Magnetically tunable colloidal nano clusters (CNC’s) have been fabricated using superparamagnetic cobalt doped iron oxide CNCs for the first time. This has the regular interparticle spacing and strongly diffracts light, which is being controlled by an external field. The size of the nanoclusters varies from 10nm–200nm. It reveals that it can be used for wide magnetic tunability. The response to the magnetic field was studied using reflection spectra by varying field sample distance. This shows good response in the UV and visible region. We obtained eight prominent peaks in the UV region which enhances the prosperity of our CNC’s sensing the UV predominately. Hysteresis behavior shows the presence of superparamagnetic nature, which is promising candidate for drug delivery, bioseperation and magnetic resonance imaging.