Paramagnetic behavior

Magnetic susceptibiUty measurements indicate that the organic part of the coal is diamagnetic, having traces of paramagnetic behavior resulting from free radicals or unpaired electrons (6). 

All atoms with an odd number of electrons must be paramagnetic. There is no way to pair all of the electrons up if there is an odd number of electrons. Many atoms with an even number of electrons are diamagnetic, but some are paramagnetic. The one of lowest atomic number is carbon (Z = 6), which has two unpaired / -electrons producing the paramagnetic behavior [He] 2s22p2. 

The magnetic moments of Fe, Co, and Ni clusters with sizes up to 700 atoms have been measured by Billas et al.1,2 Those measurements were made under conditions where the clusters exhibit super-paramagnetic behavior for low cluster temperatures (vibrational temperature Tv b = 78 K for Ni and Co clusters and 120 K for Fe clusters). Their results are shown in Figure 1. As... 

It is probable that such structures have surface analogs which are likely to exhibit differences in paramagnetic behavior and also in activity toward oxygen isotopic exchange reactions depending on their geometry. 

Fig. 11.59 Variation of magnetic susceptibility with temperature for diamagnetic, paramagnetic, ferromagnetic, and antiferromagnetic substances. Transitions to paramagnetic behavior for ferromagnetic and antiferromagnetic substances occur at the Curie (Tc) and Neel (TN) temperatures, respectively.

The Weiss constant applies a minor perturbation of the Curie Law a small FM exchange yields 6 > 0 K and an AFM exchange yields 9 < 0 K. In a Curie-Weiss plot, a negative x-intercept means AFM interactions are present, and a positive x-intercept means FM interactions. For large enough 9, these plots deviate from linearity at lower temperatures (upward for 9 < 0, downward for 9 > 0). It is best not to overinterpret small Weiss constants where 9 < 0.5 K or so, save to note that they imply nearly isolated paramagnetic behavior. From mean-field theory, one can approximate 9 = 2z JMF/k,46 where is an... 

Fig. 3 Example comparisons of X versus T behavior for a ferromagnet (left) and a sample with antiferromagnetic exchange (right) to theoretical Curie paramagnet behavior.

In this chapter, we will consider systems having small cluster exchange effects or even isolated paramagnetic behavior to be magnetic materials, because they... 

---