Ruihua Cheng

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.

Oxidation of metals at the chromium oxide interface

Metal thin-film deposition, over the Cr2O3 surface of CrO2 thin-film substrates, exhibits a redox reaction at the interface. The transition metal forms an oxide in combination with the reduction of the near-surface chromium oxide to Cr2O3. The insulating barrier layer Cr2O3 increases with the formation of Pb3O4 in Pb/Cr2O3/CrO2 and CoO in Co/Cr2O3/CrO2 junctions, respectively.

Temperature dependent induced spin polarization in Cr2O3 overlayers on epitaxial CrO2 films

Abstract Spin-resolved inverse photoemission investigations show that the native Cr 2 O 3 surface is antiferromagnetically coupled to the CrO 2 thin film substrate, with a temperature dependent induced polarization. The Cr 2 O 3 exhibits the characteristic behavior of a rigid band/spin mixing behavior (non-Stoner) of a local moment paramagnet. The strong shifts of the conduction band edge from room temperature to low temperature suggests that the extent of the induced polarization of the Cr 2 O 3 oxide surface, by the CrO 2 substrate, may be partly related to Coulomb blockade effects identified in CrO 2 /Cr 2 O 3 /CrO 2 junctions.

Magnetic coupling in Co/Cr2O3/CrO2 “trilayer” films

The ferromagnetic coupling between Co and CrO2, through an insulator (Cr2O3) was characterized by in situ magneto-optic Kerr effect. By evaporating 20–60 A Co thin films on top of epitaxial CrO2 films, a Co/Cr2O3/CrO2 trilayer system can be readily fabricated; this is possible because the native surface layer of CrO2 is Cr2O3. In situ x-ray photoemission studies show that the Co is oxidized at the interface between Co and Cr2O3, so that the system more resembles Co/CoO/Cr2O3/CrO2. The Co thickness and temperature dependence of the magnetic hysteresis loops indicate that magnetic coupling strength increases with increasing Co thickness and decreases with increasing temperature. The magnetic coupling through the insulator barrier may be related to defect states in the insulating barrier layer.

Magnetic anisotropy in epitaxial CrO2 (100) thin films

CrO2 thin films fabricated on single crystal TiO2 (100) substrates by vapor phase transport grow with strongly textured growth characteristics and strong preferential orientation of the crystallites. We have used magneto-optical Kerr effect (MOKE) and alternating gradient force magnetometry (AGFM) to study the in-plane magnetic anisotropy. Both the coercive field, HC, and remnant magnetization Mr/Ms show strong twofold in-plane symmetry and the results, vs. angle, fit a coherent rotation model of domain reversal. We verify that the magnetic easy axis is along the c-axis of the crystal and the first order anisotropy constant, K1, is estimated to be about (2.3±0.2)×105 erg/cm3, consistent with the previous results. Near Tc, there are strong indications that the contributions to the magnetic anisotropy are strongly altered.

Spin blockade effects in chromium oxide intergrain magnetoresistance

CrO2 thin films with crystallites of a significant size permit investigation of the thin intergrain conduction between a limited numbers of crystals. Phoemission techniques reveal that the CrO2 films are covered by a 1–2-nm-thick Cr2O3 insulating barrier. The electronic band gap of the surface electrons is 3.4 eV at low temperatures, decreases sharply down to 2.8 eV at ambient. Electric transport though a few junctions in series shows a remarkable zero-bias anomaly, interpreted in terms of blockade effects. We show that the magnetoresistance is governed by low-bias blockade in these junctions.

SELECTIVE AREA CHEMICAL VAPOR DEPOSITION OF CHROMIUM OXIDES

We demonstrate that two-phase CrO2 and Cr2O3 thin films can be grown by using selective organometallic chemical vapor deposition through the oxidation of Cr(CO)6 in an oxygen environment. While the magnetization measurements show that both chromium oxides are present, the relative weight of each phase depends on the oxygen partial pressure. Changes of the Curie temperature, Tc, and the saturation magnetization field may be possible by controlling the stoichiometry.

Electronic states of Fe nanostructures on Pt(997) surface

A variety of one dimensional atomic chains can be synthesized on stepped surfaces, which provide opportunities for systematically tailoring the surface properties. The electronic structure of the high vicinal surface plays an essential role in determining the physical properties of the atomic chains. The authors have used surface analysis techniques to study the surface of Pt(997) single crystal. Angle resolved ultraviolet photoemission spectroscopy was performed to characterize the electronic states of Pt(997) surface by using a synchrotron radiation light source. Different from the expected free electron model on the flat Pt(111) surface, electron confinement is observed with a wave vector perpendicular to the step direction. Fe was carefully deposited onto Pt(997) substrate at room temperature. The STM data show that Fe atomic chains were formed along the step edges due to the step decoration growth. The exchange splitting of Fe 3d bands is estimated according to the photoemission spectroscopy data of ...

Fabrication and Characterization of Chromium Oxide Nanoparticles/Thin Films

Abstract : Well-dispersed nanoscale textured chromium oxide particles/thin films can be fabricated under certain conditions by laser-induced solution deposition (LISD) from organic solutions and by using selective organometallic chemical vapor deposition (OMCVD). The fabricated nanoparticles/thin films are characterized by scanning electron microscope (SEM), EDX, X-ray diffraction, and magnetic measurements. We have successfully demonstrated that the LISD and OMCVD are unique techniques for fabricating uniformly-distributed thin films but anisotropic chromium oxide particles, which can be used in electro-magnetic devices. The magnetization measurements show that both types of chromium oxides are presented and that the Curie temperature T(sub c) and the saturation magnetization field may be adjustable by controlling the stoichiometry.

Origin of the magnetic moments in La0.65Pb0.35MnO3 epitaxial thin films

Crystalline films of La0.65Pb0.35MnO3 grown on (100) LaAlO 3 substrates by rf sputtering have been investigated using magnetic circular dichroism and inverse photoemission spectroscopy. We find evidence for strong hybridization between unoccupied levels associated with Mn 3d and O 2p states. The oxygen atoms “pick-up” a small magnetic moment through hybridization with Mn.