Ferromagnetic Spin-Ordering in Photo-active RbMn[Fe(CN)6]

Abstract

In RbMn[Fe(CN)6], 1,2) two kinds of magnetic elements, i.e., Fe and Mn, form a three-dimensional NaCl-type structure bridged by CN ions. Recently, Tokoro et al. has reported suppression of magnetization by irradiation of one-pulse laser shot (532 nm, 10mJ cm ) at 3 K. This observation suggests photo-induced ferromagnetic-to-antiferromagnetic transition, perhaps mediated by some photoinduced structural change. To clarify this novel photoinduced phenomenon, we first have to determine the spin structure of the ground state (before photo-irradiation). Crytallographically, RbMn[Fe(CN)6] belongs to the tetragonal space group (I4m2; Z 1⁄4 2) at low temperatures, in which Fe and Mn elements locate at 2a and 2b site, respectively. The low temperature valence state is considered to be high-spin Mn3þ (d; S 1⁄4 2) and low-spin Fe2þ (d; S 1⁄4 0) state. In this Note, we report the spin structure of RbMn[Fe(CN)6] determined by neutron powder diffraction by the Kinken powder diffractometer for high efficiency and high resolution measurements (HERMES). The observed magnetic Bragg reflection pattern is well reproduced by ferromagnetic alignment of the Mn3þ local spins along c. The Curie temperature is determined to be TC 1⁄4 13ð1ÞK. We will discuss a possible origin for the ferromagnetic coupling between the Mn3þ local spins. RbMn[Fe(CN)6] was prepared by reacting an aqueous solution (0.1mol dm ) of MnCl2 with a mixed aqueous solution of RbCl (1mol dm ) and K3[Fe(CN)6] (0.1mol dm ) to yield a light brown precipitate. With use of a SQUID magnetometer, we have measured temperature variation of magnetization M under a field of 10G. TheM{T curve increases steeply below 12K. The saturated magnetization is 3:3 B (at 7 T) at 3K. Neutron powder diffraction measurements were performed using the Kinken powder diffractometer for high efficiency and high-resolution measurements (HERMES) of Institute for Materials Research, Tohoku University, installed at the JRR-3M reactor at the Japan Atomic Energy Research Institute, Tokai, Japan. Neutrons with wavelength 1.8207 A were obtained by the (331) reflection of the Ge monochromator, and 120-B-Sample-180 collimation. The RbMn[Fe(CN)6] powders ( 1 g) sealed in a vanadium capsule (9mm ) with helium gas, and mounted at the cold head of a closed-cycle He-gas refrigerator. The diffraction data were integrated for 3 h. First, we investigated the lattice structure at 20K by Rietveld structural analysis (RIETAN2000) with the same model as ref. 4. Reflecting the Jahn–Teller instability of the Mn3þ (d) ions, the MnN6 octahedra are elongated along c: the Mn–N1 (out-of-plane) and Mn–N2 (in-plane) bond distances are 2.1997(37) A and 1.9581(29) A, respectively. On the other hand, distortion of FeC6 octahedra is much smaller: the Fe–C1 (out-of-plane) and Fe–C2 (in-plane) bond distances are 1.8868(42) A and 1.9167(58) A, respectively. Figure 1(a) shows neutron powder patterns at 5K ( TC; thick curve) and at 20K ( TC: thin curve) for RbMn[Fe(CN)6]. In the middle panel [Fig. 1(b)], we show magnetic components of the neutron powder pattern. Several magnetic Bragg reflections are observed at 18 , 21 , 29 and 35 . No trace of (002) magnetic Bragg reflection is observed, indicating that the easy axis is along c. This is consistent with the elongation of the MnN6 octahedra along c. In the Bottom panel [Fig. 1(c)], we plotted calculated intensities IM,cal of the magnetic Bragg reflections with ferromagnetic alignment of the Mn3þ local spins along c. IM,cal is expressed as IM.cal / mhkl Lð Þ jFMðQÞj; ð1Þ