MgO Underlayer Research Articles

Fabrication of FePt/FePt-BN/FePt-SiOx Granular Film for HAMR Media on Corning Lotus NXT Glass Substrate

In this article, we present the fabrication of FePt/FePt-BN/FePt-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> and conventional FePt/FePt-C for heat-assisted magnetic recording (HAMR) on Corning Lotus NXT glass substrates. For FePt/FePt-BN/FePt-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> media, a 2.5 nm FePt/FePt-35vol.%BN was fabricated on polycrystalline MgO underlayers at 700 °C to serve as the templated layer for the growth of 4 nm FePt-35vol.%SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> at 550 °C. For the FePt/FePt-C media, 6.5 nm FePt/FePt-C media was fabricated at 550 °C with FePt and C co-sputtered on top of 0.5 nm FePt. Both media show well-isolated FePt grains, which are fully surrounded by the grain boundaries materials. Compared with FePt/FePt-C media, the FePt/FePt-BN/FePt-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> media show distinctively smaller grain-to-grain pitch distance or higher grain areal density. Moreover, the bimodal grain-size distribution that is often observed in FePt/FePt-C media has been suppressed in the FePt/FePt-BN/FePt-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> media. In addition, both media show good magnetic properties on Corning Lotus NXT glass substrate.

Porous nickel-based catalysts for tri-reforming of methane to synthesis gas: Preparation and characterization

The effect of synthesis conditions on the phase composition and texture of porous nickel ribbon with a MgO underlayer and of nickel catalysts supported on it was studied. The MgO underlayer was prepared by impregnation of a nickel ribbon with a Mg(NO3)2 or Mg(CH3COO)2 water solutions and subsequent calcination at 750 °C in H2. The formation of more dispersed MgO was observed with the use of Mg(NO3)2 as precursor. Catalysts were prepared by supporting of nickel onto nickel ribbon with MgO underlayer by its impregnation with a mixture of solutions of Ni(NO3)2 and Mg(NO3)2 followed by thermal pre-treatment in N2 or in He. Under reduction the formation of highly dispersed nickel crystallites (3–5 nm) epitaxially bound to MgO was observed in the catalysts with MgO underlayer prepared from Mg(NO3)2 with thermal pre-treatment of the precursor of active component in N2 or He. The supported nickel catalysts showed stable activity in tri-reforming of methane throughout the test period (9 h).

Porous nickel-based catalysts for tri-reforming of methane to synthesis gas: Catalytic activity

The supported nickel catalysts based on porous nickel ribbon with MgO underlayer were tested in tri-reforming of methane to synthesis gas (1 atm, 750 °C, CH4/CO2/H2O/O2/N2 = 24/18/18/3,5/36,5, GHSV = 62.5 L/gh). The predominant part of the supported nickel particles in the used catalysts remain in the state of dispersed crystallites epitaxially bound to MgO. Besides, highly dispersed nickel crystallites (2–3 nm) epitaxially bound to MgO also appear. These catalysts showed stable activity throughout the test period (9 h); after the reaction no carbon formation was observed. The resistance of these catalysts to carbon formation was attributed both to the formation of nickel crystallites epitaxially bound to MgO and to the formation of the MgO underlayer that covers the nickel ribbon and prevents its contact with the reaction medium.

Impact of CoFeB surface roughness on reliability of MgO films in CoFeB/MgO/CoFeB magnetic tunnel junction

We investigated the impact of the interface roughness in W-CoFeB-MgO-CoFeB-W structures on MgO reliability using the atomic force microscopy images of a MgO underlayer surface and I–V measurements. As a result, the electric current in the MgO film increased as the MgO underlayer surface roughness increased. We assumed that this phenomenon is caused by the local electric field concentration, and verified it by comparing the two electric field concentration values simulated from the AFM images and calculated from the I–V measurements. These two electric field concentrations calculated from the different results have a strong correlation to each other. It is concluded that the current of the MgO film is increased by the large roughness between MgO and the electrodes. This causes the large current variation in memory cells in magnetic random access memory (MRAM), then it leads to a small margin in memory operation.

Magnetic in-plane components of FePt nanogranular film on polycrystalline MgO underlayer for heat-assisted magnetic recording media

A systematic study is conducted on the effect of MgO substrate/underlayer in terms of the surface roughness, grain boundary, and crystallographic texture on the magnetic in-plane components (medium noise) of the FePt-based heat-assisted magnetic recording (HAMR) media. Cross-sectional transmission electron microscope observations revealed that good (001) texture can be developed in FePt nanogranular films even when they are grow on a rough surface of a single crystal-MgO (001) substrate or across the grain boundaries of a (001)-textured polycrystalline-MgO underlayer. The misoriented FePt grains with magnetic easy axes (c-axes) tilted away from the film normal direction mainly originate from their epitaxial growth on the initially misoriented MgO underlayer grains. Micromagnetic simulation results revealed that the FePt grains with misorientation angle larger than ∼20° are attributed to the detected magnetic in-plane components. Based on these experimental results, we discuss how to improve the (001) texture of the polycrystalline underlayer as well as to develop FePt-based HAMR media with enhanced signal-to-noise ratio.

Modulation of carbon nanotube yield and type through the collective effects of initially deposited catalyst amount and MgO underlayer annealing temperature

ABSTRACTRecently, the millimetre-scale, highly efficient growth of single-wall carbon nanotube (SWCNT) forests from iron (Fe) catalysts has been reported through the annealing of the magnesia (MgO) underlayer. Here, we report the modulation of the CNT yield (height) and average number of CNT walls for a Fe/MgO catalyst system through the collective effects of initial Fe amount and MgO annealing temperature. Our results revealed the existence of a well-defined region for high yield SWCNT forest growth in the domain of deposited Fe thickness and MgO annealing temperature. Through topographic examinations of the catalyst surface using atomic force microscopy, we confirmed that our results stem from the collective effects of increased amounts of surface-bound Fe through the amount of deposition and suppression of Fe subsurface diffusion, together govern the amount of surface-bound catalyst. The combination of these mechanisms determined the final nanoparticle size, density, and stability and could explain the three distinctly defined regions: low yield SWCNT growth, high yield SWCNT growth, and high yield multiwall CNT growth. Furthermore, we explained the observed borders between these three regions.

Microstructure and magnetic properties of FePt-(C,SiO2) granular films deposited on MgO, MgTiO, and MgTiON underlayers

We investigated the microstructure and magnetic properties of FePt-(C,SiO2) granular films deposited on MgO, MgTiO, and MgTiON underlayers. The coercivity and grain size of the FePt-(C,SiO2) film are substantially degraded when deposited on the MgTiO underlayer compared to those on the MgO underlayer, while they become comparable when deposited on the MgTiON underlayer. Atomic resolution STEM-EDS showed Ti in the MgTiO underlayer diffuses into FePt grains, while it is suppressed using the MgTiON underlayer. This suggests that MgTiON is a good choice for a conductive underlayer for growing L10-FePt perpendicular recording media for heat assisted magnetic recording.

Surface structure of MgO underlayer with Ti diffusion for (002) oriented L1 FePt-based heat assisted magnetic recording media

Surface morphology of the MgO layer and magnetic properties of FePt-C layer deposited on the MgO were investigated for the FePt-based heat assisted magnetic recording media. Stacking structure of the underlayer for the FePt-C layer was MgO (0-5 nm)/Cr80Mn20 (0-30 nm)/Cr50Ti50 (0-50 nm)/glass sub.. Surface observation result for the MgO film by using an atomic force microscope revealed the existence of nodules with a height of about 2 nm and a network-like convex structure with a height difference of about sub nm (boundary wall, BW) on the MgO crystal grain boundary. Density of the nodules largely depends on the surface roughness of the CrTi layer, RaCrTi and it is suppressed from 10 to 2/0.5 μm2 by reducing RaCrTi from 420 to 260 pm. Height of the BW depends on thickness of the MgO layer, tMgO and it can be suppressed by reducing tMgO to less than 4 nm. From the cross-sectional energy dispersive x-ray mapping, it is clarified that the BW is formed by atomic diffusion of Ti atoms from CrTi layer due to the substrate heating process, and a compound consists of Mg, Ti and O atoms. This BW can be used as a template to magnetically isolate the FePt column in the FePt-based granular film, such as FePt-SiO2, if the size of the BW is reduced to less than 10 nm. M-H loop of the FePt-C granular film deposited on the underlayer showed that the nodule and BW induce oxidation of the FePt grains, and reduction of intergranular exchange coupling.

Preparation of c-axis perpendicularly oriented ultra-thin L1-FePt films on MgO and VN underlayers

Ultra-thin L10-FePt films of 2 nm average thickness are prepared on (001) oriented MgO and VN underlayers epitaxially grown on base substrate of SrTiO3(001) single crystal. Detailed cross-sectional structures are observed by high-resolution transmission electron microscopy. Continuous L10-FePt(001) thin films with very flat surface are prepared on VN(001) underlayer whereas the films prepared on MgO(001) underlayer consist of isolated L10-FePt(001) crystal islands. Presence of misfit dislocation and lattice bending in L10-FePt material is reducing the effective lattice mismatch with respect to the underlayer to be less than 0.5 %. Formation of very flat and continuous FePt layer on VN underlayer is due to the large surface energy of VN material where de-wetting of FePt material at high temperature annealing process is suppressed under a force balance between the surface and interface energies of FePt and VN materials. An employment of underlayer or substrate material with the lattice constant and the surface energy larger than those of L10-FePt is important for the preparation of very thin FePt epitaxial thin continuous film with the c-axis controlled to be perpendicular to the substrate surface.

Formation of $L1_{0}$ -FePt(001) Ultra-Thin Films With Flat Surfaces Using VC and VN Underlayers

FePt alloy films of 2 to 10 nm thicknesses are prepared on (001) single-crystal underlayers of MgO, VN, and VC by employing a two-step method consisting of low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. MgO, VN, and VC crystals are selected as underlayer materials, since the crystals have the same NaCl-type crystal structure and similar lattice constants of about 0.42 nm but different surface energies. The single-crystal underlayers are prepared by RF sputtering on the single-crystal substrates of MgO and SrTiO3 at 600 °C. The influences of underlayer material and FePt film thickness on the $L1_{0}$ ordering degree, the magnetic properties, and the surface morphology are systematically investigated. Epitaxial films involving $L1_{0}$ (001) crystal structure with the $c$ -axis normal to the substrate surface are formed on all the underlayers. As the film thickness decreases, the ratio of out-of-plane to in-plane lattice constant $c/a$ decreases and the ordering degree $S$ increases. Stronger perpendicular magnetic anisotropy is observed for an FePt film with a higher $S$ value. The film thickness trends of $c/a$ , $S$ , and magnetic properties are similar to those for FePt films thicker than 5 nm formed on MgO underlayers. However, when the thickness is decreased to 2 nm, de-wetting of deposited atoms occurs on MgO underlayers during annealing at 600 °C. The surfaces of 2-nm-thick films formed on MgO underlayers are composed of isolated islands. On the contrary, continuous surfaces are realized for the investigated film thickness range of 2–10 nm in the cases of VC and VN underlayers. The surface roughness of FePt film formed on VN underlayer ( $R_{a}$ : 0.1–0.2 nm) is lower than that of film formed on VC underlayer ( $R_{a}$ : 0.1–0.4 nm). This paper shows that the employment of a VN underlayer is effective in the preparation of $L1_{0}$ ordered FePt ultra-thin film with flat surface.

Improved (0 0 1)-texture of FePt-C for heat-assisted magnetic recording media by insertion of Cr buffer layer

FePt-C granular films deposited on MgO underlayers are the prototype media for heat-assisted magnetic recording. To reduce the in-plane magnetic component in the FePt-C media, we investigated the effect of Cr buffer layers on the crystallographic textures of the MgO underlayers and the resultant magnetic properties of the FePt-C layers. By growing a MgO underlayer on a Cr buffer layer, the (001) texture of the MgO underlayer is improved, on which the in-plane component of a FePt-C film is substantially reduced. We conclude that the growth in the grain size of the MgO underlayer is the key factor in reducing the in-plane component in the FePt-C recording layer.

The double-edged effects of annealing MgO underlayers on the efficient synthesis of single-wall carbon nanotube forests.

Recently, the millimetre-scale, highly efficient synthesis of single-wall carbon nanotube (SWCNT) forests from Fe catalysts has been reported through the annealing of the magnesia (MgO) underlayer. Here, we report the double-edged effects of underlayer annealing on the efficiency and structure of the SWCNT forest synthesis through a temperature-dependent examination. Our results showed that the efficiency of the SWCNT forests sharply increased with increased underlayer annealing temperatures from 600 °C up to 900 °C due to a temperature-dependent structural modification, characterized by increased grain size and reduced defects, of the MgO underlayer. Beyond this temperature, the SWCNT fraction also decreased as a result of further structural modification of the MgO underlayer. This exemplifies the double-edged effects of annealing. Specifically, for underlayer annealing below 600 °C, the catalyst subsurface diffusion was found to limit the growth efficiency, and for excessively high underlayer annealing temperatures (>900 °C), catalyst coalescence/ripening led to the formation of double-wall carbon nanotubes. As a result, three distinct regions of synthesis were observed: (i) a "low yield" region below a threshold temperature (∼600 °C); (ii) an "increased yield" region from 600 to 900 °C, and (iii) a "saturation" region above 900 °C. The efficient SWCNT forest synthesis could only occur within a specific annealing temperature window as a result of this double-edged effects of underlayer annealing.

MgO-C interlayer for grain size control in FePt-C media for heat assisted magnetic recording

Currently, the most common recording media used for heat assisted magnetic recording is granular L10 based FePt-C directly deposited on a highly textured polycrystalline MgO underlayer. In this study, a thin granular MgO-C interlayer is inserted between the MgO underlayer and the granular L10-FePt-C magnetic layer to control the grain size of the magnetic media. By varying the deposition conditions of the MgO-C interlayer, we can vary the size of the L10-ordered FePt grains from about 10.5 nm to 7.6nm, while keeping the carbon composition in the magnetic layer unchanged. With the optimized interlayer, the L10-FePt-C media of grain size to 7.6 nm shows good squareness and perpendicular coercivity of 47 kOe as well as an order parameter of S = 0.85.

Unexpected Efficient Synthesis of Millimeter-Scale Single-Wall Carbon Nanotube Forests Using a Sputtered MgO Catalyst Underlayer Enabled by a Simple Treatment Process.

An unexpected 5000% increase in growth efficiency and high (95%) single-wall selectivity synthesis of vertically aligned carbon nanotubes (CNTs) was shown from Fe catalysts supported on a sputtered MgO underlayer from a simple underlayer treatment, i.e., annealing treatment. In this way, millimeter-scale single-wall carbon nanotube "forests" could be synthesized in a 10 min time, which has never been previously reported for MgO catalyst underlayer or any underlayer besides Al2O3. This level of efficiency and characterized SWCNT properties were similar to those grown using Al2O3 underlayers. Spectroscopic and microscopic analyses revealed that the treatment improved stability of the catalyst nanoparticle array by the suppressing catalyst subsurface diffusion and retaining the metallic state of the surface Fe atoms. Taken together, these results reveal a new route in achieving highly efficient SWCNT synthesis.

Influence of MgO underlayers on the structure and magnetic properties of FePt-C nanogranular films for heat-assisted magnetic recording media

In order to optimize the nanogranular structure of FePt-C for heat-assisted magnetic recording media, we investigated the influence of MgO underlayers on the growth of FePt grains in the FePt-C layer. The FePt-C layer was deposited by using the alternating sputtering method, by which FePt and FePt-C layers were alternately deposited. To understand the growth mechanism of the FePt-C layer on the MgO underlayers deposited under various conditions, detailed plan-view and cross sectional transmission electron microscopy observations were made for different film thicknesses. We found that columnar FePt grains grow only when the deposition conditions of the MgO underlayer are optimal. Direct TEM observation of the growth process of the FePt-C layer revealed that the number density of nuclei is sufficient in the initial stage of the film deposition; however, coarsening of the grains after grain impingement causes a substantial decrease in the number density of the FePt grains.

Magnetic Anisotropy and Crystal Domain Variant in $ {L} 1_{{0}}$ -FePt Polycrystalline Films

We fabricated thin $L 1_{0}$ -Fe x Pt100– x ( $x = 46$ –54 at.%) polycrystalline perpendicular films, and examined the $x$ dependences of the first- and second-order uniaxial magnetic anisotropy, $K_{u1}$ and $K_{u2}$ , in relation to variant formation. The values of $K_{u1}$ and $K_{u2}$ were estimated from magnetic torque curves at an applied field of 70 kOe obtained using anomalous Hall effect. $K_{u1}$ for films with thickness $d$ of 10 nm showed a maximum at $x = 50$ at.%, which was consistent with the behavior of the order parameter. $K_{u1}$ decreased markedly as $x$ increased from 50 at.%, accompanied by an increase of $K_{u2}$ . In-plane X-ray diffraction analysis revealed that the variant formation increased markedly as $x$ increased, especially beyond $x = 50$ at.%. Moreover, the relative lattice strain evaluated using a Williamson–Hall plot increased remarkably. The significant reduction of $K_{u1}$ , accompanied by the enhancement of $K_{u2}$ , was coincident with variant formation. $K_{u1}$ for films with $d = 2$ nm also showed a maximum at $x = 50$ at.%. However, the $K_{u1} \sim x$ relation showed a good symmetry against Fe content variations from 50 at.%. No increase of $K_{u2}$ with increasing $x$ was observed. It is likely that a large tensile stress in the initial film growth, caused by the lattice misfit between FePt and MgO underlayers, suppressed the variant formation in these very thin films.

&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${L}{\text {1}}_{\text {0}}$ &lt;/tex-math&gt;&lt;/inline-formula&gt;-Ordered MnAl Thin Films With High Perpendicular Magnetic Anisotropy Using TiN Underlayers on Si Substrates

Ferromagnetic L1 0 -ordered τ-phase MnAl thin films have been shown to exhibit high perpendicular magnetic anisotropy (K u > 107 ergs/cm3), moderate saturation magnetization (M 0 -ordering for perpendicular anisotropy in MnAl thin films has proven challenging and highly sensitive to deposition conditions and underlayers used [2]. Using MgO underlayers, we previously demonstrated MnAl thin films with high perpendicular magnetic anisotropy (PMA) on Si substrates for the first time [3]. However, MgO is an insulator, which presents two major challenges. First, MgO must be RF-sputtered, which results in low deposition rates, re-sputtering, and therefore the potential for MgO contamination in undesirable locations. Second, since it is an insulator, MgO impedes the use of MnAl film stacks as the bottom electrode in a magnetic tunnel junction (MTJ) for spintronic applications.

Granular nanostructures and magnetic properties of FePt–C/FePt–SiO2 films

FePt–C, FePt–SiO2 and FePt–C/FePt–SiO2 films with different segregants concentration on glass substrate with polycrystalline MgO underlayer have been successfully fabricated by magnetron sputtering system. It was found that the FePt–C films had larger coercivity(Hc), chemical order parameter(S) and smaller diameter of the local interaction zone(Dn) but wider magnetic coercive force distribution(ΔHc/Hc) than that of the FePt–SiO2 films. By using FePt–C/FePt–SiO2 bilayer structure, the low Dn and ΔHc/Hc were obtained. FePt–C30vol%(4 nm)/FePt–SiO245vol%(4 nm) film with Hc higher than 29 kOe and columnar FePt grains of 10 nm in diameter and 13 nm in height were obtained, which would be very suitable for heat assisted magnetic recording media.

Effect of MgO underlayer misorientation on the texture and magnetic property of FePt–C granular film

A transmission electron microscope (TEM) based orientation mapping technique and micromagnetic simulations were applied to study the influence of easy axis distribution (EAD) on magnetic properties of FePt–C granular films which were deposited on a single crystalline MgO (001) substrate and a (001)-textured poly-crystalline MgO underlayer. The FePt–C film on the polycrystalline MgO underlayer shows smaller perpendicular coercivity, broader switching field distribution and visible in-plane minor loop compared with that deposited on the single crystalline MgO (001) substrate. Although the grain sizes and their distributions in both films look similar in TEM, orientation mapping and texture analysis revealed that the polycrystalline MgO underlayer introduces significant misorientation in the (001)-textured FePt grains. Micromagnetic simulations successfully reproduced the large hysteresis in the in-plane magnetization by introducing the specific misorientation distribution of the FePt grains obtained from the texture analysis. The misoriented FePt grains were found to be grown from misoriented MgO grains, indicating that the improvement of the (001) texture of the MgO underlayer is essential to reduce the in-plane component of FePt based recording media.

Combined steam and carbon dioxide reforming of methane over porous nickel based catalysts

The phase composition and texture of nickel catalysts supported on a porous nickel ribbon with a MgO underlayer were studied.