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Non-Fermi Liquid Behavior in Weak Itinerant Ferromagnet MnSi

Non-Fermi Liquid Behavior in Weak Itinerant Ferromagnet MnSi. Nirmal Ghimire. April 20, 2010. In Class Presentation Solid State Physics II Instructor: Elbio Dagotto. Introduction Fermi Liquid Theory Non-Fermi Liquid System Non-Fermi Liquid State in MnSi

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Non-Fermi Liquid Behavior in Weak Itinerant Ferromagnet MnSi

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  1. Non-Fermi Liquid Behavior in Weak Itinerant Ferromagnet MnSi Nirmal Ghimire April 20, 2010 In Class Presentation Solid State Physics II Instructor: Elbio Dagotto

  2. Introduction Fermi Liquid Theory Non-Fermi Liquid System Non-Fermi Liquid State in MnSi Magnetic Ordering and Spin Structure Conclusion Outline

  3. Introduction There 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) Both phenomena exist side by side: A unified theory of solid state magnetism is needed Cases of complete localization or complete ionization are hardly ever found

  4. Introduction 1957: Fermi Liquid Theory Model for metallic state: Pauli exclusion principle + screening effect Successfully described some near or weak ferromagnetic d-electron metals L. D. Landau (1908-1968) MnSi, a weakly magnetic d-electron compound, also shows Non-Fermi Liquid (NFL) behavior

  5. Outline • Introduction • Fermi Liquid Theory • Non-Fermi Liquid System • Non-Fermi Liquid State in MnSi • Magnetic Ordering and Spin Structure • Conclusion

  6. Fermi-Liquid Theory • Quasiparticle excitation of interacting Fermi system • Fermi liquids have spin 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

  7. Fermi-Liquid Theory Energy of a quasiparticle is: Mean field effect of interaction with other quasi particles Energy of quasiparticle at T =0 • Scattering amplitude of two quasi particles • Accounts for the deviation of density of states from the equilibrium value nFermi (n- nFermi)

  8. Fermi Liquid Theory Total energy: Prediction: Experimental confirmation Specific heat of CeCl3 Electrical resistivity of CeCl3

  9. Non-Fermi Liquid System Physical Properties: Experimental confirmation

  10. Outline • Introduction • Fermi Liquid Theory • Non-Fermi Liquid System • Non-Fermi Liquid State in MnSi • Magnetic Ordering and Spin Structure • Conclusion

  11. 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

  12. 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

  13. 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 15.5 Kbar • Above pc, temperature variation of resistivity is slower than quadratic 14.3 Kbar

  14. 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

  15. Outline • Introduction • Fermi Liquid Theory • Non-Fermi Liquid System • Non-Fermi Liquid State in MnSi • Magnetic Ordering and Spin Structure • Conclusion

  16. 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 Partial magnetic ordering 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

  17. 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 No experimental or theoretical support Result of polarized neutron scattering : partial order on local scale is not related to helical structure Any other possibility?

  18. 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!!

  19. Conclusion • 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.

  20. Thank You

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