a problem about the result of MTJ

[huangshenjie]

Dear,
I made a MTJ by ATK, but the results confuse me. The result of MTJ with anti-parallel spin is perfect, however, the result of parallel spin is not.
Just like the following IV curve, when the bias increases, the current tends to zero. The transmission spectrum also tends to zero. I have posted the problem several days before, but no replay.
Now I post the calculation part in my script and hope someone can help me out.
thanks a lot
 

[nori]

First of all, the k-point sampling for transmission should be increased.
Gamma point approximation is too crude for MTJ.
You should check how large k-point sampling is needed to obtain converged transmission spectrum.

[Anders Blom]

A reasonable number of k-points is 400 in each direction, A and B (x and y), but you should increase it until the results converge.

[huangshenjie]

Hi, nori, thanks for your reply. Since my MTJ is very complicated, although I take the k-points as 2, 2,10, it still takes me 2 days to calculate each point of bias (my computer has 4 CPUs). I don't how to set the k-points to get the perfect result and save my time.
Now I give you my MTJ, I hope that you can help me to set the k-points.Thanks a lot.

[huangshenjie]

Hi, Anders Blom, thanks for your reply. Since my MTJ is very complicated, although I take the k-points as 2, 2,10, it still takes me 2 days to calculate each point of bias (my computer has 4 CPUs). I don't how to set the k-points to get the perfect result and save my time.
Now I give you my MTJ, I hope that you can help me to set the k-points.Thanks a lot.

[huangshenjie]

Should I reduce the mgo to two layers to save time?

[nori]

About SCF, you can reduce calculation time by setting initial_state obtained in previous calculation.
(For instance, using 0.8V SCF result as initial_state for 1.0V SCF calculation)

About transmission, energy range and points can be reduced like linspace(-1.5, 1.5, 31).
If you find sharp and large peaks in the spectrum, you should add points around the peaks.

[huangshenjie]

Thank you, nori. I get it! Uh, but I still don't know what I should do about the k-points... 

[Anders Blom]

Note that the k-points needed for converged transmission spectrum (the k-points you give for the TransmissionSpectrum analysis option) are, most likely, very different from what you need for the density matrix (self-consistent loop) to be accurately determined. So, once you have converged all bias points (using Nori's suggestion will help a lot), you will need to experiment with the transmission spectrum calculation to determine the required k-point sampling. But for this you don't have to rerun the whole calculation, you can just restore the self-consistent state from the NC file and compute the transmission spectrum for more and more k-points, and see when the results converge.

[Anders Blom]

As a general rule, it's a bit the wrong approach to think about how to lower the accuracy of the calculation to save time. If you have decided to compute this particular structure in this model, you either have to accept the long calculation time, or increase your computational capability (this system will benefit enormously from MPI parallelization, probably you can rduce the time by a factor 5-10 if run on 10-12 nodes), or choose a simpler system. And by simpler I don't mean less MgO for instance, because in this device the functionality is very dependent on the size of the barrier. In fact, for a really serious calculation you should even investigate the dependence on the barrier thickness

[huangshenjie]

Hi, Anders Blom. Unfortuntely, I meet another problem. When I'm trying to find the adequate k-points, the log shows that :
+------------------------------------------------------------------------------+
|  75 E = -552.675 dE =  4.281446e-04 dH =  2.221776e-03                       |
+------------------------------------------------------------------------------+
| Calculation Converged in 75 steps                                            |
|                                                                              |
| Fermi Level  = -0.182751 Ha                                                  |
+------------------------------------------------------------------------------+
+------------------------------------------------------------------------------+
|                                                                              |
| Equivalent Bulk  [Finished Mon Jul 30 00:31:19 2012]                         |
|                                                                              |
+------------------------------------------------------------------------------+

                            |--------------------------------------------------|
Density Matrix Calculation : ===============================Connection to server [ 172.18.203.57 : 6200 ] lost - trying to reconnect.
Connection to server [ 172.18.203.57 : 6200 ] lost - trying to reconnect.
rank 0 in job 5  bogon_45266   caused collective abort of all ranks
  exit status of rank 0: return code 137



I have never met this problem and I'm sure that the net work is fine since I can still surf the internet…
Do you know the reason of this problem?

[Anders Blom]

Seems it lost the connection to the license server. ATK accepts that the license server goes offline (or the connection to it is broken) for 5 minutes, but longer than that and it shuts down.

[huangshenjie]

OK…Thank you. It seems that I have to calculated again. 

[Anders Blom]

In this case yes, unfortunately it was interrupted before going into the real calculation, so there is no useful checkpoint file.

[huangshenjie]

Uh…I am sorry to say that I have try again, but the problem still occur!

Density Matrix Calculation : ===============================Connection to server [ 172.18.203.57 : 6200 ] lost - trying to reconnect.
Connection to server [ 172.18.203.57 : 6200 ] lost - trying to reconnect.
rank 0 in job 5  bogon_45266   caused collective abort of all ranks
  exit status of rank 0: return code 137

Is there anything wrong with my script? I have felt disappointed about the problem…