penalizing magnetism

[Cyrille]

Hi all

I wanted to know if some kind of penalization technique is implemented in ATK for magnetic systems.
I typically would like to impose a given amplitude of magnetization (or angle of magnetic moment) on a given atom.
This type of feature is implemented in quantum espresso for instance and it can be very useful in some cases.
It uses so called penalization technique which consists in adding a positive quantity to the total energy (typically Ax(m-mO)^2 if one want to impose the amplitude of magnetization) that is zero if the constrained is obeyed and very large if it is not (A>>0).

Thanks in advance

Cyrille

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Cyrille Barreteau
CEA Saclay, IRAMIS, SPEC Bat. 771
91191 Gif sur Yvette Cedex, FRANCE
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DTU Nanotech
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DK-2800 Kgs. Lyngby,  DENMARK
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email:     cyrille.barreteau@cea.fr  /  cyrbar@nanotech.dtu.dk 
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[Jess Wellendorff]

Hi Cyrille.

We have recently implemented constrained DFT, which is what they also do in Quantum ESPRESSO. The ATK functionality is implemented in the class FixedSpin, and it allows you to constrain the noncollinear spin angles on each atom. This is useful for calculations of, e.g., the perpendicular magnetic anisotropy in magnetic tunnel junctions.

The upcoming ATK 2015 (and the present beta 3 version, http://www.quantumwise.com/products/download) will have this FixedSpin feature. We will also provide a tutorial on how to use it. In the meantime, the attached script shows a usage example for an Fe-MgO junction. Please note that it will only work with 2015+ releases of ATK.

- Jess

[Cyrille]

Thanks Jess

That's very useful when dealing with magnetic anisotropy.
Will it also be possible to constrain the amplitude of the magnetization in standard collinear calculations?

Cyrille

[Jess Wellendorff]

No, it will only constrain the angles, not the amplitude. We may implement constraints on amplitudes also, but I don't think the decision has been made yet.

[Cyrille]

It can be very interesting to be able to fix the magnetization to explore some complex magnetic situations (which can unfortunately happen more often than expected:-) or just evaluate the influence of a magnetic fluctuation.. But of course I understand that one cannot include all kind of options in ATK:-)

Cyrille

[Jess Wellendorff]

True Thanks for the input, we will certainly consider them in our continued efforts to improve ATK.