Effects of layer patterns on magnetic and other properties of single and multilayered Fe C films

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1 JOURNAL OF APPLIED PHYSICS 101, Effects of layer patterns on magnetic and other properties of single and multilayered Fe C films S. C. H. Kwok Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China, Applied Physics Group, School of Physics, A28, University of Sydney, New South Wales 2006, Australia, and Plasma Physics Group, School of Physics, A28, University of Sydney, New South Wales 2006, Australia D. R. McKenzie and M. M. M. Bilek Applied Physics Group, School of Physics, A28, University of Sydney, New South Wales 2006, Australia and Plasma Physics Group, School of Physics, A28, University of Sydney, New South Wales 2006, Australia Paul K. Chu a Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China Received 14 July 2006; accepted 23 November 2006; published online 1 February 2007 The structure and magnetic properties of Fe C films synthesized using dual cathodes and coaxial cathodes in a filtered high current pulsed cathodic vacuum arc were studied. By altering the experimental parameters such as the trigger sequence, period, and pulse length, a series of single and multilayered Fe C films were deposited. X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy were utilized to study the composition and microstructure of the films. The magnetic properties were studied using a superconducting quantum interference device magnetometer. The hysteresis loops show that the magnetic properties are controlled by the pattern of the layers and Fe concentration in the films. The reduction in the coercive field when Fe layers are mixed with ta-c layers is a key result and confirms that magnetically soft materials can be created by the multilayer method. The relationship between the magnetic properties and the structure of single and multilayered Fe C films is discussed American Institute of Physics. DOI: / I. INTRODUCTION a Author to whom correspondence should be addressed; FAX: ; electronic mail: paul.chu@cityu.edu.hk The ability to influence the magnetic coupling between ferromagnetic layers in a multilayer structure containing alternating ferromagnetic and nonferromagnetic layers can lead to useful magnetic properties such as magnetic softness. 1 An application that can make use of magnetically soft materials is high density magnetic recording. A magnetic recording medium designed to operate with perpendicular magnetization increases the recording density. The use of a magnetically soft underlayer beneath the recording medium assists in the creation of the perpendicular field. 2 As a thin film coating, tetrahedral amorphous carbon ta-c that is an amorphous carbon material with approximately 85% sp 3 bonding has superior mechanical properties including surface smoothness, rendering the materials to be useful as protective coatings against wear and corrosion in magnetic hard disks and read-write heads. 3,4 ta-c has a high density of unpaired spins and therefore has a strong paramagnetic property. 5 Some researchers have reported that iron-containing 6 8 or cobalt-containing 9,10 amorphous carbon films have attractive ferromagnetic or superparamagnetic properties, and there are potential applications in magnetic medium materials. In order to increase the density of magnetic recording, it may be advantageous to combine the ferromagnetic properties and mechanical wear resistance in one type of material. This material could then be used alone, or if magnetically soft, as a thin overlayer on another magnetic recording medium. Good mechanical performance would also be advantageous if the material is used as a magnetically soft underlayer, since it then will provide good support for the magnetic recording medium. Ferromagnetic materials that are very magnetically soft with high saturation magnetization are in demand for applications in electrical power engineering. It has recently been found that some nanocrystalline iron containing materials have exceptional magnetic softness. The first class of such materials was the melt-spun Fe Si B alloys containing small amounts of Nb and Cu. Here, we investigate the use of ta-c as a constituent layer in a magnetic multilayer structure with iron, with the aim of creating a magnetically soft material that also has good wear properties. A series of single and multilayered Fe C films were synthesized using a high current pulsed filtered cathodic vacuum arc FCVA with dual and coaxial center-triggered targets. Fe/ C multilayer films with various layer thicknesses can be produced by altering the experimental parameters such as the triggering sequence, pulse period, and pulse length. The aim of this work is to determine whether the magnetic properties of a multilayer Fe C film are influenced by the thickness of the layers and whether soft ferromagnetic properties can be achieved in very fine layers /2007/101 3 /034902/6/$ , American Institute of Physics

2 Kwok et al. J. Appl. Phys. 101, of Fe separated by C. Given that some forms of carbon deposited by the cathodic arc have a very high density of unpaired spins, magnetization of the Fe layers may interact via the paramagnetism of the carbon layers to achieve ferromagnetism. The pulsed cathodic arc presents an excellent opportunity for this study as it allows a wide range of film thicknesses to be fabricated with an accurate control of the thickness. II. EXPERIMENTAL DETAILS A. Sample preparation In our study, two special target arrangements were employed to deposit the single and multilayered films using the FCVA method. 11 The FCVA system used in this study operates in a high current pulsed mode. The power is supplied by a capacitor bank which can produce currents of up to 5 ka. The arc is triggered by surface flashover using a tungsten wire in the center of the cathode. The high currents produce multiple cathode spots which then repel each other to move outwards towards the edge of the cathode. The pulse length can be varied and is usually set equal to the time it takes for the spots to reach the outside perimeter of the cathode. Typical pulse lengths are around 500 m. Our Fe/C multilayered samples were fabricated by using dual cathodes of Fe and C triggered according to a planned sequence to produce the desired layering. Figure 1 a shows the schematic diagram of the system with the inserted photo showing the front view of the targets. The thin films were deposited on Si 100 substrates without applying a bias voltage to the sample stage. By varying the trigger shot sequence as well as the number of periods, a range of layered patterns was produced in the Fe/C films. Tables I and II reveal the experimental parameters used for our samples. Sample FeC a and FeC b are the multilayered films which were deposited with varying Fe to C ratios and periods. Sample FeC a, for instance, had a Fe to C ratio of 100 to 10, indicating that a period consists of 100 shots of Fe and 10 shots of C, with a total number of 50 periods deposited. On the other hand, FeC b had a Fe:C ratio of 500:500, indicating that a period was produced with 500 shots of Fe and 500 shots of C for a total of 10 periods. FeC c, however, is an alloy film without a multilayered pattern, which was deposited using a Fe to C ratio equal to 1 to 1. Although pulses of C and Fe were alternated in this sample, no layers were formed because a single pulse deposited a submonolayer of material and the alternating single pulses produced an alloy. Pure Fe and ta-c thin films were prepared as control samples. All of the samples synthesized by the dual target system contained a total number of 5000 Fe shots including the Fe control. This was done to try to keep constant the amount of Fe in the films in order to study the magnetism changes due only to the variation of the structure and bonding in the film rather than the Fe content. To deposit the single layered samples, a coaxial single target was used. Figure 1 b shows a schematic diagram of the system as well as the target. In this case, two coaxial components Fe and C were combined together into one target. Thus, in each trigger or shot, the cathode spots, triggered at the center, spread out on the surface, traveling from FIG. 1. Color online Illustration of the pulsed filtered cathodic vacuum arc FCVA system with a dual target and b coaxial target. the C region to the Fe region along the radii of the targets. Both the Fe and C plasmas were ignited with each pulse. The composition of the Fe C films was varied by changing the pulse durations. The experimental parameters used for the deposition of the single layered Fe C films are shown in Table II. B. Magnetic measurement and film characterization The magnetization of the films was measured by using a commercial Quantum Design superconducting quantum in- TABLE I. Experimental parameters and magnetic results of Fe C multilayered films synthesized by dual target FCVA. Note: The total shots of the Fe target of each sample are 5000; the pulse frequency of deposition is 10 Hz. Sample Fe to C ratio Period FeC a 100:10 50 FeC b 500: FeC c 1: Fe 1: ta C 0:1 5000

3 Kwok et al. J. Appl. Phys. 101, TABLE II. Experimental parameters and magnetic results of Fe C films different pulse durations synthesized by coaxial FCVA. Sample Pulse length s Pulse frequency Hz Period FeC FeC FeC FeC terference device SQUID magnetometer. The samples were initially zero-field cooled to a measuring temperature of 20 K. The hysteresis loop was obtained by increasing the field from 0 to 200 Oe and measuring the magnetization in a field decreasing from 200 to 200 Oe to give the upper magnetization branch. The field was then varied from 200 to Oe to measure the lower magnetization branch. The sweep rate of the applied field was 10 Oe/min and the sweep was paused for 5 s before each magnetic moment measurement which took 10 s. The values of coercivity and permeability were obtained from the hysteresis loop. Coercivity is the intensity of the applied magnetic field required to reduce the magnetization of the materials to zero after the magnetization of the material has been driven to saturation. The value of coercivity was obtained from the negative intersection of the applied field axis x axis when the magnetic moment was zero, while the permeability was determined from the hysteresis loop when the applied field became zero. Cross sectional transmission electron microscopy TEM was used to study the layers and their microstructure while the chemical bonding and composition depth profiles were analyzed using x-ray photoelectron spectroscopy XPS performed on a PHI model The survey spectra and depth profiles were obtained employing a monochromatic Al K radiation operated at 14 kv and 350 W. III. RESULTS AND DISCUSSION A. Cross sectional TEM The cross sectional TEM images of the samples synthesized using dual targets are shown in Figs. 2 a 2 c. Multilayered structures are observed in Figs. 2 a and 2 b. The clearest layered structure is seen in sample FeC b Fig. 2 b that was deposited with a Fe to C pulse ratio of 500:500. The layers in sample FeC a synthesized using Fe to C pulse ratios of 100:10 with 50 periods are less evident because the layers corresponding to the ten shots of carbon are very thin. The higher magnification micrograph bottom in Fig. 2 b shows that even when the same number of pulses was used for the carbon as for the iron layers, the iron layers dark regions are thicker. Figure 2 c reveals that there is no multilayered structure in sample FeC c. Because each shot deposits a submonolayer amount of material, a shot sequence of 1 to 1 mixes the Fe and C ions forming an alloylike material. B. X-ray photoelectron spectroscopy XPS Figures 3 a and 3 b display the XPS depth profiles of samples FeC b and FeC c, respectively. For sample FeC b, the first six periods of the multilayered pattern are shown. The XPS depth profiles show that the Fe layers are thicker than the C layers being in agreement with the TEM images. The Fe layers are well separated by thinner ta-c interlayers. Figure 3 b shows the XPS depth profile taken from the FeC c sample. The atomic percentages show that the pulses on Fe produce at least ten times more Fe atoms than equivalent pulses on carbon. Hence, the film is a Fe rich Fe:C alloy. A surface oxide is clearly present. XPS results acquired from the FeC a sample are not shown because the carbon layers are too thin to resolve with XPS, but based on the TEM results, we believe that the sample has a layered structure with relatively thin C layers. The XPS spectra and elemental concentrations of the single layered Fe C samples synthesized using the coaxial target are shown in Fig. 4 and Table IV, respectively. The survey scans from 0 to 1400 ev obtained from all the films are identical showing obvious C1s and Fe2p peaks. There is a broad peak at approximately ev in the C1s spectrum Fig. 4 a. Bourgoin et al. 12 have suggested that the peaks at and ev are related to sp 2 and sp 3 hybridizations, respectively. The broad C1s peak therefore indicates that both hybridization states are present. The Fe2p spectrum is shown in Fig. 4 b. The Fe2p 1/2 and Fe2p 3/2 peaks are located at 720 and 707 ev, respectively. The spectrum reveals that the Fe atoms exist close to the pure form rather than as oxide compounds. The weak signal of the O1s spectrum Fig. 4 c also supports this interpretation. C. Magnetic properties Figure 5 shows the hysteresis curves of single and multilayered samples while the results of the coercivity and permeability of the films are summarized in Table III. The multilayered films in Fig. 5 a, samples FeC a and FeC b, and the iron sample have a hysteresis behavior typical of ferromagnetic materials. The relative areas within the hysteresis loops vary with the thickness of the layers and the ratio of iron to carbon. The pure Fe film has a coercivity of 48 Oe and a permeability of emu, and both samples FeC a and FeC b have a smaller value of coercivity than Fe. FeC a has lower permeability than Fe while that of FeC b is higher. Sample FeC b has a very small area enclosed by the hysteresis loop and is therefore magnetically very soft. FeC c and ta-c are weakly magnetic antiferromagnetic or paramagnetic materials and do not produce hysteresis loops. FeC c is an amorphous material in which the Fe atoms are evenly distributed in the C matrix. The reduction in the coercive field when Fe layers are mixed with ta-c layers is a key result and confirms that magnetically soft materials can be created by the multilayer method. Liu et al. 13 studied the formation of multilayers of magnetically hard and magnetically soft materials and found that the coercive field was reduced by using a larger fraction of the soft component in this case, Fe. Since ta-c is paramagnetic with no remanent magnetization and no coercive field, it is magnetically very soft and therefore our results mirror those of Liu et al. The increase in the permeability is more surprising and is not expected on the basis of any linear mixing theory for permeability, since the permeability of

4 Kwok et al. J. Appl. Phys. 101, FIG. 2. Cross-sectional transmission electron micrographs a FeC a Fe:C=100:10; T=50, b FeC b Fe:C=500:500; T=10, and c FeC c Fe:C=1:1; T=5000. ta-c is expected to be much lower than that of Fe. An enhancement of the magnetization could be the effect of the high spin density in ta-c and the limiting of the domain size in the direction perpendicular to the layers. The size of the Fe domains within the film is reduced when the layer thickness changes and the nature of the interfaces between domains is affected when the domains fill the entire thickness of the film. This is likely to increase the permeability by allowing a more complete alignment of magnetization and also reduce the coercive field since the domain walls only need to move laterally in the film to change the magnetization The multilayers are expected to be magnetically anistropic, and so a directional measurement would be interesting although challenging to make.

5 Kwok et al. J. Appl. Phys. 101, FIG. 3. X-ray photoelectron spectroscopy depth profiles of multilayered Fe/C films: a FeC a Fe:C=100:10; T=50, b FeC b Fe:C=500:500; T=10, and c FeC c Fe:C=1:1; T=5000. Figure 5 b shows the magnetization of single-layered Fe doped C samples and all of the films except FeC 300 produce hysteresis loops. Inspection of the coaxial cathode shows that for a 300 s pulse duration, the cathode spots just reach the boundary of the C inner region but do not ablate the Fe part of the cathode. Thus, the plasma should contain almost no Fe when the pulse duration is or less than 300 s. Consequently, sample FeC 300, which was deposited with a 300 s pulse length, shows no magnetization due to a low content of Fe. XPS measurements show that the atomic concentration in this film is only 0.09% Table IV. Referring to the hysteresis loop area as well as the coercivity and permeability results Table III, the magnetization is increased with the pulse duration from 600 to 1500 s. It is because the atomic concentration of Fe is increased with the pulse length. Thus, it is believed that the magnetization is mainly created by the magnitude of the Fe magnetic moment within the films. FIG. 4. X-ray photoelectron spectra of single layered Fe C films: a C1s; b Fe 2p, and c O1s. IV. CONCLUSION We have investigated the magnetic properties of both single and multilayered Fe C thin films fabricated by a high current pulsed filtered cathodic vacuum arc FCVA with the

6 Kwok et al. J. Appl. Phys. 101, TABLE IV. Atomic concentration of Fe doped C films by XPS. Sample C 1s at. % Fe 2p at. % FeC FeC FeC FeC dual and coaxial targets. The multilayered Fe/ C films fabricated with the dual cathodes show a soft magnetic behavior similar to the Fe control. The permeability, coercivity, and shape of the hysteresis loop are found to vary with the relative thickness of the Fe and C layers. We believe that the change is due to the variation of the thickness of Fe layers as well as the Fe cluster volume within the films. The thicker Fe layers in FeC b lead to larger magnetism than FeC a, while the alloylike FeC c sample shows no magnetism. For the Fe doped C films deposited using varying pulse lengths on the coaxial target, the magnetization is found to increase with the atomic concentration of Fe in the films. Our study demonstrates two methods to synthesize superparamagnetic materials and the magnetism can be controlled by the microstructure and composition of the films. ACKNOWLEDGMENTS This research was jointly and financially supported by Hong Kong Research Grants Council RGC, Competitive Earmarked Research CERG Grant No. CityU 1120/04E, and the Australian Research Council. The authors thank Dr. S. Collocott and Dr. J. Dunlop of CSIRO of Australia for the access and assistance with the SQUIDS test and analysis. FIG. 5. Results of the hysteresis loop: a Fe/C multilayered films synthesized by dual target FCVA and b single layered Fe C films synthesized by coaxial target with different pulse length of FCVA. TABLE III. Magnetization of single and multilayered Fe C films. Sample Coercivity Hc Oe Permeability Br emu FeC a E 3 FeC b E 3 FeC c Fe E 3 ta C FeC 300 FeC E 5 FeC E 3 FeC E 4 1 R. Coehoorn, D. B. de Mooji, and C. D. E. Warrd, J. Magn. Magn. Mater. 80, A. Kikukawa, Y. Honda, Y. Hirayama, and M. Futamoto, IEEE Trans. Magn. 36, C. Casiraghi, A. C. Ferrari, J. Robertson, R. Ohr, M. V. Gradowski, D. Schneider, and H. Hilgers, Diamond Relat. Mater. 13, C. Casiraghi, A. C. Ferrari, R. Ohr, D. Chu, and J. Robertson, Diamond Relat. Mater. 13, M. M. Golzan, D. R. McKenzie, D. J. Miller, S. J. Collocott, and G. A. J. Amaratunga, Diamond Relat. Mater. 4, D. Babonneau, J. Briatico, F. Petroff, T. Cabioc h, and A. Naudon, J. Appl. Phys. 87, Y. Lee, J. H. Chen, M. S. Guo, G. M. Liu, and C. S. Lue, J. Magn. Magn. Mater. 282, Y. H. Lee and H. C. Han, Jpn. J. Appl. Phys., Part 1 43, P. Liu, Y. Huang, Y. Zhang, M. J. Bonder, G. C. Hadjipanayis, D. Vlachos, and S. R. Deshmukh, J. Appl. Phys. 97, 10J T. J. Konno, K. Shoji, K. Sumiyama, and K. Suzuki, J. Magn. Magn. Mater. 195, B. K. Gan, M. M. M. Bilek, D. R. McKenzie, P. D. Swift, and G. Mc- Credie, Plasma Sources Sci. Technol. 12, D. Bourgoin, S. Turgeon, and G. G. Ross, Thin Solid Films 357, W. Liu, Y. C. Sui, J. Zhou, X. K. Sun, C. L. Chen, Z. D. Zhang, and D. J. Sellmyer, J. Appl. Phys. 97, 10K