Structural and magnetic properties of UFe
6
Ga
6
A.P. Gonc¸alves
a,
*
, J.C. Waerenborgh
a
,S.Se
´
rio
a
, J.A. Paixa
˜
o
b
, M. Godinho
c
, M. Almeida
a
a
Departamento de Quı
´
mica, Instituto Tecnolo
´
gico e Nuclear/CFMC-UL, Estrada Nacional, P-2686-953 Sacave
´
m, Portugal
b
Universidade de Coimbra, Faculdade de Cie
ˆ
ncias e Tecnologia, Dept. Fı
´
sica, CEMDRX, 3004-516 Coimbra, Portugal
c
CFMC-UL/Dep., Fı
´
sica, FCUL, 1749-016 Campo Grande, Lisboa, Portugal
Received 11 May 2005; received in revised form 19 July 2005; accepted 11 September 2005
Available online 9 November 2005
Abstract
UFe
6
Ga
6
polycrystalline samples were prepared by arc-melting, and single crystals were grown by the Czochralski method. This compound
crystallizes in the orthorhombic ScFe
6
Ga
6
-type structure (space group Immm, aZ5.0560(4), bZ8.5484(7) and cZ8.6914(7) A
˚
), an ordered
variant of the ThMn
12
-type structure. A ferromagnetic-type transition at T
C
Z530(5) K is seen in the magnetization and A.C.-susceptibility
measurements, and no other magnetic anomaly is observed down to 5 K. Single crystal magnetization measurements along the three different
crystallographic axes indicated a as the easy direction, with a spontaneous magnetization M
S
Z12.3 m
B
/f.u. at 5 K. The analysis of the
57
Fe
Mo
¨
ssbauer spectroscopy data indicated magnetic hyperfine fields, B
hf
, significantly lower on 4f sites than on 8k sites, in agreement with the trend
already observed on UFe
x
Al
12Kx
, where the average B
hf
were found to increase with the iron–iron interatomic distances.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Magnetic intermetallics; B. Crystal chemistry of intermetallics; B. Magnetic properties; Diffraction
1. Introduction
Intermetallic compounds of f-elements with the ThMn
12
-
type structure and high iron content have been considered good
candidates for hard magnetic materials [1,2]. The interaction
between the 3d and f electrons in a tetragonal structure
frequently gives high uniaxial magnetocrystalline anisotropy,
Curie temperature and saturation magnetization. However,
binary AFe
12
(AZf-element) compounds do not exist, the
partial substitution of iron by a third element being necessary to
stabilise the ThMn
12
-type structure.
One of the most studied family of compounds with this
structure is the AFe
12Kx
Al
x
,(AZf-element) series [3–6]. The
aluminium concentration necessary to stabilise the ThMn
12
-
type structure is relatively high, usually higher than 50%, but
the study of these medium-low iron content compounds is
fundamental for a better understanding of the contribution from
the different magnetic sublattices to the magnetism.
The UFe
12Kx
Al
x
phase relations, previously explored by us,
indicate a congruent melting composition range between
UFe
3.8
Al
8.2
and UFe
5.8
Al
6.2
[7]. The UFe
6
Al
6
alloy does not
melt congruently but can be obtained by thermal treatment of
polycrystalline samples. Measurements on UFe
6
Al
6
samples
indicate an easy-plane anisotropy and a ferromagnetic
character with a Curie temperature T
C
w300 K [8]. In the
closely related UFe
6
Ga
6
compound only preliminary data
reporting a Curie temperature of T
C
Z515 K, much higher than
the aluminium equivalent, was previously reported [9]. In order
to enlighten the reason for this difference a careful
investigation of UFe
6
Ga
6
was performed. In the present
paper, we report X-ray and neutron diffraction,
57
Fe Mo
¨
ssbauer
spectroscopy and magnetisation measurements on this
compound.
2. Experimental
Samples with UFe
x
Ga
12Kx
(5%x%6.5) nominal compo-
sitions were prepared by melting the stoichiometric amount of
the elements (with purity of at least 99.9%) in an induction
furnace equipped with a levitation cold crucible and under an
argon atmosphere. The samples were turned and remelted at
least three times in order to ensure a better homogeneity. The
final weight losses were less than 0.5 wt%.
The microstructural analysis of the samples was performed
using a scanning electron microscope (JEOL-JSM 840) on
sample pieces embedded in resin and polished using SiC paper
down to 4000 mesh. Quantitative analysis of the observed
phases was made by energy dispersive spectroscopy (EDS)
Intermetallics 14 (2006) 530–536
www.elsevier.com/locate/intermet
0966-9795/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.intermet.2005.09.002
*
Corresponding author. Tel.: C351 1 9946182; fax: C351 21 9941455.
E-mail address: apg@itn.pt (A.P. Gonc¸alves).
analysis of the atomic characteristic X-rays excited by the
electron beam using an acceleration voltage of 30 keV and a
counting time of 100 s.
UFe
6
Ga
6
bulk charges with w15 g were prepared by
melting in an induction furnace the stoichiometric amounts
of the elements with purity of at least 99.9, w20% of the
material was pulled from the bulk charges using the
Czochralski method, as previously described [7], in order to
isolate small single crystals suitable for the X-ray character-
ization, and to obtain crystals appropriate for the study of the
magnetic anisotropy.
Single crystals with 0.05!0.05!0.05 mm
3
approximate
dimensions were removed from the UFe
6
Ga
6
pulled material,
glued on the top of a glass fibre and transferred to a goniometer
mounted on an Enraf-Nonius CAD-4 diffractometer with
graphite monochromatized Mo Ka radiation (lZ0.71073 A
˚
).
The unit-cell parameters were obtained from the least-squares
refinement of the 2q values of 25 strong and well centred
reflections from the various regions of the reciprocal space in
the 198!2q!308 range. The data set was collected at room
temperature in a uK2q scan mode (DuZ0.80C0.35 tan q).
Two reflections were monitored as orientation and as intensity
standards at 4 h intervals during the data collection; no
variation larger than 0.5% was observed. The intensities of
the 6476 measured reflections (with 2q!708) were corrected
for absorption using J-scan, for polarisation and the Lorentz
effects. The crystallographic and experimental data of the
structural determination are listed in Table 1.
Powder neutron diffraction measurements were made at the
TAS6 diffractometer installed at the DR3 reactor (Risø
National Laboratory, Denmark) on a finely ground UFe
6
Ga
6
sample of w30 g encapsulated in a vanadium can. The TAS6
instrument is equipped with a bank of 15 detectors covering an
angular range of 1208 and the data were collected with a
constant 2q step of 0.05298 . The incident beam was
monochromated by Bragg reflections from the (311) face of a
germanium single crystal, selecting a wavelength of 1.0697 A
˚
.
A pyrolitic graphite filter was inserted after the monochromator
to suppress high-order harmonics.
Magnetic measurements were performed on polycrystalline
samples and oriented crystals (with 0.5!0.5!0.5 mm
3
approximate dimensions) using a SQUID magnetometer
(Quantum Design, MPMS) in the 2–400 K temperature range
under fields up to 5.5 T, and a vibrating sample magnetometer
(VSM) for temperatures between 300 and 1000 K.
Mo
¨
ssbauer spectra were measured in transmission mode
using a conventional constant-acceleration spectrometer and a
25 mCi
57
Co source in a Rh matrix. The velocity scale was
calibrated using an a-Fe foil at room temperature. Mo
¨
ssbauer
absorbers were prepared from a fraction of the same sample
used for neutron diffraction, which was pressed together with
lucite powder into perspex holders, in order to obtain
homogeneous and isotropic absorbers containing about
5 mg/cm
2
of natural iron. Spectra were obtained at 295 and
5 K. Low-temperature measurements were performed using a
liquid-nitrogen/liquid-helium flow cryostat, temperature
stability G0.5 K. The spectra were fitted to Lorentzian lines
using a non-linear least-squares computer method [10].
3. Results and discussion
The microstructural study of the samples indicated that the
only nominal composition that melts congruently is UFe
6
Ga
6
.
EDS analysis shows an UFe
5.6(5)
Ga
6.4(5)
composition, close to
the nominal U:6Fe:6Ga ratio of the elements. Samples with
lower iron concentration contain an intergranular phase,
identified as FeGa
3
by EDS. For higher iron compositions the
formation of an eutectic between the UFe
6
Ga
6
grains is
observed. EDS elemental analysis indicates that this eutectic is
composed by UFe
6
Ga
6
and Fe
3
Ga
4
.
Single crystal X-ray diffraction data were used to refine the
cell parameters in the orthorhombic system to aZ5.042(4),
bZ8.510(8), cZ8.637(7) A
˚
. Careful examination of the data
led to the possible I222, I2
1
2
1
2
1
, Imm2, Im2m, I2mm and Immm
space groups [11] for the UFe
6
Ga
6
alloy. Immm was found to
be the most correct during the refinement. The structure was
solved by the Patterson function, employing the program
SHELXS-97 [12], which allowed the determination of the
uranium atoms position. The iron and gallium atoms were
located using the difference Fourier synthesis. The program
SHELXL-97 [12] was further employed to refine the UFe
6
Ga
6
crystal structure. The extinction factor, scale factor, five
occupation factors, four position parameters (y for the 4 g
Table 1
Details of single crystal X-ray diffraction measurements
Chemical formula UFe
6
Ga
6
Formula weight 991.43 g/mol
Crystal system Orthorhombic
Space group [11] Immm (No.71)
a 5.042(4) A
˚
b 8.510(8) A
˚
c 8.637(7) A
˚
V 370.6(5) A
˚
3
Z 2
D
calc
8.88 g cm
K1
m (Mo Ka) 49.43 cm
2
g
K1
Approximate crystal dimensions 0.05!0.05!0.05 mm
3
Radiation, wavelength Mo Ka, 0.71073 A
˚
Monochromator Graphite
Temperature 295 K
q range 1.5–358
u2q scan DuZ0.80C0.35 tan q
Data set K8%h%8, K13%k%13,
K13%l%13
Crystal-to-receiving-aperture distance 173 mm
Horizontal, vertical aperture 4, 4 mm
Total data 6476
Unique data 475
Observed data (IR3s(I)) 428
Number of refined parameters 35
Final agreement factors
a
RZ
P
jF
obs
KF
calc
j=
P
jF
obs
j
0.115
wRZ ½
P
ðwðjF
obs
jKjF
calc
jÞ
2
Þ=wjF
obs
j
2
1=2
0.257
SZ ½SwðjF
obs
jKjF
calc
jÞ
2
=ðmKnÞ
1=2
0.994
a
m, number of observations; n, number of variables.
A.P. Gonc¸alves et al. / Intermetallics 14 (2006) 530–536 531
and 4 h sites, and z for the 4e and 4f sites), and 24 anisotropic
displacement parameters, a total of 35 parameters, were
refined, the final results pointing to a crystallization in the
orthorhombic ScFe
6
Ga
6
-type structure, space group Immm.
However, it was not possible to reduce the residuals below
11%, and the final results had unrealistic occupation factors. A
detailed analysis of the data profiles showed that many peaks
had a double structure, indicating the existence of twining due
to the similarity between b and c cell parameters.
In order to refine the UFe
6
Ga
6
crystal structure neutron
diffraction experiments were performed. The structure was
refined by the Rietveld method [13] using as a starting model
the ScFe
6
Ga
6
-type structure and the cell parameters found in
the single crystal X-ray diffraction study (Tables 1 and 2). The
reflection profiles were modelled by a pseudo-Voigt function,
and an asymmetry parameter was included for 2q!408. The
structure refinement converged to the R
F
Z0.043, R
W
Z0.061
final residuals, with aZ5.0560(4), bZ8.5484(7) and cZ
8.6914(7) A
˚
, and the parameter values presented in Table 3.
In Fig. 1, the experimental powder neutron diffractogram is
plotted, together with the calculated and difference profiles. No
unindexed lines were found in the spectrum, as expected for a
single-phase material, confirming the ScFe
6
Ga
6
-type structure
for UFe
6
Ga
6
. The ferromagnetic ordering below 530(5) K
agrees with this (confirming the same magnetic and structural
unit cells), but points to an increase of the peaks intensity
below the Curie temperature. However, no magnetic contri-
butions were considered in the refinement due to their small
value when compared to the intensity of the crystallographic
peaks.
The ScFe
6
Ga
6
-type is a body-centred orthorhombic struc-
ture type derived from the ThMn
12
according to the sequence
I4/mmm/(t
2
; cba)/Immm. It is an ordered variant of
the ThMn
12
-type structure, which results from the splitting
8i/4e, 4 g and 8j/4f, 4 h, the 8f becoming the 8k site, the
iron atoms only occupying the 8k and 4f sites (Fig. 2).
The number of nearest neighbours and interatomic distances
(upto3.40A
˚
), obtained from the neutron diffraction
refinement of the different crystallographic positions, are listed
in Table 4. The coordination numbers (C.N.), deduced using
the maximum-gap method [14], are 20 for U(2a), 14 for
Ga1(4e) and Ga2(4 g), 13 for Ga3(4 h), and 12 for Fe1(4f) and
Fe2(8k).
Considering the metallic radii of the elements for a
coordination number of 12, 1.26 A
˚
for iron, 1.39 A
˚
for gallium
and 1.53 A
˚
for uranium [15], and the calculated interatomic
distances, all the U(2a)—(nearest neighbours) distances are
above the sum of their radii, which is partially explained by its
higher coordination number. These high U(2a)—(nearest
neighbours) distances, as well as the 5.04 A
˚
uranium–uranium
shortest distance, well above the Hill limit w3.4 A
˚
, suggest the
Table 2
Atomic positions (x, y, z), occupation factors (O.F.) and anisotropic displacement parameters (U
kl
), obtained in the UFe
6
Ga
6
structure refinement from the single
crystal X-ray data
Atom Wyckoff
position
XYz O.F. U
11
(A
˚
) U
22
(A
˚
) U
33
(A
˚
) U
23
(A
˚
) U
13
(A
˚
) U
12
(A
˚
) U
eq
(A
˚
)
U1 2a 0 0 0 1(fixed) 0.009 0.008 0.001 0 0 0 0.006
Ga1 4e 0 0 0.3465 1.00 0.014 0.006 0.000 0 0 0 0.007
Fe1 4f 0 1/2 0.3059 1.25 0.012 0.004 0.014 0 0 0 0.010
Ga2 4g 0 0.3438 0 0.99 0.014 0.010 0.015 0 0 0 0.008
Ga3 4h 0.5 0.7677 0 0.83 0.010 0.019 0.014 0 0 0 0.010
Fe2 8k 1/4 1/4 1/4 1.08 0.010 0.008 0.025 0.000 0.000 0.000 0.007
Table 3
Results of the structure refinement from the neutron data
Atom Position xYzO.F. U!10
2
(A
˚
2
)
U1 2a 0 0 0 1 0.18(6)
Ga1 4e 0 0 0.3432(5) 1 0.57(6)
Fe1 4f 0 1/2 0.2656(5) 1 0.43(6)
Ga2 4g 0 0.3468(6) 0 1 0.34(7)
Ga3 4h 0.5 0.8051(6) 0 1 0.53(7)
Fe2 8k 1/4 1/4 1/4 1 0.18(3)
Parameters defined as in Table 2.
UFe
6
Ga
6
1000
0
500
10 30 50 70 90 110
2θ (deg)
Intensity (a.u.)
Fig. 1. Experimental, calculated and differential UFe
6
Ga
6
neutron powder
diffraction profiles.
A.P. Gonc¸alves et al. / Intermetallics 14 (2006) 530–536532
possibility of a significant uranium magnetic moment in
UFe
6
Ga
6
.
The interatomic distances between the Fe atoms and the
nearest-iron/gallium positions (2.517–2.637 A
˚
) are lower or
approximately equal to the metallic radii sum, in contrast to the
interatomic distances with uranium. The remaining atoms have
interatomic distances lower, equal or higher than the sum of the
metallic radii. The gallium atoms usually have iron neighbours
at distances below and gallium neighbours at distances above
the sum of their metallic radii. It is interesting to notice that all
the iron–iron interatomic distances are very close to the sum of
the metallic radii, pointing for the possibility of predominant
Fe–Fe ferromagnetic interactions. This contrasts with the
UFe
6
Al
6
compound, where the Fe(8f)–Fe(8f) distances,
2.507 A
˚
[8], are below the sum of the metallic radii and lead
to antiferromagnetic interactions opposing the predominant
ferromagnetic Fe(8f)–Fe(8j) and Fe(8j)–Fe(8j) interactions
[16]. On the contrary, the longer iron–iron distances on the
8k sites in UFe
6
Ga
6
may reinforce the ferromagnetic
interactions, as already proposed for the 3-d magnetic
properties in UFe
x
Al
12Kx
[16] and other uranium ThMn
12
-
type compounds such as the UFe
10
X
2
(XZSi, Mo, Re) [17].
The temperature dependence of the UFe
6
Ga
6
magnetization,
measured under 100 Oe after zero field cooling a bulk
polycrystalline sample, is shown in Fig. 3. A ferromagnetic-
type transition is seen at T
ord
Z530(5) K, value corresponding
to the minimum of the temperature derivative of the
susceptibility. T
ord
was confirmed by A.C.-susceptibility
measurements that show a peak at 537(6) K in the real
component (insert in Fig. 3). No other magnetic transition is
seen down to 5 K. Ordering temperatures close to the 560 K
were previously reported for LuFe
5.8
Ga
6.2
, which has the
highest observed T
ord
in the LnFe
6Kx
Ga
6Cx
(LnZlanthanide,
Y) family of compounds [18], where magnetic order is
established at temperatures above T
C
w400 K. Moreover, for
UFe
6
Ga
6
the transition temperature is much higher than the
UFe
6
Al
6
Curie temperature, T
C
w300 K [8]. In iron-containing
R
x
Fe
y
compounds the Curie temperature is mainly determined
by the Fe–Fe sublattice interactions. In a first approximation,
the same dependence is also found in UFe
6
X
6
(XZAl, Ga). As
ThMn
12
ScFe
6
Ga
6
2a
2a
8f
8j
8i
8k
4f
4e
4g
4h
Fig. 2. Projections of the ThMn
12
and ScFe
6
Ga
6
-type structures along the c and a axes, respectively.
Table 4
UFe
6
Ga
6
interatomic distances (d) and nearest neighbours (NN) average numbers
NN Atoms d (A
˚
) NN Atoms d (A
˚
)
U(2a) 2 Ga1(4e) 2.934(3) Ga2(4g) 4 Fe2(8k) 2.622(3)
2 Ga2(4g) 3.014(4) 2 Fe1(4f) 2.632(6)
4 Ga3(4h) 3.043(3) 1 Ga2(4g) 2.663(8)
4 Fe1(4f) 3.226(3) 2 Ga3(4h) 2.852(7)
8 Fe2(8k) 3.2994(2) 1 U(2a) 3.014(4)
4 Ga1(4e) 3.156(6)
Ga1(4e) 4 Fe2(8k) 2.637(4) Ga3(4h) 4 Fe2(8k) 2.529(3)
1 Ga1(4e) 2.681(7) 2 Fe1(4f) 2.624(7)
2 Fe1(4f) 2.694(6) 2 Ga2(4g) 2.852(7)
1 U(2a) 2.934(3) 2 Ga1(4e) 2.971(6)
2 Ga3(4h) 2.971(6) 2 U(2a) 3.043(3)
4 Ga2(4g) 3.156(6) 1 Ga3(4h) 3.386(7)
Fe1(4f) 4 Fe2(8k) 2.517(4) Fe2(8k) 2 Fe1(4f) 2.517(4)
2 Ga3(4h) 2.624(7) 2 Fe2(8k) 2.5280(3)
2 Ga2(4g) 2.632(6) 2 Ga3(4h) 2.529(3)
2 Ga1(4e) 2.694(6) 2 Ga2(4g) 2.622(3)
2 U(2a) 3.226(3) 2 Ga1(4e) 2.637(4)
2 U(2a) 3.2994(2)
A.P. Gonc¸alves et al. / Intermetallics 14 (2006) 530–536 533
the number of iron nearest-neighbours is in average the same
for both compounds, the significant difference between
UFe
6
Al
6
and UFe
6
Ga
6
Curie temperatures points to a decisive
influence of the Fe–Fe interatomic distances, in agreement with
the crystallographic analysis described above. The significantly
higher ordering temperature on UFe
6
Ga
6
as compared to
UFe
6
Al
6
is probably related to the fact that the shortest iron–
iron distances in UFe
6
Ga
6
are larger than those in UFe
6
Al
6
[8].
The magnetization versus magnetic field previously
measured at 5 K for both free powder and powder fixed
samples [9] evidence a saturation magnetization (M
0
), obtained
from the linear extrapolation of M(H/0), for UFe
6
Ga
6
of
M
free
0
Z 10:5 m
B
/f.u., in the case of the free powder, and
M
fixed
0
Z 4:1 m
B
/f.u. for the fixed powder. The ratio
M
fixed
0
ðT/ 0Þ=M
free
0
ðT/ 0Þ, which gives a valuable infor-
mation on the magnetic anisotropy of the compound [19] is
in this case approximately 0.4 indicating a predominant
uniaxial anisotropy. In order to clarify this point and to better
characterize the magnetic properties of this compound single
crystal measurements were performed.
Small samples chosen from the UFe
6
Ga
6
pulled material
were tested by X-ray diffraction in order to check their single
crystalline character. Most of them had both twined domains
with similar volume, but a few showed a clear predominance of
only one of the domains (ratio bigger than 10:1). The
magnetization measurements were made on one of these single
crystals, with 0.5!0.5!0.5 mm
3
approximate dimensions.
The single crystal magnetisation measurements along the three
different crystallographic axes confirmed the uniaxial aniso-
tropy previously suggested from the M
fixed
0
ðT/ 0Þ=M
free
0
ðT/
0Þ ratio (Fig. 4). The a axis is the easy direction, the
magnetisation curves with the field parallel to this direction
presenting a typical ferromagnetic behaviour, with the
saturation being reached for fields w0.25 T. It is important
to notice that this a axis correspond to the c axis in the ThMn
12
-
type structure. Therefore, the a easy axis in UFe
6
Ga
6
is in clear
contrast with the behaviour observed in UFe
6
Al
6
, which has a
and b as the easy directions [8], and with the RFe
12Kx
Ga
x
(RZ
rare earth) alloys, where an easy-plane type anisotropy was
also reported [18].
The measurements performed with applied fields parallel to
the b direction show a linear variation of the magnetisation
with field, without saturation up to 5.5 T (Fig. 4). With an
estimated anisotropy field, given by the intersection of the
extrapolated b hard direction magnetisation curve with the a
easy one, of w25 T at 5 K, indicating a high magnetic
anisotropy. The magnetisation measurements performed with
applied fields parallel to the c direction show an initial increase
up to 1 T (2.7 m
B
/f.u.), followed by a linear variation with
magnetic field, without saturation up to 5.5 T. The predicted
anisotropy field for this direction is also w25 T at 5 K.
The variation of the spontaneous magnetisation, M
S
, with
temperature, obtained from the extrapolation to HZ0 of the
isothermal magnetisation curves measured along the easy
direction, for fields applied parallel to the a and c directions, is
presented in Fig. 5. For the measurements along a value of
12.3 m
B
/f.u.is obtained at 5 K. This value is higher than the
values obtained for the nonmagnetic rare earth YFe
6
Ga
6
and
Fig. 3. Temperature dependence of the magnetization of an UFe
6
Ga
6
polycrystalline sample for an applied field of 100 Oe. The insert show the
A.C.-susceptibility as a function of temperature curve for the same sample.
Fig. 4. Magnetization versus field along the three crystallographic directions of
the UFe
6
Ga
6
single crystal at 5 K.
Fig. 5. Spontaneous magnetization of the UFe
6
Ga
6
single crystal along a and c
axes, as a function of temperature.
A.P. Gonc¸alves et al. / Intermetallics 14 (2006) 530–536534
