Mechanism of Cu dopant in transition metal oxide Mn_1.56Co_(0.96-x)Ni_0.48Cu_xO_4+δ (0 ≤ x ≤ 0.2) thin films

Highlights

• Mn_1.56Co_(0.96-x)Ni_0.48Cu_xO_4+δ films were prepared by CSD method.

• Lattice parameters of the films become smaller with the substitution of Cu.

• Short-range correlation and cluster spin-glass state present in the films.

• Transition temperature T_P1, T_P2 shift towards lower temperature side.

• Saturation magnetization M_S decreased monotonously with the increase of Cu dopant.

Abstract

To identify the substitution of Cu element for cations in transition metal oxide AB₂O₄ structure, we propose to investigate the structural, optical and magnetic properties of Cu-doped Mn_1.56Co_(0.96-x)Ni_0.48Cu_xO_4+δ films with 0 ≤ x ≤ 0.2. The X-ray diffraction of Mn_1.56Co_(0.96-x)Ni_0.48Cu_xO_4+δ films indicates good crystallization and cubic spinel structure. Raman spectra show two peaks of F_2g and A_1g vibration modes, and the peak position changes with Cu dopant. The transition temperatures T_P1, T_P2 shift towards lower temperature by Cu doping, and the saturation magnetization M_S decreases monotonously. The results show that the substitution of Cu ions does not vary the crystal structure but changes the ratio of Mn³⁺/Mn⁴⁺. Our proposed way overcomes the difficulties to directly observe the atoms of Mn, Co, Ni, and Cu with closed sizes in microstructure, and it is instructive to further understand the conduction mechanism of doped manganese cobalt nickel oxides in theory.

Introduction

Transition metal oxides have attracted much attention because of the complex physical mechanism [1,2]. The study of spinel oxides with a general formula AB₂O₄ has great significance, because they show rich physical phenomena such as superconductor, giant magneto resistance, strong correlation, nonlinearity, ferrimagnetism, etc. [3], [4], [5]. Among them, Mn_1.56Co_0.96Ni_0.48O₄ has broad applications in negative temperature coefficient thermistors, for instance, infrared detection bolometer, surge protection devices, temperature sensors, and temperature compensation devices [6], [7], [8], [9]. But inherent big noise of the device is an urgent problem need to be solved due to its high resistivity. One effect approach to solve it is by doping. Cu is one of the important doping cations [10], [11], [12]. Cu dopant can significantly reduce the resistivity of the materials and enhance the conductivity [13, 14]. In addition, the substitution of Cu into the antiferro/ferrimagnetically ordered systems will induce disorder or distort the parent lattice. It provides pathway to interact among the magnetic ions, which leads to some complex magnetic ordering. The behavior of Cu dopant is very complex and not unified a view so far, for example what is the valence state of Cu ions and how about the occupancy of Cu ions in Mn_1.56Co_(0.96-x)Ni_0.48Cu_xO_4+δ.

Generally, atomic occupancy can be confirmed by high resolution image, such as transmission electron microscopy. While the structure of Mn_1.56Co_(0.96-x)Ni_0.48Cu_xO_4+δ system is complex, and the atom size of Mn, Co, Ni, and Cu elements are very close, then it is hard to distinguish the different atoms at position A and B. Therefore, in this article we propose a recipe of investigating the structural, optical and magnetic properties to verify that copper atoms are actually doped and clarify the mechanism of Cu dopant in Mn_1.56Co_(0.96-x)Ni_0.48Cu_xO_4+δ films.