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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Structural investigations on Co{sub 3-x}Mn{sub x}TeO{sub 6}; (0 < x ≤ 2); High temperature ferromagnetism and enhanced low temperature anti-ferromagnetism

In the quest of materials with high temperature ferromagnetism and low temperature anti-ferromagnetism, we prepare Co{sub 3-x}Mn{sub x}TeO{sub 6}; (0 < x ≤ 2) solid solutions. Room temperature structural investigations on these solid solutions as a function of Mn concentration using Synchrotron X-ray diffraction (SXRD) and X-ray absorption near edge structure measurements in corroboration with magnetism are presented. Phase diagram obtained from Rietveld Refinement on SXRD data as a function of Mn concentration indicates doping disproportionate mixing of both monoclinic (C2/c) and rhombohedral (R 3{sup ¯}) structure for x < 0.5, while only R 3{sup ¯} structure for x ≥ 0.5. Further, it shows increase in both lattice parameters as well as average transition metal-oxygen (Co/Mn-O) bond lengths for x ≥ 0.5. Co and Mn K-edge XANES spectra reveal that both Co and Mn are in mixed oxidation state, Co{sup 2+}/Mn{sup 2+} and Co{sup 3+}/Mn{sup 3+}. Relative ratios of Co{sup 3+}/Co{sup 2+} and Mn{sup 3+}/Mn{sup 2+} obtained using Linear combination fit decrease with increasing x (for x ≥ 0.5). These structural and spectroscopic evidences are used to provide possible interpretation of the observed paramagnetic to ferromagnetic transition at around 185 K followed by an enhanced antiferromagnetic transition ∼45 K for x = 0.5.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Defect-band mediated ferromagnetism in Gd-doped ZnO thin films

Gd-doped ZnO thin films prepared by pulsed laser deposition with Gd concentrations varying from 0.02–0.45 atomic percent (at. %) showed deposition oxygen pressure controlled ferromagnetism. Thin films prepared with Gd dopant levels (<Gd 0.112 at. %) at low oxygen deposition pressure (<25 mTorr) were ferromagnetic at room temperature. Negative magnetoresistance, electric transport properties showed that the ferromagnetic exchange is mediated by a spin-split defect band formed due to oxygen deficiency related defect complexes. Mott's theory of variable range of hopping conduction confirms the formation of the impurity/defect band near the Fermi level.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Active and passive kink mode studies in a tokamak with a movable ferromagnetic wall

High-resolution active and passive kink mode studies are conducted in a tokamak with an adjustable ferromagnetic wall near the plasma surface. Ferritic tiles made from 5.6 mm thick Hiperco{sup ®} 50 alloy have been mounted on the plasma-facing side of half of the in-vessel movable wall segments in the High Beta Tokamak-Extended Pulse device [D. A. Maurer et al., Plasma Phys. Controlled Fusion 53, 074016 (2011)] in order to explore ferritic resistive wall mode stability. Low-activation ferritic steels are a candidate for structural components of a fusion reactor, and these experiments examine MHD stability of plasmas with nearby ferromagnetic material. Plasma-wall separation for alternating ferritic and non-ferritic wall segments is adjusted between discharges without opening the vacuum vessel. Amplification of applied resonant magnetic perturbations and plasma disruptivity are observed to increase when the ferromagnetic wall is close to plasma surface instead of the standard stainless steel wall. Rapidly rotating m/n=3/1 external kink modes have higher growth rates with the nearby ferritic wall. Feedback suppression of kinks is still as effective as before the installation of ferritic material in vessel, in spite of increased mode growth rates.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Braking of tearing mode rotation by ferromagnetic conducting walls in tokamaks

An in-depth investigation of the braking of tearing mode rotation in tokamak plasmas via eddy currents induced in external ferromagnetic conducting structures is performed. In general, there is a “forbidden band” of tearing mode rotation frequencies that separates a branch of high-frequency solutions from a branch of low-frequency solutions. When a high-frequency solution crosses the upper boundary of the forbidden band, there is a bifurcation to a low-frequency solution, and vice versa. The bifurcation thresholds predicted by simple torque-balance theory (which takes into account the electromagnetic braking torque acting on the plasma, as well as the plasma viscous restoring torque, but neglects plasma inertia) are found to be essentially the same as those predicted by more complicated time-dependent mode braking theory (which takes inertia into account). Significant ferromagnetism causes otherwise electromagnetically thin conducting structures to become electromagnetically thick and also markedly decreases the critical tearing mode amplitude above which the mode “locks” to the conducting structures (i.e., the high-frequency to low-frequency bifurcation is triggered). On the other hand, if the ferromagnetism becomes too large, then the forbidden band of mode rotation frequencies is suppressed, and the mode frequency consequently varies smoothly and reversibly with the mode amplitude.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Coupling of microwave magnetic dynamics in thin ferromagnetic films to stripline transducers in the geometry of the broadband stripline ferromagnetic resonance

We constructed a quasi-analytical self-consistent model of the stripline-based broadband ferromagnetic resonance (FMR) measurements of ferromagnetic films. Exchange-free description of magnetization dynamics in the films allowed us to obtain simple analytical expressions. They enable quick and efficient numerical simulations of the dynamics. With this model, we studied the contribution of radiation losses to the ferromagnetic resonance linewidth, as measured with the stripline FMR. We found that for films with large conductivity of metals the radiation losses are significantly smaller than for magneto-insulating films. Excitation of microwave eddy currents in these materials contributes to the total microwave impedance of the system. This leads to impedance mismatch with the film environment resulting in decoupling of the film from the environment and, ultimately, to smaller radiation losses. We also show that the radiation losses drop with an increase in the stripline width and when the sample is lifted up from the stripline surface. Hence, in order to eliminate this measurement artefact, one needs to use wide striplines and introduce a spacer between the film and the sample surface. The radiation losses contribution is larger for thicker films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Magnetic ordering through itinerant ferromagnetism in a metal-organic framework

In this report, the materials that combine magnetic order with other desirable physical attributes could find transformative applications in spintronics, quantum sensing, low-density magnets and gas separations. Among potential multifunctional magnetic materials, metal-organic frameworks, in particular, bear structures that offer intrinsic porosity, vast chemical and structural programmability, and the tunability of electronic properties. Nevertheless, magnetic order within metal-organic frameworks has generally been limited to low temperatures, owing largely to challenges in creating a strong magnetic exchange. Here we employ the phenomenon of itinerant ferromagnetism to realize magnetic ordering at T C = 225 K in a mixed-valence chromium(II/III) triazolate compound, which represents the highest ferromagnetic ordering temperature yet observed in a metal-organic framework. The itinerant ferromagnetism proceeds through a double-exchange mechanism, which results in a barrierless charge transport below the Curie temperature and a large negative magnetoresistance of 23% at 5 K. These observations suggest applications for double-exchange-based coordination solids in the emergent fields of magnetoelectrics and spintronics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Theory of spin-coherent electrical transport through a defect spin state in a metal/insulator/ferromagnet tunnel junction undergoing ferromagnetic resonance

In this work, we describe the coherent dynamics of electrical transport through a localized spin-dependent state, such as is associated with a defect spin, at the interface of a ferromagnet and a nonmagnetic material during ferromagnetic resonance. As the ferromagnet magnetic moment precesses, charge carriers are dynamically spin filtered by the localized state, leading to a dynamic spin accumulation on the defect. Local effective magnetic fields modify the precession of a spin on the defect, which also modifies the time-integrated total charge current through the defect. Lastly, we thus identify a form of current-detected spin resonance that reveals the local magnetic environment of a carrier spin located at a defect, and thus potentially the defect’s identity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Itinerant ferromagnetism in the As 4p conduction band of Ba 0.6 K 0.4 Mn 2 As 2 identified by x-ray magnetic circular dichroism

In this study, x-ray magnetic circular dichroism (XMCD) measurements on single-crystal and powder samples of Ba 0.6 K 0.4 Mn 2 As 2 show that the ferromagnetism below T C ≈ 100 K arises in the As 4p conduction band. No XMCD signal is observed at the Mn x-ray absorption edges. Below T C , however, a clear XMCD signal is found at the As K edge which increases with decreasing temperature. The XMCD signal is absent in data taken with the beam directed parallel to the crystallographic c axis indicating that the orbital magnetic moment lies in the basal plane of the tetragonal lattice. These results show that the previously reported itinerant ferromagnetism is associated with the As 4p conduction band and that distinct local-moment antiferromagnetism and itinerant ferromagnetism with perpendicular easy axes coexist in this compound at low temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Imaging orbital ferromagnetism in a moiré Chern insulator

Capturing exotic magnetism Ferromagnetism is associated with the breaking of time-reversal symmetry, most frequently by the spin degree of freedom. Although the orbital motion of electrons can also contribute to ferromagnetism, in most materials, it is small relative to the spin contribution. Tschirhart et al. showed that the reverse is true in an unusual magnetic state hosted by twisted bilayer graphene. Their scanning magnetometry measurements were consistent with ferromagnetism of predominantly orbital origin. Science , abd3190, this issue p. 1323

Science & Technology - Other Topics↗
DOE OSTI2014

Ferromagnetic resonance of sputtered yttrium iron garnet nanometer films

Growth of nm-thick yttrium iron garnet (YIG) films by sputtering and ferromagnetic resonance (FMR) properties in the films were studied. The FMR linewidth of the YIG film decreased as the film thickness was increased from several nanometers to about 100 nm. For films with very smooth surfaces, the linewidth increased linearly with frequency. In contrast, for films with big grains on the surface, the linewidth-frequency response was strongly nonlinear. Films in the 7–26 nm thickness range showed a surface roughness between 0.1 nm and 0.4 nm, a 9.48-GHz FMR linewidth in the 6–10 Oe range, and a damping constant of about 0.001.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Important role of the non-uniform Fe distribution for the ferromagnetism in group-IV-based ferromagnetic semiconductor GeFe

We investigate the growth-temperature dependence of the properties of the group-IV-based ferromagnetic semiconductor Ge{sub 1−x}Fe{sub x} films (x = 6.5% and 10.5%), and reveal the correlation of the magnetic properties with the lattice constant, Curie temperature (T{sub C}), non-uniformity of Fe atoms, stacking-fault defects, and Fe-atom locations. While T{sub C} strongly depends on the growth temperature, we find a universal relationship between T{sub C} and the lattice constant, which does not depend on the Fe content x. By using the spatially resolved transmission-electron diffractions combined with the energy-dispersive X-ray spectroscopy, we find that the density of the stacking-fault defects and the non-uniformity of the Fe concentration are correlated with T{sub C}. Meanwhile, by using the channeling Rutherford backscattering and particle-induced X-ray emission measurements, we clarify that about 15% of the Fe atoms exist on the tetrahedral interstitial sites in the Ge{sub 0.935}Fe{sub 0.065} lattice and that the substitutional Fe concentration is not correlated with T{sub C}. Considering these results, we conclude that the non-uniformity of the Fe concentration plays an important role in determining the ferromagnetic properties of GeFe.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Laser-induced ultrafast demagnetization time and spin moment in ferromagnets: First-principles calculation

When a laser pulse excites a ferromagnet, its spin undergoes a dramatic change. The initial demagnetization process is very fast. Experimentally, it is found that the demagnetization time is related to the spin moment in the sample. In this study, we employ the first-principles method to directly simulate such a process. We use the fixed spin moment method to change the spin moment in ferromagnetic nickel, and then we employ the Liouville equation to couple the laser pulse to the system. We find that in general the dependence of demagnetization time on the spin moment is nonlinear: It decreases with the spin moment up to a point, after which an increase with the spin moment is observed, followed by a second decrease. To understand this, we employ an extended Heisenberg model, which includes both the exchange interaction and spin-orbit coupling. The model directly links the demagnetization rate to the spin moment itself and demonstrates analytically that the spin relaxes more slowly with a small spin moment. A future experimental test of our predictions is needed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Ferromagnetic resonance of a YIG film in the low frequency regime

An improved method for characterizing the magnetic anisotropy of films with cubic symmetry is described and is applied to an yttrium iron garnet (111) film. Analysis of the ferromagnetic resonance (FMR) spectra performed both in-plane and out-of-plane from 0.7 to 8 GHz yielded the magnetic anisotropy constants as well as the saturation magnetization. The field at which FMR is observed turns out to be quite sensitive to anisotropy constants (by more than a factor ten) in the low frequency (<2 GHz) regime, and when the orientation of the magnetic field is nearly normal to the sample plane; the restoring force on the magnetization arising from the magnetocrystalline anisotropy fields is then comparable to that from the external field, thereby allowing the anisotropy constants to be determined with greater accuracy. In this region, unusual dynamical behaviors are observed such as multiple resonances and a switching of FMR resonance with only a 1° change in field orientation at 0.7 GHz.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Discovery of ferromagnetism with large magnetic anisotropy in ZrMnP and HfMnP

ZrMnP and HfMnP single crystals are grown by a self-flux growth technique, and structural as well as temperature dependent magnetic and transport properties are studied. Both compounds have an orthorhombic crystal structure. ZrMnP and HfMnP are ferromagnetic with Curie temperatures around 370 K and 320 K, respectively. The spontaneous magnetizations of ZrMnP and HfMnP are determined to be 1.9 μ{sub B}/f.u. and 2.1 μ{sub B}/f.u., respectively, at 50 K. The magnetocaloric effect of ZrMnP in terms of entropy change (ΔS) is estimated to be −6.7 kJ m{sup −3} K{sup −1} around 369 K. The easy axis of magnetization is [100] for both compounds, with a small anisotropy relative to the [010] axis. At 50 K, the anisotropy field along the [001] axis is ∼4.6 T for ZrMnP and ∼10 T for HfMnP. Such large magnetic anisotropy is remarkable considering the absence of rare-earth elements in these compounds. The first principle calculation correctly predicts the magnetization and hard axis orientation for both compounds, and predicts the experimental HfMnP anisotropy field within 25%. More importantly, our calculations suggest that the large magnetic anisotropy comes primarily from the Mn atoms, suggesting that similarly large anisotropies may be found in other 3d transition metal compounds.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗