Effect of (Cd, Al) Co-doping and hydrogenation on the long-range ferromagnetic ordering of ZnO: Experimental and DFT studies

Highlights

• Synthesis of (Cd,Al) co-doped ZnO nanopowders.

• Redshift and narrowing of energy band gap.

• d⁰ ferromagnetic order in pure ZnO.

• Reinforcement of long range ferromagnetic ordering with hydrogenation.

• DFT clarify that doping and vacancies as origin for the changes in magnetic and optical properties.

Abstract

Nanopowders of ZnO codoped with Al and Cd ions (ZnO:Al:Cd) were synthesised by thermal co-decomposition of a mixture of Zn and metals complexes. X-ray diffraction analysis using the Rietveld method confirmed the formation of single nanocrystalline structure, thus, both Al and Cd ions are incorporated into ZnO host lattice forming solid solutions (SSs). Optical properties measured by diffuse reflectance spectroscopy (DRS) revealed a redshift associated with the creation of point defects and that the energy band gap is slightly affected by doping; i.e. 3.23–3.12 eV. Annealing of the as-synthesised SSs in H₂-atmosphere (hydrogenation) was conducted in order to clarify the reinforcement and/or the creation of ferromagnetic order. Magnetic measurements showed that ZnO doped with 1.2 at% Al and 0.6 at%Cd became ferromagnetic with a coercivity ∼100 Oe, remanence ∼1.4 memu/g and saturation magnetization of ∼6.5 memu/g. Moreover, hydrogenation of undoped diamagnetic (DM) ZnO transformed into ferromagnetic. To elucidate the changes on the optical properties due to presence of dopants (Al, Cd) and the vacancies, we have employed first principle calculations based on density functional theory to calculate the optical properties and the change operated in the band gap due to structural changes. The observed RT-FM was associated to the doping and vacancies created by the hydrogenation.

Introduction

Dilute magnetic semiconductor (DMS) oxides currently become of great importance because of their potential practical applications in spintronics and in the production of nano-electronic devices [[1], [2], [3]]. In fact, for practical purposes, DMS oxides should exhibit ferromagnetism at room temperature (RT-FM), while retaining their semiconducting properties. These magnetic properties could be created in the well-known semiconductor oxides like ZnO, CdO, In₂O₃, etc by doping with ferromagnetic (FM) metal/rare-earth elements such as Fe, Co, Ni, Gd or even with non-FM elements such as Al (paramagnetic-PM) [4], Cu (diamagnetic-DM) [5,6], Li (PM), Na (PM) [7]. Thus, it is important to diverse the nature of dopants aiming to improve the properties of DMSs. ZnO as a direct wide bandgap semiconductor and a member of transparent conducting oxides (TCOs), shows interesting characteristics making it very attractive for various applications such as in optoelectronics, photocatalysis, luminescence, photoconduction, etc. ZnO crystallizes within a wurtzite-type structure with a band-gap of ∼3.37 eV, a resistivity of ∼10⁻² Ω cm, and large exciton binding energy (60 meV) [8]. Furthermore, ZnO films could be prepared with high optical transparency accompanied with a good conductivity for TCO applications by appropriate doping. Therefore, it is possible to choose certain dopant ions for improving TCO functional properties as well as tailoring stable FM in order to obtain simultaneously DMS+TCO, that would diverse much more its field of applications. In general, the strength of the induced or tailored RT-FM depends on the nature and content of the dopant ions as well as the characteristics of the electronic medium of the magnetic exchange interactions (MEI) inside the host crystal for magnetic spin-spin mutual interaction between dopant ions. The quality of MEI depends on the host crystal electrical properties that also could be controlled by doping with certain ions.

In the present work, Al³⁺ ions were chosen as dopants in order to control (increase) the electronic concentration in the crystal lattice that improve the conduction properties of the host ZnO [9] as well as support the electronic medium for spin-spin mutual interaction. It was found experimentally [10] that ZnO doped with Al ions only could induce RT-FM, which was explained by the charge transfer between Zn atoms and adsorbed Al atoms (but not due to the intrinsic defects [10]). The other dopant in the present work are Cd ions. The weak magnetic properties in ZnO:Cd obtained in Ref. [11] were explained by only the existence of Zn vacancies (V_Zn), which are more stabilized with Cd doping, i.e. Cd doping is indirectly responsible for FM.

Therefore, in the present work we study the induction of FM properties in diamagnetic ZnO co-doped with two non-FM dopants Al and/or Cd. Furthermore, we study the development of the induced RT-FM properties in host ZnO by annealing in hydrogen atmosphere (referred as hydrogenation), which was found to improve the conductivity and carrier concentration of host ZnO [12] and, thus, enhance the internal crystalline medium for spin-spin interaction. The aim of the present work is to understand the relationship between structural and optical band gap changes in ZnO co-doped (Al, Cd) system in order to assess its behavior and thereby the development of novel nanomaterials with enhanced properties for optoelectronic and spintronic devices.