We observe sharp step-down discontinuities in the magnetic susceptibility of NHHPO and NHHPO- (60% deuterated) along the and -axes occurring exactly at their antiferroelectric transition temperatures. For the case of KHPO, less pronounced discontinuities occur at the ferroelectric transition temperature. To explain this, we treat the acid protons as individual oscillators that generate current elements which 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 NHHPO, the associated internal magnetic field appears to aid the NH to order at higher temperature. At 49 K, a shoulder-like anomaly in both NHHPO and KHPO is attributed to a possible onset of macroscopic quantum tunneling of protons. Our findings bring forth a new category of intrinsic multiferroic systems.

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