Neutron scattering investigation of the spin ice state in Dy2Ti2O7

T. FennellThe Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, United Kingdom.

O. A. Petrenko* and B. Fåk†

ISIS Facility, Rutherford-Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom

S. T. Bramwell‡

Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom

M. Enjalran,§ T. Yavors’kii, and M. J. P. Gingrasi

Department of Physics, University of Waterloo, Ontario N2L 3G1, Canada

R. G. MelkoDepartment of Physics, University of California, Santa Barbara, California 93106-9350, USA

G. BalakrishnanDepartment of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom

(Received 28 April 2004; published 15 October 2004)

Dy2Ti2O7 has been advanced as an ideal spin ice material. We present a neutron scattering investigation ofa single-crystal sample of162Dy2Ti2O7. The scattering intensity has been mapped in zero applied field in theh,h, l and h,k,0 planes of reciprocal space at temperatures between 0.05 and 20 K. The measured diffusescattering has been compared with that predicted by the dipolar spin ice model. The comparison is good, exceptat the Brillouin-zone boundaries where extra scattering appears in the experimental data. It is concluded thatthe dipolar spin ice model provides a successful basis for understanding Dy2Ti2O7, but that there are issueswhich remain to be clarified.

DOI: 10.1103/PhysRevB.70.134408 PACS number(s): 75.50.Lk, 75.40.Cx, 75.25.1z

I. INTRODUCTION

The concept of spin ice1 has appeared recently as a resultof a direct analogy between the statistical mechanics ofO-H bonds in Pauling’s model of water ice2,3 and those ofthe localized magnetic moments of some highly frustratedmagnets.1,4,5 These materials, typified by Ho2Ti2O7,

1

Dy2Ti2O74 and Ho2Sn2O7,

6 provide almost ideal experimen-tal realizations of spin ice, a magnetic “16 vertex” model.7

Their temperature-field phase diagrams are exceedingly richand have revealed several interesting magnetic phases andtransitions.1,8–11The magnetic dynamics of the spin ices alsoshow some unique properties including a crossover fromDebye-like spin relaxation12–14 to quantum tunnelingdynamics15,16at the unusually high temperature of 15 K. Thecurrent paper builds on earlier work1,6,17,18to characterize theneutron scattering of spin ice materials. In it, we report neu-tron scattering images of the spin ice state of Dy2Ti2O7,

19

which are compared with theoretical predictions.20–22

In Dy2Ti2O7, as well as in the other spin ice materialsHo2Ti2O7 and Ho2Sn2O7, the rare-earth moments(“spins”)are arranged on a pyrochlore lattice of linked tetrahedra(seeFig. 1). Strong crystal-field anisotropy23 confines each spinas an Ising-like doublet to the local trigonal axis that con-nects the center of the elementary tetrahedron to its vertex.With this constraint and ferromagnetic coupling, the mag-netic ground state of a single tetrahedron is defined by a “twospins in, two spins out” rule, analogous to the Bernal-Fowlerice rule that controls the proton arrangements in water ice.24

This rule minimizes only four of the six near-neighbor mag-netic interactions and so the system is highly frustrated, shar-ing with ice the property of zero-point entropy.1–4 In the spinice materials, the ferromagnetic near-neighbor couplingarises from the strong dipole-dipole interaction betweenneighboring spins.20 If the dipolar interaction is truncated atnearest neighbors one has the “near neighbor” spin icemodel1,7 (note that dipole interactions are equivalent to ex-change terms for Ising spins). This approximation capturesthe bulk properties of the spin ice materials1,4,25–27and givesa qualitative description of the microscopic spin correlations,as determined by neutron scattering.1,17

The subject of this paper, Dy2Ti2O7, has previously beeninvestigated by bulk methods.4,12,27,28In zero field, the spe-cific heat resembles a Schottky anomaly, peaking at 1.24 Kand falling to zero at lower temperatures. Unlike itsHo2Ti2O7 analog, which shows an extra peak due to freezingof the nuclear spins,17,29 Dy2Ti2O7 shows no other specificheat contributions down to 0.2 K, where the specific heatbecomes exponentially small. It has thus proved highly fa-vorable for bulk measurements, having been used to estimatethe zero-point entropy in zero field,4 and to demonstrate theexistence of “kagoméice,” a field-induced state that retainszero-point entropy.9,28 However, owing to its large neutronabsorption, Dy2Ti2O7 has not yet been extensively studied byneutron scattering,19,30 the technique that was crucial in es-tablishing the existence of the spin ice state in Ho2Ti2O7(Ref. 1 and 17) and Ho2Sn2O7.

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To address the problem of large neutron absorption innatural dysprosium based materials we have grown an isoto-pically enriched crystal of162Dy2Ti2O7 specifically for use inneutron scattering experiments. The details of the sample andthe experiments performed on it are given in Sec. II. Theexperiments were designed to examine the nature of the pre-sumed spin ice state that is established when the sample iscooled in zero applied magnetic field.4,20The data were mod-eled using the “dipolar” spin ice model that considers furtherneighbor dipole-dipole interactions as well as a small antifer-romagnetic exchange coupling that can be estimated by fit-ting the susceptibility31 or specific heat.20 Details of the di-polar spin ice model,20 as applied to Dy2Ti2O7, are given inSec. II B of the current paper, while the comparison of theoryand experiment is given in Sec. III. The paper is concludedwith a discussion of the results, Sec. IV.

II. EXPERIMENT

A. Neutron scattering

Neutron scattering measurements were carried out onPRISMA at the ISIS pulsed neutron source of theRutherford-Appleton Laboratory. A pulsed white neutronbeam is incident on the sample. PRISMA views a colds95 Kd methane moderator and a supermirror guide system,providing a high flux of long-wavelength neutrons allowingmeasurement at relatively lowuQu. The neutrons scatteredfrom the sample are recorded in 16 detectors which cover arange of 16° in scattering angle. From the neutron time offlight, the scattering angle and the orientation of the samplewith respect to the incoming beam, the scattered intensitycan be determined as a function of the position in reciprocalspace. By rotating the crystal about the vertical axis, inten-sity maps covering a large region of the scattering plane canbe constructed. This method is particularly useful for studiesof diffuse magnetic scattering. The average scattering anglef of the 16 detectors used for the measurements is chosen asa compromise between flux(at a given wave vector) andbackground(due to air scattering). The advantages of mea-suring the diffuse magnetic neutron scattering on PRISMAhave been clearly demonstrated using the example ofHo2Ti2O7, where the dipolar spin ice nature of the zero-fieldspin correlations has been unambiguously established.17 Inthat and the current experiment the neutron scattering wasmeasured in the static approximation.33

Natural dysprosium contains seven isotopes, several ofwhich absorb neutrons quite strongly. This can make the de-tection of weaker effects like diffuse scattering difficult orimpossible. To reduce this, an isotopically enriched samplewas used. The natural abundances, scattering lengths, andcross sections for natural dysprosium and our sample aregiven in Table I. Using the162Dy isotope the average absorp-tion cross sectionsa is reduced by a factor of 4.8 comparedto natural Dy. Since absorption attenuates scattered intensityby a factor of the form exps−Nsald (whereN is the numberdensity of scatterers per unit volume,sa is the average ab-

FIG. 1. Top: a unit cell of the pyrochlore lattice which consistsof four interpenetrating face centered cubic(fcc) Bravais sublat-tices. Two spins related by an fcc translation are shown with anti-ferromagnetic correlation. Correlations of this type give rise to thezone-boundary scattering observed experimentally. Bottom: thespin ice state on a single tetrahedron(bottom). A macroscopic spinice state is constructed by stacking such local arrangements on thelattice.

TABLE I. Isotopic abundances and absorption cross sectionsssad of natural dysprosium and the enriched sample used in theseexperiments(Ref. 32).

Isotope Natural Sample sa

Abundance Content (barn)

(%) (%)

Natural 994.(13.)156Dy 0.06 ,0.01 33.(3.)158Dy 0.1 ,0.01 43.(6.)160Dy 2.34 0.02 56.(5.)161Dy 19 0.47 600.(25.)162Dy 25.5 96.8 194.(10.)163Dy 24.9 2.21 124.(7.)164Dy 28.1 0.5 2840.(40.)

Sample 207.6

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sorption cross section, usually tabulated for 2200 m s−1 neu-trons, andl is the neutron wavelength scaled to the valueused insa) this makes a significant difference at long wave-lengths.

A large high quality single crystal was grown using aninfrared double mirror image furnace.34 The crystal has adiameter of<0.4 cm, length<1.5 cm, and is translucentamber-red in color. It has a cylindrical form typical of imagefurnace grown crystals. The crystal was varnished into alarge copper support to ensure good thermal contact.

Using a3He sorption refrigerator insert in an Orange cry-ostat, theh,h, l plane of reciprocal space was mapped in zerofield at 20 and 0.3 K using the average scattering anglef=48°. The same plane was mapped at 0.3 and 1.3 K usingthe average scattering anglef=32°. Using a dilution refrig-erator insert in an Oxford Instruments cryomagnet theh,k,0and h,h, l planes of reciprocal space were mapped in zerofield at 0.05 K. These maps are shown in Fig. 2. All parts ofthe sample environment in the beam were made of alu-minium (other than the copper support), which gives rise topowder lines atuQu=2.70, 3.12 and 4.40 Å−1.

Standard data reduction procedures were applied to trans-form time of flight and angle information to reciprocal space,to correct for absorption in the sample, and to normalize thedata to an absorption corrected vanadium run. The latter cor-rects for the wavelength-dependent flux profile and detectorefficiency. Despite the fivefold reduction in absorption crosssection compared to natural dysprosium, the data analysisprocedure revealed that absorption by the Dy2Ti2O7 crystalwas still significant and needed to be taken into account. Theabsorption correction was made by calculating an attenuationcoefficientfAss,fdg by which the data could be divided. Thecoefficient for a cylinder of radiusR was originally derivedby Sears35 and is

Ass,fd = f1 + 4bs2 − 0.5s2 cos2sf/2dg 3 exps− 2asd, s1d

wherea=s8/3dp, b=s1−a2d /2, s=mR, andf is the scatter-ing angle. In this formula,m, the absorption coefficient, de-pends on both the scattering cross section and the wave-length dependent absorption cross section.

FIG. 2. (Color online) Dy2Ti2O7: Diffuse scattering in theh,h, l plane measured at 20 K(top left), 1.3 K (top right), and 0.3 K(bottomleft, with zone boundaries illustrated). The sharp, intense spots are nuclear Bragg reflections with no magnetic intensity. The map at 20 Kwas recorded using an average scattering angle off=48° which gives stronger arc-shaped artefacts for smalluQu values compared tof=32°. The result of the Monte Carlo simulation of the dipolar spin ice model at 0.3 K is shown at bottom right.

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B. Monte Carlo simulations

To model the experimental data for Dy2Ti2O7, we use aHamiltonian appropriate for thek111l Ising pyrochlores:

H = − J oksi,ad,s j ,bdl

Sia ·Sj

b

+ DRnn3 o

si,ad.s j ,bdSSi

a ·Sjb

uRi jabu3

−3sSi

a ·Ri jabdsSj

b ·Ri jabd

uRi jabu5

D , s2d

where Sia= zasi

a represents an Ising moment of magnitudeuSi

au=1 at fcc lattice sitei and tetrahedral basis positiona,with its quantization axisszad oriented along the local trigo-nal axis (a member of thek111l set), and si

a= ±1. Thenearest-neighbor exchange energy isJnn=J/3 and thenearest-neighbor dipole-dipole strength isDnn=5D /3, whereD=sm0/4pdm2/Rnn

3 andm is the moment of the Dy3+ ion. Ri jab

is the vector separation between momentsSia andSj

b.The model(2), with parameters appropriate to Dy2Ti2O7,

orders magnetically at 0.18 K.21 The real material does notshow this ordering transition, but we test the hypothesis thatthe correlated paramagnetic state just above the transitiontemperature will provide an adequate description of the spinice state. Equation(2) was simulated on a pyrochlore latticeof 43434 cubic cells(N=1024 spins) with periodic bound-ary conditions. The long-range nature of the dipole-dipoleinteractions was properly handled by using standard Ewaldtechniques for real-space dipoles.20 The model employed en-ergy parameters appropriate for Dy2Ti2O7, J=−3.72 K andD=1.41 K.20

At temperatures below 1 K or so the system is only ableto access states within the spin ice manifold. Single spin-flipdynamics require the introduction of ice rule defects. Thismeans that the acceptance rate of single spin flips becomesextremely small and the ability to equilibrate the system be-low 0.3 K becomes dubious.20,21 In order to improve on ap-proach to equilibrium, dynamics were restored by using aloop algorithm.21 To simulate scattering data the system wasplaced in the ordered ground state found by Melkoet al.,21 at0.4 K. 53106 Monte Carlo steps(MCS) per spin and 53106 loop attempts were made to equilibrate the system andbring it to a paramagnetic state. Loop attempts were there-fore separated byN=1024 MCS. The system was thencooled to 0.3 K where the same number of single spin-flipand loop attempt steps were used for equilibration and thendata collection(see Refs. 17, 20, and 36 for more details).

There are 12 symmetry related ordered stuctures and tomaintain ergodicity each was used as a start point for a simu-lation. IsQd was calcuated atT=0.3 K from 200 independentspin configurations derived from each starting structure usingthe formula17

IsQd ~ufsQdu2

N osi,ad,s j ,bd

eiQ·Ri jab

kSi,'a ·Sj ,'

b l, s3d

wherek¯l denotes a thermal average,Si,'a is the spin com-

ponent perpendicular to the scattering wave vectorQ, andfsQd is the magnetic form factor for Dy3+.42 The MonteCarlo results forIsQd are compared to the experimental data

in the h,h, l and h,k,0 planes below. The simulations arenominally at 0.3 K, whereas the experimental data was col-lected at either 0.3 K(h,h, l plane) or 0.05 K (h,k,0 plane).We consider it valid to make these comparisons because attemperatures below 0.3 K the susceptibility,27 neutron scat-tering intensityIsQd, and specific heat4 do not change sig-nificantly with temperature: the system essentially appears tobe frozen with negligible thermal spin fluctuations.

III. RESULTS

In this section, the observed magnetic diffuse scattering ispresented and compared to Monte Carlo simulations wherepossible. The temperature dependence of the scattering in theh,h, l plane is discussed first(Sec. III A), in order to estab-lish the temperature regimes of interest. In the following Sec.(III B and III C) the scattering intensity measured at basetemperature is compared to Monte Carlo simulations. Fi-nally, Sec. III D is concerned with features in the experimen-tal scattering which are not reproduced by the model.

A. Temperature dependence of the diffuse scattering

The temperature dependence of the diffuse scattering dueto the development of ice rule fulfilling spin correlations wassampled at four temperatures for theh,h, l plane (20, 1.3,0.3, and 0.05 K, see Fig. 2). An important point that can beobtained from these measurements is that the diffuse scatter-ing does evolve with temperature. As the sample is cooled,no diffuse scattering characteristic of spin ice correlationshas built up by 20 K. This is entirely expected as the bulksusceptibility shows that Dy2Ti2O7 is paramagnetic at thistemperature.27 At 1.3 K strong diffuse scattering has ap-peared. The form is closely similar to that of the near-neighbor spin ice as simulated in Ref. 17. Between 1.3 and0.3 K the spin correlations evolve and additional featuresappear in the scattering pattern, as discussed below. The scat-tering at 0.05 K throughout theh,h, l plane (not illustratedhere) is not significantly different to that at 0.3 K.

B. Scattering in the h ,k ,0 plane

For the h,k,0 plane, data have only been collected at0.05 K. The simulation and the experiment seem to agreevery well indeed. The two scattering maps are shown in Fig.3. A comparison by a cut through the data is shown in Fig. 4.The Monte Carlo pattern has been fitted to the experimentaldiffuse scattering using a flat background and an overall in-tensity scale parameter.

C. Scattering in the h ,h , l plane

For theh,h, l plane direct comparison of the experimentaland simulated scattering maps reveals both close similaritiesand significant differences. The two scattering patterns areshown in Fig. 2. It is apparent that while the experimentalpattern does contain the features of the dipolar spin ice scat-tering (for example, the large diffuse feature at0,0,3) it alsocontains features that are not reproduced by the simulation.These extra features are concentrated around the boundaries

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of the Brillouin zones(the position of the zone boundariesare illustrated in the bottom left panel of Fig. 2).

The similarities and differences of the observed and cal-culated patterns are more clearly emphasized in Fig. 4. Hereslices through the data have been made. The agreement alongh,h,0 and 0,0,l is good. The slice alongh,h,2.3 is chosento highlight the differences and it can be seen that where theslice crosses the zone boundary the form of the calculatedand experimental intensities is significantly different.

D. Zone-boundary features

One possible explanation for the zone-boundary featuresis that they are due to phonons. However, the following factssuggest that the intensity is magnetic:(i) the scattering ap-pears as temperature is decreased; the converse would beexpected for a phonon;(ii ) the experimentally observed fea-tures fall off with increasinguQu, which is not expected forphonons.

The second point can be demonstrated by comparison ofthe intensity of the zone-boundary features with the magneticform factor of Dy3+. If the intensity at equivalent points onsuccessive zone boundaries is compared it would be ex-pected to follow the form factor if the scattering process ismagnetic. This is found to be the case. On the basis of thiscomparison it seems entirely reasonable to attribute the zone-boundary features to magnetic correlations.

IV. DISCUSSION

The dipolar spin ice model apparently provides a verysuccessful model for the zero-field spin correlations inDy2Ti2O7. However, some additional spin correlations do oc-cur which are not captured by the model. The model de-scribes well the bulk and microscopic data for Ho2Ti2O7,

17

and the bulk data for Dy2Ti2O7.20 One might therefore ask,

how is this further restriction of the dipolar spin ice manifoldnot manifested in the fit to the heat capacity?

In answer to this question, it should be noted that the fit tothe heat capacity20 is not perfect: the experiment and simu-lation differ in some small details. The fact that the zone-boundary features are very diffuse in reciprocal space andrun all around the zone boundaries suggests that the associ-ated spin correlations are short ranged and isotropic. A smalldifference between the experimental and theoretical heat ca-pacities is therefore understandable since the extra spin cor-relations need not make a major contribution to the entropy.We therefore conclude that the neutron scattering andspecific-heat data4,20 are consistent.

A clue to the origin of the extra spin correlations is thefact that they are much less evident in Ho2Ti2O7,

17,18 evenif they cannot be excluded completely within experimentalerror. According to the model, the principal differencebetween Ho2Ti2O7 and Dy2Ti2O7 is the more significantantiferromagnetic exchange term in the spin Hamiltonianof Dy2Ti2O7 (J=−3.72 K for Dy2Ti2O7 compared toJ=−1.56 K for Ho2Ti2O7). This exchange term was invokedto fit bulk measurements,20,27,31which are not sensitive to thedetails of a Hamiltonian in the way that diffuse neutron scat-tering is. Hence the extra scattering might reveal that extraterms are required in the model Hamiltonian: for example, itmight be necessary to include further neighbor exchange in-teractions. Further theoretical work is required to distinguishvarious possibilities.

Zone-boundary scattering has recently also been observedin theh,k,0 plane for ZnCr2O4 (Ref. 37) and theh,h, l planefor ZnFe2O4 (Ref. 38) (both frustrated antiferromagnets withthe spinel structure in which the magnetic ions also form apyrochlore lattice). We suggest that, while the three magnetsare quite different, the zone-boundary features may also be ageneric feature of frustrated magnets due to the formation oflocal spin clusters, for example those developing inZnCr2O4,

37,39 which might be at the origin of the “quantum

FIG. 3. (Color online) Dy2Ti2O7 at 0.05 K: experimental scattering intensity in theh,k,0 plane(left), compared with that calculated fromMonte Carlo simulations(right).

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protectorate” state invoked in that system. As noted by Ka-mazawaet al.,38 zone-boundary scattering implies correla-tions between the spins related by fcc translations(as illus-trated in Fig. 1). These spins are third neighbors andinteractions at this distance will clearly be important in sta-bilizing a hexagonal cluster. Simulations suggest that otherclusters can also be stabilized depending on the details of theHamiltonian.40

In conclusion, we have presented a systematic investiga-tion of the diffuse scattering of Dy2Ti2O7 in zero magneticfield. We have observed three regimes of behavior. At 20 Kthe spins are paramagnetic, without spin ice correlations,which should help interpret recent bulk susceptibility mea-surements of the dynamical properties,12,13,27 which havebeen discussed in terms of both correlated and uncorrelateddynamical processes.13,15,16At 1.3 K the spin correlations aresimilar to those of the near-neighbor spin ice model, suggest-ing that further neighbor dipole interactions are not stronglyrelevant in this temperature range. At 0.3 K the scattering is

modified to correspond quite closely to the frozen state of thedipolar spin ice model, with effective infinite range interac-tions. However, in this temperature range we have observedadditional scattering features around the zone boundarywhich for the time being remain unexplained. Further calcu-lations are in progress to clarify their origin.

ACKNOWLEDGMENTS

We thank the sample environment team at ISIS, in par-ticular Richard Down and Dave Bates, and Martyn Bull forassistance on PRISMA. T.F. acknowledges the EPSRC forfunding, and Andrew Harrison for hospitality in Edinburghwhile this work was written up. Work done at the Universityof Warwick was funded by an EPSRC grant. Work done atthe University of Waterloo has been supported by theNSERC of Canada, the Canada Research Chair Program, theProvince of Ontario, and Research Corporation.

FIG. 4. Dy2Ti2O7 at 0.03 K: comparison of experimental and Monte Carlo data forh,k,0 andh,h, l planes. The comparison in theh,k,0plane is shown at top left using a slice along 3,k,0. The Monte Carlo data was compared to the experimental data by adjusting an overallintensity scale parameter and single backward parameter in order to give the best fit. The comparisons for theh,h, l plane are slices alongh,h,0 (top right), 0 ,0 ,l (bottom left), and alongh,h,2.3 (bottom right). For slices 2 and 3 the Monte Carlo data were compared using anoverall intensity scale parameter and sloping backward, slice 4 was compared with an overall intensity scale parameter and constantbackground. The slice positions in the scattering plane are indicated in the insets, and for theh,h, l plane by numbers 2–4. Shaded areas inthe slices are Bragg peaks from the sample and were excluded from the fitting.

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*Present address: Department of Physics, University of Warwick,Coventry, CV4 7AL, United Kingdom.

†Present address: CEA Grenoble, DRFMC/SPSMS, 38054,Grenoble, Cedex 9, France.

‡Electronic address: s.t.bramwell@ucl.ac.uk§Present address: Department of Physics, Southern Connecticut

State University, New Haven, CT 06515.iAlso at Canadian Institute for Advanced Research, 180 Dundas

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