Pressure effect on valence fluctuation and magnetic ordering in YbPd

Article abstract of DOI:10.1016/j.physb.2009.07.029

The cubic CsCl type compound YbPd is known to be a valence fluctuating compound and to undergo four phase transitions at 0.5, 1.9, 105 and 125 K. In the present study, we focus on pressure effect on the transitions. The transitions at T₁ = 105

Full text of DOI:10.1016/j.physb.2009.07.029

ARTICLE IN PRESS
Physica B 404 (2009) 3002–3004
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Physica B
journal homepage: www.elsevier.com/locate/physb
Pressure effect on valence fluctuation and magnetic ordering in YbPd
Ã
A. Mitsuda , K. Yamada, M. Sugishima, H. Wada
Department of Physics, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
a r t i c l e i n f o
a b s t r a c t
The cubic CsCl type compound YbPd is known to be a valence fluctuating compound and to undergo
four phase transitions at 0.5, 1.9, 105 and 125 K. In the present study, we focus on pressure effect on the
PACS:
7
1.27.þa
transitions. The transitions at T
are of first order. With increasing pressure, the transitions shift toward lower temperature direction.
The transition at T satisfies the Clausius–Clapeyron’s relation, which also supports the first-order
1
¼ 105 K and T
2
¼ 125 K, which are accompanied by thermal hysteresis,
7
5.30.Mb
1
Keywords:
transition. T1 disappears at around 1.5 GPa. Above 1.5 GPa, some new ground state would be realized.
Ytterbium
We can observe an anomaly at 2.8 K which would be associated with magnetic ordering at 1.9 K.
Valence fluctuation
Charge ordering
Pressure effect
3þ
The anomaly becomes more distinct with pressure. Since applying pressure stabilizes magnetic Yb
the magnetic ordering becomes more conspicuous.
,
&
2009 Elsevier B.V. All rights reserved.
1. Introduction
resistivity
high pressure.
r in the temperature range between 2 and 300 K under
The competition between Kondo effect and Ruderman–Kittel–
Kasuya–Yosida (RKKY) interaction in rare-earth compounds
induces such exotic phenomena as valence fluctuation, heavy
fermion [1], Kondo insulator [2], anisotropic superconductivity [3]
and so on. Since these phenomena, which are ascribed to unstable
2. Experimental methods
To prepare a polycrystalline sample of YbPd, an ingot of
4
f electrons, are quite sensitive to pressure, pressure is a powerful
ytterbium metal and a powder of palladium metal with the purity
of 99.9% and 99.95%, respectively, were used as a starting material.
The mixture of shavings of the Yb metal and the Pd powder
in a ratio of 1.3:1 was pressed into a pellet. The pellet, which
was sealed into a tantalum tube under argon atmosphere by
tool for investigation of the unstable behavior in rare-earth
compounds.
The cubic CsCl type compound YbPd is known to be a valence
fluctuating compound and to undergo four phase transitions at
0
.5, 1.9, 105 and 125 K [4]. These transitions are confirmed by
3
using an arc-furnace, was heated at 1450 C for 40 h. Powder
X-ray diffraction pattern displays the sample is in the cubic
measurements of specific heat, electrical resistivity, ac suscept-
170
ibility and thermal expansion [4]. The
Yb M o¨ ssbauer studies
CsCl type structure with a lattice constant of a ¼ 3:439 A. As
reveal that the one at 1.9 K is due to magnetic ordering [5].
However, mechanisms of the other three phase transitions remain
˚
described in Ref. [5], a little broadening of the diffraction line
was observed. Magnetization measurement under high pressure
up to 1.0 GPa was carried out by using the magnetic properties
measurement system (MPMS) manufactured by Quantum
Design. Electrical resistivity was measured by ac four-probe
method under high pressures up to 2.4 GPa in the temperature
range from 2 K to room temperature. The pressure was generated
by using piston cylinder type pressure cell and a 1:1 mixture
of Fluorinert FC70 and FC77 as a pressure transmitting medium.
The pressure cell for magnetization measurement is made
of a non-magnetic CuTi alloy to reduce a magnetic signal of the
pressure cell. The one for electrical resistivity measurement is
a hybrid type cell which is composed of inner NiCrAl cylinder
and outer CuBe one. To calibrate pressure, pressure dependence
of superconducting transition temperature of a tin plate was
examined.
unknown. The X-ray absorption spectroscopy (XAS) at the Yb LIII
-
edge manifests Yb valence is ꢀ2:8þ and almost independent of
temperature even at 105 and 125 K [4]. This result means the
phase transitions at 105 and 125 K are not a valence transition.
The M o¨ ssbauer studies also suggest two different Yb charge states
are present in equal proportions at the lowest temperature, which
suggests charge ordering though Yb ion occupies only one
crystallographic site in the CsCl type structure [5]. In spite of
such interesting features, YbPd has not been examined for a long
time. In the present study, we focus on pressure effect on the
transitions. We have measured magnetic susceptibility electrical
Ã
Corresponding author. Tel./fax: +8192 642 2550.
E-mail address: 3da@phys.kyushu-u.ac.jp (A. Mitsuda).
0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2009.07.029

ARTICLE IN PRESS
A. Mitsuda et al. / Physica B 404 (2009) 3002–3004
3003
3
. Results and discussion
Fig.
resistivity
the resistivity decreases with decreasing temperature, which is
metallic behavior. The shape of the 2T curve is quite similar
to that reported previously [4]. The curve has two steps
accompanied by thermal hysteresis. The thermal hysteresis,
which has not been reported in the previous study, is an
evidence of a first-order phase transition. We estimated the
2
shows temperature dependence of the electrical
r
under various pressures. For ambient pressure,
Fig. 1 and its inset show temperature dependence of inverse
magnetic susceptibility and magnetic susceptibility, respectively,
at various pressures. For ambient pressure, the susceptibility
exhibits no clear anomalies at 105 and 125 K. Above 150 K, the
susceptibility obeys Curie–Weiss law with an effective Bohr
magneton number of meff ¼ 3:69mB and Weiss temperature
r
of
Y
p
¼ ꢁ95:5 K, which are in good agreement with those
value of
ð4:54mBÞ, the experimental value is quite smaller, which
phase transition temperature from the maximum of dr=dT,
reported in Ref. [6]. Compared with a theoretical
Yb
would reflect the valence fluctuation of Yb ion. Assuming that the
smaller experimental meff value is ascribed to mixture of magnetic
Yb and nonmagnetic Yb , we estimate the numerical ratio of
Yb : Yb to be 66:34 from the meff ratio squared of ð3:69=4:54Þ .
The obtained ratio corresponds to Yb valence of 2:66þ, which is
comparably close to the Yb valence determined from the XAS
ðꢀ2:8þÞ. The large negative value of the Weiss temperature,
m
as shown in the inset of Fig. 2. There exist a definite peak and a
broad shoulder at around 105 and 125 K, respectively, which
correspond very well to the transitions reported by Pott et al. [4].
For convenience, we define the lower transition temperature and
eff
3
þ
3
þ
2þ
the higher one as T
the transitions at T
1
and T , respectively. With increasing pressure,
2
and T shift toward lower temperature, which
2
3
þ
2þ
2
1
is similar to the behavior of the susceptibility under high pressure
shown in Fig. 1. Simultaneously, the shoulder at T in the d =dT2T
curve becomes more indistinct. Therefore, we give up examining
pressure dependence of T . Now we are measuring thermal
expansion under high pressure and can observe decrease in T
with pressure [7]. As shown in Fig. 3, T decreases almost linearly
2
r
p
Y ,
is also peculiar to a valence fluctuating system. Therefore, the
behavior of the magnetic susceptibility above 150 K seems to
be associated with the valence fluctuation. Below 150 K, the
susceptibility deviates upward from the Curie–Weiss law.
Mechanism of the deviation, which seems to be associated with
the transition at 105 and/or 125 K, is unknown. Actually, the
temperature, where the susceptibility begins to deviate from
the CW law, seems to shift toward lower temperature, which
corresponds to the behavior of the electrical resistivity shown
later. Under high pressure, the behavior of the susceptibility is
qualitatively unchanged, which means the Yb ion remains valence
fluctuating. With increasing pressure, the meff value increases
2
2
1
with increasing pressure up to 1.25 GPa, and shows quite different
pressure dependence at P41:25 GPa, where the thermal
hysteresis is collapsed and thus the d
denotes no longer the phase transition temperature. These results
suggest that the transition at T disappears at PZ1:62 GPa.
Extrapolation of the data at Pr1:25 GPa to T ¼ 0 K gives us a
critical pressure of ꢀ1:5 GPa. For P41:5 GPa, the high-temperature
r=dT maximum probably
1
1
phase ðT4T Þ is stabilized down to zero temperature and some
1
new ground state might be realized. For PZ1:91 GPa, the
curve is almost independent of pressure.
r2T
slightly and the
Y
p
value, of which magnitude decreases, remains
3
þ
negative, which corresponds to increase in ratio of magnetic Yb
and decrease in Kondo temperature, respectively. These results
corresponds to general trend that application of pressure
From the data plotted in Fig. 3, the dT
be ꢀ ꢁ 65 K=GPa. According to the results reported in Ref. [1],
the values of V=V and S associated with the transition
at T
are ꢁ0:3% and 1.2 J/K mol, respectively. Therefore, the
1
=dP value is found to
D
D
3
þ
stabilizes localized magnetic moment of Yb
.
1
2
2
1
1
50
00
50
00
0
0
1
peff = 3.69,Θ = -95.5 K
p
3
00
.6 peff = 3.71, Θ = -88.7 K
p
YbPd
.0 peff = 3.72, Θ = -82.6 K
p
1
.91, 2.27, 2.42 (GPa)
(
GPa)
-
1
χ-1
250
200
χ +10
0 GPa)
(
(1.0 GPa)
T2
-1
χ +5
0.6 GPa)
(
T
1
0
.2
heating process
0 (GPa)
0
0
.15
0
χ (1.0 GPa)
χ+0.02 (0.6 GPa)
1
1
50
00
0.25
0
.1
0
.64
.97
1.25
χ+0.04 (0 GPa)
5
0
T1
0
.05
YbPd
μ0H = 1 T
T2
0
100
200
300
1.62
GPa)
T (K)
(
80
100
120
140
0
100
200
300
T (K)
5
0
T (K)
0
100
200
300
Fig. 1. Temperature dependence of inverse magnetic susceptibility of YbPd at
T (K)
various pressures. The solid line shows a Curie–Weiss law. The effective Bohr
magneton number, meff
Curie–Weiss law, are indicated for each pressure. The inset shows magnetic
,
and Weiss temperature,
Y
p
,
estimated from the
Fig. 2. Temperature dependence of the electrical resistivity of YbPd under high
pressure. The white arrows show phase transition temperatures, T and T . The
inset exhibits d =dT as a function of temperature. We define T as a peak position.
1
2
susceptibility as a function of temperature.
r
1

ARTICLE IN PRESS
3
004
A. Mitsuda et al. / Physica B 404 (2009) 3002–3004
1
1
20
00
Fig. 4 shows temperature dependence of the resistivity in
the temperature range of 2–50 K. With increasing pressure,
value at the lowest temperature increases definitely.
Especially, at 0:97rPr1:62 GPa, where the transition at T is
being collapsed, the value rises drastically, which is possibly
YbPd
r
1
r
heating process
cooling process
associated with the high-temperature ðT4T Þ phase stabilized
1
down to zero temperature. In addition, a bend, which is observed
8
0
0
0
0
ꢂ
at T ¼ 2:8 K, becomes sharper and is transformed into a peak
at PZ1:25 GPa. Namely, the anomaly, which remains at 2.8 K
as shown in Fig. 3, becomes more evident with pressure. Below
2.8 K, the resistivity drops suddenly, which is reminiscent of
disappearance of magnetic scattering. Since the application of
T
1
6
4
2
dT /dP
1
3þ
=
-64.6 K/GPa
pressure generally stabilizes magnetic Yb , we speculate that the
bend is associated with the magnetic ordering reported at 1.9 K [4]
and that the magnetic ordering is more conspicuous. The ꢁlogT-
(
heating)
dT /dP
1
=
-64.7 K/GPa
cooling)
like behavior of
r at around 5 K under higher pressures than
(
0
.97 GPa reminds us of an impurity Kondo effect. Applying
pressure would lower Kondo temperature, in Yb-based
systems in contrast to Ce-based systems, which is also reported
in cubic YbCu [8,9].
K
T ,
T*
5
The possibility of charge ordering at the lowest tempe-
rature has been pointed out by Bonville et al. [5] We speculate
0
0
0.5
1
1.5
2
2.5
the transitions at T
1 2
and/or T , of which mechanism remains
P (GPa)
unknown, are associated with charge ordering. If so, the charge
ordering would be collapsed under high pressure. Hereafter, we
would like to verify the possibility.
1
Fig. 3. Pressure dependence of the transition temperature T measured during
heating and cooling. The error bar is shown only for heating process. The rate of
ꢂ
dT
1
=dP is estimated to be ꢀ ꢁ 65 K=GPa. The T , which would be associated with
magnetic ordering, is also plotted as a function of pressure.
4. Conclusions
In summary, we have reported the magnetic susceptibility and
2
2
1
1
50
00
50
00
the electrical resistivity of YbPd under high pressure. We found
the transitions at T and T are of first order. The transition at T is
1
.91, 2.27, 2.42 (GPa)
1
2
1
collapsed at around 1.5 GPa. With applying pressure, an anomaly
associated with magnetic ordering becomes more conspicuous,
the resistivity at the lowest temperature is enhanced and ꢁlogT
dependence of
r appears. In the present study, we cannot
2
investigate pressure dependence of T .
1.62
1.25
Acknowledgments
This work was partly supported by Research Foundation for the
Electrotechnology of Chubu. The authors thank Prof. H. Harima
and Prof. Yuh Yamada for useful comments and suggestions.
0.97
YbPd
.25
(
GPa)
1
0
1.62
1.91
2.27
2.42
References
0
0
0
.64
.25
0
.25
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[
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S ¼ ꢁ61 K=GPa, is
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1