Magnetization Reversal

In current longitudinal magnetic recording media, high areal density and low noise are achieved by statistical averaging over several hundred weakly coupled ferromagnetic grains per bit cell. Continued scaling to smaller bit and grain sizes, however, may prompt spontaneous magnetization reversal processes when the stored energy per particle starts competing with thermal energy, thereby limiting the achievable areal density. Charap et al. have predicted this to occur at about 40 Gbits/in 2 : This paper discusses thermal effects in the framework of basic Arrhenius-Néel statistical switching models. It is emphasized that magnetization decay is intimately related to high-speedswitching phenomena. Thickness-, temperature-and bit-densitydependent recording experiments reveal the onset of thermal decay at "stability ratios" (KuV =kB T)0 = 35 6 2. The stability requirement is grain size dispersion dependent and shifts to about 60 for projected 40 Gbits/in 2 conditions and ten-year storage times. Higher anisotropy and coercivity media with reduced grain sizes are logical extensions of the current technology until write field limitations are reached. Future advancements will rely on deviations from traditional scaling. Squarer bits may reduce destabilizing stray fields inside the bit transitions. Perpendicular recording may shift the onset of thermal effects to higher bit densities. Enhanced signal processing may allow signal retrieval with fewer grains pers bit. Finally, single grain per bit recording may be envisioned in patterned media, with lithographically defined bits. Index Terms-Arrhenius-Néel model, dynamic coercivity, high K u alloys, magnetic recording, signal decay, thermal switching. I. INTRODUCTION A N overview of thermal effect limits in magnetic recording is presented. The emphasis is on continuous dimensional scaling of granular longitudinal recording media, where these effects will first become apparent. We try to answer the important question of how thermally stable today's media are and review the assumptions behind Charap's 40 Gbits/in limit prediction [1]. In particular, we discuss a new experimental approach to reliably determine media stability by combining long time signal decay measurements with short time write experiments. A brief outlook discusses materials alternatives, in particular high anisotropy media, and points to various options to avoid thermal effects. These include reduced bit aspect ratio recording, fewer grains per bit, perpendicular recording, possibly with thermal assist, and patterned media.

Devices that show a magnetic anisotropy normal to the film surface hold great promise towards faster and smaller magnetic bits in data-storage applications. We describe an experimental demonstration of current-induced magnetic reversal of nanopillars with perpendicular anisotropy and high coercive fields. The best results are observed for Co/Ni multilayers, which have higher giant magnetoresistance values and spin-torque efficiencies than Co/Pt multilayers. The reference layers were designed to have significantly higher anisotropy allowing a complete current-field phase diagram of the free-layer reversal to be explored. The results are compared to micromagnetic modelling of the free layer that, depending on the bias current and applied field, details regions of irreversible magnetic switching, coherent and incoherent spin waves, or static non-uniform magnetization states. This ability to manipulate high-anisotropy magnetic elements could prove useful for a range of spintronic applications.

Below oxygen isotope stage 16, the orbitally derived time-scale developed by Shackleton et al. [1] from ODP site 677 in the equatorial Pacific differs significantly from previous ones [e.g., 2-5], yielding estimated ages for the last Earth magnetic reversals that are 5-7% older than the K/Ar values but are in good agreement with recent Ar/Ar dating . These results suggest that in the lower Brunhes and upper Matuyama chronozones most deep-sea climatic records retrieved so far apparently missed or misinterpreted several oscillations predicted by the astronomical theory of climate. To test this hypothesis, we studied a high-resolution oxygen isotope record from giant piston core MD900963 (Maldives area, tropical Indian Ocean) in which precession-related oscillations in t~180 are particularly well expressed, owing to the superimposition of a local salinity signal on the global ice volume signal . Three additional precession-related cycles are observed in oxygen isotope stages 17 and 18 of core MD900963, compared to the SPECUAP composite curves , and stage 21 clearly presents three precession oscillations, as predicted by Shackleton et al. [1]. The precession peaks found in the 3180 record from core MD900963 are in excellent agreement with climatic oscillations predicted by the astronomical theory of climate. Our ~180 record therefore permits the development of an accurate astronomical time-scale. Based on our age model, the Brunhes-Matuyama reversal is dated at 775 + 10 ka, in good agreement with the age estimate of 780 ka obtained by Shackleton et al.

We experimentally demonstrate that the magnetization can be reversed in a reproducible manner by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field. This optically induced ultrafast magnetization reversal previously believed impossible is the combined result of femtosecond laser heating of the magnetic system to just below the Curie point and circularly polarized light simultaneously acting as a magnetic field. The direction of this opto-magnetic switching is determined only by the helicity of light. This finding reveals an ultrafast and efficient pathway for writing magnetic bits at record-breaking speeds.

In April 1976 a group of ocean-oriented earth scientists gathered in La Jolla to discuss ways in which newly available capabilities for detailed studies of the sea floor, particularly manned small sub-crust has focused the att4 geologists on the ocean plates-the rise crests, t and trenches-as the regi vestigations would be m(

This paper presents a review of (astrobiochronological) calibration of Recent to late Oligocene calcareous nannofossil datum events. Biohorizons included in the paper are those of the widely used ''standard'' nannofossil zonations of Martini, E. [1971. Standard Tertiary and Quaternary calcareous nannoplankton zonation. ], as well as supplementary biohorizons proposed in the literature. The biohorizons have been selected on the basis of the unambiguous taxonomy of the index taxa and their biostratigraphic usefulness. We emphasise the application of rigorous methodology in nannofossil studies which permits an evaluation of biohorizons in terms of reliability, and calibrates their potential correlatability. Astrochronological age estimates rely on the Geologic timescale developed by the ICS in 2004, with some new calibrations included. We provide an overview of the relative position of biohorizons versus the astronomically calibrated ages of magnetic reversals and reference isotope stratigraphies. Surprisingly, there are still few high-resolution quantitative biostratigraphic studies of astrochronologically tuned sections in spite of the central role of such studies in addressing fundamental problems such as the tempo and mode of plankton evolution. r

We report on magnetic domain-wall velocity measurements in ultrathin Pt=Co0:5-0:8 nm=Pt films with perpendicular anisotropy over a large range of applied magnetic fields. The complete velocity-field characteristics are obtained, enabling an examination of the transition between thermally activated creep and viscous flow: motion regimes predicted from general theories for driven elastic interfaces in weakly disordered media. The dissipation limited flow regime is found to be consistent with precessional domainwall motion, analysis of which yields values for the damping parameter, .

Surface-plasmon-mediated confinement of optical fields holds great promise for on-chip miniaturization of all-optical circuits 1-4 . Following successful demonstrations of passive nanoplasmonic devices 5-7 , active plasmonic systems have been designed to control plasmon propagation. This goal has been achieved either by coupling plasmons to optically active materials 8-13 or by making use of transient optical nonlinearities in metals via strong excitation with ultrashort laser pulses 14-17 . Here, we present a new concept in which the active optical component is a metal-ferromagnet-metal structure. It is based on active magneto-plasmonic microinterferometry, where the surface plasmon wave vector in a gold-ferromagnet-gold trilayer system is controlled using a weak external magnetic field. Application of this new technique allows measurement of the electromagnetic field distribution inside a metal at optical frequencies and with nanometre depth resolution. Significant modulation depth combined with possible all-optical magnetization reversal induced by femtosecond light pulses 18 opens a route to ultrafast magneto-plasmonic switching.

A superconducting quantum interference device (SQUID) with single-walled carbon nanotube (CNT) Josephson junctions is presented. Quantum confinement in each junction induces a discrete quantum dot (QD) energy level structure, which can be controlled with two lateral electrostatic gates. In addition, a backgate electrode can vary the transparency of the QD barriers, thus permitting change in the hybridization of the QD states with the superconducting contacts. The gates are also used to directly tune the quantum phase interference of the Cooper pairs circulating in the SQUID ring. Optimal modulation of the switching current with magnetic flux is achieved when both QD junctions are in the 'on' or 'off' state. In particular, the SQUID design establishes that these CNT Josephson junctions can be used as gate-controlled pi-junctions; that is, the sign of the current-phase relation across the CNT junctions can be tuned with a gate voltage. The CNT-SQUIDs are sensitive local magnetometers, which are very promising for the study of magnetization reversal of an individual magnetic particle or molecule placed on one of the two CNT Josephson junctions.

To realize molecular spintronic devices, it is important to externally control the magnetization of a molecular magnet. One class of materials particularly promising as building blocks for molecular electronic devices is the paramagnetic porphyrin molecule in contact with a metallic substrate. Here, we study the structural orientation and the magnetic coupling of in-situ-sublimated Fe porphyrin molecules on ferromagnetic Ni and Co films on Cu(100). Our studies involve X-ray absorption spectroscopy and X-ray magnetic circular dichroism experiments. In a combined experimental and computational study we demonstrate that owing to an indirect, superexchange interaction between Fe atoms in the molecules and atoms in the substrate (Co or Ni) the paramagnetic molecules can be made to order ferromagnetically. The Fe magnetic moment can be rotated along directions in plane as well as out of plane by a magnetization reversal of the substrate, thereby opening up an avenue for spin-dependent molecular electronics.

nature materials | VOL 2 | FEBRUARY 2003 | www.nature.com/naturematerials 85 A s fabrication technology pushes the dimensions of ferromagnetic structures into the nanoscale, understanding the magnetization processes of these structures is of fundamental interest, and key to future applications in hard disk drives, magnetic random access memory and other 'spintronic' devices 1-4 . Measurements on elongated magnetic nanostructures 5,6 highlighted the importance of nucleation and propagation of a magnetic boundary, or domain wall, between opposing magnetic domains in the magnetization reversal process. Domain-wall propagation in confined structures is of basic interest 7,8 and critical to the performance of a recently demonstrated magnetic logic scheme for spintronics 9 . A previous study of a 500-nm-wide NiFe structure obtained very low domain-wall mobility in a three-layer device 10 . Here we report room-temperature measurements of the propagation velocity of a domain wall in a single-layer planar Ni 80 Fe 20 ferromagnetic nanowire 200 nm wide. The wall velocities are extremely high and, importantly, the intrinsic wall mobility is close to that in continuous films 11 , indicating that lateral confinement does not significantly affect the gyromagnetic spin damping parameter to the extreme extent previously suggested 10 . Consequently the prospects for high-speed domain-wall motion in future nanoscale spintronic devices are excellent.

Magnetic monopoles have been predicted to occur as emergent fractional quasiparticles inside pyrochlore spin ice, a frustrated magnetic insulator. Experimental signatures of such emergent monopoles accompanied by Dirac strings have been detected by means of neutron scattering in reciprocal space in pyrochlore spin ice at sub-Kelvin temperatures, but their real-space observation has remained elusive. Here we report on direct, real-space observations of emergent monopoles and their associated Dirac strings in two-dimensional (2D) artificial kagome spin ice at room temperature using synchrotron X-ray photoemission electron microscopy. Magnetization reversal proceeds through the nucleation and avalanche-type dissociation of monopole-antimonopole pairs along 1D Dirac strings. This is in sharp contrast to conventional domain growth in 2D systems, providing a striking example of dimensional reduction due to frustration. The observed hysteresis, monopole densities and 1D Dirac-string avalanches are quantitatively explained by Monte Carlo simulations.

We introduce a new class of spintronics devices in which a spin-valve like effect results from strong spin-orbit coupling in a single ferromagnetic layer rather than from injection and detection of a spin-polarized current by two coupled ferromagnets. The effect is observed in a normalmetal/insulator/ferromagnetic-semiconductor tunneling device. This behavior is caused by the interplay of the anisotropic density of states in (Ga,Mn)As with respect to the magnetization direction, and the two-step magnetization reversal process in this material.

We report on the magnetic properties of arrays of submicronic (35 nm-500 nm) Ni and Co wires fabricated by electrodeposition into the cylindrical pores of track-etched polymer membranes. This work reveals intrinsic differences between the magnetization reversal mechanisms taking place in these two systems. For Ni, the crystal anisotropy is small compared to the shape anisotropy and the magnetization lies

We study current-driven magnetization switching in nanofabricated Ni(84)Fe(16)/Cu/Ni(84)Fe16 trilayers at 295 and 4.2 K. The shape of the hysteretic switching diagram at low magnetic field changes with temperature. The reversible behavior at higher fields involves two phenomena, a threshold current for magnetic excitations closely correlated with the switching current, and a peak in differential resistance characterized by telegraph noise, with an average period that decreases exponentially with current and shifts with temperature. We interpret both static and dynamic results at 295 and 4.2 K in terms of thermal activation over a potential barrier, with a current-dependent effective magnetic temperature.

Magnetization processes are analytically described for the arrays of soft ferromagnetic polycrystalline circular dots with submicron dimensions, wherein the magnetization reversal accompanied by nucleation, displacement, and annihilation of magnetic vortices. Magnetostatic, exchange, and Zeeman energies are taken into account for the analysis. The magnetic state of each dot in an applied magnetic field is treated as an offcentered rigid vortex structure; i.e., the vortex keeps its spin distribution while being displaced. This rigid vortex model yields analytical expressions for the size-dependent initial susceptibility, the vortex nucleation, and the annihilation fields. The interdot magnetostatic interaction plays an important role in the magnetization reversal for the arrays when the interdot distance is smaller than the disk radius, where the initial susceptibility increases and both the nucleation and annihilation fields decrease. The analytical predictions are compared to the micromagnetic calculations, and limitations of the model are discussed.

A parallel finite element micromagnetics package has been implemented, that is highly scalable, easily portable and combines different solvers for the micromagnetic equations. The implementation is based on the standard Galerkin discretization on tetrahedral meshes with linear basis functions. A static energy minimization, a dynamic time integration, and the nudged elastic band method have been implemented. The details of the implementation and some aspects of the optimization are discussed and timing and speedup results are given. Nucleation and magnetization reversal processes in permalloy nanodots are investigated with this micromagnetics package.

Ferromagnetic Co nanowires have been electrodeposited into self-assembled porous anodic alumina arrays. Due to their cylindrical shape, the nanowires exhibit perpendicular anisotropy. The coercivity, remanence ratio, and activation volumes of Co nanowires depend strongly on the length, diameter, and spacing of the nanowires. Both coercivity and thermal activation volume increase with increasing wire length, while for constant center-to-center spacing, coercivity decreases and thermal activation volume increases with increasing wire diameter. The behavior of the nanowires is explained qualitatively in terms of localized magnetization reversal.

Six tetranuclear complexes ͓Fe͑III͒ 3 Ln͑ 3 -O͒ 2 ͑CCl 3 COO͒ 8 ͑H 2 O͒͑THF͒ 3 ͔ ·THF·C 7 H 16 ͓Ln= Gd͑III͒ ͑1͒, Tb͑III͒ ͑2͒, Dy͑III͒ ͑3͒, Ho͑III͒ ͑4͒, Y͑III͒ ͑5͒, and Lu͑III͒ ͑6͔͒ have been studied by magnetic susceptibility and Mössbauer spectroscopy. These isostructural molecules have a "butterfly" structure core consisting of two Fe 2 Ln͑ 3 -O͒ triangular "wings" which share a common Ln-Fe "body"; the dihedral angle between the wings is ca. 148°. The coordination spheres of the iron ions are essentially distorted octahedral. The lanthanides are eight-coordinate with coordination polyhedra that may be described as distorted tetragonal bipyramids. Variable-temperature solid-state magnetic susceptibility in the temperature range 1.8-300 K and magnetization at 1.8 K for compounds 1-6 were measured. The spin state of Fe is S =5/ 2 in all cases. In compounds 5 and 6, where Ln͑III͒ ͑Y and Lu, respectively͒ is diamagnetic, the three Fe atoms form an obtuse isosceles triangle with antiferromagnetic interactions J Fe-Fe = −50 K between the wing-tip Fe w and body Fe b atoms, and negligible interaction between the Fe w 's, resulting in a ground state of effective spin S =5/ 2 per cluster. In the complexes with paramagnetic lanthanide ions, the interaction between the Fe 3 triangle and the Ln͑III͒ center is described by an effective exchange constant which is antiferromagnetic and 1 order of magnitude weaker. Besides, at 3 K incipient spin blocking, characteristic of single molecule magnets, was found to occur in the out-of-phase component of the ac susceptibility in ͕Fe 3 TbO 2 ͖, ͕Fe 3 DyO 2 ͖, and ͕Fe 3 HoO 2 ͖. The activation energy of a Debye process describing the magnetization reversal has been determined to be, E a Ϸ 8, 9, and 10 K for the Ln= Tb, Dy, and Ho, respectively, and the prefactor 0 Ϸ 10 −7 s. The high spin states of the Fe͑III͒ centers were confirmed by the Mössbauer spectra, in which two distinguishable Fe sites could be resolved above 80 K, corresponding to the Fe w and Fe b sites, respectively. Relatively larger values of the quadrupole splitting of the Mössbauer spectra were observed for the Fe w pair as compared with that for the Fe b , and both quadrupole splittings diminished with increasing temperature. At 3 K the Mössbauer spectra showed a state with blocked spins ͑sextets͒ for the ͕Fe 3 TbO 2 ͖ and ͕Fe 3 DyO 2 ͖ cases. From the E a and 0 , determined in the ac susceptibility, the relaxation time at 3 K is estimated as Ϸ 10 −5 -10 −6 s much longer than the time window of Mössbauer spectroscopy and compatible with the single molecule magnet behavior. In the presence of a strong magnetic field the moments of the Ln͑III͒ cation and the Fe 3 triangle are polarized. For some compounds at low temperature a magnetic pattern ͑sextet͒ for each of the three Fe sites appeared, and the antiferromagnetic coupling within the Fe 3 cluster was directly proved by the opposite trend of the field dependence of the two Fe w sextets as compared with the Fe b third one.

We present an ultrafast route for a controlled, toggle switching of magnetic vortex cores with ultrashort unipolar magnetic field pulses. The switching process is found to be largely insensitive to extrinsic parameters, like sample size and shape, and it is faster than any field-driven magnetization reversal process previously known from micromagnetic theory. Micromagnetic simulations demonstrate that the vortex core reversal is mediated by a rapid sequence of vortex-antivortex pair creation and annihilation subprocesses. Specific combinations of field-pulse strength and duration are required to obtain a controlled vortex core reversal. The operational range of this reversal mechanism is summarized in a switching diagram for a 200 nm Permalloy disk.

We report here an atomic resolution study of the structure and composition of the grain boundaries in polycrystalline Sr 0.6 K 0.4 Fe 2 As 2 superconductor. A large fraction of grain boundaries contain amorphous layers larger than the coherence length, while some others contain nanometer-scale crystallites sandwiched in between amorphous layers. We also find that there is significant oxygen enrichment at the grain boundaries. Such results explain the relatively low transport critical current density (J c) of polycrystalline samples with respect to that of bicrystal films.

We present an extensive study of the magnetic reversal mechanism of Fe and Ni nanowires with diameters down to 6 nm, i.e. smaller than the domain wall width. The coercive "eld at 5 K is a factor of 3 lower than the prediction for rotation in unison. We also observe that the activation energy associated with the reversal process is proportional to the cross-section of the wires and nearly independent of the wire length. From the temperature dependence of the coercive "eld and the magnetic viscosity we can conclude that magnetization reversal takes place via a nucleation of a small magnetic domain, probably at the end of the wire, followed by the movement of the domain wall. For Co wires, we observe a di!erent behavior that is dominated by the competition between the shape anisotropy and the temperaturedependent magnetocrystalline anisotropy.

The magnetization reversal in exchange-biased ferromagnetic-antiferromagnetic (FM-AFM) bilayers is investigated. Different reversal pathways on each branch of the hysteresis loop, i.e., asymmetry, are obtained both experimentally and theoretically when the magnetic field is applied at certain angles from the anisotropy direction. The range of angles and the magnitude of this asymmetry are determined by the ratio between the FM anisotropy and the interfacial FM-AFM exchange anisotropy. The occurrence of asymmetry is linked with the appearance of irreversibility, i.e., finite coercivity, as well as with the maximum of exchange bias, increasing for larger anisotropy ratios. Our results indicate that asymmetric hysteresis loops are intrinsic to exchange-biased systems and the competition between anisotropies determines the asymmetric behavior of the magnetization reversal.

We examined the zero-field splitting of an iron(II) phthalocyanine (FePc) attached to clean and oxidized Cu(110) surfaces and the dependence on an applied magnetic field by inelastic electron tunneling spectroscopy with STM. The symmetry of the ligand field surrounding the Fe atom is lowered on the oxidized surface, switching the magnetic anisotropy from the easy plane of the bulk to the easy axis. The zero-field splitting was not observed for FePc on a clean Cu(110) surface, and the spin state converts from triplet to singlet due to the strong coupling of Fe d states with the Cu substrate, as is also confirmed by photoelectron spectroscopy. These findings demonstrate the importance of coupling at the moleculesubstrate interface for manipulating the magnetic properties of adsorbates.

The exchange bias effect, discovered more than fifty years ago, is a fundamental interfacial property, which occurs between ferromagnetic and antiferromagnetic materials. After intensive experimental and theoretical research over the last ten years, a much clearer picture has emerged about this effect, which is of immense technical importance for magneto-electronic device applications. In this review we start with the discussion of numerical and analytical results of those models which are based on the assumption of coherent rotation of the magnetization. The behavior of the ferromagnetic and antiferromagnetic spins during the magnetization reversal, as well as the dependence of the critical fields on characteristic parameters such as exchange stiffness, magnetic anisotropy, interface disorder etc. are analyzed in detail and the most important models for exchange bias are reviewed. Finally recent experiments in the light of the presented models are discussed.

We investigate the magnetization reversal in ͓Co͑4 Å͒ /Pt͑7 Å͔͒ 50 multilayers with perpendicular magnetic anisotropy both macroscopically by the first-order reversal curve (FORC) technique and microscopically by transmission x-ray microscopy (TXRM) and resonant x-ray small angle scattering (SAS). In particular, we focus on the nucleation and saturation processes. The onset of magnetization reversal is dominated by irreversible processes corresponding to the avalanchelike propagation of one-dimensional stripe domains that originate from earlier nucleated zero-dimensional bubble domains. In a second stage we observe mainly reversible behavior where the overall domain topology is preserved. Finally another irreversible process brings the sample to negative saturation, corresponding to the contraction and annihilation of domains. Interestingly, even well beyond the apparent major-loop saturation field, the FORC diagram exhibits pronounced irreversible switching and thus provides a direct measure of the true (and significantly higher) saturation field. TXRM and SAS measurements reveal on a microscopic level that some residual bubble domains with negligible moments still persist for fields well above the apparent saturation field. These residual domains, if unannihilated, significantly alter the subsequent magnetization reversal.

Four volcanic-ash beds bracket the Early-Middle Triassic boundary, as defined by conodont biostratigraphy, in a stratigraphic section in south China. High-precision U-Pb dates of single zircons allow us to place the Early to Middle Triassic (Olenekian-Anisian) boundary at 247.2 Ma. Magnetic-reversal stratigraphy allows global correlation. The new dates constrain the Early Triassic interval characterized by delayed biotic recovery and carbon-cycle instability to ϳ5 m.y. This time constraint must be considered in any model for the end-Permian extinction and subsequent recovery.

We study how micromagnetic calculations can be applied to processes that involve a singularity of the magnetization field, namely, the Bloch point. In order to allow for comparison with recent experiments, we consider Permalloy thin-film disks supporting a vortex magnetic configuration. The structure of the Bloch point at rest in the middle of the core of the vortex is studied first, comparing the evolution of the calculation results under decreasing mesh size to analytical results. The reversal of the core of the vortex under a field applied perpendicularly to the disk plane is then investigated. We apply two different procedures to evaluate switching fields and processes: direct micromagnetic time-dependent calculation, and the evaluation of the energy barrier that separates the two orientations of the vortex core in the configuration space, using a path method. Both methods show the occurrence of Bloch points during reversal. Special attention is paid to the extrapolation towards zero mesh size of the numerical results. The calculations are confronted to experimental values from Okuno et al. ͓J. Magn. Magn. Mater. 240, 1 ͑2002͔͒. We conclude that defects and thermal agitation are likely to assist Bloch-point injection, hence lowering the switching fields.

We have measured and simulated the dynamics of magnetization reversal in 5 nm by 0.8 by 1.6 mm Ni 60 Fe 40 thin films. The films measured form the upper electrode of a spin-polarized tunnel junction so that the magnetization direction of the film can be probed by measuring the tunneling resistance of the junction. When a magnetic field pulse is applied, the time to switch the film magnetization changes from greater than 10 ns to less than 500 ps as the pulse amplitude is increased from the coercive field to 10 mT and beyond. We have simulated these transitions using micromagnetic modeling of the exact experimental conditions. The simulations agree well with the experimental measurements. [S0031-9007(98)07661-3] PACS numbers: 75.70.Ak

We have carried out detailed experimental studies of the exchange bias effect of a series of CoO/Co͑111͒ textured bilayers with different Co layer thickness, using the magneto-optical Kerr effect, superconducting quantum interference device ͑SQUID͒ magnetometry, polarized neutron reflectivity, x-ray diffraction, and atomic force microscopy. All samples exhibit a pronounced asymmetry of the magnetic hysteresis at the first magnetization reversal as compared to the second reversal. Polarized neutron reflectivity measurements show that the first reversal occurs via nucleation and domain wall motion, while the second reversal is characterized by magnetization rotation. Off-specular diffuse spin-flip scattering indicates the existence of interfacial magnetic domains. All samples feature a small positive exchange bias just below the blocking temperature, followed by a dominating negative exchange bias field with decreasing temperature.

The magnetic vortex with the in-plane curling magnetization and the out-of-plane magnetization at the core is a unique ground state in nanoscale magnetic elements. This kind of magnetic vortex can be used as a memory unit for information storage, through its downward or upward core-orientation and, thus, controllable core switching deserves some special attention.

1] The aim of this paper is to investigate the relationships between the segmentation and the magnetic structure of the ultraslow-spreading Southwest Indian Ridge. Contrary to faster spreading ridges, magnetization usually decreases from high values along the neovolcanic axis to low values in the nontransform discontinuities. There is a direct correlation between the deepening of the axial valley and the decrease of the magnetization from the neovolcanic axis toward the deepest parts of the axial discontinuities. We suggest that less frequent eruptions as the distance from the segment center and the length of these discontinuities increase, result in thinner extrusive lavas and thus control the along-axis magnetization variations by thinning the magnetic source layer. A unique segment centered at 50°28 0 E shows a marked low magnetization anomaly at its center similarly to the segments of the slow-spreading Mid-Atlantic Ridge. We suggest that in this segment both the mantle temperature and the magmatic activity are high enough for the lavas not to be highly fractionated. A higher rate of melt production to the west of Gallieni transform fault may have created some form of reservoir where mixing of melts occurs and where crystalline fractionation is low producing low-magnetization lavas. To the east, magma chambers may be smaller with cooler mantle temperatures resulting in restricted mixing and significant fractionation which may lead to relatively high intensity magnetization lavas. Finally, we propose that serpentinization of peridotites has no significant contribution to the variation of the magnetization along the axial valley. Off-axis, in thin crust areas, upper mantle rocks may become progressively more altered, as distance from the axis increases. The strong faulting and alteration of a thin basaltic cap and underlying upper mantle rocks can produce the disappearance of the magnetic reversal pattern and the increase of the magnetization G 3

Simultaneous experimental determination of magnetization reversal M ¼ MðH; sÞ and magnetic-field-induced strain ¼ ðH; sÞ as a function of magnetic field, H, and external stress, s, gives full macroscopic description of the behaviour of MSM material. Moreover, by simultaneous measurements of M and ; the direct relation between magnetization and strain can be established. We measured these relations in different alloys and samples exhibiting different strain responses. The experimental dependences are interpreted within the framework of a simple energy model. The necessary conditions for the existence of magnetic shape memory effect are discussed and compared with the experiment. r

1] When five Matuyama-Brunhes (M-B) boundary records from the North Atlantic are placed on isotope age models, produced by correlation of the d 18 O record directly or indirectly to an ice volume model, the M-B boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773.1 ka (standard deviation = 0.4 kyr), ∼7 kyr younger than the presently accepted astrochronological age for this polarity reversal (780-781 ka). Two recently proposed revisions of the age of the 40 Ar/ 39 Ar Fish Canyon sanidine (FCs) standard to 28.201 ± 0.046 Ma and 28.305 ± 0.036 Ma would adjust 40 Ar/ 39 Ar ages applicable to the M-B boundary (and other reversals and excursions back to 1.2 Ma) to ages older than the new astrochronological ages by 8-24 kyr. The variables used to construct the ice volume models cannot account for the discrepancy. The FCs standard age that best fits the astrochronological ages is 27.93 Ma, which is within the uncertainty associated with the commonly used value of 28.02 (±0.16) Ma but younger than the recently proposed FCs ages. The EDC2 and EDC3 age models in the Dome C (Antarctic) ice core yield ages of 771.7 ka and 766.4 ka, respectively, for the 10 Be flux peak that denotes the paleointensity minimum at the reversal boundary, implying that the EDC2 (rather than EDC3) age model is consistent with the observations from marine sediments, at least close to the M-B boundary.

Real-time magneto-optical indicator film images reveal distinct asymmetry in the motion of a single domain wall in a wedged-NiFe /uniform-FeMn bilayer due to the nucleation and behavior of an exchange spring in the antiferromagnetic layer. Magnetization reversal from the ground state begins at the thick end of the wedge where the exchange anisotropy field ͑H E ͒ is minimal and the magnetostatic field ͑H MS ͒ is maximal, whereas reversal into the ground state begins from the thin end where H E is maximal and H MS is minimal.

We present results of detailed experimental and theoretical studies of all-optical magnetization reversal by single circularly-polarized laser pulses in ferrimagnetic rare earth-transition metal (RE-TM) alloys Gd x Fe 90−x Co 10 (20% < x < 28%). Using single-shot time-resolved magneto-optical microscopy and multiscale simulations, we identified and described the unconventional path followed by the magnetization during the reversal process. This reversal does not involve precessional motion of magnetization but is governed by the longitudinal relaxation and thus has a linear character. We demonstrate that this all-optically driven linear reversal can be modeled as a result of a twofold impact of the laser pulse on the medium. First, due to absorption of the light and ultrafast laser-induced heating, the medium is brought to a highly nonequilibrium state. Simultaneously, due to the ultrafast inverse Faraday effect the circularly polarized laser pulse acts as an effective magnetic field of the amplitude up to ∼20 T. We show that the polarization-dependent reversal triggered by the circularly polarized light is feasible only in a narrow range (below 10%) of laser fluences. The duration of the laser pulse required for the reversal can be varied from ∼40 fs up to at least ∼1700 fs. We also investigate experimentally the role of the ferrimagnetic properties of GdFeCo in the all-optical reversal. In particular, the optimal conditions for the all-optical reversal are achieved just below the ferrimagnetic compensation temperature, where the magnetic information can be all-optically written by a laser pulse of minimal fluence and read out within just 30 ps. We argue that this is the fastest write-read event demonstrated for magnetic recording so far.

Magnetic nanostructures are attracting considerable interest due to their unique properties and potential applications. There are various challenges associated with the fabrication of highly ordered large area magnetic nanostructures and the understanding of their magnetization reversal processes. This review focuses on the use of the deep ultraviolet lithography technique in fabricating arrays of magnetic nanostructures of varying geometrical parameters over a large area. Using resolution enhancement techniques such as alternating phase shift and chrome-less phase shift masks (PSMs), arrays of ferromagnetic nanostructures with lateral dimensions below the conventional resolution limit have been fabricated. Comprehensive investigation of the relationship between the swing amplitude and the pattern size using alternating PSM lithography is presented. Double patterning and double exposure with shifts are used to significantly improve the pattern density and manipulate the magnetic nanostructures. In addition, results of systematic investigations of evolution of magnetic spin states, in-plane anisotropy and magnetostatic interaction in arrays of elongated Ni 80 Fe 20 rings and their derivatives are presented. The magnetization reversal mechanism, the switching field distributions and the transition fields between different magnetic configurations are found to be strongly dependent on the inter-ring spacing, film thickness and any missing segments of the ring. A comprehensive investigation of the spin states and magnetic anisotropy in magnetic antidot nanostructures is also presented. The detailed magnetization reversal reveals a very strong pinning of domain walls in the vicinity of anti-structures, the strength of which was found to be strongly dependent on the anti-structure geometry and field orientation.

A high-resolution integrated stratigraphy is presented for the Abad marls of the Sorbas and Nijar basins in SE Spain (pre-evaporitic Messinian of the Western Mediterranean). Detailed cyclostratigraphic and biostratigraphic analyses of partially overlapping subsections were needed to overcome stratigraphic problems in particular encountered at the complex transition from the Lower to the Upper Abad. The resulting Abad composite section contains a continuous stratigraphic record from the Tortonian/Messinian boundary up to the transition to the Messinian evaporites of the Yesares Member. All together, 18 calcareous plankton events were recognized which were shown to be synchronous throughout the Mediterranean by means of detailed (bed-to-bed) cyclostratigraphic correlations. The magnetostratigraphy allowed the identification of the four magnetic reversals of chron C3An in the Upper Abad. Details in the sedimentary cycle patterns allowed the Abad composite to be astronomically calibrated. This calibration to the 65°N summer insolation curve of solution La90(1,1) yielded astronomical ages for all sedimentary cycles, calcareous plankton bioevents, ash layers and paleomagnetic reversals. Up to now, the Abad composite is the only astronomically well-calibrated section that provided a reliable cyclostratigraphy, magnetostratigraphy and calcareous plankton biostratigraphy. As such it will serve as a reference section both for the pre-evaporite Messinian in the Mediterranean as well as for the Messinian interval in the Astronomical Polarity Time Scale.

We describe nanopillar spin valves with perpendicular anisotropy designed to reduce the critical current needed for spin transfer magnetization reversal while maintaining thermal stability. By adjusting the perpendicular anisotropy and volume of the free element consisting of a ͓Co/Ni͔ multilayer, we observe that the critical current scales with the height of the anisotropy energy barrier and we achieve critical currents as low as 120 A in quasistatic room-temperature measurements of a 45 nm diameter device. The field-current phase diagram of such a device is presented.

The evolution of a magnetic ''vortex'' state in submicron ferromagnetic disks has been studied as functions of disk diameter and thickness. The vortex core displacement in the applied magnetic field was calculated by minimizing the total magnetic energy consisting of the magnetostatic, exchange, and Zeeman energies. A simple analytical expression for the initial magnetic susceptibility is deduced. The initial susceptibility increases with increasing disk diameter and decreasing thickness. The calculations agree well with the experimental data obtained for the 60 nm thick permalloy disk arrays with a variable diameter from 0.2 to 0.8 m.

Magnetization reversals through the formation of vortex state and the rotation of onion state are two processes with comparable probabilities for symmetric magnetic nanorings with radius of about 50 nanometers. This magnetic bistability is the manifestation of the competition between the exchange energy and the magnetostatic energy in nanomagnets. The relative probability of the two processes in symmetric nanorings is dictated by the ring geometry and can not be altered. In this work, we report the magnetic behavior of a novel type of nanorings -asymmetric nanorings. By tuning the asymmetry we can control the fraction of the vortex formation process from about 40% to nearly 100% by utilizing the direction of the external magnetic field. The observed results have been accounted for by the dependence of the domain wall energy on the local cross section area of nanoring for which we have provided theoretical calculations.

A novel theoretical approach to magnetization dynamics driven by spin-polarized currents is presented. Complete stability diagrams are obtained for the case where spin torques and external magnetic fields are simultaneously present. Quantitative predictions are made for the critical currents and fields inducing magnetization switching, for the amplitude and frequency of magnetization self-oscillations, and for the conditions leading to hysteretic transitions between self-oscillations and stationary states.

Article history: Available online xxx a b s t r a c t Astronomical calibration of the lower-middle Pleistocene Montalbano Jonico section located in the Lucania Basin (Southern Italy) is presented. Previous papers widely discussed the integrated stratigraphy (calcareous nannofossils, sapropel stratigraphy, benthic and planktonic oxygen stable isotopes) and the paleoenvironmental features of this section and its potential suitability for the selection of the Middle Pleistocene Global Stratotype Section and Point (GSSP). In this study, new planktonic d 18 O data, additional biostratigraphical constraints and new tephrochronology on volcaniclastic layers occurring within the studied record are reported. The new chronostratigraphic framework provides a robust base for correlation of the oxygen isotope stratigraphy for the Montalbano Jonico section with the glacial and interglacial fluctuations of the Oceanic and Mediterranean d 18 O reference deep-sea records. Specifically, the lower part of the Montalbano Jonico section (Interval A) provides correlation of the planktonic and benthic d 18 O cycles to Marine Isotope Stage (MIS) 36 to MIS 23. Interval A includes a distinct peak of left-coiled neogloboquadrinids, the Globoratalia crassaformis influx, the First Occurrence of Gephyrocapsa omega, and the First Common Occurrence and Last Common Occurrence of Reticulofenestra asanoi. These stratigraphical constraints support the tuning of five sapropel layers included in this part of the section to insolation cycles i-112, i-104, i-102, i-90 and i-86. The upper part of the section (Interval B), which includes the temporary disappearance (td2) of G. omega and tephra layer V5, Ar/Ar age dated at 719.5 AE 12.6 ka, is consistent with identification of MIS 22 to MIS 16 in the planktonic d 18 O pattern. The d 18 O time series of the whole section was reconstructed using the midpoints of individual sapropels and their correlative precession minima, visual comparison of the d 18 O pattern with the record available at the Mediterranean ODP Site 975, and, in the upper part of the section, the Ar/Ar age of tephra V5. The developed astronomical tuning revealed that the Montalbano Jonico section covers an interval from 1240 ka to 645 ka. A significant change in sedimentation rate occurs between Intervals A (0.53 m/ky) and Interval B (0.91 m/ky) at about 870 ka and is consistent with a sea-level drop from a bathyal to a circalittoral environment. Bioevents recognised in the Montalbano Jonico section have been dated according to the astronomical calibration, and age assignments of tephra V1-V4 and V6-V9 are also proposed. The Montalbano Jonico section fills the gap between the top of the Vrica section and the base of the Ionian informal Middle Pleistocene stage, and represents a Mediterranean reference section for the Mid-Pleistocene transition.

Current-induced magnetization dynamics in Co/Cu/Co trilayer nanopillars (∼100 nm in diameter) have been studied experimentally at low temperatures for large applied fields perpendicular to the layers. At 4.2 K an abrupt and hysteretic increase in resistance is observed at high current densities for one polarity of the current, comparable to the giant magnetoresistance (GMR) effect observed at low fields. A micromagnetic model, that includes a spin-transfer torque, suggests that the current induces a complete reversal of the thin Co layer to alignment antiparallel to the applied field-that is, to a state of maximum magnetic energy.