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Biocompatible suspended magneto-elastic membranes were prepared. They consist of PDMS (polydimethylsiloxane) films, with embedded arrays of micrometric magnetic pillars made with lithography techniques. For visible light wavelengths, our membranes constitute magnetically tunable optical diffraction gratings, in transmission and reflection. The optical response has been quantitatively correlated with membrane structure and deformation, through optical and magneto-mechanical models. In contrast to the case of planar membranes, the diffraction patterns measured in reflection and transmission vary very differently upon magnetic field application. Indeed, the reflected beam is largely affected by the membrane bending, whereas the transmitted beam remains almost unchanged. In reflection, even weak membrane deformation can produce significant changes of the diffraction patterns. This field-controlled optical response may be used in adaptive optical applications, photonic devices, and for biological applications.

Spin torque driven excitations in spin valves and tunnel junctions are often investigated for a two magnetic layer system for which a polarizer (fixed magnetization) and a free layer can be distinguished. In the search for improved microwave properties and to understand the role of different coupling mechanisms between the magnetic layers, here, the excitation spectrum of an exchange coupled two layer synthetic ferrimagnet (SyF) system is investigated numerically with spin momentum transfer acting on both layers simultaneously. This self-polarised two layer system does not contain an external polarizer, and excitation of coupled modes arises due to the mutual spin transfer torque and the Ruderman-Kittel-Kasuya-Yosida interlayer exchange coupling. The current-field state diagrams of static and dynamic states are reported as a function of the interlayer exchange coupling strength. The numerically determined critical boundaries are well reproduced by an analytical stability analysis. The dynamic steady states reveal an optic-like mode at low magnetic fields, which becomes progressively acoustic-like for increased magnetic fields and currents. The frequency of these modes can be tuned by the film thickness and the strength of the interlayer exchange interaction. The results presented here will provide an important guide for designing spin torque oscillators that exploit the dynamic coupling between layers and, furthermore, they will provide a basis to test analytical models of spin torque driven coupled excitations. [ABSTRACT FROM AUTHOR]

The time and field dependence of the magnetic domain structure at magnetization reversal were investigated by Kerr microscopy in a structure consisting of a hard and a soft ferromagnetic Co/Pt multilayer stack with perpendicular anisotropy, separated by a thicker nonmagnetic Pt spacer layer. Large local inhomogeneous magnetostatic stray fields appear as soon as a nonuniform magnetic area exists within one of the stacks and induce a correlated domain structure within the other. The long range nature of this magnetostatic interaction gives rise to ultraslow dynamics even in zero applied field, i.e., it affects the long time domain stability. Due to this additional interaction field, the magnetization reversal under short magnetic field pulses differs markedly from the well-known slow dynamic behavior. Namely, in high field, the magnetization of the coupled harder layer has been observed to reverse more rapidly by domain wall motion than the softer layer alone. [ABSTRACT FROM AUTHOR]

In the framework of the Valet and Fert theory of current perpendicular to plane (CPP) giant magnetoresistance (GMR) in metallic multilayers [Valet and Fert, Phys. Rev. B 48, 7099 (1993)], the calculation of the CPP resistance and magnetoresistance has been generalized to any multilayered stacks including spin valves with synthetic free layers, laminated free and pinned layers, and dual spin valves. The theory takes into account bulk and interfacial spin-dependent scattering as well as spin flip in all layers. It also considers the effect of scattering at lateral edges of submicron multilayered pillars which can be viewed as a current in plane effect intruding on CPP transport. This latter effect plays a role when the diameter of the pillar becomes of the order of the elastic mean-free paths (i.e., below ∼30 nm). Based on the theory, a code has been developed to compute the CPP resistance and CPP magnetoresistance from the transport parameters of each material involved in the stack (spin-dependent resistivities, spin-dependent interfacial resistances, and spin-diffusion length in each layers). As examples, we compare the results of the calculations with various already published CPP experiments. In particular, we interpret experiments previously carried out on (NiFe/Cu/Co/Cu)[sub N] and (Co 6 nm/Cu/Co 1 nm/Cu)[sub N] multilayers in which the order of the layers in the stack had been shown to affect the CPP resistance and magnetoresistance, a property which could not be explained in a simple two-channel serial resistance model. We also investigate the influence of the thickness of the various layers and underline the key role of the spin diffusion length in these thickness variations. Unexpected predictions are made with this theory such as the existence of a maximum in CPP–MR as a function of the thickness of the antiferromagnetic pinned layer. This type of calculations should allow a faster optimization of CPP–GMR in metallic multilayers. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Theoretical studies of spin dependent transport in Fe/Cr/MgO/Fe tunnel junctions with noncollinear alignment of magnetizations of metallic layers comprising these magnetic tunnel junctions are presented. Calculations are performed with use of nonequilibrium Green function technique in the framework of the Keldysh formalism. Electronic band structure of ferromagnetic electrodes is modeled within a two-band model with majority and minority states being s-like and d-like electrons, respectively. Furthermore, interfacial s-d hybridization is taken into account and calculated using perturbation corrections for the wave and Green functions. It is shown that in the presence of Cr layer at the Fe/MgO interface, the contribution from s-d hybridization to the total current is much stronger in the antiparallel magnetizations configuration compared to the parallel one leading to decrease in tunnel magnetoresistance values in agreement with earlier reports. [ABSTRACT FROM AUTHOR]

Metallic Mg insertions in rf deposited MgO barrier of magnetic tunnel junctions structures were investigated in a resistance-area (RA) range from 1 to 1000 Ω μm2. For the first time, investigations on Mg insertions above the MgO barrier and simultaneously on both sides of the barrier are reported. It is shown that for RA larger than 5 Ω μm2, a bottom Mg insertion does not improve the tunnel magnetoresistance (TMR) ratio compared to a sample with no Mg insertion. Furthermore, a top Mg insertion yields a lower TMR ratio decreasing as the Mg thickness is increased. On the other side, for RA lower than 5 Ω μm2, there is no significant difference between top and bottom Mg insertions indicating that in this region, the MgO crystallization occurs mainly during the annealing process. In the RA range investigated, there is no significant difference in coupling field for different insertions. In very low RA regions between 1 and 10 Ω μm2, an increase in TMR is observed for 0.3 nm insertions simultaneously below an above the barrier. [ABSTRACT FROM AUTHOR]

In this work the thermal heating of tunnel junctions is investigated doing one-dimensional numerical simulations by solving the heat equation. The temperature increase is proportional to the power density jV, which is confirmed by transport measurements on patterned μm size junctions. The observed temperature increase was 2.3 K/(mW μm-2) compared to a simulation result of 0.9 K/(mW μm-2). Tunneling AFM measurements on 4, 5.4, and 6.7 Å barrier half-junctions, show hot spots with current densities up to 13 times the average density value. The current concentration could explain the higher heating observed experimentally compared to the simulated value. The hot spot density is 600–900/μm2 occupying roughly 5% of the barrier surface.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the observation of exchange bias in (Pt/Co[sub 0.90]Fe[sub 0.10])[sub n]/FeMn multilayers, with perpendicular magnetic anisotropy. We analyze the behavior of the exchange-bias field H[sub eb] as a function of temperature, FeMn thickness, number of (Pt/Co[sub 0.90]Fe[sub 0.10]) bilayers, and annealing conditions. Measurements carried out with field applied perpendicular to the plane of the samples indicate that the hysteresis loops present 100% of remanent magnetization, with H[sub eb] reaching 200 Oe and a tunable coercive field, depending on Co and Pt thickness ratio and on Pt buffer thickness. Furthermore, spin-valves of the form (Pt/Co[sub 0.90]Fe[sub 0.10])[sub n]/NM/(Co[sub 0.90]Fe[sub 0.10]/Pt)[sub m]/Co[sub 0.90]Fe[sub 0.10]/FeMn with NM= Cu or Pt have been prepared. They exhibit two well separated hysteresis loops when the field is applied perpendicular to the plane. [ABSTRACT FROM AUTHOR]

Presents a study which examined some properties of 2π Néel walls and Néel points in the XY model. Numerical method; Line and point defects in zero magnetic field; Effect of an applied magnetic field.

This paper investigates the effect of a controlled cooling rate on magnetic field reversal assisted by spin transfer torque (STT) in thermally assisted magnetic random access memory. By using a gradual linear decrease of the voltage at the end of the write pulse, the STT decays more slowly or at least at the same rate as the temperature. This condition is necessary to make sure that the storage layer magnetization remains in the desired written direction during cooling of the cell. The influence of the write current pulse decay rate was investigated on two exchange biased synthetic ferrimagnet (SyF) electrodes. For a NiFe based electrode, a significant improvement in writing reproducibility was observed using a gradual linear voltage transition. The write error rate decreases by a factor of 10 when increasing the write pulse fall-time from ~3 ns to 70 ns. For comparison, a second CoFe/NiFe based electrode was also reversed by magnetic field assisted by STT. In this case, no difference between sharp and linear write pulse fall shape was observed. We attribute this observation to the higher thermal stability of the CoFe/NiFe electrode during cooling. In real-time measurements of the magnetization reversal, it was found that Ruderman-Kittel-Kasuya- Yosida (RKKY) coupling in the SyF electrode vanishes for the highest pulse voltages that were used due to the high temperature reached during write. As a result, during the cooling phase, the final state is reached through a spin-flop transition of the SyF storage layer. [ABSTRACT FROM AUTHOR]

This paper is focused on thermally assisted magnetic random access memories (TA-MRAMs). It explains how the heating produced by Joule dissipation around the tunnel barrier of magnetic tunnel junctions (MTJs) can be used advantageously to assist writing in MRAMs. The main idea is to apply a heating pulse to the junction simultaneously with a magnetic field (field-induced thermally assisted (TA) switching). Since the heating current also provides a spin-transfer torque (current-induced TA switching), the magnetic field lines can be removed to increase the storage density of TA-MRAMs. Ultimately, thermally induced anisotropy reorientation (TIAR)-assisted spin-transfer torque switching can be used in MTJs with perpendicular magnetic anisotropy to obtain ultimate downsize scalability with reduced power consumption. TA writing allows extending the downsize scalability of MRAMs as it does in hard disk drive technology, but it also allows introducing new functionalities particularly useful for security applications (Match-in-Place™ technology). [ABSTRACT FROM AUTHOR]

The article presents an overview of a tutorial course which focuses on the important phenomena of spinelectronics and some models in spintronics including the giant magnetoresistance (GMR). Accordingly, several experiments have been made to stimulate spin polarized electrons from a magnetic metal into semiconductors but were unsuccessful due to the density of the semiconductor and the magnetic metal. It references the study of G. Schmidt and colleagues which pointed out the impossibility of injecting spin polarized electrons from a magnetic field directly into a semiconductor.

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. [ABSTRACT FROM AUTHOR]

Coupled free layers systems used as active elements in spin-transfer nano-oscillators (STNOs) can have improved microwave performances in terms of linewidth or tunability. Here, we report a numerical study on the magnetization dynamics of a synthetic antiferromagnet (SAF) driven by a perpendicularly spin-polarized current and an in-plane applied magnetic field. Compared to the single free layer STNO, the current-field state diagrams, derived for strong and weak RKKY exchange coupling strength inside the SAF, show a more complex structure, with new static or dynamic states (chaotic dynamics), and also a larger zone of out-of-plane precession state (OPP) oscillations. The OPP frequency behaviour is generally similar to that of the single free layer STNO except for the zone near the chaotic dynamics and for fields larger than the spin-flop field of the SAF, where several frequency jumps have been observed. [ABSTRACT FROM AUTHOR]

Spin polarized current induced self oscillations have been investigated in both the free layer and the synthetic antiferromagnetic (SAF) pinned layer of spin valve nanopillars. Compared with free layer excitations, the acoustic type SAF excitations are characterized by high emitted power and much narrower linewidth. Furthermore, in contrast to free layer excitations, the SAF in-plane precession mode exhibits an unexpected crossover from redshift (df/dI<0) to blueshift (df/dI>0) in frequency f versus current I as the in-plane magnetic field is increased. From simulations we identify this crossover as a signature of large amplitude spin current induced precessional dynamics in the SAF. [ABSTRACT FROM AUTHOR]

A detailed study of the angular dependence of the magnetization reversal in polycrystalline ferromagnetic (FM)/antiferromagnetic Co/IrMn bilayers with noncollinear FM and unidirectional anisotropies shows a peculiar asymmetric magnetic behavior. The anisotropy configuration is set via a field cooling (FC) procedure with the magnetic field misaligned with respect to the easy magnetization direction of the FM layer. Different magnetization reversal modes are observed for either positive or negative angles with respect to the FC direction. The angular dependence of both coercivity and exchange bias also clearly displays the broken symmetry of the induced noncollinearity. Our findings are reproduced with a modified Stoner–Wohlfarth model including the induced anisotropy configuration. Our results highlight the importance of the relative angle between anisotropies in exchange bias systems, opening a new path for the tailoring of their magnetic properties. [ABSTRACT FROM AUTHOR]

The article discusses the course subject on magnetism which focuses on the discovery and development of spintronic devices for memory and logical applications which include the giant magnetoresistance, low magnetic film sensors, and Magnetoresistive Random Access Memory (MRAM). It notes that the discovery of the ways to manipulate the magnetization of nanostructure through a spin-polarized current offers a major breakthrough in the development of spinelectronics. It adds that electrical currents may have a direct influence on the propagation of domain walls and spin-transfer effects.

Biofilm colonisation of surfaces is of critical importance in various areas ranging from indwelling medical devices to industrial setups. Of particular importance is the reduced susceptibility of bacteria embedded in a biofilm to existing antimicrobial agents. In this paper, we demonstrate that remotely actuated magnetic cantilevers grafted on a substrate act efficiently in preventing bacterial biofilm formation. When exposed to an alternating magnetic field, the flexible magnetic cantilevers vertically deflect from their initial position periodically, with an extremely low frequency (0.16 Hz). The cantilevers' beating prevents the initial stage of bacterial adhesion to the substrate surface and the subsequent biofilm growth. Our experimental data on E. coli liquid cultures demonstrate up to a 70% reduction in biofilm formation. A theoretical model has been developed to predict the amplitude of the cantilevers vertical deflection. Our results demonstrate proof-of-concept for a device that can magneto-mechanically prevent the first stage in bacterial biofilm formation, acting as on-demand fouling release active surfaces. [ABSTRACT FROM AUTHOR]

The magnetization reversal in Permalloy (Py) and Py-IrMn disks (1 μm diameter) is investigated. The Py disks reverse their magnetization via vortex state formation. Conversely, if the Py-IrMn disks are field cooled from above the blocking temperature TB, a critical angle with respect to the cooling direction is set, beyond which the vortex no longer nucleates. This angle can be experimentally tuned by varying the magnitude of the exchange bias field. Furthermore, the coupling with IrMn can also induce an enhancement of the vortex stability when the disks are zero-field cooled from above TB. [ABSTRACT FROM AUTHOR]

The dynamic coercivity of patterned media with perpendicular magnetic anisotropy is measured over ten orders of magnitude using magnetic field pulses from magnetic recording heads with pulse widths in the range 150 ps to 1 s. The local field generated by write heads allows the study of the switching dynamics of individual bits. Existing models have been applied for a quantitative analysis of the data. As a result, we found the written bits are stable for at least 10 years. Individual dot magnetization switching has been achieved with subnanosecond pulses showing that very-high-data-rate recording is possible in patterned media. [ABSTRACT FROM AUTHOR]

A thermally assisted writing procedure is proposed in a tunnel junction based magnetic random access memory cell. The magnetic layers of the tunnel junction are both exchange-biased with antiferromagnetic layers, the reference layer having a much higher blocking temperature than the storage layer. In the operating mode, a current pulse sent through the junction generates enough heat to raise the temperature of the storage layer above its blocking temperature, without affecting the pinning of the reference layer. The concept is demonstrated here for an isolated junction using an homogeneous external magnetic field. [ABSTRACT FROM AUTHOR]

Magnetic nanoparticles are receiving an increasing interest for various biotechnological applications due to the capability that they offer to exert actuation on biological species via external magnetic fields. In this study, two types of magnetic particles recently proposed for cancer cells treatment were compared. Both are prepared by top-down approaches and imitate the properties of superparamagnetic particles. One type is made of a single magnetic layer and has a magnetic vortex configuration. The second type has a multilayered structure called synthetic antiferromagnet. Once released in solution, the agglomeration/dispersion of these particles due to their magnetostatic interactions was compared as well as the mechanical torque that they can generate when submitted to an external magnetic field. [ABSTRACT FROM AUTHOR]

Abstract: The minimum applied field HSW required to reverse the magnetic moment of the ferromagnetic/antiferromagnetic storage layer of a thermally assisted magnetic random access memory (TA-MRAM) device during the application of a heating electric pulse is investigated as a function of pulse power PHP and duration δ. For the same power of the heating pulse PHP (or, equivalently, for the same temperature of the storage layer), HSW increases with decreasing heating time δ. This behavior is consistently interpreted by a thermally activated propagating domain-wall switching model, corroborated by a real-time study of switching. The increase of HSW with decreasing pulse width introduces a constraint for the minimum power consumption of a TA-MRAM where writing combines heating and magnetic field application. [Copyright &y& Elsevier]