Laminated FeRhN films for high speed writers

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  • 3476 IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 5, SEPTEMBER 2000

    Laminated FeRhN Films for High Speed WritersY. J. Chen, S. Hossain, L. Miloslavsky, Y. Liu, C. Chien, Z. P. Shi, M. S. Miller, and H. C. Tong

    AbstractHigh moment FeRhN nanocrystalline films werelaminated by Co90Zr 9Cr (CZC) amorphous films, in order to im-prove the anisotropy definition and high frequency permeability.The experimental results also showed improved soft magneticproperties, higher resistivity, and better magnetic properties onsloped surfaces.

    Index TermsAnisotropy, high frequency performance, highmoment material, lamination.

    I. INTRODUCTION

    H IGH moment materials used for inductive write headsare required to have a low but nonzero induced uniaxialanisotropy, which is essential for controlling the magneticdomain pattern. A hard-axis state, in which the domains areoriented perpendicular to the flux path, is considered necessaryto ensure that the magnetization change is conducted viarotation and the high frequency permeability is maximized.High electrical resistance is also desirable in order to reduceeddy current loss, and therefore extend permeability roll-off tothe high frequency regime. Such high frequency characteristicsare required for disk operation at high data rate. FeRhN filmswith a value of 21.5 kG and corrosion resistivity have beenreported, and are promising writer materials [1], [2]. In thisstudy, a method of improving their high frequency performancewill be presented.

    II. EXPERIMENTAL RESULTS

    Single layer FeRhN films were reactively deposited usingDC magnetron sputtering in a N/Ar gas mixture onto glasssubstrates. Sputtering parameters that significantly change themicrostructural and magnetic properties of the FeRhN filmsinclude N /Ar flow ratio, sputtering power, gas pressure, sub-strate bias, target-to-substrate spacing during deposition. Thesedeposition parameters were chosen in order to obtain soft andhigh moment FeRhN films with nanocrystalline microstructure[3]. However, the as-deposited FeRhN films exhibited a highdegree of easy axis dispersion, even though an 80 Oe aligningfield was applied during the deposition. As indicated by thesquare symbols in Fig. 1, similar easy and hard coercivityvalues of about 23 Oe were observed in the N% range of 5%to 9% in the sputtering gas. Further increase in N% resultedin an increase of coercivity. The magnetization squareness

    in the directions both parallel and perpendicularto the aligning field are similar, as shown in Fig. 2, whichsuggest that the films have nearly isotropic in-plane magnetic

    Manuscript received February 10, 2000.The authors are with the Advanced Technology Dept., Read-Rite Corp., Fre-

    mont, CA 94539.Publisher Item Identifier S 0018-9464(00)08976-7.

    Fig. 1. Easy and hard axis coercivity of single layer and CZC laminatedFeRhN films.

    Fig. 2. Magnetization squareness(S ) of single layer and CZC laminatedFeRhN sfilms.

    properties. The values of also gradually decrease as a resultof the loss of soft magnetic properties. The definition of theanisotropy was not improved after vacuum annealing in a field.

    Co Zr Cr (CZC) amorphous films have been shown toexhibit excellent soft magnetic properties and corrosion resis-tance [4]. They have a high resistivity of about 90 cm anda of about 13.6 kG. Single layer CZC films can achievenearly perfect easy and hard axis definition. The easy and hardaxis coercivity of less than 0.3 Oe and 0.04 Oe were obtainedin the CZC films, respectively. The anisotropy field is about14.5 Oe.

    In order to improve the magnetic anisotropy of FeRhN films,100 thick CZC films were used as lamination layers for 500 thick FeRhN layers. Well-defined anisotropy axes in CZC lam-inated FeRhN films are indicated by the difference in coercivityalong the easy and hard axis. These coercivity values are alsomuch lower than in the single layer films, as indicated in Fig. 1.The change of coercivity with N% in the sputtering gas duringthe FeRhN film deposition is also shown in Fig. 1. Magneticeasy and hard axes can be clearly identified in the CZC lam-inated films, as indicated by the nearly zero hard axis square-ness values, which are shown in Fig. 2. On the other hand, thesquareness in the easy axis direction is very close to 1. The

    00189464/00$10.00 2000 IEEE

  • CHEN et al.: LAMINATED FeRhN FILMS FOR HIGH SPEED WRITERS 3477

    Fig. 3. Saturation magnetization of single layer and CZC laminated FeRhNfilms.

    Fig. 4. Coercivity of single layer and CZC laminated FeRhN films, which weredeposited onto sloped substrate surfaces.

    Fig. 5. A crossectional TEM micrograph of a FeRhN(400 )/CoZrCr(100 )laminated film.

    anisotropy field of the CZC laminated films, exhibited anincreasing trend with N%. The low values of 56 Oe re-sult in high permeability values of over 4000, which is at leasttwo times more than those of single layer films. Thevalueof CZC laminated films is close to 20 kG, as shown in Fig. 3,which is a modest reduction from that of single layer films.Such value is still very respectable and is comparable tothose of single layer FeAlN or FeTaN films. The resistivity ofsingle layer FeRhN films ranges from 20 to 30 cm withinthe N % studied here. CZC lamination also resulted in an av-erage of about 510 cm increase in resistivity.

    The magnetic properties of the films on sloped surfaces werealso characterized. During the sputtering process, Si substrateswere placed on the substrate table with selected sloping angles.

    Fig. 6. X-Ray diffraction patterns of a single layer and a CZC laminatedFeRhN film.

    Fig. 7. Frequency dependence of the normalized permeability of a single layer(squares) and a CZC laminated FeRhN film (circles), both of which are about5000 thick.

    An 80 Oe aligning field was applied in parallel to the slopedsubstrate surfaces, which defined the intended easy axis direc-tion of the films. Magnetic properties were then measured in thein-plane directions. As shown in Fig. 4, the coercivities of singlelayer films increased significantly with sloping angle; whereas,the CZC laminated films maintained a coercivity of less than2 Oe even at a sloping angle of 75 degree. It is also noted the easyaxis coercivities were lower than those of the hard axis in singlelayer FeRhN films, when the sloping angle became greater than30 .

    The crossectional transmission electron micrograph (TEM)of a laminated film is shown in Fig. 5. It is observed thatFeRhN crystallites are about 25 nm in diameter, and are wellseparated by amorphous CZC layers. The X-ray diffractionpatterns of single layer and CZC laminated FeRhN films werealso investigated. As shown in Fig. 6,-Fe (110), (200), (211),(220) and (310) diffraction peaks were observed in both filmsstudied. A broad CZC amorphous peak was seen at aangleof 44.71 , which is near the Fe (110) peak. All Fe diffractionpeaks are slightly broader in the CZC laminated FeRhN filmand the lattice constant derived from these peak positionsis slightly larger, which indicates that smaller grain size isachieved in the CZC laminated film. The grain size is estimatedto be about 20-25 nm in the laminated film by X-ray diffractiondata, which is consistent with the TEM results. It is estimated tobe 50 nm in the single layer film. It was also observed that therelative peak intensity in (211), (311), and particularly (200)reflections with respect to that of (110) peak are significantlystronger in the CZC laminated film.

  • 3478 IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 5, SEPTEMBER 2000

    High frequency permeabilities were measured in a perme-ance tester. The results showed that the permeability decreaseswith an increase in the film thickness, as a result of the eddycurrent loss. The frequency dependencies of permeability forsingle layer and CZC laminated FeRhN films of the same thick-ness were compared. The data taken from two such films ofabout 5000 thickness are shown in Fig. 7, which shows thenormalized frequency dependence of permeability. The roll-offfrequency, at which the permeability halves its low frequencyvalue, is about 200 MHz in the single layer film. It increases to350 MHz in the CZC laminated film. The solid lines in Fig. 7represent the effective permeability calculated using the eddy-current damping model [5]:

    (1)

    where , which is the ratio of the sheet thickness overthe skin depth , which is defined by:

    (2)

    where is the dc initial permeability, is the frequency, andis the electrical resistivity. Reasonable agreement between the

    model and experimental data was obtained.

    III. D ISCUSSIONS

    The poor anisotropy definition of the single layer FeRhNfilms has adverse effects on the magnetic domain structure.In order to improve the high frequency performance of thematerial, it is necessary to align the easy axis normal to thedirection in which the flux changes. The CZC laminationlayers, which have well-defined anisotropy axes, are believedto be ferromagnetically coupled with the FeRhN layers. Theimproved anisotropy of laminated film strucuture ensuresthat the magnetization switching is conducted via rotation.Lamination using high resistivity CZC layer also increasedthe overall resistivity of the film. As a result, high frequencypermeability is improved.

    The improvement in soft magnetic properties in the lam-inated films may be attributed to the smaller grain size, aswell as the change in crystallographic texture, as suggested

    by the X-ray diffraction studies. During the deposition of thelaminated films, the growth of FeRhN nanocrystalline grainswas intermittently disrupted by the CZC layer, which has amor-phous microstructure. A smaller grain size was thus obtained.In fact, grain size is controlled by the thickness of FeRhNlayers in the laminated films, as indicated by the TEM studies.In single layer FeRhN films, on the other hand, grain size iscontrolled mainly by the deposition conditions, such as N%.Consequently, the soft magnetic properties were observed tobe dependent more sensitively upon N%, or other depositionparameters. It is known that a small grain size is essentialfor achieving soft magnetic properties in nanocrystalline softmagnetic materials [6].

    Lamination also significantly improves the magnetic prop-erties on the sloped surface by disrupting the columnar filmgrowth. The magnetic and morphological properties on thesloping surfaces are essential for integrating the sputteredmaterials into the inductive writer process.

    IV. CONCLUSIONS

    In this paper, we reported nanocrystalline FeRhN highmoment films laminated with amorphous CZC magneticfilms. The laminated films exhibit well-defined anisotropy viaferromagnetic exchange coupling, and improved soft magneticproperties. Magnetic properties of the films deposited ontosloped surfaces were also improved. There is a modest increasein electrical resistivity. As a result of all, the high frequencypermeability is significantly improved.

    REFERENCES

    [1] J. Hong, K. Sin, L. Nguyentran, and S. X. Wang, Soft magnetic proper-ties and microstructures of FeRhN high moment thin films,IEEE Trans.Magn., vol. 33, pp. 28452847, 1997.

    [2] L. Nguyentran, K. Sin, J. Hong, P. P. Pizzo, and S. X. Wang, Corrosionresistance of low coercivity, high moment FeXN (X= Rh, Mo) thinfilm head materials,IEEE Trans. Magn., vol. 33, pp. 28482850, 1997.

    [3] Y. J. Chen, C. Qian, C. Y. Hung, and M. S. Miller, High resistivitynanocrystalline FeRhN films,J. Appl. Phys., vol. 87, pp. 58645866,2000.

    [4] S. A. Hossain, C. He, L. Miloslavsky, C. Qian, M. S. Miller, and H. C.Tong, Evaluation of magnetic properties of CoZrCr for high speed writeheads,J. Appl. Phys., vol. 85, pp. 59986000, 1999.

    [5] R. M. Bozorth,Ferromagnetism. Piscataway, NJ: IEEE Press, 1993, p.771.

    [6] K. Bushow, Ed.,Handbook of Magnetic Materials: Elsevier ScienceB.V., 1997, p. 10.

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