Vij D.R. Handbook of Applied Solid State Spectroscopy

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point created by a bucking coil. A zero-field region is essential if one is to

avoid magnetic splitting of the source, which will not only broaden the line,

but can also lead to significant polarization corrections to the observed line

intensities on absorption by the sample. The source is then connected to the

Mössbauer drive by a long rigid rod. Source-detector separations of 20–30 cm

are common with the J passing through a series of aluminized mylar or kapton

windows to exit through the base of the cryostat. It is essential to ensure that

the long drive rod is braced against transverse motions. Allowing the rod to

slide through several guide rings works, but this practice is prone to jamming

as the various ices build up. A more satisfactory arrangement uses two or

three disc-springs of the same kind used in the construction of Mössbauer

drives. These are flexible to longitudinal motions but extremely stiff in the

transverse direction and, as they do not involve a sliding contact, they do not

jam.

If it is necessary to apply the field perpendicular to the J direction

(transverse geometry), then some form of split solenoid is needed (two

superconducting coils with a small, ~3-cm gap to provide optical access).

Such systems come in two forms. In the first, the solenoid is mounted

horizontally and the source-sample path is vertical as in the design discussed

above. A bucking coil may be provided to create a zero-field region at the

source, but such an arrangement would be unusual. Measurements can also be

made in an axial field by providing optical access across the cryostat body,

and hence along the bore of the solenoid, through windows on the cryostat

walls. In the second, the field is vertical and a bucking coil again provides a

zero-field point for axial work, while side windows on the cryostat permit

transverse field measurements. While significantly more expensive, split-coil

magnet systems are far more versatile as they make both axial and transverse

measurements possible. The choice of magnet orientation will be dictated in

part by the expected frequency of use for the two field directions, but the

vertically oriented split-coil design is more common.

Temperature control is typically through a helium-flow system, with

thermometry provided by resistance devices such as carbon-glass or

ruthenium oxide. Si diode thermometers are unsuitable as they exhibit severe

field shifts.

5.2.6.3 Cold-Source Spectrometers

As the Mössbauer J energy exceeds about 30 keV, the recoil-free fraction

becomes progressively smaller. Mössbauer spectra cannot be recorded at

ambient temperatures and both the source and sample must be cooled. A

standard arrangement is to place the source and sample inside a conventional

helium-flow cryostat, with the source coupled to an externally mounted drive

using a long connecting rod in much the same arrangement as that used for

systems with superconducting solenoids. Issues with the calibration of these

5.2 Methodology

227

5. Mössbauer Spectroscopy

long-rod spectrometers were discussed earlier. One key difference from the

magnet systems is that the source generally needs its own thermometer and

heater, as it may be necessary to maintain the source at a slightly elevated

(with respect to the sample) temperature to avoid ordering or relaxation

effects broadening the source linewidth.

Cold-source Mössbauer isotopes often have many interfering J-photons

and X-rays, so high-rate optimized HPGe detectors would be the best choice.

There are many useful Mössbauer isotopes with J energies in the 60–80-keV

range, with the 103-keV transition in

153

Eu being about the highest practical

Mössbauer energy in use.

5.2.6.4 High Temperatures

Many Mössbauer isotopes, notably

Fe,

119

Sn,

151

Eu, and

169

Tm, exhibit

useful recoil-free fractions well above ambient temperatures, and so furnaces

can be used to heat samples and measurements can be made as high as

1000°C in some cases. Heating is typically achieved resistively, with the

sample and hot stage in vacuum, and thermocouples are used for thermo-

metry. The polymer windows used in cryostats are replaced by thin beryllium

sheets (typically 0.1 mm thick). The outer, vacuum-supporting, windows can

be made from polymers as long as they are adequately protected from the

radiated heat from the hot sample stage. A number of commercial designs are

available, but with access to a reasonable workshop, it is possible to build a

good Mössbauer furnace.

5.2.6.5 High Pressure

The effects of pressure on structural and magnetic phase transitions are of

fundamental interest. Furthermore, many iron-containing minerals undergo

geologically relevant structural changes under pressure, making high pressure

Mössbauer studies an area of active interest. The detailed methodologies are

quite specialized so only basic ideas are outlined here.

Achieving geologically relevant pressures of 100 GPa places a researcher

in the realm of diamond-anvil pressure cells. Diamonds make perfect

windows for

Fe Mössbauer spectroscopy but they are rather expensive, and

reaching such high pressures with accessible forces means that the actual

contact area is limited. As a result, samples are extremely small (less than

1 mm in diameter) and conventional sources are impractical. Special high

specific activity “point” sources ~0.5 mm in diameter are used so that they

can be placed quite close to the sample. Collimation is provided by the

diamond support structure, which is far less transparent to the 14.4-keV J.

228

Conventional drives and detectors can be used. The real experimental

challenges center on the loading and operation of the diamond-anvil pressure

cell.

5.2.7 Emission-Based Techniques

The decay from the Mössbauer level in many Mössbauer isotopes is heavily

internally converted, leading to the emission of a low energy K-shell or

L-shell conversion electron rather than the Mössbauer J. While this is often a

disadvantage from the standpoint of source preparation, it does open some

important windows onto the sample. In a conventional transmission

Mössbauer experiment, the signal appears as resonant absorption dips: When

the modulated source energy matches an allowed Mössbauer transition in the

sample, some J are absorbed and the count rate at the detector is reduced.

However, each of these resonant absorption events creates a nucleus in the

appropriate excited state. In isotopes where internal conversion is significant

(e.g.,

Fe) most of these excited nuclei will return to the ground state through

the emission of one or more conversion electrons, often followed by

characteristic X-rays. If these electrons or X-rays are detected, then an

emission Mössbauer spectrum can be obtained.

As the most common isotope used for emission studies is

Fe, most of

what follows deals specifically with Fe-Mössbauer work. However, with

suitable minor modifications,

119

Sn and

151

Eu work can be performed with

essentially the same methods.

5.2.7.1 Conversion Electron Mössbauer Spectroscopy (CEMS)

In Figure 5.15 we show the possible de-excitation processes after irradiation

with the 14.4-keV

Fe Mössbauer J-ray.

Figure 5.15 De-excitation processes in

Fe (Ryan and van Lierop, unpublished).

5.2 Methodology

229

5. Mössbauer Spectroscopy

The conversion electrons typically have energies of order 10 keV and have

penetration depths of only ~100 nm, so if the electron is detected, it must have

originated within the top 100 nm of the sample surface. Conversion electron

Mössbauer spectroscopy (CEMS) is therefore a surface-sensitive technique.

The conversion electrons are emitted following a resonant absorption event in

the sample so that the signal appears as a series of emission peaks rather than

absorption dips; however the Mössbauer pattern is unchanged and the exact

same information can be obtained from the spectrum.

The extremely low penetrating power of the conversion electrons rules out

the use of windows between the sample and detector. The sample must be

placed within the detector and this detector must be made sensitive to the

backscattered electrons but insensitive to the substantial flux of J- and X-rays

incident from the source. Gas-filled proportional detectors are the norm, with

He+10% CH

at atmospheric pressure being the standard fill gas. Detector

construction must employ low atomic number materials throughout in order to

minimize fluorescence contributions to the background. Polymers overcoated

with thin aluminium layers for conduction are common. The thickness of the

active volume is kept to a few mm to minimize J- and X-ray sensitivity and a

thin polymer filter in front of the source can be used to minimize incident

X-ray flux. These detectors provide essentially no useful energy resolution

and a simple lower-level discriminator after the amplifier is generally used

just to cut off electronic noise.

The choice of materials, thin windows and the presence of the sample

within the active volume preclude the evacuation and aggressive bake-out

normally used to out-gas a proportional counter prior to sealing, therefore a

continuous flow of counter gas is used to flush out contaminants before they

can accumulate and affect detector operation. Flow rates of a few cc/min are

typical.

With no windows between the sample and detector, non-ambient work

requires both sample and detector to be at the same temperature and the

sample is necessarily exposed to the counter gas at that temperature.

Operating temperatures are limited above by failure of the polymers used in

the construction, and below by condensation of the quench gas (CH

). Flow

systems down to 90 K are quite convenient, but the increasing gas density at

one atmosphere affects the detector efficiency so that sealed units (with

limited operating times) are preferred for lower temperatures. Alternatively,

the sample can be mounted in vacuum and electron multipliers such as

channeltrons or microchannel plates can be used.

Count rates tend to be much lower than those normally encountered in

conventional transmission experiments. However, the signal appears as

emission over a small background so that good signal-to-noise ratios can be

achieved in comparable times with a well-designed detector. CEMS is used to

study very thin samples (detection limits for a usable Mössbauer spectrum in

a few days can be as low as one monolayer of

Fe on an area of 1 cm

) such

as magnetic multilayers or spin-valve systems. It also finds applications where

230

only the surface of the sample is of interest, such as in corrosion or surface

damage studies, or where the sample is simply too thick for conventional

transmission work and cannot be thinned.

5.2.7.2 X-Ray Backscattering (XBS) Spectroscopy

The emission of a K- or L-conversion electron leaves a hole in the

corresponding atomic level. This hole can be filled radiatively through the

emission of a K

or L

X-ray. These X-rays are typically more penetrating

than the conversion electrons and have ranges of a few microns. The X-ray

backscatter (XBS) Mössbauer spectrum takes the form of a series of emission

peaks, but the information contained in the spectrum comes from a thicker

surface region than with CEMS.

Applications include deeper surface examination and comparison of

immediate and deeper surface layers. As with CEMS, XBS is also employed

where the sample cannot be made thin enough for transmission work.

An effective XBS detector can be made by simply changing the fill gas in

a CEMS detector from helium to Ar+10% CH

, also at atmospheric pressure.

This mix gives sufficient sensitivity to X-rays without appreciable sensitivity

to the 14.4-keV J-rays from the source. It is critical to attenuate fully all

X-rays from the source as these will be detected as a background and greatly

reduce the observed signal-to-noise ratio. If the sample is mounted within the

detector (as it would be for CEMS work) then about 20% of the detected

signal will be due to conversion electrons, which will introduce a slight

surface bias to the spectrum. This electron contribution can be eliminated by

replacing the back plate of the CEMS detector with a thin aluminized mylar

window, which is transparent to the X-rays, but will block electrons. The

sample is then placed outside the detector.

With the sample mounted outside the detector it is, in principle, possible to

cool or heat the sample without affecting the detector. However, the very

limited penetrating power of the X-rays puts severe constraints on the number

of windows that can be placed between the sample and the detector.

5.2.7.3 Selective Excitation Double Mössbauer (SEDM) Spectroscopy

Some fraction of the nuclei excited by a resonant absorption event will return

to the ground state through the re-emission of a J at the original Mössbauer

energy. If this re-emission is also a Mössbauer event, i.e., it is recoil-free, then

the J can be Mössbauer analyzed in a second Mössbauer experiment. This

effectively turns the sample into a Mössbauer source. Selective excitation

double Mössbauer (SEDM) spectroscopy is one of the most difficult

Mössbauer techniques, but it provides unique and unambiguous information

on dynamics.

5.2 Methodology

231

5. Mössbauer Spectroscopy

If the original source is driven in conventional constant-acceleration (or

sine) mode, then the sample is exposed to all possible J energies and the re-

emission spectrum is identical to that which would be obtained by

transmission, CEMS, or XBS. No new information is available. However, if

the source is driven at constant velocity, and this velocity is chosen to

coincide with a specific hyperfine level in the sample, then only a single state

in the sample is populated, and all re-emissions are produced by decays from

this selectively excited state. Even in the simplest case of a well-defined static

hyperfine environment, selection rules governing the decay process can lead

to multiple energies in the re-emitted J spectrum. Indeed, the strongest

emission line need not even correspond to the initial drive energy.

The key application of SEDM lies in the study of dynamics. In a static

system, the re-emitted spectrum of energies is controlled entirely by quantum

mechanical selection rules and no new information about the sample is

obtained (except that the hyperfine environment is in fact static). However,

dynamics such as superparamagnetic relaxation or valence fluctuations can

mix the nuclear levels and allow new transitions to occur that would

otherwise be forbidden. In many cases, systems that exhibit dynamic behavior

also contain some level of static disorder, and both dynamics and disorder can

lead to very similar changes in the Mössbauer spectrum. Unambiguous

separation is extremely difficult and often subject to bias. SEDM provides for

a complete and totally model-independent determination of both

contributions.

Disorder leads to broad lines in the transmission Mössbauer spectrum as a

wide range of hyperfine environments are present. This broad distribution of

environments has no effect on the SEDM spectrum as only a narrow subset of

environments is probed at any given drive energy, and the SEDM pattern is

very slightly broadened, with line intensities given by selection rules. By

contrast, a dynamic hyperfine environment mixes the nuclear levels and

“forbidden” transitions appear in the SEDM spectrum. The existence of such

lines can only be due to dynamics, while the identity and intensity of the new

lines can be used to determine the nature and rate of the fluctuations present.

SEDM requires two Mössbauer drives. The first is used to drive the source

at a constant velocity chosen to correspond to a selected hyperfine transition

(absorption line) in the sample. Since we are working with 14.4-keV J-rays,

we do not have significant window problems, and the sample can be cooled or

heated in conventional furnaces or cryostats. Straight-through windows are

needed to obtain a transmission spectrum of the sample and to allow for

tuning of the source drive to the required velocity. The emission signal is

detected at 90° to the source to minimize backgrounds from the source.

Detection is best carried out using a resonant CEMS detector containing an

Fe-enriched stainless steel sample. This resonant single-line Mössbauer

detector is driven at constant acceleration and Mössbauer analyzes the re-

emitted 14.4-keV J-photons. The detector is gated so that it is active only

232

when the source is being driven at the required constant velocity, and disabled

during the fly-back and acceleration periods. The detector drive is also

synchronized with the source drive so that it sweeps through its full velocity

range while the source is being held at constant velocity. Even with a 50-mCi

source and a large sample data rates are only a few counts per second, so

effective shielding is critical to the success of the measurement.

5.2.7.4 Absorber Thickness

One of the most important considerations one must make when undertaking a

Mössbauer experiment is thickness of the absorbing sample. The spectrum

registered during an experiment comprises both resonant and non-resonant

absorption events, and it is vital that an optimal sample thickness be chosen to

maximize the signal-to-noise of the desired Mössbauer process [27–29]. An

excellent resource is the article by Long et al. [28] in which they demonstrate

the calculation of optimal absorber thicknesses.

Basically, the optimal sample thickness lies between 1/P

and 2/P

g/cm

, where P

is the electronic mass absorption coefficient appropriate to

the radiation and sample being used. These authors provide an excellent

didactic example in which they consider two samples: FeWO

and

25 2

(by weight) of Fe (18.4 wt% and 18.7 wt%, respectively) however, the

presence of the heavy element W in the former leads to significant non-

resonant absorption of the 14.4-keV Mössbauer J-ray and demands a much

thinner sample than the latter, which only contains light elements and

consequently experiences much less non-resonant absorption. In fact, the

optimal FeWO

sample is an order of magnitude lighter than the

[Fe(CN)

(NO)]2H

O sample, despite the fact that the samples contain

the same wt% of Fe!

5.2.7.5 Data Analysis

Unlike the situation in crystallography (both X-ray and neutron) where a

veritable cornucopia of programs exists in the public domain for the analysis

of diffraction data, the field of Mössbauer data analysis seems to have

developed somewhat haphazardly, with a general trend of individual groups

writing their own programs. Some commercial programs for Mössbauer

spectroscopy exist and we mention here the programs NORMOS [30],

RECOIL/MOSMOD [31], and MOSSWIN [32]. A number of free programs

are available through the program repository associated with the journal

Computer Physics Communications. However, the task of writing a basic

least-squares fitting program to fit Lorentzian lines (or other lineshapes) to a

Mössbauer spectrum is not difficult.

5.2 Methodology

Na [Fe(CN) (NO)]

2H O. Both samples contain virtually the same amount

233

5. Mössbauer Spectroscopy

The fitting of Mössbauer spectra involves the setting up of the respective

nuclear Hamiltonians for the excited and ground states and diagonalizing,

something which can be accomplished using a variety of free routines such as

those found in CERNLIB or “Numerical Recipes” [33]. The transition

energies, and hence the Mössbauer line positions, can be found from the

energy eigenvalues while the relative line intensities can be calculated from

the eigenfunctions using the well-known Clebsch-Gordan coefficients.

However, many cases are well treated within the framework of the simpler

perturbation methods described in Sections 5.1.

The fitting of distributions of hyperfine parameters observed in the spectra

of amorphous materials, for example, can be carried out using methods such

method [13] mentioned earlier. The fitting of spectra showing the effects of

electronic relaxation is most usefully carried out using the two-state models of

In reference [34] we list a number of papers that describe some of the

processes involved in fitting Mössbauer spectra. This list is by no means

exhaustive but should provide the interested reader with a good overview of

the fitting process.

5.3 APPLICATIONS

Mössbauer spectroscopy has been widely applied in physics, chemistry,

materials science, geology, and biology. In this section we will illustrate

briefly the use of Mössbauer spectroscopy, and the information it provides,

with a selection of examples taken from our own work.

5.3.1 Magnetism

The spectrum of D-Fe shown earlier in Figure 5.4 is quite simple, due to the

single, high-symmetry, Fe crystallographic site. In Figure 5.16 we show a

more complicated example of magnetic splitting. Here, we present RT (room

temperature) spectra of the permanent-magnet material Nd

B [35]. This

intermetallic compound has a tetragonal crystal structure with six distinct Fe

crystallographic sites, in the population ratio 4:4:8:8:16:16. Each site gives

rise to a magnetically split sextet, including EFG effects due to the lower

crystal symmetry. The subspectral areas are proportional to the number of Fe

atoms in the respective crystal sites and this provides a useful constraint in the

fitting process, thereby reducing the number of free parameters. In the

example shown, we compare the

Fe spectra of a commercial Nd-Fe-B

material (upper spectrum) with that of single-phase Nd

B (lower

spectrum). Commercial permanent-magnet materials comprise the pure

B phase and smaller amounts of other phases. The upper spectrum

234

as the Hesse/Rübartsch and Le Caër/Dubois method [12] or the Window

Blume and Tjon [9] or Wickman and Wertheim [10].

5.3 Applications

clearly shows additional lines due to these other phases; particularly

noticeable is a nonmagnetic doublet, which produces a distinct line around

0.5 mm/s. This is the incommensurate Nd

1.1

minor phase. The relative

spectral areas of the various components allow us to carry out reliable phase

analysis.

Figure 5.16 RT

Fe spectra of commercial Nd Fe-B (top) and single-phase Nd

B (bottom)

[Cadogan, unpublished].

B is the basis for the world’s strongest permanent-magnets but it is

temperatures or enhancing known intermetallics, mostly by absorption of

small elements such as H, N, and C. In Figure 5.17 we show RT

Fe spectra

of the hexagonal intermetallic compound Y

and interstitially modified

. The parent compound Y

contains 4 Fe sites in the population

room temperature. Thus, its Mössbauer spectrum has almost collapsed into a

superposition of nonmagnetic doublets, with only a small broadening due to

the remaining weak magnetism. By comparison, the spectrum of Y

shows a large, well-established magnetic splitting. As shown by Coey and

Sun [36], the absorption of interstitials such as N or C leads to a dramatic

2173

is ~700 K. Thus, the room temperature of Y

is well magnetically split .

As mentioned in Section 5.2 and illustrated in the preceding two figures,

the typical

Fe magnetic spectrum is fully contained in a velocity range of

r10 mm/s, with typical hyperfine fields up to a maximum of ~52T in

hematite. Rare earth nuclei generally experience much larger hyperfine fields

and in Figure 5.18 we show

169

Tm spectra of the cubic intermetallic

compound TmFe

at 1.3 K and 295 K [11]. This cubic Laves phase compound

has a single Tm crystallographic site and the spectra show a large magnetic

splitting with a magnetic hyperfine field at the

169

Tm nuclei of around 700 T.

235



slightly limited by its modest Curie temperature (~585 K). Much subsequent

work focused on either identifying intermetallic compounds with higher Curie

temperature and in Y Feincrease in Curie

ratio 4:6:12:12 and has a Curie temperature of ~325 K, only slightly above

N the enhanced Curie temperature

5. Mössbauer Spectroscopy

The asymmetry of the spectra is a clear indication of a large EFG, in addition

to the magnetic splitting, and the temperature dependences of both hyperfine

parameters were used to derive magnetic exchange and crystal field

parameters. TmFe

is an ideal compound for calibration of

169

spectrometers. It is interesting to note the velocity scale here; on this scale of

r700 mm/s the

Fe calibration spectrum shown above in Figure 5.4 would

occupy only 2 to 3 channels.

Figure 5.17 RT

Fe spectra of Y

(top) and Y

(bottom) [Cadogan, unpublished].

Figure 5.18

169

Tm spectra of TmFe

obtained at 1.3 K (top) and 295 K (bottom) [Cadogan,

unpublished]; see also [11].

236