Hyperfine Interactions 92(1994)949-953 949

Mfissbauer and PAC studies of nanocrystalline Fe

Praveen Sinha and Gary S. Collins

Department of Physics, Washington State University, Pullman, WA 99164-2814, USA

High-purity Fe powder was mechanically milled under argon at ambient temperature using an SPEX 8000 mill. The local atomic and magnetic structure was studied using 57Co/Fe M0ssbauer and l ltln/Cd perturbed angular correlations (PAC) spectroscopies. After 32 hours of milling, X-ray diffraction revealed effective grain diameters of 18 nm and energy-dispersive X-ray analysis indicated a Cr impurity concentration of ~5%, presumably introduced by mechanical attrition of steel ball bearings used for milling. In addition to a spectral component very similar to bulk iron metal, the M0ssbauer spectra exhibited hyperfine field shifts attributed to the Cr impurities. PAC spectra on Fe milled for 5 h, with no contamination, exhibited two components: (1) A slightly broadened magnetic interaction attributed to interior, defect-free sites of ln/Cd probes with a mean hyperfine field slightly greater than in macroscopic grains. The defect-free site fraction grew appreciably during milling, even though In is essentially insoluble in Fe. (2) An indistinct signal due to mixed magnetic and quadrupole interactions attributed to probes at surface or other defect sites.

1. Introduction

Nanocrysta l l ine powders of metals and alloys can be readily produced in large quantity by mechanical attrition of bulk materials in high-energy ball mills [1]. The present work was undertaken to characterize the local a tomic and magnet ic structure of nanocrystall ine Fe (n-Fe) using two hyperfine spectroscopies: Mtissbauer effect (ME) and perturbed angular correlations of g a m m a rays (PAC). Previous 57Fe ME absorber measurements made by Herr et al. [2] on n-Fe formed by condensat ion of Fe vapor and compact ion under high pressure exhibited an interfacial signal with a reduced field H = 31.8 T at room temperature.

2. Measurements

Iron powder with a starting purity of 99.999% was used for all measurements . Prior to mill ing the PAC samples, l l l ln activity in chloride solution was dried on the surface of the starting material and heated under hydrogen at 800 ~ for one hour. This t reatment was undertaken to chemical ly reduce In chloride activity to metal form, and led to a defect-free substitutional site fraction of 27% prior to

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950 P. Sinha, G.S. Collins / Mi~ssbauer and PAC studies of nanocrystalline Fe

milling. For ME measurements, the powders were milled in the as-received state. Samples were subject to mechanical attrition in a hardened steel vial using an SPEX 8000 mill using two 1.3 cm diameter steel ball bearings. The ratio of the mass of the balls to the sample was greater than or equal to 25 : 1. The vial was always filled with high-purity argon before milling to reduce gas-solid reactions.

X-ray powder diffractograms of the samples were measured using a Siemens D-500 diffractometer. Powder diffraction peaks were computer fitted with pseudo- Voigt profiles. Lattice spacings were calibrated using diffraction peaks from Si powder [3] mixed with the milled materials. Effective grain sizes d were obtained from the inverse of the q = 0 intercept of a linear plot of Aq versus q for the (110) and (220) peaks. Values of d obtained were about a factor of two larger than using the less accurate Scherrer formula. Transmission electron micrographs of several powders exhibited roughly spherical particles and confirmed that the X-ray grain sizes agreed with observed particle diameters. Energy-dispersive X-ray (EDX) analysis was undertaken to check for possible environmental contamination.

M/Sssbauer absorbers were prepared by mixing about 10 mg/cm 2 of milled Fe powder with an adhesive [4] and drying. Measurements were made using a Ranger MS-1200 spectrometer running in constant-acceleration mode and a source of 57Co in Rh. Velocity calibration was made using absorbers of unmilled Fe. PAC measurements were made using a standard four-counter spectrometer [5]. Measurements were made at about 292 K.

3. Results

Hyperfine interaction parameters and descriptions of observed signals are summarized in table 1. After milling for 13 hours, X-ray measurements indicated a grain diameter of 28(6) nm, decreasing to 18(3) nm after 32 h. For the latter size,

Table 1

Experimental parameters.

Milling time d f H(57Fe) t5 (.OL(I 11 Cd) Description (h) (nm) (%) (T) (mm/s) (Mrad/s)

0 - 1 0 4 100 (33.0) (0.0) interior defect-free site 30 560.0(2) interior defect-free site 70 -0 quadrupole int. sites

5 -40 60 561.8(6) interior defect-free site 40 >>0 mixed-interaction sites

32 18(3) 80 33.25(1) 0.0 interior defect-free site I0 29.2(3) 0.0 Cr impurity field shifts 10 -0 + 0.6 surface oxide (FeO)

P. Sinha, G.S. Collins/MOssbauer and PAC studies of nanocrystalline Fe 951

about 5% of the atoms are in the outermost atomic layer. EDX analysis revealed contamination much less than 1% for the sample milled for 5 h, but found - 5 % Cr in the Fe after 13 or 32 h of milling. In fig. 1, the Mrssbauer spectrum after milling for 32 h (top), which was fitted well with the three spectral components listed in table 1, is shown. These include a component with hyperfine field H and isomer

1.00

t -

O

'~ .96 E t -

*~ 1.001 I

.9975

-10 -5 0 5 10

velocity (mm/s)

Fig. 1. 57Fe MOssbauer spectra of nanocrystalline Fe: (a) Spectrum obtained after mechanical attrition for 32 h; (b) spectral component attributed to Fe atoms with Cr impurities in the first or second neighbor shells, obtained by decomposition of spectrum (a).

shift t~ very similar to those for macroscopic Fe samples, a component with a reduced field of 29.2 T attributed to hyperfine field shifts due to Cr impurities in the first or second shells of the Fe probe atoms [6], and a low-field component attributed to FeO. The component attributed to nearby Cr impurities is displayed separately at the bottom of fig. 1.

In fig. 2, representative PAC spectra are shown. After diffusion of I l l ln into the Fe but before milling (top), the spectrum exhibits a pure magnetic signal com- ponent with Larmor frequency O9L = 560 Mrad/s (and a harmonic at 1120 Mrad/s) and site fraction f = 27% characteristic of In/Cd probes on defect-free substitutional sites in macroscopic Fe crystals. The remaining - 7 0 % of the sites experiences a pure but broadly-distributed quadrupole interaction with mean coupling frequency o90 - 100 Mrad/s. This latter signal is evident as a rapidly decaying low-frequency signal with an undershoot near 20 ns. After milling for 5 hours (bottom), the site fraction of the pure magnetic signal from probes on interior defect-free sites has increased to 62%. The remaining - 40% site fraction exhibits a very rapidly decaying anisotropy attributed to a mixed magnetic and quadrupole interaction which, for a spin I = 5/2 PAC level, leads to fractionation of the signal into nine incoherent frequency components.

952 P. Sinha, G.S. Collins / MOssbauer and PAC studies of nanocrystalline Fe

1.0

Gz(t)

4. Discussion

0

not milled a

milled 5 hours b

0 50 1 O0 150 200

t (ns) Fig. 2. PAC spectra of ttlln/Cd probes in Fe measured

before and after mechanical milling.

4.1. MECHANICAL ALLOYING OF PAC PROBE ACTIVITY

The defect-free substitutional site fraction increased from 27% prior to milling to 70% after 2 hours of milling time, after which it remained approximately constant. Although the mole fraction of In in the samples was only about 10 -8, In is highly insoluble in Fe. Thus, the increase in the defect-free substitutional site fraction observed after milling near ambient temperature indicates an In solubility far above the equilibrium limit. Indeed, mechanical milling may prove to be a useful new method for dissolving insoluble radioactive probes [7]. A second feature of note is the replacement in the course of milling of a non-magnetic distributed quadrupole interaction characteristic of dried In activity by a mixed magnetic/quadrupole interaction. This suggests that most or all of the In probes ultimately become incorporated in the Fe lattice, but with many remaining at defect-associated sites.

4.2. HYPERFINE INTERACTIONS AT DEFECT-FREE INTERIOR SITES IN n-Fe

The mean hyperfine fields observed at l l ICd probes are observed from table 1 to increase in magnitude by 0.3% for crystals of diameter of about 40 nm. This increase can not be attributed to contamination and is of unknown origin.

4.3. HYPERFINE INTERACTIONS AT SURFACE SITES IN n-Fe

While about 5% of the atoms in 18 nm grains should be in the outermost surface layer, it was not possible from the M6ssbauer measurements to clearly resolve any surface spectral component owing to the Cr contamination and oxidation.

P. Sinha, G.S. Collins / MiJssbauer and PAC studies of nanocrystalline Fe 953

Acknowledgements

Bruce H. Meeves assisted with some of the M6ssbauer measurements. Chris Davitt of the Electron Microscopy Center at Washington State University assisted with the TEM and EDX measurements. We thank Brent Fultz for advice on particle- size measurements using X-rays. This work was supported in part by the National Science Foundation under Grant No. DMR 90-14163 (Metallurgy Program).

References

[1] H.J. Fecht, E. Hellstern, Z. Fu and W.L. Johnson, Adv. Powder Metallurgy 1-3(1989)111. [2] U. Herr, J. Jing, R. Birringer, U. Gonser and H. Gleiter, Appl. Phys. Lett. 50(1987)472. [3] Standard reference material 640b, National Bureau of Standards. [4] 7031 vinyl phenolic adhesive, Insulating Materials Incorporated. [5] G.S. Collins, S.L. Shropshire and J. Fan, Hyp. Int. 62(1990)1. [6] I. Vincze and I,A. Campbell, J. Phys. F3(1973)647. [7] In other experiments, defect-free Illln site fractions of about 40% were obtained without diffusion

simply by drying the In activity in chloride solution on the Fe powder and milling.