Systems, devices, methods, and compositions including ferromagnetic structures

A nuclear magnetic resonance imaging composition, comprising:

• a plurality of ferromagnetic microstructures, one or more of the plurality of ferromagnetic microstructures including a first internal surface defining a void, the void being accessible to a biological sample, the one or more of the plurality of ferromagnetic microstructures configured to generate a time-invariant magnetic field within at least a portion of the void;
• and
• one or more targeting moieties attached to at least one of the one or more of the plurality of ferromagnetic microstructures.