- Email: [email protected]
Competing interaction effects in recording media
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 316 (2007) 203–205 www.elsevier.com/locate/jmmm
Competing interaction effects in recording media S.J.F. Chadwicka, M.A. Gonzalez-Fernandezb, K. O’Gradya, a
The Department of Physics, The University of York, Heslington, York YO10 5DD, UK National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, UK
b
Available online 28 February 2007
Abstract In this work we present a study of the effects of easy axis alignment on coupling effects in recording media. We have studied both particulate and thin film media representing both flexible and rigid disc materials. The texture in the systems has been determined via measurements of the angular dependence of remanence and the coupling determined using the well-established DM technique. In particulate media only dipolar coupling is present whereas in thin films intergranular exchange effects also occur. In our thin film samples the exchange coupling is weak so that dipolar effects dominate. We find that the dipolar effects in both systems increase with texture, increasing their dominance over exchange effects. r 2007 Elsevier B.V. All rights reserved. Keywords: Recording media; Dipolar coupling; Exchange interactions; Texture effects
1. Introduction In magnetic recording media it is important to understand the causes of noise production in the system in order to keep it to a minimum. A large source of noise in a recording system arises from interactions between magnetic moments, which affect the measured signal [1]. The two types of interaction of importance are intergranular exchange coupling and dipole–dipole coupling. Intergranular exchange coupling is where the exchange interaction between neighbouring grains causes them to want to be pointing in the same direction as each other. This causes problems between oppositely oriented bits of data, as it will lead to wide transitions. The dipole–dipole interaction is where moments that are oppositely oriented have a demagnetising effect on each other and moments in the same orientation have a magnetising effect on each other. However, the dipole–dipole interaction is a net demagnetising effect. The level of coupling that exists in a medium can be determined by the well-established DM technique [1], which was interpreted definitively by Zhu and Bertram [2]. It is generally understood that a positive DM is from exchange Corresponding author. Tel.: +44 1904 432289; fax: +44 1904 432247.
E-mail address: [email protected] (K. O’Grady). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.076
interactions and negative values from dipolar effects. Dipolar effects are always present in granular materials and exchange interactions occur in thin film materials depending on the degree of phase segregation at grain boundaries. Hence the two effects compete. We have compared DM curves for both particulate media and granular thin film media as a function of the degree of texture in the system. This has been done using specially made samples with carefully controlled degrees of texture. 2. Samples and experiment The thin film samples were produced by Hitachi GST (USA) and consist of alloy multilayers and a protective overcoat. The detailed structure is shown in Fig. 1. Texture is applied to the substrate via a soft pad wetted with a slurry that contains micron-sized diamonds. The rotation of the disc results in circumferential texture giving the correct orientation for growth of the easy axis. Varying the pressure on the pad or the time of exposure changes the resulting texture. There were four samples of this type that were studied whose properties are shown in Table 1. The particulate tape samples were specially prepared by Imation Corporation (USA). A metal particle tape was left to dry inside a solenoid with a maximum field at its centre
ARTICLE IN PRESS S.J.F. Chadwick et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 203–205
204
Fig. 1. Structure of conventional thin film media studied in this work. Table 1 Roughness properties of the four thin film samples Film
Spec.
Ra
Rq
Rp
Rv
1 2 3 4
None 120 GB 80 GB 60 GB
0.970.1 1.670.1 3.770.2 4.070.3
1.270.2 2.170.1 5.170.4 5.370.4
5.470.4 10.772.4 19.578.6 14.773.0
12.575.9 22.574.9 51.9712.5 52.0717.5
Ra, average roughness; Rq, RMS roughness; Rp, highest peak; Rv, deepest valley.
of 6 kOe. The field reduces progressively towards the ends of the solenoid and is essentially zero a few tens of centimeters outside the solenoid. Thus it provides a continuous sample with varying degrees of particle easy axis alignment along its length. Small samples of tape were cut at different lengths, x, along the tape. x ¼ 0 cm was from the centre of the solenoid, x ¼ 20 cm was at 20 cm from the centre and x ¼ 50 cm was 50 cm from centre and has the least alignment. For each of the thin film and particulate tape samples, magnetic measurements were made using an alternating gradient force magnetometer (AGFM). Hysteresis loops were measured followed by DM curves. DM curves are obtained from measurements of the DC demagnetisation remanence (DCD) and the isothermal remanent magnetisation (IRM). Firstly the DCD curve was measured, and then the sample was DCE demagnetised before the IRM curve was measured. The DCE consists of demagnetising the sample through minor hysteresis loops with a stepwise field reduction DH [3]. In each remanence curve, the values of Mr(H) were normalised to the maximum value, Mr(N). With the normalised values of Mr from both DCD and IRM measurements, the DM curves were found from Eq. (1) [1]. DM ¼ M DCD ð1 2M IRM Þ.
Stoner–Wohlfarth theory, which predicts that hysteresis loops are more square in more aligned systems. There is also an increase in the width of the switching field distribution as the alignment decreases, but the coercivity remains roughly constant because increased alignment increases the coercivity, which is cancelled out by an increased dipolar interaction. Fig. 3 shows the DM curves for the three tape samples with different amounts of alignment. It is clear that the sample from the centre of the solenoid with the highest alignment has a highly dipolar interaction due to the large negative DM values. Moving away from the centre of the solenoid, as the alignment gets less, the dipolar interaction becomes less pronounced. This is because dipolar interactions are stronger for highly aligned systems. Due to the discrete nature of particulate media, there is no exchange coupling interaction between particles and so the interaction is purely dipolar. M/ Ms
1.0
0.5
0.0
-0.5 x = 0cm x = 20cm x = 50cm
-1.0 -10
-8
-6
-4
-2
0 2 H (kOe)
4
6
8
10
Fig. 2. Hysteresis loops of particulate media samples with different degrees of alignment of easy axes.
0.1 ΔM
0.0
-0.1
(1)
From these measurements it was possible to examine the effect of texture on the relative amounts of exchange and dipole interactions.
-0.2
-0.3
x = 0cm x = 20cm x = 50cm
3. Results -0.4
Fig. 2 shows the hysteresis loops for samples of tape at different distances from the solenoid centre. There is a clear decrease in the squareness ratio the further the sample is from the x ¼ 0 cm position. This is in agreement with
0
1
2
3 H (kOe)
4
5
6
Fig. 3. DM curves for the particulate media samples with different degrees of alignment of easy axes.
ARTICLE IN PRESS S.J.F. Chadwick et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 203–205 M/Ms
1.0
0.5
0.0
-0.5 4 1
-1.0 -10
-8
-6
-4
-2
0 2 H (kOe)
4
6
8
10
Fig. 4. Hysteresis loops of two of the thin film samples. Sample 4 has highest degree of texture and sample 1 has no texture.
0.0
ΔM
-0.2
-0.4
205
curves. So far there has only been one report of a negative DM profile in a thin film medium which had an unusual structure [4]. The original report on DM curves [5] was followed by a detailed systematic experimental study by Mayo et al. [6], which was reproduced in detail in a computer simulation by Zhu and Bertram [2]. In both these studies the exchange coupling was varied systematically, eventually a negative DM profile resulted only when the exchange coupling he( ¼ HE/HK) was zero. Here HE is a field representing the intergranular RKKY effect. Of course for all samples and in the model the dipolar coupling was constant. For the case where he was zero a DM amplitude of 0.2 was achieved. Thus, the exact meaning of DM is that it represents in someway the difference between the effects of dipolar and exchange interactions. Thus in this case it appears that the magnitude of these effects is very similar. The data also show that for all samples in this set dipolar coupling is dominant in all cases and increases systematically with texture. Given that the dipolar interaction goes as cosy (Ei ¼ (m1, m2)/r3), where y is the angle between the moments of the interacting grains, it is apparent that the variation of he, which is normalised to HK increases less rapidly. The energy required for the exchange field to rotate the moment away from the easy axis goes as sin2 f where f is the angle between the easy axis and the moment.
-0.6
-0.8
-1.0
4. Conclusion
TFM1 TFM2 TFM3 TFM4
0
1
2
3 H (kOe)
4
5
6
Fig. 5. DM curves for all four thin film media samples that were studied.
Fig. 4 shows the hysteresis loops obtained for two of the thin film samples. The unusual shape of the loop is due to the NiP smoothing layer. This layer is deposited electrolessly and is therefore amorphous and non-magnetic; however, substrate heating during the sputtering process causes it to crystallise and become ferromagnetic. The reversal of NiP occurs at around zero field and is isotropic and so does not affect the measurements. Comparison of the DCD curves showed that the remanent coercivity and remanence are higher for the samples with the higher degree of texture. Fig. 5 shows the DM curves obtained for the thin film media samples. It is important to note the maximum magnitude of DM at 0.08 is an order of magnitude smaller than that for the particulate media shown in Fig. 3. Also it is clear that for all samples in this set, DM is negative indicating predominantly demagnetising dipolar interactions. This is a surprising and unexpected result, which has required a reinterpretation of our understanding of DM
We have seen that both interaction effects can affect the magnetic and recording properties of media. Thin film media texture increases the interactions that lead to correlated grains and thus noise. If thin film media could be made having isolated grains with no exchange interactions, then the remanence would improve and noise would decrease. Acknowledgements The samples are provided by Hitachi GST and Imation Corporation. Additional funding is from Dowa Mining Co. Ltd. References [1] P.I. Mayo, K. O’Grady, P.E. Kelly, J. Cambridge, I.L. Sanders, T. Yogi, R.W. Chantrell, J. Appl. Phys. 69 (1991) 4733. [2] J.G. Zhu, H.N. Bertram, J. Appl. Phys. 69 (1991) 4709. [3] M. El-Hilo, K. O’Grady, R.W. Chantrell, J. Magn. Magn. Mater. 120 (1993) 244. [4] J.S. Li, M. Mirzamaani, X.P. Bian, M. Doerner, S. Duan, K. Tang, M. Toney, T. Arnoldussen, M. Madison, J. Appl. Phys. 85 (1999) 4286. [5] P.E. Kelly, K. O’Grady, P.I. Mayo, R.W. Chantrell, IEEE Trans. Magn. 25 (1989) 3880. [6] P.I. Mayo, K. O’Grady, R.W. Chantrell, J.A. Cambridge, I.L. Sanders, T. Yogi, J.K. Howard, J. Magn. Magn. Mater. 95 (1991) 109.
Journal of Magnetism and Magnetic Materials 316 (2007) 203–205 www.elsevier.com/locate/jmmm
Competing interaction effects in recording media S.J.F. Chadwicka, M.A. Gonzalez-Fernandezb, K. O’Gradya, a
The Department of Physics, The University of York, Heslington, York YO10 5DD, UK National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, UK
b
Available online 28 February 2007
Abstract In this work we present a study of the effects of easy axis alignment on coupling effects in recording media. We have studied both particulate and thin film media representing both flexible and rigid disc materials. The texture in the systems has been determined via measurements of the angular dependence of remanence and the coupling determined using the well-established DM technique. In particulate media only dipolar coupling is present whereas in thin films intergranular exchange effects also occur. In our thin film samples the exchange coupling is weak so that dipolar effects dominate. We find that the dipolar effects in both systems increase with texture, increasing their dominance over exchange effects. r 2007 Elsevier B.V. All rights reserved. Keywords: Recording media; Dipolar coupling; Exchange interactions; Texture effects
1. Introduction In magnetic recording media it is important to understand the causes of noise production in the system in order to keep it to a minimum. A large source of noise in a recording system arises from interactions between magnetic moments, which affect the measured signal [1]. The two types of interaction of importance are intergranular exchange coupling and dipole–dipole coupling. Intergranular exchange coupling is where the exchange interaction between neighbouring grains causes them to want to be pointing in the same direction as each other. This causes problems between oppositely oriented bits of data, as it will lead to wide transitions. The dipole–dipole interaction is where moments that are oppositely oriented have a demagnetising effect on each other and moments in the same orientation have a magnetising effect on each other. However, the dipole–dipole interaction is a net demagnetising effect. The level of coupling that exists in a medium can be determined by the well-established DM technique [1], which was interpreted definitively by Zhu and Bertram [2]. It is generally understood that a positive DM is from exchange Corresponding author. Tel.: +44 1904 432289; fax: +44 1904 432247.
E-mail address: [email protected] (K. O’Grady). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.076
interactions and negative values from dipolar effects. Dipolar effects are always present in granular materials and exchange interactions occur in thin film materials depending on the degree of phase segregation at grain boundaries. Hence the two effects compete. We have compared DM curves for both particulate media and granular thin film media as a function of the degree of texture in the system. This has been done using specially made samples with carefully controlled degrees of texture. 2. Samples and experiment The thin film samples were produced by Hitachi GST (USA) and consist of alloy multilayers and a protective overcoat. The detailed structure is shown in Fig. 1. Texture is applied to the substrate via a soft pad wetted with a slurry that contains micron-sized diamonds. The rotation of the disc results in circumferential texture giving the correct orientation for growth of the easy axis. Varying the pressure on the pad or the time of exposure changes the resulting texture. There were four samples of this type that were studied whose properties are shown in Table 1. The particulate tape samples were specially prepared by Imation Corporation (USA). A metal particle tape was left to dry inside a solenoid with a maximum field at its centre
ARTICLE IN PRESS S.J.F. Chadwick et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 203–205
204
Fig. 1. Structure of conventional thin film media studied in this work. Table 1 Roughness properties of the four thin film samples Film
Spec.
Ra
Rq
Rp
Rv
1 2 3 4
None 120 GB 80 GB 60 GB
0.970.1 1.670.1 3.770.2 4.070.3
1.270.2 2.170.1 5.170.4 5.370.4
5.470.4 10.772.4 19.578.6 14.773.0
12.575.9 22.574.9 51.9712.5 52.0717.5
Ra, average roughness; Rq, RMS roughness; Rp, highest peak; Rv, deepest valley.
of 6 kOe. The field reduces progressively towards the ends of the solenoid and is essentially zero a few tens of centimeters outside the solenoid. Thus it provides a continuous sample with varying degrees of particle easy axis alignment along its length. Small samples of tape were cut at different lengths, x, along the tape. x ¼ 0 cm was from the centre of the solenoid, x ¼ 20 cm was at 20 cm from the centre and x ¼ 50 cm was 50 cm from centre and has the least alignment. For each of the thin film and particulate tape samples, magnetic measurements were made using an alternating gradient force magnetometer (AGFM). Hysteresis loops were measured followed by DM curves. DM curves are obtained from measurements of the DC demagnetisation remanence (DCD) and the isothermal remanent magnetisation (IRM). Firstly the DCD curve was measured, and then the sample was DCE demagnetised before the IRM curve was measured. The DCE consists of demagnetising the sample through minor hysteresis loops with a stepwise field reduction DH [3]. In each remanence curve, the values of Mr(H) were normalised to the maximum value, Mr(N). With the normalised values of Mr from both DCD and IRM measurements, the DM curves were found from Eq. (1) [1]. DM ¼ M DCD ð1 2M IRM Þ.
Stoner–Wohlfarth theory, which predicts that hysteresis loops are more square in more aligned systems. There is also an increase in the width of the switching field distribution as the alignment decreases, but the coercivity remains roughly constant because increased alignment increases the coercivity, which is cancelled out by an increased dipolar interaction. Fig. 3 shows the DM curves for the three tape samples with different amounts of alignment. It is clear that the sample from the centre of the solenoid with the highest alignment has a highly dipolar interaction due to the large negative DM values. Moving away from the centre of the solenoid, as the alignment gets less, the dipolar interaction becomes less pronounced. This is because dipolar interactions are stronger for highly aligned systems. Due to the discrete nature of particulate media, there is no exchange coupling interaction between particles and so the interaction is purely dipolar. M/ Ms
1.0
0.5
0.0
-0.5 x = 0cm x = 20cm x = 50cm
-1.0 -10
-8
-6
-4
-2
0 2 H (kOe)
4
6
8
10
Fig. 2. Hysteresis loops of particulate media samples with different degrees of alignment of easy axes.
0.1 ΔM
0.0
-0.1
(1)
From these measurements it was possible to examine the effect of texture on the relative amounts of exchange and dipole interactions.
-0.2
-0.3
x = 0cm x = 20cm x = 50cm
3. Results -0.4
Fig. 2 shows the hysteresis loops for samples of tape at different distances from the solenoid centre. There is a clear decrease in the squareness ratio the further the sample is from the x ¼ 0 cm position. This is in agreement with
0
1
2
3 H (kOe)
4
5
6
Fig. 3. DM curves for the particulate media samples with different degrees of alignment of easy axes.
ARTICLE IN PRESS S.J.F. Chadwick et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 203–205 M/Ms
1.0
0.5
0.0
-0.5 4 1
-1.0 -10
-8
-6
-4
-2
0 2 H (kOe)
4
6
8
10
Fig. 4. Hysteresis loops of two of the thin film samples. Sample 4 has highest degree of texture and sample 1 has no texture.
0.0
ΔM
-0.2
-0.4
205
curves. So far there has only been one report of a negative DM profile in a thin film medium which had an unusual structure [4]. The original report on DM curves [5] was followed by a detailed systematic experimental study by Mayo et al. [6], which was reproduced in detail in a computer simulation by Zhu and Bertram [2]. In both these studies the exchange coupling was varied systematically, eventually a negative DM profile resulted only when the exchange coupling he( ¼ HE/HK) was zero. Here HE is a field representing the intergranular RKKY effect. Of course for all samples and in the model the dipolar coupling was constant. For the case where he was zero a DM amplitude of 0.2 was achieved. Thus, the exact meaning of DM is that it represents in someway the difference between the effects of dipolar and exchange interactions. Thus in this case it appears that the magnitude of these effects is very similar. The data also show that for all samples in this set dipolar coupling is dominant in all cases and increases systematically with texture. Given that the dipolar interaction goes as cosy (Ei ¼ (m1, m2)/r3), where y is the angle between the moments of the interacting grains, it is apparent that the variation of he, which is normalised to HK increases less rapidly. The energy required for the exchange field to rotate the moment away from the easy axis goes as sin2 f where f is the angle between the easy axis and the moment.
-0.6
-0.8
-1.0
4. Conclusion
TFM1 TFM2 TFM3 TFM4
0
1
2
3 H (kOe)
4
5
6
Fig. 5. DM curves for all four thin film media samples that were studied.
Fig. 4 shows the hysteresis loops obtained for two of the thin film samples. The unusual shape of the loop is due to the NiP smoothing layer. This layer is deposited electrolessly and is therefore amorphous and non-magnetic; however, substrate heating during the sputtering process causes it to crystallise and become ferromagnetic. The reversal of NiP occurs at around zero field and is isotropic and so does not affect the measurements. Comparison of the DCD curves showed that the remanent coercivity and remanence are higher for the samples with the higher degree of texture. Fig. 5 shows the DM curves obtained for the thin film media samples. It is important to note the maximum magnitude of DM at 0.08 is an order of magnitude smaller than that for the particulate media shown in Fig. 3. Also it is clear that for all samples in this set, DM is negative indicating predominantly demagnetising dipolar interactions. This is a surprising and unexpected result, which has required a reinterpretation of our understanding of DM
We have seen that both interaction effects can affect the magnetic and recording properties of media. Thin film media texture increases the interactions that lead to correlated grains and thus noise. If thin film media could be made having isolated grains with no exchange interactions, then the remanence would improve and noise would decrease. Acknowledgements The samples are provided by Hitachi GST and Imation Corporation. Additional funding is from Dowa Mining Co. Ltd. References [1] P.I. Mayo, K. O’Grady, P.E. Kelly, J. Cambridge, I.L. Sanders, T. Yogi, R.W. Chantrell, J. Appl. Phys. 69 (1991) 4733. [2] J.G. Zhu, H.N. Bertram, J. Appl. Phys. 69 (1991) 4709. [3] M. El-Hilo, K. O’Grady, R.W. Chantrell, J. Magn. Magn. Mater. 120 (1993) 244. [4] J.S. Li, M. Mirzamaani, X.P. Bian, M. Doerner, S. Duan, K. Tang, M. Toney, T. Arnoldussen, M. Madison, J. Appl. Phys. 85 (1999) 4286. [5] P.E. Kelly, K. O’Grady, P.I. Mayo, R.W. Chantrell, IEEE Trans. Magn. 25 (1989) 3880. [6] P.I. Mayo, K. O’Grady, R.W. Chantrell, J.A. Cambridge, I.L. Sanders, T. Yogi, J.K. Howard, J. Magn. Magn. Mater. 95 (1991) 109.