Showing papers on "Magnetization published in 1996"
IBM1
TL;DR: In this paper, a new mechanism was proposed for exciting the magnetic state of a ferromagnet, where a transfer of vectorial spin accompanied an electric current flowing perpendicular to two parallel magnetic films connected by a normal metallic spacer.
5,824 citations
TL;DR: In this paper, the interaction between spin waves and itinerant electrons is considerably enhanced in the vicinity of an interface between normal and ferromagnetic layers in metallic thin films, leading to a local increase of the Gilbert damping parameter which characterizes spin dynamics.
Abstract: The interaction between spin waves and itinerant electrons is considerably enhanced in the vicinity of an interface between normal and ferromagnetic layers in metallic thin films. This leads to a local increase of the Gilbert damping parameter which characterizes spin dynamics. When a dc current crosses this interface, stimulated emission of spin waves is predicted to take place. Beyond a certain critical current density, the spin damping becomes negative; a spontaneous precession of the magnetization is predicted to arise. This is the magnetic analog of the injection laser. An extra dc voltage appears across the interface, given by an expression similar to that for the Josephson voltage across a superconducting junction. \textcopyright{} 1996 The American Physical Society.
4,433 citations
TL;DR: In this article, a new GaAs-based diluted magnetic semiconductor, (Ga,Mn)As, was prepared by molecular beam epitaxy and the lattice constant was determined by x-ray diffraction and shown to increase with the increase of Mn composition, x.
Abstract: A new GaAs‐based diluted magnetic semiconductor, (Ga,Mn)As, was prepared by molecular beam epitaxy. The lattice constant of (Ga,Mn)As films was determined by x‐ray diffraction and shown to increase with the increase of Mn composition, x. Well‐aligned in‐plane ferromagnetic order was observed by magnetization measurements. Magnetotransport measurements revealed the occurrence of anomalous Hall effect in the (Ga,Mn)As layer.
2,072 citations
TL;DR: The relaxation processes of electrons and spins systems following the absorption of femtosecondoptical pulses in ferromagnetic nickel have been studied using optical and magneto-optical pump-probetechniques and the experimental results are adequately described by a model including three interacting reservoirs.
Abstract: The relaxation processes of electrons and spins systems following the absorption of femtosecond optical pulses in ferromagnetic nickel have been studied using optical and magneto-optical pump-probe techniques. The magnetization of the film drops rapidly during the first picosecond, but different electron and spin dynamics are observed for delays in the range 0--5 ps. The experimental results are adequately described by a model including three interacting reservoirs (electron, spin, and lattice).
1,920 citations
TL;DR: In this paper, the magnetoresistance and the field dependent magnetization have been systematically examined in the low temperature ferromagnetic metallic state of single crystal and polycrystalline.
Abstract: The magnetoresistance (MR) and the field dependent magnetization have been systematically examined in the low temperature ferromagnetic metallic state of single crystal and polycrystalline ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}$. We find that the intrinsic negative MR in single crystal is due to the suppression of spin fluctuations, and magnetic domain boundaries do not dominate the scattering process. In contrast, we demonstrate that the MR in the polycrystalline samples exhibits two distinct regions: large MR at low fields dominated by spin-polarized tunneling between grains and high field MR which is remarkably temperature independent from 5 to 280 K.
1,594 citations
TL;DR: In this article, the results of low-temperature experiments on a single crystal composed of superparamagnetic manganese clusters (Mn12-ac) were reported, which clearly demonstrate the existence of quantum-mechanical tunnelling of the bulk magnetization.
Abstract: THE precise manner in which quantum-mechanical behaviour at the microscopic level underlies classical behaviour at the macroscopic level remains unclear, despite seventy years of theoretical investigation. Experimentally, the crossover between these regimes can be explored by looking for signatures of quantum-mechanical behaviour—such as tunneling—in macroscopic systems1. Magnetic systems (such as small grains, spin glasses and thin films) are often investigated in this way2–12 because transitions between different magnetic states can be closely monitored. But transitions between states can be induced by thermal fluctuations, as well as by tunnelling, and definitive identification of macroscopic tunnelling events in these complex systems is therefore difficult13. Here we report the results of low-temperature experiments on a single crystal composed of super-paramagnetic manganese clusters (Mn12-ac), which clearly demonstrate the existence of quantum-mechanical tunnelling of the bulk magnetization. In an applied magnetic field, the magnetization shows hysteresis loops with a distinct 'staircase' structure: the steps occur at values of the applied field where the energies of different collective spin states of the manganese clusters coincide. At these special values of the field, relaxation from one spin state to another is enhanced above the thermally activated rate by the action of resonant quantum-mechanical tunnelling. These observations corroborate the results of similar experiments performed recently on a system of oriented crystallites made from a powdered sample4.
1,542 citations
TL;DR: The magnetization in the ferrimagnetic region below 16 kelvin was substantially increased after illumination and could be restored almost to its original level by thermal treatment and these effects are thought to be caused by an internal photochemical redox reaction.
Abstract: Photoinduced magnetization was observed in a Prussian blue analog, K0.2Co1.4- [Fe(CN)6]·6.9H2O. An increase in the critical temperature from 16 to 19 kelvin was observed as a result of red light illumination. Moreover, the magnetization in the ferrimagnetic region below 16 kelvin was substantially increased after illumination and could be restored almost to its original level by thermal treatment. These effects are thought to be caused by an internal photochemical redox reaction. Furthermore, blue light illumination could be used to partly remove the enhancement of the magnetization. Such control over magnetic properties by optical stimuli may have application in magneto-optical devices.
1,537 citations
TL;DR: In this paper, a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin-glass-like surface layer is proposed, and the qualitative features of this model are reproduced by a numerical calculation of the spin distribution.
Abstract: Nickel ferrite nanoparticles exhibit anomalous magnetic properties at low temperatures: low magnetization with a large differential susceptibility at high fields, hysteresis loops which are open up to 160 kOe, time-dependent magnetization in 70 kOe applied field, and shifted hysteresis loops after field cooling. We propose a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin-glass-like surface layer. We find that qualitative features of this model are reproduced by a numerical calculation of the spin distribution. Implications of this model for possible macroscopic quantum tunneling in these materials are discussed.
1,407 citations
01 Mar 1996
1,123 citations
TL;DR: In this paper, a comprehensive survey of experimental studies on the magnetic anisotropy in metallic multilayers containing Fe, Co or Ni is presented and commented on, with the help of some dedicated experimental studies.
Abstract: Ferromagnetic materials exhibit intrinsic `easy' and `hard' directions of the magnetization. This magnetic anisotropy is, from both a technological and fundamental viewpoint one of the most important properties of magnetic materials. The magnetic anisotropy in metallic magnetic multilayers forms the subject of this review article. As individual layers in a multilayer stack become thinner, the role of interfaces and surfaces may dominate that of the bulk: this is the case in many magnetic multilayers, where a perpendicular interface contribution to the magnetic anisotropy is capable of rotating the easy magnetization direction from in the film plane to perpendicular to the film plane. In this review, we show that the (in-plane) volume and (perpendicular) interface contribution to the magnetic anisotropy have been separated into terms related to mechanical stresses, crystallographic structure and the planar shape of the films. In addition, the effect of roughness, often inherent to the deposition techniques used, has been addressed theoretically. Several techniques to prepare multilayers and to characterize their growth as well as methods to determine the magnetic anisotropy are discussed. A comprehensive survey of experimental studies on the perpendicular magnetic anisotropy in metallic multilayers containing Fe, Co or Ni is presented and commented on. Two major subjects of this review are the extrinsic effects of strain, roughness and interdiffusion and the intrinsic effect of the crystallographic orientation on the magnetic anisotropy. Both effects are investigated with the help of some dedicated experimental studies. The results of the orientational dependence studies are compared with ab initio calculations. Finally, the perpendicular surface anisotropy and the in-plane step anisotropy are discussed.
1,099 citations
TL;DR: In this article, experimental studies of the time decay of the nonequilibrium magnetization in high-temperature superconductors, a phenomenon known as magnetic relaxation, are reviewed from a purely experimental perspective and discussed in the context of present phenomenological theories.
Abstract: We review experimental studies of the time decay of the nonequilibrium magnetization in high-temperature superconductors, a phenomenon known as magnetic relaxation. This effect has its origin in motion of flux lines out of their pinning sites due to thermal activation or quantum tunneling. The combination of relatively weak flux pinning and high temperatures leads to rich properties that are unconventional in the context of low temperature superconductivity and that have been the subject to intense studies. The results are assessed from a purely experimental perspective and discussed in the context of present phenomenological theories. [S0034-6861(96)00403-5]
TL;DR: In this article, the authors observed superconductivity in the ladder material Sr 0.4 Ca 13.6 Cu 24 O 41.84 under pressures of 3 GPa and 4.5 GPa by means of electrical measurements.
Abstract: We have observed superconductivity in the ladder material Sr 0.4 Ca 13.6 Cu 24 O 41.84 under pressures of 3 GPa and 4.5 GPa by means of electrical measurements. The superconducting transition temperatures T c (onset) are 12 K and 9 K at 3 and 4.5 GPa, respectively. The superconducting volume fraction was obtained to be about 5% from magnetization measurement under 3.5 GPa at 4.2 K, indicating the bulk nature of the superconductivity in this system.
Patent•
IBM1
TL;DR: In this article, a nonvolatile magnetic random access memory (MRAM) is proposed, where each memory cell is a magnetic tunnel junction (MTJ) element and a diode electrically connected in series.
Abstract: A nonvolatile magnetic random access memory (MRAM) is an array of individual magnetic memory cells. Each memory cell is a magnetic tunnel junction (MTJ) element and a diode electrically connected in series. Each MTJ is formed of a pinned ferromagnetic layer whose magnetization direction is prevented from rotating, a free ferromagnetic layer whose magnetization direction is free to rotate between states of parallel and antiparallel to the fixed magnetization of the pinned ferromagnetic layer, and an insulating tunnel barrier between and in contact with the two ferromagnetic layers. Each memory cell has a high resistance that is achieved in a very small surface area by controlling the thickness, and thus the electrical barrier height, of the tunnel barrier layer. The memory cells in the array are controlled by only two lines, and the write currents to change the magnetic state of an MTJ, by use of the write currents' inherent magnetic fields to rotate the magnetization of the free layer, do not pass through the tunnel barrier layer. All MTJ elements, diodes, and contacts are vertically arranged at the intersection regions of the two lines and between the two lines to minimize the total MRAM surface area. The power expended to read or sense the memory cell's magnetic state is reduced by the high resistance of the MTJ and by directing the sensing current through a single memory cell.
TL;DR: All members of the M(II) = Mn series have magnetic susceptibilities showing broad maxima at 55 K characteristic of two-dimensional antiferromagnetism, but the magnetization of several of the salts increases sharply below 27 K due to the onset of spin canting.
Abstract: The synthesis and structural and magnetic characterization of 16 compounds AMIIFeIII(C2O4)3 (A = N(n-C3H7)4, N(n-C4H9)4, N(n-C5H11)4, P(n-C4H9)4, P(C6H5)4, N(n-C4H9)3(C6H5CH2), (C6H5)3PNP(C6H5)3, As(C6H5)4; MII = Mn, Fe) are reported. X-ray powder diffraction profiles are indexed in R3c or its subgroup P6522 or P6/mmm to derive unit cell constants. The structures of all the compounds consist of two-dimensional honeycomb networks [MIIFeIII(C2O4)3-]∞. The MII = Fe compounds behave as ferrimagnets with Tc between 33 and 48 K, but five exhibit a crossover from positive to negative magnetization near 30 K when cooled in a field of 10 mT. The compounds exhibiting this unusual magnetic behavior are those that have the highest Tc. Within the set N(n-CnH2n+1)4FeIIFeIII(C2O4)3 (n = 3−5), Tc increases with interlayer separation and the low-temperature magnetization changes from positive (n = 3) to negative (n = 4, 5). In the M = MnII compounds, the in-plane cell parameter a0 is ∼0.03 A greater than in the correspond...
TL;DR: The occurrence of giant magnetoresistance is linked to the presence of an optimal proportion of $Mn^{4+}$ ions and is found in the rhombohedral and the cubic structures where the Mn-O distance is less than 1.97 \AA.
Abstract: Results of a detailed investigation of the structure and electron-transport properties of $La_{1-x}A_xMnO_3$ (A =Ca, Sr) over a wide range of compositions are presented along with those of parent $LaMnO_3$ containing different percentages of $Mn^{4+}$. The electrical resistivity $(\rho)$ and magnetoresistance (MR) of polycrystalline pellets have been measured in the 4.2–400 K range in magnetic fields up to 6 T and the Seebeck coefficient (S) from 100 to 400 K. The electrical measurements were supplemented by ac susceptibility and magnetization measurements. MR is large and negative over a substantial range of compositions and peaks around temperatures close to the ferromagnetic transition temperatures $(T_c)$. An insulator to metal-like transition occurs near the $T_c$ and the temperature dependence of $\rho$ below $T_c$ is related to the magnetization although $\rho$ in the metallic state is generally much larger than the Mott’s maximum metallic resistivity. The occurrence of giant magnetoresistance is linked to the presence of an optimal proportion of $Mn^{4+}$ ions and is found in the rhombohedral and the cubic structures where the Mn-O distance is less than 1.97 \AA and the Mn-O-Mn angle is $170^o\pm10^o$. The field dependence of MR shows the presence of two distinct regimes. The thermopower S shows a positive peak in the composition range at a temperature where MR also peaks; S becomes more negative with increase in $Mn^{4+}$.
TL;DR: In this article, the evolution of magnetic order from the microscopic to the macroscopic regime may be studied with the use of nanometer-scale clusters and a variety of new techniques can be employed to control the size of the magnetic clusters from the atomic level.
Abstract: The evolution of magnetic order from the microscopic to the macroscopic regime may be studied with the use of nanometer-scale clusters. A variety of new techniques can be employed to control the size of the magnetic clusters from the atomic level. Molecular beams are used to construct and measure the magnetic properties of isolated metallic clusters. Superparamagnetic metallic particles embedded in a metal exhibit dramatic field-dependent changes in electrical conduction, providing a measure of spin-dependent scattering. Related efforts in semiconductor hosts with the use of ion implantation have generated room-temperature ferromagnetic clusters that can be directly imaged by magnetic force microscopy.
TL;DR: Based on a set of semiclassical equations for electrons in magnetic Bloch bands, the pattern of band splitting, the distribution of Hall conductivities, and the positions of energy subbands in the Hofstadter spectrum can be understood in a simple and unified picture.
Abstract: We have derived a set of semiclassical equations for electrons in magnetic Bloch bands. The velocity and energy of magnetic Bloch electrons are found to be modified by the Berry phase and magnetization. This semiclassical approach is used to study general electron transport in a dc or ac electric field. We also find a close connection between the cyclotron orbits in magnetic Bloch bands and the energy subbands in the Hofstadter spectrum. Based on this formalism, the pattern of band splitting, the distribution of Hall conductivities, and the positions of energy subbands in the Hofstadter spectrum can be understood in a simple and unified picture. \textcopyright{} 1996 The American Physical Society.
TL;DR: In this paper, the magnetoresistance peak occurs around the Curie point, whereas for x = 0.5 the onset of magnetoreduction is somewhat below and increases monotonically as.
Abstract: Manganites of the series , with x = 0, 0.1, 0.3, 0.5, 0.7 and 1.0, have been characterized in ceramic form and thin films have been prepared by pulsed laser deposition. Characterization techniques included x-ray diffraction, conductivity and magnetoresistance, magnetization and susceptibility, optical spectroscopy and the Faraday effect. Both the films and ceramics exhibit a maximum low-temperature conductivity at which is coexistent with ferromagnetic order. The negative magnetoresistance effect is qualitatively different for the x = 0.3 and x = 0.5 compositions. For x = 0.3 the magnetoresistance peak occurs around the Curie point, whereas for x = 0.5 the onset of magnetoresistance is somewhat below and increases monotonically as . The applied field appears to modify the magnetic order (on the scale of the spin diffusion length) down to the lowest temperatures for x = 0.5, but for x = 0.3 the ferromagnetic order is essentially complete and collinear below the Curie point.
TL;DR: In this article, alternating current (ac) magnetic susceptibility data are presented for six distorted cubane complexes of the composition [MnIVMnIII3O3X] each of these complexes has a well isolated S = 9/2 ground state where D, the axial ZFS parameter, is found to be in the range of −027 to −038 cm-1.
Abstract: Alternating current (ac) magnetic susceptibility data are presented for six distorted cubane complexes of the composition [MnIVMnIII3O3X] Each of these complexes has a well isolated S = 9/2 ground state There is zero-field splitting (ZFS) in the ground states where D, the axial ZFS parameter, is found to be in the range of −027 to −038 cm-1 As a result of the big spin ground state and appreciable magnetic anisotropy, an out-of-phase ac magnetic susceptibility signal is seen for each of the six Mn4 complexes The out-of-phase ac susceptibility signal reflects slow magnetization relaxation which is taken to indicate that individual molecules are acting as magnets Alternating current susceptibility data are presented for a frozen glass of one of the Mn4 complexes to confirm that the out-of-phase ac signal is associated with isolated molecules The factors that influence whether a given complex can function as a single-molecule magnet are described The above Mn4 complexes represent only the second type
TL;DR: It is shown that the resistivity decreases with increasing applied magnetic field because the spin-dependent tunneling increases as the relative orientation of the magnetization between grains becomes parallel.
Abstract: A mechanism for the large magnetoresistance observed recently in Co-Al-O granular magnetic films is presented. It is shown that the resistivity decreases with increasing applied magnetic field because the spin-dependent tunneling increases as the relative orientation of the magnetization between grains becomes parallel. With this mechanism we are able to account for the dependence of the magnetoresistance on the magnetization and temperature.
TL;DR: In this article, the Curie temperature demarcating the transition to the ferromagnetic state increases markedly with pressure until the magnetic order abruptly disappears at 6.8 kbar, indicating that magnetic coupling is very sensitive to intermolecular separation.
Abstract: Molecular solids that exhibit ferromagnetism are rare, and thus there is considerable interest in understanding the magnetic coupling mechanisms that operate in the few known examples(1). One such material is the charge-transfer salt NH4Ni(mnt)(2) . H2O, which consists of stacked planar metal ligands separated by ammonium cations. This salt is an insulator with localized spins that exhibit long-range ferromagnetic order at low temperatures (below 4.5 K)(2).3 Here we show that the Curie temperature demarcating the transition to the ferromagnetic state increases markedly with pressure until ferromagnetic order abruptly disappears at 6.8 kbar, indicating that the magnetic coupling is very sensitive to intermolecular separation. Using quantum-chemical calculations(3), we show that this pressure dependence arises from a competition between ferromagnetic coupling (resulting from nickel-sulphur intermolecular spin interactions), and antiferromagnetic coupling (from nickel-nickel interactions). We suggest that a similar interplay of spin-polarization effects might play a key role in determining the nature of the ground states (metallic, superconducting and so forth) observed in other molecular materials of this structural type(4,5).
TL;DR: In this article, the main magnetic characteristics regarding the domain structure and magnetization processes (axial, circular and Matteucci and inverse Wiedemann effects) of amorphous wires and glass-coated microwires are analyzed.
Abstract: The main magnetic characteristics regarding the domain structure and magnetization processes (axial, circular and Matteucci and inverse Wiedemann effects) of amorphous wires and glass-coated microwires are analysed. Magnetic bistability, spontaneously observed in samples with large enough ratio magneto-elastic anisotropy with axial easy axis to shape anisotropy, is the main source for a number of sensor applications in pulse generators, position and field sensors, encoded security tags, rotational counters, magnetostrictive delay lines, and so on. The relevant perspectives of the novel giant magneto-impedance effect recently reported and observed in non-magnetostrictive samples are also introduced.
TL;DR: This work reports the first study of isolated nanoscale wires with diameters smaller than 100 nm, for which singledomain states could be expected, and obtained unique insight into the process of magnetization reversal by measuring histograms of the switching field as a function of the orientation of the wires in the applied field, their diameter, and the temperature.
Abstract: The mechanisms of magnetization reversal in small magnetic particles have been much discussed in the last decades and prompted intense research activities, motivated in particular by applications in magnetic recording technology [1]. However, experiments were performed, in general, on large assemblies of particles, and the dispersion of morphologies, compositions, orientations, and separations of the magnetic entities limited the interpretation of the results. Furthermore, interactions between particles were difficult to take into account. Single particle studies were possible only in few cases [2]. Recently, insights into the magnetic properties of individual and isolated particles were obtained with the help of near field magnetic force microscopy [3], electron Lorentz microscopy or holography [4], and micro-SQUID (superconducting quantum interference device) magnetometry [5]. It is now possible to make a clear link between experiments performed on nanometer-sized single objects (particles, wires, etc.) and the numerical calculations based on the Brown micromagnetic equations [6]. We report the first study of isolated nanoscale wires with diameters smaller than 100 nm, for which singledomain states could be expected. The cylindrical geometry, with its large shape anisotropy, is well suited for comparison with theory. We obtained unique insight into the process of magnetization reversal by measuring histograms of the switching field as a function of the orientation of the wires in the applied field, their diameter, and the temperature. Furthermore, we measured the probability of switching as a function of the applied field and the temperature. Our studies reveal that the magnetization reversal proceeds by a distortion of the magnetization followed by a nucleation and a propagation process. The observed behavior illustrates the fundamental importance of the study of single, isolated magnetic particles in comparing models and experiments. We developed planar microbridge dc SQUID [7], made of Nb (thickness 20 nm), which were shown to be able to detect 10 4 mB [8]. The SQUID is made of a thin (20 nm) Nb layer in order to prevent flux trapping. The experimental setup allows measurements of hysteresis loops in magnetic fields of up to 0.5 T and temperatures below 6 K, with a time resolution of 100 ms. Ni wires were produced by electrochemically filling the pores of commercially available nanoporous tracketched polycarbonate membranes of thicknesses of 6 to 10 mm [9]. The pore size was chosen in the range of 30 to 100 nm [10]. In order to place one wire on the SQUID detector, we dissolved the membrane in chloroform and put a drop on a chip of some hundreds of SQUID’s. Magnetization measurements were performed on SQUID’s with a single isolated wire. Scanning electron microscopy (SEM) (Fig. 1) was used to determine the position and morphology of the wire. The surface roughness was around 5 nm, corresponding to our SEM resolution.
TL;DR: In this paper, high-frequency EPR spectroscopy was used to monitor the relaxation time of the magnetization, which was found to follow a thermally activated behavior, as in a superparamagnet, with τ 0 = 1.9 × 10−7 s and an energy barrier of 22.2 K. The authors concluded that the origin of the anisotropy in nanosize molecular clusters is associated with the single ion contributions and not with the shape of the clusters.
Abstract: Using high-frequency EPR spectroscopy we have found that a cluster comprising eight iron(III) ions, Fe8, which is essentially flat, has a ground S = 10 state and an Ising-type anisotropy. For the first time both ac susceptibility and Mossbauer spectroscopy could be used in order to monitor the relaxation time of the magnetization, which was found to follow a thermally activated behavior, as in a superparamagnet, with τ0 = 1.9 × 10−7 s and an energy barrier of 22.2 K. The set of data allowed us to conclude that the origin of the anisotropy in nanosize molecular clusters is associated with the single ion contributions and not with the shape of the clusters.
TL;DR: The current-perpendicular-to-plane magnetoresistance (CPP-MR) has been investigated for the layered manganite, La2−2xSr1+2xMn2O7 (x = 0.3), which is composed of the ferromagnetic-metallic MnO2 bilayers separated by nonmagnetic insulating block layers.
Abstract: The current-perpendicular-to-plane magnetoresistance (CPP-MR) has been investigated for the layered manganite, La2-2xSr1+2xMn2O7 (x = 0.3), which is composed of the ferromagnetic-metallic MnO2 bilayers separated by nonmagnetic insulating block layers. The CPP-MR is extremely large (10(4) percent at 50 kilo-oersted) at temperatures near above the three-dimensional ordering temperature (Tc approximately 90 kelvin) because of the field-induced coherent motion between planes of the spin-polarized electrons. Below Tc, the interplane magnetic domain boundary on the insulating block layer serves as the charge-transport barrier, but it can be removed by a low saturation field, which gives rise to the low-field tunneling MR as large as 240 percent.
TL;DR: In this article, tunneling between ferromagnetic-insulator-ferromagnet (FM-I-FM) trilayer thin-film planar junctions has been successfully studied.
Abstract: Tunneling between ferromagnet–insulator–ferromagnet (FM–I–FM) trilayer thin‐film planar junctions has been successfully studied. Tunnel current was observed to be dependent on the relative orientation of the magnetization (M). Co, CoCr, CoFe, Fe0.7Pt0.3, and NiFe were tried as the FM electrodes with Al2O3 or MgO as the barrier layers for the above studies. Large magnetoresistance (MR) was observed as the M alignment of the two ferromagnets changed from being parallel to antiparallel orientation. At room temperature, the highest change in junction MR was 18%, field sensitivity factor reaching 5%/Oe in the best cases. The MR value increased to 25.6% at 4.2 K, and decreased as the dc bias was increased to a fraction of the barrier height. The angular dependence of MR varied nearly as the cosine of the relative angle of M, as predicted by Slonczewski’s theory. The magnitude of MR agrees well with that given by Julliere’s model, which predicts that the MR varies as the product of the conduction electron spin polarization of the FMs. These trilayer junctions can find application as high‐density, nonvolatile storage media or as field sensors.
TL;DR: The Stern-Gerlach deflection technique is used to study magnetism in nickel clusters containing 5 to 740 atoms, and magnetization minima for clusters with closed geometrical shells and maxima for relatively open clusters are observed.
Abstract: We have used the Stern-Gerlach deflection technique to study magnetism in nickel clusters containing 5 to 740 atoms. These superparamagnetic particles are highly magnetic, with magnetic moments per atom that decrease toward the bulk value as their size increases. This approach toward the bulk is not monotonic; we observe magnetization minima for clusters with closed geometrical shells and maxima for relatively open clusters. This result is consistent with enhanced magnetization of the cluster surface. We also observe a decrease in magnetization with increasing cluster vibrational temperature. The implications of various cluster geometries are discussed in conjunction with existing chemical probe studies and theoretical calculations.
TL;DR: High-resolution magnetic force microscopy studies of magnetic submicrometer-sized cobalt dots with geometrical dimensions comparable to the width of magnetic domains reveal a variety of intricate domain patterns controlled by the details of the dot geometry.
Abstract: The basic magnetic properties of three-dimensional nanostructured materials can be drastically different from those of a continuous film. High-resolution magnetic force microscopy studies of magnetic submicrometer-sized cobalt dots with geometrical dimensions comparable to the width of magnetic domains reveal a variety of intricate domain patterns controlled by the details of the dot geometry. By changing the thickness of the dots, the width of the geometrically constrained magnetic domains can be tuned. Concentric rings and spirals with vortex configurations have been stabilized, with particular incidence in the magnetization reversal process as observed in the ensemble-averaged hysteresis loops.
TL;DR: The vortex-lattice phase transitions in Bi 2Sr2CaCu2O8 crystals with various oxygen stoichiometry are studied using local magnetization measurements and a very abrupt upturn in the bulk critical current occurs at the onset of the second peak in BSCCO.
Abstract: The vortex-lattice phase transitions in Bi 2Sr2CaCu2O8 crystals with various oxygen stoichiometry are studied using local magnetization measurements. Three new findings are reported: The first-order phase transition line at elevated temperatures shifts upward for more isotropic overdoped samples. At lower temperatures another sharp transition is observed that results in enhanced bulk pinning in the second magnetization peak region. The two lines merge at a multicritical point at intermediate temperatures forming apparently a continuous phase transition line that is anisotropy dependent. PACS numbers: 74.60.Ec, 74.60.Ge, 74.60.Jg, 74.72.Hs The mixed state of high-temperature superconductors (HTSC) has a very complicated phase diagram. The nature of the different vortex phases and the thermodynamic transitions between them are of fundamental interest, and are the subject of substantial recent theoretical and experimental efforts [1 ‐ 10]. It is generally accepted that the high anisotropy plays a crucial role in the richness of the phase diagram of HTSC. Nevertheless, a systematic study of the anisotropy effects is quite complicated and was limited so far by the lack of a well defined phase boundary that could be monitored as a function of the anisotropy. A recent advance in local measurements has revealed a sharp step in magnetization due to a first-order vortex-lattice phase transition in Bi2Sr2CaCu2O8 (BSCCO) crystals [10]. Such a clearly defined fundamental transition is thus a natural candidate for an investigation of the anisotropy effects in HTSC, and this paper presents a first study in this direction. Another intriguing feature of the phase diagram of many HTSC crystals, and BSCCO in particular, is the anomalous second magnetization peak at lower temperatures. The associated increase of magnetization with magnetic field has been attributed to surface barrier effects [11], crossover from surface barrier to bulk pinning [12], sample inhomogeneities [13], dynamic effects [14], and 3D to 2D transitions [15‐ 17]. Our local measurements indicate that a very abrupt upturn in the bulk critical current occurs at the onset of the second peak in BSCCO. We postulate that this behavior is triggered by an underlying thermodynamic phase transition of the flux-line lattice. Furthermore, for the different anisotropy crystals the two phase transition lines are found to form, apparently, one continuous transition line that changes from first to possibly second order at