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We study the impact of irradiation on magnetic tunnel junction (MTJ) films with perpendicular magnetic anisotropy (PMA) and spin–orbit torque (SOT) switching using magneto-optical Kerr effect and transmission electron microscopy. Our results show that the thin-film stack is robust to gamma ionizing dose up to 1 Mrad(Si) and Ta1+ ion irradiation fluences up to 1012 ions/cm2, showing SOT PMA MTJs are radiation-hard. But, at very high Ta1+ ion irradiation between 1012 and 1014 ions/cm2, reduced coercivity and eventually greatly reduced PMA are observed, corresponding with an increase in intermixing of the CoFeB–MgO layers, particularly at the lower CoFeB–MgO interface. These results agree with displacement damage modeling that predicts higher damage in the layers closer to the bottom heavy metal and substrate. Compared to spin transfer torque and in-plane anisotropy MTJs, needing a top-pinned stack, a thicker heavy metal layer, and perpendicular anisotropy that is pinned out of plane by interfaces, all could make SOT PMA MTJs more susceptible to damage at high doses. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The vector properties of a newly developed Preisach-type magnetostriction model are discussed. The model uses a modified version of the Della Torre-Pinzaglia-Cardelli model to compute the irreversible and the reversible components of the magnetization. The magnetostriction can then be simulated by assuming that its magnitude is proportional to the magnetization and its direction is dependent on the magnetization history. The modeling approach is outlined for two types of isotropic media: native and polycrystalline. The preliminary results show excellent agreement with the rotational magnetization measurements for a sample of high-strength steel. [ABSTRACT FROM AUTHOR]

To make nanoparticles useful to fit various applications, the fundamental understanding of their magnetic and structural behavior is essential. A systematic in-depth study of the effect of field, temperature, and nanoparticles size on the critical points, and the magnetic properties of chemically synthesized Y2Fe17 nanoparticles have been performed. The blocking temperature varies with the change in the applied magnetic field as well as nanoparticles’ size variations. The Curie temperature was mostly influenced by particles’ size. Saturation magnetization and coercivity were shown to be temperature, size, and shape dependent. The results are significant and especially of interest in material science as they pave the path to a better understanding of different phenomena in nanomaterials in order for them to be used in diverse scientific applications. [ABSTRACT FROM AUTHOR]

It is well known that the energy barrier for magnetization reversal, EB, varies quadratically with the magnetic field for the Stoner-Wohlfarth model. However, the enthalpy H (switching energy) required to reverse the magnetization is the sum of EB and the work Wf done by the Néel fluctuation field. The sum of these two terms gives an enthalpy of reversal. If the fluctuation field is uniaxial and parallel to the holding field, then the enthalpy barrier for switching is linear in the holding field. The linearity is consistent with certain experimental results. If the fluctuation field is anisotropic, then for the same size field, the probability of the particle’s switching will depend upon the direction of that field. The paper discusses the holding field variation of the energy barrier for different fluctuation field directions. [ABSTRACT FROM AUTHOR]

In order to determine the accuracy of an identification method of the fluctuation field, artificial data were generated. A major hysteresis loop, and its corresponding anhysteretic magnetization curve, was simulated and used to create thermal aftereffect curves as a function of a fluctuation field. Since the curves were idealized with known parameters, they could be used to test the method. In the Preisach-Arrhenius model, it was shown that the fluctuation field could be computed as the slope of the translation, in log time, of the aftereffect curves as a function of the holding fields. Gaussian noise was introduced into the curves to test the robustness of the method in the presence of experimental error. The limitations of the identification method are also discussed. [ABSTRACT FROM AUTHOR]

A systematic study of the size effect on the magnetic and structural properties of a Ni51Mn33.4In15.6 Heusler alloy converted to nanoparticles has been performed. We present new data to explain the enhanced magnetic properties of the nanostructured Heusler alloy synthesized through a high-energy ball-milling technique. The properties of the particles were characterized by X-ray diffraction, electron microscopy, and magnetometer techniques. Isothermal magnetic field variation of magnetization exhibits field hysteresis in bulk Ni51Mn33.4In15.6 alloy across the martensitic transition. As the size of the nanoparticles is reduced, there is an increase in the magnetization per unit of applied magnetic field, and a decrease in the coercivity as well as a substantial reduction in hysteresis. [ABSTRACT FROM PUBLISHER]

Bimodal magnetic behavior has been observed in a (0.3-nm Co/1.2-nm Pt)[SUB15] multilayer film. In that) film, the Kerr rotation and ellipticity exhibit magneto-optic hysteresis loops with two distinct field-dependent steps, so that there are four stable magnetization states. The magnetic aftereffects of the two distinct types of decay at the two sets of holding fields have been analyzed. Thus, it is possible to measure and calculate the two types of decay individually. We show that the shift in log time of the aftereffect curves for both the sets are linear. The fluctuation fields for the two states are calculated using these methods. We also confirm experimentally the conjecture of the Preisach-Arrhenius model that the shape of the aftereffect curve is the same as the major hysteresis curve. This helps in finding the chemical potential. [ABSTRACT FROM AUTHOR]

Magneto-optical Kerr effect measurements performed on (0.3 nm Co/x Pt)[sub 15] multilayers with x=0.3–2.0 nm showed all samples to exhibit strong perpendicular magnetic anisotropy. Recent work has shown that for compositions near (0.3 nm Co/1.2 nm Pt)[sub 15], both polar Kerr rotation and ellipticity exhibit bimodal reversals in which the major hysteresis loops have two distinct field-dependent steps due to the contributions of two magnetic phases. To better understand this bimodal magnetization phenomenon, Kerr rotation and ellipticity were determined as a function of radial position from the center to the edge of this disk sample. These measurements exhibit a continuous variation from two-step (bimodal) behavior in the center to single-step (unimodal) behavior at the edge. Since these films were deposited via MBE with a heated substrate, it is believed that this phenomenon is related to thermally induced variations in interface effects such as alloying, abruptness, and morphology. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The rate of magnetization change (magnetic after effect) which occurs after the magnetic field applied to a magnetic material is switched suddenly to a new value, is generally assumed to increase when the temperature is increased. Deviation from this temperature behavior, which has been observed in several cases, has been ascribed to changes in pertinent magnetic properties, such as magnetocrystalline anisotropy, activation volume, coercive field, etc., with temperature. We report here on the magnetic after effect temperature dependence of a magnetic recording material with magnetic properties that change very little with temperature over a wide temperature range but which, nevertheless, deviates considerably from a monotonic increase in decay rate with increasing temperature. We postulate that this behavior is explained by replacing the Maxwell–Boltzmann statistics by the Bose–Einstein statistics appropriate for the magnon energy distribution. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study which investigated the magnetization reversal and magneto-optical images of cobalt/platinum multilayer films using Kerr microscopy. Bimodal features; Magnetic images of the multilayer film.

Deals with a study which presented the Wigner-Seitz construction and its use in the analysis of the ternary compounds from an extensive family of alloys having complicated magnetic structures. Information on a magnetic ordering that has been inferred from magnetization; Factor related to the occurrence of a substantial magnetic moment; Proof for the existence of iron atoms.

Presents a study of the magnetization process of granular cobalt-silver films. Use of the magneto-optical indicator film technique and magnetic force microscopy in the study; Quality characterization of high-temperature superconductors; Analysis of magnetization domains.

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. [ABSTRACT FROM PUBLISHER]

In the vicinity of the first-order transition temperature, the \Ni50{\Mn}35{\In}15 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. [ABSTRACT FROM PUBLISHER]

Abstract: 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. [Copyright &y& Elsevier]

A magnetic medium can have different hysteresis losses when subject to a rotating field. We discuss the pertinent parameters that determine these losses and a method of identifying them. We discuss the effect of particle interaction, and isotropic and anisotropic media. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

Presents information on a study which proposed that using the Preisach-Arrhenius model, the shape of the thermal aftereffect curve is the same as that of the ascending major hysteresis curve. Decay of magnetization in the presence of large energy barriers; Description of the Preisach-Arrhenius model; Case of noninteracting hysterons.

Close to room temperature, the off-stoichiometric Ni50Mn35In15 Heusler alloy is known to undergo a first-order magnetostructural transition. This paper presents a new model that closely mimics the magnetic behavior of the virgin curve and that of the M-H loops within the temperature range where the alloy undergoes the first-order transition. The virgin curve and the M-H loops relevant to the model were measured at 280 K. Since our data show that 280 K is above the start of the transition, it implies that at this temperature the alloy is in a mixed state. The mixed state refers the presence of two distinct magnetic states. The model and mechanism we propose to explain the complex magnetic behavior of the virgin curve and of the M-H loops pertain to the action of the applied field on the transition between the two magnetic states. Both the model and the proposed mechanism provide new insight about the complex magnetic behavior displayed by the Ni50Mn35In15 alloy within the first-order transition. [ABSTRACT FROM AUTHOR]

The adiabatic temperature change (ΔT) during the magnetization and demagnetization processes of bulk gadolinium is directly measured for several applied magnetic fields in the temperature range 285 K to 305 K. In low magnetic fields, the measurements of ΔT display two maxima at 293.8 K and 291.5 K with an inflection point between 289 K and 293 K. The height of the second peak is more pronounced at low applied magnetic fields and diminishes in high fields. The occurrence of a second ΔT maximum is indicative of the complex spin dynamics in Gd and is related to the presence of metastable states near the Curie temperature. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A time-dependent model of temperature in magnetization and demagnetization processes is developed based on thermodynamic and heat transfer laws, material properties, and variable-field magnet parameters. Test results of a magnetic refrigeration test bed are compared with the model calculation. The model can be used to simulate optimum operating conditions and duty cycle of magnetic regenerative refrigeration. The effects of the thermal conductivity between the magnetocaloric material and the thermocouple on temperature measurements and thermal lag are presented. [ABSTRACT FROM AUTHOR]

Presents the abstract of the study 'Magnetic Properties of Electrochemically Produced Cu-X (X=Ni, Fe,and Co) Artificial Superlattices,' by L.H. Bennett, D.S. Lashmore, L.J. Swartzendruber, E. Soltani and C.E. Beauchamp.

Presents the abstract of an article which discussed the establishment of a magnetic aftereffect (magnetic viscosity) in texture nickel/copper multilayered alloys using a vibrating sample magnetometer at room temperature.

The adiabatic temperature change (ΔTad) during the magnetization process of polycrystalline gadolinium and Ni51Mn33.4In15.6Heusler alloy is directly measured near the Curie temperature. The cooling factor (CF) is introduced as the area under the curve of adiabatic temperature change versus ambient temperature. The CF provides more representative measure of cooling performance in the operational temperature range. Selecting different temperature abscissas qualitatively changes the interpretation of the cooling performance of a magnetocaloric material. In particular, plotting ΔTadversus initial temperature gives a measurably different CF compared to that given by plotting ΔTadversus average temperature. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]