27 research outputs found
Magneto-optic Kerr effect in a spin-polarized zero-moment ferrimagnet
The magneto-optical Kerr effect (MOKE) is often assumed to be proportional to
the magnetisation of a magnetically ordered metallic sample; in metallic
ferrimagnets with chemically distinct sublattices, such as rare-earth
transition-metal alloys, it depends on the difference between the sublattice
contributions. Here we show that in a highly spin polarized, fully compensated
ferrimagnet, where the sublattices are chemically similar, MOKE is observed
even when the net moment is strictly zero. We analyse the spectral ellipsometry
and MOKE of Mn 2 Ru x Ga, and show that this behaviour is due to a highly
spin-polarized conduction band dominated by one of the two manganese
sublattices which creates helicity-dependent reflectivity determined by a broad
Drude tail. Our findings open new prospects for studying spin dynamics in the
infra-red.Comment: 7 pages, 7 figure
Designing a fully compensated half-metallic ferrimagnet
Recent experimental work on Mn2RuxGa demonstrates its potential as a compensated ferrimagnetic
half-metal (CFHM). Here we present a set of high-throughput ab initio density functional
theory calculations and detailed experimental characterisation, that enable us to correctly describe
the nominal Mn2RuxGa thin films, in particular with regard to site-disorder and defects. We then
construct models that accurately capture all the key features of the Mn-Ru-Ga system, including
magnetic compensation and the spin gap at the Fermi level. We find that electronic doping is neccessary,
which is achieved with a Mn/Ga ratio smaller than two. Our study shows how composition
and substrate-induced biaxial strain can be combined to design a ferrimagnetic half-metal with a
compensation point close to room temperature
Experimental Determination of Momentum-Resolved Electron-Phonon Coupling
We provide a novel experimental method to quantitatively estimate the
electron-phonon coupling and its momentum dependence from resonant inelastic
x-ray scattering (RIXS) spectra based on the detuning of the incident photon
energy away from an absorption resonance. We apply it to the cuprate parent
compound NdBaCuO and find that the electronic coupling to the
oxygen half-breathing phonon mode is strongest at the Brillouin zone boundary,
where it amounts to eV, in agreement with previous studies. In
principle, this method is applicable to any absorption resonance suitable for
RIXS measurements and will help to define the contribution of lattice
vibrations to the peculiar properties of quantum materials.Comment: 6 pages, 3 figure
Fractional Spin Excitations in the Infinite-Layer Cuprate CaCuO2
We use resonant inelastic x-ray scattering (RIXS) to investigate the magnetic dynamics of the infinite-layer cuprate CaCuO2. We find that close to the (1/2,0) point, the single magnon decays into a broad continuum of excitations accounting for about 80% of the total magnetic spectral weight. Polarization-resolved RIXS spectra reveal the overwhelming dominance of the spin-flip (Delta S = 1) character of this continuum with respect to the Delta S = 0 multimagnon contributions. Moreover, its incident-energy dependence is identical to that of the magnon, supporting a common physical origin. We propose that the continuum originates from the decay of the magnon into spinon pairs, and we relate it to the exceptionally high ring exchange J(c) similar to J(1) of CaCuO2. In the infinite-layer cuprates, long-range and multisite hopping integrals are very important, and they amplify the 2D quantum magnetism effects in spite of the 3D antiferromagnetic Neel order
Mobile orbitons in CaCuO: crucial role of the Hund's exchange
We investigate the Cu edge resonant inelastic x-ray scattering (RIXS)
spectra of a quasi-1D antiferromagnet CaCuO. In addition to the
magnetic excitations, which are well-described by the two-spinon continuum, we
observe two dispersive orbital excitations, the and the
orbitons. We carry out a quantitative comparison of the RIXS spectra, obtained
with two distinct incident polarizations, with a theoretical model. We show
that any realistic spin-orbital model needs to include a finite, but realistic,
Hund's exchange eV. Its main effect is an increase in orbiton
velocities, so that their theoretically calculated values match those observed
experimentally. Even though Hund's exchange also mediates some interaction
between spinon and orbiton, the picture of spin-orbit separation remains intact
and describes orbiton motion in this compound.Comment: 13 pages, 10 figure
Crystalline and magnetic structure of Ba2CuO3+{\delta} investigated by x-ray absorption spectroscopy and resonant inelastic x-ray scattering
Motivated by the recent synthesis of BaCuO (BCO), a high
temperature superconducting cuprate with putative ground state
symmetry, we investigated its electronic structure by means of Cu x-ray
absorption (XAS) and resonant inelastic x-ray scattering (RIXS) at the Cu
edge on a polycrystalline sample. We show that the XAS profile of BCO is
characterised by two peaks associated to inequivalent Cu sites, and that its
RIXS response features a single, sharp peak associated to crystal-field
excitations. We argue that these observations are only partially compatible
with the previously proposed crystal structure of BCO. Based on our
spectroscopic results and on previously published powder diffraction
measurements, we propose a crystalline structure characterized by two
inequivalent Cu sites located at alternated planes along the axis:
nominally trivalent Cu(1) belonging to very short Cu-O chains, and divalent
Cu(2) in the oxygen deficient CuO planes. We also analyze the
low-energy region of the RIXS spectra to estimate the magnitude of the magnetic
interactions in BCO and find that in-plane nearest neighbor superexchange
exceeds 120~meV, similarly to that of other layered cuprates. Although these
results do not support the pure ground state scenario, they hint
at a significant departure from the common quasi-2D electronic structure of
superconducting cuprates of pure symmetry
Multiple-magnon excitations shape the spin spectrum of cuprate parent compounds
Thanks to high resolution and polarization analysis, resonant inelastic x-ray
scattering (RIXS) magnetic spectra of La2CuO4, Sr2CuO2Cl2 and CaCuO2 reveal a
rich set of properties of the spin 1/2 antiferromagnetic square lattice of
cuprates. The leading single-magnon peak energy dispersion is in excellent
agreement with the corresponding inelastic neutron scattering measurements.
However, the RIXS data unveil an asymmetric lineshape possibly due to odd
higher order terms. Moreover, a sharp bimagnon feature emerges from the
continuum at (1/2,0), coincident in energy with the bimagnon peak detected in
optical spectroscopy. These findings show that the inherently complex spin
spectra of cuprates, an exquisite manifestation of quantum magnetism, can be
effectively explored by exploiting the richness of RIXS cross sections.Comment: 7 pages, 3 figure
Slow magnetic relaxation of Dy adatoms with in-plane magnetic anisotropy on a two-dimensional electron gas
We report on the magnetic properties of Dy atoms adsorbed on the (001) surface of SrTiO3. X-ray magnetic circular dichroism reveals slow relaxation of the Dy magnetization on a time scale of about 800 s at 2.5 K, unusually associated with an easy-plane magnetic anisotropy. We attribute these properties to Dy atoms occupying hollow adsorption sites on the TiO2-terminated surface. Conversely, Ho atoms adsorbed on the same surface show paramagnetic behavior down to 2.5 K. With the help of atomic multiplet simulations and first-principles calculations, we establish that Dy populates also the top-O and bridge sites on the coexisting SrO-terminated surface. A simple magnetization relaxation model predicts these two sites to have an even longer magnetization lifetime than the hollow site. Moreover, the adsorption of Dy on the insulating SrTiO3 crystal leads, regardless of the surface termination, to the formation of a spin-polarized two-dimensional electron gas of Ti 3dxy character, together with an antiferromagnetic Dy-Ti coupling. Our findings support the feasibility of tuning the magnetic properties of the rare-earth atoms by acting on the substrate electronic gas with electric fields