Fabrication and Properties of Nd(Tb,Dy)Co/Cr Films with Perpendicular Magnetic Anisotropy

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1 Fabrication and Properties of (,Dy)/Cr Films with Perpendicular Magnetic Anisotropy Weiming Cheng 1,2, Xiangshui Miao * 1,2, Junbing Yan 2, Xiaomin Cheng 1,2 1 Department of Electronic cience and Technology, Huazhong University of cience and Technology, Wuhan , China 2 Wuhan National Laboratory for Optoelectronics, Huazhong University of cience and Technology, Wuhan , China ABTRACT Light rare earth-heavy rare earth-transition metal films (LRE-HRE-TM)have large saturation magnetization ( ) and are the promising media for hybrid recording. In this paper, (,Dy)/Cr films with perpendicular magnetic anisotropy were successfully fabricated onto glass substrate by RF magnetron sputtering and the effects of sputtering technology parameters and substitution for HRE atoms on the magnetic properties were investigated. It was found that when the sputtering power and sputtering time are 250W and 4min, respectively, the magnetic properties of (,Dy)/Cr films obtain optimization, perpendicular coercivity, and remanence square ratio() of /Cr film reach 3.8kOe, 247emu/cm 3 and 0.801, respectively. With the increasing of concentration, increases, while the coercivity ( )and the temperature stability of magnetic properties decrease distinctly. These results can be explained by the ferri-magnetic structure of the RE-TM alloy. Keywords: hybrid recording, (,Dy), perpendicular magnetic anisotropy, saturation magnetization, coercivity, ferri-magnetic structure 1. INTRODUCTION Heavy rare earth-transition metal (HRE-TM) films have been widely used for magneto-optical (MO) recording media because of their amorphous structure, adjustable magnetic properties, a suitable temperature dependence of magnetic properties and high perpendicular anisotropy [1,2. However, of HRE-TM film is low and hardly employed for the GMR detection of read head, therefore, can not achieve the demand for hybrid recording. Light rare earth-heavy rare earth-transition metal (LRE-HRE-TM) films have large and are the promising media for hybrid recording [3. hieh et al [4 investigated the effects of dopant on the MO properties of films and pointed out that small amount of substitution for could increase the Kerr rotation angle. Recent investigation indicated that Cr underlayer could enhance the perpendicular anisotropy of films [5. Li, Jin et al [6.7 fabricated the m(,dy)/cr perpendicular magnetization films and found that m substitution for could significantly increase and keep high. In this paper, (,Dy)/Cr films with perpendicular magnetic anisotropy were successfully prepared onto glass substrate by RF magnetron sputtering and the effects of sputtering technology and substitution on the magnetic properties of (,Dy)/Cr films were investigated. 2. EXPERIMENT (,Dy) films were deposited onto a water-cooled glass substrates with Cr underlayer from a mosaic target using a PF-430H rf sputtering system. The composition of the (,Dy) films was controlled by adjusting the area, place and number of,, Dy pellets on a mm diameter target and the films thickness was tuned with sputtering time. Before admitting high purity argon, the chamber was evacuated to a pressure below Pa. The sputtering argon pressure and the target-to-substrate distance were maintained at 0.53Pa and 90mm, respectively. The sputtering * rresponding author: Xiangshui Miao, miaoxs@mail.hust.edu.cn. Photonics and Optoelectronics Meetings (POEM) 9: Optical torage and New torage Technologies, edited by Masud Mansuripur, Changsheng Xie, Xiangshui Miao, Proc. of PIE Vol. 7517, 75170Y 9 PIE CCC code: X/09/$18 doi: / Proc. of PIE Vol Y-1

2 power varied from w to 400w. The deposition was carried out in the sequences of Cr, or Dy and io 2 layers. The io 2 layer was used for chemical and mechanical protection. The sputtering parameters, such as sputtering power, argon pressure and sputtering time, were listed in table 1. Table 1 puttering parameters for io 2 /(,Dy) /Cr films Film puttering power Argon pressure puttering time puttering rate io W 0.53Pa 2min 12.5nm/min (,Dy) W-400W 0.53Pa 2min-8min 25-58nm/min Cr 280W 0.53Pa 2min 32.5nm/min The structure of films was determined with a Philips χ Pert PRO x-ray diffraction (XRD) using Cu Kα radiation. The films composition was analysed by energy dispersive x-ray spectroscopy (EDX) on a Japan JEOL JM6700F Field Emission canning Electron Microscope. The films thickness was measured with a XP-2 surface profiler. Magnetic properties measurements were carried out using a TM-VM2050HGC vibrating sample magnetometer (VM) with applied magnetic field up to 10kOe. The magneto-optical properties were obtained from a Kerr hysteresis loop tracer with a temperature controller, which were measured with a 780nm wavelength laser. 3. REULT AND DICUION 3.1 The structure of (,Dy)/Cr films Figure1 shows the XRD patterns of io 2 / (,Dy) /Cr /glass. Only diffraction peaks at 2θ=44.4º appear, indicating the existing of Cr(110). No other defined diffraction peak for and Dy films can be detected, which confirms that (,Dy) films are in amorphous states. (110) io 2 //Cr 2 - io 2 /Dy/Cr 2500 Intensity(a.u.) θ / ( o ) Fig. 1 XRD patterns of io 2 / (,Dy) /Cr /glass P / W Fig 2. The effect of sputtering power on of Dy/Cr films 3.2 The effects of sputtering technology on the magnetic properties of (,Dy)/Cr films It is well known, that the magnetic properties of amorphous RE-TM metal alloys are influenced by sputtering conditions via sputtering power, sputter gas pressure and sputtering time dependences of such important quantities like the particle energy of sputtering atoms, magnitude of resputtering effects and hence the final composition, anisotropy constant, and etc [8. Choosing appropriate sputtering parameters is very necessary for fabricating (,Dy) /Cr films with fine magnetic properties. Figure 2 gives the effects of sputtering power on of Dy/Cr films. increases with the sputtering power in the beginning, then decreases as the further increment. The suitable value of sputtering power is 250W, however, the maximum value of is only 2.66kOe. Figure 3 presents the effects of sputtering parameters on the magnetic properties of /Cr films. As can be seen from Figure 3(a), sputtering power on and of film is significantly affected. With the increase of sputtering power, first increases and then decreases. is more than 3.5 koe with sputtering power in the range of 250W-300W. has the same changing trends as. When the sputtering power is Proc. of PIE Vol Y-2

3 250W, reaches the maximum value of puttering power determines the sputtering rate and particle energy bumping on the substrate, thus affects the film composition, structure and thermal stress, which can influence the performance of magnetic films [9. W sputtering of thin films have not enough dense structure and are far from complete film, therefore, and are not great. With the increase of sputtering power, films more dense and integrated, and increase. Further increase of sputtering power, sputtering reduces the effective ion collision, the film is not conducive to the formation of fibrous organizations and the HRE element concentration is also far from compensate component, then easily cause the and of the decline. As can be seen from Figure 3(b), with the increase of sputtering time, increases first and then decreases. increases with the sputtering time, rising from to puttering time controls film thickness, thereby affecting the structure and magnetic properties of thin films. When the sputtering time is 2min, the smaller film thickness, there is no formation of continuous films, and are small. As the sputtering time increases, film thickness increases, and gradually forms a completely continuous film. Experienced a further extension of sputtering time, thin film structure is changed from the Neel-wall domain to Block wall domain and easy to cause a decline in. As a result of thin film structure for the further integrity, continues to increase with the sputtering time P /W t / min (a) (b) Fig3. The effect of sputtering parameters on the magnetic properties of /Cr films (a) sputtering power (b) sputtering time Figure 4 shows the magnetic hysteresis loops of /Cr and Dy/Cr films under the above optimal sputtering conditions. Both and are much greater than and, indicating that perpendicular magnetic anisotropy is obtained [10. The particular magnetic properties of /Cr and Dy/Cr films are listed in the table 2. The /Cr film has larger, and K u than Dy/Cr film H // H - H H M / emu.cm M / emu.cm H H (a) (b) Fig. 4 The magnetic hysteresis loops of /Cr and Dy/Cr films under the same sputtering condition (a)/cr (b)dy/cr Proc. of PIE Vol Y-3

4 Table 2 mparison of the magnetic properties of /Cr film and Dy/Cr film Film (emu/cm 3 ) (koe) H k (T) K u (erg/cm 3 ) /Cr Dy/Cr The effects of substitution for or Dy on the magnetic properties of (,Dy)/Cr films HRE-TM film exhibits ferrimagnetic structure, the magnetic moments of HRE atoms are aligned antiparallelly with those of transition metal atoms. However, the magnetic moments of LRE atoms are aligned parallelly to those of transition metal atoms. Therefore, LRE atom substitution of HRE atom or Dy will change the magnetic properties of (,Dy)/Cr films. Figure 5 shows the relationships between the magnetic properties, such as, and, of ( x 1-x /Cr films and substitution fraction / emu.cm x / x10 4 erg/cm x Fig. 5 The relationships between, and of ( x 1-x /Cr films and substitution fraction x x As the substitution fraction x is increased to 0.338, increases from 144emu/cm 3 to 305 emu/cm 3, however, decreases from 5.20 koe to 2.65 koe. The magnetization of ( x 1-x /Cr films can be written as πm = M M + M = C 0.69μ X μ Xμ 4 s [ ( ) = C[ 0.69μ X ( μ + μ ) 0. 31μ M, M and M are the subnetwork magnetization, μ, μ and μ Where are the atomic moments of each element, X is the substitution fraction, the constant C gives the total number of atoms per unit volume. The expression indicates the magnetization will increase steadily with the increment of X. However, in order to keep the 2 2πM ( x 1-x films with perpendicular magnetization, the demagnetization energy s must be less than K u. Therefore, the substitution fraction X can not be so large. It is well-known that Hc will decrease as increases. The Ms Hc value of the magnetic medium is a key factor in high-density recording [11. As can be seen from Figure 5, with the increment of substitution of, Hc value first increases and then decreases. when x=0.265, Ms Hc value comes forth the maximum peak, indicating ( /Cr film possess the best magnetic properties. Figure 6 presents the relationships between, of ( x Dy 1-x /Cr films and substitution fraction. The and of Dy/Cr films have the same relationships of co/cr films with the substitution fraction x. When x is 0.23, Dy/Cr film gains the optimal magnetic properties. The magnetization of ( x Dy 1-x /Cr films can also be written as πm = M M + M = C 0.67μ X μ Xμ 4 ( ) s Dy [ Dy [ 0.67μ X ( μ + μ ) 0. 33μ = C (2) Dy (1) Proc. of PIE Vol Y-4

5 / emu/cm x Fig 6. The relationships between Ms, Hc of ( x Dy 1-x /Cr films and substitution fraction x θ k / ( o ) 0.2 θ k / ( o ) -T of ( T of ( Dy T of ( T of ( Dy T of ( T of ( Dy T of ( T / Fig.7 The temperature dependence of magneto-optical characteristics of ( x 1-x /Cr films 0.0 -T of ( Dy T / Fig. 8 The temperature dependence of magneto-optical characteristics of ( x Dy 1-x /Cr films Figure 7 and 8 show the influence of temperature on the magneto-optical characteristics of ( x 1-x /Cr films and ( x Dy 1-x /Cr films. As temperature increases, of the two films decreases rapidly with linearity, while θ k hardly reduces before 60. On the other hand, when temperature exceeds 60, the effects of temperature onθ k are distinct with the different substitution fraction. The θk of small x shows little reduction. The θ k with large x presents remarkable decrease, which is adverse in hybrid recording [12. Therefore, in order to keep good magneto-optical temperature characteristics, the substitution for and Dy in the and Dy films can not be more than and 0.23, respectively. 4. CONCLUION (,Dy)/Cr amorphous films with perpendicular magnetic anisotropy have been successfully fabricated onto glass substrate by RF magnetron sputtering. The effects of sputtering technology and substitution on the magnetic properties of (,Dy)/Cr films were investigated. It was found that when the sputtering power and sputtering time are 250W and 4min, respectively, (,Dy)/Cr films have the best magnetic properties. As the substitution fraction increases, the and the temperature stability of magnetic properties decreases, whereas increase steadily. The effects of substitution fraction can be explained by ferrimagnetic structure of the LRE-HRE-TM films. By adopting optimal sputtering parameters and appropriate substitution fraction, a saturation magnetization of 247emu/cm 3, a coercivity of 3.8kOe and K u of erg/cm 3 can be obtained at room temperature for ( /Cr film. Proc. of PIE Vol Y-5

6 ACKNOWLEDGEMENT This work was supported by the National Natural cience Foundation of China (No ) and the National Postdoctoral cience Foundation of China (No ). REFERENCE [1 P. Hansen, C. Clausen, G. Much, M. Rosenkranz, and K. Witter, Magnetic and magneto-optical properties of rareearth transition-metal alloys containing Gd,, Fe,, J. Appl. Phys., 66(2), (1989). [2 H. Tsujimoto, M. houjix and Y. akurai. Magnetic and Magneto-optical Properties of Amorphous Fe Magnetic Films, IEEE.Trans. Magn.,19(5), (1983). [3 Z. Y. Li, X. M. Cheng, F. Jin, et al. Investigations of LRE-HRE-TM thin films for hybrid recording, Proc. PIE, 5966, (5). [4 H. P. hieh, J. Yamasaki, M. H. Kryder. Magnetic properties of amorphous (Pr,) films, IEEE.Trans. Magn., 23(5), (1987). [5 Z.X. Huang, Z.Y. Li., F. Jin, et al. Effect of A Cr underlayer on the magnetic properties of amorphous films with perpendicular anisotropy, Chin. Phys. Lett., 19(10), (2). [6 Z.Y. Li, Z.X. Huang, G. Q. Lin, et al. Effects of m substitution on magnetic and magneto-optical properties of /Cr films, Chin. Phys. Lett., 20(6), (3). [7 F. Jin, Z.Y. Li, Z.X. Huang et al. Magneto-optical and magnetic properties of Dym/Cr thin films. Trans. Magn. oc. Jpn., 4(4-2), (4). [8 A. chilne-wamefeld. D. Weller, W. Reim, et al. Optimized Preparation nditions of RF -puttered Fe Magneto-Optical torage Films, IEEE.Trans. Magn., 24(6), (1988). [9. Yumoto, puttering power dependence of the magnetic properties of CrPt, J. Appl. Phys., 82(4), (1997). [10 J. ayama, K. Mizutani, T. Asahi, et al. Thin films of m 5 with very high perpendicular magnetic anisotropy, Appl. Phys. Lett., 85(23), (4). [11 M. Murakami and M. Birukawa, Magnetic properties and microcolumnar structure of a Fe memory layer for high-density magneto-optical recording, J. Appl. Phys., 95(11), (4). [12.Tsunashima, Magneto-optical Recording, J. Phys. D: Appl. Phys., 34, (1). Proc. of PIE Vol Y-6

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