Magnetocaloric effect in Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ thin films deposited on Si substrates

TL;DR

This study investigates the magnetocaloric behavior of Tb31Co69 and Dy31Co69 thin films deposited on naturally oxidized Si substrates, where amorphous matrices coexist with crystallized Laves-phase RCo2 inclusions. Using pulsed laser deposition, SEM/EDS, XRD, and magnetometry, the authors identify ferrimagnetic order and two distinct magnetic-transition features: the amorphous compensation temperature $T_{ m comp}$ and the crystallized Laves-phase Curie temperature $T_{ m C,Laves}$. The magnetocaloric effect, quantified via the magnetic entropy change $\\Delta S_{ m M}$ at Δμ0H = 5 T, shows two peaks per film — a larger peak at $T_{ m comp}$ for Dy-based samples (≈ 35 mJ cm^-3 K^-1) and notable peaks near $T_{ m C,Laves}$ for Tb- and Dy-based samples (Tb: ≈ 6.6 mJ cm^-3 K^-1 at $T_{ m C,Laves}$; Dy: ≈ 28 mJ cm^-3 K^-1). The results highlight the potential to tune low-temperature magnetocaloric properties in rare-earth–transition-metal thin films through controlled amorphous/crystalline phase fractions, with implications for ferrimagnetic MCE engineering in nanoscale devices.

Abstract

We present the structural, magnetic, and magnetocaloric properties of thin films with stoichiometry Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ deposited on naturally oxidized silicon Si (100) substrates. Samples with a thickness $d=50$ nm covered with a protective Au overlayer with a thickness $d_{\rm Au} =5 $ nm were produced using the pulsed laser deposition technique. X-ray diffraction analysis indicated the presence of crystallized Laves phases and amorphous phases in the prepared materials. Magnetization measurements as a function of temperature revealed ferrimagnetic behavior in both samples. We estimated the compensation temperature $T_{\rm comp}$ of the amorphous phase for Tb$_{31}$Co$_{69}$ at 81.5 K and for Dy$_{31}$Co$_{69}$ at 88.5 K, while we found the Curie temperature $T_{\rm C,\ Laves}$ of the crystallized Laves phases at 204.5 K and at 117 K, respectively. We investigated the magnetocaloric effect in a wide temperature range, covering $T_{\rm comp}$ of amorphous phases and $T_{\rm C,\ Laves}$ of crystallized Laves phases. The analysis for the magnetic field change of $Δμ_0H=5$ T showed values of the magnetic entropy change of $-ΔS_{\rm M}=4.9$ mJ cm$^{-3}$ K$^{-1}$ at $T_{\rm comp}$ and $-ΔS_{\rm M}=6.6$ mJ cm$^{-3}$ K$^{-1}$ at $T_{\rm C,\ Laves}$ for Tb$_{31}$Co$_{69}$, while for Dy$_{31}$Co$_{69}$, we determined the values of $-ΔS_{\rm M}=35$ mJ cm$^{-3}$ K$^{-1}$ at $T_{\rm comp}$ and $-ΔS_{\rm M}=28$ mJ cm$^{-3}$ K$^{-1}$ at $T_{\rm C,\ Laves}$.

Figures (4)

• Figure 1: (a) and (b) Topography of the Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ layers. (c-f) Chemical maps of specific elements for the samples with the Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ layers.
• Figure 2: (a) and (b) X-ray diffraction patterns of the Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ layers. Wide maxima at $2\theta = 38.3^{\circ}$ are related to the Au (111) peak, small maxima at $2\theta = 33.1^{\circ}$ are related to the TbCo$_2$ and DyCo$_2$ (220) peaks. (c) Schematic illustration of the samples studied. Small light blue balls indicate crystals of the $R$Co$_2$ Laves phase.
• Figure 3: Magnetic properties of the samples with the Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ layers. (a) and (b) Magnetization as a function of temperature measured in a magnetic field value of 0.1 T for for the Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ layers. The right blue axis is assigned to the derivative of the FC magnetization. The amorphous phase compensation temperatures are denoted as $T_{\rm comp}$. The Curie temperatures of crystallized Laves phases are denoted as $T_{\rm C,\ Laves}$. For Tb$_{31}$Co$_{69}$ at around 350 K $T_{\rm C,\ amorph}$ of the amorphous phase is noted. (c) and (d) Magnetization isotherms as a function of an applied magnetic field for the Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ layers. (e) and (f) Close up on the hysteresis of the magnetization isotherms as a function of an applied magnetic field for the Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ layers.
• Figure 4: (a) and (b) Magnetization isotherms as a function of magnetic field measured at exemplary temperatures for the samples with Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ layers. (c) and (d) Arrott plots for the samples with Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ layers. (e) and (f) Magnetic entropy change $\Delta S_{\rm M}$ for the Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ layers for various magnetic field values.