Cascade of Phase Transitions in GdFe3(BO3)4

Abstract

Cascade of phase transitions in GdFe3(BO3)4 at 156, 37, and 9 K has been detected by specific heat measurements and further studied by Raman scattering and Nd3+ spectroscopic probe method. A weakly first-order structural phase transition at 156 K is followed by a second-order antiferromagnetic ordering phase transition at 37 K and a first-order spin-reorientational phase transition at 9 K.

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0021-3640/04/7909-$26.00 © 2004 MAIK “Nauka/Interperiodica”0423
JETP Letters, Vol. 79, No. 9, 2004, pp. 423–426. From Pis’ma v Zhurnal Éksperimental’no
Original English Text Copyright © 2004 by Levitin, E. Popova, Chtsherbov, Vasiliev, M. Popova, Chukalina, Klimin, van Loosdrecht, Fausti, Bezmaternykh.
œ
i Teoretichesko
œ
Fiziki, Vol. 79, No. 9, 2004, pp. 531–534.
†
stands for a rare earth or yttrium and M = Al, Ga, Fe, or
Sc, have attracted considerable attention because of
their good luminescent and nonlinear optical properties
combined with excellent physical and chemical charac-
teristics. Crystals of YAl
3
doped with neodymium are used for self-frequency
doubling and self-frequency summing lasers [1–3].
Concentrated NdAl
3
(BO
3
)
4
crystals are efficient media
for minilasers [3].
Rare earth ferroborates are the least studied com-
pounds of the RM
3
(BO
3
)
4
family. At room temperature,
GdFe
3
(BO
3
)
4
crystals have the trigonal structure with
the space group (
R
32 [4]). FeO
together by their edges form spiral chains running
along the
c
axis. Gd ions reside in
tions situated between three such chains and link the
chains together. GdO
6
prisms are isolated from each
other, having no oxygen atoms in common. Each oxy-
gen atom at a vertex of the GdO
a BO
3
triangle. An indication of a structural phase tran-
sition at 174 K has been found recently by specific heat
measurements on a powder sample [5].
Measurements of the magnetic properties of
GdFe
3
(BO
3
)
4
were performed in [5, 6], and an anoma-
lous behavior of magnetization was detected at about
40 and 10 K. In the present communication, we report
on the temperature-dependent specific heat, Raman,
and optical absorption measurements on GdFe
Borates with general formula RM
3
(BO
3
)
4
, where R
(BO
3
)
4
and GdAl
3
(BO
3
)
4
6
octahedra linked
3+
D
3
symmetry posi-
6
prism belongs also to
3
(BO
3
)
4
†
¶
Deceased.
This article was submitted by the authors in English.
D3
7
single crystals, pure or doped with 1 at. % of Nd intro-
duced as a spectroscopic probe.
Crystals of GdFe
3
(BO
3
)
4
were grown using a K
2
Mo
3
O
in [6]. Big transparent single crystals of ferroborates
were green in color and had a good optical quality. Thin
plates 5–10 mm in size with different thickness
(between 2.2 mm and 150
µ
cal measurements. Specific heat in the range 5–300 K
was measured by a “Termis” relaxation-type microcal-
orimeter. Raman measurements were made on a Jobin-
Yvon T64000 spectrometer with nitrogen-cooled CCD
camera in backscattering geometry. The scattering was
excited by the second harmonic of a Nd:YAG laser at
532 nm, with a typical power of 10 mW. The sample
was attached by silver paste to a cold finger of an
Oxford Instruments helium flow cryostat “Microstat.”
Optical absorption spectra in the spectral region 4000–
20000 cm
at a resolution of 0.2 cm
by a Fourier-transform spectrometer BOMEM
DA3.002 with InSb liquid nitrogen cooled detector and
Si detector at sample temperatures between 4.2 and
300 K.
The temperature dependence of the specific heat of
GdFe
3
(BO
3
)
4
is shown in Fig. 1. Three distinct peaks
are seen in this dependence. Two of them, namely, at 9
and 156 K, are very narrow and almost symmetric,
while that at 37 K is much broader and asymmetric.
The (
zx
) polarized Raman spectra of GdFe
are shown in Fig. 2. At about
vibrational modes appear, abruptly manifesting a struc-
tural phase transition into a less symmetric phase. The
and Nd
-based flux, as described
0.01
Cd
0.99
Fe
3
(BO
3
)
4
10
m) were prepared for opti-
–1 –1
were registered
3
(BO
3
)
4
T
c
= 156 K, several new
Cascade of Phase Transitions in GdFe
R. Z. Levitin , E. A. Popova, R. M. Chtsherbov
E. P. Chukalina, S. A. Klimin
D. Fausti
1
Faculty of Physics, Moscow State University, Moscow, 119992 Russia
2
Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow oblast, 142190 Russia
3
Material Science Center, University of Groningen, 9747 AG Groningen, The Netherlands
Kirensky Institute of Physics, Siberian Division, Russian Academy of Sciences, Krasnoyarsk, 660036 Russia
e-mail: popova@isan.troitsk.ru
Received March 29, 2004
3
(BO
, M. N. Popova
3
,
3
)
4¶
†
, 111
, A. N. Vasiliev
, P. H. M. van Loosdrecht
, and L. N. Bezmaternykh
12
,
22, 3
34
4
Cascade of phase transitions in GdFe
ments and further studied by Raman scattering and Nd
structural phase transition at 156 K is followed by a second-order antiferromagnetic ordering phase transition
at 37 K and a first-order spin-reorientational phase transition at 9 K.
PACS numbers: 65.40.Ba; 71.70.Gm; 75.30.Et; 75.40.Cx; 78.30.Hv
3
(BO
3
)
4
at 156, 37, and 9 K has been detected by specific heat measure-
3+
spectroscopic probe method. A weakly first-order
© 2004 MAIK “Nauka/Interperiodica”.

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424
JETP LETTERS

Vol. 79

No. 9

2004
LEVITIN
et al
.
small difference in
Raman measurements is due to a heating of the sample
by light that excites Raman scattering. The inset to Fig.
2 illustrates the hysteretic temperature dependence of
the intensity of the lowest-frequency new mode. All the
modes shift with decreasing temperature, typically 1–2
cm in the range of temperatures between
The lowest-frequency new mode demonstrates an
unusually big shift. Its frequency changes from 26 cm
1
at
T
c
to 55 cm at 2.5 K and has a peculiarity at 37 K
(see Fig. 3).
To further study the observed phase transitions, we
used the Nd ion introduced as a spectroscopic probe.
An energy level of the Nd
tum
J
is split into (
J
+ 1/2) Kramers doublets by a crys-
tal field of any symmetry lower than a cubic one. Nd
substitutes for Gd in the lattice of GdFe
number of lines in the spectra of GdFe
corresponds to only one position for a rare earth, both
above and below the temperature 156 K of the struc-
tural phase transition. For example, for T > 40 K, there
are two spectral lines due to the optical transitions from
the ground state to the crystal-field sublevels of the
level. Figure 4 shows the lowest frequency of these two
lines at different temperatures. The growing splitting of
the line below 37 K is due to the splitting of Nd
ers doublets caused by an internal magnetic field that
appears at the sites of the Nd
ordered state of GdFe
3
(BO
should appear at an optical transition between two split
Kramers doublets, and these are clearly seen at low
temperatures. Two low-frequency ones freeze out with
a further decrease in temperature due to an emptying of
the upper component of the split ground Kramers dou-
blet of Nd. Figure 5 displays the temperature depen-
dence of the line splitting and of the relative intensities
of two high-frequency components of the split spectral
line. Sharp changes of these two quantities are observed
between 9.5 and 8 K.
Each of the three phase transitions observed in
GdFe
3
(BO
3
)
4
is of a different nature. A very strong nar-
row peak in the temperature dependence of specific
heat and an abrupt appearance of new Raman modes
exhibiting hysteretic behavior with a narrow hysteresis
loop and a strong hardening of one mode with a further
decrease in the temperature evidence a weak first-order
structural phase transition at about 156 K.
Specific heat in GdFe
3
probe data in Nd
0.01
Gd
0.99
Fe
order magnetic ordering phase transition at about 37 K.
Through the magnetoelastic interaction, the magnetic
ordering also affects Raman modes and manifests itself
as an aforementioned peculiarity in the shifts of these
modes. Judging from the temperature dependences of
magnetization given in [6], one can state that the anti-
ferromagnetic ordering takes place at 37 K. It seems,
T
c
determined from specific heat and
–1
T
c
and 2.5 K.
–
–1
3+
3+
ion with the total momen-
3+
3+
3
(BO
)
3
)
4
. The
3
(BO
34
:Nd(1%)
4
F
3/2
3+
Kram-
3+
)
4
ions in a magnetically
. Maximum four lines
3
3+
(BO
3(BO3)4 suggest a second-
3
)
4
and Nd
3+
spectral
Fig. 1. Specific heat of GdFe3(BO3)4 vs. temperature.
Fig. 2. Raman spectra of GdFe3(BO3)4 above and below the
temperature of a structural phase transition Tc = 156 K. Inset
shows the temperature dependence of the intensity of the
lowest-frequency new mode when cooling (balls) and heat-
ing (stars).
Fig. 3. Temperature dependence of the frequency of the
lowest new Raman mode that appears in GdFe3(BO3)4
below Tc = 156 K.

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JETP LETTERS Vol. 79 No. 9 2004
CASCADE OF PHASE TRANSITIONS 425
however, that this ordering does not affect the rare earth
subsystem much, which remains paramagnetic down to
lowest temperatures. In particular, this follows from the
fact that the hyperbolic increase of magnetic suscepti-
bility survives below the Neél temperature [6]. The
value of magnetic entropy released at this transition,
= 37 J/(mol K), is close to that estimated for Fe3+
Smagn
(s = 5/2) subsystem ordering only (
Taking into account that the Fe3+ subsystem in
GdFe3(BO3)4 is of a reduced dimensionality, it is possi-
ble to assume that some part of magnetic entropy is
released above the Neél temperature.
= 44 J/(mol K)).
When a magnetic ordering occurs within a d-metal
subsystem, the f-metal subsystem gets polarized due to
the f–d exchange. For a rare earth ion, the exchange
splittings and, hence, the spectral line splittings are
mainly due to the exchange interactions with d ions,
while the line width depends on the dipole–dipole inter-
actions with neighboring f ions [7, 8]. In the case of
Nd3+ probe in Nd0.01Gd0.99Fe3(BO3)4, the splitting and
narrowing of Nd3+ spectral lines manifest a magnetic
ordering of the Fe subsystem and polarization of the Gd
subsystem below 37 K.
At 9 K, a sharp peak in specific heat characteristic of
first-order phase transition is seen. The Nd3+ probe
spectrum changes at this temperature, showing the
superposition of high-temperature and low-temperature
spectra in a narrow range in the vicinity of 9 K. Such
changes are typical for a first-order spin-reorientational
phase transition when two different magnetic phases
coexist in a narrow range of temperatures [8]. The mag-
netic susceptibility data [6] evidence a sharp increase of
the signal at T < 9 K in a magnetic field oriented per-
pendicular to the c axis. At the same time, spin-flop
transitions observed below 9 K in a magnetic field par-
allel to the c axis indicate that the antiferromagnetic
sublattices in GdFe3(BO3)4 are oriented along the c
axis. Therefore, the whole of the experimental data sug-
gest that a spin-reorientational phase transition takes
place at 9 K. Above this temperature, the iron magnetic
moments are oriented perpendicular to the c axis, while
below this temperature, the iron moments are oriented
along the c axis. In accordance with this inference, the
magnetic susceptibility at T > 9 K is almost isotropic,
which is typical for an easy-plane antiferromagnet.
In summary, we have registered by specific heat
measurements three phase transitions (at 156, 37, and
9 K) in GdFe3(BO3)4. We further studied these transi-
tions by Raman and optical absorption measurements.
The absorption spectra of the Nd3+ ion introduced as a
probe into the GdFe3(BO3)4 matrix were registered. The
transition at 37 K was found to be a second-order mag-
netic-ordering phase transition. All the remaining tran-
sitions are first-order ones. At 156 K, the appearance of
new Raman modes manifests a change of the crystal
expt
Smagn
theor
structure to a less symmetric one. The spectra of Nd3+
probe evidence a spin-reorientation at 9 K.
In conclusion, we are grateful to V.I. Marchenko for
useful discussions. We acknowledge the support from
the Russian Foundation for Basic Research (project
nos. 04-02-17346, 02-02-16636, and 03-02-16286), the
Fig. 4. Spectral line of the Nd3+ probe in GdFe3(BO3)4 at
different temperatures.
Fig. 5. Temperature dependences of the line splitting (balls)
and relative intensities of the highest-frequency compo-
nents for the Nd3+ probe in GdFe3(BO3)4.

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426
JETP LETTERS Vol. 79 No. 9 2004
LEVITIN et al.
Russian Ministry of Science and Technology, and the
Russian Academy of Sciences under the Programs for
Basic Research. Part of this work was supported by the
Stichting voor Fundamenteel Onderzoek der Materie
(FOM) and by the Nederlandse Organisatie voor
Wetenschappelijk Onderzoek (NWO).
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