Energy gap formation in a valence fluctuating compound CeIrSb probed by Sb NMR and NQR

0.27 MB·English

by  Y. Kawasaki

APS/123-QED
Energy gap formation in a valence fluctuating compound CeIrSb
probed by Sb NMR and NQR
Y. Kawasaki, M. Izumi, Y. Kishimoto, and T. Ohno
Institute of Technology and Science, The University of Tokushima, Tokushima 770-8506, Japan
H. Tou, Y. Inaoka, M. Sera, and K. Shigetoh
7 ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, Japan
0
0 T. Takabatake
2
ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, Japan and

n IAMR, Hiroshima University, Higashi-Hiroshima 739-8530, Japan
a (Dated: February 6, 2008)
J
Sb-NMR/NQRstudyhasrevealedaformationofapseudogapattheFermilevelinthedensityof
4
states in a valence fluctuating compound CeIrSb. The nuclear spin-lattice relaxation rate divided
2
by temperature, 1/T1T has a maximum around 300 K and decreases significantly as 1/T1T ∼ T2,
] followed by a 1/T1T = const. relation at low temperature. This temperature dependenceof 1/T1T
l iswellreproducedbyassumingaV-shapedenergygapwitharesidualdensityofstatesattheFermi
e
level. The size of energy gap for CeIrSb is estimated to be about 350 K, which is by one order
-
r of magnitude larger than those for the isostructural Kondo semiconductors CeRhSb and CeNiSn.
t
s Despitethelargedifferenceinthesizeofenergygap,CeIrSb,CeRhSbandCeNiSnareindicatedto
. beclassifiedintothesamegrouprevealingaV-shapedgapduetoc-fhybridization. Thetemperature
t
a dependenceof theKnight shift measured in ahigh magnetic field agrees with theformation of this
m pseudogap.
-
d PACSnumbers: 71.27.+a,75.30.Mb,76.60.-k
n
o
c I. INTRODUCTION by various measurements,10,11,12,13 although CeRhAs is
[
isostructural to CeRhSb and CeNiSn at room tempera-

1 Akindofrare-earthcompoundscalledKondoinsulator ture. Recent investigation in CeRhAs indicate that suc-
v cessive structural modulations at low temperature are
or semiconductor undergoes a crossover from a metallic
2 closely related to its gap property.14,15,16,17 Thus, the
state into aninsulating groundone with decreasingtem-
8 detailedmechanismofthe anisotropicgapformationhas
5 perature, where a narrow gap or a pseudogap is formed
received new attention in connection with a crystal lat-
1 at the Fermi level (EF) in a coherent heavy fermion tice.
0 band.1,2 These energy gap openings are generally con-
7 sidered to be derived from c-f hybridization between a Recently, another isostructural compound CeIrSb
0
wide conduction band and a 4f band that is renormal- with orthorhombic ǫ-TiNiSi type structure was
/
at ized as a heavy fermion band at EF at low temperature. synthesized.19,20 The lattice parameters for CeIrSb
m ThesmallestgapKondoinsulators,CeRhSbandCeNiSn are a = 7.351, b = 4.5744, and c = 7.935 ˚A. On
with orthorhombic structure, are exotic among them: going from CeRhSb to CeIrSb, the a and b parameters
-
d they appear to develop an anisotropic energy gap, while decrease by 0.9% but the c parameter increases by a
n other systems such as Ce3Bi4Pt3 and YbB12 with cubic similar amount, thus the unit cell volume is decreased
o structure have a well-defined size of energy gap. The by 0.8%. This anisotropic shrink in the lattice may
c anisotropic energy gap has been suggested by NMR in strengthenc-fhybridization. Actually,thelargenegative
:
v CeRhSb and CeNiSn for the first time, where the power Curie-Weiss temperature of −1300 K and the gradual
Xi lawofnuclearspin-latticerelaxationrate1/T1initstem- decrease in resistivity with decreasing temperature
perature dependence is explained by a narrow V-shaped suggest that CeIrSb is a valence fluctuating compound
ar pseudogapatEF inthedensityofstates(DOS).3,4 These with stronger c-f hybridization than that for CeRhSb.20
results,togetherwithtransport,5specificheat,6 andtun- The value of C/T below 10 K is less than that of
neling spectroscopy measurements,7 indicate the forma- CeRhSb, which proves that the DOS at E is strongly
F
tion of a new type of semimetal with an anisotropic gap reduced. It is notable that the resistivity of CeIrSb
forCeRhSbandCeNiSn. Suchananisotropicgapispro- increases below 8 K, which is reminiscent of the similar
posed to originate from an anisotropic hybridization of behavior of resistivity in CeRhSb. Although this upturn
conduction band with a certain ground state of 4f elec- in the resistivity may be associated with an energy gap
tron in the crystalline field8,9 and, therefore, it may be opening, thermopower does not show an enhanced value
closely related to its crystal lattice. at low temperature, which disagrees with a distinct
On the other hand, it has been reported that CeR- characteristicofnarrowgapsemiconductors.5,21,22 Thus,
hAs shows a gap opening over the entire Fermi surface it has been still controversial whether the ground state
2
of CeIrSb is a gapped one or not. In order to make
this point clear from a microscopic viewpoint, we have (a) CeIrSb 121Sb
performed Sb-NMR/NQR measurements on CeIrSb and 123Sb
its lanthanum analog LaIrSb. )
nit
u
b.
r
a
II. EXPERIMENTAL ASPECT (
y
sit
Detailed preparation method of the polycrystalline n
e
samples was reported in the literature.20 X-ray pow- nt (b) LaIrSb
der diffraction (XRD) and electron-probe microanalysis o i 121Sb
h
(EPMA) detected impurity phases of CeSb and CeIr c
with several percent.20 The amounts of impurity phase2s –e 123Sb
n
of CeSb and CeIr were estimated to be 5% and 3%, re- pi
2 S
spectively, from the combined analysis of XRD, EPMA
and specific-heat anomaly due to the antiferromagnetic
orderofCeSbat17K.23NMR/NQRmethodis,however,
averylocalprobeandthereforeyieldsinformationonthe 0 10 20 30
majority phase, CeIrSb. In this meaning, physicalquan-
Frequency (MHz)
tities obtained by NMR/NQR method have its inherent
advantageovermacroscopiconesthatareaffectedbythe
magnetic impurity CeSb, for example.20 We grounded FIG. 1: 121,123Sb-NQRspectra of (a) CeIrSb and (b) LaIrSb
at 4.2 K.
these polycrystals into grains with smaller size than 100
µm for NMR/NQR measurement. The measurements
were made at temperatures between 1.6 K and 300 K
CeSb exhibits the magnetic order,26 has been observed,
by employing a phase-coherentpulsed NMR/NQR spec-
we consider these signals originate from CeIrSb. Any
trometer. NMR (NQR)spectrumwasmeasuredbytrac-
other signal from impurity phase is not observed in the
ing the integrated spin-echo signal as the function of an
NQRspectra,presumablybecausethesesignalintensities
external magnetic field (the frequency of pulsed rf field
are below the sensitivity of our spectrometer. The line
in zero external magnetic field). We obtained nuclear
widthforthetransition±1/2↔±3/2of123SbinCeIrSb
spin-latticerelaxationrate,1/T byfitting thelongitudi-
1
is 89 kHz, which is about four times larger than 23 kHz
nalnuclearmagnetizationrecoveryaftersaturatingpulse
for the corresponding transition observed in CeRhSb,4
to theoretical functions given in Refs. [24] and [25]. The
butsmallerthan200-300kHzobservedforCeRhAs.12 As
highqualityofthefittingswithsinglecomponentof1/T
1 forLaIrSb,theresonancelinesfor121Sb(123Sb)arefound
guaranteesthat obtained 1/T ’s will make clear intrinsic
1
at12.29and24.43MHz(7.59,14.79and22.27MHz),cor-
nature of CeIrSb, unaffected by any interruption from
the impurity phases.26 responding to 121νQ = 12.22 MHz (123νQ = 7.43 MHz)
with η = 0.078.
Figure 2 displays the temperature dependence of
1/T T for CeIrSb (closed circles) with that for CeRhSb
III. RESULTS AND DISCUSSION 1
(crosses [4]) in zero field. Here, these 1/T ’s were mea-
1
sured at the resonance line arising from the ±3/2 ↔
A. Nuclear quadrupole resonance and spin-lattice
±5/2transitionsof121Sbforbothcompounds. Thevalue
relaxation
of121(1/T )/123(1/T )=2.9at4.2KforCeIrSb,iscom-
1 1
parableto (121γ /123γ )2 = 3.4 expectedwhen magnetic
n n
Figure 1 displays the 121,123Sb-NQR spectra of (a) interactionisdominantlyresponsibleforthenuclearspin
CeIrSband(b)LaIrSbfortwoSbisotopesat4.2K.121Sb relaxation. Here,β(1/T )andβγ aretherelaxationrate
1 n
(123Sb) has a nuclear spin I = 5/2 (7/2) and therefore and the nuclear gyromagnetic ratio of Sb isotopes with
exhibits two (three) NQR transitions. In CeIrSb, the
its mass number β. This fact proves that the leading
resonance lines for 121Sb (123Sb) are found at 13.68 and contribution to the nuclear spin relaxation arises from
26.95MHz(8.50,16.30and24.56MHz),correspondingto
magnetic interactions.
121ν =13.51MHz (123ν =8.20 MHz) with η = 0.107.
Q Q The overall temperature dependence of 1/T T for
Here,thenuclearquadrupolefrequencyν andtheasym- 1
Q CeIrSbisqualitativelysimilartothatforCeRhSb. 1/T T
metryparameterηaredefinedasνQ =νz = 2I(32eI2−Q1)h∂∂Vz for CeIrSb has a maximum around Tm = 300 K and d1e-
and η = |ν − ν |/ν , respectively, with the nuclear creases significantly as 1/T T ∼ T2 below this tempera-
x y z 1
quadrupolar moment Q and the electric field gradient ture,followedbyaweakupturnbelow4K. Thisdecrease
at the position of the nucleus ∂V/∂α (α=x,y,z). Since in 1/T T with decreasing temperature indicates that
1
no change in the spectral shape across 17 K, at which CeIrSbhas an energy gapat E in the DOS at low tem-
F
3
CeIrSb
T
m
H = 0 kOe
1–1 K) 10–2 CeRHh S= b0 kOe 100 H = 1 kOe
– H = 105.5 kOe
s H = 3.6 kOe m
m T
1/TT (1 10–3 LaIrSb ) / (1/T)11
T T
m 1/
(
10–4 E) D 10–2
N( ∆ CeIrSb H = 0 kOe
H = 1 kOe CeRhSb
E H = 105.5 kOe H = 0 kOe
10–5 EF H = 3.6 kOe
100 101 102
Temperature (K)
10–4
10–2 10–1 100 101
FIG. 2: Temperature dependences of 1/T1T for CeIrSb T/T
m
(closedcircles),LaIrSb(closedsquares)andCeRhSb(crosses
[4]) in zero field. The open squares and circles represent
1/T1T for CeIrSb in magnetic fields of 1 and 105.5 kOe, re- FIG. 3: 1/T1 normalized by the value at Tm as a function of
spectively. The solid line indicates the calculation by assum- thenormalized temperatureT/Tm for CeIrSb (closed circles)
ing an effective DOS with V-shaped gap structure at EF as andCeRhSb(crosses[4]) inzerofield. Theopensquaresand
drawn in the bottom left. circlesrepresentthedataforCeIrSbatH =1and105.5kOe,
respectively. The solid curve gives the reproduction by the
best fitted V-shaped gap.
perature. The similarity in 1/T is demonstrated in Fig.
1
3, where the normalized relaxation rate (1/T )/(1/T ) is suppressed, 1/T T for CeIrSb is independent of tem-
1 1 m 1
is plotted against the normalized temperature T/T for perature below 20 K. This result indicates the existence
m
CeIrSbandCeRhSb. Here,T and(1/T ) arethetem- of residual DOS at E even though a V-shaped gap is
m 1 m F
perature at which 1/T T has a maximum and 1/T at made.
1 1
T for each compound, respectively. The temperature Since the temperature dependences of 1/T T for
m 1
dependencesof1/T forbothcompoundsarescaledwith CeIrSb and CeRhSb are quite similar, the V-shaped gap
1
T except those in low temperature region. This devi- modelusedinCeRhSb4 isapplicabletothegappedstate
m
ation from the scaling at low temperature is explained in CeIrSb. Namely, we assume an effective DOS, N(E)
by the difference in the amount of residual DOS at E havinga V-shapedgapstructure witha residualDOSat
F
as discussed later. It should be noted that this scaling E as shown in the bottom left of Fig. 2. The tempera-
F
between 1/T and T is good although the difference in ture dependence of 1/T T in CeIrSb could be well fitted
1 m 1
T ’sforCeIrSbandCeRhSbisaslargeas10times. This to the relation
m
resultindicatesthatthesetwocompoundshaveasimilar
∂f(E)
gapstructureandthatthemagnitudesofthe energygap 1/T T ∝ N(E)2 − dE, (1)
1
Z (cid:26) ∂E (cid:27)
are scaled with T .
m
The weak upturn in 1/T T below 4 K for CeIrSb is where f(E) is the Fermi distribution function, with the
1
probably due to the relaxation process by spin fluctu- bandwidthD=1800K,thegap∆=350Kandthefrac-
ations of paramagnetic Ce spins through direct mutual tion of residual DOS against a value without gap at E
F
spin flipping between electron and nuclear spins, which for the Lorentzianband N(E ) /N (E ) = 0.193. The
F res 0 F
has been also observed in CeRhSb.4 In order to sup- result of calculation, indicated by solid lines in Figs. 2
pressthisextrinsicrelaxationchannel,wehavemeasured and 3, is in good agreement with the experimental data
1/T T in a small magnetic field of H = 1 kOe, because when the above-mentioned extrinsic relaxation channel
1
the flipping may be prevented by a magnetic field which is suppressed in a small magnetic field.
leads to a different Zeeman splitting between electron These parameters for CeIrSb as well as those for
and nuclear spin levels. As a matter of fact, the upturn CeRhSb4 and CeNiSn4 are listed in Table I. The val-
is suppressed and 1/T T stays constant down to 1.6 K ues of D and ∆ for CeIrSb are much larger than those
1
in 1 kOe as indicated by open squares in Fig. 2. Sim- for CeRhSb and CeNiSn, respectively. The larger value
ilar suppression has been also observed in CeRhSb4 by of D for CeIrSb is consistent with the weaker tempera-
applying H =3.6 kOe as shownin Fig. 2. In smallmag- ture dependence of the magnetic susceptibility, suggest-
netic fields, where the extrinsic relaxation contribution ing a stronger c-f hybridization.20 Despite the values of
4
TABLEI:Listofquasi-particlebandwidth D,energygap∆
1
andthefraction ofresidualDOSagainst avaluewithoutgap
at EF for theLorentzian band N(EF)res/N0(EF) for CeIrSb,
CeRhSb4 and CeNiSn.4 %)
Samples CeIrSb CeRhSb CeNiSn hift ( nit) 10 K
s u
BENna(neErdFgy)wreigdsa/tpNh0∆D(E((FKK))) 3015.8100903 2028.10085 1014.40077 Knight 0.5 nt. (arb. 65Cu63Cu (coil)
o i
h
c
e
–
n
pi
D and ∆ for CeIrSb by one order of magnitude larger S
than those for others, it is notable that ∆ is scaled with 9 10 11 H (T)12
D among these compounds, which is expected when the 0
0 100 200 300
c-fhybridizationisessentialfortheV-shapedgapforma-
tion. In this meaning, it is considered that these com- Temperature (K)
pounds are classified into the same group revealing a V-
shaped gap due to c-f hybridization and that the much FIG. 4: Temperature dependence of 121Sb Knight shift for
larger band width for CeIrSb brings about the much CeIrSb. Inset shows the field-swept 121Sb-NMR spectrum at
larger magnitude of the energy gap. The larger value of 10 K measured with the fixedrf frequency of 107.3 MHz.
N(E ) /N (E )forCeIrSbthanthoseforCeRhSband
F res 0 F
CeNiSn may originate from a possible off-stoichiometry
in composition or effective carrier doping effect,27 which ofresonancefieldatSbsitecharacterizedbysixpeaksat
may be associated with the larger NQR line width of 89 9.34, 9.84, 10.39, 10.58, 11.00 and 11.69 T in the poly-
kHz for CeIrSb than that of 23 kHz for CeRhSb. crystalline sample, so-called powder pattern for nuclei
LaIrSb,ametallicandnon-magneticanalogofCeIrSb, with I = 5/2. Here, a second order perturbation of nu-
mayofferinformationofconductionelectronstateswith- clear quadrupole interaction added to Zeeman interac-
out4felectrons. AsindicatedbyclosedsquaresinFig.2, tion yields a splitting of the central line into two peaks.
1/T T of LaIrSb is almost independent of temperature, Neither broadening nor splitting in the resonance lines
1
characteristics of a normal metallic state, where a DOS has been observed down to 5 K (not shown), indicating
near E is finite and almost flat with a temperature- thatCeIrSbisnonmagneticatthegroundstateandthat
F
independent band structure. In such a metallic state, these resonance lines come from CeIrSb. Small peaks at
we may consider 1/T T ∝ N(E )2 as approximately 9.69 T and 11.10 T may come from CeSb.
1 F
obtained from Eq. (1). The value of 1/T1T below 10 We show the temperature dependences of 1/T1T and
K in CeIrSb, 0.134 s−1K−1, is by one order of mag- (1/T )/(1/T ) measured at the central resonance lines
1 1 m
nitude smaller than that of LaIrSb, 2.32 s−1K−1, al- byopencirclesinFigs.2and3,respectively. Apparently,
though a lanthanum has no 4f electrons. Therefore, these relaxation rates in high magnetic field are almost
if we assume a comparable hyperfine coupling constant identical to those in zero and low field, indicating that
for CeIrSb and LaIrSb, the smaller value of 1/T1T in the gapstructureis notaffectedbythe appliedmagnetic
CeIrSb is considered to be related to the reduction in field at least up to 10 T. It is a remarkable contrast to
N(EF) of CeIrSb at low temperature due to the V- thecaseforCeNiSn,inwhichthe pseudogapisknownto
shaped energy gap. The ratio of the residual DOS be progressively suppressed with increasing the applied
for CeIrSb against that for LaIrSb is estimated to be magneticfieldhigherthan2T,whilenodramaticchange
(1/T1T)CeIrSb/(1/T1T)LaIrSb = 0.134/2.31 = 0.241, wasfoundbelow 2T.1 Thesefacts havebeeninterpreted
apbout 25% larger than N(EF)res/pN0(EF) = 0.193 ob- as a resultof the presenceof a flat partatthe bottom of
tained in the previous paragraph. A possible contribu- thepseudogapandtherespectiveZeemanshiftofvalence
tionof4felectronstotheconductionbandinCeIrSbdue up-spin and conduction down-spin bands which induces
toc-fhybridizationmayberesponsibleforthisdifference. theoverlappingofthesebandsathighmagneticfield.28,29
Therobustgapunaffectedbytheexternalmagneticfield
in CeIrSb is consistent with the much larger sizes of en-
B. Nuclear magnetic resonance and Knight shift ergy gap and residual DOS than those for CeNiSn.
In the main panel of Fig. 4, we show the temperature
TogainfurtherinsightintothegappropertyofCeIrSb, dependence ofthe Knightshift, K(T),whichis obtained
we have performed 121Sb-NMR measurements in high from the two peaks of the −1/2 ↔ 1/2 transition with
magnetic field. The field-swept 121Sb-NMR spectrum takingnon-zerovalueofη =0.107intoaccount. Itisno-
measured at fixed frequency of 107.3 MHz at 10 K is table that K(T) reflects the intrinsic magnetic property
shown in the inset of Fig. 4. It indicates a distribution of CeIrSb, while χ(T) is largely affected by the impurity
5
phase CeSb giving rise to the Curie-Weiss-like upturn large value of K(T) at low temperature.
below 250 K.20 In general, K(T) is dominated by two
contributions,atemperaturedependent spinpartK (T)
s
and a temperature independent Van Vleck one K , as
VV
K(T)=K (T)+K . Thetemperaturedependentpor-
s VV IV. CONCLUSION
tion of K(T) arises from K (T) part, which is propor-
s
tional to a corresponding spin susceptibility χ (T) via a
s
dominant transferred hyperfine coupling. Therefore, the The valence fluctuating compound CeIrSb and its La
veryweaktemperaturedependenceinK(T)above200K analogLaIrSb have been investigatedby Sb-NMR/NQR
suggests that this compound is in the regime of interme- measurements. The 1/T T for CeIrSb has a maximum
1
diate valence systems. The decrease in K(T) below 200 around300Kanddecreasessignificantlyas1/T T ∼T2,
1
K is consideredto be associatedwith the decreasein the followed by a T T = const. relation at low temperature.
1
DOS at E in CeIrSb. At the lowest temperature, there This temperature dependence of 1/T T indicates a V-
F 1
remains large K(T) of 0.7 %, which may be attributed shaped energy gap with a residual DOS at E . The size
F
to the large residual DOS and the Van Vleck contribu- ofenergygapforCeIrSbisestimatedtobe about350K,
tion. By subtracting K arising from the residual DOS, which is by one order of magnitude larger than the re-
s
N(E ) /N (E )= 0.194, at low temperature, the Van spective28Kand14KforCeRhSbandCeNiSn. Despite
F res 0 F
Vleckcontributionisestimatedtobe0.65%. Suchalarge the very large difference in the size of energy gap, the
Van Vleck contribution has been also observed in other temperaturedependence of1/T hasrevealedthescaling
1
Kondoinsulatorsandsemiconductors1andexplainedina betweenthebandwidthandthemagnitudeofenergygap
strongly correlated regime, where the Van Vleck suscep- among these compounds. This fact indicates that these
tibilityisenhancedbythesamerenormalizationfactoras compounds are classified into the same group exhibiting
for the effective mass.30 Although the 4f electronic state a V-shaped gap due to c-f hybridization. The tempera-
ofCeIrSbisnotexpectedtobe highlyrenormalizedwith turedependence oftheKnightshiftforCeIrSbmeasured
theconductionbandasinaheavyfermionmaterial,some in a high magnetic field is consistent with the formation
renormalizationeffectsmightenhanceK andyieldthe of the pseudogap at E .
VV F
1 T. Takabatake, F. Iga, T. Yoshino, Y. Echizen, K. Ka- H. Namatame, M. Taniguchi, T. Suemitsu, T. Sasakawa
toh, K. Kobayashi, M. Higa, N. Shimizu, Y. Bando, and T. Takabatake, Phys.Rev.B, 66, 155202 (2002).
G. Nakamoto, H. Fujii, K. Izawa, T. Suzuki, T. Fujita, 12 M. Matsumura, T. Sasakawa, T. Takabatake, S. Tsuji,
M.Sera,M.Hiroi,K.Maezawa, S.Mock,H.v.L¨ohneysen, H. Tou and M. Sera, J. Phys. Soc. Jpn., 72, 1030 (2003).
A.Bru¨ckl, K. Neumaier and K. Andres, J. Magn. Magn. 13 D.T. Adroja, J.-G. Park, K.A. McEwen, K. Shigetoh,
Mater., 177-181, 277 (1998). T.Sasakawa,T.TakabatakeandJ.-Y.So,PhysicaB,378-
2 Z. Fisk, J.L. Sarrao, J.D. Thompson, D. Mandrus, 380, 788 (2006).
M.F. Hundley, A. Miglori, B. Bucher, Z. Schlesinger, 14 T. Sasakawa, T. Suemitsu, T. Takabatake, Y. Bando,
G.Aeppli,E.Bucher,J.F.DiTusa,C.S.Oglesby,H-R.Ott, K. Umeo, M.H. Jung, M. Sera, T. Suzuki, T. Fujita,
P.C. Canfield and S.E. Brown, Physica B 206-207, 798 M. Nakazima, K. Iwasa, M. Kohgi, Ch. Paul, St. Berger
(1995). and E. Bauer, Phys. Rev.B, 66, 041103(R) (2002).
3 M. Kyogaku, Y. Kitaoka, H. Nakamura, K. Asayama, 15 T. Sasakawa, K.Mine,K.Shigetoh and T.Takabatake,J.
T. Takabatake, F. Teshima and H. Fujii, J. Phys. Soc. Phys. Soc. Jpn., 74, 3329 (2005).
Jpn. 59, 1728 (1990). 16 M. Matsunami, H. Okamura, T. Nanba, T. Suemitsu,
4 K. Nakamura, Y. Kitaoka, K. Asayama, T. Takabatake, T. Yoshino, T. Takabatake, Y. Ishikawa and H. Harima,
H.TanakaandH.Fujii,J.Phys.Soc.Jpn.63,433(1994). J. Phys. Soc. Jpn. 71, Suppl.291 (2002).
5 T.Takabatake,H.Tanaka,Y.Bando,H.Fujii,S.Nishigori, 17 N. Ogita, M. Udagawa, T. Sasakawa, T. Suemitsu and
T. Suzuki, T. Fujita and G. Kido, Phys. Rev. B, 50, 623 T. Takabatake, Physica B, 328, 151 (2003).
(1994). 18 T. Sasakawa, H. Miyaoka, K. Umeo, S. Aoyagi, K. Kato,
6 S. Nishigori, H. Goshima, T. Suzuki, T. Fujita, F. Iga and T. Takabatake, J. Phys. Soc. Jpn. 73, 262
G. Nakamoto, H. Tanaka, T. Takabatake and H. Fujii, J. (2004).
Phys. Soc. Jpn., 65, 2614 (1996). 19 M.G. Hasse, T. Schmidt, C.G. Richter, H. Block and
7 T. Ekino, T. Takabatake, H. Tanaka and H. Fujii, Phys. W. Jeitschko, J. Solid StateChem., 168, 18 (2002).
Rev.Lett., 75, 4262 (1995). 20 T.Sasakawa,K.Shigetoh,D.Hirata,K.UmeoandT.Tak-
8 H. Ikeda and K. Miyake: J. Phys. Soc. Jpn. 65, 1769 abatake, Physica B, 359-361, 111 (2005).
(1996). 21 M.F.Hundley,P.C.Canfield,J.D.ThompsonandZ.Fisk,
9 J. Moreno and P. Coleman, Phys. Rev. Lett., 84, 342 Phys. Rev.B, 50, 18142 (1994).
(2000). 22 F. Iga, T. Suemitsu, S. Hiura, K. Takagaki, K. Umeo,
10 S. Yoshii, M. Kasaya, H. Takahashi and N. Mori, Physica M.SeraandT.Takabatake,J.Magn.Magn.Mater.,226-
B, 223-224, 421 (1996). 230, 137 (2001).
11 K. Shimada, K. Kobayashi, T. Narimura, P. Baltzer, 23 F. Hulliger, M. Landolt, H.R. Ott and R. Schmelczer, J.
6
Low. Temp. Phys., 20, 269 (1975). 6385 (1996).
24 D.E. MacLaughlin, J.D. Williamson and J. Butterworth, 28 K. Izawa, T. Suzuki, M. Kitamura, T. Fujita, T. Taka-
Phys. Rev.,4, 60 (1971). batake, G. Nakamoto, H. Fujii and K. Maezawa, J. Phys.
25 A. Narath, Phys.Rev., 162, 320 (1967). Soc. Jpn., 65, 3119 (1996).
26 Y. Kawasaki, T. Tanaka, M. Izumi, Y. Kishimoto, 29 K. Nakamura, Y. Kitaoka, K. Asayama, T. Takabatake,
T. Ohno, H. Tou, Y. Inaoka, M. Sera, K. Shigetoh and G.NakamotoandH.Fujii,Phys.Rev.B,54,6062(1996).
T. Takabatake, J. Phys.Chem. Solids, to be published. 30 H. Kontani and K. Yamada, J. Phys. Soc. Jpn., 65, 172
27 K. Nakamura, Y. Kitaoka, K. Asayama, T. Takabatake, (1996).
G. Nakamoto, H. Tanaka and H. Fujii, Phys. Rev. B, 53,