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The possibility of higher electrical efficiency in computing by operating at low temperatures raises the need for non-volatile memory cells optimized for cryogenic operation. We report a study on low temperature spin transfer torque switching of magnetic tunnel junctions with 20 to 100 nm in diameter with thermal stability adapted to low temperature operation. The evolution of magnetic and electrical properties are characterized for four different stacks from 300 to 10 K comprising insertions of Mg, Ru and permalloy (Py) in the storage layer to reduce its effective anisotropy. Two figures of merit are used to compare different devices and stacks, Δ/Ic and Δ/Esw, normalizing the thermal stability Δ by the critical current or switching energy. Devices with a Py insertion layer show a higher FOM (3.78 kBTop/μA) and switching energy Esw below 655 fJ for 100 ns pulses at Top = 10 K. A procedure to optimize the reference layer stray field was also implemented to achieve full compensation using a synthetic antiferromagnetic layer for 20 nm diameter devices. [ABSTRACT FROM AUTHOR]

Dependence of time-dependent dielectric breakdown with pulsewidth was investigated on MgO magnetic tunnel junctions. Barrier breakdown measurements were performed using different pulsewidths, from 10 ns up 1 s. The equivalence of the breakdown results between DC and pulsed voltage experiments was established using the total cumulated pulse time under electrical stress as the relevant parameter. Two different breakdown regimes were identified: the first one corresponding to pulses longer than 100 ns up to the DC limit and a second regime for pulses shorter than 100 ns. The barrier lifetime is increased for pulsewidths lower than 100 ns pulses. The breakdown process was shown to be thermally activated, but the longer lifetime at shorter pulses cannot be simply explained assuming a lower temperature during the pulse application. Index Terms--Breakdown voltage, magnetic random access memory (MRAM), magnetic tunnel junction (MTJ), pulsewidth.

The influence in the I-V characteristics and tunnel magnetoresistance (TMR) of impurities embedded into the insulating barrier separating the two ferromagnetic electrodes of a magnetic tunnel junction was theoretically investigated. When the energy of the electron bound state at the impurity site is close to the Fermi energy, the current and TMR are strongly enhanced in the vicinity of the impurity.

Spin transfer effect in spin-valve pillars developed for current-perpendicular-to-plane (CPP) magnetoresistive heads is examined. The spin transfer acts as an effective field at low current resulting in current dependent loop shift of the free layer and inverted coercivity effects.

The influence of the location of nonmagnetic impurities (Au) inserted in the NiO layer of NiO/Co bilayers on the exchange bias is investigated. This interpretation is supported by numerical simulations, which also predict the formation and annihilation of bubble-like domain walls.

This work reports experimental confirmation of the working principles of a double magnetic tunnel junction (DMTJ) to achieve highly efficient spin-transfer-torque (STT) switching. This concept uses a magnetically switchable assistance layer (ASL) acting as a top perpendicular spin polarizer. The STT-induced switching dynamics were described by macrospin simulations, while the magnetic and electrical properties of the devices were investigated in switching experiments. The reversal dynamics of the storage layer/ASL coupled system were validated by time-resolved measurements of the device resistance during write operation, confirming that the storage layer is subjected to additive STT contributions both from the reference layer and the ASL throughout its parallel-to-antiparallel and antiparallel-to-parallel transitions. The STT efficiency of the ASL-DMTJ was compared to that of single MTJ stacks comprising the same storage layer and no assistance layer. The figure of merit ∆/Ic (stability/critical current) was found to nearly double in devices of 80 and 100 nm diameter, with a smaller 30% increase obtained for 50 nm diameter cells. [ABSTRACT FROM AUTHOR]

In this work, we used the soft X-ray resonant magnetic reflectivity to study the depth-resolved out-of-plane (oop) magnetization profile of a CoFeB/MgO sample with W/Ta cap layer after annealing at 400°C. It is a powerful technique to probe buried magnetic interfaces of ultra-thin films by combining the depth-resolved information of X-ray reflectivity with the species selectivity of X-ray magnetic circular dichroism. It allowed us to resolve the oop magnetization within a 1.36 nm thick CoFeB layer by the measurement of angle-dependent specular reflectivity at large scattering angles (up to 80°). We determined a graded magnetic distribution for both Fe and Co with a 20% increase at the interface with MgO, decreasing slightly over a thickness of 0.7 nm from MgO before it rapidly decreases to 50% at the interface with W. After applying a non-saturating magnetic field in the plane of the sample, we also quantified a similar magnetization profile with an inclined moment configuration. This indicates that the magnetization gradient is a robust property of the CoFeB layer in the studied sample. [ABSTRACT FROM AUTHOR]

Spintronic rf detectors are efficient nanoscale counterparts to conventional semiconductor-based components for energy harvesting and wireless communication at low input power. Here, we report on the optimization of the rectified output dc voltage using magnetic tunnel junctions (MTJs) with strong perpendicular anisotropy of both the polarizing and free layers. The magnetization of the polarizing layer is fixed out-of-plane, while the free layer thickness is adjusted so that its magnetization orientation changes from in-plane to out-of-plane. Rectification dc output voltages in the mV range are obtained for moderate rf source powers with a signal-to-noise ratio of 26–39 dB for Prf = −25 dBm and a sensitivity ε of 300 mV/mW. The rectified signal shows a strong dependence on MTJ dimensions: it increases by a factor of 5–6 when reducing the diameter from 180 to 50 nm. Furthermore, this enhancement can be doubled when reducing the free layer thickness from 1.8 to 1.6 nm. This size-related enhancement is attributed to several jointly acting effects: the amplitude of the spin transfer torque that depends inversely on the diameter, the effective anisotropy that depends on the thickness of the excited layer, and the tunneling magneto-resistance ratio that for the devices studied here depends on diameter. The obtained results indicate that the geometry of the MTJ can be used to design spintronic based rf detectors with optimized sensitivity. [ABSTRACT FROM AUTHOR]

Sputtered [(Pt-Co).sub.n]-FeMn multilayers exhibit both perpendicular magnetic anisotropy and exchange bias at room temperature. Their magnetic properties after in-plane and perpendicular-to-plane magnetic annealing have been studied by means of extraordinary Hall effect and magnetic force microscopy or polar Kerr microscopy. A crossover from in-plane to perpendicular anisotropy was observed as a function of number of repeats n. The dynamics of magnetization switching has been investigated on both sides of the hysteresis loops. A clear asymmetry exists in the dynamics depending whether the field is parallel or antiparallel to the exchange bias field. Similarly, strong differences in the domain patterns at the coercive field were observed. Exchange-biased (Pt-Co)-Pt-(Pt-Co)-FeMn spin-valves were also investigated. Well-separated hysteresis loops were associated with the 'soft' and pinned components of the stack. These structures were used to study the interlayer coupling through the Pt spacer. Index Terms--Exchange bias, interlayer coupling, magnetic switching dynamics, perpendicular magnetic anisotropy.

Specular reflection effects on anisotropic magnetoresistance of magnetic thin films were studied.

An analytical calculation of the shape of the magnetoresistive response of spin-valve structures with synthetic antiferromagnetic pinned layers is presented. It was found that specular reflection at the CoFe/Ru interface must be introduced to account for the experimental dat.

Microstructural effects on the antiferromagnetic layer were investigated on IrMn/Co exchange biased square dots. IrMn grain size and distributions were tuned by changing Cu buffer layer or IrMn thicknesses. Lateral dimensions from 200 to 50nm were varied. Exchange bias (Hex) variability was analysed through focused Kerr measurements on small groups of dots. Patterned samples presented average exchange bias values following the trends and values of full sheet samples. Concerning the dot to dot behaviour, it resulted that IrMn microstructure variations have minor effects on Hex variability, because no particular trend is observed as a function of grain size and distribution. The variability is attributed to geometry variation and Co intrinsic variability.

Magnetic devices are often subject to thermal processing steps, such as field cooling to set exchange bias and annealing to crystallize amorphous magnetic electrodes. These processing steps may result in interdiffusion and the subsequent deterioration of magnetic properties. In this study, we investigated thermally-activated diffusion in Cu/Co/IrMn/Pt exchange biased polycrystalline thin-film structures using atom probe tomography. Images taken after annealing at 400°C for 60 min revealed Mn diffusion into Co grains at the Co/IrMn interface and along Pt grain boundaries for the IrMn/Pt stack, i.e., a Harrison type C regime. Annealing at 500°C showed further Mn diffusion into Co grains. At the IrMn/Pt interface, annealing at 500°C led to a type B behavior since Mn diffusion was detected both along Pt grain boundaries and also into Pt grains. The deterioration of the films' exchange bias properties upon annealing was correlated to the observed diffusion. In particular, the topmost Pt capping layer thickness turned out to be crucial since a faster deterioration of the exchange bias properties for thicker caps was observed. This is consistent with the idea that Pt acts as a getter for Mn, drawing Mn out of the IrMn layer. [ABSTRACT FROM AUTHOR]

Magnetic measurements have been performed on thin films with structures Ta/Pt/Co/[AlO.sub.x] to investigate the dependence of the perpendicular magnetic anisotropy induced at the Co/[AlO.sub.x] interface on both Co thickness and annealing temperature. The choice of the capping layers to protect the oxide layer is shown to be important. The interfacial anisotropy increases strongly with increasing annealing temperature, from 0.7 to 1.4 erg/[cm.sup.2], without a major change of the volume anisotropy (11 [10.sup.6] erg/[cm.sup.3]). As a consequence, the critical Co thickness (at which the magnetic moment switches from perpendicular to inplane orientation), increases a lot with annealing and reaches values larger than 3 nm after annealing at 375[degrees]C. Index Terms--Annealing effects, Co/AlO interface, extraordinary hall effect (EHE), perpendicular magnetic anisotropy.

Exchange bias without antiferromagnetic layer was observed in systems consisting of two exchange coupled ferromagnetic materials: [(Pt/Co).sub.n] multilayer (with out-of-plane anisotropy) and permalloy, NiFe, (with in-plane anisotropy). We investigated the influence of various parameters on this phenomenon: Pt buffer layer thickness, number of repeats in the multilayer, thickness of the NiFe layer. Annealing yields a doubling in the magnitude of the loop shift as a result of an improved texture and mosaicity of the films. Index Terms--Coercive field, exchange bias, perpendicular anisotropy.