The authors investigated the tunneling magnetoresistance (TMR) of CoFeB∕MgO∕CoFeB tunnel junctions by varying the thickness (tCoFeB) of the top CoFeB layer. Linear and hysteresis-free switching was observed in junctions with tCoFeB⩽10Å, while normal tunneling behavior occurred for tCoFeB>10Å. The field sensitivity and the sensing field range were found to be controlled by varying the thickness of the sensing layer. This finding means that the magnetic tunneling junction (MTJ) provides a scheme for magnetic field sensing, which has a simple sensor design and low power consumption. The magnetic properties of the sensing layer with tCoFeB⩽10Å were found to show the characteristics of superparamagnetism. Although the detailed mechanism of TMR in MTJs with a superparamagnetic layer is not fully understood at present, this phenomenon is observed repeatedly. Therefore, this sensing scheme would be an alternative method for overcoming the problems in magnetic sensors with a crossed magnetization pattern.
We have fabricated high-density arrays of individually isolated semispherical nanocapacitors consisting of porous anodic aluminum oxide (AAO) layers as dielectric materials and carbon nanotubes (CNTs) as electrodes. It is shown that the nanocapacitors made with the CNT electrodes exhibit much better C-V behaviors than those without the CNT electrodes. The improved electrical behavior is explained in terms of the use of the CNT electrodes deposited within the porous AAO layers. The capacitance calculated using semispherical capacitor formula is in agreement with the experimental value.
Variation in the quality of the interface in a CoFeB∕MgO∕CoFeB tunnel junction during thermal annealing was investigated using x-ray photoemission spectroscopy. The formation of B oxide and the reduction of Fe oxide at the bottom interface after thermal annealing near Ta=300°C were found to enhance the tunneling magnetoresistance ratio significantly. At the same time, an asymmetry of the conductance (dV∕dI) in the bias polarity and a local minimum of conductance in a positive bias state were measured which were attributed to the presence of a minority state at the bottom interface. The authors believe that the existence of the Bloch state was also responsible for the failure of the application of the Brinkman-Dynes-Rowell or Simmons models to the CoFeB∕MgO∕CoFeB junction.
Magnetostatic interactions between 360° and transverse 180° domain walls in the NiFe and Co layers of Co/Cu/NiFe multilayer stripes are investigated by micromagnetic simulations. In 200 nm wide Co (5 nm)/Cu (5 nm) /NiFe (5 nm) stripes, stray fields from 360° domain walls in the Co layer strongly influence the magnetic behavior of the NiFe layer, promoting reverse domain nucleation and providing a pinning potential of order 100 Oe which impedes domain wall propagation. 360° domain walls may be useful as programmable pinning sites in magnetoelectronic logic or memory devices.
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