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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Ferromagnetic behavior and exchange bias effect in akaganeite nanorods

We report ferromagnetic-like properties and exchange bias effect in akaganeite (β-FeOOH) nanorods. They exhibit a Néel temperature T{sub N} = 259 K and ferromagnetic-like hysteresis behavior both below and above T{sub N}. An exchange bias effect is observed below T{sub N} and represents an interesting behavior for akaganeite nanorods. These results are explained on the basis of a core-shell structure in which the core has bulk akaganeite magnetic properties (i.e., antiferromagnetic ordering) while the shell exhibits a disordered spin state. Thus, the nanorods show ferromagnetic properties and an exchange bias effect at the same time, increasing their potential for use in practical applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Toward broad-band x-ray detected ferromagnetic resonance in longitudinal geometry

An ultrahigh-vacuum-compatible setup for broad-band X-ray detected ferromagnetic resonance (XFMR) in longitudinal geometry is introduced which relies on a low-power, continuous-wave excitation of the ferromagnetic sample. A simultaneous detection of the conventional ferromagnetic resonance via measuring the reflected microwave power and the XFMR signal of the X-ray absorption is possible. First experiments on the Fe and Co L{sub 3}-edges of a permalloy film covered with Co nanostripes as well as the Fe and Ni K-edges of a permalloy film are presented and discussed. Two different XFMR signals are found, one of which is independent of the photon energy and therefore does not provide element-selective information. The other much weaker signal is element-selective, and the dynamic magnetic properties could be detected for Fe and Co separately. The dependence of the latter XFMR signal on the photon helicity of the synchrotron light is found to be distinct from the usual x-ray magnetic circular dichroism effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

On the Emergence of Ferromagnetism in LaCoO 3 Ultrathin Films

It is well known that the properties of a crystal evolve as it increases in size from a single atomic plane to that of the bulk. Such size-dependent transitions can stem from many different origins and depend on minute changes to crystal bonding and composition. Here, a model example is that of LaCoO 3 , which is non-magnetic in the bulk but can display ferromagnetism at the nanoscale. Here, the evolution of structure-property relationships is studied in the LaCoO 3-δ /SrTiO 3 (001) system as the thickness of LaCoO 3-δ is increased from a single plane to 10 unit cells. In situ synchrotron X-ray studies are performed during and post-deposition to probe changes in the interactions between structure, stoichiometry, and magnetic behavior. Structural quantification indicates that the oxygen octahedral rotation pattern evolves with thickness, due to inherent differences in crystal symmetry between the film and substrate. The change in rotation modifies the required energy barrier for the spin state transition via the Co–O bond length and Co–O–Co bond angle, affecting the appearance of ferromagnetism. Our results highlight the contributions of high spin Co 2+ and/or high spin Co 3+ to respective weak and robust ferromagnetism and the evolution of properties with size in ultrathin LaCoO 3-δ heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Single-Domain Ferromagnet of Noncentrosymmetric Uniaxial Magnetic Ions and Magnetoelectric Interaction

The feasibility of a single-domain ferromagnet based on uniaxial magnetic ions was examined. For a noncentrosymmetric uniaxial magnetic ion of magnetic moment μ at a site of local electric dipole moment p , it is unknown to date whether μ prefers to be parallel or antiparallel to μ . The nature of this magnetoelectric interaction was probed in terms of analogical reasoning based on the Rashba effect and density functional theory (DFT) calculations. Here, we show that μ and p prefer an antiparallel arrangement, predict that Fe-doped CaZnOS is a single-domain ferromagnet like a bar magnet, and find the probable cause for the ferromagnetism and weak magnetization hysteresis in Fe-doped hexagonal ZnO and ZnS at very low dopant concentrations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

d{sup 0} Half-Metallic Ferromagnetism in CaN and CaAs Pnictides: an Ab Initio Study

Conventional magnetism occurs in systems which contain transition metals or rare earth ions with partially filled d or f shells. It is theoretically predicted that compounds of groups IA and IIA with IV and V, in some structural phases, are ferromagnetic half-metals which made them new candidates for spintronics applications. Employing density functional theory (DFT), we investigate magnetism in binary compounds CaN and CaAs. Regarding the structure of analogous magnetic materials and experimental results of CaAs synthesis, we have considered two cubic structures: rocksalt (RS) and zincblende (ZB), and four hexagonal structures: NiAs, wurtzite (WZ), anti-NiAs, and NaO. The calculated results show that CaN in cubic, NiAs, and wurtzite structures, and CaAs only in zincblende phase have ferromagnetic ground states with a magnetic moment of 1 μ{sub B}. Electronic structure analysis of these materials indicates that magnetism originates from anion p states. Existence of flat p bands and consequently high density of states at the Fermi level of magnetic structures gives rise to Stoner spin splitting and spontaneous ferromagnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Determination of the Transition Temperature of a Weak Ferromagnetic Thin Film by Means of an Evolution of the Method Based on the Arrott Plots

The ferromagnetic transition temperature (T{sub c}) of a weak ferromagnetic thin film Cu{sub 0.38}Ni{sub 0.62} has been estimated by using an evolution of the Belov, Goriaga, and Arrott (BGA) model. In fact, the typically used measurements of the zero-field-cooled-field-cooled magnetic moment (m) as a function of temperature (T), the magnetic hysteresis loops (m(H)) as a function of field, and the temperature dependence of the remanent magnetic moment, performed on our sample, did not allow us to individuate precisely the value of T{sub c}, mainly due to the influence of thermal effects and noise on the weak ferromagnetic response of the sample, resulting in a large uncertainty in the estimation of T{sub c}. On the other hand, also the commonly used method of determining the T{sub c} by means of a linear extrapolation of the high-field region of m{sup 2} as a function of the normalized magnetic field (H/m) at different temperatures (Arrott curves), working though for our analyzed material, could not completely fulfill the basic assumptions of the BGA model in general. For these reasons, we have considered the analysis of the derivatives of the Arrott curves, starting from observing that the T{sub c} of the sample corresponds to the temperature where the curvature of the Arrott curves at low fields inverts. In this way, the sensitivity in the determination of the Curie point from the inductive measurements of the magnetic moment is improved while the correct assumptions of the Arrott model are also fully respected.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Ferromagnetism in Fe-doped BaTiO{sub 3} Ceramics

Polycrystalline samples of BaTi{sub 1−x}Fe{sub x}O{sub 3} (x = 0.00–0.30) are prepared by solid-state reaction method and their structural and magnetic properties are studied. Detailed investigation of XRD patterns reveal the coexistence of tetragonal (space group P4mm) and hexagonal phases (space group P6{sub 3}/mmc) for x ≥ 0.1. Magnetic measurements reveal room-temperature ferromagnetism in x = 0.15–0.3 samples, and their ferromagnetic transition temperature increases from 397 K for x = 0.15 to 464 K for x = 0.3. The initial magnetization curves for x = 0.15–0.3 are analyzed in terms of bound magnetic polaron (BMP) model. The analysis of susceptibility data in the paramagnetic region by Curie-Weiss law confirms the ferromagnetic transition for x ≥ 0.15 and the effective magnetic moment systematically increases with increase in Fe concentration.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Spin Wave Resonance Frequency in the Bilayer Ferromagnetic Film

The dependence of spin wave resonance (SWR) frequency on the surface anisotropy, the interlayer and interfacial coupling, the ferromagnetic layer thickness and the external magnetic field in the bilayer ferromagnetic film have been investigated by using the linear spin wave approximation and Green’s function technique. We find that the SWR frequency of the ferromagnetic bilayer films are shifted to higher values corresponding to those of above factors, respectively. The mode of the energetically highest can be controlled by the interfacial coupling, the thickness and the symmetry of the film. The SWR frequency of mode number m = 1 is almost flat except for applied magnetic field. Analytical details and the validity of the method are shown and discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

First-Principles Investigation of Ferromagnetic, Thermodynamic, Elastic, and Half-Metallic Behavior of ReCrTe and RuCrTe Half-Heusler Alloys

Calculations are realized within the full-potential linearized augmented plane wave (FP-LAPW) computational approach with the generalized gradient approximation (GGA) for the exchange-correlation potential. In this work, we present investigation of structural, electronic, ferromagnetic, elastic, and thermodynamic properties and half-metallic behavior of ReCrTe and RuCrTe compounds. Analysis of band structures and densities of states show the half-metallic ferromagnetic behavior of these compounds. The values of total magnetic moments µ{sub tot} are 1 µ{sub B} per unit cell for ReCrTe and 3 µ{sub B} per unit cell for RuCrTe. The contribution of Cr is the most important. The calculated values of a total magnetic moment nicely follow the rule µ{sub tot} = Z{sub t}− 18. The study of the impacts of strains on the magnetic moment reveals that the two compounds are stable ferromagnetic compounds. RuCrTe compound preserves the half-metallic behavior when the unit cell volume is changed in the range of [ − 4%, 4%], whereas, weak strain destroys the half metallic nature for ReCrTe compound. Using the quasi-harmonic Debye model, the effects of pressure P and temperature T on thermal expansion coefficient, Debye temperature, heat capacity and Grüneisen parameter for these two compounds are investigated for the first time. The study of the elastic constants shows that ReCrTe and RuCrTe are mechanically stable. RuCrTe is stiffer than ReCrTe compounds. The obtained values of Pugh’s ratio B/G and Cauchy pressure (C{sub 12} − C{sub 44}) reveal that ReCrTe and RuCrTe are brittle. The study of the electron charge density and Poisson’s ratio proves that the ionic character dominates the atomic bonding for the two compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Enhanced Room-Temperature Ferromagnetism Observed in SiO{sub 2}-Coated CuO Nanostructures

In this paper, the magnetic properties of pure CuO and SiO{sub 2}-coated CuO nanocrystals were reported. Strongly enhanced room-temperature ferromagnetism was observed in SiO{sub 2}-coated CuO nanocrystals, whose saturation magnetization is two orders of magnitude enhanced compared to that of the uncoated CuO nanocrystals. The mechanism of this enhancement was investigated. It points out the main origin of ferromagnetism in these CuO nanocrystals is the uncompensated surface state, so that the coating of SiO{sub 2} can effectively protect the surface state to improve the ferromagnetism in these CuO nanocrystals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Ferromagnetism in (X, Y)-Codoped 4H-GaN Polytype X = Na and Mg, Y = Na and Mg: Ab Initio Study

In this work, the magnetic behavior of 4H-GaN polytype codoped with (X = Na and Mg, Y = Na and Mg) is studied by means of first-principle calculations based on spin-density functional theory within the generalized-gradient approximation. It is found that the most stable configuration reveals the smallest cohesion energy and the X–Y bond length is minimal. (X, Y )-codoped 4H-GaN reveals magnetism behavior, and the total magnetic moment comes mainly from the N host atoms. Interestingly, (Na, Na)-codoped 4H-GaN is found to be half-metallic ferromagnetic with a magnetic moment of 4 μ{sub B}. Results show also that (Mg, Na)- and (Na, Mg)-codoped 4H-GaN are found to be nearly half-metallic ferromagnetic with a magnetic moment of 2.96 and 2.97 μ{sub B}, respectively. Whereas (Mg, Mg)-codoped 4H-GaN have a metallic ferromagnetic behavior with a magnetic moment of 1.87 μ{sub B}. The partial density of states denotes that magnetism is achieved through a p-p-like coupling between the impurity X-2p states of X atom, Y-2p states of Y atom, and N-2p states of N host atoms. These theoretical results make Na–Na-codoped 4H-GaN very promising materials for spintronic technology.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Half-Metallic Ferromagnetism in V-Doped FmP Binary Monopnictide Compounds: an Ab Initio Calculations

The ab initio calculations of the structural phase stability, magnetic phase stability, and electronic and magnetic properties of the FmP binary monopnictides and the half-metallic ferromagnetic properties of their Fm{sub 1−x}V{sub x}P (for x = 0.25, 0.50, 0.75, and 1)-doped alloys have been assured through the spin-polarized density functional theory (DFT), and they are performed by employing the full-potential linearized augmented plane waves plus local orbitals (FP-L/APW + lo) method that is implemented in the WIEN2k package. The exchange and correlation potential is parameterized by the generated gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) scheme. According to the studies of the magnetic phase stability, the fermium monopnictides (FmP) have a paramagnetic regime, while the ferromagnetic behavior appears when we dope this compound with vanadium. Through the ferromagnetic character of all Fm{sub 1−x}V{sub x}P alloys, the spin-polarized band structures and densities of states exhibit a nearly half-metallic character of Fm{sub 0.75}V{sub 0.25}P alloy, whereas Fm{sub 0.50}V{sub 0.50}P, Fm{sub 0.25}V{sub 0.75}P, and VP compounds showed metallic character. Moreover, the total magnetic moment of the three ternary alloys is mainly contributed by Fm and V elements, where the p-d hybridization reduces the atomic magnetic moment of V element from its free space charge and produces feeble magnetic moments on the nonmagnetic P sites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Influence of Long-Range Hopping on Ferromagnetism in the Hubbard Model on the Generalized Shastry-Sutherland Lattice

The projector quantum Monte-Carlo method is used to examine effects of long-range electron hopping on ferromagnetism in the Hubbard model on the generalized Shastry-Sutherland lattice. It is shown that the long-range electron hopping with the exponentially decaying hopping amplitudes t{sub ij}(α)∼e{sup −α|R{sub i}−R{sub j}|} stabilizes the ferromagnetic state for a wide range of electron hopping α, electron interactions U, and electron concentrations n above the half-filled band case n = 1. The critical value of the hopping parameter α{sub c}, below which the ferromagnetic state becomes stable, is calculated numerically and the ground-state phase diagram of the model in the n − α plane is presented for physically the most interesting case of the intermediate Coulomb interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Electronic Structure and Ferromagnetism in Zincblende Zn{sub 1−x}Co{sub x}S Nanoparticles

We have prepared Zn{sub 1−x}Co{sub x}S nanoparticles (NPs) and studied their local-geometric, electronic, and magnetic properties. X-ray diffraction analyses have indicated that NPs with average crystallite sizes of 2.2 ∼ 2.9 nm are single phase and crystallize in the zincblende-type structure. Cobalt ions with oxidation state + 2 (Co{sup 2+}) replaced for Zn{sup 2+} and almost unchanged the zincblende-type structure of the ZnS host lattice. Interestingly, all samples show room-temperature ferromagnetism and ferromagnetic order increases with increasing Co content (x) in Zn{sub 1−x}Co{sub x}S NPs. With the results obtained from analyzing the local-geometric structures at Zn and Co K-edges and photoluminescence spectra, we believe that ferromagnetism in Zn{sub 1−x}Co{sub x}S NPs is related to intrinsic defects and exchange interactions between Co{sup 2+} ions. We have also reviewed the recent studies on ZnS-based dilute-magnetic materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Ferromagnetism and Carrier Transport in n-type Diluted Magnetic Semiconductors Ge{sub 0.96−x}Bi{sub x}Fe{sub 0.04}Te Thin Film

The structural, Hall effect, electronic transport, and magnetic properties of Ge{sub 0.96−x}Bi{sub x}Fe{sub 0.04}Te epitaxial thin film as prepared by pulsed laser deposition technique were reported. X-ray diffraction patterns including linear scans and phi scans confirmed that all films are high quality epitaxy and crystallinity. With the substitution of high valence Bi for Ge element, we found that the previous carriers of hole were changed to electron, which was testified by the negative slopes obtained from the measurements of Hall effect under different temperatures. The electronic transports show a typical semiconductor behavior and can be understood by the small polaron hopping model because the lattice distortions increase the electron-phonon interaction. An obvious ferromagnetic properties occur in the high Bi-doping Ge{sub 0.64}Bi{sub 0.32}Fe{sub 0.04}Te rather than in that with low Bi-doping concentration, indicating that the ferromagnetic establishment is entirely dependent on carrier’s transmissions. The first-principles calculation performed on this system also reveals that the ferromagnetic state exactly exists in the present n-type diluted magnetic semiconductors with Bi co-dopants.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Enhancement of the Critical Current of Bi-2223 Superconducting Coil by Using of Soft Ferromagnetic Material

The effects of soft ferromagnetic material placed nearby the high-Tc Bi-2223 coil is analyzed. Electro-magnetic simulation have been carried out to find the distributions of the magnetic field of the Bi-2223 coil. The used soft ferromagnetic material could change the distribution of the Bi-2223 coil’s magnetic field, and the transport capability of the Bi-2223 coil changes consequently. According to the simulation results, the thickness of the soft ferromagnetic material plate shows a notable effect on the coil, and the position and width of the plate could present negative effects. Experimental result of a Bi-2223 coil with B30P105 silicon steel plate is consistent with the simulation conclusions, and the critical current of the developed Bi-2223 coil could achieve an improvement of 15% by using of a piece of silicon steel plate with a thickness of 0.3 mm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Signature of Ferromagnetic Phase at Low Temperature in Low-Doped La{sub 0.88}Ca{sub 0.12}MnO{sub 3} Single Crystal

The confirmation of ferromagnetism at low temperature in a low-doped manganites system is always debatable. It is for the first time, electrical, magnetic, and thermal properties have been measured for manganites single crystal of La{sub 0.88}Ca{sub 0.12}MnO{sub 3} by using a floating-zone method. The electrical resistivity shows a perceptible change of resistance at a low temperature which can be viewed as a signature of the ferromagnetic phase. The ferromagnetic phase of the investigated La{sub 0.88}Ca{sub 0.12}MnO{sub 3} single crystal with Curie temperature of T{sub C} = 120 K which is in closed agreement with ac-susceptibility measurement. An analysis of the specific heat suggests the transition peak (T{sub P}) coincides with the T{sub C} and attains 1.2 J mol{sup −1} K at 116 K. The magnetoresistance (MR) and temperature coefficient of resistance (TCR) were found to be 69% and 10% respectively, which expands its subtle potential for applications in spintronics and thermistor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Ferromagnetism in the Hubbard Model with a Gapless Nearly-Flat Band

We present a version of the Hubbard model with a gapless nearly-flat lowest band which exhibits ferromagnetism in two or more dimensions. The model is defined on a lattice obtained by placing a site on each edge of the hypercubic lattice, and electron hopping is assumed to be only between nearest and next nearest neighbor sites. The lattice, where all the sites are identical, is simple, and the corresponding single-electron band structure, where two cosine-type bands touch without an energy gap, is also simple. We prove that the ground state of the model is unique and ferromagnetic at half-filling of the lower band, if the lower band is nearly flat and the strength of on-site repulsion is larger than a certain value which is independent of the lattice size. This is the first example of ferromagnetism in three dimensional non-singular models with a gapless band structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗