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A magnetocaloric effect (MCE) with sizable isothermal entropy change (ΔS) maintained over a broad range of temperatures above the blocking temperature is reported for a rare earth-free superparamagnetic nanoparticle system comprising of Fe–TiN heterostructure. Superparamagnetic iron (Fe) particles were embedded in a titanium nitride (TiN) thin film matrix in a TiN/Fe/TiN multilayered pattern using a pulsed laser deposition method. High angle annular dark-field images in conjunction with dispersive energy analysis, recorded using scanning transmission electron microscopy, show a clear presence of alternating layers of Fe and TiN with a distinct atomic number contrast between Fe particles and TiN. Quantitative information about the isothermal entropy change (ΔS) and the magnetocaloric effect in the multilayer Fe–TiN system has been obtained by applying Maxwell relation to the magnetization vs temperature data at various fields. With the absence of a dynamic magnetic hysteresis above the blocking temperature, the negative ΔS as high as 4.18 × 103 J/Km3 (normal or forward MCE) is obtained at 3 T at 300 K.
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Thermodynamics and entropic inference of nanoscale magnetic structures in Gd
Abstract A bulk gadolinium (Gd) single crystal exhibits virtually zero remnant magnetization, a common trait among soft uniaxial ferromagnets. This characteristic is reflected in our magnetometry data showing virtually hysteresis free isothermal magnetization loops with large saturation magnetization. The absence of hysteresis allows to model the measured easy axis magnetization as a function of temperature and applied magnetic field, rather than a relation, which permits the application of Maxwell relations from equilibrium thermodynamics. Demagnetization effects broaden the isothermal first-order transition from negative to positive magnetization. By analyzing magnetization data within the coexistence regime, we deduce the isothermal entropy change and the field-induced heat capacity change. Comparing the numerically inferred heat capacity with relaxation calorimetric data confirms the applicability of the Maxwell relation. Analysis of the entropy in the mixed phase region suggests the presence of hitherto unresolved nanoscale magnetic structures in the demagnetized state of Gd. To support this prediction, Monte Carlo simulations of a 3D Ising model with dipolar interactions are performed. Analyzing the cluster size statistics and magnetization from the model provides strong qualitative support of our analytic approach.
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- Award ID(s):
- 2044049
- PAR ID:
- 10556895
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- Journal of Physics: Condensed Matter
- Volume:
- 37
- Issue:
- 6
- ISSN:
- 0953-8984
- Format(s):
- Medium: X Size: Article No. 065705
- Size(s):
- Article No. 065705
- Sponsoring Org:
- National Science Foundation
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