Rotating Magnetocaloric Effect in First-order Phase Transition Material Gd5Si2Ge2

TL;DR

This study demonstrates that demagnetization-induced RMCE can be substantial in a first-order magnetostructural GMCE material, Gd$_5$Si$_2$Ge$_2$, using a high-aspect-ratio sample and rotating a constant external field. The authors combine direct adiabatic temperature measurements with magnetometry and finite-element magnetostatic simulations to quantify RMCE via $ΔT_{ad}^{rot}$ and $ΔS_M^{rot}$, finding a peak $ΔT_{ad}^{rot}$ of 1.77 K at 0.8 T and a maximum $|ΔS_M^{rot}|$ of 6.42 J K$^{-1}$ kg$^{-1}$ (at 1.2 T) with non-monotonic field dependence. The work shows that RMCE can surpass conventional MCE amplitudes at low fields and that RMCE can be substantially enhanced by increasing the sample’s aspect ratio, with simulations predicting a 35% gain in $|ΔS_M^{rot}|$ for a thickness-optimized plate. Overall, the results highlight demagnetization-controlled RMCE as a viable route for low-field magnetic refrigeration in FOPT GMCE materials and underscore the critical role of sample geometry in maximizing RMCE.

Abstract

The rotating magnetocaloric effect (RMCE) induced by self-demagnetization has been investigated in the giant magnetocaloric effect (GMCE) material Gd$_5$Si$_2$Ge$_2$. This shape-dependent effect had thus far only been reported in pure Gd, marking this as the first analysis of the effect in a sample with a magnetostructural first-order phase transition. By rotating the applied magnetic field vector while keeping its intensity constant, the demagnetizing field within a high-aspect ratio sample changes significantly, resulting in a RMCE. We characterize RMCE by determining the adiabatic temperature change ($ΔT_{ad}^{rot}$) directly through temperature measurements, and the isothermal entropy change ($ΔS_M^{rot}$) via magnetometry and magnetostatic simulations. We obtain a remarkable maximum $ΔT_{ad}^{rot}$ of 1.77 K for a constant external field of 0.8 T, higher than that obtained under 1.0 T. The magnetostatic simulations not only corroborate the highly non-monotonous field-dependence of $|ΔS_{M}^{rot}|$, which reaches 95\% of its maximum value at 0.8 T, 6.12 J K$^{-1}$ kg$^{-1}$ for the experimentally measured shape, but also estimate a 35\% increase in the maximum $|ΔS_{M}^{rot}|$ up to 8.67 J K$^{-1}$ kg$^{-1}$ in a simulated shape with higher aspect ratio.

Figures (10)

• Figure 1: Photos of the sample used for direct temperature measurements (a-c) and corresponding pictures of the CAD 3D model used for the magnetostatic simulations (d-f). (g) The smaller sample measured in the MPMS, mounted on a quartz sample holder with polyimide tape. (h) The sample mounted on the cryostat, with thermocouple installed.
• Figure 2: The two direct temperature measurement protocols. (a) Continuous/cyclic protocol under cooling, in which the heat bath temperature of the sample is continuously changing and the field is applied and removed every 10 seconds. (b) The discontinuous/phase-reset protocol, in which the heat bath temperature of the sample is stabilized prior to the field being applied and removed (also every 10 seconds). The discontinuous protocol measurements are always preceded by a thermal reset taking around 30 minutes, as shown in the inset.
• Figure 3: The directly measured $\Delta T_{ad}$ values obtained for (a) the low demagnetizing field orientation ($H_{app}\parallel y$) in different measurement protocols, and (b) the high demagnetizing field orientation ($H_{app}\parallel x$). (c) The $\Delta T_{ad}^{rot}$ obtained by changing the magnetic field orientation from $H_{app}\parallel x$ to $H_{app}\parallel y$. The "cooling" and "heating" curves in (a) refer to the continuous protocol.
• Figure 4: The directly measured $\Delta T_{ad}^{rot}$ obtained through the discontinuous protocol for the rotation of different applied magnetic field intensities on the (a) first magnetic field application (irreversible), and (b) subsequent magnetic field rotations. All values correspond to rotation from high to low demagnetizing field orientations, as illustrated in figure \ref{['fig:3']}.
• Figure 5: The different magnetization datasets acquired through different protocols. Magnetization isotherms measured with (a) increasing magnetic field and (b) decreasing magnetic field. Magnetization isofield curves measured under cooling with a constant applied magnetic field are plotted (c) versus temperature and (d) versus magnetic field, to facilitate comparison with the curves measured with constant temperature. The sample temperature is raised to 350 K between each pair of isotherms and each isofield measurement.