Showing papers on "Magnetization published in 2006"

Magnetic-field-induced shape recovery by reverse phase transformation.

Abstract: Large magnetic-field-induced strains1 have been observed in Heusler alloys with a body-centred cubic ordered structure and have been explained by the rearrangement of martensite structural variants due to an external magnetic field1,2,3. These materials have attracted considerable attention as potential magnetic actuator materials. Here we report the magnetic-field-induced shape recovery of a compressively deformed NiCoMnIn alloy. Stresses of over 100 MPa are generated in the material on the application of a magnetic field of 70 kOe; such stress levels are approximately 50 times larger than that generated in a previous ferromagnetic shape-memory alloy4. We observed 3 per cent deformation and almost full recovery of the original shape of the alloy. We attribute this deformation behaviour to a reverse transformation from the antiferromagnetic (or paramagnetic) martensitic to the ferromagnetic parent phase at 298 K in the Ni45Co5Mn36.7In13.3 single crystal.

Spontaneous skyrmion ground states in magnetic metals

Abstract: Since the 1950s, Heisenberg and others have addressed the problem of how to explain the appearance of countable particles in continuous fields. Stable localized field configurations were searched for an ingredient for a general field theory of elementary particles, but the majority of nonlinear field models were unable to predict them. As an exception, Skyrme succeeded in describing nuclear particles as localized states, so-called 'skyrmions'. Skyrmions are a characteristic of nonlinear continuum models ranging from microscopic to cosmological scales. Skyrmionic states have been found under non-equilibrium conditions, or when stabilized by external fields or the proliferation of topological defects. Examples are Turing patterns in classical liquids, spin textures in quantum Hall magnets, or the blue phases in liquid crystals. However, it has generally been assumed that skyrmions cannot form spontaneous ground states, such as ferromagnetic or antiferromagnetic order, in magnetic materials. Here, we show theoretically that this assumption is wrong and that skyrmion textures may form spontaneously in condensed-matter systems with chiral interactions without the assistance of external fields or the proliferation of defects. We show this within a phenomenological continuum model based on a few material-specific parameters that can be determined experimentally. Our model has a condition not considered before: we allow for softened amplitude variations of the magnetization, characteristic of, for instance, metallic magnets. Our model implies that spontaneous skyrmion lattice ground states may exist generally in a large number of materials, notably at surfaces and in thin films, as well as in bulk compounds, where a lack of space inversion symmetry leads to chiral interactions.

Current-induced magnetization reversal in nanopillars with perpendicular anisotropy

Abstract: 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.

Ferroelectricity in spiral magnets

Abstract: It was recently observed that the ferroelectrics showing the strongest sensitivity to an applied magnetic field are spiral magnets. We present a phenomenological theory of inhomogeneous ferroelectric magnets, which describes their thermodynamics and magnetic field behavior, e.g., dielectric susceptibility anomalies at magnetic transitions and sudden flops of electric polarization in an applied magnetic field. We show that electric polarization can also be induced at domain walls and that magnetic vortices carry electric charge.

Magnetic vortex core reversal by excitation with short bursts of an alternating field

Abstract: The vortex state, characterized by a curling magnetization, is one of the equilibrium configurations of soft magnetic materials(1-4) and occurs in thin ferromagnetic square and disk-shaped elements of micrometre size and below. The interplay between the magneto-static and the exchange energy favours an in-plane, closed flux domain structure. This curling magnetization turns out of the plane at the centre of the vortex structure, in an area with a radius of about 10 nanometres-the vortex core(5-7). The vortex state has a specific excitation mode: the in-plane gyration of the vortex structure about its equilibrium position(8-10). The sense of gyration is determined by the vortex core polarization(11). Here we report on the controlled manipulation of the vortex core polarization by excitation with small bursts of an alternating magnetic field. The vortex motion was imaged by time-resolved scanning transmission X-ray microscopy(12). We demonstrate that the sense of gyration of the vortex structure can be reversed by applying short bursts of the sinusoidal excitation field with amplitude of about 1.5 mT. This reversal unambiguously indicates a switching of the out-of-plane core polarization. The observed switching mechanism, which can be understood in the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application in data storage.

Artificial ‘spin ice’ in a geometrically frustrated lattice of nanoscale ferromagnetic islands

Abstract: When a number of interactions compete within a system they can't all prevail, so the resolution of ‘frustrated’ forces is an important determinant of the overall behaviour of a system. In particular, geometrical frustration among spins in magnetic systems can lead to exotic effects such as ‘spin ice’, a state where atomic magnetic moments mimic the frustration of hydrogen ion positions in water ice. Wang et al. have created artificial spin ice using lithographically fabricated arrays of nanoscale magnets. Magnetic moments in the lattice follow the two [pointing]-in/ two-out ‘ice rule’ typical of spin ice. With this model it is possible to study frustration in great detail; this is relevant to magnetic recording, where ferromagnetic elements are being pushed to ever higher densities. On the cover, a magnetic force microscopy representation of the magnetization pattern of artificial spin ice: plateaus and valleys show regions of opposite magnetization. Frustration, defined as a competition between interactions such that not all of them can be satisfied, is important in systems ranging from neural networks to structural glasses. Geometrical frustration, which arises from the topology of a well-ordered structure rather than from disorder, has recently become a topic of considerable interest1. In particular, geometrical frustration among spins in magnetic materials can lead to exotic low-temperature states2, including ‘spin ice’, in which the local moments mimic the frustration of hydrogen ion positions in frozen water3,4,5,6. Here we report an artificial geometrically frustrated magnet based on an array of lithographically fabricated single-domain ferromagnetic islands. The islands are arranged such that the dipole interactions create a two-dimensional analogue to spin ice. Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration. We see both ice-like short-range correlations and an absence of long-range correlations, behaviour which is strikingly similar to the low-temperature state of spin ice. These results demonstrate that artificial frustrated magnets can provide an uncharted arena in which the physics of frustration can be directly visualized.

Predicted Magnetoelectric Effect in Fe / BaTiO 3 Multilayers: Ferroelectric Control of Magnetism

Abstract: An unexplored physical mechanism which produces a magnetoelectric effect in ferroelectric-ferromagnetic multilayers is studied based on first-principles calculations. Its origin is a change in bonding at the ferroelectric-ferromagnet interface that alters the interface magnetization when the electric polarization reverses. Using Fe/BaTiO3 multilayers as a representative model, we show a sizable difference in magnetic moments of Fe and Ti atoms at the two interfaces dissimilar by the orientation of the local electric dipole moments. The predicted magnetoelectric effect opens a new direction to control magnetic properties of thin-film layered structures by electric fields.

Synthesis of hematite (alpha-Fe2O3) nanorods: diameter-size and shape effects on their applications in magnetism, lithium ion battery, and gas sensors.

Abstract: We demonstrated in this paper the shape-controlled synthesis of hematite (alpha-Fe(2)O(3)) nanostructures with a gradient in the diameters (from less than 20 nm to larger than 300 nm) and surface areas (from 5.9 to 52.3 m(2)/g) through an improved synthetic strategy by adopting a high concentration of inorganic salts and high temperature in the synthesis systems to influence the final products of hematite nanostructures. The benefits of the present work also stem from the first report on the <20-nm-diameter and porous hematite nanorods, as well as a new facile strategy to the less-than-20-nm nanorods, because the less-than-20-nm diameter size meets the vital size domain for magnetization properties in hematite. Note that the porous and nonporous hematite one-dimensional nanostructures with diameter gradients give us the first opportunity to investigate the Morin temperature evolution of nanorod diameter and porosity. Evidently, the magnetic properties for nanorods exhibit differences compared with those for the spherical particle counterparts. Hematite nanorods are strongly dependent on their diameter size and porosity, where the magnetization is not sensitive to the size evolution from submicron particles to the 60-90 nm nanorods, while the magnetic properties change significantly in the case of <20 nm. In other words, for the magnetic properties of nanorods, in a comparable size range, the porous existence could also influence the magnetic behavior. Moreover, applications in formaldehyde (HCHO) gas sensors and lithium batteries for the hematite nanostructures with the diameter/surface area gradient reveal that the performance of electrochemical and gas-sensor properties strongly depends on the diameter size and Brunauer-Emmett-Teller (BET) surface areas, which is consistent with the crystalline point of view. Thus, this work not only provides the first example of the fabrication of hematite nanostructure sensors for detecting HCHO gas, but also reveals that the surface area or diameter size of hematite nanorods can also influence the lithium intercalation performances. These results give us a guideline for the study of the size-dependent properties for functional materials as well as further applications for magnetic materials, lithium-ion batteries, and gas sensors.

Magnetic Reversal of the Ferroelectric Polarization in a Multiferroic Spinel Oxide

Abstract: Ferroelectric transition has been detected in a ferrimagnetic spinel oxide of CoCr2O4 upon the transition to the conical spin order below 25 K. The direction [110] of the spontaneous polarization is normal to both the magnetization easy axis [001] and to the propagation axis [110] of the transverse spiral component, in accord with the prediction based on the spin-current model. The reversal of the spontaneous magnetization by a small magnetic field (approximately 0.1 T) induces the reversal of the spontaneous polarization, indicating the clamping of the ferromagnetic and ferroelectric domain walls.

A Family of Rare-Earth-Based Single Chain Magnets: Playing with Anisotropy

Abstract: The first family of rare-earth-based single chain magnets is presented. Compounds of general formula [M(hfac)3(NITPhOPh)], where M = Eu, Gd, Tb, Dy, Ho, Er, or Yb, and PhOPh is the nitronyl-nitroxide radical (2,4'-benzoxo-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), have been structurally characterized and found to be isostructural. The characterization of both static and dynamic magnetic properties of the whole family is reported. Dy, Tb, and Ho compounds display slow relaxation of the magnetization, and ac susceptibility shows a thermally activated regime with energy barriers of 69, 45, and 34 K for Dy, Tb, and Ho compounds, respectively, while only a frequency-dependent susceptibility is observed for Er below 2.0 K. In Gd and Yb derivatives, antiferromagnetic interactions dominate. The pre-exponential factors differ by about 4 orders of magnitude. Finite size effects, due to naturally occurring defects, affect the static and dynamic properties of the compounds differently.

Evidence of oxygen vacancy enhanced room-temperature ferromagnetism in Co-doped ZnO

Abstract: The annealing effects on structure and magnetism for Co-doped ZnO films under air, Ar, and Ar∕H2 atmospheres at 250°C have been systematically investigated. Room-temperature ferromagnetism has been observed for the as-deposited and annealed films. However, the saturation magnetization (Ms) varied drastically for different annealing processes with Ms∼0.5, 0.2, 0.9, and 1.5μB∕Co for the as-deposited, air-annealed, Ar-annealed, and Ar∕H2-annealed films, respectively. The x-ray absorption spectra indicate all these samples show good diluted magnetic semiconductor structures. By comparison of the x-ray near edge spectra with the simulation on Zn K edge, an additional preedge peak appears due likely to the formation of oxygen vacancies. The results show that enhancement (suppression) of ferromagnetism is strongly correlated with the increase (decrease) of oxygen vacancies in ZnO. The upper limit of the oxygen vacancy density of the Ar∕H2-annealed film can be estimated by simulation to be about 1×1021cm−3.

Substitution-induced phase transition and enhanced multiferroic properties of Bi1−xLaxFeO3 ceramics

Abstract: Single-phase, insulating Bi1−xLaxFeO3 (BLFOx, x=0.05, 0.10, 0.15, 0.20, 0.30, and 0.40) ceramics were prepared. An obvious phase transition from rhombohedral to orthorhombic phase was observed near x=0.30. It is found that the phase transition destructs the spin cycloid of BiFeO3 (BFO), and therefore, releases the locked magnetization and enhances magnetoelectric interaction. As a result, improved multiferroic properties of the BLFO0.30 ceramics with remnant polarization and magnetization (2Pr and 2Mr) of 22.4μC∕cm2 and 0.041emu∕g, respectively, were established.

Magnetism at the interface between ferromagnetic and superconducting oxides

Abstract: Carefully controlled interfaces between two materials can give rise to novel physical phenomena and functionalities not exhibited by either of the constituent materials alone. Modern synthesis methods have yielded high-quality heterostructures of oxide materials with competing order parameters. Although magnetic correlations at the interface are expected to be important in determining the macroscopic properties of such systems, a quantitative determination of the interfacial magnetization profile has thus far not been reported. Here we examine superlattices composed of the half-metallic ferromagnet La2/3Ca1/3MnO3 and the high-temperature superconductor YBa2Cu3O7 by absorption spectroscopy with circularly polarized X-rays and by off-specular neutron reflectometry. The resulting data yield microscopic insight into the interplay of spin and orbital degrees of freedom at the interface. The experiments also reveal an extensive rearrangement of the magnetic domain structure at the superconducting transition temperature. This methodology establishes an incisive probe of the interplay between competing electronic order parameters in oxide heterostructures.

Structural and magnetic properties of uniform magnetite nanoparticles prepared by high temperature decomposition of organic precursors

Abstract: Magnetite nanoparticles (Fe3O4) of three different sizes below the limit for single domain magnetic behaviour have been obtained by thermal decomposition of an iron precursor in an organic medium in the presence of a surfactant. Good agreement between mean particle size obtained by TEM, crystal size calculated from x-ray diffraction and magnetic diameter calculated from magnetization curves measured at room temperature shows that the samples consist of uniform, crystalline and isolated magnetite nanoparticles with sizes between 5 and 11 nm. High saturation magnetization and high initial susceptibility values have been found, the latter decreasing as the particle size decreases. The main contribution to the anisotropy is magnetocrystalline and shape anisotropy, since surface anisotropy is suppressed by the oleic acid molecules which are covalently bonded to the nanoparticle surface.

Oximate-Bridged Trinuclear Dy−Cu−Dy Complex Behaving as a Single-Molecule Magnet and Its Mechanistic Investigation

Abstract: Lanthanide ions are supposed to be promising candidates for the elements of single-molecule magnets (SMMs) because of the large magnetic momentum and anisotropy. We have established the [Dy2Cu] complex as a new SMM. A plausible mechanism for quantum tunneling of magnetization is proposed for the first time among the 4f−3d heterometallic SMMs. The magnetic coupling parameter between Dy and Cu ions was well-defined as −0.155 K.

Ferroelectric Polarization Flop in a Frustrated Magnet MnWO~4 Induced by a Magnetic Field

Abstract: The relationship between magnetic order and ferroelectric properties has been investigated for MnWO4 with a long-wavelength magnetic structure. Spontaneous electric polarization is observed in an elliptical spiral spin phase. The magnetic-field dependence of electric polarization indicates that the noncollinear spin configuration plays a key role for the appearance of the ferroelectric phase. An electric polarization flop from the b direction to the a direction has been observed when a magnetic field above 10 T is applied along the b axis. This result demonstrates that an electric polarization flop can be induced by a magnetic field in a simple system without rare-earth 4f moments.

Size‐Dependent Chemical and Magnetic Ordering in L10‐FePt Nanoparticles

Abstract: FePt nanoparticles have great application potential in advanced magnetic materials such as ultrahigh-density recording media and high-performance permanent magnets. The key for applications is the very high uniaxial magnetocrystalline anisotropy of the L10-FePt phase, which is based on crystalline ordering of the face-centered tetragonal (fct) structure, described by the chemical-ordering parameter S. Higher chemical ordering results in higher magnetocrystalline anisotropy. Unfortunately, as-synthesized FePt nanoparticles take a disordered face-centered cubic (fcc) structure that has low magnetocrystalline anisotropy. Heat-treatment is necessary to convert the fcc structure to the ordered fct structure. Several previous theoretical and experimental investigations have been reported on the size-dependent chemical ordering of FePt nanoparticles. It has been observed that the degree of ordering decreases with decreasing particle size of the sputtered FePt nanoparticles. Theoretical simulation predicted that the ordering would not take place when the particle size is below a critical value. However, there have not been systematic experimental studies on quantitative size dependence of chemical ordering of FePt nanoparticles due to the lack of monodisperse L10-FePt nanoparticles with controllable sizes. There are also few studies reported to date on the quantitative particle size dependence of magnetic properties, including the Curie temperature, coercivity, and magnetization of the L10-FePt phase, although it has been well accepted that there is a size effect on the ferromagnetism of any low-dimensional magnets. Additionally, the magnetic properties of FePt ferromagnets, as observed in thin-film samples, are affected by the degree of chemical ordering, which is in turn size dependent. It is therefore highly desirable to understand the size and chemical-ordering effects, and their influence on the magnetic properties of the nanoparticles. A major hurdle in obtaining the particle size dependence of structural and magnetic properties of the L10 phase is particle sintering during heat-treatments that convert the fcc phase to the fct phase. This long-pending problem has been solved recently by adopting the salt-matrix annealing technique. With this technique, particle aggregation during the phase transformation has been avoided so that the true size-dependent properties of the fct phase can be measured. In this paper, we report results on quantitative particle size dependence of the chemical-ordering parameter S and selected magnetic properties, including the Curie temperature, Tc, magnetization, Ms, and coercivity, Hc, with the particle size varying from 2 to 15 nm. Figure 1 shows the transmission electron microscopy (TEM) images of the FePt nanoparticles with different sizes before and after annealing in a salt matrix at 973 K for 4 h. The images, from left to right, show nanoparticles with nominal diameters of 2, 4, 6, 8, and 15 nm, respectively. The upper and lower rows are images of as-synthesized and salt-matrixannealed nanoparticles, respectively. As shown in Figure 1, the particle size is retained well upon annealing. Both the assynthesized and annealed nanoparticles are monodisperse with a standard deviation of 5–10 % in diameter. TEM observations also revealed that when the particle size is smaller than or equal to 8 nm, the fct nanoparticles are monocrystalline, whereas the 15 nm fct particles are polycrystalline. It is interesting to see that the L10 nanoparticles, tiny ferromagnets at room temperature, are dispersed very well without agglomeration despite the dipolar interaction between the particles, if a solvent with high viscosity is chosen and if the solution is diluted. Extensive TEM and X-ray diffraction (XRD) analyses have proved that the technique of salt-matrix annealing can be applied to heat-treatments of the FePt nanoparticles without leading to particle agglomeration and sintering, if a suitable salt-to-particle ratio and proper annealing conditions are chosen. Figure 2 shows the XRD patterns of the 4 nm, as-synthesized, fcc-structured nanoparticles and the particles annealed in a salt matrix at 873 K for 2 h, 973 K for 2 h, and 973 K for 4 h (from bottom to top), respectively. As shown in the figure, the positions of the (111) peaks shift in the higher-anC O M M U N IC A TI O N

Spin transfer in nanomagnetic devices with perpendicular anisotropy

Abstract: Spin momentum transfer in a nanomagnetic device with perpendicular magnetic anisotropy for both free and fixed magnetic layers is studied. The perpendicular anisotropy is induced by using CoFe∕Pt multilayer. The magnetoresistive loop shows that the perpendicular switching fields for the free and fixed layers are 170 and 380Oe, respectively, with ΔR∕R=0.47%. Resistance-current scanning clearly shows a full out-of-plane switching of the free layer magnetization under a sweeping current, which fully excludes the effect of switching by the magnetic field generated by the current. The critical current density is around 1.0×108A∕cm2, which could be tuned by changing the CoFe∕Pt multilayer structures.

Magnetic and electric phase control in epitaxial EuTiO(3) from first principles.

Abstract: We propose a design strategy---based on the coupling of spins, optical phonons, and strain---for systems in which magnetic (electric) phase control can be achieved by an applied electric (magnetic) field. Using first-principles density-functional theory calculations, we present a realization of this strategy for the magnetic perovskite ${\mathrm{EuTiO}}_{3}$.

Ultrafast spin dynamics across compensation points in ferrimagnetic GdFeCo: The role of angular momentum compensation

Abstract: Using an all-optical pump and probe technique, we have investigated the temperature dependence of the ultrafast magnetic response in a ferrimagnetic amorphous $\mathrm{GdFeCo}$ thin film. When the temperature of the sample approaches the angular momentum compensation point, both frequency and the Gilbert damping parameter of the magnetization precession increase significantly. In addition, the high-frequency exchange mode softens and becomes observable. The observed high-speed and strongly damped spin dynamics in the vicinity of the compensation of the angular momentum is ideal for ultrafast ringing-free precessional switching in magnetic and magneto-optical recording.

Multiferroicity in polarized single-phase Bi0.875Sm0.125FeO3 ceramics

Abstract: Single-phase Bi0.875Sm0.125FeO3 ceramics were prepared, and their crystal structure and multiferroic properties were studied. The triclinic structure with P1 space group was confirmed in the ceramics by refining the x-ray diffraction data. Large piezoelectric d33 coefficient of 29pC∕N, together with high remnant polarization of 15.09μC∕cm2, was measured at room temperature, suggesting the existence of long-range ferroelectric order on a macroscopic scale. The observed small remnant magnetization of 0.071emu∕g at room temperature as a result of the collapse of the space-modulated spin structure indicated the presence of long-range canted antiferromagnetic order on a macroscopic scale. The coexistence of the long-range ferroelectric and canted antiferromagnetic orders allowed the magnetoelectric effect below the antiferromagnetic Neel temperature of 265°C near which magnetoelectric coupling was obvious.

Properties of the quaternary half-metal-type Heusler alloy Co2Mn1-xFexSi

Abstract: This paper reports on the bulk properties of the quaternary Heusler alloy Co2Mn1�xFexSi with the Fe concentration x =0,1/2,1. All samples, which were prepared by arc melting, exhibit L21 long-range order over the complete range of Fe concentration. The structural and magnetic properties of the Co2Mn1�xFexSi Heusler alloys were investigated by means of x-ray diffraction, high- and low-temperature magnetometry, Mossbauer spectroscopy, and differential scanning calorimetry. The electronic structure was explored by means of highenergy photoemission spectroscopy at about 8 keV photon energy. This ensures true bulk sensitivity of the measurements. The magnetization of the Fe-doped Heusler alloys is in agreement with the values of the magnetic moments expected for a Slater-Pauling-like behavior of half-metallic ferromagnets. The experimental findings are discussed on the basis of self-consistent calculations of the electronic and magnetic structure. To achieve good agreement with experiment, the calculations indicate that on-site electron-electron correlation must be taken into account, even at low Fe concentration. The present investigation focuses on searching for the quaternary compound where the half-metallic behavior is stable against outside influences. Overall, the results suggest that the best candidate may be found at an iron concentration of about 50%.

Electrical detection of spin pumping due to the precessing magnetization of a single ferromagnet

Abstract: We report direct electrical detection of spin pumping, using a lateral normal-metal/ferromagnet/normal-metal device, where a single ferromagnet in ferromagnetic resonance pumps spin-polarized electrons into the normal metal, resulting in spin accumulation. The resulting backflow of spin current into the ferromagnet generates a dc voltage due to the spin-dependent conductivities of the ferromagnet. By comparing different contact materials (Al and/or Pt), we find, in agreement with theory, that the spin-related properties of the normal metal dictate the magnitude of the dc voltage.

A Binuclear Fe(III)Dy(III) Single Molecule Magnet. Quantum Effects and Models

Abstract: The binuclear [FeIII(bpca)(μ-bpca)Dy(NO3)4], having Single Molecule Magnet (SMM) properties, belonging to a series of isostructural FeIIILnIII complexes (Ln = Eu, Gd, Tb, Dy, Ho) and closely related FeIILnIII chain structures, was characterized in concise experimental and theoretical respects. The low temperature magnetization data showed hysteresis and tunneling. The anomalous temperature dependence of Mossbauer spectra is related to the onset of magnetic order, consistent with the magnetization relaxation time scale resulting from AC susceptibility measurements. The advanced ab initio calculations (CASSCF and spin−orbit) revealed the interplay of ligand field, spin−orbit, and exchange effects and probed the effective Ising nature of the lowest states, involved in the SMM and tunneling effects.

Spin-Transfer Torque and Dynamics

Abstract: The currents in magnetic multilayers are spin polarized and can carry enough angular momentum that they can cause magnetic reversal and induce stable precession of the magnetization in thin magnetic layers. The flow of spins is determined by the spin-dependent transport properties, like conductivity, interface resistance, and spin-flip scattering in the magnetic multilayer. When an electron spin carried by the current interacts with a magnetic layer, the exchange interaction leads to torques between the spin and the magnetization. The torque that results from this interaction excites the magnetization when the current is large enough. The qualitative features of the dynamics that result from current-induced torques are captured by a simple model in which the magnetization of the layer is assumed to be uniform. Even greater agreement results when finite temperature effects are included and the magnetization is allowed to vary throughout the film.

[Fe(bpym)(CN)4]-: a new building block for designing single-chain magnets.

Abstract: We herein present the preparation, crystal structure, magnetic properties, and theoretical study of new heterobimetallic chains of formula {[Fe III (bpym)(CN4)]2M"(H20) 2 }·6H 2 O [bpym = 2,2'-bipyrimidine; M = Zn (2), Co (3), Cu (4), and Mn (5)] which are obtained by using the building block PPh 4 [Fe(bpym)-(CN) 4 ]·H 2 O (1) (PPh 4 + = tetraphenylphosphonium) as a ligand toward the fully solvated M" ions. The structure of complex 1 contains mononuclear [Fe(bpym)(CN) 4 ] - anions. Compounds 2-5 are isostructural 4,2-ribbonlike bimetallic chains where the [Fe(bpym)(CN) 4 ] - unit acts as a bis-monodenate ligand through two of its four cyanide ligands toward the M atom. Water hexamer clusters (4) and regular alternating fused six- and four-membered water rings with two dangling water molecules (2, 3, and 5) are trapped between the cyanide-bridged 4,2-ribbonlike chains. 1 and 2 behave as magnetically isolated low-spin iron(lll) centers. 3 behaves as a single-chain magnet (SCM) with intrachain ferromagnetic coupling, slow magnetic relaxation, hysteresis effects, and frequency-dependent ac signals at T < 7 K). As expected for a thermally activated process, the nucleation field (H n ) in 3 increases with decreasing T and increasing v. Below 1.0 K, H n becomes temperature independent but remains strongly sweep rate dependent. In this temperature range, the reversal of the magnetization may be induced by a quantum nucleation of a domain wall that then propagates due to the applied field. 4 and 5 are ferro- and ferrimagnetic chains respectively, with metamagnetic-like behavior (4). DFT-type calculations and QMC methodology provided a good understanding of the magnetic properties of 3-5.

Photoinduced magnetization in copper octacyanomolybdate.

Abstract: This article describes the studies of a photomagnetic cyanide-bridged Cu−Mo bimetallic assembly, CuII2[MoIV(CN)8]·8H2O (CuII, S = 1/2; MoIV, S = 0) (1), which has an intervalence transfer (IT) band from MoIV−CN−CuII to MoV−CN−CuI around 480 nm. Wide-angle X-ray scattering and X-ray spectroscopic studies provide precise information about the 3D connectivity and the local environment of the transition metal ions. Irradiating with blue light causes solid 1 to exhibit a spontaneous magnetization (Curie temperature = 25 K). The thermal reversibility is carefully studied and shows the long-time stability of the photoinduced state up to 100 K. Photoreversibility is also observed; i.e., the magnetization is induced by irradiation with light below 520 nm, while the magnetization is reduced by irradiation with light above 520 nm. The UV−vis absorption spectrum after irradiation shows a decrease of the IT band and the appearance of the reverse-IT band in the region of 600−900 nm (λmax = 710 nm). This UV−vis absorpti...