ISSN:
1573-7357
Quelle:
Springer Online Journal Archives 1860-2000
Thema:
Physik
Notizen:
The nuclear spin system of metallic copper in a poly crystalline sample consisting of thin wires has been studied using a cryostat in which a dilution refrigerator and two copper nuclear stages operate in series. The spin system was cooled to 50 nK, which is an all-time low-temperature record. The NMR absorption curves were measured in external fields from zero to B ext = 15 mT and at entropies corresponding to polarizations up to 0.9. The line shapes reveal the effect of indirect exchange interaction corresponding to an antiferromagnetic value of Σ j J ij /γ2\ħ 2Μ 0 ϱ = −0.42 ± 0.05. This result was obtained both from the interference of the isotopic absorption lines at 15 mT and from the shift of the second harmonic in fields below 2 mT. Thus, an antiferromagnetic transition in zero field at about 200 nK is expected. The Curie—Weiss θ points to a transition temperature of 150 nK; the inverse static susceptibility vs temperature curve clearly predicts antiferromagnetism. However, no evidence of such a transition was found in NMR spectra measured with the excitation transverse to the copper wires, although the entropy was reduced to about 0.45 R ln 4, with the corresponding temperatures well below 100 nK. On the other hand, in measurements during which the excitation was parallel to the wires, a clear saturation of susceptibility to a constant value could be seen as a sudden change in the apparent relaxation rate at the lowest entropies.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1007/BF00115631
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