For distribution

[CM ICTP - Trieste]

JOINT ICTP/SISSA CONDENSED MATTER SEMINARS
Academic Year 2007/08


Seminar Room - Main Building   (first floor)



Wednesday, 17 October -     4:00 p.m.



A. YARESKO   ( Max-Planck Institute for the Physics of Complex Systems, 
Dresden )


" Calculation of effective magnetic interactions in Cr spinels "


Abstract

Recently, ACr_2O_4 (A=Zn, Cd, Hg) compounds with a spinel crystal 
structure have attracted much attention because of their unusual 
magnetic properties.  Structural transitions to low-temperature 
tetragonal phase with a complex magnetic order or the appearance of 
magnetization plateau under application of external magnetic field are 
supposedly caused by geometrical frustrations which 
anti-ferromagnetically (AFM) coupled Cr spins experience on a 
pyrochlore lattice.  On the other hand, CdCr_2Se_4 spinel is a 
ferromagnetic (FM) semiconductor.  In the present work effective 
exchange coupling constants between Cr spins in ACr_2X_4 (A=Zn, Cd, Hg; 
X=O,  S, Se) spinels are estimated by mapping the energy of spin 
spirals calculated within LSDA and LSDA+U approximations onto an 
effective Heisenberg model.  The calculations reproduce the change of 
the sign of the exchange coupling between nearest Cr neighbors from AFM 
in O-based spinels to FM in S and Se based ones.  It is shown that the 
AFM nearest neighbor coupling in ACr_2O_4$ is determined mainly by 
direct hopping between Cr t_{2g} electrons whereas the FM coupling 
S(Se)-based spinels is caused by Cr t_{2g}--X p--Cr t_{eg} 
hybridization.