellent candidate for a low cost solar absorber based on low toxicity high abundance elements. We are depositing thin films via ex situ sulfidation and reactive sputtering and attempting to understand and control doping and electronic properties, eventually for inclusion in solar cells. We are also studying FeS2 in this regard, a semiconductor that is well known to have outstanding potential as a solar absorber. In our first work on this material we have examined in considerable detail the mechanisms of electronic transport in ex situ sulfidized thin film samples. The data reveal an unexpected crossover from nanoscopic intergranular hopping to conventional charge transport, a result with important consequences in terms of previous conclusions regarding the doping type and conduction mechanism.
(vi) Novel magnetic alloys
In collaboration with Prof. Richard James (UMN Aerospace Engineering and Mechanics) and Prof. Ram Seshadri (UC Santa Barbara), we are working on specific types of off-stoichiometric Heusler alloy materials. These are materials such as Ni50-xCoxMn40Sn10, derived from full Heusler alloys such as Ni2MnSn by deliberately substituting excess Mn on the Sn sites, thus bringing ferromagnetic and antiferromagnetic interactions into competition. The resulting interplay with the martensitic phase transformation in these materials turns out to be fascinating, the result being nanoscale magnetic inhomogeneity much akin to that seen in correlated electron systems such as complex oxides. These alloys in fact resulted from theoretical work from James which identified them as having exceptionally low thermal hysteresis across the martensitic transformation, bringing another aspect of interest, particularly for shape memory, actuator, sensor, and magnetocaloric applications. Our main contribution has been to apply Small-Angle Neutron Scattering (SANS), magnetometry, and exchange bias measurements to these systems, not only establishing phase diagrams such as the one shown here, but providing the first direct evidence of nanoscale magnetic cluster formation. Recently these types of measurements have also been applied to another system (Ru2Mn1-yFeySn, with Seshadri) where ferro- and antiferro-magnetism compete, again with fascinating results. Current work is expanding to include heat capacity, transport, and magneto-transport studies.
(vii) Magnetic nanostructure arrays by block copolymer patterning
In a project with Prof. Marc Hillmyer (UMN Chemistry) aimed at developing methods for facile synthesis of extremely high density magnetic features we are using block copolymer thin films as templates for fabrication of large area arrays of magnetic nanostructures. Our prior workfocused on the important issue of characterizing and understanding the pattern transfer with these blockcopolymer lithography techniques, demonstrating 35 nm dotarrays by "lift-off" techniques, 25 nm diameter antidot arrays using pattern transfer methods, and a method for achieving spontaneous perpendicularalignment of minority phase cylinders, eliminating the need for lengthy annealing steps. In more recent work we have developed a method to fabricate 25 nm diameter magnetic nanodots with exceptional long-range order, monodispersity, and magnetization retention, at the same time avoiding lift-off and etch damage. This "damascene-type" process, which employs solvent annealing, is now being used in studies of single crystal magnetic dot arrays, as well as investigations of complex oxide antidot networks to study electronic phase separation.
(viii) Transport in conductive polymers and small molecule systems
In a collaboration with Prof. Dan Frisbie (CEMS, UMN) we are studying the details of the electronic transport mechanisms in polymeric and small molecule semiconductors. We use the recently developed ion-gel based electrostatic and electrochemical gating techniques to achieve very high charge densities in thin film transistors of these materials for low temperature and high magnetic field experiments. Our recent work on P3HT has elucidated the hopping conduction in these systems, in addition to the unique features of the approach to the insulator-metal transition in these highly-disordered conductors. We find that the onset of metallic conductivity is suppressed due to doping-induced localization at extreme electrochemically-induced charge densities, a result with important consequences for polymer semiconductor thin film transistors. Nevertheless, at the highest charge densities and mobilities attainable in our experiments we have observed the Hall effect for the first time in a polymer semiconductor transistor. Remarkably, the data are suggestive of a crossover to a regime of diffusive, “band-like”, transport in these highly disordered
solution processed polymer semiconductors.