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Spins Resonance on Strongly Correlated Electron Systems

László Mihály

Stony Brook University and National Synchrotron Light Source (BNL)

Instrument at U12IR

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.

Material: 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?

Other works

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.
Neutron scattering (magnons) by Moussa et al.
High field ESR: Mitsudo, Pimenov's group

Interpretation by D-M interaction

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 S₂)
Solve equation of motion in the presence of these terms - yields perfect fit to data

Does this make any sense?

Material: La₂CuO₄

Parent compound of high T_c materials
CuO₂ layered structure
Copper ions have localized spins, antiferromagnetic below ~120K
AF resonance has not been detected
Preliminary results on "powder" samples

Material: NaNiO₂

Frustrated antiferromagnet
High temperature structure: rhombohedral, with triangular Ni lattice
Ni³⁺ ions in t⁶_2ge¹_g configuration
Cooperative JT distortion at 475K; becomes monoclinic
Magnetic order at 20K (Magnetization only - no evidence from neutrons)
H - w mapping was done on a powder sample
Next: LaNiO₂, with no known JT distortion

L.Mihály, D. Talbayev, L.F. Kiss, J. Zhu, T. Fehér and A. Jánossy, PRB 69 024414 (2004)
D. Talbayev, L. Mihály, J. Zhu, Phys. Rev. Letters, 93 017202, 2004