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On the origin of magnetic anisotropy in 4f-free ferromagnets based on the CaCu 5 structure *
Using first-principles calculations, we investigate the origin of magnetocrystalline anisotropy in a series of 4f-electron-free intermetallics with CaCu 5 -based structures: YCo 5 , YCo 4 B, and Y 3 Co 13 B 2 . The electronic structure of these compounds is characterized by a set of narrow 3d bands near the Fermi level. In YCo 5 the easy-axis anisotropy originates primarily in the spin–orbit coupling-induced mixing of the electronic states with Co d x 2 -y 2 and d xy character. The analysis of k-resolved anisotropy shows that positive contributions accumulate from the entire Brillouin zone but are particularly large near the k z = 0 plane. The analysis of the single-site and two-site terms reveals a large positive single-site contribution to the magnetocrystalline anisotropy from the Co atoms on the honeycomb sublattice, along with two-site contributions from both honeycomb and kagome sublattices.
SAS-A spacecraft magnetic tests
Magnetic tests were conducted on the spacecraft for: (1) alignment, compensation, calibration, and bias determination for the spacecraft three-axis vector magnetometer; (2) determination of permanent, induced, and stray magnetic moments of the spacecraft and compensation of permanent magnetic moments by permanent magnets; and (3) evaluation of the spin and attitude control system.
Exploring the Limits of Overhauser Dynamic Nuclear Polarization (O-DNP) for Portable Magnetic Resonance Detection of Low γ Nuclei
Nuclear magnetic resonance (NMR) spectroscopy in portable, permanent magnet-based spectrometers is primarily limited to nuclei with higher gyromagnetic ratio, γ, such as 1 H, 19 F, and 31 P due to the limited field strength achievable in these systems. Overhauser effect dynamic nuclear polarization (O-DNP), which transfers polarization from an unpaired electron to a nucleus by saturating an electron paramagnetic resonance transition with an oscillating radio frequency magnetic field, B 1e , can increase the polarization of low γ nuclei by hundreds or even thousands, enabling detection in a portable system. We have investigated the potential for O-DNP to enhance signals using (4-amino-2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO hereafter) as a source of unpaired electrons in a homebuilt ultra-low field (ULF) O-DNP-NMR spectrometer. We have found, in general, that larger concentrations of TEMPO are required for effective O-DNP with low γ nuclei, which has a number of important effects. Spin exchange effects cause the EPR lines to overlap and ultimately merge at high concentrations of TEMPO, fundamentally increasing the maximum possible enhancement, while the electron–electron dipolar interaction reduces both longitudinal and transverse relaxation times for the electrons, dramatically increasing the required B 1e strength. The relationship between TEMPO concentration, B 1e magnitude and O-DNP enhancement is quantified, and strategies for achieving these fields are discussed.
Low-Temperature Short-Time SPS Processes to Produce Fine-Grained High-Coercivity Barium Hexaferrite Ceramics from Polyol Nanoparticles
Barium hexaferrite (BaFe{sub 12}O{sub 19}) ceramics were obtained by combination of polyol-synthesized precursors subsequently consolidated by spark plasma sintering (SPS) at a low temperature (800 {sup ∘}C). X-ray powder diffraction studies showed the influence of the experimental parameters to produce a virtually single- or multiphase material. The room temperature magnetic properties exhibited interesting variety of properties, including exchange coupling (spring magnet) between hexaferrite and other magnetic phases. The saturation magnetization and coercive field, at room temperature, are in the 34–70 emu/g and 4.1–5.0 kOe ranges, respectively. The combination of strong coercive field and magnetization resulted in a maximum energy product [(BH)max] = 10.9 kJ/m{sup 3} for the sample with the highest BaFe{sub 12}O{sub 19} content.
Effect of Sb substitution on crystal structure, texture and hard magnetic properties of melt-spun MnBi alloys
Melt-spun Mn 50 Bi 50-x Sb x alloys with x ≤ 5 were prepared at different solidification rates and characterized both in the as-spun state and after annealing. In the as-spun alloys, the Sb substitution leads to the formation of a metastable phase, similar to the binary "quenched high-temperature phase" but without the superstructure of the latter and exhibiting a different – increasing – temperature dependence of coercivity. The new nanocrystalline metastable phase is not only itself characterized by a high room-temperature coercivity (in excess of 20 kOe), but upon annealing it transforms into a high-coercivity (up to 13.5 kOe) "low-temperature" α phase. However, the advantage of obtaining a high coercivity without the otherwise required milling made possible by the Sb substitution is undermined by the absence of a local crystallographic texture in the melt-spun alloys. The best combination of the isotropic hard magnetic properties realized for the Mn 55 Bi 43.5 Sb 1.5 composition is a remanence of 35.5 emu/g and a coercivity of 8.1 kOe. The annealed Sb-free Mn 50 Bi 50 alloys did possess a local crystallographic alignment and, for high solidification rates, a moderate coercivity up to 5.6 kOe; however, they had an inadequate "rectangularity" in the demagnetization curve. The orthorhombic compound known as MnBi 0.9 Sb 0.1 and described earlier as antiferromagnetic was obtained in the annealed ribbons with x ≈ 5. The compound was found to order ferromagnetically between 200 K and ≈250 K and to exhibit a significant coercivity, 14.3 kOe at 50 K. Here, it is suggested that the MnBi 0.9 Sb 0.1 may actually be a Sb-stabilized "new phase" observed earlier as metastable in the Bi–MnBi eutectic alloys.
Effect of alloying with Sc, Nb and Zr on reduction-diffusion synthesis of magnetically hard Sm(Fe,Co,Ti) 12 -based monocrystalline powders
Powders of Sm(Fe,Co) 11.2 Ti 0.8 alloys modified with Sc, Nb and Zr, as well as with additional Ti were prepared by reducing mechanically activated raw oxides with Ca metal in the furnace preheated to 990–1250 °C. Expansion of the crystal lattice upon introduction of Nb or additional Ti implies that atoms of these elements replace the smaller Fe atoms in the tetragonal ThMn 12 -type structure. On the other hand, contraction of the lattice upon introduction of Sc or Zr was smaller than what was expected for replacement of the Sm atoms, which suggests that the Sc and Zr atoms replace both the Sm and Fe atoms. Washing away the reduction byproducts expands the crystal lattice of the 1:12 particles and increases their coercivity. The lattice expansion associated with the washing is believed to be caused by interstitial H atoms; more research, however, is needed to establish the mechanism(s) of the washing effect on the coercivity. The earlier reported development of a high coercivity in zirconium-modified monocrystalline particles achieved by increasing the reduction annealing temperature to ≈1200 °C was similarly characteristic of the particles modified with Sc (the coercivity reaches 11.5 kOe) and Nb (8.1 kOe), but not for the particles prepared with additional Ti where the maximum coercivity of 8.3 kOe develops for a lower annealing temperature. Furthermore, it is concluded that Sc, Nb and Zr modify the high-temperature phase equilibria of the Sm(Fe,Co) 11.2 Ti 0.8 alloys allowing for an effective high-temperature processing, whereas the alloy coercivity increases with the synthesis temperature through a different, still unknown mechanism which may involve suppression of the defects specific to the 1:12 crystals.
High temperature structural and magnetic properties of cobalt nanorods
We present in this paper the structural and magnetic properties of high aspect ratio Co nanoparticles ({approx}10) at high temperatures (up to 623 K) using in-situ X ray diffraction (XRD) and SQUID characterizations. We show that the anisotropic shapes, the structural and texture properties are preserved up to 500 K. The coercivity can be modelled by {mu}{sub 0}H{sub C}=2(K{sub MC}+K{sub shape})/M{sub S} with K{sub MC} the magnetocrystalline anisotropy constant, K{sub shape} the shape anisotropy constant and M{sub S} the saturation magnetization. H{sub C} decreases linearly when the temperature is increased due to the loss of the Co magnetocrystalline anisotropy contribution. At 500 K, 50% of the room temperature coercivity is preserved corresponding to the shape anisotropy contribution only. We show that the coercivity drop is reversible in the range 300-500 K in good agreement with the absence of particle alteration. Above 525 K, the magnetic properties are irreversibly altered either by sintering or by oxidation. - Graphical abstract: We present in this paper the structural and magnetic properties of high aspect ratio Co nanorods ({approx}10) at high temperatures (up to 623 K) using in-situ X-ray diffraction and SQUID characterizations. We show that the anisotropic shapes, the structural and texture properties are preserved up to 500 K. Above 525 K, the magnetic properties are irreversibly altered either by sintering or by oxidation. Highlights: Black-Right-Pointing-Pointer Ferromagnetic Co nanorods are prepared using the polyol process. Black-Right-Pointing-Pointer The structural and texture properties of the Co nanorods are preserved up to 500 K. Black-Right-Pointing-Pointer The magnetic properties of the Co nanorods are irreversibly altered above 525 K.
High temperature magnetic properties of SmCo{sub 5}/{alpha}-Fe(Co) bulk nanocomposite magnets
To find alternative high temperature magnets containing no heavy rare earths for power applications, SmCo{sub 5}/Fe bulk nanocomposite magnets with enhanced energy density and high thermal stability have been produced by using a ball-milling plus warm-compaction route. Up to 30% of the Fe soft magnetic phase has been added to the composites with grain size <20 nm distributed homogenously in the matrix of the SmCo{sub 5} hard magnetic phase. It was observed that the microstructure does not change with temperature up to 500 Degree-Sign C. It is also observed that the thermal stability of bulk nanocomposite samples is closely related to bulk density. Energy products above 11 MGOe have been obtained at 300 Degree-Sign C in fully dense bulk SmCo{sub 5}/Fe nanocomposite magnets, which is 65% higher than that of a single-phase counterpart at the same temperature.
Optimization of a Unipolar Magnetic Bearing
New designs of unipolar magnetic bearings in electrical industrial machines are optimized by means of genetic algorithms. As a result, the optimal geometric dimensions are determined. On that basis, a design is developed for an electric motor with a unipolar active magnetic bearing.
Design Considerations of a Transverse Flux Machine for Direct-Drive Wind Turbine Applications
This paper presents the design considerations of a double-sided transverse flux machine (TFM) for direct-drive wind turbine applications. The TFM has a modular structure with quasi-U stator cores and ring windings. The rotor is constructed with ferrite magnets in a flux-concentrating arrangement to achieve high air gap flux density. The design considerations for this TFM with respect to initial sizing, pole number selection, key design ratios, and pole shaping are presented in this paper. Pole number selection is critical in the design process of a TFM because it affects both the torque density and power factor under fixed magnetic and changing electrical loading. Several key design ratios are introduced to facilitate the design procedure. The effect of pole shaping on back-emf and inductance is also analyzed. These investigations provide guidance toward the required design of a TFM for direct-drive applications. The analyses are carried out using analytical and three-dimensional finite element analysis. A prototype is under construction for experimental verification.
Quadrupole wiggler for a collinear wakefield accelerator
A quadrupole wiggler consisting of a row of alternating-polarity permanent magnet-excited quadrupoles is used in a collinear wakefield accelerator under development at Argonne National Laboratory. Fine translational and angular adjustment mechanisms are implemented in all quadrupoles to obtain a better than 1−μm precision in the alignment of quadrupole wiggler assemblies. The quadrupole’s magnetic and mechanical designs are described. Two wiggler prototypes were fabricated, aligned, and characterized performing pulsed wire magnetic measurements. The sensitivity of the quadrupole wiggler alignment to the ambient temperature was measured.
Magnetic and mechanical properties of grain-refined Dy-free Nd-Fe-B sintered magnets
In this work, the effects of grain size on magnetic and mechanical properties of Dy-free Nd-Fe-B sintered magnets were studied. The grain size of the sintered magnets was varied by using different sized Dy-free Nd-Fe-B alloy feedstock powders which were ball milled to different extents. Increasing the ball milling time from 7 to 11 h caused the average particle size of the powders to decrease from 3.6 to 2.1 µm, while the average grain size of the corresponding sintered magnets decreased from 5.0 to 4.0 µm. The intrinsic coercivity Hcj and maximum energy product (BH) max of these magnets increased from 11.6 to 13.7 kOe and 41.6 to 43.6 MGOe, respectively. The finer grain magnets exhibited better thermal stability, as indicated by a smaller temperature coefficient of H cj (-0.71%/°C). With extending ball milling time to 15 h, the magnetic properties of the magnets deteriorate, while the flexural strengths first decrease and then increase. The flexural strength improvement is attributed to the increased Oxygen content in the magnets due to finer grains. The coercivity enhancement by reduction of grain size is a promising approach to develop Dy-free Nd-Fe-B sintered magnets with better thermal stability. It is important to control the Oxygen contents in the magnets to compromise both magnetic and mechanical properties.
Magnetostructural transition in Fe{sub 5}SiB{sub 2} observed with neutron diffraction
The crystal and magnetic structure of Fe{sub 5}SiB{sub 2} has been studied by a combination of X-ray and neutron diffraction. Also, the magnetocrystalline anisotropy energy constant has been estimated from magnetisation measurements. High quality samples have been prepared using high temperature synthesis and subsequent heat treatment protocols. The crystal structure is tetragonal within the space group I4/mcm and the compound behaves ferromagnetically with a Curie temperature of 760 K. At 172 K a spin reorientation occurs in the compound and the magnetic moments go from aligning along the c-axis (high T) down to the ab-plane (low T). The magnetocrystalline anisotropy energy constant has been estimated to 0.3 MJ/m{sup 3} at 300 K. - Highlights: • The crystal and magnetic structure of Fe{sub 5}SiB{sub 2} has been studied by diffraction. • At 172 K a spin reorientation occurs in the compound. • The magnetic moments are aligned along the c-axis at high T. • The magnetic moments are aligned in the ab-plane at low T. • The magnetocrystalline anisotropy energy constant has been estimated to 0.3 MJ/m{sup 3}.
Stoichiometry dependent phase transition in Mn-Co-Ga-based thin films: From cubic in-plane, soft magnetized to tetragonal perpendicular, hard magnetized
Epitaxial thin films of Mn{sub 3-x}Co{sub x}Ga were grown on MgO by magnetron co-sputtering with different Co content. Dependent on the Co content tetragonal or cubic structures are obtained. The composition dependence of saturation magnetization M{sub S} and uniaxial magnetic anisotropy K{sub u} in the epitaxial films were investigated. A high magnetic anisotropy K{sub u} of 1.2 MJ m{sup -3} was achieved for the Mn{sub 2.6}Co{sub 0.3}Ga{sub 1.1} film with low magnetic moment of 0.84 {mu}{sub B}. The valence band spectra of the films were investigated mainly by hard x-ray photoelectron spectroscopy. The evidence of sharp states in the cubic case, which are smeared out in the tetragonal case, proof the existence of a van Hove singularity that causes a band Jahn-Teller effect accompanied by a tetragonal distortion. These differences are in well agreement to the ab-initio calculations of the electronic structure.
Concept of circular-linear energy recovery accelerator to probe the energy frontier
Energy-frontier accelerators provide powerful tools performing high precision measurements confirming the fundamentals of the physics and broadening new research horizons. Such machines are either driven by circular or linear accelerators. The circular machines, having the centre-of-mass (CM) energy values reaching 200 GeV (for leptons) and above, experience beam energy loss and quality dilution, for example, due to synchrotron radiation, limiting the overall CM energy achievable and requiring a constant energy top-up to compensate the loss and the beam quality dilution. Linear colliders overcome these limitations, while the finite capabilities of generating high average current beams limits the luminosity. This is partially compensated by the quality of the colliding beams. In this work, we suggest a novel design of circular-linear accelerator based on the merging of the “non-emitting”, low-energy storage rings and energy recovery linear accelerators. We suggest using the recently considered dual-axis asymmetric cavities to enable the operation of such a system, and in particular the energy recovery from spent, high-intensity beams. The machine considered, under the scope of the SNOWMASS-2021 initiative, can be potentially used to reach ultimate energy frontiers in high-energy physics as well as to drive next generation light sources. The merging of circular and linear systems, and applications of dual axes cavities, should allow the maintaining of high beam quality, high luminosity, and high energy efficiency, while offering a flexible energy management and opening clear opportunity for reducing the running cost. We note that the numbers shown in the paper are for illustration purpose and can be improved further.
Mariner-Mars magnetic mapping tests
Mapping of permanent, and induced magnetic fields of Mariner Mars spacecraft
Fracture Strength and Surface Magnetic Flux Uniformity in an Isotropic Polymer-Bonded Ring-Shaped Nd-Fe-B Magnet
Ring-shaped (RS) polymer-bonded magnets were produced from a mixture of an isotropic nanocrystalline Nd-Fe-B powder with a nominal composition of 59.8 wt.% Fe, 29 wt.% Nd, 5 wt.% Zr, 4.8 wt.% Co and 1.4 wt.% B and a variety of epoxy resins as binders, using a compaction-molding technique. The morphology and average particle size of the powders are determined by an SEM. The magnetic properties of the magnets were measured using a permeameter. Magnetic flux on the surface of the magnets was determined by a Gauss meter. The mechanical properties of the RS specimens were determined using a tensile fracture strength (FS) test. The effects of polymer type and amount, hardener amount, applied pressing pressure and curing temperature and time on FS were investigated. The experimental results showed that at optimal conditions of 8 wt.% solid epoxy, 3.5 wt.% solid hardener, a pressing pressure of 900 MPa and a curing time of 8 h at 170 °C, a maximum tensile strength of ~ 36 MPa was achieved. Surface magnetic flux uniformity in optimal mechanical conditions was also found to be optimum at around ± 40 G, which is one of the best flux uniformities in bonded magnets.