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SUMMARY:Finite Size Kosterlitz-Thouless Transition in Fe/W(001) Ultrathin
Films
DTSTART;VALUE=DATE-TIME:20190603T204500Z
DTEND;VALUE=DATE-TIME:20190603T210000Z
DTSTAMP;VALUE=DATE-TIME:20201126T095155Z
UID:indico-contribution-3349399@indico.cern.ch
DESCRIPTION:Speakers: Jordan Atchison (McMaster University)\nIdealized two
-dimensional ferromagnetic films are predicted to undergo a Kosterlitz-Tho
uless (KT) transition that involves topological excitations termed vortice
s. Whereas the idealized system is isotropic within the plane (2DXY symmet
ry)\, real ferromagnetic films grown on single crystal substrates display
anisotropy and finite-size effects that can lead to more complicated behav
iour. In this work\, magnetic susceptibility measurements of 3-4ML Fe/W(00
1) films with four-fold in-plane anisotropy were compared to theoretical p
redictions for a finite-size KT transition\, and found to be in excellent
quantitative agreement. Susceptibility measurements on many films grown in
dependently in ultrahigh vacuum were made *in situ* using the magneto-opti
c Kerr effect (MOKE). Since the susceptibility signal can become complicat
ed by dissipation caused by domain walls at low temperatures\, the analysi
s concentrated on the high temperature\, paramagnetic tail of the signal w
here quantitative theoretical predictions apply. The paramagnetic tail was
fit to the distinctive form predicted by KT theory for a gas of unbound v
ortex/antivortex pairs: χ(T)=χ0 exp(B/(T/TKT-1)a)\, using a four-paramet
er fit. An analysis of 16 susceptibility signals from 12 independently gro
wn films gave fitted parameters of a=0.50±0.03 and B=3.48±0.16\, in exce
llent agreement with the predicted values of a=1/2 and 3.2 C(L). In these
cases\, the fitted values of the KT transition temperature\, TKT\, were t
ens of K below TC(L)\, which is quantitative agreement with finite size KT
theory. In contrast\, fitting to a power law typical of a second order ph
ase transition\, χ(T)=χ0(T/Tγ -1)γ\, gives an effective critical expon
ent of γ=3.7±0.7 and places the transition temperature Tγ ~10K below th
e peak temperature. Both of these results are unphysical.\n\nhttps://indic
o.cern.ch/event/776181/contributions/3349399/
LOCATION:Simon Fraser University SCC 9000
URL:https://indico.cern.ch/event/776181/contributions/3349399/
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