Page 397 - Book Hosokawa Nanoparticle Technology Handbook

6.6 MAGNETIC PROPERTIES FUNDAMENTALS
made small up to 4 nm with magnetic field of 1 T Shinohara et al. [6] inferred that the reason for the
(tesla). Further, it has become clear from measure- Pd nanoparticles to have a ferromagnetic component
ments of susceptibility that these particles have a mag- is that the (100) surface of the polyhedron nanocrys-
netic transition point at a temperature in the range of talline particle is ferromagnetic. This also does not
several tens of K (kelvin). The reason that zinc ferrites, contradict the results of the theoretical calculations.
which have a small magnetization value at bulk sizes, Further, it is suggested [6] that there is a possibility
have a large magnetization value at room temperature that the magnetic anisotropic energy of these
is considered to be that zinc ferrite nanoparticles nanoparticles has about the same magnitude as that of
exhibit super paramagnetism [2]. 3d transition metals.
Further, Yamamoto et al. [3] have synthesized
Co–Ni–Mn ferrite particles having a high coercivity 6.6.2.3 FePt nanoparticles
using the chemical coprecipitation method. It has While FePt is a ferromagnetic material known from
been reported that Cobalt ferrites have a large crys- the past, recently it is being studied [7] actively as a
talline magnetic anisotropy constant K compared to material having the possibility of achieving a record-
1
–2
other spinel ferrites, and is hard to become super ing density of the order of 1 Tb in because it has a
paramagnetic even when very fine particles are large crystalline magnetic anisotropy. The L1 -FePt
0
made. However it has been reported that these phase which is an ordered alloy has a multilayer struc-
Co–Ni–Mn ferrites with a chemical equation of ture of Fe atoms and Pt atoms in the C-axis direction,
[(CoO) 0.5 (NiO) (MnO) 0.1 1.125(Fe O )] exhibit and its crystalline magnetic anisotropic energy Ku
0.4
3
2
7
–3
unique magnetic characteristics; coercivity of 7.12 reaches [8] 7 10 erg cm .
–1
kOe, saturation magnetization of 44.2 emu g , average The super paramagnetism limiting diameter
particle size of 43 nm, crystalline magnetic anisotropy obtained from this Ku and the thermal fluctuation
6
6
constant K of 1.36 10 , K of 11.3 10 and energy kT is about 4 nm and is one of the smallest
2
1
anisotropic magnetic field HA of 44.7 kOe. among the magnetic materials known at present. In
Although the origin of such high coercivity is not addition, it is possible to obtain nanoparticles of these
clear, it has been reported, from measurements of the FePt particles relatively easily using the polyol
torque curve of magnetic sheets, that a large process, etc., and these particles are also superior in
anisotropic magnetic field is present. If it is possible resistance to oxidation and resistance to chemicals.
to reduce the particle size still further, it is considered However, since FePt nanoparticles that have been
that there is the possibility of use as a coated obtained using the synthesizing methods reported so
type magnetic material for high-density magnetic far are made of unordered phases, they require some
recording. form of heating process. However, since fusion
between FePt nanoparticles takes place due to the
6.6.2.2 Pd nanoparticles heating process, this has become a problem from the
In the 4d transition metals Ru, Rh and Pd, it has been point of view of dispersibility of nanoparticles and
predicted from theoretical calculations [4,5] that the their orderly arrangement.
band structure changes when there is a reduction in the In order to solve this problem, doping of a third ele-
coordination number or a change in the symmetrical ment to FePt particles or fusion prevention treatment
property due to low-dimensionalization, and there is a on the surface of FePt particles, etc. are being investi-
possibility of showing ferromagnetism. Shinohara gated. The size of L1 -FePt particles obtained so far is
0
et al. [6] obtained Pd nanoparticles (of a polyhedron 3–10 nm, and their magnetic characteristics are a
shape) with less impurities using a sample preparation coercivity of 1,000–10,000 Oe, and a saturation mag-
–1
method in an Ar gas environment. From the measure- netization of about 50 emu g . Further, research is
ment of magnetic field dependency of magnetization being made actively not only on wet type chemical
of these particles with an average diameter of 11 nm it processes but also on FePt thin film growth using a
was found that the magnetization at low magnetic vapor phase method [9,10].
fields increased suddenly, and after that it increased There is the possibility of very big technical
gradually in proportion to the magnetic field linearly. advancement in materials science and technology
Further, from the hysterisis curve it became clear that because many people from various fields are involved
they had a coercivity of about 40 Oe and a residual in this materials research. In order to realize coating
–1
magnetization of 0.1 emu g . type magnetic recording materials it is considered
On the other hand, from the result of measuring the necessary to give sufficient attention to global
temperature dependence of magnetization it became resources and economical matters.
clear that the saturation magnetization component is
not lost up to 400K. Further, there is complex 6.6.2.4 Metal nitrides
relationship between the saturation magnetization Many types of iron nitrides, which are penetration
component and the size of the Pd nanoparticles, and it type nitride materials, are present such as Fe N, Fe N,
4
3
became clear that ferromagnetic component appears Fe N , etc., and ever since Kim and Takahashi et al.
16
2
in the size range of 6–14.4 nm. [11] found that thin film Fe N has a saturation
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