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The effect of nanostructuring on the magnetic entropy of materials for room-temperature magnetic cooling is investigated by model calculations. The materials are structurally inhomogeneous with a large number of nonequivalent crystallographic sites. In the mean-field Heisenberg model, the entropy density is a unique function of the local magnetization so that the coupled set of nonlinear mean-field equations yields not only the magnetization but also the entropy density. Since most of the entropy is localized near grain boundaries, nanomagnetic cooling requires small feature sizes. Magnetic anisotropy is a substantial complication, even on a mean-field level, but the corresponding corrections are often very small [ABSTRACT FROM AUTHOR]

Nanoclusters of rocksalt TiO, anatase TiO2, and rutile TiO2 were produced by cluster deposition and examined with transmission-electron microscopy, x-ray diffraction, and magnetization measurements. The clusters are all magnetic at room temperature, but the magnetization is structure- dependent. The hysteresis loops show coercivities that are of the order of 100 Oe and all films show a preferential in-plane magnetization direction. The size dependence of the magnetization was investigated for rutile clusters with average sizes from about 15 to 40 nm. The analysis of the measurements indicates that the magnetism is predominantly located near the surface of the clusters and characterized by a nominal value of 7.6 μB/nm2. [ABSTRACT FROM AUTHOR]

We report effects of rapid thermal annealing on nanostructure, texture, and magnetic properties of granular FePt:Ag films. It was found that the orientations of FePt grains were dependent strongly on the as-deposited multilayer structure and the annealing processes. Through the control of the annealing processes, (001) textured L(1)0 granular FePt:Ag films were obtained. Magnetic measurements revealed that saturation magnetization M[SUBs], perpendicular coercivity H[SUBc], and remanence ratio M[SUBr]/M[SUBs], were also dependent on the annealing temperature, annealing time and Ag content. Perpendicular H[SUBc] values increased from 1.5 to 15 kOe when the annealing temperature changed from 400 to 600 °C for 600 s. Perpendicular H[SUBc] values remained about 11 kOe when the annealing time changed from 5 to 600 s. The effect of Ag content on magnetic properties is reported. [ABSTRACT FROM AUTHOR]

Exchange interactions between hard-magnetic particles in a nonmagnetic matrix are investigated by model calculations. A Landau-Ginzburg approach is developed to describe the net exchange interactions between spheres of arbitrary diameters. Introducing cylindrical coordinates and integrating over the surfaces of the adjacent spheres yields an exchange coupling which decreases with a decay length depending on interatomic exchange, intra-atomic exchange, and temperature. Typically, the decay length does not exceed a few interatomic distances. The decay is exponential but also contains a prefactor depending on the surface curvature of the grains. It increases with decreasing curvature, but this dependence is only a small correction to the leading exponential term. [ABSTRACT FROM AUTHOR]

The magnetic properties of nanocomposite multilayer magnets of the (Nd,Dy) (Fe,Co,Nb,B)[sub 5.5]/M (M = Co,FeCo) on Ti-buffered Si substrates have been investigated. X-ray diffraction results reveal that the Nd[sub 2]Fe[sub 14]B-type phase in almost all the films is randomly oriented. Different thickness χ (nm) for Co layers and y (nm) for FeCo layers were adopted in the multilayers. It is found that high remanence is achieved in the nanocomposite multilayer films consisting of the Nd[sub 2]Fe[sub 14]B-type phase and soft magnetic phase for Co with 6 ≤ χ ≥ 4 and for FeCo with 10 ≥ y ≥ 6. The enhancement of the remanence in the nanocomposite multilayer films is attributed to the exchange coupling between the magnetically soft and hard phases. Increasing the soft magnetic components results in a continuously decreasing coercivity. [ABSTRACT FROM AUTHOR]

It is demonstrated that the magnetostatic demagnetizing factor is not the only sample-shape dependent contribution to the hysteresis loop. There are also exchange contributions, which lead to interesting effects, particularly in nanostructures. An example is the recently discovered bulging mode in two-phase nanomagnets. The bulging mode exhibits the angular symmetry of the coherent mode, but it is incoherent due to its radial variation and gives rise to negative demagnetizing factors. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The nanostructure and magnetic properties of composite CoPt:C films at room temperature were investigated as a function of annealing temperature, carbon concentration, and film thickness. CoPt:C films with a variety of carbon concentrations were fabricated by cosputtering Co, Pt, and C onto water-cooled Si(100) substrates followed by annealing. X-ray diffraction and transmission electron microscopy analyses indicate that nanocrystallites of face-centered-tetragonal (fct) CoPt phase, which has a uniaxial magnetocrystalline anisotropy constant of about 5×10[sup 7] erg/cm[sup 3], can be formed in carbon matrix when the annealing temperature is higher than 600 °C. The grain sizes of the fct CoPt crystallites are about 10 nm and the coercivities can be as high as 12 kOe. Higher annealing temperature and lower carbon concentration generally lead to larger grain sizes and perhaps more complete formation of the fct CoPt phase, and therefore higher coercivities. The coercivity is insensitive to the film thickness until the thickness is smaller than the CoPt grain size, when the coercivity starts to decrease with film thickness. The magnetic activation volumes are typically around 1×10[sup -18] cm[sup 3]. The nanostructure and the associated magnetic properties of these composite CoPt:C films are promising as potential media for extremely high-density recording. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

There are two main thrusts towards new permanent-magnet materials: improving extrinsic properties by nanostructuring and intrinsic properties by atomic structuring. Theory—both numerical and analytical—plays an important role in this ambitious research. Our analysis of aligned hard-soft nanostructures shows that soft-in-hard geometries are better than hard-in-soft geometries and that embedded soft spheres are better than sandwiched soft layers. Concerning the choice of the hard phase, both a high magnetization and a high anisotropy are necessary. As an example of first-principle research, we consider interatomic Mn exchange in MnAl and find strongly ferromagnetic intralayer exchange, in spite of the small Mn-Mn distances. [ABSTRACT FROM AUTHOR]

Nanostructured Mn3 - x Ga ribbons with x = 0, 0.4, 0.9 and 1.1 were prepared using arc-melting, melt-spinning and annealing. As-spun samples crystallized into hexagonal D019 and cubic L21 Heusler crystal structures based on the concentration of Mn in Mn3 - xGa. Upon vacuum-annealing the samples at 450 ^\circC for about 50 hours, both the hexagonal and cubic structures transformed into a tetragonal D022 structure. High-temperature x-ray diffraction and high-temperature magnetometry showed that the samples with low Mn content (Mn1.9 Ga and Mn2.1Ga) retain their tetragonal structure up to 850 K but the samples with high Mn concentrations (Mn2.6 Ga and Mn3.0Ga) undergo a structural phase transition from tetragonal to hexagonal phases around 800 K. The magnetic properties of Mn3 - x Ga ribbons were very sensitive to Mn concentration, where the magnetization and anisotropy energy increased and the coercivity decreased as x increased from 0 to 1.1. Although the Curie temperatures of Mn2.6 Ga and Mn3.0 Ga samples could not be determined because of the structural phase transition, the Curie temperature decreased with increasing x in Mn3 - xGa. The maximum magnetization of 57 emu/g (300 emu/cm^3) and the coercivity of 6.5 kOe were measured in the Mn1.9 Ga and Mn3.0Ga ribbons, respectively. [ABSTRACT FROM AUTHOR]

The structural and magnetic properties of Pr-alloyed MnBi (short MnBi-Pr) nanostructures with a range of Pr concentrations are investigated. The nanostructures include thin films having Pr concentrations 0, 2, 3, 5 and 9 at.% and melt-spun ribbons having Pr concentrations 0%, 2%, 4% and 6%, respectively. Addition of Pr into the MnBi lattice has produced a significant change in the magnetic properties of these nanostructures including an increase in coercivity and structural phase transition temperature, and a decrease in saturation magnetization and anisotropy energy. The highest value of coercivity measured in the films is 23 kOe and in the ribbons is 5.6 kOe. The observed magnetic properties are explained as the consequences of competing ferromagnetic and antiferromagnetic interactions. [ABSTRACT FROM AUTHOR]

The structural and magnetic properties of rare-earth nanocomposite magnets of a (Nd,Dy)(Fe,Co,Nb,B)5.5 single layer and a (Nd,Dy)(Fe,Co,Nb,B)5.5/α-Fe multilayer prepared by sputtering and heat treatment have been investigated. Incomplete exchange coupling behaviour is observed in the Ti-buffered NdDyFeCoNbB + 40 wt%Fe single layer magnet. After annealing, the laminated nanostructure consisting of mutually dispersed soft and hard phases results in remanence enhancement. The optimum magnetic properties of Jr=1.11 T, μ0iHc=0.88 T and (BH)max=192 kJ m-3 are achieved in the multilayer magnet annealed at 550°C for 30 min. The designed multilayer film with thinner soft magnetic layers may favour the formation of more ideal nanostructures for exchange coupling between soft- and hard-magnetic phases. [ABSTRACT FROM AUTHOR]

Nanostructured MnyGa ribbons with varying Mn concentrations including Mn1.2Ga, Mn1.4Ga, Mn1.6Ga, and Mn1.9Ga were prepared using arc-melting and melt-spinning followed by a heat treatment. Our experimental investigation of the nanostructured ribbons shows that the material with y = 1.2, 1.4, and 1.6 prefers the tetragonal L10 structure and that with y = 1.9 prefers the D022 structure. We have found a maximum saturation magnetization of 621 emu/cm3 in Mn1.2Ga which decreases monotonically to 300 emu/cm3 as y reaches 1.9. Although both the L10- and D022-MnyGa samples show a high Curie temperature (Tc) well above room temperature, the value of Tc decreases almost linearly from 702 K for Mn1.9Ga to 551 K for Mn1.2Ga. All the ribbons are metallic between 2 K and 300 K but the Mn1.2Ga also shows a resistance minimum near 15 K. The observed magnetic properties of the MnyGa ribbons are consistent with the competing ferromagnetic coupling between Mn moments in the regular L10-MnGa lattice sites and antiferromagnetic coupling with excess Mn moments occupying Ga sites. [ABSTRACT FROM AUTHOR]

Two-phase nanostructures of hard L10-ordered FePt and soft iron-rich fcc Fe-Pt are investigated experimentally and by model calculations. The Fe-Pt thin films were produced by epitaxial co-sputtering onto MgO and have a thickness of about 10 nm. They form two-phase dots that cover a large fraction of the surface but are separated from each other. X-ray diffraction and TEM show that the c-axis of the phase FePt is aligned in the direction normal to the film plane. The experimental and theoretical hysteresis loops indicate archetypical exchange coupling, and excellent magnetic properties are obtained. The largest values of coercivity, saturation magnetization, and nominal energy product obtained in the samples studied are 51 kOe, 1287 emu/cc, and 54 MGOe, respectively. [ABSTRACT FROM AUTHOR]

We fabricated exchange-coupled composite films with perpendicular anisotropy using nanograins of L10 FePt as a magnetically hard base layer and FePt : SiO2 as a relatively soft phase. The degree of anisotropy and nanostructure in the soft layer was controlled by varying the deposition temperature. Growth of the low-anisotropy layer followed the particulate structure of the base layer at deposition temperatures of 390°C or greater. Perpendicular coercivities were strongly dependent on both the anisotropy and thickness of the softer layer. [ABSTRACT FROM AUTHOR]

A simple approach combining pyrolysis and hydrogen reduction is formulated for making ferromagnetic-branched nanostructures called Co multipods. The CoO multipods (branched nanostructures) were produced through the pyrolysis of cobalt-oleate in octadecane at 280 °C with the presence of oleic acid under the protection of pure nitrogen. The angles between the nearest arms are 900. After the reduction of the branched CoO nanostructures in flowing hydrogen gas in a tubular oven at 290 °C, the CoO is reduced to pure cobalt with hexagonal crystal structure and the branched shapes are retained. Anisotropic magnetic properties were found for cobalt multipods on silicon substrates. [ABSTRACT FROM AUTHOR]

In this letter, the authors demonstrate isotropic Fe–Pt exchange-spring nanocomposite permanent magnets with a soft magnetic phase fraction of greater than 0.5 with a coercivity of 6.5 kOe, single-phase-like magnetic behavior, and an energy product of 25.1 MG Oe. Sub-10-nm Fe–Pt clusters are formed with compositions in the two-phase Fe3Pt and FePt regions. Intracluster structuring on a scale of a few nanometers occurs after appropriate heat treatment. This ensures full exchange coupling between the two phases, allowing greater soft magnetic phase fractions. The results provide insight into developing high energy product nanostructured permanent magnets. [ABSTRACT FROM AUTHOR]

The microchemistry and magnetism of conventional and high-temperature Sm-Co permanent magnets are investigated by first-principles calculations. Particular emphasis is on the site preference for the substitution of Cu, Ti, and Zr in SmCo5 and Sm2Co17 compounds. Cu substitution is more favorable in the 1:5 phase, in agreement with experimental findings. Titanium and zirconium have positive solution energies for both the phases, with Ti(Zr) having slight preference for the 1:5 (2:17) phase. Some Zr may segregate to the phase boundaries because of its large solution energy. For Ti and Zr the dumbbell site of the 2:17 phase is preferred over the other three inequivalent cobalt sites. These results are used to discuss the observed cellular nanostructure of the high-temperature Sm-Co hard magnets with composition close to the 2:17 phase. [ABSTRACT FROM AUTHOR]

Highly textured (001) FePt:C nanocomposite thin films, deposited directly on thermally oxidized Si wafers, are obtained by multilayer deposition plus subsequent thermal annealing. Nanostructures, crystalline orientations, interactions, and magnetic properties are investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), magnetic force microscopy, and magnetic measurements. The formation of the ordered L1o phase is confirmed by XRD, and only visible (001) peaks indicate a high degree of the (001) texture. TEM observation reveals that FePt grains are embedded in the C matrix and appear to be well isolated. The FePt grains are very uniform with average sizes about 5 um. [ABSTRACT FROM AUTHOR]

The performance of hard-magnetic nanostructures is investigated by analyzing the size and geometry dependence of thin-film hysteresis loops. Compared to bulk magnets, weight and volume are much less important, but we find that the energy product remains the main figure of merit down to very small features sizes. However, hysteresis loops are much easier to control on small length scales, as epitomized by Fe-Co-Pt thin films with magnetizations of up to 1.78 T and coercivities of up to 2.52 T. Our numerical and analytical calculations show that the feature size and geometry have a big effect on the hysteresis loop. Layered soft regions, especially if they have a free surface, are more harmful to coercivity and energy product than spherical inclusions. In hard-soft nanocomposites, an additional complication is provided by the physical properties of the hard phases. For a given soft phase, the performance of a hard-soft composite is determined by the parameter (Ms - Mh)/Kh. [ABSTRACT FROM AUTHOR]

Films of aligned L10-structure (Fe,Co)Pt with fcc Fe(Co,Pt) are synthesized by co-sputtering Fe, Co, and Pt on an (001) MgO substrate with in situ heating at 830 °C. The nanostructures and magnetic properties of the films are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID). The compositions of the samples (Fe,Co)xPt1-x are designed to maintain an atomic Fe: Co ratio of 65: 35 while increasing the Fe,Co content in each successive sample. In samples with low Fe and Co concentration, the XRD patterns exhibit three strong peaks, namely L10 (Fe,Co)Pt (001), L10 (Fe,Co)Pt (002), and MgO (002). A fourth peak is observed in samples with high Fe and Co concentration and identified as fcc (002). The XRD patterns confirm the formation of L10-ordered (Fe,Co)Pt and its epitaxial growth on MgO. TEM shows that the (Fe,Co)Pt films form isolated magnetic grains of about 100 nm in diameter. Hysteresis-loop measurements show that the increase of the Fe,Co concentration from 57.3 to 68.3 at % enhances the saturation magnetization Ms from 1245 emu/cm3 to 1416 emu/cm3, and the coercivity decreases from 32 kOe to 8.9 kOe. The nominal maximum energy product per grain is 64 MGOe. [ABSTRACT FROM AUTHOR]

Artificial environment-friendly Gd-free magnetic nanostructures for magnetic cooling are investigated by temperature-dependent magnetic measurements. We consider two-phase nanocomposites where nanoclusters (Fe) are embedded in a Ni61Cu39 matrix. Several composite films are produced by cluster deposition. The average Fe cluster size depends on the deposition conditions and can be tuned by varying the deposition conditions. The quasiequilibrium Curie temperature of the Fe particles is high, but slightly lower than that of bulk Fe due to finite-size effects. Our experiments have focused on ensembles of 7.7 nm Fe clusters in a matrix with a composition close to Ni61Cu39, which has a TC of 180 K. The materials are magnetically soft, with coercivities of order 16 Oe even at relatively low temperature of 100 K. The entropy changes are modest, -ΔS = 0.05 J/kg K in a field change of 1 T and 0.30 J/kg K in a field change of 7 T at a temperature of 180 K, which should improve if the cluster size is reduced. [ABSTRACT FROM AUTHOR]

FePt/B[sub 2]O[sub 3] multilayers were deposited by magnetron sputtering onto 7059 glass substrates. By annealing the as-deposited films at 550 °C, nanostructured FePt:B[sub 2]O[sub 3] films consisting of FePt grains with L1[sub 0] structure, embedded in a glassy B[sub 2]O[sub 3] matrix, were obtained. The c axes of the FePt grains can be made to align with the film normal direction, which results in a perpendicular anisotropy constant of 3.5x10[sup 7] erg/cc. The films remain layered structures after annealing when the B[sub 2]O[sub 3] layer thickness exceeds 16 Å. The nanostructure of the films was investigated by transmission electron microscopy. The coercivities and the average grain sizes of the films are dependent on the B[sub 2]O[sub 3] concentrations, with coercivities varying from 4 to 12 kOe, while average grain sizes vary from 4 to 17 nm. Strong perpendicular anisotropy, adjustable coercivity, and fine grain size suggest this nanocomposite system might have significant potential as recording media at extremely high areal density. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]