The effect of Al 2 O 3 additive on the microstructure and magnetic properties of Co 75 Cr 13 Pt 12 /Cr thin films

Phys. Status Solidi A 210, No. 7, 1400–1406 (2013) / DOI 10.1002/pssa.201228795 p s sa

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applications and materials science

The effect of Al2O3 additive on the
microstructure and magnetic propertiesof Co75Cr13Pt12/Cr thin films
Mohammad Almasi-Kashi*,1,2, Elham Jafari-Khamse**,1, Abdolali Ramazani1,2, and Hamidreza Almasi-Kashi3

1Department of Physics, University of Kashan, 8731751167 Kashan, Iran2 Institute of Nanoscience and Nanotechnology, University of Kashan, 8731751167 Kashan, Iran3Department of ECE, University of Tehran, 14174 Tehran, Iran

Received 24 November 2012, revised 21 February 2013, accepted 22 February 2013

Published online 15 April 2013

Keywords annealing, magnetic properties, magnetron sputtering, alloys, microstructure

*Corresponding author: e-mail [email protected], Phone/Fax: þ00 983 615 552 935** e-mail [email protected], Phone/Fax: þ00 983 615 552 935

(Co75Cr13Pt12)100�x (Al2O3)x (x¼ 3, 5, 10, and 13wt.%) thin

films were deposited onto the Cr underlayer by an RF

magnetron sputtering technique. The effects of annealing

treatment on the structure and magnetic properties of the

amorphous film with 13wt.% Al2O3 content were investigated.

Increasing of aluminum oxide between 3 and 13wt.% causes a

reduction in magnetization of the films due to a decrease in the

magnetic moments in the layer. On annealing the paramagnetic

to ferromagnetic conversion of the filmwith 13wt.% aluminum

oxide occurred. Since aluminum played a sacrificial role for

oxidation of magnetic grains this prevented further reduction

of the magnetization. It was concluded that a nonmagnetic

element halo formation around the magnetic grains reduces

intergranular coupling.

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1 Introduction The Co-based alloy films have widelyovercome the use of nanocomposites as they show exclusiveproperties such as high coercivity and low intergranularcoupling. Hence, they are one of the most promisingcandidates for the ultrahigh density recording media [1–5].There are currently several methods to obtain high coercivitythrough magnetic-grain isolation. In order to reduce inter-granular coupling, magnetic grains must be segregated.Cr as underlayer causes segregation of the magneticgrains because of columnar growth on the substrate [6–8].Supplementary processes such as annealing, also improvesmagnetic properties of the films without significant effecton the structure. In our previous work [9] the granular hcp(CoCrPt)100�x(Al2O3)x thin films with Si (100) substrateswere fabricated by a sputtering technique followed by anannealing treatment. However, in the present work the effectof the Cr underlayer was investigated.

Many studies were performed on the study of magneticproperties and microstructure of the CoCrPt/ceramic thinfilms with potential application on ultrahigh density record-ing media. Li et al. [10] studied the influence of annealing onthe structural and magnetic properties of C/CoCrPt/CrTi

trilayer recording media. Yang et al. [11] investigated themagnetic properties of sputter-deposited and annealedCoCr/CoCrPt recording media. Fischer et al. [12] reportedmagnetization reversal behavior of nanogranular CoCrPtalloy thin films by transmission X-ray microscopy. In spiteof many recently published papers on characterization ofCoCr-based alloy thin films, more work is needed to make itpossible to obtain optimum characteristics. However, theaim of the present work is to study the effect of Al2O3

additive on the microstructure and magnetic properties ofCo75Cr13Pt12/Cr thin films.

2 Experimental method The (Co75Cr13Pt12)100�x-(Al2O3)x (x¼ 3, 5, 10, and 13wt.%) thin filmswere depositedonto Cr underlayer on the Si substrate with a gas mixture ofO2–Ar (5:95) by an RF magnetron sputtering techniqueat room temperature. The aluminum oxide content wasadjusted and estimated from calibration data for the powerdeposition rates of the magnetrons. To control the grain sizeand grain isolation of the magnetic layer, a gas mixture flowwas adjusted by twomass flowmeters. The base pressure was5.6� 10�6 Torr and Ar pressure during the sputtering was


Phys. Status Solidi A 210, No. 7 (2013) 1401

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7.6� 10�3 Torr. In order to improve the crystalline structureof CoCrPt (Al2O3) thin films, a post annealing treatment wasperformed at ambient pressure under Ar (99.999%) gas flowfor 15min at temperatures of 500, 600, 700, 800, and 900 8C.The heating and cooling rates were typically 250 8C/min.

X-ray diffraction (XRD) patterns were recorded in theu–2u mode with variation of 2u in the range of 35–558 witha Siemens D5000 powder diffractometer with the Karadiation of copper (l¼ 1.5406 A). The microstructuralanalysis was performed using transmission electronmicroscopy (TEM, JEOL-200CX), X-ray microanalysis(analytical JEOL) and atomic force microscopy (AFM).The magnetic measurements were performed by a LakeShore 7300 vibrating sample magnetometer (VSM) system.The AFM and magnetic force microscopy (MFM) imageswere obtained by tapping mode of a digital instrument (di),Nanoscope 3D controller Dimension 3100.

3 Results and discussion3.1 As-deposited films Figure 1 shows XRD pat-

terns of the CoCrPt thin films with various contents ofaluminum oxide in the range of 3–13wt.%. The unit cell andthe crystallographic texture of the as-deposited filmsobviously vary with the addition of aluminum oxide content.It is clearly seen that increasing nonmagnetic elementcontent leads to a decrease in the peak intensity of triplethcp Co (002), (100), and (101). Also, in the aluminum oxidecontents higher than 10wt.% the crystalline structure ofthe CoCrPt/Al2O3 layer was changed to an amorphous oneand the only clear reflection is that for the preferred 110orientation of Cr. This behavior is attributed to mismatchingbetween amorphous aluminum oxide and hcp crystallineCoCrPt structures. The Scherrer equation [13] was used tocalculate the average crystallite size (D, average diameter ofthe crystal planes). To estimate D from the growth directionof 002, deconvolution of the Cr (110) and Co (002) peakswas performed.

The results show that with addition of 5wt.% aluminumoxide to CoCrPt, the average crystallite size decreases

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Co (101)CoCrPt-13 wt.% Al2O3

CoCrPt-5 wt.% Al2O3

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Co (100)

Figure 1 XRDpatternsofCoCrPtfilmwith0, 5and13wt.%Al2O3

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(23 nm in comparison with 12 nm). Also, it significantlydecreases with more addition of Al2O3 due to formationof amorphous structure. Since the difference of theinplane atomic density of CoCrPt (101) and Cr (110)(�0.171 atoms A�2) is very small, Cr can provide amatchingbase for good nucleation and growth of the crystallineCoCrPt separated perhaps by amorphous nonmagneticAl2O3 [14].

According to the bright-field TEM images in Fig. 2, theas-deposited CoCrPt film has high crystallinity with agranular structure that changes to the nearly amorphousstructure during the oxidation process. The selected-areaelectron diffraction (SAED) patterns in the inset of Fig. 2aand b show the face-centered cubic (fcc) and nearlyamorphous structures of the aluminum and aluminum oxidefilms, respectively. The wide ring in the SAED pattern ofaluminum oxide shows the existence of small crystallitesthorough the amorphous film. Comparing the bright-fieldTEM images of CoCrPt and CoCrPt–Al2O3 indicates thataddition of low contents of a nonmagnetic element (Al2O3,up to 3wt.%) to CoCrPt alloy does not change the averagegrain size (�7 nm) and the number of grains with darkcontrast on the alloy (see Fig. 3a and b). However, moreaddition (above 3wt.%) causes a decrease in the averagegrain size and the relative number of strongly diffractinggrains with dark contrast (Fig. 3).

Adding 13wt.% Al2O3 caused almost all crystallinegrains to disappear. This behavior was confirmed by SAEDand XRD patterns (Fig. 1 and the insets of Fig. 3).

The effect of aluminum oxide addition on thesurface structure of CoCrPt alloy films is studied by AFM

Figure 2 Bright-field TEM images of (a) aluminum and (b) alumi-num oxide films (SAED patterns are presented in the insets).


1402 M. Almasi-Kashi et al.: Microstructure and magnetic properties of Co75Cr13Pt12/Cr thin filmsp

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Figure 3 Bright-field TEM image and diffraction pattern of(a) CoCrPt, (b) CoCrPt-3wt.%Al, (c) CoCrPt-5wt.%Al, (d)CoCrPt-10wt.%Al, and (e) CoCrPt-13wt.%Al films.

Figure 4 AFM image and profiles of the CoCrPt films with Alcontent of (a, f) 0wt.%, (b, g) 3wt.%, (c, h) 5wt.%, (d, i) 10wt.%and(e, j) 13wt.%.

Table 1 The obtained parameters from AFM images.

aluminum oxidecontent (wt.%)

RMS(nm)

skewness

0 2.31 –0.23 2.61 –0.465 2.17 –0.2810 2.17 –0.0713 2.03 0.02

images and displayed in Fig. 4. As can be seen, the surfaceroughness increases with addition of 3wt.% amorphousaluminum oxide to a maximum value and then reduces,which results from competition between segregation of themagnetic grains by Cr migrated from the underlayer to thegrain boundaries and/or nonmagnetic aluminum oxide andreduction of the grain size.

The addition of low contents of aluminum oxide (up to3wt.%) leads to segregation of magnetic grains [15–18] andan increase surface roughness while it reduces in the sampleswith higher contents due to formation of amorphous phase.The results are tabulated in Table 1. The presented data inTable 1 are the average of measured parameters on threepoints of the samples. It then may be said that they describewhole characteristics of the surface. Skewness describesthe asymmetry between peaks (the points with heights morethan the average height) and valleys (the points with heightsless than average height) number on the film surface [19].Its value for the films containing 5 and 10wt.% Al2O3 andsame root mean square (rms) indicates more valleys of theformer, which represents segregation between magneticgrains.

Magnetization reduces with increase in Al2O3. Additionof 13wt.% aluminum oxide leads to formation of super-paramagnetic and/or paramagnetic phase. The obtainedcoercivity shows the same treatment of surface roughness


(Fig. 5b). Initially the increase in coercivity is a result ofaggregation of amorphous aluminum oxide on the grainboundaries, which causes segregation of the grains, whichthereby increases surface roughness (see Fig. 3b). HigherAl2O3 added to the film leads to a remarkable reduction inthe magnetic grain size and the surface roughness and thena decrease in coercivity.

Magnetization may change by domain-wall motion and/or moment rotation. For thin films with a constant saturation

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CoCrPt CoCrPt-3wt.% Al2O3

CoCrPt-5wt.% Al2O3

CoCrPt-10wt.% Al2O3

CoCrPt-13wt.% Al2O3

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b)

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Al2O3 Content (wt. %)

2.0

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2.4

2.6

2.8

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RM

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Figure 5 (a) Hysteresis loops of the CoCrPt films with variouscontentsofAland(b)coercivityandRMSasa functionofaluminiumoxide content.

magnetization (Ms), coercivity caused by domain wallmovement (Hmov

c ) can be written as follows [20]:

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Hmovc ¼ 1

2Ms

Aexp2

Dtþ KvD

2tþ Dt þ 2D2

Dþ tð Þ2pM2

s

!rrms;

(1)

Figure 6 MFM images of CoCrPt, films with Al2O3 content of(a) 0wt.%, (b) 3wt.%, (c) 5wt.%, (d) 10wt.%, and (e) 13wt.%.

in which Aex, D, t, kn, and rrms are exchange constant,domain-wall thickness, film thickness, in-plane volumeanisotropy constant, and rms local slope, respectively.Therefore, for the films with the same thickness the roughersurface has a larger coercivity. However, rotationalcoercivity is independent of the surface roughness due toidentical values of demagnetizing factor in the easy and hardaxis directions.

In addition, local surface roughness induces inplanemagnetic dipoles and a demagnetizing field. Therefore, thisfield changes domain-wall thickness and domain size andcauses domain-wall pinning, which increases coercivity[21]. In the other words, decreasing the surface roughnessreduces the thickness of the domain wall and affects thedomain-wall motion, thereby decreases coercivity [20].The experimental data also indicates increasing of thecoercivity of thin films with increase in surface roughness

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[22–24]. However, intragranular migration of Cr atomstowards the periphery of the grains and the leakage of Cratoms from the underlayer into the grains causes theformation of a non- or weakly magnetic shell surroundingeach grain and increasing segregation between magneticgrains, and thereby reduces intergranular coupling. Asmentioned before [19], the presence of the shell cannot bedetected by diffraction techniques because of the largeindependency of the lattice parameters of CoCr alloys onthe layer composition.

The dark and bright regions in the MFM images presentmagnetic clusters in which the magnetic moments arealigned in different directions. The magnetic cluster size anddistribution is significantly affected with the addition ofaluminum oxide (see Fig. 6). The MFM images in Fig. 6show reducing magnetic interaction between magneticdomains with increasing Al2O3 content. As mentionedabove, this behavior is a result of reducing the magnetizationwith increasing nonmagnetic grains and increasing thesegregation between magnetic grains.

3.2 Annealed films In order to study the effect ofthermal treatment on the structure and magnetic behavior



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Figure 8 Bright-field TEM image and diffraction pattern ofannealed CoCrPt-13wt.% Al2O3 film at (a) 500, (b) 600, (c) 700,(d) 800, and (e) 900 8C.

of the films, the film with 13wt.% Al2O3 was annealed at500–900 8C with 100 8C interval for 15min.

The XRD patterns in Fig. 7 show increasing intensity ofcrystalline directions (100), (002), and (101) of Co withannealing up to 900 8C. The (100) and (101) orientations areindicators of the easy axis being nearly in the plane of thefilm. The calculated ratio of (100) and (101) to (002)orientations indicates rotation of the easy axis toward thefilm plane, which is the favored magnetic direction (seeTable 2). Calculation of crystallite size from the Scherrerequation shows that increasing of annealing temperaturecauses an increase in grain size.

Bright-field TEM images and SAED patterns of theannealed films at 500, 600, 700, 800, and 900 8Care shown inFig. 8. Due to the recrystallization process, with increase inannealing temperature the crystallinity, the average grainsize and the relative number of strongly diffracting grains(regionswith dark contrast) increase. This result is consistentwith the reducing width of the XRD peak (an indicator ofincreasing grain size of the magnetic component) withincreasing annealing temperature, while the width of thechannels (as formation of segregation between grains [25])decreases with increasing of annealing temperature andcompletely disappears at 900 8C. As shown in Fig. 9a theparamagnetic phase of the film with 13wt.% Al2O3

completely converts to a ferromagnetic phase after anneal-ing. Aluminum added to the CoCrPt prevents the oxidationof magnetic grains, therefore, an almost negligible reduction

504030

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Cr (110) Co (101)

900oC

700oC

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Figure 7 XRD patterns of annealed CoCrPt-13wt.% Al2O3 filmat various temperatures.

Table 2 The calculated ratios of (100), (002), and (101) directionsof Co from XRD patterns of annealed samples.

temperature (8C)a I100/I002 I101/I002

500 0.60 0.51700 0.78 1.02900 0.77 1.30

aAnnealing temperature.

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b)

a)

Figure 9 (a) Hysteresis loops and (b) coercivity of the film with13wt.% Al2O3 as a function of annealing temperature.

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of magnetization is found. A considerable increase incoercivity of the film containing 13wt.% Al2O3 is observedafter annealing. However, the most significant increaseoccurs on annealing at 900 8C, as shown by comparison ofthe hysteresis loops in Figs. 9a and 5a.

Figure 9b shows an almost linear relation betweencoercivity and annealing temperature. The easy-axis rotationtowards the plane of the films may be the main source of thecoercivity increment as seen in Fig. 7. It is also well-knownthat with an increase in annealing temperature morenonmagnetic atoms are depleted and pure larger magneticgrains segregated byAl2O3 (see TEM images), which resultsin an increase in coercivity.

Of course, an increase in grain size will be accompaniedby roughness in turn can increase the coercivity. It isnoticeable that the roughness contribution in coercivityimprovement of the amorphous sample is trivial as reportedby Kronmuller [26]. However, this effect in crystallinesamples is considerable due to the existence of grainboundaries as strong pinning centers. The ratio of remanentto saturation magnetization (squareness) as a function ofannealing temperature is obtained and tabulated in Table 3.As can be seen, the difference in the squareness of theannealed films is too small (in the range of calculation errors).Then they are insufficient to present any kind of trend.

The calculated deviations dM from the Wohlfarthrelationship [27] are plotted as a function of applied fieldin Fig. 10. The positive maxima indicate magnetizing,stabilizing intergranular interactions, for all the tempera-tures. A <50% height of the dM curve is an indication ofgenerally noninteracting and/or very weak interacting

Table 3 The squareness of annealed CoCrPt containing 13wt.%aluminum oxide.

temperature (8C)a 500 600 700 800 900

squareness (Mr/Ms) 0.79 0.83 0.77 0.86 0.84

aAnnealing temperature.

3.02.52.01.51.00.50.00.00

0.25

0.50

0.75

1.00 500oC 600 700 800 900

δ M

(a. u

.)

H (kOe)

Figure 10 dM curves of the annealed CoCrPt-13wt.% Al2O3

film at different temperatures.

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character. On increasing annealing temperature, the heightof the deviations dM peaks increases to a maximum <50%and shifts to the higher fields, which means increasing of theintergranular interaction. The increment of this interactiondoes not then affect the coercivity trend. Grain-sizeenhancement may also cause the increase in intergranularinteraction as seen by XRD and TEM results.

4 Conclusions The (CoCrPt)100�x(Al2O3)x (x¼ 3, 5,10, and 13wt.%) thin films were deposited onto a Crunderlayer by an RF magnetron sputtering technique. TheXRD results show reducing magnetization of the filmswith increasing Al2O3 (as a nonmagnetic element) content.Although on annealing, the paramagnetic, as-depositedsample containing 13wt.% Al2O3 has clearly converted tothe ferromagnetic phase. Magnetic and structural analysesshowed Al2O3 was more likely to be distributed at the grainboundaries in the magnetic thin film. High coercivity wasachieved by annealing treatment of the amorphous sample athigher temperatures.

Acknowledgements The authors are grateful to theUniversity of Kashan for supporting this work by Grant No.(159023/4).

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The effect of Al 2 O 3 additive on the microstructure and magnetic properties of Co 75 Cr 13 Pt 12 /Cr thin films