Tablelike magnetocaloric effect and enhanced refrigerant capacity in $\mathrm{Eu}{\mathrm{O}}_{1\text{−}\delta }$ thin films

The effect of electron doping of $\mathrm{Eu}{\mathrm{O}}_{1\text{−}\delta }$ thin films through oxygen vacancies ($\delta =0$, 0.025, and 0.09) upon the magnetocaloric response is presented. The films each showed a paramagnetic to ferromagnetic transition around 65 K, with an additional magnetic ordering transition at higher temperatures in the oxygen deficient samples. All transitions are observed to be of second order. A maximum magnetic entropy change of 6.4 J/kg K over a field change of 2 T with a refrigerant capacity of 223 J/kg was found in the sample with $\delta =0$, and in all cases the refrigerant capacities of the thin films under study were found to exceed that reported for bulk EuO. Adjusting the oxygen content was shown to produce tablelike magnetocaloric effects, desirable for ideal Ericsson-cycle magnetic refrigeration. These films are thus excellent candidates for small-scale magnetic cooling technology in the liquid nitrogen temperature range.

Figure 1

X-ray diffraction patterns for the (a) $\delta =0$, (b) $\delta =0.025$, and (c) $\delta =0.09$ $\mathrm{Eu}{\mathrm{O}}_{1\text{−}\delta }$ thin films deposited on Si substrates and capped with a Pt layer.

Figure 2

Temperature dependence of magnetization in the series of $\mathrm{Eu}{\mathrm{O}}_{1\text{−}\delta }$ films under an applied magnetic field of (a) 0.1 T and (b) 2.0 T. Insets: First derivatives of magnetization with temperature.

Figure 3

(a) Arrott-Noakes plot of magnetization isotherms acquired between 38 and 98 K for the stoichiometric thin film using 3D Heisenberg trial values of the critical exponents $\beta$ and $\gamma$. (b) Power law fit to the saturation magnetization and initial susceptibility obtained by extrapolating linear fittings of the isotherms in (a) in the high field region back to the axes.

Figure 4

Temperature dependence of magnetic entropy change over various applied field changes in the $\mathrm{Eu}{\mathrm{O}}_{1\text{−}\delta }$ films with (a) $\delta =0$, (b) $\delta =0.025$, and (c) $\delta =0.09$. Inset (a): Representation of the $RC$. Inset (b): Log-log plot of peak magnetic entropy change vs applied field change.

Figure 5

Comparison of the refrigerant capacity of the present $\mathrm{Eu}{\mathrm{O}}_{1\text{−}\delta }$ thin film samples in a 2 T applied field with previous results for bulk polycrystalline EuO reported in the literature.