Synthesis of monodisperse iron-platinum (FePt) nanoparticles by reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of oleic acid and oleyl amine stabilizers is reported. The FePt particle composition is readily controlled, and the size is tunable from 3- to 10-nanometer diameter with a standard deviation of less than 5%. These nanoparticles self-assemble into three-dimensional superlattices. Thermal annealing converts the internal particle structure from a chemically disordered face-centered cubic phase to the chemically ordered face-centered tetragonal phase and transforms the nanoparticle superlattices into ferromagnetic nanocrystal assemblies. These assemblies are chemically and mechanically robust and can support high-density magnetization reversal transitions.

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Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Physical Review B

We show a simple, robust, chemical route to the fabrication of ultrahigh-density arrays of nanopores with high aspect ratios using the equilibrium self-assembled morphology of asymmetric diblock copolymers. The dimensions and lateral density of the array are determined by segmental interactions and the copolymer molecular weight. Through direct current electrodeposition, we fabricated vertical arrays of nanowires with densities in excess of 1.9 x 10(11) wires per square centimeter. We found markedly enhanced coercivities with ferromagnetic cobalt nanowires that point toward a route to ultrahigh-density storage media. The copolymer approach described is practical, parallel, compatible with current lithographic processes, and amenable to multilayered device fabrication.

/pdf/ultrahigh-density-nanowire-arrays-grown-in-self-assembled-1mgxshd8uu.pdf

Ultrahigh-Density Nanowire Arrays Grown in Self-Assembled Diblock Copolymer Templates

Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

The microstructure and the room-temperature hysteretic magnetic properties of sputtered, 10 nm thin films of equiatomic binary alloys of CoPt and FePt were characterized using transmission electron microscopy (TEM) and a superconducting quantum interference device (SQUID) magnetometer. A transformation from an atomically disordered, face-centered-cubic structure to the L10 ordered structure occurred during postdeposition annealing and was characterized using digital analysis of dark-field TEM images. The transformation was observed to follow first-order nucleation and growth kinetics, and the ordered volume fraction transformed was quantified at numerous points during the transformation. The ordered volume fraction was then compared to the magnetic coercivity data obtained from the SQUID magnetometer. In contrast to the relationship most commonly described in the literature, that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in this study wa...

On the relationship of high coercivity and L10 ordered phase in CoPt and FePt thin films

Bulk L1/sub 0/ phase materials, such as CoPt and FePt, are known for their high magnetocrystalline anisotropy and magnetic moment, properties that are also desirable for high density magnetic recording media. This work reports the preparation, microstructural characterization, magnetic properties, and magnetic recording performance of L1/sub 0/ phase thin (25 /spl Aring/ to 200 /spl Aring/) films. The samples are prepared by sputter deposition and ex-situ anneal. The microstructural characterization is by transmission electron microscopy and X-ray diffraction. High magnetocrystalline anisotropy is apparently maintained for even the thinnest films, where coercivity, H/sub c/, >10,000 Oe is observed for a 25 /spl Aring/ sample. The H/sub c/ of films prepared for magnetic recording measurements is considerably less to ensure that test patterns could be written. Samples for recording measurements were obtained by a reduction in anneal temperature or the codeposition of an oxide with the L1/sub 0/ phase material to limit grain growth during anneal.

High anisotropy L1/sub 0/ thin films for longitudinal recording

Magnetic media using materials with high uniaxial magneto-crystalline anisotropy, KU, combined with a thermal assist to overcome write field limitations have been proposed as one of the potential technologies to extend the areal density of magnetic disk recording beyond the limitations of current technology. Here we present an investigation on structural and temperature dependent magnetic properties of chemically ordered epitaxial Fe55−xNixPt45 thin films. Increasing Ni additions result in a steady reduction of magneto-crystalline anisotropy, saturation magnetization, and Curie temperature. The ability to control thermomagnetic properties over a wide range makes Fe55−xNixPt45 and similar FePt-based pseudo-binary alloys attractive base materials for media applications in thermally assisted magnetic recording.

Temperature dependent magnetic properties of highly chemically ordered Fe55−xNixPt45L10 films

A series of sputtered multilayers of Ni80Fe20-Ag was prepared to examine the giant magnetoresistance effect before and after annealing. For a wide range of NiFe and Ag thicknesses, no giant magnetoresistance was observed in the unannealed films. After annealing, a large, negative magnetoresistance was observed of order 4 to 6% in applied fields of order 5 to 10 oersteds at room temperature. The appearance of giant managetoresistance is concurrent with the breakup of the NiFe layers, which is attributable to a magnetostatic interaction that favors local antiparallel alignment of the moments in adjacent layers. These structures may be of significant practical importance as sensors that require large changes in resistance at low fields, such as magnetoresistive heads used in magnetic recording systems.

https://science.sciencemag.org/content/sci/261/5124/1021.full.pdf

Giant magnetoresistance at low fields in discontinuous NiFe-Ag multilayer thin films

An SVMR sensor (60) having a self-pinned laminated layer (70) with at least two ferromagnetic films (72,74) antiferromagnetically coupled to one another across a thin antiferromagnetically (AF) coupling film (73). Since the two ferromagnetic films (72,74) in this laminated layer (70) have their magnetic moments aligned antiparallel, their two magnetic moments can be made to essentially cancel by making the two ferromagnetic films (72,74) of substantially the same thickness. The magnetic field energy generated by the signal field acting on this laminated layer (70) will be significantly less than the effective anisotropy energy of the laminated layer (70). As a result, the laminated layer (70) will not rotate in the presence of the signal field, but will be "self-pinned". A hard-bias or exchange bias layer in not needed, also eliminating the need for Ni-Mn and its associated high-temperature process.

Kevin R. Coffey

Papers

On the relationship of high coercivity and L10 ordered phase in CoPt and FePt thin films

High anisotropy L1/sub 0/ thin films for longitudinal recording

Temperature dependent magnetic properties of highly chemically ordered Fe55−xNixPt45L10 films

Giant magnetoresistance at low fields in discontinuous NiFe-Ag multilayer thin films

Spin valve magnetoresistive sensor with self-pinned laminated layer and magnetic recording system using the sensor.