non-fermi liquid behavior in weak itinerant ferromagnet mnsi nirmal ghimire april 20, 2010 in class...
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Non-Fermi Liquid Behavior in Weak Itinerant Ferromagnet MnSi
Nirmal Ghimire
April 20, 2010
In Class PresentationSolid State Physics II
Instructor: Elbio Dagotto
Introduction Fermi Liquid Theory Non-Fermi Liquid System Non-Fermi Liquid State in MnSi Magnetic Ordering and Spin
Structure Conclusion
OutlineOutline
IntroductionIntroductionThere are two basic mechanism for the observed magnetic moments in magnetic materials
Local magnetic moments Itinerant magnetic moments
W. K. Heisenberg(1901-1976)
E. C. Stoner(1899-1968)
Cases of complete localization or complete ionization are hardly ever found
Both phenomena exist side by
side: A unified theory of solid
state magnetism is needed
1s
c
q
q1
s
c
q
q
IntroductionIntroduction
L. D. Landau(1908-1968)
1957: Fermi Liquid Theory
Model for metallic state:
Pauli exclusion principle + screening effect
Successfully described some near or weak ferromagnetic d-electron metals
MnSi, a weakly magnetic d-electron compound, also shows Non-Fermi Liquid (NFL) behavior
Introduction
Fermi Liquid Theory
Non-Fermi Liquid System
Non-Fermi Liquid State in MnSi
Magnetic Ordering and Spin Structure
Conclusion
OutlineOutline
Fermi-Liquid TheoryFermi-Liquid Theory• Quasiparticle excitation of interacting Fermi
system• Fermi liquids have spin and obey Fermi
statistics
2
1
One to one correspondence of quasiparticle and free electron:
Interaction of the quasiparticle
Energy of the system Energy of N quasiparticles
Fermi-Liquid TheoryFermi-Liquid Theory Energy of a quasiparticle is:
Energy of quasiparticle at T =0Mean 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 TheoryFermi Liquid Theory Total energy:
Prediction:
Electrical resistivity of CeCl3 Specific heat of CeCl3
Experimental confirmation
Non-Fermi Liquid SystemNon-Fermi Liquid System
Physical Properties:
Experimental confirmation
Introduction
Fermi Liquid Theory
Non-Fermi Liquid System
Non-Fermi Liquid State in MnSi
Magnetic Ordering and Spin Structure
Conclusion
OutlineOutline
Non-Fermi Liquid State in MnSiNon-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 MnSiNon-Fermi Liquid State in MnSi
• Magnetic phase transition at Tc=29.1 from paramagnetic to helical magnetic structure
• Wavelength of spiral = 180 Å in (111) direction
Magnetic phase diagram
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
Non-Fermi Liquid State in MnSiNon-Fermi Liquid State in MnSi
Variation of resistivity with temperature
•Resistivity drops monotonically with decreasing temperature
5.55 Kbar
8.35 Kbar
8.35 Kbar
14.3 Kbar
15.5 Kbar
•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
Non-Fermi Liquid State in MnSiNon-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
Introduction
Fermi Liquid Theory
Non-Fermi Liquid System
Non-Fermi Liquid State in MnSi
Magnetic Ordering and Spin Structure
Conclusion
OutlineOutline
Magnetic Order in NFL State Magnetic Order in NFL State
Results from Neutron Scattering experiment:
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
There exists magnetic moment even above pc
Partial m
agnetic orderin
g
Non-Trivial Spin Structure!Non-Trivial Spin Structure!
Two Possible Scenarios for the partial magnetic ordering
Breaking of helical structure into multi-domain state
Unlocking of helix direction from <111> and no strict directional order
Result of polarized neutron scattering : partial order on local scale is not related to helical structure
No experimental or theoretical support
Any other possibility?
Non-Trivial Spin Structure!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!!
ConclusionConclusion
• MnSi , a weak itinerant ferromagnet, shows a 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
• Currently, there is no theoretical account for the NFL resistivity and how it is related to the partial magnetic ordering.
• There is need of more experimental evidences.
2
3
T
Thank You