Ferroelectricity in spinel solid solution Co0.8Ni0.2Cr2O

Abstract

Temperature dependencies of dielectric permittivity and loss tangent (at frequencies 0.1-200 kHz) together with thermally stimulated depolarization currents, magnetization, and Mössbauer spectra of ceramic solid solutions Co 1−x Ni x Cr 2 O 4 (x = 0, 0.2, 0.4, 0.6, 1.0) are studied at temperatures from 80 to 350 K. We find evidence of two reversible phase transitions located at T 1 and T 2 (220 and 245 K, respectively, for x = 0.2), of occurrence of pyroelectric properties below T 2 , as well as of a phase transition to magnetically ordered state of ferrimagnetic type (below 86 K for x = 0.2). The obtained results evidence that the compounds possess, additionally to magnetic, ferroelectric properties and are thus new members in the family of multiferroics.Spinel compounds with Jahn-Teller cations reveal structural phase transitions caused by removal of the orbital degeneracy leading to ordering of the d orbitals and to lowering of the crystal lattice symmetry. As a result, some low-symmetry phases become polar and reveal multiferroic properties. [1][2][3][4][5][6][7][8][9][10][11][12] Most detailed studies of such multiferroicity were performed for magnetite (Fe 2+ Fe 2 3+ O 4 ) whose low-symmetry phase is monoclinic (Cc) 5 ; here, switching between magnetic and electrical orderings can be driven by both magnetic and electric fields. 1-4 Recently, similar studies were performed also on chromites, like CoCr 2 O 4 . 7-12 Cobalt CoCr 2 O 4 and nickel NiCr 2 O 4 chromites belong to the spinel family with the general formula AB 2 O 4 in which Co 2+ (3d 7 ) or Ni 2+ (3d 8 ) bivalent cations occupy tetrahedrally coordinated by oxygen A positions and the Cr 3+ trivalent cations-octahedral positions. 6-14 These compounds crystallize, at rather high temperatures, into a cubic phase with the space group F d3m (No. 227). When lowering the temperature, the symmetry of the nickel chromite decreases to tetrahedral at 320 K, while the cobalt chromite remains cubic down to 10 K. 7-19 In NiCr 2 O 4 , the phase transition between cubic and tetragonal states (the C-T transition) is of the first-order type, near to second-order type phase transition, with the tetragonal form relating to ferroelastics. It is driven by a cooperative Jahn-Teller effect in the array of Ni 2+ cations in tetrahedral positions [for the Cr 3+ (3d 3 ) cations the Jahn-Teller effect in the octahedral crystal field is not observed]. In ACr 2 O 4 , the transition into the ferrimagnetic state takes place at T N = 93 K for A = Co and at T N = 70 К for A = Ni. 6,9,11,[20][21][22] In spite of the fact that the considered complex oxides reveal intriguing elastic, electrical, magnetic, catalytic, and other physical properties and attract much attention, 1-25 there is practically no data in the literature on their dielectric characteristics. In this paper we present our results on the first measurements of the temperature-frequency behavior of the complex dielectric permittivity ε * = ε 1 + iε 2 , loss tangent tgδ = ε 2 /ε 1

Tuning of multiferroic order with Co doping in CuCr2O4 : Interplay between structure and orbital order

“…The existence of the Cu 2+ in the series of compounds is imperative, because the Cu 2+ with the 3d 9 electronic configuration is strongly a J-T active ion. The results are rather consistent with that observed for Co 0.8 Ni 0.2 Cr 2 O 4 with T FE > T N , where the J-T active Ni 2+ led to the crucial role [4]. The delicate interplay between orbital and structural degrees of freedom has been suggested in spinel compounds, where the J-T cations led to the structural phase transitions [6][7][8].…”

“…One of the fascinating outcomes is the colossal magnetoresistance in the mixed-valent manganites [1,2]. Moreover, this intricate interplay gives rise to the diverse ranges of the interesting magnetic properties from the frustrated magnetism to the multiferroic properties [3][4][5][6][7][8]. In the above cases, the orbital ordering driven by the the Jahn-Teller (J-T) distortion has been found crucial, which was intimately correlated to the structural distortion and also, eventually, directed the magnetic properties.…”

Role of Ni substitution on structural, magnetic and electronic properties of epitaxial CoCr2O4 spinel thin films

“…Structurally, CoCr 2 O 4 is stable and remains cubic with space group Fd ̅ m at all temperatures as the sample is cooled down to 10 K [4].On the other hand for NiCr 2 O 4 the crystal structure of the system reduces from cubic to tetragonal at 310 K [5,6]. In NiCr 2 O 4 the cubic-to-tetragonal phase transition is of first order in nature [7]. It is driven by a cooperative Jahn-Teller (J-T) effect in the array of Ni 2+ cations in tetrahedral position [7].…”

“…It is driven by a cooperative Jahn-Teller (J-T) effect in the array of Ni 2+ cations in tetrahedral position [7]. For the Cr 3+ ( ) cations the J-T effect in the octahedral crystal field is absent [7]. The tetragonal structure further transforms to an orthorhombic phase, driven by a magneto-structural coupling at the Néel temperature (T N ) at about 65 K [8,9].Both magnetic susceptibility and heat capacity measurements indicate anomalies at T = 30 K resulting from the distortion occurring within the orthorhombic structure [10].…”

The cooperative Jahn–Teller effect and anti-isostructural phases in Ni1−xCoxCr

“…The system calls the additional interest due to the recent discovery of the spontaneous polarization in CoCr 2 O 4 [19,25,26,27] and, in some members of solid solutions family [28], with already existing magnetic order. Thus, these materials can be classified as multiferroics, which are of great interest from the viewpoint of thus various applications.…”