spin wave in a 2D magnet

[BaishunYang]

I am calculating the spin wave of one 2D magtnet, taking the first nearest to third nearest J1, J2, J3 and second nearest neighbor DM interaction into consideration (see the problem_temp.m file).

1. In principle, due to the DM interaction, there should be a gap in spin wave. But the results show that the gap is not opening at (1, 0, 0). And if I remove the DM interaction, only use isotropic Heisenberg interaction J1, J2 and J3, I notice that there are a lot of Qs near (1, 0, 0) with E=0, what does this mean?
2. When I am using sw_neutron to plot the neutron scattering, there is a bug with line Spectrum = sw_egrid(Spectrum,'Evect','imagChk', true, linspace(0,400,1000)); with the error, "The imaginary part of the spin wave energes is larger than the bin size! Improve your calculation or disable imagChk option!" Is there any one could help me to solve the problem? Best Regards, Baishun

The input file is as follows:

%spin wave calculation using spinw software %% Lattice - Define the crystal structure - Part 1 % We create a lattice tri = spinw; tri.genlattice('lat_const', [3.709 3.709 25], 'angled', [90 90 90], 'spgr',129) %% Atoms - Define the crystal structure - Part 2

tri.addatom('r',[0.75 0.25 0.0],'S',1,'label','Fe');

%% Define the list of bonds based on length tri.gencoupling('maxDistance', 6) % list the 1st to 3rd neighbor bonds tri.table('bond', 1:3)

%% Define the Spin-Hamiltonian (Exchange Structure)

%J matrix, positive means AFM J1=[-11.144 -0.249 0.225; -0.261 -11.132 -0.121; 0.228 -0.133 -11.754]; J2=[9.143 -0.009 -15.209; 0.004 10.602 -0.001; 15.211 -0.002 9.279]; J3=[24.263 -0.146 1.096; -0.145 24.169 -1.109; -1.096 1.109 24.38]; % J1=-11.144 % J2=9.143 % J3=24.263

tri.addmatrix('label', 'J1', 'value', J1, 'color','r') tri.addmatrix('label', 'J2', 'value', J2, 'color','g') tri.addmatrix('label', 'J3', 'value', J3, 'color','b') % tri.addmatrix('label', 'SIA', 'value', diag([0 0 -0.1])) % tri.addmatrix('label', 'biquadratic', 'value', 10)

tri.addcoupling('mat','J1','bond',1); tri.addcoupling('mat','J2','bond',2); tri.addcoupling('mat','J3','bond',3); % tri.addaniso('SIA') % tri.addcoupling('mat', 'biquadratic', 'bond',1)

%% Define the Magnetic structure %generate the magnetic structure tri.genmagstr('mode', 'helical', 'k', [0.5 0 0], 'n', [1 0 0], 'S', [0; 1; 1]) tri.table('mag') plot(tri, 'range', [2 2 1], 'cellMode', 'inside', 'magColor', 'red')

%% The energy of the ground state per spin % The spinw.energy() function gives the ground state energy per spin, the value is dinamically calculated at every call. disp('Ground state energy before optimization:') tri.energy % assert(tri.energy == -1)

%% Calculate the spin-wave dispersion

Qlist = {[3/4 1/4 0] [1/2 1/2 0] [1/2 0 0] [3/4 1/4 0] [1 0 0] [1/2 0 0] 100}; Spectrum = tri.spinwave(Qlist,'hermit',false);

figure; % subplot(2,1,1) % % plot the spin wave sw_plotspec(Spectrum,'mode',1,'axLim',[0 400],'colorbar',false,... 'colormap',[0 0 0],'dashed',true) % % plot the neutron spin wave Spectrum = sw_neutron(Spectrum); Spectrum = sw_egrid(Spectrum,'Evect',linspace(0,400,1000), 'imagChk', true); sw_plotspec(Spectrum,'mode',3,'dashed',true,'dE',0.1) caxis([0 4])

[mducle]

It looks like your magnetic structure does not agree with your exchange parameters - you should add a call to optmagsteep or optmagstr after the call to genmagstr or after where you write "Ground state energy before optimization" (you don't actually do an optimization - that's what optmagsteep / optmagstr does).

[BaishunYang]

Hello @mducle , Thank you for your reply. I tried to use the "optmagstr" tag these days. It works very well to solve my problem. Many thanks.