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Prepartion and properties of amorphous SmDyFeCo films
Materials Science and Engineering B76 (2000) 50 – 52 www.elsevier.com/locate/mseb
Preparation and properties of amorphous SmDyFeCo films Zheng-qi Lu a,*, Zuo-yi Lee b, Yuan-kai Zheng b, Zuo-qi Hu b, Gengqi Lin b a
Center for Condensed Matter Physics, State Key Laboratory for Magnetism, and Institute of Physics, Chinese Academy of Science, Bejing, 100080, PR China b Department of Electronic Science, Huazhong Uni6ersity of Science and Technology, Wuhan, 430074, PR China
Abstract The amorphous SmDyFeCo films as a practical magneto-optical (MO) recording material were successfully prepared at the optimum condition. When moderate Sm is substituted for Dy, the compensation temperature Tcomp decreases, the Curie temperature Tc remains unchanged, the Kerr rotation angle uk increases, the saturation magnetization Ms at the room temperature increases and changes slowly with the temperature, and the coercivity Hc remains high in a wide range of the temperature. It is shown that the MO performance is obviously improved in a wide range of the temperature. On the contrary, substituting Sm for FeCo, The MO performance is not improved. Moreover, the effects of the argon pressure on the properties of the amorphous SmDyFeCo films were investigated. The reflectivity R decreases with increasing the Ar pressure, uk and the anisotropy constant Ku reach their maximum at a certain argon pressure, respectively. It is plausible that there is a physically significant correlation between uk and Ku. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Amorphous; Magneto-optical; Compensation temperature
1. Introduction
2. Experimental procedure
The amorphous DyFeCo films are promising as media for the magneto-optical (MO) data storage [1,2]. As for the TbFeCo and DyFeCo films having an identical Curie point Tc, their Kerr rotation angle uk and reflectivity R are substantially equal. However, the coercivity Hc and the magnetization Ms of the DyFeCo films change with the temperature more rapidly than that of the TbFeCo films, which affect the stability of the recording domains. Studies on improving the magnetic and MO behaviours by the substitution of the light rare earth (LRE) into the heavy rare earth-transition metal (HRE-TM) films have attracted much attention recently [3–6]. But up to now, it is not clear if the addition of LRE into the HRE-TM films enhances the MO performance [7]. The present work is motivated by the desire to understand the magnetic and MO properties in the pseudoternary SmDyFeCo films and their behaviour versus the temperature. Moreover, the effect of the Ar pressure on the properties of the amorphous SmDyFeCo films is investigated.
The amorphous SmDyFeCo films were deposited by the RF- magnetron sputtering from a mosaic target onto the substrate. The mosaic target was made by bonding the pure sheets of Sm and Tb to a disc made of the FeCo alloy. The composition of the SmDyFeCo films was mainly controlled by altering the area, the place and the number of Sm and Dy chips. The sputtering conditions are as follows: the base pressure 1× 10 − 4 Pa; the Ar pressure 0.2–1.73 Pa; the sputtering power 300 W. The layer configuration is AlNSmDyFeCosubstrate. The film composition was determined by the electron microprobe analysis. A vibrating sample magnetometer (VSM) was used to characterise the films magnetically and a polar Kerr-effect hysteresis loop plotter obtained uk and R. The perpendicular anisotropy constant Ku was obtained with a torque magnetometer.
* Corresponding author. E-mail address: [email protected] (Z.-q. Lu)
3. Experimental results and discussion The temperature dependence of the magnetic properties of the amorphous SmDyFeCo films is shown in
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 3 8 9 - 5
Z.-q. Lu et al. / Materials Science and Engineering B76 (2000) 50–52
Fig. 1. The temperature dependence of the coercivity Hc and the saturation magnetization Ms of the amorphous SmDyFeCo films.
Fig. 2. The temperature dependence of the Kerr rotation angle uk and the reflectivity R of the amorphous SmDyFeCo films.
Fig. 3. The variation of the coercivity Hc and the anisotropy constant K %u1 and K %u2 of the amorphous SmDyFeCo films as a function of the Ar pressure.
Fig. 1. When Sm is substituted for Dy while holding the TM content, Ms and Hc versus the temperature change more slowly, and the product MsHc becomes large enough to be suitable for the practical use. The compensation temperature Tcomp and the Curie temperature Tc are determined from the temperature dependence of the magnetization. Tcomp decreases and Tc remains nearly unchanged as the Sm content increases. With Sm
51
replacing TM while holding the Dy content, Tcomp increases, Tc decreases, and Ms and Hc versus the temperature change more rapidly. The saturation magnetization of the RE-TM alloy is determined by a strong RE-TM exchange interaction, which leads to a parallel or antiparallel alignment of the RE and TM moments for the alloys containing LRE or HRE, respectively. When Sm is substituted for Dy, the TM subnetwork moments increase, which leads Tcomp to decrease. As the network moment of RE decreases more rapidly than that of TM, Tc mainly depends on the TM content. When the TM content is fixed, Tc remains nearly unchanged. When Sm is substituted for TM, although the net moments of Sm are larger than that of TM at the low temperature, the stronger temperature dependence of the exchange interaction of Sm results in a smaller TM subnetwork moment at the room temperature, which leads Tcomp to increase. The decrease in the TM content also results in a lower Tc. The temperature dependence of the MO properties for the amorphous SmDyFeCo films is shown in Fig. 2. When Sm is substituted for Dy while leaving the TM content fixed, a small enhancement in the MO effect is produced in the low Sm content films. However, Sm substitution for TM in the amounts reported here is not found to enhance the MO effect. In the HRE-LRE-TM films, the Kerr effect is attributed to the subnetwork magnetic moments. uk can be represented by uk = KTM + MsTM + KLREMsLRE − KHREMsHRE. Sm will contribute more in MO effect than Dy. However, substituting Sm for TM results in a rapid decrease of the TM subnetwork moment, which leads a decrease in the MO effect. Fig. 3 shows the coercivity Hc of the amorphous SmDyFeCo films as the function of the Ar pressure when the sputtering power keeps 300 W. Hc changes with the Ar pressure reflecting a variation in the composition of the thin films. As the mean free path of the argon ion is decreased with increasing the Ar pressure, the HRE-TM ratio is increased with the Ar pressure growth due to the preferential resputtering of RE. Thus, Hc increases with increasing the Ar pressure, the further increase in the Ar pressure leads to the transformation of the thin film from TM-rich to HRE-rich. Hc can also be affected by the microstructure and morphology of the thin films. Hc exhibited a rise with the film fabricated under the high pressure. Microvoids and columnar structure may act as the pinning points in the magnetic domain walls, which leads Hc to increase. The reflectivity R of the amorphous SmDyFeCo film reflects the surface microstructure of the thin films to a certain degree. As shown in Fig. 4, in the low Ar pressure, the deposited film surface is smooth and the cross-sectional features is quite fine, which is considered to be due to the high kinetic energy of the incident
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Z.-q. Lu et al. / Materials Science and Engineering B76 (2000) 50–52
netic portions of the amorphous phases. This leads to further separation between the magnetic potions, so that the interaction energy between the magnetic regions is greatly reduced. This reduction in interaction energy would give a rise to the in-plane magnetization, the anisotropy constants become negative. The less dense film microstructure caused by the less energetic bombardment during the sputtering may also lead the drop in the anisotropy at higher argon pressure [8].
4. Conclusion Fig. 4. The variation of the Kerr rotation angle uk and the reflectivity R of the amorphous SmDyFeCo films as a function of the Ar pressure.
material to promote the surface diffusion processes and further due to the bombardment of the energy species, R is high. Because of the shorter mean free paths of argon ions at the high Ar pressure, the argon ion bombardment on the growing films thereby decreases with increasing the Ar pressure. The particles impinge on the growing films less energetically at the high Ar pressure, which leads to a decrease in the surface mobility of the adatoms. The film has some texture-like structure. Thus, R decreases. With further an increase in the Ar pressure, the fibre structure develops to the columnar structure with microvoid, the density of microvoids and surface roughness increases with increasing the Ar pressure, R is further reduced. The uk, the anisotropy constants K %ul( = Ku1 − 2pM 2s ) and Ku2 show a peak as the Ar pressure is varied, respectively, as shown in Figs. 3 and 4. The argon ion bombardment on the growing films at a low Ar pressure results in not only the decrease in the RE content but also the inclusion of argon in the film and a destruction of the anisotropic structure, those lead uk and Ku to be reduced. As the argon ion bombardment on the growing films decreases with increasing the Ar pressure, the films have some columnar structure, which increases the anisotropy. As the argon ions in the thin films decrease with increasing Ar the pressure, uk becomes large, However, when the Ar pressure rises to some value, oxygen may react with Dy and Sm along the microvoids to form the nonmagnetic oxides, which leads uk to decease. Oxides may also shield the mag-
.
The magnetic and MO properties of the amorphous SmDyFeCo films were investigated with respect to their composition and temperature behaviour. When moderate Sm is substituted for Dy, Tcomp decreases, Tc remains unchanged. The MO performance is obviously improved in a wide range of the temperature. On the contrary, when substituting Sm for FeCo, the MO performance is not improved. Moreover, the effect of the sputtering Ar pressure on the properties of the amorphous SmDyFeCo films was investigated. R decreases with increasing the argon pressure, uk and Ku reach their maximum at a certain argon pressure, respectively. It is plausible that there is a physically significant correlation between uk and Ku. It is shown that the composition and microstructure of the SmDyFeCo thin films affected by the Ar pressure lead the change of these magnetic and MO properties. The amorphous SmDyFeCo films as a practical MO recording material are successfully prepared at the optimum condition.
References [1] D. Raasch, IEEE Trans. Magn. 29 (1993) 34. [2] M. Tabata, Jpn. J. Appl. Phys. 33 (1994) 5811. [3] Z.Y. Lee, X.S. Miao, P. Zhu, et al., J. Magn. Magn. Mat. 115 (1992) 44. [4] P. Hansen, D. Raasch, D. Mergel, J. Appl. Phys. 75 (1994) 5267. [5] H.P.D. Shieh, J. Yamasaki, M. Kryder, IEEE Trans. Magn 23 (1987) 3208. [6] Z.Y. Lee, Z.Q. Lu, Y.K. Zheng, et al., J. Magn, Soc. Jpn. 22 (S1) (1998) 105. [7] Z.Q. Lu, Z.Y. Lee, Y.K. Zheng, et al., SPIE 2890 (1996) 20. [8] S.C. Cheng, M.H. Kryder, M.C.A. Mather, IEEE Trans. Magn. 25 (1989) 4018.
Preparation and properties of amorphous SmDyFeCo films Zheng-qi Lu a,*, Zuo-yi Lee b, Yuan-kai Zheng b, Zuo-qi Hu b, Gengqi Lin b a
Center for Condensed Matter Physics, State Key Laboratory for Magnetism, and Institute of Physics, Chinese Academy of Science, Bejing, 100080, PR China b Department of Electronic Science, Huazhong Uni6ersity of Science and Technology, Wuhan, 430074, PR China
Abstract The amorphous SmDyFeCo films as a practical magneto-optical (MO) recording material were successfully prepared at the optimum condition. When moderate Sm is substituted for Dy, the compensation temperature Tcomp decreases, the Curie temperature Tc remains unchanged, the Kerr rotation angle uk increases, the saturation magnetization Ms at the room temperature increases and changes slowly with the temperature, and the coercivity Hc remains high in a wide range of the temperature. It is shown that the MO performance is obviously improved in a wide range of the temperature. On the contrary, substituting Sm for FeCo, The MO performance is not improved. Moreover, the effects of the argon pressure on the properties of the amorphous SmDyFeCo films were investigated. The reflectivity R decreases with increasing the Ar pressure, uk and the anisotropy constant Ku reach their maximum at a certain argon pressure, respectively. It is plausible that there is a physically significant correlation between uk and Ku. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Amorphous; Magneto-optical; Compensation temperature
1. Introduction
2. Experimental procedure
The amorphous DyFeCo films are promising as media for the magneto-optical (MO) data storage [1,2]. As for the TbFeCo and DyFeCo films having an identical Curie point Tc, their Kerr rotation angle uk and reflectivity R are substantially equal. However, the coercivity Hc and the magnetization Ms of the DyFeCo films change with the temperature more rapidly than that of the TbFeCo films, which affect the stability of the recording domains. Studies on improving the magnetic and MO behaviours by the substitution of the light rare earth (LRE) into the heavy rare earth-transition metal (HRE-TM) films have attracted much attention recently [3–6]. But up to now, it is not clear if the addition of LRE into the HRE-TM films enhances the MO performance [7]. The present work is motivated by the desire to understand the magnetic and MO properties in the pseudoternary SmDyFeCo films and their behaviour versus the temperature. Moreover, the effect of the Ar pressure on the properties of the amorphous SmDyFeCo films is investigated.
The amorphous SmDyFeCo films were deposited by the RF- magnetron sputtering from a mosaic target onto the substrate. The mosaic target was made by bonding the pure sheets of Sm and Tb to a disc made of the FeCo alloy. The composition of the SmDyFeCo films was mainly controlled by altering the area, the place and the number of Sm and Dy chips. The sputtering conditions are as follows: the base pressure 1× 10 − 4 Pa; the Ar pressure 0.2–1.73 Pa; the sputtering power 300 W. The layer configuration is AlNSmDyFeCosubstrate. The film composition was determined by the electron microprobe analysis. A vibrating sample magnetometer (VSM) was used to characterise the films magnetically and a polar Kerr-effect hysteresis loop plotter obtained uk and R. The perpendicular anisotropy constant Ku was obtained with a torque magnetometer.
* Corresponding author. E-mail address: [email protected] (Z.-q. Lu)
3. Experimental results and discussion The temperature dependence of the magnetic properties of the amorphous SmDyFeCo films is shown in
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 3 8 9 - 5
Z.-q. Lu et al. / Materials Science and Engineering B76 (2000) 50–52
Fig. 1. The temperature dependence of the coercivity Hc and the saturation magnetization Ms of the amorphous SmDyFeCo films.
Fig. 2. The temperature dependence of the Kerr rotation angle uk and the reflectivity R of the amorphous SmDyFeCo films.
Fig. 3. The variation of the coercivity Hc and the anisotropy constant K %u1 and K %u2 of the amorphous SmDyFeCo films as a function of the Ar pressure.
Fig. 1. When Sm is substituted for Dy while holding the TM content, Ms and Hc versus the temperature change more slowly, and the product MsHc becomes large enough to be suitable for the practical use. The compensation temperature Tcomp and the Curie temperature Tc are determined from the temperature dependence of the magnetization. Tcomp decreases and Tc remains nearly unchanged as the Sm content increases. With Sm
51
replacing TM while holding the Dy content, Tcomp increases, Tc decreases, and Ms and Hc versus the temperature change more rapidly. The saturation magnetization of the RE-TM alloy is determined by a strong RE-TM exchange interaction, which leads to a parallel or antiparallel alignment of the RE and TM moments for the alloys containing LRE or HRE, respectively. When Sm is substituted for Dy, the TM subnetwork moments increase, which leads Tcomp to decrease. As the network moment of RE decreases more rapidly than that of TM, Tc mainly depends on the TM content. When the TM content is fixed, Tc remains nearly unchanged. When Sm is substituted for TM, although the net moments of Sm are larger than that of TM at the low temperature, the stronger temperature dependence of the exchange interaction of Sm results in a smaller TM subnetwork moment at the room temperature, which leads Tcomp to increase. The decrease in the TM content also results in a lower Tc. The temperature dependence of the MO properties for the amorphous SmDyFeCo films is shown in Fig. 2. When Sm is substituted for Dy while leaving the TM content fixed, a small enhancement in the MO effect is produced in the low Sm content films. However, Sm substitution for TM in the amounts reported here is not found to enhance the MO effect. In the HRE-LRE-TM films, the Kerr effect is attributed to the subnetwork magnetic moments. uk can be represented by uk = KTM + MsTM + KLREMsLRE − KHREMsHRE. Sm will contribute more in MO effect than Dy. However, substituting Sm for TM results in a rapid decrease of the TM subnetwork moment, which leads a decrease in the MO effect. Fig. 3 shows the coercivity Hc of the amorphous SmDyFeCo films as the function of the Ar pressure when the sputtering power keeps 300 W. Hc changes with the Ar pressure reflecting a variation in the composition of the thin films. As the mean free path of the argon ion is decreased with increasing the Ar pressure, the HRE-TM ratio is increased with the Ar pressure growth due to the preferential resputtering of RE. Thus, Hc increases with increasing the Ar pressure, the further increase in the Ar pressure leads to the transformation of the thin film from TM-rich to HRE-rich. Hc can also be affected by the microstructure and morphology of the thin films. Hc exhibited a rise with the film fabricated under the high pressure. Microvoids and columnar structure may act as the pinning points in the magnetic domain walls, which leads Hc to increase. The reflectivity R of the amorphous SmDyFeCo film reflects the surface microstructure of the thin films to a certain degree. As shown in Fig. 4, in the low Ar pressure, the deposited film surface is smooth and the cross-sectional features is quite fine, which is considered to be due to the high kinetic energy of the incident
52
Z.-q. Lu et al. / Materials Science and Engineering B76 (2000) 50–52
netic portions of the amorphous phases. This leads to further separation between the magnetic potions, so that the interaction energy between the magnetic regions is greatly reduced. This reduction in interaction energy would give a rise to the in-plane magnetization, the anisotropy constants become negative. The less dense film microstructure caused by the less energetic bombardment during the sputtering may also lead the drop in the anisotropy at higher argon pressure [8].
4. Conclusion Fig. 4. The variation of the Kerr rotation angle uk and the reflectivity R of the amorphous SmDyFeCo films as a function of the Ar pressure.
material to promote the surface diffusion processes and further due to the bombardment of the energy species, R is high. Because of the shorter mean free paths of argon ions at the high Ar pressure, the argon ion bombardment on the growing films thereby decreases with increasing the Ar pressure. The particles impinge on the growing films less energetically at the high Ar pressure, which leads to a decrease in the surface mobility of the adatoms. The film has some texture-like structure. Thus, R decreases. With further an increase in the Ar pressure, the fibre structure develops to the columnar structure with microvoid, the density of microvoids and surface roughness increases with increasing the Ar pressure, R is further reduced. The uk, the anisotropy constants K %ul( = Ku1 − 2pM 2s ) and Ku2 show a peak as the Ar pressure is varied, respectively, as shown in Figs. 3 and 4. The argon ion bombardment on the growing films at a low Ar pressure results in not only the decrease in the RE content but also the inclusion of argon in the film and a destruction of the anisotropic structure, those lead uk and Ku to be reduced. As the argon ion bombardment on the growing films decreases with increasing the Ar pressure, the films have some columnar structure, which increases the anisotropy. As the argon ions in the thin films decrease with increasing Ar the pressure, uk becomes large, However, when the Ar pressure rises to some value, oxygen may react with Dy and Sm along the microvoids to form the nonmagnetic oxides, which leads uk to decease. Oxides may also shield the mag-
.
The magnetic and MO properties of the amorphous SmDyFeCo films were investigated with respect to their composition and temperature behaviour. When moderate Sm is substituted for Dy, Tcomp decreases, Tc remains unchanged. The MO performance is obviously improved in a wide range of the temperature. On the contrary, when substituting Sm for FeCo, the MO performance is not improved. Moreover, the effect of the sputtering Ar pressure on the properties of the amorphous SmDyFeCo films was investigated. R decreases with increasing the argon pressure, uk and Ku reach their maximum at a certain argon pressure, respectively. It is plausible that there is a physically significant correlation between uk and Ku. It is shown that the composition and microstructure of the SmDyFeCo thin films affected by the Ar pressure lead the change of these magnetic and MO properties. The amorphous SmDyFeCo films as a practical MO recording material are successfully prepared at the optimum condition.
References [1] D. Raasch, IEEE Trans. Magn. 29 (1993) 34. [2] M. Tabata, Jpn. J. Appl. Phys. 33 (1994) 5811. [3] Z.Y. Lee, X.S. Miao, P. Zhu, et al., J. Magn. Magn. Mat. 115 (1992) 44. [4] P. Hansen, D. Raasch, D. Mergel, J. Appl. Phys. 75 (1994) 5267. [5] H.P.D. Shieh, J. Yamasaki, M. Kryder, IEEE Trans. Magn 23 (1987) 3208. [6] Z.Y. Lee, Z.Q. Lu, Y.K. Zheng, et al., J. Magn, Soc. Jpn. 22 (S1) (1998) 105. [7] Z.Q. Lu, Z.Y. Lee, Y.K. Zheng, et al., SPIE 2890 (1996) 20. [8] S.C. Cheng, M.H. Kryder, M.C.A. Mather, IEEE Trans. Magn. 25 (1989) 4018.