The role of the surfaces in the inversion degree and the electronic, magnetic and hyperfine properties of ZnFe2O4 and MgFe2O4 ferrites

H. H. MEDINA CHANDUVÍ; K. L. SALCEDO RODRÍGUEZ; J. J. MELO QUINTERO; A. V. GIL REBAZA; C. RODRIGUEZ TORRES; L. A. ERRICO

Puerto Varas

Conferencia; XII Latin American Workshop on Magnetism, Magnetic Materials, and their Applications (XII LAW3M); 2023

Ferrites (MFe2O4, M: Mn, Fe, Co, Ni, Zn, among others) in bulk, thin films and nanoparticle form have a wide range of potential applications due to their chemical stability and magnetic properties, besides the interest in basic research. In the last decades, progress in synthesis process renews the interest in this kind of insulating oxides and improves their physical properties, expanding their applications to new areas [1]. Spinel ferrites crystallize into a close-packed cubic structure in which tetrahedral (A) and octahedral (B) cationic sites are occupied by metallic cations (8 A sites and 16 B sites). The magnetic properties of a given ferrite depends on their chemical compositions and are also associated with the cation distribution in the A and B sites of the structure and the crystal morphologies of the sample (structure and size). In terms of the sites, the cation distribution can be described by the formula (M2+1−xFe3+x)[M2+xFe3+2−x]O4 where round and square brackets denote A and B sites and x is the inversion parameter. For x = 0 the M2+ atoms occupy the A sites and the Fe3+ atoms the B ones (normal ferrite). For x=1, eight Fe3+ atoms occupy all the sites A and eight Fe3+ and eight M2+ atoms populate the B-sites sublattice (inverted spinel). ZnFe2O4 belongs to the class of normal ferrites, but, depending on the sample preparation method and on the thermal treatment, some degree of inversion could appear. Also, when particle size is reduced to the nano-scale some degree of inversion that may scale with the surface area of the samples is observed [2, 3]. The presence of inversion changes the magnetic interactions among Fe ions, favouring a ferromagnetic behaviour at high temperatures [1, 3]. Bulk MgFe2O4 adopts the inverted structure, and a mixed-structure can be obtained depending on synthesis methods (x ~ 0.95) [2]. When the dimensions of the samples are reduced to the nanoscale, inversion degrees as lower as 0.65 are reported. In this work we present a Density Functional Theory based study of different terminations of ZnFe2O4 and MgFe2O4. The surface reconstructions, electronic and magnetic properties and the hyperfine parameters at the Fe sites were determined. Our aim is to know how the surface termination affects the structural and electronic properties of the near surface layers, the cation inversion degree, and the magnetic response of the systems. Our calculations predict that cationic inversion become energetically favourable in the surface, explaining the high degree of inversion observed in nano-sized ZnFe2O4. The presence of some inversion degree strengthens the magnetic interactions and the ferrimagnetic behavior at high temperatures. In the case of MgFe2O4, x decrese in the surface layers and increments the total magnetic moment per unit cell., in perfect agreement with the experimental trends reported. [1] C. Rodríguez Torres, F. Golmar, M. Ziese, P. Esquinazi, S. Heluani, Phys. Rev. B 84 (2011) 064404.[2] V. Sepelák, S. Indris, P. Heitjans, K.D. Becker, J. Alloy Compd. 434–435 (2007) 776–778.[3] J. Quintero, K. Salcedo Rodríguez, C. Rodríguez Torres, L. Errico, J. Alloy Comp. 775 (2019) 1117.