J. W. Harrell

Synthesis, chemical ordering, and magnetic properties of self-assembled FePt–Ag nanoparticles

[FePt]1−xAgx nanoparticles were chemically synthesized. Self-assembled arrays were characterized by transmission electron microscopy and small angle x-ray diffraction (XRD). Particles were annealed at temperatures from 300 to 500 °C for 30 min and degree of chemical ordering was characterized by large angle XRD and by magnetometry. Compared with pure FePt nanoparticles, additive Ag reduced the A1 to L10 ordering temperature by more than 100 °C. A maximum coercivity of more than 10 kOe was measured for samples with ∼15% Ag annealed at 500 °C, compared with about 2 kOe for samples without Ag. This reduction in required annealing temperature significantly reduces particle coalescence and loss in positional order. XRD measurements suggest that the reduction of the ordering temperature is due to defects and lattice strain introduced by the Ag and the subsequent segregation of the Ag upon annealing, activating the nucleation of the ordered phase.

Easy axis alignment of chemically partially ordered FePt nanoparticles

Partially ordered Fe53Pt47 nanoparticles with size around 8nm were prepared by the simultaneous decomposition of iron pentacarbonyl and platinum acetylacetonate. The high boiling point chemical, hexadecylamine, was used as a solvent, and 1-adamantanecarboxylic acid was used as a stabilizer. X-ray diffraction measurements reveal that as-made FePt particles were partially transformed into the ordered L10 phase with some weak superlattice peaks. The room-temperature hysteresis loop and remanence curve suggest a broad distribution of anisotropies in the partially ordered particles. By coating the partially ordered FePt nanoparticles with a polyvinylchloride polymer binder, the particles could be re-dispersed in cyclohexanone. Furthermore, the easy axis of the particles coated with the polyvinylchloride polymer binder could be aligned under an external field. Easy axis alignment was confirmed from both alternating gradient magnetometer and x-ray diffraction measurements.

Model for the easy-axis alignment of chemically synthesized L10 FePt nanoparticles

Analytical and computational models of the easy-axis orientation process in magnetic nanoparticle systems are investigated and compared with experimental data. The computational model gives good agreement with experiment on the assumption of a reduced anisotropy constant, which is ascribed to the existence of aggregation. It is found that the degree of orientation depends on the parameter KV∕kT, with K the anisotropy constant and V the nanoparticle volume, which governs the coupling of the easy axis to the magnetic moment. A simple and useful analytical expression is derived, from which the primary conditions for good alignment are determined.

Size effect on L10 ordering and magnetic properties of chemically synthesized FePt and FePtAu nanoparticles

There is growing evidence that FePt nanoparticles become increasingly difficult to chemically order as the size approaches a few nanometers. We have studied the chemical ordering of FePt and FePtAu nanoparticle arrays as a function of particle size. Monodisperse Fe49Pt51 and Fe48Pt44Au8 nanoparticles with a size about 6nm were synthesized by the simultaneous decomposition of iron pentacarbonyl and reduction of platinum acetylacetonate and gold (III) acetate in a mixture of phenyl ether and hexadecylamine (HDA), with 1-adamantanecarboxylic acid and HDA as stabilizers. The nanoparticles were dispersed in toluene, films of the particles were cast onto silicon wafers from the dispersion, and the films were annealed in a tube furnace with flowing Ar+5%H2. The magnetic anisotropy and switching volumes were determined from time- and temperature-dependent coercivity measurements. By comparing with 3‐nm FePt and FePtAu nanoparticles of comparable composition, the phase transformation is easier for the larger parti...

Temperature dependence of exchange field and coercivity in polycrystalline NiO/NiFe film with thin antiferromagnetic layer: Role of antiferromagnet grain size distribution

The temperature dependence of the exchange bias Heb and coercivity Hc, has been measured for a NiO(6.5 nm)/Ni81Fe19(12 nm) bilayer which demonstrated at room temperature zero exchange bias, but an enhanced coercivity (Hc=65 Oe). Upon cooling the sample in a magnetic field (H=300 Oe), the exchange bias remains zero down to T=250 K, whereas the coercivity increases roughly linearly with decreasing temperature. Below this critical temperature an exchange shift of the hysteresis loop occurs that is accompanied by a decrease in the coercivity. Decreasing the temperature further results in an increase in both Heb and Hc. A thermal fluctuation model, where both fluctuating and stable AF grains contribute to the coercivity, with an additional assumption of increasing the density of NiO interfacial uncompensated spins at low temperature, predicts the correct temperature behavior of Heb and Hc.

Delta‐H plot evaluation of remanence behavior in barium ferrite tapes and disks

Remanence studies in recent years have used the characteristics of the Henkel plot and the delta‐M plot in the interpretation of interaction field effects in particulate magnetic media. Studies of these two plots in ordered BaFe coatings, however, indicate that both plots give inadequate description of interaction field effects. An alternative to the delta‐M plot is the delta‐H plot in which, essentially, the (normalized) susceptibility curves for the isothermal remagnetization and dc demagnetization processes are differenced horizontally (rather than vertically as in the delta‐M plot). It is shown here that the delta‐H plot is a more direct way of examining the interaction fields. Simulated delta‐H plots are presented based on a recent mean field model by Che and Bertram and are compared with measurements made on oriented BaFe coatings.

Direct synthesis and easy axis alignment of L10‐FePt nanoparticles

Partially ordered Fe53Pt47 nanoparticles with size around 8nm were prepared by the simultaneous decomposition of iron pentacarbonyl and platinum acetylacetonate. The high boiling point chemical, hexadecylamine, was used as a solvent, and 1-adamantanecarboxylic acid was used as a stabilizer. The reflux temperature of the solution could exceed 360°C, where disordered FePt particles could be partially transformed into the ordered L10 phase. A nonmagnetic mechanical stirrer was used in order to avoid agglomeration of the fct-FePt particles during synthesis. The particles were dispersed in toluene and films of the particles were cast onto silicon wafers from the solution. X-ray diffraction patterns of as-made samples showed weak superlattice peaks, indicating partial chemical ordering of the Fe53Pt47 particles. The room-temperature hysteresis loop of the as-made sample reveals a small coercivity (∼600Oe) because of thermal fluctuations; however, the loop is wide open and hard to saturate. The remanence coerciv...

FMR determination of damping constants in magnetic tapes

Abstract FMR spectra have been measured in a series of CrO 2 tapes, three metal evaporated tapes and a highly-viscous γ-Fe 2 O 3 tape in order to extract the damping constants. For CrO 2 the damping constant was approximately independent of orientation and coercivity. A strong correlation has been found between the damping constant of these and previously measured tapes and switching speed measurements made by other investigators.

Remanence studies of interparticle interactions in Ba-ferrite

Abstract Interparticle interactions in barium ferrite dispersions have been studied as a function of orienting field strenght, milling time and angle with respect to the orientation direction through the use of isothermal and dc remanence measurements. The measurements have been analyzed through the use of Henkel plots, σ M plots and σ H plots.

Time and temperature dependences of the magnetization reversal in a Co/Pd multilayer film

The time and temperature dependences of the magnetization reversal have been measured in a high-coercivity Co∕Pd multilayer film characterized by nucleation and domain wall motion. The time and field dependences of magnetic relaxation curves at room temperature were interpreted in terms of an energy barrier that depends linearly on reverse field, suggesting a Sharrock-type [M. P. Sharrock and J. T. McKinney, IEEE Trans. Magn. 17, 3020 (1981)] equation for the time and temperature dependences of coercivity with unity exponent. The sweep rate and temperature dependence of the coercivity were analyzed using the Sharrock equation to obtain the temperature dependence of the intrinsic, short-time coercivity H0 and the zero-field energy barrier E0. A single power law behavior was found for H0 versus the saturation magnetization Ms.