(spinw-addaniso)=

# spinw.addaniso method

## Syntax
  
`addaniso(obj, matrixIdx, {atomTypeIdx}, {atomIdx})`
  
## Description
  
`addaniso(obj, matrixIdx, {atomTypeIdx}, {atomIdx})` assigns the
$[3\times 3]$ matrix selected by `matrixIdx` (using either the matrix
label or matrix index) to the magnetic sites selected by `atomTypeIdx`
that can contain a name of an atom or its atom index (see [spinw.atom](#spinw-atom)).
If `atomTypeIdx` is not defined, anisotropy will be assigned to all
magnetic atoms.
  
## Examples
  
To show the effect of a fourfold axis on anisotropy, we add $A_1$
easy-axis anisotropy to atoms at $(1/4,1/4,1/2)$ and plot the result. The
3D plot shows anistropy using ellipsoid around the magnetic atoms.
 
```matlab
cryst = spinw
cryst.genlattice('lat_const',[4 4 3],'spgr','P 4')
cryst.addatom('r',[1/4 1/4 1/2],'S',1)
cryst.addmatrix('label','A1','value',diag([-0.1 0 0]))
cryst.gencoupling
cryst.addaniso('A1')
plot(cryst)
```
 
```{image} /_static/img/generated/spinw_add_1.png
:alt: plot(cryst)
```
  
## Input arguments
 
`obj`
: [spinw](#spinw) object.
 
## Name-Value Pair Arguments
  
`matrixIdx`
: Either an integer, that selects the matrix according to
  `obj.matrix.mat(:,:,matrixIdx)`, or a string identical to one
  of the previously defined matrix labels, stored in
  `obj.matrix.label`.
  
`atomTypeIdx`
: String or cell of strings that select magnetic atoms by
  their label. Also can be a vector that contains integers, the index of
  the magnetic atoms in `obj.unit_cell`, with all symmetry equivalent
  atoms. Maximum value is $n_{atom}$, if undefined anisotropy is assigned to
  all magnetic atoms. Optional.
 
`atomIdx`
: A vector that contains indices selecting some of the
  symmetry equivalent atoms. Maximum value is the number of symmetry
  equivalent atoms generated corresponding to `atomTypeIdx` site. If
  crystal symmetry is not 0, `atomIdx` is not allowed, since the
  anisotropy matrix for equivalent atoms will be calculated using the
  symmetry operators of the space group. Optional.
  
## Output Arguments
  
The function adds extra entries in the `obj.single_ion.aniso` field of the
obj [spinw](#spinw) object.
  
## See Also
  
[spinw](#spinw), [spinw.single_ion](#spinw-single-ion), [spinw.addcoupling](#spinw-addcoupling), [spinw.addg](#spinw-addg) and [spinw.addmatrix](#spinw-addmatrix)