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

The crystal and magnetic structures of SrFe2+2(PO4)2 have been determined by neutron powder diffraction data at low temperatures (space group P21/c (no. 14); Z=4; a=9.35417(13) Å, b=6.83808(10) Å, c=10.51899(15) Å, and β=109.5147(7)° at 15 K). Two magnetic phase transitions were found at T1=7.4 K (first-order phase transition) and T2=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 T1, 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. Spin canting of the outer Fe2 atoms provides a weak ferromagnetic moment of about 1 μB along the b-axis. The refined magnetic moments at 3.5 K are 4.09 μB for Fe1 and 4.07 μB for Fe2. Between T1 and T2, a few weak magnetic reflections were observed probably due to incommensurate magnetic order.

Graphical abstractTemperature dependence of the intensities of representative magnetic reflections (−101) and (−1,12,12) during cooling and heating in SrFe2(PO4)2.Figure optionsDownload full-size imageDownload as PowerPoint slide