23 results on '"Plamen, Stamenov"'
Search Results
2. Fabrication and soft magnetic properties of rapidly quenched Co-Fe-B-Si-Nb ultra-thin amorphous ribbons
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Hasan Ahmadian Baghbaderani, Valter Ström, Santosh Kulkarni, S. Cian Ó Mathúna, Plamen Stamenov, Paul McCloskey, and Ansar Masood
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Diffraction ,Amorphous metal ,Fabrication ,Materials science ,Alloy ,Single step ,engineering.material ,Amorphous metals ,equipment and supplies ,Condensed Matter Physics ,Ultra-thin ribbons ,Electronic, Optical and Magnetic Materials ,Amorphous solid ,Morphology study ,Surface roughness ,engineering ,sense organs ,Soft-magnetic properties ,Composite material ,High-frequency applications - Abstract
Ultra-thin soft magnetic amorphous ribbons of Co-Fe-B-Si-Nb alloy were synthesised by a single step rapid-quenching approach to acquire advantage of improved material performance and lower costs over commercial amorphous alloys. The amorphous ribbons of approximately 5.5 µm thicknesses were quenched by a single roller melt spinner in a single-step production process and characterised for their structural and magnetic properties. The disordered atomic structure of amorphous ribbons was confirmed by the X-ray diffraction. A surface morphology study revealed the continuity of ultra-thin ribbons without pores over a large scale. The amorphous alloy showed the ultra-soft magnetic properties in the as-quenched state. The observed thickness dependency of the magnetic properties was attributed to the increased surface roughness and possibly due to a lack of densely packed atomic structure resulting from the extremely high cooling rates experienced by ultra-thin ribbons. We propose that in-situ thinning process of amorphous ribbons significantly reduces the basic material cost and eliminates the need for post-processing steps; hence it provides the opportunity for mass production of high-performance soft magnetic amorphous ribbons at relatively lower costs. more...
- Published
- 2019
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Catalog
3. Sm-Fe-N revisited; remanence enhancement in melt-spun Nitroquench material
- Author
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Munuswamy Venkatesan, T. Iriyama, S. B. Porter, Plamen Stamenov, Rui Zhang, and J. M. D. Coey
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010302 applied physics ,Materials science ,Condensed matter physics ,02 engineering and technology ,Nitride ,Coercivity ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Electronic, Optical and Magnetic Materials ,Magnetization ,Remanence ,Magnet ,0103 physical sciences ,Thermal stability ,Crystallite ,0210 nano-technology ,Saturation (magnetic) - Abstract
Following its discovery in the aftermath of Nd2Fe14B, Sm2Fe17N3 seemed to offer intrinsic magnetic properties that were superior or comparable to those of its famous predecessor. But the promise of the new material to challenge Nd2Fe14B was not realized, mainly because the 2:17 nitride powder, prepared by a low-temperature gas-phase interstitial modification process, was unstable at the temperatures needed to process dense sintered magnets. Here we discuss the magnetic properties of Nitroquench, a melt-spun Sm-Fe-N material, which offers superior corrosion resistance and thermal stability compared to melt-spun Nd-Fe-B. The powder, with a crystallite size of approximately 30 nm is in the form of flakes 15–18 µm thick and about 100 µm in diameter. Room-temperature coercivity is 690 kAm−1 after saturation in 14 T, with a remanence of 92 Am2kg−1 and an extrapolated saturation magnetization of 160 Am2kg−1. The remanence enhancement is reflected in a preferred orientation seen in 57Fe Mossbauer spectra of magnetized isotropic powder, which exhibits different relative intensities of the ΔM = 0 absorption lines according to the direction of the field used to saturate the magnetization. When measured in zero internal field, the remanence ratio Mr/Ms is 64%. The remanence enhancement is attributed to a nanocrystallite size that is not very much greater than the exchange length. The maximum energy product for the powder, assuming full density, is 162 kJm−3. Nitroquench powder may be used to produce bonded magnets with an energy product >100 kJm−3. more...
- Published
- 2019
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4. Exfoliation of hematite: Morphological, structural and magnetic investigations
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Rui Zhang, Plamen Stamenov, J. M. D. Coey, Anup Kumar, and Munuswamy Venkatesan
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Morin transition ,Materials science ,Analytical chemistry ,Nanoparticle ,Hematite ,Condensed Matter Physics ,Exfoliation joint ,Electronic, Optical and Magnetic Materials ,symbols.namesake ,visual_art ,Mössbauer spectroscopy ,symbols ,visual_art.visual_art_medium ,Raman spectroscopy ,Superparamagnetism ,Spin canting - Abstract
Three natural specimens of hematite are characterized using X-ray diffraction, magnetometry, IR/Raman and Mossbauer spectroscopy, with focus on the Morin transition. All are exfoliated by grinding and prolonged sonication in dimethylformamide, followed by centrifuging to extract exfoliated debris from the supernatant. This material is characterized by electron microscopy and atomic force microscopy. There are three main Fe2O3 constituents: lamellae that are hundreds of nanometers in diameter and typically 1–10 nm thick, nanoparticles that are about 100 nm in size and ultra-fine nanoparticles smaller than 10 nm. Estimated yield of the lamellae is 10−3−10−4 by volume. All the iron in the debris is antiferromagnetically ordered at room temperature, including 30% that is superparamagnetic and contributes to a quadrupole doublet in the Mossbauer spectrum, but the characteristic weak net moment due to spin canting is absent. Unlike the normal hematite nanoparticles, there is no Morin transition and no net moment in the range of 100 to 300 K. The absence of a net moment is understandable because the lamellae cleave in planes that make small angles with the c-axis. more...
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- 2022
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5. CALPHAD-assisted development of in-situ nanocrystallised melt-spun Co-Fe-B alloy with high B (1.57 T)
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Ansar Masood, Paul McCloskey, Kenny L. Alvarez, Plamen Stamenov, Hasan Ahmadian Baghbaderani, and Cian O Mathuna
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Amorphous metal ,Materials science ,Mechanical Engineering ,Metals and Alloys ,Thermodynamics ,02 engineering and technology ,Coercivity ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Nanocrystalline material ,0104 chemical sciences ,Condensed Matter::Materials Science ,Electron diffraction ,Mechanics of Materials ,Phase (matter) ,Materials Chemistry ,0210 nano-technology ,CALPHAD ,Saturation (magnetic) ,Phase diagram - Abstract
A thermodynamics-based approach, Calculation of Phase Diagram (CALPHAD), combined with topological instability parameters are proposed and experimentally evaluated, in order to optimise in-situ nanocrystallisation of rapidly quenched CoFeB alloys and exploit their remarkable Bs = 1.57 T. The high Ms of the alloy is attributed to the precipitation of the metastable Co7Fe3 nanocrystalline phase dispersed heterogeneously in the amorphous matrix. High Ms of Co7Fe3 phase can also be inferred from the high hyperfine magnetic field of the Fe nuclei deduced from Mossbauer spectra. It is worth noting that the in-situ nanocrystallisation is a volume phenomenon, instead of surface crystallisation at the air-side of ribbons owning to lower cooling rates. We judge, based on nucleation theory, that the formation of the metastable phase is kinetically favoured, when compared to the equilibrium phases, hence promoting the high Ms, when compared with conventional Co-rich amorphous alloys. The local atomic order of nanocrystallised phase was confirmed by X-ray and electron diffraction techniques. Using Mossbauer spectroscopy and the extracted distribution of the hyperfine magnetic field, it is asserted that cobalt atoms form clusters, as they attract each other to form ordered structures, and boron atoms undergo only short-range ordering, likely due to covalent bond formation, governed by the size and electronegativity differences with the atoms in the amorphous matrix. We suggest the proposed CALPHAD-assisted design of nanostructured alloys, along with an in-situ nanocrystallisation, provides a practical scheme to develop novel functional alloys with the best possible balance of coercivity and saturation, exclusively aimed for a high-tech application. more...
- Published
- 2021
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6. The structural, magnetic and microwave properties of spherical and flake shaped carbonyl iron particles as thin multilayer microwave absorbers
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Morteza Zargar Shoushtari, Omid Khani, Plamen Stamenov, and Karl Ackland
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010302 applied physics ,Materials science ,business.industry ,digestive, oral, and skin physiology ,Flake ,Two layer ,Microwave permeability ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Microwave absorber ,Electronic, Optical and Magnetic Materials ,Optics ,Carbonyl iron ,Permeability (electromagnetism) ,0103 physical sciences ,Composite material ,0210 nano-technology ,business ,Microwave - Abstract
An increase in microwave permeability is a prerequisite for reducing the thickness of radar absorber coatings. The aim of this paper is to increase the magnetic loss of commercial carbonyl iron particles for fabricating wideband microwave absorbers with a multilayer structure. For this purpose, carbonyl iron particles were milled and their static and dynamic magnetic properties were studied before and after milling. A distinct morphological change from spherical to flake-like particles is measured with increased milling time, whereas no distinct changes in magnetic properties are measured with increased milling time. The imaginary part of the permeability (µ״) of the milled carbonyl iron particles increased from 1.23 to 1.88 and showed a very broad peak over the entire frequency range 1–18 GHz. The experimental results were modeled using the Rousselle effective medium theory (EMT) in the Neo formulation. The theoretical predictions showed good agreement with the experimental results. Two layer absorbers were designed according to the measured microwave parameters and the multilayer design. The results revealed that a thin multilayer with a thickness of 1.75 mm can effectively absorb microwaves in both the entire X and Ku frequency bands. The results suggest that microwave absorbers with excellent absorption properties could be mass-produced, using commercial carbonyl iron particles. more...
- Published
- 2017
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7. High-frequency power loss mechanisms in ultra-thin amorphous ribbons
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Cian O Mathuna, J.M. Blanco, Ansar Masood, Plamen Stamenov, Valter Ström, Hasan Ahmadian Baghbaderani, Zoran Pavlovic, Kenny L. Alvarez, and Paul McCloskey
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010302 applied physics ,Amorphous metal ,Materials science ,Condensed matter physics ,Magnetic domain ,Magnetostriction ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Electronic, Optical and Magnetic Materials ,Amorphous solid ,Condensed Matter::Materials Science ,Magnetization ,Remanence ,0103 physical sciences ,0210 nano-technology ,Saturation (magnetic) ,Excitation - Abstract
Soft magnetic amorphous materials with ultra-low power loss are highly desirable for high-frequency drive applications. The present work demonstrates the high-frequency power loss performance and underlying loss mechanisms in ultra-thin amorphous alloys. This is achieved by rapid-quenching amorphous alloys of Co-, CoFe- and Fe-rich systems, investigating their amorphous atomic structure, quantifying the saturation magnetostriction constants (λs), imaging magnetic domains at remanent magnetization, analyzing magnetization reversal from various magnetization levels, and finally, investigating the material loss performance over a broad frequency range (f = 50 kHz–2 MHz) at various excitation levels (Bm = 25–100 mT). The ultra-high performance of ultra-thin Co-rich amorphous ribbons, as compared to CoFe- and Fe-rich alloys, was attributed to the significantly low eddy current loss, due to the reduced thickness, and a minimal amount of excess loss, owning to minimal magnetoelastic contributions and magnetization reversal by rotation. The underlying loss mechanisms were analyzed by decomposing material loss into primary components and identifying the magnetization reversal mechanisms using minor hysteresis loops. In the Co-rich amorphous alloys, we suggest that magnetization reversal by rotation dominates, at least at low excitations, while in CoFe- and Fe-rich alloys domain wall displacement prevails and contributes significantly to the excess loss up to the MHz frequency range. Magnetization reversal by rotation in Co-rich alloys could be attributed to the zero/near-zero λs, and eventually low residual stress, leading to a homogeneous magnetic domain structure, as compared to the inhomogeneous “fingerprint-like” complex domains in highly magnetostrictive CoFe-rich alloys. more...
- Published
- 2021
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8. On the mechanisms limiting power loss in amorphous CoFeB-based melt-spun ribbons
- Author
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Plamen Stamenov, Hasan Ahmadian Baghbaderani, Kenny L. Alvarez, Paul McCloskey, Ansar Masood, Cian O'Mathuna, and Zoran Pavlovic
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010302 applied physics ,Power loss ,Amorphous metal ,Materials science ,Condensed matter physics ,Annealing (metallurgy) ,02 engineering and technology ,Limiting ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Electronic, Optical and Magnetic Materials ,law.invention ,Amorphous solid ,Magnetization ,law ,0103 physical sciences ,Eddy current ,Perpendicular ,0210 nano-technology - Abstract
The mechanisms that limit the power loss performance in melt-spun amorphous ribbons have been investigated through DC and AC magnetic characterization methods. The measured total power loss is resolved into hysteresis, eddy current, and anomalous losses. The anomalous loss is found to account for more than 90% of the total loss, which significantly reduced by annealing in a transverse magnetic field. This is attributed to the reorientation of preferred magnetisation axis perpendicular to the length of ribbons. Transverse magnetic annealing promotes the relative contribution of domain rotation over domain wall motion during magnetisation reversal process. Magnetic annealing also causes a measurable decrease in the domain width, which promotes pinning and inhibits domain wall motion, thus further favoring coherent domain rotation as the primary mechanism of magnetization. This combination accounts for a 75% decrease in the total power loss in the so-processed ribbons and renders them attractive for applications in mid-and high-frequency power supplies and inverters. more...
- Published
- 2020
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9. Spin liquids and spin glasses in Mn-based alloys with the cubic A13 (βMn) structure
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J. M. D. Coey, Munuswamy Venkatesan, Ajay Jha, Z. Gercsi, Rui Zhang, and Plamen Stamenov
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010302 applied physics ,Materials science ,Spin glass ,Spin polarization ,Condensed matter physics ,Doping ,Zero-point energy ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Magnetic susceptibility ,Electronic, Optical and Magnetic Materials ,Andreev reflection ,0103 physical sciences ,Condensed Matter::Strongly Correlated Electrons ,Quantum spin liquid ,0210 nano-technology ,Spin-½ - Abstract
As-cast ingots of Mn100−xZx with Z = Al, Ge, Mn, Fe and Co all have the A13 βMn structure up to some maximum value of x. The magnetic susceptibility of pure βMn is ≈ 9 × 10−4 from 4 to 400 K, but the field-cooled and zero-field-cooled susceptibilities of all doped samples increase at low temperature and they bifurcate at a spin-freezing temperature Tf that increases with x, linearly at first. Point contact Andreev reflection shows evidence of a net spin polarization in a spin glass sample but not in the pure βMn spin liquid. Fixed-moment density functional calculations indicate a very shallow energy minimum ( more...
- Published
- 2020
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10. Torque in open-access permanent magnet variable flux sources
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J. M. D. Coey, Bochen Li, Yangkun He, Plamen Stamenov, and Chengbao Jiang
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010302 applied physics ,Materials science ,Field (physics) ,Flux ,02 engineering and technology ,Mechanics ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Square (algebra) ,Rod ,Electronic, Optical and Magnetic Materials ,Remanence ,Magnet ,0103 physical sciences ,Perpendicular ,Torque ,0210 nano-technology - Abstract
A flux source with four rotatable permanent magnet rods having access from two perpendicular directions through gaps exceeding the rod diameter can produce a continuously-variable flux density of up to ±300 mT at the centre when high-grade Nd-Fe-B magnets are used. The trade-off between field, access and torque in this mangle design is discussed. The maximum field, achievable when the magnets touch, is equal to the remanence of the Nd-Fe-B. The torque needed to rotate the rods scales as the square of the remanence, and it increases faster than the inverse square of the separation when the separation is comparable to the rod diameter. End effects reduce the torque significantly, but appropriate gearing is needed to ensure smooth operation. Modifications of the basic design can reduce or compensate the torque by using special end pieces, and improve access to fields > 1000 mT at small rod separation. more...
- Published
- 2020
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11. Dynamic response of ammonia sensors constructed from polyaniline nanofibre films with varying morphology
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Rethi Madathil, J. M. D. Coey, and Plamen Stamenov
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Resistive touchscreen ,Materials science ,Absorption spectroscopy ,Metals and Alloys ,Analytical chemistry ,Condensed Matter Physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,chemistry.chemical_compound ,Physisorption ,chemistry ,Chemisorption ,Desorption ,Polyaniline ,Polymer chemistry ,Materials Chemistry ,Electrical and Electronic Engineering ,Thin film ,Instrumentation ,Visible spectrum - Abstract
Polyaniline is acid-doped with HCl, HNO3, HClO4, HBr or H2SO4 to yield thin films that are nanofibrous or dense. The nanofibrous films show a faster response to ammonia gas, and the response is greater when the films are in the moderately conducting green state than when they are in the highly conducting yellow state. The film resistance and optical absorption spectrum are monitored at 20 ms and 500 ms intervals, respectively during and after exposure to ammonia. Three absorption processes with characteristic times of order 10 s, 100 s, and 1000 s are observed in the resistive response. The optical spectrum is fitted with three Gaussians or derived Gaussians centred in the blue, orange and infra-red which exhibit similar response times of order 100 s and 1000 s, corresponding to the longer times in the resistive response. The data are interpreted in terms of three processes, physisorption, chemisorption and structural rearrangement of the polymer chains. Gas selectivity is demonstrated in the optical response by comparing the time-dependent desorption of H2O and NH3. These two gases may be distinguished using inexpensive on-chip two band pass filter optical detection. more...
- Published
- 2012
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12. Charge injection, transport and localization in rubrene
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J. M. D. Coey, Franklyn Burke, and Plamen Stamenov
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Magnetoresistance ,Condensed matter physics ,Band gap ,Mechanical Engineering ,Schottky barrier ,Contact resistance ,Metals and Alloys ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Weak localization ,chemistry.chemical_compound ,chemistry ,Ferromagnetism ,Mechanics of Materials ,Materials Chemistry ,Rubrene ,Ohmic contact - Abstract
Rubrene crystals bridging a 200–350 nm gap between electrodes made of gold or various ferromagnets (Ni 80 Fe 20 , Co 90 Fe 10 , Co 90 Fe 10 /AlO x ) exhibit distinct current:voltage characteristics according to their contact resistance. Fits to the nonlinear characteristics with Ni 80 Fe 20 contacts give a Schottky barrier height of 0.22(1) eV. Only for ohmic contacts with Ni 80 Fe 20 was magnetoresistance observed; it is essentially negative and linear in field, reaching 4% in 5 T at 70 K. The magnetoresistance is tentatively attributed to weak localization. There is no evidence of spin-valve magnetoresistance in the lateral geometry, where the spin diffusion length should be less than 200 nm. more...
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- 2011
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13. Magnetic susceptibility of Alq3 powder, pure and Al-doped 8-hydroxyquinoline
- Author
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Mohamed Abid, Franklyn Burke, J. M. D. Coey, and Plamen Stamenov
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Materials science ,Analytical chemistry ,chemistry.chemical_element ,Condensed Matter Physics ,Magnetic susceptibility ,Electronic, Optical and Magnetic Materials ,Nanoclusters ,Paramagnetism ,chemistry ,Ferromagnetism ,Aluminium ,Formula unit ,Orthorhombic crystal system ,Saturation (magnetic) - Abstract
Single-crystal nanowires several microns long and 100–200 nm in diameter were grown by physical vapour deposition from mixed Alq3/γ-Al2O3 powder. The crystals are orthorhombic Al-doped 8-hydroxyquinoline. The molar susceptibility is −3×10−9 at room temperature, and it shows a Curie-law upturn below about 50 K. The approach to saturation at low temperature indicates a density of S = 1 2 defects 4×10−4 per formula unit. Pure 8-hydroxyquinoline and aluminium (Alq3) behave similarly. Pressed pellets exhibit much increased paramagnetic susceptibility due to iron ions scavanged from the steel die. Subsequent melting of these samples produces a ferromagnetic signal of order 0.01 A m2 kg−1, which is attributed to metallic iron nanoclusters in the organic material. more...
- Published
- 2010
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14. On the direct magnetic detection of spin injection and adiabatic depolarization in aluminum
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Plamen Stamenov and J. M. D. Coey
- Subjects
Inductance ,Magnetization ,Materials science ,Condensed matter physics ,Spin polarization ,Ferromagnetism ,Spin diffusion ,Condensed Matter Physics ,Adiabatic process ,Gradiometer ,Electronic, Optical and Magnetic Materials ,Magnetic field - Abstract
An attempt has been made to measure directly the magnetic field created by the spin polarization injected into polycrystalline aluminum from iron injection electrodes in a geometry optimized with respect to a particular SQUID-based second-order gradiometer. Magnetizations, as low as 1 A/m, are detected independently of the influence of the ferromagnetic injectors and applied current densities of the order of 10 8 A/m 2 . The current induced changes in magnetic response are analyzed in terms of components of the coupled to the gradiometer flux, symmetric, anti-symmetric and asymmetric in current and in space, including: mutual inductance, induced spin polarization, adiabatic electronic depolarization and lattice heating effects. No evidence for induced spin polarization is found down to a spatial resolution of 1 mm and temperatures of 1.8 K for applied fields in the range 20–500 mT. more...
- Published
- 2008
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15. Magnetic susceptibility of carbon—experiment and theory
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J. M. D. Coey and Plamen Stamenov
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Materials science ,Condensed matter physics ,Graphene ,Quantum oscillations ,Carbon nanotube ,Electronic structure ,Landau quantization ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Magnetic susceptibility ,Electronic, Optical and Magnetic Materials ,law.invention ,Condensed Matter::Materials Science ,Ferromagnetism ,law ,Diamagnetism - Abstract
Carbon exists in many forms, crystalline and amorphous, with different electronic structures. These polymorphs are usually diamagnetic in the pure, defect-free state but their susceptibility spans more than two orders of magnitude. Large diamagnetic susceptibilities, found in nanotubes, and in graphite with the field applied perpendicular to the graphene sheets, are temperature-dependent and exhibit quantum oscillations at low temperatures, which are related to the distribution of conduction electrons over the Landau levels. In the case of nanotubes, the curvature of the tube acts like an effective magnetic field, limiting the thermal population to just the two lowest Landau levels at room temperature, which explains the field and temperature-dependence of the diamagnetism. Reports of ferromagnetism in some forms of carbon are briefly discussed. more...
- Published
- 2005
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16. Oriented cobalt nanowires prepared by electrodeposition in a porous membrane
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Fernando M.F. Rhen, J. M. D. Coey, Plamen Stamenov, and Nandu B. Chaure
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Materials science ,Condensed matter physics ,business.industry ,Demagnetizing field ,Nanowire ,chemistry.chemical_element ,Condensed Matter Physics ,Magnetocrystalline anisotropy ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,Magnetization ,Magnetic anisotropy ,Optics ,chemistry ,Magnet ,Texture (crystalline) ,business ,Cobalt - Abstract
Porous alumina membranes with a uniform pore size of 100 nm have been used to prepare arrays of oriented cobalt nanowires by electrodeposition in an applied magnetic field. The hexagonal c-axis lies in the plane of the membrane, perpendicular to the nanowire axis, for material deposited in 0 T or 1.5 T, but in 5 T the c-axis lies at about 70° to the axes of the wires. The degree of field-induced crystallographic orientation is insufficient to overcome the macroscopic demagnetizing field in samples with a large volume fraction of cobalt. The effective demagnetizing factors perpendicular and parallel to the plane of the membrane are N eff ⊥ = 0.7 and N eff ∥ = 0.15 , respectively. Pole figures of the vector magnetization measured in a small rotating field of fixed amplitude in a permanent-magnet vector vibrating sample magnetometer are used to deduce torque and hence the effective magnetocrystalline anisotropy. more...
- Published
- 2005
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17. Structural distortions and charge/orbital ordering in Bi0.25Ho0.25Ca0.5MnO3
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Erzsébet Sváb, Daniela Kovacheva, Kiril Krezhov, Françoise Bourée, and Plamen Stamenov
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Materials science ,Condensed matter physics ,Neutron diffraction ,Charge (physics) ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,Paramagnetism ,Nuclear magnetic resonance ,Phase (matter) ,Distortion ,Antiferromagnetism ,Condensed Matter::Strongly Correlated Electrons ,Orthorhombic crystal system ,Electrical and Electronic Engineering ,Monoclinic crystal system - Abstract
The structure and magnetic arrangement in Bi0.25Ho0.25Ca0.5MnO3 have been studied by powder neutron diffraction. At room temperature, the compound is in paramagnetic phase of small monoclinic distortion allowing for structural description in orthorhombic Pnma symmetry. At 10 K, the ground magnetic state was specified as CE-type antiferromagnetic. more...
- Published
- 2004
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18. Ultrathin (CoFe/Pt)n multilayers with tuned magnetic properties
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Munuswamy Venkatesan, Karl Ackland, J. M. D. Coey, Plamen Stamenov, Jiafeng Feng, and Karsten Rode
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Materials science ,Condensed matter physics ,Annealing (metallurgy) ,chemistry.chemical_element ,Surface finish ,Crystal structure ,Cubic crystal system ,Coercivity ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Crystallography ,Transition metal ,chemistry ,Anisotropy ,Platinum - Abstract
Perpendicular magnetic anisotropy (PMA) has been investigated in ultrathin (CoFe [0.2] nm/Pt [0.2] nm) n multilayers. The Pt layers show an fcc crystal structure with a preferred [111] orientation. The multilayers with n=3, 4 show PMA in the as-grown state, which can be enhanced by thermal annealing. However, no PMA is observed in the as-grown state with higher repetitions (n>&=5), although it is observed after thermal annealing. For 1=& 5 J/m 3 for all (CoFe [0.2]/Pt [0.2]) n stacks. The perpendicular anisotropy is related to layer thickness and interface roughness. more...
- Published
- 2012
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19. High-field anisotropy of the tunnelling magnetoresistance of CoFeB/MgO/CoFeB junctions
- Author
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J. M. D. Coey, Kaan Oguz, and Plamen Stamenov
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Materials science ,Exchange bias ,Condensed matter physics ,Magnetoresistance ,Tunnel junction ,Condensed Matter::Superconductivity ,Density of states ,Fermi surface ,Condensed Matter Physics ,Anisotropy ,Quantum tunnelling ,Electronic, Optical and Magnetic Materials ,Magnetic field - Abstract
Differential resistance data is gathered on both as-deposited and annealed tunnel junctions based on CoFeB/MgO interfaces, where one of the layers is pinned via exchange bias. When sufficiently strong magnetic field is rotated out of the plane of the junctions, a characteristic bias dependence of the anisotropic term in the differential resistance is observed at low temperature and all fields (from 5–14 T), which may be represented by a voltage dependent base function A ( V ) together with an angular sin 2 ( θ + ϕ ) dependence, where θ is the angle between the normal to the plane of the junction and ϕ is an offset phase. The base function reflects the anisotropy in the spin-polarized density of states of crystallized CoFeB and its spin-resolved Fermi surfaces. more...
- Published
- 2010
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20. Shubnikov–de Haas and Hall quantum oscillations in graphite
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V. Krstic, Plamen Stamenov, and J. M. D. Coey
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Physics ,Highly oriented pyrolytic graphite ,Condensed matter physics ,Hall effect ,Quantum oscillations ,Fermi surface ,Landau quantization ,Electron ,Quantum Hall effect ,Condensed Matter Physics ,Shubnikov–de Haas effect ,Electronic, Optical and Magnetic Materials - Abstract
Resistance and Hall effect are measured for graphite in fields of up to 23 T in the temperature range 0.3–300 K. Quantum oscillations appear below 50 K when the field is parallel to the c-axis. No oscillations are observed above 8 T, when all the electrons in the system occupy the lowest Landau level. Cyclotron masses for the lowest Landau levels range from 0.6me to 0.04me. The extremal cross section for the electron Fermi surface is 4.2×1016 m−2 and the hole cross section is similar. The Dingle temperature for the highly oriented pyrolytic graphite sample is 7 K. more...
- Published
- 2005
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21. Anisotropy of the magnetization of a dilute magnetic oxide
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Lucio Strazzabosco Dorneles, Plamen Stamenov, James G. Lunney, Munuswamy Venkatesan, C. B. Fitzgerald, and J. M. D. Coey
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Materials science ,Condensed matter physics ,Magnetism ,Doping ,Magnetic semiconductor ,Atmospheric temperature range ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,Magnetization ,Magnetic anisotropy ,Ferromagnetism ,Condensed Matter::Superconductivity ,Condensed Matter::Strongly Correlated Electrons ,Anisotropy - Abstract
High-quality (1 1 0) films of ZnO, undoped or doped with the 3d transition metals from Sc to Cu, have been prepared on R-cut sapphire. Most of the films, including the one doped with Sc, exhibit ferromagnetic moments at room temperature greater than 100 μB nm−2, which are remarkably anisotropic. The moment in 1 T is usually larger, by a factor of up to three, when the field is applied perpendicular to the plane of the film than it is in a parallel direction. Strong in-plane anisotropy is observed for Co. The data suggest a new type of ferromagnetism related to defects at the film-substrate interface. more...
- Published
- 2005
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22. Magnetic and structural properties of Co-doped ZnO thin films
- Author
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Munuswamy Venkatesan, R. Gunning, J. M. D. Coey, James G. Lunney, Jonathan Alaria, Plamen Stamenov, M. Rooney, and Lucio Strazzabosco Dorneles
- Subjects
Materials science ,Condensed matter physics ,Magnetic moment ,Magnetism ,Analytical chemistry ,chemistry.chemical_element ,Magnetic semiconductor ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,Transition metal ,chemistry ,Ferromagnetism ,Condensed Matter::Superconductivity ,X-ray crystallography ,Condensed Matter::Strongly Correlated Electrons ,Thin film ,Cobalt - Abstract
Growth temperature plays a major role on the magnetic and structural properties of Co-doped ZnO thin films. Ferromagnetism is observed only in films grown at temperatures within a narrow range around 450 °C. Metallic Co is detected in 40–300 nm thick films, and the estimated content is compatible with the magnetic moment observed. more...
- Published
- 2007
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23. Erratum to 'Ultrathin (CoFe/Pt)n multilayers with tuned magnetic properties' [J. Magn. Magn. Mater. A 324 (14) (2012) 2298–2300]
- Author
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Munuswamy Venkatesan, Karsten Rode, J. M. D. Coey, Jiafeng Feng, Plamen Stamenov, and Karl Ackland
- Subjects
Condensed Matter Physics ,Electronic, Optical and Magnetic Materials - Published
- 2012
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- View/download PDF
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