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Results for “FERROMAGNETISM”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Local vibrational modes competitions in Mn-doped ZnO epitaxial films with tunable ferromagnetism

We reported spectroscopic investigations of high quality Mn-doped ZnO (ZnMnO) films grown by oxygen plasma-assisted molecular beam epitaxy. Raman scattering spectra revealed two local vibrational modes (LVMs) associated with Mn dopants at 523 and 712 cm{sup −1}. The LVMs and magnetic properties of ZnMnO films can be synchronously modulated by post annealing processing or by introducing tiny Co. The relative intensity of two LVMs clearly shows competitions arising from uncompensated acceptor and donor defects competition for ferromagnetic and nonmagnetic films. The experimental results indicated that LVM at 523 cm{sup −1} is attributed to Mn—(Zinc-vacancy) complexes, while LVM at 712 cm{sup −1} is attributed to Mn—(Oxygen-vacancy) complexes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Off-axis electron holography of ferromagnetic multilayer nanowires

We have used electron holography to investigate the local magnetic behavior of isolated ferromagnetic nanowires (NWs) in their remanent states. The NWs consisted of periodic magnetic layers of soft, high-saturation magnetization CoFeB alloys, and non-magnetic layers of Cu. All NWs were fabricated by pulsed-potential electrodeposition in nanoporous alumina membranes. The NW composition and layer thicknesses were measured using scanning transmission electron microscopy and energy dispersive spectroscopy. The magnetization of individual NWs depended upon the thicknesses of the layers and the direction of an external magnetic field, which had been applied in situ. When the CoFeB was thicker than the diameter (50 nm), magnetization was axial for all external field directions, while thinner layers could be randomized via a perpendicular field. In some cases, magnetization inside the wire was detected at an angle with respect to the axis of the wires. In thinner Cu/CoFeB (<10 nm each) multilayer, magnetic field vortices were detected, associated with opposing magnetization in neighbouring layers. The measured crystallinity, compositions, and layer thicknesses of individual NWs were found to be significantly different from those predicted from calibration growths based on uniform composition NWs. In particular, a significant fraction of Cu (up to 50 at. %) was present in the CoFeB layers such that the measured magnetic induction was lower than expected. These results will be used to better understand previously measured effective anisotropy fields of similar NW arrays.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Ultra-narrow ferromagnetic resonance in organic-based thin films grown via low temperature chemical vapor deposition

We present the growth of thin films of the organic-based ferrimagnetic semiconductor V[TCNE]{sub x} (x ∼ 2, TCNE: tetracyanoethylene) via chemical vapor deposition. Under optimized growth conditions, we observe a significant increase in magnetic homogeneity, as evidenced by a Curie temperature above 600 K and sharp magnetization switching. Further, ferromagnetic resonance studies reveal a single resonance with full width at half maximum linewidth of 1.4 G, comparable to the narrowest lines measured in inorganic magnetic materials and in contrast to previous studies that showed multiple resonance features. These characteristics are promising for the development of high frequency electronic devices that take advantage of the unique properties of this organic-based material, such as the potential for low cost synthesis combined with low temperature and conformal deposition on a wide variety of substrates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Large anomalous Hall effect in ferromagnetic insulator-topological insulator heterostructures

We demonstrate the van der Waals epitaxy of the topological insulator compound Bi{sub 2}Te{sub 3} on the ferromagnetic insulator Cr{sub 2}Ge{sub 2}Te{sub 6}. The layers are oriented with (001)Bi{sub 2}Te{sub 3}||(001)Cr{sub 2}Ge{sub 2}Te{sub 6} and (110)Bi{sub 2}Te{sub 3}||(100)Cr{sub 2}Ge{sub 2}Te{sub 6}. Cross-sectional transmission electron microscopy indicates the formation of a sharp interface. At low temperatures, bilayers consisting of Bi{sub 2}Te{sub 3} on Cr{sub 2}Ge{sub 2}Te{sub 6} exhibit a large anomalous Hall effect (AHE). Tilted field studies of the AHE indicate that the easy axis lies along the c-axis of the heterostructure, consistent with magnetization measurements in bulk Cr{sub 2}Ge{sub 2}Te{sub 6}. The 61 K Curie temperature of Cr{sub 2}Ge{sub 2}Te{sub 6} and the use of near-stoichiometric materials may lead to the development of spintronic devices based on the AHE.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Ferromagnetism and the electronic band structure in (Ga,Mn)(Bi,As) epitaxial layers

Impact of Bi incorporation into (Ga,Mn)As layers on their electronic- and band-structures as well as their magnetic and structural properties has been studied. Homogenous (Ga,Mn)(Bi,As) layers of high structural perfection have been grown by the low-temperature molecular-beam epitaxy technique. Post-growth annealing treatment of the layers results in an improvement of their structural and magnetic properties and an increase in the hole concentration in the layers. The modulation photoreflectance spectroscopy results are consistent with the valence-band model of hole-mediated ferromagnetism in the layers. This material combines the properties of (Ga,Mn)As and Ga(Bi,As) ternary compounds and offers the possibility of tuning its electrical and magnetic properties by controlling the alloy composition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Interplay between interband coupling and ferromagnetism in iron pnictide superconductor/ferromagnet/iron pnictide superconductor junctions

An extended eight-component Bogoliubov-de Gennes equation is applied to study the Josephson effect between iron-based superconductors (SCs) with s{sub ±}-wave pairing symmetry, separated by an ferromagnet (FM). The feature of damped oscillations of critical Josephson current as a function of FM thickness, the split of the peaks induced by the interband coupling is much different from that for the junction with the s{sub ±}-wave SCs replaced by s{sub ++}-wave ones. In particular, a 0−π transition as a function of interband coupling strength α is found to always exhibit with the corresponding dip shifting toward the larger α due to enhancing the spin polarization in the FM, while there exits no 0−π transition for the SC with s{sub ++}-wave pairing symmetry. The two features can be used to identify the pairing symmetry in the iron pnictide SC different from the s{sub ++}-wave one in MgB{sub 2}. Experimentally, by adjusting the doping level in the s{sub ±}-wave SCs, one can vary α.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Microwave-induced spin currents in ferromagnetic-insulator|normal-metal bilayer system

A microwave technique is employed to simultaneously examine the spin pumping and the spin Seebeck effect processes in a YIG|Pt bilayer system. The experimental results show that for these two processes, the spin current flows in opposite directions. The temporal dynamics of the longitudinal spin Seebeck effect exhibits that the effect depends on the diffusion of bulk thermal-magnons in the thermal gradient in the ferromagnetic-insulator|normal-metal system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

High-output microwave detector using voltage-induced ferromagnetic resonance

We investigated the voltage-induced ferromagnetic resonance (FMR) with various DC bias voltage and input RF power in magnetic tunnel junctions. We found that the DC bias monotonically increases the homodyne detection voltage due to the nonlinear FMR originating in an asymmetric magnetization-potential in the free layer. In addition, the linear increase of an output voltage to the input RF power in the voltage-induced FMR is more robust than that in spin-torque FMR. These characteristics enable us to obtain an output voltage more than ten times than that of microwave detectors using spin-transfer torque.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Quantized spin waves in single Co/Pt dots detected by anomalous Hall effect based ferromagnetic resonance

Anomalous Hall effect (AHE) based ferromagnetic resonance (FMR) measurements were carried out on perpendicularly magnetized Co/Pt multilayer single dots of 0.4–3 μm in diameter. The resonance behavior was measured by detecting the decrease of perpendicular magnetization component due to magnetization precession. Resonance behavior was observed as a clear decrease of Hall voltages, and the obtained resonance fields were consistent with the results of vector-network-analyzer FMR. Spin-waves with cylindrical symmetry became significant by decreasing the dot diameter, and quantized multiple resonances were observed in the dot of 0.4 μm in diameter. The AHE based FMR proposed here is a powerful method to approach magnetization dynamics including spin waves and non-linear behavior excited in a finite nanostructure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Adjustable ferromagnetic resonance frequency in CoO/CoFeB system

Static and dynamic properties of (CoO/CoFeB){sub n} multilayers have been investigated. An anisotropy field enhancement was evidenced when the CoO layer was deposited under the CoFeB layer. Tuning the relative CoFeB and CoO layers thicknesses, high ferromagnetic resonance frequencies up to 4 GHz were achieved. The coupling effect between the CoO and CoFeB layers was induced by a dipolar coupling due to the anisotropic roughness topology of the CoO layer. This anisotropic roughness was induced by the deposition geometry and evidenced by atomic force microscopy. The strength of the dipolar interfacial coupling was calculated thanks to Schlömann's model. Multilayer stacks were fabricated and the magnetic properties observed for the trilayers could be maintained.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Rapid characterizing of ferromagnetic materials using spin rectification

Spin rectification is a powerful tool for dc electric detections of spin dynamics and electromagnetic waves. Technically, elaborately designed on-chip microwave devices are needed in order to realize that effect. In this letter, we propose a rapid characterizing approach based on spin rectification. By directly sending dynamic current into ferromagnetic films with stripe shape, resonant dc voltages can be detected along the longitudinal or transversal directions. As an example, Fe (010) films with precise crystalline structure and magnetic parameters were used to testify the reliability of such method. We investigated not only the dynamic parameters and the precise anisotropy constants of the Fe crystals but also the principle of spin rectification in this method.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Post-annealing effect on the room-temperature ferromagnetism in Cu-doped ZnO thin films

In this work, we investigated the structural and magnetic properties of both as-deposited and post-annealed Cu-doped ZnO thin films for better understanding the possible mechanisms of room-temperature ferromagnetism (RT-FM) in ZnO-based diluted magnetic oxides. All of the films have a c-axis-oriented wurtzite structure and display RT-FM. X-ray photoelectron spectroscopy results showed that the incorporated Cu ions in as-deposited films are in 1+ valence state merely, while an additional 2+ valence state occurs in post-annealed films. The presence of Cu{sup 2+} state in post-annealed film accompanies a higher magnetization value than that of as-deposited film and, in particular, the magnetization curves at 10 K and 300 K of the post-annealed film separate distinctly. Since Cu{sup 1+} ion has a filled 3d band, the RT-FM in as-deposited Cu-doped ZnO thin films may stem solely from intrinsic defects, while that in post-annealed films is enhanced due to the presence of CuO crystallites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Probing temperature-driven spin reorientation transition of GdFeCo film by Kerr loops and ferromagnetic resonance

The magnetic anisotropy is of both scientific and technological interest for magneto-optical material GdFeCo film. We characterize the magnetic anisotropy of a 20 nm GdFeCo film from 265 K to 320 K via Kerr loops and ferromagnetic resonance. With increasing temperature, both of the first-order uniaxial magnetic anisotropy and shape anisotropy increase. However, the competition between them causes a temperature-driven spin reorientation transition (SRT) and the effective perpendicular magnetic anisotropy decrease from 2.22 × 10{sup 4 }ergs/cm{sup 3} (288 K) to −1.56 × 10{sup 4 }ergs/cm{sup 3} (317 K). The positive second-order uniaxial magnetic anisotropy determines an easy-cone state as the mediated state during SRT.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Controlling spin–orbit interaction in a ferromagnetic Fe/Au double layer

Using spin-polarized single- and two-electron spectroscopy, we probe exchange and spin–orbit interaction in a double layer of Fe and Au on W(110) and measure the spin asymmetry of the Bloch spectral density function of the sample. In a 5 ML iron film, the spin-orbit contribution to the measured asymmetry of the (e,2e) spectra was not detectable, whereas a deposition of about 1 ML of gold introduced a substantial spin-orbit component in the measured asymmetry. At the same time, this double layer still exhibits ferromagnetic properties: (i) the spectral density function asymmetry demonstrate imbalance of spin-up and spin-down electron densities in the valence band and (ii) the Stoner excitation asymmetry has almost the same value as in a pure Fe film.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Proximity induced vortices and long-range triplet supercurrents in ferromagnetic Josephson junctions and spin valves

Using a spin-parameterized quasiclassical Keldysh-Usadel technique, we theoretically study supercurrent transport in several types of diffusive ferromagnetic (F)/superconducting (S) configurations with differing magnetization textures. We separate out the even- and odd-frequency components of the supercurrent within the low proximity limit and identify the relative contributions from the singlet and triplet channels. We first consider inhomogeneous one-dimensional Josephson structures consisting of a uniform bilayer magnetic S/F/F/S structure and a trilayer S/F/F/F/S configuration, in which case the outer F layers can have either a uniform or conical texture relative to the central uniform F layer. Our results demonstrate that for supercurrents flowing perpendicular to the F/F interfaces, incorporating a conical texture yields the most effective way to observe the signatures of long-ranged spin-triplet supercurrents. We also consider three different types of finite-sized two-dimensional magnetic structures subjected to an applied magnetic field normal to the junction plane: a S/F/S junction with uniform magnetization texture and two S/F/F/S configurations with differing F/F bilayer arrangements. In one case, the F/F interface is parallel with the S/F junction interfaces while in the other case, the F/F junction is oriented perpendicular to the S/F interfaces. We then discuss the proximity vortices and corresponding spatial maps of currents inside the junctions. For the uniform S/F/S junction, we analytically calculate the magnetic field induced supercurrent and pair potential in both the narrow and wide junction regimes, thus providing insight into the variations in the Fraunhofer diffraction patterns and proximity vortices when transitioning from a wide junction to a narrow one. Our extensive computations demonstrate that the induced long-range spin-triplet supercurrents can deeply penetrate uniform F/F bilayers when spin-singlet supercurrents flow parallel to the F/F interfaces. This is in stark contrast to configurations where a spin-singlet supercurrent flows perpendicular to the F/F interfaces. We pinpoint the origin of the induced triplet and singlet correlations through spatial profiles of the decomposed total supercurrents. We find that the penetration of the long-range spin-triplet supercurrents associated with supercurrents flowing parallel to the F/F interfaces is more pronounced when the thicknesses of the F strips are unequal. Finally, if one of the S terminals is replaced with a finite-sized normal metal, we demonstrate that the corresponding experimentally accessible S/F/F/N spin valve presents an effective platform in which the predicted long-range effects can be effectively generated and probed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Induced spin polarization effect in graphene by ferromagnetic nanocontact

Chemically synthesized graphene contains large number of defects which act as localized spin moments at the defect sites. Cobalt nanosheets of variable thickness are grown on graphene surface to investigate spin/magnetotransport through graphene sheets containing large number of localized spins. Negative magnetoresistance (MR) is observed over the entire temperature range (5–300 K) for thin cobalt sheets, while a cross-over from negative to positive MR with increasing temperature is noticed for thicker cobalt sheets. The observed MR results are explained on the basis of recently reported spin polarization effect in graphene due to the presence of ferromagnetic atoms on the surface considering a spin valve like Co/graphene/Co nanostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Inherent room temperature ferromagnetism and dopant dependent Raman studies of PbSe, Pb{sub 1−x}Cu{sub x}Se, and Pb{sub 1−x}Ni{sub x}Se

Polycrystalline lead selenide (PbSe) doped with copper (Cu) and nickel (Ni) was prepared to understand its magnetic behaviour and Raman activity. The processing conditions, influence of dopants (magnetically active and non-active) and their respective compositions on the magnetic properties and Raman active mode were studied. A surprising/anomalous room temperature ferromagnetism (hysteresis loop) is noticed in bulk diamagnetic PbSe, which is found to be natural or inherent characteristic of material, and depends on the crystallite size, dopant, and developed strain due to dopant/defects. The magnetic susceptibility (−1.71 × 10{sup −4} emu/mol Oe) and saturated magnetic susceptibility (−2.74 × 10{sup −4} emu/mol Oe) are found to be higher than the earlier reported value (diamagnetic: −1.0 × 10{sup −4} emu/mol Oe) in bulk PbSe. With increase of Cu concentration (2% to 10%) in PbSe, the saturated magnetic susceptibility decreases from −1.22 × 10{sup −4} to −0.85 × 10{sup −4} emu/mol Oe. Whereas for Ni dopant, the saturated magnetic susceptibility increases to −2.96 × 10{sup −4} emu/mol Oe at 2% Ni doped PbSe. But it further decreases with dopant concentration. In these doped PbSe, the shifting of longitudinal (LO) phonon mode was also studied by the Raman spectroscopy. The shifting of LO mode is found to be dopant dependent, and the frequency shift of LO mode is associated with the induced strain that created by the dopants and vacancies. This asymmetry in LO phonon mode (peak shift and shape) may be due to the intraband electronic transition of dopants. The variation in magnetic susceptibility and Raman shifts are sensitive to crystallite size, nature of dopant, concentration of dopants, and induced strain due to dopants.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Study of microstructure, dielectric and ferromagnetic properties of the (1-x)Ba{sub 0.95}Sr{sub 0.05}TiO{sub 3}-(x)CoFe{sub 1.8}Mn{sub 0.2}O{sub 4} multiferroic composites

In the present work, (1-x)Ba{sub 0.95}Sr{sub 0.05}TiO{sub 3}-(x)CoFe{sub 1.8}Mn{sub 0.2}O{sub 4} composites are prepared by standard solid state reaction method. The X-ray diffraction measurement of the composites shows that both the phases coexist in the composite, individually. The morphology of the composites were examined by field emission scanning electron microscopy and reveals homogeneous microstructure with two types of grains, smaller grains of the Ba{sub 0.95}Sr{sub 0.05}TiO{sub 3} (BST) and bigger grains of the CoFe{sub 1.8}Mn{sub 0.2}O{sub 4} (CFM). The dielectric studies show that all the composites exhibit dispersion in the lower frequency region attributable to the interfacial polarization. In addition, at lower frequencies, the dielectric constant (ε´) is found to increase with increase in CFM content in the composites. The ferromagnetic properties of the composites improve with the increase in the CFM content.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗