You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: tutorial19.m
    path of file: ./repos/spinw/tutorials/publish
    
    the code of the file until where you have to start completion: %% Spin-spin correlation function of two differ
    
function of two different coupled magnetic ion
% In this tutorial, we demonstrate how the spin wave spectrum of two
% different coupled ion can be calculated. Using the magnetic form factor,
% the spin-spin correlation function on the two ions can be separated. This
% method is also usefull for simulating RIXS (Resonant Inelastic X-ray
% Scattering) on spin waves, that typically measures only the spin waves on
% the resonant ion.

%% Cu and Fe chain
% We define a unit cell with an Fe2+ and Cu2+ ions, the size of the spin is
% determined by the code using the information stored in the  ion.dat file.
% We couple antiferromagnetically the Fe and Cu atoms in separate chains
% running along the a-axis. The chains are weakly coupled along the b-axis.


FeCuChain = spinw;
FeCuChain.genlattice('lat_const',[3 8 4],'spgr','P 1')
FeCuChain.addatom('label','MCu2','r',[0 0 0])
FeCuChain.addatom('label','MFe2','r',[0 1/2 0])

FeCuChain.gencoupling
FeCuChain.addmatrix('label','J_{Cu-Cu}','value',1,'color','r')
FeCuChain.addmatrix('label','J_{Fe-Fe}','value',1,'color','b')
FeCuChain.addmatrix('label','J_{Cu-Fe}','value',-0.1,'color','orange')

FeCuChain.addcoupling('mat','J_{Cu-Cu}','bond',1)
FeCuChain.addcoupling('mat','J_{Fe-Fe}','bond',2)
FeCuChain.addcoupling('mat','J_{Cu-Fe}','bond',[4 5])

plot(FeCuChain,'range',[3 0.7 1],'cellMode','none')

%% Define magnetic structure
% The classical ground state of the above define Hamiltonian is
% antiferromagnetic chains ordered ferromagnetically along the b-direction.
% Here we define a magnetic unit cell that is 2x1x1 of the crystal unit
% cell to incorporate the antiferromagnetic structure.

FeCuChain.genmagstr('mode','helical','S',[0 0;1 1;0 0],'k',[1/2 0 0])
FeCuChain.genmagstr('mode','helical','nExt',[2 1 1])
plot(FeCuChain,'range',[3 0.7 1],'maglHead',0.3)

%% Spin-spin correlation function including the form factor
% We calculate the spin wave intensity (withouth the polarisation factor)
% and include the magnetic form factor of both the Cu2+ and Fe2+. The
% magnetic form factor is automatically assigned to the ions based on the
% given labels in the spniw.addatom() method. The magnetic form factor is
% applied to the calculated spin-spin correlation funcion in the
% spinw.spinwave() function, when the 'formfact' option is set to true. The
% spin wave dispersion plot includes the imaginary part of the spin wave
% energies as well (plotted as red dots) to check that the magnetic ground
% state is right.

spec = FeCuChain.spinwave({[0 0 0] [1 0 0] 501},'formfact',true,'hermit',false);
spec = sw_egrid(spec,'component','Sxx+Syy+Szz');
figure
subplot(2,1,1)
sw_plotspec(spec,'mode','disp','imag',true,'axLim',[0 4.5])
subplot(2,1,2)
sw_plotspec(spec,'mode','color','dE',0.2)
legend