Electron paramagnetic resonance spectra near the spin-glass transition
in iron oxide nanoparticles
Yu. A. Koksharov,
1
S. P. Gubin,
2
I. D. Kosobudsky,
3
G. Yu. Yurkov,
2
D. A. Pankratov,
4
L. A. Ponomarenko,
1
M. G. Mikheev,
1
M. Beltran,
5
Y. Khodorkovsky,
5
and A. M. Tishin
1
1
Faculty of Physics, M. V. Lomonosov Moscow State University, 119899, Moscow, Russia
2
N. S. Kurnakov Institute of General and Inorganic Chemistry, 117091, Moscow, Russia
3
Saratov State University, Chemical Department, 410026, Saratov, Russia
4
Faculty of Chemistry, M. V. Lomonosov Moscow State University, 119899, Moscow, Russia
5
Beltran, Inc., 1133 East 35th Street, Brooklyn, New York 11210
Received 17 April 2000; published 12 December 2000
Electron paramagnetic resonance EPR in iron-oxide nanoparticles 2.5 nm embedded in a polyethylene
matrix reveals the sharp line broadening and the resonance field shift on sample cooling below T
F
40 K. At
the same temperature a distinct anomaly in the field-cooled magnetization is detected. The temperature depen-
dences of EPR parameters below T
F
are definitely different than those found for various nanoparticles in the
superparamagnetic regime. In contrast to canonical bulk spin glasses, a linear fall-off of the EPR linewidth is
observed. Such behavior can be explained in terms of the random-field model of exchange anisotropy.
DOI: 10.1103/PhysRevB.63.012407 PACS numbers: 75.50.Lk, 75.50.Tt, 76.30.-v
Oxide nanostructured media are considered as very per-
spective materials for high density magnetic recording.
1
This
is the reason for the great interest in the maghemite
( -Fe
2
O
3
) nanoparticles NP.
2
The strong decreasing of the
saturation magnetization in maghemite NP,
3
in comparison
with the bulk counterpart, gave rise to a conception of spin
canting.
4
The principle question is whether only the surface
spins of a particle resist being aligned with even a large
external magnetic field, or if such a property inheres in the
core spins as well.
5
Recent studies of low field zero field
cooled ZFC and field cooled FC magnetization curves of
-Fe
2
O
3
NP evidence the existence of a spin-glass-like sur-
face layer that undergoes a magnetic transition to a frozen
state below 42 K.
6
Analogous spin-glass-like behavior be-
low about 50 K was also found in oxygen passivated iron
NP,
7
as well as in NiFe
2
O
4
NP.
8
It seems natural to study these spin-glass-like phenomena
in NP by an EPR technique, which has been proven to be a
very useful tool for exploring spin dynamics in various fer-
romagnets and antiferromagnets
9,10
and, especially, in spin
glasses,
11
including reentrant alloys.
12,13
Whereas measure-
ments of the magnetic moment provide integral sample char-
acteristics, EPR data give information about local magnetic
properties and, in principle, about the nature of spin-spin
interactions,
13
the distribution of internal fields,
14
and spin-
spin correlations.
15
As a rule, in canonical bulk spin glasses
SG’s the EPR resonance field H
res
and the EPR linewidth
H are roughly temperature independent at high tempera-
tures, but change rapidly H exp(-T/T
g
) if T 2 T
g
,
where T
g
is the spin-freezing temperature. Contrary to usual
magnetic phase transitions, for which the linewidth diverges
at the critical temperature, bulk SG reveal a finite value of
H at T
g
. Due to the very complicated magnetism of spin
glasses, there is no completely adequate theory of the line-
width temperature dependence. The increase in the linewidth
is usually attributed either to a broadening from a distribu-
tion of internal local fields, or to a slowing down of the
spin-relaxation rate on approaching T
g
.
16
Some microscopic features of SG and NP systems are
similar, e.g., the maximum of ac and low-field ZFC suscep-
tibility at a certain temperature T
m
, as well as the irrevers-
ibility splitting between ZFC and FC curves.
17
Spin-glass-
like behavior in the NP systems is usually considered as a
result of the random dipole-dipole interaction between NP at
low enough temperatures, when all the particle moments are
blocked along the anisotropy axes.
17,18
Correlations between
the particle moments develop in a similar way to the corre-
lations between spins in spin glasses. A lot of magnetic NP
systems, similarly to SG, show a broadening and a low-field
shift of EPR lines with a temperature decreasing.
21–27
Nagata
and Ishihara
21
proposed a phenomenological description for
these anomalies in superparamagnetic systems. They derived
a simple power relation between the shift relative to a high
temperature value of the resonance field H
res
and the EPR
linewidth H . For randomly oriented particles it was found
that H
res
( H )
3
. This theory does not take into consider-
ation effects of magnetic transitions in nanoparticle systems
and, probably, it should not hold below T
g
. Some spin-glass
concepts have been used for analysis of low-temperature
anomalies of EPR spectra in NP.
23,25–27
However, qualitative
differences in behavior of EPR spectra in NP and in bulk
spin glasses are still lacking. It should be stressed, that the
present work is devoted to the study of the intrinsic spin-
glass state which takes place inside an individual particle,
resulting from interactions between spins, which form its in-
ternal magnetic structure.
We report the results of static magnetization and EPR
measurements on iron-oxide NP embedded in a polyethylene
matrix. The samples were prepared by the high-speed ther-
mal decomposition of an iron-containing compound in a
solution/melt of polyethylene in vaseline oil in an inert at-
mosphere at 220 °C. This method allows for the fabrication
of particles with bimodal lognormal diameter distribution
F ( D ) and effective sizes below 10 nm.
27
For the samples
studied, our small-angle x-ray diffraction measurements
PHYSICAL REVIEW B, VOLUME 63, 012407
0163-1829/2000/631/0124074/$15.00 ©2000 The American Physical Society 63 012407-1