MgAl2O4 Research Articles

Microstructural evolution of perpendicular magnetization films with an ultra-thin Co2FeAl/MgAl2O4(001) structure

We investigated the structure of 1-nm-thick Co2FeAl (CFA)/MgAl2O4 (MAO) epitaxial heterojunctions in order to understand the origin of perpendicular magnetic anisotropy (PMA). The CFA/MAO stacks were fabricated by plasma oxidation of Mg-Al grown on the CFA layer and subsequent post-annealing process. Before post-annealing (as-oxidized), the stack showed in-plane magnetization with poor crystallinity of the MAO layer. After post-annealing, the stack showed PMA accompanied by the improved crystallinity in the MAO layer, leading to the formation of a lattice-matched CFA/MAO(001) epitaxial interface. These behaviors indicate that the crystallization of the MAO layer is strongly promoted by atomic templating effect from a lattice-matched underlayer. In addition, we also found that this MAO crystallization effectively enhanced PMA at the interface. At the same time, significant Al atomic diffusion from CFA to MAO was confirmed. This Al diffusion was observed in the as-oxidized stack, and this was further promoted by the post-annealing. Therefore, the strong PMA at the CFA/MAO interfaces is attributed to (i) the reduced in-plane magnetocrystalline anisotropy due to the lattice-matching and (ii) the promoted hybridization between Fe and O orbitals due to the Al re-distribution near the interface. These results include important findings for constructing high performance perpendicularly-magnetized magnetic tunnel junctions for non-volatile magnetic memory applications.

Passively Q-switched Pr:YLF laser with a Co2+:MgAl2O4 saturable absorber.

We report on a 2ω-OPSL-pumped passively Q-switched Pr3+:LiYF4 laser operating at the wavelengths of 523, 607, and 640nm. For this, we utilized a Co2+-doped MgAl2O4 (MALO) crystal as a saturable absorber in the visible range for the first time, to the best of our knowledge. In the green spectral region, the pulse duration was 210ns with a pulse energy of 3.6μJ and a repetition rate of 125kHz. The minimum pulse duration of 87ns at the highest pulse energy of 8.6μJ was obtained at a repetition rate of 110kHz at 607nm. The highest Q-switched average laser power of 1.4W was obtained at an absorbed continuous-wave pump power of 3.3W with a slope efficiency of 47% at 640nm. Absorption saturation measurements with Co:MALO were performed, and ground and excited state absorption cross sections in the visible spectral range were determined.

Evolution of magnetic properties in the vicinity of the Verwey transition in Fe3O4 thin films

We have systematically studied the evolution of magnetic properties, especially the coercivity and the remanence ratio in the vicinity of the Verwey transition temperature (TV ), of high-quality epitaxial Fe3O4 thin films grown on MgO (001), MgAl2O4 (MAO) (001), and SrTiO3 (STO) (001) substrates. We observed rapid change of magnetization, coercivity, and remanence ratio at TV , which are consistent with the behaviors of resistivity versus temperature [\r{ho}(T )] curves for the different thin films. In particular, we found quite different magnetic behaviors for the thin films onMgOfrom those onMAOand STO, inwhich the domain size and the strain state play very important roles. The coercivity is mainly determined by the domain size but the demagnetization process is mainly dependent on the strain state. Furthermore, we observed a reversal of remanence ratio at TV with thickness for the thin films grown on MgO: from a rapid enhancement for 40-nm- to a sharp drop for 200-nm-thick film, and the critical thickness is about 80 nm. Finally, we found an obvious hysteretic loop of coercivity (or remanence ratio) with temperature around TV , corresponding to the hysteretic loop of the \r{ho}(T ) curve, in Fe3O4 thin film grown on MgO.

Preparation and properties of MgAl2O4 based ceramics reinforced with rod-like microcrystallines by co-doping Sm2O3 and La2O3

A rod-like microcrystalline reinforced MgAl2O4 (MA) based ceramic with high strength and good thermal shock resistance has been successfully prepared by co-doping Sm2O3 and La2O3 via a single-stage solid-state reaction sintering (SRS) process. The effects of addition amounts of Sm2O3 and La2O3 on the phase compositions, microstructures, shrinkage ratio, apparent porosity, bulk density, high temperature compressive strength and thermal shock resistance of the MA based ceramics have been investigated. The results showed that MA, SmAlO3, La10Al4O21, Sm4.67(SiO4)3O and La4.67(SiO4)3O phases could be found in the ceramics after co-doping additives. The new formed rare earth compounds could prevent the growth of MA grains leading to a densification microstructure of the MA based ceramics. The Sm4.67(SiO4)3O and La4.67(SiO4)3O were formed by the reaction between additives and impurities of raw materials, which presented as rod-like microcrystallines to effectively clean the impurities in the grain boundaries of the MA based ceramics. Accordingly, the sintering properties, high temperature compressive strength and thermal shock resistance of the MA based ceramics were improved markedly.

Endurance of magnetic tunnel junctions under dynamic voltage stress

MgAl2O4 (MAO)-based magnetic tunnel junctions (MTJs) with an MAO thickness of ∼1.25 nm are fabricated and their cycling characteristics under dynamic voltage stress are evaluated. The speed of breakdown strongly depended on the pulse polarities used, bipolar, positive (+) unipolar, and negative (−) unipolar. The bipolar condition yielded more rapid breakdown under cycling. Between the two unipolar conditions, positive bias yielded more rapid breakdown than negative bias; the difference between these is understood to arise from the conditions of the interface between the MAO and ferromagnetic layers. Among apparently normal MTJ cells showing little resistance drift, 20% were degraded during a long cycling test in the bipolar stress condition. Thus, the use of bipolar voltage stress is essential to screen for potentially defective MTJs, and the asymmetric condition at the interface is minimised by process control for application of the simple unipolar bias condition.

Bulk Single Crystal-Like Structural and Magnetic Characteristics of Epitaxial Spinel Ferrite Thin Films with Elimination of Antiphase Boundaries.

Spinel ferrite NiFe2 O4 thin films have been grown on three isostructural substrates, MgAl2 O4 , MgGa2 O4 , and CoGa2 O4 using pulsed laser deposition. These substrates have lattice mismatches of 3.1%, 0.8%, and 0.2%, respectively, with NiFe2 O4 . As expected, the films grown on MgAl2 O4 substrate show the presence of the antiphase boundary defects. However, no antiphase boundaries (APBs) are observed for films grown on near-lattice-matched substrates MgGa2 O4 and CoGa2 O4 . This demonstrates that by using isostructural and lattice-matched substrates, the formation of APBs can be avoided in NiFe2 O4 thin films. Consequently, static and dynamic magnetic properties comparable with the bulk can be realized. Initial results indicate similar improvements in film quality and magnetic properties due to the elimination of APBs in other members of the spinel ferrite family, such as Fe3 O4 and CoFe2 O4 , which have similar crystallographic structure and lattice constants as NiFe2 O4 .

In-situ formation and densification of MgAl2O4-Y3Al5O12 and MgAl2O4-MgNb2O6 ceramics via a single-stage SRS process

MgAl2O4 (MA)-Y3Al5O12 (YAG) and MA-MgNb2O6 (MN) ceramics with high density were successfully fabricated via a single-stage solid-state reaction sintering (SRS) process at 1580?C for 4 h. The effect of Y2O3 or Nb2O5 additions from 2.5 wt% to 7.5 wt% on the phase compositions, microstructures, shrinkage ratio, apparent porosity, bulk density and cold compressive strength of MA-YAG and MA-MN ceramics has been investigated. It was found that MgO and Y2O3 reacted with Al2O3 to form MA and YAG during sintering while Nb2O5 reacted with MgO to form MN. YAG and MA grains in the MA-YAG ceramics exist as granular shape, and their average grain size is about 1 ?m and 5 ?m, respectively. YAG grains distribute on the intergranular space of MA particles. Polygonal MA particles can be observed in the MA-MN ceramics, and MN grains distribute on the intergranular space of MA particles as well as on MA particles. Rod-like MN grains can be formed in the MA-MN ceramics by addition of 7.5 wt% Nb2O5. The diameter shrinkage ratio, volume shrinkage ratio, bulk density and cold compressive strength of MA-YAG and MA-MN ceramics are greatly improved by doping Y2O3 and Nb2O5, respectively.

Reliability of magnetic tunnel junctions with a spinel MgAl 2 O 4 film

We investigated the stress-induced resistance drift and breakdown characteristics in MgAl2O4 (MAO)-based magnetic tunnel junctions (MTJs) with the MAO thickness of ∼0.9 nm. The occurrence of trap sites around the tunnel barrier–electrode interfaces was characterised via interval voltage stress (IVS) and constant voltage stress (CVS) experiments, in which the occurrence of trap sites affects the resistance drift in IVS and time-dependent dielectric breakdown in CVS. The observed resistance drift ratios were almost the same for positive and negative applied voltages, indicating that the difference in the interface structure was negligibly small between the top and bottom MAO/CoFe interfaces in the MAO-based MTJs. More interestingly, the resistance drift ratios were very low compared with the results of a previous study on MgO-based MTJs. MAO-based MTJs is likely to be an effective alternative that can be used to improve MTJ device reliability.

Highly oriented epitaxial (α′′+α′)-Fe16N2 films on α-Fe(001) buffered MgAl2O4(001) substrates and their magnetization

We grew epitaxial films of α′-Fe8N by molecular beam epitaxy on α-Fe buffered MgO(001) and MgAl2O4(MAO)(001) substrates. The lattice mismatches of these substrates with α′-Fe8N are −4.0% and 0.08%, respectively. We discuss effects on the crystal orientation of the grown layers in terms of the substrate material and the thickness of an α-Fe buffer layer. The 3-nm-thick α-Fe buffer layer on MAO was compressively strained and provided a more suitable in-plane lattice constant for growth of α′-Fe8N. The full width at half maximum of the x-ray ω-scan rocking curve peak intensity of the α′-Fe8N(002) plane was 0.09° when it was formed on α-Fe(3nm)/MAO. This value was much smaller than that obtained for α-Fe(3nm)/MgO samples (1.85°). (α′′+α′)-Fe16N2 films, which had partly ordered N-sites, were formed by post-annealing of the α′-Fe8N films on α-Fe/MgO(001) and α-Fe/MAO(001) at 165°C. The degree of N-site ordering reached 0.08 after 20h for (α′′+α′)-Fe16N2 grown on α-Fe(3nm)/MAO(001). The saturation magnetizations were approximately 1650emu/cm3, regardless of the annealing time and substrate used. The saturation field of the (α′′+α′)-Fe16N2 films increased by the post-annealing.

Growth and magnetic properties of epitaxial Fe4N films on insulators possessing lattice spacing close to Si(001) plane

We grew ferromagnetic Fe4N films by molecular beam epitaxy on MgO(001), MgAl2O4(MAO)(001), SrTiO3(STO)(001), and CaF2(001) substrates, possessing the lattice spacing close to Si(001) plane. Highly oriented epitaxial growth was confirmed for the Fe4N films on the MgO, MAO, and STO by reflection high-energy electron diffraction and x-ray diffractions. The degree of orientation of the Fe4N film on the STO was the best among these samples. This was attributed to the smallest lattice mismatch of −2.8% between Fe4N(001) and STO(001). On the other hand, crystallinity of the Fe4N film on the CaF2(001) substrate was poor due to a very large lattice mismatch of −30% between Fe4N(001) and CaF2(001) arising from the unexpected epitaxial relationship as Fe4N(001)[100] || CaF2(001)[100]. The saturation magnetization of the Fe4N films was approximately 1200emu/cm3 at room temperature for all the samples, and the magnetization easy axis was in-plane Fe4N[100]. We consider that STO is the suitable buffer layer for the growth of Fe4N on Si(001), hence to realize the Si-based spintronics devices using highly spin-polarized Fe4N.

Substrate Effects on In-Plane Magnetic Anisotropy and Verwey Transition Temperatures of (100) Magnetite (Fe3O4) Films

(100) Fe3O4 films were grown on five different (100)-oriented substrates: MgO, MgAl2O4 (MAO), (PbMg0.33Nb0.67)1– x :(PbTiO3) x (PMNT), SrTiO3 (STO), and (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT), to investigate the substrate effects on structural, magnetic, and transport properties of the films. The X-ray diffraction measurements suggested that the epitaxial (100) Fe3O4 films were grown, although the lattice mismatch between the Fe3O4 and the substrates varied from +0.25% to −7.8%. Remarkably, the (100) Fe3O4 films grown on MAO, PMNT, and STO substrates exhibited a distinctive in-plane magnetic anisotropy with the magnetic easy axis along the [010] and [001] directions. In contrast, the (100) Fe3O4 films on the MgO and LSAT substrates showed a very weak anisotropy in the [011] easy direction. The Verwey transition temperature of the (100) Fe3O4 film was found to have a minimum value of $\sim 110$ K for MgO and a maximum value of $\sim 122$ K for STO. These results suggest that the magnetic anisotropy and Verwey transition behaviors of the (100) Fe3O4 film can be tuned by film growth on a specific substrate.

Perpendicular magnetic anisotropy of Pt/Co2FeAl0.5Si0.5/MgAl2O4 trilayers

Perpendicular magnetic anisotropy (PMA) has been achieved in Co2FeAl0.5Si0.5 (CFAS) based trilayers with MgAl2O4 (MAO) as oxide layer. It was found that the PMA is sensitive to the annealing conditions, the thicknesses of MAO (tMAO) and CFAS (tCFAS) layers. A strong PMA can be observed in the Pt/CFAS/MAO structure after 340 °C annealing and maintained as high as 400 °C. For a fixed CFAS thickness (tCFAS = 1.5 nm), there is a narrow range of tMAO to keep the PMA in this structure. Oxidation at the CFAS/MAO interface plays a very significant role to the PMA, which can be controlled by tuning tMAO for fixed annealing temperature and tCFAS.

Magnetization behavior of L10-ordered FePt alloy thin films prepared on MgO(100), MgAl2O4(100), and KTaO3(100) single-crystal substrates

In order to investigate the effects of lattice mismatch between FePt thin films and single-crystal substrates on the tetragonality and magnetization process, FePt thin films were fabricated on several single-crystal substrates such as KTaO3 (KTO) (100), MgAl2O4 (MAO) (100), and MgO(100) at a substrate temperature of 700 °C. The Fe content of the FePt films was varied from 45.0 to 50.8 at. %. In addition to the fundamental (002) peak, the (001) and (003) superlattice peaks were clearly observed in the X-ray diffraction patterns of all the samples, indicating the formation of the L10-ordered structure. The magnetization measurements show that all the samples were perpendicularly magnetized. Coercivities (Hc) of 57.8, 52.5, and 3.3 kOe were obtained for the films with Fe49.3Pt50.7 (at. %) deposited on the MgO, MAO, and KTO substrates. The marked reduction in Hc is considered to arise from the morphology of FePt thin films.

Facile synthesis of MgAl2O4 with high crystallinity from KCl–MgCl2 composite molten salts

MgAl2O4 (MA) powder has been synthesised by heating an equimolar composition of commercial Al2O3 and calcined MgO via a molten salt synthesis (MSS) method. Phase compositions, microstructures, elements distributions, particle size and specific surface area of as-synthesised MA at various temperatures were investigated. The synthesis reaction of MA from Al2O3 and MgO is incomplete at 1000 and 1100°C for 4 h in KCl–MgCl2 molten salts. MgAl2O4 powder can be produced via the MSS method at 1200°C for 4 h, which is ∼200°C lower than that of conventional solid–solid reaction (SSR) method. Fine MA particles can be formed at 1000, 1100 and 1200°C for 4 h, and most of them aggregate. With the increase in reaction temperature from 1000 to 1200°C, the particle size of as-synthesised MA powder decreases and specific surface area increases. The average grain size and the specific surface area of the fabricated powder at 1200°C for 4 h are 7.57 μm and 0.7566 m2 g-1, respectively. Single MA crystalline phase is detected when MA powder formed at 1200°C for 4 h was further heated at 1580°C for 4 h. The MA particles, synthesised at 1000–1200°C after the heat treatment at 1580°C for 4 h, exist in spherical shape with high crystallinity and their grain size is about 2–5 μm.

Fabrication of Nanocomposites of SnO2 and MgAl2O4 for Gas Sensing Applications

Simple solid-state and sol–gel routes have been used to synthesize nanocomposites of tin oxide and magnesium aluminate at calcination temperature of 900 K for gas sensing applications. The effects of the surface structure of magnesium aluminate on the gas response for different concentrations of tin oxide addition were investigated for potential use in gas sensors. (SnO2)x doped in small amounts x into magnesium aluminate resulted in three nanocomposite samples MAS0.25, MAS0.50, and MAS0.75 for x = 0.25, 0.50, and 0.75, respectively, plus MgAl2O4 (MA) for x = 0. The response to different pressures of gases such as oxygen (O2), carbon monoxide (CO), and ethanol (C2H5OH) was quantitatively analyzed for all samples at different operating temperatures. The temperature was varied linearly by increasing the supply to a heating pad mounted below the sensor sample, regardless of the gas pressure inside the chamber. All the sample materials showed good response at different gas pressures (1 bar to 2 bar) and operating temperatures (300 K to 600 K). It was noted that the composite samples showed enhanced and fast response to gases, at both lower and higher operating temperatures, with detection of even the smallest change in gas pressure.

Low-energy Resistive Random Access Memory Devices with No Need for a Compliance Current.

A novel resistive random access memory device is designed with SrTiO3/ La2/3Sr1/3MnO3 (LSMO)/MgAl2O4 (MAO)/Cu structure, in which metallic epitaxial LSMO is employed as the bottom electrode rather than traditional metal materials. In this device, the critical external compliance current is no longer necessary due to the high self-resistance of LSMO. The LMSO bottom electrode can act as a series resistor to offer a compliance current during the set process. Besides, the device also has excellent switching features which are originated in the formation of Cu filaments under external voltage. Therefore it provides the possibility of reducing power consumption and accelerating the commercialization of resistive switching devices.

Magnetic and Electrical Properties of Epitaxial NiFe<sub>2</sub>O<sub>4</sub> (001) Films Fabricated by Reactive Sputtering

Thin films of NiFe 2 O 4 were fabricated on MgO (001) and MgAl 2 O 4 (MAO) (001) substrates by reactive radio frequency magnetron sputtering and were evaluated with regards to their electrical and magnetic properties. A saturation magnetization of 285 emu/cm 3 was obtained with a 30 nm thick film grown on an MAO substrate at an oxygen flow rate, Q, of 9 sccm. This enhanced magnetization was found to be due to a normal spinel arrangement at the interface. From the thickness dependence of the magnetization, the intrinsic magnetization was determined as 241 emu/cm 3 with the enhancement region estimated to be around 3/4 of the lattice constant, thus providing 8 μ B /f.u. The magnetization of those films grown on an MgO substrate exhibited remarkably smaller amplitudes to those of the MAO substrates. It is suggested that this substrate-dependent magnetization can be attributed to the number of antiphase boundaries. The resistivity was found to increase with Q, with a resistivity state in the same order as that of the bulk achieved with films prepared in which Q was greater than 10 sccm.

Oxidation degree of chromite from Indian ophiolites: a crystal chemical and 57Fe Mössbauer study

Several samples of Cr-bearing spinel from Indian ophiolites have been studied by X-ray single crystal diffraction and structure refinement, electron probe microanalyses and Mossbauer spectroscopy. Differences between samples coming from massive chromitite bands and those coming from podiform chromitite deposits have been evidenced: the former have (Mg,Fe)-chromite component ≥ 75%, the latter from 69 to 74%. In both cases the complementary components are aluminate (mainly spinel sensu stricto MgAl 2O4). As magnesiochromite (MgCr 2O4) is always dominant with respect to chromite (FeCr 2O4), the studied samples are classified as magnesiochromite. The multi-analytical approach has revealed that some of the studied spinels are non-stoichiometric (due to Fe 2+ oxidation and vacancy formation), with those coming from massive chromitite being more oxidised than those from the pods. Comparison between our results and recent literature suggested that spinel oxidation is more common than usually believed: it is not restricted to ophiolites but may occur in different geological settings and may help reconstructing the complex thermooxidative history of the host rock. Consequently, the proposed multi-analytical approach is crucial for an accurate crystal chemical characterisation of spinels and other Fe-bearing mineral phases, especially when geothermobarometric calculations have to be performed.

In-situ catalytic growth of MgAl2O4 spinel whiskers in MgO–C refractories

Abstract Upon application of nano-sized metallic Ni particles as catalyst, the in-situ synthesis mechanism of spinel whiskers in MgO–C refractories was studied. Their phase composition and morphology were determined by means of X-ray diffraction and scanning electron microscopy supported by energy dispersive spectroscopy. The results show that when the catalyst of nano-sized Ni was added in MgO–C refractories, the granular MgAl2O4 (MA) spinels transformed into the shape of whiskers at 1 200 °C. The presence of Ni catalyst can accelerate the generation of Mg vapor, which can react with Al vapor to form MA spinel whiskers. Through dissolution and precipitation, MA spinel crystals nucleate directly and grow into whiskers from the catalytic droplets of nano-sized metallic Ni particles. The growth of spinel whiskers follows a typical vapor–liquid–solid (V–L–S) growth mechanism.

Epitaxial growth of spinel cobalt ferrite films on MgAl2O4 substrates by direct liquid injection chemical vapor deposition

The direct liquid injection chemical vapor deposition (DLI-CVD) technique has been used for the growth of cobalt ferrite (CFO) films on (100)-oriented MgAl2O4 (MAO) substrates. Smooth and highly epitaxial cobalt ferrite thin films, with the epitaxial relationship MAO(100)[001]∥CFO(100)[001], are obtained under optimized deposition conditions. The films exhibit bulk-like structural and magnetic properties with an out-of-plane lattice constant of 8.370Å and a saturation magnetization of 420kA/m at room temperature. The Raman spectra of films on MgAl2O4 support the fact that the Fe3+- and the Co2+-ions are distributed in an ordered fashion on the B-site of the inverse spinel structure. The DLI-CVD technique has been extended for the growth of smooth and highly oriented cobalt ferrite thin films on a variety of other substrates, including MgO, and piezoelectric lead magnesium niobate–lead titanate and lead zinc niobate–lead titanate substrates.