Search

We investigated thin La0.7Sr0.3MnO3-SrTiO3 heterostructures, where the band alignment is engineered by a variation of La/Sr stoichiometry only at the interface. In thin films, the engineered interface leads to an enhancement of the reversed spin configuration that mimics bulk behavior. Microscopically, this enhancement is closely connected with an increased magnetic anisotropy as well as intercoupling between an eg orbital reconstruction and a corresponding anisotropic lattice fluctuation. Furthermore, a reentrant-type behavior, triggered by this intercoupling, is observed in the remanent spin state. This microscopic perspective leads to insights on developing new strategies for maintaining bulk-like properties even in very thin La0.7Sr0.3MnO3 heterostructures. [ABSTRACT FROM AUTHOR]

The magnetic, transport, and electronic properties of ultrathin epitaxial La1-xSrxMnO3 (LSMO) films at near half doping (x = 0.47, 0.50, and 0.55), grown under different misfit strains on SrTiO3(001) and La0.18Sr0.82Al0.59Ta0.41O3(001) (LSAT) substrates, are investigated. We find that all films exhibit metallic behavior below the magnetic critical temperature, while the magnetic properties change markedly with both doping and strain. However, while increased doping favors antiferromagnetic ordering, strain is the driving mechanism for the change in the magnetic properties, where with increasing tensile strain the magnetic ground state changes from ferromagnetic to antiferromagnetic at a critical lattice misfit threshold of about -1%. The bulk magnetometry data are confirmed by x-ray magnetic circular dichroism spectroscopy, while x-ray magnetic linear dichroism measured at room temperature demonstrates a progressive change in the orbital occupancy with increasing misfit strain from out of plane to in-plane, leading to a preferred antiferromagnetic metallic state at larger tensile strains. [ABSTRACT FROM AUTHOR]

The magnetic properties of iron-doped cobalt ferrite (Co1-xFe2+xO4) (001) thin films grown epitaxially on MgO (001) substrates are investigated by superconducting quantum interference device magnetometry and soft x-ray magnetic linear and circular dichroisms. All Co1-xFe2+xO4 (0.01 ⩽ × ⩽ 0.63) samples have out-of-plane magnetic easy axes and large coercive fields, unlike Fe3O4, due to a large Co2+ orbital moment. The magnetic moments for those samples are significantly reduced from their bulk values; however, as x increases, the magnetic moments tend nearer to their bulk values and increase more rapidly as x approaches 1. This reduction in magnetic moment is attributed to spin canting among the Co2+ cations, owing to a small in-plane tensile strain in the film and to an increased antiferromagnetic alignment among all the cations caused by a partially inverse spinel cubic structure and the likely presence of antiphase boundaries. Our results show that small changes in stoichiometry can lead to significant changes in the magnetic moment of Co1-xFe2+xO4, especially at large values of x. [ABSTRACT FROM AUTHOR]

Here we demonstrate an experimental observation of GHz-scale spin dynamics resolved to sublattice octahedral (Oh) tetrahedral (Td) sites in a spinel ferrimagnet, in this case a Mn-ferrite thin film. X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) are used, in combination with multiplet calculations, to uniquely identify the spectral signature from Mn2+ and Fe2+,3+ on Oh and Td lattice sites. With the sample under //excitation, the spin alignment of the sublattices is tracked with time-resolved XMCD (TR-XMCD). The spin alignment of the sublattices is mostly antiferromagnetic. The phase difference between the Oh Fe2+ [Oh Fe3+] and Td Mn2+ sites is 181.2 ± 3.8° [183.3° ± 3.7°] at 150 K and 186.6 ± 2.2° [182.0° ± 2.2°] at 300 K. Such direct measurement of the dynamic coupling, exchange stiffness, and damping enabled by TR-XMCD across sublattices will be essential for optimizing the development of future-generation microwave devices. [ABSTRACT FROM AUTHOR]

Discovering new antiferromagnetic (AF) compounds is at the forefront of developing future spintronic devices without fringing magnetic fields. The AF gapless semiconducting D03 phase of V3 A1 was successfully synthesized via arc-melting and annealing. The AF properties were established through synchrotron measurements of the atom-specific magnetic moments, where the magnetic dichroism reveals large and oppositely oriented moments on individual V atoms. Density functional theory calculations confirmed the stability of a type G antiferromagnetism involving only two-thirds of the V atoms, while the remaining V atoms are nonmagnetic. Magnetization, x-ray diffraction, and transport measurements also support the antiferromagnetism. This archetypal gapless semiconductor may be considered as a cornerstone for future spintronic devices containing AF elements. [ABSTRACT FROM AUTHOR]

To address the amount of disorder and interface diffusion induced by annealing, all-Heusler multilayer structures, consisting of ferromagnetic Co2MnGe and nonmagnetic Rh2CuSn layers of varying thicknesses, have been investigated by means of hard x-ray photoelectron spectroscopy and x-ray magnetic circular dichroism. We find evidence for a 4 Å thick magnetically dead layer that, together with the identified interlayer diffusion, are likely reasons for the unexpectedly small magnetoresistance found for current-perpendicular-to-plane giant magnetoresistance devices based on this all-Heusler system. We find that diffusion begins already at comparably low temperatures between 200 and 250°C, where Mn appears to be most prone to diffusion. [ABSTRACT FROM AUTHOR]

The effect of film thickness on the magnetic properties of ultrathin Fe-doped cobalt ferrite (Co1-xFe2+xO4) grown on MgO (001) substrates is investigated by superconducting quantum interference device magnetometry and x-ray magnetic linear dichroism, while the distribution of the Co2+ cations between the octahedral and tetrahedral lattice sites is studied with x-ray absorption spectroscopy. For films thinner than 10 nm, there is a large enhancement of the magnetic moment; conversely, the remanent magnetization and coercive fields both decrease, while the magnetic spin axes of all the cations become less aligned with the [001] crystal direction. In particular, at 300 K the coercive fields of the thinnest films vanish. The spectroscopy data show that no changes occur in the cation distribution as a function of film thickness, ruling this out as the origin of the enhanced magnetic moment. However, the magnetic measurements all support the possibility that these ultrathin Fe-doped CoFe2O4 films are transitioning into a superparamagnetic state, as has been seen in ultrathin Fe3O4. A weakening of the magnetic interactions at the antiphase boundaries, leading to magnetically independent domains within the film, could explain the enhanced magnetic moment in ultrathin Fe-doped CoFe2O4 and the onset of superparamagnetism at room temperature. [ABSTRACT FROM AUTHOR]

The transition between Kondo and Coulomb blockade effects in discontinuous double magnetic tunnel junctions is explored as a function of the size of the CoPt magnetic clusters embedded between AlOx mnnel barriers. A gradual competition between cotunneling enhancement of the tunneling magnetoresistance (TMR) and the TMR suppression due to the Kondo effect has been found in these junctions, with both effects having been found to coexist even in the same sample. It is possible to tune between these two states with temperature (at a temperature far below the cluster blocking temperature). In addition, when further decreasing the size of the CoPt clusters, another gradual transition between the Kondo effect and direct tunneling between the electrodes takes place. This second transition shows that the spin-flip processes found in junctions with impurities in the barrier are in fact due to the Kondo effect. A simple theoretical model able to account for these experimental results is proposed. [ABSTRACT FROM AUTHOR]

Chromium-doped gallium sesquiselenide, Cr:Ga2Se3, is a member of a new class of dilute magnetic semiconductors exploiting intrinsic vacancies in the host material. The correlation among room-temperature ferromagnetism, surface morphology, electronic structure, chromium concentration, and local chemical and structural environments in Cr:Ga2Se3 films grown epitaxially on silicon is investigated with magnetometry, scanning tunneling microscopy, photoemission spectroscopy, and x-ray absorption spectroscopy. Inclusion of a few percent chromium in Ga2Se3 results in laminar, semiconducting films that are ferromagnetic at room temperature with a magnetic moment ⩾4μB/Cr. The intrinsic-vacancy structure of defected-zinc-blende β-Ga2Se3 enables Cr incorporation in a locally octahedral site without disrupting long-range order, determined by x-ray absorption spectroscopy, as well as strong overlap between Cr 3d states and the Se 4p states lining the intrinsic-vacancy rows, observed with photoemission. The highest magnetic moment per Cr is observed near the solubility limit of roughly one Cr per three vacancies. At higher Cr concentrations, islanded, metallic films result, with a magnetic moment that depends strongly on surface morphology. The effective valence is Cr3+ in laminar films, with introduction of Cr0 upon islanding. A mechanism is proposed for laminar films whereby ordered intrinsic vacancies mediate ferromagnetism. [ABSTRACT FROM AUTHOR]

The electronic, magnetic and transport properties of iron-doped cobalt ferrite (Co1-xFe2+xO4) thin films grown epitaxially on MgO (001) substrates are investigated by soft x-ray absorption and photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, superconducting quantum interference device magnetometry, and resistivity measurements. The crystal structure for Co1-xFe2+xO4 is determined to be nearly inverse spinel, with the degree of inversion increasing for increased doping until it becomes fully inverse spinel for Fe3O4. The doped iron cations have a valency of 2+ and reside solely on octahedral sites, which allows for conduction owing to hopping between Fe2+ and Fe3+ octahedral cations. The addition of Fe2+ cations increases the electron density of states near the Fermi energy, shifting the Fermi level from 0.75 to 0 eV with respect to the top of the valence band, as the doping increases from x = 0.01 to 1. This change in electronic structure results in a change in resistivity by over two orders of magnitude. In contrast, the magnetic properties of CoFe2O4 thin films, characterized by a significantly reduced saturation magnetization compared to the bulk and large magnetic anisotropies, are affected less significantly by doping in the range from 0 to 0.63. These results show that Co1-xFe2+xO4 has tunable electronic properties while maintaining magnetic properties similar to CoFe2O4. [ABSTRACT FROM AUTHOR]