Electronic phenomena studied in the nordic countries

Crystal and magnetic structures of Co2MTeO6 (M = Mg, Zn)

Not scheduled

15m

Max IV Lund

Speaker

Mr Christopher Røhl Andersen (Niels Bohr Institute (Copenhagen))

Description

Multiferroic compounds with the coexistence of magnetic and ferroelectric orders are well known from transition metal oxides with perovskite crystal structures. Recently, a new family of multiferroic oxides with other crystal structures has been found: *M*$_3$TeO$_6$ (*M* = Co, Mn, Ni). Co$_3$TeO$_6$ has shown a complex magnetic ordering with several positions for the magnetic cations. This study presents the results of substituting some of these magnetic cations with non-magnetic cations, which has not been reported earlier. The crystallographic and magnetic structures of Co$_2$*M*TeO$_6$ (*M* = Mg, Zn) have been studied with polycrystalline samples using neutron powder diffraction (NPD), magnetization and specific heat measurements. At room temperature, the crystal structure of Co$_2$ZnTeO$_6$ is similar to the cryolite crystal structure of Co$_3$TeO$_6$, being monoclinic space group *C2/c*, which remains stable down to 2 K. A second order phase transition is observed below $T_N$ = 13.2(3) K, which has been found to originate from antiferromagnetic interactions with an effective magnetic moment of $\mu_{eff}$ = 3.49 $\mu_B$/Co corresponding to high spin Co$^{2+}$ (S = 3/2). The magnetic structure cannot be determined from the NPD data. At room temperature, the crystal structure of Co$_2$MgTeO6 is a corundum crystal structure similar to that of Mn$_3$TeO$_6$ and Mg$_3$TeO$_6$, being trigonal space group $R\bar{3}$, which remains stable down to 2 K. It has an antiferromagnetic structure characterized by a propagation vector of *k* = (0, 0, 1/2) below $T_N$ = 29.06(10) K with a second order phase transition. The magnetic moments are oriented along the *a*- *b*-plane with 120$^\circ$ between the nearest neighbors in the plane. The effective magnetic moment has been found to be $\mu_{eff}$ = 3.97 $\mu_B$/Co corresponding to high spin Co$^{2+}$ (S = 3/2).