1. DEMONSTRATION EXPERIMENTS
Magneto-optical far-infrared spectroscopy at the NSLS
Diyar Talbayev (SUNY, Stony Brook), G. Lawrence Carr (Brookhaven National Laboratory),
Laszlo Mihaly (SUNY, Stony Brook)
INSTRUMENTS
U12IR beamline of the National Synchrotron Light Source (NSLS) is dedicated to the study of optical properties of matter in the far-infrared spectral region. A fourier transform far-infrared spectrometer, a
16 T superconducting magnet, and the VUV synchrotron ring of the NSLS are the main components of the experimental setup .
Magnet: Oxford Instruments
Tesla
Vertical optical axis
37 mm sample space
1.3K-300 K temperature range
Spectrometer: Sciencetech
Martin-Puplett, step scan
0.01cm-1 (~ 100Gauss ) resolution
2 cm-1 to 1000 cm-1 spectral range
Works with internal and (synchrotron) external sources
The available equipment allows the users to measure the spectra of samples with the infrared light propagating along the magnetic field. The possibility of sending light through samples and perpendicular to the field is being implemented.
With no external magnetic field, both reflection and transmission spectra of samples can be measured. In these measurements, a focusing mirror optics sample chamber that accommodates either a helium-flow (Helitran) or an immersion (Oxford Optistat) cryostat is used. The transition from transmission to reflection geometry is conveniently effected by moving only one mirror in the sample chamber. The mirror can be moved without disturbing any part of the chamber or venting it to the ambient pressure or. Liquid He cooled (4.2 K and 1.5 K) silicon bolometers detect the reflected or transmitted infrared light.
In magnetic field only transmission measurements are possible with the presently available equipment. The superconducting magnet replaces the sample chamber, and a length of cylindrical light-pipes carries the spectrometer output light to the bottom window of the magnet. (See the figure on the right.) The light-pipe continues along the magnet axis, and samples are mounted inside the pipe.
FUTURE PLANS
Use unique ability to cover wide range of frequencies and fields with a calibrated intensity mapping.
Identify the best magnetic field/frequency for Single Electron Spin Microscopy applications.
Screen the samples to be studied
Needed: Tenfold increase of resolution - high resolution FTIR spectrometer
DEMONSTRATION EXPERIMENTS
Antiferromagnetic resonance (AFMR) in LaMnO3
Magnetic moments on Mn3+ ions order antiferromagnetically below TN=140 K
Measurement Conditions:
Transmission geometry
Driving H field of light perpendicular to ionic magnetic moments
External field parallel to magnetic moments
K temperature range
Sample:
Oriented single crystal
~ 0.6 mm thick, 4.0 mm diameter disk
Magnetic moments of Mn3+ ions are perpendicular to surface
Time scales:
A single frequency scan took about 15 min
Complete mapping for fields 0-16 Tesla took 3 hours
AFMR theory for pedestrians
The classical picture of AFMR starts with the motion of a magnetic moment in a static magnetic field. Magnetic moment is usually associated with mechanical angular momentum, and the classical angular momentum equation of motion leads to the magnetic moment precession with frequency w=gH0 in the external magnetic field H0.
In an antiferromagnet, we see two sublattice magnetic moments moving in external magnetic field plus the antiferromagnetic exchange field He and anisotropy field Ha. The combination of these fields results in a periodic motion of sublattice moments. The frequency of this motion in zero external field is w=g(Ha(Ha+2He))1/2. Figure 1 illustrates the resonance frequency behavior in non-zero external magnetic field along the easy axis of magnetization.
The described motion of sublattice moments implies that the individual spins on each sublattice move collectively, each spin performing the same dance as any other spin. This is a particular case of the more general spin motion which is described in quantum language as magnons. Thus AFMR can alternatively be referred to as the magnon with k=0. Figure 2 shows the magnon dispersion curves in LaMnO 3 measured in neutron scattering experiments. The frequency at k=0 corresponds to the AFMR frequency in zero external field. (Neutron scattering frequency w » 0.6 THz = 20 cm-1; AFMR frequency w = 18 cm-1.)
Fig 1. Low temperature spin-wave calculation of the resonance frequencies in the external field along the easy axis. Magnetic susceptibility c|| along the easy axis is taken to be 0.
Measurement Results
Figure 3 shows the transmission spectra of LaMnO3 sample taken with the setup in the spectrometer + magnet configuration. The curves represent the transmitted intensities at T=50 K with 7 T magnetic field and at T=180 K with zero and 1 T magnetic fields. Most of the structure in these spectra is inherent to the synchrotron radiation spectrum. The lower temperature spectrum, however, exhibits two AFMR absorption lines - one at 12 cm-1 and the other at 24 cm-1.
We use the average of low field (0-2 T) high temperature (160 K, 180 K) spectra of the sample as a background spectrum and divide the spectra recorded at various sample temperatures and applied magnetic fields by this background. The procedure allows us to isolate the magnetic resonance absorption lines, and we get the frequency-field mapping of the magnetic resonance in LaMnO3. The frequency-field behavior of the resonance at different temperatures is shown in figure 4. Temperature range starts at 180 K and goes down to 15 K. The resonance frequency in the paramagnetic state, above 140 K, is proportional to the external magnetic field; this corresponds to the precession of magnetic moment in a static field. As the temperature goes below TN, the development of the sublattice magnetization gives rise to exchange and anisotropy fields, and we observe finite resonance frequency in zero field. In non-zero field at low temperatures the resonance line splits in two lines which correspond to sublattice magnetizations along and opposite the direction of the external magnetic field.
Fig. 2. Magnon dispersion curves in LaMnO3
measured by neutron scattering. F. Moussa et al, Phys. Rev. B (1996)
Fig 4. Dark purple regions correspond to AFMR absorption. Red line represents the paramagnetic resonance with g = 2.
Fig 3. Red curve: H = 0 T, T = 180 K; blue curve: H = 1 T, T = 180 K; black curve: H = 7 T, T = 50 K
LEES 02 International Conference, October 2002