Magnetic properties and granular structure of CoPtÕB films

Transcription

1 JOURNAL OF APPLIED PHYSICS VOLUME 88, NUMBER 5 1 SEPTEMBER 2000 Magnetic properties and granular structure of CoPtÕB films V. Karanasos, a) I. Panagiotopoulos, and D. Niarchos IMS, NCSR Demokritos, Ag. Paraskevi Attiki , Greece H. Okumura and G. C. Hadjipanayis Department of Physics and Astronomy, University of Delaware, Newark, Delaware Received 14 February 2000; accepted for publication 26 May 2000 Granular CoPt/B films, consisting of high anisotropy face-centered-tetragonal fct CoPt particles embedded in a boron matrix have been prepared by heat treating cosputtered films. Transmission electron microscopy and x-ray diffraction studies show that the as-deposited films consist of a mixture of an amorphous-like phase and face-centered-cubic CoPt nanocrystallites. After a heat treatment at 700 C for 5 min a microstructure consisting of interconnected particles of partially ordered fct CoPt with sizes ranging from 20 to 200 nm is formed. These samples have coercivities of 6 koe and high reduced remanence (M r /M S 0.9) due to the interparticle interactions. After prolonged annealing the particles become spherical and their size increases to around 400 nm. The coercivities of these overaged samples are higher but the hysteresis loops are constricted and the remanence is reduced. Measurements of irreversible susceptibility and viscosity as a function of temperature and applied field were used to determine the fluctuation field and activation volume in the optimally annealed sample American Institute of Physics. S INTRODUCTION Recently the ever increasing need for higher and higher recording density has triggered the search for new recording media which will meet the requirements for ultrahigh density recording. 1,2 These should combine a smaller grain size, good grain isolation, higher magnetic anisotropy and coercivity, within the limitations imposed by the write fields available. 1,2 In this respect, nanostructured materials consisting of the high anisotropy face-centered-tetragonal fct CoPt and FePt equiatomic L1 0 phases 3,4 dispersed in nonmagnetic matrices seem very promising candidates. 2 Large H C in samples consisting of isolated grains with high anisotropy have been reported in CoPt/Ag, 5,6 CoPt/C, 6,7 FePt/Si 3 N 4, 8 FePt/Al 2 O 3, 9 andcopt/sio 2. 10,11 The as-deposited material is usually in the magnetically soft face-centered-cubic fcc phase. Coercivity, grain size and particle isolation in these materials are controlled by a heat-treatment process which leads to both particle formation and to the crystallization of the hard fct phase. Thus the selection of the matrix material is very crucial to the kinetics of particle formation and consequently to the magnetic properties of the material. The results on CoPt/C nanocomposites are very promising with respect to grain size and recording characteristics. 6,7 However the transformation to the fct phase is much more sluggish and requires higher temperatures and longer processing times compared to CoPt/Ag. 6,7 Reducing the processing temperatures and times is desirable from both fabrication and grain growth aspects. 2 In this article we present results on CoPt/B nanocomposites. Boron was tried as the matrix a Electronic mail: bkaranasos@ims.demokritos.gr material instead of carbon in order to allow faster grain growth kinetics. EXPERIMENT The films were prepared by magnetron sputtering deposition from two 2 in. targets of CoPt and B. The base pressure of the chamber was about 10 7 Torr and high purity Ar % was used for deposition with a pressure of 5 mtorr. The films were deposited on Si m thick substrates and carbon coated transmission electron microscope TEM grids at ambient temperature. The CoPt and B layers were successively deposited in a multilayer fashion. The B layers were sputtered using a power of 60 W rf at a rate of 1.3 Å/s and CoPt layers with a dc power of 60 W at a rate of 5.5 Å/s. The number of bilayers was adjusted to give a total CoPt thickness of 500 Å. The boron layer thickness was kept equal to that of CoPt in all the samples considered here. The chemical composition of CoPt as-made films was checked by energy dispersive x-ray analysis. The resulting stoichiometry of the layers was verified to be around Co 50 Pt 50. X-ray diffraction XRD spectra were collected with a Siemens D500 powder diffractometer using CuK radiation. Magnetic hystersis loops were measured with a Quantum Design MPMSR2 superconducting quantum interference device SQUID magnetometer. The microstructure was examined with Philips CM20 and JEOL JEM 2000 FX transmission electron microscopes. The heat treatment was performed after deposition in situ with a homemade heater. The heating and cooling rates were 600 and 240 C/ min, respectively. RESULTS The as-made films are magnetically soft with high reduced remanence M r /M S 0.85 Fig. 1 a. Upon annealing /2000/88(5)/2740/5/$ American Institute of Physics

2 J. Appl. Phys., Vol. 88, No. 5, 1 September 2000 Karanasos et al FIG. 1. Hysteresis loops of 20 Å layer series for as-deposited a, optimized b, and overaged c samples. FIG. 2. XRD spectra of 20 Å layer series for as-deposited a, optimized b, and overaged c samples. at 700 C for several minutes the coercivity starts to increase. A summary of the hysteresis loop parameters in the CoPt/B samples with magnetic layer thickness ranging from 10 to 20 Å after annealing at different times is listed in Table I. An increase of H C with aging time can be observed for all layer thickness. However there is no clear correlation of the H C with the layer thickness. After annealing for 5 min coercivities around 6 koe and a high remanence M r /M S 0.9 can be obtained Fig. 1 b. The steep fall of the magnetization TABLE I. Summary of CoPt/B samples. CoPt Layer thickness Aging time H c Å min Oe curve at H C corresponds to a coercivity squareness parameter S* of 0.86, indicating a narrow switching field distribution. When the annealing time is prolonged up to 30 min a shoulder appears in the demagnetization curve. It seems that the optimum magnetic properties are obtained for short annealing times of about 5 min since upon further annealing the loops become constricted and the remanence is reduced Fig. 1 c. This drop in magnetization near zero field can be attributed to the existence of a soft disordered CoPt phase. 5,6,12 The XRD diffraction patterns for the as-sputtered, the optimized 5 min 700 C and the overaged 30 min 700 C samples, with layer thickness of 10 Å, are shown in Figs. 2 a 2 c, respectively. The as-sputtered sample Fig. 2 a shows only two weak and broad reflections, one corresponding to the 111 of the fcc structure with a Å and one that can be attributed to the 003 of boron. 13 This assignment is strengthened by the fact that the 003 boron peak remains unchanged with the subsequent heat treatment. Thus the as-sputtered samples seem to consist mostly of grains in the amorphous or nanocrystalline state. The optimally annealed sample 5 min at 700 C shows a strong 111 peak

3 2742 J. Appl. Phys., Vol. 88, No. 5, 1 September 2000 Karanasos et al. due to texturing along this direction Fig. 2 b. The characteristic splitting of the fct phase between the 020 and 002 reflections is not clearly observed. However, the broadening of the 002 peak can be attributed to a tetragonal phase with c/a Furthermore, the appearance of a weak 001 superstructure reflection shows that there is indeed some degree of ordering. Finally, for the overaged sample the superstructure 001 peak and the splitting of the 002 peak due to the formation of the fct structure are very clearly observed Fig. 2 c. The lattice parameters are a 3.77 Å and c Å which give a c/a ratio of compared to the bulk value of The evolution of the microstructure with aging heat treatment at 700 C as well as the fct formation was studied by transmission electron microscopy Fig. 3. The electron diffraction pattern of the as-sputtered sample, shown in Fig. 3 a, verifies that this sample is mainly comprised of an amorphous type state and some particles below 10 nm. In accordance with the XRD data shown in Fig. 2 a the bright field image BF in Fig. 3 a shows that there are very fine metastable fcc CoPt crystallites with average sizes of a few nanometers. The microstructure of the magnetically optimized sample Fig. 3 b consists of interconnected particles or grains that sometimes have planar defects. The particle size ranges from 20 up to 200 nm and the spatial distribution is homogeneous. The electron diffraction pattern from the area Fig. 3 b shows the existence of fcc CoPt, with some possible degree of ordering in accordance with the XRD observation. The electron diffraction data of Fig. 3 c for the overaged sample confirm the coexistence of an ordered CoPt with L1 0 superstructure with some small amount of fcc phase. In this sample large particles with half micron sizes coexist with smaller ones, mainly around 100 nm Fig. 3 c. The grain size in the smaller particles ranges approximately from 20 to 100 nm. In Fig. 4 the temperature dependence of the H C is shown for the optimized and overaged sample in the 20 Å layer thickness series. We have attempted to fit the data to an expression of the form 15 H C H 0 1 ln( mf 0 )k B T KV n, with H 0 2K/M S, m is the characteristic time scale of measurement, f 0 is an attempt frequency, k B the Boltzmann s constant, T is the absolute temperature, K the anisotropy constant, and V the switching volume. The exponent n usually ranges from 1/2 to 2/3, 16 but values up to 1 have been used to fit similar data. 15,17 For our SQUID measurements by assuming ln( m f 0 ) 25 the fit gives for the optimized sample n and for the overaged sample n The relatively high values of the n exponent especially in the case of the optimized sample indicate that interparticle interactions are strong and energy barriers for magnetization reversal do not follow the relations predicted for isolated single domain particles, which is consistent with the TEM observations. In order to study the thermal stability and relaxation phenomena of the samples we have performed measurements of 1 FIG. 3. Electron diffraction data and TEM photographs for as-deposited a, optimized b, and overaged c samples. magnetic viscosity and irreversible susceptibility on the optimized sample. The sample was first brought to positive saturation by applying a field of 40 koe, and then a reversed field H was applied and the magnetization was monitored as a function of time. By fitting the data to a logarithmic decay behavior of the form 18 M M 0 S ln t, 2 the magnetic viscosity coefficient S(H,T) was estimated at different values of reversed field and temperatures. The viscosity as a function of H for different temperatures from 5 to 300 K is shown in Fig. 5. The results show the following typical behavior: for each temperature S goes through a

4 J. Appl. Phys., Vol. 88, No. 5, 1 September 2000 Karanasos et al FIG. 4. Temperature dependence of the H C for the optimized open squares and overaged solid circles sample. FIG. 6. Temperature dependence of maximum magnetic viscosity S max and fluctuation field H f. maximum value S max around H C resulting a characteristic bell-shaped curve. 18,19 S max increases and the width of the bell-shaped curves decreases with temperature. 20 The value S max is plotted as a function of temperature in Fig. 6 and shows the expected linear increase due to the thermally activated demagnetization process down to cryogenic temperatures. The irreversible susceptibility irr was calculated for the same temperatures and fields by differentiating the remanence demagnetization curves. It is worth noting that the reversible part of the demagnetization curves is very small in the optimized sample and the remanence coercivity H R is almost equal to H C. This is in contrast with the CoPt/C samples prepared under similar conditions where we had an H R /H C ratio exceeding The above measurements can be combined to give the fluctuation field defined as H f S/ irr. 18,20 The fluctuation field can be considered as an effective field which has the same effect on the magnetization as that of thermal fluctuation energy. 20 The value of H f can be used to estimate the activation volume V* which represents the mean value of the volumes that take part in the thermal reversal through the expression H f kt/v*m S. 21 The values of V* correspond to a magnetic grain size D* (6/ V*) (1/3) in the range from 95 to 160 Å, depending on the temperature measured Fig. 7. This quite small value of D*, compared to the particle size, is probably due to incoherent rotation mechanisms arising from interparticle interactions and reduced surface anisotropy. CONCLUSIONS High coercivity CoPt/B granular films consisting of the highly anisotropic ordered fct CoPt embedded in a boron matrix have been successfully prepared. Samples annealed at 700 C for 5 min consist of interconnected CoPt particles with a size that ranges from 20 up to 200 nm having an H C of 6 koe. The interparticle interactions account for the high remanence M r /M S 0.9. Compared to CoPt/C films prepared under similar conditions these films acquire high coercivities at much shorter annealing times. However, the resulting particle sizes are also larger, indicating that the kinetics of the particle formation is much faster. Thus the FIG. 5. Magnetic viscosity S for the optimized sample as a function of H for different temperatures from 5 to 300 K. FIG. 7. Magnetic grain size D* vs temperature.

5 2744 J. Appl. Phys., Vol. 88, No. 5, 1 September 2000 Karanasos et al. suitability of these materials as ultrahigh density recording media will depend on further optimization efforts, probably at lower processing temperatures. ACKNOWLDGEMENTS This work was supported by the EU network, FMRX- CT Dynaspin and by NSF DMR D. N. Lambeth, E. M. T. Velu, G. H. Bellesis, L. L. Lee, and D. E. Laughin, J. Appl. Phys. 79, D. Weller and A. Moser, IEEE Trans. Magn. 35, R. A. McCurrie and P. Gaunt, Philos. Mag. 13, R. A. Ristau, K. Barmak, L, H. Lewis, K. R. Coffey, and J. K. Howard, J. Appl. Phys. 86, S. Stavroyiannis, I. Panagiotopoulos, D. Niarchos, J. A. Christodoulides, Y. Zhang, and G. C. Hadjipanayis, Appl. Phys. Lett. 73, S. Stavroyiannis, I. Panagiotopoulos, D. Niarchos, J. A. Christodoulides, and G. C. Hadjipanayis, J. Magn. Magn. Mater. 193, M. Yu, Y. Liu, A. Moser, D. Weller, and D. J. Sellmyer, Appl. Phys. Lett. 75, C.-M. Kuo and P. C. Kuo, J. Appl. Phys. 87, B. Bian, K. Sato, Y. Hirotsu, and A. Makino, Appl. Phys. Lett. 75, C. Chen, O. Kitakami, S. Okamoto, Y. Shimada, K. Shibata, and M. Tanaka, IEEE Trans. Magn. 35, K. Ichihara, A. Kikitsu, K. Yusu, F. Nakamura, and H. Ogiwara, IEEE Trans. Magn. 34, S. H. Liou, S. Huang, E. Klimek, R. D. Kirby, and Y. D. Yao, J. Appl. Phys. 85, JCPD Database No M. Hansen, Metallurgy and Metallurgical Engineering Series, Constitution of Binary Alloys McGraw-Hill, New York, M. P. Sharrock, IEEE Trans. Magn. 35, R. H. Victoria, Phys. Rev. Lett. 63, M. Alex and D. Wachenschwanz, IEEE Trans. Magn. 35, R. W. Chantrell, J. Magn. Magn. Mater. 95, Chn Mayergoyz, A. Adly, C. Korman, M. Huang, and C. Kraft, J. Appl. Phys. 85, R. Street and S. D. Brown, J. Appl. Phys. 76, D. J. Sellmyer and Z. S. Shan, NATO ASI Ser. Ser. E 338, edited by G. C. Hadjipanayis Kluwer Academic, New York, 1997.