Anomalous Magnetodielectric Effect in the Metal-Organic Framework [CH₃NH₃][Co(HCOO)₃]

We have investigated the anisotropic magnetodielectric effect in the perovskite metal-organic framework [CH₃NH₃][Co(HCOO)₃] single crystals. At 2 K, the spin reorientation along the [101] direction induces a notable dielectric peak, while the ferromagnetic ground state along the [010] direction gives rise to a pronounced positive magnetodielectric effect, which is attributed to the strong magnetic anisotropy of the [CH₃NH₃][Co(HCOO)₃] compounds. At the critical temperature of magnetic ordering, the maximum magnetodielectric effect is observed when both magnetic and electric fields are applied along the [010] or [101] directions, with Δε/ε values of -0.31% along the [101] direction and -0.23% along the [010] direction under a 9 T field. This finding suggests that strong spin fluctuations at the magnetic ordering temperature enhance the magnetodielectric effect. Even in the paramagnetic state up to 150 K, a weak magnetodielectric effect is still observed, which may be attributed to the magnetostrictive effect. Our study provides insights into the mechanisms behind the magnetodielectric effect in metal-organic frameworks, emphasizing the role of magnetic anisotropy and spin-lattice coupling.