Neel temperature

Naphthylamine, 1-phcnylazo-metal complexes structure, 6,44 1,8-Naphthyridine metal complexes, 2, 92, 93 Neel temperature, 1, 258 Neocupferron... 

Fig. 3.6. Specific heats of MnC03 and CaC03 illustrate the general effect of a magnetic transition. Data from [30], The Neel temperature for MnC03 is 29.4 K [30].

Fig. 8 Temperature dependence of din f>/d(T 1), i.e., slope of the Arrhenius plot as a function of temperature for (a) (EDT-TTFBr2)FeBr4 at various pressures - the data for 0, 5.8 and 10.1 kbar are vertically shifted up by 60, 40 and 20 K, respectively, for clarity (b) (EDO-TTFBr2)2GaCl4 and (EDO-TTFBr2)2FeCl4 at 11 kbar. TMl and TN are the metal-insulator transition temperature and the Neel temperature, respectively, hi (b) the metal-insulator transition is observed as two separate peaks...

We have described above the evolution of the magnetic properties of the [Cp2M (dmit)]AsFg salts upon isomorphous Mo/W substitution. Another possibility offered by this attractive series is the isomorphous substitution of the counter ion, that is PFg- vs AsF6 vs Sbl- fi. Electrocrystallization experiments conducted with [Cp2Mo(dmit)] and the three different electrolytes afforded an isomorphous series, with a smooth evolution of the unit cell parameters with the anion size [32], This cell expansion with the anion size leads to decreased intermolecular interactions between the [Cp2Mo(dmit)]+ radical cation, as clearly seen in Table 2 from the decreased Curie-Weiss temperatures and Neel temperatures (associated with the transition they all exhibit to an AF ground state). 

A three-dimensional set of intermolecular interactions is further confirmed by the observation of a transition to an antiferromagnetic ground state in both radical complexes, at a Neel temperatures of 8 (Mo) and 4.5 K (W), in accordance with the difference of Curie-Weiss temperatures between both complexes. Note also the spin-flop field in the antiferromagnetic state, found at 5.5 kG in [Cp M(dmit)2] and at 8 kG in [Cp W(dmit)2] , a consequence of the stronger spin orbit coupling in the latter. 

Figure 2.6 The initial dT/dp slope of selected second-order transitions relative to the transition temperature at p = 1 bar a- to /J-quartz (SiC>2) [8], the Neel temperature (T j) of Fei yO [9] and the Neel temperature of Lao.7Cao.3Mn03 [10].

Figure 2.13 Heat capacity of wiistite around the Neel temperature [19]. O Feo.990 I c0 947O V Feo.93sO + Feo.9250. Reproduced by permission of the Mineralogical Society of America.

One can measure H as a function of temperature to see where Hx vanishes. If one assumes Hx is proportional to the magnetization, one can fit a Brillouin function through the data and extrapolate to zero Hx to get the magnetic transition temperature. As an example, the Neel temperature of -Fe203 has been determined by this technique [see Figure 1 of Ref. 12]. 

Lepidocrocite is paramagnetic at room temperature. The Neel temperature of 77 K is much lower than that of the other iron oxides and is the result of the layer-like structure of this mineral. The sheets of Fe(0,0H)6 octahedra are linked by weak hydrogen bonds, hence magnetic interactions are relatively weak. The saturation hyperfine field is also lower than for any other iron oxide (Tab. 6.2). In the antiferromagnetic state, the spins are ordered parallel to the c-axis with spins in alternate layers having opposite signs. A decrease of T by 5 K was observed for Al-lepidocrocites with an Al/(Fe-i-Al) ratio of 0.1 (De Grave et al., 1995). 

At room temperature, akaganeite is, like lepidocrocite, paramagnetic. It becomes antiferromagnetic below the Neel temperature of 290 K (Murad, 1988). The value of Tn and the strength of the magnetic interactions are variable and depend upon synthesis conditions (i.e. the temperature and the length of the hydrolysis period). These influence the amount of interstitial water in the compound, which in turn induces spin reduction. Tn decreases linearly to 250 K as the H20/unit cell rises to 0.02 mol mor (Chambaere De Grave, 1984 a). 

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