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We report a comprehensive study of the temperature evolution of in-plane (IP) and out-of-plane (OOP) effective magnetic anisotropies in compensated ferrimagnetic ${\mathrm{Fe}}_{100\ensuremath{-}x}{\mathrm{Gd}}_{x}$ alloy films by employing direct current magnetometry and radiofrequency (RF) transverse susceptibility (TS) measurements. We suggest that our ${\mathrm{Fe}}_{100\ensuremath{-}x}{\mathrm{Gd}}_{x}$ system is chemically inhomogeneous and phase segregates into Fe- and Gd-enriched regions. Our IP and OOP magnetometry results indicate that the system undergoes a temperature-driven transformation from an IP-spin-configuration-dominated state to an OOP-spin-configuration-dominated state below a certain temperature (spin reorientation temperature). A two-step reversal behavior emerges in the OOP $M$($H$) loop near compensation, which we attribute to the sequential magnetization reversals of Fe- and Gd-enriched domains. Field-induced spin-flop transitions were also observed near the compensation. Our RF TS measurements indicate that the effective magnetic anisotropy for the OOP configuration dominates over that for the IP configuration below a certain spin reorientation temperature. Both IP and OOP anisotropy fields determined from our TS measurement exhibit a minimum around the compensation temperature, which has been explained in the framework of the Stoner-Wohlfarth model.

In magnetic trilayer systems, spin pumping is generally addressed as a reciprocal mechanism characterized by one unique spin-mixing conductance common to both interfaces. However, this assumption is questionable in cases where different types of interfaces are present. Here, we present a general theory for analyzing spin pumping in cases with more than one unique interface and where the magnetic coupling is allowed to be noncollinear. The theory is applied to analyze layer-resolved ferromagnetic resonance experiments on the trilayer system Ni80Fe20/Ru/Fe49Co49V2 where the Ru spacer thickness is varied to tune the indirect exchange coupling. It is demonstrated that the equation of motion of macrospins driven by spin pumping need to be modified in case of noncollinear coupling. Our analysis also shows that the spin pumping in trilayer systems with dissimilar magnetic layers, in general, is nonreciprocal.

Incorporating Fe atoms into the lattice is shown to significantly alter electronic and magnetic properties of $\mathrm{Ti}{\mathrm{O}}_{2}$ nanotubes synthesized by electrochemical anodization of Ti-Fe alloy sheets. The effects of Fe incorporation on the nanotube morphology, crystallinity, crystal structure, magnetic behavior and electronic structure were investigated with crystallographic and magnetic probes, including synchrotron-based spectroscopy. Results indicate that the iron cations predominately adopt the $\mathrm{F}{\mathrm{e}}^{3+}$ configuration, leading to a large increase of the electronic density of states at the Fermi energy. This increase is anticipated to provide enhanced catalytic action, for instance, in the degradation of water and of air pollutants. These results provide insight for tailoring the functionality of these nanostructures for energy-related applications.

The characteristics of the cooperative Mott-Peierls metal-insulator transition (MIT) of ${\mathrm{VO}}_{2}$ can be altered by employing epitaxial strain. While the most commonly used substrate for this purpose is isostructural rutile ${\mathrm{TiO}}_{2}$, thin films often suffer from interdiffusion of Ti ions near the interface. Exploiting this phenomena, we investigate the nature of interfacial ${\mathrm{V}}^{4+}/{\mathrm{Ti}}^{4+}$ cation intermixing and its effects on the MIT using scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS), soft x-ray absorption spectroscopy (XAS), and hard x-ray photoelectron spectroscopy (HAXPES), along with supporting density functional theory (DFT) calculations. We find that the reduced orbital occupancy in highly Ti incorporated ${\mathrm{VO}}_{2}$ is responsible for suppressing the MIT. Interdiffused films are found to be metallic at all measured temperatures, despite a resolute dimerization inferred from x-ray absorption data at lower temperatures. Our results demonstrate that the Mott physics can be suppressed in doped ${\mathrm{VO}}_{2}$, while a lattice dimerization remains thermodynamically favorable.

Tensile strain along the ${c}_{R}$ axis in epitaxial ${\mathrm{VO}}_{2}$ films raises the temperature of the metal insulator transition and is expected to stabilize the intermediate monoclinic M2 phase. We employ surface-sensitive x-ray spectroscopy to distinguish from the ${\mathrm{TiO}}_{2}$ substrate and identify the phases of ${\mathrm{VO}}_{2}$ as a function of temperature in epitaxial ${\mathrm{VO}}_{2}/{\mathrm{TiO}}_{2}$ thin films with well-defined biaxial strain. Although qualitatively similar to our Landau-Ginzburg theory predicted phase diagrams, the M2 phase is stabilized by nearly an order of magnitude more strain than expected for the measured temperature window. Our results reveal that the elongation of the ${c}_{R}$ axis is insufficient for describing the transition pathway of ${\mathrm{VO}}_{2}$ epitaxial films and that a strain induced increase of electron correlation effects must be considered.

Discovering new antiferromagnetic compounds is at the forefront of developing future spintronic devices without fringing magnetic fields. The antiferromagnetic gapless semiconducting D03 phase of V3Al was successfully synthesized via arc-melting and annealing. The antiferromagnetic 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-third 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 antiferromagnetic elements., Comment: Accepted to Physics Review B on 02/23/15

The magnetic, transport, and electronic properties of ultrathin epitaxial ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) films at near half doping ($x=0.47$, 0.50, and 0.55), grown under different misfit strains on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$(001) and ${\mathrm{La}}_{0.18}{\mathrm{Sr}}_{0.82}{\mathrm{Al}}_{0.59}{\mathrm{Ta}}_{0.41}{\mathrm{O}}_{3}$(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 \ensuremath{-}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.

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 angstrom 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 degrees C, where Mn appears to be most prone to diffusion.

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 AlO${}_{x}$ tunnel 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.

The magnetic properties of iron-doped cobalt ferrite (Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$) (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 Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$ (0.01 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 0.63) samples have out-of-plane magnetic easy axes and large coercive fields, unlike Fe${}_{3}$O${}_{4}$, due to a large Co${}^{2+}$ 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 Co${}^{2+}$ 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 Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$, especially at large values of $x$.

Chromium-doped gallium sesquiselenide, Cr:Ga${}_{2}$Se${}_{3}$, 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:Ga${}_{2}$Se${}_{3}$ 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 Ga${}_{2}$Se${}_{3}$ results in laminar, semiconducting films that are ferromagnetic at room temperature with a magnetic moment $\ensuremath{\geqslant}4{\ensuremath{\mu}}_{B}/\text{Cr}$. The intrinsic-vacancy structure of defected-zinc-blende $\ensuremath{\beta}\ensuremath{-}$Ga${}_{2}$Se${}_{3}$ 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 Cr${}^{3+}$ in laminar films, with introduction of Cr${}^{0}$ upon islanding. A mechanism is proposed for laminar films whereby ordered intrinsic vacancies mediate ferromagnetism.

The electronic, magnetic and transport properties of iron-doped cobalt ferrite (Co{sub 1-x}Fe{sub 2+x}O{sub 4}) 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 Co{sub 1-x}Fe{sub 2+x}O{sub 4} is determined to be nearly inverse spinel, with the degree of inversion increasing for increased doping until it becomes fully inverse spinel for Fe{sub 3}O{sub 4}. The doped iron cations have a valency of 2+ and reside solely on octahedral sites, which allows for conduction owing to hopping between Fe{sup 2+} and Fe{sup 3+} octahedral cations. The addition of Fe{sup 2+} 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 CoFe{sub 2}O{sub 4} thin films, characterized by a significantly reduced saturation magnetization compared to the bulk and large magnetic anisotropies, are affected less significantly by doping inmore » the range from 0 to 0.63. These results show that Co{sub 1-x}Fe{sub 2+x}O{sub 4} has tunable electronic properties while maintaining magnetic properties similar to CoFe{sub 2}O{sub 4}.« less

Temperature and bias voltage-dependent transport characteristics are presented for double magnetic tunnel junctions (DMTJs) with self-assembled NiFe nanoparticles embedded between insulating alumina barriers. The junctions with embedded nanoparticles are compared to junctions with a single barrier of comparable size and growth conditions. The embedded particles are characterized using x-ray absorption spectroscopy, transmission electron microscopy, and magnetometry techniques, showing that they are unoxidized and remain superparamagnetic to liquid helium temperatures. The tunneling magnetoresistance (TMR) for the DMTJs is lower than the control samples, however, for the DMTJs an enhancement in TMR is seen in the Coulomb blockade region. Fitting the transport data in this region supports the theory that cotunneling is the dominant electron transport process within the Coulomb blockade region, sequential tunneling being suppressed. We therefore see an enhanced TMR attributed to the change in the tunneling process due to the interplay of the Coulomb blockade and spin-dependent tunneling through superparamagnetic nanoparticles, and develop a simple model to quantify the effect, based on the fact that our nanoparticles will appear blocked when measured on femtosecond tunneling time scales.

Using the surface, interface, and element specificity of x-ray resonant magnetic scattering in combination with x-ray magnetic circular dichroism, we have spatially resolved the magnetic spin polarization, and the associated interface proximity effect, in a Mn-based high-susceptibility material close to a ferromagnetic Co layer. We have measured the magnetic polarization of Mn and Cu 3d electrons in paramagnetic CuMn alloy layers in [Co/Cu(x)/CuMn/Cu(x)](20) multilayer samples with varying copper layer thicknesses from x=0 to 25 angstrom. The size of the Mn and Cu L-2,L-3 edge dichroism shows a decrease in the Mn-induced polarization for increasing copper thickness indicating the dominant interfacial nature of the Cu and Mn spin polarization. The Mn polarization is much higher than that of Cu. Evidently, the Mn moment is a useful probe of the local spin density. Mn atoms appear to be coupled antiferromagnetically with the Co layer below x=10 angstrom and ferromagnetically coupled above. In contrast, the interfacial Cu atoms remain ferromagnetically aligned to the Co layer for all thicknesses studied.

The effect of the application of a magnetic field during deposition of epitaxial Co{sub 70}Fe{sub 30} onto GaAs(001) is shown; we find an initially counterintuitive result. For field applied along the interfacial uniaxial hard axis the relative effective uniaxial magnetic anisotropy is increased by a factor of two in comparison to both field along the uniaxial easy axis, or no field; usually, application of a deposition field results in a uniaxial easy axis parallel to this field direction. We show that the deposition field changes the maximal projection of the atomic orbital magnetic moments onto the easy axis, which corresponds to a deposition field induced shift in the Helmholtz free-energy landscape of the system.

Oxygen $1s$ near-edge x-ray absorption spectroscopy and x-ray diffraction investigations on ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.5)$ single crystals evidence a doping- and temperature-dependent redistribution of ${e}_{g}$ electrons between ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$, ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$, and ${d}_{3{x}^{2}\ensuremath{-}{r}^{2}}∕{d}_{3{y}^{2}\ensuremath{-}{r}^{2}}$ orbitals: For undoped $\mathrm{La}\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{4}$ the ${e}_{g}$ electrons predominantly reside on ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbitals, nevertheless a small amount of charge carriers is found on in-plane ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals. These in-plane electrons are gradually transferred to the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ states upon cooling. Sr doping, however, does not only provide holes to the system but also induces a continuous transfer of electrons from ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ to ${d}_{3{x}^{2}\ensuremath{-}{r}^{2}}∕{d}_{3{y}^{2}\ensuremath{-}{r}^{2}}$ orbitals and, finally long-range charge-exchange-type orbital ordering develops for ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}\mathrm{Mn}{\mathrm{O}}_{4}$ below $T\ensuremath{\leqslant}225\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The data indicate a short-range orbital ordering for the intermediate doping range.