  Magnetic analogue of an isolated free electric charge, i.e., a magnet with a
single north or south pole, is a long sought-after particle which remains
elusive so far. In magnetically frustrated pyrochlore solids, a classical
analogue of monopole was observed as a result of excitation of spin ice
vertices. Direct visualization of such excitations were proposed and later
confirmed in analogous artificial spin ice (ASI) systems of square as well as
Kagome geometries. However, such charged vertices are randomly created as they
are thermally driven and are always associated with corresponding emergent
antimonopoles of equal and opposite charges connected by observable strings.
Here, we demonstrate a controlled stabilisation of a robust isolated emergent
monopole state in individual square ASI vertices by application of an external
magnetic field. The excitation conserves the magnetic charge without the
involvement of a corresponding antimonopole. Well supported by Monte Carlo
simulations our experimental results enable, in absence of a true elemental
magnetic monopole, creation of electron vortices and studying electrodynamics
in presence of a monopole field in a solid state environment.
