C. Leighton

Low temperature Schottky anomalies in the specific heat of LaCoO3: Defect-stabilized finite spin states

Measurement of the low temperature specific heat of LaCoO3 single crystals reveals a previously unobserved Schottky anomaly with an energy level splitting, 0.5 meV, that is associated with the first excited spin state of the Co3+ ion. These states persist well below 2 K and have a g-factor around 3.5, consistent with the high-spin spin-orbit triplet, implying the existence of a low density (approximately 0.1% of the sites) of finite-spin Co ions even in the T=0 limit. We propose that these states are trapped at defects and are consistent with the magnetic excitons observed in earlier work.

Sulfur stoichiometry effects in highly spin polarized CoS2 single crystals

Recent experiments on polycrystalline Co1−xFexS2 demonstrated composition control over the spin polarization by Fermi level manipulation. We report here the growth and characterization of CoS2 single crystals with fine control over the stoichiometry by chemical vapor transport. At the ideal Co:S atomic ratio we observe a minimum in the low temperature resistivity and the x-ray rocking curve width, coincident with a maximum in the residual resistivity ratio and the low temperature magnetoresistance. Point contact Andreev reflection on stoichiometric crystals indicates a spin polarization at the Fermi energy of 64%, a significant increase over the 56% observed in polycrystals.

Comparison between micromagnetic simulation and experiment for the Co∕γ-Fe50Mn50 exchange-biased system

We have constructed a micromagnetic model to study the exchange coupling between Co (a ferromagnet) and γ-Fe50Mn50 (an antiferromagnet) in a thin film bilayer with (111) texturing. The intention is to compare experimental results with a micromagnetic calculation that is sufficiently sophisticated to realistically model this polycrystalline ferromagnet/antiferromagnet system. The antiferromagnet thickness dependences of exchange bias and enhanced coercivity were simulated at 10 and 300K and comparison to experiments revealed reasonable agreement. We also examined the antiferromagnet grain size dependence of exchange bias, owing to its relation to the key issue of uncompensated spin density. Simulation finds a linear relationship between exchange bias and inverse grain size for both thermally stable and thermally fluctuating antiferromagnetic grains, but with different intercepts. Experiment also finds a linear dependence, but the extrapolation to infinite diameter reveals an unexpected negative exchange bi...

The structure of the CoS2 (100)-(1 ? 1) surface

Quantitative low-energy electron diffraction (LEED) has been used to determine the structure of the cubic CoS 2 (100)-(1 × 1) surface. The clearly favored structural model from the LEED analysis is the 1S-terminated (1 × 1) surface, in which the S–S dimer is intact and the terminal surface layer retains a complete S–Co–S sandwich structure. The surface S atoms move outwards towards the vacuum while the subsurface Co atoms move towards the bulk, by approximately 0.03 and 0.11 A, respectively. In addition, the S atoms in the third sublayer relax outwards by about 0.12 A, thus providing an indication of a stronger S–S dimer bond and a denser surface region. The complete atomic coordinates of the S–Co–S surface layers are determined in this analysis. Includes “Corrigendum” from J. Phys.: Condens. Matter 19 249001

Spin-dependent intergranular transport in highly spin-polarized Co1−xFexS2 thin films

Magnetic and magnetotransport measurements on polycrystalline thin film Co1−xFexS2, a system that exhibits tunable conduction electron spin polarization in bulk, are reported. The films exhibit a low field hysteretic magnetoresistance (MR) due to spin-dependent intergranular tunneling, thus providing a simple probe of the spin polarization. The MR increases rapidly with x, saturating as the magnetization reaches 1.0μB/Co, demonstrating that high, doping tunable, spin polarization can be readily achieved in thin films synthesized by simple methods. Quantitative estimates yield spin polarizations as high as 90%.

The surface stability of CoS2(100)

The stability of various possible terminations of the CoS2 (1 × 1) surface have been explored and theoretical expectations are found to agree with experiment. With extensive annealing, there is a phase separation at the (100) surface of CoS2. Sulfur segregation to the surface leads to a significant change in the largely sulfur bands due to changes in the hybridized bands, with cobalt. Resonant photoemission spectra indicate clearly that the hybridized cobalt and sulfur bands, characteristic of the CoS2 bulk, lie at higher binding energies than those of segregated sulfur layers. This is discussed in terms of the stability of various surface structures.

Erratum: The structure of the CoS2 (100)-(1 × 1) surface (Journal of Physics: Condensed Matter 19 (156223))

Quantitative low-energy electron diffraction (LEED) has been used to determine the structure of the cubic CoS2 (100)-(1 ? 1) surface. The clearly favoured structural model from the LEED analysis is the 1S-terminated (1 ? 1) surface, in which the S?S dimer is intact and the terminal surface layer retains a complete S?Co?S sandwich structure. The surface S atoms move outwards towards the vacuum while the subsurface Co atoms move towards the bulk, by approximately 0.03 and 0.11??, respectively. In addition, the S atoms in the third sublayer relax outwards by about 0.12??, thus providing an indication of a stronger S?S dimer bond and a denser surface region. The complete atomic coordinates of the S?Co?S surface layers are determined in this analysis.