Electron Spins Resonance and Anitferromagnetic Resonance on  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
• k=0 magnon  - collective precession of spins
• Same as AF resonance: 

Electron Spin Resonance

• Resonant absorption by the electron spin when hf = 2m_BH or f = g(m_B/h)H  ; g ~ 2 
• 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
• tunable sources
Most are restricted to a single frequency, or a limited number of fixed frequencies

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

• 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
• Other works: frequency scans on pure and doped compounds 

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

• Sample: same; Orientation: same
• Measured absorption as a function of frequency at many fields
• Convert to map
• Paramagnetic state: linear relationship, as for quasi-free spins
• AF state
• Advantage of method: full mapping 
• Low temperature full agreement with theory
• Temperature dependence