Electric Field Control Of Exchange Bias By Resistive Switching

Spintronics is a newly developed interdisciplinary discipline involving magnetism,electronics,and informatics.Its core content is to combine standard microelectronics technology with spin-related effect by actively regulating the spin freedom of electrons in solid-state systems.It provides a realizable opportunity for the development of new generation of high-performance electronic devices.It is worth mentioning that compared with metal-oxide-semiconductor electronic devices which have been widely used nowadays,voltage(or electric-field)controlled spintronic devices have many advantages,such as low energy consumption,fast processing speed,high integration and multi-function.In recent years,they have been received widespread attention in the field of information technology.However,due to absence of strong direct coupling between magnetization and electric field in ordinary materials,no significant progress has been made in voltage controlled spintronics.Exchange bias is a direct coupling effect at the interface of ferromagnetic-antiferromagnetic system.Although its inherent mechanism has not been thoroughly unraveled so far,it has been widely used in many kinds of magneto-electronic devices,such as read heads and magnetic sensors.Full voltage control of exchange bias can provide a new idea in the field of voltage control of magnetism,and a possible way to achieve high energy efficiency spintronic devices.Although in the past decade,people have tried to use various methods to study the effect of voltage control of exchange bias in various materials and devices,each method has some limitations.At present,people has not yet achieved reversible,non-volatile and repeatable voltage control of exchange bias at room temperatureIn the present work concerned about this thesis,we made multilayer device samples by magnetron sputtering.The exchange coupling at theferromagnet/antiferro-magnet interface is controlled by electric field according to resistive switching effect.More specifically,the exchange bias is controlled by virtue of two types of resistive effects,i.e.conduction-filament type(bulk effect)and Schottky-like-barrier type(interface effect).In this thesis,systematic studies have been performed on the electric field control of exchange bias in the Pt/Co/NiO/Pt and Pt/Co/FeOx/ITO multilayer devices at room temperature.The main contents of this thesis are described as follows:?.Electric field control of Exchange Bias in Pt/Co/NiO/Pt Multilayer DevicesA novel approach is proposed by virtue of unipolar resistive switching effect,which realizes reversible and non-volatile electric controlled exchange bias in Si/SiO2/Pt/Co/NiO/Pt multilayer devices at room temperature.Two different NiO layers are designed for fabrication of the devices.NiO(1)layer is grown in an oxygen-free atmosphere to ensure a stable exchange bias at the Co-NiO interface,while the other layer of NiO(2)is grown in a partially oxygen-filled atmosphere to enhance the insulation level and ensure that the device is able to have a stable resistive switching effect.By applying certain voltages,the device displays obvious exchange bias(about 20 Oe)in the high-resistance-state while negligible exchange bias in the low-resistance state.By elaborate micro structure characterization,conductive filaments forming in the NiO layer and rupturing near the Co-NiO interface can be found and considered to play dominant roles in determining the combined resistive switching and exchange bias phenomena.In comparison with previous devices concerning about electric control of exchange bias,the present one possesses distinct advantages in many respects.Firstly,due to the fact that NiO has relatively high Neel temperature(?525 K),the electric control of exchange bias on the device can be operated very stably at room temperature or even higher ones.Secondly,NiO is a textbook-style resistive switching material,which exhibits distinguished HRS and LRS with on/off ratio higher than 105,leading to high reliability and stability of the device.Thirdly,the device can be fabricated by common sputtering technique at room temperature and no further magnetic and/or electric field cooling is needed.Fourthly,the multilayer film made for the device can be well grown on silicon substrate,which exhibits good compatibility with the present silicon planar technology.Finally,the electric control of exchange bias in the present device is reversible,repeatable and non-volatile at room temperature.In a word,the present work paves a new way for designing multifunctional and non-volatile magnetic-electrical random access memory devices.?.Electric field control of Exchange Bias in Pt/FeOx/Co/ITO Multilayer DevicesAs mentioned above,electric field control of exchange bias based on bulk effect mechanism has been achieved by controlling the antiferromagnetic states through conduction filaments.Whether or not the exchange bias can be controlled by interface-barrier-type resistive switching effect remains unknown.Beeause ?-Fe2O3 has a very high Neel temperature(?955 K)and is also a typical resistive switching material,we chose Co/FeOx system to control the exchange bias by using resistive switching effect.The FeOx thin films were deposited with different oxygen partial pressure and then the Pt/FeOx/Co/ITO multilayer devices were fabricated.Two types of resistive switching effects could be observed.One is the conducting-filament type and the other is the interface-barrier type.When the FeOx layer was deposited at higher oxygen partial pressure(Po2,:PAr=2:3),unipolar resistive switching effect could be observed,which had negligible influence on the exchange bias caused by the Co/FeOx interface.For this kind of devices,the micro structure of the FeOx layer was characterized carefully by TEM at low resistance state.We consider that the antiferromagnet state or interface spin structure of the FeOx layer is not able to be manipulated because of extremely small quantity of conductive filaments possibly constituted by oxygen vacancy.However,for the devices with the FeOx layer deposited at higher oxygen partial pressure(Po2:PAr=1:2),the bipolar resistive switching effect belonging to the interface-barrier type could be observed,and the on/off ratio could be achieved at about 103.The exchange bias field and coercivity in the original state are about 115 Oe and 71 Oe,respectively.If appropriate voltage was applied,the coercivity of the device increased obviously in the high resistivity state with the increment as high as 70.4%,whereas the exchange bias field remains almost unchanged.However,both the coercivity and the exchange bias field remain almost unchanged if the voltage is applied to set the device to a low resistance state.We consider that the migration of the oxygen vacancies in such devices changes the height of Schottky-like barrier at the interface,which results in bipolar resistive switching effect with significant change of resistance between the high-and low-resistance states.Therefore,the corresponding change of the exchange coupling may be caused by the modification of the antiferromagnetic structure at the Co-FeOx interface,which is resulted from the migration of the oxygen vacancies at different applied voltages.So far,the physical mechanism of such phenomena has not been fully understood and further studies are needed by virtue of more elaborate structural characterization and theoretical simulation calculation.