Lawrence H. Bennett

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.

Self-similarity in (∂M/∂T)H curves for magnetocaloric materials with ferro-to-paramagnetic phase transitions

A temperature scaling methodology to obtain a self-similar field dependence (∂M/∂T)H curve for metamagnetic material exhibiting first-order ferro-to-paramagnetic transitions is presented. The methodology extends Franco’s transformation by (i) performing the scaling methodology on the (∂M/∂T)H curve instead of the ΔSM(T,H) curve and (ii) redefining the arbitrary temperature references, Tr1 and Tr2, used by Franco, by employing the physical constants TFM and TPM, which can be determined from the (∂2M/∂T2)H curves. (∂M/∂T)H of the metamagnetic material, Gd5Si2Ge2, exhibiting first-order ferro-to-paramagnetic transition is shown as an example. Applying the new modified Franco’s transformation, Gd5Si2Ge2’s (∂M/∂T)H curves collapse onto a self-similar curve with a low index of dispersion. The collapsed curve is asymmetrical with a negative skewness, which reflects the intrinsic transition differences in the mixed-state region.

Magnetocaloric properties of metallic nanostructures

Abstract A compilation of magnetocaloric properties of metallic nanostructures with Curie temperature (TC) between 260 and 340 K has been tabulated. The tabulated data show that nanostructure plays an important role in enhancing the magnetocaloric properties of a material, namely by reducing the peak of magnetic entropy, but broadening of the magnetocaloric effect curve with an average of 10 K sliding window for Curie temperature. A second table lists all bulk metallic and intermetallic materials, in which there is no nanostructural data, with an entropy change of at least 20 J/kg K and a Curie temperature between 260 and 340 K. We propose that further experiments should be made on the nanostructured form of these materials.

Enhanced magnetic properties of yttrium-iron nanoparticles

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.

Ferri-to-ferro-magnetic and ferro-to-para-magnetic transitions in Ni48Co2Mn35In13Ga2 Heusler alloy

Heusler alloys feature both conventional and inverse magnetocaloric effects near room temperature as they undergo two different transitions. In this paper, new data are presented and analyzed and a new mechanism to explain the complex hysteretic behavior of a Ni48Co2Mn35In13Ga2 Heusler alloy is developed. This mechanism explains isothermal loops near room temperature. The various descriptions and classifications of these transitions, however, is not critical to this analysis.

Magnetization model for a Heusler alloy

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.

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.