mcphas - calculation of thermodynamic properties (Magnetisation, Susceptibility, Specific Heat, Neutron Diffraction, etc.)

ORLANDO: I understand: In order to perform calculations beyond the capabilities of singleion it is necessary to use the program mcphas. So, how can we start to do such a calculation ?

EWALD: I think by introducing more complexity step by step, this allows you to check and detect mistakes immediately:

• As a first step it is possible to calculate the thermodynamic properties such as magnetisation or specific heat considering only single ion effects. In this case all the exchange parameters have to be set to zero in mcphas.j.
• for more advanced calculations the two - ion interactions have to be considered and may lead to magnetic order. mcphas can perform calculations in the ordered state in the following way: for a given temperature $T$ and magnetic field $\mathbf H$ (vector) (and electric field and stress tensor) several possible magnetic structures are stabilised by a mean field algorithm and the free energy is calculated. The initial values for this mean-field procedure are modified randomly, the result of the mean field calculation can be used by a Monte Carlo process.

  The temperature and magnetic field (and electric field and stress) is varied during the calculation and thereby it is possible to map out the magnetic phase diagram.

The program produces a plot of the stabilised magnetic structures and the magnetisation on screen, the output files contain additional information such as calculated magnetoelastic and neutron-scattering data. Several graphic programs easy the visualisation of the calculated data¹⁷.