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Document type:
Zeitschriftenaufsatz 
Author(s):
B̈oni, P.; Martnez, J. L.; Tranquada, J. M. 
Title:
Longitudinal Spin Fluctuations in Nickel 
Abstract:
The longitudinal, transverse, and paramagnetic spin fluctuations in Ni have been measured near TC by means of polarized neutron scattering in the momentum range 0.06$<$q$<$0.18 A˚-1. In transverse scans, spin-wave peaks at $\omega$q=Dq2 appear as expected from previous measurements performed with unpolarized neutrons. The longitudinal fluctuations are quasielastic, in agreement with predictions of a recent mode-mode coupling theory and renormalization-group theory. The data indicate that the longitudinal dynamical scaling function is smaller than 1 just below TC. The scaling function for the paramagnetic scattering is shown to be in agreement with the Résibois-Piette scaling function for energy scales up to kBTC. The measured field dependence of the scattering is rather weak, indicating that the internal fields H are rather large. Therefore it was impossible to observe the H-1/2 divergence of $ḩi$L(q$̊ightarrow$0) that is predicted for the isotropic Heisenberg model in three dimensions. In contrast, we found $ḩi$L(q$̊ightarrow$0)$\propto$(1-T/TC)-$\gamma$, which also appears to be a result of the internal fields. The q dependence of $ḩi$L is Lorentzian-like. The T dependence of the correlation length indicates critical behavior. These features closely resemble the behavior of the paramagnetic fluctuations, and they are in agreement with results obtained with use of $\epsilon$-expansion techniques. 
Journal title:
Physical Review B 
Year:
1991 
Journal volume:
43 
Month:
jan 
Journal issue:
Pages contribution:
575