Magnetic Properties of Nanoscale Iron Particles Photodeposited in Glass
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519 Mat. Res. Soc. Symp. Proc. Vol. 405 01996 Materials Research Society
The absence of detectable particles immediately after photolysis suggests that the particles are very small, (•_5-10 A) in diameter. On heating to 6501C, the average size of the particles increases to about 30 A in diameter. At 12000C, when porous glass becomes non-porous uniform glass, small iron clusters aggregate to form larger precipitates in the consolidated matrix, where the average size Of the particles is about 100 A in diameter(Figure I). The absence of a Mossbauer sextuplet in the samples heated to 6500C is attributed to superparamagnetic relaxation. The superparamagnetic relaxation is a thermally activated process where the magnetization vector changes its direction. The relaxation time of a particle of volume V is given by r=,c0exp(KV/kBT) where .(= I O=sec, kB is the Boltzman constant and K is the anistropy energy constant. When particles are small, the relaxation time is fast in comparison to the measuring time of the experiment; the Mossbauer Larmor precession time. Under these conditions the average magnetization of the particles appear to be zero and the Mossbauer spectrum exhibits a paramagnetic single line or doublet depending on the existence of quadrupole interactions in the particles. This appear to be the case of unheated samples and samples heated to 6500 C, which show paramagnetic properties and the average particle size is less than 40A in diameter. On heating to the 12000C, the particles become larger and the relaxation time is longer. As a result , the average magnetization is not zero. XANES indicates that 50% of iron is in the form of metallic iron with the 100 nm balance present as iron oxide 12. We assume that each 1OA diameter particle Figure 1. TEM micrograph of Fe in PVG heated consist of a metallic iron core within an to 12000C. iron oxide envelope and the diameter of iron core is 79 A in diameter. Taking room temperature as the blocking temperature, where the sextuplet collapses to a quadrupole doublet, the anistropy energy constant for 79 A Fe particles is calculated to be 3.69x105 erg/cm3. This value is greater than that for bulk iron, 1xl05 erg/cm3 , but within the range expected for small particles. This also suggest that Fe
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particles are not directly bound to the glass matrix. If the iron particles were directly bound to the glass matrix the anistropy would be expected to be much higher than the bulk value. For example C.H.Lou et al" report an anisotropy energy constant larger than the value for bulk iron and for 45A diameter Fe particles deposited on Si0 2 matrix is two orders of magnitude higher. The assumption of room temperature as the blocking temperature for 79 A particles is consistent with the results of Dorman et al15 where they calculated 80 A in diameter as the critical size at 300 K for Fe particles deposited in Si0 2. The calculated anistropy energy for Fe 20 3 is 3.6x10 5 erg/cm3 which is an order of magnitude larger than the bulk value, 1xl0 4 erg/cm 3. Again, smaller particles e
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