Antiferromagnetic ordering in RE2C2I2 (RE = Ce, Nd, Tb)

K. Ahn (now University of Virginia, Charlottesville); B.J. Gibson (now at BOSCH, Stuttgart);R.K. Kremer, Hj. Mattausch, and A. Simon; B. Ouladdiaf (Institute Laue-Langevin, Grenoble);

L. Keller (PSI, Villingen)

The non-magnetic layered rare earth (RE = Y,La) metal carbide halides RE2C2X2 (X = Cl,Br, I) form two-dimensional metals and showsuperconductivity, e.g., Y2C2I2 at 10 K. So far,a maximumTc of 11.6 K has been reached inthe phase Y2C2Br0�5I1�5. Recently, by apply-ing hydrostatic pressure to Y2C2I2 we demon-strated thatTc can be increased to 11.7 K. Inseveral preceeding annual reports (e.g., the an-nual reports of 1994, 1995, and 1997) we havedescribed the superconducting properties of theRE2C2X2 (RE = Y, La; X = Cl, Br, I) in moredetail. With magnetic RE atoms the phasesRE2C2X2 undergo antiferromagnetic (afm) or-dering forTN� 60 K with unusual short-rangeordering behavior aboveTN which we have in-vestigated by heat capacity, magnetic and elec-tronic transport measurements. For the phaseswith RE = Ce, Nd and Tb the magnetic struc-ture was determined by detailed powder neu-tron diffraction measurements. Particularly forTb2C2I2, preceding the long-range afm order-ing at�65 K we observe diffuse magnetic scat-tering due to two-dimensional short-range or-

dering at temperatures up toT � 2.5TN whichwe ascribe to the pronounced layer characterand to magnetic frustration due to the compet-ing exchange interaction in the RE triangularlayers. The large Tb magnetic moment of about9µB and application of the new D20 high in-tensity medium-resolution powder diffractome-ter at the Institute Laue-Langevin in Grenobleallows us to observe and to analyze the dif-fuse scattering in Tb2C2I2. In addition, long-range afm ordering was also investigated withthe diffractometer DMC at the Paul-Scherrer In-stitute in Villingen.

The phases RE2C2X2 crystallize in structureswhich contain doublelayers of close-packedmetal atoms with C2 units occupying the REoctahedral voids. These doublelayers are sand-wiched by layers of halogen atoms to formX–RE–C2–RE–X slabs which connect via vander Waals forces in stacks along the crystallo-graphicc-axis (Fig. 1). Two stacking variants(1s- and 3s-type) have been discerned, whichdiffer in the number of such slabs within the unitcell.

Figure 1: Perspective view of the the crystal structures of RE2C2X2, (X = I); 1s stacking variant left and 3sstacking variant right.

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Figure 2 displays the diffraction patterns of 3s-Ce2C2I2 at 30 K (in the paramagnetic regime)and at 2 K in the afm ordered state. The diffrac-tion pattern at 30 K was analyzed and fitted withFullprof powder profile fitting programs and therefined lattice and atomic parameters are con-sistent with results obtained from room temper-ature X-ray diffraction investigations.

Figure 2: Neutron diffraction patterns of Ce2C2I2

at 30 K (top) and 2 K (middle) collected at the highintensity medium-resolution powder diffractometerD20 (ILL, Grenoble). The lower panel contains thedifference pattern (2 K–30 K) in order to highlightthe magnetic reflections. The full lines representfits to the data (Æ) and their difference. The upperand lower vertical bars mark the positions of thenuclear and magnetic reflections, respectively. Awavelength ofλ= 2.56A was used in all measure-ments.

At 2 K weak additional Bragg reflections areobserved which can be attributed to magneticscattering from an ordered afm arrangement ofthe Ce moments. These magnetic Bragg re-flections are clearly seen in a difference pat-

tern (lower panel in Fig. 2). The magneticreflections can be indexed on the basis of aτ= [0,0,1/2] propagation vector to give a mag-netic unit cell ofa = 7.4848(3)A, b = 4.0736(2)A,

c = 21.5254(7)A, andβ= 100.69(1)Æ. The finalresults of the full magnetic Rietveld refinement(Rmag= 8.8%) indicate a magnetic moment of1.3(1)µB which is significantly reduced fromthe maximum possible moment of 2.14µB forthe 2F5�2 crystal field ground state of the Ce3�

ions. The Ce moments lie within theac-planeand enclose an angle of� 30Æ with the c-axisand alternate antiferromagnetically along thea-axis. The magnetic structure is depicted forone magnetic unit cell in Fig. 3.

Figure 3: Magnetic structure of Ce2C2I2. Only themagnetic (Ce) atoms are displayed within the con-text of the magnetic unit (the crystallographic unitcell is doubled along thec-axis).

A rather similar picture is observed for the ar-rangement of the magnetic moments in a sin-gle Nd–C2–Nd slab in 3s-Nd2C2I2. The mag-netic structure was refined from high-resolutionneutron powder patterns collected at the diffrac-tometer D1A (ILL, Grenoble). AtT = 2 K, sev-

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eral additional peaks are observed in the diffrac-tion patterns (not shown here) due to coher-ent magnetic scattering from the Nd3� mo-ments. Unlike for 3s-Ce2C2I2, these peaks cannow be indexed assuming identical crystallo-graphic and magnetic unit cells (propagationvector τ= [0,0,0]). The final Rietveld refine-ments reveal that the Nd magnetic moments– as in Ce2C2I2 – lie entirely within theac-planewith a magnitude of 3.4(1)µeff, and are ar-ranged with parallel orientation across the C–Cdimers. Compared to 3s-Ce2C2I2 the Nd mo-ments, however, are significantly more rotatedtowards theab-plane.

Figure 4: (a) Magnetic susceptibility of powdersample of Tb2C2I2, (b) temperature dependence ofthe electrical resistivity of a sintered pellet (c) tem-perature dependence of the integrated magnetic in-tensity in the vicinity of the magnetic Bragg reflec-tion 100, taken from powder diffraction data col-lected at D20 (ILL, Grenoble).

In contrast to Ce2C2I2 and Nd2C2I2, the mag-netic ordering behavior of 1s-Tb2C2I2 is muchmore complex. This finding is already indicated

from the magnetic and electrical bulk proper-ties. Tb2C2I2 shows afm ordering with twotransition temperatures. A first transition at�90 K is characterized by a broad maximum inthe magnetic susceptibility followed by a sec-ond sharp transition at 65 K (Fig. 4(a)).

The broad maximum in the susceptibility is atypical signature of short-range magnetic or-dering effects preceding the long-range order-ing transition at 65 K. The paramagnetic CurietemperatureθP is found to be negative indi-cating predominant afm exchange interaction.However, the magnitude ofθP is significantlyreduced as compared to these ordering tem-peratures. This reduction points to competingexchange interactions most likely due to geo-metrical frustration in the metal atom trianglespresent in the metal atom doublelayers but alsodue to possible cancellations of the long-rangeoscillatory RKKY (Ruderman-Kittel-Kasuya-Yosida) interaction. The long-range orderingtransition at 65 K is also visible in the electricalresistivity as a local maximum while the effectof the preceding magnetic short-range orderingon the resistivity is less prominent (Fig. 4(b)).

The temperature dependence of the neutronpowder diffraction of Tb2C2I2 as measured us-ing the diffractometer D20 (ILL, Grenoble) isshown in Fig. 5. This thermodiffractogram alsoreveals two magnetic transitions at about 85 Kand at 65 K. Below 65 K sharp additional mag-netic Bragg reflections are observed. Between85 K and 65 K these reflections broaden beyondexperimental resolution and become incom-mensurate with the underlying nuclear lattice.Above 85 K the magnetic reflections form a dif-fuse but structured magnetic background whichdecays with increasing temperature. We ascribethis diffuse part to incommensurate magneticscattering from short-range ordered areas in theTb metal atom layers. A detailed quantitativeanalysis of the diffuse part of the magnetic scat-tering is currently underway. Figure 4(c) dis-plays the integrated intensity around the mag-netic Bragg reflection 100 which is the most in-tense magnetic peak observed. A sharp increase

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of the intensity at about 60 K and a broader in-tensity increase around 90 K parallels the twotransitions seen in the bulk properties.

Figure 5: Thermodiffractogram (false color repre-sentation) of 1s-Tb2C2I2 taken at the high inten-sity medium-resolution powder diffractometer D20(ILL, Grenoble). Yellow and red colors mark zonesof high intensity (coherent nuclear and magneticBragg reflections). Light blue color indicates weakdiffuse magnetic scattering above�85 K. A wave-length ofλ= 2.56A was used in all measurements.

In a first approach to analyze the afm orderingprocesses of Tb2C2I2 we determined the mag-netic structure at 10 K by profile refinementsof neutron powder diffraction patterns collected

at the diffractometer DMC (PSI, Villingen).All magnetic Bragg reflections can be indexedbased on the commensurate propagation vectorτ= [0,0,0]. The Shubnikov group isP2�m�. Thefinal results of the Rietveld refinement of thepattern at 10 K are shown in Fig. 6. The mag-netic moment amounts to 8.0(1)µB which issomewhat smaller than the possible maximummoment (J = 6, gJ = 3/2) of a Tb3� ion (9µB).The Tb moments were found to be aligned intheac-plane with a collinear antiferromagneticarrangement as shown in Fig. 6. The momentspoint along the direction of the C–C dumbbellin antiparallel orientations.

Figure 6: Magnetic structure of Tb2C2I2 as refinedfrom a diffraction data set collected at 10 K at DMC(PSI Villigen). Only the magnetic atoms (Tb) areshown.

In summary, the antiferromagnetic ordering be-havior of the layered rare earth (RE = Ce, Nd,Tb) metal carbide halides RE2C2X2 (X = Br, I)has been investigated, and the magnetic struc-tures have been established from neutron pow-der diffraction (Tab. 1).

Table 1: Compilation of magnetic parameters of the investigated RE2C2X2. For 3s-Ce2C2Br2 magneticBragg reflections could not be observed in the diffraction patterns.

TN [K] θP [K] µsat [µB] µeff [µB] gJ�

J�J�1�

3s-Ce2C2Br2 13(1) –28(1) – 2.5(1) 2.533s-Ce2C2I2 15(1) –54(1) 1.3(1) 2.6(1) 2.533s-Nd2C2I2 32(1) –32(1) 3.4(1) 3.57(5) 3.621s-Tb2C2I2 65+85 –32(3) 8.0(1) 9.6(1) 9.72

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While Ce2C2X2 and Nd2C2X2 undergo ratherstandard transitions to long-range orderedcollinear afm phases, magnetic ordering inTb2C2I2 takes place in two consecutive stepsfrom a short-range ordered phase via an incom-mensurate intermediate phase to a long-rangeordered phase, the properties of which are sim-

ilar to those of the afm phase seen for Ce2C2I2

and Nd2C2I2. Due to the exceptional magnitudeof the Tb moments the short-range order showsup as sizeable diffuse scattering. The analysisof the diffuse scattering should allow us to studyin detail the character of the unusual short-rangeordering process in Tb2C2I2.

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