Lieb-Mattis ferrimagnetism in diluted magnetic semiconductors

We show the possibility of long-range ferrimagnetic ordering with a saturation magnetisation of the order of 1 Bohr magneton per spin for arbitrarily low concentration of magnetic impurities in semiconductors, provided that the impurities form a superstructure satisfying the conditions of the Lieb-Mattis theorem. Explicit examples of such superstructures are given for the wurtzite lattice, and the temperature of ferrimagnetic transition is estimated from a high-temperature expansion. Exact diagonalization studies show that small fragments of the structure exhibit enhanced magnetic response and isotropic superparamagnetism at low temperatures. A quantum transition in a high magnetic field is considered and similar superstructures in cubic semiconductors are discussed as well.Comment: 6 pages,4 figure

Mössbauer study of the hyperfine interactions and spin dynamics in -iron(II) phthalocyanine

13 pages, 8 figures, 3 tables.-- PACS number(s): 75.50.Xx, 76.80.+y, 75.50.DdThe 57Fe Mössbauer spectroscopy on -iron(II) phthalocyanine (FePc) as a function of temperature (1.3<T<295 K) and applied field (0<B<10 T) has been used to study the peculiar magnetic properties of this ferromagnetic quasilinear chain type compound. One sextet with an internal hyperfine field Bint=66.2 T was observed at 1.3 K, a very large value for a bivalent iron with S=1 pointing to the existence of large positive orbital and dipolar contributions in the investigated FePc. Under an applied field, the experimental spectra exhibited two nonequivalent Fe positions, due to spin canting, with the values for the hyperfine fields of the split sextets increasing with increasing field, an indication that unlike most cases, Bint in -FePc is positive, i.e., parallel to the magnetic moment of iron. Therefore, the origin of the large hyperfine field is the orbital moment rather than the Fermi's contact interaction. This fact is ascribed to the orbital degeneracy of the ground state of Fe(II) in the present configuration, where an unpaired hole occupies the orbital doublet (dxz, dyz). This feature supports and explains the magnetization and susceptibility data as well as the anomalously high hyperfine field observed at 57Fe nucleus. The relaxational behavior in the ac susceptibility and Mössbauer spectra found in the region 5–20 K was ascribed to solitonlike motion of domain walls within the magnetic chains, with a single-kink activation energy of 72 K.This work has been supported by Project No. MAT05/ 1272 of the Spanish Ministry of Education and Science. M. D. Kuz’min acknowledges Grant No. SAB2000-0084. G. F. acknowledges the Royal Society Research Grant at the University of Liverpool and Grant No. SAB2003/0226 from Ministerio de Education, Cultura y Deporte of Spain. The final part of this work was facilitated and supported by EU Network of Excellence MAGMANet.Peer reviewe

Theory of magnetic domains in uniaxial thin films

For uniaxial easy axis films, properties of magnetic domains are usually described within the Kittel model, which assumes that domain walls are much thinner than the domains. In this work we present a simple model that includes a proper description of the magnetostatic energy of domains and domain walls and also takes into account the interaction between both surfaces of the film. Our model describes the behavior of domain and wall widths as a function of film thickness, and is especially well suited for the strong stripe phase. We prove the existence of a critical value of magneto-crystalline anisotropy above which stripe domains exist for any film thickness and justify our model by comparison with exact results. The model is in good agreement with experimental data for hcp cobalt.Comment: 15 pages, 7 figure

Magnetic properties of (Fe1−x_{1-x}Cox_x)2_2B alloys and the effect of doping by 5dd elements

We have explored, computationally and experimentally, the magnetic properties of \fecob{} alloys. Calculations provide a good agreement with experiment in terms of the saturation magnetization and the magnetocrystalline anisotropy energy with some difficulty in describing Co$_2$B, for which it is found that both full potential effects and electron correlations treated within dynamical mean field theory are of importance for a correct description. The material exhibits a uniaxial magnetic anisotropy for a range of cobalt concentrations between $x=0.1$ and $x=0.5$. A simple model for the temperature dependence of magnetic anisotropy suggests that the complicated non-monotonous temperature behaviour is mainly due to variations in the band structure as the exchange splitting is reduced by temperature. Using density functional theory based calculations we have explored the effect of substitutional doping the transition metal sublattice by the whole range of 5$d$ transition metals and found that doping by Re or W elements should significantly enhance the magnetocrystalline anisotropy energy. Experimentally, W doping did not succeed in enhancing the magnetic anisotropy due to formation of other phases. On the other hand, doping by Ir and Re was successful and resulted in magnetic anisotropies that are in agreement with theoretical predictions. In particular, doping by 2.5~at.\% of Re on the Fe/Co site shows a magnetocrystalline anisotropy energy which is increased by 50\% compared to its parent (Fe$_{0.7}$Co$_{0.3}$)$_2$B compound, making this system interesting, for example, in the context of permanent magnet replacement materials or in other areas where a large magnetic anisotropy is of importance.Comment: 15 pages 17 figure

Two-component {CH} system: Inverse Scattering, Peakons and Geometry

An inverse scattering transform method corresponding to a Riemann-Hilbert problem is formulated for CH2, the two-component generalization of the Camassa-Holm (CH) equation. As an illustration of the method, the multi - soliton solutions corresponding to the reflectionless potentials are constructed in terms of the scattering data for CH2.Comment: 22 pages, 3 figures, draft, please send comment

Thermomagnetic analysis of native iron from the upper sedimentary horizons of Lake Baikal, section GC-99 (Posolskaya Bank)

© 2017 We present results of a thermomagnetic analysis of Late Pleistocene-Holocene bottom sediments from the gravity core GC-99 of the borehole BDP-99 drilled at Posolskaya Bank of Lake Baikal in the framework of the Baikal Drilling Project. The results are compared with the earlier analytical data on the samples from the lower (Miocene) section of the BDP-98 drilled on the Akademichesky Ridge. Native-iron particles were found in only 14 of 61 samples. Their content varies from ~ 10 -5 to 10 -4 %, and their distribution is near-bimodal, with a distinct “zero” mode. The results of the thermomagnetic analysis are confirmed by a probe microanalysis: Only occasional native-iron particles were found. Nickel was detected in only one sample. The samples have a large number of magnetite and titanomagnetite grains. It is shown that the distribution of native-iron particles in the Baikal sediments depends on the rate of sedimentation: The rate increase is accompanied by the increase in the number of the “zero” group samples (free of iron particles). The conclusion is drawn that the native-iron particles in the studied sediments are predominantly of cosmic origin

Thermomagnetic analysis of native iron from the upper sedimentary horizons of Lake Baikal, section GC-99 (Posolskaya Bank)

© 2017 We present results of a thermomagnetic analysis of Late Pleistocene-Holocene bottom sediments from the gravity core GC-99 of the borehole BDP-99 drilled at Posolskaya Bank of Lake Baikal in the framework of the Baikal Drilling Project. The results are compared with the earlier analytical data on the samples from the lower (Miocene) section of the BDP-98 drilled on the Akademichesky Ridge. Native-iron particles were found in only 14 of 61 samples. Their content varies from ~ 10 -5 to 10 -4 %, and their distribution is near-bimodal, with a distinct “zero” mode. The results of the thermomagnetic analysis are confirmed by a probe microanalysis: Only occasional native-iron particles were found. Nickel was detected in only one sample. The samples have a large number of magnetite and titanomagnetite grains. It is shown that the distribution of native-iron particles in the Baikal sediments depends on the rate of sedimentation: The rate increase is accompanied by the increase in the number of the “zero” group samples (free of iron particles). The conclusion is drawn that the native-iron particles in the studied sediments are predominantly of cosmic origin

Antiprotons Annihilation in the Galaxy As A Source of Diffuse Gamma Background

The existence of antimatter domains in baryon asymmetrical Universe can appear as the cosmological consequence of particle theory in inflationary models with non-homogeneous baryosynthesis. Such a domain can survive in the early Universe and form globular cluster of antimatter stars in our Galaxy. The model of antimatter pollution of Galaxy and annihilation with matter gas is developed. The proton-antiproton annihilation gamma flux is shown to reproduce the observed galactic gamma background measured by EGRET. From comparison with observational data the estimation on the maximally allowed amount of antimatter stars, possibly present in our Galaxy, is found.Comment: LaTeX2e, 18 pages, 3 PostScript figures. Submitted to Yad.Fi

Shell-Model Effective Operators for Muon Capture in ^{20}Ne

It has been proposed that the discrepancy between the partially-conserved axial-current prediction and the nuclear shell-model calculations of the ratio $C_P/C_A$ in the muon-capture reactions can be solved in the case of ^{28}Si by introducing effective transition operators. Recently there has been experimental interest in measuring the needed angular correlations also in ^{20}Ne. Inspired by this, we have performed a shell-model analysis employing effective transition operators in the shell-model formalism for the transition $^{20}Ne(0^+_{g.s.})+\mu^- \to ^{20}F(1^+; 1.057 MeV) + \nu_\mu$. Comparison of the calculated capture rates with existing data supports the use of effective transition operators. Based on our calculations, as soon as the experimental anisotropy data becomes available, the limits for the ratio $C_P/ C_A$ can be extracted.Comment: 9 pages, 3 figures include