Magnetization reversal of Co/Pd multilayers on nanoporous templates.

1.7 MB·English

by  Huang, Chien-Chih

Huangetal.NanoscaleResearchLetters2012,7:41
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NANO EXPRESS Open Access
Magnetization reversal of Co/Pd multilayers on
nanoporous templates
Chien-Chih Huang1, Chin-Chung Yu1*, Shih-Yuan Chen2, Yeong-Der Yao3 and Jun-Yang Lai4
Abstract
By making use of an e-beam deposition system, the [Co(2 Å)/Pd(10 Å)] multilayers were prepared on a Si(100)
15
substrate and anodized aluminum oxide [AAO] templates with average pore diameters of around 185, 95, and
40 nm. The mechanism of magnetization reversal of the Co/Pd multilayers was investigated. Wall motion was
observed on the Co/Pd multilayers grown on the Si substrate. A combination of wall motion and domain rotation
was found in the sample grown on the AAO template with a 185-nm pore diameter. For the samples grown on
the AAO templates with pore diameters of around 95 and 40 nm, the reversal mechanism was dominated by
domain rotation. The rotational reversal was mainly contributed from the underlying nanoporous AAO templates
that provided an additional pinning effect.
PACS: 75.30.Gw, magnetic anisotropy; 78.67.Rb, nanoporous materials; 75.60.Jk, magnetization reversal mechanisms.
Keywords: Co/Pd, porous anodized aluminum oxide, magnetization reversal
Introduction mediumnoise[11-13].Asaresult,themagneticnanoma-
In the last decade, patterned magnetic nanostructures terialfabricatedthroughnanoporousanodizedaluminum
have undergone a rapid development because of their oxide [AAO] is a good candidate in the pursuit of pat-
potential applications in future high-density magnetic ternedrecordingmedia.
recording media [1-4].Nowadays,the FePtfilmwithL1 Inthisarticle,wefabricatedtheCo/Pdmultilayersona
0
phasehasreceivedsignificantattentionowingtoitsexcel- Si(100)substrateandonnanoporousAAOtemplateswith
lent magnetic properties, presenting large values of the differentporediametersusinganelectronbeamevapora-
saturationmagnetization(approximately1,100emu/cm3), tor. We attempt to study the magnetization reversal and
theperpendicularmagnetocrystallineanisotropyconstant domainstructures of the Co/Pdmultilayersonnanopor-
(approximately108erg/cm3),thecoercivity,andthehigh oustemplates.
environmentalstability[5-7].Anotherchoicefortheper-
pendicularrecordingisferromagnetic/non-ferromagnetic Experimental details
multilayers.Forexample,Co/PtandCo/Pdarealsocandi- Beforefilmdeposition,theSi(100)substratewascleanedin
dates for the perpendicular recording media of the next acetone and alcohol solutions using an ultrasonic bath.
generation.Asthearealdensityoftoday’sharddiskdrives ThreetypesofnanoporousAAOtemplateswerepurchased
keeps approaching to the limit of current technology, from Whatman International Ltd (Maidstone, UK) and
exploringnewtechniquestoobtainhigherandhigherden- havedifferentporediameters:185,95,and40nm.Priorto
sity is inevitable [8-10]. There have been significant thegrowthofCo/Pdmultilayers,a10-nm-thickPdbuffer
research efforts focusing on the patterned magnetic layerwasgrownontheSisubstrateandAAOtemplates.
recordingmediumwithitsdensitylargerthan1Tbit/in2. The[Co(2Å)/Pd(10Å)] multilayerswerethendeposited
15
In addition, the percolated recording media has been onthePdbufferlayerbyane-beamsystemwithagrowth
investigatedbecauseofitshighthermalstabilityandlow- pressureofaround5×10-8Torr.Duringthedeposition,
thetemperatureofthesubstratesandthedepositionrate
werekeptat150°Cand0.1Å/s,respectively.
*Correspondence:[email protected] The surface morphology of AAO templates was
1DepartmentofAppliedPhysics,NationalUniversityofKaohsiung,Kaohsiung
examined using a scanning electron microscope [SEM]
811,Taiwan,RepublicofChina
Fulllistofauthorinformationisavailableattheendofthearticle
©2012Huangetal;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttribution
License(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,
providedtheoriginalworkisproperlycited.
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(JSM-6390, JEOL, Tokyo, Japan). The magnetic hyster- tip radius of curvature was less than 50 nm. It guaran-
esis loops were measured at room temperature using a teedamagneticresolutionbetterthan50nm.Becausethe
vibrating sample magnetometer (7407, Lake Shore, AAOtemplatesweretoothin(thicknessofapproximately
Westerville, OH, USA) with an applied field up to 1 T. 60 μm) for handling, they were first stuck on glass sub-
The domain structures were imaged using a magnetic stratespriortotheabovemeasurements.
force microscope [MFM] (XE-100, Park Systems,
Suwon, South Korea) under a lift mode operation. The
magnetic tip used for the MFM measurements was
purchased from Nanosensors Inc. (Neuchatel, Switzer-
land) with a catalog number of ppp-MFMR. The MFM
Figure2SEMimagesofCo/PdmultilayersonAAOtemplates
Figure1SEMimagesoftheAAOtemplateswithaveragepore withaverageporediameterof185nm.(a,b,c)Thetop-view
diameters.(a,b,c)Averageporediametersaround185,95,and40 SEMimage,bird’s-eyeviewSEMimagethatwasangledat30°,and
nm,respectively. cross-sectionalSEMimage,respectively.
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Results and discussion circles sketched in Figure 3b, c, d denoted the out-of-
The nanoporous AAO templates were first examined planehysteresisloopsofthe samplesgrownontheAAO
using the SEM. Figure 1 showed the SEM images of the templateswithporediametersofaround 185, 95,and 40
AAO templates with different pore diameters. From the nm,respectively.ForthesamplesgrownontheAAOtem-
SEM images, we got the average pore diameters of the plates, they displayed a two-step magnetization reversal.
AAO templates which were around 185, 95, and 40 nm The solidarrowsinFigure3b,c,dpointedthefirstmag-
forFigure1a,b,c,respectively.Figure2showedtheSEM netization reversal of the Co/Pd multilayers. For clarity,
imagesofCo/PdmultilayersgrownontheAAOtemplate wesuperimposedthehysteresisloopofCo/Ptmultilayers
with an average pore diameter of around 185 nm. The grown on the Si substrate onto the one grown on the
top-viewSEMimageoftheCo/Pdsample,Figure2a,indi- AAOtemplate witha 40-nmpore diameter, asshownin
cated that the Co/Pd film was grown on the interpore Figure3d.Notethat,inFigure3d,thecurvewithsolidcir-
region of the AAO template. Irregular Co/Pd grains can cles denoted the hysteresis loop of Co/Pt multilayers
be observed in the bird’s-eye view SEM image that was grown on the Si substrate. In the sample grown on the
angled at30°.The cross-sectional SEM image ofthe Co/ AAOtemplate,asshowninFigure3d,therewasasecond
PdmultilayerswasshowedinFigure2c. magnetizationreversalthattookplaceatthefieldaround
The out-of-plane hysteresis loop of the Co/Pd multi- -4,000 Oe. Such a two-step magnetization reversal was
layersgrownontheSi(100)substratewasshowedinFig- alsoobservedinthesamplesgrownontheAAOtemplates
ure 3a. A well-defined perpendicular anisotropy with a with185- and 95-nmporediameters, asshowninFigure
coercivity = 695Oe was obtained. The curves with open 3b,c,respectively.Unlikethefirstreversal,thefieldrange
Figure 3 The perpendicular hysteresis loops of the Co/Pd multilayers. (a) The multilayers grown on the Si substrate. (b, c, d) The
multilayersgrownontheAAOtemplateswithporediametersofaround185,95,and40nm,respectively.
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of the second magnetization reversal was broad, and the the size (or density) of nanopores of the AAO templates
magnetizationkeptonreversinguntilthesaturationstate was getting smaller (or higher). Therefore, the magneti-
wasreached. zation rotation that took place in the Co/Pd multilayers
Thenormalizedcoercivity,H (θ)/H (⊥),versustheout- can be realized by the pinning effect provided by the
c c
of-plane angle, θ, as shown in Figure 4 can also give us underlying AAO nanopores.
informationaboutmagnetizationreversal.Curves(a),(b), The magnetic domain image of Co/Pd multilayers
(c), and (d) in Figure 4 were for the Co/Pd multilayers grown on the Si substrate, shown in Figure 5a, displayed
grown on the Si(100) substrate and the AAO templates a mazed distribution. It supported a reversal mechanism
with pore diameters of around 185, 95, and 40 nm, of domain-wall motion. Short stripe-like domains
respectively. The dashed curves in Figure 4 were two formed by joining neighboring circular domains were
ideal distributions for the domain-wall mechanism and observed in the sample grown on the AAO template
the rotation only, obeying the Stoner-Wohlfarth [S-W] with a 185-nm pore diameter, as shown in Figure 5b.
model. A typical domain wall assisted behavior was Pettier magnetic domains were observed in the samples
observed in the Co/Pd multilayers grown on the Si sub- grown on the AAO templates with pore diameters
strate. For the sample grown on AAO templates with a around 95 and 40 nm, as shown in Figure 5c, d. Mani-
185-nm pore diameter, a combination of domain-wall festly, the domain size of the Co/Pd multilayers
motion and rotation modes was found. In the cases of decreased as the size (or density) of nanopores of the
AAOtemplateswith95-and40-nmpore diameters,the AAO templates was getting smaller (or higher). We can
S-Wrotationdominatedthereversalmechanism. infer that the domain size was limited by the underlying
There was no doubt that the first magnetization rever- nanoporous structure. This is also evidence that the
sal in Figure 3b, c, d was corresponding to the domain nanoporous structure of AAO templates provided a pin-
nucleation. The plots of normalized coercivity versus ning effect.
the out-of-plane angle suggested that the second magne-
tization reversal was a magnetization rotational process. Conclusions
Figure 3b, c, d showed that the magnetization in the We systematically investigated the magnetization rever-
second reversal process kept rotating till saturation. As sal of [Co(2 Å)/Pd(10 Å)] multilayers on the AAO
15
indicated by the open arrows in Figure 3b, c, d, the templates with different pore sizes. The magnetization
saturation field of the Co/Pd multilayers increased as reversal, domain size, and saturation field of the Co/Pd
Figure4Thenormalizedcoercivity,H(θ)/H(⊥),versustheangle,θ,ofCo/Pdmultilayers.Curve(a)isforthemultilayersgrownontheSi
c c
substrate.Curves(b,c,d)areforthemultilayersgrownontheAAOtemplateswithporediametersofaround185,95,and40nm,respectively.
Huangetal.NanoscaleResearchLetters2012,7:41 Page5of6
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Figure5ThemagneticdomainimagesoftheCo/Pdmultilayers.(a)ThemultilayersgrownontheSisubstrate.(b,c,d)Themultilayers
grownontheAAOtemplateswithporediametersofaround185,95,and40nm,respectively.
multilayers were strongly influenced by the pore dia- Authordetails
meter (or density) of underlying AAO templates. The 1DepartmentofAppliedPhysics,NationalUniversityofKaohsiung,Kaohsiung
811,Taiwan,RepublicofChina2DepartmentofMaterialsScienceand
underlying AAO template provided a pinning effect that
Engineering,NationalTsing-HuaUniversity,Hsin-Chu300,Taiwan,Republic
results in a rotational reversal and a limitation of ofChina3GraduateInstituteofAppliedScienceandEngineering,FuJen
domain growth. This study demonstrated the magnetiza- CatholicUniversity,Taipei24205,Taiwan,RepublicofChina4Departmentof
AppliedPhysics,NationalPingTungUniversityofEducation,PingTung900,
tion reversal of a percolated recording media with per-
Taiwan,RepublicofChina
pendicular anisotropy mainly dominated by the
rotational mechanism. Authors’contributions
C-CHfabricatedtheCo/Ptmultilayers.C-CYcarriedouttheMFMimagesof
thesamplesanddraftedthemanuscript.S-YandY-Dparticipatedinthe
measurementsofthehysteresisloops.J-YcarriedouttheSEMimagesofthe
Acknowledgements samples.Allauthorsreadandapprovedthefinalmanuscript.
ThisworkwassupportedbytheNationalScienceCouncilofTaiwanthrough
grantnumbers96-2119-M-390-001,97-2112-M-390-003,and98-2112-M-390- Competinginterests
003. Theauthorsdeclarethattheyhavenocompetinginterests.
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Received:13September2011 Accepted:5January2012
Published:5January2012
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Citethisarticleas:Huangetal.:MagnetizationreversalofCo/Pd
multilayersonnanoporoustemplates.NanoscaleResearchLetters2012
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