Comment on Magnetic Relaxations of Antiferromagnetic Nanopar(2)

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

temperature,TB=8K,isduetothelargevariationofthenumberofunblockedmagneticmoments.Thesamephenomenawasobservedatlowertemperatureswithhigher elds[2],whereS(H)showsamaximumatagiven eldwhichdecreaseswhenthetemperatureincreases.Recently,newexperimentalresultsonmagneticrelaxationofferritincon rmbothpurequantumtunnelingbehaviorbetween0.1Kand～2Kandthermallyactivatedquantumtunnelingupto～5K[4],throughmagneticviscositymeasurementsintheabsenceofmagnetic eldsintherelaxationprocesses.

WehavealsousedboththeField-CoolingmethodanddataanalysisofMamiyaetal.

[1]tostudytimerelaxationinferritinatT=5K,whereRSTwasreported[2].WehaveusedFLUKAcommercialdilutedferritin.Wehavemeasuredtimemagneticrelaxationfromdi erentinitial elds,H0,totwo nal elds,Hf=0,-H0,fromH0=5OeuptoH0=800Oe.In gure1weshowthe elddependenceoftheratiobetweentheslopesoftherelaxationcurvesforHf=-H0andHf=0,normalizedwith1/2totakeintoaccountthedi erencebetweentheequilibriummagnetizations.Figure1isdividedintwozones:ForSHf= H0

SHf=0<1(whitezone),therelaxation

isfasteratzero eld,inagreementwithRSTinterpretation.Asonecanobservein gure1,atT=5K,zero eldrelaxationinferritinisbiggerthan eldrelaxationfor eldsbelow500Oe.Thebiggestdi erence(～80%)isobservedatH=5Oe,wheretheratiobetweenviscositiesisminimum,ingoodagreementwiththezero eldresonantwidth, Heff～5Oe,foundthroughLandau-ZenerrelaxationexperimentsbyDuranetal.[4].