Messages

1

General Questions and Answers / Re: Multiferroic tunnel junctions

« on: March 18, 2014, 14:01 »

Hello! Can anyone answer my question or help me with that problem ?

The main question is how to set ferroelectric state with ATK ? Is this possible at all ?
All my calculations lead to para phase in structure.

Maybe it connected with LCAO technique that do not allow to optimize super-cell ?

2

General Questions and Answers / Re: Multiferroic tunnel junctions

« on: November 18, 2013, 15:33 »

Some times ago we have discussion about how to set polarization (FE/NFE state) in MFTJ. My calculations based on results of that pub with doi: 10.1021/nl803318d
In result, I spent many time to set FE state in ATK software, but I dont got needed result. For NFE state I have some result but not sure in reability, one reason is that i have different results for GGA and LDA (see attachment).
To set FE state i try to displace atoms and set small bias but geometry optimization gave me constantly different wrong results.

Help me solve this problem with setting FE state!
See my structure in attachments, I build it with parametrs like in the publication, make interface in ATK builder with bothstrain option.

3

General Questions and Answers / Re: Is it possible to use the ATK to calculate the ferroelectric polarization?

« on: September 23, 2013, 15:58 »

Many thanks for your answer, but I still not quite understand how to set FE and NFE state.
Can you correct me if I do this wrong:
1. To set NFE state I must do zero-bias optimization with freezing atoms in xy plane (how set artificial freezing in ATK ?)
2. To set FE state I must set finite bias (? V) at one of electrodes ?

4

General Questions and Answers / Re: Is it possible to use the ATK to calculate the ferroelectric polarization?

« on: September 23, 2013, 10:01 »

Thank you Anders! Now i see that is two different things.

• Can you say why authors in publication above are used VASP for geometry optimization and ATK only for transport characteristics. Why they dont use ATK for optimization ?
• You say:

  the ferroelectric state (FE) by an optimization at a small electric field.

  Does it mean that for FE state I must set small bias ?

5

General Questions and Answers / Re: Is it possible to use the ATK to calculate the ferroelectric polarization?

« on: September 19, 2013, 12:23 »

can anyone answer my question please

6

General Questions and Answers / Re: Is it possible to use the ATK to calculate the ferroelectric polarization?

« on: September 17, 2013, 17:36 »

We have added it to ATK, but it's not a well tested feature yet. Also, in version 13.8 (out in beta now) you can in principle perform this calculation using Abinit.

That is great! But i want to ask you few questions about calculating polarization.
Today exists many publications about polarization of multiferroic tunnel junctions (MFTJ) (for example: Effects of ferroelectricity and magnetism on electron and spin transport in Fe/BaTiO3/Fe multiferroic tunnel junctions, DOI:10.1063/1.2828512) and I dont understand how they get ferroelectric (FE) and paraelectric (PE) states ? I only know that state is sets after optimization, but how exactly ?
1.Can you tell me how to set electric state in MFTJ (in ATK) ?
2. If this calculation may perform by Abinit, then can you give me some instructions ?

7

General Questions and Answers / Re: Is it possible to use the ATK to calculate the ferroelectric polarization?

« on: September 17, 2013, 11:38 »

Do you plan to realize calculation of the polarization of ferroelectric materials in ATK ?

8

General Questions and Answers / Re: Multiferroic tunnel junctions

« on: June 17, 2013, 13:47 »

Certainly not! As I said, the current structure is correct. 2x repeat in B would also be correct, just larger. But if you then delete part of the cell, it becomes a very different structure. If you are in doubt, try centering the structure in A and B (which doesn't change the structure, since the origin is arbitrary in an infinite periodic structure).

I see... Can you tell me does the repeating is necessary for best results in transport characteristics or i can calculate my structure with one layer and get the same result as in many layers (3 is min ) ?
And one more question.. Why in publication shows two-layers structure (They are performed All calculations in the same ATK) ?

9

General Questions and Answers / Re: Multiferroic tunnel junctions

« on: June 17, 2013, 12:33 »

The structure you sent is the one in the publication.

Thanks, but does it correct if i repeat my structure in B direction 2x times and then delete one layer and fit Cell in B direction ?

10

General Questions and Answers / Re: Multiferroic tunnel junctions

« on: June 17, 2013, 11:28 »

I doubt it will break since it's highly symmetry-constrained - at most some atoms move a bit on the Z direction. Maybe if you attach the system (what you consider the correct version) I can tell if it actually is correct or not.

Here is my system. This structure seems to be more stable and no more large forces in various directions, only Z. But I do not quite understand how to properly set the supercell and more exactly the boundaries of the supercell. Because if I set it on the atoms I have one result and if on the length of the atomic bound it will differ.

The main question is how to build up the same structure like in publication?
Does it correct if i repeat my structure in B direction 2x times and then delete one layer and fit Cell in B direction ?

11

General Questions and Answers / Re: Multiferroic tunnel junctions

« on: June 15, 2013, 13:51 »

The system is not correctly set up which you can verify by repeating it 2x in A and B. You have vacuum in the cell, and one layer of atoms is repeated.

I understand that my system is not correct, but how i can build the sytem from publication ?
If I repeat structure in A or B direction it will be wrong structure, but if I delete one atomic layer and fit cell for remove vacuum it will be structure like from pub. But optimization shows very big forces which break structure.
Help me solve this problem.

12

General Questions and Answers / Multiferroic tunnel junctions

« on: June 12, 2013, 15:26 »

Hello! I have a problem with build up that kind of junction. I try to build Fe-BaTiO₃-Fe junction by using various publications. But when i try to optimize this structure it breaks. I think that problem in lattice constant of supercell but I can be wrong.

In attachment you can see my structure and forces and structure from publication.

Can you help me build this junction?

In addition, some times if I do just fit lattice and then begun to calculate, it say error: 

Calculating Eigenvalues    :
** Back Engine Exception : Diagonalization error, overlap matrix not positive definite. This may be caused by atoms that are too close to each other or situated in equivalent positions, or (in the Extended Huckel model) a too low value of interaction_max_range
** Location of Exception : mathutils.cpp:748



This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.

13

General Questions and Answers / Re: How to calculate the I-V (spin-down and spin-up ) with the 11.2?

« on: April 19, 2013, 17:23 »

You define "old_calculation" but you never use it

Thank you! Some day i can understand this python

14

General Questions and Answers / Re: magnetic tunnel junction

« on: April 19, 2013, 17:19 »

Hello! Can anyone tell me why TMR of structure Fe-MgO-Fe in examples is exceed 9000 % ? Does it normal and how it compare with experiment ? I saw the presentation about apply ATK in spintronics and TMR in this presentation about 2300 % What value of TMR is more correct?

15

General Questions and Answers / Re: How to calculate the I-V (spin-down and spin-up ) with the 11.2?

« on: April 19, 2013, 14:27 »

Right, you would first do the parallel at zero bias, then the voltage loop, starting from the zero case. Then you start the anti-parallel zero bias from the parallel zero bias, and finally the voltage loop for anti-parallel starting from the zero-bias anti-parallel.

I use mark "old calculation" and my script look like this

Code: python

# -------------------------------------------------------------
# Initial State
# -------------------------------------------------------------
initial_spin = InitialSpin(scaled_spins=[1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0])
old_calculation = nlread('C:/Users/Alexander/Documents/VNL_tmp/CoFe-MgO-CoFe_para.nc', DeviceConfiguration)[0]

device_configuration.setCalculator(calculator, initial_spin=initial_spin, initial_state=old_calculation)

for bias in [0., 0.1, 0.3, 0.5, 0.7, 1.0, 1.3, 1.5]*Volt:  
    device_configuration.setCalculator(  
        calculator(electrode_voltages=(0.5*bias,-0.5*bias)),  
        initial_state=device_configuration
    )  
    
    device_configuration.update()
    nlsave('C:/Users/Alexander/Documents/VNL_tmp/CoFe-MgO-CoFe_para_I-V.nc', device_configuration)
    nlprint(device_configuration)

but it again shows me that " The provided initial state parameter has no pre-calculated data ..." and it calculates exceed 10 hours/