XRD and VSM Analysis of Nanostructured

Transcription

1 688 XRD and VSM Analysis of Nanostructured Cu-Co Alloys S. K. Gupta and M. G. Gartley Center for Materials Science and Engineering Rochester Institute of Technology Rochester, NY Abstract Results of an experimental study of Cu-Co alloys containing 0.3 to 5.0 wt% Co are presented. A series of solution annealed Cu-Co alloys were analyzed by x-ray diffraction (XRD) and vibrating sample magnetometry (VSM). Each Ka doublet was resolved by fitting a Pearson Type VII profile. Precise lattice parameter as a function of cobalt content deviated significantly from Vegard s law but was in reasonable agreement with King s analysis. VSM measurements showed that the alloys with cobalt I 2% were paramagnetic, and their initial magnetic susceptibility was a parabolic function of the cobalt content. Alloys containing cobalt in excess of 2% showed presence of a ferromagnetic phase. Solution annealed specimens containing 1.5% Co were aged at 700 C and 750 C for various times to precipitate sub-micron size Co-rich particles. In addition to XRD and VSM measurements, the precipitate particles were analyzed by magnetic force microscopy (MFM). The amount of magnetic precipitate determined from VSM measurements is in good agreement with Servi and Turnbull s mathematical description of the solvus line. VSM and MFM results indicate that initially the precipitate particles are superparamagnetic that grow into single domain ferromagnetic particles. At the higher aging temperature, some of the single domain particles grow and become multi-domain. XRD measurements show an unexpected trend that has features coinciding with the magnetic transitions. Introduction Interesting magnetic properties are expected when geometric dimensions of magnetic particles dispersed in a nonmagnetic medium become comparable to the size of magnetic domains. The miniaturization of magnetic technologies has fueled a renewed interest in such sub-micron size magnetic particles. This paper presents an experimental study of Cu-Co alloys containing 0.3 to 5.0 wt% co. The Cu-Co phase diagram shows that the maximum solid solubility of Co in Cu is 7 wt% at 1112 C, and the solubility decreases sharply with decreasing temperatures becoming negligible below 500 C (ASM Handbook, 1992). Similarly, the solid solubility of Cu in Co becomes negligible below 500 C. The Cu-rich solid solution is face-centred cubic (FCC) and nonmagnetic while the Co-rich solid solution is magnetic and is either FCC or hexagonal close-packed (HCP) depending on its heat treatment history. When a Cu-Co alloy containing 2 wt% Co was solution treated in nitrogen at 1010 C for 10 minutes followed by quenching into an iced NaOH solution, magnetic measurements revealed that the cobalt was completely in solution (Becker, 1957). Upon subsequent aging at 650 C for aging times up to 130 minutes, magnetization curves

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3 689 revealed that a Co-rich magnetic phase precipitates out of the nonmagnetic supersaturated solid solution. The saturation magnetization of the aged alloy disclosed the amount of magnetic phase present while the coercivity of the fine precipitate particles yielded information about their size. When the particle size of a magnetic phase is reduced, it has been found that its coercivity increases, goes through a maximum, and then tends towards zero (Cullity, 1972). Below a critical diameter D,, the coercivity of the particle is zero, and such particles are called superparamagnetic. Between D, and another critical diameter D, > D,, the particles are single domain with their coercivity increasing with increasing size. When particle size exceeds D,, the particles become multi-domain and their coercivity decreases. This paper details our attempts to corroborate results of magnetic measurements by vibrating sample magnetometry (VSM) and magnetic domain images by magnetic force microscopy (MFM) in Cu-Co alloys by x-ray diffraction (XRD) analysis. Specimen Preparation and Characterization CU,~~-~CO, powder mixtures (99.6% purity, <5~ size), for 0 < x (wt%) < 5, were massed and stored in plastic vials. 0.5 wt% zinc stereate was added to each mixture for lubrication during die compaction. The powders in the vial were mixed using a SPEX 8000 mixer/mill for 10 minutes. Cylindrical pellets, approximately 80% dense, were made by compacting the powders in a SPEX 13mm diameter die using a load of 5 tons. The pellets were solution treated at 1020 C for 60 minutes under flowing argon in a tungsten boat. After solution-treatment, the pellets were quenched in ice water to create a supersaturated solid solution. Mass density measurements revealed that the pellets had 4 to 5% porosity. Each quenched pellet was characterized by XRD and VSM techniques described below. For aging, pellets containing 1.5 wt% Co were selected. Aging was done at 700 C and 750 C under flowing argon for aging times ranging from 5 to 250 minutes. After aging, the sample was quenched in ice water. Mass density measurements revealed that porosity did not change appreciably (~1%) during the aging treatment. Each sample was analyzed by XRD and VSM technique as well as imaged by magnetic force microscopy (MFM) technique described below. XRD measurements were made on a Rigaku DMAXB diffractometer with Cu Ka radiation using a curved graphite crystal monochromator. A special sample holder was constructed to mount the pellet with its surface flush with the diffractometer axis to minimize sample displacement effects. For each peak in a diffraction pattern, (20, I) values were recorded in the step scan mode. The intensity values were corrected for Lorentz polarization and absorption effects. The Ka doublet in each peak was resolved by fitting a Pearson Type VII profile (Gupta, 1997), and the location of the Ka, maximum was used as the Bragg angle 0. The precise lattice parameter a, of the FCC Cu-rich solid solution was obtained by fitting a straight line to data in a plot of lattice parameter versus cos*o/sine for each peak in the diffraction pattern. In two-phase alloys, the diffraction peaks from the Co-rich solid solution were not discernible. VSM measurements were made on a Lakeshore Model 7300 VSM system. A thin slice of the pellet was massed and taped to the end of the vibrating beam. For each specimen, a complete

4 690 hysteresis loop (M-H curve) was recorded using magnetic fields in excess of 10,000 Oe. For a paramagnetic solid solution, the initial susceptibility x0 was determined by fitting a straight line to the M-H data. For alloys containing a magnetic precipitate, the hysteresis curve yielded the specific saturation magnetization o and coercivity H, of the magnetic phase. The specific saturation magnetization CJ is directly proportional to the amount of magnetic phase in the two phase mixture, i.e., the concentration c = o/crco, where oco = 175 emu/g is taken to be the specific saturation magnetization of FCC Co (Childress and Chien, 1991). MFM images of aged specimens were obtained using a Digital Instruments Dimension 3000 scanning probe microscope in LiftMode. Each sample was polished using a 0.3~ alumina slurry prior to MFM imaging. In LiftMode, a sharp probe tip coated with a magnetic phase and mounted on a small cantilever raster scans the specimen surface two times. During the first line scan, the tip is operated in tapping mode of atomic force microscopy to determine the surface topography. In the second scan, the tip is vibrated at a constant height (LiftHeight = 30 nm) above the specimen surface using the topography information collected during the first line scan (Gupta and Gartley, 1997). The change in the resonant frequency of the cantilever is related to the magnetic force gradient of stray magnetic fields produced by the magnetic phase in the specimen (Rugar et. al., 1990). Experimental Results and Discussion This section is divided into two parts: one contains the results and discussion of solution-treated alloys, and the other about aged alloys. Solution-treated Alloys Assuming that the magnetic phase, if present, is FCC Co, the saturation magnetization values from VSM measurements on the solution-treated alloys yield the amount of magnetic phase present. Figure 1 shows a plot of wt% of magnetic phase present as a function of wt% Co in the alloy. Also included in the figure is a plot of specific magnetization as a function of wt% Co in the alloy. The specific magnetization of pure Cu is slightly negative because it is diamagnetic. The figure shows that for alloys containing < 2% Co, all of the cobalt is in solution and the alloys have a single nonmagnetic phase. For alloys containing > 2% Co, the amount of magnetic (second) phase present is equal to the amount of Co in excess of 2%. Thus, solution treated Cu- Co alloys do not retain more than 2% Co in the solid solution after quenching.

5 a.g g! 1.5 P E s G 3 35 i; s w 2 t; E P 1 = ts.- ii wt% Co in alloy -1 Figure 1: wt% magnetic phase and specific magnetization versus wt% Co in alloy For alloys containing < 2% Co, VSM measurements reveal that the single phase present is paramagnetic. For such paramagnetic solid solutions, their initial magnetic susceptibility is proportional to the square their cobalt content (Tournier and Blandin, 1970). Figure 2 shows the experimental data supporting it. The parabolic lit to the data is excellent. In the 2% Co alloy, a small amount of magnetic phase may be present resulting in an initial susceptibility value that deviates significantly from the parabola as shown in figure 2. T 9 s E 3 Bl g e 2.OE E-06 l.oe-06 zf.. s 2 ~ E o.oe+ool~_(l.l 5.OE-07 ~, / wt% Co in solution /* n Figure 2: Initial susceptibility versus wt% Co in solid solution

6 692 In figure 3, the precise lattice parameter of the Cu-rich solid solution obtained from the XRD analysis is plotted as a function of the wt% Co in the alloy. Repeat measurements on the same pellet indicate that the experimental precision is If: A. Measurements done on different pellets of the same composition indicate that the pellet to pellet variation in the lattice parameter was A. To ensure that the alloys were solution treated for a sufficiently long time, two pellets (1.5% Co and 5% Co) were heat treated for additional 60 minutes at 1020 C. The lattice parameter did not change significantly (~0.005%) and is within the experimental precision , 5 P ;ij jj t: 3 t 6 n wt% Co in alloy Figure 3: Precise lattice parameter of solution phase versus wt% Co in alloy Figure 3 also shows the linear relation expressed by Vegard s law for solid solutions (Cullity, 1978). Clearly, Vegard s law underestimates the lattice parameters. King developed another linear relation similar to Vegard s law using atomic volumes (King, 1966). For dilute alloys, King s linear model fits the experimental data very well. Since the VSM results indicate that after quenching the Cu-Co alloys can not retain more than 2% Co in the solid solution, a least squares straight line was fitted to the experimental data for alloy compositions in the range 0 to 2 wt% Co only. The equation of the line is a = ~ (1) where a is the lattice parameter (in A ) of the solid solution and c is the wt% Co in the solid solution. Equation (1) can be used to predict the amount of cobalt remaining in solid solution after an alloy is aged. In alloys containing 2 2% Co, the diffraction peaks from the Co-rich solid solution (magnetic phase were not visible.

7 . 693 Aged 1.5% Co Alloy Specimens of 1.5% Co alloy were aged at 700 C and 750 C for aging times ranging from 5 to 250 minutes. Figure 4 shows the total amount of magnetic phase obtained from the specific saturation magnetization measurements as a function of aging time. The curves show that the precipitation is nearly complete in 20 minutes. In the alloy aged at 75O C, the amount of magnetic phase appears to be asymptotic at approximately 0.38% whereas at 700 C the corresponding amount is greater, approximately 0.62% (..~(~~ / I I Aging Time (mins) Figure 4: wt% of magnetic phase in 1.5% Co alloy versus aging time The solvus line at the copper-rich end of the Cu-Co phase diagram can be described by the equation log(c,) = (1000 / T), where c, is the solubility limit of Co in Cu at an absolute temperature T (Servi and Turnbull, 1966). Thus, at 700 C the solubility limit is 0.79% which implies that when a solution containing 1.5% Co is aged the cobalt-rich magnetic precipitate will not exceed 0.7 1%. Similarly, at 750 C the solubility limit is 1.10% which implies that the magnetic phase precipitating from the alloy aged at 750 C can not exceed 0.40%. The experimental results in figure 4 agree well with these calculations. The amount of cobalt remaining in solution at each aging time can also calculated using equation (1) and precise lattice parameter measurements by XRD. Since the composition of the alloy is 1.5% Co, the amount of Co precipitated from the solution in the form of a magnetic second phase can be computed. In the diffraction pattern, the diffraction peaks from the Co-rich magnetic phase were not visible. Figure 5 shows a plot of precise lattice parameter of the solution phase as a function of aging time at each aging temperature.

8 k z : $ 0. g P ;;; ~, ~~~ (~~ I -(- I Aging Time (mins) Figure 5: Precise lattice parameter of solution phase versus Aging Time As Co-rich magnetic phase precipitates out of the solid solution, the lattice parameter of the solution should show an increase as shown in figure 3. The curves in figures 5 were expected to be qualitatively similar to the curves in figure 4 at their respective aging temperatures. However, this is not the case (it is possible that greater precision is needed in XRD measurements). Both curves in figure 5 show a similar but an unusual trend. A local maximum appears at 40 minutes for aging at 7OO C, and at 20 minutes for aging at 750 C. Coincidentally, MFM images.show single domain magnetic particles in samples aged at 700 C for time greater than 40 minutes whereas no magnetic particles were observed for aging times less than 40 minutes. From these images, it may be concluded that the precipitate particles are superparamagnetic for aging times less than 40 minutes, and these particles grow to become single domain ferromagnetic particles at longer aging times. Similarly, for samples aged at 75O C, MFM images reveal single domain particles for aging times ranging from 20 to 80 minutes, and both single and multi-domain particles in samples aged at longer times. Thus, the precipitate particles in the alloy aged at 750 C are superparamagnetic for aging times less than 20 minutes. Upon further aging, they grow into single domain ferromagnetic particles. Figure 6 shows the magnetic force gradient image of a single-domain particle in the alloy aged for 41 minutes at 750 C. When aging time exceeds 80 minutes, some of the single domain particles grow and become multi-domain particles.

9 des 0.8 deg 0.0 deg NanoScope Tapping AFM Scan size nm Setpoint u Scan rate Hz Number of samples BOO nn 72win Figure 6: Magnetic force gradient image of a single domain particle in alloy aged for 41 minutes at 750C Even though the VSM measurements show that the precipitation is nearly complete after 20 minutes of aging at both temperatures, the precipitate particles continue to grow. The coercivity of fine magnetic particles is strongly related to their size. Figure 7 shows the coercivity of aged alloys as a function of aging time.

10 Y * , I ~1~ I I Aging Time (mins) Figure 7: Coercivity versus Aging Time At 700 C the coercivity continues to increase with aging time indicating that the magnetic precipitate grow but remain single domain up to 250 minutes of aging. Even though the VSM measurements indicate a small coercivity at aging times < 40 minutes, the precipitate particles are considered superparamagnetic as seen by magnetic relaxation during the acquisition of hysteresis loops. Ideally, hysteresis loops should be acquired very slowly because the magnetization tends relax over time. When superparamagnetic particles are present, their magnetization is expected to relax noticeably as was observed during the VSM measurements. At 75O C, the coercivity increases rapidly, reaches a maximum at about 60 minutes, and then begins to decrease. At the higher aging temperature, a precipitate particle will grow faster. After 60 minutes, it appears that the magnetic particles have become larger than the critical size D, and have become multi-domain. These conclusions from the VSM measurements are in excellent agreement with MFM observations. Conclusions 1. Precise lattice parameter measurements by XRD of solution-treated Cu-Co alloys are in reasonable agreement with King s theoretical analysis. VSM measurements show that the amount of Co that remains in solution after quenching does not exceed 2 wt%. 2. The amount of magnetic precipitate in 1.5 wt% Co aged alloys determined from VSM measurements at each of the two aging temperatures is in good agreement with Servi and Turnbull s mathematical description of the solvus line. 3. The amount of Co in solution in aged alloys as a function of aging time determined from XRD measurements show an unusual and unexpected trend. The trend appears to correlate with magnetic transitions observed in MFM images.

11 From MFM images and VSM coercivity measurements it is concluded that in alloys aged at 7OO C, the precipitate particles are superparamagnetic at aging times less than 40 minutes. Upon further aging, the particles grow into single domain ferromagnetic particles. In alloys aged at 750 C, the precipitate particles are superparamagnetic at aging times less than 20 minutes. Upon further aging, these particles grow into single domain ferromagnetic particles. After 80 minutes of aging, some of the single domain particles grow and become multidomain. References ASM Handbook, Volume 3: Alloy Phase Diagrams, ASM International, OH, (1992). Becker, J.J., Trans. AIME, (1957), ~ Childress, J. R. and Chien, C. L., Phy. Rev. B, v43#10(1991), ~ Cullity, B. D., Introduction to Magnetic Materials, Addison-Wesley, (1972). Cullity, B. D., Elements of X-Ray Diffraction, 2 d Edition, Addison-Wesley, (1978). Gupta, S. IS., to appear in J. Appl. Cryst. Gupta, S. K. and Gartley, M. G., Proceedings of ASEE Zone I Meeting, West Point, NY, Spring 1997, plc3-l:lc3-4. King, H. W., J. Mat. SC., vl( 1966), ~ Rugar, D., Mamin, H. J., Guethner, P., Lambert, S. E., Stern, J. E., McFadyen, I., and Yogi, T., J. Appl. Phy., v68#3(1990), ~ Servi, I. S. and Turnbull, D., Acta Met., v14(1966), ~ Tournier, R. and Blandin, A., Phy. Rev. Lett., v24#8(1970), ~