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FePt nanoparticles with a size of 3 nm and thermally stable room-temperature ferromagnetism are investigated. The monodisperse nanoparticles were prepared by chemical synthesis and a salt-matrix annealing technique. Structural and magnetic characterizations confirmed the phase transition from the disordered face-centered cubic structure to the L10 structure with the chemical ordering parameter of 0.62±0.05. Analysis in blocking temperature and fitting of temperature dependence of switching field reveals that the transformed 3 nm nanoparticles have a magnetic anisotropy constant of (2.8±0.2)×106 J/m3, smaller than those for the bigger particles and the fully ordered L10 bulk phase. [ABSTRACT FROM AUTHOR]

The magnetism of magnetic surface alloys such as Fe–Pt on Pt(997) has been investigated by micromagnetic model calculations. A unique feature of some of these systems is that interatomic exchange, magnetocrystalline anisotropy, and Dzyaloshinski–Moriya (DM) interactions are all of comparable magnitude, of the order of 0.5 meV. This leads to an intriguing nanoscale interplay between ordinary magnetization states and noncollinear spin structures. We identify two cases where the latter dominate. First, for sufficiently strong DM interaction, the zero-field spin structures change from uniform to a canted state with an incommensurate wave vector. Second, a similar transition occurs for weak DM interaction when the reverse external field approaches the nucleation field. [ABSTRACT FROM AUTHOR]

The effect of semihard magnetic phases and interfaces on the performance of nanostructured two-phase permanent magnets is investigated by model calculations. In addition to the trivial coercivity increase due to the replacement of soft regions by semihard regions, there is a coercivity enhancement even if the volume-averaged anisotropy is kept constant during the introduction of the semihard phase. A variational approach is used to derive analytical results for representative anisotropy profiles. The improvement is operative on length scales slightly larger than that of the soft phase in hard-soft composites, but the main challenge is to find semihard light or heavy transition metal phases with a high magnetization. There are several Fe- and Co-based phases, but most are thin-film systems and difficult to use in bulk magnets. Very hard nanostructured magnets may also be created from soft phases with negative but large anisotropy constants (hard-magnetic soft-soft magnets). [ABSTRACT FROM AUTHOR]

Structural and magnetic properties of substituted L10 compounds are investigated. Focus is on L10 structures of the general type ABC2, where each second layer of the original structure is replaced by a checkerboard of A and B atoms. At low temperatures, L10 compounds with ABC2 stoichiometry will segregate into AC and AB L10 phases, forming an ordered ABC2 compound or exhibit a transition into a more complicated structure. The ordering has a strong impact on the magnetism of the alloys, especially on the anisotropy. The hypothetical new ternary compounds are of potential interest in various areas of magnetism, including magnetic recording and permanent magnetism. [ABSTRACT FROM AUTHOR]

The temperature dependence of the magnetic anisotropy of ferromagnetic materials is analyzed. Simple ferromagnets, such as Fe and Co, obey the m=n(n+1)/2 power laws predicted by the Callen and Callen [Phys. Rev. 129, 578 (1963)] theory, but in alloys, the applicability of the theory is an exception rather than the rule. Many alloys, such as the rare-earth transition-metal intermetallics and L10 magnets, violate a basic assumption of the theory, namely, that the single-ion anisotropy and the spontaneous magnetization have the same origin. This is the reason for significant deviations from the Callen and Callen behavior, such as the m=2 law we obtained for L10 alloys. [ABSTRACT FROM AUTHOR]

The magnetic properties of collections of three soft magnetic nanodots with various aspect ratios are investigated. Permalloy films are first produced by dc magnetron sputtering. Focused ion beam milling is then used to mill dots, each with different shape anisotropy. We find that each of the three dots in the system has a unique switching field, and that there is significant magnetostatic coupling. Micromagnetic simulations suggest that for dot separations of less than 50 nm there exists strong interdot interaction, leading to the possibility of controlled switching of neighboring dots. This switching behavior is of interest in magnetic information processing. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Magnetization reversal in arrays of magnetic nanoparticles with perpendicular anisotropy is investigated. Aside from domain-wall propagation effects, the reversal involves two main aspects: the nucleation behavior of individual particles and interparticle interactions. Due to magnetostatic self-interaction effects, the interparticle interaction cannot be reduced to a stray-field correction to the external field. Exact nucleation fields and explicit stray-field and self-interaction contributions are obtained for rings of equidistant dots. An exact treatment of self-interactions in various structurally inhomogeneous but rotationally symmetric wire, sphere, and thin-film nanostructures leads to renormalization of the uniaxial anisotropy. Finally, an approximate method to calculate nucleation fields is discussed. [ABSTRACT FROM AUTHOR]

The energetics and magnetism of perfect and vacancy-containing two-dimensional NbSe 2 monolayers is investigated by first-principle calculations. It has been found the single Se vacancy has the lowest formation energy. Perfect NbSe 2 monolayers, as well as monolayers containing single-selenium and double-selenium vacancies, are nonmagnetic metallic. However, Nb vacancies create a magnetic moment of 1.5 μ B per vacancy. The moment is highly localized, confined nearly exclusively on the Se atoms adjacent to the vacancy, and mainly originates from the Se 4 p electrons. The moment distribution indicates strongly anisotropic exchange bonds between vacancy moments. [ABSTRACT FROM AUTHOR]

It is investigated how a transverse magnetic field affects the quantum-mechanical character of LiHoF4, a system generally considered as a textbook example for an Ising-like quantum-phase transition. In small magnetic fields, the low-temperature behavior of the ions is Ising-like, involving the nearly degenerate low-lying Jz = ± 8 doublet. However, as the transverse field increases, there is a substantial admixture of states having |Jz| < 8. Near the quantum-phase-transition field, the system is distinctively non-Ising like, and all Jz eigenstates yield ground-state contributions of comparable magnitude. A classical analog to this mechanism is the micromagnetic single point in magnets with uniaxial anisotropy. Since Ho3+ has J = 8, the ion's behavior is reminiscent of the classical limit (J = ∞), but quantum corrections remain clearly visible. [ABSTRACT FROM AUTHOR]

It is investigated how a transverse magnetic field affects the quantum-mechanical character of LiHoF4, a system generally considered as a textbook example for an Ising-like quantum-phase transition. In small magnetic fields, the low-temperature behavior of the ions is Ising-like, involving the nearly degenerate low-lying Jz = ± 8 doublet. However, as the transverse field increases, there is a substantial admixture of states having |Jz| < 8. Near the quantum-phase-transition field, the system is distinctively non-Ising like, and all Jz eigenstates yield ground-state contributions of comparable magnitude. A classical analog to this mechanism is the micromagnetic single point in magnets with uniaxial anisotropy. Since Ho3+ has J = 8, the ion's behavior is reminiscent of the classical limit (J = ∞), but quantum corrections remain clearly visible. [ABSTRACT FROM AUTHOR]

Material choices for permanent magnets are analyzed in terms of energy product, anisotropy and hardness parameter. Energy product is the main consideration for permanent magnets, because the purpose of a magnet is to store magnetostatic energy and create as much flux as possible in the surrounding space. Magnet processing is easiest if the hardness parameter is significantly greater than one. The anisotropy requirement becomes increasingly stringent for large M s and K 1 values of 1 to 2 MJ/m 3 may be insufficient. Some potential new magnets and alternative strategies to develop magnetic hardness are discussed. [ABSTRACT FROM AUTHOR]

Transmission electron microscopy (TEM) and electron diffraction (ED) are used to investigate the nanostructures of two ensembles of Co:CoO core–shell particles. TEM images show that particles of size about 12 nm are almost fully oxidized, while particles with size about 18 nm have a core–shell structure where a Co core is surrounded by a shell of CoO. ED simulation confirms that the larger particles have an fcc-structured Co core and a rock-salt CoO shell structure, while the smaller particles mostly have the rock-salt CoO structure. The core–shell structure is responsible for the unusual magnetic properties of the Co:CoO nanoclusters, especially the occurrence of inverted hysteresis loops (proteresis), but previous research has been indirect, largely based on magnetic measurements and on a cross-comparison with granular materials. Our measurements show that the structures have ferromagnetic fcc Co cores of varying sizes down to 1 nm which are surrounded by antiferromagnetic rock-salt CoO shells. The core radii obtained from the TEM pictures are used to estimate the exchange interactions responsible for proteresis and to pinpoint the core-size window in which proteresis occurs. [ABSTRACT FROM AUTHOR]

Abstract: Atomic and continuum effects in L10 magnets are investigated. Emphasis is on the competition between ferromagnetism, antiferromagnetism, and noncollinear order in both perfect and imperfect structures, and on the temperature dependence of the magnetic anisotropy. The applicability of micromagnetic and atomistic approaches depends on the length scales involved, but there is a broad range of phenomena where both can be used. [Copyright &y& Elsevier]

The effect of an applied electric field on the magnetic properties of L 10-ordered CoPd thin films is investigated by first-principle calculations. Both the magnetic moment and the magnetocrystalline anisotropy of the surface atoms are changed by the electric field, but the net effect depends on the surface termination. The magnetocrystalline anisotropy switches from in-plane to perpendicular in the presence of external electric field. Typical magnetic-moment changes are 0.1 \muB per eV/\AA. The main mechanism is the shift of the Fermi level, but the anisotropy change also reflects a crystal-field change due to incomplete screening. [ABSTRACT FROM AUTHOR]

The anisotropic optical dielectric functions of a metal (cobalt) slanted columnar thin film deposited by electron-beam glancing angle deposition are reported for the terahertz (THz) frequency domain using generalized spectroscopic ellipsometry. We employ a simple effective medium dielectric function homogenization approach to describe the observed optical response. The approach describes isolated, electrically conductive columns which render the thin film biaxial (orthorhombic). Our findings suggest controlled variability of dielectric polarizability and anisotropy in the THz spectral range by choice of geometry, material, and structure. [ABSTRACT FROM AUTHOR]

The electrical resistance of a constrained domain wall in a nanojunction is investigated using micromagnetic modeling and ballistic conductance calculations. The nanojunction represents two ferromagnetic electrodes connected by a ferromagnetic wire of 10 nm in length and of a few nanometers in cross section. We find that the anisotropy of the electrodes favors a localization of the domain wall within the constriction (wire) revealing a positive domain-wall resistance. An applied magnetic field moves the domain wall toward one of the electrodes and reduces its width. This compression of the domain wall leads to a sizeable enhancement of the domain-wall resistance. [ABSTRACT FROM AUTHOR]

Intergranular interactions in nanostructured FePt:Ag thin films and their effect on magnetic hysteresis are investigated. The films, produced by multilayer deposition plus rapid thermal annealing, consist of FePt nanoparticles embedded in a silver matrix. They are investigated by magnetization mea- surements and magnetic force microscopy. Analytical model - calculations, supported by micromagnetic simulations, are used to elucidate the relation between coercivity, hysteresis-loop slope, and spatial correlations during magnetization reversal. The analytical calculations yield simple expressions for the loop slope and the coercivity as a function of the intergranular exchange. Small intergranular exchange enhances the coercivity, but for strong exchange there is a self-energy cutoff, associated with the onset of cooperative reversal and preventing alpha from becoming negative. [ABSTRACT FROM AUTHOR]

FePt and Fe[sub 3]O[sub 4] nanotubes are produced by hydrogen reduction in nanochannels of porous alumina templates and investigated by electron microscopy, x-ray diffraction analysis, and magnetic measurements. Loading the templates with a Fe chloride and Pt chloride mixture followed by hydrogen reduction at 560 °C leads to the formation of ferromagnetic FePt nanotubes in the alumina pores. Using a Fe nitrate solution, thermally decomposed at 250 °C and reduced in hydrogen for 2.5 h at the same temperature, yields Fe[sub 3]O[sub 4] tubes. The length of the nanotubes is about 50 μm and their diameters range from about 150 to 220 nm, depending on the thickness of the template film and the pore diameter distribution. Reflecting the different magnetocrystalline anisotropies of the compounds, the coercivities range from 0.61 kOe for Fe[sub 3]O[sub 4] to 20.9 kOe for FePt. The hysteresis is explained in terms of a tubular random-anisotropy model, which yields a diameter and anisotropy dependent transition from a curling-type mode (Fe[sub 3]O[sub 4]) to a localized mode (FePt). © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An investigation of the magnetic properties of Co-rich nanoparticles alloyed with a small fraction of Pt and Hf is presented. Co(Hf) and Co(Pt) nanoparticles with less than 15 at% of dopants were produced using a cluster-deposition method. The nanoparticles have sizes of less than 10 nm and show improved magnetic properties upon doping. Maximum coercivities of 900 Oe (at 300 K) and 2000 Oe (at 10 K) were observed for Co nanoparticles alloyed with 14.1 at% of Hf. Doped nanoparticles also exhibit high anisotropies, such as K1 = 9.98 Mergs/cm3 (14.1 at% of Hf) and K1 = 8.24 Mergs/cm3 (9.5 at% of Pt), as compared to Co nanoparticles (K1 = 6.21 Mergs/cm3). [ABSTRACT FROM AUTHOR]

The structural and magnetic properties of Mn2+δTiSn prepared by arc melting and annealing have been investigated. Structural studies show that the compound crystallizes in the hexagonal Ni3Sn-type structure with a = 5.70 Å and c = 4.55 Å. The phase stability of Mn2TiSn in the hexagonal structure is supported by the first-principle electronic structure calculations where the total energy per unit-cell volume in the hexagonal structure is smaller than that in the cubic structure. Field and temperature dependence of magnetization show that the sample is magnetically ordered with a Curie temperature around 400 K. The anisotropy energy calculated from the high-field data is 4.0 × 105 ergs/cm3 at 300 K but increases by a factor of two (8.6 × 105 ergs/cm3) as temperature decreases to 10 K. The observed magnetic properties are explained as the consequences of competing ferromagnetic and antiferromagnetic interactions between different magnetic sublattices. [ABSTRACT FROM AUTHOR]

The structural, magnetic, and electron transport properties of Mn55-xFexBi45 (x = 0, 2, 4, 5, 8, 11, 13, 16) films prepared by multilayer deposition and annealing using e-beam evaporation have been investigated. Fe doping has produced a significant change in the magnetic properties of the samples including the decrease in saturation magnetization and magnetocrystalline anisotropy and increase in coercivity. Although the magnetization shows a smooth decrease with increasing Fe concentration, the coercivity jumps abruptly from 8.5 kOe to 22 kOe as Fe content changes from 4% to 5%, but the change in coercivity is small as the concentration goes beyond 5%. The temperature dependence of resistivity shows that the samples with low Fe concentration (≤4%) are metallic, but the resistivity increases unexpectedly as the concentration reaches 5%, where the resistance increases with decreasing temperature below 300 K. First-principle calculations suggest that the observed magnetic properties can be understood as the consequences of competing ferromagnetic and antiferromagnetic exchange interactions between the interstitial atom and the rest of the MnBi lattice. [ABSTRACT FROM AUTHOR]

The relation between orbital moment and magnetic anisotropy is investigated by model calculations, which show that only a part of the spin-orbit coupling contributes to the anisotropy. A large part of the anisotropy energy, about 50% for iron series elements and nearly 100% for rare-earths, is stored in the nonrelativistic part of the Hamiltonian. A feature important for x-ray magnetic circular dichroism is that the orbital moment of heavy atoms rotates with the spin moment, whereas in light atoms, the orbital moment is recreated in each different direction. In the discussion, we consider three examples of current interest in different areas of magnetism, namely, spin-orbit coupling in Gd3+ and Eu2+, surface anisotropy of Nd2Fe14B, and multiferroic magnetization switching using rare-earths. [ABSTRACT FROM AUTHOR]

Ab initio calculations of the biaxial anisotropy of infinite Pd zigzag chains are presented. The simulations were performed with the Vienna Ab-Initio Simulation Package, using a tetragonal unit cell with a supercell approach where the atoms are repeated along the z-direction. The anisotropy is determined from the energies along the three principal directions [001], [100], and [010]. The second-order anisotropy constants K1 and K1 ′ were extracted by fitting the calculated energy values to the phenomenological energy. The easiest magnetization direction is along the wire axis, whereas the hardest direction is perpendicular to the plane of the wire. The calculated anisotropy constants K1 and K1 ′ are 5.5 MJ/m3 and -13.9 MJ/m3, respectively These anisotropies are large, but they are smaller than those of linear chains, because the zigzag shape leads to a quenched orbital moment. [ABSTRACT FROM AUTHOR]

Ab-initio calculations are used to determine the parameters that determine magnonic band structure of PdnFem multilayers (n = 2, m ≤ 8). We obtain the layer-resolved magnetization, the exchange coupling, and the magnetic anisotropy of the Pd-Fe structures. The Fe moment is 3.0 μB close to the Pd layers and 2.2 μB in the middle of the Fe layers. An intriguing but not usually considered aspect is that the elemental Pd is nonmagnetic, similar to Cu spacer layers in other multilayer systems. This leads to a pre-asymptotic ferromagnetic coupling through the Pd (about 40 mJ/m2). Furthermore, the Pd acquires a small moment due to spin polarization by neighboring Fe atoms, which translates into magnetic anisotropy. The anisotropies are large, in the range typical for L10 structures, which is beneficial for high-frequency applications. [ABSTRACT FROM AUTHOR]

Presents a study which investigated the magnetic anisotropy of Iron (Fe) or nickel (Ni) ultrathin films. Methodology of the study; Experimental results; Theoretical interpretation; Conclusions.

Abstract: The effective mass of strained graphene is investigated by tight-binding and density-functional theory calculations. For graphene strained in the zigzag direction, we find a strong anisotropy in the effective mass near the gap opening, with an abrupt increase of the effective mass at the critical strain in one direction and a smooth variation perpendicular to it. There is no band-gap opening for isotropic strain, but at an expansive strain of about 28%, the lower edge of the s-band reaches the Fermi level and makes the graphene metallic. [Copyright &y& Elsevier]

We report HfCo7 nanoparticles with appreciable permanent-magnet properties (magnetocrystalline anisotropy K1 ≈ 10 Mergs/cm3, coercivity Hc ≈ 4.4 kOe, and magnetic polarization Js ≈ 10.9 kG at 300 K) deposited by a single-step cluster-deposition method. The direct crystalline-ordering of nanoparticles during the gas-aggregation process, without the requirement of a high-temperature thermal annealing, provides an unique opportunity to align their easy axes uniaxially by applying a magnetic field of about 5 kOe prior to deposition, and subsequently to fabricate exchange-coupled nanocomposites having Js as high as 16.6 kG by co-depositing soft magnetic Fe-Co. This study suggests HfCo7 as a promising rare-earth-free permanent-magnet alloy, which is important for mitigating the critical-materials aspects of rare-earth elements. [ABSTRACT FROM AUTHOR]

Magnetic properties of SmCo6.7−xCu0.6Tix Magnets (x=0.25 and 0.30) are studied as a function of milling time and temperature. The samples were prepared by two methods: first by arc-melting and milling, and second by mechanical alloying from powders and subsequent annealing. The X-ray diffraction analyses show that the samples consist of 1:5 and 2:17 phases. Magnetic measurements show that the coercivity for samples prepared by arc-melting and milling increases with increase in milling time and it reaches a maximum of 8.1 kOe for x=0.30 and 5.3 kOe for x=0.25, and then decreases for both samples. The coercivities for the sample with x=0.30 are higher than the coercivities for the sample with x=0.25 for all milling times. High-temperature vibrating sample magnetometer magnetic measurements show that the coercivity for all samples decreases with increasing temperature from room temperature to 600°C. The sample prepared by mechanical alloying has higher coercivity (20 kOe at room temperature) than that prepared by arc-melting and milling for all temperatures under investigation. These materials have moderate energy products (1–10 MG Oe) and can be used for high-temperature magnetic applications. [Copyright &y& Elsevier]

Abstract: Magnetic nanoparticles smaller than ∼15nm in diameter and with high magnetocrystalline anisotropies K 1 ⩾1MJm−3 can be used as building blocks for next-generation permanent magnets. Advances in processing steps are discussed, such as self-assembly, alignment of the easy axes and appropriate nanostructuring that will enable the fabrication of densely packed nanoparticle assemblies with improved permanent-magnet properties. This study also proposes an idealized nanocomposite structure for nanoparticle-based future permanent magnets with enhanced energy products. [Copyright &y& Elsevier]