Electronic phase separation in
cobaltate perovskites
Z. Németh, Z. Klencsár, Z. Homonnay,
E. Kuzmann, A. Vértes
Institute of Chemistry, Eötvös Loránd University,
Budapest 1518 P.O.Box 32, Hungary
Research Group of Chemical Research Center Hungarian Academy of Sciences
at ELTE, Budapest 1518 P.O.Box 32, Hungary
E-mail: hentes@chem.elte.hu
The ongoing interest in doped transition metal oxides is due to their complex electronic
and magnetic structures which result in a series of intriguing phenomena such as
high-temperature superconductivity and colossal magnetoresistance. One family of
the promising magnetoresistive materials are LaCoO3 based perovskites doped with
divalent ions (e.g. Ca, Sr) at the rare-earth site, the latter resulting electron holes in
the transition metal network (i.e. converting some Co3+ ions into Co4+) as well as
oxygen vacancies. Consequently, the conductivity of Sr doped LaCoO3 increases
with strontium concentration.
The classification of the magnetic states of La1-xSrxCoO3 perovskites is even more
problematic, because magnetic phase separation occurs from very low Sr
concentrations onwards. Following the observation of the coexistence of
ferromagnetic-like and spin glass properties in these perovskites, it was suggested
that the magnetic behavior of La1-xSrxCoO3 perovskites can be described as “glassy
ferromagnetism”. That is, at low Sr doping levels only short range magnetic
correlation is formed below the magnetic transition temperature, with a
characteristic coherence length of a few nanometers. At a lower temperature the
magnetization freezes out to some locally preferred direction, and the material
enters a glassy phase. The proportion of magnetic clusters as well as the magnetic
coherence length increases with increasing Sr doping level, and at about x = 0.18
the clusters coalesce, giving rise to a metallic and unconventional ferromagnetic
state, which can be described as the coexistence of coalesced, long range ordered
(with several hundred nm long magnetic coherence) magnetic clusters and isolated
magnetically disordered regions. The ferromagnetic and conductive nature of the
clusters is related to the double exchange process between intermediate-spin
trivalent and low-spin tetravalent cobalt ions.
Figure taken from J. Wu and C. Leighton PRB 67 174408 (2003)
Magnetic phase diagram of La1-xSrxCoO3 perovskites
SG: spin glass, PS: paramagnetic semiconductor,
FM: ferromagnetic metal, PM: paramagnetic metal
magnetoresistance
(MR)
of
La1-xSrxCoO3 depends strongly on the Sr
doping level. While for x ≥ 0.2 a dominant
MR peak was observed around the Curie
temperature, for x ≤ 0.18 the MR was
found to increase with decreasing
temperature. For intermediate doping rates
(0.18 ≤ x ≤ 0.2) the combination of the two
types of magnetoresistance was found.
Doping La1-xSrxCoO3 perovskites at the
transition metal site with iron ions
influences magnetoresistance, as well. The
alteration of the MR properties due to the
insertion of iron can be interpreted either
by the modification of the magnetic field
induced low-spin to high-spin transition of
Co3+ ions, or by the changes in size
distribution of the ferromagnetic clusters.
Apart from the x and y concentrations of doping
ions, the magnetic and electronic properties
of La1-xSrxFeyCo1-yO3- are also expected to
depend sensitively on oxygen deficiency.
As the procedure of preparation as well as
the type(s) and concentration(s) of applied
dopant ions may well have an influence on
the value of , the elucidation of the effect
of oxygen deficiency on the electronic and
magnetic structure of La1-xSrxCoO3-
perovskites is also highly desirable.
Figure taken from J. Wu and C. Leighton PRB 67 174408 (2003)
The
Effect of Sr on Mössbauer spectra of La1-xSrx57Fe0.025Co0.975O3- at 80 K
1.000
relative transmission
1.000
0.980
0.996
0.960
x = 0.18
x = 0.25
0.992
1.000
1.000
0.990
0.960
0.980
x = 0.20
-10 -8 -6 -4 -2 0
2
v / mm s
4
-1
6
8 10
x = 0.15
-10 -8 -6 -4 -2 0
2
v / mm s
4
6
8 10
-1
left spectra (more Sr): FM + PM ↔ right spectra: FM with relaxation (SPM) + PM
Effect of oxygen vacancy and Fe doping
on Mössbauer spectra of La0.8Sr0.2FeyCo1-yO3- at 80 K
1.00
relative transmission
1.000
relative transmission
0.995
a
1.000
0.995
0.98
d
0.96
1.00
0.98
e
b
-10 -8
1.00
-6
-4
-2
0
2
4
6
8
10
-1
v / mm s
0.99
0.98
c
-10 -8
-6
-4
-2
0
2
4
-1
v / mm s
6
8
10
a: y = 0
b: y = 0 with extra oxygen vacancies
c: y = 0.025
d: y = 0.05
e: y = 0.3
(Spectra a and b were measured via
57
Co emission Mössbauer technique)
FM component decreases and relaxation increases with increasing  and y
Effect of T on Mössbauer spectra of La0.8Sr0.257Fe0.05Co0.95O3- at 80 K
1.00
1.00
0.95
0.90
0.99
0.85
67 K
300 K
1.000
0.95
0.995
0.90
125 K
0.85
57 K
0.990
1.00
1.00
0.98
0.96
0.99
0.94
100 K
0.92
40 K
1.00
1.00
0.98
0.99
77 K
0.96
-8
0.98
relative transmission
relative transmission
1.00
-6
-4
-2
0
2
-1
v / mm s
4
6
4.2 K
-8 -6 -4 -2
0
2
4
6
8
0.98
10
-1
v / mm s
below Tb (see next slide): FM with strong relaxation (SPM) + PM
Magnetic phase diagram of La0.8Sr0.257FeyCo1-yO3-
Tb
Tf
200
PM
T/K
150
100
PM + FM
50
SG
SCG
0
0.00
0.05
0.10
0.15
0.20
y
SCG: spin cluster glass, Tb: onset temp. of FM clusters,Tf: freezing temp.
Isomer shifts vs. T of La0.8Sr0.257FeyCo1-yO3-.
Summary
• Mössbauer spectra were found to be very sensitive to the
Sr and Fe doping driven magnetic transitions.
• With the help of Mössbauer spectroscopy the magnetic
phase diagram for iron doping could be drawn.
• Besides the well-known magnetic phase separation,
isomer shifts of Mössbauer spectra proved electronic phase
separation, as well, a feature only predicted so far.
Németh Z, Klencsár Z, Kuzmann E, Homonnay Z, Vértes A, Greneche JM, Lackner B, Kellner K, Gritzner G, Hakl J, Vad K, Mészáros S,
Kerekes L: The effect of iron doping in La0.8Sr0.2Fe0.05Co0.95O3−δ perovskite, European Physical Journal B 43: 297-303 (2005)
Németh Z, Homonnay Z, Árva F, Klencsár Z, Kuzmann E, Hakl J, Vad K, Mészáros S, Kellner K, Gritzner G, Vértes A: Mössbauer and
magnetic studies of La0.8Sr0.2CoO3- CMR perovskite, Journal of Radioanalytical and Nuclear Chemistry 271(1): 11–17 (2007)
Németh Z, Homonnay Z, Árva F, Klencsár Z, Kuzmann E, Vértes A, Hakl J, Mészáros S, Vad K, de Châtel PF, Gritzner G, Aoki Y, Konno
H, Greneche JM: Response of La0.8Sr0.2CoO3- to perturbations on the CoO3 sublattice, European Physical Journal B 57: 257–263 (2007)
Klencsár Z, Németh Z, Kuzmann E, Homonnay Z, Vértes A, Hakl J, Vad K, Mészáros S, Simopoulos A, Devlin E, Kallias G, Greneche
JM, Cziráki Á, De SK: The role of iron in the formation of the magnetic structure and related properties of La0.8Sr0.2FexCo1-xO3- (x =
0.15, 0.2, 0.3), Journal of Magnetism and Magnetic Materials 320: 651-661 (2008)