Interfacial Coupling In YMnO₃ Based Multiferroic Heterostructures

As a typical multiferroic（MF）material,YMnO₃ has two special crystal structures,namely the hexagonal and orthorhombic ones.Hexagonal（h-）YMnO₃ has been much focused on for its higher Curie temperature above room temperature,which makes it possible to obtain an obvious magnetoelectric（ME）effect in ferromagnetic（FM）/ h-YMnO₃ MF heterostructures.The ferroelectricity in（o-）YMnO₃ has been found to originate from the intrinsic magnetic order,leading to a strong ME effect.Our work focuses on the interfacial coupling modulated magnetic properties in the MF heterostructures which are composed of the isomerism MF YMnO₃ and FM material.With strong correlation among charge,spin,orbital and lattice degree at the interface,it would be possible to introduce orbital reconstruction and charge transfer by applying electric or magnetic fields to the all oxide FM/MF heterostructures.Thus,we will be able to modulate their magnetic or electric properties and further clarify the ME coupling mechanisms.An anomalous thickness dependence of exchange bias（EB）effect was discovered in the Fe₃O₄/h-YMnO₃ MF heterostructures.The anomalous EB effect is considered to be associated with the joint effect from the special uncompensated moment at the antiferromagnetic domain walls in h-YMnO₃ and the interactions among antiphase domains（APDs）in Fe₃O₄.By changing t FM,the strength of the interaction among APDs can be modulated,giving rise to a modulated EB effect.Furthermore,in Fe/h-YMnO₃ MF bilayers,we realized the modulation of magnetic anisotropy by EB effect.This is due to the competition between the EB induced unidirectional anisotropy energy and Zeeman energy.In o-YMnO₃/La_0.6Sr_0.4MnO₃/SrTiO₃ MF heterostructures,the EB effect is modulated by the crystal orientations.This effect originates from the difference of Mn3+-O2--Mn4+ bond angle which has strong influence on the strength of interfacial coupling.Meanwhile,in La_0.6Sr_0.4MnO₃/o-YMnO₃/SrTiO₃ MF heterostructures,when the La_0.6Sr_0.4MnO layer was grown on o-YMnO₃ layer,the La_0.6Sr_0.4MnO₃ layer suffers a lattice distortion induced by the tensile strain.The distortion leads to an orbital reconstruction of Mn 3d orbital and brings antiferromagnetic phase to La_0.6Sr_0.4MnO₃ layers which enhances the EB effect.The tensile strain not only brings the antiferromagnetic phase,but also introduces a magnetoelastic coupling energy with two-fold symmetry to the MF heterostructures.The competition between the Zeeman energy,magnetoelastic coupling energy and EB induced unidirectional anisotropy energy leads to the hysteresis of moment rotation in the MF heterostructure.A giant magnetoresistance value of 1500% was also discovered in the MF heterojunctions.This is due to the modulation of the barrier width in the junctions.The threshold voltages decrease with the increasing magnetic field,thus at a special reading voltage,the heterostructures are in the conductive and insulating states with high and low applied fields,respectively.