Thickness-dependent transport properties of Sr4Fe6O13 epitaxial thin films

High-quality epitaxial thin films of Sr₄Fe₆O₁₃ have been deposited on NdGaO₃(001) substrates by pulsed laser deposition. The transport properties have been characterized by impedance spectroscopy. The temperature dependence of conductivity suggests a mechanism of adiabatic hopping by small polarons, in agreement with previous results in bulk samples. The transport properties show a clear dependence on the film thickness with the thinner films (10 nm) presenting conductivity values in oxygen one order of magnitude higher than the thicker ones (313 nm). We correlate this behaviour with the thickness dependence of the epitaxial strain.     

Structure and orbital ordering of ultrathin LaVO3 probed by atomic resolution electron microscopy and Raman spectroscopy

Orbital ordering has been less investigated in epitaxial thin films, due to the difficulty to evidence directly the occurrence of this phenomenon in thin film samples. Atomic resolution electron microscopy enabled us to observe the structural details of the ultrathin LaVO₃ films. The transition to orbital ordering of epitaxial layers as thin as ≈4 nm was probed by temperature‐dependent Raman scattering spectroscopy of multilayer samples. From the occurrence and temperature dependence of the 700 cm^–1 Raman active mode it can be inferred that the structural phase transition associated with orbital ordering takes place in ultrathin LaVO₃ films at about 130 K.     

Thermal and irradiation induced interdiffusion in magnetite thin films grown on magnesium oxide (0 0 1) substrates

Epitaxial Fe₃O₄(0 0 1) thin films (with a thickness in the range of 10-20 nm) grown on MgO substrates were characterized using low-energy electron diffraction (LEED), conversion electron Mössbauer spectroscopy (CEMS) and investigated using Rutherford backscattering spectrometry (RBS), channeling (RBS-C) experiments and X-ray reflectometry (XRR). The Mg out-diffusion from the MgO substrate into the film was observed for the directly-deposited Fe₃O₄/MgO(0 0 1) films. For the Fe₃O₄/Fe/MgO(0 0 1) films, the Mg diffusion was prevented by the Fe layer and the surface layer is always a pure Fe₃O₄ layer. Annealing and ion beam mixing induced a very large interface zone having a spinel and/or wustite formula in the Fe₃O₄-on-Fe film system.     

The structure and optical characters of the ZnO film grown on GaAs/Al2O3 substrate

In this paper ZnO films are grown on GaAs/Al₂O₃ substrates at different temperature by metal-organic chemical vapor deposition (MOCVD). The GaAs/Al₂O₃ substrates are formed by depositing GaAs layer (∼35 nm) on the Al₂O₃ substrate. The results of X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) demonstrate that most of the Ga and As atoms form Ga-As bond and the GaAs layer does not present any orientation. The characters of the ZnO films grown on GaAs/Al₂O₃ substrates are investigated by XRD, photoluminescence (PL), atomic force microscopy (AFM) and Raman scattering. Compared with ZnO film grown on Al₂O₃ substrate, ZnO film prepared by our fabrication scheme has good crystal and optical quality. Meanwhile its grain size becomes bigger according to the AFM image. Raman analysis indicates that the intrinsic defects and the in-plane tensile stress are obviously reduced in ZnO/GaAs/Al₂O₃ samples.     

Effect of thermal annealing of high temperature growth [(GaAs)m(Fe)n]p composite films on GaAs(001) by molecular beam epitaxy

[(GaAs) _m (Fe) _n ] _p composite films on GaAs(001) substrates obtained by molecular beam deposition methods at T _s = 580°C have been thermally annealed, and magnetic/structural changes caused by the annealing have been measured to study the relation between room-temperature photo-magnetic effect and by-products in composite films. Annealing inhomogeneous [(GaAs)₈(Fe)₅]₂₀ film, prepared by alternate beam deposition of Fe and GaAs, results in an increase in saturation magnetization, whereas the room-temperature photo-enhanced magnetization (RT-PEM) vanishes. Metamagnetic Fe₃Ga₄ is suppressed with the formation of ferromagnetic Fe₃Ga_2−x As _x . Observed results suggest two important points: firstly, metamagnetic Fe₃Ga₄ compound is most likely meta-stable bi-product and may play a principle role for RT-PEM, and secondly, ferromagnetic Fe₃GaAs and its derivatives are the stable form in Ga-As-Fe ternary system.     

Application of M?ssbauer spectroscopy in magnetism

An overview is provided on our recent work that applies ⁵⁷Fe M?ssbauer spectroscopy to specific problems in nanomagnetism. ⁵⁷Fe conversion electron M?ssbauer spectroscopy (CEMS) in conjunction with the ⁵⁷Fe probe layer technique as well as ⁵⁷Fe nuclear resonant scattering (NRS) were employed for the study of various nanoscale layered systems: (i) metastable fct-Fe; a strongly enhanced hyperfine magnetic field B_hf of ～39?T at 25?K was observed in ultrahigh vacuum (UHV) on uncoated three-monolayers thick epitaxial face-centered tetragonal (fct) ⁵⁷Fe(110) ultrathin films grown by molecular-beam epitaxy (MBE) on vicinal Pd(110) substrates; this indicates the presence of enhanced Fe local moments, μ_Fe, as predicted theoretically; (ii) Fe spin structure; by applying magnetic fields, the Fe spin structure during magnetization reversal in layered (Sm–Co)/Fe exchange spring magnets and in exchange-biased Fe/MnF₂ bilayers was proven to be non-collinear and depth-dependent; (iii) ferromagnet/semiconductor interfaces for electrical spin injection; CEMS was used as a diagnostic tool for the investigation of magnetism at the buried interface of Fe electrical contacts on the clean surface of GaAs(001) and GaAs(001)-based spin light-emitting diodes (spin LED) with in-plane or out-of-plane Fe spin orientation; the measured rather large average hyperfine field of ～27?T at 295?K and the distribution of hyperfine magnetic fields, P(B_hf), provide evidence for the absence of magnetically “dead” layers and the existence of relatively large Fe moments (μ_Fe ～ 1.8?μ_B) at the ferromagnet/semiconductor interface. - Finally, a short outlook is given for potential applications of M?ssbauer spectroscopy on topical subjects of nanomagnetism/spintronics.     

Wet‐Etching Induced Abnormal Phase Transition in Highly Strained VO2/TiO2 (001) Epitaxial Film

The metal–insulator transition (MIT) behavior in vanadium dioxide (VO₂) epitaxial film is known to be dramatically affected by interfacial stress due to lattice mismatching. For the VO₂/TiO₂ (001) system, there exists a considerable strain in ultra‐thin VO₂ thin film, which shows a lower T_c value close to room temperature. As the VO₂ epitaxial film grows thicker layer‐by‐layer along the “bottom‐up” route, the strain will be gradually relaxed and T_c will increase as well, until the MIT behavior becomes the same as that of bulk material with a T_c of about 68 °C. Whereas, in this study, we find that the VO₂/TiO₂ (001) film thinned by “top‐down” wet‐etching shows an abnormal variation in MIT, which accompanies the potential relaxation of film strain with thinning. It is observed that even when the strained VO₂ film is etched up to several nanometers, the MIT persists, and T_c will increase up to that of bulk material, showing the trend to a stress‐free ultra‐thin VO₂ film. The current findings demonstrate a facial chemical‐etching way to change interfacial strain and modulate the phase transition behavior of ultrathinVO₂ films, which can also be applied to other strained oxide films.     

Growth of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> films on the Si(111) surface covered by a thin SiO<Subscript>2</Subscript> layer

Magnetite polycrystalline films are grown by variously oxidizing a Fe film on the Si(111) surface covered by a thin (1.5 nm) SiO₂ layer. It is found that defects in the SiO₂ layer influence silicidation under heating of the Fe film. The high-temperature oxidation of the Fe film results in the formation of both Fe₃O₄ and iron monosilicide. However, the high-temperature deposition of Fe in an oxygen atmosphere leads to the growth of a compositionally uniform Fe₃O₄ film on the SiO₂ surface. It is found that such a synthesis method causes [311] texture to arise in the magnetite film, with the texture axis normal to the surface. The influence of the synthesis method on the magnetic properties of grown Fe₃O₄ films is studied. A high coercive force of Fe₃O₃ films grown by Fe film oxidation is related to their specific morphology and compositional nonuniformity.     

Development of magnetic anisotropies in ultrathin epitaxial films of Fe(001) and Ni(001)

Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asK _us = 0.96, 0.63, and 0.3 ergs/cm² respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K _1S=0.012 ergs/cm². Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.     

Phase‐controlled epitaxial growth of iron oxide thin films on MgO(001) and LaAlO3(001) substrates

We have deposited epitaxial iron oxide thin films on MgO(001) and LaAlO₃(LAO)(001) substrates, resulting in different phase stabilities. Atomic force microscopy images revealed a smooth surface. Detailed X‐ray diffraction (XRD) measurements were performed to confirm the epitaxial growth and to analyze the atomic growth configuration. We found that (00l) oriented γ‐Fe₂O₃ was the stable phase on MgO(001) substrates, whereas $ (1\bar 102) $ oriented α‐Fe₂O₃ was stable on LAO(001). Magnetic hysteresis loop measurements revealed typical ferrimagnetic behavior for γ‐Fe₂O₃ on MgO, whereas the magnetization of α‐Fe₂O₃ on LAO was relatively small and consistent with an antiferromagnetic order. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Epitaxial strain-induced magnetic anisotropy in Sm3Fe5O12 thin films grown by pulsed laser deposition

Sm₃Fe₅O₁₂ thin films of various thicknesses were grown on a (0 0 1)-oriented Gd₃Ga₅O₁₂ substrate by pulsed laser deposition. The crystal structure of the films was strongly dependent on film thickness. The lattice was strained for thinner films due to a lattice mismatch between the film and substrate. This lattice strain was relaxed when the film thickness exceeded a critical thickness of around 660 Å. It is suggested that the epitaxial strain induces uniaxial magnetic anisotropy with an out-of-plane magnetic easy axis.     

Spin-resolved photoelectron spectroscopy of the MgO/Fe(110) system

The surface structure and electronic properties of ultrathin MgO layers grown on epitaxial Fe(110) films were investigated at room temperature by means of electron diffraction, Auger electron spectroscopy, scanning tunneling microscopy, and spin-resolved photoelectron spectroscopy. The spin polarization at the Fermi level (E_F) of the Fe(110) film decreases sharply with increasing thickness of the MgO layer. This behavior arises from the formation of a thin FeO layer at the MgO(111)/Fe(110) interface, as revealed by structural and spectroscopic investigations. The strong attenuation of the intrinsic spin polarization is qualitatively attributed to the scattering of spin-polarized electrons at the unoccupied d-orbitals of Fe²⁺. PACS 68.35.-p; 68.55.-a; 73.20.r; 75.70.Cn; 79.60.-I     

Reactive sputtering of (Co,Fe) nitride thin films on TiN-bufferd Si

Epitaxial (Co,Fe) nitride films were prepared on TiN buffered Si(001) substrates by dual-target reactive co-sputtering method. With lower Co content, thin films mainly consist of (Co _x Fe_1?x )₄N phase. With higher Co content, STEM EELS found no N signal in the thin film, and, combined with XRD results, shows that fcc Co is the main phase of the thin films instead of Co₄N. The N₂ atmosphere is helpful to induce the fcc Co phase formation during dual-target reactive co-sputtering deposition. For the films with less Co content, the RT magnetization measurements show similar magnetic properties as epitaxial Fe₄N(001) films, while increasing the Co content, the resulting fcc Co thin films show biaxial anisotropy with the [110] in-plane easy axis.     

Epitaxial growth of high-Tc superconducting Tl-Ca-Ba-Cu-O films by liquid phase epataxial process

Tl-Ca-Ba-Cu-O epitaxial films have been successfully grown on (001) MgO substrate by liquid phase epitaxial (LPE) process. The as-grown films showed an onset of superconductivity at about 140 K and zero resistance at 111 K. X-ray diffraction analysis suggests the films to be highly preferentially oriented with the c-axis perpendicular to the film surface. Observation from transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) revealed the presence of both Tl-Ca₂Ba₂Cu₃O_y and TlCa₂Ba₃Cu₄O_y phases.     

Exchange coupling in ultrathin epitaxial yttrium iron garnet films

Magnetic exchange coupling has been observed for ultrathin films of yttrium iron garnet (Y₃Fe₅O₁₂ or YIG). Single-crystalline YIG films were prepared on yttrium aluminium garnet (Y₃Al₅O₁₂ or YAG) substrates by pulsed laser deposition. (111) and (110) oriented substrates were used. Film thicknesses were varied from 180 ? to 4600 ?. Epitaxial growth of YIG on YAG was obtained in spite of the lattice mismatch of 3%. Magnetic hysteresis loops recorded for ultrathin YIG films have a “bee-waist” shape and show a coupling between two different magnetic phases. The first phase is magnetically soft YIG. A composition study by secondary ion mass spectroscopy shows the second phase to be Y₃Fe_5-xAl_xO₁₂ due to the interdiffusion of Fe and Al at the film/substrate interface. This compound is known to be magnetically harder and to have weaker magnetization than YIG. The coupling of the two phases leads to a hysteresis loop displacement at low temperatures. This displacement varies differently with film thickness for two substrate orientations. Assuming an interfacial coupling, the maximal interaction energy is estimated to be about 0.17 erg/cm² at 5 K for (111) oriented sample. Received 3 June 2002 / Received in final form 7 October 2002 Published online 27 January 2003 RID="a" ID="a"Presently at LPM, Université H. Poincaré, BP 239, 54506 Vandœuvre-lès-Nancy e-mail: popova@lpm.u-nancy.fr     

Giant Hall effect in (GaAs)Fe granular films

A series of (GaAs)_1 − xFe_x (x: volume fraction) films with Fe granules embedded in GaAs matrix were prepared by magnetron sputtering. Hall Effect of the films was characterized. The largest saturated Hall resistivity of was observed in (GaAs)₃₀Fe₇₀ film at room temperature, which is over 2 orders larger than that of pure Fe, about 1 order larger than that of (NiFe)–(Al₂O₃) and (NiCo)–(SiO₂) granular films prepared under the same preparation conditions, and 150% larger than that of Ge₃₀Fe₇₀.     

Self-organized growth of pyramidal clusters in epitaxial spinel CoCr2O4 films on rock salt MgO(001) substrates

Ferrimagnetic CoCr₂O₄ epitaxial thin films have been grown on MgO(001) substrates. The surface morphology of the films shows self-organized nanometric pyramids and hut clusters. The origins of this morphology are found in the anisotropy of the surface energy of CoCr₂O₄ and the epitaxial growth. A comparison with the growth on spinel MgAl₂O₄(001) substrates is done. It turns out that pyramids on MgO are typically larger and with a higher surface coverage than on MgAl₂O₄. These results open the possibility of fabricating hybrid structures of ferrimagnetic self-organized nanometric high aspect ratio objects on substrates of selected functionality. Tailoring the size of the objects can be achieved not only by deposition temperature and film thickness but also by appropriate selection of substrates. PACS 81.07.Bc; 68.65.-k     

LP-MOCVD异质外延ZnO薄膜中的应力及对缺陷的影响

利用低压金属有机化学气相淀积(LP-MOCVD)在Si基片上外延生长ZnO薄膜，制备了两类样品 ：一类是在Si上直接外延ZnO，另一类是在Si上通过SiC过渡层来外延ZnO.根据两类样品的拉 曼光谱、x射线衍射、原子力显微图和光致发光的结果，表明ZnO外延薄膜中的张应力对薄膜 的结晶状况有着重要的影响，使用SiC过渡层能够有效缓解ZnO薄膜中的张应力，减小缺陷浓 度，提高ZnO外延层的质量；然后根据缺陷的形成机制进一步提出，对于ZnO/Si，其中较大 的张应力导致了高浓度的非辐射复合缺陷的形成，使得样品的紫外和绿峰的发射强度均大大 降低；对于ZnO/SiC/Si，其中较小的张应力导致ZnO薄膜中主要形成氧替位缺陷O_{Zn，从而使发光中的绿峰增强. 关键词： ZnO薄膜 应力 缺陷 拉曼光谱}     

Ferromagnetic resonance in epitaxial (Bi,Lu)3(Fe,Ga)5O12(210) films

Ferromagnetic resonance in epitaxial (Bi,Lu)₃(Fe,Ga)₅O₁₂ films grown on Gd₃Ga₅O₁₂(210) substrates is investigated. The spectrum contains a number of peaks, the most intense of which is related to the bulk of the film and the transition layer at the film-substrate interface. Most of the film volume is characterized by reduced magnetic anisotropy. The azimuthal and polar dependences of the resonance field exhibit 180° symmetry.     

蓝宝石基片的处理方法对ZnO薄膜生长行为的影响

采用反应射频磁控溅射方法,在经过不同方法处理的蓝宝石基片上,在同一条件下沉积了ZnO薄膜.利用原子力显微镜、X射线衍射、反射式高能电子衍射等分析技术,对基片和薄膜的结构、表面形貌进行了系统表征.研究结果显示,不同退火条件下的蓝宝石基片表面结构之间没有本质的差异,均为α-Al₂O₃ (001)晶面,但基片表面形貌的变化较大.在不同方法处理的蓝宝石基片上生长的ZnO薄膜均具有高c轴取向的织构特征,但薄膜的表面形貌差异较大.基片经真空退火处 关键词： ZnO薄膜 反应磁控溅射 基片处理 形貌分析