Theoretical investigations of tetrameric magnetic molecules for sub-kelvin cooling

Abstract

Magnetic molecules are a class of compounds that is also investigated in view of their magnetocaloric properties. The isothermal entropy change and the adiabatic temperature change are key figures of merit for magnetocaloric performance. Here, we investigate spin systems of realistic molecular structures comprising four spins. In view of potentially large spin quantum numbers as for gadolinium we model these spin systems by a combination of Heisenberg and dipolar spin-spin interactions. It turns out that a tetrahedral structure with ferromagnetic exchange interactions yields the best figures of merit.

Figures (6)

• Figure 1: Isothermal entropy change for a tetrahedron with $s=3/2$ at $T=10$ K (top), $T=0.1$ K (middle), and $T=0.1$ K (bottom); the latter including dipolar interactions for a distance between of $d=2$ Å. The magnetic field is perpendicular to the plane of the red triangle in the tetrahedron, see Tab. \ref{['tetracaloric-t-1']}. The color maps show the respective entropy changes.
• Figure 2: Isothermal entropy change for a butterfly with $s=3/2$ at $T=10$ K (top), $T=0.1$ K (middle), and $T=0.1$ K (bottom); the latter including dipolar interactions for distances of $d_1=2.3$ Å (corresponding to the exchange path $J_{13}$) and $d_2=3.3$ Å (corresponding to the exchange path $J_{24}$). The magnetic field points along the longer diagonal of the butterfly corresponding to the exchange path $J_{24}$, see Tab. \ref{['tetracaloric-t-1']}. The color maps show the respective entropy changes.
• Figure 3: Isothermal entropy change for a chain with $s=3/2$ at $T=10$ K (top), $T=0.1$ K (middle), and $T=0.1$ K (bottom); the latter including dipolar interactions for distances between adjacent spins of $d=3$ Å. The magnetic field points along the axis of the chain. The color maps show the respective entropy changes.
• Figure 4: Isothermal entropy change for a square with $s=3/2$ at $T=10$ K (top), $T=0.1$ K (middle), and $T=0.1$ K (bottom); the latter including dipolar interactions for distances of $d=2.54$ Å between adjacent spins. The magnetic field points along two parallel edges of the square. The color maps show the respective entropy changes.
• Figure 5: Adiabatic temperature change for a tetrahedron with $s=3/2$ with $B_{\text{hot}}=7$ T, $B_{\text{cold}}=0$, and $T_{\text{hot}}=10$ K. Dipolar interactions for distances of $d=2$ Å are included. The magnetic field is perpendicular to the plane of the red triangle in the tetrahedron, compare Tab. \ref{['tetracaloric-t-1']}. The color map shows the respective $T_{\text{cold}}=0$ on a logarithmic scale.