2506
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`IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 5, SEPTEMBER 2000
`
`Soft High Saturation Magnetization
`(Fe0:7Co0:3)1 xNx Thin Films For Inductive
`Write Heads
`
`N. X. Sun and S. X. Wang
`
`Abstract—(Fe0 7Co0 3)1 N (or FeCoN) alloy single layers
`and FeCoN film sandwiched between two very thin (5 nm)
`permalloy layers have been synthesized by RF diode sputtering.
`The saturation magnetization of the as-deposited FeCoN single
`layers was found to be around 24.5 kG, the same as the pure
`Fe0 7Co0 3 alloy; and the minimum hard-axis coercivity was
`5 Oe. In contrast, the sandwiched FeCoN films have a hard axis
`coercivity of 0.6 Oe, an excellent in-plane uniaxial anisotropy
`with an anisotropy field of 20 Oe. The optimized FeCoN films
`exhibit a BCC structure with a strong 110 fiber texture and the
`resistivity is 55
` cm. The combination of high saturation and
`low coercivity, makes the FeCoN films a very promising candidate
`for the write head materials for future magnetic recording.
`Index Terms—High saturation magnetization,
`iron cobalt
`nitrogen alloys, soft magnetic materials, write head materials.
`
`I. INTRODUCTION
`
`T HE AREAL density record of magnetic recording has
`
`been increasing at a compound annual growth rate of
`80% much recently, and has reached 36.5 Gb/in [1]. With
`such rapid progress of the areal density record, soft magnetic
`write head materials with high saturation magnetization are
`highly desired to write the high coercivity magnetic media. The
`unavailability of write head materials with higher saturation
`magnetization (
`kG) has been an immediate bottleneck in
`limiting the growth of areal density.
`It is well known that the binary Fe–Co alloys have a high
`saturation magnetization of 24.5 kG in the composition range
`of Fe
`Co (
`) [2]. But the Fe–Co alloys are
`highly magnetostrictive, the saturation magnetostriction con-
`stant is about
`in the composition range of
`at % of cobalt [2]. The high saturation magnetostric-
`tion makes it very difficult to achieve low coercivity or in-plane
`uniaxial anisotropy [3]–[5].
`The motivation of this work is to make the Fe–Co alloy based
`films soft, while keeping the high saturation magnetization at
`the same time. Introducing nitrogen has been shown to be very
`effective to lower the coercivity in the FeN [6], and FeMN
`(
`at % Al [7], Ta [8], etc.) alloy films. We studied the
`effects of introducing N into the Fe Co
`alloy films, and also
`
`Manuscript received February 15, 2000; revised May 15, 2000. This work is
`supported in part by NSF under Grant ECS-9710223.
`The authors are with the Department of Materials Science and Engineering
`at Stanford University, and also with the Center for Research on Informa-
`tion Storage Materials (CRISM), Stanford, CA 94305-2205 USA (e-mail:
`nxsun@stanford.edu).
`Publisher Item Identifier S 0018-9464(00)07984-X.
`
`Fig. 1. 4M versus N /Ar gas flow rate ratio in FeCoN.
`
`the effects of thin layer permalloy underlayer and overlayer on
`the FeCoN films, and successfully fabricate the soft FeCoN
`films with high saturation magnetization and low coercivity.
`
`II. EXPERIMENTAL PROCEDURE
`
`FeCoN films were synthesized through reactive RF diode
`sputtering in an argon and nitrogen atmosphere. The target com-
`position was Fe Co
`(at%) with the purity of 99.95%. Base
`pressure of the sputtering chamber was
`Torr. The
`gas flow rate of argon was set constant; while the flow rate of
`nitrogen was adjusted to get samples with different N contents
`in the films. A magnetic field of about 50 Oe was applied during
`deposition. The obtained films were characterized by vibrating
`sample magnetometer (VSM) and X-ray diffractometer (XRD),
`etc. Composition of the FeCoN films was analyzed by X-ray
`Photoelectron spectroscopy (XPS). A four-probe station was ap-
`plied to measure the resistivity of the FeCoN films.
`
`III. RESULTS AND DISCUSSION
`
`FeCoN films with the thickness of 1000 Å were deposited
`at different N /Ar gas flow rate ratio. The saturation magne-
`tization (
`) of the as-deposited FeCoN films is shown in
`Fig. 1 as a function of the N /Ar gas flow rate ratio. It is clear
`that the saturation magnetization values are almost the same as
`that of the pure Fe–Co alloy, about 24.5 kG, in a wide range
`of N /Ar gas flow rate ratio of 0% 7%. The corresponding
`nitrogen content in the FeCoN films was determined by XPS
`to be about 5 at % at a N /Ar gas flow rate ratio of 5.6%; the
`Fe/Co atomic ratio is around 2/1, and almost keeps constant
`for the FeCoN films at different N contents [9]. Similar be-
`havior was also observed in the saturation magnetization of the
`
`0018–9464/00$10.00 © 2000 IEEE
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`SUN AND WANG: SOFT HIGH SATURATION MAGNETIZATION (Fe
`
`Co
`
`)
`
`N THIN FILMS
`
`2507
`
`Fig. 2. H versus N /Ar gas flow rate ratio.
`
`Fig. 4. Resistivity of FeCoN films deposited at different N /Ar gas flow rate
`ratio.
`
`Fig. 3. XRD patterns of FeCoN films deposited at different N /Ar gas flow
`rate ratios.
`
`N films when the nitrogen content
`as-deposited (Fe Co )
`is lower than 12 at %, and the observed saturation magnetization
`is in the range of
`kG [5].
`The coercivity of these FeCoN films was measured and indi-
`cated in Fig. 2 as a function of the N /Ar gas flow rate ratio. The
`hard-axis coercivity first decreases quickly from about 100 Oe
`at a gas flow rate ratio of 2% to around 5 Oe at a N /Ar gas
`flow rate ratio of
`%, then the coercivity increases with the
`increment of the gas flow rate ratio. Similar relation between co-
`ercivity and N /Ar flow rate ratio occurs in many FeMN alloy
`systems, and was believed to be a result of the decrease of grain
`size with the increase of N content in the film [5]–[9].
`XRD patterns of the FeCoN films deposited at three N /Ar
`gas flow rate ratios, 0%, 12.5%, and 19.6%, are shown in Fig. 3.
`At low N /Ar gas flow rate ratio, the FeCoN films have a BCC
`-Fe(Co, N) structure with a strong
`fiber texture; while
`a significant amount of Fe N phase appears in the film at a high
`N /Ar flow rate ratio of 19.6%. In addition, the
`-Fe(Co, N)
`diffraction peak is shifted to lower angles and is much
`broader at higher N /Ar gas flow rate ratio, implying a higher
`N content incorporated and a much smaller grain size and/or
`micro- strain in the FeCoN films at higher N /Ar gas flow rate
`ratio. The resistivity of the FeCoN films is shown in Fig. 4. The
`resistivity increases as the increment of the N /Ar gas flow rate
`ratio. For the Fe–Co films, the resistivity is around 12
`cm,
`while the resistivity increases almost linearly with the gas flow
`rate, reaching 55
`cm at a N /Ar gas flow rate ratio of 5.6%,
`where the lowest coercivity is obtained.
`Combining Figs. 1–4, we can clearly see that the high sat-
`uration magnetization of around 24.5 kG and low coercivity
`of around 5 Oe can be realized at a N /Ar gas flow rate ratio
`
`(a)
`
`(b)
`
`(a) Typical hysteresis loop of the FeCoN single layer. (b) Hysterics
`Fig. 5.
`loop FeCoN sandwiched between two permalloy layers.
`
`%. However, the FeCoN single layer is not yet soft
`
`of
`enough.
`the
`For the purpose of lowering the coercivity further,
`FeCoN film was sandwiched with two very thin layers (5 nm)
`of permalloy (Fe Ni
`) as the underlayer and overlayer,
`respectively. The hysteresis loops of the single FeCoN layer,
`and sandwiched FeCoN film are shown in Fig. 5(a) and (b),
`respectively. It is clear the coercivity of the FeCoN single layer
`is about 5 and 18 Oe in the hard and easy axis, respectively; and
`is reduced to 0.6 and 7.8 Oe, respectively for the sandwiched
`FeCoN film.
`Furthermore, the square easy loop, and an almost linear hard
`axis loop of the sandwiched FeCoN film indicate a very good
`in- plane uniaxial anisotropy. The anisotropy field can be deter-
`mined by extrapolation to be 20 Oe, corresponding to ferromag-
`netic resonance frequency of about 1.9 GHz. Factors that may
`result in this dramatic change in the soft magnetic properties of
`the FeCoN films will be detailed in reference [9].
`
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`2508
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`IEEE TRANSACTIONS ON MAGNETICS, VOL. 36, NO. 5, SEPTEMBER 2000
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`IV. SUMMARY
`
`We successfully deposited soft FeCoN single layers with a
`high saturation magnetization of 24.5 kG, and a coercivity of
`5 Oe. A lower coercivity of 0.6 Oe, and an excellent in-plane
`uniaxial anisotropy with an anisotropy field of 20 Oe were
`achieved in the FeCoN films sandwiched between two very
`thin Permalloy layers. The obtained FeCoN films have a BCC
`lattice with strong
`fiber texture, and the resistivity is
`55
`cm. The data indicate that the FeCoN films are very
`good candidates as future write head materials.
`
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`
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