add a test related to atomistic version stt

Author: Weiwei Wang

tests/test_stt_dw_atomistic.py

@@ -0,0 +1,151 @@
+from __future__ import print_function
+import pytest
+
+import numpy as np
+
+# FIDIMAG libraries
+from fidimag.common import CuboidMesh as Mesh
+from fidimag.atomistic import Sim, UniformExchange, Anisotropy
+import fidimag.common.constant as const
+
+mu0 = 4 * np.pi * 1e-7
+
+def load_mz_npy(npy_file):
+
+    m0_z = np.load(npy_file)
+    m_magnitude = m0_z[0] ** 2 + m0_z[1] ** 2 + m0_z[2] ** 2
+
+    if np.abs(m_magnitude - 1) < 1e-6:
+        m0_z = m0_z.reshape(-1, 3)[:, 2]
+    else:
+        m0_z = m0_z.reshape(3, -1)[2]
+
+    return m0_z
+
+
+# Initial State, a rough DW in a 1D chain
+def init_m(pos):
+
+    x = pos[0]
+
+    if x < 450:
+        return (1, 0, 0)
+    elif 450 <= x < 550:
+        return (0, 1, 1)
+    else:
+        return (-1, 0, 0)
+
+
+def relax_system(mesh):
+
+    # Only relaxation
+    sim = Sim(mesh, name='relax')
+
+    # Simulation parameters
+    sim.set_tols(rtol=1e-8, atol=1e-10)
+    sim.alpha = 0.5
+    sim.gamma = 2.211e5/mu0
+    sim.mu_s = 1e-27/mu0
+    sim.do_precession = False
+
+    # The initial state passed as a function
+    sim.set_m(init_m)
+    # sim.set_m(np.load('m0.npy'))
+
+    # Energies
+    exch = UniformExchange(J=2e-20)
+    sim.add(exch)
+
+    anis = Anisotropy(0.01*2e-20, axis=(1,0,0))
+    sim.add(anis)
+
+    # dmi = DMI(D=8e-4)
+    # sim.add(dmi)
+
+    # Start relaxation and save the state in m0.npy
+    sim.relax(dt=1e-14, stopping_dmdt=1e3, max_steps=5000,
+              save_m_steps=None, save_vtk_steps=None)
+
+    np.save('m0.npy', sim.spin)
+    #sim.save_vtk()
+
+
+# This function excites the system with a
+# current in the x-direction using Spin Transfer Torque
+# formalism
+def excite_system(mesh, time=0.1, snaps=11):
+
+    # Specify the stt dynamics in the simulation
+    sim = Sim(mesh, name='dyn', driver='llg_stt')
+
+    # Set the simulation parameters
+    sim.set_tols(rtol=1e-12, atol=1e-12)
+    sim.gamma = 2.211e5/mu0
+    sim.mu_s = 1e-27/mu0
+    sim.alpha = 0.05
+
+    sim.set_m(np.load('m0.npy'))
+
+    # Energies
+    exch = UniformExchange(J=2e-20)
+    sim.add(exch)
+
+    anis = Anisotropy(0.01*2e-20, axis=(1,0,0))
+    sim.add(anis)
+    # dmi = DMI(D=8e-4)
+    # sim.add(dmi)
+
+    # Set the current in the x direction, in A / m
+    # beta is the parameter in the STT torque
+    sim.jx = -1e12
+    sim.beta = 0.1
+
+    # The simulation will run for x ns and save
+    # 'snaps' snapshots of the system in the process
+    ts = np.linspace(0, time * 1e-9, snaps)
+
+
+    for t in ts:
+        print('time', t)
+        sim.run_until(t)
+        #sim.save_vtk()
+    np.save('m1.npy', sim.spin)
+
+
+def test_stt_dw_atomistic():
+    mesh = Mesh(nx=1000, ny=1, nz=1,
+                dx=1.0, dy=1.0, dz=1.0,
+                unit_length=1e-9)
+
+    # Relax the initial state. It will save the last state
+    # to the m0.npy file
+    relax_system(mesh)
+
+    m0 = np.load('m0.npy')
+    m0.shape=(-1,3)
+    mx0 = np.average(m0[:,0])
+
+    excite_system(mesh)
+
+    m1 = np.load('m1.npy')
+    m1.shape=(-1,3)
+    mx1 = np.average(m1[:,0])
+
+    mu_s = (1e-27/mu0)
+    v = 1e-27
+    p = 0.5
+    jx = 1e12
+    alpha = 0.05
+    beta = 0.1
+    time = 0.1
+
+    u = const.g_e * const.mu_B / (2 * const.c_e) * v/ mu_s * p * jx
+    distance = (1+alpha*beta)/(1+alpha*alpha)*u*time 
+    dmx = distance/1000*2
+
+    print(mx0, mx1, dmx, abs(mx1-mx0-dmx))
+    assert(mx1-mx0>0)
+    assert(abs(mx1-mx0-dmx)<5e-5)
+
+if __name__ == '__main__':
+    test_stt_dw_atomistic()