Computer simulations of Fe3+ electron paramagnetic resonance specmaat X (9.5 GHz) and Q (34 GHz) bands in the alkali borate glass LizO-ZB203 doped with Fez03 have been carried out using nn approach b m d on the eigenfield method applied to the 'rhombic' spin Hamiltonian. which contains only the Zeeman and quadrupole fine-structure term. In order to account for the structurd disorder in the glass, two different distribution densities of finestructm parameters D md E have been tried: a two-dimensional Gaussian function of D and ). = IE/DI. and the 'Czjzek function', analogous to the one used in Mdssbauercffect studies.In simulating the experimental spectra, care has been taken to fit not only to the most prominent features arising at &t = 4.3 (at X and Q bands) and 64 2.0 (at Q band), but also to an obvious platenu of the derivative of the absorption, which extends down to the magnetic field corresponding to fisf 1 9.7 (at both bands). As a result, the Qjjzek function can be ruled out. The agreement bemeen the experimental and computer-simulated spectra found with the Gaussian distribution density suggests the existence, besides orthorhombic symmetry sites (with A 2 113). of a considerable number of Fe3+ sites with axial or feebly rhombic distortions 0 . < 0.08). The relatively high mean value of the mid finestructure parameter D is consistent with a highly distorted environment of Fe)+ ions in the glass.

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A polymer compound has been studied by optical response and electron spin resonance (EPR) using and as dopants and paramagnetic probes. The transition between low-spin (LS) and high-spin (HS) states of ions occurs with a broad hysteresis loop around room temperature and is accompanied by a colour change from bright pink in the LS state to chalky white in the HS state. With increasing doping level the hysteresis loop narrows and shifts to lower temperatures. In the HS state of the EPR spectra of and are severely broadened by spin-spin interactions with paramagnetic HS modulated by a rapid spin-lattice relaxation of the latter ions. The EPR data give evidence of the presence of domains of the LS and HS ions in the transition region.

The EPR of Fe 3 § ions has been used for the first time to evidence a low-spin (S = 0) to high-spin (S = 2) transition of Fe 2 § ions in an octahedral ferrous complex [Fe(trz)(Htrz)2](BF4). The temperature dependence of the intensity of the Fe 3 § EPR lineat g = 4.3 reveals a spin transition which occurs for the Fe 2 § ions, with hysteresis. The transition temperatures are To~ = 374 K in the warming mode and Tc~ = 345 K in the cooling mode. The analysis of the EPR spectral data indicates the presence of a structural phase transition accompanying the spin transition.

The temperature dependency of the spin-lattice relaxation rate (T 1 Ϫ1 ) in poly-4-vinylpyridine ͑P4VP͒ resin samples doped with 0.5-5.0 % weight concentrations of ions characterized by weak spin-orbit coupling, Cu 2ϩ , Cr 3ϩ , Mn 2ϩ , and Gd 3ϩ was investigated over the range 4.2-250 K. It was found to be quadratic and linear at very low and intermediate temperatures, respectively, well accounted for by the exchange interaction between ions modulated by lattice vibrations, the ions being distributed uniformly over the sample. At higher temperatures, and with a concentration of up to 3%, the relaxation rate became almost independent of temperature, described well by cross relaxation between exchange-coupled pairs of spins and individual spins. At concentrations higher than 3%, the Bloembergen-Wang three-reservoir model was found to account for the data satisfactorily at all temperatures. ͓S0163-1829͑99͒01614-8͔

The temperature dependence of the EPR spectrum of Cu 2 § in the range 293-393 K exhibits a low-spin (S = 0) to high-spin (S = 2) transition of the Fe 2+ ions, with hysteresis (ToT = 363 K, Tc~ = 343 K). At 103 K, the principal values of the g and hyperfine tensors of Cu 2 § ions are revealed by hyperfine structure.

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