content_image/374722.jpg content_image/374729.jpg content_image/374730.jpg
<img src='content_image/374722.jpg'>

have sets of planes for each of the order parameters. Namely, we have the dependencies M 2 ( H$_{a}$ M , E$_{a}$ P ) , P 2 ( H$_{a}$ M , E$_{a}$ P $^{)}$. The plane which crosses point (0 , 0) corresponds to the phase transition temperature in zero field. A representative set of the Arrot planes is drawn in Fig. 8 for the magnetization. One can see that the transition temperature can be determined from the dependence ˜ M 2 vs e/ ˜ P , not only from ˜ M 2 vs h/ ˜ M ! 

From (3) and (4) after dividing both equations by the order parameter and solving the resulting system we obtain: 

<img src='content_image/374729.jpg'>

<img src='content_image/374730.jpg'>

where x$_{e}$ = e ˜ P and x$_{h}$ = h ˜ $_{M}$. 

From Eq. (29) one can also see that for T > T$_{RM}$ we still have a nonzero value of ˜ P (cf. Eq. (20)). Therefore, the Arrot planes P $^{2}$( H$_{a}$/M, E$_{a}$/P ) are not important, since T$_{f}$ > T$_{RM}$ . 

