Vij D.R. Handbook of Applied Solid State Spectroscopy

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5.3.2 Magnetic Reorientations

As explained in Section 5.1, the Mössbauer spectrum in the presence of both

magnetic and quadrupole effect depends critically on the angle between the

quantization axes of the respective Hamiltonians, thereby allowing one to

investigate reorientations of the magnetic structures. The

119

Sn spectra of the

hexagonal intermetallic compound TbMn

5.6

0.4

, obtained over

the temperature range 12 K to 300 K and shown in Figure 5.19 [37], illustrate

clearly the effect of magnetic reorientation. At low temperatures the easy

direction of magnetization is along the hexagonal c-axis. This compound

undergoes a spin-reorientation to the hexagonal basal plane at 244 K. An axis

(T = 0°) to plane (T = 90°) reorientation leads to a change in the measured

spectroscopy. The temperature dependences of the hyperfine fields and

quadrupole splittings of the three Sn sites in this structure are also shown in

Figure 5.19. At each site we see the expected change in quadrupole splitting.

The hyperfine field “seen” by the

119

Sn nuclei is a transferred field from its

magnetic neighbors (Sn is nonmagnetic) and clear changes occur at the spin-

reorientation transition, which are related to orbital magnetism effects.

Figure 5.19 Left ––

119

5.6

. Right –– Temperature

dependences of the hyperfine field and the quadrupole splitting at each of the Sn sites [37].

Continuing this theme of transferred hyperfine fields, in Figure 5.20 we

show the

119

Sn RT spectrum of the intermetallic compound YFe

There are eight Sn crystallographic sites in this orthorhombic structure and

the spectrum shows that 1/3 of the Sn nuclei experience quite a large

hyperfine field of 24.6 T, transferred from the surrounding magnetic Fe sites,

5.3 Applications 237

quadrupole splitting by a factor of –1/2, easily observed by Mössbauer

Sn spectra of hexagonal TbMn Sn Ga

5. Mössbauer Spectroscopy

while the remaining 2/3 experience no field. This finding provided

confirmation of the suggested antiferromagnetic structure of the Fe sublattice

in YFe

, deduced by neutron diffraction [38].

Figure 5.20

119

Sn spectrum (RT) of YFe

[38].

The temperature dependences of the hyperfine parameters can also be used

to great effect to study the nature of magnetic transitions, including the

ordering transition. In Figure 5.21 we show a series of

119

Sn spectra of the

intermetallic compound Gd

obtained at various temperatures [39]. We

also show the temperature dependences of the

119

Sn hyperfine magnetic field

and the relative fraction of the magnetic component subspectra. This

compound, and related compounds, are currently of interest due to their “giant

magnetocaloric” behavior, which opens up the possibility of future

applications as refrigerant materials [40]. The

119

Sn spectra clearly

demonstrate the first-order magnetic ordering at 82 K, in addition to magnetic

inhomogeneity over a wide range of temperature.

Figure 5.21

119

Sn spectra of Gd

together with the temperature dependences of the

hyperfine field and the quadrupole splittings at the three Sn sites [39].

238

T=90K

5.3.3 Crystal Fields

We have already mentioned the use of Mössbauer spectroscopy to study

crystal-field effects. The form of the crystal-field Hamiltonian of a rare earth

ion in a compound is determined by the point-symmetry at the rare earth

crystallographic site. Thus, a study of the temperature-dependences of

hyperfine parameters, not only allows one to deduce crystal-field parameters

but can also distinguish clearly between different crystallographic models. To

illustrate this, in Figure 5.22 we show

169

Tm spectra of Tm

[41]. This

structure had been suggested to exhibit five-fold crystal symmetry, forbidden

in extended crystal structures [42]. The

169

Tm spectra allowed us to test this

claim by measuring the electric quadrupole splitting at the

169

Tm nucleus and

analyzing these data with a model for the crystal field at the parent Tm

site.

Our crystal field fit to the quadrupole splitting clearly shows that the proposed

5-fold symmetry is incorrect and the site symmetry is in fact triclinic.

Figure 5.22 Left ––

169

Tm spectra of Tm

. Right –– temperature dependence of the

169

quadrupole splitting, with a triclinic crystal-field fit. Also shown is the temperature-dependence

predicted by the 5-fold symmetry model [41].

5.3.4 Phase Analysis

Mössbauer spectroscopy is well suited to the problem of phase analysis, in

particular in Fe-bearing materials. Not only can individual phases present in a

sample be identified, but a quantitative analysis can often be achieved,

keeping in mind the caveat related to the recoil-free fractions of the individual

phases, mentioned in Section 5.1. As an example of the use of

Fe Mössbauer

spectroscopy for phase analysis, in Figure 5.23 we show a series of RT

spectra of Nd-Fe-B, with the commercial composition Nd

, after

various stages in the HDDR process (hydrogen disproportionation desorption

5.3 Applications 239

5. Mössbauer Spectroscopy

recombination) used in the preparation of permanent-magnet materials.

Heating the starting Nd-Fe-B material to about 200°C in hydrogen gas leads

to absorption, with a resulting slight increase in magnetic ordering

temperature. Further heating to above 700°C leads to the complete

disproportionation of the Nd

B phase into D-Fe, Fe

B, and NdH

(identified by X-ray diffraction) [43]. The presence of D-Fe completely

dominates the disproportionated spectrum.

Figure 5.23 RT

Fe Mössbauer spectra of Nd

in the virgin, unhydrogenated state

(spectrum (a)) and following hydrogenation up to (b) 320°C, (c) 650°C and (d) 1000°C [43].

As an example of quantitative phase analysis, we show in Figure 5.24 the

RT spectrum of the Allegan meteorite which was found in Allegan County,

Michigan, US [44]. Four distinct Fe-bearing phases are observed (Olivine

38.9 r 1.0 area-%, Pyroxene 34.1 r 0.8 area-%, Troilite 20.3 r 1.3 area-%,

and Fe-Ni metal 7.0 r 2.4 area-%).

Figure 5.24 RT

Fe spectrum of the Allegan meteorite with the individual spectral components

identified [44].

240

5.3.5 Amorphous Materials

As mentioned in Section 5.1, the Mössbauer spectra of amorphous materials

are broad due to the distribution of hyperfine parameters such as hyperfine

magnetic field or quadrupole splitting. A typical hyperfine field distribution

fit involves 15–30 subspectral components and the shape of the fitted

distribution depends on numerous factors, not only related to the actual

sample. However, the average field of such a distribution is fairly robust.

In Figure 5.25 we show a series of room temperature Mössbauer spectra of

amorphous Fe

86-x

ribbons, prepared by the melt-spinning technique

[45]. These materials are precursors for the preparation, by controlled

crystallization, of soft-magnetic materials similar to the FINEMET alloys.

These spectra are broad (refer to the crystalline D-Fe spectrum in Figure 5.4)

and their respective hyperfine field distributions clearly show the effect of the

increasing Nb content in reducing the Curie temperature and hence the

average hyperfine field.

Figure 5.25 RT

Fe spectra of amorphous Fe

86-x

ribbons [45].

The preceding set of room temperature spectra showed the effect of

by obtaining a series of spectra at various temperatures above room

temperature [46] (see Figure 5.26). The hyperfine field distributions shown in

temperature dependences are analyzed in terms of a modified Brillouin

function, taking into account the effects of the distribution of environments in

the amorphous state, which manifest themselves as a distribution of the

magnetic exchange [47].

5.3 Applications 241

have also followed the collapse of the magnetic spectrum of amorphous

Figure 5.26 provide a dramatic representation of this collapse. Often, such

reducing the Curie temperature in a series of Fe-Nb-B amorphous alloys. We

5. Mössbauer Spectroscopy

Figure 5.26

Fe spectra of amorphous Fe

ribbons over the temperature range 80 K to

170 K [46].

It is clear the

Fe Mössbauer spectroscopy is ideally suited to the

comparative study of crystalline and amorphous materials and mixtures of the

two. In Figure 5.27 we show a series of room temperature Mössbauer spectra

of the permanent-magnet material Nd-Fe-B, as previously illustrated in

Figure 5.23 [48]. Here, we prepared a series of samples by melt-spinning at

various wheel speeds, and hence quench rates, and the transition from

crystalline through microcrystalline to amorphous with increasing quench rate

is clearly reflected in the broadening of the spectrum.

Figure 5.27

Fe spectra of Nd-Fe-B prepared at various melt-spinning wheel speeds V

(m/s)

[48].

242

To illustrate the quantitative fitting of spectra comprising both broad

amorphous and sharper nano-crystalline components, we show in Figure 5.28

the room temperature Mössbauer spectra of (i) Fe

and (ii) Fe

ribbons, prepared fully amorphous by the melt-spinning technique and

subsequently partially crystallized to produce bcc-Fe(V,Nb,B) nanocrysta-

llites, embedded in an amorphous matrix [49]. These spectra show the

combined features of the broad amorphous phase and the sharper nano-

crystalline precipitates. Fitting these combinations allow one to deduce the

volume fractions of the amorphous and nano-crystalline phases in these

materials. Such data are important in the development of a comprehensive

model for the random anisotropy in such commercially important soft-

magnetic materials [16, 50].

Figure 5.28

Fe spectra of partially crystallized (top) Fe

and (bottom) Fe

ribbons [49].

Mössbauer spectroscopy also provides a simple method for estimating the

magnetic ordering temperature of a material. The “thermal scan” technique

consists of switching off the velocity drive unit and counting for fixed periods

of time in the zero-velocity channel at various temperatures. As the

temperature increases, the magnetic spectrum collapses towards a quadrupole

doublet or a singlet, depending on the crystallographic site symmetry. Thus,

the J absorption increases at zero-velocity and the number of counts registered

in the given time period falls. This is illustrated in Figure 5.29 where we show

spectra of amorphous Fe

obtained at 4.2 K and 300 K [51]. The

temperature dependence of the average hyperfine field yields a Curie

temperature of 98 K. The spectrum at 300 K is a paramagnetic, broad doublet

due to the distribution of the electric quadrupole splitting. This figure also

5.3 Applications 243

shows the Curie temperature determination using the thermal scan technique,

5. Mössbauer Spectroscopy

nonetheless useful as a quick method of obtaining a reasonable estimate of the

magnetic ordering temperature.

Figure 5.29 Left––

Fe spectra of a-Fe

at 4 K and 300 K, together with their respective

hyperfine field or quadrupole splitting distributions. Top right–– Temperature dependence of

the

Fe average hyperfine field. Bottom right–– Thermal scan determination of the magnetic

ordering temperature [51].

A further example of quantitative phase analysis in a mixed

amorphous/crystalline sample is shown in Figure 5.30, where we present a

series of RT (and 4.2 K)

Fe Mössbauer spectra obtained on amorphous and

crystallized Sc

B [52]. This experiment was an attempt to prepare

crystalline Sc

B by crystallizing an amorphous precursor, since Sc

does not form by traditional processing routes. Differential scanning

calorimetry had shown that the crystallization of amorphous Sc

B is a

two-stage process with crystallization temperatures of 600°C and 685°C.

Spectrum (a) was obtained with the amorphous, as-quenched sample at 4.2 K

and shows a well-split broad magnetic sextet, indicative of magnetic order

with a distribution of hyperfine field. Spectrum (b) is the RT spectrum of the

as-quenched amorphous sample and is a broad, very weakly magnetically split

was obtained after annealing the sample at the first crystallization stage and

clearly shows the growth of crystalline D-Fe, superimposed on a residual

amorphous component. The final spectrum (d) shows the sample after

annealing at 700°C. This fully crystallized material comprises D-Fe, ScFe

and ScB

(the latter phase being identified by X-ray diffraction).

244

yielding a value of 104 K, slightly higher than the previous value but

spectrum, obtained just below the Curie temperature of 328 K. Spectrum (c)

Figure 5.30

Fe spectra of Sc

B: (a) as-quenched amorphous ribbons at 4 K, (b) as-

quenched amorphous ribbons at 295 K, (c) partially crystallized, and (d) fully crystallized [52].

5.3.6 Electronic Relaxation

In Figure 5.31 we show the

166

Er spectra obtained in the temperature range

2–20 K of the intermetallic phase ErFe

[53]. The magnetic ordering

temperature of the Er sublattice, deduced by neutron diffraction, is 4.5 K. The

166

Er spectra show the collapse of the spectrum with increasing temperature,

but more noticeable is the fact that virtually the full magnetic splitting is seen

well above the magnetic ordering temperature. Er

is prone to slow electronic

relaxation effects, yielding the hyperfine splitting observed in the

paramagnetic regime. Similar effects have been observed by

169

spectroscopy in Tm

[54] and Tm

Al [55]. ErFe

provided a rare

opportunity to study all crystallographic sites in a compound by Mössbauer

spectroscopy, and in Figure 5.31 we also show the

Fe,

119

Sn, and

166

spectra of this intermetallic.

5.3.7 Electronic Valence

As mentioned in Section 5.1, hyperfine parameters such as quadrupole

splitting and isomer shift can be quite sensitive to the electronic valence of the

parent ion.

To illustrate this sensitivity to electronic valence, we show in Figure 5.32 a

series of

Fe Mössbauer spectra obtained at room temperature on a number of

compounds in the pseudobrookite series Fe

1+x

2–x

[56]. In the Fe

TiO

end-member, Fe is ferric, i.e., Fe

, and a broad, nonmagnetic doublet is

5.3 Applications 245

5. Mössbauer Spectroscopy

observed with a quadrupole splitting of ~ 0.55 mm/s. The other end-member

FeTi

comprises Fe

, and produces a sharper, nonmagnetic doublet with a

much larger quadrupole splitting of ~ 2.16 mm/s. Pseudobrookite contains

two crystallographic sites available to the Fe ions and we also show the site

occupancies of Fe

deduced from the Mössbauer spectra. These site

occupancies are in excellent agreement with those derived from neutron

diffraction measurements.

Figure 5.31 Left––

166

Er spectra of ErFe

. Right––

Fe,

119

Sn, and

166

Er spectra of ErFe

Figure 5.32 Left–– RT

Fe spectra of the pseudobrookite series Fe

1–x

2–x

. Right–– Relative

site occupancies as a function of x [56].

(note the different velocity scales) [53].

246