Irreversible Susceptibility Research Articles

Fabrication and application prospects of Pr–Fe–B/Alnico nanocomposite alloys

The potentials of rare earth-based nanocomposite alloys have never been realized due to strict microstructural constraints. Owing to the easy demagnetization it is challenging to increase the soft magnetic phase content. To avoid the easy demagnetization, Pr–Fe–B/Alnico magnets were fabricated and reported in this manuscript. The content of the Alnico phase is increased from 0 to 25 wt%, while the content of Pr element is reduced to below the sub-stoichiometry of the 2:14:1 main phase. The maximum magnetic energy product, which is the figure-of-merit for permanent magnets, is increased from 122 kJ/m3 for the standard alloy to 146 kJ/m3 for the alloy with 15 wt% Alnico which shows a significant improvement considering the fact that the Curie point of the magnet is also increased by ∼66 K. The special microstructure contains distinctly and heterogeneously distributed 2:14:1 and Alnico phases. The dimensions of neither the 2:14:1 nor the Alnico phases meet the dimensional requirements of the nanocomposite magnets, but still the smooth demagnetization curves are noted for the alloys. The behavior of effective anisotropy, the performance of the magnets in applied magnetic field and the magnetic interactions among the various constituent grains were quantitatively studied by reversible susceptibility, irreversible susceptibility and recoil loop openness. This study may provide some guiding principles for the development of nanocomposite magnetic alloys with excellent magnetic properties by using much less RE elements.

Enhanced magnetic properties of Ce17Fe76.5Co1Zr0.5B6 alloys by magnetic field heat treatment

In the present work, Ce17Fe76.5Co1Zr0.5B6 ribbons were prepared by a direct melt spinning method. The effects of chamber pressure and magnetic field annealing temperature on the magnetic properties and microstructures of the alloys were investigated. The grain size and content of Ce2Fe14B phase can be changed by adjusting the chamber pressure, and the optimal magnetic performance is obtained at 0.04 MPa. The magnetic properties can be influenced under magnetic field heat treatment. When the annealing temperature is lower than the Curie temperature, the refinement and a uniform distribution of the grains is obtained. The irreversible magnetic susceptibility curve reveals that magnetic field heat treatment enhances the exchange coupling interaction between grains of the Ce2Fe14B phase. When the magnetic field annealing temperature is 438 K, the alloy displays the optimal magnetic properties. Compared with the as-spun sample, the values of intrinsic coercivity (Hci), remanence (Br) and maximum energy product ((BH)max) increase by 3.4%, 9.8% and 18.7%, respectively. This work provides an effective approach by which to enhance the magnetic properties of Ce–Fe–B alloys.

Spin and valence isomerism in cyanide-bridged {FeMII} (M = Fe and Co) clusters.

Two cyanide-bridged V-shaped isostructural trinuclear complexes [{(Tp*)FeIII(CN)3}2MII(bztpen)]·Sol (M = Fe, Sol = CH3OH·3H2O, 1; M = Co, Sol = 2CH3OH·2H2O, 2; bztpen = N-benzyl-N,N',N'-tris(2-methylpyridyl)ethylenediamine; Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) were synthesized and characterized. The bztpen ligand serves as a tetradentate capping ligand around the inner metal ion, leaving one pyridyl group intact. Complex 1 exhibits a spin crossover (SCO) behavior between the {FeIIILSFeIIHSFeIIILS} and {FeIIILSFeIILSFeIIILS} spin isomers, while 2 shows both thermally- and photo-induced electron-transfer coupled spin transition (ETCST) property between the {FeIIILSCoIIHSFeIIILS} and {FeIIILSCoIIILSFeIILS} valence isomers. The total entropy changes for 1 and 2 between their corresponding two electronic states were found to be very close with the values of 87.46 and 84.49 J mol-1 K-1, respectively, indicating the comparable thermal energy barriers necessary for either an SCO or ETCST event for such a given system. Furthermore, both complexes undergo desolvation-induced irreversible and sharp magnetic change at high temperatures.

Slow magnetic relaxation in well crystallized, monodispersed, octahedral and spherical magnetite nanoparticles

Thermally activated relaxation over energy barriers concurrently related to local properties and interparticle interactions constitutes a major contribution to both the coercivity and the applied field frequency dependence of that quantity. We have measured the slow magnetic relaxation of magnetite nanoparticles (NPs), synthetized by using the oxidative precipitation technique, having spherical and octahedral shapes, monodispersed size distributions and similar transverse dimensions. From our relaxation data we evaluated the temperature dependencies of a) the irreversible demagnetization susceptibility, b) the fluctuation field (associated to the thermally induced demagnetization occurring during the measuring time range) and c) the activation volume (corresponding to the demagnetization produced by the fluctuation field). We conclude that i) the irreversible susceptibility peaks in both samples at ca. 135 K (Verwey transition temperature) and ii) the monotonically increasing temperature variation of the activation volume shows the same values in both samples for temperatures below ca. 135 K and at 290 K reaches values corresponding to 10 and 30 times the average particle volume of the spherical and octahedral NPs, respectively. Those large increases of the activation volume are compatible with a transition from local to collective of the thermally activated processes.

Irreversible magnetic processes under biaxial and uniaxial magnetic anisotropies

Irreversible magnetic processes have been investigated in magnetic systems with two different anisotropy symmetries (uniaxial and biaxial) through angular measurement of the switching field, the irreversible susceptibility and the magnetic viscosity. These two systems consist of two-dimensional cobalt ferrite hetero-structures epitaxially grown on (100) and (110) MgO substrate. It is found that for uniaxial anisotropy the irreversible characteristics of the magnetization are large and display a strong angular dependence, which exhibits its maximum at the easy axis and drops quickly to vanish at the hard axis. However, for biaxial anisotropy the magnetization irreversible characteristics are considerably reduced and are less sensitive to the field angle.

Hysteresis in single and polycrystalline iron thin films: Major and minor loops, first order reversal curves, and Preisach modeling

Hysteretic behavior was studied in a series of Fe thin films, grown by molecular beam epitaxy, having different grain sizes and grown on different substrates. Major and minor loops and first order reversal curves (FORCs) were collected to investigate magnetization mechanisms and domain behavior under different magnetic histories. The minor loop coefficient and major loop coercivity increase with decreasing grain size due to higher defect concentration resisting domain wall movement. First order reversal curves allowed estimation of the contribution of irreversible and reversible susceptibilities and switching field distribution. The differences in shape of the major loops and first order reversal curves are described using a classical Preisach model with distributions of hysterons of different switching fields, providing a powerful visualization tool to help understand the magnetization switching behavior of Fe films as manifested in various experimental magnetization measurements.

The effect of stress and incentive magnetic field on the average volume of magnetic Barkhausen jump in iron

The average volume of magnetic Barkhausen jump (AVMBJ) v¯ generated by magnetic domain wall irreversible displacement under the effect of the incentive magnetic field H for ferromagnetic materials and the relationship between irreversible magnetic susceptibility χirr and stress σ are adopted in this paper to study the theoretical relationship among AVMBJ v¯(magneto-elasticity noise) and the incentive magnetic field H. Then the numerical relationship among AVMBJ v¯, stress σ and the incentive magnetic field H is deduced. Utilizing this numerical relationship, the displacement process of magnetic domain wall for single crystal is analyzed and the effect of the incentive magnetic field H and the stress σ on the AVMBJ v¯ (magneto-elasticity noise) is explained from experimental and theoretical perspectives. The saturation velocity of Barkhausen jump characteristic value curve is different when tensile or compressive stress is applied on ferromagnetic materials, because the resistance of magnetic domain wall displacement is different. The idea of critical magnetic field in the process of magnetic domain wall displacement is introduced in this paper, which solves the supersaturated calibration problem of AVMBJ-σ calibration curve.

Spin freezing in the spin-liquid compoundFeAl2O4

Spin freezing in the $A$-site spinel FeAl$_2$O$_4$ which is a spin liquid candidate is studied using remnant magnetization and nonlinear magnetic susceptibility and isofield cooling and heating protocols. The remnant magnetization behavior of FeAl$_2$O$_4$ differs significantly from that of a canonical spin glass which is also supported by analysis of the nonlinear magnetic susceptibility term $\chi_3 (T)$. Through the power-law analysis of $\chi_3 (T)$, a spin-freezing temperature, $T_g$ = 11.4$\pm$0.9~K and critical exponent, $\gamma$ = 1.48$\pm$0.59 are obtained. Cole-Cole analysis of magnetic susceptibility shows the presence of broad spin relaxation times in FeAl$_2$O$_4$, however, the irreversible dc susceptibility plot discourages an interpretation based on conventional spin glass features. The magnetization measured using the cooling-and-heating-in-unequal-fields protocol brings more insight to the magnetic nature of this frustrated magnet and reveals unconventional glassy behaviour. Combining our results, we arrive at the conclusion that the present sample of FeAl$_2$O$_4$ consists of a majority spin liquid phase with "glassy" regions embedded.

Modelling of isothermal remanence magnetisation curves for an assembly of macrospins

Abstract We present a robust and efficient framework to compute isothermal remanent magnetisation (IRM) curves for magnetic nanoparticle assemblies. The assembly is modelled by independent, randomly oriented, uniaxial macrospins and we use a Neel model to take into account the thermal relaxation. A simple analytic expression is established for a single size, in a sudden switching approximation, and is compared to more evolved models. We show that for realistic samples (necessarily presenting a size dispersion) the simple model is very satisfactory. With this framework, it is then possible to reliably simulate IRM curves, which can be compared to experimental measurements and used in a best fit procedure. We also examine the influence of several parameters on the IRM curves and we discuss the link between the irreversible susceptibility and the switching field distribution.

Effects of vacancy and exchange-coupling between grains on magnetic properties of SrFe12O19 and α-Fe2O3 composites

All samples were synthesized via supercritical (390°C) and low-temperature (190°C) hydrothermal methods followed by annealing at 1000°C, and contained both SrFe12O19 and α-Fe2O3 phases as reported in literature. The magnetic properties sensitively depended on the relative content of each phase, which was generally attributed to phase homogeneity and microstructural characteristics in literature as far as we know. In this paper, we observed a double-peak behavior in the curve of irreversible susceptibilities χirr versus an external magnetic field H, which was assigned to the cation vacancies and their distribution. The coercivity of the sample almost changed linearly with the average value of magnetic fields at which the χirr (H) peak located. Therefore, we conclude that the magnetic parameters, at least the coercivity, are mainly determined by cation vacancies which have usually been neglected before in SrFe12O19.

Interpretation of thermal dependence of magnetic aftereffect for magnetic nanocomposite with slow decay rates

A new experimental characterization is presented of time-, field-, and temperature-dependent dynamic effects in magnetization of a nanocomposite which displays slow decay. Field and temperature variations of irreversible susceptibility, χ irr , decay coefficient, S, fluctuation field, hf , and activation volume, V, have been calculated for the nanocomposite sample (Co80Ni20) using a recently developed modified Preisach–Arrhenius (MPA) model. The sample is composed of non-interacting nanoparticles having negligible reversible magnetization. Non-Arrhenius behavior is observed in both the maximum decay coefficient, S max, and the fluctuation field, hf , as a function of temperature T. The peak of both temperature curves are identical and occur at a critical temperature Tk of ∼50 K, which agrees with our experimental results. Based on the effect of a temperature-dependent chemical potential on energy barrier, hf is studied for T < Tk and T ≥ Tk , respectively. A more complete MPA model that can predict the magnetization as function of time, field and temperature for a magnetic material with slow decay rates is proposed. This model uses a multi-variable analytical formula, , which incorporates the characteristic parameters.

Effects of the Fe3+/Sr2+molar ratio on the exchange-coupling of the composite and single-phase SrFe12O19

It has been systematically investigated about the magnetic properties of the composite and single-phase SrFe 12 O 19 prepared with the precursor of different Fe 3+ /Sr 2+ molar ratios of 12:1, 11:1 and 10:1, respectively. With increasing the calcinations temperature, the coercivity of all samples decreases which can be determined by the stress model. The δm(H) results indicate that the intergrain exchangecoupling interaction exists both in composite and in single-phase SrFe 12 O 19 , resulting in the remanence magnetization enhancement, and the magnetostatic interaction become dominant in the samples with higher calcinations temperatures. The irreversible susceptibilities χ irr (H) curves show that there may exhibit the exchange-spring behavior in the composite SrFe 12 O 19 with more soft magnetic γ -Fe 2 O 3 , and the irreversible switching field may be qualitatively used to determine the coercivity magnitude.

Study on the magnetic viscosity of Nd45Pr15Fe30−xCoxAl10 (x=0, 10) bulk amorphous Alloys

The bulk amorphous Nd45Pr15Fe30−xCoxAl10 (x=0, 10) alloys, which were prepared by arc melting and investigated by XRD and VSM, present hard magnetic behavior at room temperature. The intrinsic coercivity of the bulk amorphous increased with increasing Co addition. A single sharp and narrow peak close to the coercivity appeared in the irreversible susceptibility (χirr) curve for each alloy. The addition of Co has little influence on the magnetic viscosity of the alloys. With the substitution of Co for Fe, the fluctuation field Hf increased from 9.9mT to 11.5mT, and the activation volume Va increased from 2.9×10–18cm3 to 3.6×10–18cm3, which leads to an increase of the activation diameter Da from 17.7nm to 19.0nm. The value of Da deduced from the magnetic viscosity measurements is similar to the size of clusters of Nd-based bulk amorphous alloys, which may be responsible for the hard magnetic property exhibited at room temperature.

Maximum Susceptibility of Ferromagnetic Hysteresis

We report measured ferromagnetic susceptibility and our unifield model of hysteresis, physically describing hysteresis from minor to saturate loops and virgin and anhysteretic susceptibilities. Normal susceptibility for reversals, ΔM/ ΔH , and for the virgin curve, M/H, is the sum of anisotropic reversible and cooperative irreversible susceptibilities. Effective field is derived from Maxwell's thermodynamic and electromagnetic equations as the sum of Amperian, shape demagnetization, internal and external stress, eddy current and thermal fields. Rate dependence is modeled by time shifts and amplitude changes to our functions. We measure Faraday induction of ring and plate samples and vibrating sample magnetometer moments of a disc sample of mild steel, discussing procedures and precision. We find anhysteretic susceptibility to be our saturate susceptibility for positive magnetization, reflected in symmetry, with maximum susceptibility modified by each of the five fields.

Magnetic properties of chemical coprecipitated Sr0.8La0.2Fe11.8Co0.2O19 powders

For this work, Sr0.8La0.2Fe11.8Co0.2O19 hexaferrite powders were manufactured, using the chemical co-precipitation method. Major and minor magnetization loops recorded at room temperature show that the optima hard magnetic properties are achieved when the calcination temperature is 1000°C. SEM, TEM and XRD measurements show that the 1000°C calcined powder consists of single hexaferrite phase fine particles with a mean diameter of 90nm. From remanence magnetization, coercivity, magnetic viscosity and irreversible susceptibility measurements there can be concluded that the powder consists of single domain magnetic particles of platelet shape.

Time-dependent magnetization in epitaxial hard magnetic thin films

Time-dependent magnetic behaviour at room temperature is investigated in highly oriented epitaxial hard magnetic thin films. Single-layer films with nominal composition PrCo5 and PrCo7 were studied, along with two SmCo5/Fe/SmCo5 trilayer films, with Fe layer thicknesses of 11 and 16 nm. The films were prepared using pulsed laser deposition on Cr buffered MgO (110) substrates. For the trilayer system, judicious choice of the thickness of the hard (SmCo5) and soft (Fe) magnetic layers results in an exchange-coupled two-phase magnet. All the thin films exhibit switching behaviour, i.e. a near square hysteresis loop, and the details of this behaviour are explored through both measurement of the hysteresis loop, at very slow field sweep rates, and the irreversible susceptibility, χirr. Measurements of the sweep rate dependence of the intrinsic coercivity, and magnetic viscosity on the major hysteresis loop were performed. For each film, three techniques were used to determine the fluctuation field, Hf: from the sweep rate data; from the magnetization relaxation data using the waiting time method; and from S/χirr, where S is the magnetic viscosity coefficient. Differences in the value of Hf determined using the three different techniques, and the equivalence of the three techniques, are discussed. Further insights into the time-dependent behaviour of the magnetization are gained from observations of the spontaneous remagnetization, following dc demagnetization. The study shows the thin films to be of high-stability. Furthermore, it supports the view that magnetization reversal in these materials is most likely an incoherent process, and not a process involving the uniform reversal of a grain. The model of weak pinning provides a useful framework for understanding magnetization processes.

A new ferromagnetic hysteresis model for soft magnetic composite materials

A new ferromagnetic hysteresis model for soft magnetic composite materials based on their specific properties is presented. The model relies on definition of new anhysteretic magnetization based on the Cauchy–Lorentz distribution describing the maximum energy state of magnetic moments in material. Specific properties of soft magnetic composite materials (SMC) such as the presence of the bonding material, different sizes and shapes of the Fe particles, level of homogeneity of the Fe particles at the end of the SMC product treatment, and achieved overall material density during compression, are incorporated in both the anhysteretic differential magnetization susceptibility and the irreversible differential magnetization susceptibility. Together they form the total differential magnetization susceptibility that defines the new ferromagnetic hysteresis model. Genetic algorithms are used to determine the optimal values of the proposed model parameters. The simulated results show good agreement with the measured results.

Sweep rate dependence of the coercivity and fluctuation field in some RE-Fe(Co)–Al, RE=Nd or Pr, bulk amorphous ferromagnets

Bulk amorphous alloys of composition Nd60Fe30Al10, Nd60Fe20Co10Al10, and Pr58Fe24Al18 have been prepared, and all alloys exhibit hard magnetic properties at room temperature. Magnetic field sweep rate measurements show the intrinsic coercivity to be proportional to the logarithm of the field sweep rate. The fluctuation field, Hf, for each alloy, is determined from both sweep rate and magnetic viscosity measurements. In the case of the latter it is determined by using both the waiting time method and S/χirr, where S is the magnetic viscosity parameter and χirr the irreversible susceptibility. Differences in the value of Hf determined from the sweep rate and magnetic viscosity experiments are discussed, and the value of the activation volume for each alloy determined.

Magnetic and anomalous magnetic viscosity in the bulk amorphous ferromagnet [formula omitted] and partially amorphous ferromagnet [formula omitted

A number of ferromagnetic Pr–Fe–Al alloys have been prepared by argon arc melting and quenching into a copper mould. The alloy of composition Pr 58Fe 24Al 18 is identified as being amorphous (bulk metallic glass or bulk amorphous ferromagnet), and a range of magnetic measurements have been performed to explore differences and similarities between it and a partially amorphous alloy, containing a significant crystalline fraction, Pr 60Fe 24Al 16. For both alloys, measurements of the irreversible susceptibility, and magnetic viscosity on the major hysteresis loop are reported. From the magnetic viscosity data, the fluctuation field is determined. The behaviour of the anomalous magnetic viscosity (non-monotonic behaviour of the magnetic viscosity, where the magnetisation as a function of time is seen to increase, reach a peak, and then decrease), on the recoil curve that leads to the dc demagnetised state is investigated. Both alloys display non-monotonic behaviour. After dc demagnetisation, spontaneous remagnetisation is observed in both alloys, and some comments are made on the thermal remagnetisation behaviour of the amorphous alloy. The anomalous magnetic viscosity is interpreted in the context of the Preisach model, as it predicts a simple functional relationship between the time taken to reach a peak and the applied magnetic field. The experimental data for both alloys is in good agreement with this prediction.

Relevance of pinning, nucleation, and interaction in nanograined epitaxial hard magneticSmCo5films

Epitaxial growth of ${\text{SmCo}}_{5}$ films on substrates with two different symmetries was used to tailor film texture. A geometry where the easy axes of all grains are aligned in parallel is compared with a geometry where two sets of grains have a perpendicular alignment. For these two types of films with well defined nanoscaled microstructures, the role of pinning, nucleation, and intergrain interactions for the key permanent magnetic properties, namely, coercivity and remanence, is studied. The angular dependency of switching field and remanence reveals that the depinning of domain walls rules the magnetization reversal. In addition, these measurements indicate that the magnetization processes occur independently within each set of perpendicularly aligned grains. Although a large positive $\ensuremath{\delta}J$ indicates strong intergrain interactions, remanence enhancement is absent in both samples. An analysis of the irreversible susceptibility during the magnetizing and demagnetizing processes gives a complete picture of the relevant processes, and shows that both pinning and nucleation of domains are involved.