Page 396 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 6 EVALUATION METHODS FOR PROPERTIES OF NANOSTRUCTURED BODY
[4] M. Adachi, Y. Murata, J. Takao and J. Jiu: J. Am. In the magnetic nanomaterials used in magnetic
Chem. Soc., 126, 14943 (2004). recording media or materials for magnets, it is desir-
[5] T.E. Ivers, A. Webee and D. Herbstritt: J. Eur. Ceram. able that they have a large magnetic anisotropy so that
Soc., 21, 1805–1811 (2001). they do not become super paramagnetic. The origin of
magnetic anisotropy of magnetic nanomaterials is not
[6] A. Atokinson, S. Barnett, R.J. Gorte, J.T.S. Irvine,
only the crystalline magnetic anisotropy of bulk mate-
A.J. Mcevoy, M. Mogensen, S.C. Singhal, J. Vohs:
rials, but also the shape of the particles and the spe-
Nature Materials, 3, January, 17–27 (2004).
cific magnetic structures at the surface of the particles
[7] H. Yokogawa: Ceramics, 36(7), 472–476 (2001).
are considered to be important factors. Kawamura
[8] T. Yamaguchi, M. Matsumoto and H. Matsubara: et al. [1] have calculated the magnitudes of the coer-
Ceramics, 39(4), 281–285 (2004). cive force of cylindrical shaped nanoparticles using a
[9] M. Aizawa: Ceramics, 36(7), 493–495 (2001). core shell model with the crystalline magnetic
[10] J.-W. Moon, H. Hwang and M. Awano: J. Ceram. Soc. anisotropy constant K of bulk material and the specific
1
Jpn., 110(5), 479–484 (2002). magnetic anisotropy (surface magnetic anisotropy) edge
[11] S. Wang, M. Awano and K. Maeda: J. Ceram. Soc. K , and compared them with actually measured values.
1
From the results of these calculations and experi-
Jpn., 110(8), 703–709 (2002).
ments, it became clear that, for the same size, the
[12] T. Yamada, N. Batina and K. Itaya: J. Phys. Chem.,
intrinsic coercivity of particles increases gradually as
335, 204 (1995).
the value of the surface magnetic anisotropy edge K 1
increases, and also, that the coercivity becomes
higher as the particle size becomes small in the case
6.6 Magnetic properties of single particles [1].
Therefore, in the design of the coercivity of the
The areas of research on magnetic nanomaterials are – magnetic nanoparticles, it is necessary to consider
magnetic fluids, magnetic granular structure thin the crystalline magnetic anisotropy that is intrinsic to
films, magnetic multi-layer films, magnetic recording the material, the shape of the particles (the magnetic
media, magnetic metallic glasses, magnetic amor- anisotropy due to the shape) and the specific surface
phous alloys, permanent magnet materials, medical magnetic anisotropy.
magnetic materials, etc. While it is common to carry out the evaluation of the
While outstanding progress has been made in recent magnetic characteristics of super paramagnetic materi-
years in each of these areas, some of the areas that uti- als using a Vibrating Sample Magnetometer (VSM) or
lize the behavior of nanoparticles are – magnetic fluids a Superconducting Quantum Interference Device
and coated magnetic recording media, drug-delivery (SQUID), in recent years it has been found that even
systems in the field of medicine, etc. Magnetic fluids X-ray Magnetic Circular Dichroism (XMCD) measure-
utilize the super paramagnetism and dispersion stabil- ments using high-energy radiation X-rays are effective.
ity in liquids of nanoparticles, and in general, they use Since XMCD measurement can evaluate the mag-
materials with low crystalline magnetic anisotropy netic polarization at which a specific element is
(such as magnetite or iron-based alloys). placed from the difference between the left-polarized
On the other hand, in a magnetic recording medium, light and the right-polarized light due to the X-ray
since it is desirable that there are more magnetic parti- absorption edge of the specific element, there is the
cles per unit area, fine particles that also have a large advantage that it is possible to measure the process of
magnetic coercivity are required. In the case of med- magnetization of that specific element without being
ical magnetic materials, because they are used inside affected by impurities.
the human body, biological safety and not blocking the
blood vessels, etc., become important requirements
and hence magnetite, etc., with particle sizes of less 6.6.2 Material-specific discussion
than 50nm are being used in their research.
6.6.2.1 Nanospinel ferrite
Spinel ferrite is a material that has been used from the
6.6.1 Super paramagnetism past. Even today its research is being done actively as a
magnetic fluid, drug delivery material and as a mag-
When the size of magnetic particles is made small, in netic recording material. Tanaka et al. [2] have pre-
a region of sizes less than a certain critical diameter, pared, using the hydrothermal method, nanoparticles of
since the total magnetic energy becomes small when zinc ferrite which are a typical normal spinel ferrite,
there is no magnetic wall within the particle, it and have evaluated the magnetic characteristics.
becomes a single magnetic domain particle. In addi- Although bulk zinc ferrite is an antiferromagnetic
tion, if the particle size is made small, it exhibits material having a Neel temperature around 10 K, it has
super paramagnetism because of thermal fluctuations been reported to have a magnetization value of 11 emu
of the magnetic moment of the particle. g –1 at room temperature when the crystallite size is

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