High coercivity FeSmN thin films for longitudinal magnetic recording media

4032 IEEE TRANSACTIONS ON MAGNETICS, VOL. 30, NO. 6. NOVEMBER 1994 High Coercivity FeSmN Thin Films For Longitudinal Magnetic Recording Media D. Wang and W. D. Doyle Department of Physics and Center for Materials for Information Technology, University of Alabama, Tuscaloosa, AL 35487-0209 Abstmct-FeSm thin films with an FdSm atomic ratio of 9.2 have been deposited by dc magnetron sputtering on Ta and Cu substrates held at 400 OC. FeSmN films with in-plane easy axes were obtained by annealing the FeSm films in-situ under a nitrogen atmosphere of 300 Torr at a temperature of 400 OC. The coercivity of the FeSmN films varies from 2 kOe to as high as 7 kOe depending on the deposition conditions. Typical FeSmN films have a magnetization of -140 emdg, a squareness of 0.90 and a coercivity squareness of 0.75. It was found that when a Cr underlayer was deposited at a substrate temperature of 350 "C, FeSmN films of 150 nm thickness with an in-plane coercivity of 6.4 kOe, a squareness of 0.90 and a coercivity squareness of 0.6 were obtained. I. Introduction It is expected that future longitudinal magnetic storage media[l] will require a coercivity of several koe and a magnetic moment of 0.35 to 1.0 memdcm2. The anisotropy constant must be high in order to have magnetically stable small grains[2]. Much of the recent effort on thin film media has concentrated on CoCrX(X=Ta, Pt, Ni ...)[ 31 alloy films, which are being used commercially. These materials seem to have reached a l i i i t for which densities of 1 to 2 Gbits/in2 have been demonstrated[4],[5]. Recently Co$m films have been reported[6] to have a coercivity of 3 kOe, and a magnetization and a squareness similar to those of available media[3]. The amorphous structure of these films may limit its anisotropy constant. Very recently in-plane oriented Ba- ferrite thin films with high coercivity have been described[7],[8]. This material has excellent corrosion resistance, a moderately high anisotropy constant of -lo6 erg/cm3 but a low magnetization of 270 emdcm , and in results reported so far a very high processing temperature of 800 OC. If the coercivity, Hc, is assumed to be proportional to KIMs where K is the anisotropy constant and Ms the saturation magnetization, then Hc can be increased by increasing K as in Fe particles, or decreasing Ms as in Ba-ferrite. Low Ms may cause signal problems and will result in a high superparamagnetic size limit. Therefore we have chosen to focus on materials with high K and high M,. Nitrogenation has been found to increase the crystalline anisotropy, the magnetization and the Curie temperature for certain rare earth-Fe compounds[9]. Thick films[lO],[ll] of these compounds with coercivities more than 3 kOe and magnetization of -1100 emu/cm3 have been obtained with perpendicular anisotropy. We report here the successful preparation of in-plane oriented FeSmN films 150 nm thick with Hc as high as 6.4 kOe. cS of the films is -140 emdg, obtained by dividing the moment with the mass of the films. This corresponds to -1100 emdcm3 if an estimated density of 7.8 is used. coercivity at -2 kOe, and magnetization at -600 emdcm 3 , for 3 Manuscript received March 24,1994. 11. Experiments All the films were prepared by using dc magnetron sputtering from 10 cm diameter targets at 200 W with a source to substrate distance of 9 cm. FeSm films with an Fe/Sm atomic ratio of 9.2 were deposited at an argon pressure of 10 mTorr onto Ta and Cu substrates heated to 400 OC by using an FeSm alloy target with extra pieces of Sm to achieve the optimum composition. Cr underlayers were deposited in argon at a pressure of 5 mTorr. The Ta and Cu substrates are unpolished polycrystalline foils with a slight 2001 texture. The chamber base pressure was better than 4 situ under a nitrogen pressure of up to 300 Torr at a temperature of 400 "C for 20 minutes for films 150 nm thick and up to 100 minutes for films 600 nm thick. Substrate temperature was measured by clamping a thermal couple to the front surface of the substrate. Film thickness was varied from 100 nm to 600 nm at a deposition rate of -0.5 nm/second. Compositions were measured by using an electron microprobe and EDX. X-ray diffraction was performed on a Rigaku D/MAXB diffractometer with a thin film attachment. Magnetic properties were investigated with a Digital Measurement System VSM Model-880, with a maximum field of 14 kOe. X 10 - \ Torr. Nitrogenation was accomplished by annealing in- 111. Results and Discussion Fig. 1 gives the x-ray diffraction data for films of -600 nm thick on Ta substrates before and after nitrogenation. After nitrogenation, the FeSmN peaks shifted to lower angles, corresponding to a lattice spacing expansion of about 1.3% from (a) PeSm ( 0 ) t 30 36 40 46 60 Theta (degrees) Fig. 1. X-ray diffraction paitem for FeSm films of -600 nm thick on Ta substrates (a) before and @) after nitrogenation. 0018-9464/94$4.00 Q 1994 IEEE ~- 4033 the Bragg equation. The crystal symmetry does not change after nitrogenation as shown by the invariance of the spectral amplitudes. It is difficult to identify the crystal structure unambiguously because of the limited number of peaks observed and the several possible phases around the Fe/Sm atomic ratio of 9. These include the Zn17Th2. Mnl2Th and Cu7Tb types, which are related to each other by structural transformation[l2]. However, it is not likely to be the Zn17Th2 structure because bulk Fel7Sm2 is soft[l3] but our FeSm films are hard. The Mnl2Th type is not likely either because bulk Fe12SmNx is soft[l3] but our FeSmN films are hard. Our films are likely to have the disordered Cu7Tb hexagonal structure[ 141. A detailed electron diffraction study to c o n f i i the crystal structure is in progress. Fig. 2 shows the hysteresis loops for the same two films described in Fig. 1. The FeSm film has a coercivity higher than 2 koe for both in-plane and perpendicular loops, and has its easy axis perpendicular to the film plane. After nitrogenation the perpendicular direction changed to a hard axis and the film plane becomes an isotropic easy plane. The FeSmN film has a coercivity of 3.6 kOe. The squareness and coercivity squareness from the in- plane loop are 0.91 and 0.76 respectively, which are similar to those of typical thin film media[3]. The anisotropy field is estimated to be at least 50 koe from the extrapolated intersection of the perpendicular loop and the in-plane loop. The saturation coercivities may be larger than the given values due to the limited maximum field of 14 kOe. The magnetic and structural properties depend strongly on the substrate material and a Cr underlayer. An example is given in Fig. 3 which shows the in-plane loops for two FeSmN films of - 600 nm deposited at 400 OC on Cu substrates with and without a Cr underlayer. Without a Cr underlayer the film has a low in-plane coercivity of only 350 Oe and an a-Fe-like phase, as revealed by x-ray diffraction. With a 500 nm Cr underlayer with a slight [200] h Y \ P B Y .I 4 N 3 e 4 100 60 0 -60 -100 160 100 SO 0 -60 -100 -160 _ _ _ -16 -10 -6 0 6 10 16 Applied Field (koe ) Fig. 2. Hysteresis loops for FeSm films of -600 nm thick on Ta substrates (a) before and @) after nitrogenation. 1 1 ' I ' I ' I ' I ' I ' I I s = 0.90 . s+ = 0.77 - -16 -10 -6 0 6 10 16 Applied Field (kOe) Fig. 3. In-plane hysteresis loops for FeSmN films of -600 nm thick on Cu substrates (a) without and (b) with a Cr underlayer. texture deposited at 400 OC, the FeSmN film has a single hard magnetic phase with an in-plane Hc of 4.2 kOe, a squareness of 0.9 and a coercivity squareness of 0.77. Equivalent results(Fig. 2) were obtained on Ta substrates without the Cr underlayers. A strong thickness dependence of the properties was found for FeSmN films on Cu and Ta substrates with and without Cr underlayers. Fig. 4 shows the thickness dependence of the loops of the FeSmN films on Ta substrates with Cr underlayers. The loops Fig. 4 In-plane hysteresis loops for FeSmN films of different thicknesses on Ta substrates with 500 nm Cr underlayers deposited at a substrate temperature of 400 'C. 4034 develop from almost a single soft magnetic phase with a coercivity of 670 Oe at 100 nm to almost a single hard magnetic phase with a coercivity of 3.2 kOe at 500 nm. It is estimated from the loops that there is an initial soft magnetic transition layer of about 80 nm in all the films, assuming a softmard phase structure. The fraction of this soft region becomes less as the film becomes thicker and eventually invisible for very thick films. To illustrate this further, Fig. 5 shows the x-ray diffraction patterns for FeSm films of different thicknesses deposited under similar conditions but without a Cr underlayer and without nitrogenation. The (1 10) peak of the a-Fe-like phase increases relative to the FeSm compound peaks when the film thickness decreases. This scales roughly with the increase of the soft part in the loops. If a Cr underlayer were used, the strong Cr peaks would overlap with the peaks from the a-Fe-like phase almost exactly, making it impossible to identify the a-Fe-like phase. On Cu substrates with Cr underlayers a similar dependence of loop shape on the thickness of the FeSmN layers was found, though the initial transition layer seems to be slightly thicker than that in the case of Ta substrates. The soft transition layer must be eliminated if these materials are to be used as high density recording media. This was achieved when a Cr underlayer of 250 nm thickness was deposited at a substrate temperature 350 "C. FeSmN films of -150 nm thick with a high coercivity of 6.4 kOe, a squareness of 0.92 and a coercivity squareness of 0.60 were obtained (Fig. 6). There is no sign of the soft phase as seen for films with Cr underlayers deposited at 400 O C . The results reported here show that very high coercivity can be achieved in FeSmN thin films with in-plane anisotropy. Studies designed to understand the role of the Cr underlayer, and to optimize the coercivity and microstructure in the FeSmN films are in progress. L' ' I ' ' - ' I = - =: I ' ' = ' I . ' ' " ' A % h 2 1 500 nm 1 2 Theta (degree.) Fig. 5. X-ray diffraction patterns for FeSm films of different thicknesses on Ta substrates without a Cr underlayer. h n 2 I -16 -10 -6 0 6 10 16 Applied Field (kOe) Fig. 6. In-plane hysteresis loop for a FeSmN film of -150 nm thick deposited at a substrate temperature of 400 "C. The substrate is Ta with a Cr underlayer of 250 nm thickness deposited at a substrate temperature of 350 "C. Acknowledgments We would like to appreciate the assistance of Chuan Gao in the early stage of the experiments, also M. Shamsuzzoha and M. G. Bersch for the composition measurements. References [l]. E.S. Murdock, R.F. Simmons and R. Davidson, "Roadmap for 10 G b i d media: challenges," lEEE Trans. Magn.vol. 28,3078 (1992). [Z]. P.L.. Lu and S.H. Charap, "Magnetic viscosity in high-density recording," 1. Appl. Phys. 75,5768 (1994). [3]. R.P. Fcrrier, Noise in Digital Magnetic Recording, World Scientific, Singapore, 1992. [4]. T. Yogi, C. Tsang, T. A. 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