Developing easy and customizable strategies for the directional structure modulation of multicomponent nanosystems to influence and optimize their properties are a paramount but challenging task in nanoscience. Here, we demonstrate highly controlled eccentric off-center positioning of metal−core in metal@silica core−shells by utilizing an in situ generated biphasic silica-based intraparticle solid− solid interface. In the synthetic strategy, by including Ca 2+ -ions in silica−shell and successive oxidative and reductive annealing at high temperature, a unique hairline−biphasic interface is evolved via the heat-induced concentric radial segregation of calcium silicate phase at the interior and normal silica phase at the exterior of core−shell, which can effectively arrest the outwardly migrating metal−core within rubbery calcium silicate phase, affording various eccentric core−shells, where core-positions are flexibly controlled by the annealing time and amounts of initially added Ca 2+ -ions. In the structure−property correlation study, the strategy allows fine-tuning of dipolar interaction-based blocking temperatures and magnetic anisotropies of different eccentric core−shells as the function of variable off-center distance of magnetic core without changing the overall size of nanoparticles. This work demonstrates the discovery and potential application of biphasic solid−solid media interface in controlling the heat-induced migration of metal nanocrystals and opens the avenues for exploiting the rarely studied high-temperature solid-state nanocrystal conversion chemistry and migratory behavior for directional nanostructure engineering.