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

Over the past decade intensive research efforts have been carried out by researchers around the globe on exploring the effects of dilute doping of magnetic impurities on the physical properties of functional non-magnetic metal oxides such as TiO2 and ZnO. This effort is aimed at inducing spin functionality (magnetism, spin polarization) and thereby novel magneto-transport and magneto-optic effects in such oxides. After an early excitement and in spite of some very promising results reported in the literature, this field of diluted magnetic semiconducting oxides (DMSO) has continued to be dogged by concerns regarding uniformity of dopant incorporation, the possibilities of secondary ferromagnetic phases, and contamination issues. The rather sensitive dependence of magnetism of the DMSO systems on growth methods and conditions has led to interesting questions regarding the specific role played by defects in the attendant phenomena. Indeed, it has also led to the rapid re-emergence of the field of defect ferromagnetism. Many theoretical studies have contributed to the analysis of diverse experimental observations in this field and in some cases to the predictions of new systems and scenarios. In this review an attempt is made to capture the scope and spirit of this effort highlighting the successes, concerns, and questions.

Dilute doping, defects, and ferromagnetism in metal oxide systems.

In order to elucidate room-temperature (RT) ferromagnetism (FM) in ZnO, we have analyzed a multitude of experimental publications with respect to the ratio of grain-boundary (GB) area to grain volume. FM only appears if this ratio exceeds a certain threshold value ${s}_{\text{th}}$. Based on these important results nanograined pure and Mn-doped ZnO films have been prepared, which reveal reproducible RT FM and magnetization proportional to the film thickness, even for pure ZnO films. Our findings strongly suggest that grain boundaries and related vacancies are the intrinsic origin for RT ferromagnetism.

Magnetization study of nanograined pure and Mn-doped ZnO films: Formation of a ferromagnetic grain-boundary foam

We report here enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature Tc above room temperature. We observe a finite coercive field upto 300K and a finite thermoremanent magnetization upto 340K for the annealed sample. We propose that magnetic moments can form at anionic vacancy clusters. Ferromagnetism can occur due to either superexchange between vacancy clusters via isolated F+ centers, or through a limited electron delocalization between vacancy clusters. Isolated vacancy clusters or isolated F+ centers give rise to a strong paramagnetic like behaviour below 10K.

Enhancement of ferromagnetism upon thermal annealing in pure ZnO

Detection of chemicals and biological species is an important issue to human health and safety. In this paper, we report the hydrothermal synthesis at 95 °C of Cu-doped ZnO low-dimensional rods for room-temperature (RT) sensing applications and enhanced sensor performances. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman and photoluminescence are used to characterize the material properties. To demonstrate the suitability of the Cu-doped ZnO rods for gas sensor applications and for comparison with pure ZnO, we fabricated a double rod device using Focused Ion Beam. The responses of pure-ZnO and Cu-doped ZnO rods studied in exactly the same condition are reported. We found that Cu-ZnO sensors have enhanced RT sensitivity, faster response time, and good selectivity. Miniaturized Cu-ZnO rod-based sensors can serve as a good candidate for effective H2 detectors with low power consumption.

/pdf/synthesis-and-characterization-of-cu-doped-zno-one-3vzzo2t9rc.pdf

Synthesis and characterization of Cu-doped ZnO one-dimensional structures for miniaturized sensor applications with faster response

A series of n-type Zn1−xCuxO (x=0.02, 0.06, 0.10, and 0.12) films was prepared using direct current reactive magnetron sputtering. Magnetic measurements indicate that all the films are ferromagnetic at room temperature and the moment per Cu ion decreases with increasing copper concentration and nitrogen doping. The observed magnetic moment was 1.8μB∕Cu for Zn0.98Cu0.02O film and the transition temperature of about 350K decreased to 320K due to nitrogen doping. It can be concluded that itinerant electrons play an important role in ferromagnetism. The resistivity increased with increasing copper concentration and nitrogen doping. The anomalous Hall effect has been found for ferromagnetic materials.

Magnetic properties of n-type Cu-doped ZnO thin films

A giant exchange bias field of up to 1170 Oe was observed in the Mn2Ni1.6Sn0.4 Heusler alloy. A reentrant spin glass phase and a ferromagnetic martensitic phase coexist below the blocking temperature as confirmed by dc magnetization and ac susceptibility measurements. Exchange bias in Mn2Ni1.6Sn0.4 is thought to originate from the interface exchange interaction between the reentrant spin glass phase and the ferromagnetic martensitic phase. X-ray diffraction and selected area electron diffraction results demonstrate that excess Mn atoms occupy Ni and Sn sites randomly. In this way, Mn-Mn clusters are formed and constitute the reentrant-spin-glass phase.

/pdf/coexistence-of-reentrant-spin-glass-and-ferromagnetic-6f9anzzsjk.pdf

Coexistence of reentrant-spin-glass and ferromagnetic martensitic phases in the Mn2Ni1.6Sn0.4 Heusler alloy

A series of n-type Cu-doped ZnO thin films was prepared by magnetron sputtering. Such films have shown ferromagnetic properties at room temperature. The Cu ion is in a univalent state as identified by x-ray photoelectron spectroscopy. The moment per Cu ion decreases as the Cu concentration increases. The largest moment about 0.52 μB∕Cu was observed for Zn0.95Cu0.05O thin film. The Curie temperature about 360 K was observed for Zn0.95Cu0.05O:N film. To explore the relationship between ferromagnetism and carrier density, nitrogen was introduced into the samples which resulted magnetization and the transition temperature decreasing. These results indicate that the electron density plays an important role in ferromagnetism.

Room-temperature ferromagnetism in n-type Cu-doped ZnO thin films

Room temperature ferromagnetism in Ni-doped ZnO films

Polymorphic magnetization behavior has been observed experimentally in the Heusler alloy Mn${}_{2}$NiGa in which Co has been substituted for Ni or Ga. The magnetization of the austenitic phase can be enhanced up to 132 emu/g, when more than 50$%$ of the antiferromagnetic couplings between Mn atoms are changed to ferromagnetic couplings at the largest composition tolerance for Co substituting for Ga. The effects of the exchange interaction have been investigated based on the corresponding atomic configuration generated by the occupation selectivity of the doped Co atoms. First-principles calculations indicate that a high level of d-electron hybridization can occur when Mn atoms are the nearest neighbors of a Co atom. This causes a strong ferromagnetic exchange interaction in specific atomic configurations and produces a local ferromagnetic structure in the native ferrimagnetic structure matrix. It has been indicated that, based on theoretical work of Stearns, the interatomic distance plays a critical role in producing the local ferromagnetic structure. This has also been used to explain the magnetization behavior through the martensitic transformation in Mn${}_{2}$NiCoGa alloys.

/pdf/polymorphic-magnetization-and-local-ferromagnetic-structure-5bpvfbedtd.pdf

Congmian Zhen

Papers

Magnetic properties of n-type Cu-doped ZnO thin films

Coexistence of reentrant-spin-glass and ferromagnetic martensitic phases in the Mn2Ni1.6Sn0.4 Heusler alloy

Room-temperature ferromagnetism in n-type Cu-doped ZnO thin films

Room temperature ferromagnetism in Ni-doped ZnO films

Polymorphic magnetization and local ferromagnetic structure in Co-doped Mn2NiGa alloys