Electric-field Control Of Magnetism In Magnetic/Ferroelectric Multiferrioc Heterostructures

Multiferroic materials, which simultaneously possess two or more ferroic ordersand the coupling interaction between the different order parameters could produce neweffects. Multiferroic heterostructures composed of magnetic （M） and ferroelectric （FE）materials have attracted much attention due to their importance in exploring themagnetoelectric coupling effect. One of the key issues in the study of M/FEheterostructures by using the magnetoelectric couping interaction is the electric-fieldcontrol of magnetism. This study will not only be important in understanding themicro-mechanism of the coupling interaction, but also a far outlook for the applicationin spintronics. In this dissertation, we systemically studied some key issues in theelectric-field control of magnetism in multiferroic composite structures, including:Phase separated manganites are very sensitive to external factors, showing a varietyof unusual physical behavior. Achieveing the electric field control of magnetism withmultiferroic heterostructures composed of phase separated manganites will not only beimportant in understanding of the effects of magnetic coupling and phase separation, butalso can modulate phase separation continuously and reversibly that provides a goodopportunity to study the ef effect of strain on the evolution of phase separation ofmanganites. There is no report on the electric-field control of phase separation inPCMO/Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃（PMN-PT）. It was shown that there is a dramaticincrease in magnetization in PCMO/PMN-PT with increasing electric field and both theout-of-plane and in-plane magnetizations for M-H curves increase with applied electricfield. The results suggest that the electric-field control of magnetism inPCMO/PMN-PT is dominated by the change in phase separation in PCMO andincreases the proportion of ferromagnetic phase. More interestingly, the electric fieldwas removed in situ at low temperatures and the magnetization remains nearlyunchanged, manifesting a memory effect of strain in PCMO. By considering the modelof the energy landscape, we proposed a model for two local minima model in the freeenergy in phase separation in PCMO. The electric-field control of phase separation andmemory effect can be explained with our model.The magnetic field plays an important role on the electric-field control ofmagnetism. Study the influence of the magnetic field help us to understand the micro-mechanisms of the magnetoelectric couping interaction. We systematicallystudied the electric-field control of magnetism in Fe/PMN-PT multiferroicheterostructures at different magnetic fields. The M-V characteristics of theheterostructures show both a looklike behavior and a butterfly behavior and the sizes ofthe looklike and butterfly are closely related to the magnetic fields. In addition, thelooklike behavior is a nonvolatile behavior, which is very important for the application.We also used electron spin resonance method to investigate the magneticanisotropy of the samples in different polarization and Hr-V curve. We combine thethe reversal of109°ferroelectric domain which induced the change of uniaxialanisotropy and the converse piezoelectric effect in PMN-PT substrate which inducedthe change of Magnetic shells anisotropy to explain the experimental results.