The first principle calculation study on half-metallic spinel Presenter: Min Feng Advisor: Professor Xu Zuo College of Information Technical Science, Nankai University, Tianjin, China

Half-metallicity: an intrinsic materials property of an energy gap between valence and conduction bands for electrons of one spin polarization and continuous bands for the electrons of the other spin polarization.

Discovered by de Groot and Mueller: band structure of NiMnSb for (a) the majority spin direction and (b) the minority-spin direction. Physical Review Letters 50(25): 2024 Applications: components for MR devices, spin tunneling devices, etc. in field of spintronics.

No smoking gun experiment to prove or disprove half-metallicity. Electronic structure calculations continue to play an important role in the search for new half-metallic materials.

Spinel: An important class of ceramic compounds with a variety of interesting properties. The earliest suggestion of half metallic character was from a calculation of the spinel structure magnet CuCr 2 S 4 .

(100) direction Structure can be fully described by lattice constant (a) and oxygen parameter (u).

Tetrahedral coordination site ( A site) Octahedral coordination site ( B site) X atoms in a nearly fcc close-packed arrays

Space group O

7 h

(Fd3m).

Chemical formula: AB

2

X

4 We use GGA (generalized gradient approximation) +U method to investigate the electronic structure and magnetic properties of spinel. The calculation is accomplished by using the VASP code and the WIEN2K code. The additional on-site Coulomb repulsion (U) is applied to the d orbits of the transition metal cations. free direction

My research work:

LiCr 2 O 4 Structure optimized using VASP with a 12 × 12 × 12 k-mesh. The breaking conditions for the electronic and ionic loops are 10 -4 and 10 -3 eV .

The electronic structure and magnetic properties calculation is accomplished by using the WIEN2K code with a 21 × 21 × 21 k-mesh. The optimized structure parameters are a = 8.260

Å and u = 0.388. The dependency of DOS on U in LiCr 2 O 4 implies that the half-metallicity may be robust to U. While half-metallicity can hardly exist in CuCr 2 S 4 .

CuFe 2 O 4 The optimized structure of inverse copper ferrite is tetragonal, with a=b=8.136

Å , c=8.669

Å . Thus c/a ratio is 1.065, due to the Jahn-Teller distortion of Cu 2+ ions. The optimized structure of normal spinel copper ferrite is cubic, with a=b=c=8.062 Å . We also calculated the local magnetic moment of Cr and the interstitial magnetic moment in LiCr 2 O 4 , and their dependencies on

U are shown on the left.

When U increases, mCr increases and m int decreases due to the increasing trend of localization. Calculated density of states (DOS) of copper ferrite with U=4eV. From (a) to (e), the factor of inverse degree x=0, 0.25, 0.5, 0.75, 1, respectively. The partially inverse structures are labeled by their Cu concentration in B sites and name of space group, for example, 0.25B CM means the structure has 25% Cu ions in B sites, and its space group is CM (8).

When x=0, 0.25, 0.5, the copper ferrite systems are half-metallic. The copper ferrite system with 75% Cu ions in B site is metallic. And the inverse spinel copper ferrite, in which x=1, is semi-conducting.