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Electron Spins Resonance and Anitferromagnetic Resonance on  LaMnO3

László Mihály

National Synchrotron Light Source, Stony Brook University, Technical University

Electron Spin Resonance

  • Resonant absorption by the electron spin when hf = 2mBH or f = g(mB/h)H  ; g ~ 2
  • Needs unpaired electron (paramagnet, metal, spin label)
  • Resonance line width, integral, splitting, position measures interaction of electrons, local environment, internal magnetic and exchange fields etc.
    • "Traditional" ESR
      • Typical: f=9GHz, H=0.35Tesla
      • Resonant cavity
      • Conventional microwave source
    • High field ESR:
      • Superconducting magnet (9Tesla - 18Tesla)
      • IR laser, Gunn diode etc. source
    • Pulsed ESR:
      • Holczer/Bruker
    All of the above are restricted to a single frequency, or a limited number of fixed frequencies




Material: LaMnO3

  • Parent compound of CMR materials
  • Distorted perovskite structure
  • Mn sites have localized spins, undregoes antiferromagnetic ordering at 145K
  • Spins are ferromagnetic in ab plane, AF in c direction
  • AF resonance:  collective precession of spins
  

Theory of AF resonance

  • Large body of work in '50s, theory by Keffer, Kittel and others
  • Experiments: Richards, Tinkham, Foner
  • Three terms:  Exchange field (He), anisotropy field (Ha), external field (H0)
  • Ha << He, frequency at zero field: w ~ (HaHe)1/2
  • Uniaxial anisotropy, classical spins:
  • Zero external field:
  • Finite external field: Degeneracy is lifted, two branches
    
 

Experiment #1: High field ESR

  • Sample: Oriented single crystal, ~0.5 mm thick, 4.0mm diameter disk
  • Spins are perpendicular to surface (Susceptibility)
  • Incident EM waves perpendicular to surface
  • External field perpendicular to surface
  • Measured 3 frequencies: 75GHz, 150GHz, 225GHz
    • Paramagnetic state: g = 2 (& new lines, strongly temperature dependent)
    • AF state: More complex behavior



Instrument

Two major components: Spectrometer and magnet

  • Magnet: Oxford Instruments, 16Tesla, max 37 mm sample size
  • Temperature: 1.3K-300K
  • Spectrometer: Sciencetech, Martin-Puplett, step scan, form 2cm-1 to 2000cm-1 , 0.01cm-1 resolution, works with internal and external sources

Others

  • Coupling to light source
  • Coupling between magnet and spectrometer
  • Sample holder, Support structure, Safety devices

Experiment #2: First mapping of H - w plane


Summary

Thanks