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In this work, we present the detailed results of magnetic, structural, and magnetocaloric measurements in Ni–Mn–In-based Heusler alloys, namely, Ni51Mn32.8In16.2, Ni50.2Mn33.6In16.2, and Ni51Mn33.4In15.6. The characteristics of magnetostructural coupling play an important role in the magnetic field-driven behavior of these alloys. A small variation in the stoichiometry composition broadens the magnetic entropy change curve and shifts the critical temperatures by as much as 48 K. The effect of cobalt doping on the magnetic, structural, and magnetocaloric properties in these multifunctional alloys is investigated through characterizing Ni50Co2Mn35In13 and Ni45Co5Mn37In13 samples. Cobalt doping affects the martensitic and magnetic transformation temperatures and enhances the ferromagnetic properties of the parent austenitic phase. Therefore, the magnetization change accompanying the martensitic transformation is significantly improved, resulting in a large magnetic entropy change of 21.6 J kg $^{-1}\,\,\text{K}^{-1}$ at 1.5 T. Additionally, Co doping broadens the operating temperature window to 21 K around room temperature. Consequently, a significant effective refrigeration capacity of 254 J kg−1 for 1.5 T is obtained which is comparable or even superior to that of the high-performance, though expensive, rare-earth-based magnetocaloric refrigerants. The high performance, low cost, nontoxicity, and tunability make these alloys of great interest and promising prospect for an affordable magnetic refrigeration system.

The splitting of the (57)Fe Mössbauer spectrum in TiFe has been carefully measured in a magnetic field and compared with the zero-field quadrupolar splitting of sodium nitroprusside. The comparison gives a value of 1.699 mm/s at 295 K for the sodium nitroprusside splitting. This determination makes use of the known Knight shift of Fe in TiFe and is independent of any velocity measurement. Comparison is made with the splitting of the inner two lines of a pure Fe spectrum.

Magnetic nanoparticles have attracted a lot of interest due to their importance and potential application in a variety of fields. The complex magnetic behavior exhibited by nanoparticles is governed by many factors, including their size, composition, shape, morphology, and shell-core structure. The increased surface to volume ratio and tailored structure in nanoparticles introduces many size dependent phenomena which may be used to change their chemical and physical properties. Nanostructured magnetic materials are suitable candidates for magnetic refrigeration due to a presence of a large magnetocaloric effect (MCE) in the superparamagnetic system and reduced hysteresis losses. Magnetocaloric nanoparticles have a broader entropy change over a wider temperature span which results in a higher relative cooling power (RCP). The RCP measures how much heat can be transferred between the cold and hot heat exchangers in an ideal refrigeration cycle. Increasing the RCP not only increases the amount of refrigeration obtainable from the particular refrigerant and field excursion, but also tends to increase the thermodynamic efficiency of the cycle. Improvement in RCP mainly relies on broadening the magnetic entropy change by either coupling two phases of magnetic materials with desirable properties or nanostructure synthesis with the main motivation rooted in their inherent tendency to have distributed exchange coupling, which will broaden the magnetic entropy curve. As a result, to have a comprehensive understanding of various factors that can affect the magnetic and structural properties of materials, in this research, different-sized yttrium-iron nanoparticles were synthesized through alkalide reduction chemical synthesis. Powder X-ray diffraction measurements at room temperature were carried out to study the crystal structures. Surface morphology and size of the synthesized powder alloys were characterized by scanning and transmission electron microscopy. The composition of the powders was determined from energy dispersive X-ray fluorescence. Magnetization measurements were performed using vector vibrating sample magnetometer (VVSM) with standard zero field cooling (ZFC), field cool cooling (FCC), and field cool warming (FCW) techniques. The XRD measurements of the annealed nanoparticle samples fit the yttrium-iron hexagonal closed pack structure and a phase prototype similar to Th 2 Ni 17 , with space group P6 3 /mmc(194). The average particle size of four samples were observed to be 22, 38, 62 and 76 nm, in good agreement with crystallite size estimated from XRD data using Scherrer relation. The magnetic and structural properties of these nanoparticles were measured at different fields and temperatures to study the effect of field, temperature, and particle size variations on the critical parameters, such as Curie temperature (T C ), blocking temperature (T B ), phase transitions, saturation magnetization (M s ), remnant magnetization (M r ), and coercivity (H c ). The temperature dependent magnetization, M(T), under different applied magnetic field up to 20 KOe indicated a change in samples’ T B , which is defined as the temperature corresponding to the maximum point on the ZFC curve after its divergence from the FCC curve. As shown in Fig. 1a, for the 38 nm sample, T B increases by increasing the applied field up to 2 KOe and then decreases with further increase in the applied field. The non-monotonic field dependence of the peak temperature in nanoparticles is attributed to the anisotropic energy barrier distribution of the particles, and to the slowly decreasing magnetization above T B . Moreover, T B increased as the size of nanoparticles increased (Fig. 1b). Results display that T C of nanoparticles depends on both size and shape conditions as it decreased by reducing the particles size. The shape effect is more prominent at sizes less than 40 nm. Additionally, the phase transition became more dominant in low applied filed whereas it is smoothened by increasing the field due to particles’ magnetic saturation. Moreover, magnetic couplings between particles appear stronger for larger particles. The saturation magnetization and coercivity of these nanoparticles were size and field dependent and they both decreased by reducing nanoparticles’ size. Furthermore, as shown in Fig. 2, $H_{C}$ and $M_{S}$ are temperature dependent and they both decrease by increasing temperature since more thermal energy is supplied and individual electron spins become more likely to be in higher energy states, pointing randomly, opposite to their neighbors and less aligned, leading to a reduction in the total magnetization. Therefore, a smaller field is required to reduce remnant magnetization to zero, leading to coercivity reduction. In summary, the comprehensive results of this research are promising and novel as they suggest that the physical and chemical properties of magnetic materials can be customized to suit them in a number of applications in variety of fields such as material science, medicine, engineering, nano-electronics, information technology, etc.

The paper describes investigation of ferromagnetism at low temperatures. We explored the magneto-optical properties, influenced by photon–magnon interactions, of a ferromagnetic Co/Pd multilayered thin film below and above the magnon Bose–Einstein Condensation (BEC) temperature. Analyses of SQUID and MOKE low temperature experimental results reveal a noticeable phase transition in both magnetic and magneto-optical properties of the material at the BEC temperature.

A model for the vector magnetization of a distribution of uniaxial particles is presented. Recent work by the authors decomposed the magnetization into two components and modeled the total vector magnetization as their vector sum. Anisotropy could be ideally modeled as a constant field, but this leads to a model of unidirectional anisotropy. Considering uniaxial media, such approach will only be valid for small angles. Hence, in this paper, a model is proposed that calculates the total magnetization of a distribution of uniaxial particles as the vector sum of irreversible and reversible components. The magnitude of the irreversible component is modeled using a Preisach differential-equation approach; however, other valid models can be used. The direction of the reversible component is modeled using the minimum energy approach of the classical Stoner–Wohlfarth model. The final result of the model is an unsymmetrical lag-angle plot for intermediate applied field values, which is consistent with the measurements.

The use of Maxwell equations to calculate the entropy change (ΔS) in materials that have a first-order phase transition (FOPT) has been questioned as they are only valid at thermal equilibrium [1, 2]. Though, it has been recently shown that this artifact can be minimized after using appropriate protocol for the measurements [3]. The hysteresis losses and magnetocaloric effect of Ni 45 Co 5 Mn (37-x) In (13+x) alloy have been studied in this work. The ingots were prepared by vacuum arc melting technique under an argon atmosphere and were annealed at 900 C for 24 h followed by water quenching. The heating and cooling isothermal magnetizations, M(H), were measured up to 1.5 T and 5 T at a temperature range of 240 K to 338 K. During the heating process the temperature was raised from 240 K to 338 K and at each discrete temperature point, the M(H) was measured. The reverse procedure was carried in the cooling process. The area between the ascending and descending loops of the field-dependent magnetization curves of the Ni 45 Co 5 Mn 37 In 13 alloy was calculated for both heating and cooling processes at different temperatures and is shown in Fig. 1. These areas are interpreted as the magnetic hysteresis losses at different temperatures. The results reveal that under 1.5 T applied magnetic field, the heating process has significantly higher hysteresis compared to the cooling process. Both for the cooling and heating processes, the hysteresis losses are negligible up to 310 K. Though, after 314 K the hysteresis losses are considerable for the heating process, yet negligible for the cooling process. Furthermore, as shown in Fig. 1, by increasing the applied magnetic field up to 5 T, the hysteresis loss curve is shifted to the lower temperatures compared to the ones at 1.5 T. As depicted in this figure the maximum hysteresis losses at 5 T occurs at 312 K, whereas for 1.5 T the peak befalls at 318 K. By interpreting these results we can determine the temperature regions which indirect MCE measurements via Maxwell's and thermodynamic equations will produce the highest discrepancy with the direct MCE measurements. These areas are shown in Fig. 1, for the 1.5 T field (blue shaded temperature region) and the 5 T filed (red shaded temperature region). For instance, the indirect MCE measurements within 316 K to 322 K should be taken with caution for the heating process under 1.5 T. For the 5 T magnetic field this region falls within the temperature range of 307 K to 316 K. To study the magnetocaloric effect of Ni 45 Co 5 Mn (37-x) In (13+x) alloy family (x=0, 0.4) the isofield temperature-dependent magnetization curves, M(T), were derived from isothermal magnetization loops for both ascending and descending magnetic fields, and the entropy changes (ΔS) were derived and are shown in Fig. 2. This figure reveals three distinct results as follow. Firstly, for Ni 45 Co 5 Mn 37 In 13 sample, the peak of the entropy change curve under 5 T applied field occurs at 308 K in the descending curve and at 314 K in the ascending curve, whereas for the 1.5 T filed these peaks happen at 318 K and 320 K, respectively. This emphasizes the fact that the variation of the maximum applied magnetic field within same stoichiometry composition can very well change the critical temperature at which the peak of the entropy change curves occur. So when synthesizing the magnetocaloric materials, one should consider the maximum applied magnetic field in which the magnetic refrigeration system will be operational. Secondly, even a small change in the stoichiometry composition of a material can significantly change the material's critical temperatures. As shown in Fig. 2, the critical temperature of the Ni 45 Co 5 Mn 36.6 In 13.4 has been shifted by about 36 K to lower temperatures compared to the Ni 45 Co 5 Mn 37 In 13 . This suggests that material with high MCE at temperatures out of room temperature can be customized by changing their stoichiometry in order to shift their high MCE toward desirable temperatures. This is promising especially in finding suitable refrigerants for room temperature magnetic refrigeration systems. Thirdly, it is observed that the entropy change curves of Ni 45 Co 5 Mn 36.6 In 13.4 are broader than the ones for Ni 45 Co 5 Mn 37 In 13 . Relative cooling power (RCP) is given by ${RCP}=-\Delta S_{M}(T,H) \times \delta T_{FWHM}$, where $\Delta S_{M}$ is the refrigerant's isothermal magnetic entropy change and $\delta T_{FWHM}$ is the full-widthat-half-maximum of the peak of magnetic entropy. Therefore, the RCP of the Ni 45 Co 5 Mn 36.6 In 13.4 sample is higher than the one for Ni 45 Co 5 Mn 37 In 13 sample. This leads to obtaining a higher refrigeration capacity in a magnetic refrigeration system. In summary, the results of this research are very promising in synthesizing high-performance magnetocaloric materials that can be used to develop practical room temperature magnetic refrigeration systems.

Heusler alloys feature both regular and inverse magnetocaloric effects (MCE) near room temperature as they undergo two different transitions. A temperature scaling methodology to obtain self-similar field dependent curves for materials exhibiting one first-order transition has been previously presented. In this paper, this methodology is modified and extended to obtain self-similar curves for a Ni 51 Mn 32.8 In 16.8 Heusler alloy undergoing two transitions near room temperature. Using this method, the collapsed curve reflects the cluster compositions in the mixed-state regions. The results of characterizing the dual transitions of Heusler alloys and establishing a new model will allow the data to be better analyzed and thus more easily predicted.

The Gd5Ge2Si2 alloy and the off-stoichiometric Ni50Mn35In15 Heusler alloy belong to a special class of metallic materials that exhibit first-order magnetostructural transitions near room temperature. The magnetic properties of this class of materials have been extensively studied due to their interesting magnetic behavior and their potential for a number of technological applications such as refrigerants for near-room-temperature magnetic refrigeration. The thermally driven first-order transitions in these materials can be field-induced in the reverse order by applying a strong enough field. The field-induced transitions are typically accompanied by the presence of large magnetic hysteresis, the characteristics of which are a complicated function of temperature, field, and magneto-thermal history. In this study we show that the virgin curve, the major loop, and sequentially measured MH loops are the results of both repeatable and non-repeatable processes, in which the starting magnetostructural state, prior to the cycling of field, plays a major role. Using the Gd5Ge2Si2 and Ni50Mn35In15 alloys, as model materials, we show that a starting single phase state results in fully repeatable processes and large magnetic hysteresis, whereas a mixed phase starting state results in non-repeatable processes and smaller hysteresis.

The large inverse magnetocaloric effect (MCE) in the off-stoichiometric Heusler alloys occurs at a critical temperature near room temperature. At this temperature, the material is in a mixed-state and can have a variable ratio of two stable magnetic crystallographic-states; a high magnetization state (HM) and a low magnetization state (LM). The field-induced thermal hysteresis in the virgin curve of Ni50Mn35In15 and the virgin first-order reversal curves (VFORC) are presented. A model is introduced to describe the descending branches of these curves based on the different magnetic fields of conversion (from HM to LM). Using limited measurements, the model is used as a tool to determine the fractions of the two crystallographic-states within the mixed-state region.

A number of diverse kinds of magnetic experiments are shown to lead to the observation of a Bose-Einstein condensation of magnons in some nanostructures, especially in Fe and in Co/Pt. The experiments include 1) the measurement of magnetic aftereffect, 2) measurements of Neel’s fluctuation field, including an estimate of activation volume, 3) the measurement of the energy gap which provides a lower limit on the BEC temperature, 4) using Lagrange multipliers in the quantum thermodynamics of magnons, and 5) the observation of a visible anomaly in the Bloch T3/2 law for the temperature dependence of magnetization of nanostructured ferromagnets.

In the vicinity of the first-order transition temperature, the Ni50Mn35In15 Heusler alloy exhibits interesting magnetizing processes. The virgin curve at 280 K lies entirely outside the major loop, and the area of the MH loop formed by the virgin curve and the sequential descending curve is significantly larger than that of the major loop. At 275 K, the virgin curve lies slightly outside the major loop, and the area of the loop containing the virgin curve is slightly larger than that of the major loop. These differences are attributed to the different initial magnetic states of the alloy at the two temperatures. The initial state at 280 K is a mixed state, consisting of a low magnetization majority phase and a high magnetization minority phase. The initial state at 275 K is almost all the low magnetization phase. At both temperatures, a strong enough field converts the low magnetization phase to the high magnetization phase. The initial state affects the reverse transition from high to low magnetization phase upon cycling the field back to zero. The behavior of the virgin curve, major loop, and related properties is a complex function of the field and initial state. Similar behavior is expected in other Heusler alloys and other materials exhibiting first-order magneto structural transitions.

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 ma...

The magnetocaloric temperature change (ΔTTemp) is an important factor in the performance evaluation of the magnetocaloric material's magnetocaloric effect (MCE), which has been attracting significant interest due to its application in energy efficient near-room temperature magnetic refrigeration technology with environmentally desirable characteristics. A novel magnetocaloric temperature change test system with fully controlled field, temperature, and time capabilities is designed and analyzed. This test system allows the detailed observations of the applied field effect, sample temperature effect, and time effect on the testing magnetocaloric material. The effectiveness of this test system was evaluated by testing the MCE within sample of Gd turnings. The magnetocaloric temperature change measurements, TTemp, at various H and Temp was measured and showed numerical and characteristic agreements with literature, thus provided evidence that this test is capable of making conventional measurement as well as new measurements on the material's dynamic time effects.

The dependence of magnetic behavior of media upon the particle/grain size is studied here. A model is presented to describe the behavior of nanomagnetic materials as a function of particle size. The model is applied to two types of experiments: (1) the decay rate of the magnetic aftereffect in nanomagnets and (2) the variation of spontaneous magnetization versus temperature. Both of these experiments show a deviation from the behavior of bulk media above a critical temperature.

During the past decade the magnetic properties of Heusler alloys have been extensively studied, motivated in part by the observation of large magnetocaloric effects (MCEs) displayed by these alloys near room temperature. We present new data and develop a consistent mechanism to explain the complex hysteretic behavior of a Ni 50 Mn 35 In 15 Heusler alloy. The magnetization of this alloy is characterized by two critical temperatures. Below the lower critical temperature, the alloy is a ferrimagnetic martensite. Between the two critical temperatures, the alloy is a ferromagnetic martensite. Above the higher critical temperature, it is a paramagnetic austenite. The transitions at both critical temperatures are first order. The ferri-to-ferromagnetic transition and the crystallographic martensite-to-austenite transition explain the various facets observed in the M ZFC and M FC vs. T plots and their variations with increasing magnetic field. The model successfully explains the isothermal M vs. H loops near room temperature, whose behavior is strongly dependent on the initial magnetic state.

For magnetic materials that display slow decay rates, the entire relaxation process cannot be observed experimentally within a reasonable time interval. While magnetic aftereffect behavior has been understood and analyzed quantitatively in general, the important parameters of magnetic materials with slow decay cannot be easily extracted. We present comprehensive experimental characterization of time, field, and temperature dependence of magnetic properties of a magnetic nanocomposite, which exhibits slow decay. Based on the measurement results and previously developed Preisach–Arrhenius model, we propose an analytical model that can predict the shape of the entire aftereffect curves at different temperatures with only the major hysteresis loop and one aftereffect curve at a specific holding field within the region of interest. The model is validated with both simulated data and measured data. This model allows the derivation of an analytical formula for the time variation of the magnetization based on a Gaussian distribution.

During the past decade, the magnetic properties of metallic-based materials with first-order transitions have been extensively studied, motivated in part by the observation of large magnetocaloric effect (MCE) peaks displayed by these materials near room temperature. These large peaks are believed to be the result of the materials' magnetic properties at the metamagnetic region, characterized by (i) the thermal-induced transition from the ferromagnetic state (FM) to the paramagnetic state (PM) near the Curie temperature (TC) and (ii) the field-induced transition from PM state to FM state above TC. We developed a phenomenological model that utilizes the materials' mixed-state probability function to model the materials' complex hysteretic and properties at metamagnetic region. The approximate probability functions are obtained from the first and second derivatives of the magnetization curve. The probability functions are used to separate the materials' magnetization into a FM state component and a PM state component. The applicability of the model is demonstrated for a metallic-based metamagnetic material, Gd5Si2Ge2 compound, where the modeled behaviors show remarkable agreement with the experimental data at the metamagnetic region and provide new physical insights in this mixed-state region. Specifically, in the region of metamagnetic transition, the PM state component is non-reversible and is a function of the FM state component.

Magnetic bi-layers are becoming increasingly popular as a medium for perpendicular patterned media. In order to understand the net interactions between the layers and between adjacent magnetic elements in an array, we use a coupled Preisach model to model the hysteresis loops we measured for a sample consisting two magnetically soft layers of FeCoTaZr, separated by an 0.8-nm Ru spacer layer. The model reproduces the major hysteresis loop to decent agreement. The model was then decoupled to reveal the reversal behavior of the individual layers. This paper demonstrates that coupled Preisach modeling of the hysteresis loop is an effective tool to analyze such interacting material systems.

There have been many questions raised as to the existence of a non-zero chemical potential in a system of quasiparticles, such as magnons, which can be created and destroyed in an open system. In this paper, we spell out the reasons that there is, in fact, a non-zero chemical potential for magnons, and that it can be determined from magnetic aftereffect experiments.

A study of dynamic and reversible voltage controlled magnetization switching in ferromagnetic Co/Pt thin film with perpendicular magnetic anisotropy at room temperature is presented. The change in the magnetic properties of the system is observed in a relatively thick film of 15 nm. A surface charge is induced by the formation of electrochemical double layer between the metallic thin film and non-aqueous lithium LiClO4 electrolyte to manipulate the magnetism. The change in the magnetic properties occurred by the application of an external electric field. As the negative voltage was increased, the coercivity and the switching magnetic field decreased thus activating magnetization switching. The results are envisaged to lead to faster and ultra-low power magnetization switching as compared to spin-transfer torque (STT) switching in spintronic devices., Comment: 5 pages, 8 figures

A model for the off-axis vector magnetization of a distribution of uniaxial particles is presented. Recent work by the authors decomposed the magnetization into two components and modeled the total vector magnetization as their vector sum. In this paper, to account for anisotropy, the direction of the reversible magnetization component is specified by the vector sum of the applied field and an effective anisotropy field. The formulation of the new anisotropy field (AF) model is derived and its results are discussed considering (i) oscillation and rotational modes, (ii) lag angle, and (iii) unitary magnetization. The advantages of the AF model are outlined by comparing its results to the results of the classical Stoner–Wohlfarth model.

The critical switching field and the coercivity of a material are not necessarily the same; these field values depend upon the magnetization process. Several field-related parameters are used to identify the major hysteresis loop of a material: the saturation field, remanent coercivity, critical field, average critical field, coercivity, median coercivity, and intrinsic coercivity. The parameters vary with changes in the magnetization process, which usually is assumed to be quasi-static. If the process is not quasi-static; i.e., it is faster or slower, the results of the process will change, and the field parameters will be different.

This paper presents some results of experimental research addressing the influence of magnetic field dynamics on the copper‐constantan thermocouple performance. There are challenges in measuring temperature by thermocouples in a time‐dependent magnetic field. Although there is considerable experience on the effect of a static magnetic field, there is a lack of awareness of the outcome of a varying field on thermocouple performance. We measured the accuracy of the thermocouple response in an alternating magnetic field for various operational parameters: frequency of the magnetic field, geometry, and length of the thermocouple wire in the field, and magnetic field strength. The effect of each of the operational parameters is discussed. Test results of temperature profile measure by a copper‐constantan thermocouple in a varying magnetic field system that was used in a room temperature magnetic refrigeration test bed are demonstrated.

We report on the first experimental demonstration of the details of the transient response in the four sequential processes of active magnetic regenerative refrigeration: magnetization, warm blow, demagnetization, and cold blow. The experimental results display the details of the gradual temperature divergence due to regeneration. The temperature change of the stationary solid bed is due to the magnetocaloric effect in a periodic field. The theory for the active magnetic regenerative refrigeration is applied to the transient processes in magnetocaloric regeneration. A time and spatial dependent model of temperature profile of the magnetization and demagnetization with thermal wave regenerative processes is developed.

Results of a room temperature magnetic refrigeration test bed and an analysis using a computational model are presented. A detailed demonstration of the four sequential processes in the transient magnetocaloric regeneration process of a magnetic material is presented. The temperature profile during the transient approach to steady state operation was measured in detail. A 5 °C evolution of the difference of temperature between the hot end and the cold end of the magnetocaloric bed due to regeneration is reported. A model is developed for the heat transfer and fluid mechanics of the four sequential processes in each cycle of thermal wave propagation in the regenerative bed combined with the magnetocaloric effect. The basic equations that can be used in simulation of magnetic refrigeration systems are derived and the design parameters are discussed.

Thermal magnetic aftereffect is due to the interaction between magnons (spin waves) and the change of equilibrium magnetization states. This non-equilibrium process is controlled by (i) the height of the energy barriers between magnetization states and (ii) by the chemical potential that determines how many magnons (thermal quanta) are available. Non-Arrhenius behavior is observed in a non-monotonic variation of decay rate with temperature observed in nanoparticulate media at low temperature. We attribute this behavior to quantum statistics and the nonuniform density of states in energy space. At low temperatures, Bose–Einstein condensation of the magnons is observed in nanoparticulate systems. In these materials, the decay rate increases to a maximum at low temperatures above the condensation temperature. A standard method of evaluating permanent magnet materials is to accelerate the decay rate, by applying a reverse holding field of the order of coercivity. We show that the problem with this technique is the ratio of the decay rate in zero field to the decay rate at the coercivity is a function of the material properties. This model shows excellent agreement with measurements of the variation of the thermal magnetic aftereffect at low temperatures in both nanograin iron powders and MP tapes.

The compounds BaR2CuO5 (R=Y and lanthanides) are well-known impurities that often coexist with the high Tc superconductors, Ba2RCu3O6+x. With R=La and Nd, brown color solid solutions (Ba1+xR2−xCuO5−x) form instead of the “green phase”, BaR2CuO5 (lanthanides with ionic radii smaller than that of Sm). A complete solid solution of Ba(Nd2−xLax)CuO5 exists with the tetragonal space group P4/mbm. Crystallographic studies of selected members (x=0.2, 0.6, 1.2, and 1.6) by X-ray Rietveld refinement technique showed the structure consists of RO8 and BaO10 cages, and square planar CuO4 units. As x increases in Ba(Nd2−xLax)CuO5, the size of these RO8 and BaO10 cages increases. BaLa2CuO5 was confirmed to be ferromagnetic at ≈5 K; despite the isostructural property of BaLa2CuO5 and BaNd5CuO5, the Nd-analog was found to be antiferromagnetic. As x in Ba(Nd2−xLax)CuO5 increases between 0 and 2, the amount of ferromagnetism increases.

Dy nanoparticles have been produced at subambient temperature by alkalide reduction. In the bulk, Dy is ferromagnetic below 89 K. In contrast, the nanoparticles’ magnetization is blocked below ∼9 and ∼20 K for unheated and samples annealed at 1000 °C for 4 h, respectively. This behavior is consistent with a reduction in Tc due to finite size effects. Magnetization is linearly dependent on field at temperatures higher than ∼6 times TB and shows remanence-free hysteresis at lower temperature. Magnetic entropy curves generated from magnetization data are consistent with that expected for a paramagnet, yielding large values at low temperature due to the high effective moment of Dy.

A model for the magnetostriction of high-strength steels under uniaxial compressive stresses is presented to simulate experimental measurements. A previous Preisach-type magnetostriction model is modified to simulate the Villari reversal phenomenon. The model can also account for the crystallographic anisotropy which arises from the compressive stress magnitude as well as the machining process of the sample. The net axial magnetostriction of the sample is modeled by a Preisach-type, two-component model. The first component corresponds to the magnetostriction steep rise in the initial stage followed by rotation of magnetic domains to the direction of the external magnetic field while the second component corresponds to the counteracting restoring torques.

Aftereffect - that is, decay of magnetization with time at a fixed holding field - is often linear in log-time for a limited time window. The slope of the decay at the holding field that maximizes the decay rate (normally near the field that maximizes the irreversible susceptibility) is often used as a measurement of the long-term stability of permanent magnet media. This paper demonstrates that this measurement alone does not indicate the stability at other fields. It shows theoretically, using the Preisach-Arrhenius model, and experimentally that for materials in which there is negligible particle interaction and negligible reversible magnetization, the shape of the thermal aftereffect curve is the same as that of the ascending major hysteresis curve. For these criteria, the ratio of the decay coefficient at one field to that at another is the same as the ratio of the susceptibilities at those two fields. The paper also discusses the effect of interaction on the decay process. In general, the full identification of the Preisach parameters is necessary and sufficient to estimate the decay rate.

A systematic study of the size effect on the magnetic and structural properties of Y2Fe17 nanoparticles has been performed. We present new data to explain the enhanced magnetic properties of nanostructured yttrium-iron alloy synthesized through alkalide reduction chemical synthesis. The properties of the particles were characterized by x-ray diffraction, electron microscopy, and magnetometer techniques. As the size of the nanoparticles is reduced, there is an increase in magnetization per unit of applied magnetic field, a decrease in the coercivity and a substantial reduction in hysteresis.

The phenomenon of negative remanent magnetization (NRM) has been observed experimentally in a number of heterogeneous magnetic systems and has been considered anomalous. The existence of NRM in homogenous magnetic materials is still in debate, mainly due to the lack of compelling support from experimental data and a convincing theoretical explanation for its thermodynamic validation. Here we resolve the long-existing controversy by presenting experimental evidence and physical justification that NRM is real in a prototype homogeneous ferromagnetic nanoparticle, an europium sulfide nanoparticle. We provide novel insights into major and minor hysteresis behavior that illuminate the true nature of the observed inverted hysteresis and validate its thermodynamic permissibility and, for the first time, present counterintuitive magnetic aftereffect behavior that is consistent with the mechanism of magnetization reversal, possessing unique capability to identify NRM. The origin and conditions of NRM are explained quantitatively via a wasp-waist model, in combination of energy calculations.

Magneto-optical Kerr effect (MOKE) hysteresis loops were acquired at various radial positions on a 25 mm diameter disk of a (0.3 nm Co/1.2 nm Pt) 15 multilayer. Two distinct types of behavior: unimodal, with two stable states, and bimodal, with four stable states, were observed. At the center of the disk, the loop exhibited bimodal features. At the outer edge, the reversal behavior was significantly altered, and the loop was unimodal. The radial variations in the magnetization are attributed to diffusion and interface effects arising from nonuniform heating in the film deposition process. We model these media to understand the physics of the magnetization processes. The bimodal loops are characterized by columns of CoPt 3 and of Pt threading the multilayers, strongly coupling them, and fully polarizing the Pt layers. Then, this thread is lost due to the different growth conditions at the edge of the disk, altering the polarization of the Pt layers. The overall behavior is modeled by assuming that in the unimodal region, there are a certain number of smooth layers starting at the substrate, and the remaining layers are rougher. In the intermediate region, vertical threads start forming at random points whose length increases as the medium becomes more bimodal. When the medium is fully bimodal, it consists solely of CoPt 3 and Pt threads that pierce the entire medium.

The paper analyzes the anomalous behavior of the magnetic aftereffect decay coefficient with temperature, which has been established experimentally from a thermodynamic point of view. The Arrhenius law, which has often been used to model magnetic aftereffect, is based on the Maxwell-Boltzmann (MB) statistics. At low temperatures, however, magnons obey Bose-Einstein (BE) statistics, which have a different energy spectrum. MB statistics are a good approximation to BE statistics in the temperature range in which most devices operate. The variation in temperature, governed by chemical potential that is essentially linear, has a negative slope and is zero below a low but finite temperature. Other explanations for this anomalous behavior would predict an incorrect type of temperature dependence. This new interpretation of magnetic aftereffect may have profound implications on accelerated testing to determine life expectancy of recordings.

We report on a very interesting magnetic behavior in the intermetallic compound Zr9N11, namely, (i) spin-glass behavior and (ii) fluctuations in the magnetization versus temperature curves. The magnetic properties are consistent with structural results obtained by neutron diffraction and by scanning and transmission electron microscopies. The fluctuations are believed to arise from the Dzyaloshinskii-Moriya interactions occurring in the Zr9N11 compound. Between 7.86 kA/m and 31.85 kA/m, the temperature (Tp) of the zero-field cooled (Mzfc ) peak shifted to lower temperatures with increasing field. Tp is found to be proportional to Hb, where H is the applied field and the exponent b < 1. The value of b is found to be 0.68, which is close to the de Almeida-Thouless 2/3 power law that applies to spin glasses. Published neutron diffraction data showed that a Zr9N11 compound possessed Zr9Pt11-type tetragonal structure with columnar atomic chains parallel to the c axis and consisting of alternating Zr and Ni atoms. Small fluctuations present in the Mfc versus T plot at 31.85 kA/m are believed to be the result of atomic displacements along these chains over short distances. Another interesting featured observed in the Zr9N11 compound is the fact that at 3.98 kA/m the Mzfc values reside mostly above the corresponding Mfc values.

The magnetic aftereffect was measured for a magnetic particle tape over a temperature range of 10–380 K. Data were obtained for the saturation magnetization, the coercive field, and the remanent coercive field as a function of temperature. Switching field distributions and other Preisach parameters were also determined as a function of temperature. Based on these data, the Preisach–Arrhenius (PA) model predicts that the decay rate, S, should decrease monotonically as the temperature is reduced. It was found that, for the material studied, S is nearly proportional to temperature, T, for T between about 150 and 380 K, in good agreement with the PA model, but that there is considerable deviation for T

The aftereffect of Co/Pt multilayer films with perpendicular magnetization has been measured with a magneto-optical Kerr effect (MOKE) magnetometer and calculated with a newly developed Preisach model. Compared to materials such as traditional magnetic recording media, Co/Pt multilayer films show a more complete picture of the progression of aftereffect because the magnetization of this material decays from saturation almost all the way to a ground state in a reasonable length of time. The magnetization measurements for times equal to negative and positive infinity are asymptotically horizontal, with a transition region that is linear on a logarithmic time scale. In contrast, typical published aftereffect analyses exhibit only a very small percentage of the total aftereffect that could be observed if time were not a factor in making measurements. A Preisach–Arrhenius model is used to calculate the magnetic aftereffect in the Co/Pt multilayer. Comparisons of the model to experimental results show not only the validity of the model, but also its value in predicting very short-time and long-time aftereffect behavior, and low levels of aftereffect occurring in noisy data, all of which are difficult to observe experimentally.

The Wigner–Seitz local atomic cell construction procedure, provides a topological means for comparing series of related structures in terms of the number and type of nearest neighbors. We used this construction to further our understanding of the chemical bonding environment (and hence, the superconducting properties) in three structurally related materials, Ba 2 YCu 3 O 6+ x , ( x =1, 1/2, and 0). These are high- T c superconductors of T c ≈92 K, 56 K, and a nonsuperconductor, respectively. The local environment around Y is found to be similar in all three compounds, and the topology around Ba is generally similar to that in the ideal perovskite. The Cu sites in the Cu–O plane in all three materials show similar anisotropic bonding behavior. Significant results are: (i) that Ba contributes to the bonding with Cu sites, and (ii) the most noticeable difference in the local environment in these compounds is around the Cu sites in the Cu–O “chains”. We propose a modified charge-transfer model, involving the Cu–O “chains”, “planes” and Ba as an electronic unit, to correlate structure with the observed superconducting properties. The bonding behavior of these oxides also appears to be different from the conventional metal alloy superconductors.

Bimodal magnetic behavior, in which Kerr rotation (θK) and ellipticity (eK) each exhibit major magneto-optic hysteresis loops with two distinct field-dependent steps, was encountered in a (0.3 nm Co/1.2 nm Pt)15 multilayer film with perpendicular magnetization. A model is postulated in which there are two different interacting magnetic entities present in the layered structure of this material, each with a stable perpendicular magnetization state either parallel or antiparallel to the applied field, and which reverse quasi-independently from one another. Due to interactions between the two, the reversals occur at the intrinsic coercivities displaced by an interaction field, Hi, whose sign depends upon the magnetization of the other phase. Using major loop and first-order reversal measurements, the intrinsic coercivities, as well as Hi, were obtained.

An automated magneto-optical Kerr effect (MOKE) magnetometer was used to measure polar Kerr rotation ( θ K ) and ellipticity ( e K ) loops of (0.3 nm Co/ x Pt) 15 multilayers with x =0.3 to 1.2 nm. For most samples studied, the hysteresis loops exhibit typical perpendicular magnetization characteristics. The loops have unit squareness, and the normalized behavior of θ K and e K is identical. However, for two samples with composition near (0.3 nm Co/1.0 nm Pt) 15 , the θ K and e K loops are no longer identical, and exhibit bimodal features. Using MOKE major hysteresis loops and first-order reversal curves, a plausible explanation for this behavior is presented.

The sigmoidal decay of the magnetization in log time, which is geometrically similar to the major loop, starts at the major loop and decays to the anhysteretic magnetization for a given holding field. The linear displacement of these curves after normalization with holding field can be used to compute the fluctuation field. This paper discusses the errors in computing this field.

A non-Arrhenius temperature variation of the thermal magnetic aftereffect has been observed in some particulate media. This behavior could be understood by a quasilinear variation of the magnon chemical potential above some finite Bose-Einstein condensation temperature, and zero chemical potential below. Including the chemical potential variation into the thermal activation volumes derived from the Street and Brown formulation removes a nonphysical order of magnitude increase in the activation volume, but instead yields an activation volume that is essentially temperature independent, and is identified with the physical volume