arXiv subjects

A. Kirilyuk

Publications and source records attributed to A. Kirilyuk.

29 records · Page 2Linked to original sources

arXiv2017

Exchange interactions in transition metal oxides: The role of oxygen spin polarization

Magnetism of transition metal (TM) oxides is usually described in terms of the Heisenberg model, with orientation-independent interactions between the spins. However, the applicability of such a model is not fully justified for TM oxides because spin polarization of oxygen is usually ignored. In the conventional model based on the Anderson principle, oxygen effects are considered as a property of the TM ion and only TM interactions are relevant. Here, we perform a systematic comparison between two approaches for spin polarization on oxygen in typical TM oxides. To this end, we calculate the exchange interactions in NiO, MnO, and hematite (Fe2O3) for different magnetic configurations using the magnetic force theorem. We consider the full spin Hamiltonian including oxygen sites, and also derive an effective model where the spin polarization on oxygen renormalizes the exchange interactions between TM sites. Surprisingly, the exchange interactions in NiO depend on the magnetic state if spin polarization on oxygen is neglected, resulting in non-Heisenberg behavior. In contrast, the inclusion of spin polarization in NiO makes the Heisenberg model more applicable. Just the opposite, MnO behaves as a Heisenberg magnet when oxygen spin polarization is neglected, but shows strong non-Heisenberg effects when spin polarization on oxygen is included. In hematite, both models result in non-Heisenberg behavior. General applicability of the magnetic force theorem as well as the Heisenberg model to TM oxides is discussed.

cond-mat.str-el↗

arXiv2016

Ultrafast photo-magnetic recording in transparent medium

Finding a conceptually new way to control the magnetic state of media with the lowest possible production of heat and simultaneously at the fastest possible time-scale is a new challenge in fundamental magnetism [1-4] as well as an increasingly important issue in modern information technology [5]. Recent results demonstrate that exclusively in metals it is possible to switch magnetization between metastable states by femtosecond circularly polarized laser pulses [6-8]. However, despite the record breaking speed of the switching, the mechanisms in these materials are directly related to strong optical absorption and laser-induced heating close to the Curie temperature [9-12]. Here we report about ultrafast all-optical photo-magnetic recording in transparent dielectrics. In ferrimagnetic garnet film a single linearly polarized femtosecond laser pulse breaks the degeneracy between metastable magnetic states and promotes switching of spins between them. Changing the polarization of the laser pulse we deterministically steer the net magnetization in the garnet, write "0" and "1" magnetic bits at will. This mechanism operates at room temperature and allows ever fastest write-read magnetic recording event (< 20 ps) accompanied by unprecedentedly low heat load (< 6 J/cm3).

cs.ET↗

arXiv2016

Correlation effects and orbital magnetism of Co clusters

Recent experiments on isolated Co clusters have shown huge orbital magnetic moments in comparison with their bulk and surface counterparts. These clusters hence provide the unique possibility to study the evolution of the orbital magnetic moment with respect to the cluster size and how competing interactions contribute to the quenching of orbital magnetism. We investigate here different theoretical methods to calculate the spin and orbital moments of Co clusters, and assess the performances of the methods in comparison with experiments. It is shown that density functional theory in conventional local density or generalized gradient approximations, or even with a hybrid functional, severely underestimates the orbital moment. As natural extensions/corrections we considered the orbital polarization correction, the LDA+U approximation as well as the LDA+DMFT method. Our theory shows that of the considered methods, only the LDA+DMFT method provides orbital moments in agreement with experiment, thus emphasizing the importance of dynamic correlations effects for determining fundamental magnetic properties of magnets in the nano-size regime.

physics.atm-clus↗

arXiv2016

Treatment of 4f states of the rare-earths: the case study of TbN

The lattice constant, bulk modulus and shear constant of TbN are calculated by means of density functional theory (DFT) in the local density approximation (LDA) and generalized gradient approximation (GGA), with 4f-states treated as valence electrons or core electrons. In addition, local Coulomb repulsions U are treated both statically as in the LDA+U approach and dynamically as in the dynamical mean-field theory (DMFT) in the Hubbard-I approximation. It is shown that all methods, except DFT-LDA with 4f electrons treated as either valence states, produce lattice constants and bulk moduli in good agreement with experiment. In the LDA+U approach multiple minima are found, and we focus on the competition between a state with cubic symmetry and a state obtained from atomic Hund's rules. We find the state with cubic symmetry to be 0.59 eV lower in energy than the Hund's rules state, while the opposite was obtained in previous literature. The shear constant is shown to be rather sensitive to the theoretical method used, and the Hund's rules state obtained in LDA+U is found to be unstable towards tetragonal shear. As to the magnetism, we find that the calculation based on the Hubbard-I approximation reproduces observations with the best accuracy. Finally, the spectral properties of TbN are discussed, together with the general applicability of the different methods in describing rare-earth elements and compounds.

cond-mat.str-el↗

arXiv2016

The valence and spectral properties of rare-earth clusters

The rare-earths are known to have intriguing changes of the valence, depending on chemical surrounding or geometry. Here we make predictions from theory that combines density functional theory with atomic multiplet-theory, on the transition of valence when transferring from the atomic divalent limit to the trivalent bulk, passing through different sized clusters, of selected rare-earths. We predict that Tm clusters show an abrupt change from pure divalent to pure trivalent at a size of 6 atoms, while Sm and Tb clusters are respectively pure divalent and trivalent up to 8 atoms. Larger Sm clusters are argued to likely make a transition to a mixed valent, or trivalent, configuration. The valence of all rare-earth clusters, as a function of size, is predicted from interpolation of our calculated results. We argue that the here predicted behavior is best analyzed by spectroscopic measurements, and provide theoretical spectra, based on dynamical mean field theory, in the Hubbard-I approximation, to ease experimental analysis.

physics.atm-clus↗

arXiv2015

Geometric, electronic and magnetic structure of Fe$_{x}$O$_{y}^{+}$ clusters

Correlation between geometry, electronic structure and magnetism of solids is both intriguing and elusive. This is particularly strongly manifested in small clusters, where a vast number of unusual structures appear. Here, we employ density functional theory in combination with a genetic search algorithm, GGA$+U$ and a hybrid functional to determine the structure of gas phase Fe$_{x}$O$_{y}^{+/0}$ clusters. For Fe$_{x}$O$_{y}$ cation clusters we also calculate the corresponding vibration spectra and compare them with experiments. We successfully identify Fe$_{3}$O$_{4}^{+}$, Fe$_{4}$O$_{5}^{+}$, Fe$_{4}$O$_{6}^{+}$, Fe$_{5}$O$_{7}^{+}$ and propose structures for Fe$_{6}$O$_{8}^{+}$. Within the triangular geometric structure of Fe$_{3}$O$_{4}^{+}$ a non-collinear, ferrimagnetic and ferromagnetic state are comparable in energy. Fe$_{4}$O$_{5}^{+}$ and Fe$_{4}$O$_{6}^{+}$ are ferrimagnetic with a residual magnetic moment of 1~\muB{} due to ionization. Fe$_{5}$O$_{7}^{+}$ is ferrimagnetic due to the odd number of Fe atoms. We compare the electronic structure with bulk magnetite and find Fe$_{4}$O$_{5}^{+}$, Fe$_{4}$O$_{6}^{+}$, Fe$_{6}$O$_{8}^{+}$ to be mixed valence clusters. In contrast, in Fe$_{3}$O$_{4}^{+}$ and Fe$_{5}$O$_{7}^{+}$ all Fe are found to be trivalent.

physics.atm-clus↗

arXiv2014

All-optical magnetization switching in ferrimagnetic alloys: deterministic vs thermally activated dynamics

Using photo-emission electron microscopy with X-ray magnetic circular dichroism as a contrast mechanism, new insights into the all-optical magnetization switching (AOS) phenomenon in GdFe based rare-earth transition metal ferrimagnetic alloys are provided. From a sequence of static images taken after single linearly polarized laser pulse excitation, the repeatability of AOS can be measured with a correlation coefficient. It is found that low coercivity enables thermally activated domain wall motion, limiting in turn the repeatability of the switching. Time-resolved measurement of the magnetization dynamics reveal that while AOS occurs below and above the magnetization compensation temperature $T_\text{M}$, it is not observed in GdFe samples where $T_\text{M}$ is absent. Finally, AOS is experimentally demonstrated against an applied magnetic field of up to 180 mT.

cond-mat.mtrl-sci↗

arXiv2014

Sub-diffraction sub-100 ps all-optical magnetic switching by passive wavefront shaping

The recently discovered magnetization reversal driven solely by a femtosecond laser pulse has been shown to be a promising way to record information at record breaking speeds. Seeking to improve the recording density has raised intriguing fundamental question about the feasibility to combine the ultrafast temporal with sub-wavelength spatial resolution of magnetic recording. Here we report about the first experimental demonstration of sub-diffraction and sub-100 ps all-optical magnetic switching. Using computational methods we reveal the feasibility of sub-diffraction magnetic switching even for an unfocused incoming laser pulse. This effect is achieved via structuring the sample such that the laser pulse experiences a passive wavefront shaping as it couples and propagates inside the magnetic structure. Time-resolved studies with the help of photo-emission electron microscopy clearly reveal that the sub-wavelength switching with the help of the passive wave-front shaping can be pushed into sub-100 ps regime.

cond-mat.mtrl-sci↗

arXiv2013

Laser-Induced Magnetic Nanostructures with Tunable Topological Properties

We report the creation and real-space observation of magnetic structures with well-defined topological properties and a lateral size as low as about 150 nm. They are generated in a thin ferrimagnetic film by ultrashort single optical laser pulses. Thanks to their topological properties, such structures can be classified as Skyrmions of a particular type that does not require an externally applied magnetic field for stabilization. Besides Skyrmions, we are able to generate magnetic features with topological characteristics that can be tuned by changing the laser fluence. The stability of such features is accounted for by an analytical model based on the interplay between the exchange and the magnetic dipole-dipole interactions

cond-mat.mes-hall↗

arXiv2011

Enhancement of the Curie temperature in small particles of weak itinerant ferromagnets

Self consistent renormalization theory of itinerant ferromagnets is used to calculate the Curie temperature of clusters down to approximately 100 atoms in size. In these clusters the electrons responsible for the magnetic properties are assumed to be (weakly) itinerant. It is shown that the Curie temperature can be larger than in the bulk. The effect originates from the phenomenon of level repulsion in chaotic quantum systems, which suppresses spin fluctuations. Since the latter destroy the magnetic order the resulting Curie temperature increases, contrary to expectations of the naive Stoner picture. The calculations are done assuming that the energy levels of the cluster are described by the Gaussian Orthogonal Ensemble of random matrix theory.

cond-mat.mes-hall↗

arXiv2005

Quantitative detection of electrically injected spin accumulation in GaAs using the magneto-optical Kerr effect

Using an ultra sensitive magneto-optical Kerr rotation setup we have observed electrical spin injection from (Zn,Mn)Se into bulk GaAs and quantified the spin injection efficiency. The current induced contribution was carefully separated from possible artifacts and studied as a function of voltage and external magnetic field. Our measurements allow us to estimate the concentration of the electrically injected spins into GaAs to be approximately 0.4$\times10^{15}$cm$^{-3}$ at a reverse bias of 0.7 V.

cond-mat.mes-hall↗