Hexagonal close-packed Ni nanostructures grown on the (001) surface of MgO

Abstract

We report the in situ microscopy observation of an unnatural phase of Ni, a highly strained hexagonal close-packed ͑hcp͒ form which we believe is stabilized by heteroepitaxial growth on the ͑001͒ face of MgO. We find that the nanosized hcp nickel islands transform into the normal face-centered cubic structure when the size of the islands exceeds a critical value ͑about 2.5 nm thick with a lateral size of ϳ5 nm͒. The structural transition proceeds via a martensitic change in the stacking sequence of the close-packed planes. The formation of hcp Ni nanostructures with an unusually large crystallographic c / a ratio ͑ϳ6% larger than ideal hcp͒ is very interesting for spintronic and recording applications where large uniaxial anisotropies are desirable.

Templated Growth of Hexagonal Nickel Carbide Nanocrystals on Vertically Aligned Carbon Nanotubes

“…Interestingly, there have been reports in the literature related to the stabilization of a nonequilibrium hexagonal structure (hexagonal closepacked or HCP) in nanostructured forms of nickel, including HCP metastable nickel carbide nanocrystals. [9][10][11][12] The HCP phase is typically observed in case of far-from-equilibrium synthesis routes such as wet chemical synthesis at very low temperatures when the nanocrystals grow from an amorphous phase, 9 decomposition of neutral metal complex, 10 thin film processing via heteroepitaxial growth, 11 and mechanical alloying via highenergy ball milling. 12 These synthesis methods indicate that HCP nickel is only observed below a certain critical crystallite size (∼4 nm) and changes to the equilibrium FCC structure for larger crystalline sizes.…”

Preparation of hcp–Ni epitaxial films on Cr underlayers

“…Epitaxial films are useful to investigate the basic structural properties, since the film uniformity is well controlled. Metastable hcp-Ni and hcp-permalloy epitaxial films have been prepared by employing B1-MgO(100) [5,6] and fcc-Au(100) [7]- [10] underlayers. However, parts of these epitaxial hcp crystal also transformed into fcc structure.…”

Phase transformation accommodated plasticity in nanocrystalline nickel

“…This observation is somewhat surprising as the ␥ phase of Ni has been considered the exclusive equilibrium phase of nickel at ambient pressure and at temperatures up to its melting point, though both ␣ and hexagonal close-packed phases of Ni have been synthesized mainly as very thin films by heteroepitaxial growth. [14][15][16][17] On the other hand, molecular dynamics simulation reveals a continuous transformation even to an amorphous phase in Ni nanowires at a strain rate of 5 % ps −1 , a stress of 9.5 GPa, and a strain of 15%, respectively, with the stress level far exceeding the maximum flow stress of bulk Ni. 18 The observations reported in this letter thus reveal yet another mechanism that can accommodate plasticity of nc Ni at relatively large plastic strains.…”