Electron-spin-lattice relaxation in a one-dimensional system

Abstract

We have measured by the modulation technique at the X band the spin-lattice relaxation rate in the one-dimensional (CH3)4NMnC13, in the temperature range 20 -300 K. The experimental data are well described by the Bloembergen and Wang model. Below 4S K, an exchange-lattice relaxation process has been evidenced.The spin dynamics in one-dimensional (1D) materials has been extensively studied by magnetic resonance" and neutron diffusion techniques. ' In these materials, the exchange-narrowed electron-spin resonance shows marked properties; they concern the angular and frequency dependences of the linewidth, the line shape, and other effects such as line shifts and half-field transitions. They are consequences of the long-time diffusive decay of spin correlations in Heisenberg 1D systems. ' In this paper, we present the first study of the electron-spin-lattice relaxation time T~in a quasiperfect 1D system. We have measured T~at X band as a function of crystal orientation and temperature (Figs. 1 and 2) in tetramethylammonium trichloromanganate (CH3)4NMnC13 (TMMC). This compound consists of MnC13 chains well separated from each other. The Heisenberg exchange coupling is very strong within the chai'ns (J = -6.7 K) but about 104 smaller between the chains. 4 We took great care to operate on perfect crystals to avoid the presence of a 3D species which was described in another publication. 'In concentrated magnetic materials, the relaxation time T~is usually very short. Its measurement by conventional techniques is rather difficult and very few experimental data are available. However, precise measurements can be obtained by the "modulation method" which was originally devised by Herve and Pescia. In a "modulation spectrometer, " the amplitude of the microwave field is modulated at a frequency A. A coil placed in the cavity, around the sample and coaxial to the magnetic field, picks up a signal S proportional to the time derivative of the longitudinal magnetization dM, /dr For 0 T~(( 1,. the magnetization follows the modulation and S~0; I O I-10 50 8 (deg) 1 90 FIG. 1. Angular variation of the spin-lattice relaxation rate in TMMC at 300 K and 9 0Hz. The solid curve is obtained from Eq. (3) without adjustable parameters. 8 is the angle between the Zeeman field and the chain axis.on the other hand, for 0 T& ))1, the relaxation is effective and S tends towards a value Sp. In the general case, two data, the low-0 slope and the asymptotic value Sp are sufficient to the determination of T~. The upper values of 0 are limited by technical difficulties and Sp cannot be obtained directly in samples where T~( 10 s. Meanwhile, it can be shown' that there is a constant ratio between Sp and the ESR signal delivered by the apparatus used just like a con-23 1339

Temperature and concentration dependences of the spin-lattice relaxation rate in four borate glasses doped with Fe2O3

“…1). At temperatures below 50 K, (T~)zE begins to increase due to the approach to the antiferromagnetic order resulting in a slowing down of the decrease of Tlk It is noted that a similar relaxation behaviour was observed in TMMC by Bourdel et al [22].…”

“…Thus, the observed dependence is most likely due to the combined effect of the mechanisms involving TLS and cross relaxation. iii) In the most concentrated glass sample (3 wt.% Fe203) , exchange interaction is dominant; the relaxation is well described by the Bloembergen and Wang three-reservoir model [19][20][21][22]. Accordingly, Ti 1 is almost constant between 75 and 200 K, while it decreases rapidly with decreasing temperature below 75 K.…”

MICROWAVE AMPLITUDE MODULATION IN EPR AND SHORT T1 MEASUREMENT

“…These values were higher than those measured from the sound velocity, which indicated that the Debye model was not adequate to describe the lattice in these salts. (41,42,43) TMMC and CHAC samples are ID systems very close to the ideal ISING model. A strong exchange interaction is present in them, the coupling being antiferromagnetic in the former while it is ferromagnetic in the latter.…”

Electron paramagnetic resonance and spin-lattice relaxation in two-dimensional systems

“…The observed dependence for T < 80 K cannot be attributed to the Zeeman-exchange relaxafion, but it can be due to the exchange-lattice relaxation, because the coupling between exchange and lattice reservoirs diminishes at low temperature. The Zeeman-exchange relaxation becomes the most efficient process and we have an exchange bottleneck [16]. …”