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The high magnetocrystalline phase of L10 FePt has received considerable attention for achieving stable magnetization states in small volumes which could increase magnetic areal storage densities. When FePt is sputter-deposited, it adopts a magnetically soft A1 phase requiring annealing to phase transform to the L10 phase; this annealing results in detrimental grain growth which reduces the capacity for high areal storage densities. In the current work, a series of 10 nm thick granular FePt–SiO2 thin films with various silica contents has been annealed at different fluences using a 10 ms pulse width, 1064 nm wavelength laser to determine if the silica matrix could inhibit this grain growth. The A1 to L10 phase transformation was confirmed by selected area electron diffraction. In general, the films annealed with approximately 25 J/cm2 exhibited the highest L10 c/a tetragonality, 0.97, and coercivity of approximately 875 kA/m (11 kOe). For these films, the 38 vol.% silica incorporation resulted in a FePt grain size of approximately 8.5 nm as compared to 30 nm for films with no silica. The granular encasement of the FePt grains was effective at reducing but unable at inhibiting grain growth using single pulsed laser anneals.

The structural and magnetic properties of ∼12 nm thick FePt thin films grown on Si substrates annealed using a 1064 nm wavelength laser with a 10 ms pulse have been examined. The A 1 to L 1 0 ordering phase transformation was confirmed by electron and X-ray diffraction. An order parameter near 50% and a maximum coercivity of 12 kOe were obtained with laser energy densities of 25–32 J/cm 2 . Grain growth, quantified by dark field transmission electron microscopy, occurred during chemical ordering at the laser pulse widths studied.

A series of approximately 40 nm thick Co 80 Pt 20 thin films have been sputter-deposited onto a combination of Ta, Pt and Ru underlayers grown at different layer thicknesses. The addition of a Ta seed layer to the Pt and Ru underlayers caused the {0002} hexagonal close packed (hcp) Co 80 Pt 20 's c -axis dispersion's full width at half maximum to narrow from approximately 12° to approximately 2°. In-situ stress measurements taken during deposition showed that the Ta seed layer reduced the growth stresses for the Pt and an initial 1 nm of growth for the Ru underlayers. The Ru layer thickness controlled the c/a ratio of the hcp Co 80 Pt 20 film which regulated the degree of magnetic easy-axis alignment in the Co 80 Pt 20 film. The optimal underlayer material stack for Co 80 Pt 20 with a narrow c -axis dispersion and a high degree of magnetic easy-axis alignment was 5 nm Ta/10 nm Pt/5 nm Ru.

Ternary alloy FePt(Au) nanoparticles were prepared by the co-reduction of platinum(II) acetylacetonate and gold(III) acetate and the thermal decomposition of iron pentacarbonyl in hot phenyl ether in the presence of oleic acid and oleylamine ligands. This gave spherical particles with an average diameter of 4.4 nm with a range of diameters from approximately 1.6–9 nm. The as-synthesized particles had a solid solution, face-centered-cubic structure. Though the average composition of the particles was Fe44Pt45Au11, individual particle analysis by Scanning Transmission Electron Microscopy–X-ray Energy Dispersive Spectroscopy showed a broad distribution in composition. In general, smaller-sized particles tended to have a lower amount of Au as compared to larger-sized particles. As the Au content increased, the ratio of Fe/Pt widened.

A series of 20 and 100 nm Fe53Pt47 thin films sputter-deposited onto Si substrates have been thermally annealed using a pulsed thermal plasma arc lamp. A series of one, three or five pulses were applied to the thin films with widths of either 50 or 100 ms. The microstructure and magnetic properties of these annealed Fe53Pt47 films are discussed according to the various annealing conditions and A1 to L10 phase transformation. Upon pulse annealing, the average in-plane grain size of 15 nm (nearly equivalent for both film thicknesses) was observed to increase to values near 20 nm. In general, increasing the pulse width or number of pulses increased the L10 order parameter, tetragonality of the c/a ratio and coercivity of the specimen. The exception to this trend was for five pulses at 100 ms for both film thicknesses, which indicated a reduction of the order parameter and coercivity. This reduction is believed to be a result of the interdiffusion of Fe and Pt into the Si substrate and the formation of iron oxide clusters in the grain boundaries characterized by atom probe tomography.

Chemically synthesized FePt nanoparticles were coated with nonmagnetic SiO2 and MnO shells by sol–gel and polyol processes. TEM images show that the FePt/SiO2 nanoparticles exhibit a thick spherical shell. The size and morphology of the MnO shell can be controlled by changing the reaction temperature, the molar ratio of surfactants/Mn(acac)2, and/or the concentration of precursor. The morphology of the MnO shell can be either spherical-like or cubic-like, depending on whether the molar ratio of surfactants/Mn(acac)2 is less than or larger than 2. From XRD measurements, the spherical core/shell nanoparticles exhibit 3D random crystallographic orientation, while the cubic core/shell nanoparticles prefer (200) texture. The magnetic moment of FePt particles can be enhanced by coating with SiO2 and MnO shells. Furthermore, the agglomeration of FePt particles upon the thermal annealing can be significantly inhibited with SiO2 and MnO shells.

To explore the implications of the single family room (SFR) care environment of neonatal intensive care units (NICU) compared to Open-bay, Combination and Double-occupancy configurations, focusing on family experience, neonate outcomes, staff perceptions, cost and environmental design. This study uses a multimethod design with 11 Level III NICUs. Space allocations, construction costs, staff preferences and perceptions, and occupant behaviors were evaluated. SFR NICU design provides solutions for increasing parent privacy and presence, supporting Health Insurance Portability and Accountability Act compliance, minimizing the number of undesirable beds, increasing staff satisfaction and reducing staff stress. The analysis of this study suggests that there are benefits to SFR NICU. This study is an initial, comprehensive effort, the purpose of which is to spawn future, narrower, in-depth studies focused on SFR NICU design.

Self-organised magnetic arrays (SOMA) of high anisotropy particles are a promising candidate for ultra-high-density recording media. In principle SOMA media have the capability of storing 1 bit per particle, leading to possible reecording densities in excess of 10 Tbit/sq in. In this paper we consider two major aspects of SOMA media, namely the self-organisation process itself and the physics of the particle orientation process.

The effect of pulsed-thermal-processing with high-density plasma arc heating is discussed for 20 nm thick nanocrystalline FePt thin films. The dependence of the A1→L10 phase transformation on pulsed time and radiant energy of the pulse is quantified through x-ray diffraction and alternating gradient magnetometry. For 100 ms and 250 ms pulse widths, the phase transformation was observed. Higher radiant energy densities resulted in a larger measured coercivity associated with the L10 phase.

We have studied the effect of ion-beam irradiation on reducing the ordering temperature of FePt and FePtAu nanoparticles. FePt and FePt(Au14%) 4 nm particles dispersed on a Si-substrate were irradiated by 300 keV Al-ions with a dose of 1 × 1016 ions/cm2 at 43 °C using a water-cooled flange in order to minimize the vacancy migration and voids formation within the collision cascades. Partial chemical ordering has been observed in as-irradiated particles with coercivity of 60–130 Oe. Post-irradiation annealing at 220 °C enhanced chemical ordering in FePt nanoparticles with coercivity of 3500 Oe, magnetic anisotropy of 1.5 × 107 erg/cc, and thermal stability factor of 130. A much higher 375 °C post-irradiation annealing was required in FePtAu, presumably because Au atoms were trapped at Fe/Pt lattice sites at lower temperatures. As the annealing temperature increased, anomalous features in the magnetization reversal curves were observed that disappeared at higher annealing temperatures.

The effect of additive Au, Ag, Pd, Cr, and Cu on the chemical ordering, magnetic properties, and grain growth of chemically synthesized FePt nanoparticles has been investigated. The L1 0 phase has been obtained by solution annealing and direct synthesis. Easy axis alignment has been obtained using dispersed L1 0 FePt nanoparticles.

We have studied the ferromagnetic resonance response of as-made and annealed Fe 46 Pt 46 Au 8 self-ordered nanoparticles forming a film. As-made particles, which crystallize in a low magnetic anisotropy FCC phase, show a single isotropic resonance line close to g = 2.1 . Particles annealed at 450 ∘ C for 30 min are composed of a mixture of the FCC and the high anisotropy L 1 0 phases. In this sample the resonant spectra are anisotropic when the external applied field is rotated from the in-plane to the out-of-plane direction. This absorption also seems to be related to the minority FCC particles which have a better degree of surface order due to the surrounding ferromagnetic environment. From the line width angular dependence we obtained a large dispersion in the anisotropy constant, probably arising from the different degree of atomic ordering of particles in the L1 0 phase.

We describe a potential way to extend the ordered domain of self-assembled FePt nanoparticles. The FePt particles, with an average diameter of 3 nm, were prepared by simultaneous thermal decomposition of Fe(CO) 5 and chemical reduction of Pt(acac) 2 and then were dispersed in a mixture of hexane and octane. When self-assembling on a plain silicon wafer, FePt nanoparticles formed ordered hexagonal arrays in a range of tens to a few hundred nanometers. A silicon wafer with patterned holes of a photoresist film, made using UV-lithographing technique, was used as a template to direct the stacking direction of the FePt nanoparticles. The FePt dispersion was dropped on the patterned holes of the photoresist film. After being heat-treated at 100°C for 30 min under vacuum condition, the photoresist was stripped out by dipping the sample in acetone. The patterned disks, with an average diameter of 2.0 μm and a height of 250 nm, of self-assembled FePt nanoparticles were examined using SEM and Auger mapping. Their magnetic properties were measured using AGM. The Auger electrons of neither Fe LMM nor Pt MNN could be detected from the sample, which indicated the adsorption of oleic acid and oleylamine on the surface of FePt nanoparticles. The coercivity of patterned FePt significantly increased with the annealing temperature above 600°C.

The magnetic properties of chemically synthesized high anisotropy L1 0 [Fe 49 Pt 51 ] 88 Ag 12 nanoparticle arrays have been studied as a function of annealing temperature. Particles were prepared by the simultaneous polyol reduction of platinum acetylacetonate and silver acetate and the thermal decomposition of iron carbonyl, yielding monodispersed particles of diameter ∼3.5 nm. Addition of Ag lowers the ordering temperature of self-assembled arrays by ∼150°C. After annealing at T a =500°C for 30 min in an Ar/H 2 atmosphere, the coercivity was 13,800 Oe. TEM and delta- M measurements indicate weak particle aggregation up to T a =400°C, with evidence of sintering at higher temperatures. Large ratios of remanent to hysteresis coercivity indicate a large distribution in anisotropy energies. Anomalously large thermal stability constants, KV / k B T , and switching volumes were measured, even in samples with very little evidence of sintering. Zero field viscosity versus remanence curves show evidence of exchange interactions.

[FePt]/sub 1-x/Au/sub x/ nanoparticles were prepared by the simultaneous polyol reduction of platinum acetylacetonate and gold acetate and the thermal decomposition of iron pentacarbonyl, giving 3.5-nm-diameter FePt particles with gold atoms substituted in the lattices. The addition of gold promoted the face-centered cubic to tetragonal phase transition, thereby reducing the temperature required for this transition by more than 150/spl deg/C compared with FePt nanoparticles with no additives. This effect is even more significant than adding silver to FePt nanoparticles. For a given annealing temperature, the coercivity increases with the content of gold up to 24%, above which the coercivity starts to decrease. The mechanism for the chemical ordering acceleration may relate to the defect and strains introduced by gold atoms. Upon annealing, gold atoms leave the FePt lattice, leaving lattice vacancies that increase the mobility of the FePt atoms to rearrangement. Dynamic coercivity measurements yield thermal stability factors that are slightly higher than would be expected for noninteracting particles.

[Fe49Pt51]88Ag12 nanoparticles were prepared by the simultaneous polyol reduction of platiunum acetylacetonate and silver acetate and the thermal decomposition of iron pentacarbonyl, giving 3.5 nm diameter FePt particles with Ag atoms substituted in the lattice. The addition of Ag promoted the fcc to tetragonal phase transition, thereby reducing the temperature required for this transition by some 100 to 150 °C compared with pure FePt nanoparticles. After heat treatment at 400 °C for 30 min, the coercivity of the films containing [Fe49Pt51]88Ag12 nanoparticles was more than 3400 Oe, while the films containing FePt nanoparticles were superparamagnetic. This decrease in phase transformation temperature allowed us to decrease the particle coalescence and loss in particle positional order seen when FePt nanoparticles were transformed at temperatures above 550 °C.

The magnetic properties of L1 0 multilayer Co 47 Pt 53 /SiO 2 films have been studied as a function of bilayer thickness and SiO 2 fraction. Under identical annealing conditions, coercivities were found to decrease significantly with decreasing bilayer thickness and decreased with increasing SiO 2 fraction. Measurements on single-layer CoPt films with and without SiO 2 bottom and top layers suggest that the SiO 2 inhibits ordering.

We have measured the relaxation of the remanent magnetization in a number of granular thin films in which delta-M measurements suggest the presence of exchange coupling. The relaxation measurements were made at zero field after application of a saturation field followed by a reverse field. The films had varying compositions (CoPtCrB, Co/CN, CoPt/SiO/sub 2/) and had thermal stability factors, KV/k/sub B/T, ranging from 29 to 670. The most general behavior of the zero field relaxation, observed for the smaller thermal stability factors, was an initial increase in the remanent magnetization followed by a decrease at longer times. The maximum initial zero field viscosity is strongly correlated with the inverse thermal stability factor, k/sub B/T/KV, and generally increases with the area under delta-M curve, normalized by the remanent coercivity. A small zero field relaxation was also observed in barium ferrite and CrO/sub 2/ tapes.

The structure and magnetic properties of multilayer Co 82 Pt 18 /C films with equal CoPt and C layer thickness have been studied as a function of annealing temperature and film thickness. In general, annealing leads to the breakup of the continuous films and the formation of a laterally heterogeneous morphology consisting of separate CoPt and C regions. For individual layer thickness ⩾5.3 nm, annealing has no significant effect on the crystallographic structure and magnetic properties of the films. In both the as-deposited and annealed states, the films have HCP structure with perpendicular c -axis orientation and exhibit perpendicular magnetic anisotropy. The structure of these films appears to be associated with the growth of the CoPt on the C layers. For films with individual layer thickness of 1.3 and 2.7 nm, the structure and magnetic properties are strongly affected by annealing. In the as-deposited state, films have a fine grain structure and coercivities of a few Oe. Annealing leads to the breakup of the multilayer structure at about 300°C and the formation of ∼20 nm HCP and FCC magnetic grains, with the HCP grains exhibiting in-plane c -axis texture. Annealed films have large in-plane coercivities (maximum of 2.77 kOe) and random in-plane magnetic anisotropy. The increase in coercivity with annealing temperature above ∼375°C is primarily associated with an increase in switching volume.

The time dependence of the remanent coercivity for noninteracting Stoner-Wohlfarth particles with uniaxial anisotropy has been calculated numerically as a function of easy axis angle and distribution width. The results have been interpreted in terms of a simplified effective energy barrier function of the form /spl Delta/E=CKV(1-H/H/sub 0/)/sup n/ and a modified Sharrock equation. For a system with identical easy axis angles, Sharrock law behavior occurs at all angles over a wide range of times with C=1 and n a function of angle. For a distribution of easy axes, Sharrock law behavior also occurs with C=1 and n/spl ap/1.5 for squareness values ranging from random to at least 0.98. The dependence of the energy barrier on field and easy axis angle is compared with the Victora expansion and with the Pfeiffer model.

It has been established previously that thermal switching in high density magnetic media causes a significant time dependence of the coercivity at least for times >10/sup -8/ s. Here, the classical Arrhenius-Neel model assuming coherent rotation is applied to systems with distributions in volume, anisotropy and orientation and the numerical results for the time dependent remanent coercivity H/sub CR/(t), the viscosity S/sub R/, and the irreversible susceptibility, /spl chi//sub IRR/ compared to new experimental data on CrO/sub 2/ particulate tapes with varying orientation. The results for an anisotropy distribution are significantly different from a volume distribution and are in better agreement with the data. All distributions produce Sharrock type behavior for H/sub CR/(t), regardless of the distribution width. The model predicts that increasing the orientation distribution width will have very little effect on the fractional time dependence of H/sub CR/, in agreement with experiment which showed that S/sub R//spl chi/IRR/ and H/sub CR/(t) were essentially independent of orientation. Finally, the direct connection between S/sub R//spl chi/IRR/ and H/sub CR/(t) is confirmed theoretically and experimentally.

The effect of the AC gradient field in an alternating gradient magnetometer (AGM) on measurements of the field dependence of magnetic viscosity and irreversible susceptibility has been examined both theoretically and experimentally. Two effects are predicted: (1) The AC field produces a change in the shape of the curves by an amount which is proportional to the product of the second derivative of the curve with respect to H and the square of the effective AC field over the sample. (2) The AC field induces magnetization reversal because of the nonlinear dependence of the relaxation rate on the applied field. For symmetrically shaped curves, the first effect leads to a reduction in the peak and a broadening of the curve. The second effect causes the curves to be shifted towards lower values of H by an amount that depends on the size of the AC field relative to the anisotropy field. Measurements have been made on samples with coercive fields ranging from 64 to 1200 Oe which are in semi-quantitative agreement with the calculations. The results provide guidance about how to minimize these effects when using the AGM.

Partially ordered FePtAu nanoparticles with size around 8 nm were prepared by the simultaneous decomposition of iron pentacarbonyl and reduction of gold acetate and platinum acetylacetonate. The high boiling point chemical, hexadecylamine, was used as a solvent, and 1-adamantanecarboxylic acid was used as a stabilizer. The final temperature was controlled between 300/spl deg/C-360/spl deg/C during synthesis, where disordered FePt particles could be partially transformed into the ordered L1/sub 0/ phase. A nonmagnetic mechanical stirrer was used in order to avoid agglomeration of the fct-FePt particles during synthesis. XRD patterns of as-made samples showed weak superlattice peaks, indicating partial chemical ordering of the FePtAu particles. The coercivities of partially transformed as-made particles vary from a few hundred oersteds to a few thousand oersteds, depending on the concentration of gold additive and the temperature of synthesis. Dynamic coercivity measurements yield thermal stability factors and switching volumes that are slightly higher than would be expected for noninteracting particles.

High magnetocrystalline anisotropy CoPt particles with an average size of 8 nm were synthesized by the superhydride reduction of CoCl2 and Pt(acac)2 at a high temperature. As-made particles showed a disordered face-centered cubic lattice and were superparamagnetic. Upon heat treatment at temperatures above 600 °C, the particles transformed to the L10 phase, as indicated by the appearance of the superlattice peaks in the x-ray diffraction and high magnetocrystalline anisotropy. The temperature dependence of the coercivity of nanoparticles annealed at 650 °C was measured from 10 to 300 K and analyzed using a Sharrock formula. After annealing at 650 °C, the anisotropy of the nanoparticles was K∼1.7×107 erg/cm3.

A waterborne magnetic tape coating formulation was designed and used to prepare experimental magnetic tape samples in a pilot coating trial. The formulations contained a blend of a water-dispersed polyester and an ethylene/vinyl chloride copolymer emulsion, cross-linked with melamine-formaldehyde to give tensile properties (tensile strength 8.69 MPa and Young's modulus 245 MPa) that were comparable to a standard solvent-based binder composition. The pilot tape trial used existing processing equipment, including calendering and slitting. The tape had good magnetic properties and excellent adhesion between the pigmented magnetic layer and the base film, easily exceeding the 8 mm helical scan tape standard of 0.96 N peel force. An economic impact analysis for the case of using our waterborne video tape coating process in a conventional tape manufacturing plant showed a significant decrease in costs for adopting the waterborne process.

FePtCu nanoparticles with varying composition were prepared by the simultaneous polyol reduction of platinum acetylacetonate and copper bis(2,2,6,6-tetramethyl-3,5-heptanedionate) and the thermal decomposition of iron pentacarbonyl. As prepared the particles had a fcc structure with an average diameter of 3.5 nm and were superparamagnetic. Heat treatment of the self-assembled films at temperatures above 550 °C transformed the particles from the fcc to the L10 phase, give in-plane coercivities as high as 9000 Oe. X-ray diffraction revealed that the Cu remained in the films and the presence of an extra peak, indicating a second phase was present. Consistent with two or more phases, the magnetic hysteresis curves could be decomposed into a hard component (Hc>5000 Oe) and a soft component (Hc

In order to study the possibility of using superparamagnetic films as a hysteresis-free switching layer in magnetic tunnel junctions or spin valves we prepared thin, discontinuous NiFeCo layers on different buffer layers on Si3N4/Si wafers. The magnetic properties were measured as a function of temperature for various layer thicknesses. We observe a transition from ferromagnetic behavior via superparamagnetic to paramagnetic behavior as either the film thickness is reduced or the temperature is increased. For a 0.8 nm thick (nominal thickness) NiFeCo layer grown on 10 nm Cu the coercivity reaches essentially zero at around 260 K, indicating a transition from ferromagnetic to superparamagnetic behavior. At that point, the slope of the steepest part of the hysteresis loop diverges. A further increase in temperature causes the saturation magnetization Ms (estimated at sufficiently large fields to saturate the sample) to sharply decrease. The drop of Ms with increasing temperature points to a Curie temperatur...

[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.

Exchange-coupled composites and graded magnetocrystalline anisotropy Ku films allow a lower switching field while maintaining a given thermal stability relative to a conventional single Ku thin film. In this letter, [001] oriented single composition, multilayered single compositions, and a linearly graded composition of L10FexNi0.48-xPt0.52 thin films were sputter-deposited. The films were patterned into an array of ~70-80 nm diameter nanodots by electron beam lithography and Ar ion etching. A relative improvement in the switching field-to-thermal stability of ~41% was achieved for the continuous graded film as compared to the single composition Fe0.23Ni0.25Pt0.52 film. However, the patterned diameter size was still too large to prevent lateral magnetic domain wall motion; consequently, the absolute switching field-to-thermal stability value was low.

Summary form only given. The thermal relaxation of magnetic media is a topic of considerable importance. The stability is typically characterized by measuring the time dependence of the coercivity or by measuring the magnetization decay rate as a function of applied reverse field. For large thermal stability factors (KV/kT), the magnetization decay in zero applied field is considered to be negligible. This is not necessarily the case, however, in state-of-the-art media with KV/kT less than 100. We have recently reported on measurements of the decay of remanence in zero applied field for a series of longitudinal thin film and particulate media and for self-assembled L1/sub 0/ FePt nanoparticles. Here we report on the measurement and calculation of zero-field relaxation in a film with perpendicular anisotropy. For illustration we have used a CoCrPtB film with a coercivity of 1900 Oe. The decay of magnetization was measured at zero applied field after the application of a saturation-reverse field sequence for different reverse fields and the initial viscosity coefficient, S/sub 0/ = dm/dlog(t), was measured as a function of the initial remanent magnetization.

Thermally-activated time-decay studies have been made on a variety of BaFe and metal particle flexible media. Measurements have been made of magnetic viscosity, irreversible susceptibility and activation volumes. In oriented BaFe and MP systems the measurements have been made as a function of temperature with a view to assessing archivability.

We have studied the thermal stability of self-assembled FePt nanoparticles with partial chemical ordering. The magnetic viscosity was measured at zero field after partial dc demagnetization as a function of the initial remanent magnetization. The viscosity curves are in good agreement with a calculation using a noninteracting Stoner–Wohlfarth, Arrhenius–Neel model with a broad distribution of anisotropy fields and randomly oriented easy axes. The shape of the zero field viscosity curve differs from curves measured for thin film media with in-plane and perpendicular anisotropy, in which interactions are known to be important.

Thermal effects on delta-M curves have been studied in a series of CoCrPtB thin films. Film thicknesses ranged from 5 nm to 50 nm and K/sub u/V/k/sub B/T values ranged from 29 to 132. Delta-M curves were measured as a function of field step, wait time at each field step, and measuring conditions. If dc-demagnetization remanence (DCD) curves are measured by applying a saturating field before each reverse field, then the delta-M curves depend on field step for all films and on wait time for the film with the smallest K/sub u/V/k/sub B/T. At small field steps, the field step dependence is approximately logarithmic and the rate is approximately the same as the remanent viscosity coefficient. If a saturating field Is applied only at the beginning of the DCD measurement sequence, the field step and wait time dependences are essentially eliminated. For fixed field step and waiting time, the area under the delta-M curve normalized to the remanent coercivity increases linearly with k/sub B/T/K/sub u/V when the repeat saturation method is used. The dependence on k/sub B/T/K/sub u/V is largely eliminated if there is no repeat saturation field.

We describe an approach to synthesize monodisperse CoPt nanoparticles with dendrimer as template by a simple chemical reduction method in aqueous solution using NaBH4 as reducing agent at room temperature. The as-made CoPt nanoparticles buried in the dendrimer matrix have the chemically disordered fcc structure and can be transformed to the fct phase after annealing at 700 degrees C. This is the first report of dendrimer-mediated room temperature synthesis of monodisperse magnetic nanoparticles in aqueous solution.

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.

The 360° domain wall loops were studied using a magnetic force microscope (MFM) in a glass/Ta(10 nm)/NiFe(12 nm)/NiO(6 nm)/Ta(5 nm) multilayered film fabricated by magnetron sputtering. The film had an enhanced coercivity but no M–H loop shift due to the thin NiO layer. It was found that the local magnetic field exerted by a MFM tip can strongly affect the 360° domain wall, and this interaction was used to probe the structure and dynamics of the wall. The repeated scanning action of the tip causes the transformation from a 360° wall loop into a 180° wall loop. The magnetization distributions for these domain walls were derived from the MFM images. The mechanism of the irreversible change of wall structures caused by the tip–sample interaction was investigated based on micromagnetic analysis. The study of the transformation process provided a better understanding of the structures of the 360° walls. This work suggests that the tip–sample interaction could be a useful tool for studying the domain wall struc...

The magnetic and structural properties of a series of granular Co/CN films prepared by annealing multilayers of Co and CN have been studied. The films contained both hcp and fcc phases, and grain sizes were in the range 18–27 nm. The coercivity varied with cobalt fraction and with bilayer thickness, and a maximum room temperature coercivity of 1230 Oe was obtained for 50% cobalt and 2.7 nm bilayer thickness. Measurements of the time and temperature dependence of the highest coercivity films suggest that their coercivity is due primarily to the hcp phase and to the grain size. All films had relatively low values of magnetic viscosity (2%–3%/decade).

The temperature dependence of the magnetic viscosity S and the irreversible susceptibility χirr has been calculated for distributions of noninteracting, aligned Stoner–Wohlfarth particles. Significant differences are shown to exist for volume and anisotropy field distributions. For a volume distribution, Smax is independent of temperature and χirr,max∼T−1/2, below the superparamagnetic temperature. For an anisotropy field distribution Smax≈c1T1/2+c2T and χirr,max≈c3+c4Smax, where the c’s are positive constants. Results obtained using approximate analytical expressions that have been previously published have been compared with numerical calculations.

The maximum storage density in magnetic media is limited by the superparamagnetic size of the grains that make up the bits. The superparamagnetic size can be reduced by increasing the anisotropy of the grains; however, in conventional media, in which the anisotropy of the grains is uniform, this leads to a proportionate increase in the switching field. The switching field, however, is limited by the maximum magnetization of the core material in the write head. Recent calculations have shown that the switching field can be significantly reduced relative to the thermal stability of the grains if the anisotropy is made to vary appropriately from the bottom to the top of the grain. In this project we propose to test this concept by fabricating and characterizing anisotropy graded films. We will use the hcp CoPtX system, with X = Cr or Ru, where the anisotropy gradient is obtained by grading the composition. Both sheet films and granular films will be fabricated. The anisotropy gradients will be determined by ...

This chapter reviews recent studies of chemically synthesized FePt and related nanoparticles. Various methods for synthesizing the nanoparticles and controlling their shape are described. Thermal effects in nanoparticles near the superparamagnetic limit are discussed. Some of the methods for reducing sintered grain growth during annealing to obtain the L10 phase are described, including the use of a hard shell, annealing in a salt matrix , and flash annealing . The effect of metal additives on the ordering temperature and on sintered grain growth is discussed. Additive Ag and Au significantly not only reduce the ordering temperature but also the grain growth temperature in close-packed 3-D arrays. Preliminary experiments that show additive Ag also reduces the ordering temperature when sintering is prevented. Easy-axis alignment of L10 FePt nanoparticles can be achieved by drying a nanoparticle dispersion in a magnetic field, and the effect of thermal fluctuations on orientation is discussed. Large particle-to-particle compositional distributions in chemically synthesized FePt nanoparticles have been measured. A method of determining the anisotropy distribution is described. Theoretical and experimental works showing the size effect on chemical ordering of FePt nanoparticles are discussed.

A Monte Carlo approach and a modified nudged elastic band method are used to study the dynamic coercivity of interacting particle arrays in particular perpendicular recording media and exchange spring bilayers. Monte Carlo simulations are performed to study the effect of the interactions on the dynamic coercivity of interacting particle arrays. It is shown that the interactions in magnetic recording media only slightly influence the dynamic coercivity. The reliability of energy barrier measurements based upon Sharrock's equation for frequency-dependent coercivity data is investigated using a modified nudged elastic band method. It is shown that the extrapolated energy barrier at zero field may deviate from the correct one by up to 18% if the conventional exponent $n=1.5$ is assumed. Our micromagnetic simulations furthermore indicate that the accuracy of the extrapolated energy barrier can be improved by about a factor of 3 upon measuring the dynamic coercivity at an angle of 45\ifmmode^\circ\else\textdegree\fi{} and using the exponent $n$ as an additional fit parameter.

Chemically synthesized FePt nanoparticles were coated with nonmagnetic SiO2 and MnO shells by sol-gel and polyol processes. TEM images show that the FePt/SiO2 nanoparticles exhibit a thick spherical shell. The size and morphology of the MnO shell can be controlled by changing the reaction temperature, the molar ratio of surfactants/Mn(acac)2, and/or the concentration of precursor. The morphology of the MnO shell can be either spherical-like or cubic-like, depending on whether the molar ratio of surfactants/Mn(acac)2 is less than or larger than 2. From XRD measurements, the spherical core/shell nanoparticles exhibit 3D random crystallographic orientation, while the cubic core/shell nanoparticles prefer (200) texture. The magnetic moment of FePt particles can be enhanced by coating with SiO2 and MnO shells. Furthermore, the agglomeration of FePt particles upon the thermal annealing can be significantly inhibited with SiO2 and MnO shells.

Recent studies have shown a strong correlation between grain growth and chemical ordering in chemically synthesized FePt nanoparticles. In order to study this effect, we have prepared a series of samples in which 3.5 nm FePt nanoparticles are dispersed in a matrix of Cu nanoparticles. The samples were annealed at 600 degrees C and at 800 degrees C. Grain size was determined by XRD Scherrer analysis and time-dependent remanent coercivity measurements were made to determine the intrinsic remanent coercivity, Hcr0. For samples annealed at 600 degrees C, Hcr0 increases strongly with grain size up to approximately 5 nm and increases weakly with additional grain growth. By contrast, after annealing at 800 degrees C, Hcr0 appears nearly independent of grain size. The results suggest that isolated 3.5 nm FePt nanoparticles can be weakly ordered when annealed at 600 degrees C and sintering is necessary for significant chemical ordering.

This paper reports on the synthesis and self-assembly of FePt/MnO core/shell nanoparticles. The enhanced magnetic properties of FePt nanoparticles with MnO coating is shown. Results also demonstrated the suppression of aggregation of MnO coated FePt nanoparticles during the annealing process. The magnetic moment of the FePt nanoparticles is significantly enhanced with the MnO shell. Also, the room temperature hysteresis loops curves reveal that the annealed FePt nanoparticles without the MnO shell have a large coercivity around 14 kOe, while FePt nanoparticles with the MnO shell show a sheared loop with a small coercivity about 3 kOe.

13.1 FePt for Granular Thin-Film Magnetic Media 36913.2 FePtCu Ternary Alloy Nanoparticles 37113.3 FePtAg and FePtAu Ternary Alloy Nanoparticles 37513.4 Magnetic Properties of FePtAg and FePtAu Nanoparticle Arrays 37813.5 Significance of This Work 381Acknowledgments 382References 382

Two new methods are presented for determining the orientation distribution of magnetic tapes from measurements of the angular dependence of remanence. A closed-form expression is presented which allows a two-dimensional distribution function to be directly calculated from the remanence measurements. We have also adapted the method of maximum entropy to numerically calculate the distribution function. This method selects a distribution function which maximizes the entropy subject to the constraint that the fit cannot be better than the assumed error in the measurements. Results are presented which show very good agreement between the two methods.

In this study, the local structure around Fe and Co atoms in as-prepared and partially reduced Co,Ti,Sn-substituted Ba-hexaferrite powders are examined using extended x-ray absorption fine structure (EXAFS). It appears that the substitutional atoms in general and Co in particular show a preference for the Fe(IV) and Fe(III) sites. After partial reduction in H2 at 300 °C, both the Fe and Co EXAFS show a mixture of two phases: substituted Ba-hexaferrite and bcc metal. There does not appear to be any large preference in reduction among the five different types of Fe sites. However, a much larger fraction of the Co is reduced than is the Fe. Thus the partially reduced sample is not merely a collection of unreduced and totally reduced particles. The after-reduction spectra for both Fe and Co EXAFS were fit using a linear combination of spectra from as-prepared material and bcc metal, which enabled quantitative calculation of the relative fraction of each metal reduced.

CoPt(Ag) nanoparticles have been synthesized by high temperature polyol reduction of cobalt, platinum and silver reagents. It was found that by varying the reaction conditions, particles with different sizes and shapes can be obtained. The as-made particles were well dispersed in hydrocarbon solvents, and self-assembled particle arrays were formed when deposited on Si substrates for magnetic measurements and on carbon coated grids for transmission electron microscopy.

In this research, the improved synthesis and alignment of L1/sub 0/ FePt nanoparticles and the measured dependence of alignment on the orienting field are investigated. From the in-plane hysteresis loops of the particles after aligning and drying with a 20-kOe field, the normalized remanence values in the parallel and perpendicular in-plane directions are found to be 0.72 and 0.33 respectively. Also from the loops, the coercivities in the parallel and perpendicular directions are 1.48 kOe and 0.87 kOe, respectively. For comparison, a model for the easy-axis alignment is done assuming that the nanoparticles can freely rotate and achieve a Boltzmann orientation distribution during the drying process. Also, the anisotropy energy is assumed to be sufficiently large so that the easy axis and moment of each particle are nearly parallel during the drying process. Using an appropriate distribution of particle sizes, the model gives semiquantitative agreement with the measured field dependence of the orientation.