Giant magnetic moment in epitaxialFe3O4thin films on MgO(100)

Abstract

Magnetization studies on well characterized epitaxial magnetite ͑Fe 3 O 4 ͒ thin films grown on MgO͑100͒ show that the ultrathin films ͑Ͻ5 nm thickness͒ are ferromagnetic and their magnetic moments are much greater than those of bulk magnetite, particularly at a thickness of 20 nm or below. The observation of a ferromagnetic nature in ultrathin magnetite films ͑Ͻ5 nm͒ is in contrast to the previously accepted dead layer interface model or a superparamagnetic behavior for ultrathin films of magnetite. From detailed spin-polarized density functional theory based calculations of Fe 3 O 4 -MgO interface, we calculate a deviation in spin moment of the Fe atoms in the vicinity of the interface from their bulk values, which is insufficient to explain the observed results. Orbital moment contribution for the surface Fe atoms was found to be quite small. The noncompensation of spin moments between the tetrahedral ͑A͒ and octahedral ͑B͒ sublattices at the surface and antiphase-domain boundaries are inferred to be the main factor contributing to the observed enhanced magnetic moment.

Origin of the giant magnetic moment in epitaxialFe3O4thin films

“…͑2͒, as suggested by Arora et al, 26,27 obtaining a bulk saturation magnetization M S = 349Ϯ 41 emu/ cm 3 and a surface contribution M i = 2088Ϯ 274 emu/ cm 3 , in agreement with. 26,27 The presence of the Verwey transition down to 4-nm-thick films ͑Fig. 3͒ evidences the high quality of the films.…”

“…However, recent x-ray magnetic circular dichroism ͑XMCD͒ and resonant photoemission experiments have ruled out this hypothesis. 29 We have obtained similar magnetization results to those reported by Arora,26,27 therefore, the purpose of the present work is to investigate further the origin of the anomalous giant magnetic moment in epitaxial Fe 3 O 4 thin films grown on MgO ͑001͒. In Sec.…”

“…However, this is in contrast to the magnetic contribution from the film surface or from the interface of the film with the substrate suggested in Ref. 27. Indeed, the magnetic moment m obtained from the fit is always lower than the bulk value of 480 emu/ cm 3 .…”

“…However, recent studies performed by Arora et al 26,27 on well-characterized epitaxial Fe 3 O 4 thin films grown on MgO ͑001͒ show that the ultrathin films ͑Ͻ5 nm thickness͒ are ferromagnetic and their magnetization is much greater than that of bulk magnetite. The main factor contributing to the observed enhanced magnetic moment is proposed to be the noncompensation of spin moments between the tetrahedral and octahedral sublattices at the surface, at the interface with the substrate, and at the antiphase-domain boundaries.…”

Anomalous anisotropic magnetoresistance in epitaxialFe3O4thin films on MgO(001)

“…[7][8][9][10] However, the Fe 3 O 4 /MgO hetero-epitaxial system suffers from the formation of antiphase boundaries (APB) which is an innate outcome of the growth process due to the fact that Fe 3 O 4 has a unit cell which is twice the size and lower in symmetry compared to MgO. [7][8][9][10] The presence of APBs has a deleterious effect on the magnetic properties but they are beneficial in enhancing the magneto-resistance of films due to additional spin scattering they induce. 11,12 Magneto-transport studies in epitaxial magnetite films are mostly focused on the influence of APBs, disorder and strain.…”