The results of experimental studies of magnetoresistance oscillations are described for a HgSe monocrystal with Co impurities of low concentration (<1 at%). It has been discovered that at such impurity concentrations (~1018 cm-3) there exists two types of magnetic oscillations: Shubnikov de Haas oscillations at low temperatures (T<10 K) and magnetophonon oscillations at higher temperatures (T>10 K). The first ones are caused by the interaction of electrons with magnetic fieled inside of a Landau sublevel and the latter --- by the interaction with longitudinal optical phonons. The differences of these oscillations are characterized. A hypothesis of a nature of magnetophonon oscillations in such structures is proposed. Keywords: 3d-impurities of low concentration, zero-gap semiconductors, magnetophonon oscillations, SdH oscillations, optical and acoustic phonons.

On the single crystals of Hg1 – xFexSe (х ≤ 0.06 at %) with an extremely low concentration of the impurity iron atoms at room temperature (T = 300 K), high-temperature ferromagnetism of a new type in a impurity donor electron system was experimentally discovered.

We investigate room-temperature structural properties and spontaneous magnetism in a series of Hg1-xFexSe (0.00012 ≤ x ≤ 0.0013) diluted magnetic semiconductor bulk single crystals grown by the Bridgman method. In order to elucidate the experimentally observed high-temperature magnetism in the studied systems, the dependencies of magnetization on magnetic field strength for the entire sample series were studied. Their detailed analysis revealed impurity contributions typical of ferromagnetic materials, which are described by magnetization curves with the parameters characterizing the spontaneous spin magnetism of the donor conduction electrons of the outer d-shells of the impurity atoms. According to the previously-developed theoretical concept, such a magnetic ordering mechanism stems from the direct exchange coupling of donor conduction electrons due to hybridization of impurity states, rather than from inter-impurity interactions. The results of optical emission spectroscopy and X-ray diffraction analysis confirmed the observed ferromagnetism to be intrinsic and linked directly to the d-electrons of iron impurity atoms.

Experiments reveal a new type of high-temperature ferromagnetism in the donor impurity electron system of Hg1 − xFexSe (х ≤ 0.06 at %) single crystals with an ultralow concentration of iron impurities at room temperature (T = 300 K).

Представлены результаты экспериментального исследования осцилляций магнитосопротивления в монокристалле HgSe с примесями кобальта низкой концентрации (<1 ат%). Обнаружено, что при таких концентрациях примесей (~1018 см-3) наблюдаются два вида магнитных осцилляций: осцилляции Шубникова-де-Гааза при низких температурах (T<10 K) и магнитофононные осцилляции при температурах (T>10 K). Первые из них обусловлены взаимодействием с магнитным полем внутри подзоны Ландау, а вторые --- взаимодействием с продольными оптическими фононами. Продемонстрированы различия в свойствах этих видов осцилляций. Сделаны предположения о возможном происхождении магнитофононных осцилляций в этих структурах. Ключевые слова: магнитофононные осцилляции, осцилляции ШдГ, оптические и акустические фононы.

The given work is devoted to the experimental proof of existing the spontaneous spin polarization of the donor electron system of 3d-transition element impurity atoms of low concentration (<1 at.%) in a mercury selenide crystal. For this purpose there have been measured the dependences of the magnetization on the magnetic field strength. As a result of the analysis of the obtained dependences, there were extracted the impurity contributions, which are described by the magnetization curves typical of the ferromagnets, and by the magnetic parameters conforming to the spontaneous magnetism of the systems under study, which are unambiguously related to the donor conduction electrons of the outer d-shells of impurity atoms. By its nature, according to the developed theoretical concepts, the spontaneous spin polarization manifests itself in exchange interaction, taking place in hybridizing the electronic states of the impurity atom and the conduction band ones of the crystal.

The given work is devoted to the experimental proof of existing the spontaneous spin polarization of the donor electron system of 3d-transition element impurity atoms of low concentration (<1 at.%) in a mercury selenide crystal. For this purpose there have been measured the dependences of the magnetization on the magnetic field strength at low temperatures (T=5 K). As a result of the analysis of the obtained dependences, there were extracted the impurity contributions, which are described by the magnetization curves typical of the ferromagnets, and by the magnetic parameters conforming to the spontaneous magnetism of the systems under study, which are unambiguously related to the donor conduction electrons of the outer d-shells of impurity atoms. By its nature, according to the developed theoretical concepts, the spontaneous spin polarization manifests itself in exchange interaction, taking place in hybridizing the electronic states of the impurity atom and the conduction band ones of the crystal. Keywords: 3d-impurities of the low concentration, gapless semiconductors, hybridized electronic states, spontaneous spin polarization, low-temperature ferromagnetism

ABSTRACT The Heusler alloys are promising material for various applications, including thermoelectric power generators. There are several ways to improve their properties, among which the substitution of constituent elements, nanostructuring, temperature treatment, thin films engineering etc. This work is devoted to the experimental study of thermoelectric properties of Fe–V–Al-based Heusler alloys with different compositions under high pressure. The Seebeck coefficient and electrical resistance of several compounds were measured as a function of applied pressure up to 10 GPa at room temperature. The experimental results demonstrated a diversity of pressure responses of the thermoelectric properties that can be observed in chemically similar Heusler alloys under variation in their compositions. Moderate enhancing of the thermoelectric power factor, S 2/r (where S is the Seebeck coefficient and r is the electrical resistivity) was observed in Fe-rich Fe2.1V0.91Al0.99 compound at a pressure about ∼1 GPa. It was found that all alloys with weak deviation from stoichiometric composition had a feature in their S (P) pressure behavior at P∼2 GPa which could be related to semimetal-metal phase transition.

Features of magnetic-field-dependent microwave absorption in HgSe samples doped with Co and Ni impurities in different concentrations are investigated. The electron-spin resonance spectra of weakly coupled Co atoms and peculiarities of the magnetic-absorption variation in a magnetic field passing through the zero value are established. The main parameters of the electron-spin resonance spectra and temperature and angular dependences of microwave absorption in weak fields are determined.

The results of an experimental study and theoretical description of the magnetic field dependences of the magnetization of a mercury selenide with low concentrations of iron impurities (0.01–0.2) at% at a temperature of 5 K are presented. The contributions of the spontaneous magnetism of the electron system belonging to the iron atom donor impurity hybridized states, which are shaped like saturation magnetization curves, are highlighted by performing a detailed analysis of the obtained curves. For comparative purposes, similar measurements were carried out on mercury selenide crystals without doping and with gallium impurities (0.05 at%), which allowed confirmation of the data interpretation details. Based on previously developed theoretical concepts about the electron systems under study, formulas describing the field dependences of spontaneous magnetization were obtained, and found to be in good agreement with the experimental curves. As a result of fitting the theoretical dependences to the experimental curves, the parameters characterizing the spontaneous spin magnetism of the studied impurity systems were determined; their values agree with the experimental data obtained earlier when observing Hall effect anomalies and temperature dependences of the magnetic susceptibility in the same systems.

AbstractFeatures of magnetic-field-dependent microwave absorption in HgSe samples doped with Co and Ni impurities in different concentrations are investigated. The electron-spin resonance spectra of weakly coupled Co atoms and peculiarities of the magnetic-absorption variation in a magnetic field passing through the zero value are established. The main parameters of the electron-spin resonance spectra and temperature and angular dependences of microwave absorption in weak fields are determined.

For the first time, magnetoresistive properties of the single crystal of HgSe with a low electron concentration were studied in wide range of temperature and magnetic field. Some fundamental parameters of spectrum and scattering of electrons were experimentally determined. Two important features of magnetic transport were found - strong transverse magnetoresistance (MR) and negative longitudinal MR, which can indicate the existence of the topological phase of the Weyl semimetal (WSM) in HgSe. Taking this hypothesis into account we suggest a modified band diagram of the mercury selenide at low electron energies. The obtained results are essential for the deeper understanding of both physics of gapless semiconductors and WSMs - promising materials for various applications in electronics, spintronics, computer and laser technologies.

The results of studies of the magnetic-field dependencies of the specific magnetization M(H) of a mercury selenide single crystal with a low concentration (0.01–0.035) at.% of cobalt impurities at a temperature of 5 K are presented. The contributions of the spontaneous magnetism of the electron system of hybridized states of donor impurity cobalt atoms, which have the form of magnetization curves with saturation, were revealed for the first time as a result of detailed precision analysis of the obtained experimental data. Certain magnetic parameters characterizing the spontaneous spin magnetism of the studied system agree with previous results obtained during observation of anomalies of the Hall effect and the temperature dependencies of the magnetic susceptibility associated with the manifestation of the same system. Therefore, in the present work, a solution to the problem of justifying and describing the spontaneous spin polarization of the electron systems of impurities of transition elements to the extent of their small concentration was developed in sufficient detail.

AbstractCharacteristic effects of magnetic ordering and conduction in semiconductor heterostructures with a GaAs : Be/Ga_0.84In_0.16As/GaAs quantum well and manganese δ-layers of different thickness (from 0.4 to 2 monolayers) were studied based on analysis of magnetic field and temperature dependences, galvanomagnetic effects, and magnetization. An anomalous dependence of the conductivity on the manganese atoms concentration in the δ-layer was observed, which was due to a strong scattering of charge carriers in the structures with the low content of magnetic impurities. Magnetic properties of the heterostructures clearly indicated the magnetic ordering of the impurity system (saturation and hysteresis of the magnetization and fulfillment of the Curie–Weiss law at increasing temperature). Parameters of the magnetic subsystem allowed revealing different types of ordering in the systems with different concentrations of the magnetic impurity. Changing the concentration of the Mn admixture in the δ-layer was shown to influence significantly the conductivity and magnetism in the studied structures.

Characteristic effects of magnetic ordering and conduction in semiconductor heterostructures with a GaAs : Be/Ga0.84In0.16As/GaAs quantum well and manganese δ-layers of different thickness (from 0.4 to 2 monolayers) were studied based on analysis of magnetic field and temperature dependences, galvanomagnetic effects, and magnetization. An anomalous dependence of the conductivity on the manganese atoms concentration in the δ-layer was observed, which was due to a strong scattering of charge carriers in the structures with the low content of magnetic impurities. Magnetic properties of the heterostructures clearly indicated the magnetic ordering of the impurity system (saturation and hysteresis of the magnetization and fulfillment of the Curie–Weiss law at increasing temperature). Parameters of the magnetic subsystem allowed revealing different types of ordering in the systems with different concentrations of the magnetic impurity. Changing the concentration of the Mn admixture in the δ-layer was shown to influence significantly the conductivity and magnetism in the studied structures.

Quantum oscillations of the anomalous component of Hall resistance with an amplitude exceeding the amplitude of the Shubnikov-de Haas oscillations of transverse magnetoresistance are observed in mercury selenide crystals doped with low concentrations of cobalt impurity. In accordance with the predictions of the Hall effect theory for systems with spontaneous spin polarization of hybridized donor electrons, the observed oscillations correspond to magnetic quantum oscillations caused by the thermodynamic anomalous Hall effect.

The differential method of the study of the Hall effect in the regime of the Hall current, the implementation of which is possible due to an inhomogeneous distribution of the concentration of charge carriers in the sample, has been proposed. This method was tested in crystals of HgSe: Co and HgSe: Ga with the aim to observe the anomalous contribution to the Hall current associated with the spontaneous magnetization current and perform comparative analysis of the degree of heterogeneity of the samples.

The given report is devoted to the study of anomalous Hall resistance of donor electron system of hybridizedstates of transition element impurities of low concentration in quantum oscillation regime. There presented theoretical description of predicted specific behaviors on the base of the ideas about thermodynamic anomalous Hall effect. In experiments on mercury selenide crystals with cobalt impurities of low concentration one revealed the quantum oscillations of anomalous contribution to the Hall resistance corresponding to the developed concepts.

Предложен дифференциальный метод исследования эффекта Холла в режиме тока Холла, реализация которого является возможной благодаря неоднородному распределению в образце концентрации носителей заряда. Этот метод апробирован в кристаллах HgSe : Co и HgSe : Ga с целью наблюдения аномального вклада в холловский ток, связанного с током спонтанного намагничения, и сравнительного анализа степени неоднородности образцов.

New experimental data on the manifestation of hybridization and spin polarization effects in 3d-states of donor electrons of a low concentration of cobalt atoms in mercury selenide crystals are obtained. The Hall concentration and mobility of conduction electrons as a function of impurity concentration, and the temperature dependences of the electron mobility, specific heat, and elastic moduli at low temperatures, are investigated. A quantitative interpretation of the observed dependences based on an earlier-developed theory in a justified simplified model of a single localization peak in the electron density of states is performed. As a result, the hybridization parameters of the cobalt states are determined by consistently fitting the observed experimental dependences, and evidence for the spontaneous spin polarization of electrons in the temperature dependences of the impurity specific heat and impurity contribution to the elastic moduli is detected.