M-type　ferrite　exhibits　good　magnetic　characteristics　and　resistance　to　oxidation,　enabling　the　production　of　stable　nanoparticles　for　applications　such　as　medical　delivery,　hyperthermia　therapy,　and　magnetic　recording.　Additionally,　it　serves　as　a　valuable　tool　for　studying　the　theory　of　coercivity.　Here,　the　impact　of　surface　and　size　on　coercivity　and　associated　mechanisms　in　M-type　hexagonal　ferrites　has　been　thoroughly　analyzed　by　a　combination　of　micromagnetic　models　and　experimental　investigations.　Based　on　a　cubic　model　and　without　considering　surface　defects,　the　coercivity　still　cannot　reach　the　theoretical　value.　It　decreases　with　an　increase　in　particle　size,　and　the　quasi-coherent　and　quasi-flower　reversal　modes　appear　continuously.　Besides,　surface　defects　do　not　affect　the　reversal　mode,　but　they　do　decrease　the　coercivity　and　magnetic　hardening　effect,　hence　further　amplifying　the　difference　between　coercivity　and　its　theoretical　value.　The　simulation　results　are　strongly　supported　by　experimental　data.　This　study　extensively　examines　the　coercivity　of　M-type　ferrite　particles　produced　by　various　preparation　methods,　taking　into　account　experimental　observations　and　simulation　results.　Our　findings　can　serve　as　guidance　for　the　development　of　preparation　technology　for　permanent　magnet　nanoparticles　and　contribute　to　a　deeper　understanding　of　the　contradictions　in　coercivity　and　the　mechanisms　of　magnetization　reversal.　©　2024