Cleaved MgO Substrates Research Articles

Magnetic properties of the Fe-MgO interface studied by Mössbauer spectroscopy

Thin ${}^{57}$Fe layers evaporated onto an MgO(100) single-crystal substrate and covered by an evaporated MgO layer were studied by low-temperature conversion electron M\"ossbauer spectroscopy. The temperature dependence of the spectra indicates superparamagnetic behavior below 8 ML nominal thickness of the Fe layer signaling a cluster-type growth mode. The low-temperature hyperfine fields are consistent with a model that defines two types of metallic Fe atoms: bulklike and interfacial ones. Formation of FeO or (Fe,Mg)O at the interface layer is not observed. The sample with a 4-ML Fe layer when grown over a cleaved MgO substrate shows almost perfect perpendicular magnetization, as locally probed at 15 K by the hyperfine magnetic field, while random magnetization orientation and lower blocking temperature is observed in the case of a polished substrate. The perpendicular anisotropy observed at low temperature is attributed to mechanical stresses arising from the epitaxial relation and the different temperature dilatation of the subsequent layers.

Epitaxial growth of Ag 2S films on MgO(001)

Epitaxial films of monoclinic Ag 2S on polished and annealed surfaces and on cleaved surfaces of MgO(001) were prepared by molecular beam epitaxy. The films were characterized by X-ray diffraction method. Ag 2S films generally consisted of crystallites with orientations of (012), (−1,1,2) and (010) parallel to substrate surfaces, but for a thick film (180 nm) on cleaved MgO, the (010) orientation was not observed. In-plane orientation of the (−1,1,2) oriented crystallites was essentially the same as that of the (012) oriented crystallites in any case. That for the (010) oriented crystallites was not determined. For a film deposited on the polished MgO substrate, in-plane orientation for (012) oriented crystallites was [100]∥[110] MgO. For a film on the cleaved MgO substrate, the main in-plane orientation was [100]∥[100] MgO. For the thick film on the cleaved MgO substrate, the in-plane orientation was basically [4,−2,1]∥[100] MgO with slight modifications. There exist additional orientations due to twinning based on the monoclinic lattice of Ag 2S in each case.

An investigation of the influence of a magnetic field on the chemical potential of electrons in superconducting YBCO thin films

It is shown that measurements of the chemical potential variation in a magnetic field could give valuable information about the magnetisation of thin films. Oscillations of μ in YBCO single crystal films evaporated on cleaved MgO substrates were observed. These oscillators may be ascribed either to the de Haas-van Alphen effect or to mesoscopic effects arising due to weak link currents. Large changes of μ resulting from superconducting shielding currents were observed in films evaporated on polished substrates. For films deposited on polished NdGaO 3 substrate the change of μ was mainly reversible. For films deposited on polished MgO, SrTiO 3 and LaAlO 3 substrates μ changed irreversibly.

Investigation of the influence of a magnetic field on the chemical potential of electrons in superconducting and ferromagnetic thin films.

It is experimentally shown that measurements of the chemical-potential variation in a magnetic field could give valuable information about the magnetization of thin films. For superconducting single-crystal ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin films two kinds of behavior of \ensuremath{\mu}(H) were observed depending on the substrate used. For films deposited on polished ${\mathrm{NdGaO}}_{3}$ single-crystal substrates the change of \ensuremath{\mu} was mainly reversible. That means that no magnetic flux was trapped during a magnetic-field sweep. For films deposited on MgO, ${\mathrm{SrTiO}}_{3}$, and ${\mathrm{LaAlO}}_{3}$ single-crystal substrates \ensuremath{\mu} changed irreversibly. The estimated critical current density is ${\mathit{j}}_{\mathit{c}}$\ensuremath{\sim}${10}^{7}$ A/${\mathrm{cm}}^{2}$ at T=4.2 K in both cases. Oscillations of \ensuremath{\mu} in films evaporated on cleaved MgO substrates were observed. The effect of a magnetic field on \ensuremath{\mu} in a ferromagnetic film, predicted 60 years ago, was confirmed.

Magnetic field penetration into YBa2Cu3O7 thin film deposited on a cleaved MgO substrate

A thin film of YBa2Cu3O7 was deposited by laser ablation onto the (001) surface of a cleaved MgO substrate. The magnetic field penetration into the thin film was photographed using a magneto-optic apparatus. The field penetrates into the sample along specific directions that we identify with the cleavage steps that are more than 0.25 μm high.

Epitaxial growth study of Bi2(Sr,Ca)3Cu2Ox films on cleaved MgO substrates

In situ epitaxial growth of Bi2(Sr0.6Ca0.4)3Cu2Ox films by ion beam sputtering on cleaved MgO substrates was studied. The crystallographic structures were analyzed by in situ reflection high-energy electron diffraction, a four-circle x-ray diffractometer, and a scanning electron microscope. While the epitaxial films on polished MgO substrates commonly showed the fourfold symmetry, epitaxial films having twofold symmetry were grown on cleaved MgO substrates and the films contained two types of equivalently misoriented domains. The epitaxial relationship between these domains and cleaved MgO substrates was such that the b axis of each domain deviated approximately ±13° from [010]MgO where the cleavage steps ran along [100]MgO. The step edges seemed to play an important role in the occurrence of the twofold epitaxial relationship, where the incommensurate modulation tended to align its direction so as to avoid the step edges.

In-situ epitaxial growth study of Bi/sub 2/(Sr,Ca)/sub 3/Cu/sub 2/O/sub x/ films by ion beam sputtering on cleaved MgO substrates

In-situ epitaxial growth of Bi/sub 2/(Sr/sub 0.6/Ca/sub 0.4/)/sub 3/Cu/sub 2/O/sub x/ films on cleaved MgO substrates was studied by ion beam sputtering. The crystallographic structures were analyzed by using in-situ reflection high-energy electron diffraction, a four-circle X-ray diffractometer, and a scanning electron microscope. While the epitaxial films on polished MgO substrates commonly showed the fourfold symmetry, the epitaxial films having the twofold symmetry were successfully grown on the cleaved MgO substrates, and the films contained two types of misoriented domains. The epitaxial relationship between these domains and then cleaved MgO substrates was such that the b-axis of each domain deviated approximately +or-13 degrees from

Modulated structures in (Bi,Pb)-Sr-Ca-Cu-O films

Pb addition makes easier to form the high Tc phase in the BSCCO system. However, Pb easily vaporized at high temperature. A controlled Pb potential method has been applied to grow the high Tc phase in films. Initially, films are deposited on cleaved MgO substrates using an rf magnetron sputtering system. These amorphous as-deposited films are heat treated in a sealed gold capsule along with a large pellet of Pb-added BSCCO. Details of the process and characterization of the films have been reported elsewhere (1). Films trated for 0.5h at 850° C contain mainly the low Tc phase with a small amount of the high Tc phase. Hawever, films treated for 3h at 850°C consist mainly of the high Tc phase. This film is superconductive with a Tc(zero) of 106K. The Pb/Bi ratio of the films, analysed by SEM- EDS, are 0.12 and 0.18 for heat tratment times of 0.5 and 3h, respectively. The present study investigates the modulated structures of these films using HREM.

Low-energy ion irradiation during film growth for reducing defect densities in epitaxial TiN(100) films deposited by reactive-magnetron sputtering

Transmission electron microscopy has been used to study the defect structure in epitaxial TiN(100) films as a function of the growth temperature (Ts=550–850 °C) and negative substrate bias (Vs=0–500 V) applied during reactive-magnetron sputter deposition onto cleaved MgO substrates. For the growth conditions used in these experiments, the energy per incident N ion (N+2 was the predominant ionic species) was ∼Vs/2 and the incident ion to thermal-atom flux ratio was ∼2. The results showed that the primary defects were dislocation loops whose number density nd decreased with increasing Ts for a given Vs. However, nd decreased much more rapidly with increasing Vs at constant Ts until a minimum defect density was attained at Vs=V*s after which nd increased with further increases in Vs. Low-energy ion irradiation during film growth played at least two primary roles: it resulted in the creation of additional point defects which then condensed into dislocation loops and it enhanced atomic mobilities thereby accelerating the rate at which defects were annealed out during deposition. The residual defect density was determined by a competition between these two effects. Films grown at Ts>750 °C and Vs=V*s were essentially free of dislocation loops. For comparison, the dislocation loop density in films grown with Vs=0 ranged from 5×1012 to 1.5×1010 cm−2 as Ts was increased from 550 to 850 °C.

In-Situ Analysis of the Trajectories and Speed of Gold Clusters Moving on Electron-Beam Cleaved (001) Mgo Surfaces.

A previously described high-resolution TEM with UHV specimen chamber (10-7 Pa range), electron beam evaporation facilities, and fast speed/low electron exposure image recording facilities (1,2) was employed to observe the mobility of small gold clusters (13 Å mean diameter) on electron-beam cleaved MgO substrates (3). A deposit of 1.3 x 1015 gold atoms/cm2 was evaporated in 60 seconds immediately following the cleavage.During the first 10 minutes following the deposition, many cluster mobility events were observed. The movements can be classified in two categories. One type of cluster mobility leads to coalescence and occurs usually within a few seconds (25 Å/s speed), see Fig.l. Whereas this kind of mobility was found to be predominant in previous cluster mobility studies of silver on amorphized graphite (4), cluster mobility leading to coalescence was only found in less than 20% of the observed mobility events for gold on (001) MgO. The majority of the cluster movements in the present system were characterized by an obvious tendency to avoid the proximity of other clusters (5).

Preparation and Structural Properties of Epitaxially Grown LaB6 Films on MgO Crystals

Thin films of lanthanum hexaboride were epitaxially grown on magnesium oxide crystals by electron-beam evaporation at a pressure of 10-7 Torr. Crystals of MgO were cleaved in air or in vacuum. The substrate temperature was maintained in the range of 200°C to 850°C and the deposition rate controlled at 0.2 Å/sec or 1 Å/sec. For the vacuum cleaved MgO substrate, the parallel orientation of LaB6 film (LaB6(001)//MgO(001); LaB6[100]//MgO[100]) was established above 730°C with 0.2 Å/sec or above 790°C with 1 Å/sec. For the air cleaved substrate, the heat treatment at 1200°C for 12 hours before deposition was necessary for the parallel orientation of LaB6 film.