Wear damage of MgO single crystals

Mechanisms of surface and sub-surface wear damage in MgO single crystals were investigated by scratching with two sintered alumina sliders, having tip radii of 60 and 120m, using a simple scratching apparatus in a controlled atmosphere. The degree of surface and sub-surface cracking is dependent on the shape of the slider and the normal contact load, which are related to the penetration into the crystal. The chevron crack on the (001) plane in the [100] sliding direction consists of cracks intersecting at an angle of 90°, and with a spread angle of about 120°, and the normal crack. The nature of the sub-surface damage is investigated; a parallel crack develops in front of the slider and an oblique crack propagates towards the front of the slider. Then an internal normal crack is formed between the oblique crack and the parallel crack. In the [110] direction, the oblique crack initiates from the top of the normal crack under the surface, and the parallel crack continues from the oblique crack. This wear damage is explained by the dislocation interactions occurring due to the distribution of resolved shear stresses during sliding. The wear caused by the chevron crack is a factor of 10 higher than that with plastic flow. Internal cracks do not have a direct influence on the increase of wear.     

Cathodoluminescence at frictional damage in MgO single crystals

The frictional damage in single-crystal MgO was studied using the cathodoluminescent mode of a scanning microscope as well as selected-area electron channelling pattern analysis. With a hemispherical diamond slider, the distorted structured layer beneath the surface was noted, the cathodoluminescence of which was quenched, presumably due to the high density of defects. This non-luminescent layer was encased in the plastically deformed luminescent zone, which extended downwards to where there was a (0 0 1) cleavage crack. It was also found that with a larger ball slider, luminescent slip lines on both {1 1 0}₉₀ and {1 1 0}₄₅ were pronounced inside the track after a single to-and-fro traversal, and successive traversals were able to generate a non-luminescent, distorted structured layer inside the track.     

Exoelectron emission from ion-implanted MgO single crystals

Thermally stimulated exoelectron emission (TSEE) measurements were carried out for pure MgO single crystals, annealed and then irradiated to either ion beams of different doses and energies or X-rays. H⁺- and He⁺-ion implantation causes an increase of the TSEE yield compared to that of nonimplanted samples. The TSEE response is dependent on the dose and energy of the implanted ions.     

Optical properties of MgO doped near-stoichiometric LiTaO3 single crystals

Comparative study of the optical properties of undoped and 1-3 mol% MgO doped near-stoichiometric LiTaO₃ (SLT) crystals were undertaken. It was observed that the red shift in the absorption edge occurred with the increasing MgO doping concentration. The infrared absorption spectrum of the OH-stretch-mode in SLT was measured for crystals of undoped and 1-3 mol% MgO doped compositions. The coercive field for the crystals was measured to be 0.913, 0.610 and 0.735 kV/mm for 1-3 mol% MgO doped SLT, respectively. Photorefractive damage of SLT single crystals with 1-3 mol% MgO doping levels was measured to be 136.29, 180.25 and 222.54 MW/cm².     

Interfacial interaction in Fe/MgO/Fe magnetic tunneling junctions

We present a study on interfacial structures and tunneling magnetoresistance (TMR) in Fe/MgO/Fe junctions using a MgO(111) film with {100} facets. It is shown using X-ray photoelectron spectroscopy that a FeO layer occurs at MgO/Fe rather than Fe/MgO interface, which could be used to tune the TMR effect. At the Fe/MgO interface, such a change in electronic structure is attributed to the band bending associated with a change in thickness of Fe films. The present study provides a new understanding on the Fe/MgO/Fe interfacial behavior and metal/oxide barriers involving electron transport.     

The dielectric behaviour of single-crystal MgO,Fe/MgO and Cr/MgO

The dielectric constants and loss factors,, for pure single-crystal MgO and for Fe-and Cr-doped crystals have been measured at frequencies, , from 500 Hz to 500 kHz at room temperature. For pure MgO at 1 kHz the values of and the loss tangent, tan , (9.62 and 2.16×10^–3, respectively) agree well with the data of Von Hippel; the conductivity, , varies as ⁿ withn=0.98±0.02. In Fe-doped crystals increases with Fe-concentration (at any given frequency); for a crystal doped with 12800 ppm Fe, was about four times the value for pure MgO. At all concentrations the variation of log with log was linear andn=0.98±0.02. A decrease in with increasing Fe-concentration was also observed. A similar, although less pronounced, behaviour was found in Cr-doped crystals. The effects are discussed in terms of hopping mechanisms.     

High frequency dielectric properties of MgO,Fe/MgO and Cr/MgO

The study of the dielectric constant and loss in pure single-crystal MgO and in Fe- and Cr-doped crystals, the low frequency results of which have been reported [1], has now been extended to cover frequencies from 500 Hz to 9 GHz. Measurements were made on the same specimens, at room temperature, using bridge, cavity, slotted-line and cavity-resonator techniques. Over the whole frequency range, a good fit is obtained to the Universal Laws of dielectric response: _ac ⁿ and (–) ^n–1 with n=0.98±0.02 for both pure and doped MgO. At any point in this frequency range the addition of iron or chromium increases and decreases , the changes being more pronounced with iron. These effects are discussed in terms of hopping mechanisms.     

Thermally activated flow parameters in the deformation of MgO single crystals

Strain-rate cycling methods have been used to determine the thermally activated flow parameters in the compressive deformation of MgO single crystals. Activation volumes of the order of 100b ³ have been measured for deformation below room temperature, indicating dislocation-point defect interactions as the rate controlling mechanism for hardening at these temperatures, whileV ^* values of the order of 1000b ³ have been measured at and above room temperature. This, together with the fact that the effective stress remains constant with strain at these temperatures identifies the interaction of dislocations with dislocation dipoles as the rate controlling mechanism in this temperature range. The Gibbs free energy for deformation,G, equals the activation enthalpy,H, at low temperatures and the deviation ofG fromH above room temperature may be attributed to entropy effects.     

The temperature dependence of permittivity in MgO and Fe-MgO single crystals

The permittivities (ε′) of undoped MgO and iron-doped Fe-MgO single crystals have been measured over the temperature range 20 to 650? C for frequencies between 500 Hz and 50 kHz. From 25 to around 200? C the temperature dependence of ε′ fits well with Havinga's formula and the value of [(ε′ ? 1) (ε′ + 2)]^?1 (?ε′/?T) = 1.02 × 10^?2K^?1 found for undoped MgO agrees closely with data published for lower temperature ranges; this increases considerably with the addition of iron, rising to 2.85×10^?5K^?1 for MgO single crystals doped with 12 900 p.p.m. iron. Above 200? C the permittivity changes much more rapidly than the Havinga formula predicts, the variation being greater in iron-doped specimens. The frequency dependence ofε′ is also temperature-dependent; below 200? Cε′ follows [ε′(Ω) ? ε′ _∞] ∫Ω ^(n?1) withn=0.98±0.02 for all samples, but above 200? C the value ofε′ falls more rapidly with frequency than would be expected from this law. The effect is more pronounced for MgO with 12 900 p.p.m. iron. The results are discussed in terms of a contribution to the measured permittivity arising from temperature-enhanced conductivity.     

Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions

The tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs) is the key to developing magnetoresistive random-access-memory (MRAM), magnetic sensors and novel programmable logic devices. Conventional MTJs with an amorphous aluminium oxide tunnel barrier, which have been extensively studied for device applications, exhibit a magnetoresistance ratio up to 70% at room temperature. This low magnetoresistance seriously limits the feasibility of spintronics devices. Here, we report a giant MR ratio up to 180% at room temperature in single-crystal Fe/MgO/Fe MTJs. The origin of this enormous TMR effect is coherent spin-polarized tunnelling, where the symmetry of electron wave functions plays an important role. Moreover, we observed that their tunnel magnetoresistance oscillates as a function of tunnel barrier thickness, indicating that coherency of wave functions is conserved across the tunnel barrier. The coherent TMR effect is a key to making spintronic devices with novel quantum-mechanical functions, and to developing gigabit-scale MRAM.     

Spin filtering effects in monocristalline Fe/MgO/Fe magnetic tunnel junctions

Single-crystal magnetic tunnel junctions employing bcc (1 0 0) Fe electrodes and MgO(1 0 0) insulating barrier are elaborated by molecular beam epitaxy. Two extreme regimes have been investigated. First, for extremely thin MgO thickness we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions, mediated by the tunneling of the minority spin interfacial resonance state. Second, for large MgO barrier thickness, the tunnel transport validates specific spin filtering effect in terms of symmetry of the electronic Bloch function and symmetry-dependent wave function attenuation in the single-crystal barrier. Within this framework, we present giant tunnel magnetoresistive effects at room temperature (125–150%). Moreover, we illustrate that the interfacial chemical and electronic structure plays a crucial role in the filtering. We show that the insertion of carbon impurities at the Fe/MgO interface changes radically the voltage response of the tunnel magnetoresistance. Moreover, we provide experimental evidence for the electronic interfacial resonance states contribution to the spin polarized tunnel transport.     

The formation of hydrogen-filled cavities in MgO crystals annealed in reducing atmospheres

Defects causing cloudiness in some MgO crystals annealed in reducing atmospheres are shown to be lenticular cavities up to 200 Μm diameter, containing hydrogen gas under high pressure. A close connection between the formation of cavities and the hydroxyl impurity content of the crystals is established, and a mechanism for their formation proposed, involving the reaction of oxygen vacancies and electrons, created at the surface, with V_OH centres in the interiors of the crystals. On the basis of these results and work by Amelinckxet al. and Barret al. a general principle is postulated, in which a special gaseous environment, by reacting with a crystal surface, causes a second reaction within the crystal which converts an anion impurity into a permanent gas and produces gas-filled cavities. This principle may be applicable to many ionic crystal/reducing gas systems.     

Interface characterization of epitaxial Fe/MgO/Fe magnetic tunnel junctions

Following predictions by first-principles theory of a huge tunnel magnetoresistance (TMR) effect in epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs), measured magnetoresistance (MR) ratios of about 200% at room temperature (RT) have been reported in MgO-based epitaxial MTJs. Recently, a MR ratio of about 600% has been reported at RT in MgO-based MTJs prepared by magnetron sputtering, using amorphous CoFeB as the ferromagnetic electrode. These MTJs show great potential for application in spintronic devices. Fully epitaxial MTJs are excellent model systems that enhance our understanding of the spin-dependent tunneling process as the interface is well defined and can be fully characterized. Both theoretical calculations and experimental results clearly indicate that the interfacial structure plays a crucial role in the coherent tunneling across a single crystal MgO barrier, especially in epitaxial MgO-based MTJs grown by molecular beam epitaxy (MBE). Surface X-ray diffraction, Auger electron spectroscopy, X-ray absorption spectra, and X-ray magnetic circular dichroism techniques have been reported previously for interface characterization. However, no consistent viewpoint has been reached on the interfacial structures (such as FeO layer formation at the bottom Fe/MgO interface), and it is still an open issue. In this article, our recent studies on the interface characterization of MgO-based epitaxial MTJs by X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and spin-dependent tunneling spectroscopy, will be presented.     

Prolonged plastic deformation related to the microindentation of MgO single crystals

The plastic deformation regions near the indentations on the (0 0 1) plane of MgO single crystals were investigated for an indentor under load and after unloading. It was established that the completion of slip-line structures arising during indentor penetration occurred during indentor removal. The presence of prolonged plastic deformation has been explained by considering the impulse character of the microindentation process of MgO at room temperature.     

High-temperature creep and dislocation structure of MgO single crystals at low stresses

MgO single crystals oriented toward (100) have been compressively deformed to strains in the range of 0.04 to 0.09 at temperatures between 1948 and 2023 K using stresses under 6 MPa. Microstructural developments in the crept samples were monitored by etch pitting, SEM and HVEM. Optical microscopy revealed an equiaxed network of subgrains with 〈110〉 forming the framework for the formation of subgrains. Measurement of the dislocation density not associated with cells reveals that the stress dependence of the steady-state values of dislocation density can be described by the relationϱ ∫σ ^2.15. HVEM observations show that cell boundaries are formed by the process of knitting. Drastic unloading results in a fraction of the sub-boundaries straightening, but the majority of the boundaries are destroyed. It is concluded that creep of MgO at low stresses and high temperature is similar to those of fcc metals.     

TEM study of the recovery process of cyclically deformed MgO single crystals

Single crystals of MgO were subjected to plastic strain-controlled push-pull cyclic deformation at elevated temperatures. Below 400° C the crystals were very brittle and failed with a few fatigue cycles. At 470° C a large number of cycles could be obtained before failure, and the cyclic stress-strain response showed a period of rapid hardening followed by a period of decreasing hardening rate. TEM investigations of the lower temperature samples show structures of isolated dislocation dipoles, multipoles and debris. At 470° C dense bundles of dislocations were observed aligned perpendicular to the Burgers vector direction. The regions between the bundles were relatively dislocation free, but they contained a high density of debris. Bowed out screw dislocations are observed between the edge dislocation bundles, suggesting that screw dislocations were largely mobile. Comparisons are made with the cyclic deformation and structure of fcc metals and other NaCl structure single crystals.     

Hydroxyl impurity and the formation and distribution of cavities in melt-grown MgO crystals

Defects causing cloudiness in melt-grown magnesium oxide crystals are identified as octahedral or cubo-octahedral cavities, with sizes in the range 10³ to 10⁴ å, containing hydrogen at ～ 400 atm pressure. A close relationship between the formation of cavities and the presence of hydroxyl impurities in the crystal is established, and a mechanism for their formation is postulated, based on the reaction of oxygen vacancies and electrons produced at the surface with V_OH centres in the interior of the crystal.