ANTIFERROMAGNETIC ORDER OF THE Cu IN RBa2Cu3O6+x

Abstract

JOURNAL DE PHYSIQUE Colloque C8, Supplement au no 12, Tome 49, decembre 1988 ANTIFERROMAGNETIC ORDER OF THE Cu IN RBa2Cu3O6+, J. W. Lynn (I), W.-H. Li (I), H. A. Mook (2), B. C. Sales (2) and Z. Fisk (3) (I) Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, MD 20742 ( U S A ) and National Bureau of Standards, Gaithersburg, M D 20899 U.S.A. (') Solid State Division, Oak Ridge National Laboratory, Oak Ridge, T N 37831 U.S.A. (3) MS-K764, Los Alamos National Laboratory, Los Alamos, N M 87545 U.S.A. Abstract. - Neutron diffraction measurements have been taken on single crystals of RBa2Cu3O6+, to explore the nature of the magnetic order a s a function of oxygen concentration x. The Cu-0 planes order antiferromagnetically at T N ~ with TN1 (a: % 0) N 450 K. At lower temperatures the oxygen-deficient Cu planes also order magnetically, with a substantial low temperature moment ( N 1/2 pB) and a simple antiferromagnetic sequencing along the c-axis. Both ordering temperatures much more sensitive to x than TN1. decrease with increasing x, with T N being The magnetic properties of the oxide superconduc- tors have been of particular interest due to the intrinsic interest in magnetism as well as because of the possi- bility that the magnetic fluctuations are responsible for the superconducting pairing [I]. The RBa2CusO6+, (R = rare earth) system orders antiferromagnetically [2-81 for small x (x (, 0.4), where the system is tetrag- onal and nonsuperconducting. The magnetic struc- ture consists [3, 41 of a simple antiparallel arrangement of Cu spins both within the Cu-0 planes as well as along the tetragonal c-axis, while the oxygen deficient Cu planes possess no ordered moment. The three- ~ M 0) dimensional ordering can be quite high ( T N (x 450 K) , indicating that the magnetic interaction en- ergies are large. The 3-d transition temperature is also very sensitive [2] to the oxygen concentration x, decreasing rapidly with increasing x and approaching zero for x 0.4. A second magnetic phase transition has recently been observed [5, 61, in which the oxygen-deficient Cu chain layers also order magnetically. We have been investigating the nature of this ordering and the ef- f6ct of oxygen concentration on T N and ~ have now ob- tained results on a number of single crystal specimens + ~ find : . that at small x and low T of N d B a 2 C ~ ~ 0 ~ We the basic magnetic structure consists of a simple an- tiferromagnetic arrangement of spins both within the planes as well as along the c-axis. In particular, we find an unexpectedly large magnetic moment on the Cu chain sites. At intermediate temperatures the spin arrangement is a superposition of the high T and low T configurations, and is noncollinear in nature. We also find that both transition temperatures decrease with increasing x, with the lower transition TN2 being much more sensitive to x than the higher transition T N ~ . The neutron measurements were carried out at the National Bureau of Standards Research Reactor using standard triple-axis instruments. Unpolarized diffrac- tion data were collected with an incident energy of either 14.8 or 13.7 meV using a pyrolytic graphite N N PG(002) monochromator. Polarized neutron measure- ments were then taken to establish that the new peaks are magnetic in origin, and this procedure has been described previously [4]. The Bragg peak correspond- reflection is shown in figure 1. ing to the Even though the Cu moments are quite small, the crys- tal is sufficiently large (N 50 mg) to obtain an excel- lent signal-to-noise ratio. Polarized neutron measure- ments were employed t o establish that the observed half-integral peaks are wholly magnetic in origin. The half-integral value for the third index demonstrates that the magnetic structure is doubled along all three crystallographic directions in comparison to the chem- ical unit cell. This contrasts with the spin configura- tion observed [3, 41 in the higher temperature magnetic phase, where the t index is integral and the magnetic and chemical unit cells are the same size along the tetragonal c-axis. Sample Rotation Fig. 1. - Observed magnetic Bragg peak for the peak at low temperatures. (f f t) peak The temperature dependence of the is shown in the bottom portion of figure 2. For this value of oxygen concentration (x M 0.1) the lower phase transition occurs at T N M ~ 80 K. The top por- tion of the figure compares the intensity of the whole- integral peaks such as 2) , which are associated Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888964