PHYSICAL REVIEW B VOLUME 40, NUMBER 10 1 OCTOBER 1989

Two-dimensional spin correlations and successive magnetic phase transitions in NdzCu04

Y. Endoh, M. Matsuda, K. Yamada, and K. KakuraiDepartment of Physics, Tohoku Uniuersity, Sendai 980, Japan

Y. HidakaNippon Telegraph and Telephone Corporation Opto-electronics Laboratories, Tokai Ibaraki 319-11,Japan

G. ShiranePhysics Department, Brookhauen National Laboratory, Upton, New York 11973

R. J. BirgeneauDepartment ofPhysics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

(Received 15 May 1989)

Nd2Cu04 is found to be a quasi-two-dimensional antiferromagnet with T&=255 K. Successivephase transitions occur due to the Cu + spin reorientation as well as, at low temperatures, the parti-cipation of the Nd + spins in the three-dimensional long-range order. A sharp peak correspondingto the two-dimensional spin correlations is observed in energy-integrated scans. The magnetic prop-erties at high temperatures are closely similar to those in La2Cu04.

A new class of high-T, oxide superconductors typifiedby Ndz „Ce Cu04 (Refs. l and 2) has recently beendiscovered. These materials are especially interesting be-cause the Hall coefficient appears to be negative in thenormal state, suggesting electron rather than hole con-duction as in the well-studied oxide superconductorsexemplified by La2 Sr Cu04. A second importantdifference between these two classes is evident in the crys-tal structures shown in Fig. 1. In Nd2Cu04 each Cu ionis surrounded by four oxygen atoms located in the Cu02plane; this contrasts with the Cu06 octahedron structurein La2 „Sr Cu04. Further, unlike La2Cu04, this systemcontains Nd + ions which may couple magnetically tothe Cu +. Therefore it is essential to elucidate the role ofthe Nd moments in the overall magnetic properties, par-ticularly their effect on the strongly correlated Cu02 lay-ers. These materials thus provide a test ground for anumber of the existing models for the lamellar Cu02 su-perconductors. Specifically, the underlying mechanismfor the superconductivity is almost certainly common tothe Nd2Cu04 and La2Cu04 materials; thus one may ex-clude any model which relies on aspects of the materialproperties which differ in these two systems.

For the past two years we have been studying in de-tail the static and dynamic spin fluctuations inLa@ „Sr Cu04. In LazCu04 a two-dimensional (2D)quantum spin-Quid (QSF) state exists at high tempera-tures with spin-fluctuation energies which are enor-mous. ' The 2D spin correlations persist in the super-conducting state, although the characters of both themagnetic structure factor S(q) and the dynamicalresponse function S(q, co) are modified drastically by thedoped holes. Thus, it is our view that the magnetismand superconductivity are intimately related in theLa2 „Sr CuO„superconductors and also that the QSF

state generated by the Cu + spins provides an essentialingredient for the appearance of the novel superconduc-tivity. Based on the experimental information obtainedso far' ' we anticipate the existence of the same QSFstate in Nd2Cu04 as in La2Cu04. The superconductivityin this new class of materials thus must involve the 2Dmagnetism in a fundamental way.

We prepared large single crystals of NdzCu04 grownfrom a nonstoichiometric CuO-rich solution. The

o oNd6' o

o,

FIG. 1. Crystal structure of Nd2Cu04 and magnetic struc-tures of the Cu + spins: (a) La2Ni04 type, (b) La2Cu04 type.

40 7023 1989 The American Physical Society

7024 Y. ENDOH et al.

C:NdpCLl04 (as grown), E =50.5 meV, Z Axis

LA 1000-pp

o500-

I

O

1( — — 033 )

2

(0 6000-O

4000-

ao 2000-

CL 00

1 1o 2 2

0 )1 1

2 2

50 100 150 200 250 500 550Temperature t,'K)

FIG. 2. Temperature evolution of the peak intensities of the3D antiferromagnetic Bragg reflections and the 2D rod.

present Aux method is essentially the same as thatdeveloped in the work to grow large La2CuO4 single crys-tals. The neutron scattering experiments have been car-ried out utilizing an ingot crystal which includes someAux material due to the difficulty in removing the latter.The size of the crystal is 40X40X 1.5 mm with the c axis(in a Ilmmm representation) (Fig. 1) along the thin direc-tion.

The experiments were carried out on the Tohoku Uni-versity Neutron Spectrometer Reactor (TUNS) spectrom-eter at the Japan Research Reactor (JRR2) at the TokaiEstablishment of the Japan Atomic Energy Research In-stitute (JAERI). We employed graphite monochromator,analyzer, and filters. The quasielastic scans were per-formed with neutrons of energy 30.5 meV (k =3.83A ). In the triple axis method for either elastic or in-elastic scans the neutron energies were chosen to be pri-marily 13.7 meV (k =2.57 A ). The as-grown crystalwas oriented with a [110] axis vertical so that in thescattering plane peaks with Miller indices (hhl) could beaccessed. The lattice parameters at 300 K were a =3.941A and c = 12. 16 A; these agree with the existing data. '

We first studied the 3D antiferromagnetic ordering.The 3D Bragg reflections appear at the ( —,

'—,'1) point but

not at the ( —,'

—,'0) point just below a 7'~ of about 255 K.This indicates an La&NiO„-type magnetic structure'where the spin direction S is parallel to the propagationvector ~ of the [110]axis in the Cu02 plane. The orderedmoment could be estimated as -0.5p~ per atom, whichindicates that only the Cu + spins participate in the 3Dlong-range order (LRO) at high temperatures. This mo-ment is essentially identical to that found in the LROstate in LazCu04. " We find with decreasing temperaturetwo additional phase transitions as shown in Fig. 2. At80 K a spin reorientation transition occurs from theLazNi04-type structure (v~~ [—,

'—,'0], S~~ [—,

'—,'0] ) to the

La~Cu04 structure (r~~[ —' —'0],S~~[ 2—20]) with Cu +

moments oriented perpendicular to the propagation vec-tor as is illustrated in Fig. 1. As discussed in Ref. 12, intetragonal symmetry for both of these magnetic struc-tures the interplanar ordering may be noncollinear. At30 K a second transition occurs in which the Cu + spinsagain return to the orientation along [—,

'—,'0] and at the

same time the Nd + moments begin to participate in the3D LRO. The upper (80 K) spin reorientation phasetransition is difficult to understand in a pure tetragonalcrystal structure' and specifically a subtle distortionfrom tetragonality seems to be required. Therefore a fur-ther refinement for the determination of the crystal struc-ture for T(80 K is needed; a neutron-di8'raction studywith medium resolution led to inconclusive results. Asshown in Fig. 2, upon further cooling below 30 K we findthat the peak intensity of the ( —,

'—,'1) reflection saturates

and then decreases; eventually it disappears below 8 K.Concomitantly, the peak intensity of the ( —,

'—,'3) reflection

grows continuously below 30 K on cooling. Since there isno appreciable break in intensity of this reAection ataround 8 K where the ( —,

'—,'1) reflection disappears we

conclude that there is no true phase transition at 8 K.We are now in the process of refining the antiferromag-

netic structure below 8 K we speculate that the Nd mo-ments in each c plane couple antiferromagnetically.Specifically, since the magnetic susceptibility at low tern-peratures does not show any spontaneous magnetizationat all, each sublattice consisting of Nd + or Cu + mo-ments must satisfy the condition of no net moment. Thusthe disappearance of the ( —,

'—,'1) reflection can be inter-

preted as rejecting an increase of the staggered momentof the Nd site. There are two singularities in the inversesusceptibility curve corresponding to two phase transi-tions at around 80 and 30 K, in addition to the Neel tem-perature of about 255 K. In contrast to La2Cu04,NdzCuO4 exhibits Curie-Weiss behavior in its susceptibil-ity down to at least 30 K; this presumably originates fromthe paramagnetism of the Nd moments which may cou-ple weakly to the strongly correlated Cu moments.

On structural grounds alone, we anticipated observa-tion of the QSF state at high temperatures in NdzCu04.Indeed, we observe many features similar to those ob-served previously in the experiments in La2CuO4. Thequasielastic scattering intensity scanned along ( —,

'—,' g)

shows a prominent peak at /=0. 33, the point at whichthe outgoing neutron wave vector kf is exactly along c*;that is, kf ~~

—c* when the spectrometer is set for elasticscattering at the ( —,

'—,'0. 33) rod position [see Fig. 3(a)]. As

described in detail in Ref. 6, this phenomenon providesdirect evidence for rapidly fluctuating 2D spin correla-tions. Specifically, for 2D magnets, when kf is parallel tothe 2D magnetic rod, an approximate energy integrationfrom —kz T to the incident energy, E,-, is performed withthe momentum transfer in the Cu02 plane, Q2D, heldconstant. Therefore, the instantaneous spin-correlationfunction

(& (—Q, ,0)& (Q, 0) &

is measured directly by this special two-axis scan. The

TWO-DIMENSIONAL SPIN CORRELATIONS AND SUCCESSIVE. . . 7025

2700Nd2 Cu O„E =30.5 meV B-30'-30'-B

C:

E2500

1 1

(— —()2 2

2 Axis

I—2300

LLI

21000.0 0.2 0.4

(, (r. l.u. )

0.6 0.8

2800

2600E

v) 2400

OO

Nd2 Cu 04 E, =305 meV B-30'-30'- B

(2 +h 2+h 033)1 1

2 Axis

(2 +h 2+h 033) T = 350K

bj

00I— 2200(f)

Lll

2000

O

p 0

00' 0 0 0 00

1800-0.2 0.0

h (r. l. u. )

FIG. 3. (a) Scan along the top of the (2 z'g) magnetic rod.The line is a guide to the eye. (b) Scan across the magnetic rodat the ( —,

'—,'0.33) point (circles) and at the ( —,

'—,' —0.33) point

o

(crosses). These scans were performed with a k; of 3.85 AThe line through the l —0.33 scan is the result of a fit of aLorentzian convoluted with the experimental resolution func-tion.

-0, 1 0.2

( —,'

—,'0. 33) peak intensity as a function of temperature isshown at the top of Fig. 2. As in La2Cu04, the intensityrises slowly as T& is approached arid then drops gradual-ly with the onset of 3D LRO. This fact together withdata to be discussed suggests that the transition to 3DLRO is driven by the interplanar interaction.

As shown in Fig. 3, a rather sharp peak is observed inthe focusing scan across the 2D rod at ( —,

'—,'0. 33); in con-

trast, no magnetic signal is observed in the scan across

the rod at ( —,'

—,' —0.33). This difference demonstrates that

the scattering function is governed by fast 2D spin Auc-tuations just as in the QSF state of La2Cu04. In order toconfirm the inelasticity, we have measured directly theelastic component using three-axis scans; we find that theelastic component is barely visible as in the originalLa2Cu04 results. In addition, a single peak centeredaround the 2D rod position is observed in constant ener-

gy scans with energies up to 6 meV suggesting a large en-

ergy scale for the spin fluctuations. The result is con-sistent with the two-magnon Raman experiment' thatshows behavior similar to that in La2Cu04.

Finally we investigated the effect of heat treatment onthe antiferromagnetism, since the conductivity was foundto depend on the heat treatment as in the La2Cu04 case.We prepared two other single crystals which were cutfrom the same ingot. One was annealed in flowing Ar gasat 950 C for 15 h and the other was in an oxygen gas Aowat 600'C for 200 h. We have obtained almost identicalresults for each crystal with no significant effect of theheat treatment on the antiferromagnetic transition. Thisresult is very different from the La2Cu04 case where the3D transition temperature drops by about 100 degrees inoxygenated crystals. ' This is consistent with the @SRex-periments which show that the magnetism inNd2 „Ce Cu04 is much less sensitive to the doping thanthat in La2 „Sr Cu04. Further research is required todetermine if this reflects a fundamental difference in theway electrons and holes disturb the local magnetic struc-ture.

In summary, we find that the magnetism in Nd2Cu04is quite similar to that in La2Cu04. Therefore the presentmeasurements suggest a common mechanism for the ap-pearance of the superconductivity in Nd2 Ce Cu04 andLa2 Sr Cu04 with the magnetism playing an essentialrole. Clearly, it is now important to document the effectof Ce doping on the QSF state in Ndz „Ce,CuO„. Al-though it is not a central focus of the research on the re-lation between the magnetism and the superconductivity,the magnetic phase transitions observed here also includevery interesting concepts frequently discussed in modernmagnetism such as frustration, competing anisotropies,and exchange interactions.

ACKNOWLEDGMENTS

We thank H. Kadowaki, T. Murakami, and M. Suzukifor valuable discussions of this work. This work was sup-ported by a Cirant-In-Aid for Scientific Research fromthe Japanese Ministry of Education, Science and Culture.The work at MIT was supported by the U.S. NationalScience Foundation under Contract No. DMR85-01856.The work at Brookhaven was supported by the Divisionof Materials Science, U.S. Department of Energy underContract No. DE-AC02-CH00016.

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