Transport studies on transition-metal-oxide antiferromagnets from a perspective of applications in memory technology

Shen, Shida
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Antiferromagnetic spintronics offers a promise of more robust, stray-field-free and ultra-high-speed applications, e.g. in memory technology. Transition metal oxides (TMOs) represent a big and natural pool of antiferromagnetic materials. This thesis presents a series of transport studies with TMO antiferromagnets, i.e., Sr₂IrO₄, Ca₂RuO₄, and La₂NiO₄. We are focused on both TMO single crystals and heterostructures. For instance, we examined magnetoresistance and microwave response of a Pt/Sr₂IrO₄ double-layer structure in order to explore the spin Hall effect (SHE) enabled by the heavy-metal Pt overlayer. We have shown that the SHE-mediated spin injection from Pt into Sr₂IrO₄ can affect the magnetoresistance of Sr₂IrO₄. In a series of experiments with Ca₂RuO₄ we demonstrated that single crystals of this TMO, when doped with Cr, exhibit a non-destructive reversible resistive switching at room temperature. An applied electrical bias was shown to reduce the dc resistance of Ca₂RuO₄ crystals by as much as 75%; the original resistance of the sample could be restored by applying an electrical bias of opposite polarity. Our results of transport measurements in 2-, 3-, and 4-probe geometries suggest that the resistive switching is mostly an interfacial effect. Temperature-dependent resistivity measurements on Ca₂RuO₄ show that the activation energy of this material can be tuned by an applied dc electrical bias. A similar non-destructive, bipolar and room-temperature resistive switching is also observed in La₂NiO₄ single crystals. It is also identified by 2-, 3- and 4-porbe measurements that the switching in La₂NiO₄ happens essentially at the sample/contact probe interfaces. The wide range of observed resistance values from 10 to 80 kΩ confirmed the tunneling nature of the La₂NiO₄ contacts. We tentatively associate the resistive switching in our TMOs with electric-field induced distortions of TMO lattice and have made an attempt to probe such distortions directly in experiments with a capacitive displacement meter. The resistive switching in Ca₂RuO₄ and La₂NiO₄ may be useful for non-volatile memory devices in the future. Finally, we have performed a comparative study between recycled and virgin NdFeB magnets. The recycled magnets demonstrate an enhancement of resistivity compared to the virgin ones, but share a similar surface morphology, magnetic structure, and magnetotransport properties