Page 398 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 6 EVALUATION METHODS FOR PROPERTIES OF NANOSTRUCTURED BODY
magnetization exceeding that of
-Fe, it has attracted [5] B.V. Reddy, S.N. Khanna and B.I. Dunlap: Phys. Rev.
the attention of a large number of researchers. Also, a Lett., 3323, 70 (1993).
lot of research has been done on the synthesis of bulk [6] T. Shinohara, T. Sato: Kotai Butsuri (Solid Stage
material and thin films of Fe N . Physics), 40(8), 535–544 (2005).
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Nagatomi et al. [12] succeeded in synthesizing [7] L. Graf, A. Kussmann: Phisik. Z., 36, 544–551 (1935).
bulk material of Fe N by a process of nitriding -Fe [8] S. Sun, C.B. Murray, D. Weller, L. Folks and A.
2
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thin film or nanoparticles of -Fe under an ammo- Moser: Science, 287(5460), 1989–1992 (2000).
nium gas atmosphere at a low temperature (of 200ºC [9] Y. Takahashi, K. Hono: Magn. Soc. Jpn., 29(2), 72–79
or less). Further, Hattori et al. [13] obtained an
almost single phase Fe N by a process of nitriding, (2005).
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ultrafine particles (10–50 nm) of -Fe O under an [10] T. Shima, K. Takanashi, Y.K. Takahasi and K. Hono:
2
3
ammonium gas atmosphere. The saturation magneti- Appl. Phys. Lett., 85, 2571–2573 (2004).
zation of the obtained powder (with a specific sur- [11] T.K. Kim, M. Takahasi, Appl. Phys. Lett., 20, 492–494
–1
–1
2
face area of about 20 m g ) was about 200 emu g , (1972).
which was about 20% smaller than the predicted [12] A. Nagatomi, S. Kikkawa, T. Hinomura, S. Nasu and
–1
theoretical value (257 emu g ). F. Kanamaru: J. Jpn. Soc. Powder Powder Metall.,
The reason for the saturation magnetization being 46(2), 151–155 (1999).
lower than the predicted value is considered to be the [13] T. Hattori, N. Kamiya and Y. Kato: Magn. Soc. Jpn.,
disturbance in the particle surface due to reduction in 25, 927–930 (2001).
the particle size. On the other hand, the coercivity of
this powder is about 1,200 to 2,300 Oe and suggests a [14] Y. Sasaki, N. Usuki, K. Matsuo and M. Kishimoto:
large crystalline magnetic anisotropy [13]. Further, INTERMAG, GR02, 198 (2005).
Sasaki et al. [14] obtained single phase Fe N with a [15] T. Inoue, K. Nakiri, H. Mitsuhashi, M. Fukumoto and
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size of 17nm using a precursor of 20nm size mag- T. Doi: Y. INTERMAG, GR03, 862 (2005).
netite particles covered with an yttrium and silicon
compound. The saturation magnetization of this pow-
–1
der was 89 emu g and its coercivity was 2,900 Oe. 6.7 Optical properties
Although the saturation magnetization is much
smaller than the theoretical value, it has reached a 6.7.1 Transparency of nanoparticle
practical level as a high-density magnetic recording
material [14]. Fe N nanoparticles do not have the Free electrons in a conductor (metal) move in a col-
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large size or large magnetic anisotropy of FePt parti- lective motion called plasma oscillation. An electro-
cles, but they can be considered to be a very useful magnetic wave with a frequency that is higher than a
material considering the global resources and the plasma frequency can penetrate plasma, whereas that
manufacturing cost. with a lower frequency is absorbed or reflected. In
Although the history of development of magnetic order for a conductor to be transparent to visible light,
nanomaterials is old, need for its development has not but absorb or reflect near-infrared rays, its plasma fre-
dwindled due to the development of new functions or quency must be 800–1,000 nm, near the border sepa-
applications. If it is possible to control in the nanosize rating the visible light region and the near infrared
levels the magnetic particle size and the state of region. Because conductive metal oxides like indium
aggregation, it is considered that new applications can tin oxide (ITO) have a plasma frequency in the near
be developed. To explain the specific characteristics infrared region, they are transparent to visible light,
of nanosized magnetic materials, it is necessary to but are opaque to near infrared light. Common metals
carry out further accurate characterization of the sur- with high free electron density have a high plasma
faces of particles and the boundaries between them frequency. Therefore, they are usually opaque to both
and even theoretical developments in this field are near-infrared rays and visible light. On the other hand,
eagerly awaited. insulators such as ceramics with no free electron are
normally transparent to both.
In order for a substance to be transparent to visible
References
light, it is necessary that it absorb, reflect or scatter
[1] S. Kawamura, K. Haneda: J. Jpn. Soc. Powder Powder light only slightly. When light penetrates a substance,
some of the incident light is reflected on its surface;
Metall., 51(9), 703–707 (2004).
the remainder passes into its bulk. Reflection is a
[2] K. Tanaka, M. Katsuta, S. Nakashima and K. Fujita: J.
phenomenon by which photons rebound from the
Jpn. Soc. Powder Powder Metall., 52(4), 221–227
surface of a nanoparticle with basically no wave-
(2004).
length change. Unreflected light is absorbed or scat-
[3] H. Yamamoto, H. Nishio and N. Yoshida: Magn. Soc. tered within a substance. A fraction of the absorbed
Jpn., 29(3), 269–273 (2005). light is converted to heat or is reemitted as light
[4] S. Blugel: Phys. Rev. Lett., 68, 851 (1992). (electromagnetic waves) of a different wavelength
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