Static and dynamical properties in the Pr-based filled skutterudite compound PrFe
4
P
12
revealed
by a
31
P-NMR study
K. Ishida,
1
H. Murakawa,
1
K. Kitagawa,
1
Y. Ihara,
1
H. Kotegawa,
2
M. Yogi,
2
Y. Kitaoka,
2
Ben-Li Young,
3
M. S. Rose,
3
D. E. MacLaughlin,
3
H. Sugawara,
4
T. D. Matsuda,
4,
* Y. Aoki,
4
H. Sato,
4
and H. Harima
5
1
Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
2
Department of Physical Science, Graduate School of Engineering Science, Osaka University, Toyonaka, 560-8531, Japan
3
Department of Physics, University of California, Riverside, California 92521-0413, USA
4
Department of Physics, Graduate School of Science, Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji,
Tokyo 192-0397, Japan
5
The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan
Received 10 February 2004; revised manuscript received 23 August 2004; published 27 January 2005
31
P-NMR studies have been carried out to investigate magnetic properties in the Pr-based filled skutterudite
compound PrFe
4
P
12
. This compound shows an unusual phase transition at T
A
6.5 K, which is now regarded
as antiferroquadrupole AFQ ordering from neutron-diffraction experiments. Splitting of the P-NMR spectrum
due to the appearance of two P sites with different hyperfine fields was observed below T
A
. From the field
dependence, the splitting seems to disappear in zero magnetic field, indicating that the different hyperfine fields
are not due to magnetic order, but to the appearance of two inequivalent P sites below T
A
. This is ascribed to
the distortion of the P cage surrounding a Pr ion below T
A
, which is associated with the Pr orbital ordering. The
nuclear spin-lattice relaxation rate 1/ T
1
shows the typical behavior of Kondo systems, where onset of
local-moment screening due to the coupling between conduction and localized Pr-4 f electrons is observed
below 50 K. Far above a critical field H
A
50 kOe, the Korringa behavior T
1
T = const is observed below 2 K
in 100 kOe. The Korringa value below 2 K is one order of magnitude larger than that in LaFe
4
P
12
, which has
no 4 f electrons. Our NMR experiment shows that the heavy-fermion state is realized far above H
A
in PrFe
4
P
12
.
The magnetic fluctuations in PrFe
4
P
12
are discussed from a comparison with typical heavy-fermion com-
pounds. In magnetic fields below H
A
, 1/ T
1
shows a sharp decrease below T
A
, however, in smaller magnetic
fields less than 10 kOe, 1/ T
1
stays constant far below T
A
with a relatively large value. The temperature and
field dependence of 1 / T
1
reveals the presence of low-energy spin fluctuations in the low-temperature and
low-field region. These unusual magnetic fluctuations are considered to originate from the magnetic dynamics
of Pr-nuclear spins since the fluctuating magnetic field 1.4 Oe and frequency 3.5 MHz are so small. The
nuclear ordering temperature is estimated to be 0.08 mK using the nuclear exchange fluctuations derived
from the observed 1 / T
1
of P.We show that PrFe
4
P
12
is a quite unique compound in which nuclear magnetism
can be detected by P NMR thanks to the nonmagnetic ground state of the Pr-4 f moments.
DOI: 10.1103/PhysRevB.71.024424 PACS numbers: 76.60.-k, 74.25.Ha, 71.27.+a
I. INTRODUCTION
The rare-earth compounds with filled skutterudite struc-
ture RT
4
P
12
with R: La, Ce, Pr, Nd, Sm; T: Fe, Ru, Os; and
P: P,As, Sb have attracted much attention due to their rich
variety of physical properties.
1,2
Within these compounds,
particularly praseodymium Pr-based compounds are attrac-
tive, since the possibility of a heavy-fermion state inherent to
non-Kramers Pr
3+
state has been debated for a long time.
3
Most heavy-fermion HF compounds have a magnetic
ground state for the f ions e.g., Ce, Yb, and U, however, the
Pr ion can have a non-Kramers Pr
3+
degenerate nonmagnetic
ground state in certain crystalline electric field CEF. In
such a case, a charge-scattering analogy of Kondo spin scat-
tering can give rise to the so-called “quadrupole Kondo ef-
fect” which has been invoked to explain non-Fermi-liquid
behavior in HF systems, especially U compounds.
3
However,
experimental evidence for such a quadrupole Kondo effect
has not been confirmed and the magnetic properties in the
HF state induced by this mechanism are unclear at present.
Recently, HF superconductivity was discovered in the
compound PrOs
4
Sb
12
with T
c
1.85 K.
4
A heavy mass was
roughly estimated as m 50m
e
from the jump in the spe-
cific heat at T
c
.
4
Indeed a highly enhanced cyclotron effec-
tive mass 2.4–7.6m
0
was confirmed by the de Haas–van
Alphen dHvA experiment.
5
From the thermodynamic
properties
4
and NMR experiment,
6
the nonmagnetic doublet
3
state of Pr
3+
is suggested to be a ground state. On the
other hand, the field induced ordered state was found in the
field above 45 kOe,
7
and was identified to be an antiferro-
quadrupolar AFQ ordering from the neutron-diffraction
ND experiment.
8
The recent high-field specific-heat mea-
surements in the ordered state show the
1
singlet being the
CEF ground state and that the CEF level crossing is the
driving mechanism of the field-induced ordering.
9
As for the superconducting SC properties, the SC gap is
found to be isotropic or point node from various
measurements,
10–12
and unusual properties of the SC state
such as a double SC transition,
13
broken time-reversal
symmetry,
14
and a change of the SC gap symmetry
15
have
been suggested on the basis of the experimental results.
These are quite unusual compared with ordinary HF super-
conductors with a line-node SC gap.
PHYSICAL REVIEW B 71, 024424 2005
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