Drop Of Coercivity Research Articles

The effect of heat treatment parameters on the microstructure and torque response of a 13 wt% Cr steel

Shafts intended for magnetomechanical sensors require a combination of mechanical properties and/or characteristics in addition to good magnetic properties. In the case of X46Cr13, this can be attained by the appropriate selection of heat treatment parameters. In fact this study discusses the effect of these parameters on the microstructure, mechanical and magnetic properties of this material.The B-H curves showed that irrespective of the heat treatment condition, the material was harder to magnetize in the axial direction than circumferentially. Also, quenched and tempered structures were harder to magnetize than those in the annealed condition. Changes in the austenitising temperature or time were not observed to lead to major changes in the B-H curve. On the other hand, increasing the tempering temperature from 350 °C to 500 °C, led to a significant drop in coercivity, attributed to a decrease in M3C precipitates. This change in coercivity was comparable to that observed as the tempering temperature increased from 500 °C to 600 °C, in which case the matrix structure changed from martensite to ferrite.Martensitic structures showed a lower magnetic response to a given applied torque than their annealed counterparts. On the other hand, with martensitic structures, the signal to torque relationship remained linear even at high torque values. This was not the case with the annealed structures.The study concludes that austenitising at 1040 °C for 45 min and tempering at 350 °C and 375 °C for 2 h yields an optimal combination of properties and performance.

Magnetic properties evolution with grain boundary phase transformation and their growth in Nd-Fe-Cu-Ga-B sintered magnet during post-sinter annealing process

The formation of Nd6Fe13Ga phase promoted the continuity and non-ferromagnetism of grain boundary (GB) phases of Nd-Fe-Cu-Ga-B magnets during proper post-sintering annealing process. The coercivity increased dramatically with a decrease of remanence. After further annealing at the sintering temperature, the remanence recovered with a drop of coercivity. It indicated that Nd2Fe14B phase decomposed with the formation of Nd6Fe13Ga phase. Overextending the annealing time led to the gradual decrease of coercivity as the growth of Nd6Fe13Ga phase weakened the continuity of GB phases. Increasing the annealing temperature to the optimum temperature for Nd6Fe13Ga phase can aggravate the decrease of coercivity and remanence. Owing to the intensifying segregation of Ga and Fe in GB phases with the formation of Nd6Fe13Ga phase by dehydrogenation (HD) at 560 °C, high coercivity can be obtained by annealing at a lower temperature or for shorter time compared with HD at 480 °C. The enrichment of Cu in Nd-rich phase ensured its wettability and non-ferromagnetism when the diffusion of Fe and Ga became more local with the formation of Nd6Fe13Ga phase. Therefore, the combined effect of Cu doping and reasonable high temperature HD was conducive to obtain high coercivity in a larger range of annealing time and temperature for Nd-Fe-Ga-B magnet, which was a potential method for its mass production.

A tool to predict coercivity in magnetic materials

Magnetic coercivity is often viewed to be lower in alloys with negligible (or zero) values of the anisotropy constant. However, this explains little about the dramatic drop in coercivity in FeNi alloys at a non-zero anisotropy value. Here, we develop a theoretical and computational tool to investigate the fundamental interplay between material constants that govern coercivity in bulk magnetic alloys. The two distinguishing features of our coercivity tool are that: (a) we introduce a large localized disturbance, such as a spike-like magnetic domain, that provides a nucleation barrier for magnetization reversal; and (b) we account for magneto-elastic energy—however small—in addition to the anisotropy and magnetostatic energy terms. We apply this coercivity tool to show that the interactions between local instabilities and material constants, such as anisotropy and magnetostriction constants, are key factors that govern magnetic coercivity in bulk alloys. Using our model, we show that coercivity is minimum at the permalloy composition (Fe21.5Ni78.5) at which the alloy’s anisotropy constant is not zero. We systematically vary the values of the anisotropy and magnetostriction constants, around the permalloy composition, and identify new combinations of material constants at which coercivity is small. More broadly, our coercivity tool provides a theoretical framework to potentially discover novel magnetic materials with low coercivity.

Sense current dependent coercivity and magnetization relaxation in Gd-Fe-Co Hall bar

The understanding of the characteristics of a magnetic layer in a different environment is crucial for any spintronics application. Before practical applications, thorough scrutiny of such devices is compulsory. Here we study such a potential Hall device of MgO-capped Hf/GdFeCo bilayer (FeCo-rich) for magnetization relaxation around nucleation fields at different voltage probe line widths and dc sensing currents. The device is characterized by anomalous Hall measurements in transverse and longitudinal Hall geometries for two different probe widths A (5 µm) and B (1 µm). The coercivities of the Hall loops (ρxy-H and Rxx-H) drop with increasing the sense current for both the probes. For probe B, the sharp and large drop in coercivity (ρxy-H loops) at comparatively lower sensing currents is observed, which is attributed to the negligible current shunting and presence of pinning site at B caused by the patterning process. The average domain wall velocities at various sensing currents for probe B are found to be smaller than probe A, from the transverse and longitudinal Hall geometry magnetization relaxation measurements, which agrees with pinning sites and Joule heating effect at probe B. The notch position in the pattern and the longitudinal Hall resistance curve peak shape suggest the domain wall propagation direction from probe B to probe A in the current channel. This study highlights the domain wall propagation at different nucleation fields, sensing currents, and the Hall probe aspect ratios.

A Tool to Predict Coercivity in Magnetic Materials

Magnetic coercivity is often viewed to be lower in alloys with negligible (or zero) values of the anisotropy constant. However, this explains little about the dramatic drop in coercivity in FeNi alloys at a nonzero anisotropy value. Here, we develop a theoretical and computational tool to investigate the fundamental interplay between material constants that govern coercivity in bulk magnetic alloys. The two distinguishing features of our coercivity tool are that: (a) we introduce a large localized disturbance, such as a spike-like magnetic domain, that provides a nucleation barrier for magnetization reversal; and (b) we account for magneto-elastic energy---however small---in addition to the anisotropy and magnetostatic energy terms. We apply this coercivity tool to show that the interactions between local instabilities and material constants, such as anisotropy and magnetostriction constants, are key factors that govern magnetic coercivity in bulk alloys. Using our model, we show that coercivity is minimum at the permalloy composition at which the alloy's anisotropy constant is not zero. We systematically vary the values of the anisotropy and magnetostriction constants, around the permalloy composition, and identify new combinations of material constants at which coercivity is small. More broadly, our coercivity tool provides a theoretical framework to potentially discover novel magnetic materials with low coercivity.

Micromagnetic modelling of nanorods array-based L10-FeNi/SmCo5 exchange-coupled composites

Exchange-coupled nanocomposites are considered as the most promising materials for production of high-energy performance permanent magnets, which can exceed neodymium ones in terms of energy product. In this work, micromagnetic simulations of L10-FeNi/SmCo5 composites based on the initially anisotropic structure of nanorods array were performed. Texturing effect on magnetic properties was investigated. It was revealed that even 30% of anisotropy axes misalignment of grains in L10-FeNi phase would lead to only ≈10% drop of coercivity. To maximize magnetic properties of the composites, parameters of microstructure were optimized for 120 × 120 array of interacting nanorods and were found to be 40 nm nanorod diameter and 12–20 nm interrod distance. The estimated diameter of nanorods and the packing density of the array provide energy product values of 149 kJ m−3. Influence of interrod distance on energy product values was explored. Approaches for production of exchange-coupled composites based on anisotropic nanostructures were proposed.

Coercivity and corrosion resistance enhancement of multi-main-phase Nd-Ce-Fe-B sintered magnets by the grain boundary diffusion process using Pr81.5Ga19.5 and Pr81.5Ga14.5Cu5 alloys

In this paper, grain boundary diffusion was performed on multi-main-phase Nd-Ce-Fe-B sintered magnets using two different diffusion sources. The coercivity of the magnet was increased from 928 kA/m to 1036 kA/m and 1174 kA/m by diffusing Pr81.5Ga19.5 (PG) and Pr81.5Ga14.5Cu5 (PGC5), respectively, and the corrosion resistance of the magnet was also improved after the above diffusion process. Two kinds of continuous grain boundaries with different contrast were observed after the PG and PGC5 alloy diffusion process, and the results of line scan and quantitative analysis indicate that they originate from different intergranular phases that have been melted during the heat treatment. The distribution of Ga element after grain boundary diffusion and its effects on the coercivity of the magnet were especially investigated. It was found that the coercivity enhancement of these diffused magnets is mainly attributed to the formation of continuous grain boundary and non-ferromagnetic grain boundary phase to weaken the magnetic exchange coupling between the 2:14:1 grains, as well as the formation of the Pr-rich shell with slightly higher magnetocrystalline anisotropy due to the partial substitution of the Nd atoms by the Pr atoms, which compensates for the coercivity drop by chemical homogenization due to the heat treatment.

Magnetic properties of Co and Ti co-doped strontium hexaferrite prepared by sol–gel method

The Co and Ti co-doped SrFe12O19 hexaferrites have been successfully synthesized via the sol–gel method. The X-ray diffraction patterns demonstrate that all doped samples are single-phase hexagonal crystal confirmed by Rietveld refinement. The results of field-emission scanning electronic microscopy reveal that the particle sizes increase obviously with increasing Co–Ti substitution rate. It can be clearly observed that the magnetic properties can be effectively tuned by the partial substitution of Fe with Co and Ti. The magnetization measurement manifests both saturation magnetization and coercivity drop with increasing Co–Ti concentration. Specifically, the coercivity falls substantially from about 1697 to 20 Oe with only a little decrease in the saturation magnetization as x increases from 0.5 to 1.5. The results indicate that low-doped Co–Ti substitution rates (x) make hexaferrites suitable for high density and perpendicular recording technology. It is worthy to note that the Co–Ti substituted samples can gradually turn the magnetocrystalline anisotropy from uniaxial to planar, and the transition point is near x = 1.5, which is beneficial to the formation of a conical magnetic structure to induce electric polarization. It is expected that Co–Ti co-doped hexaferrite Sr(CoTi)1.5Fe9O19 may be potential candidates for application based on the magnetoelectric devices.

Magnetic Properties of Co/CoO Core–Shell Nanowires: Roles of Antiferromagnetic Grain Size Distribution and Interfacial Spin Glass

Exchange bias (EB) effect in aligned Co/CoO core–shell nanowires with high aspect ratio was studied. Large EB fields of about 2.2 and 1 kOe in easy axis and hard axis, respectively, were obtained after field cooling. We demonstrate that both antiferromagnetic (AFM) grains and the interfacial spin glass make strong contribution to the observed EB, which is demonstrated with both the cooling field dependence of the EB field and the training effect at different temperatures. The difference in the thermal activation temperature for these two magnetic states leads to a bimodal distribution of the blocking temperature which, in combination with the interplay between the fluctuating spin-ordered AFM grains and the demagnetizing field of Co core, is likely to explain the anomalous drop of coercivity at the temperature in range from 200 to 110 K.

Peculiar magnetism in Eu substituted BiFeO3 and its correlation with local structure

We have systematically investigated the effects of Eu substitution on chemical pressure, bond lengths, microstrain, bond angles, octahedral tilting, vibrational modes and phase transformation, in BiFeO3. Correlation between concentration, phase, local structure and magnetism has been explained. Substitution of Eu ions with contrasting magnetic moment and dissimilar size, affects the local structure by changing bond angles and hence modifies spin structure through weakening of Dzyaloshinsky–Moriya (DM) interaction which lead to destruction of spiral spin configuration. Intriguing as well as anomalous magnetic response such as weak ferromagnetism with high coercivity, stair step like loops with significant drop in coercivity and systematic decrease in the magnetic coercivity at low temperatures has been observed as a function of Eu concentration (x). These results are explained on the basis of weakening of DM interaction, field induced melting of antiferromagnetic clusters and modification in effective magnetic anisotropy including contribution of magnetoelectric coupling, for various values of x. These results were used to generate the magnetic phase diagram.

Effect of non-magnetic ions substitution on the structure and magnetic properties of Y3−xSrxFe5−xZrxO12 nanoparticles

In this work, Y3−xSrxZrxFe5O12 (0.0 ≤ x ≤ 0.7) were synthesized by citrate precursor method at 1050 °C. The structural and magnetic properties of Y3−xSrxFe5−xZrxO12 were studied by using the X-ray diffraction technique, scanning electron microscopy, transmission electron microscopy, the Fourier transform infrared spectroscopy and vibrating sample magnetometer. XRD analysis using X’Pert package show a pure garnet phase with cubic structure (space group Ia-3d) and the impurity phase SrZrO3 is observed when the range of x value is exceeded from 0.6. Rietveld refinement using Fullprof program shows the lattice volume expansion with increasing the degree of Sr/Zr substitution. The crystallite sizes remain constant in the range of x = 0.0 − 0.5 and then increase. The different morphology observed in SEM micrographs of the samples can be related to different values of the microstrain in the samples. The hysteresis loops of the samples reveal a superparamagnetic behaviour. Also, the drop in coercivity with increasing of the substitution is mainly originated from a reduction in the magneto-elastic anisotropy energy. The values of the saturation magnetization (MS) indicate a non-monotonically variant with increasing the Sr/Zr substitution and reach a maximum 26.14 emu/g for the sample x = 0.1 and a minimum 17.64 emu/g for x = 0.0 and x = 0.2. The variation of MS, in these samples results from a superposition of three factors; reduction of Fe3+ in a-site, change in angle FeT-O-FeO, and magnetic core size.

Co/Ti-substituted SrM-based composite sheets: High frequency permeability and electromagnetic noise suppression above 6 GHz

Co/Ti-substituted Sr-based M-type hexagonal ferrite containing composite sheets are used for the demonstration of electromagnetic noise suppression at and above 6 GHz. Three different compositions (SrCoxTixFe12 − 2xO19 with x = 1.0, 1.2, and 1.4) and two different size groups with the average diameter of ∼10 µm and ∼1 µm have been studied to understand the effect of Co-Ti substitution level and size on the high frequency magnetic characteristics of SrM hexaferrites. A drop in coercivity from 285 Oe to 40 Oe and 710 Oe to 90 Oe was observed as the Co/Ti substitution level, x, increases from 1.0 to 1.4 in the case of 10-µm and 1-µm particles respectively. Complex permeability was measured by two methods – by microstrip probe, and by all-shielded and shorted microstrip line. Ferromagnetic resonance frequency drops from ∼20 GHz to ∼3 GHz as x increases from 1.0 to 1.4, while it decreases a little as the particle size decreases. Electromagnetic noise suppression was demonstrated by measuring the ratio of power loss in the magnetic element to the input power by placing it on top of a microstrip line. With a 500-µm thick composite sheet, electromagnetic noise suppression up to 20% was achieved.

Hydrogen disproportionation phase diagram and magnetic properties for Nd15Fe79B6 alloy

Transformation-temperature-hydrogen pressure phase diagram was constructed for a Nd15Fe79B6 alloy in order to estimate appropriate conditions for hydrogenation, disproportionation, desorption and recombination reaction (the HDDR). Optimised recombination time (the highest coercivity) was found to be 10 min. for 5 g samples processed at 740 °C. Several HDDR processes were carried out at 30 kPa of hydrogen pressure at various temperatures. No correlation between magnetic propertiec and a direction of measurement was observed for the samples processed at 740 °C. Remanence anisotropy was induced along an alignment direction when the temperature of the HDDR process was increased up to 800 °C and 850 °C for <100 μm and 100–160 μm particles, respectively. Simultaneously, a small drop in coercivity was observed in the direction of alignment for <100 μm particles, but no for 100–160 μm particles. Furthermore, probably an ordered phase was found by TEM microstructure analysis in the bulk sample disproportionated at 850 °C under 150 kPa of hydrogen. Grains with antiphase domains were observed and corresponding electron diffraction patterns were resolved, likely indicating superlattice structures.

Magnetic Properties of Divalent (Ni,Sn,Zn)- Tetravalent (Ru,Sn) Ionic Mixtures of Substituted Ba Ferrite

Magnetic Properties of Divalent (Ni,Sn,Zn) Tetravalent (Ru,Sn) Ionic Mixtures of Substituted Ba Ferrite J. Slama, A. Gruskova, A. Gonzalez Angeles, M. ’okaa,∗, R. Dosoudil Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Ilkovi£ova 3, 812 19 Bratislava, Slovakia Facultad de Ingenieria, Universidad Autonoma de Baja California, C.P. 21900 Mexicali, B.C., Mexico The drop of coercivity, while preserving the saturation magnetic polarisation JS of substituted M-type Ba hexaferrites with composition BaFe12−2x(Me1Me2)xO19, was studied. Divalent Me1 =Ni, Zn, Sn and tetravalent Me2 =Ru, Sn ionic combinations were used in various compounds. Mossbauer spectroscopy was used to reveal sites of replacement of the Fe ions by Ru and Sn. The Ni, Zn and Sn occupation sites were identi ed as well. Strong drop of coercivity di erence ∆Hc(x) near x = 0.1 and slight decrease of ∆Hc(x) close to x = 0.3 was obtained for suitable coupling of Me 1 and Me 4+ 2 ions.

Effect of high-energy ball milling time on structural and magnetic properties of nanocrystalline cobalt ferrite powders

Cobalt ferrite nanocrystals synthesized by conventional and size-controlled coprecipitation methods were treated by high-energy ball milling, HEBM, in order to study the effect of crystal size reduction and/or strain on the resulting magnetic properties. Processed nanocrystals were characterized by X-ray diffraction, Brunauer, Emmett, and Teller surface area analysis, transmission electron microscopy (TEM), and vibrating sample magnetometry. The cobalt ferrite nanocrystals exhibited crystal size reduction from initial values (average crystallite sizes of 12±1nm and 18±3nm, respectively) down to 10nm after HEBM for 10h. The specific surface area was decreased by milling (from 96.5 to 59.4m2/g; for the 12nm cobalt ferrite nanocrystals), due to particles aggregation. TEM analyses corroborated the aggregation of the nanoparticles at such long milling times. The same cobalt ferrite nanocrystals exhibited a rise in coercivity from 394 to 560Oe after 5h ball milling which was attributed to the introduction of strain anisotropy, namely point defects, as suggested by the systematic shift of the diffraction peaks towards higher angles. In turn, the magnetic characterization of the starting 18nm-nanocrystals reported a drop in coercivity from 4506Oe to 491Oe that was attributed predominantly to size reduction within the single domain region. A correlation between particle size, cationic distribution, and HEBM processing conditions became evident.

High temperature structural and magnetic properties of cobalt nanorods

We present in this paper the structural and magnetic properties of high aspect ratio Co nanoparticles (∼10) at high temperatures (up to 623K) using in-situ X ray diffraction (XRD) and SQUID characterizations. We show that the anisotropic shapes, the structural and texture properties are preserved up to 500K. The coercivity can be modelled by μ0HC=2(KMC+Kshape)/MS with KMC the magnetocrystalline anisotropy constant, Kshape the shape anisotropy constant and MS the saturation magnetization. HC decreases linearly when the temperature is increased due to the loss of the Co magnetocrystalline anisotropy contribution. At 500K, 50% of the room temperature coercivity is preserved corresponding to the shape anisotropy contribution only. We show that the coercivity drop is reversible in the range 300–500K in good agreement with the absence of particle alteration. Above 525K, the magnetic properties are irreversibly altered either by sintering or by oxidation.

Oxygen-stoichiometry-dependent microstructural and magnetic properties of CoPt thin films capped with ion-beam-assisted deposited TiOx layers

The magnetic properties of CoPt with different percentages of oxygen in the TiOx capping layer were investigated through annealing processes. Results have shown that after annealing, the structural phase transformation from fcc to fct CoPt occurred and was confirmed by the enhanced coercivities. In addition, the isolated CoPt grains separated by grain boundary TiOx in the annealed CoPt/TiOx (30% O2/Ar) thin films exhibited the largest coercivity. The excess amount of oxygen in the TiOx layer may react with Co and Pt to form oxides (as characterized by XPS) and result in the drop of coercivity. Thus the growth of the fct CoPt phase can be enhanced by optimizing the oxygen ratio during fabrication of the TiOx layer and post annealing, which may find useful applications for future CoPt-based magnetic recording.

ToF-SIMS Study of Carbon Diffusion in Co-Pt-TiO&lt;SUB&gt;2&lt;/SUB&gt; Composite and Ruthenium Films

In this paper, the diffusion behavior of hydrogenated amorphous carbon into Co-Pt-TiO2 and Ruthenium films was first investigated. Impact of carbon on Co-Pt-TiO2 composite film and its diffusion in Co-Pt-TiO2 and ruthenium films were studied using Time-of-Flight Secondary Ion Mass Spectroscopy. Magnetic coercivity drop was observed for Co-Pt-TiO2 films coated with ruthenium and carbon hybrid overcoat at varied thickness ratio. Co-Pt-TiO2 film is widely used in perpendicular magnetic recording. Carbon depth profile showed that carbon readily diffuses in Co-Pt-TiO2 film because of its porous structure. However, carbon diffusion in ruthenium film is limited even annealing up to 200 degrees C in argon. Carbon's diffusion length and coefficient in Co-Pt-TiO2 are much higher than that in Ru film. The activation energy was calculated for both cases.

Deposition of thick Co-rich CoPtP films with high energy product for magnetic microelectromechanical applications

We report the development of Co-rich CoPtP films of thicknesses up to 82 μm for use in magnetic MEMS applications. These films have been deposited using a combination of pulse-reverse plating with the addition of stress-relieving additives to the bath. The films were electroplated on sputtered Cu/Ti seed layer on silicon with an optimized thickness of 100/20 nm. The composition, crystalline structure, grain size and magnetic properties of the CoPtP films of varying thickness are compared and analyzed. The 3-μm-thick CoPtP film showed a columnar structure and strong perpendicular anisotropy. This film shows a perpendicular coercivity of 2150 Oe, a remanence of 0.564 T and a maximum energy product of 20 kJ/m 3. As the thickness of the plated film is increased, there is a gradual decrease in the coercivity and anisotropy. The 82-μm-thick film had a perpendicular coercivity of 1150 Oe and a remanence of 0.35 T. While there is a drop in coercivity and anisotropy, the remanence and maximum energy product remain constant for film thicknesses greater than 13 μm. The reason for the drop in coercivity and the near-constant remanence for thicker CoPtP films is discussed here. The coercivity of the thick Co-rich CoPtP film reported in this work is similar to those reported in the literature; the values of remanence, maximum energy product and saturation magnetization are the highest of all the thick (>50 μm) electroplated films in the literature.

Exchange bias in Co/CoO core-shell nanowires: Role of antiferromagnetic superparamagnetic fluctuations

The magnetic properties of $\text{Co}\phantom{\rule{0.3em}{0ex}}(⟨D⟩=15\text{ }\text{nm},\text{ }⟨L⟩=130\text{ }\text{nm})$ nanowires are reported. In oxidized wires, we measure large exchange-bias fields of the order of 0.1 T below $T\ensuremath{\sim}100\text{ }\text{K}$. The onset of the exchange bias, between the ferromagnetic core and the antiferromagnetic (AFM) CoO shell, is accompanied by a coercivity drop of 0.2 T which leads to a minimum in coercivity at $\ensuremath{\sim}100\text{ }\text{K}$. Magnetization relaxation measurements show a temperature dependence of the magnetic viscosity $S$ which is consistent with a volume distribution of the CoO grains at the surface. We propose that the superparamagnetic fluctuations of the antiferromagnetic CoO shell play a key role in the flipping of the nanowire magnetization and explain the coercivity drop. This is supported by micromagnetic simulations. This behavior is specific to the geometry of a one-dimensional system which possesses a large shape anisotropy and was not previously observed in zero-dimensional (spheres) or two-dimensional (thin films) systems which have a high degree of symmetry and low coercivities. This study underlines the importance of the AFM superparamagnetic fluctuations in the exchange-bias mechanism.