Study On Resistance Switching And Magnetism In Amorphous MgO And HfO₂ Films

Binary metal oxides exhibiting the nonvolatile resistive switching effect upon applying an electric field have been known for over 50 years.Their practical application in memory devices,i.e.,resistive random access memory?RRAM?,has attracted considerable attention in recent decades due to the high density,low cost,and fast operating speed of RRAM.RRAM usually has a metal/oxide/metal capacitor-like structure and can be switched reversibly between a high resistance state?HRS?and a low resistance state?LRS?by applying an electric field.Recently,magnetic modulation accompanied by the RS effect,i.e.,magnetization changes associated with resistance states,has been found in RRAM based on binary oxide films.This phenomenon offers a new avenue for exploration of binary metal oxides in spintronic and multifunctional memory devices.In this dissertation,resistance switching and magnetism in amorphous MgO?a-MgO?and HfO₂ films are studied systematically,and the underlying mechanism of those properties are discussed finally.The mian results are as follows:?1?a-MgO based memory devices exhibited forming-free resistive switching behavior with an ultralow programming voltage?just 0.22 V?,excellent uniformity in the switching parameters?the coefficient of variation is only 1.7%and 2.2%for Set and Reset voltage,respectively?,low power consumption?less than 176.7 W?,good endurance after more than4000 consecutive cycles and long retention time of more than 10⁴ s.No obvious changes occurred in the current-voltage characteristics of the device after 6 months.The migration of O^2-ions under an electric field lead to the formation and rupture of the metallic Mg conducting filaments plays a major role in the RS process.The promising properties of the Ti/a-MgO/Pt device are attributed to its rough surface and the unstable local structures of the a-MgO layer.This work offers a new pathway for the development of low power consumption and high-performance memory devices.?2?After annealing in an oxygen atmosphere,the magnetic properties of a-MgO thin film can be controlled by ultra-low voltage.The intrinsic magnetization of a-MgO can be decreased by about 57.5%by the application of a positive bias voltage while increased by about 56.7%by a negative bias,at an ultralow voltage of just 0.2 V.More interestingly,this ultralow voltage also induces a strong magnetic anisotropy in the a-MgO film.Further analysis indicates that the migration of O^2-ions under an electric field results in a change in the Mg/O ratio and the redistribution of Mg vacancies,thus leading to the change in the magnetic properties of the film.The control of room temperature magnetic properties at ultralow voltages may find applications in multifunctional memory and ultralow-power consumption spintronics.?3?Large-magnitude magnetic modulation experimentally demonstrated in Hf/HfO₂/Pt memory device.The saturation magnetization of the HfO₂ film in a low resistance state is more than four times greater than that in a high resistance state.Moreover,the Set and Reset voltages in the Hf/HfO₂/Pt device are only 0.4 and 0.3 V,respectively.These results can attribute to the use of Hf top electrode in this device.The dual ion migration of hafnium?originating from the Hf top electrode?and oxygen in the Hf/HfO₂/Pt device can leads to the change of the concentration and distribution of oxygen vacanvy in the HfO₂ film.This work offers an opportunity to explore the application of binary oxides in multifunctional memory device by selecting suitable oxides and a suitable top electrode.