We determine experimentally the spin structure of half-metallic Co2FeAl0.4Si0.6 Heusler alloy elements using magnetic microscopy. Following magnetic saturation, the dominant magnetic states consist of quasi-uniform configurations, where a strong infl
uence from the magnetocrystalline anisotropy is visible. Heating experiments show the stability of the spin configuration of domain walls in confined geometries up to 800 K. The switching temperature for the transition from transverse to vortex walls in ring elements is found to increase with ring width, an effect attributed to structural changes and consequent changes in magnetic anisotropy, which start to occur in the narrower elements at lower temperatures.
A detailed study of the exchange bias effect and the interfacial electronic structure in Ni/Co3O4(011) is reported. Large exchange anisotropies are observed at low temperatures, and the exchange bias effect persists to temperatures well above the Nee
l temperature of bulk Co3O4, of about 40 K: to ~80 K for Ni films deposited on well ordered oxide surfaces, and ~150 K for Ni films deposited on rougher Co3O4 surfaces. Photoelectron spectroscopy measurements as a function of Ni thickness show that Co reduction and Ni oxidation occur over an extended interfacial region. We conclude that the exchange bias observed in Ni/Co3O4, and in similar ferromagnetic metallic/Co3O4 systems, is not intrinsic to Co3O4 but rather due to the formation of CoO at the interface.
The electronic valence state of Mn in Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 multiferroic heterostructures is probed by near edge x-ray absorption spectroscopy as a function of the ferroelectric polarization. We observe a temperature independent shift in th
e absorption edge of Mn associated with a change in valency induced by charge carrier modulation in the La0.8Sr0.2MnO3, demonstrating the electronic origin of the magnetoelectric effect. Spectroscopic, magnetic, and electric characterization shows that the large magnetoelectric response originates from a modified interfacial spin configuration, opening a new pathway to the electronic control of spin in complex oxide materials.
A detailed spectroscopic and structural characterization of ultrathin cobalt oxide films grown by O-assisted molecular beam epitaxy on a-Al2O3(0001) single crystals is reported. The experimental results show that the cobalt oxide films become progres
sively more disordered with increasing thickness, starting from the early stages of deposition. Low energy electron diffraction patterns suggest that the unit cell remains similar to that of a-Al2O3(0001) up to a thickness of 17 A, while at larger thicknesses a pattern identified with that of Co3O4(111) becomes visible. X-ray photoelectron spectroscopy reveals sudden changes in the shape of the Co 2p lines from 3.4 to 17 A cobalt oxide thickness, indicating the transition from an interfacial cobalt oxide layer towards [111]-oriented Co3O4. In particular, the absence of characteristic satellite peaks in the Co 2p lines indicates the formation of a trivalent, octahedrally coordinated, interfacial cobalt oxide layer during the early stages of growth, identified as the Co2O3 corundum phase.
The growth and characterization of epitaxial Co3O4(111) films grown by oxygen plasma-assisted molecular beam epitaxy on single crystalline a-Al2O3(0001) is reported. The Co3O4(111) grows single crystalline with the epitaxial relation Co3O4(111)[-12-1
]||a-Al2O3(0001)[10-10], as determined from in situ electron diffraction. Film stoichiometry is confirmed by x-ray photoelectron spectroscopy, while ex situ x-ray diffraction measurements show that the Co3O4 films are fully relaxed. Post-growth annealing induces significant modifications in the film morphology, including a sharper Co3O4/a-Al2O3 interface and improved surface crystallinity, as shown by x-ray reflectometry, atomic force microscopy and electron diffraction measurements. Despite being polar, the surface of both as-grown and annealed Co3O4(111) films are (1 * 1), which can be explained in terms of inversion in the surface spinel structure.
Temperature dependent magnetometry and transport measurements on epitaxial Fe3O4 films grown on BaTiO3(100) single crystals by molecular beam epitaxy show a series of discontinuities, that are due to changes in the magnetic anisotropy induced by stra
in in the different crystal phases of BaTiO3. The magnetite film is under tensile strain at room temperature, which is ascribed to the lattice expansion of BaTiO3 at the cubic to tetragonal transition, indicating that the magnetite film is relaxed at the growth temperature. From the magnetization versus temperature curves, the variation in the magnetic anisotropy is determined and compared with the magnetoelastic anisotropies. These results demonstrate the possibility of using the piezoelectric response of BaTiO3 to modulate the magnetic anisotropy of magnetite films.
The interface and electronic structure of thin (~20-74 nm) Co3O4(110) epitaxial films grown by oxygen-assisted molecular beam epitaxy on MgAl2O4(110) single crystal substrates have been investigated by means of real and reciprocal space techniques. A
s-grown film surfaces are found to be relatively disordered and exhibit an oblique low energy electron diffraction (LEED) pattern associated with the O-rich CoO2 bulk termination of the (110) surface. Interface and bulk film structure are found to improve significantly with post-growth annealing at 820 K in air and display sharp rectangular LEED patterns, suggesting a surface stoichiometry of the alternative Co2O2 bulk termination of the (110) surface. Non-contact atomic force microscopy demonstrates the presence of wide terraces separated by atomic steps in the annealed films that are not present in the as-grown structures; the step height of ~ 2.7 A corresponds to two atomic layers and confirms a single termination for the annealed films, consistent with the LEED results. A model of the (1 * 1) surfaces that allows for compensation of the polar surfaces is presented.
Analytical expressions for the magnetoelastic anisotropy constants of cubic magnetic systems are derived for rectangular and oblique distortions originating from epitaxial growth on substrates with lower crystal symmetry. In particular, the temperatu
re variation of the magnetic properties of magnetic films grown on barium titanate (BaTiO3) substrates are explained in terms of strain-induced magnetic anisotropies caused by the temperature dependent phase transitions of BaTiO3. Our results quantify the experimental observations in ferromagnet/bto-based structures, which have been proposed as templates for magnetoelectric composite heterostructures.
