The electron spin resonance ͑ESR͒ of (pyrene) 12 (SbF 6 ) 7 is measured as function of crystal orientation for temperatures above and below the Peierls transition temperature T P ϭ116 K. The principal values of the pyrene g tensor ͑2.00274, 2.00235, 2.00225͒ are determined. In spite of a defect concentration amounting to 2 -3ϫ10 Ϫ2 per formula unit, the ESR linewidth is below 60 mG in the metallic phase. The temperature dependence of the intrastack exchange interaction between localized defects and conduction electrons is derived. The exchange integral decreases by a factor of 8 between T P and room temperature.
The low-temperature proton spin–lattice relaxation is analysed for lanthanum hydrides
LaHx
intentionally doped with Gd or Ce. These paramagnetic impurities were also characterized
by static magnetic susceptibility and electron spin resonance measurements. A quantitative
description of the proton relaxation rate maxima, as well as of the electron spin relaxation
data, is presented. These analyses indicate that the density of states at the rare earth site
disappears only for .
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