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In spite of a major effort devoted to the mechanism of ferromagnetism with a high Curie temperature in these materials, it still remains the most controversial research topic, especially given the unexpected d0 ferromagnetism in a series of undoped wide-band-gap oxides films or nanostructures. IntroductionNowadays, people take advantage of the charge of electrons in semiconductors integrated circuits (ICs) for information processing and transmission, thanks to their high speed signal processing and good reliability, but the memory elements are volatile. 50931002, 51072094 and 51372135) and the Ministry of Education of the People's Republic of China (Grant No. Born-in-cloud enterprise solutions are redefining the digital backbone of the organizations enabling them to innovate and succeed in an increasingly connected hyper competitive market place. Ramco’s powerful single HUB with actionable insights to complete your tasks, lets you perform all transactions, from a SINGLE screen.
Ramco offered us a functionally strong and integrated solution with in-built mobility and analytics to address the end-to-end needs of our operations. In the staffing industry, it is important to ensure transparency in the processes, to build long-term engagement between the two most important stakeholders – the employee and the customer. Our business continues to expand in volume and scope of work as well—we need the system more and more as we grow. Pretec SRAM cards (with attribute memory) can be customized for specific applications, devices, and design requirements. If you reside in an EU member state besides UK, import VAT on this purchase is not recoverable. Recently, an abundance of research has shown the critical role of various defects in the origin and control of spontaneous magnetic ordering, but contradicting views from intertwined theoretical calculations and experiments require more in-depth systematic research. People utilize the spin of electrons for information storage in magnetic materials because they are non-volatile despite slow read and write speeds. However, organizations that are tied to traditional and legacy applications are struggling to cope-up with this new market reality. In the event that the primary becomes low, digital signals on the card warn the user that the primary battery needs to be replaced.
With a significant increase in demand for non-volatile, low-power and high-speed memories, researchers from the globe try to make use of both charge and spin of electrons at the same time, making it a challenging field due to its potential application for spintronic devices, such as the spin qubits for quantum computers, spin field effect transistors (spin-FETs), and spin light-emitting diodes (spin-LEDs)[1], [2], [3], [4] , [5]. The write-protect switch on the SRAM card provides a means of protecting against the inadvertent over-writing of stored data. A hardest challenge in the above devices based on spins and charges is that the material should be ferromagnetic above room temperature (RT) and have both efficient spin-polarized carrier injection and transport[4].
This review will present a summary of current experimental status of this defect-driven ferromagnetism in dilute magnetic oxides (DMOs).
However, the reported spin injection efficiency is usually too low, less than 1% with ferromagnetic metals employed as the contacts, due to the large density of states (DOS) mismatch between a metal and semiconductor[6] , [7]. Making use of ultrathin MgO or Al2O3 layers as tunnel barriers has partially alleviated the issue, which raised the injection efficiency to about 30%[8] , [9]. For this reason, developing functional ferromagnetic semiconductors as spin-injection contacts working at RT seems to be imperative because they have high-quality interfaces to semiconductors and do not have the conductivity mismatch problem, and has attracted considerable attention for several decades[10].Earliest research efforts in this field were focused on natural magnetic semiconductors (Fig. 1(a)), such as rock-salt Eu- and Mn-based chalcogenides, in 1960s, but the low Curie temperature and conductivity limited their practical application[11]. The discovery of ferromagnetism in II-VI (CdTe) and III-V (GaAs) compound semiconductors doped with magnetic impurities (Mn) in 1990s made dilute magnetic semiconductors (DMSs) a cutting-edge field[12]. The term DMS refers to a non-magnetic semiconductor wherein a homogeneous dilute concentration of a magnetic dopant (with a net spin moment) is dispersed (Fig. However, most of them also have a low Curie temperature and are difficult to synthesize and dope, which limit their interest. A major breakthrough that focused attention on wide-band-gap semiconductors as being the most promising for achieving practical magnetic ordering temperatures came with the theoretical prediction of room temperature ferromagnetism (RTFM) for 5 at.% Mn doping in GaN diamond and ZnO (see Fig. The theoretical studies were the first step to dilute magnetic oxides (DMOs) and then many groups started to work in this field. The report of RTFM in Co doped TiO2 films has obviously encouraged experimental studies[14].
People understood that the RTFM resulted from the interaction between the dopant moments and carrier spins with different theoretical models such as RKKY[39] , [40], mean field theory[13], [41] , [42], or double exchange[43] , [44] etc., and considered that doping played a key role in the RTFM of DMOs[45]. However, the observation of RTFM in undoped HfO2 thin films[46], [47] , [48] by Coey et al. Neither Hf4+ nor O2- are magnetic ions, and the d and f shells of Hf4+ ions are either empty or full, so called d0 magnetism [49], which challenges our basic understanding that magnetic order in an insulator required the cation to have partially filled shells of d or f electrons.


Triggered by the unexpected results, many groups have made considerable efforts to investigate the possible origin of RTFM in these undoped DMOs.Fig.
2 Predicted Curie temperatures in oxides and semiconductors[13].Given the high sensitivity of the observed RTFM in most DMOs to the growth conditions, researchers considered that the various intrinsic defect states (point or extended) must play a major role after tremendous theoretical and experimental work. Up to now, different kinds of intrinsic defects have been reported to be responsible for the RTFM in undoped DMOs[50], [51], [52] , [53], such as structure defects, surface defects, strain, and point defects like vacancies and interstitials, leaving it still an open question.
In the last several years, our research group has also paid considerable attention on the RTFM in undoped DMO semiconductors and insulators, such as ZnO[54], [55], [56], [57] , [58] and ZrO2[59] , [60]. Thorough analysis and characterization indicate that the ferromagnetism is induced and driven majorly by oxygen vacancy within the DMOs. Here, we present a brief review of the experimental work, which is divided into three parts. First, a brief introduction will be given about several methods used to analyze and characterize defects. Then some results on the defect-driven ferromagnetism will be present, corresponding to the two major research objects in the second and third sections, respectively.
At last, a discussion and some conclusions on defect-driven ferromagnetism in undoped DMOs will be given.2. Defect Analysis and Characterization in DMOsVarious types of intrinsic defects are very common in oxide thin films prepared by various non-equilibrium methods.
Characterizing the physical, chemical and crystallographic states is the necessary work when one wants to investigate the defect-related property.
People usually take advantage of indirect effects of defects in solids, such as electrical, optical, acoustic, and so on, to characterize them, since direct analysis is too difficult to achieve. X-ray photoelectron spectroscopyX-ray photoelectron spectroscopy (XPS) is one of the surface-sensitive quantitative spectroscopic techniques measuring the elemental composition, empirical formula, and chemical state of the elements. It is obtained with a beam of X-ray by irradiating a metal (usually Mg or Al) while simultaneously measuring the kinetic energy and number of electrons escaping from the surface (0-10nm) of the material. By a survey scan, one can know whether some certain elements exist or not within the material, and obtain the stoichiometric ratio by calculation from the survey spectra roughly. To get more details of a certain element, one can conduct a high resolution core level scan and further peak-fitting, thanks to the high sensitivity of binding energy to the chemical environment.
Taking oxygen (O) for instance, one can obtain signs of oxygen vacancy (VO) from the O1s core level spectra if the peak shifts to higher binding energy slightly.
Imagining the oxygen ions in the oxygen-deficient regions, the two electrons weakly bound by VO can be partially and fully ionized, thus decreasing the electron charge density around the VO regions and resulting in less screening of the O2- 1s electrons from their nucleus, which raises the effective nuclear charge and the binding energy of an O2- 1s electron[61] , [62]. Besides the core level spectra, Auger peak in XPS spectra can also provide some clues on defects, and is reported to be much more sensitive to the chemical environment than core level peak for some metal elements[63] , [64].
Positron annihilation spectroscopyPositron annihilation spectroscopy (PAS) is a kind of non-destructive spectroscopic techniques to study defects in solids. It operates on the principle that positrons, when injected into a solid, will annihilate through interaction with electrons, which releases gamma rays that can be detected.
For solids containing free electrons (such as metals or semiconductors), the injected positrons annihilate rapidly unless there are some vacancy defects, especially negative charged vacancy.
That is because that the positrons will reside in the vacancy defects, if they are available, and annihilate less rapidly than that in the bulk of the material. By analyzing the positron lifetime, which is between the emission of positrons from a radioactive source and the detection of gamma rays due to annihilation of positron annihilation, one can obtain some clues on the defects due to the dependence of lifetime on the type of vacancy defects[65].Besides the lifetime spectra, Doppler broadening spectra can also provide useful information of vacancy defects[66], [67] , [68]. Based on the mass-energy equivalence, the energy of gamma rays released after ideal annihilation of a pair of free positron and electron is 511keV.
However, a distribution of velocities of positrons and electrons will result in a broadening by the Doppler Effect. Regarding the Doppler broadening of 511keV line, a normal distribution curve, two line shape parameters are extracted to analyze and characterize the annihilation, i.e. When positron is trapped by a negative charged defect (cation vacancy or anion interstitial), the possibility of its annihilation with core electron will decrease and the line shape of Doppler broadening will become narrow. In other words, if the negative charged vacancy (mainly cation vacancy) defects increase in a solid, line shape parameter S will increase, while parameter W will decrease. In addition, one can also grasp some clues of defect types from the relationship between two line shape parameters of Doppler broadening.
Photoluminescence spectroscopyPhotoluminescence (PL) is one form of luminescence (light emission) after the absorption of photons (electromagnetic radiation).
In a typical PL experiment, a semiconductor is excited with a light-source that provides photons with energy larger than the band gap energy.
Once the photons are absorbed, electrons and holes are formed in the conduction and valence bands, respectively. In a defect-free semiconductor, the major luminescence comes from the near band edge (NBE) emission. If there are various types of defects, the corresponding defect bands will be created between the conduction band and valence band, which may also create conditions for the electron transitions and light emission besides the NBE emission.
By theoretical calculation and experimental analysis, one can make clear what defect state a certain emission originates from.
Taking ZnO as an example, someone can obtain the UV, violet, blue and green emissions simultaneously. The UV emission around 380nm is well understood as an NBE emission while the visible region of the PL spectra clearly illustrates the existence of point defects. Electron paramagnetic resonance spectroscopyElectron paramagnetic resonance (EPR) spectroscopy is a technique used to study materials with unpaired electrons, which can directly characterize paramagnetic defects in samples.
Under an external magnetic field, the electron's magnetic moment aligns itself either parallel or antiparallel to the field.
The splitting of the energy levels between the lower and the upper state is directly proportional to the magnetic field strength. An unpaired electron can move between the two energy levels by either absorbing or emitting a photon of energy hν once the resonance condition is obeyed.
Experimentally, a great majority of EPR measurements are conducted with microwaves at a fixed frequency.
By increasing an external magnetic field, energy separation between the two states is widened until it matches the energy of the microwaves. If there are different chemical states of various paramagnetic defects, like anion vacancy or cation vacancy binding an unpaired electron, in a material, the corresponding resonance will occur at a different g factor [72] , [73]. One can get a clue on density and type of defects referring the intensity and the position, namely the g factor of resonance peak.3. In the past decade, it has also been considered as a promising candidate of DMOs for the theoretically predicted and experimentally obtained RTFM. The discharge voltage and pulse frequency were fixed at 18.3kV and 4Hz in the process of deposition.
The magnetic measurements show a weak ferromagnetism signal existing in the pristine sample. 4 drastically increases with increasing annealing temperature and achieves a maximum after annealing at 600° C. The RT PL experiments were conducted with a 325nm He-Cd laser and revealed a strong defect-related luminescence in the visible region that can be well fitted by two Gaussian peaks centered at 520nm and 585nm respectively (see Fig.
That indicates that some certain defects created during the annealing process would be responsible for the robust luminescence and ferromagnetism. It was reported by literature that the green and yellow luminescence resulted from different states of point defects. Specifically, the emission at ~580nm can be ascribed to the doubly ionized oxygen vacancy (VO2+), while the emission at ~525nm could be assigned to either the zinc vacancy (VZn) or the singly ionized oxygen vacancy (VO+) [77] , [78].
VO2+ had no net spin magnetic moments and could make no active contribution to the ferromagnetism [79].
Excluding VO2+, it is still necessary to make clear whether VZn or VO+ generated after Ar annealing triggered the robust ferromagnetism. 6), suggesting no difference between the pristine and annealed ZnO films in quantity of the only one type of trapping site for positrons, i.e.


With the effects of VZn ignored, the significant enhancement of RTFM and the green luminescence centered at ~520nm could be ascribed to the VO+.Fig.
3 XRD patterns of pristine and annealed ZnO films, and the inset is a cross-section HRTEM image of the pristine ZnO films[56].Fig. Reasonably, under oxygen deficient or reducing annealing conditions, the oxygen vacancy is much easier to be generated as compared to zinc vacancy due to lower formation energy.
After annealing at 600° C, the crystallinity of ZnO films on both Si and quartz substrates is improved.
XPS analysis reveals that all of the samples are oxygen-deficient, but the deficiency of oxygen is very dependent on the magnetic field intensity. At a field intensity of 3T, the oxygen deficiency is at its minimum where a maximum atomic ratio of oxygen to zinc is achieved, i.e. It suggests that the external magnetic field applied during annealing could strongly affect the generation and evolution of defects. When the magnetic field is above 3T, it is strong enough to stimulate the realignments of orientation in ZnO films, thus creating more defects, specifically oxygen vacancies in an oxygen-deficient environment. While the magnetic field is below 3T, it could not cause the orientation realignments but restricts the diffusion and defect generation. In addition, EPR experiments were also conducted to investigate the paramagnetic defects binding a single electron that could contribute to ferromagnetism indeed. All of the EPR spectra of annealed samples possessed a peak with a g factor of ~2.003 (shown in Fig. 8(a)), which was reported to be from VO+, a kind of typical paramagnetic defect [61] , [73], suggesting that the quantity of VO+ generated in all annealed ZnO films is strongly dependent on the intensity of external magnetic field.
As normalized to the mass of the ZnO film, the height of the EPR signal (from peak to valley) might be used as a measure of the quantity of VO+ generated in the film. 8(b) between the normalized MS and the intensity of EPR signal is obtained, indicating that the singly ionized oxygen vacancy (VO+) should be responsible for the enhancement of RTFM observed in the annealed ZnO film. What is more, this work also provides a way to enhance the defect-driven ferromagnetism in ZnO films.Fig. XPS and Rutherford backscattering spectrometry (RBS) analysis show that there is no ferromagnetic contamination in all of the samples and no obvious lattice damage induced by thermal annealing, indicating that the observed RTFM is intrinsic and mostly resulted from defects within ZnO bulk single crystal.
Firstly, after being annealed in flowing argon at 700° C for 2h, the ZnO single crystal exhibits a robust RTFM. Then it is annealed in flowing oxygen at 700° C for 2h, and the MS drastically weakens. When it is annealed in flowing argon at 700° C for 2h again, the RTFM recovers and the MS increases to the similar extent to that annealed in Ar first time. It indicates that this switching of MS arises from the generation and compensation of oxygen vacancy during annealing in oxygen-deficient and oxygen-rich ambience, respectively. These results convincingly suggest that the RTFM in undoped ZnO bulk single crystal is driven majorly by the oxygen vacancy [57].Fig. 9 M-H curves of the ZnO single crystals measured after cyclic thermal annealing process [57].4. Magnetism in Undoped ZrO2 Zirconium dioxide (ZrO2) is one of the most important ceramic materials due to its unusual combination of strength, fracture toughness, ionic conductivity, low thermal conductivity and other distinctive physical and chemical properties, resulting in the widespread application in the fields of structure materials, solid-state electrolytes, thermal barrier coatings, catalytic supports, oxygen gas sensors, and electro-optical materials[84], [85] , [86]. ZrO2 is known to have three low-pressure crystalline structures, namely monoclinic, tetragonal and cubic phases. Synthesis of undoped ZrO2 films To study the magnetism in different phase of ZrO2 films, the high temperature phase of zirconia should be stabilized at RT firstly. As reported by lots of previous work, the most common and traditional method is doping trivalent or divalent cations into ZrO2, such as Y3+, Sc3+, Ca2+, Mg2+, etc[85], [92] , [93]. It is known that the presence of dopant (trivalent or divalent cations) can create a large number of oxygen vacancies and cause significant modification of the electronic structure of the materials, making it more complicated and difficult to clarify whether the intrinsic or the extrinsic defects dominate the physical properties of ZrO2.
Considering that the tetragonal or cubic phase can be stabilized at RT by introducing enough oxygen vacancies or confining the grain size to nanoscale in thin films and nanocrystals, different physical vapor deposition methods have been utilized to synthesize undoped ZrO2 films and achieve the manipulation of crystalline structure.Firstly, pulsed electron-beam deposition (PED) method was applied to synthesize ZrO2 films[59].
It is observed that the ZrO2 films consist of a mixture of monoclinic and tetragonal of which the ratio could be manipulated by adjusting the oxygen partial pressure during deposition (see Fig. In the ZrO2 film deposited at an oxygen partial pressure of 9 mTorr, monoclinic phase is dominant. For ZrO2 films deposited at higher oxygen partial pressures, the content of the room-temperature monoclinic phase decreases, while that of tetragonal phase increases with increasing the partial pressure of oxygen during deposition. Despite an effective regulation of the ratio of the two different phases in a large range, the pure monoclinic or tetragonal ZrO2 films are not obtained.Fig.
Then, the successful regulation and control of the crystalline structure of undoped ZrO2 films is achieved with the method of DC reactive magnetron sputtering.Fig.
Phase-dependent and defect-driven RTFM in undoped ZrO2 films Three representative undoped ZrO2 films synthesized by DC reactive magnetron sputtering was selected for further research, i.e.
These results indicate that the RTFM in undoped ZrO2 films strongly depend on the crystalline structure and that tetragonal phase could be much more active to the RTFM in undoped ZrO2 films.Fig. PL experiments reveal that the emission ascribed to oxygen vacancy drastically weakens (see Fig.
14(b), between the intensity of PL emission originating from oxygen vacancy in ZrO2 films and the normalized MS is observed.
These results indicate that the RTFM in undoped ZrO2 films is strongly driven by the oxygen vacancies.Fig. 15 reveals that a good positive correlation exists between the normalized MS and the normalized intensity of EPR signal ascribed to the singly ionized oxygen vacancy (VO+) and strongly suggest that the RTFM in undoped ZrO2 films is driven mainly by the oxygen vacancy.Fig. Firstly, the sample only consisting of tetragonal phase ZrO2 was annealed at 350° C in air for 1h. After annealing, the structure does not change and still remains in tetragonal phase, but the RTFM almost disappears. Both of Zr 3d and O 1s core level XPS spectra suggest that oxidation degree increases significantly, or that oxygen deficiency is compensated to a large extent. These results strongly indicate that the oxygen vacancy in tetragonal phase plays a key role in the origin of RTFM in undoped ZrO2. Likewise, the sample only consisting of monoclinic phase ZrO2 was also annealed at 350° C under flowing Argon for 1h. After annealing, no changes of the crystalline structure take place which still remains monoclinic, and no ferromagnetic signals appear in spite of a significant reduction of ZrO2 and vast increase of oxygen deficiency, indicating that the monoclinic undoped ZrO2 could not exhibit ferromagnetic at RT even if it possesses a similar oxygen vacancy level with the tetragonal one.
All of the above results strongly suggest that a phase-dependent and defect-driven ferromagnetism exists in undoped ZrO2 films [60].5. Actually, several different theoretical models are set up to explain the unexpected ferromagnetism in undoped wide-band-gap oxides, for example the bound magnetic polaron (BMP) theory[94] and charge transfer ferromagnetism (CTF) theory[95], both of which emphasize the critical roles that defects play in driving the RTFM of undoped DMOs[46], [50] , [96]. In terms of BMP, the spontaneous magnetization is considered to result from a considerable overall volume occupied by BMPs which is beyond the percolation threshold for magnetic ordering. When the density of oxygen vacancy exceeds a certain threshold, it will lead to an overlap of BMPs and enhancing ferromagnetic behavior.
Taking the basic idea of CTF into consideration, the ferromagnetism not only depends on the vacancy levels; it is also associated with the electronic band structure and density of states of the matrix strongly[98].
As the electronic band structure of tetragonal ZrO2 is significantly different from that of monoclinic[99] , [100], it may give us a new thinking on the phase-dependent RTFM observed in undoped ZrO2 films.In summary, the above achievements clearly indicate that the RTFM in undoped DMOs majorly originate from the intrinsic defects, specifically the oxygen vacancy. For future applications, characterizing and analyzing the microstructures and defects finely, designing methods to control the defects accurately, and building the correlations between defects and magnetization quantitatively seem to be a big challenge in this emerging field, in spite of the progress of theoretical and experimental understanding. Furthermore, making use of DMOs to design and fabricate prototype spintronic devices might draw considerable attention, which may also become a new candidate for some new physical and multifunctional devices.
We are pleased to see in the years the coming progress of understanding on the defect-driven magnetism and practical application in spintronics of DMOs.



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