Andrei Sokolov

Switchable induced polarization in LaAlO3/SrTiO3 heterostructures.

Demonstration of a tunable conductivity of the LaAlO(3)/SrTiO(3) interfaces drew significant attention to the development of oxide electronic structures where electronic confinement can be reduced to the nanometer range. While the mechanisms for the conductivity modulation are quite different and include metal-insulator phase transition and surface charge writing, generally it is implied that this effect is a result of electrical modification of the LaAlO(3) surface (either due to electrochemical dissociation of surface adsorbates or free charge deposition) leading to the change in the two-dimensional electron gas (2DEG) density at the LaAlO(3)/SrTiO(3) (LAO/STO) interface. In this paper, using piezoresponse force microscopy we demonstrate a switchable electromechanical response of the LAO overlayer, which we attribute to the motion of oxygen vacancies through the LAO layer thickness. These electrically induced reversible changes in bulk stoichiometry of the LAO layer are a signature of a possible additional mechanism for nanoscale oxide 2DEG control on LAO/STO interfaces.

Electric modulation of magnetization at the BaTiO3/La0.67Sr0.33MnO3 interfaces

We report large (>10%) magnetization modulation by ferroelectric polarization reversal in the ferroelectric-ferromagnetic BaTiO3/La0.67Sr0.33MnO3 (BTO/LSMO) heterostructures. We find that the electrically induced change in magnetization is limited to the BTO/LSMO interface but extends about 3 nm deep into the LSMO layer—far beyond the expected screening length of metallic LSMO. It is suggested that this effect is due to a metal-insulator transition occurring at the BTO/LSMO interface as a result of electrostatic doping.

Characterization of the native Cr2O3 oxide surface of CrO2

Using photoemission and inverse photoemission, we have been able to characterize the Cr2O3 oxide surface of CrO2 thin films. The Cr2O3 surface oxide exhibits a band gap of about 3 eV, although the bulk CrO2 is conducting. The thickness of this insulating Cr2O3 layer is twice the photoelectron escape depth which is about 2 nm thick. The effective Cr2O3 surface layer Debye temperature, describing motion normal to the surface, is about 370 K. From a comparison of CrO2 films grown by different techniques, with different Cr2O3 content, evidence is provided that the CrO2 may polarize the Cr2O3.

Quantized magnetoresistance in atomic-size contacts.

When the dimensions of a metallic conductor are reduced so that they become comparable to the de Broglie wavelengths of the conduction electrons, the absence of scattering results in ballistic electron transport and the conductance becomes quantized. In ferromagnetic metals, the spin angular momentum of the electrons results in spin-dependent conductance quantization and various unusual magnetoresistive phenomena. Theorists have predicted a related phenomenon known as ballistic anisotropic magnetoresistance (BAMR). Here we report the first experimental evidence for BAMR by observing a stepwise variation in the ballistic conductance of cobalt nanocontacts as the direction of an applied magnetic field is varied. Our results show that BAMR can be positive and negative, and exhibits symmetric and asymmetric angular dependences, consistent with theoretical predictions.

The electronic structure change with Gd doping of HfO2 on silicon

Gd-doped HfO2 films deposited on silicon substrates undergo a crystallographic change from monoclinic to fluorite (cubic) phase with increasing Gd concentrations. The crystallographic phase change is accompanied by a small increase in the valence bandwidth and in the apparent band offset in the surface region. Electrical measurements show pronounced rectification properties for lightly doped Gd:HfO2 films on p-Si and for heavily-doped Gd:HfO2 films on n-Si, suggesting a crossover from n-type to p-type behavior with increasing doping level.

Changing band offsets in copper phthalocyanine to copolymer poly(vinylidene fluoride with trifluoroethylene) heterojunctions

The authors have fabricated a thin film copper phthalocyanine to crystalline ferroelectric copolymer poly(vinylidene fluoride with trifluoroethylene) heterojunction diode. The formation of a diode is expected from the band offsets between the two thin film molecular systems, as ascertained from combined photoemission and inverse photoemission studies. From the temperature and field dependence of the heterojunction, dipole interactions are implicated at the interface between copper phthalocyanine and poly(vinylidene fluoride with trifluoroethylene) and affect the band offsets and resultant diode properties.

Magnetism of Cr-doped diamond-like carbon

Chromium-doped hydrogenated diamond-like carbon (Cr-DLC) and chromium carbide hydrogenated DLC alloys were synthesized by plasma-assisted vapor deposition and investigated by x-ray absorption near edge structure spectroscopy, extended x-ray absorption fine structure, and superconducting quantum interference device (SQUID) magnetometry. Structural and magnetic properties of the doped and alloy materials were investigated as a function of the Cr concentration (0.1–20 at. %). Toward the upper end of the concentration range, Cr precipitates in the form of chromium carbide (Cr3C2) nanoclusters. For low Cr concentrations, the systems are ferromagnetic at very low temperatures, whereas the chromium carbide clusters appear to be antiferromagnetic with uncompensated spins at the surface. Cr-DLC films and alloys with various Cr concentrations are used to make heterojunctions on silicon, and the produced diodes are investigated by I-V measurements. The heterojunctions exhibit negative magnetoresistance that saturate...

Magnetotransport properties of CrO2 films down to single-grain sizes

The magnetotransport properties of CrO2 films down to single-grain sizes have been investigated. The magnetoresistance of highly grain oriented CrO2 (100) films, deposited onto TiO2 (100) substrates using a high pressure cell, has been measured in the temperature range of 1.5–300 K and in magnetic fields up to 6 T. The difference in the magnetoresistance at the low temperature of these samples is related to the value of the residual resistance ratio (RRR=R273 K/R4.2 K) of the samples. The negative magnetoresistance observed in some of these films is likely due to defects (disorder) in the CrO2 crystal. We observed a large positive longitudinal magnetoresistance in the CrO2 sample when it was measured with the current perpendicular to the c axis. The nature of the large value of the longitudinal magnetoresistance may be related to the complex configuration of the Fermi surface of CrO2.

Magnetoresistance in boron carbide junctions

Photoemission and electric transport properties of ferromagnet–insulator–ferromagnet junctions with boron carbide (C2B10) dielectric barrier are presented. Using a non-oxide barrier confidence avoids oxidation of the interfaces with the ferromagnetic layers. Photoemission confirms chemical abruptness of the interface. Magnetoresistance ratios reaching 50% are observed at low temperatures, and large nonlinearity in the current–voltage curves show that impurities in the junctions play a key role.

Evidence of martensitic phase transitions in magnetic Ni-Mn-In thin films

Ni50Mn35In15 Heusler alloy thin films (with thicknesses of about 10 nm) have been grown on single crystal MgO and SrTiO3 (STO) (100) substrates using a laser-assisted molecular beam epitaxy method. Films of mixed austenitic and martensitic phases and of pure martensitic phase have been detected for those grown on MgO and STO substrates, respectively. Thermomagnetic curves were measured using a SQUID magnetometer and are consistent with those of off-stoichiometric In-based bulk Heusler alloys, including a martensitic transition at T = 315 K for films grown on MgO. The differences in the properties of the films grown on MgO and STO are discussed.