Fraction Of Ferromagnetic Phase Research Articles

High performance and exchange coupling in magnetization reversal of sintered (Nd,Dy)-Fe-B magnets

Improving the coercivity in sintered Nd-Fe-B magnets would sacrifice the remanence and the energy product. In this work it develops a method of preparing the composite (Nd,Dy)-Fe-B magnets to enhance the coercivity with a slighter decrease of remanence. The high performance (Nd,Dy)-Fe-B magnets were prepared via adding Dy-Fe-B into Nd-Fe-B powders in blending, followed by compacting and sintering. The addition of Dy-Fe-B powders gave rise to the enhancement of coercivity in the sintered magnets. The investigation of thermal activation indicates that the fluctuation field in thermal activation doesn’t obey the law of increasing with the coercivity. The grains of Dy-Fe-B crystal phase coexist in (Nd,Dy)-Fe-B magnets, which could increase the domain wall size by exchange coupling and decrease the fluctuation field in magnetization reversal. Some of Dy atoms diffuse into the grain outer-layer of Nd2Fe14B crystal phase, the grain boundary structure is probably optimized, the anisotropy increases at grain shell, and so the domain wall size would be reduced, leading to an increase of the fluctuation field. Though their effects of the exchange coupling and the anisotropy increasing are opposite on the changes of the domain wall size and fluctuation field, they are beneficial to improve the nucleation field in magnetization reversal, and the coercivity increases significantly in the sintered magnets. Since Dy-Fe-B powders mainly evolve into the main phase of RE2Fe14B crystal instead of the intergranular phase, the mass fraction of ferromagnetic phase doesn’t decrease evidently. Owing to the exchange coupling effect, the core-shell morphology of grains and the no remarkable increase in the amount of intergranular phase, the high performance with high energy product of 46.6 MGOe and high coercivity of 20.5 kOe could be obtained in the sintered (Nd,Dy)-Fe-B composite magnets.

Correlation of microstructure with magnetic properties in Pr0.67Sr0.33MnO3 thin films

Strain can be used to modulate magnetic properties of manganite thin film by affecting not only its crystal structure but also the microstructure, including the cluster size and fraction of ferromagnetic phase. In this work, epitaxial Pr0.67Sr0.33MnO3 thin films deposited on (001)-SrTiO3 single-crystal substrate are studied. The magnetization-temperature curves with zero-field cooling and field cooling show the typical feature of cluster-glass system. With the increase of film thickness, strain relaxation occurs in 80 nm Pr0.67Sr0.33MnO3 film with a wide temperature range of phase transition. By fitting the freezing temperature Tf dependence on magnetic field according to Almeida–Thouless equation, the 80 nm film has a larger local anisotropy and exchange bias compared to that of the 15 nm Pr0.67Sr0.33MnO3 film. Detailed analysis suggests that the strain-induced phase fraction and size distribution of ferromagnetic phase are responsible for the different magnetic properties of Pr0.67Sr0.33MnO3 film with different thickness.

Ultrafine-grained CrMnNi steels: Lueders phenomenon and texture inheritance

The mechanical behaviour as well as the microstructure and texture development were investigated for ultrafine-grained austenitic TRIP steels with two different austenite stabilities. The ultrafine-grained microstructure of low-carbon, high-alloy CrMnNi steels was obtained after thermo-mechanical processing (TMCP) consisting of cold rolling process up to 90% of rolling degree and subsequent reversion annealing treatment at different temperatures and holding times. The mechanical properties were evaluated during tensile tests at room temperature complemented by infrared thermography, digital image correlation, ferromagnetic phase fraction measurements as well as nanoindentation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed before and after tensile tests. In addition, the texture evolution was followed from cold rolling via reversion annealing up to tensile deformation. The results showed that an ultrafine-grained austenitic microstructure with a mean grain size between 0.7 and 1 μm was realized due to the applied TMCP. The ufg microstructure led to a significant increase in yield strength to more than 900 MPa for both steel variants. In addition, a Lueders band phenomenon was observed for steel with the lowest austenite stability, which was accompanied by dislocation-lack yield point and stress-assisted martensitic phase transformation at about 400 MPa. Texture inheritance from fcc cold rolling texture via the bcc cold rolling texture into fcc reversion annealing texture and again bcc texture after tensile deformation was observed. This texture inheritance is an indicator for the diffusionless shear transformation in this type of steels during reversion annealing.

Evolution of strain across the magnetostructural phase transition in epitaxial FeRh films on different substrates

We present a detailed x-ray diffraction study of the structural evolution of epitaxial FeRh films across the temperature-driven phase transition between antiferromagnetic and ferromagnetic order. FeRh films grown onto MgO, W/MgO, and $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ substrates show qualitatively different lattice distortions (tetragonal vs rhombohedral), while keeping a sharp transition above room temperature. Temperature-dependent x-ray reciprocal space mapping reveals the phase-specific crystal structure, giving access to both in-plane and out-of-plane lattice parameters and crystalline coherence lengths across different stages of the phase transition. Diffuse x-ray scattering from relaxed films is treated via a mosaic block model, which provides a robust data fitting scheme. It is found that the ferromagnetic phase fraction can stand a larger amount of strain before completely annihilating and transitioning to the antiferromagnetic phase upon cooling, as compared to heating. This is related to the distinct magnetic exchange correlations in the antiferromagnetic and ferromagnetic parent phases.

Study of magnetic and magneto-transport properties of nanocrystalline [formula omitted][formula omitted][formula omitted] compound: Observation of large magnetoresistance

Magnetic and magneto-transport properties of nanocrystalline Nd0.5Ca0.5MnO3 compound are presented. In contrast to its bulk counterparts, with modification of the magnetic grounds state, large magnetoresistance appeared in the presence of comparatively lower magnetic field in nanocrystalline compound. Additionally, the magnetic field induced insulator-metal transition took place in nanocrystalline compound with negligible thermal hysteresis. From application perspectives, observation of the low field magnetoresistance in this compound may be important for several uses in magnetic field sensor related technology. Fundamentally, the influences of the ferromagnetic phase fraction as well as the induced ferromagnetic interaction in nanocrystalline samples on magnetic and magneto-transport properties were addressed.

Structure, microstructure, electrical transport mechanism and magnetoresistance in La0.8Ag0.2MnO3

Bulk polycrystalline sample of La0.8Ag0.2MnO3 has been successfully synthesized by the sol gel method. Structural, morphology, electrical properties and magnetoresistance has been investigated. Refinement results from XRD at room temperature showed a single-phase material with a rhombohedral crystal structure and R-3c space group. Electrical transport showed by comparison of experimental data with theoretical model, the sample undergo a metal-insulator transition at TM-I accompanying the ferromagnetic-paramagnetic transition. Data for temperature dependence of resistivity has been fitted using the formula ρ(T) = ρFMf + (1 − f)ρPM, ρPM is resistivity of the ferromagnetic region, ρPM is resistivity of the paramagnetic region and f is the volume fraction of ferromagnetic phase. Result of fitted data is in good agreement with experimental data. MR measurements in the range of magnetic field 0-1 T, measurements were carried out at temperatures of 15 K, 100 K, 200 K and 285 K, the results showed a decrease in resistivity.

Magnetic force imaging for spatially resolved assessment of ferromagnetic phase fraction in austenitic stainless steel

This paper presents magnetic force imaging (MFI) – a method for spatially resolved quantitative assessment of the ferrite/martensite fraction in austenitic stainless steel or similar materials. This method exploits the force interaction between a magnetic tip and the surface of a specimen, similar to magnetic force microscopy (MFM). It differs from MFM in the size of the tip and close-to-saturation magnetization of the sample under investigation. In this study, AISI 347 (1.4550/ X6CrNiNb18-10) austenitic stainless steel was analyzed in the as-delivered state, i.e., 30 mm diameter round bars. The MFI results show a distinct resemblance of an inhomogeneous structure that forms bands in the rolling direction. This structure retains its initial square cross-section from which the material was hot rolled and has been detected due to the formation of α′-martensite. The information depth of MFI proved to go beyond the depth of α′ formed due to surface preparation only. The material was further characterized by standard methods, including EBSD, EDX, and hardness measurements.

First order magneto-structural transition and magnetocaloric effect in MnNiGe0.9Ga0.1

The first order magneto-structural transition (Tt≃95 K) and magnetocaloric effect in MnNiGe0.9Ga0.1 are studied via powder x-ray diffraction and magnetization measurements. Temperature dependent x-ray diffraction measurements reveal that the magneto-structural transition remains incomplete down to 23 K, resulting in a coexistence of antiferromagnetic and ferromagnetic phases at low temperatures. The fraction of the high temperature Ni2In-type hexagonal ferromagnetic and low temperature TiNiSi-type orthorhombic antiferromagnetic phases is estimated to be ∼40% and ∼60%, respectively at 23 K. The ferromagnetic phase fraction increases with increasing field which is found to be in non-equilibrium state and gives rise to a weak re-entrant transition while warming under field-cooled condition. The material shows a large inverse magnetocaloric effect across the magneto-structural transition and a conventional magnetocaloric effect across the second order paramagnetic to ferromagnetic transition. The relative cooling power which characterizes the performance of a magnetic refrigerant material is found to be reasonably high compared to the other reported magnetocaloric alloys.

Magnetic field induced low temperature upturn of magnetization in highly Ca-doped La0.1875Ca0.8125MnO3 polycrystalline compound

In our present study we have reported the magnetic properties of highly Ca-doped polycrystalline compound La0.1875Ca0.8125MnO3. Along with the conventional charge ordered antiferromagnetic ground state, a small ferromagnetic phase fraction is present at the low temperature. The effect of the external magnetic field markedly modifies the ground state of the compound. Our experimental results indicate that in addition to the ferromagnetic phase fraction, another field induced super paramagnetic phase grow at low temperature (T<50K) above H=10 kOe magnetic field within the charge ordered antiferromagnetic matrix. The nature of the temperature dependent magnetization curves influenced by the external applied magnetic field was observed and analyzed using Langevin theory of super paramagnetism.

Exchange bias phenomenon in (Nd1−xYx)2/3Ca1/3MnO3 (x = 0, 0.1) perovskites

Exchange bias phenomenon, evident of antiferromagnetic–ferromagnetic phase segregation state, has been observed in (Nd1−xYx)2/3Ca1/3MnO3 (x = 0, 0.1) compounds at low temperatures. A contribution to the total magnetization of the compounds due to the ferromagnetic phase has been evaluated. It has been found that yttrium doping leads to the growth of the ferromagnetic phase fraction. The ferromagnetic phase in the doped compound has a lower coercivity Hc and more rectangular form of the hysteresis loop. The values of the exchange bias field HEB and coercivity are found to be strongly dependent on the cooling magnetic field Hcool. In sufficiently high magnetic fields, Hcool &amp;gt; 5 kOe, HEB in the doped compound is about twice as low as in the parent compound. This difference is attributed to a lower exchange interaction and higher saturation magnetization of the ferromagnetic phase in (Nd0.9Y0.1)2/3Ca1/3MnO3.

High-Field Magnetization Behavior of Mn-Al-C Alloys

Dense Mn-Al-C permanent magnets have been produced using melt-spinning and spark plasma sintering techniques. The samples have been magnetically characterized from 4 to 300 K in a field of up to 14 T. The ahysteretic parts of magnetization loops were analyzed within a model for the approach to saturation, which allows for possible crystallographic texture. The saturation magnetization and magnetocrystalline anisotropy energy have been determined considering the estimated fraction of ferromagnetic phase. The obtained magnetic parameters have been compared with the available theoretical and experimental data.

Co-existence of Griffiths phase and ferromagnetic insulating state in electron doped nanomanganite

Detailed temperature dependent magnetic and transport properties of electron doped Sm0.09Ca0.91MnO3 nanomanganite exhibiting Griffiths phase (GP) have been investigated. A GP-like behavior having ferromagnetic spin correlation above ferromagnetic transition temperature (TC) without any spontaneous magnetization has been observed in this nanomanganite. Moreover, in this electron doped nanomanganite, the experimental large effective magnetic moment and deviation of the inverse susceptibility from Curie–Weiss law clearly indicate the possible existence of GP. The complex non-linear ac susceptibilities (2nd and 3rd harmonics) have also been studied to explore the role of nature of disorder in the GP phenomenon. The transport measurements yield a ferromagnetic insulating ground state exhibiting colossal magneto resistance (CMR) near the ferromagnetic transition temperature. It has also been reported that the temperature dependent average spin moment at the hopping site and the ferromagnetic phase fraction bear the signature of the onset of GP.

Adjustable magnetoelectric effect of self-assembled vertical multiferroic nanocomposite films by the in-plane misfit strain and ferromagnetic volume fraction

The strain-mediated magnetoelectric (ME) property of self-assembled vertical multiferroic nanocomposite films epitaxially grown on cubic substrates was calculated by a nonlinear thermodynamic theory combined with the elastic theory. The dependent relations of phase state of ferroelectric films with the in-plane misfit strain, out-of-plane misfit strain, temperature, and volume fraction of ferromagnetic phase were confirmed. The effects of in-plane misfit strain and ferromagnetic volume fraction on the polarization and dielectric constant of ferroelectric films at room temperature were elaborately analyzed for the vertical BaTiO3-CoFe2O4 and PbTiO3-CoFe2O4 nanocomposite films. Our calculated results confirmed the relationship among ME effect and in-plane misfit strain and ferromagnetic volume fraction in the nanocomposite films. The ME voltage coefficients of vertical BaTiO3-CoFe2O4 and PbTiO3-CoFe2O4 nanocomposite films displayed various maximums and abrupt points at special phases and phase transition boundaries. The ME voltage coefficients of lead-free BaTiO3-CoFe2O4 nanocomposite films epitaxially grown on different substrates could reach a comparative value of ∼2 V·cm−1·Oe−1 under the controllable in-plane misfit strain induced by substrate clamping. Our results provided an available method for the optimal design of vertical multiferroic nanocomposites with adjustable ME effect by optimizing the ferromagnetic volume fraction and substrate type.

Unusual magnetization suppression induced by higher magnetic field in (La0.83Bi0.17)0.67Ca0.33MnO3

Magnetization behavior of (La0.83Bi0.17)0.67Ca0.33MnO3 has been investigated in the temperature range from 100 to 180K. A metamagnetic transition was observed in the temperature region, where the magnetization was measured after a zero-field-cooling from room temperature to a selected temperature. Experimental results show that, after a higher magnetization route, the field-increasing branches of the magnetization curves shows an unusual training effect: below a magnetic field H0, the applied magnetic field enhances the value of magnetization; however, above H0 the magnetic field suppresses the value, and the behavior cannot be totally attributed to the enhancement effect of the applied magnetic field on ferromagnetic phase fraction. It is proposed that, in the two-phase coexistence region, the higher magnetic field promotes the phase separation and leads to both the fraction of ferromagnetic domain and the stabilization of antiferromagnetic domain increase.

STT and DTT Diagrams of Austenitic Cr–Mn–Ni As‐Cast Steels and Crucial Thermodynamic Aspects of γ → α′ Transformation

AbstractThe mechanical behavior and microstructure evolution during deformation of novel austenitic Cr–Mn–Ni as‐cast steels with varied Ni content were investigated at various temperatures using static tensile tests, optical microscopy, and the magnetic scale for the detection of ferromagnetic phase fraction. To summarize all knowledge about the deformation‐induced processes, the STT and DTT diagrams were developed for Cr–Mn–Ni steels. The diagrams illustrate the different deformation mechanisms depending on temperature and tension load, and quantify the elongation of the deformation mechanisms. The deformation‐induced ε‐ and α' martensite formation and twinning – the TRIP and TWIP effects – occur in the Cr–Mn–Ni steels depending on the chemical composition and temperature. The differences of deformation‐induced processes depend on thermodynamics and are confirmed by thermodynamic calculations. The nucleation threshold of γ → α′ transformation was determined for the investigated Cr–Mn–Ni steels.

Stress‐Temperature‐Transformation and Deformation‐Temperature‐Transformation Diagrams for an Austenitic CrMnNi as‐cast Steel

AbstractStress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) diagrams are well‐suited to characterize the TRIP (transformation‐induced plasticity) and TWIP (twinning‐induced plasticity) effect in steels. The triggering stresses for the deformation‐induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain‐induced twins and martensite formations are shown in the DTT diagram.The microstructure evolution of a novel austenitic Cr‐Mn‐Ni (16%Cr, 6% Mn, 6% Ni) as‐cast steel during deformation was investigated at various temperatures using static tensile tests, optical microscopy and the magnetic scale for the detection of ferromagnetic phase fraction. At the temperatures above 250 °C the steel only deforms by glide deformation of the austenite. Strain‐induced twinning replaces the glide deformation at temperatures below 250 °C with increasing strain. Below 100 °C, the strain‐induced martensite formation becomes more pronounced. The kinetics of the α'‐martensite formation is described according to stress and deformation temperatures. The STT and DTT diagrams, enhanced with the kinetics of the martensite formation, are presented in this paper.

Giant magnetic domain-wall resistance in phase-separated manganite films

The top and bottom electrodes of the ferromagnetic manganite La0.7Ca0.3MnO3 may be magnetically decoupled by a La0.59Ca0.41MnO3 interlayer in which ferromagnetic and paramagnetic phases coexist. In mesas fabricated from these trilayer films, in-plane magnetic domain walls associated with the antiparallel magnetic electrode configuration show a giant resistance-area product >10−10 Ω m2. This is almost two orders of magnitude larger than our devices with interlayer composition La0.6Ca0.4MnO3 [C. Israel et al., Phys. Rev. B 78, 054409 (2008)] where phase separation is less extreme. High-field treatments modify the ferromagnetic phase fraction of the interlayer.

A remarkable transformation of magnetic resonance spectra as a result of a mutualinfluence of coexisting para- and ferromagnetic phases

In this work, we carry out the analysis of the resonance absorption of electromagneticradiation for the system in which para- and ferromagnetic phases coexist over a widetemperature region. It is found that taking account of the mutual influence ofcoexisting phases gives rise to the appearance of substantial changes in the curves ofresonance absorption and values of resonance fields, as well as to making thegeometry of a phase distribution dependent on an external magnetic field. Nearthe temperature boundaries of the phase coexistence region, the expressions fordescription of the curves of the dispersive absorption of electromagnetic radiation areobtained and the rules of the behaviour of the resonance fields for each of thephases are specified. As follows from the calculations, the resonance field for theparamagnetic phase becomes dependent on the shape of the sample, the saturationmagnetization and the fraction of ferromagnetic phase. It is shown that the character ofmagnetic resonance spectra and the features of their temperature change agree wellwith the experimental data, obtained by various groups of researchers on thesingle crystalline and polycrystalline samples of doped perovskite manganites.

Structural origin of the giant oxygen isotope effect in Re 0.5Sr 0.5MnO 3 perovskites

Neutron diffraction experiments with Re 0.5Sr 0.5MnO 3 (Re= 152Sm, Nd 0.772Tb 0.228 and Nd 0.544Tb 0.456) manganites were initiated for elucidating of the structural reasons for a giant oxygen isotope effect in Sm 0.5Sr 0.5MnO 3. As a result of these experiments it was shown that in all studied compositions, two crystal phases with different type of Jahn–Teller distortions of MnO 6 octahedra and different type of magnetic ordering coexist at low temperatures. The data confirm the percolative nature of the metal–insulator transition in Sm compounds upon 16O for 18O substitution due to a sharp (from 65% to 18%) decrease of the ferromagnetic phase fraction.

Magnetization reversal and nanoscopic magnetic-phase separation inLa1−xSrxCoO3

The doped perovskite cobaltite ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ (LSCO) has been advanced as a model system for studying intrinsic magnetic phase separation. We have employed a first-order reversal curve (FORC) method to probe the amount of irreversible switching in bulk polycrystalline LSCO as a function of Sr doping, field cooling procedure, and temperature. The value of the FORC distribution $\ensuremath{\rho}$ is used as a measure of the extent of irreversible switching. For $x&lt;0.18$, the small values of $\ensuremath{\rho}$ and its ridgelike distribution along local coercivity $({H}_{c})$ and zero bias $({H}_{b})$ are characteristic of noninteracting single domain particles. This is consistent with the formation of an array of isolated nanoscopic ferromagnetic clusters, as observed in previous work. For $x\ensuremath{\geqslant}0.18$, the much larger values of $\ensuremath{\rho}$, the tilting of its distribution towards negative bias field, and the emergence of regions with negative $\ensuremath{\rho}$ are consistent with increased long-range ferromagnetic ordering. The FORC distributions display little dependence on the cooling procedure. With increasing temperature, the fraction of irreversible switching determined from the FORC distribution follows closely the ferromagnetic phase fraction measured by La nuclear magnetic resonance. Our results furthermore demonstrate that the FORC method is a valuable first-pass characterization tool for magnetic-phase separation.