X-Ray Study of Soft and Hard Magnetic Thin Films

X-Ray Study of Soft and Hard Magnetic Thin Films

Abstract :

This paper reports the correlation between crystal structures of soft and hard magnetic recording thin films and their properties. The microstruture of NiFe and cobalt-based alloy magnetic thin films were investigated using grazing incident x-ray diffraction(GIXRD).

INTRODUCTION

The recording head and disk thin film media are key components in the hrad disk industry[1].The read/write sensors are primarily made of soft NiFe magnetic material, whereas the current longitudinal media is sputtered with hard magnetic cobalt based alloys. The disk drive areal density depends upon the track and linear densities of head media performances.

In magnetic recording thin film heads, the difference in magnetic properties between the easy and the hard axes of NiFe thin film can be revealed by GIXRD[2]. The intensity and 2-theta angle of NiFe(111) reflection are slightly different in the two axes. This is observed whether the film is either dry sputtered or wet plated.

In longitudinal disks, coercivity, signal to noise ratio and off-track capability have a strong relationship with cobalt crystallographic orientation and the selection of seed layer. The seed layer characteristics such as d-mismatch, grain size were found to depend strongly on the sputter process conditions. Conventional XRD prohibits the observation of detailed cobalt structural information due to the severe noise interference of substrate background. However, GIXRD allows for measurements of cobalt (002), (101) and (100) which are parallel to the film surface. As a result, a disk with lower Co(002) and higher Co(100) peak intensities had a good magnetic performance [3-4].

EXPERIMENTAL

A Rigaku D/Max-2500 diffractometer with a 18kw rotating-anode x-ray generator and a flat crystal diffracted-beam graphite monochromator were used to collect diffraction patterns. For GIXRD, the grazing incident angle theta was fixed at 0.5 ° and the detector was scanned along either the easy or the hard axis for the NiFe films and along the disk circumferential direction of cobalt alloys films.

The bulk magnetic properties of NiFe/ Co alloys were measured by B-H Looper and M-H vibration sample magnetometer respectively.

The magnetic recording of a Head/Disk’s parametric/bit error rate were performed using a Guzic tester.

RESULTS AND DISCUSSION

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Recording Heads

NiFe is widely used in the magnetic recording thin film head industry as the pole yoke material due to its intrinsic characteristics such as high permeability, low coercivity, low anisotropy near zero magnetrostriction and good thermal stability etc. . Magnetic properties of a recording head were found to strongly dependent on how its pole yoke was fabricated. Values of bulk magnetic properties and grain size of sputtered and plated NiFe films are listed in Table 1. Diffraction patterns for the films are plotted In Fig. 1. It shows that films with different magnetic properties had different intesity ratios of (111) / (220). In addition, the (111) peaks for the films appeared at different 2 theta positions. This indicates that the lattice constants for the films were different. Figure 2 shows changes in the (111) peak intensity and 2 theta position when measured along the easy and the hard axes. Therefore, it is possible during the switching of the magnetic field that the contribution of magnetostriction energy from stress anisotropy could cause the magnetic hysteresis loop to close[5].

Table 1. Bulk Magnetic Properties and XRD grain size of NiFe(x)

Sample Easy Axis Hc(Oe) HK (Oe) Hard Axis Loop Grain size

Sputtering (250Å) 1.07 4.9 Closed

(2.4um) 0.23 2.9 Closed 195Å Plating NiFe 0.35 2.4 slightly open 124Å High Ms(NiFeX) 0.4 5.9 slightly open 84Å

Fig. 1. Diffraction patterns for plated and sputtered films

Fig. 2. GIXRD patterns measured along easy and hard directions

Fig. 3b. GIXRD patterns of cobalt-based alloy films with Hc 1790 and 2550 Oe

Recording Disks

For longitudinal recording, values of coercivity(Hc) of cobalt-based alloy films are found to depend on the crystal orientation of Co (see Fig. 3a). Hc increased with the increasing intensities of Co(100) and Co(101) while Co(002) remained unchanged. To increase Hc further, the cobalt c-axis needs to be

compressed more into the plane of film as shown in Fig. 3b where the Co(002) intensity decreased and this is same as the previous report [3].

The bit error rate(BER) which is mostly important in magnetic recording disk drives can be related to the cobalt crystal orientation. As shown in Fig. 4 and Table 2, sample A3 with a higher intensity of Co(100) and a lower intensity of Co(002) had 2 to 3 orders of magnitude lower value of BER for on-track errors, better off-track capability and better in PW50 than those of sample B1. Table 2 also shows that A3 and B1 had the same value of Hc and magnetic remanence moment thickness(Mrt) at the same value of signal to noise ratio. Since the samples A1,A2,A3 had the same crystal struture, therefore their magnetic performance remained unchanged except the overwrite(OW) and the isolation pulse width(PW50) became worse.The reason is due to the increasing physical thickness which causes the recording space to be lost.

Fig.4. GIXRD patterns of samples A3 and B1

Table 2. Parametic and Bit Error Rate of Samples with the same Hc

Sample Mrt SNRw OW AMP PW50 OnTrack Error Rate Off Track Capability

A1 2.0 34 32 287 511(nm) 7E-11 1.46(um) A2 2.5 34 29 348 528 7E-11 1.49 A3 3.0 35 24 406 555 2E-11 1.49 B1 3.0 35 23 389 628 4E-09 1.36 In recent years, the areal density of magnetic recording storage has been increasing rapidly. To obtain an adequate BER at this higher density, a lower Mrt and higher Hc are needed. Therefore, overwrite becomes an important issue due to high coercivity of the media and the head’s writability. From N. Bertram [6], OW is described as follows:

OW=20 log{1.41Mrt(d+0.5t)/4BQrHc }

where the d, B, t, Q, r are the fly height, flux change per inch, film thickness, head efficiency, write bubble respectively. This approximation, however, does not differentiate the choice of magnetic material such as cobalt based alloys, which have different saturation moment(Ms). Therefore , replacing Mrt with Ms*S*t (where S is the moment squareness) would be more appropriate. In this case, for material with a higher Ms, less spacing loss during the recording process will increase OW . However, film fabricated by using different process conditions of same alloy target material, could produce a different cobalt crystal structure which would affect OW. For instance, the same magnetic material with the same Hc/Mrt, GIXRD shows that sample B with a lower (002) and a higher (101) had overwrite 5dB better than sample A

(Fig 5a). In Figure 5b, the enhancement of Co(100) of sample C improved overwrite by 7 dB over sample D but the signal-to-noise ratios remained the same.

Fig.5a/5b. GIXRD patterns of samples A, B, C and D

CONCLUDING REMARKS

Information obtained using the GIXRD technique has enhanced understanding of the relationship of crystal structure to thin films bulk magnetic properties and their recording performance. The face centered cubic structure of soft magnetic NiFe(X) material with (111) oriented film gave better high frequency response. The magnetic rotation from easy to hard axis, the magnetostriction energy due to the stress anisotropy could cause the magnetic hysteresis loop to close. The hexagonal close packing of hard magnetic cobalt alloys with c-axis in the film’s surface and (100) oriented film showed good performance, as did the media used in Shi’s report[7]. Based on the results of this study, a demo disk with well-controlled of crystal structure characteristics was made. Fig 6 shows a magnetic force microscopy image of the disk with recording density at 250KFCI; this density is very closing to 270KFCI (5 Gb/in2) reported by Tseng [8].

Fig. 6. Magnetic force microscopy image of disk with recording density at 250KFCI ACKNOWLEDGMENT:

I would like to thank Drs. Jei-Wei Chang and Dan Parker for their technical assistance

References:

1) C. Denis Mee Magnetic Recording Vol.1,2,3 published by McGraw-Hill Book Company 1988

2) Ting Huang, Adv. X-Ray Anal. Vol.33 P91-100, 1990 3) Po-Wen Wang, Adv. X-Ray Anal. Vol.36 P197-202, 1993 4) Po-Wen Wang, J. Appl. Phys. 73(10), P6692-6694, 15 May 1993

5) Cullity Intro. To Magnetic Materials, Ch.8, published by Addison-Wesley Company 1972 6) Neal Bertram Theory of Magnetic Recording Chapter 9,Cambrige Uni. press 7) X. Shi, IEEE Tran. On Magnetics Vol.33 p2896-2901, Sept.1997