Showing papers on "Saturation (magnetic) published in 2003"

Selected room temperature magnetic parameters as a function of mineralogy, concentration and grain size

Abstract: A data set of room temperature magnetic parameters for several iron oxides and sulphides was compiled from the available literature. The aim was to propose the most effective methods for assessing mineralogy, concentration and domain state within environmental magnetic studies. Establishing the magnetic mineralogy is essential for a correct interpretation of concentration and grain-size indicative parameters. Hematite and in particular goethite are recognised by their high (remanent) coercivities. The ratio of saturation remanent magnetisation to susceptibility is suited for assessing mineralogy; for low coercivity minerals, high values of the ratio are indicative of pyrrhotite. Greigite and maghemite both have intermediate ratios, while very low values suggest the presence of (titano)magnetite. From the concentration-dependent parameters mass specific susceptibility, saturation remanent magnetisation and susceptibility of anhysteretic remanent magnetisation, susceptibility displayed the least grain-size dependence. Therefore, it is perhaps the best indicator of concentration, under the proviso of uniform mineralogy and when paramagnetic and diamagnetic contributions are taken into consideration. All minerals showed a decrease in coercivity and remanence ratios with increasing grain size for sizes larger than one micrometer. In contrast, the coercivity of goethite showed an increase with grain size. Assessment of domain state was complicated by very small and large grains displaying similar magnetic properties.

Micromagnetism and the Microstructure of Ferromagnetic Solids

Abstract: 1. Introduction 2. Magnetic Gibbs free energy 3. Basic micromagnetic equilibrium conditions 4. Domain walls in crystalline and amorphous solids 5. Interaction of domain walls with defects 6. Coercivity of modern magnetic materials 7. Statistical theory of domain wall pinning 8. Law of approach to ferromagnetic saturation and high-field susceptibility 9. Microstructure and domain patterns 10. Magnetic after-effects in amorphous alloys 11. Magnetorestriction in amorphous and polycrystalline ferromagnets 12. Micromagnetic theory of phase transitions in spatially disordered spin systems 13. Computational micromagnetism of thin platelets and small particles 14. Computational micromagnetism of dynamic magnetization processes Index.

Synthesis and magnetic characterization of Mn and Co spinel ferrite-silica nanoparticles with tunable magnetic core

Abstract: A method for coating silica on CoFe2O4 and MnFe2O4 spinel ferrite nanoparticles has been developed by using a reverse micelle microemulsion approach. The ability to controllably synthesize magnetic nanoparticulate cores independent of encapsulation provides great flexibility in tuning the magnetic properties of this magnetic nanocomposite system by controlling the magnetic properties of nanoparticulate cores. For these spinel ferrite nanoparticles, the saturation and remnant magnetizations decrease upon silica coating. The coercivity of silica-coated CoFe2O4 nanoparticles does not show any change after coating, while the coercivity of MnFe2O4 nanoparticles decreases by 10% after they have been coated with silica.

Shear properties of a magnetorheological elastomer

Abstract: This paper presents an experiment testing the damped free vibration of a system composed of a magnetorheological elastomer and a mass. The goal of this experiment was to obtain the dependence of the natural frequency and the damping ratio of the structure on the applied magnetic field. The shear properties, including the shear storage modulus and the damping factor, were therefore determined. The experimental results revealed that the shear storage modulus could reach a value of 60% of the zero-field modulus and was dominated by the magnetic field, but the change in the damping factor could be neglected. Furthermore, when the field was moderate and saturation did not occur, the shear storage modulus increased proportionally with the applied field. This interesting phenomenon was analysed, and it is suggested that the subquadratic field dependence, which arises from the saturation of the magnetization near the poles of closely spaced pairs of spheres, must be taken into consideration.

Coexistence of ferromagnetic and antiferromagnetic order in Mn-doped Ni2MnGa

Abstract: Ni-Mn-Ga is interesting as a prototype of a magnetic shape-memory alloy showing large magnetic-field-induced strains We present here results for the magnetic ordering of Mn-rich Ni-Mn-Ga alloys based on both experiments and theory Experimental trends for the composition dependence of the magnetization are measured by a vibrating sample magnetometer in magnetic fields of up to several tesla and at low temperatures The saturation magnetization has a maximum near the stoichiometric composition and it decreases with increasing Mn content This unexpected behavior is interpreted via first-principles calculations within the density-functional theory We show that extra Mn atoms are antiferromagnetically aligned to the other moments, which explains the dependence of the magnetization on composition In addition, the effect of Mn doping on the stabilization of the structural phases and on the magnetic anisotropy energy is demonstrated

Non-linear constitutive relations for magnetostrictive materials

Abstract: In this paper, non-linear deformation behavior of magnetostrictive materials is studied and three magnetoelastic coupling constitutive models are developed. The standard square (SS) constitutive model is developed by means of truncating the polynomial expansion of the Gibbs free energy. The hyperbolic tangent (HT) constitutive equations, which involve a hyperbolic tangent magnetic-field dependence, are proposed to model the magnetic-field-induced strain saturation of magnetostrictive materials in the region of intense magnetic fields. A new model based on density of domain switching (DDS) is established in terms of the basic truth that magnetic domain switching underlies magnetostrictive deformation. In this model, it is assumed that the relation between density of domain switching, defined by the quantity of magnetic domains switched by per unit magnetic field and magnetic field can be described by a density function with normal distribution. The moduli in these constitutive models can be determined by a material function that is proposed to describe the dependence of the peak piezomagnetic coefficient on the compressive pre-stress for one-dimensional cases based on the experimental results published. The accuracy of the non-linear constitutive relations is evaluated by comparing the theoretical values with experimental results of a Terfenol-D rod operated under both compressive pre-stress and bias magnetic field. Results indicate that the SS constitutive equations can accurately predict the experimental results under a low or moderate magnetic field while the HT model can, to some extent, reflect the trend of saturation of magnetostrictive strain under a high magnetic field. The model based on DDS, which is more effective in simulating the experimental curves, can capture the main characteristics of the mechanism of magnetoelastic coupling deformation of a Terfenol-D rod, such as the notable dependence of magnetoelastic response on external stress and the saturation of magnetostrictive strain under intense magnetic fields. In addition, the SS constitutive relation for a general three-dimensional problem is discussed and an approach to characterize the modulus tensors is proposed.

Fabrication and magnetic properties of ultrathin Fe nanowire arrays

Abstract: Ultrathin Fe nanowire (about 5 nm in diameter) arrays have been fabricated by electrodeposition using anodic porous alumina templates. These ultrathin nanowires exhibited uniaxial anisotropy and a quite large coercivity (4190 Oe) at 5 K. In addition, the field needed to saturate the magnetization, when the field was applied perpendicularly to the easy axis, was much larger than the shape anisotropy field (2πM S ). This saturation field increased with decreasing temperature. We believed that this enhanced saturation field was mainly due to the contribution of the surface spins.

Magnetoplastic effect: Basic properties and physical mechanisms

Abstract: This paper presents a review of the main results of investigations into the magnetoplastic effect, which manifests itself in motion of dislocations in crystals exposed to magnetic fields. The dependences of the mean free path of dislocations on the induction and direction of the magnetic field, the magnetic treatment time, the temperature, and the type and concentration of impurities are studied for NaCl, LiF, CsI, Zn, Al, and InSb crystals. The threshold magnetic field B c below which the effect is absent, the saturation field B 0 above which the mean free paths of dislocations remain constant with an increase in the magnetic induction B, and the critical frequency v c of rotation of a sample in the magnetic field above which the effect disappears are examined. The quantities B c , B 0, and v c are investigated as functions of the basic physical parameters. It is found that the magnetoplastic effect is highly sensitive to X-ray radiation at low doses and to simultaneous action of an electric field or mechanical loading. The hardening of NaCl(Pb) crystals in the magnetic field is revealed. The theoretical interpretation is proposed for all the findings and dependences observed.

Modeling of a linear PM Machine including magnetic saturation and end effects: maximum force-to-current ratio

Abstract: The use of linear permanent-magnet (PM) actuators increases in a wide variety of applications because of their high force density, robustness, and accuracy. These linear PM motors are often heavily loaded during short intervals of high acceleration, so that magnetic saturation occurs. This paper models saturation and end effects in linear PM motors using magnetic circuit models. The saturating parts of the magnetic circuit are modeled as nonlinear reluctances. Magnetomotive forces represent the currents and the magnets. This paper shows that when saturated, a negative d-axis current increases the force developed by the motor. Although the increase is not large, it is nevertheless useful, because a negative d-axis current also results in a decrease in the amplifier rating. Further, the trajectory for the maximum force-to-current ratio is derived. The correlation between the calculated and the measured force justifies the model.

Optimized pulsed laser deposited barium ferrite thin films with narrow ferromagnetic resonance linewidths

Abstract: Hexagonal M-type barium ferrite (BaM) films have been prepared by pulsed laser deposition. Optimal preparation conditions gave pure single phase films with the narrowest possible ferromagnetic resonance (FMR) linewidths. The films were deposited on c-plane sapphire substrates with a KrF excimer laser at wavelength of 248 nm and fluence of 1.7 J/cm2. The minimum linewidth films were obtained for an oxygen partial pressure of 300 mTorr and a substrate temperature of 910 °C, and were 0.85 μm thick. X-ray diffraction indicated pure single crystal BaM phase with local c-axis deviations less than 0.15°. Vibrating sample magnetometry data gave hysteresis loops with minimal hysteresis and small coercive force, saturation induction 4πMs of 4.2 kG and uniaxial anisotropy field HA of 16.4 kOe. The HA matches the anisotropy for bulk and the 4πMs is about 10% below literature values for the bulk. The FMR spectra were measured from 50 to 75 GHz by shorted wave guide techniques with the static field perpendicular to the...

Origin of Low Coercivity of Fe-(Al, Ga)-(P, C, B, Si, Ge) Bulk Glassy Alloys

Abstract: The magnetic properties of the glassy Fe-(Al, Ga)-(P, C, B, Si, Ge) alloys have been compared with those of the conventional Fe-based amorphous alloys to clarify the feature of the glassy alloys as a soft magnetic material. The glassy Fe-(Al, Ga)-(P, C, B, Si, Ge) alloys exhibit lower saturation magnetization (Js) than that of the conventional Fe-(B, Si, C) amorphous alloys with the same Fe content. The glassy alloys also have larger saturation magnetostriction constant (� s) than that of the conventional Fe-based amorphous alloys with the same Js. However, the

An integrated study of the grain‐size‐dependent magnetic mineralogy of the Chinese loess/paleosol and its environmental significance

Abstract: [1] To investigate the grain-size-dependent properties of the magnetic minerals in Chinese loess/paleosol samples (Touxiangdao, Xining, Qinghai province, China), magnetic extracts were divided into two size fractions by gravitational settling. On the basis of hysteresis measurements, thermal demagnetization of low-temperature saturation isothermal remanent magnetization, and nonmagnetic studies (SEM and XRD) we identified magnetic phases both in the grain size fractions of the magnetic extracts and in the less magnetic residues to provide more accurate and complete descriptions of all the magnetic components in the bulk natural samples. The results show that the oxidation degree (nonstoichiometry) of magnetic minerals is strongly affected by both grain size and the paleoclimatic environment in which they were deposited and altered. In ascending order of the oxidation degree of our samples, we find (1) loess-coarse particles (LC) are multidomain (MD) magnetite with slight oxidation, (2) paleosol-coarse (PC) particles are also MD magnetite but with a higher oxidation degree compared to LC, (3) loess-fine (LF) particles are pseudo-single domain (PSD) magnetite with a high oxidation degree, and (4) paleosol-fine (PF) particles are PSD maghemite. Single domain (SD) and superparamagnetic (SP) maghemite mainly stay in the residues. Further thermomagnetic analysis of PF (PSD maghemite) revealed that this natural maghemite has a Curie temperature identical to that of magnetite and that the conversion efficiency of transformation from maghemite to hematite is only about 50% after a 700°C heating/cooling cycle. These new results identify the sources of multicomponent NRM in Chinese loess sequences as well as clarify the paleoenviromental and paleoclimatic controls on the remanence components.

Some physical and magnetic properties of Mg‐Zn ferrite

Abstract: Some physical properties (such as lattice parameter, density and porosity) and magnetic properties of the system Mg 1-x Zn x Fe 2 O 4 ; where x=0,0.1,0.2,0.3,0.4,0.5and 0.6 have been studied. It was found that the lattice parameter increases with increasing the zinc concentration. The composition dependence of the physical properties is divided into two regions. The first one is for x ≤ 0.3 and the second one is for x > 0.3. From the magnetization measurements, the basic composition (MgFe 2 O 4 ) shows the lowest magnetization, while the composition of x=0.4 shows the highest one. The behaviour of magnetization M versus composition shows also two regions for x 0.3. The behaviour of M versus x was discussed in the bases of cation distribution. From the B-H loops, the remanence induction B r , saturation induction B s and the coercive force H s were determined and studied with x. The Curie temperature T c was determined from the measurements of the initial permeability μ i versus temperature. It was found T c decreases with increasing Zn-content. Also paramagnetic temperature T was determined from the behaviour of M s vs. T. In general it was found T p > T c by about 7-10 K.

Metastable states during magnetization reversal in square permalloy rings.

Abstract: The magnetic reversal process in a two-dimensional array of permalloy square rings is presented. Rings of thickness of 25 nm, of lateral size of 2.1 \ensuremath{\mu}m, and with ring width of 240 nm were microfabricated using electron-beam lithography and lift-off techniques. Analysis of the diffracted magneto-optical Kerr effect hysteresis loops, magnetic force microscopy images, and micromagnetic simulations show that the magnetization reversal path depends on the direction of the in-plane applied magnetic field. On reducing the field from saturation, for fields along an edge or a diagonal of the square, the ``onion'' state is the stable state at remanence. In a narrow field range around reversal we find that the reversal occurs via a metastable intermediate state. For fields along the diagonal this intermediate state is a magnetic vortex. When the field is applied along an edge direction the intermediate state is a ``horseshoe'' state.

Low‐temperature magnetization and AC susceptibility of magnetite: effect of thermomagnetic history

Abstract: SUMMARY Saturation isothermal remanent magnetization (SIRM) and AC susceptibility have been measured as a function of temperature between 5 K and room temperature for one multidomain and three pseudo-single-domain magnetite samples after cooling in a zero (ZFC) and in a strong magnetic field (FC), and also after three partial field coolings (PFC) when a magnetic field had been turned on in 300–150, 150–80 and 80–5 K ranges, respectively. For the multidomain sample, SIRM(5 K) after ZFC is about twice as high as after FC, while the low-field susceptibility is higher after FC. SIRM and susceptibility curves measured after PFC(300–150 K) and PFC(150–80 K) coincide with those measured, respectively, after ZFC and FC. PFC(80–5 K) curves are intermediate between the two extremes. This behaviour can be fairly well understood within the framework of a simple model, introduced back in the 1950s, which assumes that on cooling through the Verwey transition in a strong magnetic field easy magnetization axes in the low-temperature phase are set along the [001] directions of the high-temperature (cubic) phase closest to the field direction. If, on the other hand, the Verwey transition is passed in a zero field and a strong magnetic field is applied below the transition temperature, some of easy axes, initially set at random, can still be switched into the field direction, explaining observed SIRM and susceptibility versus temperature curves measured after PFC(80–5 K). Pseudo-single-domain grains show a more complex behaviour, which depends strongly on sample stoichiometry. In two samples with a relatively small non-stoichiometry (Verwey temperatures are 122 and 110 K, respectively) SIRM(5 K) is higher by 5–7 per cent after FC. After both PFC(150–80 K) and PFC(80–5 K), SIRMs are nearly equal in magnitude to SIRM acquired after FC, but are thermally demagnetized at a different rate below the Verwey transition. Low-field susceptibilities also show different temperature dependences below TV, dependent on a preceding thermomagnetic treatment. A strongly non-stoichiometric sample (TV= 95 K) shows very large, over 70 per cent, difference between SIRMs after FC and ZFC, respectively, and a 35 per cent difference between susceptibilities measured under the same conditions. These results suggest that in magnetite grains several microns in size, easy magnetization axes setting on first passing the Verwey transition from above and switching of easy axes on subsequent cooling below the transition occur, qualitatively, in the same way as in grains of larger size, but switching of easy axes is considerably facilitated. The latter process is subject to further enhancement in strongly non-stoichiometric magnetite, where it seems to be possible even in zero field, resulting in SIRM, susceptibility and magnetic hysteresis properties fairly different from more stoichiometric samples. Strongly depressed, compared with stoichiometric magnetite, magnetocrystalline anisotropy of the low-temperature phase could be a physical mechanism for this behaviour.

Range of magnetic correlations in nanocrystalline soft magnets.

Abstract: We have obtained the magnetic field dependence of static ferromagnetic correlations in nanocrystalline electrodeposited Co and Ni by means of the correlation function of the spin misalignment, determined from small-angle neutron scattering data. The approach yields a correlation length ${l}_{C}$, which is a measure for the spatial extent of inhomogeneities in the magnetization distribution. The correlation length depends strongly on the applied magnetic field with values ranging from 94 nm in nanocrystalline Co at low fields to about 15 nm at saturation. The results for ${l}_{C}$ indicate that in Co the main source of nonuniformity in the spin system is the anisotropy field of each individual crystallite, whereas in nanocrystalline Ni the main sources of spin disorder originate from twin faults or from the defect cores of grain boundaries.

Effect of radio-frequency noise suppression on the coplanar transmission line using soft magnetic thin films

Abstract: We demonstrated the radio-frequency (rf) noise suppressor using soft magnetic films on a coplanar transmission line from 0.1 to 20 GHz. The coplanar transmission line is composed of magnetic film/polyimide/Cu transmission line/seed layer (Cu/Ti)/glass substrate with the dimension of 50 μm width of the signal line and 3 μm thickness (characteristic impedance: 50 Ω). The magnetic films (CoPdAlO, CoZrO, and CoNbZr) as a noise suppressor are prepared by rf sputtering. The saturation magnetization of each magnetic film is about 10 kG. The magnetic anisotropy field and the ferromagnetic resonance frequency are 230, 89, and 6 Oe and 4.2, 2.5, and 0.7 GHz, respectively. The power loss of the coplanar line with magnetic films is significantly larger than without magnetic and nonmagnetic films due to ferromagnetic resonance losses.

Optical properties of paramagnetic ion-doped semiconductor nanocrystals

Abstract: We present a theoretical study of the optical properties of quantum dots containing a single paramagnetic ion. The eigenvalue problem of an electron-hole pair in interaction with a localized spin is solved. In Mn-doped nanocrystals of II-VI semiconductors such as CdTe the fundamental absorption line splits into six components, whose relative intensities strongly depend on the Mn spin orientation with respect to the polarization of light, suggesting the possibility of optical detection of spin. With the Mn atom at the center of the quantum dot the overall zero-field splitting is typically an order of magnitude larger than the saturation Zeeman splitting in the bulk diluted magnetic semiconductor with the same average concentration of Mn. The effects of an applied magnetic field are also investigated and recently reported magnetic circular dichroism data in high-quality ZnSe:Mn nanocrystals is discussed.

Magnetic Study of Weakly Contaminated Forest Soils

Abstract: This paper reports on magnetic and magnetomineralogical studies of forest soils from Krkonose (Giant Mountains) National Park in the Czech Republic. Low-field magnetic susceptibility was measured in 32 soil profiles using a field probe. Thermomagnetic analysis, acquisition of remanent magnetization, alternating-field demagnetization of saturation remanence and frequency-dependent magnetic susceptibility were measured in laboratory samples from individual soil horizons as well as on their magnetic extracts. X-ray diffraction and SEM were used to identify ferrimagnetic fractions. The uppermost layer, which is dominated by magnetically soft magnetite of presumably anthropogenic origin, can be reliably identified in soil profiles over the whole region of concern. Subsoil horizons are characterised by significantly different magnetic properties.

Perpendicular interlayer coupling in Ni80Fe20/NiO/Co trilayers.

Abstract: An in-plane perpendicular magnetic coupling between Ni80Fe20 and Co has been found in NiFe/NiO/Co trilayers for a NiO thickness ranging from 4 to 25 nm by magneto-optical Kerr effect and x-ray magnetic circular dichroism measurements. In the easy magnetization direction of the Co layer, the Co coercive field H(C) increases when the thickness of the NiO layer t(NiO) increases. Because of the coupling, H(C) is always larger than for NiO/Co bilayers with the same thicknesses. The saturation field of the NiFe layer H(S) decreases when t(NiO) increases, indicating a weakening of the coupling. Numerical simulations show that the presence of interface roughness combined with a small value of the NiO anisotropy can explain the observed 90 degrees coupling.

Spontaneous Generation of the Magnetic Field and Suppression of the Heat Conduction in Cold Fronts

Abstract: We have determined the physical mechanism responsible for plasma instabilities, which were first found by Ramani & Laval in 1978, associated with anisotropic velocity distributions induced by a temperature gradient in which there are growing low-frequency transverse magnetic waves, even in the absence of background magnetic fields. We have shown that the physical mechanism responsible for the growth of one of the modes is identical to the Weibel instability. The nonlinear saturation level of the instability is also provided by considering the wave-particle interactions. The nonlinear evolution of the magnetic fields after the saturation is speculated on. The results are applied to the cold fronts newly discovered by the Chandra X-Ray Observatory in clusters of galaxies. We predict the existence of a magnetic field of ~10 μG tangential to the surface over the entire region of the cold front and that the heat conduction is significantly suppressed by the trapping of electrons by the generated magnetic fields. The instability may provide a new possibility for the origin of the cosmic magnetic field.

In-plane and out-of-plane transverse susceptibility in close-packed arrays of monodisperse Fe nanoparticles

Abstract: The transverse susceptibility (TS) of arrays of self-assembled Fe nanoparticles has been studied using a sensitive radio-frequency resonant technique. Symmetrically located broad peaks in the TS data are observed below the blocking temperature as the applied field is swept from positive to negative saturation. These peaks occur at the effective anisotropy fields $(\ifmmode\pm\else\textpm\fi{}{H}_{K})$ with the peak width determined by the distribution in ${H}_{K}$ in the nanoparticle array system. These features are observed to be strongly affected by dipolar interactions as well as thermal relaxation. Systematically tracking the evolution of the TS curves across the superparamagnetic transition reveals distinct temperature ranges over which thermal activation and dipolar energy overcome the effective magnetic anisotropy energy. Hysteresis loops measured using a superconducting quantum interference device magnetometer indicate a smaller coercive field for in-plane field orientation compared to that for out-of-plane orientation. This is also reflected in the TS measurements. A comparison of the TS over a wide range in temperature and magnetic fields, applied in plane and out of plane, reveals the distinct influence of variation in dipolar interaction strengths for the two geometries.