Gross R., Sidorenko A., Tagirov L. Nanoscale Devices

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Nanocrystalline Iron-Rare Earth Alloys 381

and 1/9 systems, respectively, as compared to

= 16 obtained in the

C(N)

based compounds. The dlnJ

eff

/dlnν values were also estimated

from magnetic data by using a molecular field approximation. These values

are by about 25 – 30 % smaller compared to those determined from Fig. 6.

The differences may be attributed to the approximations used in estimating

the J

eff

values, particularly mean values of the iron moments and for

exchange interactions or a mean field approximation used in analyzing the

magnetic data. The data from Fig. 6 suggests that iron atoms show mainly a

localized behavior both in the 2/17 and 1/9 systems.

As a result of carbonation, the Sm

17-x

and SmFe

9-y

C systems

have uniaxial anisotropy. In SmFe

9-y

C nanocrystalline alloys, high

coercivities were obtained in samples annealed between 700 and 800

C [6]

(see Fig. 7). The coercive fields decrease when decreasing or increasing the

annealing temperature outside the above mentioned temperature range. A

coercive field of

= 1.3 T was found in case of the sample having

y = 0.50 and grain sizes around 22 nm. The highest coercive field

= 1.5 T was obtained for a SmFe

8.75

0.25

C alloy. Thus, these alloys

have potential technical applications. The high coercivities of mechanically

alloyed and carbonated samples originate from the presence of the 1/9

metastable phase.

Fig. 7. The dependence of coercive fields in SmFe

9-y

C alloys on the annealing

temperature.

600

Coercivity (kOe)

700 800 900 1000 1100 1200

Annealing temperature (°C)

0.25

0.5

0.75

382 E. Burzo, C. Djega–Mariadassou

Nanocrystalline Nd-Fe-B Based Alloys

The reduction of the Nd concentration in Nd-Fe-B alloys, as compared to

2:14:1 stoichiometry, provides less expensive permanent magnets, and thus

is of interest for technical applications. With respect to their microstructure,

the alloys are constituted from a matrix of a magnetically hard phase

B and

Fe and/or Fe

B particles on the grain boundaries. The

increase in the remanence due to the nanoscale structure is further enhanced

by the presence of a soft magnetic phase, which has a higher saturation

induction than that of the Nd

B phase [3, 24].

Previously, it has been shown that the addition of chromium in Nd-Fe-B

nanocomposites increases the coercivity, while the Curie temperatures and

the induction decrease [3]. In order to analyze the influence of other

substituting elements on the magnetic properties, particularly the coercivity

of nanostructure alloys, we studied the Nd

66.5-x

18.5

with M = V

or Nb system.

The Nd

66.5-x

18.5

, as melt spin ribbons, with M = V or Nb are

amorphous. The distribution of grain sizes is more homogeneous when the

vanadium content increase. The thermal treatment at 650

C leads to the

crystallization of the alloys. For short annealing time, the presence of

-Fe

was found. The mean dimensions of

-Fe crystallites increase rapidly for

an annealing time t

up to 1 min, at 650

C and then a nearly linear variation

was observed as shown in Fig. 8. For t

= 10 min the mean grain size of

Fe is d = 17 nm.

Fig. 8. The

-Fe crystallite sizes as a function of the annealing time at 650

C for

the Nd

66.5

18.5

alloy.

Nanocrystalline Iron-Rare Earth Alloys 383

Fig. 9. Remanence and coercivity for Nd

66.5

18.5

samples as a function

of the annealing time at 650

Due to the small crystallite dimensions, the as quenched samples are

superparamagnetic. The hard magnetic properties were developed after

crystallization of as quenched samples. Both the remanent induction and the

coercive field increase gradually with annealing time as shown by Fig. 9.

For samples thermally treated at 650

C during 10 min, a ratio B

= 0.7

was determined. The coercive field exceeds 0.36 MA/m.

The demagnetizing curves for the Nd

66.5

18.5

sample annealed

at 650

C for 2 and 10 min, respectively, are plotted in Fig. 10. These are

typical for an exchange spring behavior as expected in nanocomposite

magnets, when the particle sizes of

-Fe approach the domain size of the

hard Nd-Fe-B phase. The exchange interactions would suppress the

reversible rotation of the magnetization in the soft

-Fe particles and there

is a significant increase of the coercivity.

Although the Nd

66.5-x

18.5

samples contain high boron content

no evidence for the presence of a Fe

B phase was found. It was shown

previously [25] that for x > 5 at % Cr the main soft magnetic phase is

-Fe.

The presence of chromium as well as Nb and V decreases the saturation

magnetization of the alloys. Although the B

ratio is higher than 0.7, the

remanent induction is decreased as compared to Nd-Fe-B magnets.

(kOe)

0 0.5 2 5

ta (min)

M (emu/g)

384 E. Burzo, C. Djega–Mariadassou

Fig. 10. The second quadrant hysteresis loops for Nd

66.5

18.5

alloys

annealed for 2 min (a) and 10 min (b) at 650

A high value of the remanence ratio is due to exchange coupling between

the magnetic moments at the interface of the hard and soft magnetic phase

nanograins. As a consequence of this coupling, there is a large degree of

reversibility in the demagnetization behavior [26]. The better magnetic

properties are obtained when the mean grain size of

-Fe is around 20 nm.

Conclusions

The exchange interactions in R

and R

B compounds are well

described by a 4f-5d-3d model. The 5d band polarization is due to both

local 4f-5d as well as 5d-3d short range exchange interactions. The analysis

of the volume effects on the Curie temperatures show that iron has mainly a

localized behavior.

Stable Sm

17-x

and metastable SmFe

9-y

nanocrystalline alloys

have been obtained. A transition from a metastable to stable state through

an ordering process of atoms was seen on increasing the annealing

temperature. High coercivities were obtained in SmFe

9-y

C alloys

thermally treated in the temperature range between 700

C and 800

The Nd

66.5-x

18.5

nanocomposites with M = V or Nb are

constituted from hard magnetic phase based on Nd

B and

-Fe. A high

= 0.7 ratio as well as high coercive fields were found for melt

spinning samples after a thermal treatment at 650

C for 10 min. The

-Fe

dimensions are of ≅ 20 nm.

−0.6 −0.4 −0.2 0

B (G.cm

)

H (T)

Nanocrystalline Iron-Rare Earth Alloys 385

References

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Handbook. Springer Verlag Vol. III

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Rev B 69:064402

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l’Electrotechnique. Vol. 1, Politechnical University Bucharest

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Phys Rev Lett 53:2571

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12. Li ZW, Morrish AH (1997). Phys Rev B 55:3670

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Phys 92:7362

15. Burzo E (1974). Solid State Commun 14:1295; (1981) J Less Common Met

77:251

16. Burzo E, Lazar DP, Valeanu M (1976). Proc. 12

Rare-Earth Research

Conference, Colorado p 104

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19. Djega-Mariadassou C, Bessais L, Nandra A, Burzo E (2003). Phys Rev B

68:024406

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for Intermetallic Phases. ASM International

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22. Brouha M, Buschow KHJ, (1973). J Appl Phys 64:1813

23. Plugaru N, Valeanu M, Burzo E (1994). IEEE Trans Magn 30:663

24. Davies HA, Manaf A, Zhang PZ (1993). J Mater Eng Perform 2:579

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Int. Workshop Rare Earth and Their

Applications, p.87

26. Kneller EF, Hawig R (1991). IEEE Trans Magn 27:3588

The Influence of Applied Fields on the Nucleation

and Growth of Heteroepitaxial Carbon Films

B. Z. Mansurov

al-Farabi Kazakh National University, 96A, Tole be Str., 480012, Almaty,

Kazakhstan

Abstract: On the basis of a literature review and the comparison of physical and

chemical properties of materials and calculations, it is shown that

copper, saturated with hydrogen, is an appropriate substrate material

for the heteroepitaxial growth of diamond films. Our estimates have

shown that it is possible to create conditions for preferential oriented

growth of diamond films by changing the magnitude and configura-

tion of applied magnetic fields. On the basis of theoretical calcula-

tions the technological installation for the growth of carbon films by a

method of differential magnetron sputtering has been developed and

designed. The basic technological parameters of the installation are

presented. Also the mathematical algorithm describing the deposition

process of carbon films on a copper buffer layer is offered.

Keywords: carbon films, applied fields, differential magnetron sputtering

Introduction

The significant efforts devoted to the synthesis of diamonds were motivated

by the unique combination of properties of this material. They provide the

chance to produce electronic devices both in discrete and integrated form

with very high speed and power at a high operation temperature range, at

high integration density, and increased mechanical strength and reliability.

Various

techniques such as plasma-enhanced chemical vapour

deposition (CVD), ion-assisted deposition, and hot filament CVD have

387

R. Gross et al. (eds.), Nanoscale Devices - Fundamentals and Applications, 387–399.

388 B. Z. Mansurov

been developed for obtaining diamond films and are widely used today [1-

6]. However, the characteristics achieved thus far for electronic devices are

not good enough for practical applications.

The majority of the deposition processes for CVD crystallization occur

under conditions where the massive chaotic crystallization of diamond

hinders the growth of high quality crystalline material.

Therefore, the search for deposition systems, in which the rate of the

backwards process would be noticeably higher and, besides, would allow

the continuous control of film growth is an urgent problem. The solution of

this problem will determine the further development of the diamond

synthesis technology.

Evaluation of Applied Field Effects on the Nucleation of

Heteroepitaxial Carbon Films

It is generally known that specific problems such as the simultaneous

nucleation of non-diamond structures, the prevention of a flawless stable

growth of oriented diamond films, the control of growth rates, as well as the

selective elimination of highly defective areas and non-diamond structural

modifications of carbon films are characteristic for the synthesis of

diamond. These problems may be solved by suitable control of applied

fields and other control parameters (electrostatic, magnetic, optical,

temperature, field of elastic deformations) of certain limiting symmetry

group. According to the principles of crystal growth the symmetry of these

control fields should correspond to the point and space group symmetry

group of diamond and not to that of non-diamond modifications of carbon

films.

Evaluation of applied field effects on the nucleation

The nucleus formation and growth rate of a crystalline film are determined

rather by the differences in the magnitude of thermodynamic functions

under the various phase conditions than by the magnitude itself. However,

the differences are substantially less than the absolute magnitudes. Thus,

even insignificant variations of the thermodynamic functions may result in

applied field effects on the process of nucleation, as well as the rate and

direction of the crystalline structure growth.

The energy of formation of a nucleus (G

) is determined by a chemical

potential (

) of a substance, by the surface energy (

) and, in general, by

further non-considered parameters defining effects of applied

fields and

The Influence of Applied Field on the Nucleation and Growth 389

their spatial symmetry:

nEHChSESH

=++ + + + + +GWWWWWW

The additional terms take into account the following contributions: W

energy of the electric field, W

- energy of the magnetic field, W

- energy

of the field of elastic deformations, W

hν

- energy of the optical influence,

and W

- space symmetry of the applied fields (electric and magnetic

fields). Thus, it is possible to create conditions for preferential nucleation

and growth of a diamond film by changing the magnitude and the shape of

the applied fields.

Effects of the elastic deformation field

The most convenient substrates for the growth of diamond films would be

single crystalline silicon wafers with a diamond-like structure due to their

availability, low cost and perspectives for further use in semiconductor

device fabrication.

In Fig. 1 a linear model of the growth of a monolayer carbon film on a

silicon substrate is shown. Because of the difference of the lattice cell

parameters of Si (5.43 Å) and C (3.57 Å) [7] elastic deformations of the

lattices occur. In the given model the deformation forces acting on the

carbon atoms are equal to twice the deformation forces acting on the silicon

atoms, F

= 2F

. That is, the effect of the whole silicon substrate is

replaced by the effect of two silicon layers on one layer of carbon.

Fig. 1. Linear model of a monolayer carbon film grown on silicon.

390 B. Z. Mansurov

The deformation force acting on the atoms of carbon and silicon can be

written as:

cSicSi

C C Si Si C Si c Si

, , 2 , -FC FC C C ba

abab

εε ε

== = =+,

where C

and C

are the elastic constants of carbon and silicon,

and

the relative displacements, and a and b the lattice parameters of diamond

and silicon, respectively.

The energy of the elastic deformation of the diamond lattice is given by

C 11c 12c

= + UC C

εε

 

 

 

with U

= 0.569×10

or U

= 0.322eV, 1/atom.

The energy of the elastic deformation of the silicon lattice is given by

Si Si

Si 11Si 12Si

= +UC C

εε

 

 

 

with U

= 1.16 ×10

or U

= 2.247eV, 1/atom. The estimated value

of the deformation energy of the silicon and diamond lattices (U

+ U

) is

then equal to

= U

+ U

= 2.57 eV, what corresponds to a temperature

of about 30000 K required to create such deformations. That demonstrates

the impossibility of epitaxial growth of crystalline diamond films on

silicon.

Thus, to obtain high quality diamond films a buffer layer with the

following properties is required:

1. The buffer layer should have a diamond-like structure.

2. The lattice parameters of the buffer layer should be well matched to

those of diamond.

3. The thermal expansion coefficient of the buffer layer should be close

to that of diamond to minimize thermal stresses between the

substrate and the diamond films.

The analysis of the physical and chemical properties of chemical elements

shows that copper saturated with hydrogen is a suitable materials choice

acting as a buffer layer for epitaxial growth of diamond films. In Fig. 2 the

The Influence of Applied Field on the Nucleation and Growth 391

copper (111) surface is shown. The crystallographic calculations of

tetrapore and octapore volumes in the copper lattice proved that in copper

films grown in a hydrogen atmosphere the tetrapores will be filled-in with

atomic hydrogen, and octapores with molecular hydrogen. The estimated

value of the space between the atomic hydrogen atom in tetrapore and the

carbon atom deposited above tetrapore is L = 0.87 Å. This is sufficient for

C–H bond creation, because the typical C-H bond length is 1.09 Å [7].

Fig. 2. Distribution of tetrapores and octapores on the (111) surface of a copper lat-

tice.

Since copper does not create compounds with carbon, the atomic

hydrogen in the tetrapores of the copper lattice is expected to act as a center

of crystallization of carbon. As a result the difference between the lattice

parameters of substrate and the growing diamond film becomes

considerably smaller.

The distance between the carbon atoms on a (111) surface is a

D =

2.517 Å

for diamond and a

G =

2.84 Å for graphite [7]. The distance between

tetrapores on the copper (111) surface is a

Cu =

2.56 Å, i.e. the difference is:

∆

= 100

56.2

517.256.2

−

% = 1.7% for diamond

∆

= %100

56.2

84.256.2

−

= 10.94% for graphite.

Thus, the distribution of tetrapores on the copper (111) surface and,

respectively, of atomic hydrogen located in them, create suitable conditions

for the preferential formation of tetragonal carbon bonds.

Gross R., Sidorenko A., Tagirov L. Nanoscale Devices - Fundamentals and Applications

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