Comment on Magnetic Relaxations of Antiferromagnetic Nanopar

We have carried out in ferritin the Field-Cooling method and data analysis proposed by Mamiya et al. (Phys. Rev. Lett. 88, 67202 (2002) at T = 5 K in order to check the time magnetic relaxation of these antiferromagnetic nanoparticles as a function of the

Commenton”MagneticRelaxationsofAntiferromagnetic

NanoparticlesinMagneticFields”

arXiv:cond-mat/0203088v1 [cond-mat.mes-hall] 5 Mar 2002E.delBarco,M.Duran,J.M.Hernandez,andJ.TejadaDepartamentodeF´ sicaFundamental,UniversidaddeBarcelona.Diagonal647,Barcelona,08028,SpainInarecentLetterMamiyaetal.presentedanewandelegantexperimentalproceduretocontroltheinitialField-CooledMagnetization,MFC,ofthemagneticrelaxationsofferritinunderstudy.Theyuseastep-by-stepcoolingfromT=35Ktotherelaxationexperimentaltemperature,T0=8-9K,whileamagnetic eld,H=H0T/T0,isappliedateachsteptoeliminatetime-andcoolingrate-dependencesofMFC.Fromthetemperaturebehaviorofthetimemagneticrelaxationcurves,recordedattemperaturesbetween8Kand9K,theyconcludedthatrelaxationinferritinisessentiallydominatedbyclassicalactivatedprocess.Moreover,theyanalyzedthe elddependenceofthemagneticviscosityextractedfromtheslopesoftherelaxationcurvesrecordedatdi erent nal elds,Hf=0,-H0.TheyfoundthatmagneticviscosityatT=8Kwasindependentof eld.Theauthors nallyconcludedthatmagneticrelaxationsofferritininmagnetic eldsaredominatedbyclassicalsuperparamagnetic uctuationsinthetemperatureregimewherethermallyassistedResonantSpinTunneling(RST)hasbeendiscussedinrecentyears.Letus rsttopointoutthatweagreewiththeinterpretationtheauthorsdooftheirresultsinthetemperaturerange8-9Kintermsofthermalactivatedrelaxation.However,itisofgreatimportancetoclarifythatthereisnotanypaper(fromthosecitedbyMamiyaetal.)inwhichactivatedquantumtunnelinghasbeenusedtoexplainrelaxationphenomenaof

ferritininthishightemperaturerange,T～8-9K.Frommagneticviscositymeasurements,Tejadaetal.reportedresonantquantumtunnelingatzero eldattemperaturesaround4-5K.Theseresultswerecon rmedbyLuisetal.bya.c.susceptibilitymeasurementsatlowtemperature.Itisclearthatadecreaseoftheviscositywithsmall eldsaroundtheblocking