Magnetoelectric Coupling Based On Motion Of Protons In Ammonium Ferroelectrics

Current research on artificial intelligence,big data,facial recognition and automated piloting requires better and better computing performance.However,transistors and memory cells are reaching the limits of miniaturization to atomistic levels.An alternative approach is to develop multiferroic materials that can store information in both the magnetization and polarization,from such,each cell may store a value of 0,1,2 or 3.However,there are too few multiferroics and the onset of their magnetic order and ferroelectricity occurs at very low temperatures,this prohibits their practical use in memory devices.Recently,the periodic rotational motions of protons in CH₃⁺in C₆（CH₃）₆and NH₄⁺in NH₄X were identified to become magnetically ordered.These findings can effectively provide an alternative pathway to couple the ferroelectric and ferromagnetic orders because it does not involve the electronic structure.Based on this,the magnetoelectric coupling properties of ferroelectric materials containing NH₄⁺were studied in the following aspects:First,the complex dielectric constant and magnetic susceptibility of（NH₄）₂SO₄was measured in order to better understand the physics of proton-based magnetoelectric coupling.At the ferroelectric Curie temperature T_C,two successive transitions were observed in the realε（T）and imaginaryε（T）parts of the dielectric constant when the cooling rate was set to 0.1 K/min.At the same time,pronounced anomalies in the magnetic susceptibility X（T）dependent on the direction of external magnetic field confirming magnetoelectric coupling near T_C.We identified the ferroelectricity of （NH₄）₂SO₄ to arise from the ordering of the proton orbitals.The associated magnetic moment is weak,but the proton orbitals become ordered because they are linearly coupled to the dipole moment arising from distorted tetrahedra NH₄⁺which become constrained at T_C.Hence,with decreasing thermal energy down to T_C,the NH₄⁺reorientations end up only rotating about their least energetic orbitals which render the magnetic moments to establish long-range order.These are essentially quantum rotors that become resonant with each other at T_Cso the lattice ends up having to distort to lift the energy degeneracies.There are two inequivalent NH₄⁺sites in（NH₄）₂SO₄so the distortions do not cancel each other out and the lattice becomes non-centrosymmetric.Second,we measured the magnetic susceptibilities of NH₄H₂PO₄and KH₂PO₄to compare any differences between K⁺and NH₄⁺ions.In NH₄H₂PO₄and its deuterated analogues,discontinuities inX（T）along all axes occur at antiferroelectric transition temperature T_N.For the case of KH₂PO₄,less pronounced discontinuities only occur along the b-axes inX（T）at T_C.With the isotope effect of NH₄H₂PO₄,the transition temperatures ofX（T）shift upward with increasing deuteration.We employed the one dimensional vibrator model to describe the role of the H atoms on the nonpolar to polar phase transitions.Finally,we measured the ac and dc magnetic susceptibilities,the specific heat and the dielectric constant of NH₄Al（SO₄）₂·12H₂O（AASD）.In AASD,the site of each μ_pis equally distanced to 12 other nearest neighbors and the ground state is4-fold degenerate so competition exists between a long-range ordered configuration and a spin glass state.If the cooling rate is 1 K/min,then the proton orbitals are in a non-equilibrium state.If the cooling is slow,the proton orbitals become ordered at T_C=58 K and the magnetization becomes positive.These two phases result in opposite polarities in the magnetic susceptibility gives rise to an entire new generation of non-volatile memory devices.Research and development of such technology will provide a shortcut to better understanding the ordering processes of glass systems and an alternative pathway toward sustaining Moore’s Law.