[Wien] AFM structure for double perovskite oxide

 

BUSHRA SABIR Tue, 13 Oct 2020 19:31:13 -0700

Hi Wien2k users,
I want to perform AFM calculations for double perovskite oxide La2FeNiO6For 
this, I have a cubic 225-Fm3m structure. My question is ..Do I need to flip the 
spin of Ni only during instgen_lapw -ask?or should I make a supercell with two 
Fe and then choose spin up for one Fe and spin dn for second Fe.
Need help for this
Bushra
 

The answer

 

Gavin Abo Tue, 13 Oct 2020 22:36:44 -0700

If you look at the AFM Cr example of Exercise 7 on slide 17 of [1], the struct file has Cr1 atom with a multiplicity of 1 at a non-equivalent atomic position of (0,0,0) and it also has Cr2 atom with a multiplicity of 1 at an non-equivalent atomic position of (0.5, 0.5, 0.5).  Since multiplicity is 1, there are no equivalent atomic positions in this example. At [2], you can see during the "instgen_lapw -ask" step during execution of "init_lapw" that "u" was entered for Cr1 and "d" was entered for Cr2.  After the scf calculation, the Cr1 moment from the calculation was 1.13184 for MMI001 and the Cr2 moment was -1.13184 for MMI002.  The Cr1 and Cr2 are antiferromagnetic [3] with their moments having equal magnitude and opposite sign resulting zero total moment (MMTOT). Similar for your case, I think you need to ask and answer yourself the same questions as you would for the above example. In other words:  What are the non-equivalent atomic positions for Cr (but for Fe and Ni in your case)?  What is the multiplicity of each non-equivalent atomic position?  What are the equivalent atomic position for Cr (i.e, Fe and Ni in your case), if any? What atoms at the non-equivalent and equivalent [4] atomic positions give the AFM configuration(s) of interest and are expected to cancel to give a zero total moment?  Of note, if you not familiar with what the multiplicity is in the struct file, refer to section "4.3. THE CASE.STRUCT.FILE" on page 40 in the usersguide for WIEN2k 19.1 and 19.2 [5]. I realize the use of runafm_lapw in the above Cr example might be misleading as runsp_lapw typically has to be used in most cases as explained in previous posts in the mailing list archive such as [6-8]. Also, since the configuration set in case.inst with instgen_lapw is just the initial one that could change during the scf as explained in [9,10], don't forget to check the final magnetic order that comes out of the scf to see if you have AFM or not [11]. Though, in some cases when the scf does not result in the expected spin state, it might be possible to force a particular state as described in previous posts in the mailing list archive such as [12-14]. There used to be a presentation online that I think illustrated WIEN2k calculations on a structure for different AFM configurations like Type G, Type A, and Type C that might been of interest to you, but unfortunately I could not find it again in a Google search.  So something might have happened to the website that was hosting it.  It might have been the presentation "P. Blaha: “Magnetic and charge order phase transition in YBaFe2O5" seen in [15].

[1] http://susi.theochem.tuwien.ac.at/events/ws2015/Exercises_15.pdf

[2] http://wien2k-algerien1970.blogspot.com/2016/02/how-to-do-antiferromagnetic-calculation.html [3] https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg13159.html [4] https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg17879.html

[5] http://www.wien2k.at/reg_user/textbooks/usersguide.pdf

[6] https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg16805.html

[7] http://susi.theochem.tuwien.ac.at/reg_user/faq/afm.html

[8] https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg17307.html [9] https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg03243.html [10] https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg06739.html [11] https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg17516.html [12] https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg18051.html [13] https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg18895.html [14] https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg19462.html

[15] https://publik.tuwien.ac.at/files/PubDat_175934.pdf