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

• The resonance line in "pure" LaMnO₃ is too broad for Q-band
• In doped samples spin diffusion leads to motional narrowing, yields narrower line. Paramagnetic state studied in great detail by Oseroff, Muller and others.
• High field ESR: Mitsudo, Pimenov's group

• 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

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