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Thermal effect on recording quality in heat-assisted recording
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 303 (2006) e34–e38 www.elsevier.com/locate/jmmm
Thermal effect on recording quality in heat-assisted recording B.X. Xu, H.X. Yuan, S.B. Hu, R. Ji, Y.J. Chen, J. Zhang, X.S. Miao, J.S. Chen, T.C. Chong Data Storage Institute, 5 Engineering Drive 1, Singapore 117608, Singapore
Abstract Heat-assisted magnetic recording is a promising approach to overcome the superparamagnetic limit, but the thermal effect will bring a lot of new issues in implementation of it. In this paper, the thermal effect on the recording mark and readout signal quality are investigated with heat-assisted magnetic recording (HAMR) platform experimentally. The differences of recorded marks and readout signals between heat-assisted recording and conventional recording are studied. It is also observed that after the saturation write current, the readout signal will decrease as the write current increases and laser power increases. r 2006 Published by Elsevier B.V. PACS: 75.60.Nt; 75.70.i Keywords: Heat-assisted magnetic recording; Hybrid recording; Magnetic recording media; Thermal effect
1. Introduction As magnetic recording density increases, density limitation occurs due to weakening of the thermal stability, the so-called superparamagnetism. In respect of magnetic material, this effect is an insurmountable barrier. It is true that media with higher coercivity may push this limitation a little further; however, as the material’s coercivity reaches certain value, another problem has to be tackled: the stateof-art writing technology, even in the predictable future, cannot provide sufficient magnetic field to squeeze user data into such media. As a promising approach to overcome this problem, heat-assisted recording was proposed [1] and has attracted more efforts on such researches [2–12]. Besides the writing strategy and writing head researches, the studies on thermal effects of the magnetic media and recording performance are also badly desired to build a pragmatic system. Some papers dwelled on this issue with the aim to disclose the transition location and transition length [13,14]. Theoretically, a thermal Williams– Comstock model was proposed to predict the transition lengths and the experimental results showed some agreeCorresponding author. Tel.: 65 68748512; fax: 65 67778517.
E-mail address: [email protected] (B.X. Xu). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.01.245
ment with theoretical results [13]. But the more detail differences of the readout signal shapes and recorded mark shapes between the conventional recording and heatassisted recording were not studied yet. In this paper, the more detail of difference of the readout signal between conventional recording and thermal-assisted recording will be presented. 2. Experiment The media used in the experiment is a kind of longitudinal media other than perpendicular media due to the unavailability of perpendicular writing head, and its structure is designed as Galss/CrMo(10 nm)/CoCr(3 nm)/ CoCrPtB(20 nm)/C(4 nm). The temperature dependences of the coercivity H c and saturation magnetization M s of the media are measured by vibrating sample magnetometer (VSM) and shown in Fig. 1. All H c and M s reduce with the temperature increase, and H c reduces more quickly. The heat-assisted magnetic recording (HAMR) experiments are conducted with far-field HAMR platform [15]. In the platform, the laser with wavelength of 405 nm is focused on the magnetic recording layer through the glass substrate by an objective lens with numerical aperture of 0.6. The
ARTICLE IN PRESS B.X. Xu et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e34–e38
3000
250
2500
200
2000 150 1500 100
1000
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500 0 250
TAA (mV)
300
Ms (emu/cc)
Coercivity (Oe)
Hc, Ms .Vs. T 3500
300
350 400 Temperature (K)
450
0 500
e35
Saturation curves with/without laser 0.6 0.55 0.5 19.46mw 0.45 0.4 16.68mw 13.90mw 0.35 8.34mw 0.3 5.56mw 0.25 2.78mw 0.2 0mw 0.15 0 10 20 30
40
Write current (mA)
(a)
diameter of the focused laser beam spot is estimated to be around 450 nm. The GMR magnetic head is used in the experiments. The write head width and write gap length are around 200 and 100 nm, respectively, which all are smaller than the diameter of the focused laser beam spot. In order to obtain good alignment between laser spot and write magnetic head, a two-step process is adopted: first, conduct alignment of them through a mounted glass substrate with the same thickness and character of material as the interested media on the platform spindle very well; then, replace the glass substrate with magnetic media and optimize the laser spot position by monitoring the readout signal from written track.
3. Results and discussion Write current saturation curve measurement is a direct approach to visualize the thermal effect on the media in the HAMR experiment. The saturation curve experimental results for different laser powers at linear velocity of 6.5 m/ s are shown in Fig. 2(a). As the laser power increases, the saturation current shifts towards low current. This implies that the laser has heated the media locally, pulled down its coercivity, and thus lower writing current can make data impressed into the media. In order to evaluate the temperature change of the media caused by laser shinning, the following method is adopted: assuming (i) the magnetic write head has a linear relationship between generated magnetic field and writing current and (ii) the writing current I c , at which the readout signal amplitude is half of the saturation amplitude, corresponds to media’s coercivity value, when the laser power increases from zero to certain value, the percentage of I c change corresponds to percentage of the coercivity change, based on the dependence of coercivity on temperature (Fig. 1), the temperature change of the media can be obtained. Fig. 2(b) shows the results of the media temperature change with the laser power, the local temperature increases with the laser
Disk temperature change (K)
Disk temperature change Vs. Laser power Fig. 1. Dependences of coercivity ðH c Þ and saturated magnetization ðM s Þ on temperature measured by VSM.
160 140 120 100 80 60 40 20 0 0
(b)
5
10
15
20
Laser power (mW)
Fig. 2. (a) Track-averaged amplitudes of readout signals with writing currents at different laser powers, (b) the local temperature change of the media with laser power.
power. Around 140 K temperature change can be obtained under this experimental condition. To investigate the performance difference between conventional recording and heat-assisted recording, lowfrequency signals (5 MHz) are recorded with and without laser shining. Fig. 3 shows the readout signals at large writing current (25 mA) without laser (Fig. 3a) and at low writing current (10 mA) with laser (13.9 mW) (Fig. 3b). There is a very pronounced difference between these two set of signals, i.e., an extended shoulder in the signal with laser shining can be found, which is actually the embodiment of the big thermal profile and low cooling speed. This phenomenon could be explained by thermal Williams–Comstock model very well [13]. Since the thermal gradients existed, the transition length has changed with the thermal gradient and dependences of coercivity and saturation magnetization on temperature. In the case of increase of the ratio of saturation magnetization to coercivity with temperature, the transition length increase is expected. The more shoulders in the tailing edges, we believe, are caused by asymmetry thermal profile due to the moving medium, which is not included in the model. The magnetic force microscopy (MFM) images of the recorded marks in the conditions of Fig. 3a and b are shown in Fig. 4. It is clearly indicated that the transition area is
ARTICLE IN PRESS e36
B.X. Xu et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e34–e38
higher laser powers, as the writing current increases beyond the saturation point. This has been observed through the readout signal in heat-assisted recording at large write current (beyond saturation current), where the amplitude of the readout signal at large writing current is smaller than that at small writing current in the fixed laser power when the laser beam spot is aligned with write head gap [13]. Actually this is far from all; it is obvious from Fig. 2 that, the readout signal will decrease further as the write current increase, and the higher the laser power, the more significant the signal decrease. This is also supposed to result from the existence of the thermal gradient which generates the longer magnetic transition, further cause the readout signal decrease. As results shown in Ref. [13], the alignment situation can affect the readout signal amplitude due to the asymmetry thermal profile along the track of moving media, higher laser power will cause more asymmetry, therefore, the readout signal drops will be more significant. Fig. 6 shows the readout signal at writing current of 25 mA and laser power of 19.46 mW. The shoulder is still there but the amplitude of signal is very small. In the actual heat-assisted recording, for benefiting the recording density mostly, the operating write current should be very high and a little higher than the saturation current. Higher laser power is desired to pull the coercivity down lower enough so that the data can be impressed readily at the obtainable writing current. Within the reachable intensity of the writing field, as low as possible the laser power is expected to reduce the unexpected thermal effect. Even for delivering laser power, there is still a basic confinement to obtain higher laser power with nano-size spot since in this case near-field rather than far field will be utilized. 4. Conclusions
Fig. 3. Readout signals (a) at large writing current (25 mA) without laser shining and (b) at low writing current (10 mA) with laser shinning (13.9 mW).
longer in the heat-assisted recording than that in conventional recording. For lower shoulder, it may not be reflected in PW50 measurement, but it will affect the signal-to-noise ratio (SNR). The SNR test results at different laser power and different writing current are plotted in Fig. 5, it is obviously shown that the SNR in heat-assisted recording are less than that in conventional recording. In order to flatten and narrow the shoulder, the media must have uniform temperature distribution in the writing area at writing moment, but is not easy to get it due to the Gaussian laser profile in most cases. In Fig. 2, another phenomenon worthwhile to highlight is the track averaged amplitude (TAA) tends to decrease at
Heat-assisted recording can overcome the superparamagnetic effect, but the recorded performance is not as good as that of conventional recording, especially for the non-optimized media where thermal profile and lower cooling speed is unavoidable. The extended shoulder along the trailing edge of readout signal hints the observable transition change in the heat-assisted recording compared with conventional recording. This will definitely lead to some affects on the SNR. The experimental results show that the SNR at heat-assisted recording is lower than that at conventional recording. After saturation point, the amplitude of readout signal will be dwarfed as the write current increase. The higher the laser power, the more pronounced the signal decrease. Though this experiment is done with longitudinal media, the conclusions, however, is still applicable for perpendicular recording. In typical heat-assisted recording strategy, the track width is determined by laser beam spot and the bit length is determined by magnetic head. In the case of circle laser beam spot, the thermal profile is always bigger than bit length
ARTICLE IN PRESS B.X. Xu et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e34–e38
e37
Fig. 4. MFM imagines of the recorded marks with laser shining and without laser shining.
SNR (dB)
SNR at different write current 25 23 21 19 17 15 13 11 9
Laser power: 13.9mW Rotation speed: 2000rpm
10
30
50
70
90
Frequency (MHz)
(a) 8mA
9mA
10mA
11mA
12mA
50mA, w/o laser
SNR at different laser power 25 Write current: 10mA Rotation speed: 2000rpm
SNR (dB)
23 21 19 17
Fig. 6. Readout signals at write current of 25 mA and with laser shining (19.46 mW).
15 13
References
11 9 10
30
50
70
90
Frequency (MHz)
(b) 5.56mW
8.34mW
13.9mW
16.68mW
50mA, w/o laser
Fig. 5. Signal-to-noise ratio with writing frequency: (a) at different writing current and fixed laser power, (b) at different laser power and fixed writing current.
and its situation is similar to the situation in this investigation. Therefore, the results have significant reference meaning.
[1] M.B. Hintz, J. Appl. Phys. 69 (1991) 5767. [2] J.J.M. Ruigrok, R. Coehoorn, S.R. Cumpson, H.W. Kesteren, J. Appl. Phys. 87 (2000) 5398. [3] S.R. Cumpson, P. Hidding, R. Coehoorn, IEEE Trans. Magn. 36 (2000) 2271. [4] H. Katayama, M. Hamamoto, J. Sato, Y. Murakami, K. Kojima, IEEE Trans. Magn. 36 (2000) 195. [5] M. Hamamoto, K. Kojima, J. Sato, H. Katayama, Appl. Phys. Lett. 77 (2000) 415. [6] H. Sukeda, H. Saga, H. Nemoto, Y. Itou, C. Haginoya, T. Matsumoto, IEEE Trans. Magn. 37 (2001) 1234. [7] M. Alex, A. Tselikov, T. Mcdaniel, N. Deeman, T. Valet, D. Chen, IEEE Trans. Magn. 37 (2001) 1244.
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B.X. Xu et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e34–e38
[8] K. Kojima, M. Hamamoto, J. Sato, K. Watanabe, H. Katayama, IEEE Trans. Magn. 37 (2001) 1406. [9] P. Herget, T. Rausch, A.C. Shiela, D.D. Stancil, T.E. Schlesinger, J.-G. Zhu, Appl. Phys. Lett. 80 (2002) 1835. [10] S. Miyanishi, K. Kojima, J. Sato, K. Takayama, H. Fuji, A. Takahashi, K. Ohta, J. Appl. Phys. 93 (2003) 7801. [11] C.T. Lie, P.C. Kuo, C.L. Shen, J. Appl. Phys. 94 (2003) 2538.
[12] A. Lyberatos, K.Y. Guslienko, J. Appl. Phys. 94 (2003) 1119. [13] T. Rausch, J.A. Bain, D.D. Stancil, T.E. Schlesinger, IEEE Trans. Magn. 40 (2004) 137. [14] M.F. Erden, T. Rausch, W.A. Challener, IEEE Trans. Magn. 41 (2005) 2189. [15] B. Xu, S. Hu, H. Yuan, Y. Chen, J. Zhang, R. Ji, X. Miao, J. Chen, T.C. Chong, ISOM/ODS’2005.
Journal of Magnetism and Magnetic Materials 303 (2006) e34–e38 www.elsevier.com/locate/jmmm
Thermal effect on recording quality in heat-assisted recording B.X. Xu, H.X. Yuan, S.B. Hu, R. Ji, Y.J. Chen, J. Zhang, X.S. Miao, J.S. Chen, T.C. Chong Data Storage Institute, 5 Engineering Drive 1, Singapore 117608, Singapore
Abstract Heat-assisted magnetic recording is a promising approach to overcome the superparamagnetic limit, but the thermal effect will bring a lot of new issues in implementation of it. In this paper, the thermal effect on the recording mark and readout signal quality are investigated with heat-assisted magnetic recording (HAMR) platform experimentally. The differences of recorded marks and readout signals between heat-assisted recording and conventional recording are studied. It is also observed that after the saturation write current, the readout signal will decrease as the write current increases and laser power increases. r 2006 Published by Elsevier B.V. PACS: 75.60.Nt; 75.70.i Keywords: Heat-assisted magnetic recording; Hybrid recording; Magnetic recording media; Thermal effect
1. Introduction As magnetic recording density increases, density limitation occurs due to weakening of the thermal stability, the so-called superparamagnetism. In respect of magnetic material, this effect is an insurmountable barrier. It is true that media with higher coercivity may push this limitation a little further; however, as the material’s coercivity reaches certain value, another problem has to be tackled: the stateof-art writing technology, even in the predictable future, cannot provide sufficient magnetic field to squeeze user data into such media. As a promising approach to overcome this problem, heat-assisted recording was proposed [1] and has attracted more efforts on such researches [2–12]. Besides the writing strategy and writing head researches, the studies on thermal effects of the magnetic media and recording performance are also badly desired to build a pragmatic system. Some papers dwelled on this issue with the aim to disclose the transition location and transition length [13,14]. Theoretically, a thermal Williams– Comstock model was proposed to predict the transition lengths and the experimental results showed some agreeCorresponding author. Tel.: 65 68748512; fax: 65 67778517.
E-mail address: [email protected] (B.X. Xu). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.01.245
ment with theoretical results [13]. But the more detail differences of the readout signal shapes and recorded mark shapes between the conventional recording and heatassisted recording were not studied yet. In this paper, the more detail of difference of the readout signal between conventional recording and thermal-assisted recording will be presented. 2. Experiment The media used in the experiment is a kind of longitudinal media other than perpendicular media due to the unavailability of perpendicular writing head, and its structure is designed as Galss/CrMo(10 nm)/CoCr(3 nm)/ CoCrPtB(20 nm)/C(4 nm). The temperature dependences of the coercivity H c and saturation magnetization M s of the media are measured by vibrating sample magnetometer (VSM) and shown in Fig. 1. All H c and M s reduce with the temperature increase, and H c reduces more quickly. The heat-assisted magnetic recording (HAMR) experiments are conducted with far-field HAMR platform [15]. In the platform, the laser with wavelength of 405 nm is focused on the magnetic recording layer through the glass substrate by an objective lens with numerical aperture of 0.6. The
ARTICLE IN PRESS B.X. Xu et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e34–e38
3000
250
2500
200
2000 150 1500 100
1000
50
500 0 250
TAA (mV)
300
Ms (emu/cc)
Coercivity (Oe)
Hc, Ms .Vs. T 3500
300
350 400 Temperature (K)
450
0 500
e35
Saturation curves with/without laser 0.6 0.55 0.5 19.46mw 0.45 0.4 16.68mw 13.90mw 0.35 8.34mw 0.3 5.56mw 0.25 2.78mw 0.2 0mw 0.15 0 10 20 30
40
Write current (mA)
(a)
diameter of the focused laser beam spot is estimated to be around 450 nm. The GMR magnetic head is used in the experiments. The write head width and write gap length are around 200 and 100 nm, respectively, which all are smaller than the diameter of the focused laser beam spot. In order to obtain good alignment between laser spot and write magnetic head, a two-step process is adopted: first, conduct alignment of them through a mounted glass substrate with the same thickness and character of material as the interested media on the platform spindle very well; then, replace the glass substrate with magnetic media and optimize the laser spot position by monitoring the readout signal from written track.
3. Results and discussion Write current saturation curve measurement is a direct approach to visualize the thermal effect on the media in the HAMR experiment. The saturation curve experimental results for different laser powers at linear velocity of 6.5 m/ s are shown in Fig. 2(a). As the laser power increases, the saturation current shifts towards low current. This implies that the laser has heated the media locally, pulled down its coercivity, and thus lower writing current can make data impressed into the media. In order to evaluate the temperature change of the media caused by laser shinning, the following method is adopted: assuming (i) the magnetic write head has a linear relationship between generated magnetic field and writing current and (ii) the writing current I c , at which the readout signal amplitude is half of the saturation amplitude, corresponds to media’s coercivity value, when the laser power increases from zero to certain value, the percentage of I c change corresponds to percentage of the coercivity change, based on the dependence of coercivity on temperature (Fig. 1), the temperature change of the media can be obtained. Fig. 2(b) shows the results of the media temperature change with the laser power, the local temperature increases with the laser
Disk temperature change (K)
Disk temperature change Vs. Laser power Fig. 1. Dependences of coercivity ðH c Þ and saturated magnetization ðM s Þ on temperature measured by VSM.
160 140 120 100 80 60 40 20 0 0
(b)
5
10
15
20
Laser power (mW)
Fig. 2. (a) Track-averaged amplitudes of readout signals with writing currents at different laser powers, (b) the local temperature change of the media with laser power.
power. Around 140 K temperature change can be obtained under this experimental condition. To investigate the performance difference between conventional recording and heat-assisted recording, lowfrequency signals (5 MHz) are recorded with and without laser shining. Fig. 3 shows the readout signals at large writing current (25 mA) without laser (Fig. 3a) and at low writing current (10 mA) with laser (13.9 mW) (Fig. 3b). There is a very pronounced difference between these two set of signals, i.e., an extended shoulder in the signal with laser shining can be found, which is actually the embodiment of the big thermal profile and low cooling speed. This phenomenon could be explained by thermal Williams–Comstock model very well [13]. Since the thermal gradients existed, the transition length has changed with the thermal gradient and dependences of coercivity and saturation magnetization on temperature. In the case of increase of the ratio of saturation magnetization to coercivity with temperature, the transition length increase is expected. The more shoulders in the tailing edges, we believe, are caused by asymmetry thermal profile due to the moving medium, which is not included in the model. The magnetic force microscopy (MFM) images of the recorded marks in the conditions of Fig. 3a and b are shown in Fig. 4. It is clearly indicated that the transition area is
ARTICLE IN PRESS e36
B.X. Xu et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e34–e38
higher laser powers, as the writing current increases beyond the saturation point. This has been observed through the readout signal in heat-assisted recording at large write current (beyond saturation current), where the amplitude of the readout signal at large writing current is smaller than that at small writing current in the fixed laser power when the laser beam spot is aligned with write head gap [13]. Actually this is far from all; it is obvious from Fig. 2 that, the readout signal will decrease further as the write current increase, and the higher the laser power, the more significant the signal decrease. This is also supposed to result from the existence of the thermal gradient which generates the longer magnetic transition, further cause the readout signal decrease. As results shown in Ref. [13], the alignment situation can affect the readout signal amplitude due to the asymmetry thermal profile along the track of moving media, higher laser power will cause more asymmetry, therefore, the readout signal drops will be more significant. Fig. 6 shows the readout signal at writing current of 25 mA and laser power of 19.46 mW. The shoulder is still there but the amplitude of signal is very small. In the actual heat-assisted recording, for benefiting the recording density mostly, the operating write current should be very high and a little higher than the saturation current. Higher laser power is desired to pull the coercivity down lower enough so that the data can be impressed readily at the obtainable writing current. Within the reachable intensity of the writing field, as low as possible the laser power is expected to reduce the unexpected thermal effect. Even for delivering laser power, there is still a basic confinement to obtain higher laser power with nano-size spot since in this case near-field rather than far field will be utilized. 4. Conclusions
Fig. 3. Readout signals (a) at large writing current (25 mA) without laser shining and (b) at low writing current (10 mA) with laser shinning (13.9 mW).
longer in the heat-assisted recording than that in conventional recording. For lower shoulder, it may not be reflected in PW50 measurement, but it will affect the signal-to-noise ratio (SNR). The SNR test results at different laser power and different writing current are plotted in Fig. 5, it is obviously shown that the SNR in heat-assisted recording are less than that in conventional recording. In order to flatten and narrow the shoulder, the media must have uniform temperature distribution in the writing area at writing moment, but is not easy to get it due to the Gaussian laser profile in most cases. In Fig. 2, another phenomenon worthwhile to highlight is the track averaged amplitude (TAA) tends to decrease at
Heat-assisted recording can overcome the superparamagnetic effect, but the recorded performance is not as good as that of conventional recording, especially for the non-optimized media where thermal profile and lower cooling speed is unavoidable. The extended shoulder along the trailing edge of readout signal hints the observable transition change in the heat-assisted recording compared with conventional recording. This will definitely lead to some affects on the SNR. The experimental results show that the SNR at heat-assisted recording is lower than that at conventional recording. After saturation point, the amplitude of readout signal will be dwarfed as the write current increase. The higher the laser power, the more pronounced the signal decrease. Though this experiment is done with longitudinal media, the conclusions, however, is still applicable for perpendicular recording. In typical heat-assisted recording strategy, the track width is determined by laser beam spot and the bit length is determined by magnetic head. In the case of circle laser beam spot, the thermal profile is always bigger than bit length
ARTICLE IN PRESS B.X. Xu et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e34–e38
e37
Fig. 4. MFM imagines of the recorded marks with laser shining and without laser shining.
SNR (dB)
SNR at different write current 25 23 21 19 17 15 13 11 9
Laser power: 13.9mW Rotation speed: 2000rpm
10
30
50
70
90
Frequency (MHz)
(a) 8mA
9mA
10mA
11mA
12mA
50mA, w/o laser
SNR at different laser power 25 Write current: 10mA Rotation speed: 2000rpm
SNR (dB)
23 21 19 17
Fig. 6. Readout signals at write current of 25 mA and with laser shining (19.46 mW).
15 13
References
11 9 10
30
50
70
90
Frequency (MHz)
(b) 5.56mW
8.34mW
13.9mW
16.68mW
50mA, w/o laser
Fig. 5. Signal-to-noise ratio with writing frequency: (a) at different writing current and fixed laser power, (b) at different laser power and fixed writing current.
and its situation is similar to the situation in this investigation. Therefore, the results have significant reference meaning.
[1] M.B. Hintz, J. Appl. Phys. 69 (1991) 5767. [2] J.J.M. Ruigrok, R. Coehoorn, S.R. Cumpson, H.W. Kesteren, J. Appl. Phys. 87 (2000) 5398. [3] S.R. Cumpson, P. Hidding, R. Coehoorn, IEEE Trans. Magn. 36 (2000) 2271. [4] H. Katayama, M. Hamamoto, J. Sato, Y. Murakami, K. Kojima, IEEE Trans. Magn. 36 (2000) 195. [5] M. Hamamoto, K. Kojima, J. Sato, H. Katayama, Appl. Phys. Lett. 77 (2000) 415. [6] H. Sukeda, H. Saga, H. Nemoto, Y. Itou, C. Haginoya, T. Matsumoto, IEEE Trans. Magn. 37 (2001) 1234. [7] M. Alex, A. Tselikov, T. Mcdaniel, N. Deeman, T. Valet, D. Chen, IEEE Trans. Magn. 37 (2001) 1244.
ARTICLE IN PRESS e38
B.X. Xu et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e34–e38
[8] K. Kojima, M. Hamamoto, J. Sato, K. Watanabe, H. Katayama, IEEE Trans. Magn. 37 (2001) 1406. [9] P. Herget, T. Rausch, A.C. Shiela, D.D. Stancil, T.E. Schlesinger, J.-G. Zhu, Appl. Phys. Lett. 80 (2002) 1835. [10] S. Miyanishi, K. Kojima, J. Sato, K. Takayama, H. Fuji, A. Takahashi, K. Ohta, J. Appl. Phys. 93 (2003) 7801. [11] C.T. Lie, P.C. Kuo, C.L. Shen, J. Appl. Phys. 94 (2003) 2538.
[12] A. Lyberatos, K.Y. Guslienko, J. Appl. Phys. 94 (2003) 1119. [13] T. Rausch, J.A. Bain, D.D. Stancil, T.E. Schlesinger, IEEE Trans. Magn. 40 (2004) 137. [14] M.F. Erden, T. Rausch, W.A. Challener, IEEE Trans. Magn. 41 (2005) 2189. [15] B. Xu, S. Hu, H. Yuan, Y. Chen, J. Zhang, R. Ji, X. Miao, J. Chen, T.C. Chong, ISOM/ODS’2005.