Shubnikov-de Haas and high-field magnetoresistance effects in the A15 compound Nb3Sb

High-field magnetoresistance and Shubnikov-de Haas (SdH) effects were studied in the A15 compound Nb₃Sb in fields up to 215 kG. A change in the field dependence of the magnetoresistance for certain field directions above about 150 kG appears to signal the onset of magnetic breakdown. Five sets of SdH frequencies were observed, four of them closely corresponding to de Haas-van Alphen (dHvA) frequencies observed by Arko et al. The fifth frequency had an extremely large amplitude, about 20% of the backgroud magnetoresistance, and it is suggested that this also is due to magnetic breakdown. The results are compared with the ab initio band calculations of van Kessel et al., which can explain many of the observed features of the dHvA and SdH frequencies.Research performed at the Francis Bitter National Magnet Laboratory at MIT, which is supported by the NSF. This work has been supported by NSF Grants DMR-7810781 and DMR-7916187 (DJS and DL), the U.S. Department of Energy (AJA), and NSF Grant DMR 77-08469 (ZF).     

Cluster synthesis and direct ordering of rare-earth transition-metal nanomagnets

Rare-earth transition-metal (R-TM) alloys show superior permanent magnetic properties in the bulk, but the synthesis and application of R-TM nanoparticles remains a challenge due to the requirement of high-temperature annealing above about 800 °C for alloy formation and subsequent crystalline ordering. Here we report a single-step method to produce highly ordered R-TM nanoparticles such as YCo(5) and Y(2)Co(17), without high-temperature thermal annealing by employing a cluster-deposition system and investigate their structural and magnetic properties. The direct ordering is highly desirable to create and assemble R-TM nanoparticle building blocks for future permanent-magnet and other significant applications.     

Size dependence of the magnetic properties of electrochemically self-assembled Fe quantum dots

A quasi-periodic array of Fe quantum dots of cylindrical shape has been synthesized by electrodeposition of Fe in porous alumina. By controlling the fabrication parameters, we have controlled the length, diameter, and spacing of the dots. The magnetic properties are shown to depend on these parameters. It has been found that at room temperature, there exists a critical diameter of the dots for which the coercivity is a maximum. The largest value of coercivity obtained at room temperature is 2640 Oe which rises to 2900 Oe on annealing. At a low temperature of 5K, an increase in coercivity is observed for most of the samples. The largest value is 3800 Oe which rises to a value of 4100 Oe in the corresponding annealed counterpart. At 5K, no maximum is seen in the coercivity as a function of diameter. Instead, the coercivity is found to decrease with increasing diameter. This dependence of the coercivity on diameter is discussed in terms of localized reversal effects.     

Magnetoresistance of UIr3 and UGe3 to 220 kG

Topological properties of the Fermi surfaces of UIr₃ and UGe₃ were investigated. UIr₃ is an uncompensated metal with open orbits along 110. This is in agreement with a proposed model of Arko and Koelling, based on augmented-plane-wave (APW) calculations. UGe₃ is a compensated metal, which showed nearly quadratic magnetoresistance behavior for all field directions studied. The results are compared with the APW model of Arko and Koelling, which contains a multiply connected sheet of Fermi surface. The model, or slight variations thereof, is consistent with the available data.Research at the University of Nebraska supported by National Science Foundation Grants DMR 72-03208 A01 and DMR 76-17417, and at Argonne National Laboratory by the United States Energy Research and Development Administration. The work was performed at the Francis Bitter National Magnet Laboratory, which is supported at MIT by the National Science Foundation.     

Effect of Exchange Interactions on the Coercivity of SmCo5 Nanoparticles Made by Cluster Beam Deposition

Single crystal SmCo₅ nanoparticles with an average size of 3.5 nm are produced by cluster‐beam deposition. When deposited without matrix, the nanoparticles showed a super‐paramagnetic behavior with a blocking temperature of 145 K. Dispersion of the SmCo₅ nanoparticles in a carbon matrix results in an increase in both the coercivity and the blocking temperature. Room temperature coercivities as high as 12 kOe are obtained for the first time in mono‐layers of SmCo₅ nanoparticles dispersed in C matrix. δM plots show that the interactions in the samples are of exchange type, which can decrease the overall effective anisotropy and coercivity according to the random‐anisotropy model. Coercivity is found to be inversely proportional to the packing density of the particles. SmCo₅ nanoparticles with high coercivity are potential candidates for the next generation ultra high‐density magnetic recording media.     

FePt nanocluster films for high-density magnetic recording

High anisotropy L1(0) ordered FePt thin films are considered to have high potential for use as high areal density recording media, beyond 1 Tera bit/in2. In this paper, we review recent results on the synthesis and magnetic properties of L1(0) FePt nanocomposite films. Several fabrication methods have been developed to produce high-anisotropy FePt films: epitaxial and non-epitaxial growth of (001)-oriented FePt:X (X = Au, Ag, Cu, C, etc.) composite films that might be used for perpendicular media; monodispersed FePt nanocluster-assembled films grown with a gas-aggregation technique and having uniform cluster size and narrow size distribution; self-assembled FePt particles prepared with chemical synthesis by reduction/decomposition techniques, etc. The magnetic properties are controllable through variations in the nanocluster properties and nanostructure. FePt and related films show promise for development as heat-assisted magnetic recording media at extremely high areal densities. The self-assembled FePt arrays show potential for approaching the ultimate goal of single-grain-per-bit patterned media.