The invention provides a liquid-dispersible powder comprising nanoscale grains of matrix embedded with one or more isolated nanoparticles and a composition for the magnetic nanoparticle hyperthermia (MNH) treatment of tumours comprising nanoscale grains of matrix material containing one or more isolated nanoparticles. The invention also provides a method of production of a liquid-dispersible powder described herein, the method comprising the steps of providing nanoparticles prepared under ultra-high vacuum (UHV) gas phase conditions; co-depositing the nanoparticles within a matrix material under UHV gas phase conditions; and grinding the film to a fine powder comprising grains of groups of matrix material isolated nanoparticles. The invention also provides a method of reducing the agglomeration of nanoparticles in liquid, the method comprising isolating nanoparticles in nanoscale grains of matrix material, and the use of a liquid-dispersible powder comprising nanoscale grains of matrix material containing one or more isolated nanoparticles in the manufacture of a medicament for the MNH treatment of tumours.