Showing papers on "Magnetic anisotropy published in 2006"

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.

Magnetic sensors and their applications

Abstract: Magnetic sensors can be classified according to whether they measure the total magnetic field or the vector components of the magnetic field. The techniques used to produce both types of magnetic sensors encompass many aspects of physics and electronics. Here, we describe and compare most of the common technologies used for magnetic field sensing. These include search coil, fluxgate, optically pumped, nuclear precession, SQUID, Hall-effect, anisotropic magnetoresistance, giant magnetoresistance, magnetic tunnel junctions, giant magnetoimpedance, magnetostrictive/piezoelectric composites, magnetodiode, magnetotransistor, fiber optic, magnetooptic, and microelectromechanical systems-based magnetic sensors. The usage of these sensors in relation to working with or around Earth's magnetic field is also presented

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.

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.

Intrinsic distribution of magnetic anisotropy in thin films probed by patterned nanostructures.

Abstract: We demonstrate that the switching field distribution (SFD) in arrays of 50 nm to $5\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ $\mathrm{Co}/\mathrm{Pd}$ elements, with perpendicular anisotropy, can be explained by a distribution of intrinsic anisotropy rather than any fabrication related effects. Further, simulations of coercivity and SFD versus element size allow the distribution of intrinsic anisotropy to be quantified in highly exchanged coupled thin films where the reversal mechanism is one of nucleation followed by rapid domain wall motion.

Signatures of Molecular Magnetism in Single-Molecule Transport Spectroscopy

Abstract: We report single-molecule-transistor measurements on devices incorporating magnetic molecules. By studying the electron-tunneling spectrum as a function of magnetic field, we are able to identify signatures of magnetic states and their associated magnetic anisotropy. A comparison of the data to simulations also suggests that sequential electron tunneling may enhance the magnetic relaxation of the magnetic molecule.

Magnetic multilayers on nanospheres

Abstract: Thin-film technology is widely implemented in numerous applications1. Although flat substrates are commonly used, we report on the advantages of using curved surfaces as a substrate. The curvature induces a lateral film-thickness variation that allows alteration of the properties of the deposited material2,3. Based on this concept, a variety of implementations in materials science can be expected. As an example, a topographic pattern formed of spherical nanoparticles4,5 is combined with magnetic multilayer film deposition. Here we show that this combination leads to a new class of magnetic material with a unique combination of remarkable properties: The so-formed nanostructures are monodisperse, magnetically isolated, single-domain, and reveal a uniform magnetic anisotropy with an unexpected switching behaviour induced by their spherical shape. Furthermore, changing the deposition angle with respect to the particle ensemble allows tailoring of the orientation of the magnetic anisotropy, which results in tilted nanostructure material.

Signatures of Molecular Magnetism in Single-Molecule Transport Spectroscopy

Abstract: Single-molecule transistors provide a unique experimental tool to investigate the coupling between charge transport and the molecular degrees of freedom in individual molecules. One interesting class of molecules for such experiments are the single-molecule magnets, since the intramolecular exchange forces present in these molecules should couple strongly to the spin of transport electrons, thereby providing both new mechanisms for modulating electron flow and also new means for probing nanoscale magnetic excitations. Here we report single-molecule transistor measurements on devices incorporating Mn12 molecules. By studying the electron-tunneling spectrum as a function of magnetic field, we are able to identify clear signatures of magnetic states and their associated magnetic anisotropy. A comparison of the data to simulations also suggests that electron flow can strongly enhance magnetic relaxation of the magnetic molecule.

Chiral Magnetic Metal-Organic Frameworks of Dimetal Subunits: Magnetism Tuning by Mixed-Metal Compositions of the Solid Solutions

Abstract: The isostructural, chiral molecular magnetic materials with the formula [MxM'(2-x)(ca)2(1,4-dimb)]n [H2ca = D-(+)-camphoric acid, 1,4-dimb = 1,4-di-(1-imidazolyl-methyl)-benzene, M = Ni(II), M' = CoII, 0 < or = x < or = 2] consist of ca-bridged (4,4) layers with [M2(O2CR)4] as secondary building units that are pillared by the 1,4-dimb ligands into a unique 3D framework. The high-spin octahedral symmetry and the proportions of the mixed-metal ions were characterized by UV-vis spectroscopy. The compounds exhibit the onset of antiferromagnetic ordering at 7.5 approximately 23 K, as well as weak ferromagnetism, spin-flop, and glassy behavior that result from the randomness of the mixed-metal pairs, magnetic anisotropy of the metallic cations, and antisymmetric exchange. The composites should be regarded as molecular alloys of the pure Ni(II) and Co(II) compounds. The magnetic behavior of the solid solutions shows unambiguously that the organic bridges, bond angles, and bond distances greatly influence the effective interactions and bring about cooperative magnetic behavior in the chiral 3D frameworks.

Effect of Nd dopant on magnetic and electric properties of BiFeO3 thin films prepared by metal organic deposition method

Abstract: Polycrystalline Bi1−xNdxFeO3 (x=0–0.15) thin films were prepared on (111) Pt∕Ti∕SiO2∕Si substrates via metal organic deposition method. The effect of Nd dopant on the structural, electric, and magnetic properties was studied. It was found that the ferroelectric polarization and saturation magnetization of the films were enhanced by appropriate Nd doping due to the structural distortion and the suppressed cycloidal spin structure. Meanwhile, Nd-doped BiFeO3 thin films exhibited magnetic anisotropy because of the magnetocrystalline anisotropy.

Multilayer exchange spring media for magnetic recording

Abstract: The concept of exchange spring media is extended from two layers to N layers. The coercive field of the multilayer structure decreases with 1∕N while the energy barrier is only determined by the magnetic properties of the hardest layer. A 25nm thick trilayer has a 7.5 times smaller coercive field (1.4T) as a single layer at the same thermal stability and for the same value of the magnetization. A continuous variation of the anisotropy in the recording media reduces the coercive field by a factor of 10 for a layer thickness of 25nm.

Magnetic anisotropy of Co2+ as signature of intrinsic ferromagnetism in ZnO:Co.

Abstract: We report on the magnetic properties of thoroughly characterized Zn(1-x)Co(x)O epitaxial thin films, with low Co concentration, x = 0.003-0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co2+ ions in ZnO possess a strong single ion anisotropy which leads to an "easy plane" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials.

Tuning anisotropy barriers in a family of tetrairon(III) single-molecule magnets with an S=5 ground state

Abstract: Tetrairon(III) Single-Molecule Magnets (SMMs) with a propeller-like structure exhibit tuneable magnetic anisotropy barriers in both height and shape. The clusters [Fe4(L1)2(dpm)6] (1), [Fe4(L2)2(dpm)6] (2), [Fe4(L3)2(dpm)6].Et2O (3.Et2O), and [Fe4(OEt)3(L4)(dpm)6] (4) have been prepared by reaction of [Fe4(OMe)6(dpm)6] (5) with tripodal ligands R-C(CH2OH)3 (H3L1, R = Me; H3L2, R = CH2Br; H3L3, R = Ph; H3L4, R = tBu; Hdpm = dipivaloylmethane). The iron(III) ions exhibit a centered-triangular topology and are linked by six alkoxo bridges, which propagate antiferromagnetic interactions resulting in an S = 5 ground spin state. Single crystals of 4 reproducibly contain at least two geometric isomers. From high-frequency EPR studies, the axial zero-field splitting parameter (D) is invariably negative, as found in 5 (D = -0.21 cm(-1)) and amounts to -0.445 cm(-1) in 1, -0.432 cm(-1) in 2, -0.42 cm(-1) in 3.Et2O, and -0.27 cm(-1) in 4 (dominant isomer). The anisotropy barrier Ueff determined by AC magnetic susceptibility measurements is Ueff/kB = 17.0 K in 1, 16.6 K in 2, 15.6 K in 3.Et2O, 5.95 K in 4, and 3.5 K in 5. Both |D| and U(eff) are found to increase with increasing helical pitch of the Fe(O2Fe)3 core. The fourth-order longitudinal anisotropy parameter B4(0), which affects the shape of the anisotropy barrier, concomitantly changes from positive in 1 ("compressed parabola") to negative in 5 ("stretched parabola"). With the aid of spin Hamiltonian calculations the observed trends have been attributed to fine modulation of single-ion anisotropies induced by a change of helical pitch.

Valence, spin, and orbital state of Co ions in one-dimensional Ca3Co2O6 : An x-ray absorption and magnetic circular dichroism study

Abstract: We have investigated the valence, spin, and orbital state of the Co ions in the one-dimensional cobaltate Ca3Co2O6 using x-ray absorption and x-ray magnetic circular dichroism at the Co-L2,3 edges. The Co ions at both the octahedral Cooct and trigonal Cotrig sites are found to be in a 3+ state. From the analysis of the dichroism we established a low-spin state for the Cooct and a high-spin state with an anomalously large orbital moment of 1.7B at the Co trig ions. This large orbital moment along the c-axis chain and the unusually large magnetocrystalline anisotropy can be traced back to the double occupancy of the d2 orbital in trigonal crystal field.

Magnetic interactions in a ferromagnetic honeycomb nanoscale network

Abstract: The magnetic structure and magnetization process in a permalloy wire-based honeycomb network have been investigated by means of magnetic-force microscopy (MFM) and magnetoresistance measurement. The MFM measurements show the remanent magnetic structures to be governed by magnetic interaction similar to the ice rule, which provides a direct analogy between the present honeycomb network and an Ising system on a kagom\'e lattice. The magnetoresistance measurements reveal that this interaction also dominates the magnetization processes in the network. By decreasing the exchange energy at the vertices of the network, the ice-rule type of interaction causes a transition of the magnetization process in the network.

Spin-polarized current-induced magnetization reversal in perpendicularly magnetized L10-FePt layers

Abstract: Current-induced magnetization reversal of perpendicularly magnetized layers was studied in current-perpendicular-to-plane giant magnetoresistance pillars with L10-FePt (001) layers. The FePt layers exhibited strong perpendicular magnetic anisotropy of the order of 107erg∕cm3. A series of magnetoresistance curves after applying pulse currents with different current densities showed that current-induced magnetization reversal from an antiparallel to a parallel alignment occurred at the current density of the order of 108A∕cm2 with the assistance of magnetic field.

Spin disorder and magnetic anisotropy in Fe3O4 nanoparticles

Abstract: We have studied the magnetic behavior of dextran-coated magnetite (Fe3O4) nanoparticles with median particle size ⟨d⟩=8nm. Magnetization curves and in-field Mossbauer spectroscopy measurements showed that the magnetic moment MS of the particles was much smaller than the bulk material. However, we found no evidence of magnetic irreversibility or nonsaturating behavior at high fields, usually associated to spin canting. The values of magnetic anisotropy Keff from different techniques indicate that surface or shape contributions are negligible. It is proposed that these particles have bulklike ferromagnetic structure with ordered A and B sublattices, but nearly compensated the magnetic moments. The dependence of the blocking temperature with frequency and applied fields, TB(H,ω), suggests that the observed nonmonotonic behavior is governed by the strength of interparticle interactions.

Spinodal Decomposition under Layer by Layer Growth Condition and High Curie Temperature Quasi-One-Dimensional Nano-Structure in Dilute Magnetic Semiconductors

Abstract: We show that spinodal decomposition under layer by layer crystal growth condition leads to characteristic quasi-one-dimensional nano-structures in dilute magnetic semiconductors (DMS). It is found that the DMS systems can form rather large clusters with highly anisotropic shape even for low concentrations. It is suggested that the blocking phenomena in the super-paramagnetism, the magnetic dipole–dipole interaction and the network of the one-dimensional structures should be considered to understand the magnetism in DMS. Based on the present simulations, we propose that the delta-doping method can be effective approach to realize high Curie temperature.

Temperature dependence of magnetic anisotropy in Mn-substituted cobalt ferrite

Abstract: The temperature variation of magnetic anisotropy and coercive field of magnetoelastic manganese-substituted cobalt ferrites (CoMnxFe2?xO4 with 0 ? x ? 0.6) was investigated. Major magnetic hysteresis loops were measured for each sample at temperatures over the range 10–400 K, using a superconducting quantum interference device magnetometer. The high-field regimes of the hysteresis loops were modeled using the law of approach to saturation equation, based on the assumption that at sufficiently high field only rotational processes remain, with an additional forced magnetization term that was linear with applied field. The cubic anisotropy constant K1 was calculated from the fitting of the data to the theoretical equation. It was found that anisotropy increases substantially with decreasing temperature from 400 to 150 K, and decreases with increasing Mn content. Below 150 K, it appears that even under a maximum applied field of 5 T, the anisotropy of CoFe2O4 and CoMn0.2Fe1.8O4 is so high as to prevent complete approach to saturation, thereby making the use of the law of approach questionable in these cases.

Magnetic antidot nanostructures: effect of lattice geometry.

Abstract: We investigate the effect of lattice geometry on the magnetic anisotropy and transport properties of Ni80Fe20 antidot nanostructures. The structures were fabricated using deep ultra-violet lithography at 248 nm exposure wavelength. For an antidot array with a square lattice, a fourfold magnetic anisotropy with alternating hard axis and easy axis every 45° was observed. The honeycomb and rhomboid antidot lattice, however, both show a sixfold anisotropy, conforming well to the symmetry of their respective lattices. The magnetic hysteresis and micromagnetic simulation of the spin states at remanence show that the magnetization reversal process is very sensitive to the lattice arrangement of the holes. From the magnetotransport measurements, both the current density distribution and the magnetoresistance behaviour are markedly dependent on the antidot lattice geometry, in agreement with our transport simulations.

Temperature dependence of magnetic anisotropy: An ab initio approach

Abstract: We present a first-principles theory of the variation of magnetic anisotropy, K, with temperature, T ,i n metallic ferromagnets. It is based on relativistic electronic structure theory and calculation of magnetic torque. Thermally induced local moment magnetic fluctuations are described within the relativistic generalization of the disordered local moment theory from which the T dependence of the magnetization, m, is found. We apply the theory to a uniaxial magnetic material with tetragonal crystal symmetry, L10-ordered FePd, and find its uniaxial K consistent with a magnetic easy axis perpendicular to the Fe/Pd layers for all m and proportional to m 2 for a broad range of values of m. This is the same trend that we have previously found in L10-ordered FePt and which agrees with experiment. We also study a magnetically soft cubic magnet, the Fe50Pt50 solid solution, and find that its small magnetic anisotropy constant K1 rapidly diminishes from 8 eV to zero. K1 evolves from being proportional to m 7 at low T to m 4 near the Curie temperature. The accounts of both the tetragonal and cubic itinerant electron magnets differ from those extracted from single ion anisotropy models and instead receive clear interpretations in terms of two ion anisotropic exchange.

Origin of orbital ferromagnetism and giant magnetic anisotropy at the nanoscale.

Abstract: The origin of orbital magnetism recently observed in different nanostructured films and particles is discussed as a consequence of spin-orbit coupling. It is shown that contact potentials induced at the thin film surface by broken symmetries, as domain boundaries in self-assembled monolayers, lead to orbital states that in some cases are of large radius. The component of the angular momentum normal to the surface can reach very high values that decrease the total energy by decreasing spin-orbit interaction energy. Intraorbital ferromagnetic spin correlations induce orbital momenta alignment. The estimated values of the magnetic moments per atom are in good agreement with the experimental observations in thiol capped gold films and nanoparticles.

Symmetry-Based Magnetic Anisotropy in the Trigonal Bipyramidal Cluster [Tp2(Me3tacn)3Cu3Fe2(CN)6]4+

Abstract: Reaction of [(Me3tacn)Cu(H2O)2]2+ (Me3tacn = N,N‘,N‘ ‘-trimethyl-1,4,7-triazacyclononane) with [TpFe(CN)3]- (Tp- = hydrotris(pyrazolyl)borate) in a mixture of ethanol and acetonitrile affords the pentanuclear cluster [Tp2(Me3tacn)3Cu3Fe2(CN)6]4+. Single-crystal X-ray analysis reveals a trigonal bipyramidal structure featuring a D3h-symmetry core in which two opposing FeIII (S = 1/2) centers are linked through cyanide bridges to an equatorial triangle of three CuII (S = 1/2) centers. Fits to variable-temperature dc magnetic susceptibility data are consistent with ferromagnetic coupling to give an S = 5/2 ground state, while fits to low-temperature magnetization data indicate the presence of a large axial zero-field splitting (D = −5.7 cm-1). Frequency dependence observed in the ac magnetic susceptibility data confirms single-molecule magnet behavior, with an effective spin reversal barrier of Ueff = 16 cm-1. When compared with the much lower anisotropy barrier previously observed for the face-centered cubi...

Quantum-tunneling-induced kondo effect in single molecular magnets

Abstract: We consider transport through a single-molecule magnet strongly coupled to metallic electrodes. We demonstrate that, for a half-integer spin of the molecule, electron and spin tunneling cooperate to produce both quantum tunneling of the magnetic moment and a Kondo effect in the linear conductance. The Kondo temperature depends sensitively on the ratio of the transverse and easy-axis anisotropies in a nonmonotonic way. The magnetic symmetry of the transverse anisotropy imposes a selection rule on the total spin for the occurrence of the Kondo effect which deviates from the usual even-odd alternation.

Orbital and Bonding Anisotropy in a Half-Filled GaFeO 3 Magnetoelectric Ferrimagnet

Abstract: We investigated the orbital anisotropy of GaFeO3 using the Fe L2,3-edge x-ray magnetic circular dichroism and the polarization dependent O K-edge x-ray absorption spectroscopy. We found that the system shows a considerably large orbital momentum and anisotropic Fe-O bonding, which are unexpected in a half-filled d5 system such as GaFeO3. The orbital and bonding anisotropies, which turn out to be induced by the lattice distortions with exotic off-centering site movements, contribute the large magnetocrystalline energy and magnetoelasticity. These results provide critical clues on the microscopic understanding of the magnetoelectricity.

Nonthermal ultrafast optical control of the magnetization in garnet films

Abstract: We demonstrate coherent optical control of the magnetization in ferrimagnetic garnet films on the femtosecond time scale through a combination of two different ultrafast and nonthermal photomagnetic effects and by employing multiple pump pulses. Linearly polarized laser pulses are shown to create a long-lived modification of the magnetocrystalline anisotropy via optically induced electron transfer between nonequivalent ion sites while circularly polarized pulses additionally act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday effect. Due to the slow phonon-magnon interaction in these dielectrics, thermal effects of the laser excitation are clearly distinguished from the ultrafast nonthermal effects and can be seen only on the time scale of nanoseconds for sample temperatures near the Curie point. The reported effects open exciting possibilities for ultrafast manipulation of spins by light, and provide insight into the physics of magnetism on ultrafast time scales.

Colloidal Synthesis and Characterization of Tetrapod-Shaped Magnetic Nanocrystals

Abstract: Tetrapod-shaped maghemite nanocrystals are synthesized by manipulating the decomposition of iron pentacarbonyl in a ternary surfactant mixture under mild thermal conditions. Adjustment of the reaction parameters allows for the systematic tuning of both the width and the length of the tetrapod arms, which grow preferentially along the 〈111〉 easy axis direction. Such degree of control leads to modulation of the magnetic behavior of the nanocrystals, which evolves systematically as their surface magnetization phase and shape anisotropy are progressively increased.

Correlation between spin-orbital coupling and the superparamagnetic properties in magnetite and cobalt ferrite spinel nanocrystals.

Abstract: The superparamagnetic properties of CoFe2O4 and Fe3O4 nanocrystals have been systematically investigated. The observed blocking temperature of CoFe2O4 nanocrystals is at least 100 deg higher than that of the same sized Fe3O4 nanocrystals. The coercivity of CoFe2O4 nanocrystals at 5 K is over 50 times higher than the same sized Fe3O4 nanocrystals. The drastic difference in superparamagnetic properties between the similar sized spherical CoFe2O4 and Fe3O4 (or FeFe2O4) spinel ferrite nanocrystals was correlated to the coupling strength between electron spin and orbital angular momentum (L−S) in magnetic cations. Compared to the Fe2+ ion, the effect of much stronger spin−orbital coupling at Co2+ lattice sites leads to a higher magnetic anisotropy and results in the dramatic discrepancy of superparamagnetic properties between CoFe2O4 and Fe3O4 nanocrystals. These results provide some insight to the fundamental understanding of the quantum origin of superparamagnetic properties. Furthermore, they suggest that i...