Effect of Ti⁴⁺ doping on magnetism and magnetodielectric properties of Ca₃Co₂O₆

Abstract

As a quasi-one-dimensional spin frustrated material, Ca₃Co₂O₆ has a series of interesting physical properties such as low-temperature spin freezing and multiple magnetized steps due to its unique structure. The magnetic properties of Ca₃Co₂O₆ mainly come from Co ions, and the doping of different elements at the Co site has a great effect on the magnetic structure of Ca₃Co₂O₆. At present, the magnetic research of Ca₃Co₂O₆ and its related compounds mainly focuses on exploring the influence of other elements replacing Co sites. For example, non-magnetic Sc³⁺ can dilute the intrachain ferromagnetic exchange, while the doping of magnetic ions Mn⁴⁺, Fe³⁺ or Cr³⁺ can inhibit the intrachain ferromagnetic interaction and enhance the antiferromagnetic interchain interaction. Doping Ti⁴⁺ ions, which are high-valence non-magnetic ions, not only dilutes the magnetic interaction of Ca₃Co₂O₆, but also changes the valence state of cobalt ions. i.e. it can convert part of Co³⁺ ions into Co²⁺ ions. Therefore, comparing with other doped ions, their introduction may have a more significant effect on the magnetoelectric properties of Ca₃Co₂O₆. In this study, a series of Ca₃Co_2–<i>x</i>Ti_<i>x</i>O₆ (<i>x</i> = 0, 0.02, 0.04, 0.06) polycrystalline samples is prepared by sol-gel method. Their magnetic, dielectric and magnetodielectric properties are measured. The XRD patterns show that a small number of Ti⁴⁺ ions do not change the crystal structure of Ca₃Co₂O₆. Due to the destruction of the long-range ferromagnetic correlation of Ca₃Co₂O₆ by non-magnetic Ti⁴⁺ ions, the ferromagnetic interaction is inhibited to some extent. Because Ti⁴⁺ ions are non-magnetic ions, they cannot form antiferromagnetic coupling with Co ions, resulting in the decrease of the Curie-Weiss temperature(<i>θ</i>). The positive <i>θ</i> value and exchange constant still indicate that the ferromagnetic interaction is dominant in Ti⁴⁺ doped Ca₃Co_2–<i>x</i>Ti_<i>x</i>O₆ (<i>x</i> = 0, 0.02, 0.04, 0.06) samples. The substitution of non-magnetic ions Ti⁴⁺ for Co³⁺ ions also makes the effective magnetic moment of Ca₃Co_2–<i>x</i>Ti_<i>x</i>O₆ (<i>x</i> = 0, 0.02, 0.04, 0.06) monotonically decrease from <i>μ</i>_eff = 5.42<i>μ</i>_B for <i>x</i> = 0 to <i>μ</i>_eff = 5.18<i>μ</i>_B for <i>x</i> = 0.06. Accompanying the introduction of Ti⁴⁺ ions, the spin frustration of Ca₃Co₂O₆ is released partly, thus gradually fading the magnetization steps of Ca₃Co₂O₆. As the Ca₃Co₂O₆ is a typical magnetodielectric material, the released spin frustration in Ti⁴⁺ doped samples and the variation of the subtle magnetic structure exert a large influence on the magnetodielectric coupling effect of Ca₃Co_2–<i>x</i>Ti_<i>x</i>O₆ (<i>x</i> = 0, 0.02, 0.04, 0.06) compounds.