Computational approach to finite size and shape effects in iron nanomagnets

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1、ComputationalapproachtofinitesizeandshapeeffectsinironnanomagnetsMichaelMcGuiganJamesDavenportComputationalScienceCenterBrookhavenNationalLaboratoryUptonNY11973JamesGlimmStonyBrookUniversitymcguigan@bnl.govjdaven@bnl.govglimm@ams.sunysb.eduAbstractWedev

2、elopandvalidateacomputationalapproachtonanomagnets.ItisbuiltonthespinwaveapproximationtoaHeisenbergferromagnetwhosepa-rameterscanbecalculatedfromafirstprinciplestheory(e.g.densityfunctionaltheory).Themethodcanbeusedforhighthroughputanalysisofavarietyof

3、nanomagneticmaterials.Wecomputethedependenceofthemagnetizationofanironnanomagnetontemperature,sizeandshape.Theapproachisappliedtonanomagnetsintherangeof432atomsto59millionatoms,asizewhichisseveralordersofmagnitudebeyondthescalabilityofdensityfunctiona

4、ltheory.1IntroductionNanomagnetsarepromisingmaterialsforhighdensitydatastoragedevices,astheyrepresentabitbyasingledotormagneticdomain.InsuchsystemsdatastoragearXiv:cond-mat/0701225v1[cond-mat.mtrl-sci]10Jan2007densitiesgreaterthan1012bits/in2maybeposs

5、iblewhilebeingthermallystableatroomtemperature[1].Importantdesignchoicesforsuchsystemsarethematerialcomposition,thenumberofatoms,andtheshapeofthedot.Allofthesechoicescanstronglyaffectthemagneticpropertiesofthesystem.However,anexperimen-talsearchofdiffer

6、entcombinationsistimeconsuming.Thus,itisimportanttoinvestigateefficientcomputationalapproachestocalculatingmagneticproperties.Onemightexpectsuchcomputationalapproachestoreducethecostofbringingnanomagneticstoragedevicestomarket.1Asignificanteffortisdevoted

7、tothecalculationofmagneticpropertiesofmateri-alsfromfirstprinciples.Thesestudiestypicallydescribezerotemperaturepropertiesofasmallnumberofatoms(lessthanathousand).Aeffectivespinmodelwithpa-rameterstakenfromsuchafirstprinciplemodelcanbesimulatedtopredictt

8、hebehavioroflargersystemsathighertemperatures.Thusfrombothfundamentalandpracticalpointsofview,anefficientcompu-tationalapproachtomagnetsisimportant.OnesuchapproachisgivenbytheHeisenbergmodelwiththeHamiltonian:XXH=−J(i−j)S~i·S~j+gµBB~ext·S~i+Hcry