Showing papers on "Magnetic anisotropy published in 2010"

A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junction

Abstract: Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing a perpendicular magnetic easy axis are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips with high thermal stability and low critical current for current-induced magnetization switching. To attain perpendicular anisotropy, a number of material systems have been explored as electrodes, which include rare-earth/transition-metal alloys, L1(0)-ordered (Co, Fe)-Pt alloys and Co/(Pd, Pt) multilayers. However, none of them so far satisfy high thermal stability at reduced dimension, low-current current-induced magnetization switching and high tunnel magnetoresistance ratio all at the same time. Here, we use interfacial perpendicular anisotropy between the ferromagnetic electrodes and the tunnel barrier of the MTJ by employing the material combination of CoFeB-MgO, a system widely adopted to produce a giant tunnel magnetoresistance ratio in MTJs with in-plane anisotropy. This approach requires no material other than those used in conventional in-plane-anisotropy MTJs. The perpendicular MTJs consisting of Ta/CoFeB/MgO/CoFeB/Ta show a high tunnel magnetoresistance ratio, over 120%, high thermal stability at dimension as low as 40 nm diameter and a low switching current of 49 microA.

Ferroelectric Control of Spin Polarization

Abstract: A current drawback of spintronics is the large power that is usually required for magnetic writing, in contrast with nanoelectronics, which relies on "zero-current," gate-controlled operations. Efforts have been made to control the spin-relaxation rate, the Curie temperature, or the magnetic anisotropy with a gate voltage, but these effects are usually small and volatile. We used ferroelectric tunnel junctions with ferromagnetic electrodes to demonstrate local, large, and nonvolatile control of carrier spin polarization by electrically switching ferroelectric polarization. Our results represent a giant type of interfacial magnetoelectric coupling and suggest a low-power approach for spin-based information control.

Electric-field effects on thickness dependent magnetic anisotropy of sputtered MgO/Co40Fe40B20/Ta structures

Abstract: We have investigated the effect of applied electric field EG on thickness dependent magnetic anisotropy of sputtered Co40Fe40B20 sandwiched with MgO and Ta. The range of CoFeB thickness explored is 2 nm and below. As the thickness is reduced, the easy axis of magnetization becomes perpendicular from in-plane. We show that perpendicular magnetic anisotropy of in-plane samples and coercivity of perpendicular samples can be modified by applying EG at room temperature. Furthermore, superparamagnetic behavior is observed for CoFeB layers with further reduced thickness below ≈0.9 nm, where electric-field effect is also observed below their blocking temperature.

Slow Magnetic Relaxation in a High-Spin Iron(II) Complex

Abstract: Slow magnetic relaxation is observed for [(tpaMes)Fe]−, a trigonal pyramidal complex of high-spin iron(II), providing the first example of a mononuclear transition metal complex that behaves as a single-molecule magnet. Dc magnetic susceptibility and magnetization measurements reveal a strong uniaxial magnetic anisotropy (D = −39.6 cm−1) acting on the S = 2 ground state of the molecule. Ac magnetic susceptibility measurements indicate the absence of slow relaxation under zero applied dc field as a result of quantum tunneling of the magnetization. Application of a 1500 Oe dc field initiates slow magnetic relaxation, which follows a thermally activated tunneling mechanism at high temperature to give an effective spin-reversal barrier of Ueff = 42 cm−1 and follows a temperature-independent tunneling mechanism at low temperature. In addition, the magnetic relaxation time shows a pronounced dc-field dependence, with a maximum occurring at ∼1500 Oe.

Inverse spin-galvanic effect in the interface between a topological insulator and a ferromagnet.

Abstract: When a ferromagnet is deposited on the surface of a topological insulator the topologically protected surface state develops a gap and becomes a two-dimensional quantum Hall liquid. We demonstrate that the Hall current in such a liquid, induced by an external electric field, can have a dramatic effect on the magnetization dynamics of the ferromagnet by changing the effective anisotropy field. This change is dissipationless and may be substantial even in weakly spin-orbit coupled ferromagnets. We study the possibility of dissipationless current-induced magnetization reversal in monolayer-thin, insulating ferromagnets with a soft perpendicular anisotropy and discuss possible applications of this effect.

Slow magnetic relaxation in a family of trigonal pyramidal iron(II) pyrrolide complexes.

Abstract: We present a family of trigonal pyramidal iron(II) complexes supported by tris(pyrrolyl-α-methyl)amine ligands of the general formula [M(solv)(n)][(tpa(R))Fe] (M = Na, R = tert-butyl (1), phenyl (4); M = K, R = mesityl (2), 2,4,6-triisopropylphenyl (3), 2,6-difluorophenyl (5)) and their characterization by X-ray crystallography, Mossbauer spectroscopy, and high-field EPR spectroscopy. Expanding on the discovery of slow magnetic relaxation in the recently reported mesityl derivative 2, this homologous series of high-spin iron(II) complexes enables an initial probe of how the ligand field influences the static and dynamic magnetic behavior. Magnetization experiments reveal large, uniaxial zero-field splitting parameters of D = -48, -44, -30, -26, and -6.2 cm(-1) for 1-5, respectively, demonstrating that the strength of axial magnetic anisotropy scales with increasing ligand field strength at the iron(II) center. In the case of 2,6-difluorophenyl substituted 5, high-field EPR experiments provide an independent determination of the zero-field splitting parameter (D = -4.397(9) cm(-1)) that is in reasonable agreement with that obtained from fits to magnetization data. Ac magnetic susceptibility measurements indicate field-dependent, thermally activated spin reversal barriers in complexes 1, 2, and 4 of U(eff) = 65, 42, and 25 cm(-1), respectively, with the barrier of 1 constituting the highest relaxation barrier yet observed for a mononuclear transition metal complex. In addition, in the case of 1, the large range of temperatures in which slow relaxation is observed has enabled us to fit the entire Arrhenius curve simultaneously to three distinct relaxation processes. Finally, zero-field Mossbauer spectra collected for 1 and 4 also reveal the presence of slow magnetic relaxation, with two independent relaxation barriers in 4 corresponding to the barrier obtained from ac susceptibility data and to the 3D energy gap between the M(S) = ±2 and ±1 levels, respectively.

Electric field effect on magnetization at the Fe/MgO(001) interface

Abstract: Density-functional calculations are performed to explore magnetoelectric effects originating from the influence of an external electric field on magnetic properties of the Fe/MgO(001) interface. It is shown that the effect on the interface magnetization and magnetocrystalline anisotropy can be substantially enhanced if the electric field is applied across a dielectric material with a large dielectric constant. In particular, we predict an enhancement of the interface magnetoelectric susceptibility by a factor of the dielectric constant of MgO over that of the free standing Fe (001) surface. We also predict a significant effect of electric field on the interface magnetocrystalline anisotropy due to the change in the relative occupancy of the 3d-orbitals of Fe atoms at the Fe/MgO interface. These results may be interesting for technological applications such as electrically controlled magnetic data storage.

Tilting of the spin orientation induced by Rashba effect in ferromagnetic metal layer

Abstract: We devised a method to measure the virtual magnetic field induced by Rashba effect in ferromagnetic metal layer. Transverse Rashba magnetic field makes the magnetization direction tilted out of the easy axis, which could be detected by the change in anomalous hall resistances. Through a specified measurement of the second harmonics of the hall resistance, the Rashba field could be obtained with high sensitivity even at low current regime. The results are compared with the prior reports based on the measurement of the transverse field required for the nucleation of reversed domain.

Ultrathin Co/Pt and Co/Pd superlattice films for MgO-based perpendicular magnetic tunnel junctions

Abstract: Ultrathin [Co/Pt]n and [Co/Pd]n superlattice films consisting of 0.14–0.20-nm-thick Co and Pt(Pd) layers were deposited by sputtering. A large perpendicular magnetic anisotropy [(3–9)×106 ergs/cm3] and an ideal square out-of-plane hysteresis loop were attained even for ultrathin superlattice films with a total thickness of 1.2–2.4 nm. The films were stable against annealing up to 370 °C. MgO-based perpendicular magnetic tunnel junctions with this superlattice layer as the free layer showed a relatively high magnetoresistance ratio (62%) and an ultralow resistance-area product (3.9 Ω μm2) at room temperature. The use of these films will enable the development of gigabit-scale nonvolatile memory.

Voltage-induced perpendicular magnetic anisotropy change in magnetic tunnel junctions

Abstract: A voltage-induced perpendicular magnetic anisotropy change in an ultrathin FeCo layer was observed in an epitaxial magnetic tunnel junction (MTJ) structure. A spin-transfer induced ferromagnetic resonance measurement technique was used under various bias voltage applications to evaluate the anisotropy change. From the peak frequency shifts, we could estimate that a surface magnetic anisotropy change of 15 μJ/m2 was induced by an electric field application of 400 mV/nm in the MTJ with a 0.5 nm thick FeCo layer. The realization of voltage-induced anisotropy changes in an MTJ structure should have a large impact on the development of electric-field driven spintronic devices.

Giant magnetoelectric coefficients in (Fe90Co10)78Si12B10-AlN thin film composites

Abstract: Thin film magnetoelectric (ME) two–two composites consisting of AlN and amorphous (Fe90Co10)78Si12B10 layers were fabricated by magnetron sputtering on Si (100) substrates. Upon magnetic field annealing they show an extremely high ME coefficient of 737 V/cm Oe at mechanical resonance at 753 Hz and 3.1 V/cm Oe out of resonance at 100 Hz. These are the highest reported ME coefficients in thin film composites ever. Furthermore, the induced magnetic anisotropy by field annealing serves the possibility to obtain a sensor element with a pronounced sensitivity in only one dimension, which allows the realization of a three-dimensional vector field sensor.

Spin-orbit coupling induced anisotropy effects in bimetallic antiferromagnets: A route towards antiferromagnetic spintronics

Abstract: Magnetic anisotropy phenomena in bimetallic antiferromagnets ${\text{Mn}}_{2}\text{Au}$ and MnIr are studied by first-principles density-functional theory calculations We find strong and lattice-parameter-dependent magnetic anisotropies of the ground-state energy, chemical potential, and density of states, and attribute these anisotropies to combined effects of large moment on the $\text{Mn}\text{ }3d$ shell and large spin-orbit coupling on the $5d$ shell of the noble metal Large magnitudes of the proposed effects can open a route towards spintronics in compensated antiferromagnets without involving ferromagnetic elements

High speed low power magnetic devices based on current induced spin-momentum transfer

Abstract: A high speed and low power method to control and switch the magnetization direction and/or helicity of a magnetic region in a magnetic device for memory cells using spin polarized electrical current. The magnetic device comprises a reference magnetic layer with a fixed magnetic helicity and/or magnetization direction and a free magnetic layer with a changeable magnetic helicity. The fixed magnetic layer and the free magnetic layer are preferably separated by a non-magnetic layer, and the reference layer includes an easy axis perpendicular to the reference layer. A current can be applied to the device to induce a torque that alters the magnetic state of the device so that it can act as a magnetic memory for writing information. The resistance, which depends on the magnetic state of the device, is measured to thereby read out the information stored in the device.

Magnetic anisotropy, exchange and damping in cobalt-based full-Heusler compounds: an experimental review

Abstract: Cobalt-based full-Heusler compounds with composition Co2M'Z (where M' is a transition metal and Z is a main group element) are attracting attention due to their predicted half-metallic behaviour, a greatly desired property for spin-dependent electron transport devices. Knowledge of the basic magnetic properties of these materials, in particular in the form of thin films, is required both to fully exploit these promising materials and to understand their underlying electronic structure and establish structure–property relationships. In this topical review, we present a survey of the magnetic anisotropy, exchange and damping of Co2M'Z compounds. These properties are directly related to spin–spin and spin–orbit interactions.

Structure-related frustrated magnetism of nanosized polyoxometalates: aesthetics and properties in harmony

Abstract: The structural versatility characterizing polyoxometalate chemistry, in combination with the option to deliberately use well-defined building blocks, serves as the foundation for the generation of a large family of magnetic clusters, frequently comprising highly symmetric spin arrays. If the spin centers are coupled by antiferromagnetic exchange, some of these systems exhibit spin frustration, which can result in novel magnetic properties of purely molecular origins. We discuss here the magnetic properties of selected nanosized polyoxometalate clusters featuring spin triangles as their magnetic ‘building blocks’ or fragments. This includes unique porous Keplerate clusters of the type {(Mo)Mo5}12M30 (M = FeIII, CrIII, VIV) with the spin centers defining a regular icosidodecahedron and the {V15As6}-type cluster sphere containing a single equilateral spin triangle; these species are widely discussed and studied in the literature for their role in materials science as molecular representations of Kagome lattices and in relation to quantum computing, respectively. Exhibiting fascinating and unique structural features, these magnetic molecules allow the study of the implications of frustrated spin ordering. Furthermore, this perspective covers the impact of spin frustration on the degeneracy of the ground state and related problems, namely strong magnetic anisotropy and the interplay of antisymmetric exchange and structural Jahn–Teller effects.

[ReCl4(CN)2]2-: a high magnetic anisotropy building unit giving rise to the single-chain magnets (DMF)4MReCl4(CN)2 (M = Mn, Fe, Co, Ni).

Abstract: An S = 3/2, high-anisotropy building unit, trans-[ReCl4(CN)2]2−, representing the first paramagnetic complex with a mixture of just cyanide and halide ligands, has been synthesized through the reaction of (Bu4N)CN with ReCl4(THF)2. This species is characterized in detail and employed in directing the formation of a series of one-dimensional coordination solids of formula (DMF)4MReCl4(CN)2 (M = Mn (2), Fe (3), Co (4), Ni (5)). Variable-temperature dc magnetic susceptibility measurements demonstrate the presence of intrachain antiferromagnetic (2) and ferromagnetic (3−5) exchange coupling within these solids. In addition, probing the ac magnetic susceptibility as a function of both temperature and frequency reveals that all of the chain compounds exhibit slow relaxation of the magnetization. The relaxation time is shown to be thermally activated, with energy barriers to relaxation of Δτ = 31, 56, 17, and 20 cm−1 for 2−5, respectively. Notably, the field-dependent magnetization of the iron congener exhibits ...

Role of an interfacial FeO layer in the electric-field-driven switching of magnetocrystalline anisotropy at the Fe/MgO interface

Abstract: The electric-field-induced switching of magnetocrystalline anisotropy (MCA) between in-plane and out-of-plane orientations is investigated by first-principles calculations for the prototypical Fe on MgO(001) system. Our results predict that an ideal abrupt Fe/MgO interface gives rise to a large out-of-plane MCA due to weak Fe-O hybridization at the interface, but the MCA switching by an applied electric field is found to be difficult to achieve. Instead, the existence of an interfacial FeO layer plays a key role in demonstrating the MCA switching that accompanies an electric-field-induced displacement of Fe atoms on the interfacial FeO layer.

Spin-glass-like freezing and enhanced magnetization in ultra-small CoFe2O4 nanoparticles

Abstract: The magnetic properties of ultra-small (3 nm) CoFe2O4 nanoparticles have been investigated by DC magnetization measurements as a function of temperature and magnetic field. The main features of the magnetic behaviour are blocking of non-interacting particle moments (zero-field-cooled magnetization Tmax≈40 K), a rapid increase of saturation magnetization (up to values higher than for the bulk material) at low T and an increase in anisotropy below 30 K due to the appearance of exchange bias. The low temperature behaviour is determined by a random freezing of surface spins. Localized spin-canting and cation distribution between the two sublattices of the spinel structure account quantitatively for the observed increase in saturation magnetization.

Realization of giant magnetoelectricity in helimagnets

Abstract: We show that low field magnetoelectric (ME) properties of helimagnets Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22 can be efficiently tailored by the Al-substitution level. As x increases, the critical magnetic field for switching electric polarization is systematically reduced from approximately 1 T down to approximately 1 mT, and the ME susceptibility is greatly enhanced to reach a giant value of 2.0x10{4} ps/m at an optimum x=0.08. We find that control of the nontrivial orbital moment in the octahedral Fe sites through the Al substitution is crucial for fine-tuning the magnetic anisotropy and obtaining the conspicuously improved ME characteristics.

Saturation magnetization and crystalline anisotropy calculations for MnAl permanent magnet

Abstract: Theoretical saturation magnetization and magnetocrystalline anisotropy energy (MAE) of τ-phase (face-centered tetragonal) Mn50Al50 alloy were obtained by first principles calculations, and the alloy was fabricated to compare the experimental values with the theoretical predictions. The calculated magnetic moment and MAE for τ-phase Mn50Al50 were 2.37 μB/f.u. and 0.259 meV/f.u. (1.525×106 J/m3), respectively, which result in the maximum energy product (BH)max of 12.64 MG Oe and the magnetocrystalline anisotropy field of 38 kOe. The saturation magnetization for τ-phase Mn54Al46 alloy was measured to be 98.3 emu/g, which gives 4.7 MG Oe of (BH)max. The magnetization is about 70% of the theoretical value of 144 emu/g.

Coherent magnetization precession in ferromagnetic (Ga,Mn)As induced by picosecond acoustic pulses.

Abstract: We show that the magnetization of a thin ferromagnetic (Ga,Mn)As layer can be modulated by picosecond acoustic pulses. In this approach a picosecond strain pulse injected into the structure induces a tilt of the magnetization vector M, followed by the precession of M around its equilibrium orientation. This effect can be understood in terms of changes in magnetocrystalline anisotropy induced by the pulse. A model where only one anisotropy constant is affected by the strain pulse provides a good description of the observed time-dependent response.

Electric field effects on magnetocrystalline anisotropy in ferromagnetic Fe monolayers

Abstract: The magnetocrystalline anisotropy (MCA) for an Fe monolayer on MgO substrate (Fe/MgO) and that sandwiched by MgO (MgO/Fe/MgO) is investigated by means of the first principles full-potential linearized augmented plane-wave method, and the effects of an external electric field on the MCA is discussed. In both systems, the MCA is found to be modified through a change in the d band structure around Fermi level by the external electric field.

Magnetic nanowires by electrodeposition within templates

Abstract: Magnetic nanowire arrays allow studying magnetism at the nanoscale and have broad application areas. Here we review our recent experiments on tailoring the structure and microstructure of electrochemically grown elemental (Fe, Ni, Co) and alloy (Co-Pt) magnetic nanowires. The comparison of these different materials allows identifying the role of shape, magnetocrystal-line and magnetoelastic anisotropies as well as magnetostatic interactions.

Electrical tuning of magnetism in Fe3O4/PZN-PT multiferroic heterostructures derived by reactive magnetron sputtering

Abstract: Strong magnetoelectric (ME) coupling was demonstrated in Fe3O4/PZN–PT (lead zinc niobate–lead titanate) multiferroic heterostructures obtained through a sputter deposition process. The dependence of the magnetic anisotropy on the electric field (E-field) is theoretically predicted and experimentally observed by ferromagnetic resonance spectroscopy. A large tunable in-plane magnetic anisotropy of up to 600 Oe, and tunable out-of-plane anisotropy of up to 400 Oe were observed in the Fe3O4/PZN–PT multiferroic heterostructures, corresponding to a large ME coefficient of 100 Oe cm/kV in plane and 68 Oe cm/kV out of plane, which match well with predicted results. In addition, the electric field manipulation of magnetic anisotropy is also demonstrated by the electric fields dependence of magnetic hysteresis loops, showing a large squareness ratio change of 44%. These Fe3O4/PZN–PT multiferroic heterostructures exhibiting large E-field tunable magnetic properties provide great opportunities for novel electrostatic...

Electric field controlled magnetic anisotropy in a single molecule.

Abstract: We have measured quantum transport through an individual Fe4 single-molecule magnet embedded in a three-terminal device geometry. The characteristic zero-field splittings of adjacent charge states and their magnetic field evolution are observed in inelastic tunneling spectroscopy. We demonstrate that the molecule retains its magnetic properties and, moreover, that the magnetic anisotropy is significantly enhanced by reversible electron addition/subtraction controlled with the gate voltage. Single-molecule magnetism can thus be electrically controlled.

Electric Field Controlled Magnetic Anisotropy in a Single Molecule

Abstract: We have measured quantum transport through an individual Fe$_4$ single-molecule magnet embedded in a three-terminal device geometry. The characteristic zero-field splittings of adjacent charge states and their magnetic field evolution are observed in inelastic tunneling spectroscopy. We demonstrate that the molecule retains its magnetic properties, and moreover, that the magnetic anisotropy is significantly enhanced by reversible electron addition / subtraction controlled with the gate voltage. Single-molecule magnetism can thus be electrically controlled.

Initiation of spin-transfer torque by thermal transport from magnons

Abstract: As now practiced in experimental nanomagnetic spintronics, spin-transfer torque acting on a free metallic moment is driven by electric current flowing serially through it and a metallic reference magnet. I propose driving spin-transfer torque by flow of heat serially through the free magnet and an insulating reference ferrite. The needed spin current initiates from magnons present in the ferrite. A quantum yield of heat-driven in-plane spin-transfer torque can be substantially greater, in principle, than that achievable using electric current in a magnetic tunnel junction. A Bloch-type dynamical equation for the conduction-electron-spin polarization excited by a paramagnetic-monolayer model of the ferrite/metal interface predicts the dependence of this yield on material parameters. In practice, achieving a high yield beneficial to applications will require strong exchange coupling of the local $3d$-electron atomic spins in this monolayer to both to the ferrite moment with ferromagnetic sign and also with either sign to the conduction $s$ electrons in a normal metallic spacer. Advantageous will be suppression of the interfacial heat flow diverted to phonons within the ferrite. If a nonmagnetic electrically insulating layer additionally adjoins the free magnet, the theory also predicts a perpendicular spin-transfer torque component whose angular dependence mimics conventional uniaxial magnetic anisotropy.

Hysteresis and unusual magnetic properties in the singular Heusler alloy Ni45Co5Mn40Sn10

Abstract: The alloy Ni45Co5Mn40Sn10 is shown to be singular relative to nearby alloys in three following ways: (1) The austenite has remarkably high magnetization (1170 emu/cm3) and low magnetic anisotropy. (2) The thermal hysteresis is near minimum. (3) The transformation temperature ∼135 °C is unusually high. Because the unusually large magnetization and low hysteresis is seen at relatively small applied fields, applications such as magnetic shape memory, energy conversion, and solid state refrigeration may become practical.

Core-Shell Magnetic Morphology of Structurally Uniform Magnetite Nanoparticles

Abstract: A new development in small-angle neutron scattering with polarization analysis allows us to directly extract the average spatial distributions of magnetic moments and their correlations with three-dimensional directional sensitivity in any magnetic field. Applied to a collection of spherical magnetite nanoparticles 9.0 nm in diameter, this enhanced method reveals uniformly canted, magnetically active shells in a nominally saturating field of 1.2 T. The shell thickness depends on temperature, and it disappears altogether when the external field is removed, confirming that these canted nanoparticle shells are magnetic, rather than structural, in origin.