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 th
Trang 1A M ossbauer study of the spin reorientation transition
a
Groupe de Physique des Mat !eriaux, UMR CNRS 6634, Universit!e de Rouen, Site Universitaire du Madrillet, avenue de l’Universit!e-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
M.ossbauer 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)12compounds
r2003 Elsevier Science B.V All rights reserved
PACS: 75.30.m; 75.50.Bb; 76.80.+y
Keywords: DyFe 11 Mo; Spin reorientation; M ossbauer spectrometry
1 Introduction
Because they exhibit interesting magnetic
prop-erties, 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
crystal-lize in the ThMn12type 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
anisotropy The competitive anisotropy contribu-tions from the two sublattices can induce rotacontribu-tions
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 magnetiza-tion and susceptibility measurements either on magnetically non-oriented[3,6,7]or oriented[4–6] powders However, M.ossbauer spectrometry
*Corresponding author Tel.: 95-50-39; fax:
+2-32-95-50-32.
E-mail address: jean-marie.lebreton@univ-rouen.fr
(J.M Le Breton).
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V All rights reserved.
doi:10.1016/S0304-8853(03)00077-5
Trang 2analysis 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 M.ossbauer
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
tempera-ture Tsr¼ 220 K[7]
2 Experimental
A DyFe11Mo sample was prepared by
arc-melting the constituents in the nominal
stoichio-metric 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
proce-dure The ingot obtained was then annealed at
1000C 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
diffrac-tion, and the pattern shows typical reflections of
the ThMn12structure, and the presence of some
a-Fe, as an impurity phase The lattice parameters of
the ThMn12structure indicate that the Mo content
in the ThMn12 phase is very close to the nominal
composition[7], i.e DyFe11Mo
The M.ossbauer spectra were recorded in the
temperature range from 77 to 300 K in
transmis-sion geometry using a 57Co source in a rhodium
matrix The M.ossbauer sample contains about
10 mg cm2 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
errors s given by the fitting program[9] The error bars indicate 3sU
3 Results and discussions
The M.ossbauer spectra of the DyFe11Mo powder have to be fitted with the contributions
of both the ThMn12phase and the a-Fe phase, in agreement with the results of X-ray diffraction analysis The contribution of the ThMn12phase is fitted according to a model that accounts for both the content of Mo and the crystal structure 3.1 Fitting model
The magnetic properties of the R–Fe com-pounds 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] Con-sequently: B(8i)>B(8j)>B(8f) This order corre-sponds to that of the average Fe–Fe distances for each Fe ion site (dFeFeð8iÞ > dFeFeð8jÞ XdFeFeð8fÞ )[2] Neutron diffraction experiments showed that in R(Fe,M)12compounds, 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
Trang 3for the M.ossbauer 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
envir-onments around each Fe site is simulated by two
broad sextets, having the same relative intensity,
and each hyperfine parameter d; 2e or B
corre-sponding to each Fe site is the mean value of the
corresponding distribution The contribution of the
ThMn12phase 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 M.ossbauer relative
inten-sities 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 M.ossbauer 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
considera-tions, and the spectra are reported inFig 2
3.2 Analysis of the M.ossbauer 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 inFigs 3–
5 In each curve, the spin reorientation
tempera-ture (Tsr¼ 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
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 (dFeFeð8iÞ > dFeFeð8jÞ XdFeFeð8fÞ ) [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 magne-tisation and the electric field gradient [15], a discontinuity of the corresponding curves is expected in the region around the spin reorienta-tion temperature From the temperature depen-dence of the quadrupolar shift reported in Fig 4, this discontinuity is only suggested The
Velocity (mm/s) 0
0.97 1.00
0.97 1.00
0.97 1.00
0.97 1.00
8i site
8j site
8f site
α-Fe
Fig 1 M ossbauer spectrum at 77 K of the DyFe 11 Mo powder The contributions of the Fe sites of the ThMn 12 phase and the a-Fe phase are displayed.
Trang 4discontinuity is not clearly evidenced probably because the value of 2e for each site is obtained from a distribution of M.ossbauer 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
Velocity (mm/s) 0
77 K
120 K
170 K
220 K
240 K
270 K
300 K
0.97
1.00
0.97
1.00
0.97
1.00
0.97
1.00
0.97
1.00
0.97
1.00
0.97
1.00
Fig 2 M ossbauer spectra of the DyFe 11 Mo powder in the
temperature range from 77 to 300 K.
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
Temperature (K)
δ (mm/s)
8i 8j 8f
T sr
Fig 3 Temperature dependence of the isomer shift d for each
Fe site of the DyFe 11 Mo compound The full lines are guides for the eye.
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15
Temperature (K)
2ε
8i 8j 8f
T sr
Fig 4 Temperature dependence of 2e (e is the quadrupolar shift) for each Fe site of the DyFe 11 Mo compound The lines (full for 8j and dotted for 8i and 8f) are guides for the eye.
Trang 5temperature increases and, on each curve, an
obvious discontinuity is evidenced in the
tempera-ture 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
½BðMJcÞ2BðMconicalÞ > 0 in case DK1¼ ½K1ðT >
TsrÞ2K1ðToTsrÞ > 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,
respec-tively[16] A close inspection of the data presented
DBð8iÞE0:2; DBð8jÞE0:4; and DBð8fÞE1:7 T: This
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 M.ossbauer results reported for RFe12xTix com-pounds [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)) hy-bridization may be stronger than the 3d(Fe(8f))– 3d(Ti(8i)) hybridization It is worthwhile to men-tion 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 The spin reorientation transition in DyFe11Mo around the spin reorientation temperature (220 K) was investigated by M.ossbauer spectrometry, focusing on the temperature dependence of the hyperfine parameters for each Fe site No dis-continuity 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 M.ossbauer spectrum However, the results show an obvious discontinuity of the hyperfine field, which is related to the temperature dependence of the second-order anisotropy con-stant 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 aniso-tropy 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)) hybri-dization in R(Fe,Ti) compounds
0
5
10
15
20
25
30
35
40
Temperature (K)
8i
8j
8f
15
17
19
21
23
25
27
29
Temperature (K)
T sr
T sr
(a)
(b)
Fig 5 Temperature dependence of: (a) the hyperfine field B at
each Fe site and (b) the mean hyperfine field /BS of the
DyFe 11 Mo compound The full lines are guides for the eye.
Trang 6This work is partly supported by the Program of
the Fundamental Research of Vietnam, nr 420 301
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