Reduction In Coercivity Research Articles

Soft magnetic properties and corrosion resistance of the annealed (Fe0.7Co0.15Ni0.15)75B21Nb4 metallic glasses

The phase structure, magnetic properties and corrosion resistance in the as-spun and annealed (Fe0.7Co0.15Ni0.15)75B21Nb4 metallic glasses (MGs) were investigated. Annealing below glass transition temperature at 693 K causes a reduction in coercivity and ameliorates the soft magnetic properties, originating from the formation of a homogeneous glassy structure. The hysteresis loops of the as-annealed ribbons, especially the 888 K-annealed one, appear to be more rectangular in their geometry, showing that the approach to saturation becomes more expeditious. The synergy of amorphous phase and nano-size magnetic α-Fe(Co) phase in the 753 K-annealed ribbon leads the improvement of saturation magnetization. The corrosion behaviors in seawater solution have been investigated by electrochemical polarization measurement. By evaluating the corrosion potential and current density, annealing treatment induces the detrimental influence in the general corrosion resistance of MG ribbons. However, compared with as-spun amorphous state, the 693 K-annealed relaxation ribbon exhibits superior passivity with lower passive current density and higher pitting resistance as revealed by wider passive region.

Reactive magnetron sputtered–assisted deposition of nanocomposite thin films with tuneable magnetic, electrical and interfacial properties

In this work, different magnetic thin films of Ni, NiFe and NiFe2O4 are deposited on the SiO2 substrate using sputtering technique. Our experiments confirmed that thin films possess a good nanocrystalline structure. The key deposition parameters controlling their magnetic properties are sheet resistivity, crystalline structure and microtopography of the sputtered thin film. Besides, the reactive gas oxygen (O2) also plays a leading role in transforming the phase and structure of the ferrite film. The nanocrystalline nature of the ferrite film results in the reduction of overall coercivity (HC). The thickness of the sputtered thin film is in the range of 800–1000 A. The prepared film exhibits roughness in the range of (~ 0.60 to ~ 0.98 nm). Furthermore, the structural transformation study is performed with X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The quite low roughness, high resistivity and low Hc make NiFe2O4 thin film as a potential candidate for the future spintronics, optoelectronics, photocatalysis and solar cell applications.

On the temperature-dependent coercivities of anisotropic Nd-Fe-B magnet

We have studied the thermal stability of coercivity in anisotropic Nd-Fe-B magnet from 300 K to 500 K by combining the micromagnetic simulation with the numerical estimation of coercivity reduction caused by thermal activation. The numerically achieved coercivities agree well with the measured value of a conventional sintered magnet. The concave shape of Hc(T) which is different from linear shape of HA(T) in anisotropic Nd-Fe-B magnets, as well as the break of linearity in the fitting of Hc/Msvs. HA/Ms are found both originated from the temperature dependence of magnetization of the ferromagnetic grain boundary phase. This curving of Hc(T) can be eliminated via transformation of grain boundary magnetism into non-ferromagnetic. We have demonstrated this hypothesis experimentally in the hot-deformed magnet heavily infiltrated with Nd–Cu alloy. The parameters, α and Neff, achieved in the fitting of Kronmüller equation are found to be largely deviated from those achieved in micromagnetic method, which is due to the ignorance of temperature dependent grain boundary magnetization and the thermal activation effect in the fitting.

MFM Observation of High Coercivity in Nanostructured Tetragonally Distorted FeCo Films

The hard magnetic properties of nanostructured tetragonally distorted FeCo films, which have a relatively high magnetic anisotropy of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> J·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , were investigated to reveal the mechanism of coercivity. FeCo nanodots with a diameter of 50 nm and thickness of 20 nm were fabricated using electron beam lithography. Magnetic force microscopy (MFM) revealed that the FeCo nanodots were magnetized perpendicularly, and that magnetization reversal occurred individually. The magnetization curve of the nanodots was obtained from the MFM results, and the coercivity was found to be 0.60 T, which was lower than the theoretically obtained value. The mechanism of the coercivity reduction is discussed.

Achievement of high performance in multi-main-phase (Pr,Nd,MM)-Fe-B sintered magnets by regulating microstructure

Misch-metal (MM) containing four kinds of rare earth elements of La, Ce, Pr and Nd, leads to the complex multi-main-phase (MMP) structure of MM-based sintered magnets. In order to prepare high-performance MM-based sintered magnets, it is necessary to explore the correlation between the microstructure and properties. Here, the MMP [(Pr, Nd)0.65MM0.35]-Fe-B sintered magnets were prepared by the mixtures of MM-free Pr-Fe-B powders and MM-containing (Nd0.5MM0.5)-Fe-B powders, and the microstructures of the magnets were controlled by tuning annealing conditions to optimize the magnetic properties. The results show that the continuous RE-rich grain boundary (GB) phases are formed between the adjacent main phase grains by annealing, which is beneficial to weaken the short-range ferromagnetic coupling and improve the coercivity. On the other hand, the inter-diffusion in the main phase grains will take place during the annealing, which makes the composition of the main phase grains to homogenize gradually. Thus, the magnetic impedance effect of the MM-rich grains by the Pr-rich grains is weakened, leading to the reduction of coercivity. Combining these two factors, the MMP [(Pr, Nd)0.65MM0.35]-Fe-B magnet annealed at 420 °C possessing optimal microstructure could obtain excellent magnetic properties of Hcj = 11.03 kOe and (BH)max = 38.21 MGOe.

Benchmarking of Spin–Orbit Torque Switching Efficiency in Pt Alloys

AbstractA magnetic heterostructure with good thermal stability, large damping‐like spin–obit torque (DL‐SOT), and low power consumption is crucial to realize thermally stable, fast, and efficient magnetization manipulation in SOT devices. This work systematically investigates on PtxCu1‐x/Co/MgO magnetic heterostructures with perpendicular magnetic anisotropy (PMA), and reports a promising spin Hall material, Pt–Cu alloy, possessing large DL‐SOT efficiency and moderate resistivity. The optimal Pt0.57Cu0.43 has a large DL‐SOT efficiency of about 0.44, as determined by hysteresis loop shift measurements, with a relatively low resistivity (82.5 µΩ cm at 5 nm thickness). Moreover, this large DL‐SOT efficiency and the coercivity reduction accompanying with proper alloying contribute to a low critical switching current density (2.37 × 106 A cm−2 in the Pt0.57Cu0.43 layer) in current‐induced magnetization switching measurements. Finally, the thermal stability of the Co layer can be preserved under alloying, whereas the switching power consumption can be significantly reduced, being the best performance among reported Pt‐based spin current sources. This systematic study on SOT switching properties suggests that Pt0.57Cu0.43 is an attractive spin current source with moderate resistivity, large DL‐SOT efficiency, good thermal stability, and low power consumption for future SOT applications.

Spin transfer and proximity effects in case of FePt (L1) nanoparticles coated with P3HT

Nanocomposites based on half-metallic FePt (L10) magnetic nanoparticles coated with the semiconducting conjugated polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) show a significant reduction in the magnetic coercivity. This study adopts a physical approach based on chemical potential equalization at the interface. The underlying charge/spin transfer mechanism unveils an imbalance: only spin-down polarized electrons are allowed to be transferred from the semiconductor to the half-metal (spin-down) conduction band, while spin-up states remain blocked at the interface. This process determines an excess of spin-up states on the P3HT side, and due to a RKKY mechanism, this effective spin system becomes ferromagnetic polarized. Due to this proximity effect, the conjugate polymer becomes exchange coupled to the hard magnetic FePt (L10) phase, thus reducing the coercivity of the half-metal. These processes make this type of composite suitable for magnetic recording applications.

Effects of copper and zirconium contents on microstructure and magnetic properties of Sm(Co,Fe,Cu,Zr)z magnets with high iron content

The evolution of the microstructure, microchemistry and magnetic properties of the Sm(CobalFe0.28CuyZrx)7.6 magnets with different Zr and Cu contents was investigated. It is found that the coercivity of the Sm(Co,Fe,Cu,Zr)z magnets is sensitive to Zr content. The deficiency of Zr content causes heterogeneity of Cu and Fe distributions, while an excessive Zr content leads to the formation of a SmCoZr impurity phase. The cellular structure and distribution of Cu concentration gradient between the cell boundary phase and cell phase are destroyed by inappropriate Zr content, which results in a reduction of coercivity. The Cu concentration difference between the cell boundary phase and cell phase increases with increasing Cu content. The coercivity of the Sm(CobalFe0.28CuyZr0.02)7.6 magnets increases from 10.4 to 25.4 kOe for y = 0.05 and y = 0.07. However, the excess of Cu element destroys the cell boundary phase and enlarges the cell size, resulting in a significant decrease of squareness and energy density. The optimum performance (remanence of 11.4 kG, coercivity of 25.4 kOe, maximum magnetic energy product of 30.4 MGOe) was obtained for the Sm(Co0.63Fe0.28Cu0.07Zr0.02)7.6 magnet.

Substitution effect of Y3+ ions on the structural, magnetic and electrical properties of cobalt ferrite nanoparticles

Abstract Y3+-doped cobalt ferrite nanoparticles, CoYxFe2-xO4 (x=0.00, 0.05, 0.10, 0.15), were synthesized using the sol-gel auto-combustion technique. Single-phase cubic spinel structure was confirmed by the X-ray diffraction technique. The lattice parameter increased with the increase of Y3+ content in the cobalt ferrite and varied from 8.3876 to 8.4271 Å. FE-SEM images showed that the samples had distinct crystalline nanoparticles of spherical shape with small agglomeration. EDAX results showed good agreement with the particularized composition. Magnetic measurement using a vibrating sample magnetometer (VSM) recorded a reduction in saturation magnetization (Ms) and coercivity (Hc) by Y3+ substitution. The values of Ms and Hc ranged from 66.61 to 17.52 emu/g and 1168.9 to 801.9 Oe, respectively. The electrical properties of Y3+-substituted cobalt ferrite were studied by measuring the dielectric constant and loss. With increasing frequency, the value of the dielectric constant decreased, as a natural behavior of the ferrite material.

Magnetoelastic effects and random magnetic anisotropy in highly strained ultrathin Ni nanowires epitaxied in a SrTiO3 matrix

We analyze the magnetic anisotropy of Ni nanowires with diameters smaller than 5 nm. The nanowires are vertically epitaxied in a SrTiO3(001) matrix which generates huge tensile strains up to 3.6% along the nanowire axis. This leads to an unusual anisotropy, characterized by an easy magnetization plane perpendicular to the nanowire axis. Hysteresis cycles M(H) unveil an overall in-plane isotropy, while an opening of the M(H) cycles and thermal activation measurements indicate the presence of local energy barriers inside the nanowires. Surprisingly, the coercive field Hc(T) decays exponentially with increasing temperature, for both the easy plane and the hard axis. Based on these findings, we provide an analysis of magnetoelastic effects in the nanowires. By considering global averaging over the anisotropy distribution and local averaging according to the Random Magnetic Anisotropy model, we find that the global anisotropy, with its hard axis and isotropic easy plane, is related to the mean strain, while coercivity arises from local strain variations. We evidence that a thermally activated anisotropy softening occurs in the nanowires, in addition to Sharrock’s law of thermal reduction of coercivity. Possible mechanisms responsible for this thermal softening of anisotropy are proposed and discussed. Our study eventually allows to identify two major competing effects at play in the present system: an increasing magnetic anisotropy with increasing strain and a reduction of the anisotropy with increasing local strain fluctuations.

Microstructural evolution, recovery and recrystallization kinetics of isothermally annealed ultra low carbon steel

The recovery and recrystallization kinetics of 80% cold rolled ultra low carbon steel are investigated during isothermally annealing for temperature ranges 350–640 °C as a function of different annealing time. The recovery is assessed by magnetic coercivity (Hc), while the recrystallization is determined by mechanical hardness. At low temperature (350 to 520 °C) annealing, recovery dominates for long time (∼12 000 s), while the annealing at 550 °C/ 900s and 580 °C/ 300s causes the recrystallized nuclei formation . The recovery kinetics is introduced by differential rate equation, explaining the reduction in coercivity with the recovery progress and the variation of an activation energy from 41–113 kJ mol−1. The recrystallization kinetics is found faster at high annealing temperature 640 °C than 550 and 580 °C based on hardness measurement, justifying by apparent activation energy within 114–190 kJ mol−1. Furthermore, the recovery and recrystallization rate increase with different annealing time, consistent to the change of microstructures and grain boundary characteristics evaluated by the orientation imaging microscopy (OIM) of electron backscattered diffraction (EBSD).

Study of structural and magnetic properties of Zinc-Substituted Cadmium ferrite nanocrystals

Polycrystalline CdxZn1-xFe2O4 ferrites with (x = 0.2 and 0.8) have been prepared by conventional microwave induce sol–gel method. The Calcination of samples was performed at 800 OC for 4 h. The prepared powder samples were structural and compositional characterized by XRD and FE-SEM. Rietveld refined XRD patterns exposed the cubic spinel phase. The magnetic property of prepared samples was investigated using VSM. Magnetization measurements have shown that the particles have superparamagnetic behavior at room temperature. The hysteresis curves of the sample exhibited the reduction of saturation magnetization and coercivity by Zn2+ ions substitution.

Ni addition induced modification of structural, magnetic properties and bandgap of Ni-Zn nano ferrites

We report synthesis, characterization of Zn1-xNixFe2O4 (x = 0.0–1.0) nano ferrites, by using x-ray diffraction 'XRD', vibration sample magnetometery, UV–Vis spectroscopy. XRD validates the formation of fcc spinel structure. Ni-addition leads to: grain diameter modification; linear decrease of lattice parameter; un-equal distribution of both Ni2+, Zn2+, Fe3+ ions on A, B sites; changes in the degree of inversion, and oxygen parameter showing changes in disorder; linear variation of bandgap; significant reduction of coercivity with simultaneous increase of saturation magnetization. Results clearly demonstrate that Ni-addition in Ni-Zn ferrite can be effectively used to tune magnetic properties, bandgap.

Experimental Assessment of Coercivity Microstructural Parameters of a Nd-Fe-B Sintered Magnet

We examined the microstructural parameter α in order to investigate the influence of its components on the intrinsic coercivity of an isotropic Nd-Fe-B sintered magnet. By applying boundary conditions for the assessment of grain (mis)alignment α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ψ</sub> and exchange coupling α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">exc</sub> and combining with magnetic measurements and structural characterization results obtained by magnetic small angle neutron scattering, the parameter α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</sub> -associated with the reduction of the magnetocrystalline field-plays a major role on the reduction of coercivity of the case study. Approaches in determining (α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</sub> ) are also discussed.

Maintaining high saturation magnetic flux density and reducing coercivity of Fe-based amorphous alloys by addition of Sn

The effect of minor Sn on crystallization behavior, soft magnetic properties and corrosion resistance of Fe83B10-xC3Si3P1Snx (x = 0.25, 0.5 and 0.75) amorphous alloys was investigated. Sn added alloys exhibit excellent saturation magnetic flux density (Bs) of 1.63–1.72 T, low coercivity of 2.5–5.4 A/m and higher corrosion resistance. Furthermore, coercivity of as-spun alloys is lowered by Sn addition, and mechanisms behind the change of soft magnetic properties were discussed. The finding suggests that soft metal Sn may act as a stress reliever in as-spun alloys, leading to low coercivity while maintaining ductility. This theory was then confirmed in high Co content amorphous alloys with Bs of 1.75 T, in which case Sn addition causes significant reduction of coercivity but no decline of Bs. The present strategy of Sn addition may pave a new and universal way for Fe-based amorphous alloys to simultaneously obtain high Bs, lower Hc and high ductility.

Synthesis and Characterisation of Copper Substituted Cobalt Ferrite Nanoparticles by Sol-Gel Auto Combustion Route

In the present study, the structural, functional, morphological and magnetic characteristics of copper substituted cobalt ferrite nanoparticles were investigated. The samples of Co1-xCuxFe2O4 at x = 0.06, 0.08 and 0.1 M were prepared by Sol-gel auto combustion route. X-ray diffraction technique was used to confirm the phase formation and structural analysis which matches with the JCPDS Data. The average crystallite size was found to be ∼25, ∼19 and ∼18 nm for x = 0.06, 0.08 and 0.1 respectively. The Micro Raman Spectroscopy revealed the stretching vibrations at 274 cm−1, 660 cm−1, and 466 cm−1, which are characteristic of Spinel Ferrites. From the FTIR analysis, the band observed at 3457cm−1 and 1650 cm−1 is assigned to hydrogen bonded O-H group and ionic stretching of C-H bond. The band assigned at 1105 cm−1, is due to Co – O and Cu - O or Fe – O vibrations. The existence of water adsorption band and metal oxygen band confirms the existence of Co and Cu in the synthesized sample. The surface morphology of samples was imaged by the field emission scanning electron microscope. The substitution of Cu2+ in the parent systems caused a significant reduction in particle size. The compositional analysis was done, which confirmed that the concentration of copper was increased in the samples. The samples were subjected to magnetic characterization because magnetic behavior is also affected by substitution of Copper in Cobalt ferrite. Magnetic hysteresis study at room temperature confirmed the reduction in saturation magnetization (MS = 14.25 to 8.33 emu/g.) and reduction in coercivity (HC = 602.64 to 380.94 Oe) when size is reduced. As the concentration of Cu into CoFe2O4 matrix increases, particle size decreases and the saturation magnetization decreases.

FORC investigation of Co-Ni bulk ferrite consolidated by spark plasma sintering technique

Bulk samples of Co1−xNixFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 1.0) were shaped by a spark plasma sintering (SPS) process. The initial ferrite nano-powders which were prepared using a co-precipitation method were characterized by high-resolution transmission electron microscopy (HRTEM) and Mössbauer spectroscopy. The HRTEM showed nanoparticles with almost 5–35 nm average size. Mössbauer spectrometer is employed to study the magnetic properties of ferrite nano-powders at room temperature. X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM) coupled with EDS detector for the chemical composition analysis and vibration sample magnetometer (VSM) equipped with FORC software were used to characterize the bulk samples. Without any structural changes to powder samples, a single-phase spinel structure was obtained. In the FE-SEM micrograph, the porosity decrease and consequently the density increase are clearly visible and the EDS study confirms the presence of Fe, Co, Ni and O ions in the fabricated samples by SPS process. The magnetic parameters such as saturation magnetization and the coercivity showed a decreasing behavior with an increase in Ni concentration from 67 emu/g and 787 Oe to 42.77 emu/g and 151 Oe, respectively. FORC analysis implies the coercivity reduction of bulk samples comparing with powders state in cobalt-rich ferrites is due to the multi-domain formation. The coercivity unavoidable in nickel-rich ferrites is due to the occurrence of the multi-domain state along with the disappearance of the superparamagnetic phase.

Bulk Depolarization Fields as a Major Contributor to the Ferroelectric Reliability Performance in Lanthanum Doped Hf0.5Zr0.5O2 Capacitors

AbstractAfter the discovery of ferroelectricity in HfO2, many dopants have been incorporated into the material to improve the ferroelectric properties. The binary mixture of HfO2 and ZrO2, HfxZrx−1O2, showed the widest process window in terms of polarization, but other memory related aspects still need improvement. Recently, the co‐doping of La into a mixed Hf0.5Zr0.5O2, La:HZO, was reported to improve the endurance properties further but the explanation spanning both structural and electrical characteristics of La:HZO and their interaction is still missing. In this work, an extensive study of La:HZO with La content ranging from 0 to 4.3 mol% is conducted and resultant stabilization of nonpolar tetragonal phase, coercive field reduction, endurance improvement, stronger retention loss, and less imprinted hysteresis loop is reported with increasing La concentration. The model simultaneously explaining the electrical and structural properties is presented. In ferroelectric capacitor structures, the depolarization fields originating from nonferroelectric layers at the metal/ferroelectric interface are discussed extensively in previous studies but, here, for the first time, the impact of depolarization fields from nonferroelectric regions in the bulk of the ferroelectric material is reported, which is an important element to explain all the observed trends.

Microwave-assisted switching in CoCrPt granular medium under continuous microwave fields

We have fabricated a device for microwave-assisted switching (MAS) experiments with perpendicularly magnetized CoCrPt-SiO2 granular film with anisotropy field Hk = 18.8 kOe. The device was carefully designed to be able to apply a microwave field of 0.95 kOe in amplitude continuously, allowing direct evaluation of microwave-assisted switching properties of the medium without any assumptions about time-dependent switching behavior. The coercivity of CoCrPt-SiO2 medium linearly decreases with increasing microwave frequency, and the maximum coercivity reduction ratio reaches 80% for hrf = 0.95 kOe at frf = 25 GHz after subtracting the effect of temperature rise due to microwave field application. The linear slope of coercivity against the frequency becomes steeper with increasing microwave field amplitude, which is a unique frequency and amplitude dependencies in granular media. Correlation length obtained by X-ray magnetic circular dichroism microscopy measurement decreases by applying a microwave field with higher frequency, suggesting that the magnetic clustering dimensions of the medium possibly depend on microwave frequency during microwave-assisted switching.

High temperature spin reorientation, magnetization reversal and magnetocaloric effect in 50% Mn substituted polycrystalline ErFeO3

The structural, magnetic and magnetocaloric properties of 50% Mn substituted ErFeO3 are studied by X-ray diffraction, magnetization and specific heat measurements. ErFe0.5Mn0.5O3 crystallizes in an orthorhombic structure (Pbnm). DC magnetization and specific heat studies point to two magnetic transitions: (i) Fe3+/Mn3+ ordering, TN ~ 245 K and (ii) spin re-orientation transition, TSR ~ 218 K. Below TSR, ErFe0.5Mn0.5O3 exhibits a reduction in magnetization and coercivity which could be due to a Γ4-Γ1 magnetic transition. Interestingly, we have observed a negative magnetization in field cooled curves at lower temperatures under lower applied fields (<50 Oe) as a result of the polarization of paramagnetic Er3+ moments with the internal field created by Mn3+ ordering from the Mn-rich regions. A significant magnetic entropy change (-ΔSm) of 13.4 J/kg.K is also observed at 9.5 K for an applied field of 7 T.