Magnetic and Charge Ordering Transitions of PrBaCo2O5

Abstract

In R2 xAxCo2O5þ (A = alkali earth elements; R 1⁄4 Y and rare earth elements), the spin state change can be easily induced by varying temperature T and/or by changing R2 xAx and . 1–9) The local O atom arrangement around the Co sites is also important for the determination of the spin state. Because their physical properties are sensitive to the spin state, we can control them by properly choosing these material parameters. To investigate how the spin state depends on these parameters, we have carried out neutron diffraction studies on single crystals of RBaCo2O5:5 (R 1⁄4 Tb and Nd), having the linkages of CoO6 and CoO5 polyhedra 6,8) and reported the magnetic structures of their magnetically ordered phases. We have also reported results of similar neutron studies on NdBaCo2O5 with the linkage of CoO5 pyramids 7) and shown that the system exhibits the antiferromagnetic ordering at 360K and the charge ordering at 250K. Results of these studies and other kinds of studies carried out by the authors’ group indicate that for the smaller ionic radius of R, the magnitude of the Co moment is smaller. It has been also found that in the CoO6 octahedra is smaller than that in the CoO5 pyramids. These results can be understood by considering the crystal field strength at the Co sites. In the present work, neutron studies on a single crystal of PrBaCo2O5 with the similar structure to that of NdBaCo2O5 7) have been carried out, where successive transitions to the antiferromagnetic and the charge ordered state have been observed with decreasing T at TN 360K and TCO 255K, respectively. The magnetic structures have been analyzed at T 1⁄4 300 and 15K. The crystal was grown by the floating zone method. The measurements were carried out by using the T1-1 triple axis spectrometer installed at JRR-3 of JAERI. We have observed several superlattice reflections as well as the fundamental ones for the unit cell with the size of ap ap 2ap, where ap is the lattice parameter of the pseudo cubic perovskite cell. Because the sample has the a and b -domains, the indices hkl may be khl. The integrated intensities of two superlattice reflections, for example, are plotted against T in Fig. 1. The superlattice reflections at Q 1⁄4 ðh=2; h=2; lÞ with odd h, which correspond to the antiferromagnetic order, appear at TN 360K with decreasing T . With further decreasing T , another set of peaks at Q 1⁄4 ðh=2; 0; lÞ or ð0; h=2; lÞ with odd h appears at TCO 255K. As is shown later, the latter peaks indicate the existence of two distinct Co-sites with different magnetic moments, implying that the charge ordering takes place at TCO. The observed profile widths are found to be broadened below TCO due to the structural distortion induced by this charge ordering. The magnetic and crystal structures have been optimized simultaneously, because the crystal structure changes at the temperatures TN and TCO. 3–5) (Space groups are P4/mmm, Pmmm and Pmma at T > TN, TCO < T < TN and T < TCO, respectively.) In the present analyses, the Co moments at crystallographically equivalent sites are assumed to have equal magnitudes. It is also assumed that the a and b domains have equal weights. For the magnetic form factors, the isotropic value is used. The absorption corrections are made. At 300K, the chemical unit cell with the size of ap ap 2ap is used in the analysis, where the G-type antiferromagnetic structure is found to be realized. (At this temperature, the white and gray pyramids in Fig. 2 are crystallographically equivalent.) The Co moment is within the ab plane and 1⁄4 2:28 0:05 B for each Co2:5þ. Because the lattice parameters a and b are very close to each other, we cannot determine its direction within the ab plane because of the domain distribution. In Table I, the calculated intensities of the magnetic reflections are compared with the observed data, where the agreement is found to be reasonably well. At 15K, we use the space group Pmma, where the chemical unit cell is doubled along a, as shown in Fig. 2 by the white and gray pyramids. Because the magnetic reflections, which appear at TN with decreasing T do not exhibit any anomalies at TCO, we consider that the G-type magnetic structure is basically preserved below TCO, though the magnitudes of at the distinct sites may not be equal. If the magnitudes of the Co moments in the white and gray pyramids in Fig. 2 are equal, the h=200 and 0h=20 magnetic reflections with odd h are forbidden for the G-type 0 2 10 4 4 10 4 6 10 4 8 10 4 1 10 5 1.2 10 5