Constant Weiss

Above a critical temperature Tc, the Curie temperature, a ferromagnetic material becomes paramagnetic, since thermal motion inhibits the parallel orientation of the magnetic moments. The susceptibility then follows the Curie-Weiss law with a positive value of the Weiss constant, 0 > 0 (Fig. 19.6). 

Table 1 Cation and anion S values basic structural motives Weiss constants transition temperatures, magnetic ordering critical fields of the salts based on metallocenium and on metal bisdichalcogenate complexes ... 

Table 2 Cation and anion S values Weiss constants of the metallocenium salts of metal bisdichalcogenate complexes based on segregate chains of anions...

The Curie-Weiss constant (or Weiss constant), 0, is positive and has the dimensions of temperature. The Curie-Weiss constant is a measure of the departure of the system from the ideal behavior represented by the Curie law. 

The Curie-Weiss constant, 0, is positive, has the dimensions of temperature, and a value usually close to, but not quite identical to, the Curie temperature, TC- The transition is reversible, and on cooling, ferromagnetism returns when the magnetic dipoles align parallel to one another as the temperature drops through the Curie temperature. 

The EPR data are of course intimately related to magnetic susceptibility data. The Weiss constant of the 5-phase is proportional to D which will amount to only a few degrees Kelvin at most, while the Weiss constant of the /8-phase is large due to the large exchange effects in this phase. The Weiss constant obtained from a magnetic susceptibility measurement is thus due predominantly to the /8-phase. 

The paramagnetic susceptibility of pure FeAl204 (x = 2) gives a Weiss constant 0 = -144 K and a molar Curie constant (emu) = 3.80 K, somewhat large for high-spin Foa ions with a spin-only moment however, there is no apparent antiferromagnetic ordering of the FeA -ion spins down to 9.5 K, where a peak in the susceptibility has been associated with a local Jahn-Teller distortion about the Fe ions ... 

The value of the Weiss constant 0 was 43 1°K and suggested that Eu2RuH6 becomes ferromagnetic at some temperature below 77°K. The magnetic susceptibility for Yt RuHe appeared to be consistent with di-valent ytterbium, which theoretically has zero atomic moment. The small paramagnetism observed experimentally was assigned to Yb23 impurity. 

Curie-Weiss behaviour from 309 to 83 K with a Weiss constant 6 s 10 K. The 3-carboxylate was studied in the range 275-4.2 K and the data were interpreted in terms of an antiferromagnetic interaction between silver ions using the Ising linear chain model. This gave an exchange energy of / = —30.8 1.0 cm-1 and g = 2.5 0.02 which was somewhat smaller than the experimentally determined (g) of 2.08.513... 

The structure of the pyrazine complex was suggested to contain polymeric square planar cations with bridging pyrazines and the magnetic properties (300-80K) were interpreted in terms of an antiferromagnetic exchange interaction. A Curie-Weiss constant of 84 K was calculated. At room temperature, the magnetic moment was found to be 1.61 BM. 

Here, 9 is the Weiss constant and xtip is a temperature-independent paramagnetic contribution, often neglected as it may be even smaller than Xdiam-For some purposes it is more convenient to deal with magnetic properties per unit mass rather than per unit volume. We define... 

In general, the Curie constant C from the slope of l/x versus Tcan be used with the above equation to find the spin quantum number S of the spin carriers, assuming g 2 for organic spin units. The // T criterion for the Curie law is readily achieved in an external field of 1000 Oe at 1 K for calibration, 10,000 Oe (1 T) — 1 cm-1 — 1.33 cal/mol 5.6 J/mol — 0.7 K. If there is a weak, generalized interaction between spin units, one can apply a mean-field, generalized correction, 9 (the Weiss constant), to the Curie law to get the so-called Curie-Weiss law as follows in Equation (4) ... 

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... 

---