Showing papers on "Magnetic anisotropy published in 2009"
TL;DR: Simulations confirm that voltage-controlled magnetization switching in magnetic tunnel junctions is possible using the anisotropy change demonstrated here, which could be of use in the development of low-power logic devices and non-volatile memory cells.
Abstract: A voltage-induced symmetry change in a ferromagnetic material can change its magnetization or magnetic anisotropy, but these effects are too weak to be used in memory devices. Researchers have now shown that a relatively small electric field can cause a large change in the magnetic anisotropy of a few atomic layers of iron. The results could lead to low-power logic devices and non-volatile memory cells.
1,201 citations
TL;DR: The magnetocaloric effects of Ni-Mn-based Heusler alloys are surveyed and their relation with the magnetic shape-memory and magnetic superelasticity reported in these materials are discussed.
Abstract: Magnetic Heusler alloys which undergo a martensitic transition display interesting functional properties. In the present review, we survey the magnetocaloric effects of Ni-Mn-based Heusler alloys and discuss their relation with the magnetic shape-memory and magnetic superelasticity reported in these materials. We show that all these effects are a consequence of a strong coupling between structure and magnetism which enables a magnetic field to rearrange martensitic variants as well as to provide the possibility to induce the martensitic transition. These two features are respectively controlled by the magnetic anisotropy of the martensitic phase and by the difference in magnetic moments between the structural phases. The relevance of each of these contributions to the magnetocaloric properties is analysed.
886 citations
TL;DR: In this paper, a systematic magnetic force microscope study indicates that ferromagnetism in graphite is the result of localized spins that arise at grain boundaries, which usually only occurs in materials containing elements that form covalent 3d and 4f bonds.
Abstract: Ferromagnetism usually only occurs in materials containing elements that form covalent 3d and 4f bonds. Its occurrence in pure carbon is therefore surprising, even controversial. A systematic magnetic force microscope study indicates that ferromagnetism in graphite is the result of localized spins that arise at grain boundaries.
505 citations
TL;DR: The magnetization of a magnetic random access memory is usually controlled by the injection of an externally polarized spin-current as mentioned in this paper, which can be manipulated with local fields generated by spin-orbit interactions of an unpolarized current.
Abstract: The magnetization of a magnetic random-access memory is usually controlled by the injection of an externally polarized spin-current. A proof-of-principle demonstration shows that this could instead be manipulated with local fields generated by spin–orbit interactions of an unpolarized current.
428 citations
TL;DR: In this paper, a review of microwave ferrites is presented, with a brief discussion of fundamentals of magnetism, particularly ferrimagnetism, and chemical, structural, and magnetic properties of ferrites of interest as they pertain to net magnetization, especially to self biasing.
Abstract: Ferrimagnets having low RF loss are used in passive microwave components such as isolators, circulators, phase shifters, and miniature antennas operating in a wide range of frequencies (1–100 GHz) and as magnetic recording media owing to their novel physical properties. Frequency tuning of these components has so far been obtained by external magnetic fields provided by a permanent magnet or by passing current through coils. However, for high frequency operation the permanent part of magnetic bias should be as high as possible, which requires large permanent magnets resulting in relatively large size and high cost microwave passive components. A promising approach to circumvent this problem is to use hexaferrites, such as BaFe12O19 and SrFe12O19, which have high effective internal magnetic anisotropy that also contributes to the permanent bias. Such a self-biased material remains magnetized even after removing the external applied magnetic field, and thus, may not even require an external permanent magnet. In garnet and spinel ferrites, such as Y3Fe5O12 (YIG) and MgFe2O4, however, the uniaxial anisotropy is much smaller, and one would need to apply huge magnetic fields to achieve such high frequencies. In Part 1 of this review of microwave ferrites a brief discussion of fundamentals of magnetism, particularly ferrimagnetism, and chemical, structural, and magnetic properties of ferrites of interest as they pertain to net magnetization, especially to self biasing, are presented. Operational principles of microwave passive components and electrical tuning of magnetization using magnetoelectric coupling are discussed in Part 2.
362 citations
TL;DR: The effects of an external electric field on the magnetocrystalline anisotropy in ferromagnetic transition-metal monolayers are demonstrated to show that the MCA in an Fe(001) monolayer can be controlled by the electric field through a change in band structure.
Abstract: Controlling and designing quantum magnetic properties by an external electric field is a key challenge in modern magnetic physics. Here, from first principles, the effects of an external electric field on the magnetocrystalline anisotropy (MCA) in ferromagnetic transition-metal monolayers are demonstrated which show that the MCA in an Fe(001) monolayer [but not in Co(001) and Ni(001) monolayers] can be controlled by the electric field through a change in band structure, in which small components of the p orbitals near the Fermi level, which are coupled to the d states by the electric field, play a key role. This prediction obtained opens a way to control the MCA by the electric field and invites experiments.
344 citations
TL;DR: In this article, the authors performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1−xCox)2As2.
Abstract: We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1−xCox)2As2. Unlike the high-temperature superconductor cuprates and 1111-phase oxypnictides, Ba(Fe1−xCox)2As2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. We report the temperature dependencies of the upper critical field Hc2 both parallel and perpendicular to the c-axis, the irreversibility field Hirrc(T), and a rather unusual symmetric volume pinning force curve Fp(H) suggestive of a strong pinning nanostructure. The anisotropy parameter γ=Hc2ab/Hc2c deduced from the slopes of dHc2ab/dT=4.9 T/K and dHc2c/dT=2.5 T/K decreases from ∼2 near Tc, to ∼1.5 at lower temperatures, much smaller than γ for 1111pnictides and high-Tc cuprates.
333 citations
TL;DR: In this article, the chiral magnetic conductivity of a high-temperature plasma for nonzero frequencies has been computed for time-dependent magnetic fields, such as produced in heavy ion collisions.
Abstract: Gluon field configurations with nonzero topological charge generate chirality, inducing P- and CP-odd effects. When a magnetic field is applied to a system with nonzero chirality, an electromagnetic current is generated along the direction of the magnetic field. The induced current is equal to the chiral magnetic conductivity times the magnetic field. In this article we will compute the chiral magnetic conductivity of a high-temperature plasma for nonzero frequencies. This allows us to discuss the effects of time-dependent magnetic fields, such as produced in heavy ion collisions, on chirally asymmetric systems.
329 citations
TL;DR: It is demonstrated that introducing pores smaller than the grain size further reduces constraints and markedly increases MFIS to 2.0-8.7%.
Abstract: The magnetic shape-memory alloy Ni-Mn-Ga shows, in monocrystalline form, a reversible magnetic-field-induced strain (MFIS) up to 10%. This strain, which is produced by twin boundaries moving solely by internal stresses generated by magnetic anisotropy energy, can be used in actuators, sensors and energy-harvesting devices. Compared with monocrystalline Ni-Mn-Ga, fine-grained Ni-Mn-Ga is much easier to process but shows near-zero MFIS because twin boundary motion is inhibited by constraints imposed by grain boundaries. Recently, we showed that partial removal of these constraints, by introducing pores with sizes similar to grains, resulted in MFIS values of 0.12% in polycrystalline Ni-Mn-Ga foams, close to those of the best commercial magnetostrictive materials. Here, we demonstrate that introducing pores smaller than the grain size further reduces constraints and markedly increases MFIS to 2.0-8.7%. These strains, which remain stable over >200,000 cycles, are much larger than those of any polycrystalline, active material.
324 citations
TL;DR: The revealed necessity of the additional orbital physics leads to a correlated electronic structure fundamentally distinct from that of the cuprates, and the strong coupling to the magnons advocates active roles of light orbitons in spin dynamics and electron pairing in iron pnictides.
Abstract: The puzzling nature of magnetic and lattice phase transitions of iron pnictides is investigated via a first-principles Wannier function analysis of representative parent compound LaOFeAs. A rare ferro-orbital ordering is found to give rise to the recently observed highly anisotropic magnetic coupling, and drive both phase transitions---without resorting to widely employed frustration or nesting picture. The revealed necessity of the additional orbital physics leads to a correlated electronic structure fundamentally distinct from that of the cuprates. In particular, the strong coupling to the magnons advocates active roles of light orbitons in spin dynamics and electron pairing in iron pnictides.
288 citations
TL;DR: Although the magnetic anisotropy energy (MAE) density reveals large variation around the atoms, the intrinsic contribution to the MAE is found to mainly come from the Fe layer, and the surface without the capping Pt layer also shows similar linear dependence.
Abstract: We investigate crystalline magnetic anisotropy in the electric field (EF) for the FePt surface which has a large perpendicular anisotropy, by means of the first-principles approach. Anisotropy is reduced linearly with respect to the inward EF, associated with the induced spin density around the Fe layer. Although the magnetic anisotropy energy (MAE) density reveals large variation around the atoms, the intrinsic contribution to the MAE is found to mainly come from the Fe layer. The surface without the capping Pt layer also shows similar linear dependence.
TL;DR: The zero-field splitting was not observed for FePc on a clean Cu(110) surface, and the spin state converts from triplet to singlet due to the strong coupling of Fe d states with the Cu substrate, as confirmed by photoelectron spectroscopy.
Abstract: We examined the zero-field splitting of an iron(II) phthalocyanine (FePc) attached to clean and oxidized Cu(110) surfaces and the dependence on an applied magnetic field by inelastic electron tunneling spectroscopy with STM. The symmetry of the ligand field surrounding the Fe atom is lowered on the oxidized surface, switching the magnetic anisotropy from the easy plane of the bulk to the easy axis. The zero-field splitting was not observed for FePc on a clean Cu(110) surface, and the spin state converts from triplet to singlet due to the strong coupling of Fe d states with the Cu substrate, as is also confirmed by photoelectron spectroscopy. These findings demonstrate the importance of coupling at the molecule-substrate interface for manipulating the magnetic properties of adsorbates.
TL;DR: The effective Hamiltonian procedure enables us to confirm the validity of the standard model Hamiltonian to produce the magnetic anisotropy of monometallic complexes and the calculated ZFS parameters are in good agreement with high-field, high-frequency electron paramagnetic resonance spectroscopy and frequency domain magnetic resonanceSpectroscopy data.
Abstract: Monometallic Ni(II) and Co(II) complexes with large magnetic anisotropy are studied using correlated wave function based ab initio calculations Based on the effective Hamiltonian theory, we propose a scheme to extract both the parameters of the zero-field splitting (ZFS) tensor and the magnetic anisotropy axes Contrarily to the usual theoretical procedure of extraction, the method presented here determines the sign and the magnitude of the ZFS parameters in any circumstances While the energy levels provide enough information to extract the ZFS parameters in Ni(II) complexes, additional information contained in the wave functions must be used to extract the ZFS parameters of Co(II) complexes The effective Hamiltonian procedure also enables us to confirm the validity of the standard model Hamiltonian to produce the magnetic anisotropy of monometallic complexes The calculated ZFS parameters are in good agreement with high-field, high-frequency electron paramagnetic resonance spectroscopy and frequency domain magnetic resonance spectroscopy data A methodological analysis of the results shows that the ligand-to-metal charge transfer configurations must be introduced in the reference space to obtain quantitative agreement with the experimental estimates of the ZFS parameters
Spanish National Research Council1, Catalan Institution for Research and Advanced Studies2, École Polytechnique Fédérale de Lausanne3, Max Planck Society4, Chalmers University of Technology5, Utrecht University6, Indian Institute of Science7, European Synchrotron Radiation Facility8, Pierre-and-Marie-Curie University9, Technische Universität München10
TL;DR: It is shown that the supramolecular assembly of Fe and 1,4-benzenedicarboxylic acid molecules on a Cu surface results in ordered arrays of high-spin mononuclear Fe centres on a 1.5 nm square grid, opening a way to control the magnetic anisotropy in supramolescular layers akin to that used in metallic thin films.
Abstract: Magnetic atoms at surfaces are a rich model system for solid-state magnetic bits exhibiting either classical(1,2) or quantum(3,4) behaviour. Individual atoms, however, are difficult to arrange in regular patterns(1-5). Moreover, their magnetic properties are dominated by interaction with the substrate, which, as in the case of Kondo systems, often leads to a decrease or quench of their local magnetic moment(6,7). Here, we show that the supramolecular assembly of Fe and 1,4-benzenedicarboxylic acid molecules on a Cu surface results in ordered arrays of high-spin mononuclear Fe centres on a 1.5nm square grid. Lateral coordination with the molecular ligands yields unsaturated yet stable coordination bonds, which enable chemical modification of the electronic and magnetic properties of the Fe atoms independently from the substrate. The easy magnetization direction of the Fe centres can be switched by oxygen adsorption, thus opening a way to control the magnetic anisotropy in supramolecular layers akin to that used in metallic thin films(8-11).
TL;DR: Results are in agreement with ab initio calculations performed using the complete active space self-consistent field (CASSCF) method, validating the predictive power of this theoretical approach for complex systems containing rare-earth ions, even in low-symmetry environments.
Abstract: A mixed theoretical and experimental approach was used to determine the local magnetic anisotropy of the dysprosium(III) ion in a low-symmetry environment. The susceptibility tensor of the monomeric species having the formula [Dy(hfac)(3)(NIT-C(6)H(4)-OEt)(2)], which contains nitronyl nitroxide (NIT-R) radicals, was determined at various temperatures through angle-resolved magnetometry. These results are in agreement with ab initio calculations performed using the complete active space self-consistent field (CASSCF) method, validating the predictive power of this theoretical approach for complex systems containing rare-earth ions, even in low-symmetry environments. Susceptibility measurements performed with the applied field along the easy axis eventually permitted a detailed analysis of the temperature and field dependence of the magnetization, providing evidence that the Dy ion transmits an antiferromagnetic interaction between radicals but that the Dy-radical interaction is ferromagnetic.
TL;DR: Understanding of the interaction of a single magnetic impurity with the conduction electrons of the nonmagnetic host has been considerably deepened and new insight has emerged by taking advantage of quantum size effects in the metallic support and decoupling the magnetic adatom from the supporting host metal.
Abstract: The present topical review focuses on recent advances concerning an intriguing phenomenon in condensed matter physics, the scattering of conduction electrons at the localized spin of a magnetic impurity: the Kondo effect. Spectroscopic signatures of this effect have been observed in the past by high-resolution photoemission which, however, has the drawback of averaging over a typical surface area of 1 mm(2). By combining the atomic-scale spatial resolution of the scanning tunneling microscope (STM) with an energy resolution of a few tens of mu eV achievable nowadays in scanning tunneling spectroscopy (STS), and by exposing the magnetic adatom to external magnetic fields, our understanding of the interaction of a single magnetic impurity with the conduction electrons of the nonmagnetic host has been considerably deepened. New insight has emerged by taking advantage of quantum size effects in the metallic support and by decoupling the magnetic adatom from the supporting host metal, for instance by embedding it inside a molecule or by separating it by an ultrathin insulating film from the metal surface. In this way, Kondo resonances and Kondo temperatures can be tailored and manipulated by changing the local density of states of the environment. In the weak coupling limit between a Kondo impurity and a superconductor only a convolution of tip and sample DOS is observed while for strongly coupled systems midgap states appear, indicating superconducting pair breaking. Magnetic impurities with co-adsorbed hydrogen on metallic surfaces show pseudo-Kondo resonances owing to very low-energy vibrational excitations detected by inelastic tunneling spectroscopy. One of the most recent achievements in the field has been the clarification of the role of magnetic anisotropy in the Kondo effect for localized spin systems with a spin larger than S = 1/2.
TL;DR: A new structural type in mixed Mn–Ln SMMs having a {Mn12Gd} 38+ core is reported, in which clear QTM steps have been observed in the hysteresis loops of a mixed 3d–4f SMM for the first time.
Abstract: Single-molecule magnets (SMMs) are individual molecules that function as single-domain nanoscale magnetic particles. A SMM derives its properties from a combination of a high-spin ground state (S) and an easy axis type of magnetoanisotropy (negative zero-field splitting parameter,D), which results in a significant energy barrier to the reversal of the magnetization vector. Such species display both classical magnetization hysteresis, quantum tunneling of magnetization (QTM), and quantum phase interference. Thus, SMMs represent a molecular (“bottom-up”) route to nanoscale magnetism, with potential technological applications in information storage and spintronics at the molecular level, and use as quantum bits (qubits) in quantum computation by exploiting the QTM through the anisotropy barrier. The upper limit to the barrier (U) is given by S jD j or (S-1/4) jD j for integer and half-integer S, respectively. In practice, QTM through upper regions of the barrier makes the true or the effective barrier (Ueff) lower than that of U. Ideally, the QTM can be observed and studied in magnetization vs. DC (direct current) field hysteresis loops, appearing as distinct step-like features at periodic field values, at which levels on either side of the anisotropy barrier to relaxation are in resonance. The steps are thus field positions at which the magnetization relaxation rate increases owing to the onset of QTM. Such steps are a diagnostic signature of resonant QTM, and have been clearly seen only for a few classes of compounds, such as manganese, iron, and nickel SMMs. 7, 8] The most fruitful source of SMMs is the manganese carboxylate chemistry. The prototype was the [Mn12O12(O2CR)16(H2O)4] family, [2,4, 9] and a number of others have since been discovered; almost all have been transition metal clusters, and the vast majority of them have been manganese clusters containing at least some manganese(III) ions. As the search for new SMMs expanded, several groups explored mixed transition metal/lanthanide (Ln) compounds, and particularly Mn–Ln ones, as an attractive area; these efforts were greatly stimulated by the Cu2Tb2 SMM reported by Matsumoto and co-workers. The strategy is obviously to take advantage of the lanthanide ion s significant spin, and/or its large anisotropy, as reflected in a largeD value, to generate SMMs distinctly different from the homometallic ones. Indeed, there are now several Mn–Ln SMMs, including Mn11Ln4, [11] Mn11Gd2, [12] Mn5Ln4, [13a] and Mn6Dy6 [13b] . Many of them have exhibited magnetization hysteresis loops, but unfortunately none of them have displayed resolved QTM steps in these loops. Thus, the incorporation of lanthanide ions has led to a degradation of the quantum properties, as reflected in the QTM steps. The likeliest reason for the degradation of the quantum properties is the step broadening owing to the low-lying excited states resulting from very weak exchange interactions involving the 4f metal ion(s). Herein we report a new structural type in mixed Mn–Ln SMMs having a {Mn12Gd} 38+ core, in which clear QTM steps have been observed in the hysteresis loops of a mixed 3d–4f SMM for the first time. As a result, the D value of a 3d–4f SMM can be measured directly for the first time from the hysteresis data, that is, from magnetic field separation between the steps. The reaction of Mn(O2CPh)2, nBu4NMnO4, Gd(NO3)3, and PhCO2H in a 4:1:4:32 molar ratio in nitromethane produced a dark brown solution, which upon filtration and slow evaporation of the solvent resulted in crystals of [Mn12GdO9(O2CPh)18(O2CH)(NO3)(HO2CPh)] (1) in 40% yield. The structure of 1 consists of a {MnMn11} 35+ cluster with a central Gd ion (Figure 1). The {Mn12Gd} 38+ core is held together by seven m4-O 2 and two m3-O 2 ions. Peripheral ligation is provided by a m4-, three m3-, fourteen m-benzoate groups, a m3-formate group, a chelating NO3 on Mn12, and a terminal benzoic acid on Mn5. The formate probably comes from oxidation of nitromethane by the highly oxidizing MnO4 reagent. The metal oxidation states and the protonation levels of O ions were established by bond valence sum (BVS) calculations and the observation of manganese(III) Jahn–Teller (JT) elongation axes (Figure S1). All manganese atoms are six-coordinate, whereas the gadolinium [*] Dr. T. C. Stamatatos, Prof. Dr. G. Christou Department of Chemistry, University of Florida Gainesville, FL 32611-7200 (USA) Fax: (+1)352-392-8757 E-mail: christou@chem.ufl.edu
TL;DR: The resistivity minimum is strongly dependent on the magnetic field, suggesting that the upturn of the resistivity at low temperatures should be related to spin fluctuation, and the twofold symmetry of susceptibility in the ab plane indicates a stripelike spin structure as observed by neutron scattering.
Abstract: We report that sizable single crystals of BaFe2As2 have been grown with the self-flux method. Measurements and anisotropy of intrinsic transport and magnetic properties from high-quality single crystal are first presented. The resistivity anisotropy (rho{c}/rho{ab}) is as large as 150 and independent of temperature. In contrast to the susceptibility behavior observed in polycrystalline samples, no Curie-Weiss behavior is observed, and a linear-T dependent susceptibility occurs from the spin-density-wave transition temperature, (T{s}), to 700 K. This result suggests that strong antiferromagnetic correlations are present well above T{s}. A twofold symmetry of susceptibility in the ab plane indicates a stripelike spin structure as observed by neutron scattering. The resistivity minimum is strongly dependent on the magnetic field, suggesting that the upturn of the resistivity at low temperatures should be related to spin fluctuation.
Journal Article•
TL;DR: In this paper, the magnetic anisotropy has been shown to play a decisive role in the physics of Kondo screening, and it was shown that a Kondo resonance emerges for large-spin atoms only when the magnetic aisotropic effect creates degenerate ground-state levels that are connected by the spin flip of a screening electron.
Abstract: Localized magnetic moments on surfaces can be screened through the Kondo effect by forming a correlated system with the surrounding conduction electrons. Measurements now show that the orientation of the magnetic moment’s spin relative to the surface has a decisive role in the physics of Kondo screening. In the Kondo effect, a localized magnetic moment is screened by forming a correlated electron system with the surrounding conduction electrons of a non-magnetic host1. Spin S=1/2 Kondo systems have been investigated extensively in theory and experiments, but magnetic atoms often have a larger spin2. Larger spins are subject to the influence of magnetocrystalline anisotropy, which describes the dependence of the magnetic moment’s energy on the orientation of the spin relative to its surrounding atomic environment3,4. Here we demonstrate the decisive role of magnetic anisotropy in the physics of Kondo screening. A scanning tunnelling microscope is used to simultaneously determine the magnitude of the spin, the magnetic anisotropy and the Kondo properties of individual magnetic atoms on a surface. We find that a Kondo resonance emerges for large-spin atoms only when the magnetic anisotropy creates degenerate ground-state levels that are connected by the spin flip of a screening electron. The magnetic anisotropy also determines how the Kondo resonance evolves in a magnetic field: the resonance peak splits at rates that are strongly direction dependent. These rates are well described by the energies of the underlying unscreened spin states.
TL;DR: It is shown that an electric field-induced change of electric polarization is able to toggle the direction of anisotropy in the ferromagnet through the magnetoelectric effect, which links the antiferromagnetic spins to the local polarization in BiFeO3.
Abstract: We report here that a Permalloy layer deposited on top of a multiferroic ${\mathrm{BiFeO}}_{3}$ single crystal acquires an easy magnetic direction along the propagation vector of the cycloidal arrangement of antiferromagnetic moments in ${\mathrm{BiFeO}}_{3}$. This anisotropy originates from a direct magnetic coupling with the canted spins forming the cycloid. Moreover, we show that an electric field-induced change of electric polarization is able to toggle the direction of anisotropy in the ferromagnet through the magnetoelectric effect, which links the antiferromagnetic spins to the local polarization in ${\mathrm{BiFeO}}_{3}$.
TL;DR: In this paper, the epitaxial growth and magnetic properties of Mn2.5Ga thin films, which were deposited on Cr/MgO single crystal substrates by magnetron sputtering, were reported.
Abstract: We report on epitaxial growth and magnetic properties of Mn2.5Ga thin films, which were deposited on Cr/MgO single crystal substrates by magnetron sputtering. X-ray diffraction results revealed the epitaxial relationships as Mn2.5Ga(001)[100]∥Cr(001)[110]∥MgO(001)[100]. The presence of (002) and (011) superlattice peaks indicates that the films were crystallized into DO22 ordered structures. The perpendicular magnetic anisotropy (PMA) properties were found to be related to the extent of DO22 chemical ordering. A giant PMA (Kueff=1.2×107 erg/cm3) and low saturation magnetization (Ms=250 emu/cm3) can be obtained for the film with highest chemical ordering parameter (S=0.8).
TL;DR: In this paper, a voltage-induced magnetization switching in the perpendicular direction under the assistance of magnetic fields was demonstrated, which may open a new window of electric-field controlled spintronics devices.
Abstract: Growing demands for the voltage-driven spintronic applications with ultralow-power consumption have led to new interest in exploring the voltage-induced magnetization switching in ferromagnetic metals. In this study, we observed a large perpendicular magnetic anisotropy change in Au(001)/ultrathin Fe80Co20(001)/MgO(001)/polyimide/indium tin oxide (ITO) junctions, and succeeded in realizing a clear switching of magnetic easy axis between in-plane and perpendicular directions. Furthermore, employing a perpendicularly magnetized film, voltage-induced magnetization switching in the perpendicular direction under the assistance of magnetic fields was demonstrated. These pioneering results may open a new window of electric-field controlled spintronics devices.
TL;DR: In this paper, the spin-torque diode effect in submicron-scale Co60Fe20B20∕MgO∕(CoxFe1−x)80B20 (0⩽x ⩽0.9) magnetic tunnel junctions (MTJs) under perpendicular magnetic fields Hext up to 10kOe was investigated.
Abstract: We investigated the spin-torque diode effect in submicron-scale Co60Fe20B20∕MgO∕(CoxFe1−x)80B20 (0⩽x⩽0.9) magnetic tunnel junctions (MTJs) under perpendicular magnetic fields Hext up to 10kOe. A single peak was clearly observed in every spin-torque diode spectrum and the dependence of resonant frequency fres on Hext was well explained by using Kittel’s formula. It was found that effective demagnetizing fields in the perpendicular-to-plane direction of the Fe-rich CoFeB free layers obtained from the spectra were considerably smaller than those expected from the magnetizations of the free layers. This suggested that the Fe-rich CoFeB free layers exhibited a perpendicular magnetic anisotropy, which agreed well with the reduced switching current density in the MTJs.
TL;DR: In this paper, the influence of thermal annealing on Pt/Co/AlOx trilayers has been investigated up to $450 in terms of magnetic anisotropy as a function of the plasma oxidation time of the AlOx layer.
Abstract: The influence of thermal annealing on Pt/Co/AlOx trilayers has been investigated up to $450\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ as a function of the plasma oxidation time of the AlOx layer. Both magnetic properties of the Co layer and transport properties are strongly modified upon annealing. The perpendicular magnetic anisotropy reaches very large values, while the Hall angle increases with annealing temperature. This study reveals that this trilayer system possesses tunable magnetic anisotropy properties, which can be controlled by varying either oxidation time or annealing temperature. These results are compared with those obtained on standard Pt/Co/Pt trilayers which show, on the contrary, a continuous degradation of their magnetic properties upon annealing.
TL;DR: In this article, the authors used the transport and magneto-optic polar Kerr effect to study ultrathin films of the itinerant ferromagnet and found that below 4 monolayers the films become insulating and their magnetic character changes as they loose their simple ferromagnetic behavior.
Abstract: Ultrathin films of the itinerant ferromagnet ${\text{SrRuO}}_{3}$ were studied using the transport and magneto-optic polar Kerr effect. We find that below 4 monolayers, the films become insulating and their magnetic character changes as they loose their simple ferromagnetic behavior. We observe a strong reduction in the magnetic moment which for 3 monolayers and below lies in the plane of the film. Exchange-bias behavior is observed below the critical thickness and may point to induced antiferromagnetism in contact with ferromagnetic regions.
TL;DR: In this article, the authors proposed a spin valve with perpendicular anisotropy to reduce the critical current needed for spin transfer magnetization reversal while maintaining thermal stability and achieved critical currents as low as 120μA in quasistatic room-temperature measurements of a 45 nm diameter device.
Abstract: We describe nanopillar spin valves with perpendicular anisotropy designed to reduce the critical current needed for spin transfer magnetization reversal while maintaining thermal stability. By adjusting the perpendicular anisotropy and volume of the free element consisting of a [Co/Ni] multilayer, we observe that the critical current scales with the height of the anisotropy energy barrier and we achieve critical currents as low as 120 μA in quasistatic room-temperature measurements of a 45 nm diameter device. The field-current phase diagram of such a device is presented.
TL;DR: In this paper, a voltage-induced magnetization switching in ferromagnetic metals was demonstrated in the direction of the magnetic easy axis between in-plane and perpendicular directions by employing a perpendicularly magnetized film.
Abstract: Growing demands for the voltage-driven spintronic applications with ultralow-power consumption have led to new interest in exploring the voltage-induced magnetization switching in ferromagnetic metals. In this study, we observed a large perpendicular magnetic anisotropy change in Au(001) / ultrathin Fe80Co20(001) / MgO(001) / Polyimide / ITO junctions, and succeeded in realizing a clear switching of magnetic easy axis between in-plane and perpendicular directions. Furthermore, employing a perpendicularly magnetized film, voltage-induced magnetization switching in the perpendicular direction under the assistance of magnetic fields was demonstrated. These pioneering results may open a new window of electric-field controlled spintronics devices.
TL;DR: In this paper, a multiferroic heterostructures were fabricated by growing ferrimagnetic CoFe2O4 films on ferroelectric Pb(Mg1/3Nb2/3)0.7Ti0.3O3 substrates using pulsed laser deposition.
Abstract: Multiferroic heterostructures were fabricated by growing ferrimagnetic CoFe2O4 films on ferroelectric Pb(Mg1/3Nb2/3)0.7Ti0.3O3 substrates using pulsed laser deposition. Upon applying an electric field, the in-plane magnetization of the heterostructures increases and the out-of-plane magnetization decreases. Sharp and reversible changes in magnetization under electric field were also observed for the poled sample. The relative change in magnetization-electric field hysteresis loops were obtained for both the in-plane and out-of-plane magnetizations. Analysis of the results suggests that the electric field induced change in magnetic anisotropy via strain plays an important role in the magnetoelectric coupling in the heterostructures.
Patent•
10 Aug 2009TL;DR: In this article, the magnetic element is used in a magnetic device that includes a contact electrically coupled to the magnetic elements, and a non-magnetic spacer layer is provided between the pinned and free layers.
Abstract: A method and system for providing a magnetic element and a magnetic memory utilizing the magnetic element are described. The magnetic element is used in a magnetic device that includes a contact electrically coupled to the magnetic element. The method and system include providing pinned, nonmagnetic spacer, and free layers. The free layer has an out-of-plane demagnetization energy and a perpendicular magnetic anisotropy corresponding to a perpendicular anisotropy energy that is less than the out-of-plane demagnetization energy. The nonmagnetic spacer layer is between the pinned and free layers. The method and system also include providing a perpendicular capping layer adjoining the free layer and the contact. The perpendicular capping layer induces at least part of the perpendicular magnetic anisotropy in the free layer. The magnetic element is configured to allow the free layer to be switched between magnetic states when a write current is passed through the magnetic element.
TL;DR: In this paper, the behavior of charge carriers in graphene in inhomogeneous perpendicular magnetic fields was studied and two types of one-dimensional magnetic profiles, uniform in one direction, were considered.
Abstract: We study the behavior of charge carriers in graphene in inhomogeneous perpendicular magnetic fields. We consider two types of one-dimensional magnetic profiles, uniform in one direction: a sequence of N magnetic barriers, and a sequence of alternating magnetic barriers and wells. In both cases, we compute the transmission coefficient of the magnetic structure by means of the transfer matrix formalism, and the associated conductance. In the first case the structure becomes increasingly transparent upon increasing N at fixed total magnetic flux. In the second case we find strong wave-vector filtering and resonant effects. We also calculate the band structure of a periodic magnetic superlattice, and find a wave-vector-dependent gap around zero-energy.