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Magnetic viscosity analysis is a widely studied topic in understanding the stability and magnetization mechanisms of magnets. However, observing the minute changes in magnetic domains during the magnetic viscosity process poses a challenge in research. To address this issue, a self-made in-situ magnetized sample holder was used in this study to observe the dynamic evolution of domain structures in alnico alloy during magnetic viscosity. The findings indicate that the domain reversal during magnetic viscosity is largely irreversible, with only minor reversible changes observed in the inhomogeneous region. The magnetization reversal in alnico alloy occurs through the reversal of α1 phases, which follows a single domain particle reversal mode. The zero-field activation energy (E 0) of alnico alloy is determined as 3.30 × 10−17 J, and the coercivity field without thermal disturbance (H 0) is measured at 1779.19 Oe. This research method proves to be a valuable tool in the study of magnetic materials. [Display omitted] [ABSTRACT FROM AUTHOR]

• Effect of magnetic field intensity during spinodal decomposition on microstructure of alnico magnets. • The randomly oriented a1 phase reduces the decay rate of magnetization. • The main factor affecting the decay rate of magnetization is irreversible magnetic domain reversal. The slow decay of permanent magnet magnetization with storage time will greatly affect the accuracy of precision instruments. The alnico is a traditional permanent magnetic material with excellent stability, and the research on its stability cannot be ignored. Magnetic field heat treatment is a key process that affects the microstructure of the alnico alloy. The alnico alloys with different α1 phase orientations and aspect ratios were prepared by adjusting the magnetic field intensity during spinodal decomposition. With the increase of magnetic field strength, the aspect ratio of the α1 phase increases significantly, and the orientation becomes better, which leads to the increase of coercive force and remanent magnetization, but the magnetic viscosity coefficient decreases. The demagnetization process of alnico samples was characterized by Lorentz transmission electron microscopy (LTEM) and electron holography. At the magnetic field interval of 200 Oe, the area of irreversible magnetization reversal of the S3 is significantly larger than that of the S0. The first-order reversal curves (FORC) show that most the randomly oriented α1 phases are reversible magnetization reversal, and the interaction field is smaller than that of neatly arranged α1 phases. This structure results in better stability. The alnico alloy without a magnetic field during spinodal decomposition sacrifices some magnetic properties but obtains better stability, which provides a research direction for stabilizing treatment by adjusting microstructure. [ABSTRACT FROM AUTHOR]

The magnetic viscosity, S , of Mn-Ga and Mn-Al alloys with the L1 0 type crystal structure has been measured at temperatures in the range 2–300 K. The values of the fluctuation field, S v , and the coercive field, H c , of the two types of materials are comparable to each other. The observed dependence of S v on the applied field, H , is not in accordance with the theory of magnetic viscosity of particulate recording media. The relationship between H c and S v , the latter measured at H = − H c , fits the Barbier relationship, S v ∼ H c α , rather well. [ABSTRACT FROM AUTHOR]

Noble/transition bimetallic nanowires of nominal composition Fe 90 Rh 10 are AC electrodeposited into 20 nm diameter hexagonally self-assembled alumina nanopores. Nanowires about 18 nm in diameter and 1 μm long are polycrystalline and multiphase. Wires contain α-Fe grains and very small grains of the ClCs-type α′-Fe(Rh) phase. The room temperature magnetization mechanism and the thermal stability of nanowire magnetic configurations are further investigated by measuring the dependence of the coercive field on the applied field sweeping rate R. From these data a mean fluctuations field value of μ 0 H FR = (9.0 ± 0.5) mT is obtained (at coercivity) and an effective activation magnetic moment μ ac = 5400 μ B is estimated, with μ B the Bohr magneton. At the coercive field (about 45 mT) the resulting activation lengths become l AC ≈ 6.4 and 6.7 nm for α-Fe and α′-Fe(Rh), respectively. Assuming an effective magnetic anisotropy, considering magnetostatic shape contributions in addition to the magnetocrystalline one, the domain wall thickness δ w in α-Fe grains and in the α′-Fe(Rh) become δ wFe = 13.4 nm and δ wFeRh = 10.9 nm respectively. These values are comparable to the activation lengths estimated at the coercive field in each phase. These facts strongly indicate that irreversible polarization reversal in these nanowires takes place by local curling, involving localized nucleation modes. [ABSTRACT FROM AUTHOR]

The magnetic properties and magnetic entropy changes of DyCo 2 B x ( x =0, 0.05, 0.1, and 0.2) alloys were investigated. The Curie temperature ( T C ) increases with increasing B concentration. The frequency dependence of ac magnetic susceptibility of DyCo 2 caused by the narrow domain wall pinning effect is depressed by B doping, but the coercivity and the magnetic viscosity are prominently increased in the B doped alloys. The magnetic transition nature of DyCo 2 B x changes from the first-order to the second-order with increasing x , which leads to the decrease of the maximum magnetic entropy change. However, the relative cooling power (RCP) of DyCo 2 and the B doped alloys remains nearly constant. [ABSTRACT FROM AUTHOR]

The exponential model of ferromagnetic susceptibility for steel has been extended to include ferromagnetic viscosity by comparing measured eddy current free susceptibility with the static exponential model. Viscous susceptibility and viscous field advance are derived from the definition of ferromagnetic viscosity. Measurements were performed on both a pipe in an isothermal yoke and a disk in a vibrating sample magnetometer. Yoke design is described and disk corrections for magnetized volume fraction and field demagnetization were computed using a quasistatic magnetization volume integral equation. Accuracy of this model suggests that the four parameters of the static model plus two parameters of the Gaussian model plus one eddy current delay parameter form a complete description of ferromagnetic hysteresis. • Designed and fabricated high field yoke to apply stress to isothermal pipe sample. •Verified computed volume and demagnetization corrections to VSM M(H) data. •Conceived of viscous Gaussian gain factor to precisely fit hysteresis and susceptibility data. •Achieved complete four process seven parameter ferromagnetic hysteresis model. [ABSTRACT FROM AUTHOR]

Abstract: The bulk amorphous Nd45Pr15Fe30−x Co x Al10 (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 (χ irr) 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 H f increased from 9.9mT to 11.5mT, and the activation volume V a increased from 2.9×10–18 cm3 to 3.6×10–18 cm3, which leads to an increase of the activation diameter D a from 17.7nm to 19.0nm. The value of D a 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. [Copyright &y& Elsevier]

Abstract: The quasilogarithmic magnetic viscosity of the perpendicularly anisotropic Nd–Fe–B films is determined by measuring the time dependent magnetization in coercive fields at 300K–4.2K. The distribution of energy barrier heights in the films is believed to be the origin of this stretched exponential magnetic relaxation. Another feature exhibited in the anisotropic Nd–Fe–B films is the nonmonotonic temperature dependence of the magnetic viscosity coefficients, which are extracted by the decay slopes of the linear quasilogarithmic magnetic aftereffect curves. [Copyright &y& Elsevier]

Abstract: This work is an investigation of the experimental method used for measuring the magnetic viscosity in a hard ferromagnetic material, i.e. the recording of the magnetization under constant applied field and temperature, after the material has been magnetically saturated. It investigates how the experimental results are affected by the initial conditions of the method (saturation field, field change rate and field oscillation prior to its stabilization), and by minor variations of field and temperature during the recording. Based on the arising conclusions and the use of a more complex fitting function of measurements, the accuracy and repeatability of experimental results is improved. [Copyright &y& Elsevier]

Abstract: This article presents an anomalous magnetic viscosity in exchange-spring magnet. A similar effect has been observed in non-interacting or weakly interacting systems but not in a strong interacting magnetic systems. We reported a new procedure to measure magnetic relaxation under various magnetic fields. Changing the applied magnetic field by different field protocols during the reversal process, we found that a memory effect of the magnetization appeared if the field change is large enough. The mechanism of the phenomenon can be explained in the model of conventional magnetic reversal in strong ferromagnetic systems with an energy barrier distribution. The study of such magnetic relaxations can provide some information related to the energy barrier distribution function. [Copyright &y& Elsevier]

Abstract: We have developed a simple numerical model for simulating domains as well as remanence and viscosity curves in the slow dynamics regime, for thin films characterized by perpendicular magnetization and irregular domain configurations due to strong disorder. The physical system is represented as constituted of identical switching units, described by proper switching field distributions and energy barrier laws for pinning and nucleation processes. The model also includes an effective field which accounts for magnetic forces proportional to magnetization, on average. Simulations of DCD curves show that when the reversal of magnetization is governed by pinning, the coercive field depends on the physical size of the film area on which the external field is applied. In the case of viscosity phenomena described by a linear energy barrier law associated with a single predominant reversal process (pinning or nucleation), universal viscosity curves can be generated by properly transforming the DCD curve of the system. We also demonstrate that a reduction of the maximum viscosity coefficient can coexist with a reduction of the energy barrier heights. [Copyright &y& Elsevier]

Abstract: 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 Pr58Fe24Al18 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, Pr60Fe24Al16. 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. [Copyright &y& Elsevier]

Abstract: The correspondence between the crystallographic texture and intergranular exchange coupling interactions, with the switching mechanism and the thermal response of the magnetization in CoCrPt–SiO2 perpendicular recording media was investigated. Virgin hysteresis and isothermal remanence magnetization measurements both showed a three-stage process, which was interpreted to indicate that the Stoner–Wohlfarth coherent reversal mode is the dominant switching mechanism irrespective of the texture. For media samples with similar degree of texture, improvement in exchange decoupling of the media grains caused an increase in the onset field for the virgin magnetization process. The thermal decay of the magnetization, evaluated via the field-dependent viscosity coefficient peaked near the nucleation field, and the peak value showed a strong dependence on the strength of the exchange coupling interactions. A model establishing the role of the texture and exchange interactions in perpendicular recording media is put forth. [Copyright &y& Elsevier]

Abstract: The variation of the applied field results in a subsequent change of magnetization with time. There is a relationship between the coercivity (H c), as the equilibrium characteristic of the system, and its magnetic stability (1/S), as a parameter characterizing the time dependence. 1/S as a function of H c has been measured and studied for different Fe1− x Co x samples. We synthesized several samples with different values of x by applying various magnetic fields during the grains’ growth, and observed a linear relationship between 1/S and H c. [Copyright &y& Elsevier]

Abstract: Low-temperature viscous acquisition and decay measurements above and below the Verwey transition have been carried out for a selection of natural and synthetic multidomain magnetite samples. A strong correlation between the viscosity spectra and published disaccommodation spectra was found, where disaccommodation reflects electron mobility. Assuming the viscosity is controlled by identical mechanisms as disaccommodation, the reduction in electron mobility below the Verwey transition is found to significantly increase viscous acquisition and decay rates over the time scales measured (1–3000s). Although strongly affecting the viscosity, disaccommodation processes do not appear to control the rate of change of viscosity with time, i.e. the viscosity curvature. It is suggested that the curvature is controlled by the shape of relaxation-time distributions, which is approximately the same for all the magnetite samples studied. In addition, the acquisition and decay curvature parameters mirror each other when plotted as a function of temperature, inferring that at any given temperature, the acquisition and decay processes are identical. [Copyright &y& Elsevier]

Abstract: The strong influence of Cu substitution on the magnetic properties of SmCo5, especially on the coercive field, is well known. Induction-melted samples () show a phase separation within the 1:5 structure. An unusual jump from a low to a high magnetisation state in an increasing external field is observed. Using pulse field, high coercive forces up to about μ 0 J H c≈30T and a high critical field μ 0 H U for the unusual jump reaching 45T at 4.2K for are observed for the first time. H c and H U increase with decreasing temperature. An influence of the field changing rate on the measured H U is found. [Copyright &y& Elsevier]

Abstract: Five specially prepared elongated metal particle pigments have been analysed to determine the detailed switching behaviour and magnetic viscosity. The particles varied in length from about 130nm down to 40nm. Hysteresis loops and time dependence measurements were made which led to a calculation of the activation volume of reversal. The activation volumes reduce with the particle size and are comparable for the smallest pigments. Physical particle sizes were measured by TEM. The objective of the work is to find the limiting size of the current technology determined by superparamagnetic behaviour. [Copyright &y& Elsevier]

The scope of this article is to report very detailed results of the measurements of magnetic relaxation phenomena in the new Cu0.5Fe2.5O4 nanoparticles and known CuFe2O4 nanoparticles. The size of synthesized particles is (6.5±1.5) nm. Both samples show the superparamagnetic behaviour, with the well-defined phenomena of the blocking of magnetic moment. This includes the splitting of zero-field-cooled and field-cooled magnetic moment curves, dynamical hysteresis, slow quasi-logarithmic relaxation of magnetic moment below blocking temperature. The scaling of the magnetic moment relaxation data at different temperatures confirms the applicability of the simple thermal relaxation model. The two copper-ferrites with similar structures show significantly different magnetic anisotropy density and other magnetic properties. Investigated systems exhibit the consistency of all obtained results. [Copyright &y& Elsevier]

The magnetic viscosity coefficient in YxSm1-xCo3Cu2 alloys, with x=0, 0.25, 0.5, 0.75, and 1, was determined from measurements of the magnetization reversal relaxation close to the coercive field using a vibrating sample magnetometer and from the coercive field relaxation by the sweep-rate method using a pulse-field magnetometer, respectively. The coercive field and the magnetic viscosity coefficient, as determined from both methods, decrease with increasing yttrium concentration. However, the magnetic viscosity coefficient obtained from magnetization relaxation is always smaller (about six times) than that obtained by means of pulsed field measurements. These results are explained on the basis of different time windows of the measurements, proving different flat distributions of energy barriers due to a complex distribution of activation energies. [Copyright &y& Elsevier]

A magnetic heat treatment is employed for [Sm&z.sbnd;Co (21 nm)/Fe65Co35 (7 nm)]10 multi-layered films prepared by magnetron sputtering. It is found that applying magnetic field along the plane of films during annealing (magnetic heat treatment) is favorable for enhancing the coercivity of the films, compared with annealing under no field. The exchange interaction is analyzed in detail by measuring the remanence magnetization. The contribution of the exchange interaction to the magnetic reversal can be deduced from the δm curves and the annealing temperature dependence of coercivity. Additionally, the magnetic viscosity studies reveal that the magnetic heat treatment improves the stability of magnetic properties of the films. [Copyright &y& Elsevier]

The magnetic properties of chemically synthesized high anisotropy L10 [Fe49Pt51]88Ag12 nanoparticle arrays have been studied as a function of annealing temperature. Particles were prepared by the simultaneous polyol reduction of platinum acetylacetonate and silver acetate and the thermal decomposition of iron carbonyl, yielding monodispersed particles of diameter ∼3.5 nm. Addition of Ag lowers the ordering temperature of self-assembled arrays by ∼150°C. After annealing at Ta=500°C for 30 min in an Ar/H2 atmosphere, the coercivity was 13,800 Oe. TEM and delta-M measurements indicate weak particle aggregation up to Ta=400°C, with evidence of sintering at higher temperatures. Large ratios of remanent to hysteresis coercivity indicate a large distribution in anisotropy energies. Anomalously large thermal stability constants, KV/kBT, and switching volumes were measured, even in samples with very little evidence of sintering. Zero field viscosity versus remanence curves show evidence of exchange interactions. [Copyright &y& Elsevier]

Zero- and finite-temperature coercivity mechanisms in disordered nanostructures are investigated by model calculations. Three different aspects are considered. First, it is shown that strongly reduced exchange in grain-boundary regions changes the power-law scaling exponent for the coercivity of random-anisotropy magnets. Second, it is analyzed how random interatomic exchange affects the thermal blocking and therefore the coercivity of spin glasses. Third, a master-equation approach is used to quantitatively elucidate the relation between the magnetic-viscosity regime and the sweep-rate dependence of the coercivity. A common feature of the considered mechanisms is that interatomic exchange creates nanoscale cooperative units which realize the coercivity in real space. [Copyright &y& Elsevier]

Reverse domain nucleation time measurements have been performed on two Tb/Fe multilayer magneto-optic films exhibiting different degrees of domain wall pinning. A linear relationship between ln (reverse domain nucleation time) and the applied field has been predicted and observed for a sample exhibiting weak domain wall pinning. This is in agreement with theoretical work presented which addresses time dependence in systems possessing weak domain wall pinning. A non-linear relationship applicable over a restricted field range has been derived for a sample exhibiting strong domain wall pinning. Experimental results have indicated that this relationship is also valid. [Copyright &y& Elsevier]

We introduce a dynamic exponential model of ferromagnetic hysteresis that unifies the cooperative exponential model with the Néel fluctuating field, Jiles-Atherton approach to the anhysteretic and a Preisach convolution integral over a field distribution function. Dynamic effects such as viscous field advance and eddy current field delay are modeled directly by the model parameters. The dynamic effect of creeping in the direction of the previous maximum susceptibility due to viscosity as predicted by Street is observed and is modeled using the proposed method. Negative tilting of asymmetric minor hysteresis loops (lower susceptibility) with cycling is also observed and is also modeled using the proposed model as increased Néel's "opposition" in our cooperative field, H coop. Shearing of hysteresis loops is modeled as an averaging of the exponential model over viscous advance and eddy current delay fields, causing truncation and broadening. Detailed measurement procedures are described for a hollow cylinder and thin and thick rings of Fe94Ni3Cr2Mo1 steel with variable frequency sine, triangle and step field waveforms. • Unifies exponential, Néel, Jiles-Atherton and Preisach models. • Models creeping and negative tilting with field cycling. • Averages susceptibility over viscous and eddy current fields. • Reversible susceptibility approaches anhysteretic and returns. • Describes minor and saturate hysteresis loops with same parameters. [ABSTRACT FROM AUTHOR]

Abstract: When a uniaxial magnetic field is applied to a non-magnetic dispersive medium filled with magnetic nanoparticles, they auto-assemble into thin needles parallel to the field direction, due to the strong dipolar interaction among them. We have prepared in this way magnetically oriented nanocomposites of nanometer-size CoFe2O4 particles in a polydimethylsiloxane polymer matrix, with 10%w/w of magnetic particles. We present the characteristic magnetic relaxation curves measured after the application of a magnetic field forming an angle α with respect to the needle direction. We show that the magnetic viscosity (calculated from the logarithmic relaxation curves) as a function of α presents a minimum at α=0, indicating slower relaxation processes associated with this configuration of fields. The results seems to point out that the local magnetic anisotropy of the nanoparticles is oriented along the needles, resulting in the macroscopic magnetic anisotropy observed in our measurements. [Copyright &y& Elsevier]

Abstract: Doping magnesium oxide with cobalt and nickel by ion implantation, results in the formation of nanosized metallic clusters [Pinto, et al., Eur. Phys. J. B 45 (2005) 331]. Single crystals of [100] MgO were implanted with the transition ions, Co and Ni, with 250keV energy and fluence of 1×1017 cm−2. After vacuum annealing at 1270K the implanted samples exhibit superparamagnetic behaviour associated with the formation of metallic clusters. The magnetic relaxation of these clusters was studied using magnetization measurements, in order to characterize the nanosized aggregates. [Copyright &y& Elsevier]

Abstract: We have studied the thermal stability of L10 FePt thin films and Fe–FePt exchange-spring (ES) bilayers grown on (100) MgO by RF sputtering. The viscosity curves showed both for FePt films and bilayers a clear logarithmic decay of magnetization. Moreover, it was possible to evaluate the viscosity coefficient S for different applied reverse fields and the activation volumes at the coercivity. The latter values were then related to structural, magnetic and morphological measurements performed on the samples. [Copyright &y& Elsevier]

Abstract: The interactions in ternary and quaternary Co-based alloy thin films for longitudinal recording media, with different thickness, are studied. The analysis is performed through the measurement of the initial magnetization and ordinary hysteresis curves. The interactions result stronger in quaternary than in ternary alloy films and when the film thickness is smaller. These findings are discussed in relationship with the evolution of the magnetization switching, characterized by a tendency towards a more coherent rotation of the magnetization in single-domain grains of thinnest and quaternary films, owing to the complex structure of these films. The impact of this evolution on the thermal stability of the magnetic properties is also discussed. [Copyright &y& Elsevier]

Abstract: A system of particle pairs with partial exchange coupling is studied, considering identical particles and a fixed angle between their anisotropy axes. The energy of each pair is calculated in terms of the extent of interaction, , as a function of the applied demagnetizing field. Using the probability per unit time for the inversion of magnetization, the coercive field H c and the viscosity S of the system are calculated. An unexpected result is that fully coupled particles are more stable against temperature than the uncoupled particles. [Copyright &y& Elsevier]