We investigate the second-order phase transitions in non-magnetic wurtzite ZnO and magnetic doped with cobalt. Using reformulated Landau-Lifshitz theory of second-order phase transitions and our computer program, we have found all possible lower space group symmetries of ZnO and ZnO doped with Co as well as symmetries of vibration modes which may cause structural phase transitions. We interpret the Raman phonon modes of magnetic doped ZnO according to corepresentations of the magnetic space group P6'₃m'c (ZnO magnetic). Some experimental techniques like X-ray diffraction, reflectivities and Raman spectroscopies can verify our theoretical results.
Magnetoreflectance and magnetization of ZnCrSe were measured for B ≤ 5 T and T = 2 K. A linear dependence between exciton splitting and magnetization was found. The s, p-d exchange parameter was estimated: N_{0}α-N_{0}β=0.67eV.
In this paper the present understanding of the origin of ferromagnetic response that has been detected in a number of diluted magnetic semiconductors and diluted magnetic oxides at room temperature is outlined. It is argued that in these systems, owing to a typically small solubility of magnetic ions, crystallographic or chemical phase separation into regions with a large and a small concentration of magnetic component takes place. The ferromagnetic signatures then come from the regions with a large concentration of magnetic ions, which show non-zero spontaneous magnetization up to the blocking temperature, whose magnitude is proportional to the nanocrystal volume and magnetic anisotropy. Novel methods enabling a control of nano-assembling of magnetic nanocrystals in non-conducting matrices as well as possible functionalities of these spatially modulated magnetic systems are described. We also discuss phase separations into paramagnetic and ferromagnetic regions, which are driven by the Anderson-Mott localization and/or competing ferromagnetic and antiferromagnetic interactions. Finally, the question whether the high temperature ferromagnetism is possible in materials without magnetic ions is addressed.
Light induced remanent changes in magnetization of n-type Cd_{1-x}Mn_{x} Te (x=0.01, 0.05) single crystals have been measured at T=2 and 5 K and in magnetic fields of up 0.5 T. The effect observed gradually saturates with increasing magnetic field. It also correlates with light induced increase in shallow donor concentration measured on the same samples. The bound magnetic polaron theory accounts for the temperature and magnetic field variations of the persistent magnetization. There are no fitting parameters.
The effective magnetic momentum p_{eff} of Gd-impurities in the Pb_{1-x}Sn_{x}Te⟨Gd⟩ crystals as a function of crystal composition x with 0 ≤ x ≤ 0.25 is studied. It is shown that with the increase in tin content in the crystals from 0 to 0.15, p^{2}_{eff}(Gd) increases slowly from 37 to 43. Then p^{2}_{eff}(Gd) increases abruptly to 63 value when x = 0.16±0.01. For subsequent increase x ≥ 0.17 p^{2}_{eff}(Gd) remains constant at 63. Possible reasons of observed changes of p_{eff}(Gd) and possible mechanisms of influence of the Pb_{1-x}Sn_{x}Te crystal field on the effective magnetic momentum value of Gd-impurities in the crystals under study are analysed.
We report results of magnetization study of EuS/PbS superstructures with different thicknesses of magnetic and nonmagnetic layers. Reduction of ferromagnetic phase transition temperature was found with decreasing EuS thickness. Reasonable description of this effect is obtained within the model based on the mean field approximation.
The energy gap and magnetic susceptibility of Cd_{0.85}Fe_{0.15}Se were measured in function of temperature. Experiments showed that the magnetic contribution to the variation of the energy gap in Cd_{1-x}Fe_{x}Se is not proportional to the product of magnetic susceptibility and temperature as it has been observed in Mn^{++}-containing semiconductors.
Possible symmetry of modes those may cause transitions in magnetic and non magnetic crystals as well as lower space subgroup symmetries of the crystals have been found. The Landau-Lifshitz theory for non magnetic crystals has been reformulated for magnetic crystals and has been applied to Ca₃Al₂(SiO₄)(Oₕ¹⁰). Some experimental data confirm our results.
In recent years transition metal ion doped ZnO has been the focus of intense scientific enquiry. ZnO:Co is by far the most studied member of this family. However, despite the many experimental claims for ferromagnetism, a definitive theoretical explanation for long range magnetic interaction has so far been lacking. Here we present a mechanism which describes not only the origin of this previously inexplicable magnetism but also explains the experimental findings to date and in addition, suggests a recipe for tailoring the magnetic properties. We demonstrate that the magnetism originates from a Co^{2+} oxygen vacancy pair with a partially filled level close to the ZnO conduction band minimum. The long range coupling then occurs via conduction electrons at moderate n doping. This however is not sufficient for room temperature ferromagnetism due to the prohibitive concentrations of O vacancies needed. The experimental findings may then be explained by a combination of our proposed mechanism and the formation of blocked superparamagnetic clusters.
The magnetoabsorption and the Faraday rotation of ZnCrSe were measured for B ≤ 5 T and T = 2 K in the region of photon energies lower than the fundamental absorption edge. The circular dichroism of the photoionization absorption bane was observed and analyzed. The Faraday rotation was interpreted as due to the sum of contributions from interband and photoionization optical transitions.
Investigations of the photoluminescence of PbS-EuS superlattices deposited on (111)BaF_{2} substrates are presented. Quantum-size and deformation effects in photoluminescence spectra are observed. The strain-induced gap shift and valence-band offset is determined from experimental results. A strong stimulated photoluminescence with relatively low threshold was observed. It was found that the photocarriers generated in EuS barrier strongly affect the population of PbS subbands.
Magnetization of 1 μm thick ferromagnetic IV-VI (Ge, Mn)Te semiconductor layers with 10 at.% of Mn was studied by SQUID magnetometry method up to the magnetic fields of 70 kOe. The layers were grown on BaF₂ (111) substrates by molecular beam epitaxy with varying Te molecular flux, which permitted the control of layer stoichiometry and conducting hole concentration. X-ray diffraction and in situ electron diffraction characterization of layer growth and crystal structure revealed two-dimensional mode of growth and monocrystalline rhombohedral crystal structure of (Ge, Mn)Te layers. Controlling the layer stoichiometry influences the temperature dependence of magnetization with the ferromagnetic Curie temperature varying in Ge_{0.9}Mn_{0.1}Te layers from T_c=30 K (low Te flux) to T_c=42 K (high Te flux).
The first direct measurement of the magnetization of donor bound magnetic polarons in diluted magnetic semiconductors is reported. The experiment has been performed taking advantage of photomemory effect found in n-type Cd_{1-x}Mn_{x}Te_{1-y}Se_{y} crystals doped with In. Good agreement between experimental results and theory of bound magnetic polarons is observed.
Resonant Raman scattering was measured for (Zn,Fe)Se and (Zn,Cr)Se with the magnetic ion mole fraction below 0.02. LO phonons and intra-Fe^{++} transitions were observed. Polarization measurements versus magnetic field allowed to obtain polarization lifetimes (polariton flight times) in the sub-picosecond range.
The ac magnetic susceptibility (χ) was measured in the temperature range T = (1.5÷50) K for Pb_{1-x-y}Sn_{y}Mn_{x}Te samples with a composition y = 0.72, x = 0.015÷0.04 and carrier concentrations in the range p = (2.85÷14) × 10^{20} cm^{-3}. The breakdown of ferromagnetism and the formation of a spin glass phase are observed with an increasing carrier concentration. It is experimentally evidenced as continuous deviation of χ(T) from the characteristic critical behavior observed for ferromagnetic samples.
Monocrystalline thin layers of (Eu,Gd)Te, n-type ferromagnetic semiconductor, were grown by molecular beam epitaxy technique on BaF_2 (111) substrates. Reflection high-energy electron diffraction, X-ray diffraction, and atomic force microscopy characterization proved epitaxial mode of growth and high crystal quality of the layers. Magnetic susceptibility and magnetic resonance measurements showed that in (Eu,Gd)Te layers ferromagnetic transition takes place at about 13 K. Electrical characterization carried out by the Hall effect and resistivity measurements revealed very high electron concentration of 10^{20}~cm^{-3} and sharp maximum of resistivity at transition temperature.
Percolation thresholds, p_{c}, for site diluted spin systems on the fcc lattice are determined for exchange interactions extending up to the shell of the fourth nearest neighbors. If the interactions include the nearest, second, third, and fourth neighbors, p_{c} is equal to 0.198, 0.136, 0.061, and 0.05 respectively. These results agree with the Roberts approximate formula for p_{c}. Estimation of p_{c} to even longer-ranged couplings is presented. For instance for p_{c} = 0.01 the range of the couplings should extend at least to the eight shell of neighbors.
We present the results of the low temperature electron paramagnetic resonance (EPR) and transport investigations of the crystals of Pb_{1-x}Cr_{x}Te (x ≤ 0.01). The samples with chromium concentrations x ≥ 0.0015 are all n-type. For these samples we observe the single EPR line with the g-factor decreasing from g = 1.97 till g = 1.93 with increasing carrier concentration. This resonance can be attributed to electrically and magnetically active Cr^{3+} ions. The crystals with Cr concentration x ≤ 0.0015 may be both n- and p-type. The EPR spectrum of these samples consists of two lines: the one discussed above and the other one with g = 1.99 observed only for samples with electron concentration n ≤ 10^{18} cm^{-3}.
We present the results of the experimental studies of the low temperature transport and magnetic properties of PbTe, Pb_{1-x}Sn_{x}Te (x ≤ 0.3) and SnTe crystals doped with 0.5 at.% of chromium. Cr was found to be a resonant donor in PbTe and PbSnTe. Magnetic susceptibility measurements revealed that PbTe:Cr and Pb_{1-x}Sn_{x}Te:Cr (x ≤ 0.2) are Curie paramagnets whereas SnTe:Cr exhibits van Vleck paramagnetism.
We study magnetic effects in a trilayer formed of magnetic layers separated by a spacer with a parabolic potential profile. The focus is on mechanisms of indirect magnetic interactions within the spacer. We show existence of magnetic oscillations and novel type of magnetoelectric effect.
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