Electronic and Magnetic Properties of YCrO 3 and YFeO 3 – A First Principles Study

1. Electronic and Magnetic Properties of YCrO3 and YFeO3–A First Principles Study Vidhya G Nair Department of Physics IIT Madras HPC Symposium 2014 - April 25, 2014

2. Overview of the talk • Introduction to multiferroics • Computational details • Results and Discussion • YFeO3 • YCrO3

3. Introduction Multiferroics more than one ferroic order coexist and are coupled (magnetic, electric or elastic) (usually refers specifically to) • Rare earth chromites and ferrites shows multiferroic behavior. • YCrO3 and YFeO3 • orthorhombic structure • canted antiferromagnet • Neel temperature (TN) of ~140 K and ~655 K [1, 2] Daniel Khomskii, Physics2, 20 (2009) • [1] J. R. Sahuet al., J. Mater. Chem., 17, (2007) 42. • [2] M. Shang et al., Appl. Phys. Lett., 102, (2013) 062903.

4. Computational details • First principles calculation of the electronic and magnetic properties of YCrO3 and YFeO3 is performed within generalized gradient approximation. • The calculations are executed by employing the Cambridge Serial Total Energy Package (CASTEP) code based on density functional theory. • The calculations were performed by the ultrasoft pseudopotential method with plane-wave basis which describes the interaction of electrons with ion cores. • Structural optimizations are carried out for both samples with all possible magnetic structures.

5. Different magnetic spin structures G-type AFM C-type AFM A-type AFM FM

6. YFeO3 Space group - Pnma Lattice parameter a = 5.7909 Å b = 7.7354 Å c = 5.4407 Å α = β = γ = 90o G-type AFM

7. Comparison of Total energies

8. Band structure and Density of states (DOS) of YFeO3 Band structure and density of states of G-type antiferromagnet YFeO3 shows the insulating behavior.

9. Effect of Hubbard parameter (U) Density of states without U parameter. Density of states with U = 5 eV.

10. Effect of Hubbard U parameter for G-AFM Density of states of G-type AFM with U = 1 to 5 eV. Partial density of states contribution to total DOS

11. Estimation of magnetic – ordering temperature • To estimate the magnetic-ordering temperature for YFeO3, the Heisenberg exchange constants corresponding to the nearest-neighbor magnetic couplings for the magnetic configuration is determined. • The calculated energies are mapped onto a simple Heisenberg model, • From the coupling constants, the magnetic-ordering temperature is calculated using the mean-field approximation. • The magnetic transition temperature for G-type magnetic structure is calculated to be 700 K which is close to that of the experimental value TN = 655 K. Magnetic moment Fe3+ : 3.82 μB (5 μB)

12. YCrO3 Space group - Pnma Lattice parameter a = 5.5157 Å b = 7.5301 Å c = 5.2409 Å α = β = γ = 90o G-type AFM

13. Comparison of Total energies

14. G-type Antiferromagnetic ordering Band Gap = 1.435 eV Magnetic moment Cr3+ : 2.98 μB (3 μB) • The magnetic transition temperature for G-type magnetic structure is calculated to be 137 K (TN = 140 K).

15. Hubbard U = 1 (G-type) Band Gap = 1.648 eV

16. Comparison of experimental and theoretical results

17. LIBRA cluster • We are extensively using LIBRA cluster for running our programs. • The total time used for each calculation depends on the sample. • For the present samples, the run time for each structural optimization is approximately one week, if 4 processors are used for calculation.

18. Acknowledgment • High Performance Computing Environment (HPCE), IIT Madras. • DBT for funding (CASTEP). • Mr. V. Ravichandran, HPCE • Mrs. P. Gayathri, HPCE • Mr. C. Ganeshraj

19. Thank You

21. Ferromagnetic ordering Band Gap = 0.759 eV

22. C-type antiferromagnetic ordering Band Gap = 1.082 eV

23. A-type antiferromagnetic ordering