Characterization of the natural barriers of intergranular tunnel junctions: Cr2O3 surface layers on CrO2 nanoparticles

Abstract

Cold-pressed powder compacts of CrO2 show large negative magnetoresistance (MR) due to intergranular tunneling. Powder compacts made from needle-shaped nanoparticles exhibit MR of about 28% at 5 K. Temperature dependence of the resistivity indicates that the Coulomb blockade intergranular tunneling is responsible for the conductance at low temperature. In this letter we report direct observation and characterization of the microstructure of the intergranular tunnel barriers, using transmission electron microscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). A very thin native oxide layer with a thickness of 1–3 nm on the surface of CrO2 powders has been observed. The composition and crystal structure of this surface layer has been determined to be Cr2O3 by XPS and XRD. The dense and uniform Cr2O3 surface layers play an ideal role of tunnel barriers in the CrO2 powder compacts.

Spin blockade effects in chromium oxide intergrain magnetoresistance

“…8 Discrepancies with the expected high spin polarization value are apparent in the small ͑1%͒ MR, found on CrO 2 tunnel junctions at 70 K, 9 or the small negative values ͑Ϫ8%͒ found at 4 K. 10 We present a combination of photoemission and transport measurements demonstrating spin blockade effects in chromium oxide samples. Complementary photoemission and inverse photoemission studies confirm the presence of Cr 2 O 3 insulating oxide at the surface, consistent with the studies of Dai et al 11 on commercial CrO 2 powders. We show that the apparent density of states at the conduction band edge is strongly temperature dependent.…”

Zero-bias anomaly in CrO 2 junctions

“…It is known that Cr 2 O 3 is thermodynamically more stable than CrO 2 [1,15] and is the generally accepted stable surface of CrO 2 [12]. This insulating surface oxide also plays the role of a thin tunnel barrier between adjacent crystallites.…”

The finite-temperature densities of states for half-metallic ferromagnets

“…The negligible DOS at the Fermi level in the spin-polarized photoemission spectra of La 2/3 Sr 1/3 MnO 3 [3] and CrO 2 [4] suggests that either the surface is not the correct stoichiometry in those studies (see [15] for La 2/3 Sr 1/3 MnO 3 and [16,17] for CrO 2 ) or that the Fermi level crossings are well away from Γ (or some combination of both effects). To a small extent, final-state effects in photoemission can also contribute [18].…”