[Wien] Magnetic moments converging in a different direction to the one they are defined

[Penny, Charles]

Dear all,


I am running spin-polarised calculations on a range of iron-spinel structures (namely, magnetite (Fe3O4), maghemite (gamma-Fe2O3) and greigite (Fe3S4)) with the objective of calculating magnetic exchange energies in these minerals. This requires calculating total energies of lot of different spin configurations. This process has worked well for magnetite and maghemite, but I have encountered a problem with greigite.


When I run a calculation on a spin configuration of greigite that isn’t the ferrimagnetic ground state (e.g. a ferromagnetic configuration) the calculation converges to the ferrimagnetic solution, with the sublattice moments pointing in opposing directions.


In the examples below, I have used a low-symmetry unit cell with eight unique iron atoms which allows me to calculate the required number of spin configurations for estimating J_ij. Atoms 1-4 correspond to A site iron atoms in the spinel structure, atoms 5-8 correspond to B site iron atoms and atoms 9-16 are sulphur atoms. In a ferrimagnetic system the A and B sites have opposing moments and sulphur atoms are non-magneitc.


When I define a ferrimagnetic spin configuration, the calculation proceeds as expected, with the final moments looking like;


rkmax_8_k_500.scf::MMINT:  MAGNETIC MOMENT IN INTERSTITIAL  =   -0.05116

rkmax_8_k_500.scf::MMI001: MAGNETIC MOMENT IN SPHERE   1    =    2.47349

rkmax_8_k_500.scf::MMI002: MAGNETIC MOMENT IN SPHERE   2    =    2.47348

rkmax_8_k_500.scf::MMI003: MAGNETIC MOMENT IN SPHERE   3    =    2.47348

rkmax_8_k_500.scf::MMI004: MAGNETIC MOMENT IN SPHERE   4    =    2.47348

rkmax_8_k_500.scf::MMI005: MAGNETIC MOMENT IN SPHERE   5    =   -3.01699

rkmax_8_k_500.scf::MMI006: MAGNETIC MOMENT IN SPHERE   6    =   -3.01699

rkmax_8_k_500.scf::MMI007: MAGNETIC MOMENT IN SPHERE   7    =   -3.01699

rkmax_8_k_500.scf::MMI008: MAGNETIC MOMENT IN SPHERE   8    =   -3.01699

rkmax_8_k_500.scf::MMI009: MAGNETIC MOMENT IN SPHERE   9    =   -0.03675

rkmax_8_k_500.scf::MMI010: MAGNETIC MOMENT IN SPHERE  10    =   -0.03675

rkmax_8_k_500.scf::MMI011: MAGNETIC MOMENT IN SPHERE  11    =   -0.03675

rkmax_8_k_500.scf::MMI012: MAGNETIC MOMENT IN SPHERE  12    =   -0.03675

rkmax_8_k_500.scf::MMI013: MAGNETIC MOMENT IN SPHERE  13    =   -0.03675

rkmax_8_k_500.scf::MMI014: MAGNETIC MOMENT IN SPHERE  14    =   -0.03675

rkmax_8_k_500.scf::MMI015: MAGNETIC MOMENT IN SPHERE  15    =   -0.03675

rkmax_8_k_500.scf::MMI016: MAGNETIC MOMENT IN SPHERE  16    =   -0.03675

rkmax_8_k_500.scf::MMTOT:  SPIN MAGNETIC MOMENT IN CELL     =  -14.88108


Final energy;


rkmax_8_k_500.scf::ENE  : ********** TOTAL ENERGY IN Ry =       -43322.30312592


However, when I define a ferromagnetic spin configuration the system converges to a ferrimagnetic solution with final moments;


k_500_rkmax_8.scf::MMINT:  MAGNETIC MOMENT IN INTERSTITIAL  =    0.05118

k_500_rkmax_8.scf::MMI001: MAGNETIC MOMENT IN SPHERE   1    =   -2.47348

k_500_rkmax_8.scf::MMI002: MAGNETIC MOMENT IN SPHERE   2    =   -2.47347

k_500_rkmax_8.scf::MMI003: MAGNETIC MOMENT IN SPHERE   3    =   -2.47346

k_500_rkmax_8.scf::MMI004: MAGNETIC MOMENT IN SPHERE   4    =   -2.47346

k_500_rkmax_8.scf::MMI005: MAGNETIC MOMENT IN SPHERE   5    =    3.01697

k_500_rkmax_8.scf::MMI006: MAGNETIC MOMENT IN SPHERE   6    =    3.01697

k_500_rkmax_8.scf::MMI007: MAGNETIC MOMENT IN SPHERE   7    =    3.01697

k_500_rkmax_8.scf::MMI008: MAGNETIC MOMENT IN SPHERE   8    =    3.01697

k_500_rkmax_8.scf::MMI009: MAGNETIC MOMENT IN SPHERE   9    =    0.03675

k_500_rkmax_8.scf::MMI010: MAGNETIC MOMENT IN SPHERE  10    =    0.03675

k_500_rkmax_8.scf::MMI011: MAGNETIC MOMENT IN SPHERE  11    =    0.03675

k_500_rkmax_8.scf::MMI012: MAGNETIC MOMENT IN SPHERE  12    =    0.03675

k_500_rkmax_8.scf::MMI013: MAGNETIC MOMENT IN SPHERE  13    =    0.03675

k_500_rkmax_8.scf::MMI014: MAGNETIC MOMENT IN SPHERE  14    =    0.03675

k_500_rkmax_8.scf::MMI015: MAGNETIC MOMENT IN SPHERE  15    =    0.03675

k_500_rkmax_8.scf::MMI016: MAGNETIC MOMENT IN SPHERE  16    =    0.03675

k_500_rkmax_8.scf::MMTOT:  SPIN MAGNETIC MOMENT IN CELL     =   14.88103


Final energy is the same as in the ferrimagnetic case;


k_500_rkmax_8.scf::ENE  : ********** TOTAL ENERGY IN Ry =       -43322.30312578


Charge distance looks like it converges in both cases. Note that whilst the two calculations have the same saved name, they are in completely different CASE files. To outline my procedure, I initially call,


instgen -ask


And define the moments as ‘u u u u u u u u n n n n n n n n’ for a ferromagnetic calculation and ‘u u u u d d d d n n n n n n n n’ for a ferrimagnetic calculation.


Then,


init -b -sp -numk 500 -rkmax 8.00

runsp -ec 0.0001 -cc 0.0001 -fc 1.0 -p -i 200


I am at a loss as to what is going on and can’t find anything in the mailing list to explain this. Any help on this matter would be greatly appreciated.


Regards,


Charlie

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