ESR millimeter-band spectroscopy of magnetic ordering in the low-dimensional magnet CuGeO3

Resonant absorption of microwaves in CuGeO 3 single crystals in a frequency band of 40 to 120 GHz, in magnetic field B ⩽15 T, at temperatures ranging between 0.5 and 300 K, and in the configuration B ∥ a has been investigated. Several absorption lines ( S 0 , S a , and S b ) whose parameters strongly depend on temperature have been detected close to ESR. The temperature dependence of the total absorption in the main line S 0 with the Landé g -factor g 0 =2.154 at temperatures above the spin-Peierls transition temperature is in good agreement with Bonner and Fisher’s theoretical prediction for a one-dimensional Heisenberg spin chain. In addition to the main resonance, a resonance of smaller amplitude, S a , with the g -factor g a =2.72 has been detected at temperatures ranging down to a characteristic temperature T ≃1 K, below which the amplitude of this feature drops to zero. A radical restructuring of the magnetoabsorption spectrum occurs at the temperature of the spin-Peierls transition T SP ≈14 K. At T <12 K new features emerge in the spectrum, namely, a broad absorption line overlapping with the narrow lines S 0 and S a , and a line S b with g b =1.83, which is not detected at temperatures above T SP . An analysis of amplitudes and total absorption of ESR lines as functions of temperature has shown that the temperature range below 1 K is anomalous, which may be caused by an additional ordering in the CuGeO 3 magnetic subsystem at low temperatures.