Magnetic frustration in intermetallic compounds Introductory remarks Frustrated systems with localized moment Itinerant magnetic systems with frustration ! Intermetallic compounds, not metallic oxides ! No spin glass C. Geibel Max-PIanck Institute for Chemical Physics of solids Dresden, Germany
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C. Geibel- Magnetic frustration in intermetallic compounds
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Magnetic frustration in intermetallic compounds
Introductory remarks
Frustrated systems with localized moment
Itinerant magnetic systems with frustration
! Intermetallic compounds, not metallic oxides
! No spin glass
C. Geibel
Max-PIanck Institute for Chemical Physics of solids
Dresden, Germany
Frustration in intermetallic compounds
Rare earth
localized moments
large total angular moment at high T
! but at low T reduced by
crystal field effects
Ce, Yb effective doublet at low T
very strong spin orbit coupling
often large anisotropy
not a problem
Exchange mechanism: RKKY
! long range (1/r3)
Unlikely that only NN (and NNN)
exchange is relevant
Probably a severe problem
Two types of magnetic moments
3d metals: Itinerant magnetism
no stable local moment
Effective interaction defined by
Fermi surface
description in terms of NN-
interactions not appropriate?
completely different approaches
needed?
Maximum of generalized
susceptibility (Q)
Sharp peaks on Fermi surface
or broad hills?
Appropriate structures?
Many structures with topological frustrated sublattices
Example: CaCu5 structure type, here CeCo3B2
- Co site: kagome lattice
- Ce site: triangular
huge number of compounds with magnetic atoms
on different sites
But evidence for frustration rather exceptional
Problem:
no directional bonding in metals
Very difficult to get well defined
2- or 1-dimensional systems
Ce
Co
B
RTX compounds with ZrNiAl structure
ZrNiAl structure : Hexagonal
R: Rare earth element Magnetism
T: transition metal non-magnetic
X: p-metal: Al, In, Sn, Bi
R -atoms distorted Kagome lattice
equivalent bonds but triangle tilted
Very complex antiferromagnetic structures
evidence for frustration
B. Fåk et al.
G. Ehlers et al. (1997) YbNiAlH. Maletta et al. (1994) TbNiAl
C(T) and ( )T:
Pronounced transition at TN = 3.6 K
paramagnetic to antiferromagnetic
Further transition at TN2= 1.4 K
Specific heat (Trovarelli et al., 2000)YbPtIn
Mössbauer spectroscopy (Bonville et al., 2007):
TN2 < T < TN:
only tiny ordered moment along hard c axis
no ordered moment along easy basal plane
T < TN2:
large ordered moment on 2/3 of Yb-sites
no ordered moment on 1/3 of Yb sites
! absence of ord. moment persist until 60 mK
Mössbauer spectra
Umeo et al. (2004), Morosan et al.(2004)
Transition at TM1 = 0.8 K
strong fluctuations above TN
Efluc 10K >> TN
Kondo or frustration?
1rd order transition at TM2= 0.6 K
YbAgGe: Frustration and Kondo?
B-T phase diagram
(T), ( )T and C(T)
Elastic neutron scattering:
Fåk et al. (2005), Fåk et al. (2005)
TM2 < T < TM1:
incommensurate along c
sum of basal plane components = 0
T < TM2: commensurate along c
AF-order in basal plane
YbAgGe: magnetic structure
neutron peak intensity
Mössbauer (Bonville et al., 2007)
inhomogeneous line broadening
modulated structure down to 60 mK?
Lines are still broaden above TM1
evidence for fluctuating Yb moments
fl 1 B 3·109 s-1
modulation of Yb moment
YbAgGe: dynamic magnetic susceptibility
Inelastic neutron scattering (Fåk et al., 2005)
dynamic suscept. dominated by quasi elastic spin fluctuations, no spin waves
no Q dependence within basal plane strong Q dependence along c*