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
• Spins are ferromagnetic in ab plane, AF in c direction
• AF resonance:  collective precession of spins

• Sample: Oriented single crystal, ~0.5 mm thick, 4.0mm diameter disk
• Spins are perpendicular to surface ((Susceptibility by L. Kiss, Budapest)
• Incident light perpendicular to surface
• External field perpendicualr to surface
• Measured at two temperatures:  200K and 4.2K
• Paramagnetic state: linear relationship as expected for free spins
• AF state: More complex behavior with finite frequency resonance at zero field

• 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