Temperature-dependent multi-k magnetic structure in multiferroic Co3TeO6

Abstract

A complex magnetic order of the multiferroic compound Co 3 TeO 6 has been revealed by neutron powder diffraction studies on ceramics and crushed single crystals. The compound adopts a monoclinic structure (s.g. C2/c) in the studied temperature range 2 K -300 K but exhibits successive antiferromagnetic transitions at low temperature. Incommensurate antiferromagnetic order with the propagation vector k 1 = (0, 0.485, 0.055) sets in at 26 K. A transition to a second antiferromagnetic structure with k 2 = (0, 0, 0) takes place at 21.1 K. Moreover, a transition to a commensurate antiferromagnetic structure with k 3 = (0, 0.5, 0.25) occurs at 17.4 K. The magnetic structures have been determined by neutron powder diffraction using group theory analysis as a preliminary tool. Different coordinations of the Co 2+ ions involved in the low-symmetry C2/c structure of Co 3 TeO 6 render the exchange-interaction network very complex by itself. The observed magnetic phase transformations are interpreted as an evidence of competing magnetic interactions. The temperature dependent changes in the magnetic structure, derived from refinements of highresolution neutron data, are discussed and possible mechanisms connected with the spin reorientations are described. 1.IntroductionThe origin and understanding of the coupling phenomena between different physical properties within a material is a central subject of solid state science. A great deal of theoretical and experimental attention in this field is currently focused on the coupling between magnetism and ferroelectricity, as can be encountered in the so-called multiferroics [1,2]. These compounds present opportunities for a wide range of potential applications [3,4] in addition to the fact that the fundamental physics of multiferroic materials is rich and fascinating [5][6][7][8]. The coexistence of ferromagnetism and ferroelectricity is difficult to achieve for many reasons [2,9] and only very few multiferroic materials are known [5]. First, and most fundamentally, the cations which are responsible for the electric polarization in conventional ferroelectrics have filled d-electron shells. In contrast, ferromagnetism requires unpaired electrons, which in many materials are provided by d electrons of transition metal ions. Therefore the coexistence of the two phenomena, although not prohibited by any physical law or symmetry consideration, is discouraged by the local chemistry that favors one or the other but not both. In practice, alternative mechanisms for introducing both the polar ion displacements and the spin ordering are still needed. Theory gives us a good guide in what type of materials we can expect a large coupling effect. There are several important conclusions that we can draw regarding future directions of multiferroic research [2,5,9].1 However, experimental activity has been limited due to a lack of novel materials. Existing multiferroic compounds belong to different crystallographic classes, and although some general rules governing their behaviour are already estab...

Exploring the Corundum Structure as a Host for Colored Compounds – Synthesis, Structures, and Optical Studies of (MM′)3TeO6 (M = Mg, Mn, Co, Ni, Zn; M′ = Mg, Mn, Co, Ni, Cu)

“…The structure offers considerable scope for the exploration of color as there are differences in the coordination environment as well as the overall space group. Thus, the compounds Mg 3 TeO 6 and Mn 3 TeO 6 are isostructural, and are in the centrosymmetric space group ( R 3), Ni 3 TeO 6 is non‐centrosymmetric ( R 3), and Zn 3 TeO 6 and Co 3 TeO 6 are centrosymmetric ( C 2/ c ) and also have different coordination environments (octahedral, square pyramidal, and tetrahedral) for the Zn/Co ions. Of these, Mn 3 TeO 6 has been investigated for possible multiferroic behavior, and Ni 3 TeO 6 exhibits spontaneous polarization under an applied field (ferroelectric behavior) .…”

Complex magnetic incommensurability and electronic charge transfer through the ferroelectric transition in multiferroic Co3TeO6

“…Apparently, the Co spins in Layer A and in Layer B order separately, reflecting a weak interlayer magnetic coupling for Co 3 TeO 6 . The two magnetic transitions found 2, 9, 13, 17 are the result of the ordering of the Co spins in the zigzag chains in Layer B into an ICM spin arrangement at T M1  = 26 K, and the separate ordering of the Co spins in the honeycomb web in Layer A into a CM spin structure at T M2  = 19.5 K. Clearly, the modulation vector q  = ( q h , q k , q l ) of the satellite reflections is very sensitive to temperature as previously discussed 14 .

Figure 4Neutron-diffraction intensity maps in the ( h 0 l ) scattering plane near the (203) Bragg position observed at ( a ) 22.5 K, ( b ) 16.5 K, and ( c ) 3 K. The strong powder ring is from the Al sample holder. Below the initial ordering at T M1  = 26 K there is no detectable intensity at the commensurate (203) Bragg position at 22.5 K ( a ), but two satellite magnetic reflections are revealed arising from the development of incommensurate magnetic order.

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“…We note that T M4 was defined as 16 K in our previous reports 14, 17 , while the present results reveal that this transition is better defined by 15 K, the temperature below which the k component of the ICM structure reappears. These transitions are naturally linked to the crystal geometry, where Co 3 TeO 6 crystallizes into a monoclinic symmetry with the space group C2 / c , with 36 Co ions in a chemical unit cell 9, 17 . The complex crystalline structure of Co 3 TeO 6 can be viewed as being composed of six distorted Co 3 Te layers that are interconnected through O ions, forming significantly distorted corner-, edge-, and face-sharing CoO 6 octahedra and CoO 4 tetrahedra.…”

“…There are five crystallographically distinct Co ions that occupy two very different layered arrangements (Fig. 1) 9, 17 . One is comprised of distorted zig-zag Co-O-Co chains (marked Layer B, Fig.…”

Crystal Chemistry, Optical Spectroscopy and Crystal Field Calculations of Co3TeO6 and Solid Solutions Co3–xZnxTeO6

“…Cobalt(II) oxotellurate(VI), Co 3 TeO 6 , is a type‐II multiferroic material that has been extensively investigated for its magnetic and electric properties . Its monoclinic crystal structure adopts the β‐Li 3 VF 6 structure type .…”