The Experts below are selected from a list of 2109 Experts worldwide ranked by ideXlab platform
Gang Xiao - One of the best experts on this subject based on the ideXlab platform.
-
Magnetotransport properties of polycrystalline and epitaxial Chromium Dioxide nanowires
Journal of Applied Physics, 2008Co-Authors: Xiaojing Zou, Gang Xiao, Sunxiang Huang, Tingyong Chen, Chia-ling ChienAbstract:Temperature dependent magnetotransport measurements were performed on polycrystalline and epitaxial Chromium Dioxide (CrO2) nanowires fabricated using the selective-area growth technique. Polycrystalline nanowires showed a negative temperature coefficient of resistivity at low temperatures because of strong grain boundary scattering. The magnetoresistance (MR) value exhibited a width dependence, reaching a maximum of 20% for a 150nm wide wire. In contrast, the MR response of single crystal CrO2 wires was mainly determined by magnetocrystalline and shape anisotropy.
-
Half-metallic Chromium Dioxide (CrO2) nanostructures and field-dependent magnetic domain evolution
Journal of Applied Physics, 2008Co-Authors: Xiaojing Zou, Gang XiaoAbstract:Both polycrystalline and epitaxial Chromium Dioxide structures in nanoscale have been fabricated using selective-area growth technique. Multiple grains were formed in the polycrystalline CrO2 dots larger than 100nm, however, below which only a single grain was allowed to exist. The lateral growth rate of epitaxial CrO2 nanocrystals was found to be highly anisotropic, which is affected by both the orientation and the deposited thickness. Field-dependent magnetic force microscopy was used to study the domain structures of single crystal CrO2 nanowires. The magnetization states were found to be closely related to the wire width. A stripelike domain structure with alternating magnetization parallel to the magnetic easy axis was observed within the CrO2 nanowires aligned along the [010] directions, indicating the existence of a strong uniaxial magnetocrystalline anisotropy.
-
Magnetic domain configurations of epitaxial Chromium Dioxide (CrO2) nanostructures
Applied Physics Letters, 2007Co-Authors: Xiaojing Zou, Gang XiaoAbstract:Magnetic domain structures of submicrometric epitaxial CrO2 fabricated using selective-area growth technique were studied by magnetic force microscopy. In-plane, lamellar domain structure with fragmented walls aligned along the magnetic easy axis direction is observed, indicating the existence of a large magnetocrystalline anisotropy. A classical model for ferromagnetic materials with a uniaxial anisotropy was used to explain this domain configuration. Estimates of the domain wall energy density and exchange stiffness constant for CrO2 were obtained.
-
Properties of epitaxial Chromium Dioxide films grown by chemical vapor deposition using a liquid precursor
Journal of Applied Physics, 2002Co-Authors: A. Anguelouch, Gang Xiao, A. Gupta, Guo-xing Miao, David W. Abraham, Snorri Ingvarsson, Chia-ling ChienAbstract:Epitaxial Chromium Dioxide (CrO2) films have been grown using chemical vapor deposition on (100) TiO2 substrate with chromyl chloride (CrO2Cl2) as a liquid precursor. The films are extremely smooth (rms roughness less than 4.6 A for a 1000-A-thick film) and have the largest spin polarization (P=98.4%) yet observed, as determined by point contact Andreev reflection. Magnetization switching properties of the films are close to those of a single-domain particle. Preliminary results on the in situ growth of exchange-biased CrO2/Cr2O3 multilayers are also reported. Although a bias field is observed, it is much smaller in comparison with the coercivity of the CrO2 film.
-
Near-complete spin polarization in atomically-smooth Chromium-Dioxide epitaxial films prepared using a CVD liquid precursor
Physical Review B, 2001Co-Authors: A. Anguelouch, Gang Xiao, David W. Abraham, Snorri Ingvarsson, Arunava Gupta, Chia-ling ChienAbstract:We have fabricated single crystal Chromium Dioxide $({\mathrm{CrO}}_{2})$ films using chemical vapor deposition with chromyl chloride $({\mathrm{CrO}}_{2}{\mathrm{Cl}}_{2})$ as a liquid precursor. Their electrical and magnetic properties have been investigated. These films are atomically smooth with a rms roughness of less than 5 \AA{} for 1000 \AA{} thick films. We have obtained a spin polarization of $P=98.4%,$ as determined by the technique of point-contact Andreev reflection. Magnetization and resistivity measurements on these films are in good agreement with those measured on films made with the ${\mathrm{CrO}}_{3}$ solid precursor. The process using the liquid precursor is superior to other existing techniques for the preparation of single crystal single- and multilayers containing ${\mathrm{CrO}}_{2},$ especially the structure of magnetic tunnel junctions.
A. Gupta - One of the best experts on this subject based on the ideXlab platform.
-
Activation of additional energy dissipation processes in the magnetization dynamics of epitaxial Chromium Dioxide films
Physical Review B, 2008Co-Authors: Georg M. Müller, Markus Münzenberg, Guo-xing Miao, A. GuptaAbstract:The precessional magnetization dynamics of a Chromium Dioxide (100) film is examined in an all-optical pump-probe setup. The frequency dependence on the external field is used to extract the uniaxial in-plane anisotropy constant. The damping shows a strong dependence on the frequency, but also on the laser pump fluence, which is revealed as an important experimental parameter in this work: above a certain threshold further channels of energy dissipation open and the damping increases sharply. This behavior might stem from spin-wave instabilities.
-
Properties of epitaxial Chromium Dioxide films grown by chemical vapor deposition using a liquid precursor
Journal of Applied Physics, 2002Co-Authors: A. Anguelouch, Gang Xiao, A. Gupta, Guo-xing Miao, David W. Abraham, Snorri Ingvarsson, Chia-ling ChienAbstract:Epitaxial Chromium Dioxide (CrO2) films have been grown using chemical vapor deposition on (100) TiO2 substrate with chromyl chloride (CrO2Cl2) as a liquid precursor. The films are extremely smooth (rms roughness less than 4.6 A for a 1000-A-thick film) and have the largest spin polarization (P=98.4%) yet observed, as determined by point contact Andreev reflection. Magnetization switching properties of the films are close to those of a single-domain particle. Preliminary results on the in situ growth of exchange-biased CrO2/Cr2O3 multilayers are also reported. Although a bias field is observed, it is much smaller in comparison with the coercivity of the CrO2 film.
-
Inverse magnetoresistance in Chromium-Dioxide-based magnetic tunnel junctions
Applied Physics Letters, 2001Co-Authors: A. Gupta, Gang XiaoAbstract:Magnetic tunnel junctions have been fabricated using half-metallic Chromium-Dioxide (CrO2) epitaxial film with a Co counterelectrode. The native insulating layer formed on the surface of CrO2 after air exposure is used as the tunneling barrier. These junctions exhibit nonlinear current–voltage characteristics, and the changes in junction resistance with applied field correspond to the coercivities of the two magnetic layers. The maximum observed magnetoresistance (MR) is about 8% at 4.2 K and has a negative sign, i.e., the resistance of the junction with parallel alignment of the electrodes is higher than with antiparallel alignment. This is opposite of what is normally observed with transition-metal electrodes and an Al2O3 barrier. Possible reasons for the inverse MR are discussed based on previous results on manganite/Co junctions.
-
Thickness dependence of magnetic and transport properties of Chromium Dioxide (CrO/sub 2/) strained epitaxial thin films
IEEE Transactions on Magnetics, 2001Co-Authors: A. Anguelouch, A. Gupta, Gang XiaoAbstract:Chromium Dioxide (CrO/sub 2/) thin films of different thicknesses (110-2600 /spl Aring/) have been grown epitaxially on [100]-oriented TiO/sub 2/ substrates by the chemical vapor deposition technique. The thicker films, with a Curie temperature of 395 K, exhibit in plane magnetocrystalline anisotropy with the magnetic easy axis along the c-axis direction. At low temperatures, the easy axis is deflected from the c- toward the b-axis in the thickness range 1500-600 /spl Aring/. For even smaller thicknesses, the c-axis and the magnetic easy axis coincide again. However, at 300 K, the c-axis is always the magnetic easy axis in every film studied. Resistivity measurement in the temperature range of 4.2 to 420 K has also been performed. Temperature dependence of the resistivity is highly anisotropic between the b- and the c-axis, due to thickness induced anisotropic strain in the CrO/sub 2/ films.
-
Influence of strain on the magnetic properties of epitaxial (100) Chromium Dioxide (CrO2) films
Applied Physics Letters, 1999Co-Authors: A. Gupta, Giang XiaoAbstract:Chromium Dioxide (CrO2) films have been grown epitaxially on (100) TiO2 substrates using chemical vapor deposition and their magnetic properties were studied as a function of film thickness (500 A–1.2 μm). Because of the lattice mismatch with the substrate, the films are strained as evidenced by x-ray diffraction measurements. The amount of strain depends on the thickness and also on the substrate cleaning conditions used prior to growth. Independent of their thickness, the films exhibit a sharp ferromagnetic transition with a Curie temperature in the range of 390–395 K. In-plane magnetic anisotropy is observed for the films, with [001] and [010] being the easy axis and hard axis directions, respectively, for the thicker films. The anisotropy field decreases with decreasing thickness, with the easy and hard axes switching directions for the thinnest films. The results are explained in terms of the competition between magnetocrystalline and strain anisotropies that favor the [001] and [010] magnetization d...
Lalgudi Venkataraman Ramanathan - One of the best experts on this subject based on the ideXlab platform.
-
Cyclic Oxidation Resistance of Rare Earth Oxide Gel Coated Fe-20Cr Alloys
Materials Research, 2007Co-Authors: Stela M.c. Fernandes, Lalgudi Venkataraman RamanathanAbstract:Rare earths (RE) have been used to increase high temperature oxidation resistance of Chromium Dioxide and alumina forming alloys. The RE can be added as elements (or oxides) to the alloys or applied as an oxide coating to the alloy surface. RE oxide gels, obtained by the sol-gel technique, were used to coat Fe-20Cr specimens prior to cyclic oxidation tests in the temperature range RT-900 °C. The cyclic oxidation resistance of the alloy increased with time required to reach a specific Chromium Dioxide layer thickness, and this was influenced by RE ion radius, the coating morphology and coverage. Extended cyclic oxidation tests from peak temperatures of 900, 1000 and 1100 °C at cooling rates of 330 and 1000 °C/s revealed the marked influence of La2O3 coatings. The role of RE in increasing overall oxidation resistance of Chromium Dioxide forming alloys is discussed.
-
Cyclic oxidation behaviour of rare earth oxide coated Fe–20Cr alloys
Surface Engineering, 2006Co-Authors: Stela M.c. Fernandes, Lalgudi Venkataraman RamanathanAbstract:AbstractThe use of rare earths (REs) to increase high temperature oxidation resistance of Chromium Dioxide and alumina forming alloys is well known. The REs can be added as elements (or oxides) to the alloys or applied as an oxide coating to the alloy surface. The sol gel technique is very efficient to apply fine oxide particle coatings. The influence of various RE oxide gels such as La2O3, CeO2, Pr2O3, Nd2O3, Sm2O3, Gd2O3, Dy2O3, Y2O3, Er2O3 and Yb2O3 on cyclic oxidation behaviour (RT-900°C) of Fe–20Cr alloy has been studied. The morphology and coverage of the RE oxide gels varied with the type of RE. The cyclic oxidation resistance increases with increase in time at temperature required to reach a specific Chromium Dioxide layer thickness and this is influenced by RE ion radius and characteristics of the RE oxide coating such as morphology, stability, coverage, resistance to thermal stresses and consequently adhesion.
-
Effect of surface deposited rare earth oxide gel characteristics on cyclic oxidation behavior of Fe20-Cr alloys
Materials Research, 2006Co-Authors: Stela M.c. Fernandes, Lalgudi Venkataraman RamanathanAbstract:Rare earths have been used to increase high temperature oxidation resistance of many Chromium Dioxide and alumina forming alloys. These rare earths can be added as elements (or as oxide dispersions) to the alloys or applied as an oxide coating to the alloy surface. The sol-gel technique is considered to be very efficient to apply fine oxide particle coatings. Oxide gel coatings of various rare earths such as lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, dysprosium, yttrium, erbium and ytterbium have been applied to an iron-Chromium alloy to determine their influence on the cyclic oxidation behavior (RT-900 °C) of the alloy. The morphology and coverage of the rare earth oxide gels varied with the type of rare earth. The cyclic oxidation resistance of the alloy increased with increase in time at temperature required to reach a specific Chromium Dioxide layer thickness and this in turn was influenced by the rare earth ion radius and characteristics of the rare earth oxide coating such as morphology, stability, coverage, resistance to thermal stresses and consequently adhesion.
-
The effect of rare earth additions on the oxidation behaviour of Fe-20Cr alloys
Corrosion Prevention & Control, 2002Co-Authors: Stela M.c. Fernandes, Lalgudi Venkataraman RamanathanAbstract:HIGH-TEMPERATURE oxidation resistance of iron-Chromium alloys is due to the formation of a protective layer of Chromium Dioxide. The addition of small quantities (< 1%) of certain reactive elements to these alloys improves their overall oxidation resistance. A number of mechanism have been proposed to explain the reactive element effect, without consensus as yet. This paper attempts a better understanding of this effect and reports the influence of three different rare earth (RE) elements - Pr, Dy, and Y - on the oxidation behaviour of Fe-20Cr alloy at 1000° and 1100°C. Specimens were oxidized for prolonged periods in a muffle furnace, and for storter durations in a thermobalance. The oxidized specimens were examined in a scanning-electron microscope coupled to an energy-dispersive spectrometer, and the oxide scales were analysed by x-ray diffraction. A thin layer of fine-grained Chromium Dioxide formed on the RE-containing alloy surface and the higher oxidation resistance of this alloy attributed to stifling of Chromium ion diffusion in the oxide layer by the rare-earth ions. The extent to which the RE elements influenced scale growth decreased in the following order: Pr, Dy, and Y. Direct correlation between RE ion radius and extent of influence on Chromium Dioxide growth rate has been observed.
Arunava Gupta - One of the best experts on this subject based on the ideXlab platform.
-
Epitaxial film growth of Chromium Dioxide by low pressure chemical vapor deposition using Chromium carbonyl
Thin Solid Films, 2010Co-Authors: Jinwen Wang, M. Pathak, Xing Zhong, Patrick Leclair, Tonya M. Klein, Arunava GuptaAbstract:Abstract Epitaxial Chromium Dioxide (CrO 2 ) thin films have been deposited by low pressure chemical vapor deposition (LPCVD) on (100) TiO 2 substrates using the precursor Chromium hexacarbonyl (Cr(CO) 6 ) within a narrow temperature window of 380–400 °C. Normal θ –2 θ Bragg x-ray diffraction results show that the predominant phase is CrO 2 with only a small amount of Cr 2 O 3 present, mostly at the film surface. The LPCVD films have a reasonably smooth surface morphology with a root mean square roughness of 4 nm on a scale of 5 μm. Raman spectroscopy confirms the existence of rutile CrO 2 in the deposited films, while transmission electron microscopy confirms the single-crystalline nature of the films. The LPCVD films showing a dominant CrO 2 phase exhibit clear uniaxial magnetic anisotropy with the easy axis oriented along the c direction.
-
Growth and Properties of Epitaxial Chromium Dioxide (CrO2) Thin Films and Heterostructures
Nanoscale Magnetic Materials and Applications, 2009Co-Authors: Guo-xing Miao, Arunava GuptaAbstract:\({\rm CrO}_2\) is a remarkable ferromagnetic material that is simultaneously an excellent metal for majority spin electrons and an insulator for minority spin electrons [1–3]. For this reason, \({\rm CrO}_2\) is called a half-metal, and in fact, it is the only one experimentally demonstrated [4–6]. Because of this, \({\rm CrO}_2\) has received considerable interest for spintronic applications in recent years. Band structure calculations have shown that the conduction bands in the spin minority channel of this system are completely shifted away from the Fermi level, resulting in 100% spin polarization. This makes it an attractive choice as a ferromagnetic material for spin-dependent devices such as spin injectors and spin detectors. In this chapter, we briefly describe the bonding characteristics in \({\rm CrO}_2\), based on first principles band structure calculations, as well as discuss some of its intrinsic structural, electrical, and magnetic properties. The strain-induced magnetic anisotropy resulting from lattice mismatch with the substrates is also discussed. Finally, we provide some details regarding the fabrication of epitaxial rutile-based heterostructures and their transport properties in micron-sized tunnel junction and GMR devices.
-
Near-complete spin polarization in atomically-smooth Chromium-Dioxide epitaxial films prepared using a CVD liquid precursor
Physical Review B, 2001Co-Authors: A. Anguelouch, Gang Xiao, David W. Abraham, Snorri Ingvarsson, Arunava Gupta, Chia-ling ChienAbstract:We have fabricated single crystal Chromium Dioxide $({\mathrm{CrO}}_{2})$ films using chemical vapor deposition with chromyl chloride $({\mathrm{CrO}}_{2}{\mathrm{Cl}}_{2})$ as a liquid precursor. Their electrical and magnetic properties have been investigated. These films are atomically smooth with a rms roughness of less than 5 \AA{} for 1000 \AA{} thick films. We have obtained a spin polarization of $P=98.4%,$ as determined by the technique of point-contact Andreev reflection. Magnetization and resistivity measurements on these films are in good agreement with those measured on films made with the ${\mathrm{CrO}}_{3}$ solid precursor. The process using the liquid precursor is superior to other existing techniques for the preparation of single crystal single- and multilayers containing ${\mathrm{CrO}}_{2},$ especially the structure of magnetic tunnel junctions.
-
Magnetic and transport properties of epitaxial and polycrystalline Chromium Dioxide thin films (invited)
Journal of Applied Physics, 2000Co-Authors: Arunava Gupta, Gang XiaoAbstract:The magnetic and transport properties of epitaxial and polycrystalline Chromium Dioxide (CrO2) thin films have been investigated. They are grown epitaxially on single crystal TiO2 (100) substrates, and with multiple grain orientations on polycrystalline TiO2 substrates, by chemical vapor deposition. The films have a Curie temperature (TC) of 390–395 K, with the epitaxially grown CrO2 (100) films exhibiting in-plane uniaxial magnetic anisotropy. While the epitaxial samples display metallic characteristics, the polycrystalline films are semiconducting with a dominant grain boundary contribution to the resistance at low temperatures. The magnetoresistance (MR) properties have also been measured with the magnetic field applied in the plane. For the epitaxial films, the MR is negative at temperatures near TC and is positive at low temperatures. A negative MR is also observed near TC for the polycrystalline samples. However, unlike the epitaxial films, the MR is found to be negative also at low temperatures, wi...
-
Strain dependence and magnetic anisotropy in Chromium Dioxide thin films
MRS Proceedings, 2000Co-Authors: Leonard Spinu, Arunava Gupta, Hariharan Srikanth, Gang XiaoAbstract:AbstractField-dependent transverse susceptibility measurements based on a novel tunnel-diode oscillator (TDO) method were used to probe the magnetic anisotropy and switching in epitaxially grown CrO2 films on TiO2 substrates. The static field was applied in three different orientations (0°, 45° and 90° ) with respect to the magnetic easy axis. Singular peaks in the transverse susceptibility were observed that are associated with the anisotropy and switching fields in CrO2. Theoretical calculations based on a coherent rotation model display remarkable agreement with the experimental data. A new peak emerges at low temperatures and is identified to be a consequence of temperature-dependent magnetoelastic contributions resulting from the in-plane tensile strain in the films due to varying coefficients of thermal expansion in the film and the substrate.
Stela M.c. Fernandes - One of the best experts on this subject based on the ideXlab platform.
-
Cyclic Oxidation Resistance of Rare Earth Oxide Gel Coated Fe-20Cr Alloys
Materials Research, 2007Co-Authors: Stela M.c. Fernandes, Lalgudi Venkataraman RamanathanAbstract:Rare earths (RE) have been used to increase high temperature oxidation resistance of Chromium Dioxide and alumina forming alloys. The RE can be added as elements (or oxides) to the alloys or applied as an oxide coating to the alloy surface. RE oxide gels, obtained by the sol-gel technique, were used to coat Fe-20Cr specimens prior to cyclic oxidation tests in the temperature range RT-900 °C. The cyclic oxidation resistance of the alloy increased with time required to reach a specific Chromium Dioxide layer thickness, and this was influenced by RE ion radius, the coating morphology and coverage. Extended cyclic oxidation tests from peak temperatures of 900, 1000 and 1100 °C at cooling rates of 330 and 1000 °C/s revealed the marked influence of La2O3 coatings. The role of RE in increasing overall oxidation resistance of Chromium Dioxide forming alloys is discussed.
-
Cyclic oxidation behaviour of rare earth oxide coated Fe–20Cr alloys
Surface Engineering, 2006Co-Authors: Stela M.c. Fernandes, Lalgudi Venkataraman RamanathanAbstract:AbstractThe use of rare earths (REs) to increase high temperature oxidation resistance of Chromium Dioxide and alumina forming alloys is well known. The REs can be added as elements (or oxides) to the alloys or applied as an oxide coating to the alloy surface. The sol gel technique is very efficient to apply fine oxide particle coatings. The influence of various RE oxide gels such as La2O3, CeO2, Pr2O3, Nd2O3, Sm2O3, Gd2O3, Dy2O3, Y2O3, Er2O3 and Yb2O3 on cyclic oxidation behaviour (RT-900°C) of Fe–20Cr alloy has been studied. The morphology and coverage of the RE oxide gels varied with the type of RE. The cyclic oxidation resistance increases with increase in time at temperature required to reach a specific Chromium Dioxide layer thickness and this is influenced by RE ion radius and characteristics of the RE oxide coating such as morphology, stability, coverage, resistance to thermal stresses and consequently adhesion.
-
Effect of surface deposited rare earth oxide gel characteristics on cyclic oxidation behavior of Fe20-Cr alloys
Materials Research, 2006Co-Authors: Stela M.c. Fernandes, Lalgudi Venkataraman RamanathanAbstract:Rare earths have been used to increase high temperature oxidation resistance of many Chromium Dioxide and alumina forming alloys. These rare earths can be added as elements (or as oxide dispersions) to the alloys or applied as an oxide coating to the alloy surface. The sol-gel technique is considered to be very efficient to apply fine oxide particle coatings. Oxide gel coatings of various rare earths such as lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, dysprosium, yttrium, erbium and ytterbium have been applied to an iron-Chromium alloy to determine their influence on the cyclic oxidation behavior (RT-900 °C) of the alloy. The morphology and coverage of the rare earth oxide gels varied with the type of rare earth. The cyclic oxidation resistance of the alloy increased with increase in time at temperature required to reach a specific Chromium Dioxide layer thickness and this in turn was influenced by the rare earth ion radius and characteristics of the rare earth oxide coating such as morphology, stability, coverage, resistance to thermal stresses and consequently adhesion.
-
The effect of rare earth additions on the oxidation behaviour of Fe-20Cr alloys
Corrosion Prevention & Control, 2002Co-Authors: Stela M.c. Fernandes, Lalgudi Venkataraman RamanathanAbstract:HIGH-TEMPERATURE oxidation resistance of iron-Chromium alloys is due to the formation of a protective layer of Chromium Dioxide. The addition of small quantities (< 1%) of certain reactive elements to these alloys improves their overall oxidation resistance. A number of mechanism have been proposed to explain the reactive element effect, without consensus as yet. This paper attempts a better understanding of this effect and reports the influence of three different rare earth (RE) elements - Pr, Dy, and Y - on the oxidation behaviour of Fe-20Cr alloy at 1000° and 1100°C. Specimens were oxidized for prolonged periods in a muffle furnace, and for storter durations in a thermobalance. The oxidized specimens were examined in a scanning-electron microscope coupled to an energy-dispersive spectrometer, and the oxide scales were analysed by x-ray diffraction. A thin layer of fine-grained Chromium Dioxide formed on the RE-containing alloy surface and the higher oxidation resistance of this alloy attributed to stifling of Chromium ion diffusion in the oxide layer by the rare-earth ions. The extent to which the RE elements influenced scale growth decreased in the following order: Pr, Dy, and Y. Direct correlation between RE ion radius and extent of influence on Chromium Dioxide growth rate has been observed.