Showing papers on "Ferromagnetism published in 2012"

Spin-torque switching with the giant spin hall effect of tantalum

Abstract: Spin currents can apply useful torques in spintronic devices. The spin Hall effect has been proposed as a source of spin current, but its modest strength has limited its usefulness. We report a giant spin Hall effect (SHE) in β-tantalum that generates spin currents intense enough to induce efficient spin-torque switching of ferromagnets at room temperature. We quantify this SHE by three independent methods and demonstrate spin-torque switching of both out-of-plane and in-plane magnetized layers. We furthermore implement a three-terminal device that uses current passing through a tantalum-ferromagnet bilayer to switch a nanomagnet, with a magnetic tunnel junction for read-out. This simple, reliable, and efficient design may eliminate the main obstacles to the development of magnetic memory and nonvolatile spin logic technologies.

Spin-half paramagnetism in graphene induced by point defects

Abstract: T he possibility to induce a magnetic response in graphene by the introduction of defects has been generating much interest, as this would expand the already impressive list of its special properties and allow novel devices where charge and spin manipulation could be combined. So far there have been many theoretical studies (for reviews, see refs 1‐3) predicting that point defects in graphene should carry magnetic moments B and these can in principle couple (anti)ferromagnetically 1‐12 . However, experimental evidence for such magnetism remains both scarce and controversial 13‐16 . Here we show that point defects in graphene—(1) fluorine adatoms in concentrations x gradually increasing to stoichiometric fluorographene CFxD1:0 (ref. 17) and (2) irradiation defects (vacancies)—carry magnetic moments with spin 1=2. Both types of defect lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene’s low-temperature magnetic properties, despite the fact that the maximum response we could achieve was limited to one moment per approximately 1,000 carbon atoms. This limitation is explained by clustering of adatoms and, for the case of vacancies, by the loss of graphene’s structural stability. Our work clarifies the controversial issue of graphene’s magnetism and sets limits for other graphitic compounds. The emerging consensus that magnetism in carbon-based systems can exist is based mostly on a large body of work on magnetic measurements of highly-oriented pyrolytic graphite (HOPG) and carbon films, with many reports of weak ferromagnetic signals at room temperature (T) observed in both pristine HOPG and after itsionirradiation(see,forexample,refs18,19).However,thewhole subject remains controversial, especially concerning (1) the role of possible contamination and (2) the mechanism responsible for the strong interaction required to lead to ferromagnetic ordering at room temperature. Some observations of ferromagnetism are probably artefacts, doing little justice to the subject (one frequent artefact is identified and described in the Supplementary Information, where we show that commonly used HOPG crystals contain micrometre-sized magnetic particles). Adatom magnetism in graphite is also contentious and, for example, different studies of fluorinatedgraphitehavereportedinconsistentresults 20,21 .

Evidence of the existence of magnetism in pristine VX₂ monolayers (X = S, Se) and their strain-induced tunable magnetic properties.

Abstract: First-principles calculations are performed to study the electronic and magnetic properties of VX2 monolayers (X = S, Se). Our results unveil that VX2 monolayers exhibit exciting ferromagnetic behavior, offering evidence of the existence of magnetic behavior in pristine 2D monolayers. Furthermore, interestingly, both the magnetic moments and strength of magnetic coupling increase rapidly with increasing isotropic strain from −5% to 5% for VX2 monolayers. It is proposed that the strain-dependent magnetic moment is related to the strong ionic–covalent bonds, while both the ferromagnetism and the variation in strength of magnetic coupling with strain arise from the combined effects of both through-bond and through-space interactions. These findings suggest a new route to facilitate the design of nanoelectronic devices for complementing graphene.

Transport magnetic proximity effects in platinum.

Abstract: Platinum (Pt) metal, being nonmagnetic and with a strong spin-orbit coupling interaction, has been central in detecting the pure spin current and establishing most of the recent spin-based phenomena. Magnetotransport measurements, both electrical and thermal, conclusively show strong ferromagnetic characteristics in thin Pt films on the ferromagnetic insulator due to the magnetic proximity effects. The pure spin current phenomena measured by Pt, including the inverse spin Hall and the spin Seebeck effects, are thus contaminated and not exclusively established.

Chiral magnetic soliton lattice on a chiral helimagnet.

Abstract: Using Lorenz microscopy and small-angle electron diffraction, we directly present that the chiral magnetic soliton lattice (CSL) continuously evolves from a chiral helimagnetic structure in small magnetic fields in ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_{2}$. An incommensurate CSL undergoes a phase transition to a commensurate ferromagnetic state at the critical field strength. The period of a CSL, which exerts an effective potential for itinerant spins, is tuned by simply changing the field strength. Chiral magnetic orders observed do not exhibit any structural dislocation, indicating their high stability and robustness in ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_{2}$.

Exchange bias in LaNiO3-LaMnO3 superlattices.

Abstract: Interfaces between insulating oxides have revealed exotic electronic and magnetic properties. It is now shown that a complex magnetic structure can emerge in an oxide superlattice, and that specific interfaces can unexpectedly exhibit exchange bias. The observations reveal the induction of antiferromagnetism in a material that is usually paramagnetic.

The Physics of Ferromagnetism

Abstract: Part I: Foundation of magnetism.- Basis of magnetism.- Magnetism of atoms.- Magnetism of solids.- Exchange interaction.- Magnetic anisotropy.- Magnetostrictive effects.- Magnetic domain.- Micromagnetism.- Part II: Magnetic materials.- Soft magnetism.- Hard magnetism.- Part III: Spintronics.- Magnetoresistance effect.- Tunnel magnetoresistance effect.- Magnetoresistive random access memory (MRAM).- Technologies that accompany the development of spintronics devices.

Spin and valley quantum Hall ferromagnetism in graphene

Abstract: The extra states sometimes observed in graphene’s quantum Hall characteristics have been presumed to be the result of broken SU(4) symmetry. Magnetotransport measurements of high-quality graphene in a tilted magnetic field finally prove this is indeed the case.

Tensile strain switched ferromagnetism in layered NbS2 and NbSe2.

Abstract: Developing approaches to effectively induce and control the magnetic states is critical to the use of magnetic nanostructures in quantum information devices but is still challenging. Here we have d...

A universal critical density underlying the physics of electrons at the LaAlO 3 /SrTiO 3 interface

Abstract: The two-dimensional electron system at the interface between the insulating oxides LaAlO3 and SrTiO3 exhibits ferromagnetism, superconductivity and a range of unique magnetotransport properties. An open experimental challenge is to identify, out of the multitudinous energy bands predicted to exist at the interface, the key ingredients underlying its emergent transport phenomena. Here we show, using magnetotransport measurements, that a universal Lifshitz transition between d orbitals of different symmetries lies at the core of the observed phenomena. We find that LaAlO3/SrTiO3 systems generically switch from one- to two-carrier transport at a universal carrier density, which is independent of the LaAlO3 thickness and electron mobility. Interestingly, the maximum superconducting critical temperature occurs also at the Lifshitz density, indicating a possible connection between the two phenomena. A simple band model, allowing for spin-orbit coupling at the atomic level, connects the observed transition to a variety of previously reported properties. Our results demonstrate that the fascinating behaviour observed so far in these oxides follows from a small but fundamental set of bands. When lanthanum aluminate and strontium titanate are brought together, a 2D electron gas with many interesting properties forms at the interface. Magnetotransport results obtained by Joshuaet al. suggest that the behaviour of this interface is governed by a small but fundamental set of electronic bands.

Resistive Switching and Magnetic Modulation in Cobalt‐Doped ZnO

Abstract: A combination of resistive switching and magnetic modulation gives rise to the integration of room temperature ferromagnetism (spin) and electrical properties (charge) into a simple Pt/Co:ZnO/Pt structure due to the formation of oxygen vacancy-based conductive filaments. This is promising for broadening the applications of random access memories to encode quaternary information.

First-Principles Study of Ferromagnetism in Two-Dimensional Silicene with Hydrogenation

Abstract: We performed first-principles simulation on the electronic structure and magnetic properties of two-dimensional hexagonal silicene, which was recently synthesized. The results show that the weak ov...

Theory of laser-induced ultrafast superdiffusive spin transport in layered heterostructures

Abstract: Femtosecond laser excitation of a ferromagnetic material creates energetic spin-polarized electrons that have anomalous transport characteristics. We develop a semiclassical theory that is specifically dedicated to capture the transport of laser-excited nonequilibrium (NEQ) electrons. The randomly occurring multiple electronic collisions, which give rise to electron thermalization, are treated exactly and we include the generation of electron cascades due to inelastic electron-electron scatterings. The developed theory can, moreover, treat the presence of several different layers in the laser-irradiated material. The derived spin-dependent transport equation is solved numerically and it is shown that the hot NEQ electron spin transport occurs neither in the diffusive nor ballistic regime, it is superdiffusive. As the excited spin majority and minority electrons in typical transition-metal ferromagnets (e.g., Fe, Ni) have distinct, energy-dependent lifetimes, fast spin dynamics in the femtosecond (fs) regime is generated, causing effectively a spin current. As examples, we solve the resulting spin dynamics numerically for typical heterostructures, specifically, a ferromagnetic/nonmagnetic metallic layered junction (i.e., Fe/Al and Ni/Al) and a ferromagnetic/nonmagnetic insulator junction (Fe or Ni layer on a large band-gap insulator as, e.g., MgO). For the ferromagnetic/nonmagnetic metallic junction where the ferromagnetic layer is laser-excited, the computed spin dynamics shows that injection of a superdiffusive spin current in the nonmagnetic layer (Al) is achieved. The injected spin current consists of screened NEQ, mobile majority-spin electrons and is nearly 90$%$ spin-polarized for Ni and about 65$%$ for Fe. Concomitantly, a fast demagnetization of the ferromagnetic polarization in the femtosecond regime is driven. The analogy of the generated spin current to a superdiffusive spin Seebeck effect is surveyed.

Diffusive spin dynamics in ferromagnetic thin films with a Rashba interaction

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.

Ultrafast spin dynamics in multisublattice magnets.

Abstract: We propose a general theoretical framework for ultrafast laser-induced spin dynamics in multisublattice magnets. We distinguish relaxation of relativistic and exchange origin and show that when the former dominates, nonequivalent sublattices have distinct dynamics despite their strong exchange coupling. Even more interesting, in the exchange dominated regime sublattices can show highly counterintuitive transitions between parallel and antiparallel alignment. This allows us to explain recent experiments with antiferromagnetically coupled sublattices, and predict that such transitions are possible with ferromagnetic coupling as well. In addition, we predict that exchange relaxation enhances the demagnetization speed of both sublattices only when they are antiferromagnetically coupled.

Magnetic properties of MoS 2 : Existence of ferromagnetism

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.

Probing the timescale of the exchange interaction in a ferromagnetic alloy.

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.

Size Dependence of Defect-Induced Room Temperature Ferromagnetism in Undoped ZnO Nanoparticles

Abstract: We report the intrinsic room-temperature ferromagnetism in undoped ZnO nanoparticles with different sizes synthesized by a wet chemical method at different temperatures. Electron paramagnetic resonance, X-ray photoelectron spectroscopy, and photoluminescence measurements demonstrate clearly the singly charged oxygen vacancies are the main defects, and the relative occupancy of that decreases with increasing sizes and annealing temperatures. Importantly, a direct correlation between the ferromagnetism and the relative concentration of the singly charged oxygen vacancies is established, which suggests that the singly charged oxygen vacancies play a crucial role in modulating ferromagnetic behaviors. Moreover, the size-dependent ferromagnetism can be manipulated conveniently by changing of the surface–volume ratio, which is in favor of future electronic and spintronic application.

Coexisting Ferromagnetic and Ferroelectric Order in a CuCl4-based Organic–Inorganic Hybrid

Abstract: We investigate the structural, magnetic, and dielectric properties of the organic–inorganic hybrid material CuCl4(C6H5CH2CH2NH3)2 and demonstrate that spontaneous ferroelectric order sets in below 340 K, which coexists with ferromagnetic ordering below 13 K. We use X-ray diffraction to show that the electric polarization results from the spatial ordering of hydrogen bonds that link the organic block comprised of phenylethylammonium cations to the inorganic copper chloride block. The hydrogen bond ordering is driven by buckling of the corner-linked copper chloride octahedra. Because the magnetic exchange pathways are also determined by this octahedral buckling, a potentially large magnetoelectric coupling is induced. Our results imply that such hybrids form a new family of multiferroic materials.

Superconducting and ferromagnetic phases in SrTiO3/LaAlO3 oxide interface structures: possibility of finite momentum pairing.

Abstract: We introduce a model to explain the observed ferromagnetism and superconductivity in LAO/STO oxide interface structures. Because of the polar catastrophe mechanism, 1/2 charge per unit cell is transferred to the interface layer. We argue that this charge localizes and orders ferromagnetically via exchange with the conduction electrons. Ordinarily, this ferromagnetism would destroy superconductivity, but, due to strong spin-orbit coupling near the interface, the magnetism and superconductivity can coexist by forming a Fulde-Ferrell-Larkin-Ovchinikov-type condensate of Cooper pairs at finite momentum, which is surprisingly robust in the presence of strong disorder.

Critical thickness for ferromagnetism in LaAlO3/SrTiO3 heterostructures

Abstract: The interface within heterostructures consisting of LaAlO3 and SrTiO3 has been reported to give rise to magnetism, in addition to a two-dimensional electron gas. Kalisky et al. observe that magnetism can occur only above a critical thickness, and that it occurs in heterogeneous patches.

Two-Dimensional Tetragonal TiC Monolayer Sheet and Nanoribbons

Abstract: We report a two-dimensional tetragonal Titanium Carbide (TiC) monolayer sheet with distinguished structure and properties based on comprehensive first-principles calculations. The TiC sheet exhibits a novel zigzag-shaped buckling structure with all atoms being quasiplanar tetracoordinate, as favored by strong in-plane C2p–Ti3d bonding and synergetic out-of-plane electronic delocalization. This unique structure endows the sheet with high kinetic stability and anisotropic mechanical properties. Moreover, the TiC sheet displays orientation-dependent electronic properties derived from its special rectangular symmetry, with indirect band gap of ∼0.2 eV and substantial ferromagnetism along its edges, thus promising for wide applications in nanoelectronics.

Stability, Electronic and Magnetic Properties of In-Plane Defects in Graphene: A First-Principles Study

Abstract: The electronic and magnetic properties of graphene can be modified through combined transition-metal and nitrogen decoration of vacancies. In this study, we used density functional theory to investigate the following defect motifs: nitrogen doping, nitrogen decoration of single and double vacancies (SVs and DVs), TM doping (TM = Co, Fe), TM adsorption on nitrogen-doped graphene, and combined TM–nitrogen chemistries in SV and DV (TM–Nx) configurations. The results show that the highest magnetic moments are supported in TM–Nx defect motifs. Among these defects, Co–N3, Fe–N3, and Fe–N4 defects are predicted to show ferromagnetic spin structures with high magnetic moments and magnetic stabilization energies, as well as enhanced stability as expressed by favorable formation energies, and high TM binding energies.

Giant reversible magnetocaloric effect in ErMn2Si2 compound with a second order magnetic phase transition

Abstract: The magnetic properties and magnetocaloric effect (MCE) in the ternary intermetallic compound ErMn2Si2 have been studied by magnetization and heat capacity measurements. A giant reversible MCE has been observed, accompanied by a second order magnetic phase transition from paramagnetic to ferromagnetic at ∼4.5 K. Under a field change of 5 T, the maximum value of magnetic entropy change (−ΔSMmax) is 25.2 J kg−1 K−1 with no thermal and field hysteresis loss, and the corresponding maximum value of adiabatic temperature change (ΔTadmax) is 12.9 K. Particularly, the values of −ΔSMmax and ΔTadmax reached 20.0 J kg−1 K−1 and 5.4 K for a low field change of 2 T, respectively. The present results indicate that the ErMn2Si2 compound is an attractive candidate for low temperature magnetic refrigeration.