Spin-orbit torque

[zangyuzhe]

Hello,

We want to use spin orbit torque in the Spirit. Bu we found there is only zhang-li model. Are we able to use Slonczewski model for spin torque?

And when setting the magnitude of the current strength for the STT, is it actually setting u, which has a unit of m/s?

Many thanks

[GPMueller]

The documentation is lacking in this regard... Thanks for pointing it out and sorry that this basic info is not obviously accessible.

In the input file, you can specify llg_stt_use_gradient 0 to switch from the Zhang-Li model to Slonczewski.
The same can be set in the GUI and you can also use the corresponding API.

---

Regarding the units, I'm not 100% sure right now, as I haven't used the feature in quite a while. But I believe when setting the magnitude in the Zhang-Li model, you are setting the value of u * norm(j_e) in equation (10) of our paper

spirit/core/src/engine/Method_LLG.cpp

Lines 187 to 195 in 14ed778

	if (parameters.stt_use_gradient)
	{
	auto& boundary_conditions = this->systems[0]->hamiltonian->boundary_conditions;
	// Gradient approximation for in-plane currents
	Vectormath::directional_gradient(image, geometry, boundary_conditions, je, s_c_grad); // s_c_grad = (j_e*grad)*S
	Vectormath::add_c_a ( dtg * a_j * ( damping - beta ), s_c_grad, force_virtual); // TODO: a_j durch b_j ersetzen
	Vectormath::add_c_cross( dtg * a_j * ( 1 + beta * damping ), s_c_grad, image, force_virtual); // TODO: a_j durch b_j ersetzen
	// Gradient in current richtung, daher => *(-1)
	}

The implementation of Slonczewski's model (monolayer approximation) in Spirit is according to Chureemart et al:

spirit/core/src/engine/Method_LLG.cpp

Lines 198 to 200 in 14ed778

	// Monolayer approximation
	Vectormath::add_c_a (-dtg * a_j * ( damping - beta ), s_c_vec, force_virtual);
	Vectormath::add_c_cross(-dtg * a_j * ( 1 + beta * damping ), s_c_vec, image, force_virtual);

so the magnitude corresponds to a_j in this equation, as the spin polarisation vector S_p is normalised: