Physica B 325 (2003) 401–409

Perpendicular magnetic anisotropy, hysteresis and structuralproperties of nanostructured FeCoV/Ti multilayers

M. Senthil Kumara,*, P. B .onib, M. Horisbergerc

aDepartment of Physics, Indian Institute of Technology Bombay, Mumbai 400 076, IndiabPhysik Department E21, Technische Universit .at M .unchen, D-85748 Garching, Germany

cLaboratory for Neutron Scattering, ETH Zurich and Paul Scherrer Institute, CH-5232 VilligenPSI, Switzerland

Received 31 August 2002; accepted 13 October 2002

Abstract

FeCoV/Ti multilayers are investigated in order to study perpendicular magnetic anisotropy, variation of saturation

magnetization, remanence, coercivity and structural properties. Five samples with tTi¼ 1:04 nm and

tFeCoV¼ 0:8924:47 nm are prepared by DC magnetron sputtering. X-ray diffraction measurements carried out on

the samples show a strong FeCoV(1 1 0) texture for larger tFeCoV: The grain size determined from the FeCoV(1 1 0)

peaks displays a decreasing trend with decreasing tFeCoV indicating that the grain size is mainly determined by the

tFeCoV: We have performed magnetic hysteresis measurements in order to investigate various magnetic properties. Theinterface/surface and volume anisotropy constants determined from the measurements are Ks¼ 0:826 mJ=m2 and

Kv¼ �1:84MJ=m3; respectively. The data analysis indicates that the samples may have their easy direction of

magnetization perpendicular to the film plane for tFeCoVo0:89 nm:The saturation magnetization is observed to decreasewith decreasing tFeCoV and from a plot of MstFeCoV vs. tFeCoV we have determined a dead layer thickness of 0.33 nm at

the FeCoV interfaces. The variation of coercivity and remanence with tFeCoV may be due to both random anisotropy

and superparamagnetism. The decrease in remanence with decreasing tFeCoV can also be attributed to the progressive

change of the easy direction of magnetization away from the film plane.

r 2002 Elsevier Science B.V. All rights reserved.

PACS: 75.70; 75.30.G; 76.60.E

Keywords: FeCoV/Ti multilayers; Perpendicular magnetic anisotropy; Coercivity; Remanence; Grain size

1. Introduction

Multilayers with ferromagnetic layers separatedby nonmagnetic spacer layers are interestingsystems from both fundamental and technological

viewpoints [1–6]. Particularly, perpendicular mag-netic anisotropy in multilayers plays an importantrole in magneto-optical recording [1,7,8]. Thisanisotropy is due to the so-called interface orsurface anisotropy that results from the loweredsymmetry and competes with the volume aniso-tropy of the sample. By varying the thickness ofthe individual layers and choosing appropriatematerials, it is possible to tailor the magnetic

*Corresponding author. Tel.: +91-22-576-7581; fax: +91-

22-576-7552.

E-mail address: senthil@phy.iitb.ac.in (M. Senthil Kumar).

0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 6 9 3 - 9

anisotropy. As individual layers in a multilayerbecome thinner, the role of the interfaces andsurfaces may dominate that of the bulk so that theperpendicular interface contribution to the mag-netic anisotropy is capable of rotating the easydirection of magnetization from the plane of thefilm to the perpendicular direction. In this articlewe have investigated a commercially availableferromagnetic alloy with a composition ofFe50Co48V2 (hereafter FeCoV). This compositionis close to the Fe50Co50 alloy which in bulk formpossesses high saturation magnetization, highmagnetostriction and soft magnetic behaviourwhich are some of the interesting properties forapplications. Currently the films and multilayers ofthis alloy are studied in order to understandvarious magnetic properties [4,6,9–12]. TheFeCoV=TiNx multilayers and supermirrors areused for the polarization of neutron beam [6]. Inthis paper, we present the investigations performedon the nanostructured FeCoV/Ti multilayersby X-ray diffraction (XRD) and magneticmeasurements in order to study their structure,perpendicular magnetic anisotropy, saturationmagnetization, remanence and coercivity.

2. Experimental

The commercially available ferromagnetic alloywith a composition Fe50Co48V2 (Vacoflux50,Vacuumschmelze GmbH, Germany) is chosen forour investigations. Five samples of FeCoV/Tihaving 75 bilayers each are prepared by DCmagnetron sputtering using a Leybold Z600sputtering system having three target holders[13]. One of the targets is FeCoV and another isTi. The FeCoV layers are sputtered in Ar at apressure pAr¼ 0:73� 10�3 mbar: The Ti layers aresputtered at pAr¼ 0:44� 10�3 mbar: The multi-layers are deposited onto glass and Si substrates atambient temperatures. The substrates are coatedby linearly translating them under the targets. Thethickness of the Ti layers, tTi; is the same for allthe samples by keeping the target power and thetranslation speed constant. The thickness of theFeCoV layers, tFeCoV; is varied from sample tosample by keeping the power constant and varying

the translation speed in steps of equal intervals.X-ray reflectivity and high angle diffraction meas-urements are carried out using a Siemens D500diffractometer with CuKa radiation. Magnetichysteresis measurements are performed at roomtemperature using an extraction magnetometerthat is part of the physical property measurementssystem (PPMS) of quantum design equipped witha superconducting magnet delivering magneticfields of 0–9T. Intrinsic stress measurements areperformed using a Tencor P2 Long scan profilo-meter. The in-plane stress in the layers along (sjj)and perpendicular (s>) to the substrate motion isdetermined from the radii of curvatures of the Siwafers before and after coating.

3. Results and discussion

The FeCoV/Ti multilayers are investigated bylow angle X-ray reflectivity, high angle XRD,magnetic hysteresis and stress measurements.From the X-ray reflectivity data we have deter-mined the bilayer period L of the multilayers thatare listed in Table 1. Since the substrate speed, asalready mentioned, for the FeCoV layers issystematically varied while keeping the speed forthe Ti layers unchanged the data points for the Lplotted as a function of the expected thickness fallon a straight line. A straight line fit to the datayields tTi¼ 1:04 nm (y-intercept). The tFeCoV valuesare obtained by subtracting tTi from L for eachsample. The values of tFeCoV and L obtained fromthis analysis are listed in Table 1.

Table 1

Thickness of FeCoV and Ti layers in the FeCoV/Ti multilayers,

deduced from X-ray reflectivity measurements, tTi¼ 1:04 nm

Sample identification tFeCoV [nm] Bilayer period L [nm]

tTi = 1.04mm

A 0.89 1.93

B 1.80 2.84

C 2.64 3.68

D 3.53 4.57

E 4.47 5.51

M. Senthil Kumar et al. / Physica B 325 (2003) 401–409402

In Fig. 1, the data obtained from the high angleXRD measurements are shown. In the range of 2yvalues shown, all the FeCoV peaks correspondingto bulk FeCoV are present for our multilayers aswell; however, the relative intensities are differentfrom that of the bulk. The rise in the backgroundat about 2y ¼ 351 is due to the scattering from theglass substrates. Furthermore, we also see satellitepeaks around the main FeCoV(1 1 0) peaks. Thesatellites vanish for the samples with smallertFeCoV: A gradual broadening of the FeCoV peakswith decreasing tFeCoV is also seen. The Ti peaks ofthe samples are not observed in the XRD data.From Fig. 1 we see a gradual shift in the

FeCoV(1 1 0) peaks towards lower angles astFeCoV decreases. This shift may be due to thestrains at the interfaces. Such shifts in the peakpositions are also observed by Lafford et al. inFeCo/Ag multilayers [14]. The strong Fe-CoV(1 1 0) peaks for large tFeCoV seen in Fig. 1indicate a (1 1 0) texture of the layers. Zuberek et al.[4] have reported that in MBE grown Fe50Co50/Aumultilayers the Fe50Co50 layers are polycrystallinewith (1 1 0) texture. Whereas their sputteredFe50Co50(6.4 nm)/Ag(2 nm) multilayers show pat-terns closer to isotropic polycrystalline Fe50Co50[14,15]. The same absence of texture is observedfor their 600 nm single Fe50Co50 layers as well. Thesatellites observed for large tFeCoV in the XRD

patterns are due to the structural coherence at theFeCoV and Ti interfaces [12,16,17]. The satellitesare found to gradually vanish as tFeCoV decreases.Lafford et al. [14,15] did not observe any satellitepeaks in Fe50Co50/Ag and Fe50Co50/Au multi-layers. This may be due to the large interfaceroughness of their samples leading to the loss ofcoherency at the interfaces. Their multilayers aregrown on Kapton substrates that were probablyrougher than our glass substrates that have an rmsroughness of about 0.3 nm. From Fig. 1 we alsosee broadening of the FeCoV peaks with decreas-ing tFeCoV indicating a decrease in grain size. Thegrain size determined using the Scherrer formula[18] from the FeCoV(1 1 0) peaks is shown inFig. 2. As the intensities of the other FeCoV peaksare low we could not determine the grain size fromthose peaks. The nearly linear, decreasing trend ofthe grain size with decreasing tFeCoV as seen in thefigure indicates that the grain size is mainlydetermined by tFeCoV: In the XRD data we havenot observed any peaks corresponding to the Tilayers. The absence of peaks may be due toamorphization of the Ti layers. We also noticed asimilar absence of the Ti peaks in Ni/Ti multi-layers (with thin Ti layers) in which the Ti layerswere amorphous [19]. However, Ti(0 0 2) peakswere observed by us in the FeCoV/Ti multilayersfor tTiX3 nm; studied for polarized neutronmirrors [12].

40 50 60 70 80 90 100 1100

10000

20000

30000

E

D

C

B

sampleA

FeC

oV(2

20)

FeC

oV(2

11)

FeC

oV(2

00)

FeC

oV(1

10)

X-r

ay in

tens

ity [a

rb. u

nits

]

2θ [deg.]

Fig. 1. X-ray diffraction patterns of the FeCoV/Ti multilayers.

The vertical, dashed lines represent the positions of the FeCoV

peaks for bulk samples, plotted for comparison.

0 1 2 3 4 50

1

2

3

4

5

6

7

8

9

10

Gra

in s

ize

[nm

]

tFeCoV [nm]

Fig. 2. Grain size determined from the FeCoV(1 1 0) peaks as a

function of tFeCoV: The solid line indicates the trend.

M. Senthil Kumar et al. / Physica B 325 (2003) 401–409 403

The data obtained from magnetic hysteresismeasurements in magnetic fields ranging from �4to +4T is shown in Fig. 3. The solid and opensymbols represent the data for the perpendicular(out of plane) and in-plane measurements, respec-tively. It can be readily seen that the samples areeasily saturated for in-plane magnetic fields.Furthermore the in-plane hysteresis data alsodisplay variations in remanence and coercivity ofthe magnetic layers that will be discussed later. Wehave measured the intrinsic stress in the layers andthe data obtained is as shown in Fig. 4(a). Thestress sjj measured in the direction of the substratemotion and s> measured in the perpendiculardirection of the substrate motion are shown. Thesedata indicate the presence of a stress anisotropywithin the plane of the layers. The difference stressDs ¼ s> � sjj shown in Fig. 4(b) is tensile for allthe samples. Since s> is more tensile than sjj andFeCoV has a positive magnetostriction an easyaxis of magnetization develops within the plane ofthe FeCoV layers, in the direction perpendicular tothe substrate motion. Such a stress anisotropy andeasy axis within the plane of the layers were

-30 -20 -10 0 10 20 30-2000

-1500

-1000

-500

0

500

1000

1500

2000

A

in-planeperpendicular

M [e

mu/

cm3 ]

M [e

mu/

cm3 ]

-30 -20 -10 0 10 20 30

sample samplesampleB

H [kOe]H [kOe]

-30 -20 -10 0 10 20 30-2000

-1500

-1000

-500

0

500

1000

1500

2000

sampleD

H [kOe] H [kOe]

-30 -20 -10 0 10 20 30

C

H [kOe]

-30 -20 -10 0 10 20 30

sampleE

Fig. 3. In-plane (open squares) and perpendicular (solid circles) magnetic hysteresis data of the FeCoV/Ti multilayers.

0.0

0.2

0.4

0.6

(a)

(b)

σ⊥

σ

Str

ess

[GP

a]0 1 2 3 4 5

-0 .2

0.0

0.2

0.4

0.6

∆σ [G

Pa]

tensile

tensile

compressive

compressive

tFeCoV [nm]

Fig. 4. (a) Stress in the FeCoV/Ti multilayers as a function of

tFeCoV: sjj and s> are the stress measured along and

perpendicular to the substrate movement. (b) The difference

stress Ds ¼ s> � sjj as a function of tFeCoV:

M. Senthil Kumar et al. / Physica B 325 (2003) 401–409404

observed by us in the samples having large tFeCoV[12]. The in-plane hysteresis curves shown in Fig. 3are measured along the perpendicular direction ofthe substrate motion, i.e. along the in-plane easyaxis.In the following, let us discuss the magnetic

anisotropy in the multilayers determined from thehysteresis loops. The effective magnetic anisotropyKeff is given by the relation,

Keff ¼ Kv þ2Ks

tFeCoV;

where Ks is the surface/interface anisotropy con-stant and Kv is the volume anisotropy constantthat includes the contribution of the shapeanisotropy [1]. Experimentally, Keff is obtainedfrom the area enclosed between the in-plane andperpendicular magnetization curves in the firstquadrant of the graph [1]. The area can beobtained from the third quadrant as well. Wehave determined the areas from the magnetizationcurves shown in Fig. 3, in both first and thirdquadrants separately and then the average of thetwo areas is taken as Keff : The Keff values thusobtained are plotted in Fig. 5 in a Keff tFeCoVvs. tFeCoV representation based on the aboverelation. The graph indicates that the magnetiza-tion of the FeCoV layers may have aneasy direction perpendicular to the film planefor tFeCoV smaller than about 0.89 nm. Thestraight line fit as shown in the figure yields

Ks¼ 0:83 mJ=m2 and Kv¼ �1:84MJ=m3: Thesevalues are comparable with those reported onvarious Fe-based multilayers [1]. The Ks valuesreported by Zuberek et al. [4] for Fe50Co50/Au is0.65mJ/m2 and for Kv is 0.69MJ/m

3. The Ks andKv values obtained for Ce/Fe by M .ossbauermeasurements are 0.98mJ/m3 and 0.11MJ/m3

[11]. For Ce/FeCoV system they have predicted alarger Ks value than that of the Ce/Fe system. TheKv values of our data are not comparable withthese data as their values do not include thecontribution from shape anisotropy. The angles ofthe easy direction of magnetization reported byCase et al. [11] for the Ce/Fe are y ¼ 901; 601 and391 for tFe ¼ 2; 1.5 and 1 nm, respectively. Herey ¼ 01 and 901 corresponds to the perpendicularand in-plane easy directions, respectively. In thecase of the Ce/FeCoV the angles are y ¼ 831; 341and 281 for tFeCoV ¼ 2; 1.5 and 1 nm, respectively.This data shows a progressive change in the easydirection from in-plane to the out of planeorientation. The data also further shows that themagnetization of the FeCoV layers in Ce/FeCoVare more out of plane than in the Ce/Fe inferring alarger interface anisotropy in the former case.Thus the results of the Ce/Fe and Ce/FeCoVsystems support our observation that the easydirection may lie out of plane for small tFeCoV:From Fig. 4 we see that the samples exhibit tensilestress. Since FeCoV has positive magnetostrictioncoefficient the tensile stress would tend to align themagnetization in the plane of the layers. Thus theinterface/surface contribution is responsible forthe rotation of the magnetization away from theplane of the films observed in the present study.The variation of the saturation magnetization,

Ms; with tFeCoV determined from Fig. 3 is shown inFig. 6. The inset of the figure shows a decrease inMs with decreasing tFeCoV: It is generally observedthat Ms follows the relation, Ms ¼ M0ð1� 2d=tÞ;where t is the thickness of the magnetic layer, d isthe dead layer thickness at the interfaces and M0 isthe saturation magnetization of the magnetic layermaterial in bulk form. We therefore plotted Ms ofour samples in a MstFeCoV vs. tFeCoV representationin Fig. 6. From the linear relation we determinedd¼ 0:33 nm: This value is higher than d¼ 0:21 nm;determined in our previous investigation on

0 1 2 3 4 5

-8

-6

-4

-2

0

2

4

Kef

ft FeC

oV[m

J/m

2 )]

tFeCoV [nm]

Fig. 5. Effective anisotropy constant in a Keff tFeCoV vs. tFeCoVrepresentation. The values of Ks and Kv determined from this

plot are 0.826mJ/m2 and �1.84MJ/m3, respectively.

M. Senthil Kumar et al. / Physica B 325 (2003) 401–409 405

FeCoV/Ti with large tTi (3�15 nm) and tFeCoV(3�70 nm) [12]. The reason for this difference inthe d values is probably due to the difference intFeCoV and tTi: Zuberek et al. [4] have reported,in the case of Fe50Co50ðtÞ=Auð2 nmÞ witht¼ 2211:1 nm; that Ms determined using a vibrat-ing sample magnetometer shows no significantvariation. The presence of the dead layers at theinterfaces are reported by many authors forvarious multilayer systems. For example, Ankneret al. [20] have reported in the case of Fe/Simultilayers d¼ 0:62 nm from polarized neutronreflectometry. H�g�hj et al. [21] have reported inthe case of Fe/Si multilayers that Ms vs. tFe curvefollows a linear behaviour in the rangetFeE2:5218:5 nm: From this data they have foundthat the Fe layers would be nonmagnetic below1.02 nm which corresponds to d¼ 0:51 nm: Theyexamined the same samples by polarized neutronreflectometry and determined the value of d to be0.5 nm.In Fig. 7 and in the inset, we have plotted the

coercivity Hc of the FeCoV/Ti multilayers as afunction of tFeCoV: As can be seen from the figure,the Hc lies between 12 and 15Oe. In the samefigure we have also plotted the Hc data obtained inour previous investigation on the FeCoV/Ti layers,for comparison [12]. Even though tFeCoV for somesamples in both the investigations is in the samerange, Hc observed in the present study is

appreciably smaller when compared with ourprevious study. However, the MBE grownFe50Co50(3.2 nm)/Au(2 nm) bilayer shows a Hc of20Oe as reported by Zuberek et al. [4] whichcompares well with our data.Various reports on the variation of Hc in

multilayers are available in the literature. In as-deposited Fe–N/Al multilayers Barnard et al. [22]have observed an increase in Hc with tFe2N from2Oe for 3.2 nm to about 115Oe for 13 nm. Gaoet al. [23] also reported an increase of Hc in Co/Cuand Co95B5/Cu systems with tCo and tCo95B5;respectively. Similar behaviour is also noticed byDirne et al. [24] for Fe/CoNbZr multilayers. Thereare reports that contradict this behaviour bydisplaying a decrease in Hc with increasingthickness of the magnetic layers. Wang et al. [10]have reported a decrease in Hc with increasingtFeCo for 4–17 nm in Fe50Co50/Ag multilayers.Lafford et al. [14] have also reported that Hc

decreases with increasing tFeCo in Fe50Co50/Agmultilayers. Matsumoto et al. [25] have reportedthat in the case of Fe/Ti, Fe/Ta, Fe/Al, Fe/Ag andFe/Cu systems, Hc decreases with increasing tFe:Haftek et al. [26] have reported a decrease in Hc

from 80 to 7Oe with increasing tNi in the Ni/Tisystem.L .offler et al. [27] have reported coercivity in

nanostructured Fe. In this case, Hc increases from

1 2 3 4 50

500

1000

1500

2000M

s[e

mu/

cm3]

tFeCoV [nm]

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00

1000

2000

3000

4000

5000

6000

7000

8000

Mst F

eCoV

[em

u-nm

/cm

3 ]

tFeCoV [nm]

Fig. 6. Saturation magnetization of the FeCoV/Ti multilayers

in an MstFeCoV vs. tFeCoV representation. The value of dead

layer thickness d determined from the linear fit is 0.33 nm.The

inset shows the variation of Ms with tFeCoV:

0.0 2.5 5.0 7.5 10.00

50

100

150

200

present studyFeCoV/Ti

(Senthil Kumar et al. [12])

Hc[O

e]

tFeCoV [nm]

0 1 2 3 4 50

5

10

15

20

Hc[O

e]

tFeCoV [nm]

Fig. 7. Coercivity Hc as a function of tFeCoV in the FeCoV/Ti

multilayers. The solid squares are the data of the present study.

The open circles are the data taken from Senthil Kumar et al.

[12]. The inset shows the magnified form of the data of the

present study.

M. Senthil Kumar et al. / Physica B 325 (2003) 401–409406

5Oe for the grain size of 10 nm to 100Oe for about35 nm. The Hc decreases above 35 nm and attains avalue of 10Oe for 100 nm. Thus a maximum of Hc

at about 35 nm is observed. They have explainedthe behaviour of Hc for the grain sizes between 10and 35 nm by a random anisotropy model [27].For random orientation of the anisotropy axes ofthe grains the effective anisotropy is remarkablyreduced. This model shows a decrease of theanisotropy energy and consequently a decrease ofHc with decreasing grain size. L .offler et al. [27]have explained the increase in Hc with decreasinggrain size till the maximum, in nanostructured Fe,by domain wall pinning at grain boundaries thatbecomes progressively more efficient as the volumefraction of the grain boundaries increases. In ourcase, the pinning effect can come from bothFeCoV grain boundaries and interfaces of themultilayer structure. It should be noted that the Hc

values observed for both systems are much largerthan the value HcE3 Oe for bulk FeCoV (datasupplied by the manufacturer VacuumschmelzeGmbH, Germany). This indicates that the Hc

values we have obtained in the present as well asprevious studies are below the expected maximumbecause we see an over all increasing trend of Hc

with tFeCoV when the data from these studies areput together as can be seen from Fig. 7 and hencethe trend can be explained using the randomanisotropy model. The smaller Hc obtained in thepresent study when compared with our previousstudy is due to the smaller grain size of FeCoVthat result from the smaller tTi: The bilayers of theprevious investigation are FeCoV(3 nm)/Ti(3 nm),FeCoV(5 nm)/Ti(5 nm) and FeCoV(10 nm)/Ti(8 nm). In these samples, the grain size of theFeCoV layers determined from the FeCoV(1 1 0)peaks are about 10 nm. This value is considerablylarger than that of the present study wherein thegrain sizes are significantly smaller as can be seenfrom Fig. 2. The tTi values of our previousinvestigations are larger than 3 nm and hence thegrain size in the Ti layers is expected to be larger.This larger grain size of the Ti layers in turninfluence the growth of the FeCoV layers leadingto larger grain size in the FeCoV layers. Thus theHc observed in our present study is smaller thanthe ones of the previous study.

The behaviour of Hc as a function of tFeCoVobserved in the present study may be due to thecompeting effects from the layers and interfaces.The interface contribution to the pinning may bedominant as tFeCoV becomes smaller whereas wecan expect a decrease in Hc with decreasing tFeCoVaccording to the random anisotropy model. Thusthe resultant Hc does not show a large variation asa function of tFeCoV: The random anisotropymodel discussed above considers that the particlesare in contact and that magnetic exchange inter-actions take place across the boundaries [27].However, in the present study, the FeCoV layersmay be discontinuous for samples with smalltFeCoV: Hence, superparamagnetism can also playa significant role in determining the hysteresisproperties. The superparamagnetism would alsolead a decrease in Hc with decreasing tFeCoV[28,23]. Thus, both the random anisotropy andthe superparamagnetism are responsible for thebehaviour of Hc in our system.Fig. 8 shows the remanence of the FeCoV/Ti

multilayers as a function of tFeCoV, determinedfrom the in-plane hysteresis loops. No significantremanence is observed from the perpendicularloops. The figure shows a high remanence ofMr=Ms ¼ 1 for samples D and E for which thetFeCoV is larger. For these samples the easy

0.0 2.5 5.0 7.5 10.00.0

0.2

0.4

0.6

0.8

1.0

Mr/

Ms

tFeCoV [nm]

present studyFeCoV/Ti(Senthil Kumar et al. [12])

Fig. 8. Remanence of the FeCoV/Ti multilayers as a function

of tFeCoV for in-plane hysteresis data. The solid squares are the

data of the present study The open circles are the data taken

from Senthil Kumar et al. [12].

M. Senthil Kumar et al. / Physica B 325 (2003) 401–409 407

direction of magnetization is expected to liecompletely in the plane of the film. When tFeCoVdecreases Mr=Ms also decreases to 0.25 for sampleA. The remanence observed by us in the FeCoV/Tisystem with larger tFeCoV is also shown in thisfigure, for comparison [12]. We notice that theMr=Ms ratio is high, i.e. in the range 0.85–0.95, inthis case as well. From both the studies, we see anoverall decrease in the remanence with decreasingtFeCoV: The decrease in remanence may be due tothe following reasons. As already mentioned, thereis no significant remanence observed for the harddirection, i.e. the perpendicular direction. Since thein-plane direction is progressively becoming harderthe remanence in our FeCoV/Ti multilayers gra-dually decreases with decreasing tFeCoV. Alterna-tively, the decrease in remanence can be explainedbased on the random anisotropy and superpar-amagnetism, mentioned above. We presume thatall the three factors, i.e. the gradual rotation of theeasy direction of magnetization, random anisotro-py and superparamagnetism result in the reductionin remanence with decreasing tFeCoV:

4. Conclusion

The FeCoV/Ti multilayers show strong Fe-CoV(1 1 0) texture for large tFeCoV: The grain sizeas a function of tFeCoV shows a decreasing trenddue to decreasing tFeCoV: A linear behaviour isobserved from the Keff tFeCoV vs. tFeCoV plot and weexpect that for tFeCoVo0:89 nm the samples mayhave their easy direction of magnetization perpen-dicular to the film plane. The Ks and Kv valuesdetermined from the plot are in good agreementwith those reported in the literature. The satura-tion magnetization decreases with decreasingtFeCoV exhibiting a linear relation betweenMstFeCoV and tFeCoV: The value of dead layerd¼ 0:33 nm determined falls in the range of valuesreported in the literature. The variation of Hc withtFeCoV observed in the present study is comparedwith our previous study. The over all decreasingtrend of Hc with decreasing tFeCoV is explainedusing a random anisotropy model for nanocrystal-line materials. The nearly flat curve in the Hc vs.tFeCoV plot observed in the present study may be

due to the competing interactions from the domainwall pinning at the interfaces and from the grainsize according to the random anisotropy model.The behaviour of Hc can also be explained basedon superparamagnetism. The decrease in Mr=Ms

with decreasing tFeCoV can be attributed to thegradual rotation of the easy direction from the in-plane to the perpendicular direction, randomanisotropy and superparamagnetism.

Acknowledgements

One of the authors (MS) acknowledges Councilof Scientific and Industrial Research (CSIR), Indiafor the financial support.

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