Long Periodic Magnetic Structure in CeB2C2

Abstract

In this paper, we provide information on the magnetic structure in tetragonal CeB2C2, which is isostructural with the antiferroquadrupolar (AFQ) ordering compound DyB2C2, and discuss peculiarities of CeB2C2 among the RB2C2 (R = rare earth) system. Some of the RB2C2 compounds, which have the tetragonal LaB2C2 type structure, 1) exhibit characteristic AFQ orderings with antiferromagnetic (AFM) orderings. For instance, DyB2C2 undergoes an AFQ ordering at TQ 1⁄4 24:7K, which is nearly ten times higher than those in other AFQ ordering compounds, and an AFM ordering at TN 1⁄4 15:3K with a complicated magnetic structure. Thus, the RB2C2 compounds are attracted our interests by characteristic AFQ and AFM orderings with tetragonal symmetry. Of this system, HoB2C2, TbB2C2 and ErB2C2 have long periodic magnetic ordering states with close periodicity in the c-plane. Moreover, in the long periodic states of HoB2C2 and TbB2C2, which show characteristic AFQ orderings, similar and anomalous magnetic diffuse scattering which can not be understood by only magnetic correlations was observed in the region surrounded by the satellite peaks by neutron diffraction. The close periodicity and the similar anomalous scattering in spite of the different 4f states implies that the long periodic states originate in some common mechanism in RB2C2; therefore, the long periodic states is worthy of clarifying in details to understand properties in RB2C2. Our preliminary neutron powder diffraction experiments suggest that CeB2C2 also has a long periodic magnetic ordering state; however, the periodicity and magnetic structure are not determined yet because of weakness of magnetic scattering intensity. To obtain more accurate information on the magnetic structure in CeB2C2, we performed neutron diffraction experiments on a single crystalline sample of CeB2C2. CeB2C2 undergoes a magnetic ordering at TN 1⁄4 7:3K with a sharp peak in the temperature dependence of specific heat CpðTÞ, and a successive transition at Tt 1⁄4 6:5K with a broad hump in CpðTÞ. The temperature dependence of magnetic susceptibility ðTÞ also shows an peak at TN when magnetic field is applied along the a and [110] directions without anisotropy in the c-plane, while no obvious anomaly was observed at Tt in ðTÞ. Since the c-plane is the magnetic easy plane, the magnetic moments lie in the c-plane. The single crystal for the neutron diffraction experiments was grown by the Czochralski method with a tri-arc furnace from the mixtures of 99.9% pure Ce, 99.5% pure B and 99.999% pure C. Natural B was enriched by B so that we avoided the strong absorption effect by B in natural B. The neutron diffraction experiments were performed on the Kinken (Institute for Materials Research) neutron diffractometer KSD installed at the reactor, JRR-3M, in Japan Atomic Energy Research Institute; the wave length was 1⁄4 1:52 A, and the collimation condition was 120-opensample-300. Figure 1 shows a Bragg peak found in this work at T 1⁄4 2:2K below Tt 1⁄4 6:5K. The magnetic origin is evident because the peak disappears above TN. From Gaussian fitting procedure, the centre of the peak was determined as (0.161(2), 0.161(2), 0.100(2)). The result indicates that the magnetic structure of CeB2C2 at 2.2K is a long periodic one with a propagation vector of k 1⁄4 ð ; ; 0Þ, where 1⁄4 0:161ð2Þ 1=6, 0 1⁄4 0:100ð2Þ 1=10. In fact, all the Bragg peaks observed at T 1⁄4 2:2K can be indexed with this propagation vector; therefore, the magnetic structure below Tt is represented with the single propagation vector, k 1⁄4 ð ; ; 0Þ. We can not discuss the correlation length of the magnetic ordering, because of the bad mosaicity of the sample. Figure 2 shows the temperature dependence of the integrated intensity of the peak in Fig. 1 with careful 150