Coherent magnetic structures in terbium/holmium superlattices
C. Bryn-Jacobsen and R. A. Cowley
Oxford Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom
D. F. McMorrow
Department of Solid State Physics, Riso ” National Laboratory, DK-4000 Roskilde, Denmark
J. P. Goff, R. C. C. Ward, and M. R. Wells
Oxford Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom
Received 23 December 1996
Neutron-scattering techniques have been used to investigate the magnetic properties of three Tb/Ho super-
lattices grown by molecular-beam epitaxy. It is revealed that for temperatures in the range T =10 to
T
N
(Ho) 130 K, there is a basal-plane ferromagnetic alignment of Tb moments within Tb blocks that is
coherent with a basal-plane helical ordering of Ho moments. Between T T
N
Ho and 200 K, the Tb moments
remain ferromagnetically aligned within Tb blocks, with adjacent Tb blocks antiferromagnetically coupled. As
the temperature is raised from T 200 to 230 K, two samples retain this magnetic structure while the third
undergoes a transition first to a mixed phase of helically and ferromagnetically ordered Tb moments, then to a
phase with only helically ordered Tb moments. In all cases, the magnetic ordering is found to be long ranged,
with coherence lengths extending over three to six bilayers. The results are discussed with a consideration of
previous rare-earth superlattice studies, and the possible mechanisms for interlayer coupling.
S0163-18299701421-5
I. INTRODUCTION
Early investigations of the magnetic properties of rare-
earth superlattices concentrated on magnetic/nonmagnetic
combinations. The long-range coherent ordering of elements
such as Gd Ref. 1 and Dy Ref. 2 over nonmagnetic inter-
layers encouraged the development of models for the inter-
layer coupling mechanisms. These were originally based
upon a conventional Ruderman-Kittel-Kasuya-Yosida
RKKY interaction, in which the localized 4 f moments in
the magnetic blocks spin polarize the conduction electrons in
adjacent nonmagnetic blocks. The peak in the conduction-
electron susceptibility ( q) for the nonmagnetic layers was
considered to determine the form of the induced moments,
with the spin-density wave in the nonmagnetic region medi-
ating the coupling required for coherent magnetic ordering. It
was suggested that this model was able to describe the mag-
netic properties of superlattices such as Gd/Y Ref. 3 and
Dy/Y.
3
For some superlattice combinations it has also been
necessary to consider the influence of dipolar interactions.
These interactions were reported to have been particularly
important in Dy/Lu Ref. 4 superlattices, and to affect the
magnetic ordering in for example, Ho/Lu Ref. 5 and
Ho/Sc.
6
The studies of rare-earth superlattices have since been ex-
tended to include magnetic/magnetic combinations. When
both superlattice constituents are magnetically ordered,
nearest-neighbor exchange interactions at the interfaces are
an additional factor that may influence the magnetic order-
ing, and extensive modeling was used to illustrate their im-
portance in some Gd/Dy Ref. 7 samples. The results from
these more complex systems have sustained an interest in the
possible mechanisms for interlayer coupling. For example,
an approach has been proposed recently that suggests in
some cases long-range coherence is achieved by the estab-
lishment of a coherent band structure for the whole
superlattice.
8,9
A primary necessary condition for the estab-
lishment of this overall band structure is believed to be the
matching at the interfaces of the wave functions associated
with the magnetic structures of the constituent elements.
These ideas have been used to explain for instance, the lack
of ordering of the c -axis components of Er moments over Ho
blocks in Ho/Er,
8
and the suppression of the ferromagnetic
phase of Dy in Dy/Ho Ref. 9 superlattices.
Despite continuing activity in the field,
10
there are still
relatively few reports on magnetic/magnetic rare-earth com-
binations. Here we present an investigation of the magnetic
properties of Tb/Ho superlattices. The bulk structural char-
acteristics of Tb and Ho are similar, with both elements
adopting the hexagonal close-packed hcp crystal structure,
with a mismatch in lattice parameters of less than 1.5%.
However, the magnetic properties differ considerably both in
the temperature range of the magnetic ordering, and in the
details of the magnetic structures themselves. Two of the
earliest reports of the magnetic properties of Tb are given by
Koehler et al.,
11
and Dietrich and Als-Nielsen;
12
with a more
recent study undertaken by Gehring et al.
13
It was found that
at T
N
226 K,
12
the Tb 4 f
8
moments align ferromagnetically
in the basal plane, with an orientation that rotates between
successive planes along the c axis. The wave vector q asso-
ciated with this helical structure reduces on cooling from
q=0.115 c* at T
N
to a minimum of q=0.091 c* at
T
C
216 K. At this temperature, there is a first-order transi-
tion to a phase where the Tb moments are ferromagnetically
aligned in the basal plane. However, Dietrich and
Als-Nielsen
12
detected some helical ordering of Tb moments
PHYSICAL REVIEW B 1 JUNE 1997-I VOLUME 55, NUMBER 21
55 0163-1829/97/5521/1436010/$10.00 14 360 © 1997 The American Physical Society