- Email: [email protected]
Curie temperature and substructure of Ni nanocrystals
Volume 154, number 1,2
PHYSICS LETTERS A
25 March 1991
Curie temperature and substructure of Ni nanocrystals M.W. Belous, Yu.N. Rudoj Department of General Physics and Solid State Physics, Polytechnicalinstitute ofKiev, KPI-1013 Kiev 252056, USSR
L.I. Trusov and V.!. Novikov ULTRAM InternationalResearch Center, 115230 Moscow, USSR Received 2 January 1991; accepted for publication 25 January 1991 Communicated by J. Flouquet
The substructure of Ni nanocrystals compacted from ultrafine particles at different pressures and their Curie point is studied, using the FMR method. A dependence of T~on the compacting pressure is found. This fact could beexplained as the result ofthe formation in the compacts ofa submicroporous structure which appeared during the evolution of the real structure ofthe material which was created by the motion ofthe grain boundaries duringrecrystallization.
A number, of size effects was observed in nanocrystals as a result of the comparable roles of surface and volume energy of ultrafine grains [1]. For example, such size effects were found experimentally for an ensemble of nonmteracting magnetic particles near the Curie point (Tc) [2]: a monotone decrease of T~as a result of the decrease of particle size was observed. It was found in compact ensembles of small (less than 106 m) Ni particles created by compacting under high pressure that as the grain size decreased T~and the saturation of magnetization also decreased (in comparison with bulk Ni) [31.It was however noticed that this dependence could hardly be explained just by size magnetic effects and it is necessary to take into account the structural size effects. Therefore it is of interest to study the Curie point of nanocrystals in dependence of the compacting pressure. In this study the magnetic properties of Ni nanocrystals generated by compacting Ni particles with initial dispersion of 80 nm were investigated. The particles were prepared by evaporation-condensation technology in inert atmosphere, the purity of bulk Ni was 99.99%. The compacting of the specimens was carried out by using axial loading, the pressure was 0.2—2.0 GPa. The magnetic properties were measured by the Elsevier Science Publishers B.V. (North-Holland)
FMR method (x-band at 9.8 GHz). A special device
gave the possibility of heating the specimens from room temperature to T~in inert atmosphere. The accuracy of the temperature measurement was 0.5 K. The time individual measurement during the heating from room temperature to T~and the time of isothermal measurement was the same for all specimens. The basis experimental parameters used for T~determinations were the resonance field and the line shape of FMR spectra [41.The Curie point could be determined by fixinga sharp maximum ofB0 and by catching the changes of the FMR line shape during the transfer from FMR to EPR. It was found that there is a dependence of T~on the compacting pressure P (fig. 1). The minimumvalue of T~was found for the specimen that was pressed using a pressure of 2.0 GPa; this value was 607 Kwhich essentially differs from T~of bulk Ni (631 K). It is difficult to explain such a large decreaseof Tc just by the high dispersion ofinitial Ni powderbecause in this case the corresponding change of T~could be no more than 10 K [2]. It is not possible to explain this effect as the result ofhigh heterogeneous elastic stresses which appear in interparticle contacts during compacting. Firstly, even ifit is accepted that on the contacts between the particles there could exist “accumulated” 81
Volume 154, number 1,2
PHYSICS LETTERS A
a disturbance of long range order. Using the results of ref. [21 the corresponding values of the effective dispersion (Deff) were calculated for the assumed microporousstructure (fig. 1). Inourcasethisvalue
b40 -
U 0—U ~0
~ 60
—
25 March 1991
is the mean distance between vacancy clusters. The value of Deff obtained for the nanocrystals
stresses which are essentially higher than the cornpacting pressure it is necessary to take into account that the small value ofdT~/dPdoes not give the possibility to accept the explanation given above. Secondly, for Ni the compressive elastic stresses lead to an increase of T~[5]. The model of cover of ultrafine grains [6] does not find any experimental confirmation for the specimens under study. According to that model the nanocrystal in a more simple approximation is to be considered as a two-phase magnetic system that has two Curie points corresponding to the casing of the particle and to their internal volume. However, a careful analysis of the temperature dependence of
compacted at low pressure was equal to the dispersion of the initial Ni particles. The same result was obtained for T~of the specimens which were prepared by hydrostatic loading using a pressure of 5.5 GPa. This method is characterized by the absence of recrystallization during compacting. In the case of axial loading starting with a definite characteristic pressure which is more than 1.2 GPa [8] the process of grain boundary migration (recrystallization) begins; it is accompanied by the kinetic effect of superfluous vacancy formation, their concentration being about 1 0—a. The decrease of T~ begins in our experiments namely at these pressures. Such a high concentration of superfluous vacancies provides the velocity of mass transfer in nanocrystals including the high velocity of cluster formation which leads to the formation of a hypothetic submicroporous structure. The data obtained on the specimens that were subjected to the heat treatment confirmed the above conclusion (fig. 2). For the investigation the speci-
FMR parameters did not give any evidence of two
men was chosen compacted under a pressure of 0.8
or more different magnetic phases. It is possible that in that case structural size effects in nanocrystals take place. It is typical that the minimum of T~corresponds to the interval of dynamic relaxation [7] in which the principal processes of the evolution of defect structure take place which are initiated by the grain boundary migration (recrystallization). We think that this process leads to the formation of a submicroporous structure and that this is a result of cluster forming through the vacation mechanism; this submicroporous structure in common could be present as an increase of the range of system dispersion. Since the fluctuations of the spontaneous intensity of magnetization show themselves in a scale much larger than an atomic interaction the critical behaviour of a ferromagnet is essentially sensitive to the formation ofsuch a structure. These fluctuations being unsensitive to the details of the interatomic potential react only on the high scale changes, for instance on
GPa; in that case the recrystallization develops in a very small degree. The annealing was run in vacuum, the time ofannealing at the definite temperature was no more than 1.5 mm. It is clearly seen that the heat treatment at temperatures to 750 K leads to a de-
I20
~
1.o 1~° p (GPa)
Fig. 1. Plot of Tc and Doff versus compacting pressure.
82
——--------—-—--
--
// 0---(.:
-.
~ .
--
-
1
lOft
70)
lU.)
T (K)
Fig. 2. Dependence of T~on annealing temperaturefor the specimen compacted under 0.8 GPA.
Volume 154, number 1,2
PHYSICS LETTERS A
crease of T~.Higher annealing temperatures cause an intensive grain growth leading to the disappearance of the submicroporous structure. The offered model of a submicroporous structure is a logical continuation of the vacancy model. Its further development could facilitate essentially the physical understanding of such processes in nanocrystals as agglomeration, compaction, solid phase synthesis and recrystallization.
25 March 1991
References [1] I.D. Morochov, L.I. Trusov and V,N. Lapovok, Fizicheskie javienija v ultradispersnych sredach (Energoatomizdat, [2] ~ (USSR) 27 (1985) 3147. [31R.Z. Valiev et al., Pis’ma Zh. Tekh. Fiz. 15 (1989) 78. [41T. Sekiguchi, T. Miyadai and K. Manabe, J. Magn. Magn. Mater. 28 (1982) 154. [5] M. Brouha and A.G. Rijnbeek, High Temp. High Pressures 6(1974)519. [61 R.Z. Valiev etal., Phys. Stat. Sol. (a) 117 (1990) 549. [7] Li. Trusov et a)., in: Svoistva i primenenie dispersnych poroshkov (Naukova Dumka, Kiev, 1986). [8]v.N:Lapovoketal.,FMM (USSR) 57 (1984) 718.
83
PHYSICS LETTERS A
25 March 1991
Curie temperature and substructure of Ni nanocrystals M.W. Belous, Yu.N. Rudoj Department of General Physics and Solid State Physics, Polytechnicalinstitute ofKiev, KPI-1013 Kiev 252056, USSR
L.I. Trusov and V.!. Novikov ULTRAM InternationalResearch Center, 115230 Moscow, USSR Received 2 January 1991; accepted for publication 25 January 1991 Communicated by J. Flouquet
The substructure of Ni nanocrystals compacted from ultrafine particles at different pressures and their Curie point is studied, using the FMR method. A dependence of T~on the compacting pressure is found. This fact could beexplained as the result ofthe formation in the compacts ofa submicroporous structure which appeared during the evolution of the real structure ofthe material which was created by the motion ofthe grain boundaries duringrecrystallization.
A number, of size effects was observed in nanocrystals as a result of the comparable roles of surface and volume energy of ultrafine grains [1]. For example, such size effects were found experimentally for an ensemble of nonmteracting magnetic particles near the Curie point (Tc) [2]: a monotone decrease of T~as a result of the decrease of particle size was observed. It was found in compact ensembles of small (less than 106 m) Ni particles created by compacting under high pressure that as the grain size decreased T~and the saturation of magnetization also decreased (in comparison with bulk Ni) [31.It was however noticed that this dependence could hardly be explained just by size magnetic effects and it is necessary to take into account the structural size effects. Therefore it is of interest to study the Curie point of nanocrystals in dependence of the compacting pressure. In this study the magnetic properties of Ni nanocrystals generated by compacting Ni particles with initial dispersion of 80 nm were investigated. The particles were prepared by evaporation-condensation technology in inert atmosphere, the purity of bulk Ni was 99.99%. The compacting of the specimens was carried out by using axial loading, the pressure was 0.2—2.0 GPa. The magnetic properties were measured by the Elsevier Science Publishers B.V. (North-Holland)
FMR method (x-band at 9.8 GHz). A special device
gave the possibility of heating the specimens from room temperature to T~in inert atmosphere. The accuracy of the temperature measurement was 0.5 K. The time individual measurement during the heating from room temperature to T~and the time of isothermal measurement was the same for all specimens. The basis experimental parameters used for T~determinations were the resonance field and the line shape of FMR spectra [41.The Curie point could be determined by fixinga sharp maximum ofB0 and by catching the changes of the FMR line shape during the transfer from FMR to EPR. It was found that there is a dependence of T~on the compacting pressure P (fig. 1). The minimumvalue of T~was found for the specimen that was pressed using a pressure of 2.0 GPa; this value was 607 Kwhich essentially differs from T~of bulk Ni (631 K). It is difficult to explain such a large decreaseof Tc just by the high dispersion ofinitial Ni powderbecause in this case the corresponding change of T~could be no more than 10 K [2]. It is not possible to explain this effect as the result ofhigh heterogeneous elastic stresses which appear in interparticle contacts during compacting. Firstly, even ifit is accepted that on the contacts between the particles there could exist “accumulated” 81
Volume 154, number 1,2
PHYSICS LETTERS A
a disturbance of long range order. Using the results of ref. [21 the corresponding values of the effective dispersion (Deff) were calculated for the assumed microporousstructure (fig. 1). Inourcasethisvalue
b40 -
U 0—U ~0
~ 60
—
25 March 1991
is the mean distance between vacancy clusters. The value of Deff obtained for the nanocrystals
stresses which are essentially higher than the cornpacting pressure it is necessary to take into account that the small value ofdT~/dPdoes not give the possibility to accept the explanation given above. Secondly, for Ni the compressive elastic stresses lead to an increase of T~[5]. The model of cover of ultrafine grains [6] does not find any experimental confirmation for the specimens under study. According to that model the nanocrystal in a more simple approximation is to be considered as a two-phase magnetic system that has two Curie points corresponding to the casing of the particle and to their internal volume. However, a careful analysis of the temperature dependence of
compacted at low pressure was equal to the dispersion of the initial Ni particles. The same result was obtained for T~of the specimens which were prepared by hydrostatic loading using a pressure of 5.5 GPa. This method is characterized by the absence of recrystallization during compacting. In the case of axial loading starting with a definite characteristic pressure which is more than 1.2 GPa [8] the process of grain boundary migration (recrystallization) begins; it is accompanied by the kinetic effect of superfluous vacancy formation, their concentration being about 1 0—a. The decrease of T~ begins in our experiments namely at these pressures. Such a high concentration of superfluous vacancies provides the velocity of mass transfer in nanocrystals including the high velocity of cluster formation which leads to the formation of a hypothetic submicroporous structure. The data obtained on the specimens that were subjected to the heat treatment confirmed the above conclusion (fig. 2). For the investigation the speci-
FMR parameters did not give any evidence of two
men was chosen compacted under a pressure of 0.8
or more different magnetic phases. It is possible that in that case structural size effects in nanocrystals take place. It is typical that the minimum of T~corresponds to the interval of dynamic relaxation [7] in which the principal processes of the evolution of defect structure take place which are initiated by the grain boundary migration (recrystallization). We think that this process leads to the formation of a submicroporous structure and that this is a result of cluster forming through the vacation mechanism; this submicroporous structure in common could be present as an increase of the range of system dispersion. Since the fluctuations of the spontaneous intensity of magnetization show themselves in a scale much larger than an atomic interaction the critical behaviour of a ferromagnet is essentially sensitive to the formation ofsuch a structure. These fluctuations being unsensitive to the details of the interatomic potential react only on the high scale changes, for instance on
GPa; in that case the recrystallization develops in a very small degree. The annealing was run in vacuum, the time ofannealing at the definite temperature was no more than 1.5 mm. It is clearly seen that the heat treatment at temperatures to 750 K leads to a de-
I20
~
1.o 1~° p (GPa)
Fig. 1. Plot of Tc and Doff versus compacting pressure.
82
——--------—-—--
--
// 0---(.:
-.
~ .
--
-
1
lOft
70)
lU.)
T (K)
Fig. 2. Dependence of T~on annealing temperaturefor the specimen compacted under 0.8 GPA.
Volume 154, number 1,2
PHYSICS LETTERS A
crease of T~.Higher annealing temperatures cause an intensive grain growth leading to the disappearance of the submicroporous structure. The offered model of a submicroporous structure is a logical continuation of the vacancy model. Its further development could facilitate essentially the physical understanding of such processes in nanocrystals as agglomeration, compaction, solid phase synthesis and recrystallization.
25 March 1991
References [1] I.D. Morochov, L.I. Trusov and V,N. Lapovok, Fizicheskie javienija v ultradispersnych sredach (Energoatomizdat, [2] ~ (USSR) 27 (1985) 3147. [31R.Z. Valiev et al., Pis’ma Zh. Tekh. Fiz. 15 (1989) 78. [41T. Sekiguchi, T. Miyadai and K. Manabe, J. Magn. Magn. Mater. 28 (1982) 154. [5] M. Brouha and A.G. Rijnbeek, High Temp. High Pressures 6(1974)519. [61 R.Z. Valiev etal., Phys. Stat. Sol. (a) 117 (1990) 549. [7] Li. Trusov et a)., in: Svoistva i primenenie dispersnych poroshkov (Naukova Dumka, Kiev, 1986). [8]v.N:Lapovoketal.,FMM (USSR) 57 (1984) 718.
83