Magnetic　materials　with　significant　permeability　and　high　resonance　frequency　are　a　challenge　due　to　the　Snoek　limit,　but　it　is　possible　to　achieve　breakthroughs　by　inducing　superparamagnetism　in　magnetic　nanoparticles.　Here,　size-controlled　monodisperse　CoFe2O4　nanocubes　(NCs)　were　successfully　synthesized　via　a　facile　high-temperature　organic-phase　method.　The　superparamagnetic/ferrimagnetic　relaxation　induced　high　frequency　properties　of　CoFe2O4　NCs　controlled　by　particle　size　has　been　investigated.　The　resonance　frequency　of　CoFe2O4　NCs　increases　from　6.0　GHz　to　6.3　GHz　with　decreasing　particle　size　from　40　to　19　nm.　With　further　decreasing　particle　size　to　13　nm,　no　resonance　peak　can　be　observed　in　the　measured　frequencies　from　4　GHz　to　10　GHz,　which　can　be　attributed　to　the　superpara　magnetic/ferromagnetic　relaxation　tuned　by　particle　sizes.　This　finding　opens　up　a　straightforward　avenue　for　optimizing　high　frequency　properties　of　magnetic　nanomaterials.　(C)　2017　Elsevier　B.V.　All　rights　reserved.