Resistive Switching Properties And Magnetism In The Ti/Pr(Sr_0.1Ca_0.9)₂Mn₂O₇/Pt Heterostructures

With the rapid development of semiconductor technology, various new non-volatilememory(NVM) devices such as ferroelectric random access memory (FRAM), magneticrandom access memory (MRAM), phase-change random access memory (PRAM) andresistance random access memory (RRAM) have been investigated. RRAM has beenconsidered one of the most promising candidates for the next generation new non-volatilememories due to its simple structure, high storage density, low power consumption, fastread/write speed and better compatibility with conventional CMOS processes. RRAM hasbecome one of hotspots in physics, materials science and other research in the field ofelectronics, increasing widespread concern of researchers. RRAM memory based on theelectric-pulse-induced resistance switching effect (EPIR) to read data, the device canreversible switch between high impedance state (HRS) and low resistance state (LRS) byapplying a pulsed electric field in order to achieve binary data "0" and "1" been stored.However, the physical mechanism of EPIR still exist a lot of controversy, it’s a majorobstacle for RRAM commercial applications. Currently, the research has mainly focus onmetal oxide material, binary transition metal oxides, perovskite oxides and rare earthmanganese oxides. In this paper, bilayer perovskite manganese oxides Pr(Sr0.1Ca0.9)2Mn2O7(PSCMO) chosen as resistive switching film layer, the resistance swiching characteristics andmagnetism changes after EPIR have been investigated. The main research in the followingtwo aspects:1. Preparation and resistive switching characteristics in Ti/PSCMO/Pt memory devicesTwo films (PSCMO-1and PSCMO-2) deposited on Pt/Ti/SiO2/Si(100) substrate bypulsed laser deposition (PLD) under different deposition time. The XRD analysis showed thatthe two films are polycrystalline, the (215) diffraction peak of PSCMO-2film reveals higherand sharper, showing an improved crystallinity. From the AFM and SEM measurements, thetwo samples have nearly the same grain size, but the grain boundaries are obviously different.PSCMO-1shows that some grains are embedded in amorphous background, forming a loosegrowth along the surface. In contrast, PSCMO-2displays tightly connected grains withoutamorphous background, forming a considerably denser film. The film thickness was about 50nm (PSCMO-1) and60nm (PSCMO-2). Ti electrodes with diameter of200μm wereprepared by magnetron sputtering, the two Ti/PSCMO/Pt memory devices both showedbipolar resistive switching characteristics, but the I-V characteristic curves showed significantdifferences. Analyzed the differences of I-V characteristic curve combined with the differentgrain boundary of two samples, the loose grain boundaries may be related to the formation offilaments in the PSCMO-1film, the sharp change of I-V curve caused by conductivefilament’s formation and rupture. In contrast, for PSCMO-2film, it is difficult to formfilamentary conduction paths in the denser and more regular grain boundaries. By fitting theI-V curves, the slow changes of I-V curve due to the change of Schottky barrier at Ti/PSCMOinterface. Different resistance changes were explained by two resistive switching mechanisms.Our results suggest that the different grain boundaries may play a critical role in oxygenvacancy movement, and hence result in the two different resistive switching properties.2. Resistance switching dependence of magnetism in Ti/PSCMO/Pt memory devicesThe magnetic properties of the PSCMO films after EPIR treament. PSCMO film havebeen studied. A PSCMO film about60nm deposited on Pt/Ti/SiO2/Si(100) substrate by PLDand then prepared different top electrodes using magnetron sputtering and thermalevaporation coating respectively. Magnetization curves of the sample in LRS and HRSmeasured by PPMS, showing a change from paramagnetic at initial state (IS) to ferromagneticat LRS in the parallel or perpendicular to the magnetic field direction. Saturationmagnetization(Ms) in LRS and HRS are50emu/cm3and80emu/cm3respectively, and thechange is60%in the parallel to the magnetic field direction. In the perpendicular to themagnetic field direction, the Ms reaches30emu/cm3and60emu/cm3in HRS and LRS, changerate of Ms reaches100%. This shows that the resistance switching between HRS and LRS caninduce a great vary on the magnetism of the device. Ag top electrode has been prepared bymagnetron sputtering and thermal evaporation coating respectively. Two samples showsignificantly different. The sample in the initial state shows ferromagnetic due to the effect ofion bombardment in magnetron sputtering process and then gradually decays after EPIRtreatments. But the magnetism of the device sputtered top electrodes by thermal evaporationcoating is consistant with previous studies, which magnetization curves shows paramagneticat IS to ferromagnetic at LRS. We speculate that the change of magnetism come from the migration of oxygen vacancies, which lead to the change of spin coupling interaction betweenMn3+and Mn4+, thus affecting the lattice structure and resulting in magnetic transform.