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Unique Ferromagnetism in α-Mn Nanorods
In the periodic table, Mn is located just before the ferromagnetic family of metals, namely, iron, cobalt and nickel. However, it's allotrope at room temperature (a-Mn) is known to be paramagnetic. Thus, at room temperature, Mn cannot achieve the magnetic strength of its fellow ferromagnetic metals. Magnetism in ferromagnetic metals can be described in terms of direct exchange interaction, where the coupling strength depends on the ratio (l) of the inter-atomic distance to the radius of d-shell of an atom in a material. In case of Mn, lvalue is just below 1.5, which is slightly less than that for its ferromagnetic neighbours. This implies that, increasing the separation between Mn atoms will favourably alter the ratio and leading to ferromagnetism. This is normally achieved either by alloying Mn with a non-magnetic element or by growing epitaxial layer of Mn on Fe or Co or Ni. The ferromagnetic ternary alloys containing Mn, known as Heusler alloys (such as Cu2MnAl) are typical examples of the first type. The other way of imparting ferromagnetism in Mn is by a technique in which an epitaxial layer of Mn grown on Fe (100) exhibits a net magnetic moment.
Figure: (a) Morphology of a-Mn nanorod and schematic representation; (b) VSM measurements of the cryomilled nanorods showing ferromagnetic hysteresis loop and (c ) Actual spin density calculated using DFT simulations. A vertical cross-section of the nanorod is shown in the figure, where the atoms in top layer (red box) belongs to (110) surface. Ferrmoagnetic and antiferromagnetic region is denoted by the red and blue box, respectively.