Magnetic Properties of Nh 4 Al(SO 4 ) 2 ·12H 2 O: Evidence of Spin Glass Behavior Based on Proton Orbitals

The polarity of the magnetic susceptibility χ(T) of ammonium alum Nh4Al(SO4)2·12H2O single crystals below its ferroelectric Curie temperature TC is identified to depend only on the cooling rate. To explain this, we consider reorienting distorted NH4+ to carry weak magnetic and electric dipole moments that are linearly coupled. Slow cooling allows the dipole moments to align which enhances χ(T) to become positive. However, due to energy degeneracy of rotational motion and geometric frustration of the NH4+, faster cooling rates render the system to become an orientational glass state causing χ(T) to remain negative. These weak and misaligned molecular magnets are 10-1000 times more sensitive to ac magnetic fields than conventional spin glasses according to ac susceptibility measurements. Specific heat measurements also show the presence of excess entropy below a negative anomaly near 53 K. Our findings identify a new class of non-equilibrium system with potential applications in non-volatile memory devices.