Spin-orientation phase transition

The spin-orientation transitions can be distinguished by their unique features from the large family of magneto-structural phase transitions. Unfortunately, the thermal conductivity of RECs has not been investigated for all groups of magnetic phase transitions. Reliable data are available for the MPT of the order-disorder type and for the spin-orientation phase transition (SOPT). 

Moving from structure to dynamics, we recall the NMR results, [Ty91, Ya91a] which indicate that the molecules in solid Ceo tumble rapidly at room temperature but freeze at lower temperatures. Considering the structural data reviewed above, one would expect a more-or-less continuous rotational diffusion in the room temperature fee phase, and either small oscillations or molecular jumps between favorable orientations in the low temperature phase. This difference was very clearly seen in the overall spin-lattice relaxation time near the orientational phase transition.[Ty91b] It was also found that the... 

It was shown above that, apparently, individual spins in FeCC>3 may have only two opposite directions, intermediate orientations are forbidden. Therefore we will start directly with assuming a sharp phase boundary, in which the phase transition from one phase to another occurs over one interatomic distance. Fig. 19 depicts the boundary in the perpendicular plane (111). At the left-hand side the crystal stays in the antiferromagnetic state, and at the right-hand side it is in the paramagnetic state the external magnetic field h = —7 is aligned along the [111] axis. 

Regarding the relative intensities of the observed spin polarization mechanisms, it is also important to note that the [3-hyperfine interactions in these radicals are conformationally modulated, and this process can also quench RPM polarization. In a qualitative way, we can consider the modulation process to be a relaxation mechanism that exchanges magnetization between different nuclear spin orientations. Since these different orientations can have opposite phases of RPM polarization, the exchange of emissive and absorptive lines can cancel the intensity of the transitions. 

Copper-63 NMR/NQR spectra for CuS, from 4.2K up to ambient temperatures, show two distinct lines based on the abrupt change in the spectrum at 55K. This is ascribed to a structural phase transition for both the powder and the oriented CuS. A more recent study of powdered samples at temperatures as low as 1.5K under a magnetic field of 6.5 T revealed more detail. An intense resonance with satellites was observed at all temperatures up to the ambient. This peak consisted of two resonance lines above 60K and four lines below 50K. The splittings observed correspond to crystal distortions below 55K. The central peaks with satellites were assigned to the metallic Cu(l) and Cu(2) in the crystal structure of CuS, for which spin-lattice relaxation times of 4 ms and 55 ms, respectively, were measured at 15K. The authors suggest that the anomalously short Ti for Cu(l) is indicative of the metallic character in the plane formed by Cu(l)-S bonds. 

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