1063-7834/03/4511- $24.00 © 2003 MAIK “Nauka/Interperiodica”
2152
Physics of the Solid State, Vol. 45, No. 11, 2003, pp. 2152–2154. Translated from Fizika Tverdogo Tela, Vol. 45, No. 11, 2003, pp. 2048–2050.
Original Russian Text Copyright © 2003 by Bezlepkin, Kuntsevich.
1. INTRODUCTION
Hexaferrites of the structure type
M
, namely,
Me
Fe
12
O
19
(where
Me
= Ba
2+
, Sr
2+
, or Pb
2+
), contain only
one type of magnetoactive ions, Fe
3+
ions. These ions are
located at lattice sites with octahedral (4
f
2
, 2
a
, 12
k
), tet-
rahedral (4
f
1
), and bipyramidal (2
b
) oxygen environ-
ments [1]. The bipyramidal environment has the form of
a trigonal bipyramid. This environment is not character-
istic of iron oxides and has been the subject of many
investigations. In particular, Townes
et al.
[2] performed
an x-ray diffraction study of the BaFe
12
O
19
hexaferrite
and drew the conclusion that, at room temperature, the
iron ions occupying the 2
b
positions in this structure are
located not in the mirror plane of the bipyramid but are
randomly arranged on each side of this plane at a mean
distance of 0.16 Å. Mössbauer investigations of
BaFe
12
O
19
(BaM) and SrFe
12
O
19
(SrM) hexaferrites [1,
3] revealed strong anisotropy of thermal vibrations of
iron ions in the 2
b
positions at high temperatures. It was
also inferred that, at low temperatures (
T
< 100 K), these
ions occupy two pseudohedral positions inside the bipyr-
amid. The specific dynamics of iron ions in the 2
b
posi-
tions can affect the distribution of the electron and spin
densities and, correspondingly, the local magnetic fields
on
57
Fe nuclei. In our earlier work [4], the BaM hexafer-
rite was studied using the nuclear magnetic resonance
(NMR) method in the temperature range 4.2–77 K. It
was found that a decrease in temperature in this range is
accompanied by an anomalous decrease in the local
magnetic field on iron nuclei in the 2
b
positions. In the
present work, we carried out an NMR investigation of
the local magnetic fields on
57
Fe nuclei of Fe
3+
ions in
bipyramidal (2
b
) and tetrahedral (4
f
1
) positions of the
BaM hexaferrite in the temperature range 4.2–295 K.
Moreover, we analyzed the applicability of the Bloch
spin wave theory to the description of the changes
observed in local magnetic fields in the 4
f
1
and 2
b
posi-
tions with variations in temperature.
2. SAMPLES AND EXPERIMENTAL TECHNIQUE
The experiments were carried out using BaM single
crystals enriched to 95% with a
57
Fe isotope. The single
crystals were grown from a BaO · B
2
O
3
melt according
to the solution-growth technique. The synthesis was
performed through crystallization on a rotating seed in
the temperature range 1420–1300 K. The phase compo-
sition of the crystals grown was checked using x-ray
diffraction. The lattice parameters at room temperature
are
a
= 0.589 nm and
c
= 2.319 nm. The
57
Fe nuclear
magnetic resonance was examined in domain regions
of the Bloch domain walls according to the stationary
technique with the use of an instrument described in
[5]. The rf voltage across the
LC
circuit containing the
studied sample did not exceed 10
–2
V. The frequencies
of NMR signals from domain layers (domain-wall
edges), which were determined by the stationary tech-
nique at a temperature of 77 K, coincided accurate to
within 0.2% with the frequencies of nuclei signals in
the domains, which were determined using the spin-
echo method at a high level of the excitation voltage
(~1.5 kV). The error in determining the frequencies of
NMR signals did not exceed 0.1%.
3. RESULTS AND DISCUSSION
Figure 1 shows the temperature dependences of
local magnetic fields
H
loc
on
57
Fe nuclei of iron ions in
bipyramidal 2
b
and tetrahedral 4
f
1
positions. It can be
seen from Fig. 1 that, for the 2
b
positions, a decrease in
the temperature from 77 to 4.2 K leads to an anomalous
decrease in the local magnetic field
H
loc
. For the 4
f
1
positions, no anomaly is observed and the local field
increases monotonically as the temperature decreases
from 295 to 4.2 K.
As is known, the dominant contribution to the local
magnetic field on nuclei of Fe
3+
ions is made by uncom-
Changes in Local Magnetic Fields on
57
Fe Nuclei in BaFe
12
O
19
with Temperature Variations
A. A. Bezlepkin and S. P. Kuntsevich
Kharkov National University, pl. Svobody 4, Kharkov, 61077 Ukraine
Received April 22, 2003
Abstract
—Changes in local magnetic fields on
57
Fe nuclei of Fe
3+
ions in bipyramidal (2
b
) and tetrahedral (4
f
1
)
positions of the BaFe
12
O
19
hexaferrite with variations in temperature are investigated using nuclear magnetic
resonance in the temperature range 4.2–295 K. The applicability of the Bloch spin wave theory to the descrip-
tion of the thermal behavior of local magnetic fields on
57
Fe nuclei is considered. Low-temperature deviations
from the Bloch law are revealed for bipyramidal positions. The model describing the dynamics of an iron ion
in these positions is refined.
© 2003 MAIK “Nauka/Interperiodica”.
MAGNETISM
AND FERROELECTRICITY