Uniform metal iron ellipsoidal particles of around 200 nm in length were obtained by reduction and passivation of alumina-coated α–Fe2O3 (hematite) particles under different conditions of temperature and hydrogen flow rate. The monodispersed hematite particles were prepared by the controlled hydrolysis of ferric sulfate and further coated with a homogeneous thin layer of Al2O3 by careful selection of the experimental conditions, mainly pH and aluminum salt concentration. The reduction mechanism of α–Fe2O3 into α–Fe was followed by x-ray and electron diffraction, and also by the measurements of the irreversible magnetic susceptibility. The transformation was found to be topotactic with the [001] direction of hematite particles, which lies along the long axis of the particles, becoming the [111] direction of magnetite and finally the [111] direction of metal iron. Temperature and hydrogen flow rate during the reduction have been found to be important parameters, which determine not only the degree of reduct...

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Structural and magnetic transformation of monodispersed iron oxide particles in a reducing atmosphere

Nanopores with a wide range of aspect ratios were fabricated in an anodized aluminium oxide layer on bulk metallic aluminium. The aspect ratios (L/D) were around 20?1000 (pore length, L, 1000?60?000?nm; pore diameter, D, 50?100?nm). For comparison, nanopores in polymer films were also prepared using heavy-ion-track etched polyimide films (L, 70?000?nm; D, 70?nm; L/D = 1000) and polycarbonate films (L, 30?000?nm; D, 100?nm; L/D = 300). The pore diameter of the anodized aluminium oxide layer was controlled by the anodization voltage, while the pore diameter of the heavy-ion-tracked polyimide and polycarbonate films was controlled by the etching time in a sodium hydroxide alkaline solution. Ni and Co homogeneous single nanowires were fabricated using the electrodeposition and in situ contacted techniques in the nanoporous templates. The Ni and Co nanowires with the largest aspect ratios (L/D = 1000) showed around 2.3% and 1.6% of the typical anisotropic magnetoresistance (AMR), and the effects of aspect ratio on the resistance and AMR were investigated.

http://iopscience.iop.org/article/10.1088/0022-3727/36/24/003/pdf

Template synthesis and magnetoresistance property of Ni and Co single nanowires electrodeposited into nanopores with a wide range of aspect ratios

The time-dependent properties of arrays of elongated and mutually aligned iron particles in alumite (anodized aluminum oxide film) have been studied. At a constant particle diameter of 11 nm, the magnetic viscosity is nearly independent of the particle length (varied from 140 to 440 nm). The activation volume (1.1 to 1.2×10−18 cm3) is almost independent of the particle length and fields: this nearly constant value is much smaller than previously reported values, but very close to the theoretical superparamagnetic limit 1.0×10−18 cm3 at room temperature.

Activation volume of α-Fe particles in alumite films

Characterisation of advanced metal particle recording media pigments

Relevant aspects concerning the ultimate achievable recording densities for particulate as well as for thin-film media are discussed This review covers the entire range starting from micromagnetics of individual single domain particles, moving on to their magnetic behavior in a particle assembly under particular consideration of the structure being actually obtained in the process of manufacturing recording media, and finally embarking on an outline of recording physics These considerations are not only carried out for longitudinally and perpendicularly oriented recording media but also for media having an arbitrary orientation of the easy axis of magnetization All aspects are discussed and illustrated for the first commercially available thin-film medium on a flexible substrate, which is the metal evaporated tape, ie, the obliquely deposited Co-Ni-O layer for the Hi8 video system >

Media requirements and recording physics for high density magnetic recording

Chromium dioxide was introduced in the later 1960s as the first magnetic material capable of high density recording. Since that time a continuous improvement of material properties has been made possible. The most recent improvement concerns particles with coercivities well above 70 kA/m which can be prepared by a modified hydrothermal process without using expensive Ir doping. Although CrO/sub 2/ has a somewhat higher specific magnetization, the static magnetic properties of CrO/sub 2/ and cobalt-modified iron oxides are comparable. There are, however, some significant differences in recording performance. CrO/sub 2/ pigments show better print-through to noise ratios than Co-modified iron oxides of the same mean magnetic volume. There are no problems with erasability, which becomes increasingly difficult to control for high-coercivity cobalt-modified iron oxides. Finally, CrO/sub 2/ is the only pigment material which allows thermomagnetic duplication. >

Chromium dioxide particles for magnetic recording

With recorded wavelengths well under 1 micron the geometric dimensions of the particles start to have significant influence on the output of particulate media. The recorded wavelengths of well under 1 micron being used in video storage approach the length dimensions of standard recording pigments. As a single domain particle cannot support a magnetic transition along its length, the finite length of the particle leads to a reduction in the maximum output signal analogous to the gap loss function and known as length loss. At the same time also the magnetic homogeneity as expressed by (1-S*) becomes increasingly important. The authors experimentally verify the theoretical predictions regarding the influence of the particle parameters by measuring different sets of experimental tapes prepared with appropriate pigment samples. >

Experimental study of the influence of particle size and switching field distribution on video tape output

The process of thermally assisted switching in small metallic particles is studied by measuring the field dependent fluctuation fields. A correlation is found between the size of crystallographic sub-units rather than the total volume of the particles and the fluctuation fields. Therefore, very small particles can be more stable magnetically than large particles, if they contain only few large crystal grains. The grain volumes are determined by X-ray line broadening measurements on random and on oriented samples. The effect of thermal activation on the switching process is confirmed by measuring the recording currents needed to optimize high frequency output in tape samples prepared with the same particles. >

Thermal effects in small metallic particles

A modification of hydrothermal synthesis has led to a new class of iron-doped CrO/sub 2/ pigments with coercivities significantly higher than the previous 'limit' of 56 kAm/sup -1/ (700 Oe). It is demonstrated that enhanced incorporation of iron is the reason for the increased coercivity. A summary of crystallographic and magnetic properties is presented. >

Magnetic analysis of high coercivity chromium dioxide particles

Samples of chromium dioxide particles with a range of remanent coercivities H/sub r/, but with identical particle size distributions were prepared in order to study the effect of changes in the magnetocrystalline anisotropy on the time-dependent magnetization. Comparisons have been made with three cobalt modified gamma-Fe/sub 2/O/sub 3/ samples. The field-dependent fluctuation field H/sub f/, derived from measurements of magnetic viscosity as a function of applied field H, has been used to characterize the time-dependent magnetic switching of the particles. Plots of H/sub f//H/sub r/ against H/H/sub r/ are unaffected by anisotropy variations in either pigment type. The interpretation is that the details of the magnetics and the reversal mode are not primarily responsible for the more pronounced time-dependent magnetization effects observed in CrO/sub 2/ and that the more likely explanation is the high state of magnetic excitation associated with the low Curie temperature. >

E. Schwab

Papers

Chromium dioxide particles for magnetic recording

Experimental study of the influence of particle size and switching field distribution on video tape output

Thermal effects in small metallic particles

Magnetic analysis of high coercivity chromium dioxide particles

Viscous magnetization of chromium-dioxide particles: effect of increased magnetocrystalline anisotropy