6.1.1.3 Device Description

The device description consists of the geometry and a special list of keywords necessary for describing the properties of the ferroelectric material. An overview of these keywords and their default values is given in Table 6.2.

Table 6.2: Device description

Keyword	Values	Default
PsatFerro	Real	0
ECoercFerro	Real	0
ShapeFerro	Real	0
FerroHysteresisModel	1,2	1
FerroAxesNumber	1,2	1
FerroAxisI	0...1	0
FerroAxisII	0...1	0
RelaxFerro	Real	0
RelaxFerroEf	Real	0
RelaxFerroExp	Real	0

The geometry is stored in a separate file which is specified in the Input section of the input deck. In order to enter a ferroelectric material into the device, the command ferro = segname in the Phys section has to be added. The ferroelectric properties of the specified segment have to be declared in the Solve section. According to the implemented model, three parameters are necessary to describe the hysteresis: PsatFerro for the saturation polarization $P_\mathrm{Sat}$, ECoercFerro for the coercive field $E_\mathrm{c}$, ShapeFerro for the shape of the hysteresis. Using these parameters the remanent polarization $P_\mathrm{Rem}$ and the parameter $w$ are calculated.

If the material is anisotropic, the axes properties are specified with the keyword FerroHysteresisModel. It is used to toggle between the different shape functions. If this keyword is set to 1 the $\textsf {tanh}$ is selected, if set to 2, the $\textsf {arctan}$ shape function is selected.

The number of axes is specified with the keyword FerroAxesNumber. Default is one.
The keyword FerroAxisI = 0...1 specifies the first anisotropy axis. The value entered is the cosinus of the angle to the x-axis, and default is zero. FerroAxisII = 0...1 specifies the second axis. The value entered is defined the same way as for the first axis.

The three parameters for the transient simulation are :
RelaxFerro, the time constant for the polarization $\tau_\mathrm{pol}$, RelaxFerroEf, the time constant for the electric field $\tau_\mathrm{ef}$, and RelaxFerroExp, the value of the nonlinear parameter $k_\mathrm{nonlin}$. All of these keywords are set to zero by default.