Structural, Optical, and Magnetic Properties of Highly Ordered Mesoporous MCr₂O₄ and MCr_2–<i>x</i>Fe_<i>x</i>O₄ (M = Co, Zn) Spinel Thin Films with Uniform 15 nm Diameter Pores and Tunable Nanocrystalline Domain Sizes

Abstract

Herein is reported the synthesis and characterization of nanocrystalline cobalt chromite (CoCr₂O₄) and zinc chromite (ZnCr₂O₄) thin films with highly ordered cubic networks of open pores averaging 15 nm in diameter. We also show that the synthesis method employed in this work is readily extendable to solid solutions of the type MCr_2–<i>x</i>Fe_<i>x</i>O₄ (M = Co, Zn), which could pave the way for innovative device design. All of these materials can be prepared by facile coassembly of hydrated nitrate salts with an amphiphilic diblock copolymer, referred to as KLE. The as-made materials are amorphous thin films with face-centered-cubic close-packed pore structures. Electron microscopy, X-ray diffraction, grazing incidence small-angle X-ray scattering, krypton physisorption, UV–vis spectroscopy, time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and Raman spectroscopy studies collectively verify that both the transition metal chromites and the solid solutions are well-defined at the nanoscale and the microscale. In addition, the data show that the different thin film materials are nanocrystalline after annealing in air at 600 °C, adopt the spinel structure in phase-pure form, and that the conversion of the initially amorphous frameworks comes at little cost to the ordering of the cubic pore-solid architectures. Magnetization studies as a function of temperature and field further reveal the high quality of the KLE-templated CoCr₂O₄ thin films with both long-range ferrimagnetic order and spiral magnetic order at low temperatures, in agreement with previous findings