Showing papers on "Magnetization published in 2012"
TL;DR: It is suggested that the SHE torque also affects current-driven magnetic domain wall motion in Pt/ferromagnet bilayers and can enable memory and logic devices with similar critical currents and improved reliability compared to conventional spin-torque switching.
Abstract: We show that in a perpendicularly magnetized Pt/Co bilayer the spin-Hall effect (SHE) in Pt can produce a spin torque strong enough to efficiently rotate and switch the Co magnetization. We calculate the phase diagram of switching driven by this torque, finding quantitative agreement with experiments. When optimized, the SHE torque can enable memory and logic devices with similar critical currents and improved reliability compared to conventional spin-torque switching. We suggest that the SHE torque also affects current-driven magnetic domain wall motion in Pt/ferromagnet bilayers.
1,455 citations
TL;DR: How currents can generate torques that affect the magnetic orientation and the reciprocal effect in a wide variety of magnetic materials and structures is explained.
Abstract: The magnetization of a magnetic material can be reversed by using electric currents that transport spin angular momentum. In the reciprocal process a changing magnetization orientation produces currents that transport spin angular momentum. Understanding how these processes occur reveals the intricate connection between magnetization and spin transport, and can transform technologies that generate, store or process information via the magnetization direction. Here we explain how currents can generate torques that affect the magnetic orientation and the reciprocal effect in a wide variety of magnetic materials and structures. We also discuss recent state-of-the-art demonstrations of current-induced torque devices that show great promise for enhancing the functionality of semiconductor devices.
1,049 citations
TL;DR: Electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy is reported, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities.
Abstract: The advent of spin transfer torque effect accommodates site-specific switching of magnetic nanostructures by current alone without magnetic field. However, the critical current density required for usual spin torque switching remains stubbornly high around 10(6)-10(7) A cm(-2). It would be fundamentally transformative if an electric field through a voltage could assist or accomplish the switching of ferromagnets. Here we report electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities. These results represent a crucial step towards ultralow energy switching in magnetic tunnel junctions, and open a new avenue for exploring other voltage-controlled spintronic devices.
956 citations
TL;DR: A coherent precessional magnetization switching using electric field pulses in nanoscale magnetic cells with a few atomic FeCo (001) epitaxial layers adjacent to a MgO barrier is demonstrated and the realization of bistable toggle switching using the coherentPrecessions is demonstrated.
Abstract: The magnetization direction of a metallic magnet has generally been controlled by a magnetic field or by spin-current injection into nanosized magnetic cells. Both these methods use an electric current to control the magnetization direction; therefore, they are energy consuming. Magnetization control using an electric field is considered desirable because of its expected ultra-low power consumption and coherent behaviour. Previous experimental approaches towards achieving voltage control of magnetization switching have used single ferromagnetic layers with and without piezoelectric materials, ferromagnetic semiconductors, multiferroic materials, and their hybrid systems. However, the coherent control of magnetization using voltage signals has not thus far been realized. Also, bistable magnetization switching (which is essential in information storage) possesses intrinsic difficulties because an electric field does not break time-reversal symmetry. Here, we demonstrate a coherent precessional magnetization switching using electric field pulses in nanoscale magnetic cells with a few atomic FeCo (001) epitaxial layers adjacent to a MgO barrier. Furthermore, we demonstrate the realization of bistable toggle switching using the coherent precessions. The estimated power consumption for single switching in the ideal equivalent switching circuit can be of the order of 10(4)k(B)T, suggesting a reduction factor of 1/500 when compared with that of the spin-current-injection switching process.
725 citations
TL;DR: This work demonstrates the generation of single-mode coherent auto-oscillations in a device that combines local injection of a pure spin current with enhanced spin-wave radiation losses, and suggests a new route for the implementation of nanoscale microwave sources for next-generation integrated electronics.
Abstract: The dynamical processes associated with the magnetization of a material can be drastically altered by the application of a spin current. This study now demonstrates the feasibility of selectively exciting coherent auto-oscillation modes in magnetic nanostructures.
638 citations
TL;DR: Numerically and experimentally a novel mechanism of deterministic magnetization reversal in a ferrimagnet driven by an ultrafast heating of the medium resulting from the absorption of a sub-picosecond laser pulse without the presence of a magnetic field is shown.
Abstract: The question of how, and how fast, magnetization can be reversed is a topic of great practical interest for the manipulation and storage of magnetic information. It is generally accepted that magnetization reversal should be driven by a stimulus represented by time-non-invariant vectors such as a magnetic field, spin-polarized electric current, or cross-product of two oscillating electric fields. However, until now it has been generally assumed that heating alone, not represented as a vector at all, cannot result in a deterministic reversal of magnetization, although it may assist this process. Here we show numerically and demonstrate experimentally a novel mechanism of deterministic magnetization reversal in a ferrimagnet driven by an ultrafast heating of the medium resulting from the absorption of a sub-picosecond laser pulse without the presence of a magnetic field.
600 citations
TL;DR: It is shown that antiferromagnetic nanostructures, composed of just a few Fe atoms on a surface, exhibit two magnetic states, the Néel states, that are stable for hours at low temperature.
Abstract: Control of magnetism on the atomic scale is becoming essential as data storage devices are miniaturized. We show that antiferromagnetic nanostructures, composed of just a few Fe atoms on a surface, exhibit two magnetic states, the Neel states, that are stable for hours at low temperature. For the smallest structures, we observed transitions between Neel states due to quantum tunneling of magnetization. We sensed the magnetic states of the designed structures using spin-polarized tunneling and switched between them electrically with nanosecond speed. Tailoring the properties of neighboring antiferromagnetic nanostructures enables a low-temperature demonstration of dense nonvolatile storage of information.
553 citations
TL;DR: Four 52-metal-ion 3d-4f cluster complexes featuring a common core of Ln(42)M(10) were obtained through self-assembly of the metal ions templated by mixed anions (ClO(4)(-) and CO(3)(2-)).
Abstract: Four 52-metal-ion 3d–4f cluster complexes featuring a common core of Ln42M10 (Ln = Gd3+, Dy3+; M = Co2+/3+, Ni2+) were obtained through self-assembly of the metal ions templated by mixed anions (ClO4– and CO32–). Magnetic studies revealed that the Gd42Co10 and Gd42Ni10 clusters exhibit the largest magnetocaloric effect (MCE) among any known 3d–4f complexes. Replacement of Gd3+ ions with anisotropic Dy3+ ions caused significant changes in the magnetic behavior of the clusters; both Dy42Co10 and Dy42Ni10 displayed slow relaxation of the magnetization.
414 citations
TL;DR: It is shown that owing to the coupling between magnetization and ferro electric polarization at the interface between the electrode and barrier of a multiferroic tunnel junction, the spin polarization of the tunnelling electrons can be reversibly and remanently inverted by switching the ferroelectric polarization ofThe barrier.
Abstract: Spin-polarized transport in ferromagnetic tunnel junctions, characterized by tunnel magnetoresistance, has already been proven to have great potential for application in the field of spintronics and in magnetic random access memories. Until recently, in such a junction the insulating barrier played only a passive role, namely to facilitate electron tunnelling between the ferromagnetic electrodes. However, new possibilities emerged when ferroelectric materials were used for the insulating barrier, as these possess a permanent dielectric polarization switchable between two stable states. Adding to the two different magnetization alignments of the electrode, four non-volatile states are therefore possible in such multiferroic tunnel junctions. Here, we show that owing to the coupling between magnetization and ferroelectric polarization at the interface between the electrode and barrier of a multiferroic tunnel junction, the spin polarization of the tunnelling electrons can be reversibly and remanently inverted by switching the ferroelectric polarization of the barrier. Selecting the spin direction of the tunnelling electrons by short electric pulses in the nanosecond range rather than by an applied magnetic field enables new possibilities for spin control in spintronic devices.
409 citations
TL;DR: It is visualize that cube has higher magnetization value than sphere with highest coercivity at 60 nm, and its hybridization into core-shell (CS) structure brings about a 14-fold increase in the coercivity with an exceptional energy conversion of magnetic field into thermal energy, the largest reported to date.
Abstract: With the aim of controlling nanoscale magnetism, we demonstrate an approach encompassing concepts of surface and exchange anisotropy while reflecting size, shape, and structural hybridization of nanoparticles. We visualize that cube has higher magnetization value than sphere with highest coercivity at 60 nm. Its hybridization into core–shell (CS) structure brings about a 14-fold increase in the coercivity with an exceptional energy conversion of magnetic field into thermal energy of 10600 W/g, the largest reported to date. Such capability of the CS-cube is highly effective for drug resistant cancer cell treatment.
384 citations
TL;DR: In this paper, a sputtered CoFeB/MgO-based magnetic tunnel junction with a perpendicular magnetic easy axis in a static external magnetic field is realized for a ∼180° magnetization reversal, where the bias voltage pulse duration is adjusted to a half period of the precession.
Abstract: The electric field-induced ∼180° magnetization reversal is realized for a sputtered CoFeB/MgO-based magnetic tunnel junction with perpendicular magnetic easy axis in a static external magnetic field. Application of bias voltage with nanoseconds duration results in a temporal change of magnetic easy axis in the free layer CoFeB to in-plane, which induces precessional motion of magnetization in the free layer. The magnetization reversal takes place when the bias voltage pulse duration is adjusted to a half period of the precession. We show that the back and forth magnetization reversal can be observed by using successive application of half-period voltage pulses.
TL;DR: A facile magnetization of MOF MIL-101(Cr) is shown for rapid magnetic solid-phase extraction of polycyclic aromatic hydrocarbons (PAHs) from environmental water samples and the results showed that hydrophobic and π-π interactions between the PAHs and the framework terephthalic acid molecules, and the ρ-complexation between PAHS and the Lewis acid sites in the pores of MIL- 101 play a significant role in the ads
Abstract: The unusual properties such as high surface area, good thermal stability, uniform structured nanoscale cavities and the availability of in-pore functionality and outer-surface modification make metal–organic frameworks (MOFs) attractive for diverse analytical applications. However, integration of MOFs with magnets for magnetic solid-phase extraction for analytical application has not been attempted so far. Here we show a facile magnetization of MOF MIL-101(Cr) for rapid magnetic solid-phase extraction of polycyclic aromatic hydrocarbons (PAHs) from environmental water samples. MIL-101 is attractive as a sorbent for solid-phase extraction of pollutants in aqueous solution due to its high surface area, large pores, accessible coordinative unsaturated sites, and excellent chemical and solvent stability. In situ magnetization of MIL-101 microcrystals as well as magnetic solid-phase extraction of PAHs was achieved simultaneously by simply mixing MIL-101 and silica-coated Fe3O4 microparticles in a sample solution under sonication. Such MOF-based magnetic solid-phase extraction in combination with high-performance liquid chromatography gave the detection limits of 2.8–27.2 ng L−1 and quantitation limits of 6.3–87.7 ng L−1 for the PAHs. The relative standard deviations for intra- and inter-day analyses were in the range of 3.1–8.7% and 6.1–8.5%, respectively. The results showed that hydrophobic and π–π interactions between the PAHs and the framework terephthalic acid molecules, and the π-complexation between PAHs and the Lewis acid sites in the pores of MIL-101 play a significant role in the adsorption of PAHs.
TL;DR: In this paper, the authors present a compact model of the CoFeB/MgO PMA MTJ, a system exhibiting the best tunnel magnetoresistance ratio and switching performance.
Abstract: Magnetic tunnel junctions (MTJs) composed of ferromagnetic layers with perpendicular magnetic anisotropy (PMA) are of great interest for achieving high-density nonvolatile memory and logic chips owing to its scalability potential together with high thermal stability. Recent progress has demonstrated a capacity for high-speed performance and low power consumption through current-induced magnetization switching. In this paper, we present a compact model of the CoFeB/MgO PMA MTJ, a system exhibiting the best tunnel magnetoresistance ratio and switching performance. It integrates the physical models of static, dynamic, and stochastic behaviors; many experimental parameters are directly included to improve the agreement of simulation with experimental measurements. Mixed simulation based on the 65-nm technology node of a magnetic flip-flop validates its relevance and efficiency for MTJ/CMOS memory and logic chip design.
TL;DR: A long-wavelength helimagnetic superstructure in bulk samples of the ferrimagnetic insulator Cu2OSeO3 is reported, providing putative evidence for effective spin currents as the origin of enhancements of the magnetodielectric response instead of atomic displacements considered so far.
Abstract: We report a long-wavelength helimagnetic superstructure in bulk samples of the ferrimagnetic insulator Cu2OSeO3. The magnetic phase diagram associated with the helimagnetic modulation inferred from small-angle neutron scattering and magnetization measurements includes a skyrmion lattice phase and is strongly reminiscent of MnSi, FeGe, and Fe1-xCoxSi, i.e., binary isostructural siblings of Cu2OSeO3 that order helimagnetically. The temperature dependence of the specific heat of Cu2OSeO3 is characteristic of nearly critical spin fluctuations at the helimagnetic transition. This provides putative evidence for effective spin currents as the origin of enhancements of the magnetodielectric response instead of atomic displacements considered so far.
TL;DR: This work demonstrates an effective way to manipulate the quantum transport properties of the topological insulators by breaking time-reversal symmetry.
Abstract: We report transport studies on magnetically doped Bi(2)Se(3) topological insulator ultrathin films grown by molecular beam epitaxy. The magnetotransport behavior exhibits a systematic crossover between weak antilocalization and weak localization with the change of magnetic impurity concentration, temperature, and magnetic field. We show that the localization property is closely related to the magnetization of the sample, and the complex crossover is due to the transformation of Bi(2)Se(3) from a topological insulator to a topologically trivial dilute magnetic semiconductor driven by magnetic impurities. This work demonstrates an effective way to manipulate the quantum transport properties of the topological insulators by breaking time-reversal symmetry.
TL;DR: In this article, a combined experimental and theoretical study of the macroscopic response of a particular MRE consisting of a rubber matrix phase with spherical carbonyl iron particles is presented.
Abstract: Magnetorheological elastomers (MREs) are ferromagnetic particle impregnated rubbers whose mechanical properties are altered by the application of external magnetic fields. Due to their coupled magnetoelastic response, MREs are finding an increasing number of engineering applications. In this work, we present a combined experimental and theoretical study of the macroscopic response of a particular MRE consisting of a rubber matrix phase with spherical carbonyl iron particles. The MRE specimens used in this work are cured in the presence of strong magnetic fields leading to the formation of particle chain structures and thus to an overall transversely isotropic composite. The MRE samples are tested experimentally under uniaxial stresses as well as under simple shear in the absence or in the presence of magnetic fields and for different initial orientations of their particle chains with respect to the mechanical and magnetic loading direction. Using the theoretical framework for finitely strained MREs introduced by Kankanala and Triantafyllidis (2004) , we propose a transversely isotropic energy density function that is able to reproduce the experimentally measured magnetization, magnetostriction and simple shear curves under different prestresses, initial particle chain orientations and magnetic fields. Microscopic mechanisms are also proposed to explain (i) the counterintuitive effect of dilation under zero or compressive applied mechanical loads for the magnetostriction experiments and (ii) the importance of a finite strain constitutive formulation even at small magnetostrictive strains. The model gives an excellent agreement with experiments for relatively moderate magnetic fields but has also been satisfactorily extended to include magnetic fields near saturation.
TL;DR: This work provides a route to realize large and nonvolatile magnetoelectric coupling at room temperature and is significant for applications.
Abstract: New iron selenide superconductors by intercalating smaller-sized alkali metals (Li, Na) and alkaline earths using high-temperature routes have been pursued ever since the discovery of superconductivity at about 30 K in KFe2Se2, but all have failed so far. Here we demonstrate that a series of superconductors with enhanced T-c=30 similar to 46 K can be obtained by intercalating metals, Li, Na, Ba, Sr, Ca, Yb, and Eu in between FeSe layers by the ammonothermal method at room temperature. Analysis on their powder X-ray diffraction patterns reveals that all the main phases can be indexed based on body-centered tetragonal lattices with a similar to 3.755-3.831 angstrom while c similar to 15.99-20.54 angstrom. Resistivities show the corresponding sharp transitions at 45 K and 39 K for NaFe2Se2 and Ba0.8Fe2Se2, respectively, confirming their bulk superconductivity. These findings provide a new starting point for studying the properties of these superconductors and an effective synthetic route for the exploration of new superconductors as well.
TL;DR: In this paper, X-ray diffraction patterns and transmission electron microscopy images confirmed the formation of PANI coating and incorporation of BaTiO3 or Fe3O4 nanoparticles.
Abstract: Composite absorbers based on conducting fabrics possessing moderate conductivity and dielectric/magnetic properties were prepared by in situ incorporation of nanoparticles of BaTiO3 (15–25 nm) or Fe3O4 (25–40 nm) within coated poly(aniline) (PANI) matrix. The X-ray diffraction patterns and transmission electron microscopy images confirmed the formation of PANI coating and incorporation of BaTiO3 or Fe3O4 nanoparticles. Scanning electron microscopy images show formation of thick and uniform coating of PANI over individual fibers and in interweave regions. The dielectric studies show that incorporation of BaTiO3 lead to enhancement of dielectric properties of PANI whereas magnetization measurements revealed that incorporation of Fe3O4 resulted in noticeable improvement in magnetic properties with saturation magnetization of 17.9 emu/g. The Ku-band (12.4–18.0 GHz) shielding studies revealed that pure PANI-coated fabric show total shielding effectiveness (SET) of −15.3 dB which enhanced to −16.8 and −19.4 dB ...
TL;DR: In this paper, the spin dynamics in magnetic trilayers can be driven by ultrafast light pulses, resulting in transient magnetization changes on femtosecond timescales.
Abstract: Spin dynamics in magnetic materials can be driven by ultrafast light pulses, resulting in transient magnetization changes on femtosecond timescales. Rudolph et al. find that in magnetic trilayers the magnetization of one layer can be enhanced by superdiffusive spin currents from adjacent layers.
TL;DR: A series of Mn doped cobalt ferrite compounds with the formula Mn x Co 1− x Fe 2 O 4 where x ǫ = 0.0, 0.2, 0.4, 0., 0.8 and 1.0 were successfully synthesized by polyethylene glycol-assisted hydrothermal method as discussed by the authors.
TL;DR: Two ways to relax: A defect-dicubane Co2Dy2 single-molecule magnet displays slow relaxation of magnetization with a blocking temperature of 22 K (at 1500 Hz), the highest reported for a 3d–4f-based SMM.
Abstract: Two ways to relax: A defect-dicubane Co2Dy2 single-molecule magnet (SMM) displays slow relaxation of magnetization with a blocking temperature of 22 K (at 1500 Hz), the highest reported for a 3d–4f-based SMM. Analysis of the relaxation reveals two distinct blocking regimes, one of which is intraionic, localized on the DyIII ions, while the other is exchange-based.
TL;DR: In this paper, the authors examined the trends in structural, electronic properties and relative stability of a representative group of 2D (graphene-like, GL) materials: titanium carbides and nitrides.
TL;DR: An approach to electrically control local magnetic properties, including the writing and erasure of regular ferromagnetic domain patterns and the motion of magnetic domain walls, in CoFe-BaTiO3 heterostructures with negligible magnetocrystalline anisotropy is reported on.
Abstract: Spintronic devices currently rely on magnetic switching or controlled motion of domain walls by an external magnetic field or spin-polarized current. Achieving the same degree of magnetic controllability using an electric field has potential advantages including enhanced functionality and low power consumption. Here we report on an approach to electrically control local magnetic properties, including the writing and erasure of regular ferromagnetic domain patterns and the motion of magnetic domain walls, in CoFe-BaTiO3 heterostructures. Our method is based on recurrent strain transfer from ferroelastic domains in ferroelectric media to continuous magnetostrictive films with negligible magnetocrystalline anisotropy. Optical polarization microscopy of both ferromagnetic and ferroelectric domain structures reveals that domain correlations and strong inter-ferroic domain wall pinning persist in an applied electric field. This leads to an unprecedented electric controllability over the ferromagnetic microstructure, an accomplishment that produces giant magnetoelectric coupling effects and opens the way to electric-field driven spintronics.
TL;DR: In this paper, the authors investigated the magnetic properties of electron-doped La0.23Ca0.77MnO3 manganite nanoparticles, with average size of 12 and 60 nm, prepared by the glycine-nitrate method, in the temperature range 5-300k and magnetic fields up to 90kOe.
Abstract: Magnetic properties of electron-doped La0.23Ca0.77MnO3 manganite nanoparticles, with average size of 12 and 60 nm, prepared by the glycine–nitrate method, have been investigated in the temperature range 5–300 K and magnetic fields up to 90 kOe. It is suggested that weak ferromagnetic moment results from ferromagnetic shells of the basically antiferromagnetic nanoparticles and from domains of frustrated disordered phase in the core. Assumption of two distinct sources of ferromagnetism is supported by the appearance of two independent ferromagnetic contributions in the fit of the T
3/2 Bloch law to spontaneous magnetization. The ferromagnetic components, which are more pronounced in smaller particles, occupy only a small fraction of the nanoparticle volume and the antiferromagnetic ground state remains stable. It is found that the magnetic hysteresis loops following field cooled processes, display size-dependent horizontal and vertical shifts, namely, exhibiting exchange bias effect. Time-dependent magnetization dynamics demonstrating two relaxation rates were observed at constant magnetic fields upon cooling to T < 100 K.
TL;DR: The theory provides an explanation for the mechanism driving the magnetization switching in a single ferromagnet as observed in the recent experiments.
Abstract: In a ferromagnetic metal layer, the coupled charge and spin diffusion equations are obtained in the presence of both Rashba spin-orbit interaction and magnetism. The misalignment between the magnetization and the nonequilibrium spin density induced by the Rashba field gives rise to Rashba spin torque acting on the ferromagnetic order parameter. In a general form, we find that the Rashba torque consists of both in-plane and out-of-plane components, i.e., T=T(perpendicular)y^×m^+T(parallel)m^×(y^×m^). Numerical simulations on a two-dimensional nanowire consider the impact of diffusion on the Rashba torque and reveal a large enhancement to the ratio T(parallel)/T(perpendicular) for thin wires. Our theory provides an explanation for the mechanism driving the magnetization switching in a single ferromagnet as observed in the recent experiments.
TL;DR: Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron.
Abstract: The air stable complex [(PNP)FeCl2] (1) (PNP = N[2-P(CHMe2)2-4-methylphenyl]2–), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh3, displays an unexpected S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex 1 to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of Ueff = 32–36 cm–1 (47–52 K). Variable-temperature Mossbauer data also corroborate a significant temperature dependence in δ and ΔEQ values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state th...
TL;DR: The magnetic properties of MoS2 measured from room temperature down to 10 K and magnetic fields up to 5 T have been investigated in this article, where it was shown that single crystals display ferromagnetism superimposed onto large temperature-dependent diamagnetic diameters.
Abstract: We report on the magnetic properties of MoS2 measured from room temperature down to 10 K and magnetic fields up to 5 T. We find that single crystals of MoS2 display ferromagnetism superimposed onto large temperature-dependent diamagnetism and have observed that ferromagnetism persists from 10 K up to room temperature. We attribute the existence of ferromagnetism partly to the presence of zigzag edges in the magnetic ground state at the grain boundaries. Since the magnetic measurements are relatively insensitive to the interlayer coupling, these results are expected to be valid in the single layer limit.
TL;DR: This work uses extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (permalloy), and shows that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni.
Abstract: The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies.
TL;DR: In this paper, the authors present results of detailed experimental and theoretical studies of all-optical magnetization reversal by single circularly-polarized laser pulses in ferrimagnetic rare earth transition metal alloys.
Abstract: We present results of detailed experimental and theoretical studies of all-optical magnetization reversal by single circularly-polarized laser pulses in ferrimagnetic rare earth---transition metal (RE--TM) alloys Gd${}_{x}$Fe${}_{90\ensuremath{-}x}$Co${}_{10}$ ($20%lxl28%$). Using single-shot time-resolved magneto-optical microscopy and multiscale simulations, we identified and described the unconventional path followed by the magnetization during the reversal process. This reversal does not involve precessional motion of magnetization but is governed by the longitudinal relaxation and thus has a linear character. We demonstrate that this all-optically driven linear reversal can be modeled as a result of a two-fold impact of the laser pulse on the medium. First, due to absorption of the light and ultrafast laser-induced heating, the medium is brought to a highly nonequilibrium state. Simultaneously, due to the ultrafast inverse Faraday effect the circularly polarized laser pulse acts as an effective magnetic field of the amplitude up to $\ensuremath{\sim}$20 T. We show that the polarization-dependent reversal triggered by the circularly polarized light is feasible only in a narrow range (below 10%) of laser fluences. The duration of the laser pulse required for the reversal can be varied from $\ensuremath{\sim}$40 fs up to at least $\ensuremath{\sim}$1700 fs. We also investigate experimentally the role of the ferrimagnetic properties of GdFeCo in the all-optical reversal. In particular, the optimal conditions for the all-optical reversal are achieved just below the ferrimagnetic compensation temperature, where the magnetic information can be all-optically written by a laser pulse of minimal fluence and read out within just 30 ps. We argue that this is the fastest write-read event demonstrated for magnetic recording so far.
TL;DR: A comprehensive tutorial-style review of the achievements of fifty years of development and generalizations of the seminal work of Brown [Phys. Rev. 130, 1677 (1963)] on thermal fluctuations of magnetic nanoparticles is presented in this article.
Abstract: The reversal time, superparamagnetic relaxation time, of the magnetization of fine single domain ferromagnetic nanoparticles owing to thermal fluctuations plays a fundamental role in information storage, paleomagnetism, biotechnology, etc. Here a comprehensive tutorial-style review of the achievements of fifty years of development and generalizations of the seminal work of Brown [Phys. Rev. 130, 1677 (1963)] on thermal fluctuations of magnetic nanoparticles is presented. Analytical as well as numerical approaches to the estimation of the damping and temperature dependence of the reversal time based on Brown's Fokker-Planck equation for the evolution of the magnetic moment orientations on the surface of the unit sphere are critically discussed while the most promising directions for future research are emphasized.