IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 2, FEBRUARY 2009 839

Development of ��� FePt:C (001) Thin Films With High Coercivity andSmall Grain Size for Ultra-High-Density Magnetic Recording Media

J. S. Chen�, J. F. Hu�, B. C. Lim�, Y. F. Ding�, G. M. Chow�, and G. Ju�

Department of Materials Science and Engineering, National University of Singapore, 117574 SingaporeData Storage Institute, 117608 Singapore

Seagate Technology, Pittsburgh, PA 15222 USA

The FePt:C films with different volume fractions of carbon were epitaxially grown on the CrRu(200) underlayer with Pt and MgOintermediate layers. The magnetic properties and microstructure of these FePt:C films were investigated. The FePt:C films grown onPt intermediate layer consisted of a continuous layer of FePt, with overlying granular FePt grains. The FePt:C films grown on MgOintermediate layer consisted of a granular FePt:C layer with overlying granular grains. The formation of the overlying granular FePtgrains was attributed to carbon diffusion to the surface, which resulted in the second nucleation of FePt. The different interface energyand surface energy of FePt on Pt and MgO intermediate layers caused the formation of initial continuous FePt layer on Pt intermediatelayer and initial granular FePt layers on MgO intermediate layer. The out-of-plane coercivity measured at room temperature increasedfrom 10.2 to 19 kOe when C concentrations increased from 0 to 15%. Further increasing the C contents to 20% and 25% caused thedecrease of coercivity to 17.2 and 14.8 kOe, respectively. The coercivity of the exchange coupling assisted FePt:C perpendicular mediawas indeed substantially reduced.

Index Terms—FePt, high coercivity, microstructure, perpendicular media.

I. INTRODUCTION

ordered FePt films with large magnetocrystallineanisotropy (7 10 erg/cm ) have attracted much atten-tion in recent years due to their potential application forultra-high-density recording media. The large noise of FePtmedia has, however, became the main obstacle to its applicationfor ultra-high-density media. In order to reduce the noise, apinned FePt medium was proposed to trap the domain walland decrease the domain size [1], [2]. Using this method, thenoise was indeed decreased, and signal-to-noise ratio (SNR)was increased. However, it remains very difficult to control thedensity and uniformity of the pinning sites. The conventionalapproach to noise reduction and increase of SNR involvesreducing grain size and narrowing grain size distribution.FePt (001) textured films with small grain size have beenobtained by postannealing FePt/SiO , [3] FePt/C [4], andFePt/MgO [5] multilayers at high temperature (above 550C), but the long time and high-temperature process were

not favored for practical application. Significant progress hasbeen made to fabricate FePt (001) textured films withcontinuous film structure at the temperature lower than 400C by the epitaxial growth on Ag, Cr/MgO, CrRu/Pt, and

CrRu/MgO underlayers [6]–[10]. It is therefore desirable toepitaxially grow granular FePt (001) textured film withsmall grain size on the underlayers at low temperature bydoping. Although the highly chemically ordered FePt filmswith perpendicular anisotropy were obtained by some of abovemethods at below 400 C, the coercivity was less than 8 kOe.For 1 Tbit/in , the required lowest coercivity was 12 kOe[11]. However, the writability of such high coercivity mediabecomes a key challenge. Heat assisted magnetic recording was

Manuscript received November 10, 2008. Current version published February11, 2009. Corresponding author: J. S. Chen (e-mail: msecj@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.2008.2010648

proposed to solve the problem. However, the system design ofthis technology was quite different from those used in currenthard disk industry. Recently, the exchange coupled composite(ECC) media, exchange spring media, and domain wall assistedmedia have been proposed to solve the problem by maintainingthe necessary thermal stability and reducing the switchingfield of the hard recording layer at the same time [12]–[15].Experimental evidence of domain wall assisted switching incomposite media was observed. However, to achieve highestgain of decrease in switching field, the magnetocrystallineanisotropy field of the hard layer has to be as large as 400 kOeand saturation magnetization as low as 200 emu/cm . No mate-rial with such kind of properties is known to exist at this time.Experimental works on Co/Pd multilayer-based ECC mediawere also reported, and the switching field was decreased whilemaintaining comparable thermal stabilities [16].

In this paper, investigations on the microstructure and mag-netic properties of FePt:C (001) textured composite films grownon CrRu underlayer with Pt and MgO intermediate layers arecarried out. Furthermore, the FePt:C-based exchange-couplingassisted (ECA) perpendicular media are fabricated and the mi-crostructure and magnetic properties of the ECA media wereinvestigated.

II. EXPERIMENTAL DETAILS

FePt and FePt:C films with structure of FePt:C (4–12 nm)/Ptor MgO(2 nm)/CrRu (30 nm)/glass were prepared by a mag-netron sputtering system with a base pressure of less than3 10 torr. The FePt:C films were cosputtered using FePtalloy and C targets. The volume fraction of C varied up to 20%by adjusting sputtering power of C and keeping the sputteringpower of FePt target as 15 W. The FePt:C, Pt, and CrRu layerswere deposited at 350 C and MgO layers at 90 C, usingAr pressures of 2 mtorr for both CrRu and Pt and 10 mtorrfor MgO and FePt:C layers. The films were characterized byX-ray diffraction and transmission electron microscopy (TEM).

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840 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 2, FEBRUARY 2009

Fig. 1. Plan-view transmission electron microscopy images of FePt:C films onPt intermediate layer with (a) 10 vol% and (b) 20 vol% C doping.

Fig. 2. Cross-section transmission electron microscopy images of FePt:C filmson Pt intermediate layer with 20 vol% C doping: (a) low magnification and (b)higher magnification.

The magnetic properties were measured by polar magneto-op-tical Kerr effect and vibrating sample magnetometer at roomtemperature.

III. RESULTS AND DISCUSSIONS

A. Microstructure and Magnetic Properties of FePt:C FilmsWith Pt Intermediate Layer

Fig. 1(a) and (b) shows the plan-view transmission electronmicroscopy images of FePt:C film with 10 and 20 vol% C grownon Pt intermediate layer, respectively.[17] For 10 vol.% C, gran-ular film was formed with some grain aggregation. The grainsize roughly followed a Gaussian distribution with a mean di-ameter 9.9 nm and a standard deviation of 2.3 nm. When Cincreased to 20 vol%, well-separated grains with relatively uni-form grain size were obtained. The average grain size and stan-dard deviation were 5.6 and 1.6 nm, respectively. The well-separated FePt grains and uniform size were also confirmedfrom the cross-section transmission electron microscopy imageas shown in Fig. 2(a). Note that the FePt grains appeared to bespherical. The layer with dark contrast between granular layerand CrRu underlayer was around 7-8 nm and thicker than 2 nm(as-deposited Pt layer). It has been reported that the Cr diffusioncould be effectively blocked by a thin Pt buffer layer.[18], [19]This layer with dark contrast should consist of the Pt buffer layerand FePt continuous layer, which was also confirmed by subse-quent experiments. The regions with light contrast between thecontinuous FePt layer and overlying granular FePt grains couldbe attributed to carbon. Fig. 2(b) shows the image of Fig. 2(a)at higher magnification. At the tangent of FePt grains in contactwith the FePt continuous layer, epitaxial growth of FePt (001)textured grains occurred.

Fig. 3. Polar Kerr hysteresis loops of the FePt:C films on Pt intermediate layerwith (a) 10 vol% C doping and (b) 20 vol% C doping.

Fig. 4. Cross-section transmission electron microscopy images of FePt:C filmson Pt intermediate layer with 15 vol% C doping and the thickness (a) 4, (b) 12,and (c) 20 nm.

The polar Kerr hysteresis loops of FePt:C films with 10 vol%and 20 vol% C doping are shown in Fig. 3. The hysteresis loopsindicated the perpendicular magnetic anisotropy of the FePt:Cfilms. The slopes of perpendicular loops at coercivity remainedvery steep and showed no obvious change with C concentration.These results indicated exchange coupling did not decrease evenfor the well-isolated FePt grains with 20 vol% C concentration.[20]

In order to clarify the origin of large exchange coupling forthe well-isolated FePt grains and further confirm the nature ofthe dark contrast layer between CrRu underlayer and FePt:Cgranular layer, FePt:C films with 15 vol% C and differentthicknesses were deposited on Pt (2 nm)/CrRu (30 nm)/glass atsubstrate temperature of 350 C. The cross-section transmis-sion electron microscopy images of FePt:C films with differentthicknesses are shown in Fig. 4. When the FePt:C film was 5nm thick, it appeared continuous and good FePt (001) epitaxialgrowth was clearly seen, indicated by lattice fringes of FePt(001) plane. X-ray photoelectron spectroscopy investigationshowed that C diffused to the film surface. Increasing the thick-ness of FePt:C film to 12 nm, 4–5 nm of FePt grains weredeposited. The continuous layer ( 7–8 nm thick in figure) withdark contrast consisted of two parts: 2 nm Pt layer and 5–6 nmFePt layer. Further increasing the thickness of FePt:C film to20 nm, the thickness of the continuous layer with dark contrastremained unchanged; only granular FePt grains grew in thefilm normal direction. Associated with the above transmissionelectron microscopy results, the steep slope of hysteresis loopof FePt:C film at coercivity can be interpreted as follows:the FePt grains in the continuous layer had strong exchangecoupling. This continuous layer also strongly coupled with theoverlying granular FePt grains. The magnetization of granularFePt grains switched together with the continuous FePt layerwhen an external magnetic field was applied.

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CHEN et al.: DEVELOPMENT OF FEPT:C (001) THIN FILMS 841

Fig. 5. Cross-section transmission electron microscopy images of the 4 nmFePt:C films (15% C concentration) on MgO intermediate layer.

B. Microstructure and Magnetic Properties of FePt:C FilmsWith MgO Intermediate Layer

For practical application of FePt film as recording media,it is necessary to modify the properties of interface, on which theFePt film epitaxially grows in three-dimensional island modewith carbon segregating to the boundaries of FePt grains in-stead of the layer-by-layer mode. It is known that island growth(Vomer–Weber growth) is preferred when the sum of the sur-face energy of the film material and interface energy was largerthan the surface energy of the substrate. The surface energyof MgO is 1.1 J/m , and FePt and Pt were 2.9 J/m . Fur-thermore, the interface energy between FePt and MgO shouldbe larger than that between FePt and Pt since there are somedangling bonds of the surface of sputter-deposited MgO layer,which will react with Fe or Pt atoms, and the reaction (ionicbond) is stronger than the bonds between Pt layer and FePt layer(metal bond). It is, therefore, expected that the granular FePt:Cnanocomposite film can be obtained when using MgO interme-diate layer. FePt:C films (15% C concentration) with the thick-ness of 4 nm were deposited on CrRu underlayer with 2 nmMgO intermediate layer. Its cross-section transmission electronmicroscopy image is illustrated in Fig. 5. It was clearly shownthat the FePt:C layer was well-isolated granular structure andthe grain size was around 8 nm, confirming that by changingthe intermediate layer, island growth of FePt:C film may occur.

The microstructure and magnetic properties of FePt:C films(12 nm) with various C concentration on MgO intermediatelayer were investigated.[21], [22] The cross-section transmis-sion electron microscopy images of FePt:Cfilms with variousC concentrations are shown in Fig. 6. Without C doping, eventhough the grain boundary was not clear in the image, it wasobvious that it was island growth and the grain size was largerthan 10 nm [Fig. 6(a)]. For the sample with 15% C concentra-tion, the FePt:C film consisted of two layers [Fig. 6(b)]. Onelayer was epitaxially grown on MgO intermediate layer. In thislayer, the isolated FePt grains with carbon at grain boundarieswere found. The other layer consisted of individual sphere-likeFePt particles above the first layer of FePt:C film. Increase of theC concentration to 20% resulted in the increase of the numberof FePt particles in layer 2 [Fig. 6(c)]. With further increase inC concentration of 25%, FePt:C films could still be divided intotwo layers [Fig. 6(d)]. The grains in the first layer epitaxiallygrown on MgO intermediate layer became smaller. The amountof FePt particles in the second layer increased. Although thethinner FePt:C films formed a one-layer granular structure, withincrease of the thickness of the FePt:C films the second nucle-ation still occurred and two granular structured films formed.

Fig. 6. Cross-section TEM images of FePt:C films grown on MgO intermediatelayer with carbon concentration of (a) 0, (b) 15 vol%, (c) 20 vol%, and (d) 25vol%.

The formation of such microstructure of FePt:C films can be in-terpreted as follows: At the initial stage of FePt:C film growth,FePt preferred to epitaxially grow on MgO/CrRu and part ofC diffused onto the surface. The diffusion of C to the surfaceincreased with the increasing thickness of FePt:C film. Withthe increasing coverage of C on the surface, subsequent de-posited FePt atoms would renucleate on the diffused C coverage,and sphere-like FePt particles formed to reduce the surface en-ergy. For the sample with higher C concentration, the C contentremaining in the first layer would increase, and thus granularstructure with distinguished boundaries formed. Furthermore,high-resolution TEM (HRTEM) investigation showed that layer1 of FePt:C films was of phase and layer 2 consisted of bothface-centered cubic (fcc) and phase. For example, particle“1” in Fig. 6(d) was L1XS FePt single crystalline particle andparticle “2” was fcc polycrystalline particle. The formation of

phase FePt in layer 1 was due to the strain-induced phasetransformation. The formation of FePt particles in layer 2may be attributed to that FePt nucleated on some FePt parti-cles without C coverage, followed by the homoepitaxial growth.The formation of fcc FePt particles was due to FePt nucleatedon C coverage, and thus epitaxial growth was damaged.

The out-of-plane hysteresis loops of FePt:C films with var-ious C concentrations are shown in Fig. 7 [22]. The square-ness ratio (remanent to saturated magnetization) of all loops was

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842 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 2, FEBRUARY 2009

Fig. 7. Polar Kerr hysteresis loops of FePt:C films with various carbon concen-trations grown on MgO intermediate layer.

one, which was required to reduce the dc noise of perpendicularmedia. With the increase of C concentration from 0 to 10%, theslope of hysteresis loop at coercivity slightly increased; then theslope decreased with further increase of the carbon concentra-tion to 15 vol%. Further increasing the C concentrations to 20%and 25%, the slope remained constant. The out-of-plane coer-civity and nucleation field of the FePt:C films with various Cconcentrations are summarized in Fig. 10. With C concentra-tions ranging from 0 to 10%, the coercivities were around 10kOe. As the C concentration increased to 15%, the coercivitysubstantially increased to 19 kOe. Further increasing the C con-centrations to 20% and 25% caused the decrease of coercivity to17.2 and 14.8 kOe, respectively. Note that the hysteresis loopswere not saturated, and the real coercivity of these FePt:C filmsshould be larger than those currently measured at the maximumapplied field of 30 kOe. It is worth noting that as C concentra-tion was 25%, the hysteresis loop showed a small positive kink.A similar hysteresis loop was reported in the film with struc-ture CoCr/Ru/CoCrPt-SiO and was attributed to the magneticreversal of magnetically soft CoCr layer [23]. Therefore the pos-itive kink of hysteresis loop of FePt:C film with 25 vol% C wasconsidered to be the magnetization reversal of magnetically softfcc FePt phase in layer 2 of the FePt film.

For high-density perpendicular magnetic recording, largenegative nucleation field is desirable. The nucleation field isgenerally defined as the reversing field at which the magne-tization starts to drop from its value at saturation. The more“negative” the nucleation field, the more stable the remanentmagnetic state (stored data) should be since a larger reversingfield is required to alter the magnetization. Here the nucleationfield is defined as the field that the saturated magnetizationdecreased by 5%. The negative nucleation field of FePt:C filmsincreased from 4.4 to 9.0 kOe as C concentrations varied from0 to 15%. Further increasing C concentration to 20% gave riseto the decrease of the negative nucleation field to 6.8 kOe. Thenucleation field of FePt film with 25 vol% C was 0 kOe, wherethe fcc FePt phase started to reverse. The decrease of the slopeof hysteresis with C concentrations above 10 vol% indicatedthe decrease of the exchange coupling. It is well establishedthat the coercivity increased with the decrease of the exchangecoupling. As the C concentration increased to 15%, the increaseof the coercivity was due to the exchange decoupling of FePtgrains. With carbon concentration further increased from 15%

Fig. 8. Out-of-plane coercivity and nucleation field of FePt:C films with var-ious carbon concentrations on MgO intermediate layer.

Fig. 9. (a) Cross-section transmission electron microscopy image. (b) Plan-view images of FePt-C film on MgO intermediate layer with carbon concentra-tion of 15 vol% and FePt sputtering power of 50 W.

Fig. 10. Polar Kerr hysteresis loops of FePt:C films on MgO intermediate layerwith carbon concentration of 15 vol% and FePt sputtering power of 15 and 50 W.

to 25%, the decrease of coercivity may be due to the decreaseof FePt grain size and the existence of the soft fcc FePtphase, which were not fully exchange decoupled with FePtgrains [16]. The existence of soft fcc FePt phase and FePtwith smaller grain size had a lower energy barrier and wouldresult in lower reversal field. Therefore, the negative nucleationfield decreased with carbon concentration increasing from 15%to 25%.

Although the high coercivity and (001) textured FePt:C filmswere fabricated at a temperature of 350 C, the microstructureof the FePt:C films such as coexistence of fcc FePt and FePtparticles and formation of two layers of FePt:C films would

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CHEN et al.: DEVELOPMENT OF FEPT:C (001) THIN FILMS 843

still not be favored for high-density perpendicular recording.Sputtering condition such as the sputtering power was variedto optimize the microstructure. High sputtering power of FePttarget would increase the deposition rate. Fe or Pt atoms there-fore had less time to move on the substrate, and small grainwas expected. Furthermore, the carbon diffusion to the surfacealso might be restricted due to less diffusion time; one-layercolumnar growth is expected. The cross-section TEM imageof the sample with carbon concentration of 15% deposited bycosputtering FePt and carbon targets with 50 and 290 W isshown in Fig. 9(a). It was evident that only one layer of well-iso-lated columnar FePt grains with 7.5 nm in diameter was epi-taxially grown on the MgO intermediate layer. The plane-viewTEM images [Fig. 9(b)] of the sample deposited at high sput-tering power showed that the grain boundaries were more dis-tinguished, indicating that the exchange coupling was furtherreduced. The crystallographic structure and magnetic propertyof the sample deposited at high sputtering power also were ex-amined. X-ray diffraction results showed that the (001) tex-tured FePt:C film remained. The coercivity slightly decreased to14.4 kOe compared with the sample deposited at low sputteringpower (15.1 kOe) measured at the maximum applied field of 20kOe (Fig. 10).

C. Exchange Couple Assisted (ECA) FePt:C PerpendicularMedia

Although high coercive FePt:C films with small grain sizehave been obtained by using an MgO intermediate layer, ahigher writing field was required to write data on such media.In order to reduce the switching field of the FePt:C film andmaintain its thermal stability at the same time, exchangecoupled assisted perpendicular media were fabricated thatconsisted of an FePt:C hard layer deposited on an MgO in-termediate layer at 350 C and FePt:C soft layer deposited atroom temperature [24]. Here, FePt:C films were fabricated bysputtering FePtC (with 21 vol% of C) composite target. Fig. 11shows the out-of-plane perpendicular hysteresis loops of thesingle hard layer FePt:C film and the ECA media with differenthard and soft layer thickness. The coercivity c of the singleFePt:C layer was about 10 kOe (10 nm) and 6.6 kOe (7 nm),respectively. After introducing the top soft FePtC layer, thevalues of c reduced to about 7 and 4 kOe, respectively. Forthe ECA media with magnetically soft FePt overlayer, a smallkink around the applied field was observed, indicatingpart of the soft FePt overlayer was independently reversed andnot exchange coupled with the FePt hard magnetic layer.

The angle dependences of the coercivity and the remnant co-ercivity were measured for ECA media as shown in Fig. 12.To measure the angular dependence of the remnant coercivity,the sample was saturated at every angle by an applied magneticfield of 29 kOe; the field was then decreased to zero to mea-sure the remnant moment. The remnant moment as a functionof a magnetic field was measured for each angle by applying apositive magnetic field with a step of 400 Oe, until the appliedfield reached 29 kOe. The angle dependences of coercivity andremnant coercivity of ECA media were consistent with the sim-ulation results of Victora [12]. For the FePt single hard layer,

Fig. 11. Out-of-plane hysteresis loops of (a) FePt:C 7 nm (350 C), (b) FePt:C7nm (350 C)/FePt:C 3 nm (RT), (c) FePt: C 10 nm (350 C), and (d) FePt:C10 nm (350 C)/FePt:C 5 nm (RT).

Fig. 12. Angle dependence of the coercivity and the remnant coercivity ofFePt single layer (10 nm) and ECA media with structure of FePt:C 10 nm(350 C)/FePt:C 5 nm (RT).

though the minimum remnant coercivity was at around 45 , theangle dependence of remnant coercivity tremendously deviatedfrom the Stoner–Wohlfarth (S-W) model. The angular depen-dence of coercivity was consistent with that of both exchangedecoupled S-W model and ECC model. This may be attributedto deteriorated (001) texture [rocking curve of FePt (001) peakwas around 12 ] and not fully exchange-decoupled grains.

The cross-section TEM image of a representative ECAmedium is shown in Fig. 13. The ECA medium was composedof two granular layers, and the grain size of the bottom hardFePt:C layer was larger than that of the soft overlayer. It can beseen that the soft FePt grains were located both on top of thebottom FePt grains and the grain boundaries. This indicatedthat the soft fcc FePt layer did not epitaxially grow on theFePt hard layer and the spherical particles were formed toreduce the surface energy due to the diffusion of carbon ontothe surface of FePt hard layer. HRTEM showed that some fccgrains nucleate on some positions without carbon coverage andepitaxially grew on FePt (001). In this case, the exchangecoupling between the fcc grains and grains was strong.Some grains directly nucleated on the carbon coverage, andthus fcc FePt (111) grains formed. In this case, the exchangecoupling between the fcc magnetically soft grain and hardgrains was weak. The strength of exchange-couple dependedon the thickness of the carbon coverage. When the FePt hard

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