Electron Spins Resonance and Anitferromagnetic Resonance on  LaMnO₃

• Magnet: Oxford Instruments, 16Tesla, 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

• Parent compound of CMR materials
• Distorted perovskite structure
• Mn sites have localized spins, undregoes antiferromagnetic ordering at 145K
• Susceptibility by László Kiss, Budapest: Spins are perpendicular to surface 
• Spins are ferromagnetic in ab plane, AF in c direction
• Magnons: dispersion and order parameter (Moussa et al., 1996)
• Spin resonance: Broad signal (Mitsudo et al., 1998)
• AF resonance:  collective precession of spins

• Large body of work in '50s, theory by Keffer, Kittel
• Three terms:  Exchange field, anisotropy field, external field
• 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 extrenal: Degeneracy is lifted, two branches
• Field dependence

• Sample: Oriented single crystal, ~0.5 mm thick, 4.0mm diameter disk
• Incident light perpendicular to surface
• Externalfield perpendicualr to surface
• Frequencies: 75GHz, 150GHz, 225GHz
• 225GHz: high temp, around transition, low temp
• 150GHz
• Summary:
• Paramagnetic state: Broad, "regular"  ESR line + anomalous signal 
• Around critical temperature: Very complex behavior
• Low temperature:  mostly out of range

• Sample: Same
• Orientation: Same
• Zero field: AF resonance temperature dependence
• Full map: Measured at two temperatures (200K and 4.2K)
• Paramagnetic state: linear relationship as expected for free spins
• AF state: Kittel's theory works