0428同步年報-2021-全

Neutron Science 079 New Cu-Based Multiferroic Materials Neutron powder diffraction is a useful tool to characterize the magnetic structures of multiferroic materials. M ultiferroics, exhibiting two or more ferroic orders, are extensively investigated in condensed-matter physics because of their potential applications in spintronics and low-power-based microelectronics, especially multiferroics type II, of which the magnetic order and electric order are mutually coupled. Among all currently known multiferroics, materials containing Cu 2+ spin-1/2 are placed in a special class due to their unconventional spin structures, involving spin frustrations or quantum effects. CuO and CuCl 2 are well known multiferroic materials; compounds from the CuO-CuCl 2 -SeO 2 ternary phase diagram have also been reported to exhibit exotic quantum magnetic states and multiferroic properties. Hung-Duen Yang (National Sun Yat-sen University) and his co-workers studied the properties of various Cu-based materials, such as Cu 2 OSeO 3 and Cu 2 OCl 2 . They recently reported the structural, magnetic and dielectric properties of α-Cu 5 O 2 (SeO 3 ) 2 Cl 2 and Cu 9 O 2 (SeO 3 ) 4 Cl 6 , 1,2 which belong to the CuO-CuCl 2 -SeO 2 ternary system. Single crystals of α-Cu 5 O 2 (SeO 3 ) 2 Cl 2 and Cu 9 O 2 (SeO 3 ) 4 Cl 6 were synthesized using a chemical vapor-transport method. The mixture of raw materials was sealed in an ampoule (length 10 cm), which was placed horizontally in a tubular two-zone furnace, as illustrated in Fig. 1(a) . Single crystals of α-Cu 5 O 2 (SeO 3 ) 2 Cl 2 and Cu 9 O 2 (SeO 3 ) 4 Cl 6 formed in the hot and cold ends, respectively. The crystal structures of α-Cu 5 O 2 (SeO 3 ) 2 Cl 2 and Cu 9 O 2 (SeO 3 ) 4 Cl 6 are both monoclinic with space groups P2 1 /c and C2/m, respectively. Both compounds were characterized with synchrotron radiation X-ray powder diffraction ( TPS 09A ), magnetometry, specific heat, dielectric measurements and neutron powder diffraction. The magnetic Bragg peaks from the Cu 2+ spin structures are typically weak because of small magnitudes of magnetic moment and quantum fluctuation. The magnetic structures of these two compounds were hence studied with the high-intensity neutron powder diffractometer, WOMBAT . Figure 1(b) shows magnetic diffraction patterns of α-Cu 5 O 2 (SeO 3 ) 2 Cl 2 obtained on subtracting the high-temperature diffraction pattern from the low- temperature one. The propagation vector is determined as q = (1/2 0 0), which indicates that the magnetic Fig. 1 : (a) Experimental two-zone growth of α-Cu 5 O 2 (SeO 3 ) 2 Cl 2 and Cu 9 O 2 (SeO 3 ) 4 Cl 6 single crystals: pictorial representation and photograph of the quartz ampoule. A red line denotes the temperatures of the hot and cold ends of the two-zone furnace. Brownish α-Cu 5 O 2 (SeO 3 ) 2 Cl 2 and green Cu 9 O 2 (SeO 3 ) 4 Cl 6 single crystals grown at hot and cold ends, respectively. (b) and (d) show the fit of magnetic neutron diffraction patterns of α-Cu 5 O 2 (SeO 3 ) 2 Cl 2 and Cu 9 O 2 (SeO 3 ) 4 Cl 6 , respectively. (c) and (e) show the order magnetic peak intensities as a function of temperature. [Reproduced from Ref. 1 and Ref. 2]