Ferromagnetism of Nanoparticles and Secondary Crystal Structure
Yu.I. Vesnin
yu_vesnin@ngs.ru
It is known that magnetism is a universal property of substance [1]. In the work [2] it is
shown that nanoparticles of inorganic substances of different classes (metals, oxides, various
salts) reveal ferromagnetic properties. Suvbstances dia- and paramagnetic in massive state
become ferromagnetic in superdispersed state (nanoparticles, size ~10-6 cm and less).
Appearance of ferromagnetism of such particles is associated with various reasons (surface state
of atoms, vacancies and other defects, magnetic anisotropy, Hund's rule etc.) [2].
In [3-5] the theory of secondary crystal structure (SCS) is developed as well as
application examples of this theory in chemistry and physics of solids are given. According to
the SCS theory, a crystal consists of elementary units of the size dm ~ 30 nm. This unit (the
minimal crystal – a mic) is an analog of molecule – a giant crystalline molecule. A particle of
smaller size (dsc  30 nm) is a subcrystal – an analog of molecule-radical, i.e. a free radical of
crystal solid. It is known that magnetic susceptibility of radicals increases, and almost all known
molecule-radicals are paramagnetic particles, i.e. they have magnetic moments [6]. Therefore,
the crystal particles with sizes dsc < dm ~ 30 nm (subcrystals), like other radicals, should have
magnetic moment as well. This moment takes place because of mass deficit (or quantity of atoms)
relative to the mass of the elementary crystal unit (the main molecule). In this connection other
properties of the crystal particle change as well [4].
It is known that magnetic structure of ferromagnetics consists of domains – magnetic
ordered areas with macroscopic sizes. Each domain has a constant magnetic moment
independent of an external field [1]. Without the external magnetic field these moments are
mutually compensated, and magnetization is absent. In an ensemble of nanoparticles each
particle can be a domain. Its magnetic moment is obtained owing to the transition to the state of
the radical-molecule at the particle size dsc < 30 nm. Therefore, ferromagnetism can be expected
for nanoparticles with the size dsc  30 nm. Magnetoactive form of substance appears here owing
to the size effect [4].
In [2] it is shown that ferromagnetism of nanoparticles of various substances exists at
temperatures 300 К and higher. It can be supposed that the ferromagnetic Curie point K will
depend on of nanoparticle sizes with the maximum at a certain size.
The ferromagnetism of nanoparticles has a principal meaning for the SCS theory. Earlier
it was shown [4] that subcrystals (nanoparticles with sizes dsc < 30 nm) have the external force
field with the extension in tens nm. The electron microscopic observations of crystal particles
movement at annealing of discontinuous films evidence of the presence of these fields [7].
Ferromagnetism of nanoparticles allows supposing that observed force fields have magnetic
nature. It can be checked in experiments on annealing of discontinuous films in a magnetic field
(see Fig. 1). The movement character of particles will change depending on the field orientation.
Determination of the magnetic nature of force fields of nanoparticles (subcrystals) can be of
fundamental importance for chemical bond theory.
Fig. 1 Movement of gold particles on a carbon substrate at t=264C under force magnetic fields
of nanoparticles [7]. A similar experiment carried out in magnetic field will enable to
determine the nature of nanoparticle force fields.
References
S.V. Vonsovsky. Magnetism. Nauka, Мoscow; 1971 (In Russian).
A.Sundaresan, C.N.R. Rao. Nano Today, (2009), v. 4, p. 96.
Yu.I. Vesnin. Jurn. Struct. Chem., (1995), v. 36, p. 724.
Yu.I. Vesnin. Secondary structure and properties of crystals. SB RAS Edition
Novosibirsk, 1997; www.nanometer.ru (In Russian)
5. Yu.I. Vesnin. Chemistry for sustainable development, (2000), v. 8, No. 1-2, p. 61.
6. V.N. Kondratyev. Free radicals as an active form of substance. Academy of Sciences
UdSSR Edition, Мoscow; 1960 (In Russian).
7. W.B. Phillips, E.A. Desloge, J.G. Skofronik. J. Appl. Phys., (1968), v. 39, p. 3210.
1.
2.
3.
4.