The “nano” stands for dwarf in Greek. Quantitatively, it is a prefix for one billionth. For length, 1 nm means one billionth of a meter. By comparison, 1 nm is approximately 1/50,000–1/100,000 as thick as a human hair, or 3.5 atoms of gold lined up in a row equal 1 nm. All particles with at least one dimension between 1 and 100 nm fall in the category of nanoparticles (NPs). NPs that show magnetic behavior are defined as magnetic NPs. Magnetic NPs are abundant in nature, in the human brain, in bacteria, algae, birds, ants, and bees8 and are also found in many other biological objects. Interestingly, some vertebrates use NP-assisted natural navigation system for their long distance migration. The size at which magnetic materials exhibit superparamagnetic behavior is called critical size. The critical size is a strong function of the saturation magnetization of the particles, anisotropy energy, and exchange interactions between individual spins.