Coercivity Ratio Research Articles

Effect of Phosphate Treatment on the Corrosion Resistance of Nd–Fe–B Anisotropic Magnetic Powder

Anisotropic Nd–Fe–B hydrogenation disproportionation desorption recombination powder with high corrosion resistance was prepared by phosphoric acid treatment. The treated samples baked at temperatures higher than 250 °C exhibited high corrosion resistance. In addition, the change ratio of coercivity before and after the anticorrosion test was within 5%. Microstructural observations indicated the formation a uniform phosphate layer with a small amount of Nd oxide at high baking temperature.

Excellent magnetic properties in multiferroic BiFeO3 based thin films for magnetic devices application

The operation of magnetic devices with a magnetic field or spin current causes high energy consumption. Very low energy consumption can be achieved by using multiferroic thin films, where we can control the magnetization direction by applying an electric field. Good magnetic properties in multiferroic thin films are needed for these magnetic device applications. However, multiferroic thin films generally do not have such good magnetic characteristics, like high saturation magnetization (Ms), perpendicular magnetic anisotropy (PMA, the ratio of coercivity between the alignment of film plane perpendicular and parallel to the applied field (Hc⊥/Hc//)), magneto-optical Kerr rotation angle (Θk) and Curie temperature (Tc). In this research, we have looked into improving magnetic characteristics such as Ms, Hc⊥/Hc//, Θk and Tc in (Bi,L)(Fe,Co)O3(L = Lanthanides) thin films by substituting different lanthanides with cobalt. The highest Ms value of 140 emu/cm3 and large Hc⊥/Hc// ratio of 2.6 were found in (Bi,Nd)(Fe,Co)O3 thin film, and maximum Θk of 0.67° and relatively high Hc⊥/Hc// of 1.6 were found for the (Bi,La)(Fe,Co)O3 thin film. These properties are suitable for magnetic recording devices and optical devices, respectively. A very small coercivity of 0.8 kOe, along with a high Ms value of 110 emu/cm3 and Tc of 450 °C, were observed in the (Bi,Eu)(Fe,Co)O3 thin film. So, the (Bi,Eu)(Fe,Co)O3 thin film is suitable for magnetic sensor applications because of its high sensitivity to magnetic fields. Moreover, the domain size of the above-mentioned thin films was observed in the small nano range, which is suitable for magnetic nano device applications.

Greigite (Fe3S4) Formation in Artificial Sediments via Solid‐State Transformation of Lepidocrocite

AbstractGreigite (Fe3S4) is a ferrimagnetic iron‐sulfide mineral that forms in sediments during diagenesis. Greigite growth can occur diachronously within a stratigraphic profile, complicating or overprinting environmental and paleomagnetic records. An important objective for paleo‐ and rock‐magnetic studies is to identify the presence of greigite and to discern its formation conditions. Greigite detection remains, however, challenging and its magnetic properties obscure due to the lack of pure, stable material of well‐defined grain size. To overcome these limitations, we report a new method to selectively transform lepidocrocite to greigite via the intermediate phase mackinawite (FeS). In‐situ magnetic characterization was performed on discrete samples with different sediment substrates. Susceptibility and chemical remanent magnetization increased proportionally over time, defining two distinct greigite growth regimes. Temperature dependent and constant initial growth rates indicate a solid‐state FeS to greigite transformation with an activation energy of 78–90 kJ/mol. Low and room temperature magnetic remanence and coercivity ratios match with calculated mixing curves for superparamagnetic (SP) and single domain (SD) greigite and suggest ∼25% and ∼50% SD proportions at 300 and 100 K, respectively. The mixing trend coincides with empirical data reported for natural greigite‐bearing sediments, suggesting a common SP endmember size of 5–10 nm that is likely inherited from mackinawite crystallites. The average particle size of 20–50 nm determined by X‐ray powder diffraction and electron microscopy accords with theoretical predictions of the SP/SD threshold size in greigite. The method constitutes a novel approach to synthesize greigite and to investigate its formation in sediments.

Rock magnetic evidence for a middle Holocene transition in marine sediments from La Paz basin, southern Gulf of California

Results of a rock magnetic study of a marine sediment core from La Paz Basin are used to investigate the Holocene paleoclimate in the Gulf of California. Chronological control based on AMS radiocarbon dating shows that the sequence covers the past ~7650 cal yr BP. The coercivity, magnetization intensity, susceptibility and magnetic hysteresis loops point to dominant fine-grained magnetite and titanomagnetites as main magnetic carriers. In the magnetic hysteresis ratio plot, magnetization and coercivity ratios fall in the pseudo-single domain field. High magnetic susceptibilities and magnetization intensities characterize the upper core section, with lower values downcore on a low amplitude oscillation trend. The late Holocene is marked by low susceptibilities, frequency-dependent, paramagnetic slope factors and variable hysteresis parameters with low Mr/Ms magnetization ratios, low Bc and Bcr coercivities and high Bcr/Bc ratios. The lower core section in the ~7650-5000 cal yr BP interval shows increased susceptibilities as compared to the upper sections with lower values. The magnetic hysteresis logs for the interval around 5000-4000 cal yr BP show some samples with distinct lower magnetization (Mr/Ms) and higher coercivity (Bcr/Bc) ratios, which characterize the interval from the downcore variation trend. The pattern in the magnetic properties is interpreted in terms of a change in precipitation from high to low humidity from the middle to the late Holocene. The sediment section records a middle Holocene climate transition in the southern Gulf ~5000-4000 cal yr BP, supporting a northward migration of climate patterns along the western North America margin, from the southern Gulf of California to Alaska.

Technogenic magnetic particles from steel metallurgy and iron mining in topsoil: Indicative characteristic by magnetic parameters and Mössbauer spectra

Technogenic magnetic particles (TMPs), produced during various industrial processes, are released into the atmosphere as dust and get deposited on the surrounding topsoil. The mineralogical and structural differences of TMPs produced in different technological processes should be reflected in their magnetic properties and therefore should be indicative for industrial pollution sources. The goal of this study was to characterize the TMPs by novel methodological approach, based on combination of magnetic methods and Mössbauer spectroscopy to indicate parameters that are discriminative enough to be used as environmental indicators for iron metallurgy, steel production, and iron mining. We collected the topsoil samples in the vicinity of 4 European iron- and steelworks, located in three different countries (Poland, Norway, and Czech Republic) and operating for minimum 40 years. We sampled also topsoil close to the opencast iron mine, iron ore dressing plant, and over strongly magnetic natural background. Analysis of the hyperfine parameters of the Mössbauer spectra revealed that TMPs are “magnetite-like” minerals with low stoichiometry. It is indicated by ratio of iron ions contributions in B sites (octahedral) and A sites (tetrahedral) in magnetite spinel structure, which is much lower than 2.0 (theoretical value for stoichiometric magnetite). The characteristic feature of TMPs collected from the vicinity of old metallurgical plants (>180 years) was the high contribution of surface components probably related to the surface oxidation/maghemitization. We found that, TMPs can be easily differentiated from geogenic magnetite based on their magnetic parameters. The TMP produced by the iron and steel metallurgy had relatively narrow ranges of magnetic parameters (saturation ratio Mrs/Ms, <0.15, coercivity ratio Bcr/Bc 2.5–6.0 and saturation to susceptibility ratio Mrs/χ 3.5–15). These magnetic parameters may be indicative for TMPs emitted by these pollution sources and helpful in the study of historical pollution sources in topsoil in urban and post-industrial areas.

Controllable fabrication and magnetic properties of Nd/Co core/shell nanowires

Ordered Nd/Co100 and Nd/Co200 core/shell nanowire arrays with lengths of ~ 10 μm and diameters of 100 nm and 200 nm were fabricated by two-step electrodeposition into anodic alumina oxide templates. Co-hcp phases were observed in the Nd/Co100 and Nd/Co200 nanowires. Hysteresis curves obtained from the Nd/Co100 and Nd/Co200 nanowires showed that the squareness ratio of the parallel nanowire long axis was greater than that of the perpendicular nanowire long axis. The magnetic properties indicated that the easy axis was parallel to the long axis. The coercivity and parallel squareness ratio in the Nd/Co200 nanowires were greater than those in the Nd/Co100 nanowires, which demonstrated that the Nd/Co200 nanowires exhibited hard magnetism behavior.

Studies on the B-H Loop Properties of Microwave Processed Li-Zn-Cd Ferrites Nanocrystallites

Lithium zinc cadmium ferrite nanocrystallites having the compositional formula Li1+0.4-x/2Zn2+0.2Cd2+xFe3+2.4-x/2O2−4 where x = 0.02, 0.03 and 0.04 were prepared by microwave process heating at 1050ºC for 25 min. X Ray Diffraction (XRD) technique confirmed the formation of single phase with spinel structure in all the samples and compared with samples of same composition prepared by conventional process heating at 1050ºC for 6 h. Scanning Electron Microscopy (SEM) photomicrograph revealed the microstructures of the samples and showed finer microstructures in the microwave processed samples. The B-H loop traced from the samples showed significant improvements in the various hysteresis parameters such as coercivity and remanance ratio for the microwave processed samples. The Curie temperature and saturation magnetization also increased for these samples. The microstructural properties such as grain, grain boundaries, pores, and their distribution due to microwave processing contributed to the observed hysteresis properties. The results of the various measurements are discussed in the paper.

Structural, magnetic, and fluorescence properties of Nd/Ni/PA66 multilayer coaxial nanocables

Abstract One-dimensional (1D) Nd/Ni/PA66 multilayer nanocables were prepared in anodic aluminum oxide (AAO) template from outer layer to inner layer with physical wetting method and two-step electrochemical deposition method, respectively. The length of each layer is almost the same. SEM, TEM, XRD were employed to investigate the morphology and structure and chemical compositions. Magnetization measurements revealed that the Nd/Ni/PA66 nanocable arrays have greater coercivity and remanence ratio than the Ni and Ni/PA66 nanotubes and exhibited hard magnetism behavior. The easy axis was parallel to the long axis of nanocables. The interaction between each layer and the synergistic effect between iron and neodymium were explained in detail. In addition, the nanocables showed fluorescence properties due to the presence of Nd, and obvious emission peak at 401–472 nm was detected. We believe that these methods may provide an idea for ferromagnetic alloys and permanent magnet material and suitable for future applications in perpendicular recording media.

Rock magnetic and paleomagnetic investigations of Sylhet traps, Shillong Plateau, NE India

Magnetic signals in basalts from the Sylhet traps (ST), Shillong Plateau, India is investigated using rock magnetism as a proxy to explain the potentiality of the ST basalts as an indicator of magnetic mineralogy. A paleomagnetic study was carried out in order to unravel the Paleopole. In detail, rock magnetic and Anisotropy of Magnetic susceptibility (AMS) results have been reported for the first time on the ST basalts. A total of 38 oriented block samples were collected from 8 sites along Cherrapunji-Shella bazaar road section, Meghalaya, India. Samples were subjected to rock magnetic measurements such as magnetic susceptibility (K), Isothermal Remanent Magnetization (IRM), hysteresis loops, backfield (coercivity remanence) and k-T (Susceptibility vs. Temperature) analysis to identify the magnetic properties of basalts. Advanced Variable Field Translation Balance (AVFTB) was used for some rock magnetic studies. A few samples were also subjected to Anisotropy of Magnetic susceptibility (AMS) study to understand the petrofabric orientations and shape anisotropy in the ST basalts. The Remanence ratio (Mrs/Ms) and the Coercivity ratio (Bcr/Bc) ranging from 0.105 to 0.333 and from 1.25 to 1.87 respectively indicate sizes of magnetic grains are mainly in the range of Pseudo Single Domain (PSD) limits with minority of them in the ranges of Single Domain (SD) limits. The results of k-T study in combination with other rock magnetic results shows that magnetite (Fe3O4) [Curie Temperature = 565-590 °C] as the dominant magnetic carrier with a subordinate content of titanomagnetite Fe2+(Fe3+Ti)2O4 [Curie Temperature = 380–450 °C]. From the AMS study, parameters like lineation, foliation are calculated, and results define all the three shape anisotropies present in the ST basalts. The ST basalts display paleomagnetic directions with mean declination = 255°, mean inclination = -57° (α95 = 3.44°; k = 16.74). The pole position of the ST at 117 Ma is -26.79 °N, 330.75 °E (dp = 3.6; dm = 5) with a paleolatitude 37.6 °S. This infers that the ST is a later volcanic event than Rajmahal traps (RT) which is considered contemporary to the ST.

Characterization of Nd(Tb)–Fe–B-based exchange-spring nanocomposite magnets

The authors attempt to optimize for performance the composition of R 2Fe14B/Fe3B-based hard nanocomposite alloys with R = neodymium (Nd) and terbium (Tb) by optimizing phases in order to develop hysteresis parameters such as maximum magnetization, coercivity and maximum energy product through variation in composition and heat treatment. The samples were prepared from arc-melted ingots of different compositions. The crystallization temperatures were obtained from the differential scanning calorimeter traces of the samples with the composition Nd4−x Tb x Fe71Co5Cu0·5Nb1B18·5 (x = 0·0, 0·2, 0·4, 0·6, 0·8 and 1·0). Depending on their crystallization temperatures, the samples were annealed at 600, 625, 650, 675 and 700°C for 10 min. Amorphosity and crystallization behavior were studied by X-ray diffraction using copper (Cu) Kα radiation (1·5418 Å). The Mössbauer spectra of all the samples were recorded at room temperature by using iron-57 (57Fe) Mössbauer spectrometry in transmission geometry with a constant-acceleration spectrometer. Magnetic properties were determined by a vibrating-sample magnetometer in the as-cast and annealed conditions. Although the highest values of coercivity and remanent ratio were found to be 4·18 kOe and 0·72, respectively, for the sample of composition Nd3Tb1Fe71Co5Cu0·5Nb1B18·5 annealed at 650°C with a higher concentration of terbium, the highest maximum energy product was determined to be 12·67 MGOe for the sample of Nd3·8Tb0·2Fe71Co5Cu0·5Nb1B18·5.

Phase structure of Al doped Ce-rich alloys and its effect on magnetic properties of sintered Ce-Fe-B magnets

In this article, the phase constitution, microstructures and melting points of Ce35.58Fe63.47-xAlxB0.95 alloys (at.%; x = 0, 3, 5, 10) have been investigated. It indicated that the Ce-rich alloy with x = 0 was mainly composed of CeFe2, Ce2Fe17 and γ-Ce phases. The AlCe3 phase was found in the alloy with x = 3. The microstructures and phase distribution characteristics of the alloys have obviously changed with increasing Al content (x = 5,10). The volume fractions of CeFe2 and Ce2Fe17 decreased. Some dispersed Ce-Al eutectic structures with low melting point were found in the alloy with x = 10. It is showed that the melting points of Ce-rich alloys have a decreasing tendency with increasing Al content, the melting points reduced from 942 °C at x = 0–856 °C at x = 10. Furthermore, the influences of the Ce35.58Fe53.47Al10B0.95 alloy addition on the microstructures and magnetic properties of sintered Ce-Fe-B magnets, were also discussed. The remanence Br slightly decreased, while the intrinsic coercivity Hcj, maximum energy product (BH)m and squareness Hk/Hcj (the ratio of knee coercivity to intrinsic coercivity) of the magnets had significant improvements, and these indexes increased by 8%, 22%, 24%, respectively.

Inverse Magnetoresistance Characteristic of Hybrid-Type Multilayer Structure of IrMn-Based Giant-Magnetoresistance Spin Valve and High-Tc Superconductor YBa2Cu3O7−x Film

An antiferromagnetic-IrMn-based giant-magnetoresistance spin-valve (GMR-SV) multilayer on a high-temperature superconductor YBa2Cu3O7−x (YBCO) film was prepared using an ion-beam deposition system. The exchange bias coupling field (Hex), coercivity (HC), and magnetoresistance ratio (MR (%)) of the hybrid YBCO/GMR-SV multilayer structure were measured and compared at room temperature and 77 K. The MR ratio (%) of the hybrid YBCO/GMR-SV structure measured by the four-probe method at 77 K was − 9.3%, different from that of the conventional GMR-SV structure. The phenomenon of a negative MR (%) below the critical temperature (Tc) can be explained by the current-in-plane tunneling effect, when the resistance of the middle gap (G) layer between the high-temperature superconductor YBCO and GMR-SV multilayer reaches a value comparable to the plane resistance of the upper metal layer. In order to analyze the inversed MR (%) of the hybrid YBCO/GMR-SV multilayer structure, the equivalent circuit for each layer was constructed and an equation of the output voltage was derived.

Preparation and characterization of dip coated cobalt oxide thin films

In this work, cobalt oxide thin films were prepared by sol–gel dip coating technique by varying the withdrawal speed of the substrate in the sol and their optical properties were analyzed, using the UV–vis spectrophotometer. The optical constants (n and k) for these films, in the 300–900 nm range and band gap of thin films are evaluated. FTIR confirmed formation of Co3O4 thin films. X-ray diffraction showed polycrystalline films consisting of the Co3O4 phase. Magnetic properties, such as saturation magnetization, remanence, coercivity and squareness ratio were evaluated from the hysteresis loop. Saturation magnetization was in the range of 16·26–114·30 emu/cm3 and coercivity range was from 143·73–6·03 Oe.

Phase formation and magnetic property evolution processes of the hexaferrite with composition BaO⋅0.9Sc2O3⋅5.1Fe2O3

The phase formation and magnetic property evolution processes of the hexaferrite with composition BaO⋅0.9Sc2O3⋅5.1Fe2O3 have been investigated. Results show that when the calcination temperature is lower than 1000 °C, the spinel phase BaFe2O4 and M-type hexaferrite phase BaFe12O19 dominate. The M-type hexaferrite BaFe12−xScxO19 (0 < x ≤ 1.8) appears above 1050 °C and becomes a single phase BaFe10.2Sc1.8O19 above 1200 °C. A two-step decrease of both the coercivity and remanence ratio is observed above 1050 °C, which agrees well with the appearance of soft magnetic phase BaFe12−xScxO19 (0 < x ≤ 1.8). The saturation magnetization of the sample increases with calcination temperature until 1100 °C and then decreases. Raman spectra results show that the above magnetic property evolutions can be explained by a temperature dependent incorporation of Sc3+ into the lattice sites nearby the magnetic blocks’ interfaces. This weakens the local magnetic exchange interactions between Fe3+ and thus leads to a change in the magnetic structure.

Promotion of L10 Ordered Phase Transformation by the Si3N4 Top Layer on FePd Thin Films

Structure and magnetic properties of FePd thin films substrate on the glass with Si3N4 to player have been investigated. The results suggested that ceramic Si3N4 component distributed around the FePd nanograins, which inhibits the growth of the FePd particles and refined grain size of FePd films. Compared with the pure FePd thin films the coercivity and remanence ratio of FePd/Si3N4 films reached to the maximum value of 3.3 and 0.98 kOe, respectively. With further increase of the annealing time, magnetic properties of FePd/Si3N4films do not drop rapidly, but keep high magnetic properties at a relatively wide temperature range. Defects, inclusions and intermixing phenomena occurred between the FePd layers and the Si3N4 top layers promoted the improvement of L10 ordered phase transformation.

Low-temperature magnetic properties of monoclinic pyrrhotite with particular relevance to the Besnus transition

Monoclinic pyrrhotite (Fe7S8) owes its ferrimagnetism to an ordered array of Fe vacancies. Its magnetic properties change markedly around 30 K, in what is known as the Besnus transition. Plausible explanations for the Besnus transition are either due to changes in crystalline anisotropy from a transformation in crystal symmetry or from the establishment of a two-phase system with magnetic interaction between the two phases. To help resolve this discrepancy, we measured hysteresis loops every 5° and backfield curves every 10° in the basal plane of an oriented single crystal of monoclinic pyrrhotite at 300 K and every 2 K from 50 K through the Besnus transition until 20 K. Between 50 and 30 K, hysteresis loops possess double inflections between crystallographic a-axes and only a single inflection parallel to the a-axes. Magnetization energy calculations and relative differences of the loops show a sixfold symmetry in this temperature range. We propose that the inflections stem from magnetic axis switching, which is both field and temperature dependent, in a manner somewhat analogous to an isotropic point where magnetocrystalline constants change their sign. The Besnus transition is best characterized by changes in magnetic remanence and coercivity over a 6° temperature span (28–34 K) with a maximum rate of change at 30 K. A surprising yet puzzling finding is that the coercivity ratio becomes less than unity below the transition when fourfold symmetry arises. Because the changes in magnetic parameters are linked to the crystal structure, we conclude the Besnus transition owes its origin to a distortion of the crystallographic axes below 30 K rather than an apparition of a two-phase system. An isothermal magnetization of natural pyrrhotite cycled from room temperature to successively lower temperatures through the Besnus transition decreases 2–4 times less than equivalent grain sizes of magnetite, with less than a 10 per cent loss in remanence between 300 and 150 K in pseudo-single-domain (PSD) pyrrhotite. As PSD monoclinic pyrrhotite carries the magnetic remanence in some meteorites, it is likely that low-temperature cycling in space then warming to ambient conditions at the Earth's surface will have only a minor influence on palaeointensity values derived from those meteorites.

Magnetometry as a Tool to Estimate the Pollution of Marine Environment Around Small Shipwrecks (Gulf of Gdańsk) — Preliminary Results

Magnetic properties of deposits around the shipwrecks (“Munin” and “Abille”) in the Gulf of Gdansk were investigated. Values of magnetic susceptibility (χ) are relatively low; however, they reveal significant differences between investigated sites. The values of χ around “Abille” wreck were 5-8×10-8 m3/kg. Around “Munin” wreck results were more diversified reaching value of χ between 3.07×10-8m3/kg and 12.92×10-8 m3/kg. The spatial variability of χ coincided with near-bottom water currents distribution in the Gulf of Gdansk. Magnetic minerals were identified by thermomagnetic analysis. Around “Abille” wreck we have found magnetite with small amount of maghemite or hematite. The “Munin” sediments include only one magnetic phase; in several samples it is magnetite, in the others — maghemite. Day—Dunlop plot shows that “Abille” set is shifted towards lower magnetization ratio and higher coercivity ratio. The correlation between the distribution of χ and hydrodynamic condition around shipwrecks allows to determine the direction of contaminant transport.

Manipulation of Magnetic Properties of Cr-Substituted Ni Ferrite Synthesized by Conventional Ceramic Technique

A series of polycrystalline Ni–Cr ferrites, with the composition NiCr x Fe 2−x O 4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0), were synthesized by conventional solid state ceramic method. The effect of Cr substitution on the structural, morphological, and magnetic properties of Ni ferrite has been investigated. Formation of single-phase cubic spinel structure of the compositions has been confirmed by X-ray diffraction. The morphological study was performed by scanning electron microscopy, and the average grain size was found to decrease with increasing Cr content. The magnetic properties of the samples were measured using an impedance analyzer and a vibrating sample magnetometer. Saturation magnetization, coercivity, and remanent magnetization have been measured from the hysteresis curves. The value of the saturation magnetization was found to decrease with increasing Cr concentration. The values of coercivity, remanent magnetization, and remanence ratio (M r/M s) ratio were also reported. The Curie temperatures (T c) were determined from the plots of the real part of permeability ( $\mu _{\mathrm {i}}^{/}$ ) versus temperature, and the values were found to be 511, 465, 415, 364, and 338 ∘C for x = 0.0, 0.2, 0.4, 0.6, and 0.8, respectively. The frequency dependence of the $\mu _{\mathrm {i}}^{/}$ , loss factor, and quality factor has been studied as a function of composition in the frequency range 1 kHz–120 MHz. The real part of permeability was observed to be steady up to ∼10 MHz for all the samples. The loss factor (tan δ) of the samples decreased rapidly before 40 kHz and became almost constant with low values after 40 kHz. The values of quality factor have gradually been increased for samples with x ≤ 0.4, whilst the values diminished for samples with x ≥ 0.6.

Experimental mixtures of superparamagnetic and single‐domain magnetite with respect to Day‐Dunlop plots

AbstractDay‐Dunlop plots are widely used in paleomagnetic and environmental studies as a tool to determine the magnetic domain state of magnetite, i.e., superparamagnetic (SP), stable single‐domain (SD), pseudosingle‐domain (PSD), multidomain (MD), and their mixtures. The few experimental studies that have examined hysteresis properties of SD‐SP mixtures of magnetite found that the ratios of saturation remanent magnetization to saturation magnetization and the coercivity of remanence to coercivity are low, when compared to expected theoretical mixing trends based on Langevin theory. This study reexamines Day‐Dunlop plots using experimentally controlled mixtures of SD and SP magnetite grains. End‐members include magnetotactic bacteria (MSR‐1) as the SD source, and a commercial ferrofluid or magnetotactic bacteria (ΔA12) as the SP source. Each SP‐component was added incrementally to a SD sample. Experimental results from these mixing series show that the magnetization and coercivity ratios are lower than the theoretical prediction for bulk SP magnetic size. Although steric repulsion was present between the particles, we cannot rule out interaction in the ferrofluid for higher concentrations. The SP bacteria are noninteracting as the magnetite was enclosed by an organic bilipid membrane. Our results demonstrate that the magnetization and coercivity ratios of SD‐SP mixtures can lie in the PSD range, and that an unambiguous interpretation of particle size can only be made with information about the magnetic properties of the end‐members.

Study of ball milled nanocrystalline Fe-50%Al alloy: structural and magnetic properties

Structural, morphological and magnetic changes induced by ball milling in Fe–50%Al alloy have been investigated using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer techniques. The MA process induces a progressive dissolution of Al into Fe, resulted in the formation of an extended nanostructured Fe(Al) solid solution with BCC structure only after 5 h of milling. The grain size of constituents was decreased to the nanometre range (between 6 and 8 nm) and the constituents dissolved at the nanograin boundaries. Expansion in lattice parameter accompanied by reduction in crystallite size occurs during transition to nanostructured alloy. The corresponding magnetic measurements show continuous decrease in saturation magnetisation and increase in coercivity, retentivity and squareness ratio with increase in milling time. The systematic variation of the magnetic properties has been correlated with the evolution of microstructure, reduction in grain size and enhanced intergranular exchange coupling.