arXiv subjects

V. A. Ryzhov

Publications and source records attributed to V. A. Ryzhov.

6 recordsLinked to original sources

arXiv2017

Second-harmonic magnetic response characterizing magnetite-based colloid

Nonlinear second-harmonic magnetic response (M2) was used to characterize an aqueous colloidal solution of dextran-coated magnetite (Fe3O4) nanoparticles. Data analysis with the formalism based on Gilbert-Landau-Lifshitz equation for stochastic dynamics of superparamagnetic (SP) particles ensured extensive quantifying of the system via a set of magnetic and magnetodynamic parameters, such as the mean magnetic moment, the damping constant, the longitudinal relaxation time, the magnetic anisotropy field and energy, and others. Combined with transmission electron microscopy and dynamic light scattering, M2 technique allowed obtaining additional parameters, viz., the dextran-coating thickness and the interparticle magnetic dipolar energy. Aggregated colloidal nanoparticles were shown to be magnetically correlated inside the aggregate due to magnetic dipole-dipole (d-d) coupling within the correlation radius ~50 nm. With the d-d coupling account, the volume distribution of the aggregates recovered from M2 measurements is well consistent with electron microscopy results. From electron magnetic resonance, abrupt change of SP dynamics with increasing external magnetic field was observed and explained. The presented study exemplifies a novel M2-based procedure of comprehensive quantitative characterization applicable for a wide variety of SP systems.

cond-mat.soft↗

arXiv2015

Temperature evolution of superparamagnetic clusters in single-crystal La0.85Sr0.15CoO3 from nonlinear magnetic ac response and neutron depolarization

The representative measurements of the second harmonic in ac magnetization complemented by neutron depolarization have been performed for single-crystal La0.85Sr0.15CoO3 in the temperature range 97 K < T < 230 K, where occurrence of a small fraction (~ 0.001) of nanoscale ferromagnetic clusters (FMC) has been found. Magnetic, geometrical and dynamical parameters of the FMC system have been evaluated in the temperature range T < 140 K, where superparamagnetic regime installs, by means of the formalism involving the Fokker-Planck equation (FPE). With lowering the temperature, the amount of clusters fraction, the cluster size and magnetic moment along with its diffusion relaxation time strongly increase, each in its own temperature interval. Below 130 K, FMC contribute essentially to the total linear magnetic susceptibility. The damping factor of the order 0.1 proves the importance of precession in thermal relaxation of the cluster magnetic moment. The FMC are a precursor of long-range ferromagnetic correlations seen below 100 K with neutron-scattering techniques. The employed technique supplemented with FPE-based data-treatment formalism is a novel method for studying superparamagnetic systems.

cond-mat.mtrl-sci↗

arXiv2014

Unusual scenario of the temperature evolution of magnetic state in novel carbon-based nanomaterials

Two porous carbon-based samples doped with Au and Co are investigated. The neutron diffraction study reveals an amorphous structure of both samples. The Co-doped sample exhibits a long-range ferromagnetic (FM) ordering at 2.6 K. The NMR investigations demonstrate, that the samples are obtained with a partial carbonization of initial aromatic compounds and do not reach a state of glassy carbon. The magnetization, longitudinal nonlinear response to a weak ac field and electron magnetic resonance data give evidences for presence of FM clusters in the samples well above 300 K. A short-range character of the FM ordering in the Au-doped sample transforms below T$_C \approx$ 210 K into another inhomogeneous FM state. Besides the FM clusters, this state contains a subsystem with a long-range FM ordering (matrix) formed by paramagnetic centers, existing outside the clusters. The nonlinear response data suggest a percolative character of the long-range FM matrix, which is connected probably with a porous sample structure. The magnetization data give evidence for the formation of an inhomogeneous state in the Co-doped sample, similar to that in the Au-doped one. However, this state is formed at higher temperatures, lying well above 350 K, and exhibits a more homogeneous arrangement of the FM nanoparticles and the FM matrix. Temperature dependence of the magnetization in the Au-doped sample is attributable to changes of the domain formation regime in the FM matrix on cooling, connected with the inhomogeneous character of its FM state. Such peculiarity is absent in the Co-doped sample below 350 K, which is in agreement with formation of the FM state in this sample at much higher temperatures. Further cooling below T ~ 3(10) K leads to a steep increase of the magnetization in both samples. This is attributable to the domain rearrangement in the inhomogeneous FM state at low temperatures.

cond-mat.mes-hall↗

arXiv2009

Estimation of characteristic size of ferromagnetic clusters forming above T$_C$ in Nd$_{0.75}$Ba$_{0.25}$MnO$_3$ manganite

We present the data on depolarization of polarized neutron beam and second harmonic of magnetization ($M_2$) for Nd$_{1-x}$Ba$_x$MnO$_3$ ($x$ = 0.23, the Curie temperature $T_C \approx$ 124 K; $x$ = 0.25, $T_C \approx$ 129 K) manganites. The depolarization starts to develop below $T$* $\approx$ 147 K $> T_C$ for both samples, being larger in x = 0.25 compound. This evidences the arising of a ferromagnetic (F) cluster phase below $T$* and a growth of its relative volume fraction with increasing doping concentration that agrees with the previously published results of $M_2$ study. A characteristic size of the F clusters and their concentration are estimated combining the neutron depolarization and $M_2$ data for $x$ = 0.25 manganite.

cond-mat.mes-hall↗

arXiv2005

Neutron diffraction, magnetization and ESR studies of pseudocubic Nd(0.75)Ba(0.25)MnO3 and its unusual critical behavior above Tc

Results of structural neutron diffraction study, magnetization and ESR measure-ments are presented for insulating Nd0.75Ba0.25MnO3, Tc = 129 K. The crystal structure is refined in the range 4.2-300 K. The compound is found to exhibit the Jahn-Teller (JT) transition at 250 K. The field cooled (FC) magnetization data are in a reasonable agreement with the predictions for a 3D isotropic ferromagnet above Tc. However, these measurements reveal a difference between the FC and zero FC data in the paramagnetic region. ESR results are also in a correspondence with behavior of a cubic ferromagnet above T* = 143 K. It is shown that an anisotropic exchange coupling of the Mn and Nd magnetic moments can give a substantial contribution in ESR linewidth masking its critical enhan-cement. The different temperature treatments of the sample reveal a temperature hysteresis of the ESR spectra below T* indicating an anomalous response in the paramagnetic region. The study of phase transition in this manganite suggests change in its character from the second to first order at T*. The conventional free energy including the magnetization and magnetic field is not found to describe the first order transition. This suggests that the charge, orbital and JT phonon degrees of freedom, in addition to magnetization, may be the critical variables, the unusual character of the transition being determined by their coupling. Unconventional critical behavior is attributed to orbital liquid metallic phase that coexists with the initial orbital ordered phase below T*.

cond-mat.str-el↗

arXiv2003

Slowing heavy, ground-state molecules using an alternating gradient decelerator

Cold supersonic beams of molecules can be slowed down using a switched sequence of electrostatic field gradients. The energy to be removed is proportional to the mass of the molecules. Here we report deceleration of YbF, which is 7 times heavier than any molecule previously decelerated. We use an alternating gradient structure to decelerate and focus the molecules in their ground state. We show that the decelerator exhibits the axial and transverse stability required to bring these molecules to rest. Our work significantly extends the range of molecules amenable to this powerful method of cooling and trapping.

physics.atom-ph↗