Hence the overall effect of this morphology on the magnetic properties is exhibited through the high coercivity, relatively high temperature magnetic irreversibility, lack of magnetic saturation, high blocking temperature, etc. This random orientation of the crystallographic domains also causes randomization of the local anisotropy axes. Structurally, hollow polycrystalline nanoparticles are composed of multiple crystallographic domains. CoFe2O4, NiFe2O4 and γ-Fe2O3 hollow nanoparticles exhibit strikingly contrasting magnetic behavior compared to bulk and conventional solid particles similar behavior was also observed in core/shell nanoparticles. For such cases, surface spin glass transition enhancements have been reported due to the presence of the additional inner surface. ☐ In addition to the core/shell morphology, nanoparticles with hollow morphology are also of interest to the scientific community. Similar to traditional core/shell nanoparticles, inverted core/shell having a FiM or FM order above the Curie temperature (TC) of the shell has been reported where the Néel temperature (TN) is comparable with the bulk value and there is nonmonotonic dependence of the coercive field (HC) and exchange bias (HEB) on the core diameter. which helps enhance their magnetic properties. ☐ More specifically, the morphology of core/shell nanoparticles provides added degrees of freedom compared to conventional solid magnetic nanoparticles, including variations in the size, phase and material of the core and shell of the particle, etc. In these cases, interesting and strikingly different features, such as unusually high spin glass transition temperature, large exchange bias, finite size effects, magnetic proximity effects, unusual trend of blocking temperature as function of average crystal size, etc., have been reported. For example, in the case of core/shell NPs various ferromagnetic (FM), ferrimagnetic (FiM), and antiferromagnetic (AFM) core and shell combinations have been reported. Achieving the right crystal structure with required morphology and the magnetic behavior of the nanoparticle phases determines the magnetic response of the structure. It has been shown that morphology affects the magnetic response. Morphology wise, there have been numerous reports on magnetic nanoparticles where morphologies such as core/shell, hollow, solid, etc., have been explored. Synthesis of magnetic NPs with the right morphology, phase, size and surface functionality, as well as their usage for specific applications are challenging in terms of efficiency and safety. These nanoparticles exhibit remarkably different properties compared to their bulk counter parts. Magnetic nanoparticles (NPs) have the potential to be useful in a variety of applications such as biomedical instruments, catalysis, sensing, recording information, etc.
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