Temperature Dependence of the Magnetic Hyperfine Field of 181Ta

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Temperature Dependence of the Magnetic Hyperfine
Field of 181Ta in Rare Earth-Cobalt Laves Phases
RCo2 H. Saitovitch, P. R. J. Silva, J. Th.
Cavalcante, Centro Brasileiro de Pesquisas
Fisicas, Rio de Janeiro, Brazil P. de la Presa,
M. Forker Helmholtz Institut für Strahlen-
und Kernphysik, Universität Bonn, Germany
Support
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MOTIVATION

• Rare Earth-Cobalt Laves Phases RCo2 may be a very
  suitable series of compounds to study magnetic
  interactions
• well known compounds
• Interesting interaction of 3d - 4f electrons
• Easy to prepare
• Easy doping
• Good theoretical knowledge
• Here we will focus on the nature of magnetic
  transition order

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The C15 lattice structure of RCo2
Co non-cubic point symmetry, combined magnetic
and electric hfi Rare earth cubic point
symmetry, pure magnetic interaction
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Experimental detail
Probe isotopes Sources preparations PAC
equipment Temperature variation
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The temperature dependence of the magnetic
hyperfine field of 111Cd in RCo2 Previous
measured spectra
PrCo2
GdCo2
DyCo2
Second-order transition
First-order transition
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Theory of itinerant electron magnetism
Wohlfarth and Rhodes , 1962 Landau expansion of
the free energy
a1 lt 0 , a3 gt 0
a1 lt 0 , a3 lt 0
One minimum Continuous, second order phase
transition (SOT)?
Two minima Discontinuous, first order phase
transition (FOT)?
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Temperature dependence of the Landau
coefficients a1 and a3
Based on the analysis of the susceptibility of
YCo2 Bloch et al. predict a change of sign of a3
at T0 150 K
Consequence The order of the transition depends
on the order temperature TC
TC lt T0 First-order transition (FOT), TC lt T0
Second-order transition (SOT)
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Curie temperatures of RCo2
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The order of the magnetic phase transitions of
RCo2 deduced from the magnetic hyperfine field
at 111Cd
SOT
FOT
First results of these studies M. Forker et al.,
PHYS. REV. B 68, 014409 (2003)?
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PAC spectra of TbCo2
111CdTbCo2
181TaTbCo2
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The temperature dependence of the magnetic
hyperfine field of the probe
nuclei 111Cd and 181Ta in TbCo2
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PAC spectra of HoCo2
181TaHoCo2
111CdHoCo2
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The temperature dependence of the magnetic
hyperfine field of the probe nuclei 111Cd and
181Ta in HoCo2
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SUMMARY / REMARKS

• For T/Tc 0.5 the MHF of 181Ta decreases much
  faster with temperature than that of 111Cd, both
  in SOT compounds (TbCo2) and FOT compounds
  (HoCo2) probably because 111Cd is closed shells,
  so MHF is mainly caused by the spin polarization
  of the s-conduction electrons.
• In contrast, 181Ta has an open 5d-shell with a
  finite 5d spin which may produce a core
  polarization contribution to the MHF. It is
  conceivable that the expectation values of this
  core polarization depends on coupling of the 5d
  spin to the host magnetization.
• If the coupling is weak, the 5d spin possibly
  fluctuates, resulting in a decrease of the time
  averaged core polarization part of the field
  which should be stronger the closer one comes to
  Tc.
• Probe isotope 140La/140Ce

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Magnetic elements
3d elements MnNi - unclosed 3d shell (Ar)
3dm
Rare earth (4f) elements (Xe) 4fn 5d2 6s1 in
solids mostly R3 - (Xe) 4f n

except Ce, Eu, Yb
 
n 0 1 2 3 4 5 6
7 8 9 10 11 12 13 14
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Itinerant electron metamagnetism
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Magnetic properties of trivalent rare earth (R)
ions
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RCo2 metamagnetism driven by the 4f-exchange
field (g-1)J