IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010 1955

��� Phase CoPt-TiO�/FePt-TiO� Exchange Coupled Media With SmallSwitching Field

K. K. M. Pandey, J. S. Chen, and G. M. Chow

Department of Materials Science and Engineering, National University of Singapore, Singapore 119260

The effects of ��� CoPt-TiO�/FePt-TiO� bilayer and (Co�Fe��� �)��Pt��-TiO� (� � ��� at.%) multilayer exchange cou-pled media on switching field and magnetization reversal mechanism were studied. Results showed that both bilayer and multilayerexchange coupled media with out-of-plane anisotropy in the hard and soft layers were effective to reduce the switching field of singlelayer FePt-TiO� media. First order reversal curve (FORC) distribution indicated that switching field distribution in the bilayer and mul-tilayer exchanged coupled media was significantly narrowed down compared to the single layer and magnetization reversal was ascribedto domain formation followed by domain wall motion.

Index Terms—Exchange coupled media, high anisotropy, FePt.

I. INTRODUCTION

T HE phase FePt has been considered as a potentialcandidate for high density magnetic recording media due

to its large magnetocrystalline anisotropy constant (erg/cc). But it exhibits a large coercivity and therefore requires alarge switching field. Exchange spring media [1] and exchangecoupled composite (ECC) media [2] (It is further extended tomultilayer [3] and graded media [4]) consisting of magneti-cally hard and soft regions in each grain have been proposedto reduce the switching field. Strong interlayer exchange cou-pling between the hard and soft layers [5] and large satura-tion magnetization soft-layer [6] are required for the maximumbenefit of ECC media. FePt (deposited at high tempera-ture) and fcc FePt (deposited at room temperature) have beenused as a hard-layer and a soft-layer, respectively, in the ECCmedia [7]. However, the shape-induced in-plane magnetizationof fcc soft-layer would reduce the magnetization squarenessof composite media and undesirably generate noise during areading process. Hence, a soft-layer with magnetic anisotropyexceeding its shape anisotropy is required. Deposition of suc-cessive layer of FePt at gradually reduced temperature can con-trol the anisotropy. However, it is difficult to deposit subsequentlayers at gradually reduced temperature without affecting theprevious layers.

It is known that equiatomic CoPt and FePt posses or-dered phase, and extent of their ordering determine their mag-netocrystalline anisotropy. Ordering temperature of CoPt is sig-nificantly larger than that of FePt [8]. As a result, at fixed depo-sition temperature, ordering and hence the magnetocrystallineanisotropy can be manipulated by changing the relative compo-sition of Co and Fe in (Co Fe ) Pt ternary alloy. There-fore, in this experiment CoPt-TiO /FePt-TiO bilayer and(Co Fe ) Pt -TiO ( at.%) multilayer ex-change coupled media are fabricated. CoPt was chosen as

Manuscript received October 31, 2009; revised December 29, 2009; acceptedJanuary 21, 2010. Current version published May 19, 2010. Corresponding au-thor: K. K. M. Pandey (e-mail: msepkkm@nus.edu.sg).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2010.2042146

a soft-layer due to its relatively smaller and out-of-planeanisotropy. It has same atomic arrangement and close latticeparameter in the film plane as FePt and therefore allows for astrong exchange coupling between the hard and soft layers. Theeffects of bilayer and multilayer media on switching field dis-tribution, switching field reduction and magnetization reversalmechanism are studied.

II. EXPERIMENTAL METHOD

FePt-TiO (12 nm) (hereafter called single layer),CoPt-TiO (8 nm)/FePt-TiO (12 nm) bilayer and(Co Fe ) Pt -TiO ( at.%) multilayerexchange coupled media were fabricated using FePt-TiO andCoPt-TiO composite targets on MgO (001) single crystalat 500 C. TiO content was 15 at. % ( 30 vol. %) in thecomposite targets. During deposition, substrate was rotatingwith 20 revolutions per minute to achieve the uniformityin the film. The base pressure of sputtering chamber was

5 10 Torr. The films were deposited at 15 mTorrArgon. The microstructural and structural characterizationswere performed using transmission electron microscopy(TEM) and x-ray diffraction (XRD) using Cu K radiation,respectively. Hysteresis loops of all samples were measuredusing vibrating sample magnetometer (VSM). Magnetizationreversal was studied using first order reversal curve (FORC)measured from AGFM as it gives the information about theirreversible magnetization reversal [9], [10]. Total of 40FORC was measured with equal field spacing. During FORCmeasurement, samples were first saturated in the field of20 kOe. Thereafter field was reduced to reverse field ,and then magnetization was measured with increasing fieldstarting from reverse field to positive saturation field.The process was repeated at equal interval for decreasingfield until it reached to kOe. The nonzero value of FORCdistribution function, which shows only irreversible componentof magnetization, was derived from the mixed second-orderderivative . In orderto reduce the noise in the FORC, the raw data were smoothenusing Savitzky-Golay method using smoothing factor two andaveraging over five nearest points.

0018-9464/$26.00 © 2010 IEEE

1956 IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010

Fig. 1. X-ray � � �� scans of (Co Fe ) Pt -TiO for � � �� ��� �����

and ���.

III. RESULTS AND DISCUSSION

In order to investigate the effects of different compo-sitions of Co and Fe in (Co Fe ) Pt -TiO (where

) on structural and magnetic properties, 12 nm(Co Fe ) Pt -TiO films were deposited on MgO (001)single crystal for varying composition of Co and Fe. Fig. 1shows the x-ray scans of (Co Fe ) Pt -TiO ,where .

No peak was detected corresponding to TiO indicating itsamorphous nature in the film. For all samples only (001) su-perlattice peak and combined (200) and (002) peaks were de-tected for magnetic layer. The combined (200) and (002) peaksshowed shifting toward a lower value with increasing the Cocomposition. The smaller atomic size of Co (2.50 ) comparedto Fe (2.52 ) discard the possibility of increased interplanerspacing responsible for peak shifting. In order to investigate thereason behind the observed peak shifting towards a lowervalue, the ordering parameter (S), defined by equation,

(where and are the integrated peak in-tensity of (001) and (002) peaks, respectively) [11] was calcu-lated, and shown in Fig. 2. It was observed that with increasingthe Co doping in FePt, ordering parameter decreased monoton-ically. A slight enhancement in ordering was observed for pureCoPt-TiO compared to the (Co Fe ) Pt -TiO . The reasonfor this enhancement is currently unclear and requires furtherinvestigation. The reduction in ordering parameter with Co ad-dition indicated that addition of Co in FePt-TiO deterioratedthe ordering which favored the growth of (200) peak, and ledto appear the combined peak shifted towards the lower valueafter superposition with (002) peak. The reduced peaks inten-sity for CoPt-TiO is attributed to the poor crystallinity.

Magnetic measurement indicated that all samples for dif-ferent Co composition showed out-of-plane anisotropy. Theout-of-plane coercivity extracted from the out-of-planehysteresis loops was plotted in Fig. 2 along with orderingparameter. It showed that with increasing the Co composition,both and ordering parameter varied in similar fashionindicating that change in coercivity was attributed to the cor-responding change in ordering. The decreased from 9.6kOe to 1.2 kOe with increasing the Co composition from 0to 75 at.%, and further increased to 2.2 kOe for CoPt-TiO .

Fig. 2. Variation of ordering parameter and out-of-plane coercivity of(Co Fe ) Pt -TiO for � � �� �������� and ���.

Fig. 3. Out-of-plane hysteresis loops of CoPt-TiO , single layer, bilayer andmultilayer media.

Therefore, a multilayer media was fabricated by first depositing12 nm FePt-TiO , and thereafter successive layers of 2 nmeach were deposited by adding 25, 50, 100 and 75 at.% Coin (Co Fe ) Pt -TiO in the order of reducing coercivityalong the growth direction. Whereas the bilayer media was fab-ricated by depositing CoPt-TiO (8 nm) on FePt-TiO (12 nm),keeping the total thickness same as multilayer media. Fig. 3shows the out-of-plane hysteresis loops of CoPt-TiO , singlelayer, bilayer and multilayer media. The coercivity of singlelayer media (9.6 kOe) was reduced to 5.0 kOe in bilayerand multilayer media without deteriorating the magnetizationsquareness (ratio of remanent magnetization to saturationmagnetization ). It indicated that bilayer and multilayermedia were effective to reduce the switching field of singlelayer. However, the performance of multilayer media in this ex-periment was not better than the bilayer media as theoreticallypredicted [3], [4] and experimentally observed [12]. Probablythis may be due to the large anisotropy of soft layers.

Fig. 4(a)–(c) shows the FORC measurement of single layer,bilayer and multilayer media, and corresponding contour plotof FORC distribution function versus H and are shownin Figs. 5(a)–(c). Fig. 5(a) shows that the prominent sign of nu-cleation of irreversible switching started at reverse field

kOe (marked by line1) and annihilated atkOe (marked by line 2) with maximum of irreversible peak lo-cated around coercivity. However, in Fig. 5(b) and (c) corre-sponding to bilayer and multilayer exchange coupled media,the nucleation (annihilation) field was kOe ( kOe)and kOe ( kOe), respectively. The difference betweennucleation and annihilation field for single layer was 10 kOe,

PANDEY et al.: PHASE COPT-TIO /FEPT-TIO EXCHANGE COUPLED MEDIA WITH SMALL SWITCHING FIELD 1957

Fig. 4. FORC measurement of (a) single layer, (b) bilayer and (c) multilayermedia.

Fig. 5. Contour plot of FORC distribution for (a) single layer, (b) bilayer and(c) multilayer media.

which reduced to 4.9 kOe and 5.6 kOe for bilayer and mul-tilayer, respectively. It indicated that the switching field dis-tribution (SFD) narrowed down in the bilayer and multilayer

Fig. 6. Plane-view TEM images of (a) single layer and (b) multilayer media.

exchange coupled media compared to the single layer, whichis consistent with the theoretical prediction [13]. To quantifythe SFD, DC demagnetization curve was measured, and theSFD was extracted from the relation , where isthe full width-at-half maximum of versus H curveand is the remanent coercivity ( is obtained bytaking first derivative of DC demagnetization curve). The SFDof single layer, bilayer and multilayer without correcting for de-magnetization effect were 0.64, 0.34 and 0.33, respectively. Theplane-view TEM images of single layer and multilayer shownin Fig. 6 indicated the same grain size showing the columnargrowth of successive layer over the bottom layer. AmorphousTiO was presumably segregated to the grain boundaries (brightcontrast in the TEM images). Note that the amount of grainboundaries as shown in the images did not correspond to theconcentration of TiO in the target. Possible explanations in-clude low sputtering yield of TiO arriving at substrate and/ordiffusion of oxide to the surface of the deposited films [14].

The onset of irreversibility in bilayer and multilayer media atkOe, smaller than the soft-layer coercivity of CoPt (

kOe) in bilayer and softest layer coercivity of (Co Fe ) Pt(1.2 kOe) in multilayer showed that soft-layers were stronglyexchanged coupled with the hard-layer. However, once the nu-cleation starts due to applied field, an abrupt switching occur,which leads to reduce the switching field. A negative-positivepair of FORC distribution observed in the bilayer and multi-layer media (it can be seen above line 2 in the FORC contourin Fig. 5(b) and (c)) is a clear indication of domain formationfollowed by domain wall motion [15], which is responsible forthe reduced switching field of single layer [4].

IV. SUMMARY

In this paper switching field reduction and magnetizationreversal mechanism of bilayer and multilayer exchange coupledmedia were studied. Both bilayer and multilayer media reducedthe switching field of single layer media to approximately halfwithout affecting the out-of-plane magnetization squareness.FORC distribution curve showed that the magnetization re-versal in bilayer and multilayer media was initiated by domainwall formation followed by domain propagation and assist toreduce the switching field of hard layer. Bilayer and multilayerexchange coupled media were also effective to narrow downthe switching field distribution.

1958 IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010

ACKNOWLEDGMENT

This work was supported by A*STAR Singapore under Grant062 101 0021 and the Office of Naval Research under GrantN00014-06-1-0157.

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