Novel lowdimensional spin 1/2 antiferromagnets: Twohalide exchange pathwaysin A2CuBr4 saltsP. Zhou, J. E. Drumheller, G. V. Rubenacker, K. Halvorson, and R. D. Willett Citation: Journal of Applied Physics 69, 5804 (1991); doi: 10.1063/1.347883 View online: http://dx.doi.org/10.1063/1.347883 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/69/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetic Phase Diagram of the QuasiTwoDimensional S = 1/2 Antiferromagnet Cs2CuBr4 AIP Conf. Proc. 850, 1093 (2006); 10.1063/1.2355085 Perturbative approximation scheme for isolated impurity bonds in the twodimensional spin1/2 Heisenbergantiferromagnet J. Appl. Phys. 75, 5532 (1994); 10.1063/1.355679 Raman spectra of twodimensional spin1/2 Heisenberg antiferromagnets J. Appl. Phys. 75, 6340 (1994); 10.1063/1.355393 Isolated ferromagnetic bonds in the twodimensional spin1/2 Heisenberg antiferromagnet (abstract) J. Appl. Phys. 69, 4902 (1991); 10.1063/1.348220 Observation of the dynamic behavior of the antiferromagneticferromagnetic phase transition in the onedimensional spin1/2 antiferromagnet αCuNSal J. Appl. Phys. 50, 1859 (1979); 10.1063/1.327145

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Novel Ilow-dimensional spin l/2 antiferromagnets: Two-halide exchange pathways in A,CuBr~ salts

P. Zhou and J. E. Drumheller Department of Physics, Montana State University, Bozeman, Montana 59707

G. V. Rubenacker, K. Halvorson, and R. D. Willett Department of Chemistry, Washington State University, Pullman, Washington 99164

Magnetic susceptibility studies of several AzCuBr4 salts, where A is an organoammonium cation, are reported. These salts contain tetrahedrally distorted CuBri - anions. Magnetic exchange pathways exist- through interanionic Br* . * Br pathways. In this manner, various types of new antiferromagnetic spin l/2 magnetic systems have been obtained. As examples, a one-dimensional chain, a coupled pair of chains or “ladder,” a two-dimensional square lattice, and a distorted “honeycomb” lattice are discussed.

INTRODUCTION

The design of new and novel low-dimensional mag- netic insulators continues to be a challenge in lattice engi- neering. Most strategies for design of transition metal com- plexes focus on the manipulation of the local electronic structure of the metal ion or the electronic and geometric nature -of the metal-ligand-metal linkages.’ In our labora- tories, much of the attention has focused on copper (II) halide salts.’ The simplicity of the electronic structure of the halide ion, coupled with the inherent flexibility of the Cu( II) coordination sphere (due to the Jahn-Teller ef- fect), has led to a host of oligomeric and low-dimensional spin l/2 systems. Included have been a series of 1D and 2D systems with varying degrees of isolation and varying anisotropy” as well as oligomeric species containing from 2 to 7 Cu atoms.7-10 In all of these systems, the magnetic coupling occurs via CLI-X-Cu linkages (X = Cl and Br), with FM behavior when the magnetic orbitals are orthog- onal and AFM behavior when substantial overlap occurs.

cent CuBrz - anions. Among these parameters will be the interionic Br*0 *Br distance and the dihedral angle r be- tween the two Cu-Br* * -Br planes. The purpose of this pa- per is to present representative results on four of these systems to demonstrate their versatility for developing new and novel AFM spin l/2 magnets. Included are com- pounds for which the Cu-Br. * *Br-Cu contacts lead to a one-dimensional chain, a ladder (coupled pair of chains ) , a square lattice, and a distorted honeycomb lattice.

EXPERIMENT

Powder magnetic susceptibility data were taken on a EG&G Model 155 vibrating sample magnetometer in ap- plied fields up to 5000 Oe from 4.2 to 60 K. No field dependence was found down to 4.2 K. The magnetometer with a variable-temperature helium cryostat system was calibrated with a nickel crystal obtained from the National Bureau of Standards. The data are shown in Figs. 1-3 and plotted as x~T vs T.

More recently, it has been realized that significant magnetic coupling, of an antiferromagnetic nature,- can be achieved via Cu-X- * *X-Cu contacts, particularly when X is Br. Thus the layer perovskite ‘series (NHsRNHs) CuBr, transforms from 2D FM-behavior for large inter- layer separations to 1D AFM behavior when the interlayer Br- * *Br contacts become very short.’ 1**2 Similar examples can be found in a wide variety of copper (II) halides.13-15

RESULTS

We have begun an investigation of a series of A,CuBr4 salts, where A is an organoammonium cation, containing distorted tetrahedral CuBrtL anions. Interan- ionic Bra * *Br contacts, with distances from 3.7-4.5 A-, pro- vide magnetic exchange pathways. defining a variety of magnetic system with AFM values of IJ/kl ranging up to 20 K. While Br * * Br contacts will define the type of mag- netic lattice to be found, the. actual value of the exchange constant will depend on the extent of overlap of the mag- netic orbitals from the two centers. Thus, it will be a func- tion of the local geometry, primarily the extent of flat- tening of the distorted tetrahedral coordination sphere, measured by the average value (0) of the two larger ( 1 109”) Br-Cu-Br angles. It will also be a complicated function of the distance between and orientation of adja-

All four of the representative A$uBr, salts exhibit antiferromagnetic .coupling, with values of J/k ranging from approximately - 3 to - 22 K. The structural and magnetic properties are summarized in Table I, while the magnetic susceptibility data are given in Figs. l-3. The data were, in each case, fit to a model for nearest-neighbor Heisenberg spin l/2 AFM systems, with the Hamiltonian defined by

The symbol Jir- within the summation over nearest- neighbor spins is ‘to indicate that more than one Cu- Br- * -Br-Cu exchange may be present in a single structure.

%@= - 2 $ ‘JjjSj*Sja I

The 1D system in the NMPH salt is embedded in a monoclinic C-centered lattice, l6 with adjacent anions in the chain related by a 2i screw axis symmetry operation. The Br- * *Br distance is 4.129 A. A fit of the data to the Bonner-Fisher model for a ID spin l/2 AFM17 yielded a value of J/k= - 4.0 K (solid curve in Fig. 1).

5804 J. Appl. Phys. 69 (8), 15 April 1001 0021-8979/91/085804-03$03.00 (03 1991 American Institute of Physics 5804

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__~_ .I.._I...__I- - o.5 i

__ __~. _~--I -I

0.4

3

i

a 0.3 i

j I

2 0.2

I- E i

x O.ij

i o.o& mrrrrrpm-~s ..,... ~I ,........,. . . . ..Yr-T-r-- J

0 10 20 30 40 50 60

TEMPERATURE (K)

FIG. 1. Plot of xmT vs T for the NMPH (X ) and 5MAP (0) salts. FIG. 2. Plot of x,T vs T for the 2AP salts.

The 2AP system has a more complex structure” which defmes a magnetic ladder system, as seen schematically in Fig. 4(a). In the triclinic system, chains of anions are re- lated by unit cell translations with a Bra . *Br distance of 4.137 A. This exchange pathway (Jz) defines the rails in the ladder. Pairs of chains are in contact to form the lad- der. The Br* * *Br distance of 4.369 L% between chains de- fines the pathway JI, corresponding to the rungs of the ladder. The susceptibility data can be fit reasonably well to a model consisting of Bleaney-Bowers expression for spin l/Z dimers, with a mean-field correction for interdimer coupling (dashed line in Fig. 2), to yield J,/k = - 3.1 K and J,/k = - 0.2 K. We have also derived19

a power series (to fourth order) for a ladder system and obtain values of J,/k = - 3.4 K and Jg‘k = - 2.1 K for the fit shown by the solid line in Fig. 2. A substantial improvement in the quality of the fit is evident. The ex- pression for the susceptibiIity is

,,-,,,kr[l+~(~J,+J,)+(~)‘( -;J;

+JIJz) + (&)‘i

TABLE I. hlagnetic and structural characteristics of selected A,CuBq salts.

A’ Br...Br distance e (cWb 7 (deg)” J/k(K)

NMPH 4.129 142.1 180 - 4.0 2AP 4.137 m 132.5 54.5 - 2.1

4.369 (J,) 0 - 3.4 5MAP 4.545 137.2 15.5 - 3.6 MOR 3.819 (Jd 134.6 53.0 - 4.4

4.100/4.221(J,) 20.2/o - 21.3

‘NMPH = N-methylphenethylammonium, (C-&W+ 1; 2AP = 2- aminopyridinium, (C,H,N: ); 5MAP = S-methyl-2-aminopyridinium, (C,HoN2f ); MOR = morpholinium, (C!.+HIoNO+ ).

bAverage value for the larger pair of Br-Cu-Br angles in the CuBr$- anion.

“Dihedral angle between the two Cu-Br . . *Br planes.

z ?i E 0.3

2

E 0.2 I- E

x 0.1

0 10 20 30 40 50 60

TEMPERATURE (K)

-iJi) + (&)*( +;J‘&-fJ:J;+$J;J;

- fJ,J:&J: )I .

The SMAP salt defines a square 2D magnetic system with neighboring CuBri - anions occupying C-centered sites in the monoclinic lattice2’ to give Bra *Br contact distances.of 4.545 A. A maximum is observed in XM at a temperature just above 4.2 K. The data, fit to a 2D spin l/2 AFM Heisenberg mode1,21 yielded J/k= - 3.2 K, as shown in Fig. 1 (dashed curve).

The final example is the distorted honeycomb lattice in the MOR salt, 4(b) shown in Fig. 4(b). Pairs of CuBr$ -, located about centers of inversion in the structure, have multiple Br. . .Br contacts (three total) to define an ex- change pathway designated as J1. Contacts between neigh- boring pairs, designated as the J2 pathway, complete the distorted honeycomb lattice. The susceptibility reaches a maximum near 25 K. Assuming that the J1 pathway is dominant, the data was fit to a dimer model but with power series corrections to fourth order for the inclusion of the J2 pathway. The expression for XM is

0.4 3 ‘; E ‘; 0.3 E 2

% “’ x

0.1

0.0 ----w-v -Y.-F-- -y--

TEMPERATURE IKI

FIG. 3. Plot of xmT vs T for the MOR salt.

5805 J. Appl. Phys., Vol. 69, No. 8,15 April 1991 Zhou et a/. 5805 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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!L! 121 I .==xz-.

!-!

/--*(-*y*~2 / \ i-

1-1 >-, / “.,

/ -.‘, ;-

FIG. 4. (a) Diagram illustrating exchange pathways for the magnetic ladder system in the 2AP salt. (b) Diagram illustrating exchange path- ways for the distorted honeycomb system in the MOR salt.

3 + exp( _ 2J,,T) + J,/kT+

which yields a value of J,/k = - 21.3 K and J,/k = - 4.4 K.

DISCUSSION

It has been shown that the magnetic exchange path- ways associated with Br. **Br contacts between CuB&- anions can lead to a variety of novel low-dimensional spin l/2 Heisenberg antiferromagnets. The type of magnetic lattice obtained depends upon the manner in which space- group symmetry elements replicate the significant ex- change pathways. The value of the exchange coupling will, in turn, depend upon the parameters defining the geomet- rical relationship between the two interacting CuBri- an- ions. A strong dependence upon the Br. ..Br distance is anticipated, and is found experimentally to vary as d- ‘, where d is the Bra * *Br distance. I2 Because of the shape of the magnetic orbital containing the unpaired electron in the CuBr$- anions, the value of the exchange coupling should also depend upon the dihedral angle (7) between Cu-Br* . *Br planes in the Cu-Br. . *Br-Cl linkage. It can be argued the 1 J/k] will be a maximum when r = 0” or 1 80”.i8 These two factors provide a crude rationalization for the J/k values reported in Table I. Other angular parameters,

such as the Cu-Bra . *Br angle will also affect the value of J. These effects will be discussed in subsequent papers.

ACKNOWLEDGMENTS

This work was supported by the National Science Foundation under Grant Nos. DMR x7-02933 and DMR 88-03382.

LMagneto-Structural Correlations in Exchange Coupled Systems, edited by R. D. Willett, D. Gatteschi, and 0. Kahn, NATO ASI Series Cl40 (Reidel, Dordrecht, 1985).

‘H. A. Groenendijk, H. W. J. Bliite, A. J. van Duyneveldt. R. Gaura, C P. Landee, and R. D. Willett, Physica B/C 106,45 (1981); R. Hooger- beets, S. A. J. Weigers, A. J. van Duyneveldt, R. D. Willett, and U. Geiser, ibid. 125, 135 (1984).

‘U. Geiser, R. M. Gaura, R. D. Willett, and D. X. West, Inorg. Chem. 25, 4203 (1986).

4 (a) R. D. Willett, R. J. Wong, and M. Numata, Inorg. Chem. 22, 3184 (1983): (b) R. D. Willett, H. Place, and M. Middleton, J. Am. Chem. Sot. 110, 8639 (1988).

‘M. R. Bond, R. D. Willett, and R. Rubenacker, Inorg. Chem. 29,2713 (1990); C P. Landee, A. Djili, D. F. Mudgett, M. Newhall, H. Place, B. Scott, and R. D. Willett, ibid. 27, 620 (1988).

‘L. 0. Snively, P. L. Seifert, K. Emerson, and J. E. Drumheller, Phys. Rev. B 20, 2101 (1979).

‘R. D. Willett, T. Grigereit, K. Halvorson, and B. Scott, Proc. Indian Acad. Sci. Chem. Sci. 98, 147 (1987):

sB. Scott and R. D. Willett, J. Appl. Phys. 61, 3289 (1987). ‘C. E. Zaspel, G. V. Rubenacker, S. L. Hutton, J. E. Drumheller, R. D.

Willett, and M. R. Bond, J. Appl. Phys. 63, 3028 (1988); M. R. Bond, R. D. Willett, R. S. Rubins, P. Zhou, C. S. Zaspel, S. L. Hutton, and J. E. Drumheller, Phys. Rev. B (in press).

‘OP. Zhou, J. E. Drumheller, G. V. Rubenacker, M. Bond, and R. D. Willett, J. Phys. (Paris) 49, C8-147 (1988).

‘*G V Rubenacker, D. N. Haines, J. E. Drumheller, and K. Emerson, J. Magi. Magn. Mater. 43, 238 (1984).

“K Halvorson and R. D. Willett, Acta Crystallogr. Sect. C 44, 2071 (1988).

13J. T. Blanchette and R. D. Willett, Inorg. Chem. 27, 843 (1988). 14D. B. Brown, J. W. Hall, H. M. Helis, E. G. Walton, D. J. Hodgson,

and W. E. Hatfield, Inorg. Chem. 16, 2675 (1977). “B R. Patyal, B. Scott, and R D. Willett, Phys. Rev. B 41, 1657 (1990). 16H. Place and R. D. Will&t, Acta Crystallogr. Sect. C 44, 34 (1988). “J C. Bonner and M. E. Fisher, Phys. Rev. 135, 640 (1965). 18K. Halvorson, M. S. thesis, Washington State University (1989). “G S Rushbrooke, G. A. Baker, and P. J. Wood, Phase Transitions . .

Critical Phenomena, edited by C. Domb and M. S. Green (Academic, New York, 1974), Vol. 3, pp. 245-356.

“H. Place and R. D. Willett, Acta Crystallogr. Sect. C 43, 1050 (1987). “G. S. Rushbrooke and P. T. Wood, Mol. Phys. 1, 257 (1958); M. E.

Lines, J. Phys. Chem. Solids 31, 101 (1976).

5806 J. Appl. Phys., Vol. 69, No. 8.15 April 1991 Zhou eialr 5806

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