Emergence of magnetic long-range order in frustrated pyrochlore Nd$_2$Ir$_2$O$_7$ with metal-insulator transition

0.61 MB·English

by  K. Tomiyasu

Emergence of Magnetic Long-range Order in Frustrated Pyrochlore Nd Ir O with
2 2 7
Metal-insulator Transition
K. Tomiyasu,1,∗ K. Matsuhira,2 K. Iwasa,1 M. Watahiki,1 S. Takagi,2 M.
Wakeshima,3 Y. Hinatsu,3 M. Yokoyama,4 K. Ohoyama,5 and K. Yamada6
1Department of Physics, Tohoku University, Aoba, Sendai 980-8578, Japan
2Faculty of Engineering, Kyusyu Institute of Technology, Kitakyusyu 804-8550, Japan
3Division of Chemistry, Graduate School of Science, Hokkiado University, Sapporo 060-0810, Japan
4Faculty of Science, Ibaraki University, Mito, Ibaraki 310-8512, Japan
5Institute of Materials Research, Tohoku University, Aoba, Sendai 980-8577, Japan
6WPI Advanced Institute of Materials Research, Tohoku University, Aoba, Sendai 980-8577, Japan
2
(Dated: January 4, 2012)
1
0 In this study,we performed powder neutron diffraction and inelastic scattering measurements of
2 frustratedpyrochloreNd2Ir2O7,whichexhibitsametal-insulatortransitionatatemperatureTMI of
33K.Thediffractionmeasurementsrevealedthatthepyrochlorehasanantiferromagneticlong-range
n
a structurewithpropagationvectorq0 of(0,0,0)andthatitgrowswithdecreasingtemperaturebelow
J 15 K. This structure was analyzed to be of the all-in all-out type, consisting of highly anisotropic
3 Nd3+ magnetic moments of magnitude 2.3±0.4µB, where µB is the Bohr magneton. The inelastic
scatteringmeasurementsrevealedthattheKramersgrounddoubletofNd3+ splitsbelowTMI. This
] suggeststheappearanceofastaticinternalmagneticfieldattheNdsites,whichprobablyoriginates
l from amagneticorderconsisting ofIr4+ magneticmoments. Here,wediscussamagnetic structure
e
- model for the Ir order and the relation of the order to the metal-insulator transition in terms of
r frustration.
t
s
. PACSnumbers: 71.30.+h,75.25.-j,75.30.-m,75.50.-y
t
a
m
- I. INTRODUCTION from Nd to Ho.16,17 The R dependence of TMI suggests
d thatthe MI transitionsaresensitive tothe changeinthe
n Since Pauling’sinitialproposalforwaterice,1 geomet- ionic radius of R3+ (i.e. lanthanide contraction) but are
o independent of the 4f electronic states.16,17
ricalfrustrationhasplayedanimportantroleinthefields
c
[ of both chemistry and solid-state physics. Only some Magnetic susceptibility measurements of R2Ir2O7 in

classical-spinpairsinfrustratedmagnetscanbearranged a previous study revealed that the zero-field-cooled
2
antiferromagnetically on a triangular lattice.2,3 There- and field-cooled curves do not follow each other below
v
5 fore, frustration suppresses magnetic ordering and pro- TMI.16,17 This observation suggests that the MI transi-
0 motes intriguing phenomena in insulators, such as spin- tionsareaccompaniedbyamagneticanomaly.16,17 Band
6 liquid-like fluctuations of spin ice and spin molecules,4,5 calculationsof the iridate in anotherstudy revealedthat
6 and the formation of complex magnetic structures with the conduction band consists of Ir 5d and O 2p orbitals
0. multiferroics.6,7 inthemetallicphase.18Therefore,themagneticanomaly
1 Metallicfrustratedsystemscanexhibitnoveltransport seems to originate from the Ir magnetic moments. The
1 properties. Examples of such properties include heavy Ir atoms are expected to be tetravalent magnetic ions in
v:1 pfehramsieonY(bSech)aMvino2r,s8,i9nssuppineerlcoLniVdu2cOt4ivaitnydiinnptyhreoCch1l5orLeaovxes- t(hIre4+in:su5dla5t,inJgeffp=ha1se/,2a,sLienfft=he1c,alosewo-sfpiinnsustlaattiengSS=r21I/rO2)4;
Xi ide Cd2Re2O7,10 anomalous Hall effects in pyrochlores here, Jeff is the effective total angular momentum, Leff
Nd Mo O andPr Ir O ,11,12metal-insulator(MI)tran- is the effective orbital momentum to describe triply de-
2 2 7 2 2 7
ar sitions in pyrochlores Cd2Os2O7 and Hg2Ru2O7, and so generated t2g states under spin-orbit coupling, S is the
on.13,14 Allthesesystemsconsistofcorner-sharingtetra- spin angular momentum, and Jeff =Leff +S.19
hedral lattices of magnetic atoms, called pyrochlore lat- Recently, several microscopic experiments have fur-
tices, which provide an ideal platform for the occurrence ther investigated Nd Ir O , and almost ensured that its
2 2 7
offrustration. Thus,one ofthe challengingissuesincur- MI transition originates from magnetic ordering at T .
MI
rent condensed matter physics is to determine the rela- Forexample,RamanscatteringexperimentsofNd Ir O
2 2 7
tion between the transport properties and geometrical suggested that there is no structural phase transition
frustration. down to 4.2 K and that lattice distortion does not inter-
Pyrochlore iridates R Ir O (R = Nd, Sm, Eu, Gd, estingly cause the MI transition.20 Further, in studies of
2 2 7
Tb, Dy, or Ho) also exhibit MI transitions,15–17 wherein temperaturevariationoftheNd4f-electronstatesacross
bothRandIratomsformpyrochloresublattices. Allthe T ,crystallineelectricfieldexcitationsofNd3+ wereob-
MI
MI transitions are of the second order.16,17 The transi- servedby inelastic neutronscattering,specific heatmea-
tiontemperatureT monotonicallyincreasesfrom33to surements, and magnetic susceptibility measurements,
MI
141 K with increasing atomic number of the R element and the results were used to deduce the Ir magnetic
2
states.17,21 The tenstatesofNd3+ (4f3, J =9/2,L=6, maximum) was 0.13 meV under the elastic condition.
S = 3/2) split into five Kramers doublets. The first ex- A powder sample of Nd Ir O was synthesized by a
2 2 7
cited doublet exists at around 26 meV (300 K), which solid-state reaction method.16,17 The sample (4.5 g) was
is much higher than TMI. The ground doublet possesses placed in a thin aluminum foil and shaped to a hollow
in/out-type Ising moments with 2.37µB along the 111 cylinder with a thickness of 0.7 mm and diameter of 20
h i
direction, where µB is the Bohr magneton. With de- mm in order to mitigate the effect of the strong neutron
creasing temperature, the ground doublet energetically absorption of Ir nuclei as much as possible. Then, the
splits at TMI like an order parameter of the MI transi- cylinder was kept in an aluminum container that was
tion, and the splitting energy is about 1.3 meV at 3 K. placed under a cold head in a 4He or 3He closed-cycle
This phenomenonsuggeststhe appearanceofa staticin- refrigerator.
ternalmagneticfieldatthe Ndsites,probablycausedby
the magnetic ordering of Ir at T .
MI
Thus, it is most probable that Nd2Ir2O7 simultane- III. RESULTS
ously undergoes the MI transition and the magnetic or-
deringwithoutstructuralphasetransition. However,the
Figure 1(a) shows the diffraction pattern measured at
followingmajorquestionsarestillopen: howthefrustra-
9 K, i.e. below T . In this figure, only fundamental
MI
tionisreleasedwithoutstructuraltransitionandhowthe
reflections of the pyrochlore structure and a tiny peak
MI transition is related to its release. To clarify them,
of the IrO impurity phase are observed; no superstruc-
2
informationofthemagneticstructure,whichisprimefor
ture lines are detected. Figure 1(b) shows the differ-
the study of magnetic frustration, will be necessary.
encebetweendiffractiondatameasuredbelowandabove
In this study, we conducted neutron diffraction and
T , i.e. at 9 K and 40 K, respectively. No apprecia-
MI
inelastic scattering experiments of Nd Ir O in powder
2 2 7 ble magnetic diffuse scattering is observed, unlike that
form. We also studied a magnetic structure model both
often seen as the spin-ice state in magnetic pyrochlore
for the Nd and Ir moments below TMI and determined oxides.23 In contrast, a component characterized by the
its relation to the MI transition in terms of frustration.
propagationvectorq =(0,0,0)wasobservedbelowT .
0 MI
Figure 1(c) shows the (220) fundamental reflection lines
measured below and above T . A weak yet noticeable
MI
II. EXPERIMENTAL increase in the intensity is observed as the temperature
decreases from above T to below it, and no broaden-
MI
Neutron diffraction experiments were performed on ing of the line width is observed. These results suggest
the powder diffractometer HERMES (T1-3) at the In- a long-range structure of the pyrochlore with q . Fur-
0
stitute for Materials Research (IMR), Tohoku Univer- ther, since previous Raman scattering experiments have
sity, installed in the thermal guide tube of the JRR-3 shownthat structuraltransitionis not observeddownto
reactor at the Japan Atomic Energy Agency (JAEA).22 4.2 K,20 the q structure is most probably magnetic.
0
Incident neutrons with an initial energy of 24.5 meV Table I summarizes the difference between integrated
(λ=1.8204(5)˚A)wereextractedby a(331)reflectionof intensities at 40 K and 9 K for several fundamental re-
aGemonochromatorandhorizontalcollimationsequence flections in barn/unit cell, where a unit cell refers to a
′
ofguide-blank-sample-22. Elasticneutronscatteringex- crystallographic unit cell denoted by 2(Nd Ir O ). The
2 2 7
periments were performed on the triple-axis spectrome- scale conversion factor for the units was estimated by
ter TOPAN (6G) of Tohoku University, also installed in analyzing the fundamental nuclear reflection intensities
the JRR-3 reactor. The final energy of the neutrons was at 40 K, where the lattice parameter x = 0.33(0) co-
fixedto30.5meVwithahorizontalcollimationsequence inciding with those for isomorphic materials Sm Ir O
2 2 7
′ ′
of blank-60-sample-60-blank. A sapphire filter and a and Eu Ir O as estimated by room-temperature X-ray
2 2 7
pyrolytic graphite filter efficiently removedfast neutrons diffraction was obtained.24,25 The conversion factor also
and higher-order contamination, respectively. includes the absorption correction, since the correction
Inelastic neutron scattering experiments were per- is canceled in the ratio to the nuclear reflection inten-
formed on the triple-axis spectrometer HER (C1-1) at sities. Further, the reflection-angle dependence of the
the Institute for Solid State Physics (ISSP), University correction can be approximately ignored, since the sam-
of Tokyo, installed in the cold guide tube of the JRR- ple shape is cylindrical. We confirmed that the nuclear
3 reactor. The final energy of the neutrons was fixed reflection intensities at 40 K are well analyzed without
to 3.6 meV with a horizontal collimation sequence of the reflection-angle dependence of the correction.
guide-blank-sample-radial-blank, where the radial colli- Figure 1(d) shows the temperature dependence of the
mator has three blank channels. A horizontal focusing summationoftheintegratedintensities113and222;this
analyzer, which covers the range of scattering angles by summationshowsthelargestintensityincrement(seeTa-
◦
5 and permits acquisition of higher statistics of data, bleI).Withdecreasingtemperature,theintensitybegins
was used. A cooled Be filter and a pyrolytic graphite to increase at around 15 5 K, and not T . A simi-
MI
±
Bragg-reflectionfilterefficientlyremovedthehalf-lambda lar temperature dependence is observed in the inelastic
contamination. The energy resolution(full width at half scattering data. Figure 2(a) shows the energy (E) spec-
3
TABLE I: Integrated intensities of magnetic components of
several fundamental reflections in barn/unit cell. Here, a
unit cell refers to a crystallographic unit cell denoted by
2(Nd2Ir2O7). Theexperimentalintensitieswereevaluatedby
subtractingtheintegratedintensitiesat40Kfrom thoseat9
K.Thebest-fitcalculatedintensitieswereobtainedfortheall-
in all-out structureshown in Figs. 3(b) and 3(c). The exper-
imental and calculated intensities are compared in Fig. 3(a)
as well.
hkl Exp. Cal. (Nd only) Cal. (Nd and Ir)
111 −5.9±5.4 0 0
200 0±2.5 0 0
220 13.6±3.5 16.8 13.7
113 + 222 23.3±13.7 17.4 20.6
400 −2.4±3.9 0 0
331 5.0±5.8 5.5 6.4
420 9.3±6.7 9.8 8.1
422 0.9±4.5 3.3 2.7
135 4.9±2.9 8.6 10.0
tra at a fixed scattering wavenumber (Q). The follow-
FIG. 2: (Color online) Measured inelastic neutron scattering
raw data for powder Nd2Ir2O7. (a) Scan data measured at
constant Q of 0.8 ˚A−1 at different temperatures.21 The ver-
tical origins shift to the height indicated by the horizontal
dottedlines. (b)Inelasticscatteringintensitydistributionsin
(Q,E) space at 3 K. (c) Comparison results of Constant-E
FIG. 1: (Color online) Measured neutron diffraction datafor scans measured at E = 1.2 and 1.4 meV in (b). In (a) and
powder Nd2Ir2O7. (a) Diffraction data measured at 9 K. (b) (c), thesolid curves are a guide to theeye.
Diffraction data obtained by subtracting the 40 K data from
the9 K data (a). (c) Bragg reflection lines measured around
the (220) reciprocal lattice point at 9 K, 40 K, and 102 K. ingbehavioris observedwith decreasingtemperature. A
(d) Temperature dependence of summation of the integrated broad inelastic signal appears at around T , as shown
MI
intensities 113 and 222. In (a), (c), and (d), all the lines are
bythe dottedarrows;it changesinto a sharppeakat1.1
a guide to theeye.
meVataround15K;below15K,thepeaksharpensfur-
ther,asshownbythesolidarrows;inthepreviousstudy,
it was not important for us to know at what tempera-
ture the peak begansharpening.21 The emergence of the
sharp inelastic peak can be attributed to the splitting of
4
the grounddoublet of Nd 4f electrons.21 The agreement in Fig. 3(a).
between the data in Figs. 1(d) and 2(a) strongly sug- Further, from Fig. 1(d), the ratio of magnetic scatter-
gests that the q structure mainly originates from Nd3+ ing intensity at 0.7 K to that at 9 K is estimated to be
0
magnetic moments. about 3.2. Since the magnetic intensity is proportional
Figure2(b)showstheinelasticscatteringintensitydis- to the square of m,28 the value of m(0.7 K) is estimated
tributionsin(Q,E)spacemeasuredbelowTMI. Thehor- to be 2.3 0.4 (=√3.2 m(9 K))µB. This value is in
± ×
izontallyspreadingexcitationmodeisobservedataround good agreement with the value of 2.37µ estimated by
B
1.3 meV; it corresponds to the splitting of the Nd3+ the crystalline field analysis.21
ground doublet. The scattering intensity decreases with To summarize Secs. 3 and 4, we found that the an-
increasing Q, confirming that the excitations are mag- tiferromagnetic long-range structure with q grows with
0
netic. However,themodeappearsslightlydispersive. To decreasingtemperaturebelowT =15 5K.Thestruc-
Nd
±
verify the dispersion, we comparedthe constant-E scans ture can be approximately described by the all-in all-
measured at 1.2 and 1.4 meV in Fig. 2(b); the compar- out type of model for Nd moments with a magnitude of
ison results are shown in Fig. 2(c). The Q dependences 2.3 0.4µ at 0.7 K. No direct signals for Ir moments
B
±
atthese energiesareclearlydifferent, indicatingthatthe were obtained in the present experiments.
excitations are not completely flat but dispersive, with a
bandwidth of 0.1 meV (1 K).
∼
V. DISCUSSION
IV. ANALYSES A. Magnetic structure below TMI
We analyzed the magnetic structure on the basis of The ground doublet of Nd3+ splits when the temper-
the q intensities given in Table I. Crystalline field anal- ature decreases to below T , as shown by the dotted
0 MI
ysis21 revealed the magnitude of Nd3+ moments to be arrows in Fig. 2(a); however, the Nd magnetic structure
about2.37µB, whereasthat ofIr4+ moments is expected grows mainly below TNd, as shown in Fig. 1(d) and by
to be 1µB at most. Therefore, as the first approxima- the solid arrows in Fig. 2(a). This discrepancy in tem-
tion, we assumed that the q intensities consist of only perature suggests that not Nd moments exhibit static
0
the Nd moments. In fact, the statistical errors of our magnetic order at T . In isomorphic Nd Mo O , the
MI 2 2 7
data would be too large to resolve the Nd and Ir mo- magnetic moments of d and f electrons do not order si-
ments. Furthermore,later inthis section,the magnitude multaneously, as revealed by neutron diffraction experi-
of moment estimated by the magnetic structure analysis ments; the Mo structure grows below Curie temperature
is confirmed to be in agreement with that estimated by T =93KandthentheNdstructuregrowsmainlybelow
C
the crystalline field analysis. 20 K with decreasing temperature.26 In analogy with
∼
The crystalline field analysis also strongly suggests Nd Mo O , Ir magnetic ordering is expected to occur in
2 2 7
that the Nd moments are highly anisotropic along the Nd Ir O at T . The ordered Ir moment is estimated
2 2 7 MI
111 directions (in/out-type Ising moments),21 as ex- to be as smallas the errorsof m later in this subsection.
hpecteid from available data on other Nd pyrochlore ox- The splitting ofthe grounddoubletofNd3+ is slightly
ides.26 In addition, the measured magnetic susceptibil- dispersive, with a width of 0.1 meV (1 K), as shown in
ity shows a small value of only 10−3µ /formula un- Fig. 2(b); this width roughly corresponds to the magni-
B
∼
der a magnetic field of 1 kOe, and no hysteresis curve is tude of Nd-Nd interactions. The isomorphic pyrochlore
observed at 5 K.16,17,21,24 Therefore, among the various insulator Nd Sn O with a similar magnitude of Nd3+
2 2 7
magnetic structure models described by the in/out-type magneticmoments(2.6µ )alsoexhibitsaNe`eltempera-
B
moments,the3-in1-outandthe2-in2-outtypesaremost tureof0.9KandaCurie-Weisstemperatureof 0.3K,29
−
probablyruledout,becausethesemodeltypesareessen- againindicating that the magnitude is about 1 K. Thus,
tially ferromagnetic. A unique possible solution is the the Nd structure growing below T is not formed by
Nd
all-inall-outtype ofmodel, asshowninFig.3(b). Thus, Nd-Nd interactions alone. The structure probably para-
we examined the consistency between the all-in all-out sitizesthe aforementionedhidden IrorderthroughNd-Ir
model and the q intensities. exchange interactions, whose magnitude is estimated to
0
The remaining fitting parameter is the absolute value be about the splitting width of 1.3 meV 15 K; this
of magnetic moments of Nd3+ (m). Therefore, we eval- width is in excellent agreement with the va∼lue of T .
Nd
uated it by the least-squares method, by employing the We next discuss the Ir magnetic structure. First, as
magneticformfactorofNd3+ calculatedbyFreemanand shown in Fig. 2(a), a single sharp peak appears at 3
Desclaux.27 Since the sample was cylindrical in shape, K. This fact indicates that the internal magnetic field
the reflection-angle dependence of the absorption factor is uniform at all Nd3+ sites in the entire sample; this
was ignored. The evaluation results showed that the all- uniformity in turn suggests that the Ir structure is also
in all-out model had the best-fit calculated intensities, describedbyq withoutany modulation. Indeed, q has
0 0
with m(9 K) = 1.3 0.2 µ (Table I); these intensities no specialsymmetricaldirectionsandthereforeformsno
B
±
are in agreement with the experimental ones, as shown domains,andisconsistentwiththe absenceofstructural
5
FIG. 3: (Color online) Magnetic structure modelling. (a) Comparison between experimental and best-fit calculated magnetic
reflectionintensitiesgiveninTableI.(b)All-inall-outmagneticstructure. ThefilledcirclesrepresentNd3+ions,andthearrows
representtheNdmoments. (c)TrigonallydistortedO2− ligandaroundIr4+ ion. AlltheIr-Obondsareofthesamelength(2.01
˚A).(d)Relationbetweenmagneticmoments(bluethinarrows)ofIr4+ ions(bluesmallballs)andmagneticmoments(redthick
arrows)ofNd3+ions(redlargeballs). BoththeIrandtheNdmomentsformtheall-inall-outstructures. Alternativedirections
of Nd moments in the case of a ferromagnetic Nd-Ir interaction are depicted: when the moments are antiferromagnetic, the
directions of all thered arrows are reversed,and theall-in all-out typestructure is retained.
transition. Second, as mentioned above, the antiferro- 0.1µ , which correspondto m (0.7 K)=2.3µ and
B Nd B
−
magnetic nature of the magnetic structure is clear from m (0.7 K) = 0.2 µ . The negative sign means the
Ir B
−
themagneticsusceptibility. Third,asshowninFig.3(c), magneticstructureshowninFig.3(c)(ferromagneticNd-
the localenvironmentofIr4+ is trigonal,suggestingthat Ir interaction). These calculated intensities are again in
Ir moments with J = 1/2 also favour the 111 direc- agreement with the experimental ones within the statis-
eff
h i
tions(in/outtype). Thus,fromthesethreeobservations, tical errors, as shown in Fig. 3(a). Thus, the very small
alongthesamelinesasfortheNdstructurementionedin value of m supports the validity of the approximation
Ir
4, the Ir moments are also expected to form the all-in with only Nd moments.
§
all-out type of antiferromagnetic structure.
We canconfirm that the all-in all-out type of Ir struc-
ture generates the all-in all-out type of Nd structure, B. Relation between frustration and MI transition
when only nearest-neighbor Nd-Ir exchange interactions
aretakenintoaccount. Figure3(d)showsthe relationof The present experiments revealed that the antiferro-
Ir and Nd crystallographic sites and magnetic moments magnetic long-range structure with q exists below T
0 MI
with the all-in all-out structures. The single Nd site is in Nd Ir O . This fact strongly suggests that the frus-
2 2 7
positioned at the centre of the hexagon on the kagome tration is released below T . That is, the system is ex-
MI
planeintheIrsublattice. AttheNdsite,thesurrounding pected to changefrom the magnetically frustrated metal-
six nearest-neighbor Ir moments generate the exchange lic phase to the antiferromagnetic insulating phase at
field, which is perpendicular to the (111) kagome plane. T . The metallic phase will gain orbital hybridization
MI
The alternative upward or downward direction depends energy (band formation energy), whereas the insulating
on whether the Nd-Ir interaction is ferromagnetic or an- phase will gain magnetic ordering energy.
tiferromagnetic.
Then, the following question arises: how is the frus-
To examine the consistency between the approxima- tration released without lattice distortion at T ? In
MI
tion with only Nd moments in 4 and the aforemen- a pyrochlore lattice, antiferromagnetic nearest-neighbor
§
tioned model with both Nd and Ir structures, it will be interactions with isotropic moments induce strong frus-
worth reanalyzing the experimental reflection intensities tration, as is the case in the highly frustrated spinel
(Table I),thoughthe statisticalerrorsarelarge. The fit- MgCr O (Cr3+: 3d3),31 whereas those with anisotropic
2 4
ting parameters are the absolute value of magnetic mo- moments induce the all-in all-out type long-range order
ments of Nd3+ (m > 0) and that of Ir4+ (m ) with without frustration.32,33 The anisotropy is based on the
Nd Ir
sign corresponding to the alternative upward or down- spin-orbit-coupledJ states. The spin-orbitcouplingis
eff
ward direction. We evaluated them by the least-squares normallysuppressedina metallic phase (unlike in anin-
method,byemployingthemagneticformfactorsofNd3+ sulating phase), since the coupling constant λ is roughly
andIr4+.27,30 Thebest-fitcalculatedintensities(TableI) proportional to 1/r3, where r is the size of an unpaired
were obtained at m (9 K) = 1.3 µ and m (9 K) = electron cloud. Thus, in Nd Ir O , above T , an anti-
Nd B Ir 2 2 7 MI
6
ferromagneticinteractionwithrelativelyisotropicIrmo- dispersionofthe splitting is estimatedto be only 1 K,
∼
ments is considered to hamper magnetic ordering owing whichismuchbelowT ;thissuggeststhattheNdmag-
Nd
to frustration. Below T , the spin-orbit coupling will netic structure cannot be formed by Nd-Nd interactions
MI
become relatively active and the Ir moments will restore alone. Therefore, we concluded that a hidden magnetic
the in/out-type anisotropy; therefore, the all-in all-out orderofIrmomentsappearsatT , anditisparasitized
MI
type of order will emerge without frustration. by the Nd structure;however,we couldnotdetectdirect
It is open how the electron/hole bands change below signals for the Ir moments in the experiments, probably
and above T . We remark that the Mott-type antifer- because of their very small magnitude.
MI
romagnetic insulator with J = 1/2 half-filled bands TheIrmagneticstructurecanalsobedescribedbythe
eff
was recently established in Sr IrO .19,34 The J = 1/2 all-in all-out type of structure. We proposed a scenario
2 4 eff
Mott insulator might be related to the insulating phase that the in/out type of anisotropy based on spin-orbit
in Nd Ir O . coupling plays a key role in releasingthe frustrationand
2 2 7
inducing the MI transition. The scenariowill providean
exampleofthe relationbetweenthe transportproperties
VI. CONCLUSIONS and geometrical frustration.
In summary,to study a relationamong magnetic frus-
tration, the absence of structural phase transition, and Acknowledgments
theMItransitioninNd Ir O ,weconductedneutronex-
2 2 7
perimentsofthismaterialinpowderform. Astheresults, We thank Mr. M. Ohkawara, Mr. K. Nemoto, and
magneticBraggreflectionswithq =(0,0,0)werefound Mr. T. Asami for providing assistance at the JAEA,
0
by diffraction. The propagationvectoris consistentwith and Mr. M. Onodera for providing assistance at To-
theabsenceofstructuraltransition. Thereflectioninten- hoku University. The neutron experiments at the JAEA
sity of Nd Ir O increases with decreasing temperature were performed under User Programs conducted by
2 2 7
belowT =15 5K.Themagneticstructureofthissys- ISSP. This study was financially supported by Grants-
Nd
±
tem can be described by the all-in all-out type of model in-Aid for Young Scientists (B) (22740209);Priority Ar-
for Nd moments with a magnitude of about 2.3 0.4µ eas (22014001 and 19052005); Scientific Researches (C)
B
±
at 0.7 K. The magnitude of moments is consistent with (23540417),(S)(21224008),and(A) (22244039);andIn-
the results of a previous crystalline field analysis. novativeAreas(20102005and21102518)fromtheMEXT
The Kramers ground doublet of Nd3+ begins to split of Japan. The study was also supported by the Inter-
at T with decreasing temperature, suggesting another universityCooperativeResearchProgramoftheInstitute
MI
magneticorderingatthistemperature. Themagnitudeof for Materials Research at Tohoku University.
∗ Electronic address: [email protected] Solid State Commun. 14, 357 (1974).
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