Multiferroicity In The Manganite-based Multilayer Structure

Magnetism and ferroelectricity are two basic properties of materials.How to integrate them into the same system and realize the induction of the magnetism?ferroelectricity?by electric field?magnetic field?is an important research topic in the field of condensed matter physics and material science nowadays.Perovskite manganese oxide is a typical strong associated electronic system,where has strong coupling between the spin,orbital,charge and lattice.So it shows many novel peculiar physical phenomena,such as colossal magnetoresistance,magnetostriction and magnetic anisotropy,magnetoelectric coupling,photoinduced voltages effect,optical magnetoelectric effect,etc.In addition,by changing the angle and length of Mn-O-Mn bond in the manganese oxide,the electrical and magnetic properties of the manganese oxide can be effectively regulated.In recent years,the discovery of the huge magnetoelectric effect of the rare-earth manganese oxide TbMnO₃ with the quadrature structure has spawned a new type of multiferroics.Many researchers have paid much attention to the multiferroicity of manganese oxide.Even so,the performances of many multiferroics are still too difficult to meet the application.Therefore,it is still a great challenge to obtain considerable magnetoelectric effect and even optical magnetoelectric effect at room temperature.Previous studies have shown that composite materials,including ferroelectric/ferromagnetic heterojunctions and artificial superlattices,are an effective method to solve the above problems.In this paper,the low-dimensional system of manganese oxide are the main research object.By using laser molecular beam epitaxy technology,the manganese oxide films are grown,including tri-color superlattice film,ferroelectric/ferromagnetic heterojunction film.The magnetoelectric,magnetodielectric and optical magnetoelectric effects of thin films are investigated by doping,interface effect,and growth direction.The study on perovskite manganese oxides is of great and practical significance for the design and development of new multiferroic materials.The key contents in our reasearch include four parts:?1?The structural and ferroelectric properties of[?LSMO?_n/?PCMO?_n/?LSbMO?_n]_m SLs films are grown on Nb:SrTiO₃ single crystal substrate and investigated by a laser molecular-beam epitaxy technology at 1053 K.We obtain high quality films with a remnant polarization Pr¹8.9?C/cm² and a saturation polarization Ps⁸8.7?C/cm² at 30 K,respectively.We demonstrate the emergence of ferroelectricity in artificial SLs composed of materials that are non-ferroelectrics in their bulk form.The ferroelectric properties of the multilayer structures are enhanced with increasing SL periodicity m,which is attributed to the spatial variations of magnetizations induced by the frustration in multilayer films.Meanwhile,the frustration enhances with increasing SL periodicity m,and then the exchange interaction between Mn3+and Mn4+is weakened.This leads to the reduction of magnetic properties in superlattice films.Thus,the strong periodic dependence of magnetism of SLs presents the presence of frustration,and we contribute the frustration in the films as the extremely critical part to the presence of multiferroicity.Our results further verify the previous theoretical research of generating multiferroics experimentally paving a way for designing or developing the novel magnetoelectric devices based on manganite ferromagnets.?2?We successfully fabricated[?LSMO?_n/?PCMO?_n/?LSbMO?_n]_m tricolor SL films on?001?-oriented Nb:SrTiO₃ single-crystal substrates with?L-MBE?at 1053 K to investigate the ME and MD effects of SL films.According to the electric-polarization hysteresis loops at different external magnetic fields,both coercive electric field and remnant polarization intensity of the SLs show strong dependences on the magnetic field,suggesting a strong negative ME effect that does not appear in the single-phase compounds.Further,a negative MD effect is observed at low temperatures,which indirectly supports the ME effect.Both ME and MD effects depend on?n,m?.Consequently,the interfaces contribute to an increased ME effect.The maximum ME coupling coefficient in the SL is obtained with?n,m?=?3,15?at 30K.Our results confirm that the ME effect can be increased by employing asymmetric tricolor SLs—even though the SL components are non-multiferroic materials.These results pave the way for the design of novel ME devices.?3?We fabricate the tricolor manganese oxide SLs[?LSMO?_m/?PCMO?_m/?LSbMO?_m]_s composed of Pr_0.9Ca_0.1MnO₃,La_0.9Sr_0.1MnO₃,and La_0.9Sb_0.1MnO₃ with the simultaneous breakings of the spatial inversion and timereversal symmetries.The grating structures with different d=4,6,8,and 10?m are patterned on the SLs with?m,s?=?3,15?,?7,6?,?9,5?,and?15,3?.The OME effect is investigated by the Bragg diffraction,yielding the relative change of diffracted light intensity.We find that the interfaces in SLs play an extremely critical part to the giant OME effect by comparing with the results of samples with different?m,s?.The OME effect in the sample with d=4?m and?m,s?=?3,15?reaches 8.27%at 40K and H=5 kOe.Our work achieves the tunable OME effect,paving the way for the application of OME devices.?4?We have successfully fabricated the ferroelectric/ferromagnetic composite heterostructure composed of BTO and LSMO on the?100?and?111?LSAT single-crystal substrate by using an L-MBE technology.The P-E hysteresis loops and magnetization curves of thin films are investigated,suggesting that both?100?-and?111?-oriented samples have ferroelectric and ferromagnetic properties at room temperature.Then,a grating structure with d=4?m is patterned on the thin film,and a Bragg diffraction technology is applied to detect the OME effect,yielding the relative change of diffracted light intensity.Both the?100?-and?111?-oriented BTO/LSMO thin films present the room-temperature OME effect,which is in line with our expectations.Otherwise,the?111?-oriented thin film has a stronger OME effect compared to the?100?-oriented thin film,which is attributed to the fact that the?111?-oriented thin film has a stronger ME coupling.Our work achieves the room-temperature OME effect,paving way to the designing of novel OME devices based on the ferroelectric/ferromagnetic heterostructure.