A first-principles study of Mg/Ni induced magnetic properties of Zn_0.95−xMg_xNi_0.05O

Highlights

• Magnetic properties of Mg/Ni doped ZnO were investigated by the first principles study.

• DFT with GGA-PBE scheme utilizing plane-wave pseudo-potential method was used.

• Oxygen vacancies (VO) enhanced FM state; distant vacancies led to HMF for all Mg ratios.

• Zn-d, O-p and Ni-d (dominates) control the spin-up/down channels by hybridization.

Abstract

Magnetic properties of Mg/Ni doped ZnO were investigated by the first-principles study. The generalized gradient approximation (GGA) in Perdew–Burke–Ernzerhov of the scheme as a form of density functional theory (DFT) utilizing the plane-wave pseudo-potential method was used. Calculations were performed for a constant Ni doping ratio as 5% and different concentrations of Mg varying from 1% to 5%. It was shown that Mg concentrations helped to tune band gap and mediate the ferromagnetic property. 1% Mg-doped structure had a half-metallic ferromagnetic (HMF) state. Meanwhile, metallic behavior (MB) was observed for higher concentrations of Mg (>1%) impurities. It was revealed that Mg-doped ZnNiO possesses ferromagnetic behavior solely for 1% Mg while other doping ratios were showing distinctive phases including antiferromagnetism (AFM). Besides, there is no evidence of a clear connection between the doping concentration of the Mg and the magnetic phase. Ni distant/near oxygen vacancies ($V_o$) enhanced the FM state; however, distant vacancies led to HMF state for all Mg concentrations. Zn-d, O-p, and Ni-d (dominates) control the spin-up/down channels by hybridization.

Introduction

Due to its interesting electrical, optical, mechanical, and magnetic properties, a big effort has been given to study zinc oxide for the last twenty years. ZnO exhibits a large excitation binding energy, an inherently n-type II-VI semiconductor with a wide band gap, a refractive index, high thermal conductivity and photoconductivity [1], [2], [3], [4], [5], [6]. The application fields include optoelectronic devices, solar cells [3], transparent conductor [4], UV-absorbing material in sunscreens [5], the active material in varistors [6], catalysts, chemical sensors, piezoelectric transducers, etc. ZnO can be incorporated into different morphologies including nanoparticles, nanowires, nanorods [1], [2]. Moreover, ZnO nanomaterials are favorable candidates for photonics and nanoelectronics.

ZnO materials having a high Curie temperature are potential materials for the diluted magnetic semiconductors (DMS) and DMS doped with varying transition metal (TM) exhibits varying magnetic properties [7], [8], [9]. Sato et al. [10] have researched transition metal atoms (Mn, V, Cr, Fe, Co, Ni) doped ZnO materials through first-principle calculations, and they found that the doping system had ferromagnetic (FM) properties. Ueda et al. [11] magnificently prepared 3d transition metal doped ZnO thin film materials. Recent studies showed that in order to improve device technologies such as magnetic memories (MRAM), spin LED, logic devices zinc oxide semiconductors must be doped with different transition metals and elements using magnetic chemical assets at room temperature (RT) [12], [13], [14].

Ni-doped ZnO material researches showed that at RT this material had FM property with different preparation techniques such as the sol-gel technique [15], the pulsed laser deposition (PLD) [16]. Cui et al. [17] synthesized Ni- and Co-doped ZnO nanowire arrays with an electrochemical method at 90^◦C, and they found that magnetic nanowires have anisotropic FM properties. He et al. [18] prepared Ni-doped ZnO nanowire arrays by means of metal vapor vacuum arc (MEVVA) ion source doping technology and pointed out that the electron transport ability increased by 30 times and the absorption peak exhibits red-shift phenomena. Wakano et al. [19] synthesized Ni-doped ZnO thin film materials and found that FM features appeared at 2 K, while PM properties appeared at 300 K. Al-Harbi [20] synthesized a Zn_1−xNi_xO nanorod with excellent UV emissive power by means of a low-temperature hydrothermal method. Cheng et al. [21] prepared a Zn_1−xNi_xO nanorod at RT FM features by means of a low-temperature hydrothermal method. On the contrary, Yin et al. [22] reported that no FM feature was observed in Zn_1−xNi_xO nanomaterials.

Arda et al. [23] showed that the existence of ferromagnetism in Ni²⁺ doped ZnO does not only depend on lattice defects but also on growth techniques. Dogan et al. [24] pointed out that the grain sizes of the ZnNiO nanoparticles were measured to be approximately 90 nm by means of a Scanning Electron Microscope (SEM). The XRD and SEM measurements showed that Ni-doped ZnO had wurtzite structures with NiO secondary phases. RT ferromagnetism was observed for highly Ni-doped ZnO nanoparticles. Heiba et al. [25] founded that NiFe_2−xGd_xO₄ (0.0 ≤ x ≤ 0.4) nanoferrites exhibited superparamagnetic behavior at RT while pure nickel ferrite exhibited ferromagnetic (FiM) behavior. The saturation of magnetization (Ms) at 10 K first decreased due to the presence of Gd ions in A site, after that it increased as the amount of Gd substitution increased due to substitution of Fe³⁺ ions by Gd³⁺ ions which had a larger magnetic moment. The coercive field increased with the Gd content due to the decrease in crystallite size.

Viswanatha et al. [26] produced Mg-doped ZnO nanoparticles in the range of 60–90 nm and they found that the morphology structure changed when the concentration of dopant was increased. In our previous work, it was pointed out that as Mg concentration value increased, the magnetization decreased considerably and the loop M−H curve at 300 K exhibited the strong ferromagnetism in Zn_0.94Mg_0.01Ni_0.05O nanoparticles in the range of 24–27 nm within 0.01% and 0.05% of Mg [7]. It was concluded that the size and shape of the nanoparticles depended on the preparation materials and methods [27]. El Foulani et al. [28] produced Co_0.5Zn_0.5Fe₂O₄ spinel ferrite nanoparticles by a novel synthetic technique and showed that the saturation magnetization and remanent magnetization increased gradually as the crystallite size decreased, although all samples were well above the reported critical grain sizes of ~ 20–25 nm for CFO by Rajendran et al. [29]. They also contributed a useful theoretical basis for the application of ZnO DMSs. Vachani et al. [30] deduced that the concentration of oxygen vacancies is not the only important parameter for the occurrence of ferromagnetism in Cu:ZnO but that the position of oxygen vacancies also plays an important role. Single-crystalline ZnNiO nanoparticles were produced to investigate room temperature FM originating from oxygen vacancies and magnetic polaron formation [31].

The structural, electronic and magnetic properties of Mg/Ni doped ZnO for various concentrations by DFT calculations based on generalized gradient approximation (GGA) with PBE scheme were investigated. Mg was doped to the system instead of Zn atoms up to 5% and Ni concentration was chosen as 5%. Origins of magnetism and electronic behavior that were mediated by intrinsic effects such as structural formation (variation of bond lengths, bond angles and dihedral angles) and orbital hybridizations were revealed including Ni distant/near oxygen vacancies in which spin density intrinsically changed causing orbital hybridizations and unpaired electrons.