V. A. Ryzhov

Low-energy librational excitations in vitreous poly(methyl methacrylate)

The infrared (IR) and Raman spectra of vitreous poly(methyl methacrylate) (PMMA) are measured and investigated in the frequency range 10–150 cm−1. A comparison of the results obtained from IR and Raman spectroscopic measurements permits the assignment of the low-frequency anomaly (boson peak) observed in the spectra to librational vibrations occurring in a segment of the main chain that is comparable in length to the statistical chain segment. It is demonstrated that coherent librational excitations are associated with the relaxation processes proceeding in the polymers.

The nonlinear magnetic properties of the pseudocubic Nd0.77Ba0.23MnO3 single crystal in the critical paramagnetic region and phase separation

The second magnetization harmonic was studied for a moderately doped Nd0.77Ba0.23MnO3 neodymium manganite single crystal in parallel constant and harmonic magnetic fields in the critical paramagnetic region. According to the neutron and X-ray diffraction data, the crystal was crystallographically single-phase and had a pseudocubic structure both at room temperature and below the Curie point TC=124.1 K. Although the specific resistance of this compound had a singularity near TC and exhibited giant magnetoresistance, it remained an insulator in the ferromagnetic state. Nonlinear response measurements in the TC<T<T*≈146.7 K paramagnetic region were indicative of the existence of two magnetic phases. Above T*, the crystal was magnetically single-phase, and its critical behavior was well described by dynamical similarity theory for isotropic 3D ferromagnets. The unexpected appearance of a new magnetic phase in the structurally homogeneous crystal was discussed based on phase separation ideas; such a phase separation could occur in moderately doped cubic manganites experiencing orbital ordering.

Electron spin resonance study of the dynamic magnetic susceptibility of CuO, Cu1−xZnxO, and Cu1−xLixO single crystals

Results are presented of studies of the dynamic magnetic susceptibility of CuO, Cu1−xZnxO (x ≈ 1.5%), and Cu1−xLixO (x ≈ 1%) single crystals. The orientational dependence of the ESR spectra was investigated at room temperature. The results for CuO are analyzed using a model of a quasi-one-dimensional antiferromagnet (S = 1/2) with anisotropic exchange interaction between Cu2+ spins in the chains and exchange coupling between the chains allowing for one-dimensional spin diffusion and spinon excitations. The estimated line width is of the same order of magnitude as the experimental data. Substituting Cu with Zn scarcely alters the spin dynamics of the Cu2+ ions, as in weakly diluted magnets. Lithium doping substantially increases the ESR line width and this is attributed to excess holes forming rapidly relaxing spin complexes with copper ions.

Temperature evolution of superparamagnetic clusters in single-crystalLa0.85Sr0.15CoO3characterized by 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.

Neutron diffraction and ESR studies of pseudocubic Nd0.77Ba0.23MnO3 and its anomalous critical behavior above TC

Abstract Structural neutron diffraction study and electron spin resonance (ESR) measurements are presented for insulating Nd 0.77 Ba 0.23 MnO 3 with the Curie temperature T C ≈124 K. Its pseudocubic structure exhibits the definite distortions to a low symmetry. A detailed analysis of the data is given for a Pbnm space group in the temperature range 4.2–500 K. A systematic transformation of the crystalline structure, which is accompanied by the peculiarities in the temperature variations of structural parameters, is found. The ESR data are in agreement with the predictions for 3D isotropic ferromagnets in the paramagnetic critical region at τ ∗ ≈0.2≤τ C )/T C . The main disagreement observed below τ ∗ is that the ESR linewidth reveals no the characteristic critical enhancement. The anomalous critical behavior, which is found in the present and our previous studies in this range, is consistent with the development of some structural changes. Different temperature treatments (slow/fast cooling/heating, with/without external magnetic field) of the sample, which usually reveal the possible hysteretic phenomena, have a little effect on the ESR spectra. These results present a challenge to the understanding of the magnetic phase transition in these manganites.

Paramagnetic-ferromagnetic and insulator-metal phase transitions in La0.88MnO2.95

We present the results a study of structure by neutron diffraction and data on the magnetic properties (linear and nonlinear (second and third order) susceptibilities) of polycrystalline La0.88MnO2.95. This compound exhibits an insulator-metal (IM) phase transition at TIM ≈ 253 K (above the Curie temperature, TC ≈ 244 K) and reveals colossal magnetoresistance. The crystal structure is found to be rhombohedral, and the space group is R3c. Analysis of magnetic properties shows that at T* ≈ 258 K > TC, isolated paramagnetic clusters occur in the paramagnetic matrix; their concentration increases upon cooling. We observed no noticeable differences between the temperature evolution of the clustered state of this manganite with its insulator-metal transition and in the insulator La0.88MnO2.91. Possible scenarios of the paramagnet-ferromagnet and I-M transitions in a self-organized clustered structure are discussed.

Nonlinear properties and paramagnet-to-ferromagnet transition in Nd0.7Ba0.3MnO3 single crystal with metallic ground state

We present the results from studying the magnetic properties (linear and nonlinear susceptibilities and the depolarization of polarized neutrons) of Nd1 − xBaxMnO3 manganite, x = 0.3, with Curie temperature TC ≈ 140 K and dielectric-to-metal transition temperature TDM ≈ 129 K. Its critical behavior corresponds to that of an isotropic 3-D ferromagnet at temperatures above T*≈ 144 K, but a strong nonlinear response in weak magnetic fields and depolarization are observed at temperatures below T*. It is shown that this nontraditional behavior is related to the generation of ferromagnetic clusters in the paramagnetic matrix that form a conducting percolative network at temperatures near TDM.

Low-frequency librational vibrations, boson peak, and interchain interactions in a vitreous polymer

The IR and Raman spectra of plasticized poly(methyl methacrylate) (PMMA) are measured and analyzed in the frequency range 10–120 cm−1, in which the absorption and scattering due to the individual (85–90 cm−1) and correlated librational vibrations (15–20 cm−1, boson peak) preceding the appearance of relaxation dynamics manifest themselves. It is demonstrated that the presence of the boson peak as an indication of the solid-state behavior of the polymer material in the low-frequency spectra is associated with the correlation of librational vibrations not only inside macromolecules (in segments corresponding in length to the Kuhn segment) but also in segments of neighboring chains.

Study of fast relaxation in a glassy polymer by far-IR spectroscopy

The temperature dependence of the far-IR spectra of glassy polymethylmethacrylate is studied in the range of ν = 10–130 cm–1 at temperatures from 90 to 420 K. The analysis of the spectra revealed absorption corresponding to two rapid relaxation processes: (1) 180°-rotation of the OCO plane of the side groups and (2) reorientation of the links of the main chain. The activation energies of these processes are determined.

Magnetic and transport properties of the charge-ordered manganite Pr2/3Ca1/3MnO3

The transport properties, ac susceptibility χ, and EPR spectra have been studied for the insulator polycrystalline manganite Pr2/3Ca1/3MnO3 undergoing the orbital (O) and charge (C) orderings below TCO ∼ 250 K that lead to antiferromagnetic (AF) ordering below TN ≈ 155 K. Above TC ∼ 110 K, the χ′(T) dependence indicates that the sample contains a phase undergoing a second-order transition from the paramagnetic to ferromagnetic state. In the vicinity of TC, the colossal magnetoresistance effect is observed. The EPR spectrum characteristics are sensitive to the development of the O/C and AF orderings, and they show peculiarities at TCO and TN. Between TCO and TN, the temperature dependence of the g factor exhibits a characteristic point that can be related to the appearance of the electric polarization revealed in manganites of this class.