Selective Excitation Double Mössbauer Spectroscopy

McGill
Mössbauer Group

Selective Excitation Double Mössbauer Spectroscopy
(SEDM).

Selective excitation double Mössbauer Spectroscopy (SEDM) is a variant of Mössbauer Spectroscopy that is uniquely sensitive to slow (0.1 - 20 MHz) fluctuations. We have made substantial improvements to the technique that have allowed us to study a variety of materials exhibiting static and dynamic disorder:

Amorphous Fe₈₀B₂₀, is a magnetic metallic glass that shows substantial static chemical disorder.
Fe₆₅Ni₃₅ ``INVAR'' is a chemically disordered crystalline alloy that has been claimed to exhibit magnetic relaxation effects.
Magnetite ferrofluids are fine magnetic particles suspended in kerosene that exhibit superparamagnetic fluctuations above 50 K.
Amorphous Fe₉₂Zr₈, is a frustrated magnetic system that combines substantial chemical disorder with magnetic fluctuation effects at the transverse spin freezing transition.

Energy level diagram for ⁵⁷Fe in a static magnetic field.	By tuning a source to drive a specific transition in the sample, one selected hyperfine level can be populated. When the excited nucleus returns to its ground state, it will emit radiation that can be Mössbauer analysed.
Top: Transmission spectrum of Fe metal showing the six absorption limes that correspond to the six transitions identified in the level diagram above. Bottom: SEDM spectra obtained while driving lines 1 - 3 (red arrows).	If the magnetic environment is static (as here in alpha-Fe), then the radiation re-emitted is just that allowed by the selection rules for the populated state. Thus driving line 1 (red arrow) simply yields line 1, while driving lines 2 or 3 yields two lines as there are two allowed transitions from the m_e = +1/2 and -1/2 states.

Our system has been thorougly tested on Fe-metal and amorphous-Fe₈₀B₂₀ at room temperature.
We have run experiments at elevated temperatures (RT-600 K) on Invar (Fe₆₅Ni₃₅) looking for evidence of magnetic relaxation that has been claimed to affect the magnetic properties of this material. Our data show absolutely no evidence for relaxation up to 0.99 T_c.
We added a low-temeprature system based around a closed-cycle refrigerator that cools to 10K, and have used it to study superparamagnetism in ferrofluids, and dynamics around the transverse spin freezing transition in a partially frustrated ferromagnetic glass.

Our room/high - temperature SEDM system.

The drive at the left runs at constant velocity and is tuned to drive the selected transition in the sample.
The vertical cylinder is a vacuum enclosure for the sample and to its right there is a proportional counter used to obtain a transmission spectrum and allow tuning of the constant velocity drive.
A second Mössbauer drive with a resonant conversion electron detector is housed in the box at the upper right. The detector is run in constant acceleration mode and serves to Mössbauer-analyse the radiation scattered from the sample.

Search for relaxation in INVAR

Top: Transmission Mössbauer spectrum for amorphous-Fe₈₀B₂₀ at room temperature.
Bottom: SEDM spectra obtained on the same sample while driving lines 1 and 2 (red arrows).

Chemical disorder leads to substantial line broadening in the transmission spectrum, and this static disorder also leads to increased linewidth in the SEDM spectra. The SEDM patterns can be fitted by assuming that the line broadening is just due to the static disorder seen by transmission.

Top: Transmission spectra of INVAR at various temperatures.
Bottom: SEDM spectra obtained while driving line 1 at the same temperatures.
The lowest SEDM pattern shows a fit calculated assuming that the form of the transmission spectrum at 496 K is due to relaxation effects.

The transmission spectra of Fe₆₅Ni₃₅ ``INVAR'' can be fitted equally well by assuming either static or dynamic effects are dominant.
However the SEDM spectra shown here can be fitted by assuming only static broadening is present. Dynamic effects would lead to an additional line (as shown for the 496 K spectrum) and this feature allows us to rule out any dynamic contribution faster than 0 ± 0.1 MHz.
The order is purely static.

Our low-temperature SEDM system.

The sample is cooled by a closed-cycle refrigerator.
The constant-velocity drive is at the right and there is a proportional counter at the left to obtain transmission spectra and allow tuning of the constant velocity drive.
The analyser consists of a resonant conversion electron detector mounted on a constant-acceleration drive and is visible behind the shielding at the lower centre of the picture.

The electronics stack, at the left, is a conventional Mössbauer setup, and the sample is cooled by the closed-cycle fridge at the lower right.
The spectrometer is completely vibration-isolated to prevent external line broadening effects from destroying the resolution.
The system was designed and built by Johan van Lierop who is standing behind the system trying to not look surprised that it all works.

Ferrofluids: a demonstration sample

Ferrofluids (6nm particles of Fe₃O₄ in kerosene) provide the simplest example of a system that exhibits magnetic fluctuations. Above about 50 K, they are superparamagnetic, i.e. the particle moments undergo rapid 180^o flips. These magnetisation reversals mix the m_e = +3/2 and m_e = -3/2 levels and allow an extra line to develop in the SEDM spectrum.

At 20 K the system is static and slight line broadening due to chemical disorder is observed.

As the temperature is raised, collective excitation develop (the moments rock in the potential well defined by the anisotropy of the particle) and the line at -7 mm/s broadens.

SEDM spectra of a 6 nm ferrofluid obtained by driving line 1. With increasing temperature, first the direct line at -7mm/s broadens as collective excitations develop, then a new line (6) appears at +7 mm/s as moment reversals start.	By 70 K superparamagnetic spin-flips have started and a new line develops at +7 mm/s. This line can only be due to moment reversals and is clear evidence of magnetic dynamics. It was the absence of this feature from the INVAR data that allowed us to rule out dynamic effects in that system.
Relaxation rates derived from the SEDM spectra shown above. Moment reversals occur above the blocking temperature of 54 ± 3 K.

Dynamics at T_xy
Numerical simulations indicated that there would be a significant fluctuation peak as the transverse spin components ordered at T_xy in a partially frustrated ferromagnetic system. Detecting such behaviour in the presence of ferromagnetic order led us to use both µSR and SEDM.

SEDM spectra a-Fe₉₂Zr₈ at 80K obtained driving line 1 (top) and line 2 (bottom). The additional lines due to moment reversals associated with the fluctuations at T_xy are clearly evident.	As with the ferrofluids shown above, moment reversals lead to additional lines in the SEDM spectrum. Line 6 appears when line 1 is driven, while a more complex 4-line pattern is observed when line 2 is driven. Even without further analysis, the spectra clearly demonstrate that relaxation is taking place.
Comparison of relaxation rates derived from µSR and SEDM around T_xy for a-Fe₉₂Zr₈. Insets show the static (left) and dynamic (right) signals derived from µSR for both T_c and T_xy.	The comparison with µSR is particularly instructive. µSR is an interstitial probe that is affected by any fluctuations in its magnetic environment. By contrast, SEDM probes the behaviour of the iron moments directly, and is dominated by moment reversals. The almost perfect agreement in derived relaxation rates around T_xy indicates that moment reversals are the dominant relaxation mechanism.

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Updated: 09/May/03 by Dominic Ryan, ERP 429, (514) 398-6534.