- Email: [email protected]
Magnetic ordering in the erbium honeycomb lattices of ErX3 (X=Cl, Br, I)
Physica B 276}278 (2000) 674}675
Magnetic ordering in the erbium honeycomb lattices of ErX (X"Cl, Br, I) K.W. KraK mer *, H.U. GuK del , B. Roessli, P. Fischer, A. DoK nni, N. Wada, F. Fauth, M.T. Fernandez-Diaz, T. Hauss
Department for Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3000 Bern 9, Switzerland Laboratory for Neutron Scattering, Paul Scherrer Institute and ETH Zu( rich, CH-5232 Villigen PSI, Switzerland Department of Physics, Niigata University, Niigata 950-2181, Japan Institute of Physics, Graduate School of Arts and Sciences, University of Tokyo, Tokyo 153-8902, Japan Institute Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France Hahn-Meitner Institute, Glienicker Strasse 100, D-14109 Berlin, Germany
Abstract Unusual two-dimensional (2D) and three-dimensional (3D) magnetic ordering phenomena were observed in the insulating, transparent erbium trihalides ErX (X"Cl, Br, I) in the milli-Kelvin temperature range. The layer-type crystal structures of ErCl (AlCl -type structure) and ErBr /ErI (BiI -type structure) are closely related. All three compounds show a new type of two-sublattice 1203 antiferromagnetic order in the erbium honeycomb layers with an in"nite rotational degeneracy. The magnetic structures were determined by powder and single-crystal neutron di!raction. ErCl shows short-range 2D magnetic order above 350 mK to several Kelvin and a transition to 3D magnetic order at 350 mK with a k-vector of (, 0, ! ). ErBr and ErI display short-range 2D magnetic order from 400 mK to several Kelvin, long-range 2D order between 280 and 400 mK, and short-range 3D order below 280 mK with a correlation length of 15 As along the c-axis and k"(, , 0). The di!erences between ErCl and ErBr /ErI are due to di!erent geometrical layer stackings. For ErBr /ErI a threefold ambiguity in the magnetic layer stacking causes the disorder along the c-axis. The ordered magnetic moments at saturation increase from 3.3 to 4.7 and 5.5 l /Er> for ErCl , ErBr and ErI , respectively. 2000 Elsevier Science B.V. All rights reserved. PACS: 75.25.#Z; 75.40.!s; 61.12.Ld Keywords: Antiferromagnetism; Triangular systems; Two-dimensional systems
The anhydrous rare-earth halides MX are an interest ing group of compounds to study magnetic interactions between rare-earth ions in insulating lattices. The magnetic cations M> as well as the anions X\ can be varied to investigate di!erent cations in the same host lattice or the in#uence of the crystal lattices on a selected cation. The latter was done for the erbium halides ErX with X"F, Cl, Br, and I. Results on ErF were reported * Corresponding author. Tel.: #41-31-631-4248; fax: #4131-631-4399. E-mail address: [email protected] (K.W. KraK mer)
earlier [1] and a paper on ErX with X"Cl, Br, and I was published recently [2]. A detailed account on ErCl is submitted for publication and one on ErBr /ErI in preparation. Due to the shielding of the 4f electrons and long Er}Er distances the magnetic interactions are weak. This is the "rst magnetic structure determination of layer-type rare-earth halides. Magnetic susceptibility data down to the ordering temperature were published for ErCl [3]. ErCl and ErBr were prepared according to the NH X-route [4]. ErI was synthesized from the ele ments. All compounds were sublimed in a silica apparatus under vacuum for puri"cation. Single crystals were
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 7 2 4 - X
K.W. Kra( mer et al. / Physica B 276}278 (2000) 674}675
Fig. 1. 2D magnetic structure of ErX (X"Cl, Br, and I) in the a}b plane. The two magnetic sublattices are denoted by full and open symbols. The dashed and full lines mark the size of the crystallographic and magnetic unit cells for ErBr /ErI , respec tively.
grown in sealed silica ampoules by the Bridgman technique. Powder neutron measurements were done on the D1A and D1B di!ractometers at the Institute LaueLangevin in Grenoble. Neutron single-crystal investigations were performed at the V1 di!ractometer at the Hahn-Meitner Institute in Berlin. Heat capacity measurements on an ErBr crystal were performed at the University of Tokyo. The low-ordering temperatures require He/He dilution cryostats and the hygroscopicity of the halides absolutely moisture free handling. ErCl crystallizes in the AlCl -type structure with the monoclinic space group C2/m. ErBr and ErI both crystallize in the BiI -type structure with the rhom bohedral space group R-3. Both structures contain distorted ErX -octahedra which are connected to three neighboring ones by common edges. These ErX -layers are stacked along the c-axis and held together by vander-Waals forces. The Er>-ions form honeycomb nets with the same 2D magnetic structure for all the three compounds as shown in Fig. 1. The two magnetic sublattices are denoted by full and open symbols, respectively.
675
Each sublattice represents a triangular net with a 1203 antiferromagnetic spin arrangement in the a}b plane. The absolute in-plane orientation of the spins is not "xed. The rotation of all individual spins of one sublattice by the same arbitrary in-plane angle u accompanied by the rotation of the spins of the second sublattice by the angle!u yields 2D spin arrangements which are indistinguishable with respect to di!raction and energy. The geometrical layer stacking of ErCl is di!erent from ErBr /ErI and results in dissimilar magnetic be havior. ErCl undergoes a transition to 3D order at ¹ "350 mK. The higher symmetric ErBr /ErI -latti, ces show a threefold ambiguity for the relative spin orientation between adjacent layers. This leads to a short-range 3D magnetic order below ¹ "280 mK , with a correlation length of 15 As along the c-axis. The temperature dependence of the magnetic neutron peak intensity of the powder measurements compared to those of the single crystals and the heat capacity measurement on ErBr reveals that only 20% of the magnetic entropy is dissipated by 3D order. The major part occurs from ¹ upwards as long-range and short-range 2D order. , This study was "nancially supported by the Swiss National Science Foundation. References [1] K. KraK mer, H. Romstedt, H.U. GuK del, P. Fischer, A. Murasik, M.T. Fernandez-Diaz, Eur. J. Solid State Inorg. Chem. 33 (1996) 273. [2] K. KraK mer, H.U. GuK del, B. Roessli, P. Fischer, A. DoK nni, N. Wada, F. Fauth, M.T. Fernandez-Diaz, T. Hauss, Phys. Rev. (B) 60 (1999) R3724. [3] C.W. Fairall, J.A. Cowen, E. Grabowski, Phys. Lett. A 35 (1971) 405. [4] G. Meyer, Inorg. Synth. 25 (1989) 146.
Magnetic ordering in the erbium honeycomb lattices of ErX (X"Cl, Br, I) K.W. KraK mer *, H.U. GuK del , B. Roessli, P. Fischer, A. DoK nni, N. Wada, F. Fauth, M.T. Fernandez-Diaz, T. Hauss
Department for Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3000 Bern 9, Switzerland Laboratory for Neutron Scattering, Paul Scherrer Institute and ETH Zu( rich, CH-5232 Villigen PSI, Switzerland Department of Physics, Niigata University, Niigata 950-2181, Japan Institute of Physics, Graduate School of Arts and Sciences, University of Tokyo, Tokyo 153-8902, Japan Institute Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France Hahn-Meitner Institute, Glienicker Strasse 100, D-14109 Berlin, Germany
Abstract Unusual two-dimensional (2D) and three-dimensional (3D) magnetic ordering phenomena were observed in the insulating, transparent erbium trihalides ErX (X"Cl, Br, I) in the milli-Kelvin temperature range. The layer-type crystal structures of ErCl (AlCl -type structure) and ErBr /ErI (BiI -type structure) are closely related. All three compounds show a new type of two-sublattice 1203 antiferromagnetic order in the erbium honeycomb layers with an in"nite rotational degeneracy. The magnetic structures were determined by powder and single-crystal neutron di!raction. ErCl shows short-range 2D magnetic order above 350 mK to several Kelvin and a transition to 3D magnetic order at 350 mK with a k-vector of (, 0, ! ). ErBr and ErI display short-range 2D magnetic order from 400 mK to several Kelvin, long-range 2D order between 280 and 400 mK, and short-range 3D order below 280 mK with a correlation length of 15 As along the c-axis and k"(, , 0). The di!erences between ErCl and ErBr /ErI are due to di!erent geometrical layer stackings. For ErBr /ErI a threefold ambiguity in the magnetic layer stacking causes the disorder along the c-axis. The ordered magnetic moments at saturation increase from 3.3 to 4.7 and 5.5 l /Er> for ErCl , ErBr and ErI , respectively. 2000 Elsevier Science B.V. All rights reserved. PACS: 75.25.#Z; 75.40.!s; 61.12.Ld Keywords: Antiferromagnetism; Triangular systems; Two-dimensional systems
The anhydrous rare-earth halides MX are an interest ing group of compounds to study magnetic interactions between rare-earth ions in insulating lattices. The magnetic cations M> as well as the anions X\ can be varied to investigate di!erent cations in the same host lattice or the in#uence of the crystal lattices on a selected cation. The latter was done for the erbium halides ErX with X"F, Cl, Br, and I. Results on ErF were reported * Corresponding author. Tel.: #41-31-631-4248; fax: #4131-631-4399. E-mail address: [email protected] (K.W. KraK mer)
earlier [1] and a paper on ErX with X"Cl, Br, and I was published recently [2]. A detailed account on ErCl is submitted for publication and one on ErBr /ErI in preparation. Due to the shielding of the 4f electrons and long Er}Er distances the magnetic interactions are weak. This is the "rst magnetic structure determination of layer-type rare-earth halides. Magnetic susceptibility data down to the ordering temperature were published for ErCl [3]. ErCl and ErBr were prepared according to the NH X-route [4]. ErI was synthesized from the ele ments. All compounds were sublimed in a silica apparatus under vacuum for puri"cation. Single crystals were
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 7 2 4 - X
K.W. Kra( mer et al. / Physica B 276}278 (2000) 674}675
Fig. 1. 2D magnetic structure of ErX (X"Cl, Br, and I) in the a}b plane. The two magnetic sublattices are denoted by full and open symbols. The dashed and full lines mark the size of the crystallographic and magnetic unit cells for ErBr /ErI , respec tively.
grown in sealed silica ampoules by the Bridgman technique. Powder neutron measurements were done on the D1A and D1B di!ractometers at the Institute LaueLangevin in Grenoble. Neutron single-crystal investigations were performed at the V1 di!ractometer at the Hahn-Meitner Institute in Berlin. Heat capacity measurements on an ErBr crystal were performed at the University of Tokyo. The low-ordering temperatures require He/He dilution cryostats and the hygroscopicity of the halides absolutely moisture free handling. ErCl crystallizes in the AlCl -type structure with the monoclinic space group C2/m. ErBr and ErI both crystallize in the BiI -type structure with the rhom bohedral space group R-3. Both structures contain distorted ErX -octahedra which are connected to three neighboring ones by common edges. These ErX -layers are stacked along the c-axis and held together by vander-Waals forces. The Er>-ions form honeycomb nets with the same 2D magnetic structure for all the three compounds as shown in Fig. 1. The two magnetic sublattices are denoted by full and open symbols, respectively.
675
Each sublattice represents a triangular net with a 1203 antiferromagnetic spin arrangement in the a}b plane. The absolute in-plane orientation of the spins is not "xed. The rotation of all individual spins of one sublattice by the same arbitrary in-plane angle u accompanied by the rotation of the spins of the second sublattice by the angle!u yields 2D spin arrangements which are indistinguishable with respect to di!raction and energy. The geometrical layer stacking of ErCl is di!erent from ErBr /ErI and results in dissimilar magnetic be havior. ErCl undergoes a transition to 3D order at ¹ "350 mK. The higher symmetric ErBr /ErI -latti, ces show a threefold ambiguity for the relative spin orientation between adjacent layers. This leads to a short-range 3D magnetic order below ¹ "280 mK , with a correlation length of 15 As along the c-axis. The temperature dependence of the magnetic neutron peak intensity of the powder measurements compared to those of the single crystals and the heat capacity measurement on ErBr reveals that only 20% of the magnetic entropy is dissipated by 3D order. The major part occurs from ¹ upwards as long-range and short-range 2D order. , This study was "nancially supported by the Swiss National Science Foundation. References [1] K. KraK mer, H. Romstedt, H.U. GuK del, P. Fischer, A. Murasik, M.T. Fernandez-Diaz, Eur. J. Solid State Inorg. Chem. 33 (1996) 273. [2] K. KraK mer, H.U. GuK del, B. Roessli, P. Fischer, A. DoK nni, N. Wada, F. Fauth, M.T. Fernandez-Diaz, T. Hauss, Phys. Rev. (B) 60 (1999) R3724. [3] C.W. Fairall, J.A. Cowen, E. Grabowski, Phys. Lett. A 35 (1971) 405. [4] G. Meyer, Inorg. Synth. 25 (1989) 146.