On the Correlation between Nanoscale Structure and Magnetic Properties in Ordered Mesoporous Cobalt Ferrite (CoFe₂O₄) Thin Films

Abstract

In this work, we report the synthesis of periodic nanoporous cobalt ferrite (CFO) that exhibits tunable room temperature ferrimagnetism. The porous cubic CFO frameworks are fabricated by coassembly of inorganic precursors with a large amphiphilic diblock copolymer, referred to as KLE. The inverse spinel framework boasts an ordered open network of pores averaging 14 nm in diameter. The domain sizes of the crystallites are tunable from 6 to 15 nm, a control which comes at little cost to the ordering of the mesostructure. Increases in crystalline domain size directly correlate with increases in room temperature coercivity. In addition, these materials show a strong preference for out-of-plane oriented magnetization, which is unique in a thin film system. The preference is explained by in-plane tensile strain, combined with relaxation of the out-of-plane strain through flexing of the mesopores. It is envisioned that the pores of this ferrimagnet could facilitate the formation of a diverse range of exchange coupled composite materials.

Mesoporous bismuth ferrite with amplified magnetoelectric coupling and electric field-induced ferrimagnetism

“…Both in-plane and out-of-plane q values are consistent with previous reports employing related polymer templates (Fig. 1a) 39 . The elliptical nature of the diffraction pattern indicates that the pores are anisotropic, with significant out-of-plane pore shrinkage during heating 39 that is not balanced by in-plane contraction because the films are anchored to the silicon substrate.…”

“…Porous materials thus offer interesting potential for strain engineering using wet chemical methods without the need for expensive epitaxy. Strain induced by porosity can be reproducibly generated and can be tuned, though not with the precision of epitaxial films 24 , by controlled thermal annealing 39 . The observation of electrically induced changes in both strain and M S in porous BFO further demonstrates novel magnetoelectric coupling not previously observed in epitaxially strained materials.…”

“…1a) 39 . The elliptical nature of the diffraction pattern indicates that the pores are anisotropic, with significant out-of-plane pore shrinkage during heating 39 that is not balanced by in-plane contraction because the films are anchored to the silicon substrate. On further thermal annealing to achieve crystallization of the framework, some rearrangement of the mesoporous structure is observed (Fig.…”

“…The dense and porous films were then annealed at the exact same temperatures and using the same heating rates to insure comparable crystallization of the films. Previous studies of mesoporous thin films composed of magnetostrictive cobalt ferrite revealed that flexing and contracting of the pores on high-temperature crystallization produced anisotropic strain states that resulted in magnetic anisotropy 39 . In this report, we demonstrate that strains achieved in templated nanoporous BFO films allow for magnetoelectric coupling that exceeds that reported previously for epitaxial thinfilm counterparts in samples that do not contain significant levels of oxygen vacancies.…”

Anomalous second ferromagnetic phase transition as a signature of spinodal decomposition in Fe-doped GeTe diluted magnetic semiconductor

“…At low temperatures, a large bifurcation between the ZFC and FC curves and a cusp in ZFC curve are observed, which brings on the central issue in magnetic system: whether the magnetic ground state is a conventional ferromagnet or spin glass or superparamagnetic system. [11][12][13] It is previously reported that the material with magnetic anisotropy has an intrinsic long range ferromagnetic property. 14 Next, we will demonstrate that the ferromagnetic behavior is dominated by magnetic anisotropy-related long range ferromagnetic exchange interaction.…”