Grain-size-dependent magnetic properties of nanocrystalline Gd
R. Kruk,
1,2
M. Ghafari,
3
H. Hahn,
1,3
D. Michels,
4
R. Birringer,
4
C. E. Krill III,
5
R. Kmiec,
2
and M. Marszalek
2
1
Institute of Nanotechnology, Forschungszentrum Karlsruhe, D-76021 Karslruhe, Germany
2
Institute of Nuclear Physics PAN, Radzikowskiego 152, 31-342 Krakow, Poland
3
Joint Research Laboratory Nanomaterials Forschungszentrum Karlsruhe/Technische Universität Darmstadt,
Institute of Materials Science, D-64287 Darmstadt, Germany
4
FR. 7.3 Technical Physics, Universität des Saarlandes, Postfach 151150, Geb. 43, D-66041 Saarbrücken, Germany
5
Materials Division, Universität Ulm, Albert-Einstein-Allee 47, D-89081 Ulm, Germany
Received 28 October 2005; revised manuscript received 3 January 2006; published 13 February 2006
Microscopic magnetic and electronic properties of nanocrystalline Gd were studied by
155
Gd Mössbauer
spectroscopy. This technique made it possible to distinguish the microstructure-dependent properties of Gd
located in nanocrystal interiors from the properties of Gd in the grain boundaries. For the grain interiors a
correlation between the induced magnetic anisotropy and the grain size was observed; this anisotropy can be
attributed to the internal pressure resulting from the interface stress of the grain boundaries. The magnetic and
electronic structure of the atoms in the grain boundaries differs distinctively from that in the grain interiors: the
Gd magnetic moments at the grain boundaries are randomly oriented with respect to the local crystallographic
axes, and the density of conduction s electrons is reduced, perhaps as a result of a lower number of Gd nearest
neighbors.
DOI: 10.1103/PhysRevB.73.054420 PACS numbers: 75.50.Tt, 75.75.a, 76.80.y
I. INTRODUCTION
Bulk hexagonal Gd has been studied extensively in past
decades and still attracts considerable experimental and the-
oretical attention, particularly concerning the electronic
structure and fnite-temperature magnetic properties.
1–5
Ex-
perimentally, many of the ground-state properties of elemen-
tal Gd are well understood.
6–11
It crystallizes in the hcp struc-
ture with a lattice constant a = 0.3629 nm and c / a ratio
= 1.597. Magnetic properties originate almost entirely from
the half filled 4 f shell L =0, J =S=7/2, which contributes
strictly localized magnetic moments. The zero-temperature
moment T =0 = 7.63
B
indicates that an induced polar-
ization of the conduction bands of at least 0.63
B
arises from
an interband exchange coupling Ruderman-Kittel-Kasuya-
Yosida type between itinerant 5d /6s conduction-band elec-
trons and the localized 4 f electrons.
12,13
Gd is a ferromagnet
with a Curie temperature T
C
=293 K. Magnetization
measurements,
6
neutron diffraction,
9
and crystalline
anisotropy
14
show that the angle between the c axis and the
easy axis of magnetization increases from around 32° at
10 K to around 65° at 183 K and drops abruptly to zero at
T 232 K. Using first-principles theory
15
it has recently
been shown that the magnetic anisotropy energy MAE of
Gd metal originates from classical dipole-dipole interactions
between the large 4 f spins 7 eV/atom and from the
MAE of the conduction electrons 16 eV/atom. The latter
contribution results from the spin-orbit splitting of the con-
duction electrons and is transferred to the 4 f spin. The direc-
tion of the magnetic moment calculated for low temperature
lies at an angle of 20° to the c axis, in good agreement with
the experimental results.
Recent developments in the fields of magnetism and mag-
netic materials
16,17
have focused increasingly on nanostruc-
tures as a particularly interesting class of materials for both
scientific and technological exploration. Current research is
directed towards studying and understanding the influence of
spatial confinement and order, topological arrangement, as
well as the proximity of magnetic nanoscale building blocks
nanocrystals, nanorods, chains of nanocrystals, layers hav-
ing a thickness of a few nm, etc. on fundamental and ap-
plied magnetism. A key issue of fundamental research is de-
voted to understanding the effect of imperfections defects
and structural disorder—which is present in any real nano-
structured material—on extrinsic magnetic properties. So far,
the overwhelming number of investigations of size-reduced
systems with rare earth elements have been carried out on
thin films and multilayer systems
18–23
or on clusters contain-
ing up to a few tens of atoms.
24,25
Studies of isolated or
embedded nanocrystals or single or multicomponent bulk
nanocrystalline materials are rare.
26–29
Nanocrystalline nc
materials are thermodynamically unstable against grain
growth, which takes place at elevated temperatures.
30
Con-
sequently, annealing of as-prepared kinetically frozen nc
specimens results in the evolution of microstructure toward
the coarse-grained conventional polycrystalline state of a
given material, which may in turn serve as a reference state.
This latter feature enables nc materials to be model systems
for the study of the influence of reduction in the structural
correlation length grain size and the concomitant buildup
of internal interface concentration on magnetic properties.
Nanocrystalline Gd can be considered to be a polycrystal
with randomly oriented nanometer-sized grains the crystal-
line phase embedded in a network of grain boundaries
GBs. It has recently been shown that in nanocrystalline Gd
the fraction of GB atoms is sufficiently large to affect inter-
nal magnetic properties.
27
Therefore it is the aim of this
study to investigate the effect of nanoscale polycrystallinity,
as represented in nanocrystalline Gd, on the extrinsic mag-
netic and electronic properties of Gd. Mössbauer spectros-
copy using the
155
Gd isotope provides a unique local probe
PHYSICAL REVIEW B 73, 054420 2006
1098-0121/2006/735/0544206/$23.00 ©2006 The American Physical Society 054420-1