This review article first presents a summary of current understanding of the magnetic properties and intrinsic ferromagnetism of transition-metal (TM)-doped ZnO films, which are typical diluted magnetic oxides used in spintronics. The local structure and magnetic behavior of TM-doped ZnO are strongly sensitive to the preparation parameters. In the second part, we discuss in detail the effects of doping elements and concentrations, oxygen partial pressure, substrate and its orientation and temperature, deposition rate, post-annealing temperature and co-doping on the local structure and subsequent ferromagnetic ordering of TM-doped ZnO. It is unambiguously demonstrated that room-temperature ferromagnetism is strongly correlated with structural defects, and the carriers involved in carrier-mediated exchange are by-products of defects created in ZnO. The third part focuses on recent progress in TM-doped ZnO-based spintronics, such as magnetic tunnel junctions and spin field-effect transistors, which provide a route for spin injection from TM-doped ZnO to ZnO. Thus, TM-doped ZnO materials are useful spin sources for spintronics.

Ferromagnetism and possible application in spintronics of transition-metal-doped ZnO films

The detailed science and technology of crystalline oxide film growth using vacuum methods is reviewed and discussed with an eye toward gaining fundamental insights into the relationships between growth process and parameters, film and interface structure and composition, and electronic, magnetic and photochemical properties. The topic is approached first from a comparative point of view based on the most widely used growth methods, and then on the basis of specific material systems that have generated very high levels of interest. Emphasis is placed on the wide diversity of structural, electronic, optical and magnetic properties exhibited by oxides, and the fascinating results that this diversity of properties can produce when combined with the degrees of freedom afforded by heteroepitaxy.

Epitaxial Growth and Properties of Doped Transition Metal and Complex Oxide Films

ZnO doped with a few per cent (<10%) of magnetic ions such as Co exhibit room temperature (RT) ferromagnetism, transforming it into a very promising candidate for future spin electronic applications. We present x-ray magnetic circular dichroism (XMCD) spectroscopy, which has been used in total electron yield, total fluorescence yield, and reflection mode to investigate the origin of ferromagnetism in such diluted magnetic semiconductor materials in a surface, bulk and interface sensitive way, respectively. We investigated three different types of samples: ZnO doped with 5% Co, artificially layered films, and layered films with additional co-doping of 10% Li. These films are prepared by pulsed laser deposition (PLD) and characterized by superconducting quantum interference device (SQUID) magnetometry. Extensive XMCD measurements at the Co L2,3 and the O K edges, done with a very good signal-to-noise ratio, have revealed only cobalt paramagnetism and non magnetic oxygen signatures at low and high temperatures. We do not see any element specific signature for ferromagnetism at all. By exclusion, our results suggest oxygen vacancies as the intrinsic origin for RT ferromagnetism in doped ZnO.

https://iopscience.iop.org/article/10.1088/1367-2630/10/5/055009/pdf

XMCD studies on Co and Li doped ZnO magnetic semiconductors

Increase of Co solubility with decreasing grain size in ZnO

Indentation size effect and shear transformation zone size in a bulk metallic glass in two different structural states

Here we report on systematic studies of the epitaxial growth and properties of Co and Cu codoped ZnO thin films deposited onto sapphire c-plane single crystals using pulsed-laser deposition. The films display ferromagnetic behavior at room temperature. Detailed atomic scale characterization rules out the presence of clusters and secondary phases as the source of ferromagnetism. Optical measurements and x-ray photoelectron spectroscopy confirm the direct substitution of dopant atoms into Zn lattice sites. At low concentrations of Cu (∼5%) the magnetic moment of Zn1−(0.05+x)Co0.05CuxO materials appears to be additive. At higher concentrations of Cu the net magnetic moment per atom drops off sharply and seems to be relatively insensitive to the Co content. There is a dramatic increase in resistivity of the Co-doped films that accompanies Cu doping. Yet, this change of resistivity does not affect the magnetic moment, suggesting that free carrier mediated mechanism is not a feasible explanation for ferromagnet...

Magnetic, electrical, and microstructural characterization of ZnO thin films codoped with Co and Cu

Here we report a systematic study of magnetic, electronic and microstructural properties of Cu-doped and Cu and Al codoped ZnO thin films, grown epitaxially on a (0 0 0 1) sapphire substrate by the pulsed laser deposition technique. The films were annealed in oxygen at high temperature (600 °C) and their properties were compared with the as deposited films in order to study the role of defects like oxygen vacancies on the ferromagnetic properties. The doping of ZnO : Cu specimens with Al increased the carrier concentration by three orders of magnitude (from 1017 to 1020 cm−3) without altering the ferromagnetic ordering. On the other hand, a reduction in oxygen vacancies concentration brought about by high temperature annealing in oxygen had a large detrimental effect on ferromagnetism. These results tend to eliminate a free carrier mediated mechanism and point towards a defect mediated mechanism, such as a bound-magnetic-polaron mediated exchange, as being responsible for stabilizing long-range ferromagnetic ordering in these systems.

https://iopscience.iop.org/article/10.1088/0022-3727/40/24/002/pdf

The effect of oxygen annealing on ZnO : Cu and ZnO: (Cu,Al) diluted magnetic semiconductors

We present findings on the effect of microstructure on magnetic properties of the epitaxial FePt system. Epitaxial c-axis oriented FePt was synthesized on Si (100) using TiN as a template buffer. The microstructure was progressively varied from a 9nm nanoparticle system to a 30nm thick continuous thin film. We found that individual nanoparticles showed higher coercivity than the continuous thin film. Within the nanoparticle regime, coercivity increased with increasing particle size. The highest coercivity of 13500Oe was realized for a beadlike microstructure, when the individual nanoparticles just begin to merge to form a continuous thin film.

Structure-magnetic property correlations in the epitaxial FePt system

The effect of Al doping on the magnetic properties of Zn(Cu)O based dilute magnetic semiconducting thin films has been systematically investigated. Epitaxial thin films have been deposited onto sapphire c-plane single crystals using pulsed laser deposition technique. X-ray diffraction and high resolution transmission electron microscopy studies show that the Zn(Cu,Al)O films are epitaxially grown onto (0001) sapphire substrates with a 30°/90° rotation in the basal plane. The large lattice misfit of the order of 16% is accommodated by matching integral multiples of lattice and substrate planes. In these large mismatch systems, the resulting films are fully relaxed following deposition of the first complete monolayer of ZnO (consistent with a critical thickness that is less than one monolayer). Magnetic hysteresis measurements indicate that the pure Zn(Cu)O thin films are ferromagnetic at room temperature. Doping with up to 5% Al (n type) does not significantly affect the ferromagnetism even though it resul...

G. R. Trichy

Papers

Magnetic, electrical, and microstructural characterization of ZnO thin films codoped with Co and Cu

The effect of oxygen annealing on ZnO : Cu and ZnO: (Cu,Al) diluted magnetic semiconductors

Structure-magnetic property correlations in the epitaxial FePt system

Effect of Al doping on the magnetic and electrical properties of Zn(Cu)O based diluted magnetic semiconductors

Ball indentation tests for a Zr-based bulk metallic glass