Rotating Magnetocaloric Effect in Sintered La(Fe,Mn,Si)$_{13}$H$_z$ Plates

TL;DR

The study addresses enabling room-temperature magnetic refrigeration via the rotating magnetocaloric effect (RMCE) in La(Fe,Mn,Si)$_{13}$H$_z$ alloys produced by powder metallurgy. It combines direct RMCE measurements on a thin plate with magnetostatic simulations to compute $ΔS_{iso}^{rot}$, leveraging demagnetizing-field anisotropy in a high-aspect-ratio geometry. Key findings include $ΔT_{ad}^{rot}$ values up to $1.17$ K at $H_{ext}=1.0$ T (and $1.12$ K at $0.6$ T) and $ΔS_{iso}^{rot}$ up to $3.97$ J kg$^{-1}$ K$^{-1}$ (1 T) and $3.68$ J kg$^{-1}$ K$^{-1}$ (0.6 T), with internal-field maps showing strong demagnetization effects that drive RMCE. The work highlights the potential of RMCE-based cooling at relatively low field amplitudes and discusses device-design implications, including magnet-volume reductions via Halbach geometries, while noting practical considerations such as a moderate $ΔT_{ad}^{rot}$ and alpha-Fe content.

Abstract

La-Fe-Si-based alloys are among the most application-ready magnetocaloric materials for room-temperature magnetic refrigeration. Powder metallurgy methods have been previously demonstrated to successfully produce structures with sub-mm features for magnetic refrigerators in a scalable method. In this work, we explore the rotating magnetocaloric effect (RMCE) present in a 0.27 mm thin plate of sintered and hydrogenated La(Fe,Mn,Si)$_{13}$. The high aspect ratio ($\sim$50) of the thin plate leads to an anisotropic magnetocaloric effect (MCE), dependent on the relative orientation of the external magnetic field, and an RMCE when the external field is rotated. We find a maximum rotating adiabatic temperature change ($ΔT_{ad}^{rot}$) of 1.17 K with the rotation of a 1 T magnetic field and 1.12 K when rotating a 0.6 T magnetic field, a reduction of only 4% for a 40% reduction in applied field strength. Magnetostatic computations revealed a considerable rotating isothermal entropy change ($ΔS_{iso}^{rot}$), comparable to the conventional MCE of Gd for similar fields, reaching 3.97 J K$^{-1}$ kg$^{-1}$ for 1 T and 3.68 J K$^{-1}$ kg$^{-1}$ for 0.6 T (7% reduction), highlighting La-Fe-Mn-Si alloys as high potential candidates for a magnetic refrigerator based on the RMCE utilizing relatively low external magnetic field amplitudes, such as 0.6 T.

Figures (8)

• Figure 1: Schematic view of the temperature measurement setup (left), and procedure to measure the conventional (center) and rotating magnetocaloric effect (right) with a Halbach cylinder providing 1 T.
• Figure 2: Magnetization versus temperature and magnetic field (a) as-measured, in iso-field curves obtained on heating for different external magnetic field intensities, and (b) after demagnetization correction, versus internal magnetic field, shown for temperatures between 280 K and 320 K with a step of 0.5 K.
• Figure 3: Directly measured $\Delta T_{ad}$ when applying an external magnetic field (a) along the low demagnetizing factor orientation and (b) along the high demagnetizing factor orientation of the sample, as shown in the inset schematic. (c) Directly measured $\Delta T_{ad}^{rot}$ when rotating the external magnetic field. The relative orientation of the sample (not to scale) and external magnetic field is schematically shown.
• Figure 4: Cross section of the magnetic field strength in the La(Fe,Mn,Si)$_{13}$H sample and near surroundings at 280 K with 1.0 T applied field in the (a) high demagnetization, and (b) low demagnetization orientations of the applied magnetic field ($H_{ap}$). The results for all other temperature and applied field intensities for (c) the high demagnetization, and (d) low demagnetization orientations.
• Figure 5: Isothermal entropy change, $\Delta S_{iso}$ as obtained through the Maxwell relation in conjunction with the results of the magnetostatic simulations for the (a) low demagnetization and (b) high demagnetization orientations of the applied magnetic field, $H_{ap}$, as shown in the inset schematic. (c) Rotating isothermal entropy change, $\Delta S_{iso}^{rot}$, occurring upon rotating $H_{ap}$ from the high to the low demagnetization orientation, corresponding to the difference between $\Delta S_{iso}$ of each orientation.