Bias Voltage Dependence Of Magnetoresistance Research Articles

Abnormal bias dependence of magnetoresistance in CoFeB/MgO/Si spin-injection tunnel contacts

We report a strong bias voltage dependence of magnetoresistance (MR) in CoFeB/MgO/Si spin-injection tunnel contacts using the three-terminal Hanle geometry. When a bias voltage is relatively large, the MR is composed of two characteristic signals: a conventional Hanle signal observed at a low magnetic field, which is due to the precession of injected spins, and another signal originating from the rotation of the magnetization at a larger magnetic field. In contrast, for a small bias voltage, additional signals appear at a wide range of magnetic fields, which occasionally overwhelms the conventional Hanle signals. Because the additional signals are pronounced at a low bias and are significantly reduced by annealing at moderate temperatures, they can be attributed to multi-step tunneling via defect states at the interfaces or tunnel barrier. Our results demonstrate that the spin injection signal caused by the defect states can be evaluated by its bias voltage dependence.

Large magnetoresistance at high bias voltage in double-layer organic spin valves

We report studies of magnetoresistance (MR) in double-layer organic spin valves (DOSV) using tris (8-hydroxyquinolinato) aluminum (Alq3) spacers. The device exhibits three distinct resistance levels depending on the relative magnetizations of the ferromagnetic electrodes. We observed a much weaker bias voltage dependence of MR in the device compared to that in the conventional organic spin valve (OSV). The MR magnitude reduces by the factor of two at 0.7V bias voltage in the DOSV compared to 0.02V in the conventional OSV. Remarkably, the MR magnitude reaches 0.3% at 6V bias in the DOSVs, the largest MR response ever reported in OSVs at this bias. Our finding may have a significant impact on achieving high efficient bipolar OSVs strictly performed at high voltages.

Temperature dependence of magnetoresistance in magnetic tunnel junctions with different free layer structures

The temperature and bias voltage dependence of magnetoresistance and the resistance of two types of magnetic tunnel junction (MTJ) samples were studied. These two types of MTJ samples have different free layer structures, while having the same pinned layer structures and with the same material for free and reference layers. The layer structure for type 1 MTJs is $80\mathrm{Ru}\text{\penalty1000-\hskip0pt}8\mathrm{Co}\mathrm{Fe}\mathrm{B}\text{\penalty1000-\hskip0pt}15{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\text{\penalty1000-\hskip0pt}50\mathrm{Co}\mathrm{Fe}\mathrm{B}\text{\penalty1000-\hskip0pt}9\mathrm{Ru}\text{\penalty1000-\hskip0pt}54\mathrm{Fe}\mathrm{Co}\text{\penalty1000-\hskip0pt}350\mathrm{Cr}\mathrm{Mn}\mathrm{Pt}$ (in angstroms). The layer structure for type 2 MTJs is $80\mathrm{Ru}\text{\penalty1000-\hskip0pt}40\mathrm{Co}\mathrm{Fe}\mathrm{B}\text{\penalty1000-\hskip0pt}50\mathrm{Ru}\mathrm{Ta}\text{\penalty1000-\hskip0pt}40\mathrm{Co}\mathrm{Fe}\mathrm{B}\text{\penalty1000-\hskip0pt}15{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\text{\penalty1000-\hskip0pt}50\mathrm{Co}\mathrm{Fe}\mathrm{B}\text{\penalty1000-\hskip0pt}9\mathrm{Ru}\text{\penalty1000-\hskip0pt}54\mathrm{Fe}\mathrm{Co}\text{\penalty1000-\hskip0pt}350\mathrm{Cr}\mathrm{Mn}\mathrm{Pt}$. The tunneling magnetoresistance (TMR) ratio $[({R}_{\mathit{AP}}\text{\penalty1000-\hskip0pt}{R}_{P})∕{R}_{P}]$ is about 26% and 69% at room temperature for type 1 and type 2 MTJs, respectively. A TMR as high as 107% has been observed for type 2 MTJ samples at $13\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. By analysis of the voltage and temperature dependence of the resistance and magnetoresistance in these MTJs, we discuss the effects of the magnetic behavior of the free layers, barrier qualities, and barrier interfaces. The results clearly indicate that the micromagnetization orientation at the interface between the free layer and the barrier layer is one of the important factors that determines the TMR ratio.