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Correlation of fluctuation field and temperature coefficient of coercivity in a CoCrPtB thin film
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 320 (2008) 2992–2995
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Correlation of fluctuation field and temperature coefficient of coercivity in a CoCrPtB thin film Hiroaki Nishio , Hiroshi Yamamoto Meiji University, 1-1-1 Higashi-mita, Tama-ku, Kawasaki 214-8571, Japan
a r t i c l e in f o
a b s t r a c t
Available online 6 August 2008
The effect of the ratio of fluctuation field (Hf) to coercivity (Hc) on the temperature coefficient of coercivity [a(Hc)] was investigated for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10 Ta4/Cr thin films (longitudinal recording media) with very small average grain volume (Vphy). a(Hc) increases markedly with increase in temperature between near 250 and 350 K for Co55Cr15.5Pt28 B1.5/Co63Cr37/Cr thin films. a(Hc) is approximately proportional to the ratio Hf/Hc for all thin films, as in the case of advanced data backup tapes prepared from ultrafine particles. a(Hc) and the ratio Hf/Hc increase as Vphy decreases. Smaller Hf/Hc values are necessary for small a(Hc) values, which is very important for the thermal stability of high-density recording media with very small Vphy. & 2008 Elsevier B.V. All rights reserved.
Keywords: Temperature coefficient of coercivity Thermal stability Fluctuation field CoCrPtB thin film
1. Introduction It is particularly important to reduce the average grain volume (Vphy) and the intergranular interactions in thin films for longitudinal recording media with high coercivity (Hc) to achieve high recording density with low noise. However, excessive reduction of Vphy degrades both the magnetization decay [DM/(ln t1–ln t2)] due to thermal fluctuation, and also the thermal stability factor, where DM is the change in magnetization during the time interval from t1 to t2 [1,2]. The magnetic viscosity is known to depend on the fluctuation field (Hf), irreversible susceptibility (Xirr), and the logarithmic time interval (ln t1–ln t2). Hf can be expressed by the following equations [1–4]: Hf ¼ DM=½X irr ðln t 1 ln t 2 Þ ¼ kT=ðM S V act Þ
(1)
where k is Boltzmann’s constant, T the absolute temperature, Ms the saturation magnetization, and Vact the activation volume. This paper reports on the effect of the ratio of Hf to Hc on the temperature coefficient of coercivity [a(Hc)], in the temperature range from 200 to 350 K for CoCrPtB/CoCr/Cr, CoCrPtTa/Cr, and CoCrTa/Cr longitudinal recording media with very small Vphy.
2. Experimental The compositions of the magnetic thin films were Co55Cr15.5 Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr from Tel.: +81 44 934 7171; fax: +81 45 741 6979.
E-mail address: [email protected] (H. Nishio). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.08.007
electron spectroscopy for chemical analysis. Table 1 shows the average grain diameter (D) and thickness (d) obtained by transmission electron microscopy (TEM), Vphy, and fundamental magnetic properties for the thin films used in this study. The magnetic thin films were prepared with Vphy ranging from 0.9 to 5.5 1018 cm3. The listed magnetic properties of thin films are Ms, squareness (SQ), Hc, anisotropy field (HA), Vact around Hc, rotational hysteresis integral (Rh), which corresponds to the mechanism of magnetization, and maximum value in delta M plots (delta Mmax) obtained from the differential remanence measured by a highly sensitive vibrating sample magnetometer as a function of the applied field [5,6]. The delta M plots are used to investigate the interactions between crystal grains. The magnetic properties and DM/(ln t1–ln t2) were measured using a SQUID magnetometer in the same way as in Ref. [7]. The change in magnetization with a waiting time from 60 to 2000 s was measured in the reverse field after applying the maximum field of +20 kOe. Reversible susceptibility (Xrev) and total differential susceptibility (Xtot) were also measured, and Xirr was obtained by Xtot–Xrev. The rotational hysteresis loss (Wr) was measured using a highly sensitive torque magnetometer to study HA and Rh. Wr falls to zero above the HA; extrapolation of the decreasing Wr yields an intercept with the abscissa that characterizes the largest anisotropy field of the materials [8].
3. Results and discussion Magnetization curves of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69 Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films were measured in the temperature range from 200 to 350 K. The value of Hc for all thin
ARTICLE IN PRESS H. Nishio, H. Yamamoto / Journal of Magnetism and Magnetic Materials 320 (2008) 2992–2995
2993
Table 1 Average grain sizes, volume, and fundamental magnetic properties for CoCr thin films at 300 K used in this study Material
D d (nm)
Vphy (1018 cm3)
Ms (emu/cm3)
SQ
Hc (kOe)
HA (kOe)
Vact (1018 cm3)
Rh
Delta Mmax
Co55Cr15.5Pt28B1.5/Co63Cr37/Cr Co69Cr19Pt9Ta3/Cr Co86Cr10Ta4/Cr
6.5 (8.7/11.6) 15 17 17 19
0.9 3.8 5.5
430 400 580
0.61 0.68 0.88
4.40 3.15 2.32
12.2 6.4 4.9
1.4 2.8 3.7
0.90 0.54 1.12
0.25 0.17 0.17
Coercivity (kOe)
6
ΔM/(ln t1 - ln t2) (emu/cm3)
7 CoCrPtB/CoCr/Cr
5 CoCrPtTa/Cr
4 3 2
CoCrTa/Cr
40
250 300 Temperature (K)
20 10 0 2 3 4 Magnetic field (kOe)
5
6
Fig. 3. Reverse field dependence of the change in magnetization for the logarithmic time interval DM/(ln t1–ln t2) of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films at 300 K.
0.8
2
0.6
Xirr (Oe emu/cm3)
α (Hc) (%/K)
1
0
350
Fig. 1. Temperature dependence of coercivity for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films in the temperature range between 200 and 350 K.
CoCrPtB/CoCr/Cr CoCrPtTa/Cr
1 200
CoCrTa/Cr
30
CoCrPtB/CoCr/Cr
0.4 0.2 CoCrTa/Cr
CoCrPtTa /Cr
CoCrPtTa/Cr 0.5 0
350
Fig. 2. Temperature dependence of the temperature coefficient of coercivity a(Hc) for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films in the temperature range between 200 and 350 K.
films decreases as the temperature increases, as shown in Fig. 1. The value of Hc at 350 K is approximately 22% lower than the value at 300 K for the Co55Cr15.5Pt28B1.5/Co63Cr37/Cr thin film. The dependence of the temperature variation of coercivity (DHc/DT) on the temperature was approximately linear in the temperature range of 50 K. The value of a(Hc) was derived from the –DHc/DT value dividing by the Hc at 200, 250, 300, and 350 K. Fig. 2 shows a(Hc) for these thin films obtained using the values shown in Fig. 1. In Co69Cr19Pt9Ta3/Cr and Co86Cr10Ta4/Cr thin films, the value of a(Hc) monotonically increases as the temperature increases. However, the value of a(Hc) increases markedly with increase in temperature between near 250 and 350 K for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr thin films with the smallest Vphy in this study. a(Hc) at 300 K for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films were 0.43, 0.35, and 0.26%/K, respectively. The dependence of DM of all thin films on the logarithm of time was approximately linear from 60 to 2000 s. The time dependence of DM in these thin films obeys a logarithmic law. A comparison of the reverse field dependence of DM/(ln t1–ln t2) and Xirr shows that the shape of all thin films at 300 K is similar as shown in Figs. 3 and 4. DM/(ln t1–ln t2) and Xirr have maximum values around the Hc point for all thin films. Fig. 5 shows the
1
0
2 3 4 Magnetic field (kOe)
5
6
Fig. 4. Reverse field dependence of the irreversible susceptibility Xirr of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films at 300 K.
80 Fluctuation field (Oe)
250 300 Temperature (K)
CoCrPtB/CoCr/Cr
1
0 200
CoCrTa/Cr
1.5
CoCrPtB/CoCr/Cr 60 Hc CoCrPtTa/Cr
40
Hc CoCrTa/Cr
20 Hc
0 0
1
2 3 4 Magnetic field (kOe)
5
6
Fig. 5. Reverse field dependence of the fluctuation field of Co55Cr15.5Pt28B1.5/ Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films at 300 K.
reverse field dependence of Hf as obtained from DM/(ln t1–ln t2) and Xirr at 300 K. Hf is not constant, but depends on the reverse field. Hf approximately remains a constant value around Hc for all thin films. The value of Vact obtained from Eq. (1) is a major factor affecting the media noise and recorded signal decay for highdensity recording media [9,10]. Vact is expected to be equal to Vphy
ARTICLE IN PRESS 2994
H. Nishio, H. Yamamoto / Journal of Magnetism and Magnetic Materials 320 (2008) 2992–2995
0.8
CoCrPtB/CoCr/Cr 60 α(Hc) (%/K)
Fluctuation field (Oe)
80
CoCrPtTa/Cr
40
CoCrTa/Cr
20
CoCrTa /Cr 0.4 0.2
0
CoCrPtB/CoCr/Cr
0
200
250 300 Temperature (K)
350
5
10
15 Hf / Hc
Fig. 6. Temperature dependence of the fluctuation field of Co55Cr15.5Pt28B1.5/ Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films.
20
(10-3)
Fig. 8. Trend of the temperature coefficient of coercivity a(Hc) with the ratio Hf/Hc of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films.
20
25 20
CoCrPtB/CoCr/Cr
Hc /Hf (10-3)
Hf / Hc (10-3)
CoCrPtTa/Cr
0.6
15 10 5
CoCrTa/Cr
250 300 Temperature (K)
12 8
CoCrPtTa/Cr
0 200
16
4
350
Fig. 7. Temperature dependence of the ratio Hf/Hc for Co55Cr15.5Pt28B1.5/Co63Cr37/ Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films.
for a magnetically isolated single domain particle, and is the minimum unit volume of magnetization reversal [3,11]. In the case of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr thin films with the smallest Vphy, Vphy was smaller than Vact as shown in Table 1. For all thin films, delta M increases to a peak around Hc and then decreases as the magnetic field increases. It exhibits positive interactions between particles, and the value of delta Mmax for Co55Cr15.5Pt28 B1.5/Co63Cr37/Cr double-layered thin films is larger than those of the other thin films. Fig. 6 shows the temperature dependence of Hf around Hc for these thin films. The change of slope of Hf with temperature is small for Co69Cr19Pt9Ta3/Cr and Co86Cr10Ta4/Cr thin films. However, the value of Hf increases markedly with increase in temperature for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr thin films with very small Vphy. In order to decrease the Hf value, it is necessary to improve both the distribution of grain volume and the ratio Hc/HA [12]. Fig. 7 shows the temperature dependence of the ratio Hf/Hc using the data presented in Figs. 1 and 6. A comparison of the temperature dependence of a(Hc) and the ratio Hf/Hc shows that the shape is similar for all thin films. The value of Hf/Hc increases markedly with increase in temperature between near 250 and 350 K as in the case of a(Hc) for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr thin films with very small Vphy. The variation in Hf/Hc of Co55Cr15.5Pt28 B1.5/Co63Cr37/Cr thin films was larger than those of the other thin films. The values of a(Hc) are plotted as a function of the ratio Hf/Hc in Fig. 8. a(Hc) is approximately proportional to the ratio Hf/Hc for all thin films, as in the case of advanced data backup tapes prepared from ultrafine acicular metal particulate composite and hexagonal platelet Ba-ferrite particles [13]. The value of the ratio Hf/Hc increases as Vphy decreases, as shown in Fig. 9. The following
0
1
2
3
4 -18
Mean volume (10
5
6
3
cm )
Fig. 9. Trend of the ratio Hf/Hc with mean volume of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films.
relation of a(Hc) to Hf/Hc also had been found to hold in these thin films, as in the case of advanced data backup tapes, rare earth and SrLaCo ferrite magnets [10,13,14]. a(Hc) was expressed as [28.5k(qE/qT)H]Hf/(HckT), where E and H were the activation energy and the magnetic field, respectively [10,13,14]. The equation shows that smaller Hf/Hc values are necessary for small a(Hc) values, which is very important for the thermal stability of high-density recording media with very small Vphy. 4. Conclusion The effect of the ratio Hf/Hc on a(Hc) was investigated for CoCr thin films with very small average Vphy. The value of a(Hc) is approximately proportional to the ratio Hf/Hc for all thin films. a(Hc) and the ratio Hf/Hc increase as Vphy decreases. Smaller Hf/Hc values are necessary for small a(Hc) values, which is very important for the thermal stability of high-density recording media with very small Vphy. Acknowledgements The authors would like to thank Tamakawa Co., Ltd. for the use of a highly sensitive torque magnetometer and Dr. Mu-Pei Chen for chemical analysis and TEM observation of thin films. References [1] [2] [3] [4]
R. Street, J.C. Woolley, Proc. Phys. Soc. A 62 (1949) 562. L. Neel, J. Phys. Rad. 11 (1950) 49. E.P. Wohlfarth, J. Phys. F 14 (1984) L155. P. Gaunt, J. Appl. Phys. 59 (1986) 4129.
ARTICLE IN PRESS H. Nishio, H. Yamamoto / Journal of Magnetism and Magnetic Materials 320 (2008) 2992–2995
[5] D.E. Speliotis, W. Lynch, J. Appl. Phys. 69 (1991) 4496. [6] 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. [7] H. Nishio, IEEE Trans. Magn. 24 (1988) 3188. [8] D.M. Paige, S.R. Hoon, B.K. Tanner, K. O’Grady, IEEE Trans. Magn. 20 (1984) 1852. [9] H. Nishio, J. Magn. Soc. Jpn. 21 (S22) (1997) 301.
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[10] H. Nishio, J. Magn. Soc. Jpn. 22 (1998) 3. [11] H. Nishio, Y. Ota, M. Maekawa, H. Yamamoto, J. Magn. Magn. Mater. 287 (2005) 234. [12] R. Street, S.D. Brown, J. Appl. Phys. 76 (1994) 6386. [13] H. Nishio, H. Yamamoto, J. Appl. Phys. 101 (2007) 123904. [14] H. Nishio, H. Yamamoto, J. Magn. Soc. Jpn. 31 (2007) 67.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Correlation of fluctuation field and temperature coefficient of coercivity in a CoCrPtB thin film Hiroaki Nishio , Hiroshi Yamamoto Meiji University, 1-1-1 Higashi-mita, Tama-ku, Kawasaki 214-8571, Japan
a r t i c l e in f o
a b s t r a c t
Available online 6 August 2008
The effect of the ratio of fluctuation field (Hf) to coercivity (Hc) on the temperature coefficient of coercivity [a(Hc)] was investigated for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10 Ta4/Cr thin films (longitudinal recording media) with very small average grain volume (Vphy). a(Hc) increases markedly with increase in temperature between near 250 and 350 K for Co55Cr15.5Pt28 B1.5/Co63Cr37/Cr thin films. a(Hc) is approximately proportional to the ratio Hf/Hc for all thin films, as in the case of advanced data backup tapes prepared from ultrafine particles. a(Hc) and the ratio Hf/Hc increase as Vphy decreases. Smaller Hf/Hc values are necessary for small a(Hc) values, which is very important for the thermal stability of high-density recording media with very small Vphy. & 2008 Elsevier B.V. All rights reserved.
Keywords: Temperature coefficient of coercivity Thermal stability Fluctuation field CoCrPtB thin film
1. Introduction It is particularly important to reduce the average grain volume (Vphy) and the intergranular interactions in thin films for longitudinal recording media with high coercivity (Hc) to achieve high recording density with low noise. However, excessive reduction of Vphy degrades both the magnetization decay [DM/(ln t1–ln t2)] due to thermal fluctuation, and also the thermal stability factor, where DM is the change in magnetization during the time interval from t1 to t2 [1,2]. The magnetic viscosity is known to depend on the fluctuation field (Hf), irreversible susceptibility (Xirr), and the logarithmic time interval (ln t1–ln t2). Hf can be expressed by the following equations [1–4]: Hf ¼ DM=½X irr ðln t 1 ln t 2 Þ ¼ kT=ðM S V act Þ
(1)
where k is Boltzmann’s constant, T the absolute temperature, Ms the saturation magnetization, and Vact the activation volume. This paper reports on the effect of the ratio of Hf to Hc on the temperature coefficient of coercivity [a(Hc)], in the temperature range from 200 to 350 K for CoCrPtB/CoCr/Cr, CoCrPtTa/Cr, and CoCrTa/Cr longitudinal recording media with very small Vphy.
2. Experimental The compositions of the magnetic thin films were Co55Cr15.5 Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr from Tel.: +81 44 934 7171; fax: +81 45 741 6979.
E-mail address: [email protected] (H. Nishio). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.08.007
electron spectroscopy for chemical analysis. Table 1 shows the average grain diameter (D) and thickness (d) obtained by transmission electron microscopy (TEM), Vphy, and fundamental magnetic properties for the thin films used in this study. The magnetic thin films were prepared with Vphy ranging from 0.9 to 5.5 1018 cm3. The listed magnetic properties of thin films are Ms, squareness (SQ), Hc, anisotropy field (HA), Vact around Hc, rotational hysteresis integral (Rh), which corresponds to the mechanism of magnetization, and maximum value in delta M plots (delta Mmax) obtained from the differential remanence measured by a highly sensitive vibrating sample magnetometer as a function of the applied field [5,6]. The delta M plots are used to investigate the interactions between crystal grains. The magnetic properties and DM/(ln t1–ln t2) were measured using a SQUID magnetometer in the same way as in Ref. [7]. The change in magnetization with a waiting time from 60 to 2000 s was measured in the reverse field after applying the maximum field of +20 kOe. Reversible susceptibility (Xrev) and total differential susceptibility (Xtot) were also measured, and Xirr was obtained by Xtot–Xrev. The rotational hysteresis loss (Wr) was measured using a highly sensitive torque magnetometer to study HA and Rh. Wr falls to zero above the HA; extrapolation of the decreasing Wr yields an intercept with the abscissa that characterizes the largest anisotropy field of the materials [8].
3. Results and discussion Magnetization curves of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69 Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films were measured in the temperature range from 200 to 350 K. The value of Hc for all thin
ARTICLE IN PRESS H. Nishio, H. Yamamoto / Journal of Magnetism and Magnetic Materials 320 (2008) 2992–2995
2993
Table 1 Average grain sizes, volume, and fundamental magnetic properties for CoCr thin films at 300 K used in this study Material
D d (nm)
Vphy (1018 cm3)
Ms (emu/cm3)
SQ
Hc (kOe)
HA (kOe)
Vact (1018 cm3)
Rh
Delta Mmax
Co55Cr15.5Pt28B1.5/Co63Cr37/Cr Co69Cr19Pt9Ta3/Cr Co86Cr10Ta4/Cr
6.5 (8.7/11.6) 15 17 17 19
0.9 3.8 5.5
430 400 580
0.61 0.68 0.88
4.40 3.15 2.32
12.2 6.4 4.9
1.4 2.8 3.7
0.90 0.54 1.12
0.25 0.17 0.17
Coercivity (kOe)
6
ΔM/(ln t1 - ln t2) (emu/cm3)
7 CoCrPtB/CoCr/Cr
5 CoCrPtTa/Cr
4 3 2
CoCrTa/Cr
40
250 300 Temperature (K)
20 10 0 2 3 4 Magnetic field (kOe)
5
6
Fig. 3. Reverse field dependence of the change in magnetization for the logarithmic time interval DM/(ln t1–ln t2) of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films at 300 K.
0.8
2
0.6
Xirr (Oe emu/cm3)
α (Hc) (%/K)
1
0
350
Fig. 1. Temperature dependence of coercivity for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films in the temperature range between 200 and 350 K.
CoCrPtB/CoCr/Cr CoCrPtTa/Cr
1 200
CoCrTa/Cr
30
CoCrPtB/CoCr/Cr
0.4 0.2 CoCrTa/Cr
CoCrPtTa /Cr
CoCrPtTa/Cr 0.5 0
350
Fig. 2. Temperature dependence of the temperature coefficient of coercivity a(Hc) for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films in the temperature range between 200 and 350 K.
films decreases as the temperature increases, as shown in Fig. 1. The value of Hc at 350 K is approximately 22% lower than the value at 300 K for the Co55Cr15.5Pt28B1.5/Co63Cr37/Cr thin film. The dependence of the temperature variation of coercivity (DHc/DT) on the temperature was approximately linear in the temperature range of 50 K. The value of a(Hc) was derived from the –DHc/DT value dividing by the Hc at 200, 250, 300, and 350 K. Fig. 2 shows a(Hc) for these thin films obtained using the values shown in Fig. 1. In Co69Cr19Pt9Ta3/Cr and Co86Cr10Ta4/Cr thin films, the value of a(Hc) monotonically increases as the temperature increases. However, the value of a(Hc) increases markedly with increase in temperature between near 250 and 350 K for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr thin films with the smallest Vphy in this study. a(Hc) at 300 K for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films were 0.43, 0.35, and 0.26%/K, respectively. The dependence of DM of all thin films on the logarithm of time was approximately linear from 60 to 2000 s. The time dependence of DM in these thin films obeys a logarithmic law. A comparison of the reverse field dependence of DM/(ln t1–ln t2) and Xirr shows that the shape of all thin films at 300 K is similar as shown in Figs. 3 and 4. DM/(ln t1–ln t2) and Xirr have maximum values around the Hc point for all thin films. Fig. 5 shows the
1
0
2 3 4 Magnetic field (kOe)
5
6
Fig. 4. Reverse field dependence of the irreversible susceptibility Xirr of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films at 300 K.
80 Fluctuation field (Oe)
250 300 Temperature (K)
CoCrPtB/CoCr/Cr
1
0 200
CoCrTa/Cr
1.5
CoCrPtB/CoCr/Cr 60 Hc CoCrPtTa/Cr
40
Hc CoCrTa/Cr
20 Hc
0 0
1
2 3 4 Magnetic field (kOe)
5
6
Fig. 5. Reverse field dependence of the fluctuation field of Co55Cr15.5Pt28B1.5/ Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films at 300 K.
reverse field dependence of Hf as obtained from DM/(ln t1–ln t2) and Xirr at 300 K. Hf is not constant, but depends on the reverse field. Hf approximately remains a constant value around Hc for all thin films. The value of Vact obtained from Eq. (1) is a major factor affecting the media noise and recorded signal decay for highdensity recording media [9,10]. Vact is expected to be equal to Vphy
ARTICLE IN PRESS 2994
H. Nishio, H. Yamamoto / Journal of Magnetism and Magnetic Materials 320 (2008) 2992–2995
0.8
CoCrPtB/CoCr/Cr 60 α(Hc) (%/K)
Fluctuation field (Oe)
80
CoCrPtTa/Cr
40
CoCrTa/Cr
20
CoCrTa /Cr 0.4 0.2
0
CoCrPtB/CoCr/Cr
0
200
250 300 Temperature (K)
350
5
10
15 Hf / Hc
Fig. 6. Temperature dependence of the fluctuation field of Co55Cr15.5Pt28B1.5/ Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films.
20
(10-3)
Fig. 8. Trend of the temperature coefficient of coercivity a(Hc) with the ratio Hf/Hc of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films.
20
25 20
CoCrPtB/CoCr/Cr
Hc /Hf (10-3)
Hf / Hc (10-3)
CoCrPtTa/Cr
0.6
15 10 5
CoCrTa/Cr
250 300 Temperature (K)
12 8
CoCrPtTa/Cr
0 200
16
4
350
Fig. 7. Temperature dependence of the ratio Hf/Hc for Co55Cr15.5Pt28B1.5/Co63Cr37/ Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films.
for a magnetically isolated single domain particle, and is the minimum unit volume of magnetization reversal [3,11]. In the case of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr thin films with the smallest Vphy, Vphy was smaller than Vact as shown in Table 1. For all thin films, delta M increases to a peak around Hc and then decreases as the magnetic field increases. It exhibits positive interactions between particles, and the value of delta Mmax for Co55Cr15.5Pt28 B1.5/Co63Cr37/Cr double-layered thin films is larger than those of the other thin films. Fig. 6 shows the temperature dependence of Hf around Hc for these thin films. The change of slope of Hf with temperature is small for Co69Cr19Pt9Ta3/Cr and Co86Cr10Ta4/Cr thin films. However, the value of Hf increases markedly with increase in temperature for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr thin films with very small Vphy. In order to decrease the Hf value, it is necessary to improve both the distribution of grain volume and the ratio Hc/HA [12]. Fig. 7 shows the temperature dependence of the ratio Hf/Hc using the data presented in Figs. 1 and 6. A comparison of the temperature dependence of a(Hc) and the ratio Hf/Hc shows that the shape is similar for all thin films. The value of Hf/Hc increases markedly with increase in temperature between near 250 and 350 K as in the case of a(Hc) for Co55Cr15.5Pt28B1.5/Co63Cr37/Cr thin films with very small Vphy. The variation in Hf/Hc of Co55Cr15.5Pt28 B1.5/Co63Cr37/Cr thin films was larger than those of the other thin films. The values of a(Hc) are plotted as a function of the ratio Hf/Hc in Fig. 8. a(Hc) is approximately proportional to the ratio Hf/Hc for all thin films, as in the case of advanced data backup tapes prepared from ultrafine acicular metal particulate composite and hexagonal platelet Ba-ferrite particles [13]. The value of the ratio Hf/Hc increases as Vphy decreases, as shown in Fig. 9. The following
0
1
2
3
4 -18
Mean volume (10
5
6
3
cm )
Fig. 9. Trend of the ratio Hf/Hc with mean volume of Co55Cr15.5Pt28B1.5/Co63Cr37/Cr, Co69Cr19Pt9Ta3/Cr, and Co86Cr10Ta4/Cr thin films.
relation of a(Hc) to Hf/Hc also had been found to hold in these thin films, as in the case of advanced data backup tapes, rare earth and SrLaCo ferrite magnets [10,13,14]. a(Hc) was expressed as [28.5k(qE/qT)H]Hf/(HckT), where E and H were the activation energy and the magnetic field, respectively [10,13,14]. The equation shows that smaller Hf/Hc values are necessary for small a(Hc) values, which is very important for the thermal stability of high-density recording media with very small Vphy. 4. Conclusion The effect of the ratio Hf/Hc on a(Hc) was investigated for CoCr thin films with very small average Vphy. The value of a(Hc) is approximately proportional to the ratio Hf/Hc for all thin films. a(Hc) and the ratio Hf/Hc increase as Vphy decreases. Smaller Hf/Hc values are necessary for small a(Hc) values, which is very important for the thermal stability of high-density recording media with very small Vphy. Acknowledgements The authors would like to thank Tamakawa Co., Ltd. for the use of a highly sensitive torque magnetometer and Dr. Mu-Pei Chen for chemical analysis and TEM observation of thin films. References [1] [2] [3] [4]
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ARTICLE IN PRESS H. Nishio, H. Yamamoto / Journal of Magnetism and Magnetic Materials 320 (2008) 2992–2995
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