Magnetic Properties of NH$_4$H$_2$PO$_4$ and KH$_2$PO$_4$: Emergence of Multiferroic Salts

TL;DR

This study uncovers magnetic discontinuities in NH$_4$H$_2$PO$_4$ and KH$_2$PO$_4$ at their phase transitions, revealing a new class of intrinsic multiferroic salts driven by proton oscillations and resonant magnetic forces.

Contribution

It introduces a novel explanation for magnetic anomalies in these salts based on proton oscillators and resonance, establishing a new category of multiferroic materials.

Findings

Magnetic susceptibility shows sharp discontinuities at phase transitions.

Proton oscillations generate resonant magnetic forces influencing structural changes.

Possible macroscopic quantum tunneling of protons observed at 49 K.

Abstract

We observe sharp step-down discontinuities in the magnetic susceptibility of NH$_4$H$_2$PO$_4$ and NH$_4$H$_2$PO$_4$-$d$$_{60}$ (60% deuterated) along the $a$ and $c$-axes occurring exactly at their antiferroelectric transition temperatures. For the case of KH$_2$PO$_4$, less pronounced discontinuities occur at the ferroelectric transition temperature. To explain this, we treat the acid protons as individual oscillators that generate current elements which translate to magnetic forces in near resonance with each other. With decreasing temperature, the resonant forces become more commensurate which amplifies a disproportionate drop off of two types of magnetic forces to eventually trigger the structural phase transitions. For the case of NH$_4$H$_2$PO$_4$, the associated internal magnetic field appears to aid the NH$_4$$^+$ to order at higher temperature. At 49 K, a shoulder-like anomaly in both NH$_4$H$_2$PO$_4$ and KH$_2$PO$_4$ is attributed to a possible onset of macroscopic quantum tunneling of protons. Our findings bring forth a new category of intrinsic multiferroic systems.