"Energetics of Nanomaterials" Page: 10 of 14
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.03'15'2005 TEE 09:26 FAv -307529207 VCD NEAT ORU THERUt0
1993; Wolf 1995; Shrivastava 2002; Oya 1999]. (2) The lower hyperfine field H in CoO nanoparticles indicates a
decrease in the coupling between the electronic and nuclear spins. Mssbauer investigations on 7-FeO3 [Hancda
1977, 1987] and BaFe201 [Gajbhiye 19991 nanoparnicles have also reported such reductions. (3) The greatly
reduced Debyc temperature of the nanoparticles indicates a softening of the phonons in the naneparticles.
Using our single crystal results as a baseline, we have calculated C for nano-Co. BElow 250 K, C,
is positive, but it goes negative at higher temperatures because of the larger magnetic anomaly in the single crystal.
The excess entropy of the nanoparticle (S, - Scna1) is positive at all tempcraturem. At 250 K, the excess entropy is
about 2.4 JIK-'mol- and it drops to 1.5 JKF-mo at 298 K. Taking the S at 250 K as arising primarily from
surface contributions, we calculate a surface entropy of 2.8 mJ IK-m. which is in excclient agreement with the
result of Jura and Garland [Tura 1952] for MgO.
We have also measured the specific heat of vapor-deposited thin film Coo. In these films, there are two
different structural lengths. each of which can be controlled; (1) film (or layer) thickness, which can be reduced
easily to the nanometer regime and accurately prepared and measured, with very little dispersion (in a multilayer
film); and (2) grain size for a polycrysalline film, which is controlled by growth conditions (substrate choice,
growth temperature and rate, partial pressure, ion bombardment, growth technique) and any subsequent annealing.
The nature of the grain boundaries and any ir.terface structure control whether these grain boundaries limit the
magnetic interactions (and less likely, the phonons). Hence films can be either 2 dimensional strucires, limited by
thickness, or can (at least in principle) exhibit GD to 3D crossover effects.
Figure I1 shows low temperature Cp for films of CoO with different grain sizes, prepared by growing at
different temperatures; data is plotted as Cp.7 vs. T2. These films weigh only a few micrograms and arc measured
using Si-micromachined calorimeters. As seen in the 7 nm bulk sample of nanocomposite-CoO, significant softening
of the lattice (larger T' term and reduced Debye temperature) is seen in the film grown at room temperature
compared to that grown at 100 C. Also as seen in the nanocomposite-CoO, there is a small linear term in the data
on the 'ms that increases with decreasing grain size and is not present in the single crystal data-
Figure 11 Low temperaturee Cp for 300 nim thick CcO films
-gmwn at 100' (- average grain sire from XRD 74 nin) and room
temperature (average grain size 34 nm) compared to single
f' "'' crystal date. The small bump near 900 K in one fim reflects the
anti ferromagnatic ordering of a small amount of CosO4 in this
Sample (wn order of a few mo.%f). Work at UCSD (fIlms) and
r IBY (single crysrrl)
ar RT
J Fat I * RT To further reduce grain size in the firms, the specific
heat of vapor deposited Coo films layered with amorphous SiOC
No -*a 86 00 s 49 (Figure 12a) was studied as a function of CoG layer thickness, as
-r Na comparison study to our earlier work [Abarra 1996] on CoO
layered with MgO. The previous work with MgO intervening layers had shown broadening of the Coo peak, but
very little suppression of the N4ei temperature even for layers as thin as 1.6 nir (Figure 12b). The specific heat of
CoO layers separated by SiO2 demonstrates a dramatic difference. Here we see strong suppression and broadening,
such that the material is not antiferromagne: c at any temperature for thicknesses below 2 nr. The explanation lies
in the structure of the CoO layers: CoO/MgC grows as a coherent superlattice, with grains extending through the
thickness of the fim, which also results in large grain sire in-plane, while for CoO/SiO2, the amorphous nature of
the itervening SiO2 layers causes grain sizes to be strictly limited to the CoO thickness, causing the in-plane grain
size to be similarly small (see Figure 5.13). For extremely thin CoO layers (<2 nm), high resolution cross-sectional
TEM images showed that the CoO was no longer structurally ordered but had become amorphous.ER 1 237. "Enercerics of Nanonaterials"
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Navrotsky, Professor Alexandra. "Energetics of Nanomaterials", report, January 31, 2005; United States. (https://digital.library.unt.edu/ark:/67531/metadc778992/m1/10/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.