Correlation between crystal structure and magnetism in PLD grown epitaxial films of ε-Fe₂O₃ on GaN

Abstract

In the present paper we discuss correlations between crystal structure and magnetic properties of epitaxial ε-Fe 2 O 3 films grown on GaN. The large magnetocrystalline anisotropy and room temperature multiferroic properties of this exotic iron oxide polymorph, make it a perspective material for the development of low power consumption magnetic media storage devices. Extending our recent progress in PLD growth of ε-Fe 2 O 3 on the surface of technologically important nitride semiconductors, we apply reciprocal space tomography by electron and x-ray diffraction to investigate the break of crystallographic symmetry occurring at the oxide-nitride interface resulting in the appearance of anisotropic crystallographic disorder in the sub-100 nm ε-Fe 2 O 3 films. The orthorhombic-on-hexagonal nucleation scenario is shown responsible for the development of a peculiar columnar structure observed in ε-Fe 2 O 3 by means of HRTEM and AFM. The complementary information on the direct and reciprocal space structure of the columnar ε-Fe 2 O 3 films is obtained by various techniques and correlated to their magnetic properties. The peculiar temperature dependence of magnetization studied by the small-field magnetization derivative method and by neutron diffraction reveals the existence of a magnetic softening below 150 K, similar to the one observed earlier solely in nanoparticles. The magnetization reversal in ε-Fe 2 O 3 films probed by X-ray magnetic circular dichroism is found different from the behavior of the bulk averaged magnetization measured by conventional magnetometry. The presented results fill the gap between the numerous studies performed on randomly oriented ε-Fe 2 O 3 nanoparticles and much less frequent investigations of epitaxial epsilon ferrite films with lattice orientation fixed by the substrate.

Structural peculiarities of ε-Fe2O3/GaN epitaxial layers unveiled by high-resolution transmission electron microscopy and neutron reflectometry

“…As shown in the present paper, the defects related to the nitrideoxide mismatch are mostly accommodated by the transition layer while the bulk of the ε-Fe2O3 layer grows with the native iron oxide lattice structure. Importantly, the huge magneto-crystalline anisotropy which is the most crucial property of ε-Fe2O3 is present in the epitaxial ε-Fe2O3 / GaN films despite the large lattice mismatch observed in this system [19].…”

“…Epitaxial films of ε-Fe2O3 iron oxide were grown on GaN (0001) surface by means of PLD following the routine described in detail in [6,19]. The growth was performed using the PLD system produced by Surface (Germany).…”

“…The polarization of the direct beam was measured by the analyzer for each value of the magnetic field used in the experiment and the amplitudes of the PNR curves were corrected for polarization efficiency. For quantitative discussions of the structure and magnetic properties of the studied samples the PNR data was fitted using the Parratt algorithm by the GenX software package [19].…”

Technological Peculiarities of Epsilon Ferrite Epitaxial Stabilization by PLD

“…The magnetic properties of ε-Fe 2 O 3 /GaN films have been later described in detail in Refs. [10,25]. The peculiar columnar crystal structure of ε-Fe 2 O 3 films has been investigated by high-resolution transmission electron microscopy (HRTEM) in planar [25] and transversal [26] geometries.…”

“…[10,25]. The peculiar columnar crystal structure of ε-Fe 2 O 3 films has been investigated by high-resolution transmission electron microscopy (HRTEM) in planar [25] and transversal [26] geometries. Being metastable, ε-Fe 2 O 3 is very sensitive to growth conditions such as the composition and pressure of the background gas, substrate temperature, and growth rate.…”