Ferromagnetic EuS Research Articles

The crossover from atomistic to continuous dynamic symmetry at the magnetic phase transition

It is suggested that the change from Curie–Weiss susceptibility to critical susceptibility has to be identified as crossover from atomistic to continuous dynamic symmetry. At this crossover the correlation length becomes of the order of the near neighbour distance. This enables dynamic percolation for lower temperatures and crossover to continuous symmetry. Continuous dynamic symmetry implies universality, i.e. a temperature dependence that is independent of the atomistic structures. This is in disagreement with the material specific magnon dispersion relations evaluated by inelastic neutron scattering. On the basis of these magnon dispersions the observed universality at the stable fixed point T = 0 cannot be explained. Instead a linear dispersion has to be postulated for all magnets. According to Goldstone, Salam and Weinberg (GSW) the quasi particles of this linear dispersion are non-interacting and massless. No interaction between the quasi particles requests that they have no magnetic moment and, as a consequence, must be spin compensated, in a similar way as is known from the Cooper pairs of superconductivity. Observation by neutron scattering therefore is impossible. The magnetic specific heat provides, so far, the only indirect access to the hypothesized spin-less magnetic quasi particles. We investigate experimental data of the ferromagnet EuS as a simple model material.

Correlation effects in the valence bands of ferromagnetic semiconductor EuS

AbstractWe present a many body analysis of the multi‐band Kondo lattice model. The study is then combined with the first principles TB‐LMTO band structure calculations, in order to investigate the temperature dependent correlation effects in the 3p valence bands of the ferromagnetic semiconductor EuS. Some of the physical properties of interest like the quasi‐particle density of states (Q‐DOS), spectral density (SD) and quasi‐particle band structure (Q‐BS) are calculated and discussed. Therewith, we propose a spin resolved ARPES of the valence bands of EuS to be performed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spontaneous spin-filter effect across EuS∕GaAs heterojunction

We study current transport across a EuS∕GaAs heterojunction (HJ) modulated by the filtering action of the ferromagnetic semiconductor EuS in zero external magnetic field. Analysis of the current-voltage (I-V) characteristics of the current injected from EuS into GaAs across the HJ yields a value for the Zeeman splitting of the EuS conduction band of (0.48±0.12)eV at 5 K. The change in the barrier height at the HJ mimics the change of the spontaneous magnetization of EuS, i.e., it has Brillouin like characteristics with a TC of 17 K. Utilizing the experimentally obtained values for the Zeeman splitting as input parameters, we analyze the I-V characteristics for unpolarized electrons injected from GaAs, to estimate the polarization detection efficiency as a function of bias and temperature below 30 K.

Temperature dependence of the spontaneous magnetisation of ferromagnetic insulators: Does it obey the [formula omitted] law?

Temperature dependence of the spontaneous magnetisation is analysed for three archetypal ferromagnetic insulators, EuS, EuO and CrBr 3. The main point is the shape, rather than scale, of the curve M s ( T ) . As far as the available data enable us to judge, this shape is adequately described by the simple “ 3 / 2 – 5 / 2 – β ” expression ( m = [ 1 − s τ 3 / 2 − ( 1 − s ) τ 5 / 2 ] β ), proposed previously for metallic ferromagnets, and depends on a single dimensionless parameter s. The lack of direct experimental data makes this conclusion a preliminary one. In order to resolve the question definitively, accurate magnetic measurements on single crystals are required as a matter of priority.

II‐VI and IV‐VI Diluted Magnetic Semiconductors — New Bulk Materials and Low‐Dimensional Quantum Structures

AbstractFor Abstract see ChemInform Abstract in Full Text.

Some applications of polarized inelastic neutron scattering in magnetism

A brief account of applications of polarized inelastic neutron scattering in condensed matter research is given. We show that full polarization analysis is the only tool allowing to discriminate unambiguously between different magnetic modes in various magnetic materials. We show by means of recent results in the Heisenberg ferromagnet EuS that the effects of dipolar interactions can be studied on a microscopic scale. Moreover, we have found for the first time indications for the divergence of the longitudinal fluctuations belowT c. In the itinerant antiferromagnet chromium we demonstrate that the dynamics of the longitudinal and transverse excitations are very different, resolving a long standing puzzle concerning the slope of their dispersion. Finally, we show that a measurement of the polarization-dependent part of the cross section of non-centrosymmetric MnSi proves directly that the chirality of the magnetic fluctuations is left-handed.

Magnetic and structural properties of EuS for magnetic tunnel junction barriers

In view of potential applications as a spin filter in spintronic devices, we systematically studied the growth by sputtering of ferromagnetic EuS barriers. The relationship between growth and magnetic and transport properties, also in combination with magnetic and nonmagnetic materials, was investigated. We demonstrate that growth at lower substrate temperatures (200 °C), followed by an anneal step at elevated temperatures (430 °C), leads to improved magnetic and transport properties of the barrier layer. We tentatively attribute the observed low-temperature magnetoresistance of high-resistive Al/PbS/EuS/PbS/Gd devices to spin filtering.

Ferromagnetic EuS films: Magnetic stability, electronic structure, and magnetic surface states

We present the temperature and layer dependent electronic structure of a 20-layer EuS(100)-film using a combination of first-principles and model calculation, the latter based on the ferromagnetic Kondo-lattice. The calculated thickness-dependent Curie temperature agrees very well with experimental data. The projected 5d-bandstructure is at finite temperatures strongly influenced by damping effects due to spin exchange processes. Spin-split unoccupied 5d-surfaces states are found with a Stoner-like collapsing for increasing temperature towards the Curie point and with an exponential decay of spectral weight with increasing distance from the surface.

Antiferromagnetic interlayer coupling in EuS/YbSe superlattices

Semi-insulating EuS/YbSe superlattices were fabricated and studied both experimentally and theoretically. Antiferromagnetic correlations between ferromagnetic EuS layers were observed in neutron reflectivity spectra for samples with thin spacer layers (up to 20 Å). This is explained by a model in which the interlayer exchange coupling is dependent upon the electronic band structure of the superlattice. The calculated strength and the range of the coupling are in qualitative agreement with the experimental findings.

Ferromagnetic Kondo-lattice model

We present a many-body approach to the electronic and magnetic properties of the (multiband) Kondo-lattice model with ferromagnetic interband exchange. The coupling between itinerant conduction electrons and localized magnetic moments leads, on the one hand, to a distinct temperature-dependence of the electronic quasiparticle spectrum and, on the other hand, to magnetic properties, as e.~g.the Curie temperature T_C or the magnon dispersion, which are strongly influenced by the band electron selfenergy and therewith in particular by the carrier density. We present results for the single-band Kondo-lattice model in terms of quasiparticle densities of states and quasiparticle band structures and demonstrate the density-dependence of the self-consistently derived Curie temperature. The transition from weak-coupling (RKKY) to strong-coupling (double exchange) behaviour is worked out. The multiband model is combined with a tight-binding-LMTO bandstructure calculation to describe real magnetic materials. As an example we present results for the archetypal ferromagnetic local-moment systems EuO and EuS. The proposed method avoids the double counting of relevant interactions and takes into account the correct symmetry of atomic orbitals.

Vertical Electron Transport through PbS-EuS Structures

Temperature dependence of current-voltage I-V characteristics and resistivity is studied in ferromagnetic PbS-EuS semiconductor tunnel structures grown on n-PbS (100) substrates. For the structures with a single (2-4 nm thick) ferromagnetic EuS electron barrier we observe strongly non-linear I-V characteristics with an effective tunneling barrier height of 0.3-0.7 eV. The experimentally observed non-monotonic temperature dependence of the (normal to the plane of the structure) electrical resistance of these structures is discussed in terms of the electron tunneling mechanism taking into account the temperature dependent shift of the band offsets at the EuS-PbS heterointerface as well as the exchange splitting of the electronic states at the bottom of the conduction band of EuS.

Interlayer Coupling in EuS-Based Superlattices (Dependence on the Energy Structure of Non-Magnetic Layer)

The interlayer coupling between ferromagnetic EuS layers separated by spacer layers of diamagnetic insulators, YbSe and SrS, is studied within a 3D tight-binding model. The dependencies of the coupling strength on the energy structure of the spacer, on strains resulting from the lattice mismatch between the superlattice constituents, as well as on an applied hydrostatic pressure and lattice deformations, are presented. The sign and the range of the obtained coupling agree with the behavior of magnetic correlations observed recently in neutron reflectivity spectra of EuS/YbSe superlattices.

Search for Spin Filtering By Electron Tunneling Through Ferromagnetic EuS Barriers in Pbs

Perpendicular transport through single- and double-barrier eterostructures made up of ferromagnetic EuS layers embedded into a PbS matrix was investigated. Both resonant tunneling and probably spin filtering through EuS barrier were observed.

Controlling the magneto-transport properties of EuS thin films

The growth of thin films of the ferromagnetic semiconductor EuS on [100] oriented GaAs substrates by electron-beam evaporation is demonstrated. Structural characterization by X-ray diffraction reveals [100] oriented growth. In general, the magnetic and transport properties can strongly be influenced by the deposition parameters. We systematically investigate the influence of the growth temperature T/sub S/ in the range T/sub S/=235 K-675 K on the above properties. The Curie temperature increases with decreasing growth temperature, while the magnetic transition becomes broader. The resistivity decreases over four orders of magnitude with decreasing substrate temperature. The behavior of the magnetic properties and the resistivity can be explained by a change in stoichiometry, leading to higher carrier concentrations for lower substrate temperatures. Hall-effect measurements, magnetoresistance, and temperature dependence of the resistivity show a qualitative behavior known from bulk EuS samples. The demonstrated tunability of the magnetic and transport properties opens a wide range of possible applications for EuS in spin-electronics.

Temperature-dependent quasiparticle band structure of the ferromagnetic semiconductor EuS

We present calculations for the temperature-dependent electronic structure of the ferromagnetic semiconductor EuS. A combination of a many-body evaluation of a multiband Kondo-lattice model and a first-principles $T=0$--band-structure calculation [tight-binding linear muffin-tin orbital (TB-LMTO)] is used to get realistic information about temperature- and correlation effects in the EuS energy spectrum. The combined method strictly avoids double-counting of any relevant interaction. Results for EuS are presented in terms of spectral densities, quasiparticle band structures, and quasiparticle densities of states, and that over the entire temperature range.

Studies of Interlayer Magnetic Coupling in All-Semiconductor Superlattices by Means of Neutron Scattering Techniques

An overview of neutron scattering studies of ferromagnetic and antiferromagnetic all-semiconductor superlattices is presented. Diffraction experiments on MnTe/CdTe, MnTe/ZnTe and EuTe/PbTe superlattices show pronounced correlations between the MnTe and EuTe layers across the non-magnetic spacers, even though these layers are antiferromagnetic and the systems are nearly-insulating. Current theory status of these systems is discussed. Diffractometry and reflectometry data from EuS/PbS superlattices reveal pronounced antiferromagnetic coupling between the ferromagnetic EuS block. First polarized neutron reflectometry data from superlattices prepared of a novel ferromagnetic spintronics material, Ga(Mn)As, are also presented.

Damping of spin waves and singularity of the longitudinal modes in the dipolar critical regime of the Heisenberg ferromagnet EuS

By inelastic scattering of polarized neutrons near the (200)-Bragg reflection, the susceptibilities and linewidths of the spin waves and the longitudinal spin fluctuations were determined separately. By aligning the momentum transfers q perpendicular to both \delta S_sw and the spontaneous magnetization M_s, we explored the statics and dynamics of these modes with transverse polarizations with respect to q. In the dipolar critical regime, where the inverse correlation length kappa_z(T) and q are smaller than the dipolar wavenumber q_d, we observe:(i) the static susceptibility of \delta S_sw^T(q) displays the Goldstone divergence while for \delta S_z^T(q) the Ornstein-Zernicke shape fits the data with a possible indication of a thermal(mass-)renormalization at the smallest q-values, i.e. we find indications for the predicted 1/q divergence of the longitudinal susceptibility; (ii) the spin wave dispersion as predicted by the Holstein-Primakoff theory revealing q_d=0.23(1)\AA^{-1}in good agreement with previous work in the paramagnetic and ferromagnetic regime of EuS; (iii) within experimental error, the (Lorentzian) linewidths of both modes turn out to be identical with respect to the q^2-variation, the temperature independence and the absolute magnitude. Due to the linear dispersion of the spin waves they remain underdamped for q<q_d. These central results differ significantly from the well known exchange dominated critical dynamics, but are quantitatively explained in terms of dynamical scaling and existing data for T>=T_C. The available mode-mode coupling theory, which takes the dipolar interactions fully into account, describes the gross features of the linewidths but not all details of the T- and q-dependencies. PACS: 68.35.Rh, 75.40.Gb

Kerr Effect Investigations of Magnetic Interlayer Interactions in EuS-PbS Multilayers

K err magnet ometr y was employe d to study the temp erature dependence of magnet izati on and magnetic hysteresis loops in ferromagnetic EuS{ PbS semiconductor multilayers in the temp erature range T = 3 A 35 K at low mag - netic Uelds H c 1 5 0 Oe. For EuS{PbS /K Cl( 100) structures with ultrathi n non- magnetic PbS spacer of 1 nm , we observed a maximum on the temp erature dep endence of magnetizati on at low Uelds H c 30 Oe. For higher Uelds, we found for these structures a regular mean- Ueld-li ke increase in magnetization with decreasing temp erature. T he same regular beha vior was also found for EuS{PbS/K Cl structures with thicker PbS spacer, as well as for all EuS{PbS/ Ba F2 (111) multilayers indep endently of spacer thickness. For qualitati ve interpretation of these Undings, we consider two magnetic contributions to the total energy of EuS{PbS multilayers : the Zeeman energy and the antif erromagnetic interlayer exchange coupling betw een ferromagnetic EuS layers via diamagnetic PbS spacer.

Experimental identification of fourth-order exchange interactions inmagnets with pure spin moments

It is shown experimentally, that fourth-order exchange interactions, i.e. biquadratic, three-spin and four-spin interactions, are able to create aparticular order parameter which we have called O4. Consistently, theordering type of O4 always conforms to the sign of the fourth-orderinteraction sum evaluated from measurements of the cubic susceptibilityχ3. Earlier investigations suggest that O4 can be identified withthe expectation value of the transverse spin component, ⟨Sx⟩, while the conventional (Heisenberg) order parameter, O2, isgiven by ⟨Sz⟩. Therefore, the observed ordering temperatureof O4 is never larger than the ordering temperature of O2. Theexperimental signatures of O4 are illustrated using the cubic pure spinmagnets Eu0.75Sr0.25Te, GdAg, GdMg, EuS and EuO as examples.These materials provide all ferromagnetic and antiferromagneticcombinations for O2 and O4. For the ferromagnets EuS and EuO,second-order and fourth-order interactions are known to be ferromagnetic.This is the most complicated situation for the identification of O4. Acsusceptibility measurements performed at different angles to an appliedstatic magnetic field reveal the conventional rotational symmetric statearound the field axis. However, measurements in the critical temperaturerange indicate a discontinuous rise of O2 which is in contrast to allhitherto reported results but in agreement with mean field predictions. Inmost antiferromagnets in which susceptibility measurements revealferromagnetic O4 the associated ordered moment is usually too small tobe detected with neutron scattering without polarization analysis.

Magnetic polaritons in Fibonacci quasicrystals

A macroscopic theory is employed to investigate the magnetic polariton spectra in pure and generalized Fibonacci magnetic quasicrystals. The polariton spectra are evaluated in the geometry where the wave vector and the applied field are in the same plane (Voigt geometry). They are calculated for both ferromagnetic and antiferromagnetic orders by using a transfer-matrix approach. Numerical results for the ferromagnets EuS and for the antiferromagnet MnF 2 are presented to discuss the fractal aspect of the spectra, including the localization of the modes as well as their scaling properties.