Magnetic Properties of NH₄H₂PO₄ and KH₂PO₄: Emergence of Multiferroic Salts

We observe sharp step-down discontinuities in the magnetic susceptibility of NH₄H₂PO₄ and NH₄H₂PO₄-d₆₀ (60% deuterated) along the a- and c-axes occurring exactly at their antiferroelectric transition temperatures. For the case of KH₂PO₄, less pronounced discontinuities occur at the ferroelectric transition temperature. To explain this, we treat the acid protons as individual oscillators that generate current elements that translate to magnetic forces in near resonance with each other. With decreasing temperature, the resonant forces become more commensurate, which amplifies a disproportionate drop off of two types of magnetic forces to eventually trigger the structural phase transitions. For the case of NH₄H₂PO₄, the associated internal magnetic field appears to aid the NH₄⁺ to order at a higher temperature. At 49 K, a shoulder-like anomaly in both NH₄H₂PO₄ and KH₂PO₄ is attributed to a possible onset of macroscopic quantum tunneling of protons. Our findings bring forth a new category of intrinsic multiferroic systems.