Preparation And Magnetic Properties Of Manganite/BiFeO₃ Heterostructures

Manganite heterostructures are arousing considerable attention due to their innovative and peculiar physical phenomena,which are fascinating from the fundamental physics perspectives as well as potential applications in magnetic memory,sensors and spintronics.More novel physical phenomena are expected due to the strong interfacial coupling of spin,orbit,charge and lattice degrees of freedom in manganite heterostructures.Many potential physical phenomena in manganite heterostructures need to be further discovered and the physical mechanism of many discovered physical phenomena need to be deeply studied.In this work,the manganite/BiFeO3(manganite/BFO)heterostructures were prepared by pulsed-laser deposition.The origin of the exchange bias effect in manganite/BFO heterostructures has been discussed.A new interfacial phenomenon,which is the interface-induced transition from cluster glass state(CGS)to spin glass state in LaMnO3/BiFeO3 heterostructures,was reported and the reason of the transition was investigated.It was found that the CGS is formed in LMO films,and the formation mechanism of the CGS was investigated.All the contents are show as follows:1.Charge transfer and exchange bias effect in LSMO/BFO and LMO/BFO heterostructures.La0.7Sr0.3MnO3/BiFeO3(LSMO/BFO)and LaMnO3/BiFeO3(LMO/BFO)heterostructures were prepared on(001)-oriented SrTiO3(STO)substrates by pulsed-laser deposition,and the exchange bias effect and the charge transfer procedure at the heterointerfaces were investigated.The LSMO/BFO and LMO/BFO heterostructures exhibited an exchange bias with a magnetic field HEB=46 Oe and HEB=30 Oe,respectively.The/HEB and TB values in the LSMO/BFO heterostructure are larger/higher than those in the LMO/BFO heterostructure,indicating that the magnetic coupling at the LSMO/BFO interface is stronger than that at the LMO/BFO interface.The charge transfer procedure is determined from Fe ions to Mn ions at the LSMO/BFO interface,conversely from Mn ions to Fe ions at the LMO/BFO interface.The charge transfer from Fe ions to Mn ions can lead to stronger interfacial magnetic coupling.According to the charge transfer mode,the possible microscopic scenarios of orbital reconstruction at the LSMO/BFO and LMO/BFO interfaces were proposed.The charge transfer,which is associated with orbital reconstruction,is also the origin of the exchange bias effect in the manganite/BFO heterostructures.2.Charge transfer and strain induced transition of cluster glass state in LMO/BFO heterostructures.LMO/BFO heterostructures and pure LMO films were deposited on(001)-oriented STO substrates by pulsed laser deposition,and the low temperature glassy magnetic state in the LMO/BFO heterostructures and the LMO films was investigated,respectively.It is found that the cluster glass state is formed in the LMO films,while the spin glass state is formed in the LMO/BFO heterostructures.The LMO/BFO interface induced the transition from cluster glass state to spin glass state in LMO/BFO heterostructures.And the spin glass state in the LMO/BFO heterostructure is fundamentally confirmed using susceptibility measurements.It is found that the Vogel-Fulcher model can also apply to the spin glass state in LMO/BFO heterostructures.In addition,compared with the LMO film,the decrease of the coercive field(HC)of the LMO/BFO heterostructure indicates the weaker pinning effect of spin glass state than cluster glass state.The charge transfer from Mn ions to Fe ions described as Fe3++Mn3+?Fe2++Mn4+occurs at the interface region of LMO/BFO heterostructures;the LMO layer of LMO/BFO heterostructure undergoes the out-of-plane tensile strain state,in contrast to the out-of-plane compressive strain state in the LMO films.The interface-induced transition from cluster glass state to spin glass state in LMO/BFO heterostructures can be attributed to the combined effects of interfacial charge transfer and strain effect.3.Strain-induced cluster glass state in LMO films.LMO thin films were deposited on(001)-oriented LaAlO3(LAO),SrTiO3(STO)and MgO single crystal substrates by pulsed laser deposition,and the formation of CGS in the LMO films was studied.The results show that the CGS is formed in the LMO films grown on the STO and LAO substrates,but is not formed in the LMO films grown on MgO substrate.It was demonstrated that the formation of CGS is due to the biaxial tensile strain,which induces the increase of oxygen vacancy concentration by reducing the formation energy of oxygen vacancy.To maintain the electric neutrality,the number of Mn++ions increases in the LMO film grown on STO and LAO substrate in comparison with that of the LMO film grown on MgO substrate.Therefore,the ferromagnetic interactions between Mn3+and Mn4+ions are weaken due to the changed ratuo of Mn3+/Mn4+.However,the antiferromagnetic interactions between Mn3+ ions are enhanced due to the increase of Mn3+ ions.Therefore,the competition between ferromagnetic interaction and antiferromagnetic interaction is formed in the LMO films grown on LAO and STO substrates,thus forming CGS in these films.The pinning effect of CGS induces the enhanced coercivity in the LMO films grown on STO and LAO substrates.In addition,compared with the LMO film grown on STO substrate,the higher oxygen vacancy concentration induced by the larger biaxial tensile strain of the LMO film grown on LAO substrate leads to the stronger competition between ferromagnetic and antiferromagnetic interactions,which lead to the stronger pinning effect of the CGS.As a result,the exchange bias and larger coercivity are achieved in the LMO film grown on LAO substrate.