DSpace at VNU: A Mossbauer study of the spin reorientation transition in DyFe11Mo

Journal of Magnetism and Magnetic Materials 262 (2003) 452–457

.
A Mossbauer
study of the spin reorientation transition
in DyFe11Mo
J.M. Le Bretona,*, N.H. Ducb, V.T. Hienb, N.P. Thuyb,c, J. Teilleta
a

Groupe de Physique des Mat!eriaux, UMR CNRS 6634, Universit!e de Rouen, Site Universitaire du Madrillet, avenue de l’Universite!-B.P.
12, 76801 Saint Etienne du Rouvray Cedex, France
b
Cryogenic Laboratory, Faculty of Physics, National University of Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
c
International Training Institute for Materials Science (ITIMS), 1 Dai Co Viet, DHBK Hanoi, Hanoi, Viet Nam

Abstract
The spin reorientation transition in DyFe11Mo around the spin reorientation temperature (220 K) is investigated by
.
Mossbauer
spectrometry. The temperature dependence of the hyperfine parameters for each Fe site reveals an obvious
discontinuity of the hyperfine field. The magnitude of the discontinuity is more important for the 8f site than for the 8i
and 8j sites, indicating that the most prominent contribution to the overall anisotropy in the Fe sublattice should be
from the Fe ion at the 8f site. This is attributed to the 3d(Fe(8f))–3d(Mo(8i)) hybridization, which may play a quite
important role in R(Fe,Mo)12 compounds.
r 2003 Elsevier Science B.V. All rights reserved.
PACS: 75.30.Àm; 75.50.Bb; 76.80.+y
.
Keywords: DyFe11Mo; Spin reorientation; Mossbauer
spectrometry

1. Introduction
Because they exhibit interesting magnetic properties, the R(Fe,M)12 compounds (R=rare earth;
M=Ti, V, Cr, Mn, Mo, W, Al or Si) have been
extensively studied [1,2]. These compounds crystallize in the ThMn12 type tetragonal structure. In the
compounds containing R elements with a negative
second–order Stevens factor aJ ; the rare-earth (4f)
sublattice shows a planar anisotropy whereas the
Fe (3d) sublattice has a uniaxial easy axis
*Corresponding author. Tel.: +2-32-95-50-39; fax: +2-3295-50-32.
E-mail address:
(J.M. Le Breton).

anisotropy. The competitive anisotropy contributions from the two sublattices can induce rotations
of the resultant magnetic moments with respect to
the crystallographic directions, leading to spin–
reorientation phase transitions as the temperature
changes. This is related to a change of the sign of
the overall magnetic anisotropy constant at a
temperature Tsr (spin reorientation temperature).
This is the case, for example, for the Nd(Fe,Mo)12
[3,4], Tb(Fe,Mo)12 [4,5] and Dy(Fe,Mo)12 [5–7]
compounds.
This phenomenon is generally evidenced by
means of magnetic measurements, e.g. magnetization and susceptibility measurements either on
magnetically non-oriented [3,6,7] or oriented [4–6]
.
powders. However, Mossbauer
spectrometry

0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0304-8853(03)00077-5


J.M. Le Breton et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 452–457

analysis can provide useful information on local
properties, as the discontinuity in the hyperfine
fields at the 3d sites at Tsr reflects the orbital
contribution to the 3d magnetic moment at a given
site [1]. This information, therefore, enables one to
consider the local anisotropy at each 3d site in this
type of compound [8].
.
In this paper, we present a detailed Mossbauer
study of the spin reorientation transition in
DyFe11Mo. The magnetic properties of this
compound have been reported in a previous paper
[7]. We focus here on the changes of the hyperfine
parameters around the spin reorientation temperature Tsr ¼ 220 K [7].

453

errors s given by the fitting program [9]. The error
bars indicate 3sU

3. Results and discussions
.
The Mossbauer
spectra of the DyFe11Mo
powder have to be fitted with the contributions

of both the ThMn12 phase and the a-Fe phase, in
agreement with the results of X-ray diffraction
analysis. The contribution of the ThMn12 phase is
fitted according to a model that accounts for both
the content of Mo and the crystal structure.
3.1. Fitting model

2. Experimental
A DyFe11Mo sample was prepared by arcmelting the constituents in the nominal stoichiometric composition in a protective atmosphere of
pure argon (99.99%). Pure metals (Dy of 99.9%,
Fe and Mo both of 99.99% purity) were used. In
order to ensure its homogeneity, the as–melted
sample was several times turned over and melted
again. We have added about 2 wt% excess of Dy
to compensate the rare-earth loss caused by
evaporation during the repeated melting procedure. The ingot obtained was then annealed at
1000 C for 70 h in a pure argon atmosphere. At
the end of the annealing procedure the sample was
quenched in water down to room temperature.
The powder was characterized by X-ray diffraction, and the pattern shows typical reflections of
the ThMn12 structure, and the presence of some aFe, as an impurity phase. The lattice parameters of
the ThMn12 structure indicate that the Mo content
in the ThMn12 phase is very close to the nominal
composition [7], i.e. DyFe11Mo.
.
The Mossbauer
spectra were recorded in the
temperature range from 77 to 300 K in transmission geometry using a 57Co source in a rhodium
.
matrix. The Mossbauer

sample contains about
10 mg cmÀ2 of natural iron. The isomer shift
(relative to metallic a-Fe at room temperature),
quadrupolar shift and hyperfine field are denoted
d, e and B; respectively. Estimated errors for the
hyperfine parameters originate from the statistical

The magnetic properties of the R–Fe compounds being mainly governed by the Fe–Fe
exchange interactions, the hyperfine field at one
Fe site mainly depends on both the number of Fe
nearest neighbours (NN) and the corresponding
interatomic distances. The 8i site having 13 Fe
NN, the corresponding hyperfine field B(8i) is
higher than at the 8j and 8f sites, both having 10
Fe NN. The Fe–Fe interatomic distances around
the 8f site being weaker than those around the 8j
site [2], this should result in a lower magnetic
moment (and consequently, a lower hyperfine
field) at the 8f site than at the 8j site, in agreement
with neutron diffraction experiments [2,10]. Consequently: B(8i)>B(8j)>B(8f). This order corresponds to that of the average Fe–Fe distances for
ð8iÞ
ð8jÞ
ð8fÞ
each Fe ion site (dFeÀFe
> dFeÀFe
XdFeÀFe
) [2].
Neutron diffraction experiments showed that in
R(Fe,M)12 compounds, the M atoms substitute to
Fe preferentially on the 8i site [11,12]. This is in

agreement with the positive enthalpy contribution
associated with R and Mo, the R atoms having
four nearest 8i neighbours compared with eight
nearest 8j and 8f neighbours [12]. Consequently, it
is considered here that the M atoms occupy the 8i
sites only. The ramdom occupancies of the Fe
atoms in the ThMn12 unit cell can be calculated
according to a binomial distribution, which gives
the probability of finding Mo atoms in the vicinity
of a given Fe site. According to the Mo content,
the calculation shows that 13 sextets must be used


J.M. Le Breton et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 452–457

.
for the Mossbauer
contribution of the DyFe11Mo
phase: five sextets for 8i, four sextets for 8j and
four sextets for 8f. This fitting procedure results in
a high number of contributions, with numerous
hyperfine parameters, and was not used here. With
the aim to look for a simpler model, with a lesser
number of contributions, the distribution of environments around each Fe site is simulated by two
broad sextets, having the same relative intensity,
and each hyperfine parameter d; 2e or B corresponding to each Fe site is the mean value of the
corresponding distribution. The contribution of the
ThMn12 phase is thus fitted with six sextets. As an
example, the contributions of the different Fe sites
for the spectrum recorded at 77 K are presented in

Fig. 1. The relative intensities are constrained to the
values calculated from the atomic distribution of Fe
and Mo atoms in the crystal structure. According
.
to the Mo content, the Mossbauer
relative intensities of the different Fe sites in the DyFe11Mo
phase were thus constrained to the following values:
27.2% for 8i, 36.4% for 8j and 8f.
.
The Mossbauer
relative intensity of the a-Fe
contribution can be deduced from the fitting of the
room temperature spectrum, as its contribution is
clearly distinguishable from that of the pure
DyFe11Mo phase: the obtained value is 5%. Thus,
the contribution of a-Fe is fitted in each spectrum
with a relative intensity fixed to 5%. At each
temperature, the contribution of the DyFe11Mo
phase thus represents 95% of the intensity of the
spectrum.
The spectra were fitted consistently in the whole
temperature range according to these considerations, and the spectra are reported in Fig. 2.
3.2. Analysis of the Mossbauer
data
.
From the fittings, the hyperfine parameters of
each Fe site contribution in the DyFe11Mo
compound and the mean hyperfine field /BS of
the DyFe11Mo compound were obtained. Their
temperature dependences are presented in Figs. 3–

5. In each curve, the spin reorientation temperature (Tsr ¼ 220 K [7]) is evidenced.
The order sequence of the magnitudes of the
isomer shift is d(8i)>d(8j)Ed(8f) in the whole
temperature range (Fig. 3). This is in good

-10

Velocity (mm/s)
0

+10

1.00

8i site

0.97
1.00

8j site
Absorption (%)

454

0.97
1.00

8f site

0.97

1.00

α-Fe

0.97

.
Fig. 1. Mossbauer
spectrum at 77 K of the DyFe11Mo powder.
The contributions of the Fe sites of the ThMn12 phase and the
a-Fe phase are displayed.

agreement with the literature [13,14]. This order
can be understood as the consequence of the order
of the average Fe–Fe distances for each Fe ion site
ð8iÞ
ð8jÞ
ð8fÞ
(dFeÀFe
> dFeÀFe
XdFeÀFe
) [2]. For each Fe site, a
continuous decrease of the curve is observed, and
no obvious discontinuity can be evidenced.
As the behaviour of 2e is connected with the
change of angle between the easy axis of magnetisation and the electric field gradient [15], a
discontinuity of the corresponding curves is
expected in the region around the spin reorientation temperature. From the temperature dependence of the quadrupolar shift reported in
Fig. 4, this discontinuity is only suggested. The

J.M. Le Breton et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 452–457

-10

Velocity (mm/s)
0

455

0.2

+10

8i

0.15

8j

0.1

1.00

77 K

δ (mm/s)

8f

0.05
0
-0.05
-0.1

Tsr

-0.15

0.97
1.00

-0.2
0

50

100

150

200

250

300

350

Temperature (K)

120 K

Fig. 3. Temperature dependence of the isomer shift d for each
Fe site of the DyFe11Mo compound. The full lines are guides
for the eye.

0.97
1.00

0.15
0.1

Absorption (%)

170 K
2ε (mm/s)

0.05

0.97
1.00

8i

0

8j

-0.05

8f

-0.1
-0.15

220 K

-0.2

Tsr
0

50

100

150

200

250

300

350

Temperature (K)

0.97

1.00

Fig. 4. Temperature dependence of 2e (e is the quadrupolar
shift) for each Fe site of the DyFe11Mo compound. The lines
(full for 8j and dotted for 8i and 8f) are guides for the eye.

240 K

0.97
1.00

270 K

0.97
1.00

300 K

0.97
.
Fig. 2. Mossbauer
spectra of the DyFe11Mo powder in the
temperature range from 77 to 300 K.

discontinuity is not clearly evidenced probably
because the value of 2e for each site is obtained
.
from a distribution of Mossbauer
sextets (related
to Mo/Fe substitution effects on the 8i site) which

simulates the contribution of the corresponding Fe
atoms to the spectrum. Each site contribution
being not easily resolved from the distributions
corresponding to the other sites, and 2e being
treated by the fitting program as a perturbation of
B; this does not allow to measure 2e with the
highest possible accuracy.
The temperature dependence of the hyperfine
field of each Fe site, and that of the mean
hyperfine field of the DyFe11Mo compound
are reported in Figs. 5a and b, respectively.
The hyperfine field gradually decreases as the


J.M. Le Breton et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 452–457

456
40
35
30
B (T)

25
20
15

8i

10

8j
8f

5

Tsr

0
0

50

100

(a)

150

200

250

300

350

Temperature (K)

29
27

<B> (T)

25
23
21

Tsr

19
17
15
0

(b)

50

100

150
200
Temperature (K)

250

300

350

Fig. 5. Temperature dependence of: (a) the hyperfine field B at

each Fe site and (b) the mean hyperfine field /BS of the
DyFe11Mo compound. The full lines are guides for the eye.

means that the most prominent contribution to the
overall anisotropy in the Fe sublattice should be
from the Fe ion at the 8f site (if one takes 1.7 T/
3=0.57 T, so rather closely to 0.6 T). The fact that
the Fe ion at the 8f site gives the largest
contribution to the overall 3d anisotropy in the
DyFe11Mo compound is contradictory to the
.
Mossbauer
results reported for RFe12ÀxTix compounds [1,2]. This effect is associated with the
preferential substitution of Mo for Fe at the 8i site
and suggests that the 3d(Fe(8f))–3d(Mo(8i)) hybridization may be stronger than the 3d(Fe(8f))–
3d(Ti(8i)) hybridization. It is worthwhile to mention that among the Fe–Fe distances around the
(8i) site, the mean Fe(8f)–Fe(8i) distance is the
shortest one. Consequently, the 3d(Fe(8f))–
3d(Mo(8i)) hybridization would be the strongest
and the density of the negative 3d(Mo) spin
around Fe(8f) site would be the highest [18]. As
the change of hyperfine field is related to a change
in the spin density, this leads to a strong reduction
of B at the 8f-site.

4. Conclusion
temperature increases and, on each curve, an
obvious discontinuity is evidenced in the temperature region around the spin reorientation phase
transition. The discontinuity in the temperature
dependence of both the average and the individual

Fe site hyperfine field observed at the spin
reorientation temperature Tsr is closely related to
the second-order anisotropy constant which is
determined by the residual orbital moment
quenched by the crystal field [16,17]. At Tsr ; the
sign of the hyperfine field change is positive: DB ¼
½BðMJc Þ2BðMconical ފ > 0 in case DK1 ¼ ½K1 ðT >
Tsr Þ2K1 ðToTsr ފ > 0: This is in good agreement
with what was found in many other reports
concerning the discontinuity of the hyperfine field
at Tsr [16,17]. The magnitude of the discontinuity
in the average (DB) and in the individual site
(DBðiÞ ) hyperfine field is proportional to the overall
Kl and the individual site K1ðiÞ anisotropy, respectively [16]. A close inspection of the data presented
in Fig. 5 shows that, DBE0:6 T whereas
DBð8iÞ E0:2; DBð8jÞ E0:4; and DBð8fÞ E1:7 T: This

The spin reorientation transition in DyFe11Mo
around the spin reorientation temperature (220 K)
.
was investigated by Mossbauer
spectrometry,
focusing on the temperature dependence of the
hyperfine parameters for each Fe site. No discontinuity was observed for the isomer shift. A
discontinuity is only suggested for the quadrupolar
shift, in relation with the fitting procedure used to
.
fit a complex Mossbauer
spectrum. However, the
results show an obvious discontinuity of the

hyperfine field, which is related to the temperature
dependence of the second-order anisotropy constant. The magnitude of the discontinuity, which is
proportional to the individual site first-order
anisotropy constant, is more important for the 8f
site than for the 8i and 8j sites, indicating that the
most prominent contribution to the overall anisotropy in the Fe sublattice should be from the Fe
ion at the 8f site. This is attributed to the
3d(Fe(8f))–3d(Mo(8i)) hybridization, which may
be stronger than the 3d(Fe(8f))–3d(Ti(8i)) hybridization in R(Fe,Ti)12 compounds.


J.M. Le Breton et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 452–457

Acknowledgements
This work is partly supported by the Program of
the Fundamental Research of Vietnam, nr. 420 301

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