Magnetic structure of Sm2IrIn8
Magnetic structure of Sm2IrIn8
C. Adriano,1, ∗ R. Lora-Serrano,1 C. Giles,1 F. de Bergevin,2 J. C.
Lang,3 G. Srajer,3 C. Mazzoli,2 L. Paolasini,2 and P. G. Pagliuso1
Instituto de F́ısica ”Gleb Wataghin”,UNICAMP,13083-970, Campinas-São Paulo, Brazil.
European Synchrotron Radiation Facility, Grenoble 38043, France.
Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439.
(Dated: December 28, 2018)
The magnetic structure of the intermetallic antiferromagnet Sm2IrIn8 was determined using x-ray
resonant magnetic scattering (XRMS). Below TN = 14.2, Sm2IrIn8 has a commensurate antiferro-
magnetic structure with a propagation vector ~η = (1/2, 0, 0). The Sm magnetic moments lie in
the ab plane and are rotated roughly 18o away from the a axis. The magnetic structure of this
compound was obtained by measuring the strong dipolar resonant peak whose enhancement was
of over two orders of magnitude at the L2 edge. At the L3 edge both quadrupolar and dipolar
features were observed in the energy line shape. The magnetic structure and properties of Sm2IrIn8
are found to be consistent with the general trend already seen for the Nd-, Tb- and the Ce-based
compounds from the RmMnIn3m+2n family (R = rare earth; M=Rh or Ir, m = 1, 2; n = 0, 1),
where the crystalline electrical field (CEF) effects determine the direction of magnetic moments and
the TN evolution in the series. The measured Néel temperature for Sm2IrIn8 is slightly suppressed
when compared to the TN of the parent cubic compound SmIn3.
PACS numbers: 75.25.+z, 75.50.Ee, 75.30.-m, 75.30.Kz
I. INTRODUCTION
The microscopic details of 4f -electron magnetism play
a fundamental role in the physical properties of vari-
ous classes of rare-earth based materials such as heavy
fermions, magnetically ordered alloys and permanent
magnets. The existence of structurally related families of
rare-earth based compounds provides a great opportunity
to explore how the details of the 4f -electrons magnetism
evolve as a function of changes in the dimensionality, lo-
cal symmetry and electronic structure along each related
family. The recently discovered1,2,3,4,5,6,7,8 family of in-
termetallic compounds RmMnIn3m+2n (M = Co, Rh or
Ir, m = 1, 2; R = La, Ce, Pr, Nd, Sm, Gd) have proved
to be very promising in this regard, since it possesses
many members of structurally related heavy-fermions su-
perconductors (HFS), for R = Ce, antiferromagnets (R =
Nd, Sm, Gd and Tb) and paramagnetic metals (R = La,
Pr). Within this family, the physical properties of a par-
ticular R-member can also be compared to compounds
based on the same R with three different related struc-
tures [the cubic RIn3 and the tetragonal RMIn5(1-1-5)
and R2MIn8 (2-1-8)]
9,10,11 and/or to the same R formed
with three distinct transition metals (M = Rh, Ir and Co
- not for all R -) in the same structure.
For the Ce-based HFS in this family, extensive
investigation has revealed fascinating physical prop-
erties such as quantum criticality, non-fermi-liquid-
behavior and an intriguing interplay between mag-
netism and superconductivity, reflected in very rich phase
diagrams.12,13,14,15,16,17,18,19,20 Because the HFS mem-
bers of this family are structurally related, its investiga-
tion has been used to provide some insights on the ques-
tion why some structure types are favorable to host many
superconductors. A possible relationship between the su-
perconducting critical temperature Tc and the crystalline
anisotropy13,21,22, the role of the 4f -electron hybridiza-
tion with the conduction electrons in the occurrence of
superconductivity23,24,25 and the effects of quasi-2D elec-
tronics structures26,27,28are some of the physical phenom-
ena that have been brought to the scenario to answer the
question above. Further, motivated by this experimental
trend, new materials search based on the 1-1-5 structures
has led to the discovery of the Pu-based HFS PuMGa5
(M = Rh and Co).29,30
On the other hand, as these HFS are presumably mag-
netically mediated, others studies5,6,7,21,31,32,33,34,35,36,37
have been focused in understanding the evolution of the
4f local magnetism, not only for the magnetically or-
dered Ce-based members of this family such as CeRhIn5
and Ce2RhIn8, but also for their antiferromagnetic coun-
terparts RmMnIn3m+2n (M = Rh or Ir, m = 1, 2;) for
R = Nd, Sm, Gd and Tb. From these studies, it was es-
tablished the role of tetragonal crystalline electrical field
(CEF) in determining the spatial direction of the ordered
R-moments with respect to the lattice and the evolution
of the Néel temperature, TN , in the series.
5,6,7,32,37
A key set of experiments allowing the above conclu-
sions was the experimental determination of the magnetic
structures of various members of the RmMnIn3m+2n (M
= Rh or Ir, m = 1, 2;) family.7,31,36,38,39,40,41 Up to date,
however, none of the Sm-based compounds from this fam-
ily have had their magnetic structures determined. In
fact, the compounds of this series containing Sm ions
may be particularly important in testing the extension
of the CEF trends in this family because the presence
of excited J-multiplet states in Sm3+ and quadrupolar
interactions have to be taken into account in order to
understand their magnetic phase diagrams.42,43,44,45 Es-
pecially interesting is Sm2IrIn8 which presents a first or-
http://arxiv.org/abs/0704.1480v2
der antiferromagnetic transition at TN = 14.2 K.
5 This
value is slightly smaller than the TN ∼ 16 K of the cubic
SmIn3
11 which according to the CEF trends observed in
other members of this family7,37 suggest that the ordered
Sm-moments should lie the ab-plane.
To further explore the magnetic properties of Sm2IrIn8
and to check the extension of the CEF trends observed
for R = Nd, Tb, and Ce,5,6,7,32,37 to the Sm-based
compounds, we report in this work the solution of the
magnetic structure of the intermetallic antiferromagnet
Sm2IrIn8 by means of the x-ray resonant magnetic scat-
tering (XRMS) technique. The XRMS technique has
proved to be a very important tool for the investigation
of microscopic magnetism in condensed matter, specially
for highly neutrons absorber ions such as Sm.
Sm2IrIn8 presents, below TN = 14.2 K, a commensu-
rate antiferromagnetic structure with a propagation vec-
tor ~η = (1
, 0, 0). The Sm magnetic moments lie in the
ab plane. In terms of relative orientation, the propa-
gation vector ~η indicates that the Sm-spins are ordered
antiferromagnetically along the a axis and ferromagnet-
ically along the b axis and, because of the presence of
two Sm ions per unit cell along c axis, some calculations
have to be performed in order to determine the type of
ordering along this direction. Furthermore, as it could be
expected for such spin arrangement in a tetragonal com-
pound, antiferromagnetic domains were observed in the
ordered state of Sm2IrIn8. These domains were removed
by field-cooling the sample at a field of H = 10 T.
II. EXPERIMENT
Single crystalline samples of Sm2IrIn8 were grown from
Indium flux as described previously.5,46 The crystal struc-
ture, unit cell dimensions and macroscopic properties of
the Sm2IrIn8 single crystals used in this work were in
agreement with the data in Ref. 5. For the XMRS exper-
iments of this work, selected crystals were extracted and
prepared with polished (0,0,l) flat surfaces, and sizes of
approximately 4 mm x 3.4 mm x 1.5 mm. The preferred
crystal growth direction of this tetragonal compound is
columnar along the [00l ] direction and the (001) facet is
relatively large. The mosaic spread of the sample was
found to be < 0.08◦ by a rocking curve (θ scan) on a
Phillips four circle diffractometer.
XRMS studies were performed at the 4-ID-D beam-
line at the Advanced Photon Source (APS) and at the
ID-20 beamline at the European Synchrotron Radiation
Facility (ESRF). The 4-ID-D x-ray source is a 33 mm
period planar undulator and the energy is selected with
a double crystal Si(111) monochromator. A toroidal mir-
ror focuses the beam to a 220 µm (horizontal) x 110 µm
(vertical) spot, yielding an incident flux of ∼3.5 x 1013
photons/s with an energy resolution of δE/E = 1.4 x
10−4. The sample was cooled in a closed-cycle He refrig-
erator (with a base temperature of 4 K) with a dome Be
window. Our experiments were performed in the copla-
nar geometry with σ-polarized incident photons, i.e., in
the vertical scattering plane, using a four-circle diffrac-
tometer. Except for azimuthal scans, the sample was
mounted with the b axis perpendicular to the scattering
plane.
In most measurements, we have performed a polar-
ization analysis, whith Cu(220), Graphite (006) and
Au(111) crystal analysers, appropriate for the energies of
Sm L2 and L3 edges. The diffractometer configuration
at the APS allowed measurements at different azimuthal
angles (φ) by rotating the sample around the scatter-
ing vector Q. This was particularly useful to probe the
magnetic moment components at the dipolar resonant
condition with σ incident polarization.
The x-ray source on the ID-20 beamline was a linear
undulator with a 32 mm period. The main optical com-
ponents are a double Si(111) crystal monochromator with
sagital focusing and two meridional focusing mirrors on
either side of the monochromator. At 7.13 keV using the
first harmonic of the undulator u32, the standard inci-
dent flux at the sample position was approximately 1 x
1013 ph/s at 200 mA with a beam size of 500 µm (hori-
zontal) x 400 µm (vertical). The sample was mounted on
a cryomagnet (with a base temperature of 2 K), installed
on a horizontal six-circle diffractometer, with the b axis
parallel to the cryomagnet axis and perpendicular to the
scattering plane. This configuration allowed π-polarized
incident photons in the sample and the application of an
external magnetic field up to 10 T perpendicular to the
scattering plane.
III. RESULTS
A. Temperature dependence and resonance
analysis
Magnetic peaks were observed in the dipolar resonant
condition at temperatures below TN = 14.2 K at recip-
rocal lattice points forbidden for charge scattering and
consistent with an antiferromagnetic structure with prop-
agation vector (1
, 0, 0). Their temperature dependence
was studied for increasing and decreasing temperature
sweeps. Figure 1 shows the temperature dependence of
(0, 1
, 9) magnetic reflection at an incident photon energy
of 7.313 keV (L2 edge) and measured at π incident polar-
ization without polarization analysis. The squared root
of the integrated intensity, which is proportional to a Sm
sub-lattice magnetization, is displayed. A pseudo-voigt
peak shape was used to fit transversal θ scans through the
reciprocal lattice points in order to obtain the integrated
intensities of the reflection peak. This peak intensity de-
creases abruptly to zero for T > 13 K and its critical be-
havior can not be described by a power-law function with
a critical exponent β. This result is in agrement with the
first order character of the magnetic transition at 14.2 K,
revealed by heat capacity data, from which a latent heat
of ∼ 10 J/mol was extracted.5 Consistently, we found
4 6 8 10 12 14
1.2 (0,1/2,9) Peak
 Heating
 Cooling
T (K)
FIG. 1: Temperature dependence of one Sm2IrIn8 sub-lattice
magnetization measured with transverse (θ) scans at the
(0, 1
, 9) peak.
evidence of small hysteresis for T . 14.2 when changing
from warming to the cooling temperature sweep.
The energy line shape curves for the polarization chan-
nels σ - π’ and σ - σ’ of the (1
,0,9) diffraction peak at
(a) the L2 and (b) the L3 absorption edges of Sm
3+ ion
at T = 5.9 K are shown in Figure 2. The solid lines
in both panels represent the absorption spectrum, µ(E),
extracted from fluorescence yield. The data of Figure 2
were collected at the 4-ID-D beamline of APS by count-
ing the photons reaching the detector at a fixed Q while
changing the incident energy. The strong resonant en-
hancement of the x-ray scattering at this reciprocal space
position provide clear evidence of the magnetic origin of
the observed peaks.
The energy scan curve in Figure 2(a) has a maximum
at 7.312 keV which is only ∼2.5 eV larger than the L2
absorption edge (defined by the inflection point of the
absorption spectrum), revealing the electric dipolar char-
acter (E1) of this transition (from 2p to 5d states). Fig-
ure 2 also shows the polarization analysis performed to
unambiguously confirm the magnetic origin of the super-
structure peaks. Polarization analysis was also used to
verify whether the anomaly at approximately 8 eV be-
low the dipolar peak in Figure 2(a) could be associated
with a quadrupolar transition47 or it simply represents
an enhanced interference between the non-resonant and
the resonant part of the scattering amplitude. For the ex-
perimental configuration used (incident σ-polarization),
the electric dipole transitions E1 rotate the plane of po-
larization into the scattering plane (π-polarization). Our
data in Figure 2(a) reveals a strong enhancement of the
scattered intensities at the σ - π’ channel (closed cir-
cles) and no enhancement at the σ − σ’ channel for the
same energy range. These results confirm the magnetic
origin of the (h, 0, l)±(1
, 0, 0) reflections due to the ex-
6.68 6.70 6.72 6.74 6.76
7.28 7.30 7.32 7.34
Energy (keV)
Sm L3 edge
6.716 keV
Sm L2 edge
7.312 keV
FIG. 2: Energy scan of the ( 1
,0,9) magnetic peak at T = 5.9 K
for σ - π’ (closed circles) and σ - σ’ (open circles) polarization
channels at the L2 (top) and L3 (bottom) absorption edges.
The data have been corrected for absorption, µ(E), using the
measured fluorescence yield. Arrows indicate the scales for
the fluorescence yield (right) and the observed data (left).
istence of an antiferromagnetic structure doubled along
the crystallographic â direction, with a propagation vec-
tor ~η = (1
, 0, 0).
The energy profile around the Sm L3 edge is pre-
sented in Figure 2(b). Firstly, the observed intensities
are roughly one order of magnitude weaker than those
obtained at the L2 resonance, in agreement with previ-
ous measurements on pure Sm.45 Secondly, there are two
peaks in the σ - π’ channel signal, as also observed for
other light rare-earth48,49 and Sm-based compounds.45,50
A high energy peak appears at 6.716 keV, while a low en-
ergy and more intense enhancement can be observed at
6.708 keV. Interestingly, Stunault et al.45 have demon-
strated that for pure Sm the quadrupolar E2 resonance
is more intense than the dipolar E1 at the L3 edge and
they found that the energy difference between the E2
and the E1 resonances is of the order of 8 eV, the same
as the one found in this work. Furthermore, in the σ - σ’
channel only an enhancement at 6.708 keV could be ob-
served which is consistent with the quadrupolar character
of this resonance, since scattering signal in σ - σ’ chan-
nel for dipolar transitions is strictly forbidden.47,51 Thus,
the presence of this pre-edge enhancement in the energy
curves of Figure 2 confirms an expected quadrupole (E2)
2p to 4f contribution to the resonant x-ray scattering in
Sm2IrIn8.
B. The magnetic structure
The magnetic structure of the Sm2IrIn8 was experi-
mentally investigated using dipolar resonant x-ray mag-
netic scattering with polarization analisys. In general,
the magnetic scattering intensities are given by:47,50
µ∗sin(2θ)
~k, ǫ̂, ~k′, ǫ̂′)ei
~Q·~Rn
, (1)
where µ∗ is the absorption correction for asymmetric re-
flections, 2θ is the scattering angle, ~Q = ~k′ − ~k is the
wave-vector transfer, ~k and ~k′ (ǫ̂ and ǫ̂′) are the incident
and scattered wave (polarization) vectors, respectively.
~Rn is the position of the nth resonant atom in the lattice,
and ẑn is the moment direction of this atom. The reso-
nant scattering amplitude contains both dipole (E1) and
quadrupole (E2) contributions. For the determination
of the magnetic structure of this work we have used the
second term of the electric dipole transition (E1) form
factor which produces magnetic peaks. In this case we
have:
nE1 ∝
0 k̂′ · ẑn
−k̂′ · ẑn (k̂ × k̂′) · ẑn
0 z1cosθ + z3sinθ
−z1cosθ + z3sinθ −z2sin(2θ)
, (2)
where θ is the Bragg angle, z1, z2 and z3 are the compo-
nents of the magnetic moment at the nth site, according
to the commonly used geometry convention of Ref. 52;
σ, π, σ’ and π’ describe the incident (non-primed terms)
and scattered (primed) photon polarizations.
As described previously, two experimental setups have
been used in this work, in the vertical (4-ID-D beamline)
and horizontal (ID-20) scattering configurations. This
permitted us to access all four polarization channels of
the 2x2 matrix in (2) and to determine the magnetic mo-
ment orientations through their polarization dependence
at the E1 resonance by comparing the relative intensities
of experimental (1
, 0, l) magnetic peaks with the calcu-
lated ones using the appropriate terms of matrix (2).50
In the case of Sm2IrIn8 the magnetic propagation vec-
tor ~η = (1
, 0, 0) does not unequivocally determine the
magnetic structure due to the presence of two magnetic
Sm atoms per chemical unit cell along the ĉ direction.
Therefore, as stated above, we have an antiparallel or-
dering of the Sm moments along the â direction and a
parallel ordering along b̂. Along ĉ there are, however, two
possibilities of coupling that can take place: a parallel
arrangement (Model I), in which the moments of neigh-
boring Sm ions along c axis are parallel to each other
(sequence ++ ++ ++ . . . ), or the antiparallel coupling
(Model II), with the sequence (+− +− +− . . . ). These
two possibilities have been considered into the calculated
magnetic structure factor while orienting the magnetic
moment along the three crystallographic directions for
five different (1
, 0, l) magnetic Bragg peaks, with l = 6,
7, 8, 9, 10. The calculated intensities are strongly de-
pendent on the projections of magnetic moments along
the crystallographic axis through the product k̂′ · ẑn of
equation (2). Therefore, they were compared to the rel-
ative observed intensities for each case. This evaluation
was performed at the vertical geometry of the 4-ID-D
beamline at 9 K by performing rocking scans with the
crystal analyzer and numerically integrating the data.50
We show this analysis in Table I, where “Model I” stands
for the ++ ++ ++ . . . sequence and “Model II” for the
+−+−+−. . . one. This comparison shows that the model
which best fits the experimental data is the one assum-
ing antiparallel coupling along c axis (Model II) with the
magnetic moments approximately oriented along the a
axis (according to matrix (2), for a σ polarized incident
beam and peaks at reciprocal space positions with the
(001) normal surface contained in the scattering plane,
contributions from an oriented moment along b̂ direction
cannot be detected).
In addition, we have also measured the π − σ′ and
π − π′ polarization channels at the horizontal geometry
of the ID-20 beamline. Measuring these two channels we
gained access to the z1 and z3 components (in equation
2) of magnetic moment vector in one case [π − σ′, Fig-
ure 3(a)] and to z2 in the other [π − π
′, Figure 3(b)].
There is a clear indication that for the π − σ′ channel
the observed data are well fit when considering the mo-
ments along the â direction [dotted curve in Figure 3(a)]
instead of ĉ direction [short dashed curve]. Also in this
case the E1 terms are not sensitive to the component
of the ordered moment perpendicular to the scattering
plane, i.e. along b axis. Further, when measuring the
channel (π − π′) we are only allowed to measure the b
component, which is confirmed by the good fit of exper-
imental data when assuming magnetic moments along
such direction [dash-dotted curve in Figure 3(b)]. These
two last results indicate that the Sm moments actually
have components along both a and b real space axis and
not perfectly aligned along any of these two directions.
To determine the exact orientation of the magnetic mo-
ments within the ab plane, we have performed azimuthal
scans (φ scan) through the (1
, 0, 9) reflection (Figure 4)
TABLE I: Comparison between observed and calculated intensities of magnetic Bragg reflections, assuming either parallel
(model I) or antiparallel (model II) alignment between the moments of two Sm ions along the c axis in the same chemical unit
cell.
MODEL I MODEL II
(h, k, l) Exp. Data m//c m//a m//c m//a
(1/2,0,6) 66 13 29 24 55
(1/2,0,7) 78 17 29 39 68
(1/2,0,8) 5 77 100 3.4 4.5
(1/2,0,9) 100 3 3 100 100
(1/2,0,10) 12 100 68 32 23
( ')channel   z
//a-axis
                                z
//c-axis
 ( ')channel   z
//b-axis
Q  (r.l.u)
FIG. 3: Analysis of the possible magnetic moment directions
for Sm2IrIn8 at the L2 resonance. Q-dependence of the inte-
grated intensities of: (a) six satellite peaks signal in the π−σ′
channel with the moments along â and ĉ, and (b) in the π−π′
with moments parallel to b̂.
at the E1 resonance. At the σ − π′ polarization channel
this procedure warrants the determination of moments
directions with no ambiguity because the magnetic cross
section is strongly dependent of the magnetic moment
direction and the polarization of the incoming and scat-
tered radiation, the maximum (minimum) intensity in
the curve will occur with the magnetic moment being
parallel (perpendicular) to the diffraction plane. With
the experimental setup of 4-ID-D beamline we had ac-
cess to record points at azimuthal angles φ between -50o
-120 -80 -40 0 40 80 120
//18o
Azimuth angle (degree)
, (1/2,0,9), T = 6.5 K
 Exp. Data ( ')
FIG. 4: Azimuth scan analysis. Normalized integrated in-
tensities of the ( 1
, 0, 9) magnetic peak at T = 6.5 K (open
circles). The other curves represent the integrated intensi-
ties behavior considering the magnetic moments along the â
(dotted line), b̂ (dashed) and 18o away from â (solid line)
direction.
and 60o. In order to compare with the observed data,
one can calculate the intensities for the σ − π′ channel
using the expressions (1) and (2) and a reasonably simple
geometry analysis considering the projections of both k̂′
and ẑn on the coordinate system of Ref. 52 when the
azimuth angle is changed. Then, the calculated inten-
sity is proportional to Iσπ
∝ |-cosθ cosφ cosα + sinθ
sinα|2, where α represents the assymetry angle between
the scattering and the normal surface vector.50 Figure 4
shows the experimental and the calculated relative in-
tensities considering the moment along the a and b axis,
as well as 18o tilted from the a axis, which is the value
that nicely adjust the experimental data. Considering
the experimental errors we can then conclude that the
magnetic moment is in the ab plane making (18o ± 3o)
with the â direction of the sample. Using all the above
results, a model of the magnetic unit cell of Sm2IrIn8 can
be constructed and is shown in Figure 5.
FIG. 5: Magnetic structure of Sm2IrIn8 below TN = 14.2 K
(left) and a Sm-In plane top view (right) showing the in-plane
arrangement of Sm moments.
As it was observed in the magnetic structure of other
members of the RmMIn3m+2 series such as NdRhIn5
TbRhIn5,
7 GdRhIn5,
41 and Gd2IrIn8
36 the magnetic
structure of Sm2IrIn8 presents a lower symmetry than
the crystallographic structure, as the Sm spins present
different relative orientations along the â and b̂ direc-
tions even though a and b are indistinguishable. This
spin arrangement was explained by considering the first
(J1) and second (J2) R-neighbors exchange interactions
in the case of a small J1/J2 ratio.
Considering the observation of this kind of magnetic
structure in tetragonal compounds, it may be expected
that at zero magnetic field the antiferromagnetic ordering
takes place with the formation of antiferromagnetic do-
mains where the relative orientation of the magnetic mo-
ments along a given direction (â or b̂) changes from paral-
lel to antiparallel between the domains. The presence of
a twinned magnetic structure with symmetry-related do-
mains was evidenced by the observation of both (1
, 0, l)
and (0, 1
, l) reflection-types in this work. To further in-
vestigate the presence of antiferromagnetic domains in
the ordering state of Sm2IrIn8 we follow the behavior
of the magnetic (1
, 0, l) and (0, 1
, l) reflections under an
applied magnetic field.
Figure 6 presents the behavior of the (1
, 0, 9) and
(0, 1
, 9) intensities as a function of the applied magnetic
field of 10 T along one of the tetragonal axis in the plane
(defined as b̂ direction). At zero field and T = 6 K,
both (1
, 0, 9) [open circles] and (0, 1
, 9) [closed squares]
intensities can be observed with comparable magnitude
[Figure 6(a)]. The (1
, 0, 9) intensity is roughly 66% that
of the (0, 1
, 9) peak. The sample was then field cooled
(H = 10 T) from the paramagnetic (16 K) to the ordered
state (6 K) with the field applied along the b̂ direction.
As can be seen in Figure 6(b) the (0, 1
, 9) diffraction peak
disappears as the magnetic field favors the parallel spin
orientation along the b axis. The same effect was also ob-
served for the other five (0, 1
, l) reflections (not shown).
The results under applied magnetic field shown in Fig-
38.7 38.8 38.9 39.0 39.1 39.2
0.000
0.075
0.150
45.9 46.0 46.1 46.2 46.3 46.4
 (1/2,0,9)
H = 0
T = 6 K
Degrees
 (0,1/2,9)
H = 10 T
Field Cooled
uo to T = 6 K
 (0,1/2,9)
Degrees
alized
sity x 10
-3 (a.u
 (1/2,0,9)
FIG. 6: Field-dependence of the integrated intensities of the
, 0, 9) and (0, 1
, 9) magnetic peaks taken with transverse
(θ) scans around each reciprocal space lattice points. (a) For
H = 0 applied field at T = 6 K, (b) for H = 10 T and (c) field
cooled from 16 K to 6 K at H = 10 T.
ure 6 confirm the existence of a twinned magnetic struc-
ture for Sm2IrIn8 which allows the observation of both
(0, 1
, l) and (1
, 0, l) magnetic reflections at zero field.
IV. DISCUSSION
Early studies on the antiferromagnetic cubic com-
pound SmIn3 have shown multiple magnetic transitions
associated with quadrupolar ordering, magnetoelastic
and magnetocrystalline competitive effects at 14.7, 15.2
and 15.9 K (the former two temperatures being associ-
ated with successive magnetic dipolar, antiferromagnetic,
orders and the last one due to quadrupolar ordering).42,43
For the tetragonal Sm2IrIn8, the insertion of two addi-
tional SmIn3 atomic layers into the crystalline structure
slightly decreases TN compared to that of SmIn3 (14.2
and 15.2 K for the Sm2-1-8 and Sm1-0-3 TN ’s, respec-
tively) and an additional anomaly at 11.5 K has been ob-
served in the specific heat and resistivity measurements,5
probably related to the successive transitions seen in the
ordered phase of the SmIn3.
Following the investigation of the isostructural mag-
netic non-Kondo compounds from the RmMIn3m+2 fam-
ily, where the details the 4f magnetism along the series
may be important to understand the possible magnetic-
mediated superconductivity in the compounds with R =
Ce, we have studied the magnetic structure of Sm2IrIn8,
which is the only compound from this family with a clear
first order antiferromagnetic transition and now it is the
first Sm-member from this family with a solved magnetic
structure, which is the main result of this work. The de-
termination of the Sm2-1-8 magnetic structure allows for
the investigation of the CEF driven trends of magnetic
properties within the RmMIn3m+2 family to be extended
to the Sm-based members.
Our results confirm the complex resonance profile of
Sm-based compounds (at one satellite reciprocal point,
Figure 2), as seen in previous studies of pure Sm.45 It has
been argued that the larger intensity of E2 resonance at
Sm L3 edge compared to its intensity at the L2 edge may
be explained qualitatively by the spin-orbit splitting of
the intermediate 4f levels involved.45 The L3 transitions
connect the j = 7
state while L2 involves transitions to
the j = 5
level, which lie lower in energy and therefore
can be preferentially populated by the five 4f Sm elec-
trons. This reduces the number of vacant j = 5
states
from 6 to 1, in contrast to the 8 states available for the
j = 7
level, which increases the transition probability of
the E2 resonance at Sm L3 in Sm2IrIn8.
Considering the additional magnetic transitions ob-
served for SmIn3,
42,43 and the additional anomaly at T
= 11.5 K in heat capacity and electrical resistivity mea-
surements for Sm2IrIn8,
37 we did not observe any dis-
continuities, within the resolution of our experiment, in
the integrated intensities of the (0, 1
, 9) magnetic peak
from roughly 4 K up to 16 K (Figure 1). Therefore we
conclude that there are no changes of the magnetic prop-
agation vector ~η = (1
, 0, 0) below TN . For completeness,
on going field-dependent heat capacity and thermal ex-
pansion measurements (not shown and will be published
elsewhere) have revealed no field-induced transitions up
to H=9 and 18 T, respectively, similarly to SmIn3 where
no additional transition was found with applied field up
to H=32 T.44
On the other hand, recent works have shown that the
low temperature CEF configuration plays a fundamen-
tal role on the behavior of TN and the magnetic mo-
ment directions within the RmMIn3m+2 family.
7,32,37,40
Further, Kubo et al.53 has also proposed an orbital con-
trolled mechanism for superconductivity in the Ce-based
compounds from this family. For the Sm members, CEF
effects confine the magnetic moments to the ab plane,
consistent with the experimental CEF trends observed
for R = Ce, Nd and Tb5,6,7,32 and also by the predictions
of a recently developed mean field theoretical model.7,37
If the magnetic ordered moments lie in the ab-plane but
they are more magnetically susceptible along the c axis
the magnetic order can be frustrated to lower TN val-
ues than for their cubic relatives. The mean-field model
of Ref. 37, however, only includes the contributions of
tetragonal CEF and first neighbor isotropic dipolar ex-
change interaction. Therefore, it may not be expected
to work for Sm containing compounds, because for the
Sm3+ ion the first excited J -multiplet lying just above
the ground state is closer in energy. Thus, the tetragonal
CEF splitting can mix both the excited and ground state
CEF scheme and this particular effect should be consid-
ered into the calculations. Indeed, this is the responsible
for the non-linear response of the inverse of magnetic
susceptibility at high temperatures on SmIn3 and other
Sm-based compounds,11,54 as well as in Sm2IrIn8.
5 Fur-
thermore, as it was found for SmIn3,
42,43 quadrupolar
magnetic interactions also have to be considered in order
to achieve a complete description of the magnetic prop-
erties of the Sm-based compounds in the RmMIn3m+2
family.
Apart from the higher complexity of the magnetic
properties of the Sm-compounds, it was found exper-
imentally that TN is decreased (roughly ∼ 10%) for
the tetragonal compounds when compared to the cu-
bic SmIn3. In addition, we have found that the mag-
netic structure of Sm2IrIn8 shows the ordered Sm mo-
ments in the ab plane, as expected in the case of TN
suppression.7,37 Although the changes in TN for the Sm
compounds are much smaller (perhaps due to the partic-
ularities of the Sm3+ ion discussed above) than that ob-
served for R = Ce, Nd and Tb in the RmMIn3m+2 family,
we can conclude with the solution of the magnetic struc-
ture reported here, that the general CEF trend of the
RmMIn3m+2 is also qualitatively present in Sm2IrIn8.
V. CONCLUSION
In summary, we have presented the results of the mag-
netic structure determination of the intermetallic anti-
ferromagnet Sm2IrIn8. The magnetic order is commen-
surate with propagation vector ~η = (1
, 0, 0) and the
Sm moments oriented in the ab plane. We used differ-
ent scattering geometries (exploring the polarization de-
pendences of magnetic intensities) and azimuthal scans
around a magnetic reciprocal space point to determine
without ambiguity that the moments are aligned approx-
imately 18o away from the a axis. The temperature be-
havior of the magnetic satellites have been probed at the
(0, 1
, 9) reciprocal node and show no evidence of changes
in the magnetic structure within the studied temperature
range. Besides, an abrupt (non-power law) decrease of
magnetic intensities at TN was found, consistent with the
first order character of the antiferromagnetic transition of
Sm2IrIn8. The resonance properties at the Samarium L2
and L3 absorption edges revealed both resonant E1 and
E2 process with roughly one order of magnitude more in-
tense resonance peaks at the L2 edge and a much stronger
quadrupole resonance in the L3 edge. The orientation of
Sm moments in the ab plane and the small decrease of
TN compared to its value for SmIn3 agrees with a general
CEF trend found in the RmMIn3m+2 family.
Acknowledgments
This work was supported by FAPESP (SP-Brazil)
Grants No. 05/55272-9, 05/00962-0, 04/08798-2 and
03/09861-7, CNPq (Brazil) Grants No. 307668/03,
04/08798-2, 304466/20003-4 and 140613/2002-1, and
FAEPEX (SP-Brazil) Grant No. 633/05. Use of
the Advanced Photon Source was supported by the
U. S. Department of Energy, Office of Science, Office
of Basic Energy Sciences, under Contract No. DE-
AC02-06CH11357. The staff at the 4-ID-D and ID-20
beam lines are gratefully acknowledged for providing an
outstanding scientific environment during these experi-
ments.
∗ Electronic address: cadriano@ifi.unicamp.br
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