Room temperature ferromagnetism in HfO2 films
TL;DR: In this article, HfO2 films were produced by sputter deposition in the substrate temperature (Ts) range of room temperature (RT)−300 C and their structural, magnetic, and electrical properties were evaluated.
Abstract: HfO2 films were produced by sputter deposition in the substrate temperature (Ts) range of room temperature (RT)−300 °C and their structural, magnetic, and electrical properties were evaluated. The results indicate that the HfO2 films crystallize in the monoclinic structure and are oriented along the (−111) direction. Magnetization measurements (300–1.8 K) evidence their RT ferromagnetism. The effect of Ts is significant on the magnetic moment (M) and coercivity (Hc). M and Hc values enhanced with increasing Ts due to formation of oxygen vacancies. Increase in the temperature from 150 to 300 K decreases Hc without any transition, indicating that the Curie temperature of HfO2 films is higher than RT. Electrical measurements indicate that the HfO2 films are semiconducting.
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TL;DR: Y-doped HfO2 nanopowders, produced by metathesis synthesis, exhibit ferromagnetism at room temperature as mentioned in this paper, and X-ray diffraction and Raman measurements have shown that Y doping is followed by increased ferromagnetic ordering because of the increased concentration of oxygen vacancies in different charge states.
19 citations
Cites background or result from "Room temperature ferromagnetism in ..."
...Having in mind theoretical calculations [7,36] and experimental observations [30–33] and knowing from the Raman and XPS results that Hf1 xYxO2 δ nanopowders are oxygen deficient, we concluded that the oxygen vacancies can be attributed to be the main source of ferromagnetism in undoped and Y-doped HfO2 nanopowders....
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...The experimental reports [30–33] confirmed the existence of RTFM in pure and doped hafnia films and clearly proved that the RTFM originates from the presence of oxygen vacancies....
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19 citations
TL;DR: In this article, monoclinic HfO2 nanorods with an average size of about 33.7 ± 3.1nm in length and 8.9µ± 0.7µnm in width were synthesized via a chemical solution method.
Abstract: Whether or not nanoscale HfO2 has ferromagnetism is a debatable issue. In this study, monoclinic HfO2 nanorods with an average size of about 33.7 ± 3.1 nm in length and 8.9 ± 0.7 nm in width were synthesized via a chemical solution method. In comparison with many HfO2 nanoparticles/nanoclusters synthesized by physical methods, the as-prepared HfO2 nanorods were characterized by their better crystallinity and anisotropic shape. Interestingly, although the pristine HfO2 nanorods showed overall paramagnetic characteristics, when the nanorods were annealed in reducing environment, they exhibited clear room-temperature ferromagnetism (RTFM). The observed RTFM probably resulted from oxygen vacancies generated in the annealing process. In the mean time, it is suggested that factors such as the dimension, specific shape, and crystallinity of the HfO2 materials also should not be ignored when correlating the occurrence of ferromagnetism with defects.
7 citations
TL;DR: In this article, a SiO 2 nanoarray with a hole-bridge structure was constructed using a modified Stober method, which was obtained through three-dimensional self-organization from a siO 2 aqueous suspension by vertical deposition.
7 citations
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TL;DR: Stable electron and hole polaron states in perfect monoclinic HfO2 are predicted by means of ab initio calculations to suggest relatively high mobility of trapped charges.
Abstract: We predict, by means of ab initio calculations, stable electron and hole polaron states in perfect monoclinic HfO2. Hole polarons are localized on oxygen atoms in the two oxygen sublattices. An electron polaron is localized on hafnium atoms. Small barriers for polaron hopping suggest relatively high mobility of trapped charges. The one-electron energy levels in the gap, optical transition energies and ESR g-tensor components are calculated.
132 citations
TL;DR: In this paper, the physical conditions required for the creation of collective ferromagnetism in nonmagnetic oxides by intrinsic point defects such as vacancies were discussed, and the minimum percolation concentration needed to achieve wall-to-wall percolations in a given lattice was derived.
Abstract: We discuss the physical conditions required for the creation of collective ferromagnetism in nonmagnetic oxides by intrinsic point defects such as vacancies. We use ${\mathrm{HfO}}_{2}$ as a case study because of recent pertinent calculations and observations. It was previously noted theoretically that charge-neutral Hf vacancies in ${\mathrm{HfO}}_{2}$ have partially occupied electronic levels within the band gap, and thus the vacancies carry a nonvanishing local magnetic moment. Such density functional supercell calculations have further shown that two such vacancies interact ferromagnetically if they are separated by up to third-neighbor distance. This suggested to the authors that Hf vacancies could explain the observed collective ferromagnetism in thin ${\mathrm{HfO}}_{2}$ films. Here we use our previously developed more complete methodology [Phys. Rev. Lett. 96, 107203 (2006)] to inquire if such vacancies can lead to collective ferromagnetism. Applying this methodology to ${\mathrm{HfO}}_{2}$, we find the following: (i) Hf vacancies appear in a few possible charge states but not all of these have a local magnetic moment. (ii) We calculate the energy required to form such vacancies in ${\mathrm{HfO}}_{2}$ as a function of the chemical potential and Fermi energy, and from this we compute, as a function of growth temperature and oxygen pressure, the equilibrium concentration of those vacancies that have a nonvanishing local magnetic moment. We find that under the most favorable equilibrium growth conditions the concentration of Hf vacancies with magnetic moment at room temperature does not exceed $6.4\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ (fractional composition of ${x}_{\mathrm{eq}}=2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}%$). (iii) Independently, we calculate the minimum Hf vacancy concentration needed to achieve wall-to-wall percolation in the ${\mathrm{HfO}}_{2}$ lattice, given the range of the magnetic ${V}_{\mathrm{Hf}}\text{\ensuremath{-}}{V}_{\mathrm{Hf}}$ interaction (five neighbors) obtained from our supercell calculations. It turns out that the minimum percolation concentration ${x}_{\mathrm{perc}}=13.5%$ needed for collective ferromagnetism is eight orders of magnitude higher than the equilibrium vacancy concentration ${x}_{\mathrm{eq}}$ in ${\mathrm{HfO}}_{2}$ under the most favorable growth conditions. We conclude that equilibrium growth cannot lead to ferromagnetism and that ferromagnetism can be established only if one beats the equilibrium Hf vacancy concentration during growth by as much as eight orders of magnitude. This paper presents also an Appendix that gives the Monte Carlo--calculated percolation thresholds of various lattices as a function of the percolation radius of the interaction.
99 citations
TL;DR: In this paper, it was shown that Fe doping is not the main cause for the ferromagnetic properties of HfO2 thin films, but only acts as a catalyst.
Abstract: Fe-doped HfO2 thin films are room temperature ferromagnetic. In comparison with results of the undoped HfO2 films, it seems that the Fe doping is not the main cause for the ferromagnetism but only acts as a catalyst. Experimental results of oxygen annealing and vacuum heat treatments have proven that in this family of compounds, magnetism might originate from oxygen vacancies or defects. Removing oxygen enhances the magnetic moment, while reversibly filling up oxygen vacancies can destroy the ferromagnetic ordering of the system.
74 citations
TL;DR: In this article, the structural, magnetic and transport properties of polycrystalline Fe doped SrSnO3 samples were investigated and the Reitveld refinement of the X-Ray data for all the compositions was performed.
27 citations
TL;DR: In this article, the authors used vacuum UV spectroscopic ellipsometry (VUVSE) to characterize new high dielectric materials and showed that VUVSE can detect the crystalline character of the layers and their composition in addition to the layer thickness.
Abstract: In this study, we use vacuum UV spectroscopic ellipsometry (VUVSE) to characterize new high dielectric materials. All the candidates for high- k dielectrics become strongly absorbent when the wavelength is reduced down to 140 nm. Due to the high dispersion of the refractive index in the VUV range, it is easier to determine the thickness and the refractive index independently than in the visible range and much more precise structural information can be deduced. HfO 2 , Al 2 O 3 and mixed HfAlO x layers have been studied with and without thin SiO 2 oxide at the interface. X-ray reflectometry (XRR) has been used to measure precisely the layer thickness and roughness. The two techniques are included in the same automated metrology system dedicated to 300 mm technology which is also presented. We show in particular that VUVSE can detect the crystalline character of the layers and their composition can be measured in addition to the layer thickness. Results are compared to those obtained by transmission electron microscopy (TEM), X-ray fluorescence analysis (XRF) and X-ray photoemission (XPS).
19 citations