Hatem ElBidweihy

Design and Instrumentation of an Advanced Magnetocaloric Direct Temperature Measurement System

The magnetocaloric temperature change (ΔTTemp) is an important factor in the performance evaluation of the magnetocaloric materials 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 materials dynamic time effects.

Adiabatic magnetocaloric temperature change in polycrystalline gadolinium - A new approach highlighting reversibility

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. During the magnetization process, ΔT measurements display the same maximum for each applied field when plotted against the initial temperature (Ti). However, during the demagnetization process, the maximum ΔT varies for each applied field. This discrepancy between the magnetization and demagnetization measurements appears inconsistent with the reversibility of the magnetocaloric effect. A new approach is undertaken to highlight the reversibility of the magnetocaloric effect by plotting ΔT against the average temperature change (Tavg) instead of Ti. The value of Tavg which corresponds to the maximum ΔT is found to increase linearly with the applied magnetic field, consistently for both the magnetization and demagnetization measurements. Solving the linear-fitting equations of these measurements gives...

Physical Justification for Negative Remanent Magnetization in Homogeneous Nanoparticles

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.

Implicit measurement of the latent heat in a magnetocaloric NiMnIn Heusler alloy

The latent heat linked with the first-order transformation of a NiMnIn Heusler alloy has been studied through direct measurements of the adiabatic temperature change, ΔTad, during magnetization process. The experimental procedure used guarantees independent data points and negates any contribution of hysteretic losses to the magnetocaloric effect. Thus, the differences between the magnitudes of ΔTad measurements during the magnetization with the initial temperature change directions from low-to-high and high-to-low are solely attributed to the latent heat exchange, which accompanies the irreversible structural first-order transformation. An estimate of the latent heat inducing such differences is about 0.292 J/g.

Metastability in the Magnetic Structure of Ni 51 Mn 33.4 In 15.6 Heusler Alloy

The metastability of the nonstoichiometric Ni₅₁Mn_33.4In_15.6 Heusler alloy in the first-order transition region is studied using direct adiabatic temperature change (ΔT_ad) measurements for heating and cooling magnetization processes. During the heating process, ΔT_ad shows an unusual pronounced peak at 294 K in low applied magnetic fields. The height of this peak diminishes and eventually vanishes at higher fields. Kinks are as well observed in the 290-295 K temperature interval for both processes. The existence of the peak and kinks is indicative of the complex magnetic dynamics in Ni₅₁Mn_33.4In_15.6 and is attributed to a spin-reorientation transition near the first-order phase transition temperature.

Vector Magnetization of a Distribution of Uniaxial Particles

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.

Vector properties of magnetostriction

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.

Magnetocaloric effect in NiMnInSi Heusler alloys

The NiMnInSi Heusler alloy family is analyzed, and a self-similarity based method is used to analyze the first-order transition of Ni-Mn Heusler alloys. This method is appropriate to determine magnetic characteristics of the Magnetocaloric Effect providing that there is sufficient separation in their phase transition temperatures. A temperature scaling methodology is used to model the cluster compositions in the mixed-state regions where two stable magnetic states co-exist. The various descriptions and classifications of these transitions, however, are not critical to this analysis.

A Preisach-Type Magnetostriction Model for Materials Exhibiting Villari Reversal

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.

Vector magnetization of a distribution of cubic particles

A model for the vector magnetization of a distribution of particles with cubic anisotropy is presented. Recent work by the authors modeled the vector magnetization of a distribution of uniaxial particles by decomposing the total magnetization into reversible and irreversible components. In this paper, using an energy approach applicable to a generic plane, the model is extended to include cubic anisotropy projected to the (100) plane. 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 and taking into account the anisotropy field. The formulation of the generalized model is derived and its results are discussed considering (i) oscillation and rotational modes, (ii) lag angle, and (iii) magnetization trajectories.