Standard Problem 4 (sim_with)

link: https://www.ctcms.nist.gov/~rdm/mumag.org.html/

In this example, we demonstrate how to use MicroMagnetic.jl to simulate standard problem 4, which involves a thin film geometry. We will run the simulation on a GPU, process the results, and visualize the magnetization dynamics.

Import MicroMagnetic.jl

using MicroMagnetic

Enable GPU acceleration

@using_gpu()

Define the system geometry: a film with thickness t = 3 nm, length L = 500 nm, and width d = 125 nm. Gather all the parameters related to standard problem 4:

args = (name="std4", task_s=["relax", "dynamics"],           # List of tasks
        mesh=FDMesh(; nx=200, ny=50, nz=1, dx=2.5e-9, dy=2.5e-9, dz=3e-9), Ms=8e5, # Saturation magnetization
        A=1.3e-11,                              # Exchange constant
        demag=true,                             # Enable demagnetization
        m0=(1, 0.25, 0.1),                      # Initial magnetization
        alpha=0.02,                             # Gilbert damping
        steps=100,                              # Number of steps for dynamics
        dt=0.01ns,                              # Time step size
        stopping_dmdt=0.01,                     # Stopping criterion for relaxation
        dynamic_m_interval=1,                   # Save magnetization at each step
        H_s=[(0, 0, 0), (-24.6mT, 4.3mT, 0)]);

Run the simulation using the sim_with function:

sim = sim_with(args);

[ Info: MicroSim has been created.
[ Info: Exchange has been added.
[ Info: Static Zeeman has been added.
[ Info: Running Driver : MicroMagnetic.EnergyMinimization{Float64}.
[ Info: max_dmdt is less than stopping_dmdt=0.01 @steps=370, Done!
[ Info: The driver LLG is used!
....................................................................................................
[ Info: step =  100  t = 1.000000e-09

With the above code, the simulation for standard problem 4 is complete. Next, we proceed to process the data, such as visualizing the magnetization distribution or creating a movie from the simulation results.

Visualize the magnetization dynamics using CairoMakie

using CairoMakie

Generate a movie from the simulation results stored in the jld2 file

jld2movie("std4.jld2"; output="std4.mp4", component='x');

Display the generated movie

Finally, we plot the time evolution of the magnetization components to compare the results with OOMMF simulations.

Import necessary functions to plot the time evolution of magnetization

using DelimitedFiles
using MicroMagnetic
using CairoMakie

function plot_m_ts()
    #Load data
    data, unit = read_table("std4_llg.txt")
    oommf = readdlm("assets/std4_oommf.txt")

    #Create a figure for the plot
    fig = Figure(; size=(800, 480))
    ax = Axis(fig[1, 1]; xlabel="Time (ns)", ylabel="m")

    #Plot OOMMF results
    lines!(ax, oommf[:, 1] * 1e9, oommf[:, 2]; label="OOMMF")
    lines!(ax, oommf[:, 1] * 1e9, oommf[:, 3])
    lines!(ax, oommf[:, 1] * 1e9, oommf[:, 4])

    #Plot MicroMagnetic results
    scatter!(ax, data["time"] * 1e9, data["m_x"]; markersize=6, label="MicroMagnetic.jl")
    scatter!(ax, data["time"] * 1e9, data["m_y"]; markersize=6)
    scatter!(ax, data["time"] * 1e9, data["m_z"]; markersize=6)

    #Add legend to the plot
    axislegend()

    return fig
end

plot_m_ts (generic function with 1 method)

Plot the magnetization time series

fig = plot_m_ts()