Neutron powder diffraction study of the magnetic and crystal structures of SrFe2(PO4)2

The crystal and magnetic structures of SrFe22+(PO4)(2) have been determined by neutron powder diffraction data at low temperatures (space group P2(1)/c (no. 14); Z=4: a=9.35417(13)angstrom, b = 6.83808(10) angstrom, c = 10.51899(15) angstrom, and beta = 109.5147(7)degrees at 15 K). Two magnetic phase transitions were found at T-1 = 7.4 K (first-order phase transition) and T-2 = 11.4 K (second-order phase transition). The transition at T2 was hardly detectable by dc and ac magnetization measurements, and a small anomaly was observed by specific heat measurements. At T1, strong anomalies were found by dc and ac magnetization and specific heat. The structure of SrFe2(PO4)(2) consists of linear four-spin cluster units, Fe2-Fe1-Fe1-Fe2. Below T-1, the propagation vector of the magnetic structure is k [0,0,0]. The magnetic moments of the inner Fe1-Fe1 atoms of the four-spin cluster unit are ferromagnetically coupled. The magnetic moment of the outer Fe2 atom is also ferromagnetically Coupled with that of the Fe1 atom but with spin canting. The four-spin cluster units form ferromagnetic layers parallel to the [-101] plane, while these layers are stacked antiferromagnetically in the [-101] direction. Spill canting of the Outer Fe2 atoms provides a weak ferromagnetic moment of about 1 (mu B) along the b-axis. The refined magnetic moments at 3.5 K are 4.09 (mu B) for Fe1 and 4.07 (mu B) for Fe2. Between T-1 and T-2, a few weak magnetic reflections were observed probably due to incommensurate magnetic order. (C)2008 Elsevier Inc. All rights reserved.