Fitting of magnetic susceptibility data as a function of temperature of various spin systems-a FORTRAN program

Abstract

A FORTRAN program, SUSCEP.FOR, was developed for the fitting of the magnetic susceptibility curve measured as a function of temperature into a selected expression. It can be executed on an IBM PC compatible. The simplex method was used for least squares fit of a wide range of expressions for various spin systems. The calculated and observed susceptibility data are compared graphically for choosing initial values of unknown parameters and to examine the final fit. The input is given interactively through menus and the output is saved on disk file. Standard deviations of parameters are estimated. SUSCEP.FOR includes the following functions and the expressions resulted by linear combinations among them: (1) Curie law, (2) ions with zero-field splitting term (D) for 1≤ S ≤ 5/2, (3) Ising equation, (4) Bleaney-Bower equation, (5) magnetization equation for S = 1/2 dimer, (6) Bonner-Fisher equation, (7) modified Bonner-Fisher equation, (8) dimers with a variety of values for S₁ and S₂, (9) a few other selected functions from the literature, (10) a constant as a correction term. The program also considers molecular field correction for functions 2-9 and the Weiss constant for the Curie law. It provides a routine for user-specified function which can be modified to user requirements. Also there is provision to add more functions to enhance the capability. The program can handle up to 300 data points and 12 parameters. These limits can be increased if necessary by modifying the array dimensions.