Transcript Presentation

Non-Fermi Liquid Behavior in
Weak Itinerant Ferromagnet MnSi
Nirmal Ghimire
April 20, 2010
In Class Presentation
Solid State Physics II
Instructor: Elbio Dagotto
Outline
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


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Introduction
Fermi Liquid Theory
Non-Fermi Liquid System
Non-Fermi Liquid State in MnSi
Magnetic Ordering and Spin Structure
Conclusion
Introduction
There are two basic mechanism for the observed magnetic moments in
magnetic materials
Local magnetic moments
qc
1
qs
W. K. Heisenberg
(1901-1976)
Cases of complete
localization or
complete
ionization are
hardly ever found
Itinerant magnetic moments
qc
 1
qs
E. C. Stoner
(1899-1968)
Both phenomena
exist side by side: A
unified theory of
solid state
magnetism is needed
Introduction
1957: Fermi Liquid Theory
Model for metallic state:
Pauli exclusion principle + screening effect
L. D. Landau
(1908-1968)
Successfully described some near or weak
ferromagnetic d-electron metals
MnSi, a weakly magnetic d-electron compound, also shows NonFermi Liquid (NFL) behavior
Outline

Introduction
 Fermi Liquid Theory
 Non-Fermi Liquid System
 Non-Fermi Liquid State in MnSi
 Magnetic Ordering and Spin Structure
 Conclusion
Fermi-Liquid Theory
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Quasiparticle excitation of interacting Fermi system
Fermi liquids have spin 12 and obey Fermi statistics
One to one correspondence of quasiparticle and free electron:
Interaction of the quasiparticle
Energy of the system
Energy of N quasiparticles
Fermi-Liquid Theory
Energy of a quasiparticle is:
Energy of quasiparticle at T =0
Mean field effect of interaction
with other quasi particles
•Scattering amplitude of two quasi particles
•Accounts for the deviation of density of states from the
equilibrium value nFermi (n- nFermi)
Fermi Liquid Theory
Total energy:
Prediction:
Experimental confirmation
Specific heat of CeCl3
Electrical resistivity of CeCl3
Non-Fermi Liquid System
Physical Properties:
Experimental confirmation
Outline

Introduction
 Fermi Liquid Theory
 Non-Fermi Liquid System
 Non-Fermi Liquid State in MnSi
 Magnetic Ordering and Spin Structure
 Conclusion
Non-Fermi Liquid State in MnSi
Structure of MnSi
•B20 Cubic structure with a =4.588 Å
•Lacks space inversion symmetry
Consequence of the broken inversion symmetry
Helical spin density wave
Non-Fermi Liquid State in MnSi
Magnetic properties:
•Curie-Weiss fit of susceptibility: Effective magnetic moment = 1.4 μB
•Observation: spontaneous magnetic moment of 0.4 μB at 0K.
Weak itinerant
ferromagnet
Magnetic phase diagram
•
Magnetic phase transition at Tc=29.1 from
paramagnetic to helical magnetic structure
•
Wavelength of spiral = 180 Å in (111)
direction
Non-Fermi Liquid State in MnSi
Variation of resistivity with temperature
8.35
Kbar
8.35
Kbar
5.55
Kbar
•Resistivity drops monotonically
with decreasing temperature
•Peak position indicates the
transition temperature
•Below pc=14.6 Kbar, there is
quadratic behavior
•At pc, quadratic behavior
collapses
•Above pc, temperature variation
of resistivity is slower than
quadratic
15.5
Kbar
14.3
Kbar
Non-Fermi Liquid State in MnSi
Comparison between experiment and FFL Theory
High T: FFL model in agreement
with experiment
Low T: T dependence deviates
from experimental observation
Outline

Introduction
 Fermi Liquid Theory
 Non-Fermi Liquid System
 Non-Fermi Liquid State in MnSi
 Magnetic Ordering and Spin Structure
 Conclusion
Magnetic Order in NFL State
Results from Neutron Scattering experiment:
There exists magnetic moment even above pc
Critical pressure = 14.6 Kbar
Magnetic Ordering above critical pressure?
Helical with same periodicity and long range order
Unusual thing:
Considerable degree of disorderness in the
direction of magnetic propagation vector
A broad angular distribution around <110>:
not expected to be favored by the crystal field
in cubic symmetry
Non-Trivial Spin Structure!
Two Possible Scenarios for the partial magnetic ordering
Breaking of helical structure
into multi-domain state
Result of polarized
neutron scattering : partial
order on local scale is not
related to helical structure
Unlocking of helix direction from
<111> and no strict directional
order
No experimental or
theoretical support
Any other
possibility?
Non-Trivial Spin Structure!
Quantum critical phenomena?
NFL resistivity emerges under pressure without quantum criticality
Spin ordering other than plain pining of the
helix or a multi-domain state
A non-trivial spin structure!!
Conclusion
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MnSi , a weak itinerant ferromagnet, shows a T 2 behavior of
resistivity which is not consistent with current model of itinerant
ferromagnetism
•
Temperature dependence of resistivity may lie in the novel form of
magnetic ordering
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Currently, there is no theoretical account for the NFL resistivity and
how it is related to the partial magnetic ordering.
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There is need of more experimental evidences.
Thank You