Ferrimagnetic nanoparticles of CoFe2O4 with dimensions of 4-16 nm were synthesized by pyrolysis of a
mixture of acetylacetonates of iron and cobalt. In the temperature range 300-500 K investigated field dependence
of magnetization up o 18 kOe. Found a significant contribution of "surface" anisotropy to the effective
anisotropy of the nanoparticles.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3536

Ishchenko, A.

Kryshtal, A.P.

Mozul, K.

Olkhovik, L.P.

Sizova, Z.I.

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 1 No 4, 04MFPN14(2pp) (2012)   2304-1862/2012/1(4)04MFPN14(2) 04MFPN14-1  2012 Sumy State University Magnetic Anisotropy of Ultra-small Nanocrystals of CoFe2O4  A.V. Ishchenko, A.P. Kryshtal’, K.A. Mozul’, L.P. Ol'khovik, Z.I. Sizova  V.N. Karazin Kharkiv National University, Svobody Sq. 4, 61022, Kharkiv. Ukraine  (Received 04 July 2012; published online 06 August 2012)  Ferrimagnetic nanoparticles of CoFe2O4 with dimensions of 4-16 nm were synthesized by pyrolysis of a mixture of acetylacetonates of iron and cobalt. In the temperature range 300-500 K investigated field de-pendence of magnetization up o 18 kOe. Found a significant contribution of "surface" anisotropy to the ef-fective anisotropy of the nanoparticles.  Keywords: Ferrimagnetic, Nanoparticles, Effective anisotropy, Cobalt spinel.   PACS numbers: 75.75. – c, 75.30.Gw, 75.50.Gg   1. INTRODUCTION  To study the magnetic anisotropy field various methods, static and resonance, are used. Analysis of the available published data on the problem of the ef-fective magnetic anisotropy demonstrated the ambigui-ty of the results obtained by different methods of meas-urements on powder samples. In the present work to determine the effective magnetic anisotropy field (Haef) two well-known method based on the analysis of mag-netization curves (method of Asti-Rinaldi [1] and the Akulov method [2]) are used, and the assess by coerci-tive force were made.  2. ANISOTROPY FIELD OF COBALT FERRITE  The object of the study was a CoFe2O4 ferrite. The se-lecting of this object first of all caused by that the cobalt ferrite has the highest magnetocrystalline anisotropy due to the large single-ion contribution of Co2+ ions, which are localized in the octahedral sites of the crystal lattice of ferrite [2]. In addition, in the temperature range (300-550) K anomalous sharp decrease of magne-tocrystalline anisotropy field from 20 kOe to near zero is observed. It is interesting to investigate the effective magnetic anisotropy field, where, along with the contri-bution of the magnetocrystalline anisotropy specific for small particles of the other two contributions - the "sur-face" anisotropy (Hasurf) and the anisotropy of particle shape (Hadem) can play an important role:   ef surf dema a aakH H H H  (1)  Nano-dispersed powder sample was obtained by py-rolysis of a mixture of acetylacetonates of iron and co-balt [3]. The parameters of the crystal structure, phase composition and dispersion of the synthesized samples were investigated using X-ray analysis and the method of transmission electron microscopy (Fig. 1). Investigation of the basic magnetization curves and hysteresis loops was carried out on laboratory-type pendulum magnetometer (H = 0.1-18 kOe, T = 300-500K). At each fixed temperature the measurements were made on thermally demagnetized samples. For all magnetization curves in the range 300 - 500 K two effects that are characteristic for small particles were observed. It's – high-field unsaturation and un-derestimate (compared with the macroscopic analogue)   Fig. 1 – Electron micrograph and distribution by particle diam-eter of CoFe2O4 ferrite  values of the magnetization in fields that are comparable or even much higher than the magnetocrystalline anisot-ropy field (Fig. 2). The table shows the experimental values of the effec-tive magnetic anisotropy field.  Table 1 – Experimental results   Т, К method of determination Haef, kOe by Нс Asti-Rinaldi method Akulov meth-od 300 4,0 3,2 4,4 350 3,3 3,5 3,6 400 2,5 1,5 2,9 450 1,7 1,0 1,7 500 1,2 1,3 1,6  As the table shows, the data obtained by different methods correspond within acceptable limits. In contrast to the macroscopic analogue (Hak) the dependence of Haef (T) in the temperature range (300-375) K does not detect anomalous sharp decrease (Fig. 3).  A.V. ISHCHENKO, A.P. KRYSHTAL’, K.A. MOZUL’, L.P. OL'KHOVIK, Z.I. SIZOVA PROC. NAP 1, 04MFPN14 (2012)   04MFPN14-2   Fig. 2 – The dependence of specific magnetization of CoFe2O4 nanopowder from magnetic field in comparison with the mac-roscopic analogue    Fig. 3 – Dependence of the effective anisotropy field of CoFe2O4 nanocrystalline powder of the temperature in com-parison with macroscopic analogue.   3. CONCLUSIONS  Considering negligibly small anisotropic contribu-tion Hadem for the octahedral form particles it is pos-sible to make the following conclusion. From two speci-fied in the formula (1) specific for small particles aniso-tropic contributions, the determining role in the for-mation of the temperature dependence of Haef (300-375) K plays structurally defective near-surface region of the nanoparticles. The same area which has "skewed" magnetic structure [5], is responsible for the lower value of magnetization in the field H = Hak and high-field unsaturation of main magnetization curves.   REFERENCES  1. G. Asti, S. Rinaldi. J. Appl. Rhys. 45, 8 (1974). 2. Tikadzumi S. Physics of ferromagnetism. Magnetic charac-teristics and practical applications. (Moscow: Mir: 1987). 3. Sh. Sun, H. Zeng, D. B. Robinson J. Am. Chem. Sos. 126, 273 (2004). 4. J. Smith, H. Wayne, Ferrites (Moscow: IL:1962). 5. A. H. Morrish, K Haneda. J. Appl. Phys. 63, 4258 (1988).  

Magnetic Anisotropy of Ultra-small Nanocrystals of CoFe2O4

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 1 No 4, 04MFPN14(2pp) (2012) 
 
 
2304-1862/2012/1(4)04MFPN14(2) 04MFPN14-1  2012 Sumy State University 
Magnetic Anisotropy of Ultra-small Nanocrystals of CoFe2O4 
 
A.V. Ishchenko, A.P. Kryshtal’, K.A. Mozul’, L.P. Ol'khovik, Z.I. Sizova 
 
V.N. Karazin Kharkiv National University, Svobody Sq. 4, 61022, Kharkiv. Ukraine 
 
(Received 04 July 2012; published online 06 August 2012) 
 
Ferrimagnetic nanoparticles of CoFe2O4 with dimensions of 4-16 nm were synthesized by pyrolysis of a 
mixture of acetylacetonates of iron and cobalt. In the temperature range 300-500 K investigated field de-
pendence of magnetization up o 18 kOe. Found a significant contribution of "surface" anisotropy to the ef-
fective anisotropy of the nanoparticles. 
 
Keywords: Ferrimagnetic, Nanoparticles, Effective anisotropy, Cobalt spinel. 
 
 PACS numbers: 75.75. – c, 75.30.Gw, 75.50.Gg 
 
 
1. INTRODUCTION 
 
To study the magnetic anisotropy field various 
methods, static and resonance, are used. Analysis of 
the available published data on the problem of the ef-
fective magnetic anisotropy demonstrated the ambigui-
ty of the results obtained by different methods of meas-
urements on powder samples. In the present work to 
determine the effective magnetic anisotropy field (Haef) 
two well-known method based on the analysis of mag-
netization curves (method of Asti-Rinaldi [1] and the 
Akulov method [2]) are used, and the assess by coerci-
tive force were made. 
 
2. ANISOTROPY FIELD OF COBALT FERRITE 
 
The object of the study was a CoFe2O4 ferrite. The se-
lecting of this object first of all caused by that the cobalt 
ferrite has the highest magnetocrystalline anisotropy 
due to the large single-ion contribution of Co2+ ions, 
which are localized in the octahedral sites of the crystal 
lattice of ferrite [2]. In addition, in the temperature 
range (300-550) K anomalous sharp decrease of magne-
tocrystalline anisotropy field from 20 kOe to near zero is 
observed. It is interesting to investigate the effective 
magnetic anisotropy field, where, along with the contri-
bution of the magnetocrystalline anisotropy specific for 
small particles of the other two contributions - the "sur-
face" anisotropy (Hasurf) and the anisotropy of particle 
shape (Hadem) can play an important role: 
 
 ef surf dema a aakH H H H  (1) 
 
Nano-dispersed powder sample was obtained by py-
rolysis of a mixture of acetylacetonates of iron and co-
balt [3]. The parameters of the crystal structure, phase 
composition and dispersion of the synthesized samples 
were investigated using X-ray analysis and the method 
of transmission electron microscopy (Fig. 1). 
Investigation of the basic magnetization curves and 
hysteresis loops was carried out on laboratory-type 
pendulum magnetometer (H = 0.1-18 kOe, T = 300-
500K). At each fixed temperature the measurements 
were made on thermally demagnetized samples. 
For all magnetization curves in the range 300 - 500 
K two effects that are characteristic for small particles 
were observed. It's – high-field unsaturation and un-
derestimate (compared with the macroscopic analogue) 
 
 
Fig. 1 – Electron micrograph and distribution by particle diam-
eter of CoFe2O4 ferrite 
 
values of the magnetization in fields that are comparable 
or even much higher than the magnetocrystalline anisot-
ropy field (Fig. 2). 
The table shows the experimental values of the effec-
tive magnetic anisotropy field. 
 
Table 1 – Experimental results  
 
Т, К 
method of determination 
Haef, kOe 
by Нс 
Asti-Rinaldi 
method 
Akulov meth-
od 
300 4,0 3,2 4,4 
350 3,3 3,5 3,6 
400 2,5 1,5 2,9 
450 1,7 1,0 1,7 
500 1,2 1,3 1,6 
 
As the table shows, the data obtained by different 
methods correspond within acceptable limits. In contrast 
to the macroscopic analogue (Hak) the dependence of Haef 
(T) in the temperature range (300-375) K does not detect 
anomalous sharp decrease (Fig. 3). 
 A.V. ISHCHENKO, A.P. KRYSHTAL’, K.A. MOZUL’, L.P. OL'KHOVIK, Z.I. SIZOVA PROC. NAP 1, 04MFPN14 (2012) 
 
 
04MFPN14-2 
 
 
Fig. 2 – The dependence of specific magnetization of CoFe2O4 
nanopowder from magnetic field in comparison with the mac-
roscopic analogue 
 
 
 
Fig. 3 – Dependence of the effective anisotropy field of 
CoFe2O4 nanocrystalline powder of the temperature in com-
parison with macroscopic analogue. 
 
 
3. CONCLUSIONS 
 
Considering negligibly small anisotropic contribu-
tion Hadem for the octahedral form particles it is pos-
sible to make the following conclusion. From two speci-
fied in the formula (1) specific for small particles aniso-
tropic contributions, the determining role in the for-
mation of the temperature dependence of Haef (300-
375) K plays structurally defective near-surface region 
of the nanoparticles. The same area which has 
"skewed" magnetic structure [5], is responsible for the 
lower value of magnetization in the field H = Hak and 
high-field unsaturation of main magnetization curves. 
 
 
REFERENCES 
 
1. G. Asti, S. Rinaldi. J. Appl. Rhys. 45, 8 (1974). 
2. Tikadzumi S. Physics of ferromagnetism. Magnetic charac-
teristics and practical applications. (Moscow: Mir: 1987). 
3. Sh. Sun, H. Zeng, D. B. Robinson J. Am. Chem. Sos. 126, 
273 (2004). 
4. J. Smith, H. Wayne, Ferrites (Moscow: IL:1962). 
5. A. H. Morrish, K Haneda. J. Appl. Phys. 63, 4258 (1988). 
 


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Magnetic Anisotropy of Ultra-small Nanocrystals of CoFe2O4

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