MgO Single Crystal Substrates Research Articles

Static and dynamic strain-driven photoluminescence tuning in rare-earth doped perovskite oxide thin films

Recently, there have been significant interest in the wide potential of photoluminescence (PL) response of rare-earth doped perovskite oxide thin films in the field of optoelectronics. This work reports an effective method to modulate the PL properties of 0.8 mol%Sm3+-doped BaTiO3 (BTO:Sm) thin films through static and dynamic strain. BTO:Sm films were epitaxially grown on LaAlO3 (LAO), 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT), MgO, and Mica single-crystal substrates by pulsed laser deposition technique. Compared with the BTO:Sm/PMN-PT heterostructure which experiences minimal lattice mismatch effect, the PL intensity of BTO:Sm/LAO and BTO:Sm/MgO is enhanced by 818.1% and 1014.7%, respectively, due to static tensile/compressive strain (-0.429% and 0.461%) during the epitaxial growth process. In the flexible BTO:Sm/Mica heterostructure, dynamic strain is introduced through mechanical bending. Compared with the unbent state, the u-type and n-type bending modes increase the relative changes in PL intensity (ΔI/I) by 599.0% and 768.5%, respectively. Particularly, BTO:Sm/Mica also exhibits a remarkable optical transmittance exceeding 75% (500–2500 nm). The enhancement of PL intensity is closely related to the adjustment of lattice strain, which changes the crystal field strength of the BTO:Sm and the radiative transition probability. This work provides valuable insights for designing future optoelectronic devices and environmental monitoring equipment. It has the potential to advance photonics and sensor technologies, paving the way for innovation and exploration in these fields.

Growth of multiferroic γ-BaFe2O4 thin films by Pulsed Electron Deposition technique

Multiferroic materials have attracted significant attention due to their potential to revolutionize logic and memory devices by reducing energy consumption or increasing information density. Barium monoferrite (γ-BaFe2O4 or BaFeO) is a promising lead- and bismuth-free multiferroic material with a stuffed tridymite-like structure, exhibiting both ferroelectricity and antiferromagnetism at room temperature. BaFeO thin films with engineered grain orientation, composition, elastic strain, and magnetic interlayer couplings could open new avenues for novel devices. However, synthesizing the stoichiometric "1–2–4" phase in the BaO-Fe2O3 system remains a challenge. This study focuses on the deposition of pure and crystalline γ-BaFe2O4 thin films using the Pulsed Electron Deposition (PED) method. An analysis of the plasma plume revealed that two main mechanisms govern the dynamics of BaFeO deposition: a congruent ablation process occurring far from thermodynamic equilibrium and an incongruent low-energy evaporation process. The regions of the film with maximized ablative/evaporative ratios are free from the secondary Ba2Fe2O5 phase, suggesting that the thermal evaporative contribution is the primary factor responsible for the out-of-stoichiometry composition. The influence of the substrate type on the structural properties of BaFeO films was also evaluated. It was confirmed that (h00)-oriented BaFeO tends to grow on amorphous quartz and polycrystalline Pt-coated quartz in the 700–800°C temperature range. Conversely, the growth on SiO2/Si leads to the preferential grain orientation along the (0k0) and (0kl) directions by increasing the temperature. These findings demonstrate the potential of PED for producing high-quality γ-BaFe2O4 polycrystalline thin films. A fully heteroepitaxial (h00)-BaFeO/(111)-MgO system was even achieved by depositing on a single-crystal MgO substrate. This unprecedented result paves the way for utilizing such thin films in multiferroic applications, where phase purity, structural orientation, and defect-free structures can be exploited to enhance the magnetoelectric properties of the device.

Fabrication and characterization of sub-micron high current density NbN Josephson junctions for large-scale digital circuits

We have developed a fabrication process based on Chemical Mechanical Polishing (CMP) for the sub-micron NbN Josephson junctions on single-crystal MgO substrates. The NbN junctions were fabricated with epitaxial NbN/AlN/NbN trilayers deposited by reactive DC magnetron sputtering. The top electrode of the junction was exposed using CMP to contact the NbN wire without opening the submicron via through the dielectric. It has produced submicron NbN junctions with areas as small as 0.09 μm2. We have fabricated NbN tunnel junctions with a wide range of critical current density Jc. The NbN junctions show excellent Josephson tunneling characteristics, the gap voltage Vg is 5.8 mV and the quality factor Rsg/Rn is 12 for the junctions with a Jc of 10.7 kA/cm2, and Vg is 5.3 mV and Rsg/Rn is 4 for the junctions with a Jc of 150.5 kA/cm2. The 1000-junction arrays were fabricated and their I-V characteristics were measured, critical current uniformity was obtained and the standard deviation is about 5%. The high Jc sub-micron NbN junctions will be helpful in developing large-scale NbN SFQ circuits with high operating frequency.

NaCl flux growth of non-polar m-plane ZnO epitaxial thin film on c-plane sapphire substrate

We have developed a technique to fabricate non-polar m-plane ZnO thin films: the post NaCl flux treatment of Zn-containing precursors, such as amorphous zinc carbonate hydroxide, Zn5(CO3)2(OH)6. Conventionally, single-crystal m-plane sapphire or MgO (001) substrates have been utilized to fabricate m-plane ZnO thin films. In contrast, this method facilitates the growth of m-plane ZnO thin films on various substrates, such as sapphire and quartz glass, despite its c-plane growth habit. Furthermore, the m-plane ZnO thin film was demonstrated to epitaxially grow even on a c-plane sapphire substrate. Owing to the good lattice matching with the substrate, the m-plane ZnO epitaxial film had high crystallinity comparable to a c-plane ZnO epitaxial film that was created using a conventional pulsed laser deposition method, and exhibited similar photocatalytic activity to the c-plane ZnO film. This research offers valuable insights into the fundamental mechanisms of flux growth orientation control, which can be beneficial in designing ZnO-based electronic devices and photocatalysts.

Large magnetostriction in γʹ-Fe4N single-crystal thin film

An Fe4N(110) single-crystal film of 50 nm thickness with γʹ phase is prepared through hetero-epitaxial growth on an MgO(110) single-crystal substrate at 400 °C by reactive sputtering. The out-of-plane and in-plane lattice constants agree with those of bulk within small differences less than 0.2% and the orientation dispersions are about 1.2°. The degree of N site ordering in Fe4N structure is estimated to be 0.995. The arithmetical mean surface roughness is as small as 0.3 nm. These data show that a high-quality Fe4N single-crystal film is successfully formed. The Fe4N film shows in-plane magnetic anisotropy with the easy magnetization direction parallel to [001] and with the hard direction parallel to [1_11], which is reflecting the positive magnetocrystalline anisotropy, K1. A large negative λ100 value of –40 × 10–6 and a fairly-large positive λ111 value of +150 × 10–6 are observed. The present study has shown that γʹ-Fe4N compound is one of the strong candidates for rare-metal-free magnetostrictive materials.

Realization of tunable fabrication from Cu nanoparticles film towards Cu2O thin film by pulse laser deposition method

In this paper, the tunable fabrication from Cu nanoparticles (NPs) film to Cu2O thin film on single-crystal MgO (100) substrates via pulse laser deposition (PLD) has been realized by precisely adjusting the oxygen pressure. In-situ X-ray photoelectron spectroscopy confirms that the chemical state of Cu is altered from Cu0 to mixed state of Cu0 and Cu1+, and then to Cu1+. X-ray diffraction results reveal the main crystal structure of the films is changed from Cu (200) to Cu2O (110) with the increase of oxygen pressure. The morphology of three typical samples (Cu NPs film, Cu@Cu2O core-shell NPs films and Cu2O thin film) was record by transmission electron microscopy. According to the analysis, the mechanism of oxygen is to absorb on the surface of MgO (100) substrate and act as an oxidant in the growth of the films. The increase of oxygen pressure is beneficial to the formation of Cu@Cu2O core-shell NPs with larger diameters and the transition from Cu@Cu2O core-shell NPs film to continuous Cu2O (110) thin film. This work provides essential information for the fabrication of Cu2O thin films by PLD with Cu target at low oxygen pressure.

Crystallization features of YBa2Cu3O7-δ single crystals in 2YBa4Cu3O9-δ + BaCu2O2 + CuO2 system

In this article, a consistent study of phase transformations during the crystallization of YBa2Cu3O7-δ compound was carried out using XRD, thermogravimetric and differential thermal analyzes, as well as optical microscopy. When studying the microstructure and elemental composition in the reaction zone in the process of obtaining single crystals by the crucible-less method, the products of chemical reactions were identified depending on the composition of the reacting components and synthesis conditions. It has been established that the use of precursors Y2BaCuO5, YBa4Cu3O9-δ and ВаCu2O2 as initial reagents has made it possible to carry out the direct synthesis of YBa2Cu3O7-δ single crystals without the formation of intermediate phases. The superconductor has been synthesized at 1270 K on single-crystal MgO substrates with the (001) orientation, since their surface is poorly wetted by the melt solution and stimulates the YBa2Cu3O7-δ nucleation process. This ensures the minimum loss of the liquid fraction formed in the sample. The growth conditions for YBa2Cu3O7-δ single crystals have been studied and optimized. It has been experimentally revealed that the use of combined cooling conditions leads to an increase in the size of single crystals and a reduction in the time of their growth without changing the quality and crystal structure. The investigation showed that the largest volume (50 mm3) was achieved for single-phase YBa2Cu3O7-δ single crystals grown at a cooling rate of 0.5 deg/h in the temperature range 1260–1240 K and at a rate of 1.2 deg/h in the range 1240–1210 K. An analysis of the Laue rotation lines obtained in this work indicates the presence of blocks in single crystals cooled in the temperature range 1243–1193 K at a cooling rate of 1.5 deg/h and their absence in crystals cooled at 1.2 deg/h. An assessment of the degree of perfection of the structure by the width of the rocking curves at half-height of the X-ray reflection (006) showed that the width of the rocking curves of 0.36 deg indicates the absence of structural defects, such as twins, blockiness, and shear defects.

Fabrication of β-Mn type Co–Zn–Mn(001) film on MgO single crystal substrate

β-Mn type Co–Zn–Mn is a chiral magnet hosting Bloch type magnetic skyrmions and has been extensively studied because the magnetic skyrmions are observed above room temperature depending on the composition of Co, Zn, and Mn. So far, epitaxially grown film of Co–Zn–Mn on single crystal substrates has not been not realized whereas the epitaxial film is important from the fundamental and practical point of view. In this study, Co–Zn/Mn multilayered film was sputter-deposited on MgO(001) single crystal substrate and then annealed in vacuum for the crystallization of Co–Zn–Mn alloy. β-Mn type Co–Zn–Mn epitaxial film with (001) orientation was successfully fabricated although the film consisted of twinned crystal grains. We also showed that the annealing of 300–350 °C was effective to drive the crystallization, and that the possibility to enhance the crystallinity by optimizing the fabrication conditions.

Introduction of VN underlayer and caplayer for preparation of Mn4N(001) single-crystal thin film with perpendicular magnetic anisotropy

A Mn4N thin film is prepared on MgO(001) single-crystal substrate by introducing VN underlayer and caplayer which are respectively expected to inhibit the oxidation from substrate and surface. The thin film is prepared by ultra-high vacuum radio-frequency magnetron sputtering. The growth, the structure, and the magnetic properties are investigated. A fully epitaxial VN/Mn4N/VN film with sharp interfaces is formed on the substrate. The Mn4N film has low out-of-plane and in-plane orientation dispersions of about 1° and high N site order degree of 0.88. The lattice of Mn4N film is slightly deformed along the perpendicular direction (c/a = 0.9872) possibly due to accommodation of the lattice mismatch at Mn4N/VN interfaces. The film shows a low saturation magnetization of 85 kA/m (85 emu/cm3) and a strong perpendicular magnetic anisotropy. The present study has shown that introduction of VN underlayer and caplayer is useful for preparation of well-defined Mn4N thin films with perpendicular magnetic anisotropy.

Preparation and electrical transport properties of TiO thin films

Titanium monoxide has attracted great attention due to its unique superconducting characteristics and potential applications in microelectronics. In this work, a series of TiO thin films are prepared at room temperature by using the radio frequency magnetron sputtering method through changing the oxygen partial pressures. The crystal structures, valences of the elements, and electrical transport properties of the films are investigated systematically. X-ray diffraction results indicate that the films are epitaxially grown on MgO single crystal substrates along the [001] direction. After the surface of the TiO film is treated by ion etching, it is found that the Ti element mainly exists in a divalent form. For all films, the temperature coefficients of resistance are negative above superconducting transition temperature <i>T</i><sub>c</sub>. The films prepared under high oxygen partial pressures reveal insulator characteristics, and the temperature dependence of resistivity obeys the Mott-type variable-range-hopping law in low temperature regime. The films prepared under low oxygen partial pressures exhibit metallic properties, and enter into a superconducting state at low temperature. The superconducting transition temperature <i>T</i><sub>c</sub> in our film can be as high as 3.05 K. The carrier concentrations of the films are in the vicinity of ~2.0×10<sup>22</sup> cm<sup>–3</sup>, which is comparable to those for the typical metals. Interestingly, it is found that the main species of the charge carriers in the films transforms from electrons to holes with the oxygen partial pressure decreasing. The crossover of the species of the charge carriers could related to the changing of energy-band structure with the oxygen content in TiO film.

Low-temperature electrical transport properties of La doped BaSnO<sub>3</sub> films

A series of Ba<sub>0.94</sub>La<sub>0.06</sub>SnO<sub>3</sub> thin films are deposited on MgO(001) single crystal substrates by RF magnetron sputtering method, and their structure and electrical transport properties are systematically investigated. All films reveal degenerate semiconductor (metal) characteristics in electrical transport properties. In the high-temperature region (<inline-formula><tex-math id="M8">\begin{document}$T > {T_{\min }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M8.png"/></alternatives></inline-formula>, where <inline-formula><tex-math id="M9">\begin{document}${T_{\min }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M9.png"/></alternatives></inline-formula> is the temperature at which the resistivity reaches a minimum value), the resistivity of each film increases with temperature, and exhibits a linear relationship with the square of the temperature. In the low-temperature region (<inline-formula><tex-math id="M10">\begin{document}$T < {T_{\min }}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M10.png"/></alternatives></inline-formula>), the resistivity increases with decreasing temperature and varies linearly with <inline-formula><tex-math id="M11">\begin{document}$ \ln T $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M11.png"/></alternatives></inline-formula>. This temperature dependent behavior of resistivity cannot be explained by the general electron-electron interaction or weak localization effects in homogeneous disordered conductors and nor by Kondo effect. After quantitative analysis, it is found that the <inline-formula><tex-math id="M12">\begin{document}$ \ln T $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M12.png"/></alternatives></inline-formula> behavior of resistivity at low temperatures can be explained by the electron-electron Coulomb interaction effect in the presence of granularity. In addition, it is found that the Hall coefficient <inline-formula><tex-math id="M13">\begin{document}$ {R_{\text{H}}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M13.png"/></alternatives></inline-formula> also varies linearly with <inline-formula><tex-math id="M14">\begin{document}$ \ln T $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20231082_M14.png"/></alternatives></inline-formula> for the Ba<sub>0.94</sub>La<sub>0.06</sub>SnO<sub>3</sub> film, which also quantitatively accords with the theoretical prediction of the electron-electron Coulomb interaction effects in the granular metals. The results of cross-section high-resolution transmission electron microscope indicate that although the films have epitaxial structures as a whole, there are many strip-shaped amorphous regions in films, which makes the films have electrical transport properties similar to those of metal granular films. Our results provide strong support for the validity of the theory concerning the effects of Coulomb interaction on the conductivity and Hall coefficient in granular metals.

Microstructure study of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-<i>δ</i></sub> thin film with synchrotron-based three-dimensional reciprocal space mapping

High-temperature superconducting films can be used for fabricating the cutting-edge high-temperature superconducting microwave devices because of their low microwave surface resistances. However, the microwave surface resistances of high-temperature superconducting materials are particularly sensitive to microstructure due to their special two-dimensional superconducting mechanisms and extremely short superconducting coherence lengths. To investigate the correlations between microstructure and microwave surface resistance of high-temperature superconducting materials, YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-<i>δ</i></sub> (YBCO) films with different thickness are grown on (00l)-oriented MgO single-crystal substrates by using the pulsed laser deposition (PLD) technique. Electrical measurements reveal that their superconducting transition temperatures and room temperature resistances do not show significant difference. However, their microwave surface resistances in superconducting state display a significant difference. The characterizations of the microstructures of YBCO films by synchrotron radiation three-dimensional reciprocal space mapping(3D-RSM) technique show that the number of the grains with CuO<sub>2</sub> face parallel to the surface (c crystals), and the consistency of grain orientation are the main causes for the difference in microwave surface resistance.

Compositional dependence of anisotropic magnetoresistance effects in Weyl semimetal Co2MnAl Heusler alloy epitaxial thin films

Anisotropic magnetoresistance (AMR) effect on a Weyl semimetal Co2MnAl Heusler alloy was experimentally and theoretically investigated by changing the Co content, the direction of electric current against the crystal axis, and the measurement temperature. The experimentally fabricated Co2MnAl thin films epitaxially grown on a MgO single-crystal substrate showed the positive AMR ratios independent of the Co content, the direction of electric current, and the measurement temperature. The AMR ratios for the direction of electric current along Co2MnAl[100] were larger than those along Co2MnAl[110]. The maximum AMR ratios for Co2MnAl[100] were 0.576% at 5 K and 0.349% at 300 K for a Co content of 47 at. %, which is close to the stoichiometric composition. Furthermore, the theoretical calculations of AMR ratios based on the s-d scattering process of the electrons at the Fermi energy, including the information on the density of states (DOS) obtained from first-principles calculations, exhibited trends similar to the experimental results. Compared with the AMR results of Co2MnGa[110], we found that the magnitude relation between the partial DOS of the ε and γ orbitals of the d state of Co atoms determined the sign of the AMR ratios. These results suggest that the AMR effect of Weyl semimetal Co2MnAl can be elucidated by the s-d scattering process of the electrons.

Structural, magnetic, and transport properties of epitaxial thin films of equiatomic quaternary CoFeCrGa Heusler alloy

We present the first report on the structural, magnetic, and transport properties of epitaxial thin films of equiatomic quaternary CoFeCrGa, grown on a single crystal MgO(001) substrate. From the structure and surface morphology analyses, we have shown that the films grown are ordered, epitaxial, and homogeneous. Magnetic measurements confirm the soft ferromagnetic nature of the film along the in-plane direction with a coercivity of 4.6 Oe. The resistivity measurements indicate semiconducting nature with a very low temperature coefficient of resistivity (TCR) value, suggesting an almost temperature independent resistivity, similar to other reported spin-gapless semiconductors (SGSs) among the Heusler alloys. A non-saturating, quantum linear magnetoresistance is observed even in high fields of up to 12 kOe, another prerequisite for gapless materials. Hall measurements are also performed, and a detailed analysis has been carried out to estimate the ordinary and anomalous Hall contributions. The conductivity value (σxx) at 300 K is found to be 4280 S cm−1, which is comparable with the other reported SGS materials. We have also studied the effect of L21 and B2 type disorder on the electronic properties based on the first principle calculations and found that the SGS nature in CoFeCrGa is quite robust against the Co–Fe swap disorder (L21 disorder); however, with swap disorder (≥25%) between Cr and Ga sites, the system transits from SGS to half-metallic state. All these results indicate the possibility of spin-gapless semiconducting nature in the CoFeCrGa film, making it highly suitable for spin-based device applications.

Epitaxial growth of LaMnO3 thin films on different single crystal substrates by polymer assisted deposition

Structure of epitaxial LaMnO3 thin films grown on different single crystal substrates by unconventional polymer assisted deposition (PAD) method was investigated. Epitaxial films were prepared from lanthanum manganite water based solutions deposited by spin coating on single crystal MgO (001), SrTiO3 (001) and SrTiO3 (110) substrates, and the influence of substrate type on the film structure was analysed. Better uniformity of the epitaxial LaMnO3 films on SrTiO3 substrates was obtained, but a non-stoichiometric La1-xMnO3 phase was formed after the heat treatment at 750 °C. In addition, the prepared thin films were multiple annealed at different temperatures up to 900 °C, in order to investigate importance of post-annealing treatment. Epitaxial nature of the prepared films was preserved after annealing at up to 900 °C and the structure rearrangement through formation of cell closer to bulk stochiometric LaMnO3 phase was observed.

Fabrication and characterization of monocrystalline Zn2TiO4 films on MgO (111) substrates by PLD

Cubic Zn2TiO4 films were deposited on MgO (111) single crystal substrates using pulsed laser deposition (PLD). With the assistance of post-annealing process, the best epitaxial Zn2TiO4 film with only (111) single plane was obtained at 950[Formula: see text]C. For the best Zn2TiO4 single crystalline film, the full width at half maximum (FWHM) of the XRD [Formula: see text] rocking curve for the Zn2TiO4 (111) diffraction peak was only 0.12[Formula: see text]. From the TEM spectrum, the film had a regular lattice arrangement and strong periodicity. Structural analysis indicated that the out-plane and in-plane epitaxial arrangement of the film/substrate separately were Zn2TiO4 (111)//MgO (111) and Zn2TiO4 [10[Formula: see text]]//MgO <10[Formula: see text]>. The optical properties of the films were investigated in detail. After removing the influence of the transmittance of the substrate, the relative transmittance of the films exceeded 90% and the optical band gap of the films varied between 3.84 and 4.38 eV.

Effects of various seed layers on the agglomeration phenomenon on a MgO substrate

In this study, samples composed of various materials were fabricated using direct current magnetron sputtering on a single-crystal MgO(001) substrate. Ti, Cr, and Co were used as the seed layers, and Ag was used as the upper layer. Atomic force microscopy confirmed that Ag nanodots with regular shapes were formed when Ti was used as a seed layer, but when other materials were used, irregularly shaped nanodots were formed. Furthermore, it was confirmed that thermal agglomeration was disrupted if the thickness exceeded the critical point. Moreover, X-ray diffraction analysis indicated that the use of Ti in the seed layer promoted the epitaxial growth of Ag nanodots along the plane of the MgO(001) substrate; in contrast, Ag/Cr nanodots grew non-epitaxially. Based on the results of this study, it can be concluded that Ti is the most suitable seed layer material for Ag nanodot formation among the three materials examined herein.

Formation of FePt–MgO Nanocomposite Films at Reduced Temperature

The MgO nanolayer effect on the microstructure and magnetic characterizations added into Fe/Pt stacked films directly deposited onto MgO (001) single-crystal substrates at the reduced temperature of 380 °C using electron-beam technology was investigated in this present work. The nanograin isolation and exchange decoupling for the FePt–MgO system is attributed to the magnetic FePt isolated grains that originate from MgO atoms with a spreading behavior mostly along grain boundaries owing to its weaker surface energy than that of a single Fe or Pt element. The grain and domain size decreased when the MgO nanolayer was applied due to the interpenetration of MgO and created a strain-energy variation at the MgO/FePt interface. Measuring angular-dependent coercivity indicates a general trend of a domain-wall motion, and changes to the rotation of the reverse-domain model occurred as the MgO nanolayers were added into FePt films. The intergrain interaction is confirmed by the Kelly–Henkel plot, which shows that there is strong intergrain exchange coupling (positive δM type) between neighboring grains in the continuous Fe/Pt stacked films without MgO nanolayers. In addition, a negative δM type occurred when the Fe/Pt stacked films were added into MgO nanolayers, showing that the MgO nanolayer can be applied to adjust the force of intergrain exchange coupling between the adjacent FePt nanograins, and the addition of MgO nanolayers change into magnetic decoupling; thus, there was a formed dipole interaction in our claimed FePt–MgO composite structure of stacked ultrathin films at a reduced temperature of 380 °C.

Structural and magnetic properties of FeRh films grown on MgO(0 0 1), MgO(0 1 1) and MgO(1 1 1) substrates

FeRh thin films are grown on MgO(0 0 1), MgO(0 1 1) and MgO(1 1 1) single-crystal substrates at 600 °C by dc magnetron sputtering and post-annealed at the same temperature for 1 h. Effect of substrate orientation on structural and magnetic properties of FeRh films is studied using X-ray diffraction (Cu-Kα and synchrotron radiation), temperature, and field dependent magnetization measurements. FeRh(0 0 1), (1 1 2), (0 1 1) orientated films with CsCl ordering are obtained on MgO(0 0 1), MgO(0 1 1) and MgO(1 1 1) substrates, respectively. Epitaxial quality of FeRh(0 0 1) is relatively better than FeRh(1 1 2) and FeRh(0 1 1) films. The FeRh(0 1 1) film grows on two orientation relationships: Nishiyama-Wassermann (NW) and Kurdjumov-Sachs (KS). Temperature-dependent out-plane XRD measurements indicate an increase in c-parameter ∼0.56% to 0.62% across the AF-FM phase transition. The c/a ratio indicate that FeRh(0 0 1) film has in-plane compressive strain, while FeRh(1 1 2) and FeRh(0 1 1) films have tensile strain. The c/a ratio and temperature-dependent magnetization measurements (M–T) indicate that AF-FM phase transition can be tuned by appx. 52 K (388 K to 440 K) by variation of lattice strain at FeRh/MgO interface. Thermal hysteresis of appx. 12.8 K, 17 K and 22.9 K is observed for FeRh(0 0 1), FeRh(1 1 2) and FeRh(0 1 1) thin films. Larger thermal hysteresis is expected due to relatively poor epitaxial quality of the FeRh(1 1 2) and FeRh(0 1 1) films as compared to FeRh(0 0 1) film. The larger remnant FM component is observed in nominal AF state (room temperature) in all the samples, and is expected due to combined effect of composition inhomogeneity, substrate-induced strain and defects.

Superconductivity in Ti2O3 films on MgO substrate

Transition metal oxide films exhibit strong structure-property correlations, which foster innovative physical properties that are not present in their bulk counterparts. This study has successfully prepared different thickness-dependent epitaxial Ti2O3 thin films on MgO (001) single crystal substrate using the pulsed laser deposition method. The results reveal a strong correlation between the crystal structure and superconductivity in Ti2O3 thin films. Synchrotron x-ray diffraction and high resolution transmission electron microscope measurements confirmed that the appearance of superconductivity is closely related to the coexistence of the cubic-Ti2O3 (γ-Ti2O3) and orthorhombic-Ti2O3 (o-Ti2O3) phases. Superconductivity is absent in 80 nm film, which exhibits only a single γ-Ti2O3 phase. The highest superconducting transition temperature (T C) ∼ 7.2 K is achieved in 1200 nm film, which shows about 55% o-Ti2O3 phase and 45% γ-Ti2O3 phase. Room temperature Raman data suggest that E g modes at ∼330 and ∼430 cm−1 may associate with superconductivity, and an E g mode at ∼660 cm−1 is related to the insulating behavior observed in 80 nm film.