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Electron Spins Resonance and Anitferromagnetic Resonance on LaMnO₃

László Mihály

Stony Brook University and National Synchrotron Light Source (BNL)

LaMnO₃

Parent compound of CMR materials
Distorted perovskite structure
Mn sites have localized spins, undergoes antiferromagnetic ordering at 145K
Spins are ferromagnetic in ab plane, AF in c direction
AF resonance: collective precession of spins
Precession mode depends on orientation of external fields

AF resonance: history

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, zero external field:
- precession around local field
- two degenerate modes
Finite external field: Degeneracy is lifted, two branches
Splitting depends on direction of the external field

Goal #1: How much of this applies to LaMnO₃?
Goal #2: LaMnO₃ has ferromagnetic moment in the c direction. Why?

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

Measured absorption as a function of frequency at many fields; convert to map of H - w plane.

Results
Field parallel to spins (b direction): Kittel theory seems to work

Paramagnetic state: linear relationship, as for quasi-free spins

Low temperature full agreement with theory
Temperature dependence
Field perpendicular to spins (a and c directions): no agreement

Staggered anisotropy and Dzyalushinski-Moriya coupling

Two structural transitions: rotation of octahedra and Jahn-Teller disortion
Explains ferromagnetic coupling within layers, antiferromagnetic between layers
Staggered anistropy: anisotropy axis points along Mn orbital; orbital is tilted.
Results in ferromagnetic moment in c direction
Tilt angle of anisotropy axis, f and strength, H_a
Dzyalushinski-Moriya coupling: D (S₁ x S₁)
Solve equation of motion in the presence of these terms - yields perfect fit to data

Does this make any sense?
Thanks