The advent of spin transfer torque effect accommodates site-specific switching of magnetic nanostructures by current alone without magnetic field. However, the critical current density required for usual spin torque switching remains stubbornly high around 10(6)-10(7) A cm(-2). It would be fundamentally transformative if an electric field through a voltage could assist or accomplish the switching of ferromagnets. Here we report electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities. These results represent a crucial step towards ultralow energy switching in magnetic tunnel junctions, and open a new avenue for exploring other voltage-controlled spintronic devices.
The tunneling magnetoresistance and perpendicular magnetic anisotropy in CoFeB(1.1-1.2 nm)/ MgO/CoFeB(1.2-1.7 nm) junctions were found to be very sensitively dependent on annealing time. During annealing at a given temperature, decay of magnetoresistance occurs much earlier compared to junctions with in-plane magnetic anisotropy. Through a rapid thermal annealing study, the decrease of magnetoresistance is found to be associated with the degradation of perpendicular anisotropy, instead of impurity diffusion as observed in common in-plane junctions. The origin of the evolution of perpendicular anisotropy as well as possible means to further enhance tunneling magnetoresistance is discussed. V
In order to incorporate the extraordinary intrinsic thermal, electrical, mechanical, and optical properties of graphene with three dimensional (3D) flexible substrates, we introduce a solvent-driven self-folding approach using graphene-polymer bilayers. A polymer (SU-8) film was spin coated atop chemically vapor deposited graphene films on wafer substrates and graphene-polymer bilayers were patterned with or without metal electrodes using photolithography, thin film deposition, and etching. After patterning, the bilayers were released from the substrates and they self-folded to form fully integrated, curved, and folded structures. In contrast to planar graphene sensors on rigid substrates, we assembled curved and folded sensors that are flexible and they feature smaller form factors due to their 3D geometry and large surface areas due to their multiple rolled architectures. We believe that this approach could be used to assemble a range of high performance 3D electronic and optical devices of relevance to sensing, diagnostics, wearables, and energy harvesting.
Perpendicular CoFeB/MgO/CoFeB magnetic tunnel junctions with diameters under 100 nm are investigated by conductive atomic force microscopy. Minor loops of the tunnel magnetoresistance as a function of applied magnetic field reveal the hysteresis of the soft layer and an offset due to the magnetostatic field of the hard layer. Within the hysteretic region, telegraph noise is observed in the tunnel current. Simulations show that in this range, the net magnetic field in the soft layer is spatially inhomogeneous, and that antiparallel to parallel switching tends to start near the edge, while parallel to antiparallel reversal favors nucleation in the interior of the soft layer. As the diameter of the tunnel junction is decreased, the average magnitude of the magnetostatic field increases, but the spatial inhomogeneity across the soft layer is reduced.
Magnetic tunnel junctions with perpendicular magnetic anisotropy are investigated using a conductive atomic force microscope. The 1.23 nm Co 40 Fe 40 B 20 recording layer coercivity exhibits a size dependence which suggests single domain behavior for diameters ≤ 100 nm. Focusing on devices with diameters smaller than 100 nm, we determine the effect of voltage and size on the effective device anisotropy K eff using two different techniques. K eff is extracted both from distributions of the switching fields of the recording and reference layers, and from measurement of thermal fluctuations of the recording layer magnetization when a field close to the switching field is applied. The results from both sets of measurements reveal that K eff increases monotonically with decreasing junction diameter, consistent with the size dependence of the demagnetization energy density. We demonstrate that K eff can be controlled with a voltage down to the smallest size measured, 64 nm. PACS numbers: 85.75.-d,73.40.Gk,75.78.-n,75.70.-i 1 I. INTRODUCTION Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) are an attractive building block for non-volatile memories. PMA MTJs (p-MTJs) show promisein terms of the key requirements for implementation into products competitive with current data storage and memory technologies: large tunnel magnetoresistance (TMR), low writing energy cost, non-volatility over ∼ 10 years, and scalability of these properties toward ∼ 1 Tbit/inch 2 densities. Room temperature TMR ratios greater than 100% have long existed in in-plane MTJs 1,2 . In state of the art in-plane MTJs, TMR in excess of 600% is achieved by controlling the diffusion of Ta in the film stack through the addition of boron to the magnetic electrodes 3 . Despite these achievements, in-plane MTJs suffer from scalability issues due to their dependence on shape anisotropy for thermal stability and the high energy cost of switching the magnetization by the spin transfer torque (STT) effect 4,5 . For in-plane MTJs, switching energies E sw = I 2 c Rt, where I c is the critical switching current, R is the resistance, and t is the length of the pulse, of approximately 10 µJ/bit were achieved for current pulses on the order of 10 ms 6 . This value was drastically reduced using nanosecond pulses, yielding E sw on the order of single pJ/bit in purely in-plane MTJs 7 . In high TMR p-MTJs the large out-of-plane demagnetization energy does not contribute to E sw 8 . Recently, TMR ratios up to 162% were obtained in p-MTJs by further controlling interlayer diffusion through the substitution of Ta with Mo in the film stack 9 . PMA is achieved when the CoFeB thickness is less than about 1.5 nm, so that the effective anisotropy K eff is dominated by the interfacial anisotropy between Fe in the CoFeB and oxygen in the MgO 10 . In such p-MTJs, switching energies of hundreds of fJ/bit were achieved in 60 nm × 170 nm ellipses 11 . One of the most promising aspects of p-MTJs is that the interface anisotropy can be controlled by applying a...
We demonstrate a voltage-controlled exchange bias effect in CoFeB/MgO/CoFeB magnetic tunnel junctions that is related to the interfacial Fe(Co)Ox formed between the CoFeB electrodes and the MgO barrier. The unique combination of interfacial antiferromagnetism, giant tunneling magnetoresistance, and sharp switching of the perpendicularly-magnetized CoFeB allows sensitive detection of the exchange bias. It is found that the exchange bias field can be isothermally controlled by magnetic fields at low temperatures. More importantly, the exchange bias can also be effectively manipulated by the electric field applied to the MgO barrier due to the voltage-controlled antiferromagnetic anisotropy in this system.
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