Hi All
Curie temperature (TC) of the materials depends on exchange energy and interatomic distance of the ions. In (A70B30)1-XCX alloys, TC can be changed by changing the value of X. Is there any easy way (software package) by which we can get the exact value of TC theoretically? I heard that exchange energy can be obtained by Quantum Espresso. Is it user friendly? Any opinion?
Thank you
Regards
It is rare and/or difficult to obtain magnetic behaviour by ab-initio methods. In this particular case, dealing with Curie temperature has to be not only with magnetic interactions but also with variation of them with Temperarture. I recall that ab-initio methods in general are designed for absolute cero kelvin, and temperature effects are not included in ab-initio codes.
In addition to Ferrari's answer, I would like to point out that in some particular cases, such as Rare Earth intermetallics, using Landau theory one can determine the critical temperature if the first-order magnetic susceptibility is known, according to the expression:
1-n Khi(TC)=0, n being the bilinear Exchange parameter.
You can find additional information in Chap. 1 "Quadrupolar intereactions and magnetoelastic effects in Rare Earth intermetallic compounds", P. Morin and D. Schmitt, Ferromatic Materials, Vol. 5, Ed. K. H. J. Buschow and E.P. Wohlfarth, 1990
Thanks for the answer Mr. Blanco. I have been reading the reference and it is very interesting although a little bit too theoretical to my personal taste. As I see for determining the Curie temperature you must not only have the bilinear Exchange parameter (not quite sure how to do it by ab-initio methods) but also the behaviour of the first-order magnetic susceptibilty Xhi with temperature (i.e. Xhi(T)) to solve the equation. So again, we need a temperature dependence of something, out of the scope of Density Functional Theory (base of most ab-initio methods)
Yes It is true what you said, but for experimental data you can estimate the exchange bilinear coefficient and the Khi you need to know the Crystal field paramenters. It is true that for the momento no ab initio information can be obtained, but semi-empirical methods can be used to estimate those parameters and then to derive the TC.
Dear Ferrari and Blanco,
Thank you for your kind comments! I went through with the ref. and found interesting.
Our materials are actually rare earth free transition metal alloys which have second order phase transition. R. O’Handley (Journal of Applied Physics 62,1987, R15–R49) has explained the magnitude of the Curie temperature in terms of a mean field model by
the following expression TC = J(r)ZT S(S+1)/3kB, where J(r) is the distance-dependent interatomic exchange integral, ZT is the coordination number, S is the atomic spin quantum number.
In (A70B30)1-XCX alloys, the alloying of C with (A70B30) will change the ZT which can easily get from the XRD data. On the other hand, J(r) will also change and i think the contribution of J(r) will be more than that of ZT. But, I don't know how to calculate the J(r) for binary and ternary alloys.
I only use two ab-initio codes: ABINIT and SIESTA. As far as I know these codes do not calculate the exchange integral J(r), although it may be possible to do so by making a small program for reading the wavefunctions and calculate that integral from them. It is out of my scope how real is that possibility.
Hi Varun,
I have seen several groups calculate the Curie temperature of systems using first principle approaches based on linear muffin tin orbital approaches (LMTO) or the Korringa-Kohn-Rostoker formalism. There is a nice overview by Turek et al in one of the Psi-k newsletters. It is a bit technical, but it does address alloys and should provide you with a good start. Here is the link.
www.psi-k.org/newsletters/News_59/Highlight_59.pdf
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