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Impurity atoms in small metallic particles
0038-1098/160327-03 $03.00[0 Pergamon Press Ltd.
Solid State Communications, Vol. 42, No. 4, pp. 327-329, 1982. Printed in Great Britain.
IMPURITY ATOMS IN SMALL METALLIC PARTICLES P.E. Chizhov, V.I. Petinov and A.V. Grigorevski Institute of Chemical Physics, Academy of Sciences, 142432, Chernogolovka, U.S.S.R. (Received 2 4 N o v e m b e r 1981 by G.S. Zhdakov)
The effect of self-purification has been discovered in small metallic particles containing impurity atoms. Due to small dimension of the particles impurities were shown to get out of the particles and to reach its surface in relatively short times. The phenomenon has been studied for small particles of Li containing Na atoms as impurities and for Ni-Cu alloy particles. The distribution of atoms in lithium particles was studied by the ESR method and the information about the distribution of Ni atoms within Cu-Ni alloy particles was obtained by comparing the structure date with magnetic properties. THE SURFACE of any solid can be in principle considered as an efficient trap for various structural defects and impurities. If there are any defects in the lattice they should be "repaired" sooner in the vicinity of the surface. Due to the fact that all structure defects in the lattice have diffusional mobility, sometimes they may appear at the surface. The back way for defects might be hindered since defects may annihilate at the surface and impurity atoms may undergo chemical reactions and/or form their own crystal lattice. For defects and impurities at the distance from the surface the characteristic "lifetime" is given by the well known formula r "" 12[D, where D is the diffusion coefficient. Usually diffusion coefficient are very small in solid state. Due to considerable vibrational mobility in the surface layer, the D value near the surface may exceed that of a bulk material by many orders. That is why lifetimes r for defects and impurities near the surface may occur to be rather small. It is natural that the phenomenon could be conveniently studied in the samples with the large surface• volume ratio. Therefore small metallic particles provide a desirable situation. In this work the small lithium particles with Na atoms as impurities and the small particles of Cu-Ni alloy have been studied. The distribution of Na atoms was studied by ESR method. The necessary information for Cu-Ni alloys was gained from the magnetic and structure data. In small particles the relaxation of conductivity electron spins may occur either due to interaction of electrons with lattice ions and impurity atoms, or due to the collisions with the surface. These processes may be considered as independent, so their contribution to the ESR linewidth must be additive. Therefore, the relaxation time determined from the linewidth, may be 327
calculated according to [ 1]: r -l = r/"1 + r? 1 + r~ 1
(1)
1 rl -
o'n'VF
2
(2)
d
r. = 3 e'VF
(3)
where rl, ri, r, are the relaxation times due to electron interactions with the lattice, impurity atoms and the surface respectively; VF is the Fermi velocity; n is the impurity concentration; o is the efficient electron crosssection on the impurity with the change of spin; d is the particle diameter and e is the spin change probability upon the interaction with the surface. Using equation (1)--(3) and varying the particle dimension we may separate the surface and volume particle contributions into the spin relaxation process. In this case the electron spins may be regarded as a specific probe providing information on the state of the particle surface and impurities within the bulk of the small metallic particles. The validity of equations (1) and (3) has been confirmed previously [2]. However, the validity of equation (3) for small particles with impurities was questioned later in [3, 4]. It was proposed [3] that the equation (4) a
d2
r, = 3 v~.t
(4)
should be used instead of equation (3) when the particle size is less than the free path l of conduction electrons. Hence it follows that for small particles the presence of impurities may effect not only the bulk spin relaxation time but also the dependence of rs upon d as well. Small particles of I_i with the size less than 500 nm
328
IMPURITY ATOMS IN SMALL METALLIC PARTICLES
were prepared by the vapour condensation technique described in details elsewhere [2]. Li was evaporated from the crucible in an inert gas flow through a quartz tube. Upon cooling, Li vapour condensed forming small spherical particles. Their dimension was in the region 50-500 nm by using gases with various molecular weights (argon, helium) and by evaporating rate. To be sure that the particles composition corresponded to the initial impurity concentration in Li, the selection of particles for preparation of the samples was made after the establishment of the dynamic equilibrium for the evaporation process. Such conditions were maintained by the constant income rate of lithium wire with the appropriate impurity concentration into crucible. While Na vapour pressure at the evaporation temperature is greater by order than that of Li, the constant rate of Li wire input guaranteed the equal ratio Na/Li in lithium wire and particles thus obtained. The particles were condensed together with paraffin which prevented their coalescence and oxidation.
. ..
30
/ ",-4
/
,o
Fig. 1. Reciprocal relaxation time vs reciprocal diameter for Li particles containing the different concentrations of the impurity Na atoms. (1) Less than 0.03%; (2)
0.5%;(3) 2%; (4) 4%.
Experimentally determined dependences of r on d for particles containing varied quantities of Na are presented in Fig. 1. All of them are linear plots with different slope, except dependences 3 and 4 having the same slopes. It is interesting that when d rushes to infinity, all four plots intercept in one point. Experimental dependences o f t - t o n d -1 are in full accordance with equation (3), but the plot r -1 vs d -~ shows that equation (4) does not hold true [5]. Hereby, the surface relaxation is affected by the presence of Na atoms, but it is manifested only in decreasing the spin change probability upon their encounters with the surface and has no influence on the r6 dependence upon d [5]. From these data the important conclusion may be drawn: atoms in small Li particles do not affect
Vol. 42, No. 4
relaxation processes within the particle volume, ~ut do have effect on the spin change probability at electron encounters with the surface. It is the consequence of the fact that Na atoms get out to the surface of the particles. It is worth noting one more interesting feature. The e value proved to remain unchanged practically upon increase Na contents from 2 to 4% and, moreover, to be almost equal to that found in the study of spin relax. ation in Na particles [6]. Probably, this is mere coincidence, but it may be assumed also that it is Na atoms at the particle surface which are responsible for the mechanism of electron scattering accompanied by spin change. Na has the poor solubility in Li, hence the phenomenon observed might have been accounted for as a specificity in the behaviour of the small particles of Na-Li alloy. Therefore the effect had to be checked on the other systems in particle on those whose components are mutually well dissolved. From this point of view Cu-Ni alloy seemed to be most convenient and promising. In contrast to Na-Li system it consists of the components having essentially the same atomic radia, it has no structure peculiarities, its lattice parameters are smoothly changed together with Cu/Ni ratio [7]. This alloy has advantageously high "sensitivity" of its magnetic properties to alloy composition [8]. We assumed that if the self.purification phenomenon took place for this alloy, this would have resulted in the deviation of the local concentrations of components in the surface layers from the normal, and accordingly, in the change of magnetic properties that could be registered easily. The small particles of Cu-Ni alloy were prepared by essentially the same method as described above. The alloy composition which was used for our study corresponded to Cu~Nis2. X-ray analysis confirmed the identity of the small particles composition and that of initial material. Macroscopic samples of the alloy are paramagnetic at room temperatures, they display magnetic ordering only upon cooling down to 70 K [8]. Small particles of this alloy behave quite the other way. This is illustrated by Fig. 2 which represents magnetization curves at 273 K and 4.2 K for 56 nm particles. The hysteresis seen in the curve for the particles of 56 nm size disappears at T > 120 K. As is seen, the magnetism of the particles consists of two components, paramagnetic and ferromagnetic, which are readily separated. The ferromagnetic fraction, nf, is given by the interest of the ordinate axis with approximated linear magnetization plot within the range of large magnetic fields. Experimental data on the dependence of n r upon T for Cu-Ni small particles are given in Fig. 3. At high temperatures n r for particles of 56 nm size is not approaching zero. These data imply that there exist ferromagnetic areas with Curie point
Vol. 42, No. 4
IMPURITY ATOMS IN SMALL METALLIC PARTICLES
I
I
l .,~nm 4,e~
5 ,j
Fig. 2. Magnetization per alloy atom vs magnetic field for Cu,mNis2 alloy particles, d = 56 nm, T = 4.2 K and 273 K; d = 30 nm, T = 273 K.
5
le
9~
329
noting that the temperature dependences of paramagnetic susceptibility of quenched and normally obtained particles are practically coincident (see inset on Fig. 3). For quenched particles just as for macroscopic alloy samples the non-linearity in the dependence n (H) (Fig. 2) appears only at T < 170K. Therefore, magnetic properties of quenched particles are nearly the same as for macroscopic sample, while particles formed from vapours under low temperature gradients display remarkable differences due to impurity ferromagnetism with high Curie temperature (To > 300 K). The fraction of this impurity ferromagnetism is rather large (0.4% of atoms Ni take part in ferromagnetism). We relate it to the outlet of Ni atoms to the surface. It happened due to relatively prolonged stay of the particles in high temperature zone, where they formed. At the surface the escaped Ni atoms have probably formed magnetic clusters. We failed to propose any reasonable alternative explanation. Hence the obtained data leads to the conclusion that the particle surface play an important role in the self-purification of the under-surface layers from impurities and structural defects. REFERENCES
R
~k.
w
--.,a~ Z'.A" Ia ,z-K
Fig. 3. Ferromagnetic part of the magnetization per alloy atom (nf = n(H) --X(I-I)I-I, H = 1.5 mAm -1) vs temperature for Cu~Nsa alloy particles with diameters 56 and 30 rim. Insertion: susceptibility per alloy atom vs temperature for the same samples, which were warmed in d.c. field 5.73 kAm -1 . above 300 which are more rich in Ni than average particle and have smaller dimensions (supermagnetism at T > 120K). The question arises: what is the nature of these areas? There exist separate components in vapour phase and particles formation occurs upon vapour condensation, hence, the particles thus formed may have different compositions. To cheek this possibility, we prepared the particles of the same composition under conditions of quenching, i.e. in the presence of strong temperature gradient due to cooling of gas flow. This promoted sharp depressing of diffusion processes in particles, which are difficult already at T < 400 ° C [9]. The quenching influenced the particle magnetism strongly and even changed their dimension from 56 to 30rim, but did not affect lattice parameters and eompostion. For quenched particles (d = 30 nm) the magnetization is a linear function of the field (see Fig. 2), that corresponds to their paramagnetie state. It is worth
F. Dyson, Phys. Rev. 98,349 (1955). V.I. Petinov & M.Ya. Gen, Zh. Eksperim. i Teor. F/z. 48, 29 (1965). 3. V.A. Zhikharev & A.P. Kessel, Zh. Eksperim. i Teor. Fiz. 64,356 (1973). 4. S.P. Shumacher & P. Wang,Phys. Rev. B8,4119 (1973). 5. V.I. Petinov, Coll. Abstr. Conf. Amorph. Met. Mater., p. 114. Smolenice (1978). 6. V.I. Petinov & Yu.A. Ardashev, Fiz. 79erd. Tela 11, 3 (1969). 7. M. Hansen & K. Anderko, Constitution o f Binary Alloys, p. 646. McGraw-Hill New York, Toronto and London (1958). 8. J.S. Ododo & B.R. Coles, Z Phys. F7, 2393 (1977). 9. M.V. Belous, L.O. Zvorykin, N.V. Serdoukhova & I.Ya. Khemdors, Metallofizika 71,73 (1978). 10. H.C. Van Elst, B. Cubach & G.I. Van der Bery, Phy sica 28, 1287 (1962). 1. 2.
Solid State Communications, Vol. 42, No. 4, pp. 327-329, 1982. Printed in Great Britain.
IMPURITY ATOMS IN SMALL METALLIC PARTICLES P.E. Chizhov, V.I. Petinov and A.V. Grigorevski Institute of Chemical Physics, Academy of Sciences, 142432, Chernogolovka, U.S.S.R. (Received 2 4 N o v e m b e r 1981 by G.S. Zhdakov)
The effect of self-purification has been discovered in small metallic particles containing impurity atoms. Due to small dimension of the particles impurities were shown to get out of the particles and to reach its surface in relatively short times. The phenomenon has been studied for small particles of Li containing Na atoms as impurities and for Ni-Cu alloy particles. The distribution of atoms in lithium particles was studied by the ESR method and the information about the distribution of Ni atoms within Cu-Ni alloy particles was obtained by comparing the structure date with magnetic properties. THE SURFACE of any solid can be in principle considered as an efficient trap for various structural defects and impurities. If there are any defects in the lattice they should be "repaired" sooner in the vicinity of the surface. Due to the fact that all structure defects in the lattice have diffusional mobility, sometimes they may appear at the surface. The back way for defects might be hindered since defects may annihilate at the surface and impurity atoms may undergo chemical reactions and/or form their own crystal lattice. For defects and impurities at the distance from the surface the characteristic "lifetime" is given by the well known formula r "" 12[D, where D is the diffusion coefficient. Usually diffusion coefficient are very small in solid state. Due to considerable vibrational mobility in the surface layer, the D value near the surface may exceed that of a bulk material by many orders. That is why lifetimes r for defects and impurities near the surface may occur to be rather small. It is natural that the phenomenon could be conveniently studied in the samples with the large surface• volume ratio. Therefore small metallic particles provide a desirable situation. In this work the small lithium particles with Na atoms as impurities and the small particles of Cu-Ni alloy have been studied. The distribution of Na atoms was studied by ESR method. The necessary information for Cu-Ni alloys was gained from the magnetic and structure data. In small particles the relaxation of conductivity electron spins may occur either due to interaction of electrons with lattice ions and impurity atoms, or due to the collisions with the surface. These processes may be considered as independent, so their contribution to the ESR linewidth must be additive. Therefore, the relaxation time determined from the linewidth, may be 327
calculated according to [ 1]: r -l = r/"1 + r? 1 + r~ 1
(1)
1 rl -
o'n'VF
2
(2)
d
r. = 3 e'VF
(3)
where rl, ri, r, are the relaxation times due to electron interactions with the lattice, impurity atoms and the surface respectively; VF is the Fermi velocity; n is the impurity concentration; o is the efficient electron crosssection on the impurity with the change of spin; d is the particle diameter and e is the spin change probability upon the interaction with the surface. Using equation (1)--(3) and varying the particle dimension we may separate the surface and volume particle contributions into the spin relaxation process. In this case the electron spins may be regarded as a specific probe providing information on the state of the particle surface and impurities within the bulk of the small metallic particles. The validity of equations (1) and (3) has been confirmed previously [2]. However, the validity of equation (3) for small particles with impurities was questioned later in [3, 4]. It was proposed [3] that the equation (4) a
d2
r, = 3 v~.t
(4)
should be used instead of equation (3) when the particle size is less than the free path l of conduction electrons. Hence it follows that for small particles the presence of impurities may effect not only the bulk spin relaxation time but also the dependence of rs upon d as well. Small particles of I_i with the size less than 500 nm
328
IMPURITY ATOMS IN SMALL METALLIC PARTICLES
were prepared by the vapour condensation technique described in details elsewhere [2]. Li was evaporated from the crucible in an inert gas flow through a quartz tube. Upon cooling, Li vapour condensed forming small spherical particles. Their dimension was in the region 50-500 nm by using gases with various molecular weights (argon, helium) and by evaporating rate. To be sure that the particles composition corresponded to the initial impurity concentration in Li, the selection of particles for preparation of the samples was made after the establishment of the dynamic equilibrium for the evaporation process. Such conditions were maintained by the constant income rate of lithium wire with the appropriate impurity concentration into crucible. While Na vapour pressure at the evaporation temperature is greater by order than that of Li, the constant rate of Li wire input guaranteed the equal ratio Na/Li in lithium wire and particles thus obtained. The particles were condensed together with paraffin which prevented their coalescence and oxidation.
. ..
30
/ ",-4
/
,o
Fig. 1. Reciprocal relaxation time vs reciprocal diameter for Li particles containing the different concentrations of the impurity Na atoms. (1) Less than 0.03%; (2)
0.5%;(3) 2%; (4) 4%.
Experimentally determined dependences of r on d for particles containing varied quantities of Na are presented in Fig. 1. All of them are linear plots with different slope, except dependences 3 and 4 having the same slopes. It is interesting that when d rushes to infinity, all four plots intercept in one point. Experimental dependences o f t - t o n d -1 are in full accordance with equation (3), but the plot r -1 vs d -~ shows that equation (4) does not hold true [5]. Hereby, the surface relaxation is affected by the presence of Na atoms, but it is manifested only in decreasing the spin change probability upon their encounters with the surface and has no influence on the r6 dependence upon d [5]. From these data the important conclusion may be drawn: atoms in small Li particles do not affect
Vol. 42, No. 4
relaxation processes within the particle volume, ~ut do have effect on the spin change probability at electron encounters with the surface. It is the consequence of the fact that Na atoms get out to the surface of the particles. It is worth noting one more interesting feature. The e value proved to remain unchanged practically upon increase Na contents from 2 to 4% and, moreover, to be almost equal to that found in the study of spin relax. ation in Na particles [6]. Probably, this is mere coincidence, but it may be assumed also that it is Na atoms at the particle surface which are responsible for the mechanism of electron scattering accompanied by spin change. Na has the poor solubility in Li, hence the phenomenon observed might have been accounted for as a specificity in the behaviour of the small particles of Na-Li alloy. Therefore the effect had to be checked on the other systems in particle on those whose components are mutually well dissolved. From this point of view Cu-Ni alloy seemed to be most convenient and promising. In contrast to Na-Li system it consists of the components having essentially the same atomic radia, it has no structure peculiarities, its lattice parameters are smoothly changed together with Cu/Ni ratio [7]. This alloy has advantageously high "sensitivity" of its magnetic properties to alloy composition [8]. We assumed that if the self.purification phenomenon took place for this alloy, this would have resulted in the deviation of the local concentrations of components in the surface layers from the normal, and accordingly, in the change of magnetic properties that could be registered easily. The small particles of Cu-Ni alloy were prepared by essentially the same method as described above. The alloy composition which was used for our study corresponded to Cu~Nis2. X-ray analysis confirmed the identity of the small particles composition and that of initial material. Macroscopic samples of the alloy are paramagnetic at room temperatures, they display magnetic ordering only upon cooling down to 70 K [8]. Small particles of this alloy behave quite the other way. This is illustrated by Fig. 2 which represents magnetization curves at 273 K and 4.2 K for 56 nm particles. The hysteresis seen in the curve for the particles of 56 nm size disappears at T > 120 K. As is seen, the magnetism of the particles consists of two components, paramagnetic and ferromagnetic, which are readily separated. The ferromagnetic fraction, nf, is given by the interest of the ordinate axis with approximated linear magnetization plot within the range of large magnetic fields. Experimental data on the dependence of n r upon T for Cu-Ni small particles are given in Fig. 3. At high temperatures n r for particles of 56 nm size is not approaching zero. These data imply that there exist ferromagnetic areas with Curie point
Vol. 42, No. 4
IMPURITY ATOMS IN SMALL METALLIC PARTICLES
I
I
l .,~nm 4,e~
5 ,j
Fig. 2. Magnetization per alloy atom vs magnetic field for Cu,mNis2 alloy particles, d = 56 nm, T = 4.2 K and 273 K; d = 30 nm, T = 273 K.
5
le
9~
329
noting that the temperature dependences of paramagnetic susceptibility of quenched and normally obtained particles are practically coincident (see inset on Fig. 3). For quenched particles just as for macroscopic alloy samples the non-linearity in the dependence n (H) (Fig. 2) appears only at T < 170K. Therefore, magnetic properties of quenched particles are nearly the same as for macroscopic sample, while particles formed from vapours under low temperature gradients display remarkable differences due to impurity ferromagnetism with high Curie temperature (To > 300 K). The fraction of this impurity ferromagnetism is rather large (0.4% of atoms Ni take part in ferromagnetism). We relate it to the outlet of Ni atoms to the surface. It happened due to relatively prolonged stay of the particles in high temperature zone, where they formed. At the surface the escaped Ni atoms have probably formed magnetic clusters. We failed to propose any reasonable alternative explanation. Hence the obtained data leads to the conclusion that the particle surface play an important role in the self-purification of the under-surface layers from impurities and structural defects. REFERENCES
R
~k.
w
--.,a~ Z'.A" Ia ,z-K
Fig. 3. Ferromagnetic part of the magnetization per alloy atom (nf = n(H) --X(I-I)I-I, H = 1.5 mAm -1) vs temperature for Cu~Nsa alloy particles with diameters 56 and 30 rim. Insertion: susceptibility per alloy atom vs temperature for the same samples, which were warmed in d.c. field 5.73 kAm -1 . above 300 which are more rich in Ni than average particle and have smaller dimensions (supermagnetism at T > 120K). The question arises: what is the nature of these areas? There exist separate components in vapour phase and particles formation occurs upon vapour condensation, hence, the particles thus formed may have different compositions. To cheek this possibility, we prepared the particles of the same composition under conditions of quenching, i.e. in the presence of strong temperature gradient due to cooling of gas flow. This promoted sharp depressing of diffusion processes in particles, which are difficult already at T < 400 ° C [9]. The quenching influenced the particle magnetism strongly and even changed their dimension from 56 to 30rim, but did not affect lattice parameters and eompostion. For quenched particles (d = 30 nm) the magnetization is a linear function of the field (see Fig. 2), that corresponds to their paramagnetie state. It is worth
F. Dyson, Phys. Rev. 98,349 (1955). V.I. Petinov & M.Ya. Gen, Zh. Eksperim. i Teor. F/z. 48, 29 (1965). 3. V.A. Zhikharev & A.P. Kessel, Zh. Eksperim. i Teor. Fiz. 64,356 (1973). 4. S.P. Shumacher & P. Wang,Phys. Rev. B8,4119 (1973). 5. V.I. Petinov, Coll. Abstr. Conf. Amorph. Met. Mater., p. 114. Smolenice (1978). 6. V.I. Petinov & Yu.A. Ardashev, Fiz. 79erd. Tela 11, 3 (1969). 7. M. Hansen & K. Anderko, Constitution o f Binary Alloys, p. 646. McGraw-Hill New York, Toronto and London (1958). 8. J.S. Ododo & B.R. Coles, Z Phys. F7, 2393 (1977). 9. M.V. Belous, L.O. Zvorykin, N.V. Serdoukhova & I.Ya. Khemdors, Metallofizika 71,73 (1978). 10. H.C. Van Elst, B. Cubach & G.I. Van der Bery, Phy sica 28, 1287 (1962). 1. 2.