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NanCoO

Surface Energies, Entropies and Magnetic Properties of NanoCoO

CoO is of interest for both practical and theoretical reasons. It is used widely as the magnetic material on recording devices. While CoO has a simple face-centered cubic (NaCl) structure at room temperature in its paramagnetic phase, it undergoes a coupled structural-magnetic transition near 290 K into an antiferromagnetic monoclinic phase. The Co2+ ion is unusual in that the orbital angular momentum associated with the unpaired electrons is not fully quenched by the crystal field so that the magnetism is more complicated than it might be otherwise. There are also some indications that the bonding in CoO is primarily ionic rather than covalent and a claim has been made that the superexchange mechanism may not be operative here.

We have made heat capacity measurements on particles with diameters of (7.0 ± 1.0) nm. The nanoparticle Cp has a broad anomaly with a rounded maximum at 265 K, a reduction of 23 K from the Néel temperature TN of single crystal CoO. The sharp peak that appears in single crystal CoO is indicative of long-range ordering of the magnetic spins. The rounding of the anomaly in the nanoparticles tells us that the long-range magnetic order is pretty much lost. The shape of the nanoparticle heat capacity and the location of TN is in excellent agreement with thin films of CoO of comparable dimensions.

Comparison of Single Crystal and 7 nm CoO

From an analysis of the excess heat capacity, (Cexc = C(nano) - C(single crystal), we can calculate an excess entropy: . The magnetic and surface contributions to the excess entropy cannot be resolved uniquely, but we can take the behavior of Cexc in the region from 50 to 180 K as being typical of surfaces, and extrapolate that behavior through the transition region. From Sexcat 245 and the surface area of the particle, we estimate the surface entropy as (0.28 " 0.03) mJ AK -1 Am -2 which is in line with the only literature report (for MgO). Then, depending on how we extrapolate Cexc we get negative excess magnetic entropies that range from (-1.7 to -0.8) J · K-1 · mol-1. The negative entropy supports the conclusion that long-range magnetic order is not established in the nanoparticles as compared to the single crystals.

Plot of Cex = C(nano)-C(single) as a function of temperature; Inset is the excess entropy as a function of temperature.

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