O. P. Smirnov

Inelastic neutron scattering study of spin waves in the garnet with a triangular magnetic structure

The spin-wave spectrum of the antiferromagnet with the garnet structure has been studied by inelastic neutron scattering on the triple-axis spectrometers IN12 and IN14 at ILL. Magnon dispersion curves were measured along and directions up to energies of 0.3 THz. The spin-wave symmetry analysis, which reduced the number of independent exchange parameters to six, was performed to ensure correct description of the spin-wave branches. As determined by the fitting, five of the parameters are statistically significant and have been found to be , and . This quite unusual sequence of exchange parameters clearly demonstrates the importance of the superexchange chain geometry.

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.