Anisotropic magnetoresistance in antiferromagnetic Sr2IrO4

We report point-contact measurements of anisotropic magnetoresistance (AMR) in a single crystal of antiferromagnetic (AFM) Mott insulator Sr2IrO4. The point-contact technique is used here as a local probe of magnetotransport properties on the nanoscale. The measurements at liquid nitrogen temperature revealed negative magnetoresistances (MRs) (up to 28%) for modest magnetic fields (250 mT) applied within the IrO2 a-b plane and electric currents flowing perpendicular to the plane. The angular dependence of MR shows a crossover from four-fold to two-fold symmetry in response to an increasing magnetic field with angular variations in resistance from 1-14%. We tentatively attribute the four-fold symmetry to the crystalline component of AMR and the field-induced transition to the effects of applied field on the canting of AFM-coupled moments in Sr2IrO4. The observed AMR is very large compared to the crystalline AMRs in 3d transition metal alloys/oxides (0.1-0.5%) and can be associated with the large spin-orbit interactions in this 5d oxide while the transition provides evidence of correlations between electronic transport, magnetic order and orbital states. The finding of this work opens an entirely new avenue to not only gain a new insight into physics associated with spin-orbit coupling but also better harness the power of spintronics in a more technically favorable fashion.Comment: 13 pages, 3 figure

Electrically Tunable Band Gap in Antiferromagnetic Mott Insulator Sr2IrO4

The electronic band gap in conventional semiconductor materials, such as silicon, is fixed by the material's crystal structure and chemical composition. The gap defines the material's transport and optical properties and is of great importance for performance of semiconductor devices like diodes, transistors and lasers. The ability to tune its value would allow enhanced functionality and flexibility of future electronic and optical devices. Recently, an electrically tunable band gap was realized in a 2D material - electronically gated bilayer graphene [1-3]. Here we demonstrate the realization of an electrically tunable band gap in a 3D antiferromagnetic Mott insulator Sr2IrO4. Using nano-scale contacts between a sharpened Cu tip and a single crystal of Sr2IrO4, we apply a variable external electric field up to a few MV/m and demonstrate a continuous reduction in the band gap of Sr2IrO4 by as much as 16%. We further demonstrate the feasibility of reversible resistive switching and electrically tunable anisotropic magnetoresistance,which provide evidence of correlations between electronic transport, magnetic order, and orbital states in this 5d oxide. Our findings suggest a promising path towards band gap engineering in 5d transition-metal oxides that could potentially lead to appealing technical solutions for next-generation electronic devices.Comment: 14 pages, 6 figure

Magnetic switching and magnetodynamics driven by spin transfer torque

textIn the scope of this thesis spin transfer torque (STT) driven switching and resonances in point contact experiments are investigated. In the first part, the focus is on STT driven switching events in magnetic devices with different tilt of the magnetization with respect to the thin film sample plane. Varying tilt is reached by different magnetic multilayers as Co/Ni and Co/Pt and the e efficiency of STT is compared by measuring the magneto resistance (MR) traces. As expected it was observed that tilting the magnetization of one layer with respect to the other, can improve STT efficiency. This was confirmed by micromagentic simulations using OOMMF. In the second part of this thesis, STT driven resonances in an exchange-biased spin valve (EBSV) were investigated by applying ac (microwave) and dc currents while sweeping the applied magnetic field. The resulting magnetodynamics were observed by measuring the rectified voltage which appears across the sample. To characterize the sample first the well known and understood ferromagnetic resonance (FMR) was excited. After that the power of the applied ac current was increased and a second resonance at a smaller magnetic field could be observed. This resonance structure was investigated and shown to be due to parametric resonance. This non-linear excitation appears in oscillator systems, if one or both parameter (damping, eigen frequency) oscillate in time. In the STT driven resonance experiments, the accurrent causes the damping to oscillate and therefore drives the system into parametric resonance.Physic

Current-driven non-linear magnetodynamics in magnetic nano-devices

Spintronis is an emerging electronic technology that is built on interconnections between the electron’s electric charge and its quantum-mechanical spin. The interconnections allow altering the electrical transport properties of magnetic nano-devices by changing the magnetic configuration, and vice versa. It opens up a possibility of denser and faster magnetic memory and logic devices. In this work, we conducted electronic transport studies using nanco-scale point-contacts in CoSiBFeNb, exchange-biased spin valves, and the antiferromagnetic Mott insulator Sr2IrO4 and Sr3Ir2O7. Magnetic domain switching and evidence of spin-transfer torque in CoSiBFeNb were observed. Furthermore, by simultaneously measuring the rectification signal and microwave absorption, we were able to directly compare electrical detection of ferromagnetic resonance and conventional absorption measurements. We found a good agreement between the methods and showed that here the point-contact acts as a nano-scale bolometer, monitoring the absorption of microwave current. Measurements in exchange-biased spin valves showed that parametric resonance can be excited next to ferromagnetic resonance. These non-linear excitations are driven by spin-transfer torque and due to the field-like component shift with applied dc bias. Parametric resonance can potentially be used as a new and faster method to switch the magnetization in magnetic memory and logic devices. Last, we studied electrical transport in Sr2IrO4 and Sr3Ir2O7. Both compounds revealed a decrease in activation energy with increasing dc bias, which was well fitted by a field-effect model and explained by small lattice distortions. Moreover, a small resistive switching due to the transition between meta-stable states at a critical current was observed. High-frequency measurements in Sr3Ir2O7 showed a resonance-like peak structure in the rectification signal as a function of dc bias at sufficiently high microwave power. We attribute these features to magnonics that can be excited in Sr3Ir2O7 when the lattice is distorted in an ac electric field. Our results show that transition metal oxides such as Sr2IrO4 and Sr3Ir2O7 are a new class of materials that allow for modifying band structures via dc and ac currents in spintronic applications.Physic