Magnetic properties of NH4Al(SO4)2•12H2O: Evidence of glass behavior based on proton orbitals

The polarity of the magnetic susceptibility χ( T ) of ammonium alum NH 4 Al(SO 4 ) 2 •12H 2 O single crystals below its ferroelectric Curie temperature T C is identified to depend only on the cooling rate. To explain this, we consider reorienting distorted NH 4 + 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 NH 4 + , faster cooling rates render the system to become an orientational glass state causing χ( T ) to remain negative. Specific heat measurements also show the presence of excess entropy below a negative anomaly near 53 K. Our findings identify a possible new class of non-equilibrium system with potential applications in non-volatile memory devices.