Magnetization Reversal

We have investigated spin accumulation in Ni/Au/Ni single-electron transistors assembled by atomic force microscopy. The fabrication technique is unique in that unconventional hybrid devices can be realized with unprecedented control, including real-time tunable tunnel resistances. A grid of Au disks, 30 nm in diameter and 30 nm thick, is prepared on a SiO2 surface by conventional e-beam writing. Subsequently, 30 nm thick ferromagnetic Ni source, drain, and side-gate electrodes are formed in similar process steps. The width and length of the source and drain electrodes were different to exhibit different coercive switching fields. Tunnel barriers of NiO are realized by sequential Ar and O2 plasma treatment. By use of an atomic force microscope with specially designed software, a single nonmagnetic Au nanodisk is positioned into the 25 nm gap between the source and drain electrodes. The resistance of the device is monitored in real time while the Au disk is manipulated step-by-step w...

We report on the magnetic properties of arrays of submicronic ͑35 nm-500 nm͒ 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 along the wire axis. In contrast, the strong crystal anisotropy of Co and the orientation of the crystal easy axis ͑nearly perpendicular to the wire axis͒, allows for the appearance of a multidomain magnetization configuration, each domain being oriented partially along the normal to the wire axis. Experimental evidence for the existence of this multidomain configuration has been obtained from resistivity and magnetization measurements. Large scale micromagnetic calculations for Co and Ni wires with high aspect ratios corroborate the strong influence of the crystal anisotropy on the overall properties of Co wires and provide an accurate microscopic description of the nucleation fields and the magnetization reversal mechanism for Ni wires. ͓S0163-1829͑97͒08045-4͔

Iron layers are grown on GaAs(001) single crystal wavers by magnetron sputtering and characterized by magneto-optical Kerr effect (MOKE) and X-ray photoelectron spectroscopy (XPS). It is found that all deposited layers, starting with 2 nm thickness, are ferromagnetic at room temperature with a relatively high coercitive field (500 to 750 Oe). The average Fe magnetic moment is larger for the thinnest Fe film deposited, of about 0.52 Bohr magnetons per Fe atom. The average Fe magnetic moment decreases to 0.26 Bohr magnetons, then to 0.17 Bohr magnetons for 4 and 8 nm, resepctively. XPS evidenced that the outermost Fe layers are oxidized; however, the thinnest Fe film presented the lower oxidation state. We may connect this reaction with Fe-As bonds, resulting in a nonferromagnetic interface compound. Indeed, XPS depth profiling of the 8 nm sample evidenced the formation of a very thin (about 1.4 nm) Fe metal layer immediately at the interface with GaAs. In this layer, Fe may reach an ...

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Magnetic anisotropy plays an important role in determining the magnetization direction of ferromagnetic electrodes in spintronic devices. In Mn-doped III-V ferromagnetic semiconductor (FMS) (Ga,Mn)As, control of the magnetic anisotropy between in-plane and perpendicular magnetization using epitaxial growth and strain effects has been demonstrated [1]. However, due to the low Curie temperature (TC < 300K), the (Ga,Mn)As is not suitable for room temperature applications. To overcome this problem, recently we have successfully grown Fe-doped FMS (Ga,Fe)Sb, which shows TC > 300K when the Fe concentration is higher than 23% [2,3]. Therefore, this new material is considered as a good candidate for device applications at room temperature. In this work, we investigate the magnetic anisotropy of Ga1-xFexSb (x = 0.2, 15nm) grown by low temperature molecular beam epitaxy (LT-MBE) on various different buffer layers: AlSb, GaSb, (In0.5,Ga0.5)As and GaAs. All of the buffer layers are thick ...

Magnetization reversal in a ferromagnetic nanowire which is much narrower than the exchange length is believed to be accomplished through the thermally activated growth of a spatially localized nucleus, which initially occupies a small fraction of the total volume. To date, the most detailed theoretical treatments of reversal as a field-induced but noise-activated process have focused on the case of a very long ferromagnetic nanowire, i.e., a highly elongated cylindrical particle, and have yielded a reversal rate per unit length, due to an underlying assumption that the nucleus may form anywhere along the wire. But in a bounded-length (though long) cylindrical particle with flat ends, it is energetically favored for nucleation to begin at either end. We indicate how to compute analytically the energy of the critical nucleus associated with either end, i.e., the activation barrier to magnetization reversal, which governs the reversal rate in the low-temperature (Kramers) limit. Our treatment employs elliptic functions, and is partly analytic rather than numerical. We also comment on the Kramers prefactor, which for this reversal pathway does not scale linearly as the particle length increases, and tends to a constant in the low-temperature limit.

Magnetization reversal in a ferromagnetic nanowire which is much narrower than the exchange length is believed to be accomplished through the thermally activated growth of a spatially localized nucleus, which initially occupies a small fraction of the total volume. To date, the most detailed theoretical treatments of reversal as a field-induced but noise-activated process have focused on the case of a very long ferromagnetic nanowire, i.e., a highly elongated cylindrical particle, and have yielded a reversal rate per unit length, due to an underlying assumption that the nucleus may form anywhere along the wire. But in a bounded-length (though long) cylindrical particle with flat ends, it is energetically favored for nucleation to begin at either end. We indicate how to compute analytically the energy of the critical nucleus associated with either end, i.e., the activation barrier to magnetization reversal, which governs the reversal rate in the low-temperature (Kramers) limit. Our treatment employs elliptic functions, and is partly analytic rather than numerical. We also comment on the Kramers prefactor, which for this reversal pathway does not scale linearly as the particle length increases, and tends to a constant in the low-temperature limit.

We report the scaling behavior of Barkhausen avalanches for every small field step along the hysteresis loop in CoCrPt alloy film having perpendicular magnetic anisotropy. Individual Barkhausen avalanche is directly observed utilizing a high-resolution soft Xray microscopy that provides real space images with a spatial resolution of 15 nm. Barkhausen avalanches are found to exhibit power-law scaling behavior at all field steps along the hysteresis loop, despite their different patterns for each field step. Surprisingly, the scaling exponent of the power-law distribution of Barkhausen avalanches is abruptly altered from 1 ± 0.04 to 1.47 ± 0.03 as the field step is close to the coercive field. The contribution of coupling among adjacent domains to Barkhausen avalanche process affects the sudden change of the scaling behavior observed at the coercivity-field region on the hysteresis loop of CoCrPt alloy film.

Activation volumes of the wall-motion and nucleation processes in Co-based multilayer films were characterized from time-resolved domain evolution patterns. These activation volumes were both sensitive to the multilayer structure as well as the film preparation condition. The two activation volumes were generally unequal with each other and the inequality directly influenced on magnetization reversal behavior.

Magnetic transmission soft X-ray microscopy has been used to study element-specifically the magnetization reversal behavior of (Co 84 Cr 16 ) 87 Pt 13 alloy thin films with a lateral resolution of 35 nm. Our results indicate that the magnetization switching is carried out by a random nucleation process that can be attributed to the reversal of individual grains. We found evidence of a large distribution of the switching fields at the nanogranular length scale, which has to be considered seriously for applications of CoCrPt systems as magnetic high-density storage materials.

We report a non-deterministic nature in the magnetization reversal of nanograins of CoCrPt alloy film. Magnetization reversal process of CoCrPt alloy film is investigated using high resolution soft X-ray microscopy which provides real space images with a spatial resolution of 15 nm. Domain nucleation sites mostly appear stochastically distributed within repeated hysteretic cycles, where the correlation increases as the strength of the applied magnetic field increases in the descending and ascending branches of the major hysteresis loop. In addition, domain configuration is mostly asymmetric with inversion of an applied magnetic field in the hysteretic cycle. Nanomagnetic simulation considering thermal fluctuations of the magnetic moments of the grains explains the nearly stochastic nature of the domain nucleation behavior observed in CoCrPt alloy film. With the bit size in high-density magnetic recording media approaching nanometer length scale, one of the fundamental and crucial is...

Conditions for Triggering Avalanches in Mn 12 -acetate. YOKO SUZUKI, S. MCHUGH, R. JAAFAR, M.P. SARACHIK, City College of New York/CUNY, Y. MYASOEDOV, H. SHTRIKMAN, E. ZELDOV, The Weizmann institute of Science, R. BAGAI, N.E. CHAKOV, G. CHRISTOU, University of Florida at Gainesville. -Recent measurements in Mn 12 -acetate have shown that magnetic avalanches (corresponding to fast magnetization reversal) propagate as a narrow front with a velocity that is roughly two orders of magnitude smaller than the speed of sound. This phenomenon is closely analogous to the propagation of a flame front through a flammable chemical substance (deflagration) [1]. The conditions for nucleation of avalanches triggered in response to a time-varying (swept) magnetic field were studied for different fields and temperatures. In these crystals, avalanches happened only at low temperatures and were found to occur stochastically at fields ranging from 1.0 T to 4.5 T. There is no apparent structure in the distribution of avalanches for fields below ≈ 3.5 T; at higher fields we find evidence that the probability is lower at "nonresonant" magnetic fields where tunneling across the anisotropy barrier is suppressed. This provides evidence that lowering the barrier by quantum mechanical tunneling facilitates the ignition of avalanches. Based on these and other measurements, we suggest that avalanches are triggered below 3.5 T by defects with lower energy barriers. [1] Y. Suzuki, et al., Phys. Rev. Lett. 95, 147201 (2005).

Local time-resolved measurements of fast reversal of the magnetization of single crystals of Mn12acetate indicate that the magnetization avalanche spreads as a narrow interface that propagates through the crystal at a constant velocity that is roughly two orders of magnitude smaller than the speed of sound. We argue that this phenomenon is closely analogous to the propagation of a flame front (deflagration) through a flammable chemical substance.

The magnetization reversal processes in Fe 20 Ni 80 dot arrays have been studied both experimentally and theoretically. The circular-shaped dots with thickness of 60 nm and variable diameters of 0.2-0.8 m were fabricated by using electron-beam lithography and lift-off technique. A size dependent vortex nucleation, annihilation and initial susceptibility were determined from magneto-optical hysteresis loops. With increasing dot diameter the nucleation and annihilation fields decrease, whereas the initial susceptibility exhibits a linear increase. The experimental data were compared to the results of analytical calculations taking into account magnetostatic interactions, exchange and Zeeman energies for a planar array of magnetically soft circular dots.

The 140 m loess-paleosol profile at Luochuan in the central Chinese Loess Plateau was sampled at 5-cm intervals in loess units and at 3 cm in paleosol units, in order to obtain a high resolution climatic record covering the past 2.5 million years. All samples were measured for magnetic susceptibility, which is regarded as a good proxy index of the East Asian summer monsoon strength. On the basis of the astronomical theory of Pleistocene climatic change, an age model of the Luochuan loess-paleosol sequence was developed by tuning the magnetic susceptibility record to time-series of insolation changes. The results show that the ages of the boundaries between the Malan and Lishi, and Lishi and Wucheng loess formations are 71 and 1320 kyr BP, respectively. The onset of loess accumulation is at 2470 kyr BP. Our age model was tested by comparing the orbitally derived ages with absolute age determinations of magnetic reversals, and cross-spectrum analyzing with solar radiation variations for summer at 65ºN. These indicate that the calibration provides a reliable time scale for the Luochuan loess-paleosol deposit.

Scattering of polarized soft x-rays is applied to study the magnetic and structural properties of Co/ Cu GMR ͑giant magnetoresistance͒ layer stacks. The Co/ Cu multilayers with layer thicknesses tuned to the second maximum of the GMR show a pronounced series of half-integer-order magnetic Bragg peaks in reflectometry curves measured at the Co L 2,3 absorption edges. These peaks of purely magnetic origin arise due to the antiparallel alignment of neighboring magnetic layers caused by interlayer exchange coupling. In applied magnetic fields the antiferromagnetic coupling is reduced, as proved by a reduction of the magnetic peaks in both specular and diffuse scattering. The magneto-optical constants at the Co L edges were determined from the energy dependence of the Bragg peak positions and utilized for modeling the scattering curves. Upon magnetization reversal, the reflectometry curves exhibit angular-dependent asymmetry effects, which indicate a high sensitivity of the magnetic signal on the probing depth. Comparison of x-ray magnetic hysteresis loops measured at the one-half and higher-order half-integer Bragg peaks shows that for detailed structural and magnetic information, the higher-order peaks have to be evaluated, too. Through the angular dependence of the hysteresis loops, they can be tuned to show antiferromagnetic and ferromagnetic features separately.

During ODP Leg 124, late middle Eocene to Quaternary sediment sequences were recovered from 13 holes drilled at five sites in the Celebes and Sulu basins. Paleomagnetic measurements and biostratigraphic studies using calcareous nannofossils, planktonic and benthic foraminifers, radiolarians, and diatoms were completed and summarized here. Two Neogene sediment sections recovered in the Sulu Basin yielded excellent core recoveries and magnetic reversal records, allowing direct magnetobiostratigraphic correlations for the Pliocene and Quaternary at Site 768 and for the middle Miocene to Quaternary at Site 769. The interpolated ages of biohorizons are not consistent between sites and only a few of them are in good agreement with previous calibrations. The differences may be the results of redeposition by turbidity currents and selective dissolution of key fossils.

Casablanca,the metropolis and economic center of Morocco, contains in its old quarries a unique archeological and paleontological heritage of great scientific, pedagogic and touristic value. Reflecting a rich past recounting the events preserved in these geological formations as well as the prehistoricalevolutionof the mankind in Casablanca. Other quarries tend to become bio-ecological sites. The inventory of these quarries shows an alarming situation due to anthropogenic threats resulting from human activities such as urban development and pollution. The preservation of this heritage for the future generations is a shared responsibility between authority, citizens, researchers andsocio-economic sector. Their valorization, such as the development of a geotourism circuit will promote even local ecotourism.

A coexistence of lateral and in-depth domain walls in antiferromagnet/ferromagnet (AF/FM) thin films exhibiting double hysteresis loops (DHLs) is demonstrated. Comparison of single and DHLs together with local and global measurements confirms the formation of two oppositely oriented domains in the AF that imprint a lateral domain structure into the FM layer. Most significantly, the magnetization reversal mechanism within each opposite domain takes place by incoherent rotation of spring-like domain walls extending through the Ni thickness. Therefore, complex three-dimensional domain walls are created perpendicular and parallel to the AF/FM interface in exchange biased systems.

Positively and negatively exchange biased (PEB and NEB) magnetoresistance (MR) loops in Ni/FeF2 ferromagnetic/antiferromagnetic (AF) heterostructures proceed through the same reversal mechanisms. The MR curves exhibit mirror symmetry: the increasing (decreasing) field branch of the PEB (NEB) loop is identical to the decreasing (increasing) branch of the NEB (PEB) loop, suggesting that the interfacial areal density of pinned uncompensated AF spins responsible for PEB and NEB is similar. Micromagnetic simulations are in agreement with experimental results and imply the coexistence of EB domains of opposite sign for all cooling fields, which results in a reversal mechanism not previously reported.

Magnetic frustration effects in artificial kagomé arrays of nanomagnets are investigated using X-PEEM microscopy and monte carlo simulations. Spin configurations of demagnetized networks reveal unambiguous signatures of long range, dipolar interaction between the nanomagnets. As soon as the system enters the spin ice manifold, the kagomé dipolar spin ice model captures the observed physics, while the short range kagomé spin ice model fails.

Ni nanohole arrays are prepared by a replication process involving sputtering, polymer molding pressing, and electroplating techniques, using anodic alumina membranes as templates. Nanohole diameter to interhole distance ratio is engineered by suitable template processing. From the analysis of the magnetization curves for increasing nanohole diameter, it is concluded that coercivity increases due to the pinning of domain walls to nanoholes, while in-plane anisotropy decreases owing to local shape anisotropy effects.

A coexistence of lateral and in-depth domain walls in antiferromagnet/ferromagnet (AF/FM) thin films exhibiting double hysteresis loops (DHLs) is demonstrated. Comparison of single and DHLs together with local and global measurements confirms the formation of two oppositely oriented domains in the AF that imprint a lateral domain structure into the FM layer. Most significantly, the magnetization reversal mechanism within each opposite domain takes place by incoherent rotation of spring-like domain walls extending through the Ni thickness. Therefore, complex three-dimensional domain walls are created perpendicular and parallel to the AF/FM interface in exchange biased systems.

Magnetization of an array of 16 elongated iron nanoparticles with diameter of ∼7 nm and height of ∼100 nm has been studied with a submicron semiconductor Hall magnetometer. Details of the magnetization hysteresis curves and the angular dependence of switching fields are examined. The results indicate that the magnetization reversal in these cylindrical nanoparticles cannot be described with a single coherent rotation mode.

The dependence of magnetoresistance (MR) on the magnetization reversal processes was studied in Co and NiFe line structures with pads. Conventional patterning and deposition processes are applied for the fabrication of a device that consists of five-line structures with a line-width of 2 lm. It is demonstrated that the large asymmetry of AMR loops in Co-lines, which are observed after a field-cooling process, might pave the way for applications in magnetoelectronic devices where the magnetic field direction parallel or antiparallel to dc-current direction inside the magnetic line structures really matters.

Temperature dependent Ferromagnetic Resonance measurements performed as a function of diameter on Nickel nanowire arrays reveal several interesting features in these systems. With diameter decrease from 100 nm to 15 nm, a transition induced by surface anisotropy increase is observed at 50 nm in easy axis orientation from parallel to perpendicular with respect to individual nanowire geometric axis. Analysis of resonance field H res temperature variation (between liquid Helium and room temperature) reveals underlying strong magneto-elastic effects in small and large diameter nanowire arrays with potential applications in recording and spintronics.

The magnetostratigraphic and biostratigraphic results of Ocean Drilling Program Sites 881-884 and 887 are summarized and combined to form age vs. depth models for these sites. The recovery of continuous magnetostratigraphic records of the upper lower Miocene through Pleistocene section at Sites 884 and 887 allows the first direct calibration of diatom and radiolarian datum levels with magnetostratigraphy between magnetic Subchrons C5En and C3Bn (18.317-6.744 Ma) in the North Pacific. Based on these records and temporally shorter magnetostratigraphic records at Sites 881-883, at least 25 diatom and 22 radiolarian datum levels appear to be isochronous in the late early Miocene through Pleistocene of the North Pacific. The use of calcareous nannofossils for biostratigraphy of the post-early middle Miocene appears to be limited to the late Pliocene and Quaternary in sections sampled by Leg 145. Biostratigraphy indicates that a composite section of Sites 883 and 884 represents a relatively complete record of the lower Eocene through the lower Miocene, although carbonate preservation is best and the section is less complicated by reworking at Site 883. Primary biostratigraphic control is provided by calcareous nannofossils. A large part of the upper middle Eocene and upper Eocene is missing at hiatuses at both Sites 883 and 884. A condensed upper Paleocene section is present at Site 883, unconformably overlying uppermost Cretaceous sediments. Late Maestrichtian calcareous nannofossils are documented in Site 883 immediately above the basalt.

Mut HavzasÝÕnda yapÝlan bu incelemede, Kšselerli formasyonunun havza kenarÝna ait olan •škellerinin Ÿst seviyelerinde šl•Ÿlen 3 stratigrafi kesitinden derlenen 72 šrnekte nannoplankton zonlarÝndan Reticulofenestra pseudoumbilica (Zankliyen), Dictyococcites productus (Gelasiyen), Pseudoemiliana lacunosa ZonlarÝ, Calcidiscus macintyrei, Helicosphaera sellii ve KŸ•Ÿk Gephyrocapsa Alt zonlarÝ (Gelasiyen-Kalabriyen) ile planktik foraminifer zonlarÝndan Globorotalia puncticulata (Zankliyen-Piasenziyen), Globorotalia inflata (Piasenziyen-Gelasiyen) zon-larÝ ve Globigerina calida calida Alt Zonu (Kalabriyen) tanÝmlanmÝßtÝr. Bu •alÝßmada tanÝmlanan biyozonlar ile daha šnceki araßtÝrmalarda havza •škellerindeki yaßÝ ge• Burdigaliyen-Langiyen olarak belirlenen Kšselerli formasyonunun yaßÝnÝn havza kenarÝnÝ temsil eden •škellerinde erken Pleyistosen (Kalabriyen)Õe kadar •ÝktÝÛÝ belirlenmißtir.

Femto-second laser pulses are becoming an important tool that allows us to explore non-equilibrium spin dynamics at short time (high frequency) scales [1]. It has therefore become apparent [2] that more rigorous treatments are needed to correctly address spin relaxation at these energies. I will show how functional-methods of calculations of correlation energies in electron gas [3] can be successfully

Magnetization reversal by a femtosecond circularly polarized laser pulse has been recently demonstrated in rare-earth doped transition metals (RE-TM). The switching mechanism has been attributed to an inverse Faraday effect and thermal effects. Based on the parameters provided in the experimental work, we show that this claim is unlikely to give rise to femtosecond reversal. Using a hybrid itinerant-localized picture

We investigated the influence of the Gd doping element to the magnetocaloric effect in the Pr 0:68Àx Gd x Ca 0:32 MnO 3 manganite ðx ¼ 0:25Þ: Isothermal magnetization measurements up to 40 kOe from 7oTo300 K were performed and the data were used to calculate the magnetic entropy change. The results show a low magnetocaloric effect when compared to the compound with x ¼ 0:

We have studied the evolution of domain wall dynamics during the devitrification of amorphous and nanocrystalline Fe 73.5 Cu 1 Nb 3 Si 11.5 B 11 microwires. Depending on the annealing temperature range, three different regimes in the domain wall dynamics can be considered for these materials. Annealing below the Curie temperature, T c , of the alloy leads to the stabilization of the domain structure and a strong temperature dependence of the domain wall dynamic parameters. However, annealing above T c leads to a destabilization of domain patterns and a decrease in the domain wall damping in one order of magnitude. Finally, annealing above the crystallization temperature, T x , leads to the appearance of the nanocrystalline state that is structurally very stable. In this case, the domain wall velocity is very fast due to the lack of magnetic anisotropy and extremely stable with the temperature.

The magnetic domain structure and magnetic properties of a ferromagnetic (Ga,Mn)As epilayer with perpendicular magnetic easy-axis are investigated. We show that, despite strong hysteresis, domain theory at thermodynamical equilibrium can be used to determine the micromagnetic parameters. Combining magneto-optical Kerr microscopy, magnetometry and ferromagnetic resonance measurements, we obtain the characteristic parameter for magnetic domains λ c , the domain wall width and specific energy, and the spin stiffness constant as a function of temperature. The nucleation barrier for magnetization reversal and the Walker breakdown velocity for field-driven domain wall propagation are also estimated.

It is shown that for existent natural materials the Faraday rotation is far below the theoretical limit [Steel et al. in J. Lightwave Technol. 18:1297, 2000]. Under this condition the value of the Faraday rotation is primarily determined by the Q-factor, while the low group velocity value, multipass traveling and energy concentration in magneto-optical material play a secondary role. A comparative analysis of the efficiency of different schemes employing defect modes, Tamm surface states, the Borrmann effect and plasmon resonance is presented.

Spin valve structures of Co/Cu/Co, Co/Au/Co, and Fe/Co/Cu/Co types were fabricated by electron beam deposition. Cr or Cr/M (M=Ag, Au, Cu) layers were used as buffer layers. Layer thicknesses and interface roughnesses were determined by the X-ray reflectivity while internal structure of the layers was analyzed by the X-ray diffraction. Magnetoresistance (MR) data indicate the effect of both buffer and magnetic layer thicknesses on the magnetic field dependence of the MR. We investigated the influence of Cu, Ag and Au layers on the value of magnetic field necessary to reverse the polarization of magnetic layers of different thicknesses. The largest value of such a magnetic field was obtained for 5nm Cr/2.5nm Au/5nm Co/2.2nm Au/2nm Co/1Au multilayer.

Spin crossover multiferroic [Fe(PM-BiA)2(NCS)2] is investigated for the first time with the first order reversal curve (FORC) diagram method for its hysteretic pressure behavior. The experimental setup allows the measurements of the high spin fraction as the function of temperature, light intensity, and pressure. The experimental FORC diagrams obtained in compression and releasing modes show significant differences which are in disagreement with the classical Preisach model. Also, stronger kinetic effects are evidenced in the compression mode and possible explanations of this effect considering the viscoelastic behavior are provided.

The domain con"gurations in permalloy wires (30 nm thick, 1}50 m wide and 210 m long) with a central &bowtie' (10 m long) were investigated in both their demagnetized and remanent states using magnetic force microscopy (MFM) and the results were con"rmed by micromagnetic calculations. Domain walls were found to be trapped at both ends of the bowtie, suggesting that domain-wall trapping is the dominant process in the magnetization reversal of these structures.

The domain configuration in permalloy wires ͑30 nm thick, 10 m wide, and 205 m long͒ with a wide size range of a narrow central bridge ͑5 m long and w m wide; 0.5рwр10 m͒ were investigated in both their demagnetized and remanent states using magnetic force microscopy and the results were confirmed by micromagnetic calculations. At the bridge region, domain walls were found to be shifted by a small external field. Scanning magneto-optical Kerr effect revealed that the coercivity in these structures are the same as that in a straight wire, suggesting that domain wall movement is the dominant process in the magnetization reversal of these structures.

The reversal of the magnetization under the influence of a field pulse has been previously predicted to be an incoherent process with several competing phenomena such as domain wall relaxation, spin wave-mediated instability regions, and vortex-core mediated reversal dynamics. However, there has been no study on the direct observation of the switching process with the aid of a microwave signal input. We report a time-resolved imaging study of magnetization reversal in patterned magnetic structures under the influence of a field pulse with microwave assistance. The microwave frequency is varied to demonstrate the effect of resonant microwave-assisted switching. We observe that the switching process is dominated by spin wave dynamics generated as a result of magnetic instabilities in the structures, and identify the frequencies that are most dominant in magnetization reversal.

We present monsoon variability records for the Holocene using multi-proxy approach (environmental magnetism, carbon isotope, and total organic carbon) from a 146 cm thick sedimentary profile in the Kotikanasar meadow (Chakrata), Northwest Himalaya. The chronology of the record was constrained by five AMS 14 C ages. The carbon isotope (δ 13 C) and Total Organic Carbon (TOC) data highly variable which vary between − 26.62‰ and − 22.46‰ (C 3-plants) and 0.1-~4%, respectively, indicating paleo-vegetation history and productivity of the studied area. The environmental magnetism is highly fluctuating in the Early Holocene with high concentrations of magnetic minerals during the high monsoon conditions and vice-versa. Intense Indian Summer Monsoon (ISM) phases were identified during the Early and Late Holocene i.e., ~9.2 to 7.4 ka, and ~4.8 ka to Modern which shows warm and wet climate. While decline in the ISM intensity during ~7.4 to 4.8 ka which indicates cold and dry climatic condition in the Northwest Himalayan regions. From ~9.2 to 7.4 ka, highly fluctuating climate linked with the Early Holocene warming. Sediment profile exhibits aridity in climate accompanying with the high influence of mid-latitude westerlies during ~7.4 to 4.8 ka from Northwest Indian regions. Hence the longterm fluctuation in the climate governed by the changes in the North Atlantic Ocean circulation as well as variations in the incoming solar radiations.

MnF 2 ͞Fe bilayers exhibit asymmetric magnetization reversal that occurs by coherent rotation on one side of the loop and by nucleation and propagation of domain walls on the other side of the loop. Here, we show by polarized neutron reflectometry, magnetization, and magnetotransport measurements that for samples with good crystalline "quality" the rotation is a two-stage process, due to coherent rotation to a stable state perpendicular to the cooling field direction. The result is remarkably asymmetrically shaped hysteresis loops.

Polarized neutron reflectometry is used to probe the in-plane projection of the net-magnetization vector M of polycrystalline Fe films exchange coupled to twinned (110) MnF 2 or FeF 2 antiferromagnetic (AF) layers. The magnetization reversal mechanism depends upon the orientation of the cooling field with respect to the twinned microstructure of the AF, and whether the applied field is increased to (or decreased from) a positive saturating field; i.e., the magnetization reversal is asymmetric. The reversal of the sample magnetization from one saturated state to the other occurs via either domain wall motion or magnetization rotation on opposite sides of the same hysteresis loop.