We have studied the effect of annealing on the interface magnetization in a CoFeB/MgO structure which models the lower electrode in a magnetic tunnel junction device. We find that MgO deposition causes Fe to diffuse toward the CoFeB/MgO interface, where it preferentially bonds with oxygen to form a Fe-O-rich interfacial region with reduced magnetization. After annealing at 375 °C the compositional inhomogeneity remains; Fe is reduced back to a ferromagnetic metallic state and the full interfacial magnetization is regained.
Single electron electronics is now well developed, and allows the manipulation of electrons one-by-one as they tunnel on and off a nanoscale conducting island. In the past decade or so, there have been concerted efforts in several laboratories to construct single electron devices incorporating ferromagnetic components in order to introduce spin functionality. The use of ferromagnetic electrodes with a non-magnetic island can lead to spin accumulation on the island. On the other hand, making the dot also ferromagnetic introduces new physics such as tunnelling magnetoresistance enhancement in the cotunnelling regime and manifestations of the Kondo effect. Such nanoscale islands are also found to have long spin lifetimes. Conventional spintronics makes use of the average spin-polarization of a large ensemble of electrons: this new approach offers the prospect of accessing the quantum properties of the electron, and is a candidate approach to the construction of solid-state spin-based qubits.
We have inserted nonmagnetic impurity layers of Au into sputtered AlOx-based magnetic tunnel junctions (F/I/F) and Meservey–Tedrow junctions (S/I/F) in order to study their effect on the tunneling magnetoresistance (TMR) and spin polarization (TSP). Both room temperature TMR and the TSP at 250mK decay exponentially as an interfacial Au layer is introduced between the barrier and one Co electrode, with 1∕e decay lengths λTMR=11±3Å and λTSP=14±2Å. We also inserted a 1Å thick Au layer at a variable distance from the barrier/Co interface and find that both the TMR and TSP recover to the undoped value with the shorter exponential length scales of λTMR=7±4Å and λTSP=6±2Å.
Double magnetic tunnel junctions (DMTJs) have been fabricated using alumina barriers with NiFe particles (∼1.8 nm) embedded within. The junctions exhibit spin-dependent transport properties and Coulomb blockade effects. We study differences between control samples and the DMTJs; specifically I-V characteristics and tunnel magnetoresistance (TMR) versus bias voltage characteristics. Clear differences in the systems are evident: the DMTJ with NiFe particles shows a marked peak in TMR at low bias, whereas the dependence of TMR on bias is much weaker for the control MTJ without embedded particles. Hence the TMR at low bias is enhanced by the Coulomb blockade effects.
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