Inelastic neutron scattering measurements in a quadrupolar ordering compound YbSb

Article abstract of DOI:10.1016/j.jmmm.2006.10.167

Inelastic neutron scattering measurements were performed to study the excitation spectra of a quadrupolar ordering compound YbSb. We observed the three peaks (12.5, 17 and 21 meV) around Γ₆ - Γ₈ excitations far above the quadrupolar ordering temperature (T_Q). This result suggests that the magnetic excitations have a dispersion rather than the splitting of Γ₈ states at high temperatures. We also observed the strong q dependence in q scans at energy = 1 meV above T_Q. The observed q dependence above T_Q suggests that the theoretical approach beyond the molecular field approximation is required in order to understand the magnetic properties of Yb monopnictides.

Full text of DOI:10.1016/j.jmmm.2006.10.167

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 766–767
www.elsevier.com/locate/jmmm
Inelastic neutron scattering measurements in a quadrupolar ordering
compound YbSb
Ã
A. Oyamadaa, , K. Kuwaharab, K. Ohoyamac, K. Iwasad, M. Kohgib, S. Maegawaa
aGraduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan
bDepartment of Physics, Tokyo Metropolitan University, Tokyo 192-0397, Japan
cInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan
dDepartment of Physics, Tohoku University, Sendai 980-8578, Japan
Available online 30 October 2006
Abstract
Inelastic neutron scattering measurements were performed to study the excitation spectra of a quadrupolar ordering compound YbSb.
We observed the three peaks (12.5, 17 and 21 meV) around G62G8 excitations far above the quadrupolar ordering temperature (TQ).
This result suggests that the magnetic excitations have a dispersion rather than the splitting of G8 states at high temperatures. We also
observed the strong q dependence in q scans at energy ¼ 1 meV above TQ. The observed q dependence above TQ suggests that
the theoretical approach beyond the molecular field approximation is required in order to understand the magnetic properties of
Yb monopnictides.
r 2006 Elsevier B.V. All rights reserved.
PACS: 61.12.ꢀq; 71.20.Eh
Keywords: Quadrupolar ordering; YbSb; Neutron scattering
1. Introduction
renewed interest of those compounds. A molecular field
approximation including quadrupolar interactions demon-
strates that the mixing-type quadrupolar ordering occurs
even in the ground states which do not have a quadrupolar
moment and the first excited states are at 150 K. The model
calculations explained the magnetic properties of Yb
monopnictides qualitatively, however, there are still some
problems which have not been resolved. The splitting of the
first excited states ðG8Þ of the crystal field splittings is
the most anomalous property. Keller et al. proposed that
the unexpected splitting is explained by the occurrence of a
quadrupolar ordering around 80 K in YbN, YbP and
YbAs [5]. Kuwahara et al. has observed the similar
splitting in YbSb [6]. However, there is no other evidence
of the quadrupolar ordering around 80 K. In this study, we
concentrate on the excited states in YbSb to study the
dynamical properties above the quadrupolar ordering
temperature ðTQ ¼ 5 KÞ and to clarify the origin of the
unexpected splitting below 80 K. The powder samples of
YbSb were prepared by a reaction of the constituent
elements in an evacuated quartz tube.
Yb monopnictides (YbN, YbP, YbAs and YbSb) have
attracted much interest as they show heavy fermion
characteristics such as a large g value of specific heat and
reduced antiferromagnetic ordered moments although the
carrier concentration is only several percents per Yb atom.
The antiferromagnetic ordering temperatures ðTNÞ of
YbN, YbP and YbAs are 0.74, 0.41 and 0.49 K [1] while
the estimated exchange interactions are of the order of 10 K
[2]. In contrast to other Yb monopnictides, YbSb shows
two successive phase transitions; an antiferromagnetic
ordering at 0.32 K and another phase transition at 5 K
[2]. On the other hand, neutron diffraction measurements
did not observe any long range ordering down to 7 mK.
Recent observations by specific heat and NMR measure-
ments reveal the phase transition at 5 K is of a mixing-type
quadrupolar ordering [3,4]. Thus, YbSb has attracted
Ã
Corresponding author. Tel.: +81 75 753 2943; fax: +81 75 753 2999.
E-mail address: f52017@sakura.kudpc.kyoto-u.ac.jp (A. Oyamada).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.10.167

ARTICLE IN PRESS
A. Oyamada et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 766–767
767
1500
2. Results and discussions
The energy spectra below 4 meV were measured on the
triple axis spectrometer C1-1(HER) at the JRR-3M of
JAEA. Fig. 1 shows the results of q scans with a fixed
YbSb
ꢀ1
˚
kf ¼ 1:5061 A
at E ¼ 1 meV. There is a strong q
dependence of intensity in q scans at energy ¼ 1 meV as
shown in Fig. 1. The decrease of intensity in small q vectors
is noticeable although we used powder samples. The q
dependence develops with decreasing temperature below
20 K. The another peak at h ¼ 2:0 is a contamination from
the nuclear incoherent peak. The energy spectra can be
interpreted as the Gaussian function (an incoherent peak at
0 meV) and a Lorentzian function (a quasi-elastic peak)
convoluted by the energy resolution. The temperature
factor and ki=kf factor are calibrated. The fitting of the
energy spectra shows that the decrease of the intensity at
h ¼ 0:6 is ascribed to the decrease of the width of the
Lorentzian function. Ohoyama et al. reported that q
dependence of the quasi-elastic peak in powder samples
and a single crystal of YbAs exist up to 20 K [7,8]. The q
dependence was ascribed to the short range magnetic
correlations of an ordering wave vector ½1; 0; 12ꢁ. We should
note that the magnetic correlations still exist above 40TN
ðTN ¼ 0:49 KÞ. Therefore the q dependence observed in this
study in YbSb seems to be the common feature of the Yb
monopnictides. The magnetic correlations survive up to
20 K, which is in contrast to the TN ¼ 0:32 K in YbSb. The
measurements of the correlations below TQ are in progress.
Fig. 2 shows the energy spectra between 1 and 27 meV of
powder samples of YbSb at 20 and 100 K measured on the
triple axis spectrometer 6G(TOPAN) at the JRR-3M
research reactor of JAEA. Three peaks at 12.5, 17 and
21 meV around G62G8 excitations were observed. Two
peaks are consistent with the previous results, however, in
addition to these two peaks we found a new peak at
12.5 meV. This new peak develops below 80 K with
decreasing temperature. Here, we note that the temperature
where the intensity of the new peak starts to develop is
similar to the quadrupolar ordering temperatures in YbN,
1000
500
0
20 K
100 K
0
5
10
15
Energy (meV)
20
25
30
Fig. 2. Inelastic neutron scattering spectra measured at q ¼ (2.2, 0, 0).
YbP, YbAs which Keller et al. proposed although it is very
difficult to understand that the localized states like G8
quartet split into three. If we assume that a quadrupolar
ordering occurs at 80 K, the molecular field approxima-
tions show that G8 states should split into two. Further-
more, we found the small q dependence of intensity at the
energies of three peaks. These results may offer a new
interpretation of the long-standing mystery of G8 states;
there is a dispersion in G8 states rather than the splitting.
Rather large magnetic exchange interactions and/or quad-
rupolar interactions between G8 states are suggested.
Further investigations of the nature of G8 states are
urgently required.
The observed q dependence above TQ and the suggested
strong interaction between G8 states require the theoretical
approach beyond the molecular field approximation in order
to understand the magnetic properties of Yb monopnictides.
1000
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E = 1 meV
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ꢀ1
˚
Fig. 1. The q scans at 1 meV with a fixed kf ¼ 1:5061 A
.