Magnetic Properties of the Quasi-1D Magnesium Lanthanide Borates Mg$Ln$B${}_5$O${}_{10}$

Abstract

Lanthanide borates are widely studied for their optical and magnetic properties. A wide variety of structures are known with 3, 2, 1 and 0 dimensional connectivity of lanthanide ions. Here, we explore Mg$Ln$B${}_5$O${}_{10}$, with a quasi-1D arrangement of the $Ln$ ions. Polycrystalline samples of Mg$Ln$B${}_5$O${}_{10}$ ($Ln$ = La, Pr, Nd, Sm--Er) were synthesised with high purity via a sol-gel method. Powder X-ray diffraction data confirmed the reported monoclinic space group ($P{2}_1/c$). The magnetic $L{n}^{3+}$ ions in Mg$Ln$B${}_5$O${}_{10}$ form relatively isolated zig-zag chains parallel to the $b$ axis. Magnetic susceptibility and isothermal magnetisation were measured: all samples except $Ln=$ (Eu, Sm) fit the Curie-Weiss Law in isothermal magnetisation at high temperatures, in broad agreement with theoretical expectations. $Ln=$ (Nd, Tb, Dy, Ho) exhibit signatures characteristic of Ising spin saturation, implying single ion anisotropy, while Gd exhibits characteristics of Heisenberg spins. Estimation of magnetic interactions suggests that Mg$Ln$B${}_5$O${}_{10}$ are candidate materials for quasi-1D magnetism. The magnetocaloric entropy change was also calculated, with MgGdB${}_5$O${}_{10}$ showing promise for application to solid-state refrigeration at liquid helium temperatures.

27 pages, 11 figures

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