High Field Electron Spin Resonance at the National Synchrotron Light Source (Brookhaven National Laboratory)

László Mihály

Quasi-optical detection: Measure transmission/reflection. Quantitative analysis in terms of the complex index of refraction, n=Öem=Öe(1+4pc)

Magnet: Oxford Instruments, 16Tesla, max 37 mm sample size
Temperature: 1.3K-300K
Spectrometer: Sciencetech, Martin-Puplett, 2cm^-1 to 2000cm^-1 , 0.01cm^-1 resolution, works with internal and external sources

Also important:

Synchrotrom source is a must; brightness and absolute intensity advantage are needed to make measurements possible.

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

Budapest results: measured 3 frequencies: 75GHz, 150GHz, 225GHz

Experiments: Richards, Tinkham, Foner

Fields in effective Hamiltonian (uniaxial anisotropy):
Exchange (H_e), anisotropy (H_a), external (H₀), excitation (H_rf)

Zero external field:

Finite external field lifts degeneracy.
Parallel to easy axis: two branches (circular polarization)

Perp to easy axis: one mode is independent of the external field (linear polariztion)

Sample: Oriented single crystal, ~0.5 mm thick, 4.0mm diameter disk
Easy axis is perpendicular to surface (Susceptibility)
Incident EM waves perpendicular to surface
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
- Clear evidence for Dzyalushinskii-Moriya field, H_d = 3.5T
Temperature dependence