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Using electron spin resonance spectroscopy and SQUID-magnetometry we obtained direct evidence of the occurrence of magnetic domains in the antiferromagnetically ordered state of a $${\text{EuF}}{{{\text{e}}}_{{\text{2}}}}{\text{A}}{{{\text{s}}}_{{\text{2}}}}$$ single crystal. The resonance spectra of europium ions were measured in the temperature range from 4 to 200 K. Using an equation for the resonance field in an antiferromagnet that takes into account the exchange and anisotropy fields, we have performed an analysis of the angular dependence of the spectrum at a temperature of 4.8 K, measured upon the crystal rotation around the c axis. Data analysis showed that $${\text{EuF}}{{{\text{e}}}_{{\text{2}}}}{\text{A}}{{{\text{s}}}_{{\text{2}}}}$$ is the antiferromagnet with easy anisotropy plane. Besides, we found in the $$ab$$-plane the second order axes of easy magnetization for each of the two types of magnetic domains, related to the structural transition and the formation of twins. Magnetic anisotropy caused by the exchange interaction of europium ions with iron ions indicates the occurrence of nematic magnetic ordering in the basal $$ab$$ plane. An estimate of the magnitude of the exchange field and the anisotropy field is obtained from the angular dependence of the resonance fields.

The $\rm Fe_{1+y}Te_{1-x}Se_x$\ single crystals with the various Se/Te ratios were studied by the microwave absorption and direct current resistivity measurements. The comparison of the microwave absorption data and the resistivity versus temperature made it possible to separate the contributions of two types of spin fluctuations. One of them is due to the anisotropic magnetic (nematic) fluctuations. It is observed over the wide temperature range from 30K to 150 or 200K. In FeSe it has the maximum close to the structural transition temperature. Another MWA anomaly is located in the narrow temperature range above the superconducting transition. It is likely due to the antiferromagnetic fluctuations. Annealing of a sample at the temperature around 300 C in the oxygen atmosphere made it possible to exclude the effect of excess iron on the observed anomalies., 6 pages, 4 figures

The results of studying the La2–xSrxCuO4 (x = 0.077−0.195) crystals with the microwave absorption, direct current resistance and magnetic susceptibility measurements are presented. The doping-temperature phase diagram with the boundaries of the areas with charge density waves, superconducting fluctuation and the bulk superconductivity state is plotted. The doping range of the charge density wave existence is found to be broader than that obtained from the X-Ray Diffraction study. It spreads at least down to x = 0.077.

Crystals of 3D topological insulators, bismuth telluride Bi2Te3, doped with manganese were studied using electron spin resonance (ESR) spectroscopy together with the SQUID magnetometry, transport measurements, and X-ray characterization. The obtained ESR data, such as the temperature and the angular dependence of the resonance field, reveal the specific critical behavior and confirm the ferromagnetic ordering of Mn spins even at modest doping. In addition to the studies of the critical behavior of diluted ferromagnet Bi2−x Mn x Te3, we also discuss the effects of the limited solubility of Mn ions giving rise to microscopic inclusions of the spurious magnetic phases which were revealed using ESR technique.

Fluctuations of various order parameters are considered as the significant components of understanding the mechanism of high-Tc superconductivity. To study these fluctuations in the cuprate and Fe-based superconductors we use the joint measurements of direct current resistivity and non-resonant microwave absorption. Comparing data obtained with both methods allowed us to extract the contribution of dynamic charge density waves in La2−xSrxCuO4, superconducting fluctuations in Bi2Sr2Ca1−xYxCu2O8 and nematic fluctuations in FeTe1−xSex. © Kazan Federal University (KFU). Government Council on Grants, Russian Federation Government Council on Grants, Russian Federation Russian Academy of Sciences, RAS Kazan Federal University This work was supported by the Russian Academy of Sciences via the grant of Program 1.12 “Fundamental Problems of High-Temperature Superconductivity”. The work of D.Ch. is supported by the program 211 of the Russian Federation Government, agreement No. 02.A03.21.0006 and by the Russian Government Program of Competitive Growth of Kazan Federal University. The work of A.V. has been supported by Act 211 of the Government of Russian Federation, Contracts No. 02.A03.21.0006 and No. 02.A03.21.0011.

Basing on electron spin resonance (ESR) data for Bi2Te3 doped by Mn ions we argue that this compound can be inhomogeneous and consists of two components with the different structures. Its main phase Bi 2−x Mn x Te 3 is intertwined with the microscopical inclusions of MnBi phase. The integral volume of these intermetal clusters is less than 1 % but nevertheless they exert the serious impact on the dynamic magnetic properties of the entire system. These inclusions are ferromagnetic with the Curie temperature of 630 K, while the main bulk phase Bi 2−x Mn x Te 3 has x= 0.05 orders at T c= 10 K (qualitatively this twophase picture is valid not only for this given x). Below this temperature two ferromagnetic phases coexist. Since the integral spontaneous polarization in MnBi phase is averaged out due to its random orientations in different clusters the time-reversal symmetry of Bi 2Te 3 doped by Mn ions is violated only at the low-temperature ferromagnetic transition.

The magnetic field distribution on the surface of the high-Tc superconductor crystals Bi2Sr2Ca1−x Y x Cu2O8+y has been studied in a wide temperature range using electron paramagnetic resonance (EPR) of the surface probe. The changes of the EPR signal broadening and shift with temperature and the probe-sample distance have been investigated. The analysis of obtained data made it possible to conclude that in the large temperature range above the critical temperature, some of the studied crystals are magnetized inhomogeneously. In the samples underdoped with charge carriers, the range is 40 K. Such samples contain inclusions with diamagnetic and paramagnetic magnetization. They are probably formed due to the electron phase separation.

Bi2Sr2CaCu2O8 thin films 200 nm thick were irradiated with 120 keV trivalent boron ions with the dose ranging from 1011 to 1014 ion/cm2. The critical parameters of the samples prior and after irradiation were monitored by non-resonance modulated microwave absorption and Hall-probe techniques. For low doses of 1011–1012 ion/cm2, a slight increase in the critical current density and expansion of the area of the non-dissipative transport current flow were revealed. Such results are explained by the formation of separate areas of displaced atoms, which serve as effective pinning centers. The positive effects of irradiation faded away with dose increase of up to 1013–1014 ion/cm2. This is due to overlap of radiation-induced defects and weak pinning on them.

Using electron spin resonance (ESR) technique we have obtained data evidencing the existence of magnetic vortices in high-temperature superconductors at temperatures above the critical one Tc. We have studied magnetic excitations in Bi2Sr2Ca2Cu3O10 single crystals above Tc with the method of surface spin decoration. The surface layer of diphenylpicrylhydrazyl was used as a sensitive probe of magnetic field distortions. The temperature dependence of the ESR signal parameters has indicated that far above Tc the magnetic flux of a sample is affected by the superconducting order parameter fluctuations while close to Tc its changes are due to vortex-type excitations.

Vortex excitations have been detected at temperatures both below and above the critical temperature when investigating local magnetic fields on the surface of a Bi2Sr2Ca2Cu3O10 single crystal by means of an electron paramagnetic resonance (EPR) probe. A thin layer of a diphenyl picrylhydrazyl organic radical deposited on the crystal surface is used as the EPR probe. A narrow EPR signal makes it possible to detect weak distortions of the magnetic field appearing at T ≿ Tc. The analysis of the temperature dependences of the resonance field and the EPR linewidth is thebasis of the assumption of the vortex nature of magnetic excitations in this temperature range.

Electron spin resonance is studied in films of La1−x Ca x MnO3 manganites with compositions in the vicinity of the calcium content x = 0.5, in which the phase separation is most clearly pronounced. It is found that the La0.5Ca0.5MnO3 manganite undergoes different types of phase separation: (i) at temperatures above the Curie point, ferromagnetic regions exist in the paramagnetic phase; and (ii) at temperatures below the Neel point, antiferromagnetic microregions coexist with ferromagnetic microregions. Two types of ferromagnetic domains with different magnetization orientations are revealed In the temperature range between the Curie and Neel points.

Measurements of the irreversibility line in Bi2Sr2CaCu2Oy single crystals are made by three experimental methods: by dc magnetization measurements, microwave absorption measurements, and registration of the harmonics in the high-frequency response of the superconductor. It is found that the position of the irreversibility line on the magnetic phase diagram depends substantially on the method of measurement and the frequency used. Analysis of the results shows that the discrepancy in the irreversibility lines is due to both a difference in the mechanisms of hysteresis of the magnetic properties and to the different scales of the measurement times. It is shown that the position of the irreversibility line is determined at low temperatures by the limiting value of the vortex creep rate for the given method of registration and at high temperatures by the time required for two-dimensional vortices to overcome the surface barrier. It is concluded that the irreversibility line is a dynamic phase transition. The ...

Excitations of the vortex type in the regions of temperatures both below and above critical temperature Tc are found by studying the local magnetic fields on the surface of Bi2Sr2Ca1 − xYxCu2O8 + y single crystals by means of EPR. The narrow EPR signal of the surface paramagnetic probe allows us to observe weak distortions of the magnetic field that arise when T > Tc. The behavior of the signal broadening agrees with the hypothesis about the existence of vortices in this temperature region.

The results of experimental and theoretical studies of the hysteretic microwave absorption (MWA) in the superconducting Bi2Sr2CaCu2O8 thin films are presented. It has been found experimentally that the hysteresis loop manifests some unusual features such as the non-monotone hysteresis variation and the change of a hysteresis sign. We have shown that such unusual behavior is due to the special nature of the bulk pinning in a superconducting film with a thickness comparable with the field penetration depth. The theoretical model of the MWA hysteresis has been developed taking into account the inhomogeneous distribution of centers with different symmetry of a pinning potential and their variation with the magnetic field value.

The field distribution in the Bi:2212 rectangular strip, placed in a perpendicular magnetic field, was studied using the ESR microprobe scanned over the sample surface. The crossover of the magnetic-field profile from the dome shape, typical for a geometrical barrier (Tg30 K), to the bulk pinning picture (T25 K), was observed upon variation of temperature. The analysis of field distribution made it possible to determine the temperature dependence of the lower critical field. \textcopyright{} 1996 The American Physical Society.

The distribution of a magnetic field in a superconducting sample is studied by a scanning ESR microprobe. The main attention is focused to the penetration of a perpendicular magnetic field into an YBaCuO single crystal strip. The field profile in the interior of the sample is mapped from the data on the ESR-signal shift of the spin probe measured at different points over the plane surface. The penetration of the magnetic flux is shown to be delayed and this gives rise to the formation of steep fronts in the profile of the field near the center of the strip. The profile observed is highly dependent on the magnetic history. The demagnetizing effects and the distribution of the current are discussed. The analysis of the experimental data obtained gives evidence that the critical state picture differs considerably from the well known Bean model.

Studies of the flux lattice dynamics via microwave absorption measurements with the help of an ESR spectrometer are presented. The experiments give evidence that for the temperatures below 35 K the absorbed power depends on the input microwave power in a non-linear way. For the case of absorption due to the motion of vortexes via thermally activated flux creep this corresponds to the dependence of the activation barrier on the current density. The measurements lead to the conclusion, that the activation barrier should grow with decreasing current. Fitting of the expression for the dissipated power obtained in terms of the collective creep theory to the obtained experimental data enable us to determine the character of pinning and creep at small amplitudes of the external exciting current. The obtained numerical values give evidence that the flux lattice is frozen in a vortex-glass state and in the limit of zero current the resistance at finite temperatures vanishes.

The mixed state of superconducting Yba 2 Cu 3 O x single crystals has been studied by the observation of the electron spin resonance (ESR) of a paramagnetic probe on their surface. To investigate the distribution of the magnetic field at the vicinity and on the surface of a superconductor the evolution of the ESR spectrum was observed upon elevating the probe above the crystal by means of a buffer layer of varying thickness placed in between. The field inhomogeneity was found to extend to a distance of the order of the crystal size. The spatial scale of inhomogeneity on the superconductor surface was of the same magnitude. From the dependences of ESR signal parameters on the cooling temperature the features of the distribution of magnetic flux inside a sample have been revealed and the values of the critical current density have been determined on the base of the Bean's model of the critical state of a superconductor.

Measurements of nonresonance microwave absorption have been used to determine the position of the irreversibility line and to estimate the critical current density for the three SmO1?xFxFeAs samples with fluorine concentrations x = 0.06, 0.08, and 0.1. The irreversibility lines of all samples are characterized with a sharp slope, indicating a strong pinning up to Tc. A weak field dependence of the critical current density and an estimate of parameters characterizing the vortex matter allow us to suggest the presence of additional pinning centers in underdoped samples. It is likely that these pinning centers are nanoscale inclusions of a possibly magnetic phase occurring in the fluorine concentration range of 0.06?0.08.

The field distribution in the Bi2Sr2CaCu2Ox rectangular strip, placed into perpendicular magnetic field, was studied. The measurement of local field magnitude was made using the ESR microprobe moved over the sample surface. Our results confirm the existence of geometrical barrier for vortex motion, which is due to edge shielding current, even at high magnetic fields. Changing the temperature we were able to observe the crossover of the magnetic field profile from the dome shape, typical for geometrical barrier (T>30K), to the steep front penetration picture (T

The new procedure of the magnetic field mapping with the help of ESR microprobe scanning over the surface is applied for investigation of the perpendicular magnetic field profile in the superconducting YBa 2 Cu 3 O x single crystal strip.

The studies of the flux lattice dynamics via the microwave absorption measurements with the help of ESR spectrometer are presented. For low temperatures the nonlinear dependence of the absorbed power on the input microwave power is discovered. The analysis of this phenomenon in terms of the collective creep theory gives evidence, that the flux lattice is frozen in a vortex-glass state.

The time dependent anomalies of the microwave absorption due to the trapped magnetic flux are studied with the help of standard EPR spectrometer. The displacement of the observed extremum is described in terms of flux creep.

Temperature dependent intrinsic spin relaxation of conduction electrons in lithium observed via a spin-echo method is reported. Experimental results obtained are likely to correspond to a spin-current relaxation mechanism predicted earlier by Overhauser.

Conduction electrons spin echo (CESE) and CESR in the ensemble of small Li particles are studied in the X-band at temperatures 77–450K. By using a pulse method, we have succeeded to reconstruct the particles size distribution and so to describe the revealed resonant properties of a system.

A new method for the study of the distribution of the magnetic field in a superconducting sample is developed. The field profile in the interior of the sample is mapped from the data on the spin probe ESR-signal shift measured at different points over the plane surface. The main attention is focused to the penetration of a perpendicular magnetic field into single crystal strips. The penetration of magnetic flux into an YBCO crystal is shown to be delayed and this gives rise to formation of the steep fronts in the profile of the field near the center of the strip. The profile observed differs considerably from the well-known Bean picture. It is highly dependent on the magnetic history. For the case of a Bi:2212 rectangular crystal, the crossover of the magnetic field profile from a dome shape, typical for a geometrical barrier (T > 30 K), to a bulk pinning picture (T < 25 K) was observed upon variation of temperature. The analysis of the field distribution made it possible to determine the temperature dependence of the lower critical field.