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We experimentally investigated the influence of the location of nonmagnetic impurities (Au) inserted in the NiO layer of NiO/Co bilayers on the exchange bias. The inserted Au layer actually consists of a discontinuous plane of Au islands leading to local disruption of the exchange interactions within the NiO layer or at the Co/NiO interface. When the Au layer is introduced at the Co/NiO interface, this results in a significant initial increase in the exchange bias field. If the Au layer is moved deeper within the NiO layer, the exchange bias progressively decreases towards its value without Au. Our experiments are interpreted in terms of a reduced dragging of the NiO magnetization during Co magnetization reversal due to the reduced interfacial interaction. This interpretation is supported by numerical simulations which also predict the formation and annihilation of bubble-like domain walls. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the effect of specular reflection on the anisotropic magnetoresistance of magnetic thin films. Calculation of the sheet conductance; Combination of specular reflection on the film interfaces with mean-free paths.

Presents information on a study which described an analytical calculation of the magnetoresistive (MR) response of spin-valve structures with synthetic antiferromagnetic pinned (AP) layer. Semiclassical calculation of the giant MR amplitude in spin valves with synthetic AP layers; Results and discussion; Conclusions.

Presents a study which examined anisotropy and angular variation of the giant magnetoresistance in magnetic multilayers. Intrinsic anisotropy of the giant magnetoresistance in magnetic multilayers; Angular variation of the multilayers; Conclusion.

Describes the magnetic and magnetotransport properties of several series of ferromagnetic layers. Spin-valve effect in NiFe/Cu/NiFe and Co/Cu/Co sandwiches; Dependence of the magnetoresistance amplitude on the thickness of the free ferromagnetic layer; Dependence of the magnetoresistance amplitude on the thickness of the noble-metal layer.

Focuses on the time and frequency evolution of the magnetization vector of a rotating disk of amorphous DyNi. Macroscopic variables that determine the dynamics of the system; Measurements of the magnetization vector of a rotating sample as a function of time or rotating frequency; General features of the magnetism of amorphous magnets.

Many spintronic devices such as thermally-assisted magnetic random access memories take advantage of the ferromagnetic(F)/antiferromagnetic(AF) interaction to pin the magnetization of a reference layer. However, they suffer from detrimental blocking temperatures distributions from memory cell to memory cell. A low-temperature contribution to the distribution was ascribed to spin-glass like regions which are randomly spread over the film. We report on an attempt to reduce the amount of these spin-glass like regions due to interdiffusion of species by adding a diffusion barrier at the F/AF interface. [ABSTRACT FROM AUTHOR]

Square arrays of silicon dots with feature sizes from 30 to 400 nm and periodicities from 60 to 500 nm were obtained by standard lithography or nanoimprint and reactive ion etching. Co/Pt multilayers with perpendicular anisotropy were subsequently deposited by magnetron sputtering onto these prepatterned wafers. The deposition was performed either at normal or oblique incidence. The magnetic domain structures were characterized by magnetic force microscopy after zero field cooling (ZFC) from the (Co/Pt) Curie temperature or by alternative demagnetization (AD). It appears that the angle of deposition of the Pt influences the existence of direct exchange coupling between neighboring dots due to the formation of a magnetic layer on the sidewalls of the dots. [ABSTRACT FROM AUTHOR]

From a phenomenological point of view, we have investigated the effect of scattering at lateral edges on the current-perpendicular-to-plane (CPP) transport properties of submicronic magnetic multilayer pillars. This scattering can be considered as a current-in-plane contribution in the CPP geometry. As a first attempt to calculate the CPP resistance and GMR in this situation, we have combined the serial resistance network model of CPP transport with an exponential variation of the local conductivity as a function of the distance from the edges of the pillars assuming that the scattering is perfectly diffusive at the edges of the pillars. As the lateral size of the pillars decreases below 100 nm, it is shown that this scattering leads to a strong increase in the CPP resistivity as well as a decrease in both ΔR/R and ΔR[sup *] area. Furthermore, due to the difference in mean-free paths from layer to layer, the current lines are no longer uniform in the pillar ! even if the latter is the shape of an infinite cylinder. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies spin-polarized electron tunneling by growing MOM junctions where M is a magnetic transition metal and O a reactively sputtered oxide. Range of oxide thickness; Structural, magnetic, and transport properties of tunnel junctions.

Investigates the structural, magnetic, and transport properties of discontinuous CoFe/HfO2 multilayers consisting of layers of ferromagnetic particles dispersed in an insulating matrix. Negative magnetoresistance caused by spin-polarized tunneling.

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.

Presents a study that focused on the magnetization processes in amorphous sandwiches. Preparation of samples using direct current triode sputtering; Association of the creation or annihilation of planar Bloch walls parallel to the surface with magnetic transitions; Magnetic properties of the amorphous sandwiches.

In this work we demonstrate explicit analytical expressions for both charge and spin currents which constitute the 2 × 2 spinor in magnetic tunnel junctions with noncollinear magnetizations under applied voltage. The calculations have been performed within the free electron model in the framework of the Keldysh formalism and WKB approximation. We demonstrate that spin/charge currents and spin transfer torques are all explicitly expressed through only three irreducible quantities, without further approximations. The conditions and mechanisms of deviation from the conventional sine angular dependence of both spin currents and torques are shown and discussed. It is shown in the thick barrier approximation that all tunneling transport quantities can be expressed in an extremely simplified form via Slonczewski spin polarizations and our effective spin averaged interfacial transmission probabilities and effective out-of-plane polarizations at both interfaces. It is proven that the latter plays a key role in the emergence of perpendicular spin torque as well as in the angular dependence character of all spin and charge transport considered. It is demonstrated directly also that for any applied voltage, the parallel component of spin current at the FM/I interface is expressed via collinear longitudinal spin current components. Finally, spin transfer torque behavior is analyzed in a view of transverse characteristic length scales for spin transport. [ABSTRACT FROM AUTHOR]

Using first-principles calculations, we investigated the impact of chromium (Cr) and vanadium (V) impurities on the magnetic anisotropy and spin polarization in Fe/MgO magnetic tunnel junctions. It is demonstrated, using layer resolved anisotropy calculation technique, that while the impurity near the interface has a drastic effect in decreasing the perpendicular magnetic anisotropy (PMA), its position within the bulk allows maintaining high interfacial PMA while reducing the bulk magnetization and correlatively the easy-plane demagnetizing energy. As a result, the effective magnetic anisotropy tends to increase as a function of the Cr or V concentration resulting in an increase in the critical magnetic thickness at which the crossover from out-of-plane to in-plane anisotropy takes place. At the same time, the interfacial spin polarization is not affected by the magnetic layer bulk doping by Cr or V impurities and even enhanced in most situations thus favoring an increase of tunnel magnetoresistance (TMR) amplitude. [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.

Abstract: In this paper, a detailed study of the magnetic behaviour particularly at the Verwey transition is presented for some stoichiometric magnetite samples, as a function of their shape (film/bulk) and crystalline structure (single/polycrystal). Travelling solvent floating zone technique was first used to obtain crystalline bulk magnetite samples (110) oriented, while molecular beam epitaxy allowed us to grow a 68nm Fe3O4 (111) crystalline film on -Al2O3(0001) substrate. A 64nm Fe3O4 polycrystalline film grown on an Si/SiO2 substrate using DC reactive magnetron sputtering method completed the sample series. An anomalous magnetisation behaviour has been evidenced in the Verwey temperature vicinity that is shown to depend on the applied magnetic field. A magneto-electronic model explains the origin of this behaviour. The maximum in the demagnetisation at the Verwey transition has been noticed very near 0.1T (i.e. 90kA/m, as found out on bulk magnetite). The amount and nature of structural defects have been proven to be determinant in the magnetisation process, and the approach to saturation is well described by the relation, with in bulk magnetite crystal and in epitaxial magnetite film, even across and below the Verwey transition after paraprocess corrections. These dependences exist in the 10–300K temperature range, excluding isotropic point (near 130K), and demonstrate that the antiphase boundaries (APBs) are responsible for the much more progressive approach to saturation observed in thin films than in bulk samples which is consistent with previous observations. On the epitaxial film, n is found to decrease down to 0.33 at isotropic point due to an enhanced super-exchange interaction through APBs. [Copyright &y& Elsevier]

An array of spin torque nano-oscillators (STNOs), coupled by dipolar interaction and arranged on a ring, has been studied numerically and analytically. The phase patterns and locking ranges are extracted as a function of the number N, their separation, and the current density mismatch between selected subgroups of STNOs. If N ≥ 6 for identical current densities through all STNOs, two degenerated modes are identified an in-phase mode (all STNOs have the same phase) and a splay mode (the phase makes a 2 π turn along the ring). When inducing a current density mismatch between two subgroups, additional phase shifts occur. The locking range (maximum current density mismatch) of the in-phase mode is larger than the one for the splay mode and depends on the number N of STNOs on the ring as well as on the separation. These results can be used for the development of magnetic devices that are based on STNO arrays. [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.

The effect of patterning on the shift of the hysteresis loop HE and coercivity HC in a system composed of two perpendicularly exchange-coupled ferromagnets (NiFe sputtered onto a [Pt/Co] multilayer) is investigated in long stripes and square dots. Setting the exchange bias coupling along the stripes results in a threefold increase of HE compared to the continuous films. HC increases dramatically when the coupling is set perpendicular to the stripes and also in the dots. Magnetic force microscopy studies and micromagnetic simulations suggest that differences in the number and orientation of the magnetic domains can account for the observed effects. [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]

Exchange bias effects have been induced along the perpendicular-to-film direction in nanostructures prepared by electron beam lithography, consisting of a ferromagnetic [Pt/Co] multilayer exchange coupled to an antiferromagnet (FeMn). As a general trend, the exchange bias field and the blocking temperature decrease, whereas the coercivity increases, as the size of the nanostructures is reduced. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a study which focused on a code that calculated the conductance and magnetoresistance of spin-valve samples from the microscopic transport parameters. Description of the code; Examples of use of the code; Conclusions.

Presents a study which described the magnetic and structural properties of sputtered NiO based spin-valves. Experimental setup; Results of the study; Conclusions.

We identified reactively sputtered HfO2 as a particularly good material for making thin insulating barriers for spin-dependent tunnel junctions. This material allows one to form pinhole-free tunnel barriers with good transmission of the spin polarization of the tunneling electrons. Magnetic tunnel junctions consisting of a thin layer of HfO2 sandwiched between transition metal electrodes (Co and Fe, for instance) exhibit changes of tunnel resistance up to 30% at low temperature as a function of applied field. This effect can be used in a variety of magnetic field sensing applications or in magnetic random access memory. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the importance of sheet conductance change in spin-valve multilayer magnetoresistance measurements. Relation between sheet conductance and Fermi sphere changes; Use of buffer layer to improve structure growth; Representation of the conductance change thermal variation.

The temperature required to set the exchange bias of a ferro/antiferromagnetic (F/AF) storage bilayer as a function of the heating pulse width was studied on magnetic tunnel junctions (MTJs) of thermally assisted magnetic random access memories. Heating is produced by a pulse of electric current flowing through the junction. For sufficiently long heating pulse (>20 ns), a quasiequilibrium temperature profile is reached in the MTJ. In this stationary regime, a relationship between the temperature of the storage layer and the power of the pulse was established by using an Arrhenius–Néel model of thermal relaxation. The introduction of thermal barriers between the junction tunnel barrier and the electrodes allows a significant reduction of the power required to achieve a given temperature rise of the storage layer. When the heating pulse duration is reduced from 1 s to 2 ns, the heating power required for setting the F/AF storage bilayer increases by about 80%. This experimental observation is quantitatively interpreted by combining the Arrhenius–Néel model with thermodynamic simulations of heat diffusion with source term given by the experimentally known heating power dissipated in the tunnel barrier by Joule effect. [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]

Nanostructured tunnel junctions with composition Al2O3/Ta/Ru/Mn51.5Pt48.5/Co90Fe10/Al oxidized/Co90Fe10/Ni81Fe19/Ta were prepared on prepatterned silicon substrates, consisting of rectangular dots with dimensions as small as 100×400 nm2 and different spacings. Reduced lateral dimensions lead to an enhancement of the exchange bias field by comparison to continuous films. Magnetic force microscopy images taken at remanence indicate a mixture of magnetic states corresponding to the different relative orientations of the magnetizations of the pinned and the free layers. Significant dipolar interactions between the dots also proved to contribute to the complexity of the magnetization reversal of the stacks. [ABSTRACT FROM AUTHOR]

Pt/Co/M sandwiches (M=Al, Ta, Cr) were prepared by sputtering. The M capping layer was oxidized either by natural oxidation in air or by exposure to an oxygen plasma. By measuring the extraordinary Hall effect in these systems, we observed that the magnetic anisotropy of the Co electrode is extremely sensitive to the degree of oxidation of the capping layer. In particular, when M5Al, the magnetic anisotropy is found in plane when the AlO[SUBx] layer is under or overoxidized whereas it is out of plane when this oxide layer is stochiometric alumina. The conditions of oxidation which give the perpendicular anisotropy in Pt/Co/AlO[SUBx] are found to be the same than those which lead to a maximum of specular reflection at Co/AlO[SUBx] interface in Co/Ru/Co/Cu/Co/AlO[SUBx] synthetic spin-valves. This crossover of anisotropy therefore provides a very useful way for controlling the oxidation of tunnel barriers. Furthermore, this phenomenon can be used to study the aging of naturally oxidized tunnel junctions exposed to air, and it is also observed with other capping materials such as Ta or Cr. [ABSTRACT FROM AUTHOR]

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]

An introduction is presented in which the editors discuss various articles within the issue on the topics including the spin-transfer torque random access memory (RAM), the application for complementary metal oxide semiconductor and the resistive RAM based on migration of cations.

Exchange bias and coercivity are investigated in antiferromagnetic (AFM)/ferromagnetic (FM) bilayers by steepest descent and Monte Carlo simulations. For a given range of the AFM anisotropy, a magnetic domain wall parallel to the interface is created in the AFM layer. This domain wall propagates along the AFM thickness and eventually gets pinned at the negative saturation due to the intrinsic Peierls potential of the AFM material. We demonstrate that this pinning and its associated dissipation is one possible source of coercivity in such exchange coupled bilayers. Monte Carlo simulations were also carried out to investigate the influence of thermal fluctuations, which induce a random walk of the domain wall throughout the AFM layer, leading to magnetic aftereffect. Depending on the experimental characteristic measurement time and temperature, the domain wall can eventually be expelled from the AFM layer leading to the reversal of the entire AFM spin lattice. This contribution to the exchange bias phenomenon particularly explains quite well the enhancement of coercivity in exchange coupled bilayers at low temperature, even in polycrystalline samples. [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]

A new contribution to the exchange bias phenomenon in antiferromagnetic-ferromagnetic bilayers is presented, based on the intrinsic Peierls potential experienced by narrow magnetic domain walls moving through the antiferromagnet crystal lattice. Although the model is developed in the ideal case of a perfect crystalline structure with an atomically flat interface, this new contribution is expected to play a significant role at the grain scale in polycrystalline realistic samples. The thermal variation of the hysteresis loss in such bilayers is interpreted by a random walk process of the domain wall through the antiferromagnet. This new contribution yields a relationship between exchange bias and coercivity. [ABSTRACT FROM AUTHOR]

Using first-principles calculations, we elucidate microscopic mechanisms of perpendicular magnetic anisotropy (PMA) in Fe/MgO magnetic tunnel junctions through evaluation of orbital and layer resolved contributions into the total anisotropy value. It is demonstrated that the origin of the large PMA values is far beyond simply considering the hybridization between Fe-3d and O-2p orbitals at the interface between the metal and the insulator. Onsite projected analysis shows that the anisotropy energy is not localized at the interface but it rather propagates into the bulk showing an attenuating oscillatory behavior which depends on orbital character of contributing states and interfacial conditions. Furthermore, it is found in most situations that states with dyz(xz) and dZ2 character tend always to maintain the PMA while those with dxy and dx2-y2 character tend to favor the in-plane anisotropy. It is also found that while MgO thickness has no influence on PMA, the calculated perpendicular magnetic anisotropy oscillates as a function of Fe thickness with a period of 2 ML and reaches a maximum value of 3.6 mJ/m2. [ABSTRACT FROM AUTHOR]

Field-programmable gate arrays (FPGAs) are mainly composed of memory cells which store the configuration data defining the implemented functionality of the circuit. Today, advanced memories are facing a number of issues such as increased static power consumption and radiation sensitivity. These can be solved by the use of a new memory technology: magnetic random access memory (MRAM) refid="ref1"/. It combines the reliability against radiation induced soft errors, the non-volatility of Flash memory, fast writing cycle, and the endurance of static random access memory (SRAM). MRAMs therefore appear as a good candidate for replacing today's memories in FPGAs refid="ref2"/. To take advantage of all of its assets, a new FPGA architecture has been designed by taking into account all the specificities of MRAMs. This paper presents a silicon proven innovative MRAM-based look-up table (LUT) intended for space applications. It has been designed to validate the concept and measure the performance. It combines MRAM and dynamic logic for lower power consumption while increasing density and reliability to soft errors. [ABSTRACT FROM AUTHOR]

In exchange biased spintronic devices such as magnetic random access memories, spin-glass-like regions are randomly spread over ferromagnetic/antiferromagnetic interfaces. They contribute to detrimental distributions of exchange bias properties from memory cell to memory cell. The integrated area of the low-temperature (T) contribution of the blocking temperature (TB) distribution provides information about the amount of these spin-glass like regions. In this paper, we report on the influence of various growth conditions on the low-T contribution of the TB distributions for cobalt/iridium-manganese (IrMn) based bilayers. Top, bottom, on- and off- axis depositions were compared. We show that off-axis deposition, i.e., when the target and substrate are off-centred, does not reduce the low-T contribution despite an expected smoother F/AF interface. We observe that bottom deposition, i.e., when IrMn is deposited first, allows reducing AF spin-glass like regions as compared to top deposition. This contributes to improved exchange bias properties. [ABSTRACT FROM AUTHOR]

The perpendicular magnetic anisotropy (PMA) arising at the interface between ferromagnetic transition metals and metallic oxides was investigated via first-principles calculations. In this work very large values of PMA, up to 3 erg/cm2, at Fe∣MgO interfaces are reported, in agreement with recent experiments. The origin of PMA is attributed to overlap between O-pz and transition metal dz2 orbitals hybridized with dxz(yz) orbitals with stronger spin-orbit coupling-induced splitting around the Fermi level for perpendicular magnetization orientation. Furthermore, it is shown that the PMA value weakens in the case of over- or underoxidation due to the fact that oxygen pz and transition metal dz2 orbital overlap is strongly affected by disorder, in agreement with experimental observations in magnetic tunnel junctions. [ABSTRACT FROM AUTHOR]

Abstract: Spin-torque effects were investigated in double barrier magnetic tunnel junctions of the form F1/I1/F/I2/F2 where F is a soft ferromagnetic layer, I1 and I2 are two tunnel barriers, and F1 and F2 are two ferromagnetic layers whose magnetization are pinned in-plane, in either antiparallel or parallel magnetic configuration. We show that for some particular thickness of the F layer, resonant effects take place in the F layer, which enhance the net spin-torque amplitude by more than an order of magnitude. The spin-torque was calculated as a function of distance from the barriers within the F layer. It has an oscillatory behavior with very large amplitude when the resonance conditions are fulfilled. [Copyright &y& Elsevier]

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]

Since the first experimental observation of all-optical switching phenomena, intensive research has been focused on finding suitable magnetic systems that can be integrated as storage elements within spintronic devices and whose magnetization can be controlled through ultra-short single laser pulses. We report here atomistic spin simulations of all-optical switching in multilayered structures alternating n monolayers of Tb and m monolayers of Co. By using a two temperature model, we numerically calculate the thermal variation of the magnetization of each sublattice as well as the magnetization dynamics of [ Tb n / Co m ] multilayers upon incidence of a single laser pulse. In particular, the condition to observe thermally-induced magnetization switching is investigated upon varying systematically both the composition of the sample (n,m) and the laser fluence. The samples with one monolayer of Tb as [ Tb 1 / Co 2 ] and [ Tb 1 / Co 3 ] are showing thermally induced magnetization switching above a fluence threshold. The reversal mechanism is mediated by the residual magnetization of the Tb lattice while the Co is fully demagnetized in agreement with the models developed for ferrimagnetic alloys. The switching is however not fully deterministic but the error rate can be tuned by the damping parameter. Increasing the number of monolayers the switching becomes completely stochastic. The intermixing at the Tb/Co interfaces appears to be a promising way to reduce the stochasticity. These results predict for the first time the possibility of TIMS in [Tb/Co] multilayers and suggest the occurrence of sub-picosecond magnetization reversal using single laser pulses. [ABSTRACT FROM AUTHOR]

The all-optical magnetization reversal of magnetic layers, by picosecond optical pulses, is of particular interest as it shows the potential for energy-efficient and fast magnetic tunnel junction (MTJ) elements. This approach requires memory elements that are optically and electronically accessible, for optical writing and electronic read-out. In this paper, we propose the integration of indium tin oxide (ITO) as a transparent conducting electrode for magnetic tunnel junctions in integrated spintronic–photonic circuits. To provide light with sufficient energy to the MTJ free layer and allow electrical read-out of the MTJ state, we successfully integrated indium tin oxide as a top transparent electrode. The study shows that ITO film deposition by physical vapor deposition with conditions such as high source power and low O2 flow achieves smooth and conductive thin films. Increase in grain size was associated with low resistivity. Deposition of 150 nm ITO at 300 W, O2 flow of 1 sccm and 8.10−3 mbar vacuum pressure results in 4.8 × 10−4 Ω.cm resistivity and up to 80% transmittance at 800 nm wavelength. The patterning of ITO using CH4/H2 chemistry in a reactive ion etch process was investigated showing almost vertical sidewalls for diameters down to 50 nm. The ITO based process flow was compared to a standard magnetic tunnel junctions fabrication process flow based on Ta hard mask. Electrical measurements validate that the proposed process based on ITO results in properties equivalent to the standard process. We also show electrical results of magnetic tunnel junctions having all-optical switching top electrode fabricated with ITO for optical access. The developed ITO process flow shows very promising initial results and provides a way to fabricate these new devices to integrate all-optical switching magnetic tunnel junctions with electronic and photonic elements. [ABSTRACT FROM AUTHOR]

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]

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]

Abstract: We theoretically studied spin-dependent electron transport perpendicular-to-plain (CPP) in magnetic laminated multilayered structures by using Kubo formalism. We took into account not only bulk scattering, but the interface resistance due to both specular and diffuse reflection and also spin conserving and spin-flip processes. It was shown that spin-flip scattering at interfaces substantially reduces the value of giant magnetoresistance (GMR). This can explain the experimental observations that the CPP GMR ratio for laminated structures only slightly increases as compared to non-laminated ones even though lamination induces a significant increase in CPP resistance. [Copyright &y& Elsevier]

We studied current-induced magnetic switching and excitations in structures comprising a free layer with in-plane magnetization traversed by a current with perpendicular-to-plane spin polarization. We derived analytical solutions from the Landau–Lifshitz–Gilbert equation including the spin-torque term, and compared them to numerical simulations within the single domain assumption. Taking into account the criterion of thermal stability, the magnetization switching in nanostructures of typical size below 100 nm comprising a perpendicular polarizer is found to require larger current density but to be much faster than with a longitudinal polarizer. Furthermore, a steady precession of magnetization can be generated in this geometry; those frequencies can be tuned from about 1 to 20 GHz by only changing the current without applying any external field. This opens a promising application as microwave sources. [ABSTRACT FROM AUTHOR]

We have written and read back bits on perpendicular pre-patterned media prepared by electron-beam lithography. Using a contact tester equipped with a standard longitudinal head, we are able to write different bit lengths on 100×200 nm[sup 2] dots spaced 100 nm apart. These tests demonstrate the role played by the distribution of the coercive fields of the dots during the write process. Signal-to-noise ratios (SNRs) are obtained by numerical analysis and are found to be on the same order of magnitude as in continuous perpendicular media, but remain constant at decreasing bit lengths. This property of the SNR is due to a partial reduction of transition noise. The potential to pattern these media with nanoimprint lithography and their good recording performance make them candidates for ultrahigh density recording. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Since the advent of the MOS transistor, the performance of microelectronic circuits has followed Moore's law, stating that their speed and density would double every 18 months. Today, this trend tends to get out of breath: the continuously decreasing size of devices and increasing operation frequency result in power consumption and heating issues. Among the solutions investigated to circumvent these limitations, the use of non-volatile devices appears particularly promising. It allows easing, for example, the power gating technique, which consists in cutting-off the power supply of inactive blocks without losing information, drastically reducing the standby power consumption. In this approach, the advantages of magnetic tunnel junctions (MTJs) compared with other non-volatile devices allow one to design hybrid CMOS/magnetic circuits with high performance and new functionalities. Designing such circuits requires integrating MTJs in standard microelectronics design suites. This is performed by means of a process design kit (PDK) for the hybrid CMOS/magnetic technology. We present here a full magnetic PDK, which contains a compact model of the MTJ for electrical simulation, technology files for layout and physical verifications, and standard cells for the design of complex logic circuits and which is compatible with standard design suites. This PDK allows designers to accurately and comfortably design high-performance hybrid CMOS/magnetic logic circuits in the same way as standard CMOS circuits. [ABSTRACT FROM AUTHOR]