Evidence of ferromagnetically coupled Nd
3¿
ion pairs in weakly doped Nd:LiYF
4
and Nd:YVO
4
crystals as revealed by high-resolution optical and EPR spectroscopies
O. Guillot-Noe
¨
l,* V. Mehta,
†
B. Viana, and D. Gourier
Ecole Nationale Supe ´rieure de Chimie de Paris (ENSCP), Laboratoire de Chimie Applique ´e de l’Etat Solide,
UMR CNRS 7574, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France
M. Boukhris and S. Jandl
Centre de Recherche en Physique du Solide, Departement de Physique, Universite de Sherbrooke, Sherbrooke, Quebec, Canada J1K2R1
Received 22 December 1999
Electron paramagnetic resonance EPR and high-resolution fluorescence spectra of Nd
3+
ions in weakly
doped LiYF
4
and YVO
4
crystals are analyzed. A simple model, based on an effective spin-Hamiltonian
approach is proposed to explain the general features of the EPR and optical neodymium spectra in these
matrices. Pairs of satellite lines whose intensities grow with neodymium content are observed on each side of
the main EPR signals of isolated Nd
3+
ions. These satellites are assigned to Nd
3+
-Nd
3+
ion pairs coupled by
magnetic dipolar interaction. The calculated Nd-Nd distances are found to be in good agreement with the
Y
3+
-Y
3+
distances in LiYF
4
and YVO
4
hosts. The concentration-dependent satellites accompanying the neody-
mium
4
F
3/2
→
4
I
9/2
transition are quantitatively explained as being due to several types of ferromagnetically
coupled pairs of Nd
3+
ions, each ion of a pair being located at the regular Y
3+
site with S
4
and D
2d
point
symmetry in LiYF
4
and YVO
4
, respectively. The exchange coupling values J are found in the range +0.8 to
+4.9 cm
-1
. From literature data, it appears that such ferromagnetically coupled Nd
3+
pairs with J =
+3 cm
-1
also quantitatively explain the optical satellite structure in Nd:Y
3
Al
5
O
12
YAG.
I. INTRODUCTION
Optical transitions of rare-earth ions in condensed matter,
even at very low doping levels, often exhibit a multisite char-
acter which manifests itself as i complex structures in ab-
sorption or emission spectra consisting of more 4 f -4 f tran-
sitions than expected and/or ii inhomogeneously broadened
transitions. In particular, it is the case of trivalent neody-
mium ions in yttrium lithium fluoride (LiYF
4
), yttrium
orthovanadate (YVO
4
), and yttrium aluminate garnet
(Y
3
Al
5
O
12
,YAG), three very important laser crystals.
1–6
LiYF
4
and YVO
4
matrices are investigated in the present
work. LiYF
4
belongs to the scheelite-type structure with
space group I 4
1
/ a ( C
4 h
6
). The Nd
3+
ions substitute for triva-
lent yttrium ions at S
4
point site symmetry. The zircon-type
matrix YVO
4
is also tetragonal space group I 4
1
/ amd and
Nd
3+
ions substitute eightfold coordinated Y
3+
ions, forming
YO
8
bisdisphenoid with D
2 d
point site symmetry. Despite
these relatively simple crystal structures, which allow only
one substitution site for rare-earth ions, a much more com-
plex behavior is observed in optical spectra.
7–11
The optical
transitions of isolated Nd
3+
ions are accompanied by several
‘‘satellites’’ whose intensities grow strongly with neody-
mium content. It is generally recognized that these extra
lines are due to pairs or clusters of ions.
Several experimental works have already been devoted in
the past to Nd
3+
-Nd
3+
pair spectra in different matrices.
12–20
For example, Pelletier-Allard and Pelletier have performed
studies on neodymium satellite structure around the
4
I
9/2
→
4
G
5/2
transition in LaCl
3
using absorption and up-
conversion techniques.
16
Additional studies on Nd
3+
pairs
have been reported in LaF
3
by Buisson and co-workers,
17
in
YAlO
3
by Lupei, Lupei, and Georgescu,
14
and in CaF
2
by
Basiev et al.
18
Energy transfer processes between Nd
3+
ions,
in pairs or in clusters, in CsGd
2
F
7
have been discussed by de
Barros, Barthem, and Khaidukov.
19
The interaction between
pairs of Nd
3+
ions in CsCdBr
3
was measured by Ramaz,
Vial, and Macfarlane using high-resolution spectral hole-
burning spectroscopy.
20
However, up to now it was not pos-
sible to quantify the respective role of exchange interactions
and crystal-field effects in pair spectra, as we have done in
the present work.
In previous studies on Nd:LiYF
4
,
7,8
it was shown that for
weakly doped crystals, Nd
3+
ions are inhomogeneously dis-
tributed in the host and a coupled pair of Nd
3+
ions is well
isolated from other ions or other pairs. The extra optical lines
are assigned to specific types of Nd
3+
pairs.
8
However, the
interaction mechanism between two ions of a pair is still
debated. The same behavior was observed for Nd:Y
3
Al
5
O
12
.
6
For the vanadate host also, the origin of the rich satellite
structure, accompanying the transitions of isolated Nd
3+
ions
in unperturbed sites, is still not clear. In all these matrices,
these additional optical lines could be due to i a fraction of
Nd
3+
ions occupying distorted sites perturbed by neighbor-
ing lattice defects such as oxygen vacancies F-type centers;
ii a fraction of Nd
3+
ions occupying distorted sites per-
turbed by a neighboring Nd
3+
ion, i.e., there is a mutual
crystal-field perturbation between the pair ions; iii ferro- or
antiferromagnetically coupled pairs of Nd
3+
ions in unper-
turbed sites that give rise to exchange or superexchange
splitting of the isolated ions optical transitions; iv a com-
bination of all these mechanisms, where both exchange split-
ting and crystal-field shifts are responsible for the appearance
of the optical satellites.
PHYSICAL REVIEW B 1 JUNE 2000-II VOLUME 61, NUMBER 22
PRB 61 0163-1829/2000/6122/153389/$15.00 15 338 ©2000 The American Physical Society