Nanomagnets with high shape anisotropy and strong crystalline anisotropy: perspectives on magnetic force microscopy

We report on a new approach for magnetic imaging, highly sensitive even in the presence of external, strong magnetic fields. Based on FIB-assisted fabricated high-aspect-ratio rare-earth nanomagnets, we produce groundbreaking magnetic force tips with hard magnetic character where we combine a high aspect ratio (shape anisotropy) together with strong crystalline anisotropy (rare-earth-based alloys). Rare-earth hard nanomagnets are then FIB-integrated to silicon microcantilevers as highly sharpened tips for high-field magnetic imaging applications. Force resolution and domain reversing and recovery capabilities are at least one order of magnitude better than for conventional magnetic tips. This work opens new, pioneering research fields on the surface magnetization process of nanostructures based either on relatively hard magnetic materials-used in magnetic storage media-or on materials like superparamagnetic particles, ferro/antiferromagnetic structures or paramagnetic materials.     

Magnetic multilayers on nanospheres

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

Decoherence in grazing scattering of fast H and He atoms from a LiF(001) surface

The influence of decoherence on the diffraction during grazing scattering of fast hydrogen and helium atoms from a LiF(001) single crystal surface with projectile energies of some keV, is investigated by two-dimensional angular distributions for scattered projectiles in coincidence with their energy loss and emitted electrons from the target surface. For keV hydrogen atoms, we identify the excitations of electrons and excitons of the target surface as the dominant mechanisms for decoherence, whereas for keV helium atoms these contributions are negligibly small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.     

Interface effects on the magnetism of CoPt-supported nanostructures

The magnetism of CoPt nanostructures supported on the MgO(100) surface is investigated via first-principles simulations using 1D models. Nanostructures with L1(0) chemical ordering and cube-on-cube epitaxy are predicted to possess large magnetic moments and easy magnetization axis perpendicular to the surface. However, their magnetic anisotropy energy is roughly halved with respect to the bulk alloy due to a peculiar mixing of particle and support electronic states. The general factors at play in determining this behavior and the implications of these findings are discussed in view of designing room-temperature magnetic bits.     

Templates as Shadow Masks to Tune the Magnetic Anisotropy in Nanostructured CoCrPt/Ti Bilayer Films

Off‐axis deposition of Ti and CoCrPt films onto lithographically patterned templates has been used to make nanostructures with a lateral thickness variation that allows the tuning of the magnetic anisotropy. CoCrPt rectangles of 1 μm × 725 nm without a thickness variation show an out‐of‐plane easy axis and a single‐domain configuration after demagnetization. On the other hand, rectangles with a thickness variation along their longer dimension show an out‐of‐plane multidomain state, but an in‐plane vortex configuration occurs when the thickness variation is along the shorter dimension. The evolution of the magnetic behavior is understood from the change in both Ti and CoCrPt thicknesses and their effects on the magnetic anisotropy, and provides a simple method for controlling the magnetic state and reversal process of patterned nanostructures.     

Anisotropic and high magnetization rare earth transition metal compounds containing metalloids

The structural and magnetic behavior is presented for selected metalloid (B,C) containing hexagonal and tetragonal rare earth-transition metal compounds and compound series. Focus is on materials with high Fe content and resulting high magnetizations. The unusual axial ratios and features of the sheet type structures of these materials have pronounced consequences on such properties as magnetic anisotropy and magnetic hardness. Individual site anisotropy contributions are studied by temperature dependence of magnetization along easy and hard magnetic axes. As an example it is found that tetragonal Nd₂Fe₁₄B has axial anisotropy with H_A = 76 kOe at 300 K but shows tendencies for a spin reorientation around 150 K. Y₂Fe₁₄B has axial anisotropy with H_A = 25 kOe but does not exhibit a similar spin reorientation. This indicates that the two crystallographic Nd sites (4f and 4g) have axis and plane preference respectively, with different temperature dependencies. Axial Nd anisotropy is a consequence of the lack of Nd coordination along the z axis due to intervening thick Fe layers. Both extrinsic (fine particle) and intrinsic magnetic hardness is observed. Crystallographically disordered materials show intrinsic hardness based on domain wall pinning by local fluctuations of magnetic parameters. Strong nucleation phenomena are characteristic for ordered materials in bulk and powder form. The unusually high achievable ratios of extrinsic coercivities to anisotropy fields in the metalloid stabilized materials are related to their chemically relatively inert layer structure. This appears to lead to less corrugated surface structures and is so responsible for the characteristic domain wall nucleation processes.     

Magnetic anisotropy variation of Fe single atom on Ti/Al(001) surface by the change of Ti-Al surface phase

Using density functional theory based ab initio calculations, we investigated the effects of Ti/Al(001) surface phase variation on the Fe adatom magnetism. The symmetry of the in-plane magnetic anisotropy of the Fe adatomcorresponded to the symmetry of the Ti and Al atomic configurations on the top surface. When B2 or L1(2) structures of Ti and Al atoms were formed on the surface, the energy barriersfor the Fe in-plane magnetization rotations were smaller than the case of the bare Al(001) surface. The out-of-plane magnetization of Fe adatoms were induced only on the Al-terminated substrates while the Fe on the Ti-appearing surface had its magnetic easy axis in the in-plane directions. The magnetic anisotropy energy magnitude was, on the other hand, largely determined by the underlayer composition of Ti-Al alloy. The decomposed 3d-electron density of states showed that the 3d(xy) and 3d(z2) orbitals of Fe adatoms provide the main contribution to the variation of the magnetic anisotropy energy.     

Selective sensitivity of ellipsometry to magnetic nanostructures

Magneto-optic (MO) ellipsometry of ferromagnetic materials is extremely sensitive to ultra-thin films, multilayers, and nanostructures. It gives a possibility to measure all components of the magnetization vector in the frame of the magneto-optic vector magnetometry and enables us to separate magnetic contributions from different depths and materials in nanostructures, which is reviewed in this article. The method is based on ellipsometric separation using the selective MO Kerr effect. The figure of merit used to quantify the ellipsometric selectivity to magnetic nanostructures is defined on the basis of linear matrix algebra. We show that the method can be also used to separate MO contributions from areas of the same ferromagnetic materials deposited on different buffer layers. The method is demonstrated using both: (i) modeling of the MO ellipsometry response and (ii) MO measurement of ultra-thin Co islands epitaxially grown on self-organized gold islands on Mo/Al₂O₃ buffer layer prepared using the molecular beam epitaxy at elevated temperatures. The system is studied using longitudinal (in-plane) and polar (perpendicular) MO Kerr effects.     

On calculating the magnetic state of nanostructures

We review some of our recent work on first principles calculations of the magnetic structure of surface and bulk nanostructures. The calculations are based on implementation of relativistic density functional theory within state of the art surface embedding and order-N multiple scattering Green’s function methods. First principles spin-dynamics and the constrained local moment approximation are reviewed as they relate to optimization of moment configurations in highly inhomogeneous materials such as surface and bulk nanostructures. Results are present for three prototypical nanostructures – short Co-chains adjacent to a Pt{1 1 1}-surface step-edge, a Cr-trimer on the Au{1 1 1}-surface, and Fe-chains and impurities in Cu – that illustrate the need to treat the underlying electronic interactions on a fully self-consistent basis in which the very different energy scales appropriate to exchange coupling and magneto-crystalline anisotropy are treated on an equal footing.     

Magnetocrystalline anisotropy of zinc-blende CrTe (001) surface: A first-principles study

By using the highly precise all-election full potential linearized augmented plane-wave method based on density functional theory within the generalized gradient approximation, we investigated magnetocrystalline anisotropy and magnetism of zinc-blende CrTe (001) surface. We observed that both of the Cr- and Te-terminated zinc-blende CrTe (001) surfaces maintain the half-metallicity by analyzing the density of states. The magnitudes of the calculated magnetic moments for the Cr(S) and Cr(S-1) atoms are calculated to be 3.92 and 3.16 μ_B for the Cr- and Te-terminated surfaces, respectively. The Te atoms show significantly induced negative spin polarizations of 0.13-0.30 μ_B. The spin orientations at the Te-terminated (Cr-terminated) surfaces were found to be in-plane (out-of-plane) regardless of its thickness. Since a Te-terminated surface is known as to be more stable than a Cr-terminated one, our result is consistent with an experiment which observed in-plane magnetic anisotropy at a CrTe (001) surface.     

The physics of magnetic materials from ab-initio calculations

Recent advances on ab-initio computation of physical properties of magnetic materials are reviewed. The emphasis is put forward regarding the new development of the electronic structure methods, namely the calculation of magnetic anisotropy energy, X-ray magnetic dichroism, non-collinear magnetism, spin density wave, and spin fluctuations in materials. These theoretical advances have lead to new levels of understanding of magnetic materials. In particular, new results on magnetic anisotropy, surface and interface magnetism, and magnetic alloys will be briefly discussed.     

Nanopatterning the electronic properties of gold surfaces with self-organized superlattices of metallic nanostructures

The self-organized growth of nanostructures on surfaces could offer many advantages in the development of new catalysts, electronic devices and magnetic data-storage media. The local density of electronic states on the surface at the relevant energy scale strongly influences chemical reactivity, as does the shape of the nanoparticles. The electronic properties of surfaces also influence the growth and decay of nanostructures such as dimers, chains and superlattices of atoms or noble metal islands. Controlling these properties on length scales shorter than the diffusion lengths of the electrons and spins (some tens of nanometres for metals) is a major goal in electronics and spintronics. However, to date, there have been few studies of the electronic properties of self-organized nanostructures. Here we report the self-organized growth of macroscopic superlattices of Ag or Cu nanostructures on Au vicinal surfaces, and demonstrate that the electronic properties of these systems depend on the balance between the confinement and the perturbation of the surface states caused by the steps and the nanostructures' superlattice. We also show that the local density of states can be modified in a controlled way by adjusting simple parameters such as the type of metal deposited and the degree of coverage.     

Effects of AlN layer thickness on magnetic anisotropy and transport phenomena of CoPt/AlN layered structure

The magnetic anisotropy was studied as a function of the AlN layer thickness in [AlN(x nm)/CoPt(2 nm)]5/AlN(x nm) layered structure (x is AlN layer thickness, and 5 is the number of multilayer series). The multilayered film was deposited by a sputtering apparatus equipped with two pairs of facing targets. It was found that, in the range of AlN layer thickness below 30 nm, CoPt/AlN multilayers transform from an enhanced in-plane magnetic anisotropy to perpendicular magnetic anisotropy (PMA) through thermal annealing in vacuum, with an optimized AlN thickness of 10 nm for strong PMA. However, beyond this thickness range, the PMA did not occur, and thermal annealing only results in magnetic isotropy in both parallel and perpendicular directions. The related structure analysis revealed that smooth interface and good texture of CoPt (111) make positive contributions to interface anisotropy energy and magnetocrystalline anisotropy energy for producing PMA in CoPt/AlN layered structure. In addition, the transport phenomena were also studied by using a four-probe method.     

Transition-metal dimers and physical limits on magnetic anisotropy

Recent advances in nanoscience have raised interest in the minimum bit size required for classical information storage. This bit size is determined by the necessity for bistability with suppressed quantum tunnelling and energy barriers that exceed ambient temperatures. In the case of magnetic information storage, much attention has centred on molecular magnets with bits consisting of about 100 atoms, magnetic uniaxial anisotropy energy barriers of about 50 K and very slow relaxation at low temperatures. Here, we draw attention to the remarkable magnetic properties of some transition-metal dimers, which have energy barriers approaching 500 K with only two atoms. The spin dynamics of these ultrasmall nanomagnets is strongly affected by a Berry phase, which arises from quasi-degeneracies at the electronic highest occupied molecular orbital energy. In a giant-spin approximation, this Berry phase makes the effective reversal barrier thicker.     

Numerical experiments on effect of morphology on wave propagation in nanoporous materials

Mechanics of nanoporous metallic materials have been an interest due to their applications in a wide range of areas such as sensors, energy conversion, and smart materials design. In this study, the effects of morphological parameters, such as ligament diameter and length, on wave propagation in bicontinuous porous nanostructures is studied by using the discontinuous Galerkin method. Computational results show that energy is localized on the surface independent of the morphological parameters. It is observed that localization length increases with the increase in frequency. In addition, surface roughness parameter and ligament diameter do not have a significant influence on localization length.     

Surface anisotropy broadening of the energy barrier distribution in magnetic nanoparticles

The effect of surface anisotropy on the distribution of energy barriers in magnetic fine particles of nanometer size is discussed within the framework of the Tln(t/τ(0)) scaling approach. The comparison between the distributions of the anisotropy energy of the particle cores, calculated by multiplying the volume distribution by the core anisotropy, and of the total anisotropy energy, deduced by deriving the master curve of the magnetic relaxation with respect to the scaling variable Tln(t/τ(0)), enables the determination of the surface anisotropy as a function of the particle size. We show that the contribution of the particle surface to the total anisotropy energy can be well described by a size-independent value of the surface energy per unit area which permits the superimposition of the distributions corresponding to the particle core and effective anisotropy energies. The method is applied to a ferrofluid composed of non-interacting Fe(3-x)O(4) particles of 4.9?nm average size and x about 0.07. Even though the size distribution is quite narrow in this system, a relatively small value of the effective surface anisotropy constant K(s) = 2.9 × 10(-2)?erg?cm(-2) gives rise to a dramatic broadening of the total energy distribution. The reliability of the average value of the effective anisotropy constant, deduced from magnetic relaxation data, is verified by comparing it to that obtained from the analysis of the shift of the ac susceptibility peaks as a function of the frequency.     

Nanoscale Magnetic Characterization of Tunneling Magnetoresistance Spin Valve Head by Electron Holography

Nanostructured magnetic materials play an important role in increasing miniaturized devices. For the studies of their magnetic properties and behaviors, nanoscale imaging of magnetic field is indispensible. Here, using electron holography, the magnetization distribution of a TMR spin valve head of commercial design is investigated without and with a magnetic field applied. Characterized is the magnetic flux distribution in complex hetero‐nanostructures by averaging the phase images and separating their component magnetic vectors and electric potentials. The magnetic flux densities of the NiFe (shield and 5 nm‐free layers) and the CoPt (20 nm‐bias layer) are estimated to be 1.0 T and 0.9 T, respectively. The changes in the magnetization distribution of the shield, bias, and free layers are visualized in situ for an applied field of 14 kOe. This study demonstrates the promise of electron holography for characterizing the magnetic properties of hetero‐interfaces, nanostructures, and catalysts.     

Self-assembled ferromagnetic cobalt/yttria-stabilized zirconia nanocomposites for ultrahigh density storage applications

We report on a low-cost, innovative approach for synthesizing prepatterned, magnetic nanostructures, the shapes and dimensions of which can be easily tuned to meet requirements for next-generation data storage technology. The magnetic nanostructures consist of self-assembled Co nanodots and nanowires embedded in yttria-stabilized zirconia (YSZ) matrices. The controllable size and aspect ratio of the nanostructures allows the selection of morphologies ranging from nanodots to nanowires. Co nanowires show strong shape anisotropy and large remanence at 300 K. In contrast, Co nanodots display minimal effects of magnetocrystalline anisotropy and superparamagnetic relaxation above the blocking temperature. These prepatterned magnetic nanostructures are very promising candidates for data storage technology with an ultrahigh density of 1 terabit in(-2) or higher.     

Electric-field-assisted switching in magnetic tunnel junctions

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.