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richard-evans / vampire

Atomistic simulator for magnetic materials
GNU General Public License v2.0
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MC for 2D systems #80

Open chengyangXu opened 2 years ago

chengyangXu commented 2 years ago

I am designing 2D ferromagnetic insulators. For testing, I construct a 2D system with only isotropic Heisenberg term. but it yields the curie temperature, which i hope to be very close to zero, to be nearly 30K. I'm wondering where is wrong in my input file. Thank you

-.mat material:num-materials=1

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Material 1 Cr

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material[1]:material-name=Cr material[1]:damping-constant=0.01 material[1]:atomic-spin-moment=1 !muB material[1]:material-element=Cr material[1]:uniaxial-anisotropy-constant=0.0

-.ucf

Unit cell size

3.48792 3.48792 26.5639534098312744

Unit cell lattice vectors:

 1.0000    0.0000    0.0000
 0.0000    1.0000    0.0000
 0.0000    0.0000    1.0000

Atoms num_atoms num_materials; id cx cy cz

1 1 0 0.50000 0.50000 0.44055

Interactions n exctype; id i j dx dy dz Jij

4 0 0 0 0 1 0 0 4.066e-22 1 0 0 0 1 0 4.066e-22 2 0 0 -1 0 0 4.066e-22 3 0 0 0 -1 0 4.066e-22

-input

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Creation attributes:

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create:full create:periodic-boundaries-x create:periodic-boundaries-y create:periodic-boundaries-z

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material:file= test.mat material:unit-cell-file = "test.ucf"

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System Dimensions:

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dimensions:unit-cell-size-x = 3.48792 !A dimensions:unit-cell-size-y = 3.48792 !A dimensions:unit-cell-size-z = 26.5639534098312744 !A dimensions:system-size-x = 15.0 !nm dimensions:system-size-y = 15.0 !nm dimensions:system-size-z = 2.6 !nm

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Simulation attributes:

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sim:minimum-temperature=0 sim:maximum-temperature=50 sim:temperature-increment=0.1 sim:time-steps-increment=1 sim:equilibration-time-steps=8000 sim:loop-time-steps=2000 sim:time-step=1e-16

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Program and integrator details

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sim:program=curie-temperature sim:integrator=monte-carlo

sim:two-temperature-electron-heat-capacity

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Data output

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output:temperature output:mean-magnetisation-length output:mean-susceptibility

screen:temperature screen:mean-magnetisation-length screen:mean-susceptibility

richard-evans commented 2 years ago

There is nothing wrong with your input file - the Mermin-Wagner theorem (2D magnetism has no order above zero K) only applies to an infinite system. Spin-spin correlations always give a finite Tc for a finite system.