Structural and magnetic properties of Co-doped Gd₂O₃ nanorods

Highlights

• Co-doped Gd₂O₃ nanorods are prepared by hydrothermal method.

• Lattice parameter decreases with increasing doping concentration.

• Magnetism of Co-doped Gd₂O₃ nanorods is intrinsic property.

Abstract

Cobalt-doped Gd₂O₃ (Gd_2−xCo_xO₃, 0≤x≤0.06) nanorods of about 100 nm diameter and 2 µm length were synthesized using a simple hydrothermal method. XRD, Raman, XPS, and TEM measurements showed the samples to have a single cubic phase structure of Gd₂O₃ doped with Co²⁺ cations, without any cobalt clusters. All the samples showed paramagnetism at room temperature as well as at 5 K. The samples’ high magnetization values at 5 K were due to reduction of the thermal randomization of the magnetic spins. The Curie–Weiss fitting of the magnetic data reflected antiferromagnetism along with paramagnetism due to the exchange interactions of Gd³⁺ via O²⁻ ions and coupling between Co²⁺–Co²⁺ pairs.

Introduction

Diluted magnetic semiconductors (DMSs) have attracted a considerable amount of attention owing to their promising applications in spintronics [1] and magneto-optics [2]. Over the past decades, the enhancement of DMS magnetic properties such as Curie temperature has been the focus of intense research. In the DMS materials, transition metal (TM)-ion-doped ZnO, In₂O₃, SnO₂, and TiO₂, among others, are well known spintronic materials that have been exhaustively studied in the last ten years or so [3], [4], [5], [6], [7], [8], [9], [10]. However, their ferromagnetism, which is to say its existence and underlying origin, remains controversial. Whereas Torres et al. [3] have reported paramagnetism in Fe-doped TiO₂ samples, Xing et al. [7] have reported ferromagnetism in Cr-doped In₂O₃ samples. In any case, most recent reports support the existence of intrinsic ferromagnetisms due to the presence of oxygen vacancies or metal interstitial defects [4], [5], [6], [7], [10], or alternatively, to the formation of magnetic clusters [8], [9]. Thus, both positive (due to vacancies) and negative (formation of magnetic clusters) results on the magnetism of TM-ion-doped semiconductors have been reported. Curiously, in this context, TM-ion-doped rare-earth (R³⁺) oxides have yet to be given significant consideration. Binary rare-earth oxides are the most stable rare-earth compounds, in which rare-earth ions typically hold the trivalent state. In TM ions, magnetic moment arises from the partially filled outermost 3d electrons, whereas in R³⁺ ions, magnetic moment arises from the inner 4 f incomplete sub-shell. TM-ion doped R³⁺ rare-earth oxides, therefore, as both the dopant and doping ions have high values of magnetic moments, promise to become an exciting field for DMS systems.

Among rare-earth oxides, Gd₂O₃ is a transparent wide-band-gap (5.9 eV) and thermodynamically stable semiconductor. Recently, doping of rare-earth ions such as Eu³⁺, Yb³⁺, and Er³⁺ has been proposed as a way to introduce magnetism to Gd₂O₃ nanomaterials [11], [12]. Considering the importance of R³⁺ rare-earth oxides, particularly the broad interest in Gd-based oxides, Gd₂O₃ may be an advanced potential DMS candidate after doping with TM ions.

Recently, intensive research attention has been devoted to one-dimensional (1D) nanostructures instead of thin films, for applications in various nanoscale spintronic devices owing to their high aspect ratio, flexibility, single-crystallinity, shape anisotropy, and unique electronic features due to the 1D quantum confinement effect [13], [14], [15]. The ferromagnetism of the Gd atom below 289 K drives the study of Gd₂O₃'s magnetic properties. To the best of our knowledge, there are no reports yet available of systematic studies of room-temperature magnetic properties of Co-doped Gd₂O₃ nanorods with respect to various Co doping concentrations.