Is it possibility do collinear spin transport with Fixed spin Ni (electrode)?

[gayani2025]

Dear All,

I am performing spin-polarized transport calculations using Ni electrodes — one with spin-up and the other with spin-down initialization. However, I observe that the spin-down Ni electrode relaxes to spin-up during the calculation, as confirmed by the Mulliken population analysis.

To address this, I understand that I need to fix the spin direction during the calculation. I came across the FixedSpin class, which provides a way to fix spins by applying an energy penalty.
https://docs.quantumatk.com/manual/Types/FixedSpin/FixedSpin.html

 However, the documentation and examples I found (such as the Cr dimer case) are based on non-collinear spin calculations.

I would like to know:

How can I apply fixed spin directions in a collinear spin transport calculation?

Is there an example or tutorial available for using FixedSpin in the collinear spin framework, especially in the context of transport calculations?

I’ve attached the input script I used so far. Any guidance or example code would be greatly appreciated.

Thank you in advance!


Gayani

[Anders Blom]

Spin up and down have no meaning in an absolute sense, so in the example you attached, flipping the spin of the whole system does not change any physics, since there is no reference magnetic field that would mean spin up/down have different energy. This is where the fixed spin comes in, it can sort of mimic a magnetic field.

Relative spins do matter, and can be used in noncollinear and collinear. So if you had the two electrodes with opposite spins, or same spin, these would be different physical situations, with different transport (parallel and antiparallel, as needed to compute TMR for instance).

[gayani2025]

Hi,


Thank you for your reply.

Please see the attached slide, where I have explained both the experimental I–V curve and the results I obtained from the simulation. As you mentioned, I don’t think I have fixed the spin correctly in my setup.

Could you please advise which option I should use in this type of situation?  I found this. https://docs.quantumatk.com/manual/Types/FixedSpin/FixedSpin.html() . But it is for non collinear situation.

 I have also attached the Python files of the setup I have been using.

As I understand it, I need to perform a collinear spin transport calculation and be able to fix the spin of the Ni electrode for both up and down orientations. Is that correct?

Thank you for your guidance.

[Anders Blom]

As mentioned before, there is no difference between up and down spin unless you have a magnetic axis in the system, which defines up/down relative some reference direction. And in the collinear case there is no such direction, and since Gold is non-magnetic there is not even a parallel or antiparallel spin setup, so your up and down for Ni will remain degenerate and cannot be distinguished. In fact, I don't think you would see a different even in a noncollinear simulation since the magnetic field is not actually part of the simulation, and I honestly don't see physically how it would change anything in the experiment either, if the field is turned off before the measurement?