Ferromagnetic Semiconductor EuO Research Articles

The effect of electron-magnon interaction on the band structure of ferromagnetic semiconductors with application to EuO and EuS

The exact nature of the conduction band states in the ferromagnetic semiconductors EuO and EuS at T=0 is investigated. No corrections to standard band theory occur for majority spin states, but shifts and broadening occur in minority spin states due to electron-magnon interaction. Previous work incorporating this effect assumes a non-degenerate s band instead of the more realistic d band used here, but qualitative comparison is possible.

Modified sf model for mixed-valence Eu chalcogenides

EuO exhibits at room temperature a pressure-induced metal-insulator transition of first order into a mixed-valence phase (pc approximately=300 kbar), which is followed by a structural NaCl-CsCl transition at about 400 kbar. For EuS and EuSe only the structural transitions are observed. The author proposes a modified sf model for describing the mixed valency in the ferromagnetic semiconductors EuO, EuS and EuSe. It turns out that the temperature-dependent shift of the conduction band edge, occurring below Tc due to the sf exchange interaction, drives the metal-insulator transition point to substantially lower pressure values. As a consequence of the dynamic interplay between the red shift of the band edge and the 'dilution' of the Heisenberg magnet, caused by f electron fluctuations, the electronic phase transitions change from first to second order with decreasing temperature. At T=0 the authors find the following critical regions: EuO, 200-260 kbar; EuS, 130-160 kbar; euSe, 125-150 kbar. For EuS the electronic collapse happens in the low-temperature region clearly before the NaCl-CsCl transition and should therefore be observable as in EuO.

Problems of the OPW Method III. Rare Earths

AbstractThe application of the modified OPW method for calculating the band structure of rare earths is considered. The effect of the parameters of the MOPW method on the energy bands obtained is systematically studied for Eu and general rules on the choice of these parameters are found. The spin‐polarized band structure of the ferromagnetic semiconductor EuO is determined taking into account these rules. The effect of f‐states of the metal and the (m‐t) approximation of the crystal potential on the compound energy bands is studied.

S−fmodel in magnetic semiconductors

In the $s\ensuremath{-}f$ model electrons in the conduction band are coupled with the lattice of localized magnetic moments by exchange interaction. In this paper the influence of the exchange interaction on electron bands and electrical resistivity is studied. An intermediate-coupling theory is presented, valid up to the region where the level broadening exceeds the average thermal energy but is small compared to the bandwidth. Using the functional-derivative method the spin correlations are expressed in terms of the connected correlation functions. The functional-derivative method also provides a decoupling recipe for the electron Green's functions. Concentrating on the two-spin correlations the single-particle Green's function is derived by a decoupling method and is shown to be equivalent to a perturbation expansion. The finite lifetime is obtained for all band energies. The absorption edge, derived from density of states, shows in ferromagnetic semiconductors the familiar red shift and in addition a blue shift of magnetic origin in the paramagnetic region. Mobility is derived from the two-particle Green's function calculated by a decoupling method. Level broadening affects both the acceleration and the scattering part, resulting generally in smaller mobilities than predicted by the weak-coupling theory. In addition, corrections to the ordinary acceleration term are obtained. Results for the ferromagnetic semiconductors EuS and EuO are presented. The narrow mobility minimum occurring at ${T}_{C}$ in the weak-coupling case is considerably broadened in temperature. The minimum mobilities are around 3 ${\mathrm{cm}}^{2}$/Vsec in both materials. The low mobility near ${T}_{C}$ is partly caused by the intense scattering and partly by the decrease in the acceleration term. The results compare reasonably well with experiments.

Comments on the paper “anti-stokes luminescence in europium monochalcogenides”

We have reported earlier magnon assisted transition in the ferromagnetic semiconductor EuO. Recently, Merlin et al. have published similar experiments, but due to an error in line assignments, they disagree with our interpretation. We recall briefly the experimental features and develop the arguments supporting our point of view.

Surface spin-exchange-scattering of polarized electrons emitted from magnetic materials

It is known that a paramagnetic surface sheet persists far below the Curie temperature in the ferromagnetic semiconductors EuO and EuS. We show that the observed spin-polarization of the photo- and field-emitted electrons is largely determined by spin-exchange collisions in the surface sheet. A single paramagnetic atomic layer may have a depolarizing effect of up to 50 per cent. Other circumstances in which the proposed model might apply are suggested.

Direct determination of Hall mobility of photoelectrons in the ferromagnetic semiconductor EuO

The importance of applying the pulsed Redfield technique in the direct determination of the Hall mobility μH of photocarriers in ferromagnetic semiconductors is exemplified, for the first time, by using pure EuO crystals. The value of μH is obtained directly from the measurement of the Hall angle θ = μHB/c, where B is the magnetic induction and c is the light velocity. Thus, it is shown that a large negative-magnetoresistance effect in undoped EuO crystals near the Curie temperature Tc can be attributed mainly to the mobility change. The photocarriers are identified to be electrons, and a mobility value as high as 250 cm2/V sec is observed at 26 °K.

Optical Reflectance Study of Magnetic Ordering Effects in EuO, EuS, EuSe and EuTe

The reflectivity of the ferromagnetic semiconductors EuO, EuS and EuSe has been measured as a function of temperature and light polarization in an orienting magnetic field. In the energy range from just above the absorption edges to about 5.0 eV, there are two prominent features, E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> and E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , which change with the magnetic order, indicating exchange splittings of the 5d states. Antiferromagnetic EuTe shows a different structure in the E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> and E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> peaks. However, with a large magnetic field applied (>40 kOe) the spectrum becomes characteristic of the other ferromagnetic europium chalcogenides suggesting a phase transition to parallel spin alignment at H ≈ 80 kOe. The effect provides direct experimental confirmation of the band theory prediction that there are superlattice splittings in the band structure of an antiferromagnetic crystal.

Magnetic Ordering Effects in the Reflectance of EuO, EuS, EuSe, and EuTe

In the reflectance spectra of the ferromagnetic semiconductors EuO,1 EuS,2,3 and EuSe2,3 and antiferromagnetic EuTe,4 there are two prominent peaks, E1 and E2, which involve 4f7→4f65d(t2g) and 4f7→4f65d(eg) transitions respectively. Measurements of E1 and E2 in the ferromagnetic compounds as a function of temperature and polarization in a magnetic field show changes with magnetic ordering which indicate exchange splittings of the 5d states of about 0.25 eV. In antiferromagnetic EuTe above 40 kOe applied field, the spectrum becomes characteristic of the ferromagnetic chalcogenides suggesting a transition to parallel spin alignment which saturates at H∼80 kOe. The observed effects also indicate confirmation of the prediction that there are superlattice splittings in the band structure of an antiferromagnetic crystals.