Oxygen vacancy controlled tunable magnetic and electrical transport properties of (Li, Ni)-codoped ZnO thin films
TL;DR: In this article, the electrical, magnetic, and magnetotransport properties of Li-Ni codoped ZnO thin films in the electron dominated, hole dominated, and insulating regimes were investigated.
Abstract: We investigated the electrical, magnetic, and magnetotransport properties of Li–Ni codoped ZnO thin films in the electron dominated, hole dominated, and insulating regimes. In a narrow window of oxygen growth pressure, 10−3–10−2 mbar, the films exhibited p-type conductivity with a maximum hole concentration ∼8.2×1017 cm−3. Magnetoresistance exhibited by the films is attributed to scattering of charge carriers due to localized magnetic moments. Insulating films showed superparamagnetic behavior, whereas both n-type and p-type films showed room temperature ferromagnetism. Our findings suggest that oxygen vacancies and Ni ions in cation site are jointly responsible for ferromagnetism that is not dependent on the carrier type.
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TL;DR: In this paper, undoped and lithium (Li) doped MoO 3 thin films were deposited over ITO-coated glass substrates by spray pyrolysis at a substrate temperature of 325 ˚C.
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TL;DR: In this paper, a comprehensive investigation of ZnO:Co thin films based on the combined measurement of macroscopic and microscopic properties was conducted, and no evidence for carrier-mediated itinerant ferromagnetism was found.
Abstract: During the past years, there has been renewed interest in the wide-bandgap II–VI semiconductor ZnO, triggered by promising prospects for spintronic applications. First, ferromagnetism was predicted for dilute magnetic doping. In a comprehensive investigation of ZnO:Co thin films based on the combined measurement of macroscopic and microscopic properties, we find no evidence for carrier-mediated itinerant ferromagnetism. Phase-pure, crystallographically excellent ZnO:Co is uniformly paramagnetic. Superparamagnetism arises when phase separation or defect formation occurs, due to nanometer-sized metallic precipitates. Other compounds like ZnO:(Li,Ni) and ZnO:Cu do not exhibit indication of ferromagnetism. Second, its small spin–orbit coupling and correspondingly large spin coherence length makes ZnO suitable for transporting or manipulating spins in spintronic devices. From optical pump/optical probe experiments, we find a spin dephasing time of the order of 15 ns at low temperatures, which we attribute to electrons bound to Al donors. In all-electrical magnetotransport measurements, we successfully create and detect a spin-polarized ensemble of electrons and transport this spin information across several nanometers. We derive a spin lifetime of 2.6 ns for these itinerant spins at low temperatures, corresponding well to results from an electrical pump/optical probe experiment.
27 citations
TL;DR: In this article, the long-rang interaction between bound-magnetic-polarons led by the singly charged oxygen vacancy (Vo+) was found to increase the magnetization of hydrogenated Zn1−xMnxO nanoparticles.
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TL;DR: Zener's model of ferromagnetism, originally proposed for transition metals in 1950, can explain T(C) of Ga(1-)(x)Mn(x)As and that of its II-VI counterpart Zn(1)-Mn (x)Te and is used to predict materials with T (C) exceeding room temperature, an important step toward semiconductor electronics that use both charge and spin.
Abstract: Ferromagnetism in manganese compound semiconductors not only opens prospects for tailoring magnetic and spin-related phenomena in semiconductors with a precision specific to III-V compounds but also addresses a question about the origin of the magnetic interactions that lead to a Curie temperature (T(C)) as high as 110 K for a manganese concentration of just 5%. Zener's model of ferromagnetism, originally proposed for transition metals in 1950, can explain T(C) of Ga(1-)(x)Mn(x)As and that of its II-VI counterpart Zn(1-)(x)Mn(x)Te and is used to predict materials with T(C) exceeding room temperature, an important step toward semiconductor electronics that use both charge and spin.
7,062 citations
TL;DR: It is proposed thatferromagnetic exchange here, and in dilute ferromagnetic nitrides, is mediated by shallow donor electrons that form bound magnetic polarons, which overlap to create a spin-split impurity band.
Abstract: Dilute ferromagnetic oxides having Curie temperatures far in excess of 300 K and exceptionally large ordered moments per transition-metal cation challenge our understanding of magnetism in solids. These materials are high-k dielectrics with degenerate or thermally activated n-type semiconductivity. Conventional super-exchange or double-exchange interactions cannot produce long-range magnetic order at concentrations of magnetic cations of a few percent. We propose that ferromagnetic exchange here, and in dilute ferromagnetic nitrides, is mediated by shallow donor electrons that form bound magnetic polarons, which overlap to create a spin-split impurity band. The Curie temperature in the mean-field approximation varies as (xdelta)(1/2) where x and delta are the concentrations of magnetic cations and donors, respectively. High Curie temperatures arise only when empty minority-spin or majority-spin d states lie at the Fermi level in the impurity band. The magnetic phase diagram includes regions of semiconducting and metallic ferromagnetism, cluster paramagnetism, spin glass and canted antiferromagnetism.
2,743 citations
TL;DR: In this paper, the authors used a new technique to fabricate p-type ZnO reproducibly, and showed high-quality undoped films with electron mobility exceeding that in the bulk.
Abstract: Since the successful demonstration of a blue light-emitting diode (LED)1, potential materials for making short-wavelength LEDs and diode lasers have been attracting increasing interest as the demands for display, illumination and information storage grow2,3,4. Zinc oxide has substantial advantages including large exciton binding energy, as demonstrated by efficient excitonic lasing on optical excitation5,6. Several groups have postulated the use of p-type ZnO doped with nitrogen, arsenic or phosphorus7,8,9,10, and even p–n junctions11,12,13. However, the choice of dopant and growth technique remains controversial and the reliability of p-type ZnO is still under debate14. If ZnO is ever to produce long-lasting and robust devices, the quality of epitaxial layers has to be improved as has been the protocol in other compound semiconductors15. Here we report high-quality undoped films with electron mobility exceeding that in the bulk. We have used a new technique to fabricate p-type ZnO reproducibly. Violet electroluminescence from homostructural p–i–n junctions is demonstrated at room-temperature.
1,964 citations
TL;DR: It is shown that thin films of hafnium dioxide (HfO2), an insulating oxide better known as a dielectric layer for nanoscale electronic devices, can be ferromagnetic even without doping.
Abstract: It is generally accepted that magnetic order in an insulator requires the cation to have partially filled shells of d or f electrons. Here we show that thin films of hafnium dioxide (HfO2), an insulating oxide better known as a dielectric layer for nanoscale electronic devices, can be ferromagnetic even without doping. This discovery challenges our understanding of magnetism in insulators, because neither Hf4+ nor O2- are magnetic ions and the d and f shells of the Hf4+ ion are either empty or full.
1,046 citations
TL;DR: Results are interpreted in terms of a spin-split donor impurity-band model, which can account for ferromagnetism in insulating or conducting high-k oxides with concentrations of magnetic ions that lie far below the percolation threshold.
Abstract: Room-temperature ferromagnetism is observed in (110) oriented ZnO films made from targets containing 5 at. % of Sc, Ti, V, Fe, Co, or Ni, but not Cr, Mn, or Cu ions. There are large moments, $2.6{\ensuremath{\mu}}_{B}$ and $0.5{\ensuremath{\mu}}_{B}/\mathrm{\text{dopant atom}}$ for Co- and Ti-containing oxides, respectively. There is also a moment of $0.3{\ensuremath{\mu}}_{B}/\mathrm{S}\mathrm{c}$. Magnetization is very anisotropic, with variations of up to a factor of 3 depending on the orientation of the applied field relative to the substrate. Results are interpreted in terms of a spin-split donor impurity-band model, which can account for ferromagnetism in insulating or conducting high-$k$ oxides with concentrations of magnetic ions that lie far below the percolation threshold. Magnetic moments are associated with two-electron defects in the films as well as unpaired electrons of the $3d$ ions.
979 citations