Microscopic theory for the fluctuation-driven phase transition in weak itinerant magnets Page: 1 of 5
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Boston APS Conference October 1973
November 13-16, 1973 BNL 18281
MICROSCOPIC THEORY FOR THE FLUCTUATION-DRIVEN PHASE
TRANSITION IN WEAK ITINERANT MAGNETS*
K. K. Murata
Brookhaven National Laboratory, Upton, New York 11973
The microscopic basis for a recent phenomenological
theory for the thermodynamics near Tc of weak itinerant
magnets is presented. Calculation of dynamic renormaliza-
tion effects within paramagnon theory justifies the pre-
vious assumption of weakly temperature dependent Ginsburg-
Landau a and 0 coefficients. The cutoff on the fluctua-
tion phase space is found to go as qm~(Tc)1/3 rather than
the usual inverse coherence length. In spite of the phase
space reduction, as Tc 0, the phenomenological descrip-
tion becomes exact in a temperature region of order Tc.
However, it is found that the a and B coefficients must be-
determined phenomenologically since no small parameter
enters the calculation of their overall magnitude.
Recent work * on spin fluctuations has explained why the random
phase approximation for the Hubbard model U E nit nil for a conduc-
tion band gives incorrect predictions for the experimental suscepti-
bilities of extremely weak itinerant magnets. One faces the problem
in deriving the Ginsburg-Landau functional
qm ~ qm
' . I 2 h qI2 (aX+u2 2) + 4 h h qh qh qqiqei(1)
where T = TN(ep) is the reduced temperature, h is a unit normalized
field representing magnetization fluctuations, and the partition
function Z/Z0 - < e > is given as an ensemble average over the h
The comparison with experiment fails if one assumes, as in the
theory of superconductivity, that the order parameter susceptibility
X cc < IhOJ2 > is given by a-l calculated in the ladder approximation
(RPA). The point is that oPAel-2UN(EF) contains temperature depen-
dence due only to the thermal average N(eF) of the density of states.
This results in the- extremely weak dependence acFA-cPA(T=0)+(T2)..
Now in weak magnets X1 is observed to have linear3 rather
than quadratic dependence on .T over several Tc. Further the Curie
coefficient obtained by expanding %RPA around its zero is propor-
tional to CF/Tc, whereas experimentally this number should be more
* Work performed under the auspices of U.S. Atomic Energy Commission.
This report was prepared. as an account of work
sponsored by the united States Government. Neither
the United States nor the United States Atomic Energy
Commission, nor any of their employees, nor any of
their contractors, subcontractors, or their employees,
makes warranty, exprs or impid o asum a ny
legal liability or responsibility for 'the accuracy, corn-
pleteness or usefulness of any inform stion, apparatus,
product or process disclosed, or represents that its use DISTRIBUTION OF THIS DOCUMENT IS UNLIMI D
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Murata, K.K. Microscopic theory for the fluctuation-driven phase transition in weak itinerant magnets, article, October 1, 1973; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc1023502/m1/1/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.