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C multilayers
Journal of Magnetism and Magnetic Materials 231 (2001) 231–240
Magnetic and structural properties of annealed CoPt/C multilayers J. Dua, S. Wanga, C. Rubya, A. Khapikova, W.J. Liua, J.A. Barnarda,*, J.W. Harrellb a
Center for Materials for Information Technology, Department of Metallurgical and Material Engineering, Box 870 209, 205 Bevill Research Building, Tuscaloosa, AL-35487-0209, USA b Materials for Information Technology, Departments of Physics and Astronomy, The University of Alabama, Tuscaloosa, AL 35487-0209, USA Received 17 November 2000; received in revised form 7 February 2001
Abstract The structure and magnetic properties of multilayer Co82Pt18/C films with equal CoPt and C layer thickness have been studied as a function of annealing temperature and film thickness. In general, annealing leads to the breakup of the continuous films and the formation of a laterally heterogeneous morphology consisting of separate CoPt and C regions. For individual layer thickness 55.3 nm, annealing has no significant effect on the crystallographic structure and magnetic properties of the films. In both the as-deposited and annealed states, the films have HCP structure with perpendicular c-axis orientation and exhibit perpendicular magnetic anisotropy. The structure of these films appears to be associated with the growth of the CoPt on the C layers. For films with individual layer thickness of 1.3 and 2.7 nm, the structure and magnetic properties are strongly affected by annealing. In the as-deposited state, films have a fine grain structure and coercivities of a few Oe. Annealing leads to the breakup of the multilayer structure at about 3008C and the formation of 20 nm HCP and FCC magnetic grains, with the HCP grains exhibiting in-plane c-axis texture. Annealed films have large in-plane coercivities (maximum of 2.77 kOe) and random in-plane magnetic anisotropy. The increase in coercivity with annealing temperature above 3758C is primarily associated with an increase in switching volume. # 2001 Elsevier Science B.V. All rights reserved. PACS: 61.16.Bg; 68.55.a; 75.50.Ss; 75.70.i; 81.05.Ys Keywords: Magnetic granular films; Transmission electron microscopy; Sputtering; Nanolamination; Magnetic thermal stability
1. Introduction Substantial work has been done in recent years on Co-based granular alloys for potential applica*Corresponding author. Tel.: +1-205-348-9399; fax: +1205-348-2346. E-mail address: [email protected] (J.A. Barnard).
tions as magnetic media. The ideal granular medium should consist of high anisotropy magnetic grains embedded in a robust non-magnetic matrix. The grains should be small with a uniform size distribution, yet large enough for adequate thermal stability. The grains should also be sufficiently isolated to minimize exchange interactions and yet tightly packed to enable sharp
0304-8853/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 1 7 5 - 5
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transitions. A common way to produce the granular films is to co-sputter the magnetic and non-magnetic phases. Depending on deposition conditions, subsequent annealing may be required. Alternatively, multilayers of the magnetic and non-magnetic phases can be deposited, followed by annealing. The coercivity of a granular media is determined by the magnetic anisotropy of the grains, the grain size, and intergranular interactions. The magnetic anisotropy of an individual grain is primarily determined by its crystal phase and shape. Small pure Co particles are typically a mixture of FCC and HCP phases, depending on particle size and processing conditions [1]. The anisotropy of the HCP phase (K1 þ 2K2 ) is 7 106 erg/cm3 [2], which is large enough in principle to yield HC0.5HA5 kOe. The anisotropy of the FCC phase is about 10 times smaller than the HCP phase [3]. Consequently, coercivities greater than 1 kOe are difficult to achieve in granular films with pure Co. The addition of platinum enhances the HCP fraction, and coercivities greater than 2 kOe can be achieved in Co1xPtx granular films, where x 0:2 [4,5]. Further addition of elements such as Cr, Ta, and B can enhance grain separation and lead to still larger coercivities. However, this may be at the expense of K1 þ 2K2 and Ms . Coercivities of 4400 and 5600 Oe have recently been reported in annealed multilayer granular CoCrPt/C [6] and CoCrPt/SiO2 films [7], respectively. Much larger coercivities can be obtained in high-anisotropy equi-atomic ordered CoPt alloy films [8]. In this paper, we report the results of a study of the Co82Pt18/C (hereafter CoPt/C) multilayer system. The structural and magnetic properties, including the thermal stability of the magnetization, are examined as a function of layer thickness, number of layers, and annealing temperature.
2. Sample preparation and experimental details CoPt/C multilayers were made at ambient temperature by sequential DC magnetron sputtering from a pure C target and a CoPt mosaic target, both 5 cm in diameter. Power densities of 9.9 and
1.7 W/cm2 were used to achieve deposition rates of 4.4 and 7.2 nm/min for C and the CoPt alloy, respectively. The base pressure of the chamber was 5 107 Torr. High purity Ar, at a pressure of 2 mTorr, was used as the processing gas. The substrates were carbon coated copper grids for transmission electron microscopy observation of as-deposited samples and Corning 7059 glass for all other measurements. Magnetic hysteresis loops were measured with a PMC 2900 alternating gradient magnetometer, with in-plane external field up to 10 kOe. The microstructure was examined with a Hitachi 8000 transmission electron microscope (TEM). X-ray photoelectron spectroscopy measurements (KRATOS AXIS 165 system) were used to determine the composition of the CoPt alloy (Co82Pt18 for all samples) using the relative areas of the Co2p and Pt4f spectra (or the Co3p and Pt4f spectra) and the corresponding standard atomic sensitivity factors [9]. X-ray reflectivity (XRR) and conventional high angle diffraction were measured with Philips X’pert and Rigaku diffractometers, respectively, using Cu Ka radiation. Multilayers with equal thickness C and CoPt layers were prepared which, according to our previous experience with Co/CN multilayers [10], yield the maximum coercivities. The geometry of the multilayers was ðtCoPt=tCÞn with t ¼ 8, 2.7, 1.3 nm and n ¼ 1, 3, 6, respectively, for a set of films with a total thickness of 16 nm; and t ¼ 16, 5.3, 2.7 nm and n ¼ 1, 3, 6 for a set of films with a total thickness of 32 nm. To minimize any potential differences between films grown on TEM grids and those on glass, C was the first layer sputtered in all cases. All films were capped with a thin layer of C. Annealing was carried out in a vacuum of 106 Torr at different temperatures for 2 h. XRR measurements were used to check the layered structure of sample (1.3/1.3)6 in the asdeposited state and after annealing at 3008C, see Fig. 1. The measured reflectivities decay by 6 orders of magnitude for a small increase in the ( 1. Information scattering vector q from 0 to 0.6 A on the thickness, mass density, and roughness of each layer can be deduced from XRR data, based on optical reflectivity theory. In the as-deposited
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Fig. 1. Grazing angle X-ray reflectivity from (1.3/1.3)6 films in as-deposited and after annealing at 3008C. The reflected intensity was measured as a function of incident angle y, and plotted as a function of the scattering vector q ¼ 4psiny=l, where l is X-ray wavelength.
state, the dominant oscillations have a period of ( 1, from interference between the film Dq 0:04 A surface and the bottom. These fringes are modulated by another oscillation with a longer period of ( 1, corresponding to the superlattice Dq 0:27 A unit (CoPt/C). The 1st and 2nd order superlattice peaks can clearly be seen, confirming the expected periodicity in the as-deposited state to within 10% of the nominal value. After annealing at 3008C, the layered structure is totally lost.
3. Results and discussion 3.1. Structural characterization In this section attention is generally limited to the thinner set of samples, since this set exhibits the maximum coercivities. Annealing at 4008C was found to be sufficient to produce drastic changes in magnetic properties (see below). The structure of these films is discussed here. As-deposited bright field (BF) images and the corresponding selected area diffraction (SAD) patterns for samples (8/8)1, (2.7/2.7)3, and (1.3/1.3)6 appear in Fig. 2. The BF image from (8/8)1 is consistent with a continuous crystalline CoPt layer although grain-to-grain contrast is not distinct. The broken rings in the
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SAD pattern from (8/8)1 can be indexed sequentially as (1 0 0)HCP, (0 0 2)HCP or (1 1 1)FCC, (1 0 1)HCP, (1 0 2)HCP, (1 1 0)HCP or (2 2 0)FCC, (1 0 3)HCP, (2 0 0)HCP, (1 1 2)HCP or (3 1 1)FCC reflections, according to the lattice parameters given by Aboaf (HCP phase: a0 ¼ 0:260 nm, c0 ¼ 0:422 nm; FCC phase: a0 ¼ 0:366 nm) [11]. The strong intensity of the (1 0 0)HCP and (1 1 0)HCP rings indicates a [0 0 1]HCP texture perpendicular to the film. Six strong (1 0 0)HCP spots distributed with six-fold symmetry along with the corresponding six (1 1 0)HCP spots indicate that hexagonal CoPt grains with [0 0 1]HCP orientation also exhibit in-plane texture. In fact some regions of this sample display a single crystal CoPt [0 0 1] diffraction pattern with (1 0 0)HCP and (1 1 0)HCP spots and without the (0 0 2)HCP, (1 0 1)HCP, (1 0 2)HCP, and (1 0 3)HCP rings. The [0 0 1]HCP perpendicular texture in (8/8)1 is also confirmed by the strong (0 0 2)HCP peak found in the conventional y–2y X-ray diffraction pattern (Fig. 3). The second peak in the spectrum is probably due to CoPt oxidation. Considering the fact that the CoPt film is only 8 nm thick in this sample, the (0 0 2)HCP peak is quite strong. Strong perpendicular and in-plane texture is observed in all of the multilayers with thicker component CoPt layers, i.e., (5.3/5.3)3, (8/8)1, and (16/16)1. To observe such definitive texture in layers only 5.3 nm thick deposited without substrate heating on amorphous C underlayers indicates that the preferred orientation is determined in the early stages of film growth, and may perhaps be enhanced by the C underlayer. To explore this issue a single layer CoPt reference film, 15 nm thick, was sputtered directly on glass (no C underlayer) under the same experimental conditions as the CoPt layers in the CoPt/C multilayers. This sample was prepared for TEM observation by mechanical polishing and ion-thinning from the glass side. Fig. 4 is the electron diffraction pattern from this sample. The pattern displays only slight in-plane texture, in complete contrast to that found for 5.3, 8, and 16 nm thick CoPt layers grown on C underlayers, and confirms the importance of amorphous C in generating highly oriented growth. As the 8 nm thick CoPt layer is subdivided to produce samples (2.7/2.7)3, and
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Fig. 2. Plan-view TEM bright field images and electron diffraction patterns from as-deposited CoPt/C samples; (a) (8/8)1, (b) (2.7/2.7)3, and (c) (1.3/1.3)6.
(1.3/1.3)6, atomic disorder clearly increases (Fig. 2b, c). The BF images are now quite homogeneous and the electron diffraction patterns consist of broad halos. No individual rings are resolvable. The microstructural evolution of samples (8/8)1, (2.7/2.7)3, and (1.3/1.3)6 on annealing is summarized in the BF images in Fig. 5. All three samples are now well crystallized with good grain-to-grain contrast. However, the most dramatic change is
the clear evidence that the CoPt layers have now broken up to form laterally heterogeneous two phase CoPt/C films. In this sequence of BF images the uniform light areas are C and the darker areas with substantial internal contrast are CoPt alloy. Broadly speaking, sample (8/8)1 (Fig. 5a) is a contiguous network-like CoPt film with numerous large (200–300 nm) irregularly shaped C ‘holes’. When the initial structure is further divided,
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Fig. 3. y–2y X-ray diffraction from as-deposited (8/8)1 sample.
Fig. 4. Electron diffraction pattern from 15 nm CoPt reference film on glass sputtered under the same experimental conditions as the CoPt layers in the CoPt/C multilayers.
samples (2.7/2.7)3, and (1.3/1.3)6, annealing produces a ramified structure, now well crystallized, but the scale of the features decreases consistently while the number of isolated, small CoPt grains and C holes increases. One can only conclude that during annealing atomic rearrangements over long distances have converted the component layers in the as-grown structures from a perpendicularly heterogeneous two-phase structure into a laterally heterogeneous two-phase structure. The plan view area fractions of CoPt are 0.5 in all three samples. Many of the CoPt grains in samples (2.7/ 2.7)3, and (1.3/1.3)6 are clearly striated, a contrast feature associated with stacking faults which are
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common in Co and Co-alloy films and Co-based granular films. The absence of striations in (8/8)1 is due to the fact that the CoPt grains in the single layer sample are mostly [0 0 1]HCP oriented. To further quantify the structures of the annealed nanolaminates through grain size analysis, DF images obtained using a segment of the four innermost diffraction rings were made (not shown here). The presence of stacking fault striations was particularly clear in the DF images. Grains were hand-traced and digitally scanned and the areas of each measured grain were converted to equivalent area circles. The diameters of the circles were used as the grain diameter in the statistical analysis. Samples (2.7/2.7)3 and (1.3/1.3)6 have average grain sizes of 23 and 19 nm with standard deviations of 13 and 10 nm, respectively. As for the highly textured sample (8/8)1, features in the DF image obtained using a diffraction spot actually contain multi-grains. Individual grains are difficult to resolve. However, the bright field image of this sample (Fig. 5a) does show distinct grain boundaries for some grains and a rough grain size of 30 nm is obtained. The effect of annealing on the electron diffraction patterns from samples (8/8)1, (2.7/2.7)3, and (1.3/1.3)6 is also seen in Fig. 5. Sample (8/8)1 retains the as-deposited texture. The pattern shown in Fig. 5a consists of several sets of [0 0 1]HCP zone axis patterns oriented differently in the plane. The observation of a (2 0 0)FCC ring is an unequivocal indication of the presence of the FCC phase, all other FCC rings overlap with the HCP phase. The (8/8)1 specimen has a very strong [0 0 1] hexagonal and/or [1 1 1] cubic texture and in order to try to observe the (2 0 0)FCC reflection, the specimen was tilted 358 about the horizontal. However, after tilting the (2 0 0)FCC ring was not observed. Thus, we conclude that this film, in both as-deposited and annealed states, is in fact composed of the HCP phase only. It has been demonstrated by other authors that Co rich Co–Pt alloys with more than 14 atomic % Pt can exhibit only the HCP phase [12,13]. Electron diffraction rings with uniform circumferential intensity from samples (2.7/2.7)3 and (1.3/1.3)6 indicate grains randomly oriented in the plane. For these two samples the sequence of rings is consistent with a
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Fig. 5. Plan-view TEM bright field images and electron diffraction patterns from annealed Co/CN samples; (a) (8/8)1, (b) (2.7/2.7)3, and (c) (1.3/1.3)6.
mixture of HCP and FCC phases. The strong intensity of the (0 0 2)HCP diffraction ring indicates that grains with the c-axis lying in the film plane predominate (Fig. 5b, c). In sputtered Co/Pt multilayers [14] a transition from FCC to HCP beginning at six to eight monolayers equivalent Co thickness has been reported while in high pressure sputtered Co particles [1] the same transition is noted beginning
at diameters of 20 nm. Thus it appears that there is a dimensional effect on the stability of these two phases in Co with FCC favored in thinner films and smaller particles. If this trend also holds for Co rich CoPt films then the HCP nature of sample (8/8)1 in both as-deposited and annealed states is at least qualitatively understandable. For this film, the crystal structure as well as the strong perpendicular and in-plane texture is selected
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Fig. 6. In-plane coercivities as a function of annealing temperature.
during growth. Similar results were found for samples (5.3/5.3)3 and (16/16)1. By contrast, for the samples with thinner component layers, the CoPt is essentially amorphous in the as-deposited state. During annealing both HCP and FCC CoPt crystallites nucleate with their close-packed planes preferentially aligned parallel to the film plane but without in-plane texture.
Fig. 7. Effect of annealing on the hysteresis loop of sample (2.7/ 2.7)3. (a) In-plane loop for as-deposited sample. (b) In-plane and perpendicular loops after annealing at 3758C for 2 h.
3.2. Magnetic properties Fig. 6 shows the in-plane coercivities of the films on glass as a function of annealing temperature. Magnetically, the samples fall into two categories depending on the thickness of the individual Co layers. The three samples with the thinnest Co layers (1.3 and 2.7 nm) were very soft as deposited and partially superparamagnetic. These samples exhibited strong annealing effects, and the coercivities increased to 1.5–2.2 kOe when annealed at the maximum temperature of 5008C for 2 h. The structure (2.7/2.7)3 yielded the largest coercivity, both on glass and on the TEM grid. Fig. 7 shows in-plane loops for sample (2.7/2.7)3 on glass as deposited and when annealed at 3758C for 2 h. Inplane coercivities for these thinnest samples were significantly larger than out-of-plane coercivities, consistent with the TEM measurements that show a predominance of grains with the c-axis lying in the film plane. The three samples with the thickest Co layers (5.3, 8, and 16 nm) exhibited perpendi-
cular anisotropy both as deposited and after annealing. Fig. 8 shows in-plane and perpendicular loops for sample (8/8)1 as-deposited on glass. The loops for the annealed films were virtually identical. Similarly shaped loops were observed for samples (16/16)1 and (5.3/5.3)3. However, the perpendicular loops for these two samples were more sheared resulting in much lower remanence. Although the TEM diffraction for sample (8/8/)1 showed in-plane texture (on the scale of the electron beam spot), no in-plane magnetic anisotropy was observed. The perpendicular coercivities for these films were significantly smaller than those measured in-plane. This fact along, with the shape of the perpendicular loops, suggests magnetization reversal by irreversible domain wall motion. The perpendicular magnetic anisotropy of these films is consistent with the perpendicular c-axis texture obtained from the TEM and XRD measurements.
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Fig. 8. In-plane and perpendicular loops for sample (8/8)1 as deposited. Loops after annealing are nearly the same.
Fig. 9. Activation volume as a function of annealing temperature for samples (2.7/2.7)3, (1.3/1.3)6, and (2.7/2.7)6.
Activation volumes were obtained from measurements of the maximum remanent viscosity (Sr ) and maximum irreversible susceptibility (wirr ) using the relationship Vac ¼ kTwirr =Ms Sr . These values for the films on glass are shown in Fig. 9. The large values of Vac for samples (2.7/2.7)3 and (1.3/1.3)6 annealed at 3008C are likely associated with some domain structure involving several grains. These samples have very high loop squareness and inplane anisotropy, suggesting reversal by domain wall motion. The activation volumes for the other samples are more closely related to actual grain sizes. The values of Vac for the 4008C annealed samples (3–3.5 1018 cm3) are slightly smaller than the grain volumes calculated using the film
Fig. 10. Thermal stability factor KV=kT as a function of annealing temperature for samples (2.7/2.7)3, (1.3/1.3)6, and (2.7/2.7)6.
Fig. 11. Switching field H0 as a function of annealing temperature for samples (2.7/2.7)3, (1.3/1.3)6, and (2.7/2.7)6.
thickness and measured grain diameters for samples (2.7/2.7)3 and (1.3/1.3)6 (4.4–6.9 1018 cm3). The thermal stability of the three films that exhibited strong annealing effects was measured as a function of annealing temperature. The remanent coercivity was measured as a function of time and fit to a Sharrock formula of the form [15] ( 2=3 ) kT lnð f0 t=lnð2ÞÞ Hcr ðtÞ ¼ H0 1 : ð1Þ KV Figs. 10 and 11 show the thermal stability factor KV=kT and the switching field H0 determined from the fits (using f0 ¼ 109 Hz) as a function of annealing temperature for the films on glass. The
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sharp increase in H0 between 3008C and 3758C for films (2.7/2.7)3 and (1.3/1.3)6 is presumably associated with the breakup of the multilayer structure, the formation of grains, and the change in reversal mechanism. For non-interacting uniaxial particles undergoing coherent rotation, one would expect H0 0:5Ha ¼ ðK1 þ 2K2 Þ=Ms . Assuming K1 þ 2K2 6:5 106 erg/cm3 [2] and Ms 1140 emu/cm3 [16] for HCP Co82Pt18 gives H0 5:7 kOe. This is larger than the maximum value of H0 ¼ 2:77 kOe measured for film (2.7/ 2.7)6 on the TEM grids annealed at 4008C. Intergrain interactions, incoherent reversal, and the presence of some FCC phase and stacking faults could account for the lower measured values of H0 . The gradual increase in H0 with increasing annealing temperature above 3758C could be due to the increase of CoPt grain size, or diffusion of C and/or Pt into grain boundaries, reducing the intergranular interactions. The thermal stability factor, KV=kT, of the films generally increases with increasing annealing temperature, most significantly at the highest annealing temperatures. The smaller variation of H0 suggests that this increase in KV=kT is mainly associated with an increase in switching volume. Approximate values of switching volume can be obtained from H0 and KV=kT by assuming K H0 Ms . These values range from 3.6 1018 to 8.4 1018 cm3 in the annealing range 3758C– 5008C and are quite similar to measured values of Vac , which also increase with annealing temperature in this range. Generally, the TEM grid samples had larger coercivities than the samples on glass. The largest coercivity (2.77 kOe) was measured for sample (2.7/2.7)3 on a TEM grids annealed at 4008C for 2 h. (Higher annealing temperatures were not attempted for the TEM grid samples.) This larger value of Hc can be ascribed to both larger values of H0 (2.95 kOe) and KV=kT (859) relative to the same sample structure deposited on glass. The fact that both the switching volume obtained from KV=kT and H0 and the activation volume are slightly smaller than the grain size suggests weak intergranular coupling. This is supported by the fact that all delta-M measurements for films (2.7/ 2.7)3, (1.3/1.3)6, and (2.7/2.7)6 are negative.
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4. Summary and conclusions We have examined the structure and magnetic properties of multilayer CoPt/C films as a function of annealing temperature and film thickness. In general, annealing leads to the breakup of the continuous films and the formation of a laterally heterogeneous morphology consisting of separate CoPt and C regions. No significant annealing effect is observed on the crystallographic structure and magnetic properties of the films with individual CoPt and C layer thickness 55.3 nm. These films exhibit perpendicular c-axis orientation and perpendicular magnetic anisotropy in both the asdeposited and annealed states, which appears to be related to the growth of the CoPt on the carbon layers. By contrast, the films with 1.3 and 2.7 nm CoPt (and C) layer thickness exhibit strong annealing effects, and the coecivities increase from a few Oe in the as-deposited state to a maximum of 2.2 kOe when annealed at 5008C (2.77 kOe on a TEM grid at 4008C). The strong increase in coercivity is associated with the breakup of the multilayer structure at about 3008C and the formation of 20 nm HCP magnetic grains with random in-plane c-axis texture. These films exhibit strong random in-plane magnetic anisotropy. The largest in-plane coercivity enhancement is obtained from the film with the structure (2.7/2.7)3 (both on glass and on the TEM grid). The increase in coercivity with annealing temperature beginning at 3758C is primarily associated with an increase in switching volume. The morphology of the annealed films on glass is not ideal for high-density recording because of the non-uniform distribution of CoPt grains in the carbon. A more uniform, classical grain structure is obtained on all of the films deposited on a TEM copper grid, with the exception of film (2.7/2.7)3, which has a morphology similar to that on glass (Fig. 1b). Somewhat surprisingly, this film has the highest coercivity in spite of the apparent lack of individual isolated grains. A similar morphology was observed in annealed multiplayer Co/CN films. A more uniform grain structure has been reported in CoC and CoPtC films in which the metal and carbon have been codeposited.
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In spite of the fact that the TEM images show interconnecting CoPt grains in the annealed thinlayer films, exchange coupling between the grains is not observed and the films exhibit classical granular magnetic behavior. Although not seen at the resolution of our TEM micrographs, small amounts of C or Pt may exist in the grain boundaries to provide the apparent grain isolation. The reduction in coercivity below that expected for non-interacting uniaxial particles undergoing coherent rotation may be related to magnetostatic coupling between grains, multidomain behavior of larger grains, and the existence of some FCC phase material. Acknowledgements This work was supported by the MRSEC program of the NSF under Award No. DMR9809423.
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Magnetic and structural properties of annealed CoPt/C multilayers J. Dua, S. Wanga, C. Rubya, A. Khapikova, W.J. Liua, J.A. Barnarda,*, J.W. Harrellb a
Center for Materials for Information Technology, Department of Metallurgical and Material Engineering, Box 870 209, 205 Bevill Research Building, Tuscaloosa, AL-35487-0209, USA b Materials for Information Technology, Departments of Physics and Astronomy, The University of Alabama, Tuscaloosa, AL 35487-0209, USA Received 17 November 2000; received in revised form 7 February 2001
Abstract The structure and magnetic properties of multilayer Co82Pt18/C films with equal CoPt and C layer thickness have been studied as a function of annealing temperature and film thickness. In general, annealing leads to the breakup of the continuous films and the formation of a laterally heterogeneous morphology consisting of separate CoPt and C regions. For individual layer thickness 55.3 nm, annealing has no significant effect on the crystallographic structure and magnetic properties of the films. In both the as-deposited and annealed states, the films have HCP structure with perpendicular c-axis orientation and exhibit perpendicular magnetic anisotropy. The structure of these films appears to be associated with the growth of the CoPt on the C layers. For films with individual layer thickness of 1.3 and 2.7 nm, the structure and magnetic properties are strongly affected by annealing. In the as-deposited state, films have a fine grain structure and coercivities of a few Oe. Annealing leads to the breakup of the multilayer structure at about 3008C and the formation of 20 nm HCP and FCC magnetic grains, with the HCP grains exhibiting in-plane c-axis texture. Annealed films have large in-plane coercivities (maximum of 2.77 kOe) and random in-plane magnetic anisotropy. The increase in coercivity with annealing temperature above 3758C is primarily associated with an increase in switching volume. # 2001 Elsevier Science B.V. All rights reserved. PACS: 61.16.Bg; 68.55.a; 75.50.Ss; 75.70.i; 81.05.Ys Keywords: Magnetic granular films; Transmission electron microscopy; Sputtering; Nanolamination; Magnetic thermal stability
1. Introduction Substantial work has been done in recent years on Co-based granular alloys for potential applica*Corresponding author. Tel.: +1-205-348-9399; fax: +1205-348-2346. E-mail address: [email protected] (J.A. Barnard).
tions as magnetic media. The ideal granular medium should consist of high anisotropy magnetic grains embedded in a robust non-magnetic matrix. The grains should be small with a uniform size distribution, yet large enough for adequate thermal stability. The grains should also be sufficiently isolated to minimize exchange interactions and yet tightly packed to enable sharp
0304-8853/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 1 7 5 - 5
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transitions. A common way to produce the granular films is to co-sputter the magnetic and non-magnetic phases. Depending on deposition conditions, subsequent annealing may be required. Alternatively, multilayers of the magnetic and non-magnetic phases can be deposited, followed by annealing. The coercivity of a granular media is determined by the magnetic anisotropy of the grains, the grain size, and intergranular interactions. The magnetic anisotropy of an individual grain is primarily determined by its crystal phase and shape. Small pure Co particles are typically a mixture of FCC and HCP phases, depending on particle size and processing conditions [1]. The anisotropy of the HCP phase (K1 þ 2K2 ) is 7 106 erg/cm3 [2], which is large enough in principle to yield HC0.5HA5 kOe. The anisotropy of the FCC phase is about 10 times smaller than the HCP phase [3]. Consequently, coercivities greater than 1 kOe are difficult to achieve in granular films with pure Co. The addition of platinum enhances the HCP fraction, and coercivities greater than 2 kOe can be achieved in Co1xPtx granular films, where x 0:2 [4,5]. Further addition of elements such as Cr, Ta, and B can enhance grain separation and lead to still larger coercivities. However, this may be at the expense of K1 þ 2K2 and Ms . Coercivities of 4400 and 5600 Oe have recently been reported in annealed multilayer granular CoCrPt/C [6] and CoCrPt/SiO2 films [7], respectively. Much larger coercivities can be obtained in high-anisotropy equi-atomic ordered CoPt alloy films [8]. In this paper, we report the results of a study of the Co82Pt18/C (hereafter CoPt/C) multilayer system. The structural and magnetic properties, including the thermal stability of the magnetization, are examined as a function of layer thickness, number of layers, and annealing temperature.
2. Sample preparation and experimental details CoPt/C multilayers were made at ambient temperature by sequential DC magnetron sputtering from a pure C target and a CoPt mosaic target, both 5 cm in diameter. Power densities of 9.9 and
1.7 W/cm2 were used to achieve deposition rates of 4.4 and 7.2 nm/min for C and the CoPt alloy, respectively. The base pressure of the chamber was 5 107 Torr. High purity Ar, at a pressure of 2 mTorr, was used as the processing gas. The substrates were carbon coated copper grids for transmission electron microscopy observation of as-deposited samples and Corning 7059 glass for all other measurements. Magnetic hysteresis loops were measured with a PMC 2900 alternating gradient magnetometer, with in-plane external field up to 10 kOe. The microstructure was examined with a Hitachi 8000 transmission electron microscope (TEM). X-ray photoelectron spectroscopy measurements (KRATOS AXIS 165 system) were used to determine the composition of the CoPt alloy (Co82Pt18 for all samples) using the relative areas of the Co2p and Pt4f spectra (or the Co3p and Pt4f spectra) and the corresponding standard atomic sensitivity factors [9]. X-ray reflectivity (XRR) and conventional high angle diffraction were measured with Philips X’pert and Rigaku diffractometers, respectively, using Cu Ka radiation. Multilayers with equal thickness C and CoPt layers were prepared which, according to our previous experience with Co/CN multilayers [10], yield the maximum coercivities. The geometry of the multilayers was ðtCoPt=tCÞn with t ¼ 8, 2.7, 1.3 nm and n ¼ 1, 3, 6, respectively, for a set of films with a total thickness of 16 nm; and t ¼ 16, 5.3, 2.7 nm and n ¼ 1, 3, 6 for a set of films with a total thickness of 32 nm. To minimize any potential differences between films grown on TEM grids and those on glass, C was the first layer sputtered in all cases. All films were capped with a thin layer of C. Annealing was carried out in a vacuum of 106 Torr at different temperatures for 2 h. XRR measurements were used to check the layered structure of sample (1.3/1.3)6 in the asdeposited state and after annealing at 3008C, see Fig. 1. The measured reflectivities decay by 6 orders of magnitude for a small increase in the ( 1. Information scattering vector q from 0 to 0.6 A on the thickness, mass density, and roughness of each layer can be deduced from XRR data, based on optical reflectivity theory. In the as-deposited
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Fig. 1. Grazing angle X-ray reflectivity from (1.3/1.3)6 films in as-deposited and after annealing at 3008C. The reflected intensity was measured as a function of incident angle y, and plotted as a function of the scattering vector q ¼ 4psiny=l, where l is X-ray wavelength.
state, the dominant oscillations have a period of ( 1, from interference between the film Dq 0:04 A surface and the bottom. These fringes are modulated by another oscillation with a longer period of ( 1, corresponding to the superlattice Dq 0:27 A unit (CoPt/C). The 1st and 2nd order superlattice peaks can clearly be seen, confirming the expected periodicity in the as-deposited state to within 10% of the nominal value. After annealing at 3008C, the layered structure is totally lost.
3. Results and discussion 3.1. Structural characterization In this section attention is generally limited to the thinner set of samples, since this set exhibits the maximum coercivities. Annealing at 4008C was found to be sufficient to produce drastic changes in magnetic properties (see below). The structure of these films is discussed here. As-deposited bright field (BF) images and the corresponding selected area diffraction (SAD) patterns for samples (8/8)1, (2.7/2.7)3, and (1.3/1.3)6 appear in Fig. 2. The BF image from (8/8)1 is consistent with a continuous crystalline CoPt layer although grain-to-grain contrast is not distinct. The broken rings in the
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SAD pattern from (8/8)1 can be indexed sequentially as (1 0 0)HCP, (0 0 2)HCP or (1 1 1)FCC, (1 0 1)HCP, (1 0 2)HCP, (1 1 0)HCP or (2 2 0)FCC, (1 0 3)HCP, (2 0 0)HCP, (1 1 2)HCP or (3 1 1)FCC reflections, according to the lattice parameters given by Aboaf (HCP phase: a0 ¼ 0:260 nm, c0 ¼ 0:422 nm; FCC phase: a0 ¼ 0:366 nm) [11]. The strong intensity of the (1 0 0)HCP and (1 1 0)HCP rings indicates a [0 0 1]HCP texture perpendicular to the film. Six strong (1 0 0)HCP spots distributed with six-fold symmetry along with the corresponding six (1 1 0)HCP spots indicate that hexagonal CoPt grains with [0 0 1]HCP orientation also exhibit in-plane texture. In fact some regions of this sample display a single crystal CoPt [0 0 1] diffraction pattern with (1 0 0)HCP and (1 1 0)HCP spots and without the (0 0 2)HCP, (1 0 1)HCP, (1 0 2)HCP, and (1 0 3)HCP rings. The [0 0 1]HCP perpendicular texture in (8/8)1 is also confirmed by the strong (0 0 2)HCP peak found in the conventional y–2y X-ray diffraction pattern (Fig. 3). The second peak in the spectrum is probably due to CoPt oxidation. Considering the fact that the CoPt film is only 8 nm thick in this sample, the (0 0 2)HCP peak is quite strong. Strong perpendicular and in-plane texture is observed in all of the multilayers with thicker component CoPt layers, i.e., (5.3/5.3)3, (8/8)1, and (16/16)1. To observe such definitive texture in layers only 5.3 nm thick deposited without substrate heating on amorphous C underlayers indicates that the preferred orientation is determined in the early stages of film growth, and may perhaps be enhanced by the C underlayer. To explore this issue a single layer CoPt reference film, 15 nm thick, was sputtered directly on glass (no C underlayer) under the same experimental conditions as the CoPt layers in the CoPt/C multilayers. This sample was prepared for TEM observation by mechanical polishing and ion-thinning from the glass side. Fig. 4 is the electron diffraction pattern from this sample. The pattern displays only slight in-plane texture, in complete contrast to that found for 5.3, 8, and 16 nm thick CoPt layers grown on C underlayers, and confirms the importance of amorphous C in generating highly oriented growth. As the 8 nm thick CoPt layer is subdivided to produce samples (2.7/2.7)3, and
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Fig. 2. Plan-view TEM bright field images and electron diffraction patterns from as-deposited CoPt/C samples; (a) (8/8)1, (b) (2.7/2.7)3, and (c) (1.3/1.3)6.
(1.3/1.3)6, atomic disorder clearly increases (Fig. 2b, c). The BF images are now quite homogeneous and the electron diffraction patterns consist of broad halos. No individual rings are resolvable. The microstructural evolution of samples (8/8)1, (2.7/2.7)3, and (1.3/1.3)6 on annealing is summarized in the BF images in Fig. 5. All three samples are now well crystallized with good grain-to-grain contrast. However, the most dramatic change is
the clear evidence that the CoPt layers have now broken up to form laterally heterogeneous two phase CoPt/C films. In this sequence of BF images the uniform light areas are C and the darker areas with substantial internal contrast are CoPt alloy. Broadly speaking, sample (8/8)1 (Fig. 5a) is a contiguous network-like CoPt film with numerous large (200–300 nm) irregularly shaped C ‘holes’. When the initial structure is further divided,
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Fig. 3. y–2y X-ray diffraction from as-deposited (8/8)1 sample.
Fig. 4. Electron diffraction pattern from 15 nm CoPt reference film on glass sputtered under the same experimental conditions as the CoPt layers in the CoPt/C multilayers.
samples (2.7/2.7)3, and (1.3/1.3)6, annealing produces a ramified structure, now well crystallized, but the scale of the features decreases consistently while the number of isolated, small CoPt grains and C holes increases. One can only conclude that during annealing atomic rearrangements over long distances have converted the component layers in the as-grown structures from a perpendicularly heterogeneous two-phase structure into a laterally heterogeneous two-phase structure. The plan view area fractions of CoPt are 0.5 in all three samples. Many of the CoPt grains in samples (2.7/ 2.7)3, and (1.3/1.3)6 are clearly striated, a contrast feature associated with stacking faults which are
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common in Co and Co-alloy films and Co-based granular films. The absence of striations in (8/8)1 is due to the fact that the CoPt grains in the single layer sample are mostly [0 0 1]HCP oriented. To further quantify the structures of the annealed nanolaminates through grain size analysis, DF images obtained using a segment of the four innermost diffraction rings were made (not shown here). The presence of stacking fault striations was particularly clear in the DF images. Grains were hand-traced and digitally scanned and the areas of each measured grain were converted to equivalent area circles. The diameters of the circles were used as the grain diameter in the statistical analysis. Samples (2.7/2.7)3 and (1.3/1.3)6 have average grain sizes of 23 and 19 nm with standard deviations of 13 and 10 nm, respectively. As for the highly textured sample (8/8)1, features in the DF image obtained using a diffraction spot actually contain multi-grains. Individual grains are difficult to resolve. However, the bright field image of this sample (Fig. 5a) does show distinct grain boundaries for some grains and a rough grain size of 30 nm is obtained. The effect of annealing on the electron diffraction patterns from samples (8/8)1, (2.7/2.7)3, and (1.3/1.3)6 is also seen in Fig. 5. Sample (8/8)1 retains the as-deposited texture. The pattern shown in Fig. 5a consists of several sets of [0 0 1]HCP zone axis patterns oriented differently in the plane. The observation of a (2 0 0)FCC ring is an unequivocal indication of the presence of the FCC phase, all other FCC rings overlap with the HCP phase. The (8/8)1 specimen has a very strong [0 0 1] hexagonal and/or [1 1 1] cubic texture and in order to try to observe the (2 0 0)FCC reflection, the specimen was tilted 358 about the horizontal. However, after tilting the (2 0 0)FCC ring was not observed. Thus, we conclude that this film, in both as-deposited and annealed states, is in fact composed of the HCP phase only. It has been demonstrated by other authors that Co rich Co–Pt alloys with more than 14 atomic % Pt can exhibit only the HCP phase [12,13]. Electron diffraction rings with uniform circumferential intensity from samples (2.7/2.7)3 and (1.3/1.3)6 indicate grains randomly oriented in the plane. For these two samples the sequence of rings is consistent with a
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Fig. 5. Plan-view TEM bright field images and electron diffraction patterns from annealed Co/CN samples; (a) (8/8)1, (b) (2.7/2.7)3, and (c) (1.3/1.3)6.
mixture of HCP and FCC phases. The strong intensity of the (0 0 2)HCP diffraction ring indicates that grains with the c-axis lying in the film plane predominate (Fig. 5b, c). In sputtered Co/Pt multilayers [14] a transition from FCC to HCP beginning at six to eight monolayers equivalent Co thickness has been reported while in high pressure sputtered Co particles [1] the same transition is noted beginning
at diameters of 20 nm. Thus it appears that there is a dimensional effect on the stability of these two phases in Co with FCC favored in thinner films and smaller particles. If this trend also holds for Co rich CoPt films then the HCP nature of sample (8/8)1 in both as-deposited and annealed states is at least qualitatively understandable. For this film, the crystal structure as well as the strong perpendicular and in-plane texture is selected
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Fig. 6. In-plane coercivities as a function of annealing temperature.
during growth. Similar results were found for samples (5.3/5.3)3 and (16/16)1. By contrast, for the samples with thinner component layers, the CoPt is essentially amorphous in the as-deposited state. During annealing both HCP and FCC CoPt crystallites nucleate with their close-packed planes preferentially aligned parallel to the film plane but without in-plane texture.
Fig. 7. Effect of annealing on the hysteresis loop of sample (2.7/ 2.7)3. (a) In-plane loop for as-deposited sample. (b) In-plane and perpendicular loops after annealing at 3758C for 2 h.
3.2. Magnetic properties Fig. 6 shows the in-plane coercivities of the films on glass as a function of annealing temperature. Magnetically, the samples fall into two categories depending on the thickness of the individual Co layers. The three samples with the thinnest Co layers (1.3 and 2.7 nm) were very soft as deposited and partially superparamagnetic. These samples exhibited strong annealing effects, and the coercivities increased to 1.5–2.2 kOe when annealed at the maximum temperature of 5008C for 2 h. The structure (2.7/2.7)3 yielded the largest coercivity, both on glass and on the TEM grid. Fig. 7 shows in-plane loops for sample (2.7/2.7)3 on glass as deposited and when annealed at 3758C for 2 h. Inplane coercivities for these thinnest samples were significantly larger than out-of-plane coercivities, consistent with the TEM measurements that show a predominance of grains with the c-axis lying in the film plane. The three samples with the thickest Co layers (5.3, 8, and 16 nm) exhibited perpendi-
cular anisotropy both as deposited and after annealing. Fig. 8 shows in-plane and perpendicular loops for sample (8/8)1 as-deposited on glass. The loops for the annealed films were virtually identical. Similarly shaped loops were observed for samples (16/16)1 and (5.3/5.3)3. However, the perpendicular loops for these two samples were more sheared resulting in much lower remanence. Although the TEM diffraction for sample (8/8/)1 showed in-plane texture (on the scale of the electron beam spot), no in-plane magnetic anisotropy was observed. The perpendicular coercivities for these films were significantly smaller than those measured in-plane. This fact along, with the shape of the perpendicular loops, suggests magnetization reversal by irreversible domain wall motion. The perpendicular magnetic anisotropy of these films is consistent with the perpendicular c-axis texture obtained from the TEM and XRD measurements.
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Fig. 8. In-plane and perpendicular loops for sample (8/8)1 as deposited. Loops after annealing are nearly the same.
Fig. 9. Activation volume as a function of annealing temperature for samples (2.7/2.7)3, (1.3/1.3)6, and (2.7/2.7)6.
Activation volumes were obtained from measurements of the maximum remanent viscosity (Sr ) and maximum irreversible susceptibility (wirr ) using the relationship Vac ¼ kTwirr =Ms Sr . These values for the films on glass are shown in Fig. 9. The large values of Vac for samples (2.7/2.7)3 and (1.3/1.3)6 annealed at 3008C are likely associated with some domain structure involving several grains. These samples have very high loop squareness and inplane anisotropy, suggesting reversal by domain wall motion. The activation volumes for the other samples are more closely related to actual grain sizes. The values of Vac for the 4008C annealed samples (3–3.5 1018 cm3) are slightly smaller than the grain volumes calculated using the film
Fig. 10. Thermal stability factor KV=kT as a function of annealing temperature for samples (2.7/2.7)3, (1.3/1.3)6, and (2.7/2.7)6.
Fig. 11. Switching field H0 as a function of annealing temperature for samples (2.7/2.7)3, (1.3/1.3)6, and (2.7/2.7)6.
thickness and measured grain diameters for samples (2.7/2.7)3 and (1.3/1.3)6 (4.4–6.9 1018 cm3). The thermal stability of the three films that exhibited strong annealing effects was measured as a function of annealing temperature. The remanent coercivity was measured as a function of time and fit to a Sharrock formula of the form [15] ( 2=3 ) kT lnð f0 t=lnð2ÞÞ Hcr ðtÞ ¼ H0 1 : ð1Þ KV Figs. 10 and 11 show the thermal stability factor KV=kT and the switching field H0 determined from the fits (using f0 ¼ 109 Hz) as a function of annealing temperature for the films on glass. The
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sharp increase in H0 between 3008C and 3758C for films (2.7/2.7)3 and (1.3/1.3)6 is presumably associated with the breakup of the multilayer structure, the formation of grains, and the change in reversal mechanism. For non-interacting uniaxial particles undergoing coherent rotation, one would expect H0 0:5Ha ¼ ðK1 þ 2K2 Þ=Ms . Assuming K1 þ 2K2 6:5 106 erg/cm3 [2] and Ms 1140 emu/cm3 [16] for HCP Co82Pt18 gives H0 5:7 kOe. This is larger than the maximum value of H0 ¼ 2:77 kOe measured for film (2.7/ 2.7)6 on the TEM grids annealed at 4008C. Intergrain interactions, incoherent reversal, and the presence of some FCC phase and stacking faults could account for the lower measured values of H0 . The gradual increase in H0 with increasing annealing temperature above 3758C could be due to the increase of CoPt grain size, or diffusion of C and/or Pt into grain boundaries, reducing the intergranular interactions. The thermal stability factor, KV=kT, of the films generally increases with increasing annealing temperature, most significantly at the highest annealing temperatures. The smaller variation of H0 suggests that this increase in KV=kT is mainly associated with an increase in switching volume. Approximate values of switching volume can be obtained from H0 and KV=kT by assuming K H0 Ms . These values range from 3.6 1018 to 8.4 1018 cm3 in the annealing range 3758C– 5008C and are quite similar to measured values of Vac , which also increase with annealing temperature in this range. Generally, the TEM grid samples had larger coercivities than the samples on glass. The largest coercivity (2.77 kOe) was measured for sample (2.7/2.7)3 on a TEM grids annealed at 4008C for 2 h. (Higher annealing temperatures were not attempted for the TEM grid samples.) This larger value of Hc can be ascribed to both larger values of H0 (2.95 kOe) and KV=kT (859) relative to the same sample structure deposited on glass. The fact that both the switching volume obtained from KV=kT and H0 and the activation volume are slightly smaller than the grain size suggests weak intergranular coupling. This is supported by the fact that all delta-M measurements for films (2.7/ 2.7)3, (1.3/1.3)6, and (2.7/2.7)6 are negative.
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4. Summary and conclusions We have examined the structure and magnetic properties of multilayer CoPt/C films as a function of annealing temperature and film thickness. In general, annealing leads to the breakup of the continuous films and the formation of a laterally heterogeneous morphology consisting of separate CoPt and C regions. No significant annealing effect is observed on the crystallographic structure and magnetic properties of the films with individual CoPt and C layer thickness 55.3 nm. These films exhibit perpendicular c-axis orientation and perpendicular magnetic anisotropy in both the asdeposited and annealed states, which appears to be related to the growth of the CoPt on the carbon layers. By contrast, the films with 1.3 and 2.7 nm CoPt (and C) layer thickness exhibit strong annealing effects, and the coecivities increase from a few Oe in the as-deposited state to a maximum of 2.2 kOe when annealed at 5008C (2.77 kOe on a TEM grid at 4008C). The strong increase in coercivity is associated with the breakup of the multilayer structure at about 3008C and the formation of 20 nm HCP magnetic grains with random in-plane c-axis texture. These films exhibit strong random in-plane magnetic anisotropy. The largest in-plane coercivity enhancement is obtained from the film with the structure (2.7/2.7)3 (both on glass and on the TEM grid). The increase in coercivity with annealing temperature beginning at 3758C is primarily associated with an increase in switching volume. The morphology of the annealed films on glass is not ideal for high-density recording because of the non-uniform distribution of CoPt grains in the carbon. A more uniform, classical grain structure is obtained on all of the films deposited on a TEM copper grid, with the exception of film (2.7/2.7)3, which has a morphology similar to that on glass (Fig. 1b). Somewhat surprisingly, this film has the highest coercivity in spite of the apparent lack of individual isolated grains. A similar morphology was observed in annealed multiplayer Co/CN films. A more uniform grain structure has been reported in CoC and CoPtC films in which the metal and carbon have been codeposited.
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In spite of the fact that the TEM images show interconnecting CoPt grains in the annealed thinlayer films, exchange coupling between the grains is not observed and the films exhibit classical granular magnetic behavior. Although not seen at the resolution of our TEM micrographs, small amounts of C or Pt may exist in the grain boundaries to provide the apparent grain isolation. The reduction in coercivity below that expected for non-interacting uniaxial particles undergoing coherent rotation may be related to magnetostatic coupling between grains, multidomain behavior of larger grains, and the existence of some FCC phase material. Acknowledgements This work was supported by the MRSEC program of the NSF under Award No. DMR9809423.
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