High Neel temperature

In an ideal case we should expect very high Neel temperatures since for transitional-metal oxides B 1 eV, if 2z = 10 then we expect 1200 K. But in transitional-metal compounds, as we shall see in Chapter 6, the equation that we must use to determine the condition for the transition is... 

Interestingly, the blocking temperature (Tb) increases with the increase in particle size as expected from theory in the case NiO nanoparticles, but shows the opposite relation in the case of MnO nanoparticles. It is not clear why such an inverse relation between 7b and particle size manifests itself. It is noteworthy that supermagnetism at low temperatures is the general feature of small nanoparticles of transition metal oxides, which are otherwise antiferromagnetic with fairly high Neel temperatures. 

Hyperfine interactions at U nuclei in uranium dipnictides are correlated with the magnetic sequence in their antiferromagnetic states. Because of high Neel temperature, the lattice contribution in the EFG tensor is difficult to estimate. Therefore, it has not been clarified experimentally whether the nuclear quadrupole interaction is caused by the cancellation between the 5f quadrupole moments and lattice contributions or not Considering the crystal structure of the series of the uranium dipnictides, the pnictogen dependence of the lattice contribution in the EFG tensor is small. The results ofthe U Mossbauer spectroscopy imply that the electronic structure thataffects the hyperfine interactions at nuclei... 

The high pressure form of FeOOH is more compact than any other iron oxide hydroxide, hence it has a higher than usual Neel temperature of 470 K. At room temperature, high pressure FeOOH is antiferromagnetic with a collinear spin arrangement parallel to the c-axis (Fernet et al., 1973). High-pressure FeOOH is completely miscible with CrOOH. Substitution with Cr reduces T to the extent that with 80%... 

The addition of 2.5% Cr02 leads at intermediate temperature to a phase i n which only half the V ions are paired the others form a zig-zag chain (Marezio et al 1972, Pouget et al 1974). At low temperatures pairing takes place, and at higher temperatures the usual transition to the metallic rutile form. This intermediate phase has high susceptibility, and the zig-zag chains are interpreted as onedimensional Mott-Hubbard insulators above their Neel temperature. Since the transition temperature is little changed, this shows that U is the most important quantity in determining the gap. 

The resistivities of the mixed crystals are shown in Fig. 6.22. It can be seen that (as indeed follows from the phase diagram, Fig. 6.20) the transition to the insulating state occurs with larger x for increasing temperature, probably because of the high entropy of the AF insulator above its Neel temperature. 

Another anomaly is to be found in the complicated changes that occur very close to JN. For instance, the maximum negative values of A/cH are found for a-Cr203 and CoO slightly above the Neel temperature, but this occurs only at fairly high fields. 

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