# Curie temperature using SPIN package

Dear all

I am a newbie. I want to calculate the Curie temperature of some simple systems i.e. iron using the LAMMPS SPIN package. I need some help in preparing the input file. What I have understood is I have to equilibrate the system at different temperatures using magnetic exchange calculated from DFT. Is it correct? Is the example for Fe given in the examples for the SPIN package good to start with?

Best regards
Lokanath

Can you be more specific as to what kind of help you need exactly?

These are questions for @julient, the author of the SPIN package.

Basically, I want to calculate the magnetization at various temperatures e.g. 0 to 1500 K at an interval of 100 K. Taking the iron input file from examples, how should I modify it? Here is the input file from the examples folder.

clear
units metal
atom_style spin

dimension 3
boundary p p p

# necessary for the serial algorithm (sametag)

atom_modify map array

lattice bcc 2.8665
region box block 0.0 5.0 0.0 5.0 0.0 5.0
create_box 1 box
create_atoms 1 box

# setting mass, mag. moments, and interactions for bcc iron

mass 1 55.845
set group all spin 2.2 -1.0 0.0 0.0
velocity all create 100 4928459 rot yes dist gaussian

pair_coeff * * eam/alloy Fe_Mishin2006.eam.alloy Fe
pair_coeff * * spin/exchange/biquadratic biquadratic 3.4 0.02726 0.2171 1.841 0.0 0.0 2.0 offset yes
neighbor 0.1 bin
neigh_modify every 10 check yes delay 20

fix 1 all precession/spin cubic 0.001 0.0005 1.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0 1.0
fix_modify 1 energy yes
fix 2 all langevin/spin 0.0 0.0 21

fix 3 all nve/spin lattice moving
timestep 0.0001

# compute and output options

compute out_mag all spin
compute out_pe all pe
compute out_ke all ke
compute out_temp all temp

variable magx equal c_out_mag[1]
variable magy equal c_out_mag[2]
variable magz equal c_out_mag[3]
variable magnorm equal c_out_mag[4]
variable emag equal c_out_mag[5]
variable tmag equal c_out_mag[6]

thermo_style custom step time v_magx v_magy v_magz v_magnorm v_tmag v_emag pe etotal
thermo 50

compute outsp all property/atom spx spy spz sp fmx fmy fmz
dump 1 all custom 100 dump_iron.lammpstrj type x y z c_outsp[1] c_outsp[2] c_outsp[3]

run 1000

# minimize 1.0e-16 1.0e-16 10000 10000

Thanks and regards
Lokanath

Hello Lokanath,

This should be rather straightforward. The easiest way is to run different simulations, each one of them at a different temperature.

To set the spin thermostat at a given temperature, you can have a look at the documentation of the fix Langevin/spin command:
https://docs.lammps.org/fix_langevin_spin.html

If you want to also add lattice vibrations (and so, lattice temperature) to your calculation, you can add a simple fix Langevin to do that.

Once one of those simulations has been running for long enough, take the time average of the spin norm (the variable referred to as “magnorm” in your script) over a “long-enough” number of timesteps. Of course, you need to converge and determine what “long-enough” means for your system.

Repeat this operation for each temperature you need. Note that you can easily create scripts to run all those temperatures in parallel. Then, your last step is to plot the averages of the “magnorm” quantity as a function of temperature.

Note that you can find an example of this calculation (performed with the Spin package of Lammps) on the Figure 4a) of this manuscript:
https://www.nature.com/articles/s41524-021-00617-2

Hope this helps! Let me know if you have additional questions!

All the best,
Julien.