Anomalous Magnetic Properties of Nanoparticles Arising from Defect Structures: Topotaxial Oxidation of Fe_1–xO|Fe_3−δO₄ Core|Shell Nanocubes to Single-Phase Particles

Abstract

Here we demonstrate that the anomalous magnetic properties of iron oxide nanoparticles are correlated with defects in their interior. We studied the evolution of microstructure and magnetic properties of biphasic core|shell Fe(1-x)O|Fe(3-δ)O4 nanoparticles synthesized by thermal decomposition during their topotaxial oxidation to single-phase nanoparticles. Geometric phase analysis of high-resolution electron microscopy images reveals a large interfacial strain at the core|shell interface and the development of antiphase boundaries. Dark-field transmission electron microscopy and powder X-ray diffraction concur that, as the oxidation proceeds, the interfacial strain is released as the Fe(1-x)O core is removed but that the antiphase boundaries remain. The antiphase boundaries result in anomalous magnetic behavior, that is, a reduced saturation magnetization and exchange bias effects in single-phase nanoparticles. Our results indicate that internal defects play an important role in dictating the magnetic properties of iron oxide nanoparticles.

The influence of oxidation process on exchange bias in egg-shaped FeO/Fe3O4 core/shell nanoparticles

“…The hysteresis loops recorded at 2 K are presented on Fig.5c [19], H EB = 0.6 and 0.5 kOe (10 K, particle size 9 nm-spherical and 45 nm-cubic) [7], H EB = 3.1 kOe (10 kOe cooling field, 2 K, 32 nm cubes) [32], H EB = 0.79, 2.97 and 5.1 kOe (40 kOe cooling field, T= 5K, corresponding particle size 9, 16 and 22 nm, cubes) [33], H EB = 1.6 kOe (50 kOe cooling field, T= 10 K, particle size 23 nm, cubes) [27].…”

Applications of exchange coupled bi-magnetic hard/soft and soft/hard magnetic core/shell nanoparticles

“…Moreover, scanning TEM (STEM) using the so-called high angular annular dark field mode (HAADF) can enhance the elemental contrast as the image intensity, I, scales with near the square of the atomic number, i.e., IZ 1.7 [181]. For instance, Hai et al [187] and Wetterskog et al [188] visualized the core and the shell of Fe 1-x O/Fe 3-δ O 4 nanocubes by comparing DF-TEM images taken using diffracted beams corresponding only to the spinel structure (Fe 3-δ O 4 ) or both the spinel and rock salt structure (Fe 1-x O) (Fig. 2a,b) whereas Pichon et al [189] and Liu et al [190] used STEM-HAADF imaging to highlight the core from the shell (see Fig.2c-d).…”

“…Wetterskog et al showed the presence of internal defects (antiphase boundaries) in Fe 1-x O/Fe 3-δ O 4 nanocubes and how these internal defects could be directly correlated to the interface between the core and the shell (see Fig. 2e,f) [188]. Alternatively, spectroscopic TEM-based methods can also be employed to study core/shell nanoparticles.…”

Мессбауэровские исследования наночастиц FeO/Fe-=SUB=-3-=/SUB=-O-=SUB=-4-=/SUB=- типа ядро/оболочка

“…Перспективным [14] и ссылки там), тем не менее, их аномальные магнитные свойства, такие как пониженная намагниченность насыщения, по сравнению с объем-ными величинами, обменное смещение в однофазных наночастицах и высокополевые восприимчивости все еще плохо изучены и обычно приписываются скосу спи-новой структуры или дефектам на внешней поверхности частиц [19,20]. Отсюда следует, что необходимы иссле-дования, связывающие структурные особенности (напри-мер, дефекты) внутри магнитных наночастиц (МНЧ) с наблюдаемыми аномальными магнитными свойствами.…”