Magnetism example

Magnetic structures can be anchored to the layer boundaries.

Defining a magnetic model starts as usual.

from refl1d.names import *

We still need the nuclear structure, so define the materials.

Si = SLD(name="Si", rho=2.0737, irho=2.376e-5)
Cu = SLD(name="Cu", rho=6.5535, irho=8.925e-4)
Ta = SLD(name="Ta", rho=3.8300, irho=3.175e-3)
TaOx = SLD(name="TaOx", rho=1.6325, irho=3.175e-3)
NiFe = SLD(name="NiFe", rho=9.1200, irho=1.032e-3)
CoFe = SLD(name="CoFe", rho=4.3565, irho=7.986e-3) # 60:40
IrMn = SLD(name="IrMn", rho=-0.21646, irho=4.245e-2)

The materials are stacked as usual, but the layers with magnetism have an additional magnetism property specified. This example use refl1d.magnetism.Magnetism to define a flat magnetic layer with the given magnetic scattering length density rhoM and angle thetaM.

The magnetism is anchored to the corresponding nuclear layer, and by default will have the same thickness and interface. The magnetic interface can be shifted relative to the nuclear interface using dead_below and dead_above. These can be negative, allowing the magnetism to extend beyond the nuclear layer. The magnetic interface can also be varied independently by using interface_above and interface_below as in the example below. Note that interface_below is ignored # in consecutive layers, much like the nuclear layers, for which the interface attribute indicates the interface above. Using extent=2, the single magnetism definition can extend over two consecutive layers.

The refl1d.magnetism.MagnetismTwist allows you to define a magnetic layer whose values of theta and rho change linearly throughout the layer. There are additional magnetism types defined in reflid.magnetism. Note that the current definition of interface only transitions smoothly into and out of layers with constant magnetism. This behaviour may change in newer releases.

sample = (Si(0,2.13) | Ta(38.8,2)
          | NiFe(25.0,5, magnetism=Magnetism(rhoM=1.4638, thetaM=270,
          | CoFe(12.7,5, magnetism=Magnetism(rhoM=3.7340, thetaM=270,
          | Cu(28,2)
          | CoFe(30.2, 5, MagnetismTwist(rhoM=[4.5102,1.7860], thetaM=[270,85],
          | IrMn(4.74,1.7)
          | Cu(5.148,2) | Ta(55.4895,2) | TaOx(47.42,3.5) | air

Define the fittable parameters as usual, including the magnetism attributes.



Load the data

instrument = NCNR.NG1(slits_at_Tlo=0.1)
probe = instrument.load_magnetic("n101Gc1.reflA")

We are going to compare the calculated reflectivity given two different step sizes on the profile. Steps of dz=0.3 are good enough for this example in that finer steps will not significantly change \(\chi^2\) Steps of dz=2 however are significantly different. You can see the difference by looking at the spin asymmetry curves for the model rendered with dz=2 and dz=0.3 as we do below. The reflectivity calculation time scales linearly with the step size, so you may want to use a large step size for your initial fits and a smaller step size later. The dA parameter ought to give the best of both worlds, using a finer step size where the profile is changing quickly and coarser step size elsewhere, but it is currently broken and disabled below.

experiment = Experiment(probe=probe, sample=sample, dz=0.3, dA=None)
experiment2 = Experiment(probe=probe, sample=sample, dz=2, dA=None)
problem = FitProblem([experiment,experiment2])