Engineering Papers⌕ Search

SEARCH · Engineering Papers

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

A first principles investigation of ferromagnetic EuFe{sub 4}As{sub 12}

Density functional theory (DFT) based calculations of electronic and magnetic properties of filled skutterudites EuFe{sub 4}As{sub 12} have been performed using FP-LAPW method within the framework of the LSDA approach. The rare-earth filled skutterudites have attracted much attention because of the presence of the highly localized f-electrons and d- electrons of rare-earth and transition metal respectively, with high density of states near Fermi level. The calculation performed near the Fermi level of density of states shows the compound to be suitable for thermoelectric application. The exchange-splitting of Eu-4f states were analysed to explain the ferromagnetic behaviour of EuFe{sub 4}As{sub 12} with magnetic moment value 5.18 μ{sub B}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Annealing effects on the ferromagnetic resonance linewidths of sputter-deposited Fe{sub 100−x}Co{sub x}(001) thin films (x < 11)

Effects of post-growth annealing on the magnetic damping of 3d transition alloy thin films were investigated. Fe{sub 100−x}Co{sub x} (x < 11 at. %) thin films were epitaxially deposited on GaAs(001) substrates by rf magnetron sputtering, and some of them were annealed without exposing to atmosphere. Electrical measurement showed that in-plane resistivity was smaller in the annealed films than in the as-deposited ones, indicating that the annealing mitigates crystalline imperfections and leads to reduced electron scattering rates. Magnetic damping was evaluated by the peak widths of ferromagnetic resonance (FMR) spectra obtained by a conventional Q-band spectrometer. Comparison of as-deposited and annealed specimens showed that the damping was decreased by annealing. Combined with the electrical and FMR measurements, these observations are consistent with the theoretical predictions that crystalline imperfections strongly influence the magnetic damping, both in intrinsic and extrinsic origins.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Systematic study of the spin stiffness dependence on phosphorus alloying in the ferromagnetic semiconductor (Ga,Mn)As

We study the dependence of the spin stiffness constant on the phosphorus concentration in the ferromagnetic semiconductor (Ga,Mn)(As,P) with the aim of determining whether alloying with phosphorus is detrimental, neutral, or advantageous to the spin stiffness. Time-resolved magneto-optical experiments are carried out in thin epilayers. Laser pulses excite two perpendicular standing spin wave modes, which are exchange related. We show that the first mode is spatially uniform across the layer corresponding to a k≈0 wavevector. From the two frequencies and k-vector spacings we obtain the spin stiffness constant for different phosphorus concentrations using weak surface pinning conditions. The mode assessment is checked by comparison to the spin stiffness obtained from domain pattern analysis for samples with out-of-plane magnetization. The spin stiffness is found to exhibit little variation with phosphorus concentration in contradiction with ab-initio predictions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Room temperature long range ferromagnetic ordering in Ni{sub 0.58}Zn{sub 0.42}Co{sub 0.10}Cu{sub 0.10}Fe{sub 1.8}O{sub 4} nano magnetic system

The structural and magnetic behavior of sol-gel autocombustion synthesized nanocrystalline Ni{sub 0.58}Zn{sub 0.42}Co{sub 0.10}Cu{sub 0.10}Fe{sub 1.8}O{sub 4} have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), Mössbauer spectroscopy and vibrating sample magnetometer(VSM). Sample of high purity and high homogeneity was obtained by calcination at low temperature (500°C) resulting in nanoparticles of average diameter ∼15nm as determined by XRD and further confirmed by TEM. X-ray diffraction (XRD) and selective area diffraction (SAED) confirmed the single phase of the sample. Mössbauer results are supported by magnetization data. Well defined sextets and appearance of hysteresis at room temperature indicate the existence of ferromagnetic coupling at room temperature finding material utility in magnetic storage data. The existence of iron in ferric state confirmed by isomer shift is a clear evidence of improved magnetic properties of the present system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Room temperature ferromagnetism in Mg-doped ZnO nanoparticles

Zn{sub 1-x}Mg{sub x}O (x = 0, 0,10) nanoparticles were successfully synthesized using sol-gel method. X-ray diffraction (XRD) confirms that the synthesized nanoparticles possess wurtzite phase having hexagonal structure. Morphological analysis was carried out using transmission electron microscopy (TEM) which depicts the spherical morphology of ZnO nanoparticles. Energy dispersive spectroscopy (EDS) showed the presence of Mg in ZnO nanoparticles. Electron spin resonance (ESR) signal was found to be decreasing with increasing of Mg-doping concentration. The room temperature ferromagnetism was observed in undoped and Mg-doped ZnO nanoparticles. The increase of Mg-doping concentration resulted in decrease of saturation magnetization value which could be attributed to decrease of oxygen vacancies present in host nanoparticles.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Room temperature ferromagnetism in undoped and Ni doped In{sub 2}O{sub 3} thin films

Undoped and Ni (5 at.%) doped In{sub 2}O{sub 3} thin films were deposited on glass substrate using electron beam evaporation technique and Ni doped In{sub 2}O{sub 3} thin films were annealed at 450 oC. A systematic study was carried out on the structural, chemical and magnetic properties of the as deposited and annealed thin films. X-ray diffraction analysis revealed that all the films were cubic in structure and exhibied ferromagnetism at room temperature. The undoped In{sub 2}O{sub 3} thin films exhibited a saturation magnetization of 24.01 emu/cm3. Ni doped In{sub 2}O{sub 3} thin films annealed at 450 oC showed a saturation magnetization of 53.81 emu/cm3.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Crossover from antiferro-to-ferromagnetism on substitution of Co by V in RE(Co{sub 1-x}V{sub x}){sub 2}Si{sub 2} (0 ≤ x ≤ 0.35)

In PrCo{sub 2}Si{sub 2} and NdCo{sub 2}Si{sub 2}, Co has been partially substituted by V. Vanadium having a larger atomic radius than cobalt, the substitution results in a negative pressure affecting the magnetic properties of the compound. The samples RE(Co{sub 1-x}V{sub x}){sub 2}Si{sub 2} (RE = Pr, Nd; x = 0, 0.20, 0.35) were prepared by melting the corresponding elements in arc furnace and characterized using x-ray diffraction. Magnetometric measurements show that the parent compounds PrCo{sub 2}Si{sub 2} and NdCo{sub 2}Si{sub 2} are antiferromagnetic, as reported. With doping, ferrimagnetic behaviour is observed from temperature dependence of inverse susceptibility at about 50 K. At lower than 30 K, the magnetizations tend to saturate at high fields. From the field dependence of magnetization, the hysteresis loops and also coercive fields as observed, the samples exhibit ferromagnetism below ∼ 30 K. Exchange bias effect is also observed in the high V containing sample. The specific heat studies of the samples show transitions consistent with the magnetization data. Pr(Co{sub 0.65}V{sub 0.35}){sub 2}Si{sub 2} and Nd(Co{sub 0.65}V{sub 0.35}){sub 2}Si{sub 2} show magnetoresistance (MR) of ∼ 25% and 15%, respectively, at 4 K and 9 tesla.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Enhancing ferromagnetic resonance absorption for very thin insulating magnetic films with spin plasmonics

We consider enhancing the ferromagnetic resonance (FMR) absorption of very thin insulating magnetic films by placing it on top of a dielectric. We find that the signal is enhanced by at least an order of magnitude due to a new nonreciprocal interface resonance that is a mixture of the magnetic surface plasmon mode and a wave guide mode. This resonance occurs over a wide range of thicknesses of the dielectric that is still much less than the wavelength and is made possible by the negative magnetic susceptibility of the magnetic layer. The line width of absorption is reduced by an order of magnitude less than the Gilbert damping parameter. At some frequency, the group velocity of this resonance is negative. Experimentally, very thin yttrium iron garnet (YIG) films are grown on a Gadolinium Gallium Garnet (GGG) substrate which can be considered the dielectric. Our model applies to experiments performed in the YIG/GGG system. Indeed, our picture resolves the disagreement on the magnitude of the spin diffusion lengths obtained with the FMR and the Brillouin scattering techniques. It also provides for a way to make new adaptive thin film miniaturized photonic nonreciprocal devices with low loss.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Nonlinear dynamics of three-magnon process driven by ferromagnetic resonance in yttrium iron garnet

We report an investigation of the dynamics of the three-magnon splitting process associated with the ferromagnetic resonance (FMR) in films of the insulating ferrimagnet yttrium iron garnet (YIG). The experiments are performed with a 6 μm thick YIG film close to a microstrip line fed by a microwave generator operating in the 2–6 GHz range. The magnetization precession is driven by the microwave rf magnetic field perpendicular to the static magnetic field, and its dynamics is observed by monitoring the amplitude of the FMR absorption peak. The time evolution of the amplitude reveals that if the frequency is lowered below a critical value of 3.3 GHz, the FMR mode pumps two magnons with opposite wave vectors that react back on the FMR, resulting in a nonlinear dynamics of the magnetization. The results are explained by a model with coupled nonlinear equations describing the time evolution of the magnon modes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Multilayered graphene acquires ferromagnetism in proximity with magnetite particles

Anisotropic diamagnetism of pristine graphite and graphene is well known. Here, evidence of significant induced ferromagnetism in multilayer graphene (MLG) decorated with ferrimagnetic Fe{sub 3}O{sub 4} particles is reported. This MLG-Fe{sub 3}O{sub 4} nano-composite was prepared by a one-step ultrasonic treatment at 75 °C in the surfactant sodium dodecyl-benzene-sulfonate. To verify the phase structure and morphology of the composite, X-ray diffraction, scanning and transmission electron microscopy, scanning tunneling electron microscopy, and Raman spectroscopy were employed. Room temperature data of magnetization versus magnetic field showed that the saturation magnetization M{sub S} = 58.6 emu/gm for pristine Fe{sub 3}O{sub 4} increased to M{sub S} = 158.4 emu/gm for a 1:1 composite of Fe{sub 3}O{sub 4} to MLG. These results lead to induced M{sub S} = 253 emu/gm in MLG resulting from its proximity to Fe{sub 3}O{sub 4}. Similar experiments on Fe{sub 3}O{sub 4} to single walled carbon nanotubes (SWNT) composite did not show any induced magnetism in SWNT.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Ferromagnetic properties of fcc Gd thin films

Magnetic properties of sputtered Gd thin films grown on Si (100) substrates kept at two different temperatures were investigated using X-ray diffraction, ac magnetic susceptibility, and dc magnetization measurements. The obtained Gd thin films have a mixture of hcp and fcc structures, but with their fractions depending on the substrate temperature T{sub S} and film thickness x. Gd fcc samples were obtained when T{sub S} = 763 K and x = 10 nm, while the hcp structure was stabilized for lower T{sub S} (300 K) and thicker film (20 nm). The fcc structure is formed on the Ta buffer layer, while the hcp phase grows on the fcc Gd layer as a consequence of the lattice relaxation process. Spin reorientation phenomenon, commonly found in bulk Gd species, was also observed in the hcp Gd thin film. This phenomenon is assumed to cause the magnetization anomalous increase observed below 50 K in stressed Gd films. Magnetic properties of fcc Gd thin films are: Curie temperature above 300 K, saturation magnetization value of about 175 emu/cm{sup 3}, and coercive field of about 100 Oe at 300 K; features that allow us to classify Gd thin films, with fcc structure, as a soft ferromagnetic material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effects of antiferro-ferromagnetic phase coexistence and spin fluctuations on the magnetic and related properties of NdCuSi

Polycrystalline NdCuSi is found to show co-existence of antiferromagnetic (AFM) and ferromagnetic (FM) phases at low temperatures, as revealed by neutron diffraction data. The coexistence is attributed to the competing exchange interactions and crystal field effect. The compound shows a large, low-field magnetoresistance (MR) of ∼ − 32% at 20 kOe below T{sub N} (3.1 K), which becomes ∼ − 36% at 50 kOe. The MR value at 50 kOe is found to be the highest among the RTX compounds. Magnetocaloric effect (MCE) is also found to show a large value of ∼11 J/kg K close to T{sub N}. Resistivity data show the presence of spin fluctuations, which get suppressed by the applied field. Large MR and MCE in this compound arise due to the coexistence of the two phases. The field dependencies of MR and MCE show quadratic behavior, confirming the presence of spin fluctuations.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Study of structural and ferromagnetic resonance properties of spinel lithium ferrite (LiFe{sub 5}O{sub 8}) single crystals

The effect of B-site cation ordering on the room temperature structural and ferromagnetic resonance (FMR) properties of single crystal spinel lithium ferrite (LiFe{sub 5}O{sub 8}, LFO) have been investigated. A detailed microstructural analysis is done through X-ray diffraction, polarized Raman spectroscopy, and transmission electron microscopy (TEM) to examine the effect of post-annealing on the B-site cation ordering. The X-ray diffraction pattern of the as-grown crystal indicates a disordered state of the crystal. However, the annealed sample shows additional superlattice reflections corresponding to the ordered phase. This ordering is further confirmed by Raman spectra and TEM images, which reveal ordering of Li and Fe ions at the octahedral sites contrasting with the relatively high degree of octahedral site disorder in the as-grown crystal. To study the effect of B-site ordering on the magnetic properties and FMR linewidth, vibrating sample magnetometry and broadband FMR measurements have been performed for both the ordered and disordered phases of lithium ferrite. The value of saturation magnetization for both phases is ∼290 emu/cm{sup 3}. A single mode FMR profile is observed for both phases with little distortion. The linewidth characteristics of the ordered and disordered phases of lithium ferrite phases are compared, and it is observed that the linewidth is independent of the cation ordering. Both the phases exhibit a low linewidth (∼26 Oe at 30 GHz) and the effective damping parameter for the as-grown and annealed samples is determined to be 0.0021 ± 0.0001.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Magnetic coupling in ferromagnetic semiconductor (Ga,Mn)As/(Al,Ga,Mn)As bilayers

We report on a study of ferromagnetic semiconductor (Ga,Mn)As/(Al,Ga,Mn)As bilayers using magnetometry and polarized neutron reflectivity (PNR). From depth-resolved characterization of the magnetic structure obtained by PNR, we concluded that the (Ga,Mn)As and (Al,Ga,Mn)As layers have in-plane and perpendicular-to-plane magnetic easy axes, respectively, with weak interlayer coupling. Therefore, the layer magnetizations align perpendicular to each other under low magnetic fields and parallel at high fields.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Coexistence of electric field controlled ferromagnetism and resistive switching for TiO{sub 2} film at room temperature

The Ag/TiO{sub 2}/Nb:SrTiO{sub 3}/Ag device exhibits the coexistence of electric field controlled ferromagnetism and resistive switching at room temperature. The bipolar resistive switching in Ag/TiO{sub 2}/Nb:SrTiO{sub 3}/Ag device may be dominated by the modulation of Schottky-like barrier with the electron injection-trapped/detrapped process at the interface of TiO{sub 2}/Nb:SrTiO{sub 3}. We suggest that the electric field-induced magnetization modulation originates mainly from the creation/annihilation of lots of oxygen vacancies in TiO{sub 2}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Magnetic anisotropy induced by crystal distortion in Ge{sub 1−x}Mn{sub x}Te/PbTe//KCl (001) ferromagnetic semiconductor layers

Ferromagnetic resonance (FMR) study of magnetic anisotropy is presented for thin layers of IV-VI diluted magnetic semiconductor Ge{sub 1−x}Mn{sub x}Te with x = 0.14 grown by molecular beam epitaxy on KCl (001) substrate with a thin PbTe buffer. Analysis of the angular dependence of the FMR resonant field reveals that an easy magnetization axis is located near to the normal to the layer plane and is controlled by two crystal distortions present in these rhombohedral Ge{sub 1−x}Mn{sub x}Te layers: the ferroelectric distortion with the relative shift of cation and anion sub-lattices along the [111] crystal direction and the biaxial in-plane, compressive strain due to thermal mismatch.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Evidence of dilute ferromagnetism in rare-earth doped yttrium aluminium garnet

This work demonstrates strong coupling regime between an erbium ion spin ensemble and microwave hybrid cavity-whispering gallery modes in a yttrium aluminium garnet dielectric crystal. Coupling strengths of 220 MHz and mode quality factors in excess of 10{sup 6} are demonstrated. Moreover, the magnetic response of high-Q modes demonstrates behaviour which is unusual for paramagnetic systems. This behaviour includes hysteresis and memory effects. Such qualitative change of the system's magnetic field response is interpreted as a phase transition of rare earth ion impurities. This phenomenon is similar to the phenomenon of dilute ferromagnetism in semiconductors. The clear temperature dependence of the phenomenon is demonstrated.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Visible photoluminescence and room temperature ferromagnetism in high In-content InGaN:Yb nanorods grown by molecular beam epitaxy

We report the growth of high indium content InGaN:Yb nanorods grown on c-plane sapphire (0001) substrates using plasma assisted molecular beam epitaxy. The in situ reflection high energy electron diffraction patterns recorded during and after the growth revealed crystalline nature of the nanorods. The nanorods were examined using electron microscopy and atomic force microscopy. The photoluminescence studies of the nanorods showed the visible emissions. The In composition was calculated from x-ray diffraction, x-ray photoelectron spectroscopy, and the photoluminescence spectroscopy. The In-concentration was obtained from photoluminescence using modified Vegard's law and found to be around 37% for InGaN and 38% for Yb (5 ± 1%)-doped InGaN with a bowing parameter b = 1.01 eV. The Yb-doped InGaN showed significant enhancement in photoluminescence properties compared to the undoped InGaN. The Yb-doped InGaN nanorods demonstrated the shifting of the photoluminescence band at room temperature, reducing luminescence amplitude temperature dependent fluctuation, and significant narrowing of excitonic emission band as compared to the undoped InGaN. The magnetic properties measured by superconducting quantum interference devices reveals room temperature ferromagnetism, which can be explained by the double exchange mechanism and magnetostriction.

36 MATERIALS SCIENCE↗