Antiferromagnet–ferromagnet transition in the cobaltites
The three series oxygen-deficient cobaltites La₀.₅Ba₀.₅CoO₃₋δ, LnBaCo₂O₅.₅ and Sr₂YCo₄O₁₀.₅ have been studied. It has been shown that La₀.₅Ba₀.₅CoO₃ is an insulating ferromagnet whereas La₀.₅Ba₀.₅CoO₂.₇₅ is a pure antiferromagnet in which the oxygen vacancies are disordered. The oxygen-vacancies o...
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Troyanchuk, I.O. Bushinsky, M.V. Karpinsky, D.V. Sirenko, V.A. 2017-05-21T17:19:48Z 2017-05-21T17:19:48Z 2012 Antiferromagnet–ferromagnet transition in the cobaltites / I.O. Troyanchuk, M.V. Bushinsky, D.V. Karpinsky // Физика низких температур. — 2012. — Т. 38, № 7. — С. 833-841. — Бібліогр.: 53 назв. — англ. 0132-6414 PACS: 75.50.Dd, 75.30.–m, 75.60.Ej https://nasplib.isofts.kiev.ua/handle/123456789/117271 The three series oxygen-deficient cobaltites La₀.₅Ba₀.₅CoO₃₋δ, LnBaCo₂O₅.₅ and Sr₂YCo₄O₁₀.₅ have been studied. It has been shown that La₀.₅Ba₀.₅CoO₃ is an insulating ferromagnet whereas La₀.₅Ba₀.₅CoO₂.₇₅ is a pure antiferromagnet in which the oxygen vacancies are disordered. The oxygen-vacancies ordering leads to appearance of the ferromagnetic component apparently due to a formation of the noncollinear magnetic structure. The antiferromagnet–“ferromagnet” transition is accompanied by a giant magnetoresistance. It is suggested that in the ferromagnetic oxidized compounds Co³⁺ and Co⁴⁺ ions adopt intermediate spin state whereas for antiferromagnetic (Co⁴⁺-free) compositions Co³⁺ ions have high-spin state (pyramids CoO₅) and dominant low-spin state (octahedra CoO₆). In both ferromagnetic and antiferromagnetic compounds the superexchange via oxygen plays an essential role in a formation of the magnetic properties. The authors would like to acknowledge the financial support of the BRFFI (Grant Nos. T11D-003). en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Магнетизм Antiferromagnet–ferromagnet transition in the cobaltites Article published earlier |
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Antiferromagnet–ferromagnet transition in the cobaltites |
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Antiferromagnet–ferromagnet transition in the cobaltites Troyanchuk, I.O. Bushinsky, M.V. Karpinsky, D.V. Sirenko, V.A. Магнетизм |
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Antiferromagnet–ferromagnet transition in the cobaltites |
| title_full |
Antiferromagnet–ferromagnet transition in the cobaltites |
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Antiferromagnet–ferromagnet transition in the cobaltites |
| title_full_unstemmed |
Antiferromagnet–ferromagnet transition in the cobaltites |
| title_sort |
antiferromagnet–ferromagnet transition in the cobaltites |
| author |
Troyanchuk, I.O. Bushinsky, M.V. Karpinsky, D.V. Sirenko, V.A. |
| author_facet |
Troyanchuk, I.O. Bushinsky, M.V. Karpinsky, D.V. Sirenko, V.A. |
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Магнетизм |
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Магнетизм |
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2012 |
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English |
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Физика низких температур |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Article |
| description |
The three series oxygen-deficient cobaltites La₀.₅Ba₀.₅CoO₃₋δ, LnBaCo₂O₅.₅ and Sr₂YCo₄O₁₀.₅ have been
studied. It has been shown that La₀.₅Ba₀.₅CoO₃ is an insulating ferromagnet whereas La₀.₅Ba₀.₅CoO₂.₇₅ is a pure
antiferromagnet in which the oxygen vacancies are disordered. The oxygen-vacancies ordering leads to appearance
of the ferromagnetic component apparently due to a formation of the noncollinear magnetic structure. The
antiferromagnet–“ferromagnet” transition is accompanied by a giant magnetoresistance. It is suggested that in
the ferromagnetic oxidized compounds Co³⁺ and Co⁴⁺ ions adopt intermediate spin state whereas for antiferromagnetic
(Co⁴⁺-free) compositions Co³⁺ ions have high-spin state (pyramids CoO₅) and dominant low-spin state
(octahedra CoO₆). In both ferromagnetic and antiferromagnetic compounds the superexchange via oxygen plays
an essential role in a formation of the magnetic properties.
|
| issn |
0132-6414 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/117271 |
| citation_txt |
Antiferromagnet–ferromagnet transition in the cobaltites / I.O. Troyanchuk, M.V. Bushinsky, D.V. Karpinsky // Физика низких температур. — 2012. — Т. 38, № 7. — С. 833-841. — Бібліогр.: 53 назв. — англ. |
| work_keys_str_mv |
AT troyanchukio antiferromagnetferromagnettransitioninthecobaltites AT bushinskymv antiferromagnetferromagnettransitioninthecobaltites AT karpinskydv antiferromagnetferromagnettransitioninthecobaltites AT sirenkova antiferromagnetferromagnettransitioninthecobaltites |
| first_indexed |
2025-11-24T20:17:21Z |
| last_indexed |
2025-11-24T20:17:21Z |
| _version_ |
1850495139812737024 |
| fulltext |
© I.O. Troyanchuk, M.V. Bushinsky, D.V. Karpinsky, and V.A. Sirenko, 2012
Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 7, pp. 833–841
Antiferromagnet–ferromagnet transition in the cobaltites
I.O. Troyanchuk, M.V. Bushinsky, and D.V. Karpinsky
Scientific-Practical Materials Research Centre of NAS of Belarus
19 P. Brovka Str., Minsk 220072, Belarus
V.A. Sirenko
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine
47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: sirenko@ilt.kharkov.ua
Received March 13, 2012
The three series oxygen-deficient cobaltites La0.5Ba0.5CoO3–δ, LnBaCo2O5.5 and Sr2YCo4O10.5 have been
studied. It has been shown that La0.5Ba0.5CoO3 is an insulating ferromagnet whereas La0.5Ba0.5CoO2.75 is a pure
antiferromagnet in which the oxygen vacancies are disordered. The oxygen-vacancies ordering leads to appear-
ance of the ferromagnetic component apparently due to a formation of the noncollinear magnetic structure. The
antiferromagnet–“ferromagnet” transition is accompanied by a giant magnetoresistance. It is suggested that in
the ferromagnetic oxidized compounds Co3+ and Co4+ ions adopt intermediate spin state whereas for antiferro-
magnetic (Co4+-free) compositions Co3+ ions have high-spin state (pyramids CoO5) and dominant low-spin state
(octahedra CoO6). In both ferromagnetic and antiferromagnetic compounds the superexchange via oxygen plays
an essential role in a formation of the magnetic properties.
PACS: 75.50.Dd Nonmetallic ferromagnetic materials;
75.30.–m Intrinsic properties of magnetically ordered materials;
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects.
Keywords: antiferromagnet, oxygen vacancies, spin state transition, layered perovskite.
1. Ferromagnetic and antiferromagnetic phases in the
anion deficient cobaltites with disordered oxygen
vacancies
Rare earth cobaltites Ln1–xAxCoO3 (Ln = lanthanide,
A = alkaline earth metal: Ca, Sr or Ba) with perovskite
structure attract much interest as they exhibit a variety of
unusual magnetic and transport properties [1–5]. The Co
ions in octahedral symmetry may have either high, inter-
mediate or low-spin state as the energies of the crystalfield
splitting of both the Co 3d states and the Hund’s rule ex-
change energy are comparable. In the ground state at low
temperature, LaCoO3 contains Co3+ ions with the low-spin
electronic configuration 6 0
2g gt e [1–3]. Upon heating, the
spin state of Co ions thermally activates to the intermediate
state (IS, 5 1
2g gt e , S = 1) or high-spin state (HS, 4 2
2g gt e ,
S = 2) [1–3].
In the hole-doped cobaltites, La1–xAxCoO3, the addi-
tional Co4+ ion increases the complexity of the system as it
can also be in the different spin states. Among doped
cobaltites, the system La1–xSrxCoO3 is the most extensive-
ly investigated. A spin glass behavior was reported for
x < 0.18 as well as a ferromagnetic long-range ordering
that coincides with concentration insulator-to-metal transi-
tion for x ≈ 0.18 [6]. Similar metallic ferromagnetic state
was observed in barium-doped cobaltites with barium con-
tent x > 0.2 [7–9]. The structural studies performed on the
cubic oxygen-stoichiometric perovskite La0.5Ba0.5CoO3
have revealed an onset of a long-range tetragonal phase
accompanying a para-ferromagnetic transition at
TC ≈ 180 K [10,11]. The tetragonal distortion has been
discussed in terms of cooperative Jahn–Teller distortions
of the CoO6 octahedra. It was assumed that the Jahn–Teller
effect is favored by the intermediate spin-state configura-
tion of the Co3+(d6) and Co4+(d5) ions derived from the
measured ferromagnetic moment –1.9 μB per cobalt ion.
However, the Sr-doped ferromagnetic cobaltites have ap-
proximately the same magnetic moment value and do not
exhibit any structural transition at the Curie point [12,13].
Moreover the extended x-ray absorption fine structure
(EXAFS) and neutron diffraction studies do not reveal any
appreciable local Jahn–Teller distortion in La1–xSrxCoO3
[14]. The nature of the ferromagnetic state in cobaltites
was a subject of debates for a long time [15–17]. Three main
I.O. Troyanchuk, M.V. Bushinsky, D.V. Karpinsky, and V.A. Sirenko
834 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 7
mechanisms explaining magnetic properties of mixed-va-
lence cobaltites were suggested: the superexchange model
based on the localized electron interaction via oxygen ion,
the Zener double exchange via charge transfer and the itin-
erant electron ferromagnetism [15–17].
Besides the alkaline earth doping there is another way
to manipulate the physical properties of the rare earth
cobaltites. In accordance with [18,19] the oxygen deficien-
cy in La0.5A0.5CoO3–δ (A = Sr, Ba) perovskites is accom-
panied by a strong decrease of magnetization and phase
separation phenomenon. The crystal structure of the
La0.5Ba0.5CoO3–δ has been analyzed using x-ray, neutron
and synchrotron diffraction techniques. The XRD data
obtained at room temperature verify the cubic structure of
the La0.5Ba0.5CoO3–δ up to concentration x = 0.45. The
neutron diffraction data for the La0.5Ba0.5CoO3–δ sample
cooled at a rate of 300 °C/h from 1200 °C have been rec-
orded at temperatures of 300, 150, 80 and 2 K. Rietveld
refinement performed for the 300 K NPD pattern assumes
cubic symmetry of the compound ( 3Pm m space group).
The structural analysis performed by using the tetragonal
space group (P4/mmm), rhombohedral ( 3R c), and ortho-
rhombic one (Pnma) did not result in any essential im-
provement of the reliability factors and cubic structure has
been assumed as a more feasible one. The value of the oxy-
gen occupation refined from the NPD data at 300 K is
about 2.88, the cubic symmetry supposes a random distri-
bution of the oxygen vacancies as well as of La and Ba
ions. The crystal structure has been additionally studied by
using synchrotron powder diffraction. In order to estimate
the structural parameters in detail, the SPD patterns were
recorded in the range from 5 K up to 300 K at the 5 K step.
The refinement of the SPD pattern recorded at 300 K, has
confirmed the macroscopic cubic symmetry ( 3 )Pm m of the
La0.5Ba0.5CoO2.88. However, close inspection of the SPD
patterns has revealed a very small asymmetric broadening
of the diffraction peaks. The asymmetry becomes more
pronounced with temperature decreasing. Similar behavior
of the NPD and SPD diffraction peaks excludes possible
instrumental faults and/or texturing effects that could lead
to an asymmetry of the peaks. Analysis of the NPD and
SPD patterns recorded at temperatures below 150 K clari-
fies a cause of the phase instability. The most probable
reason for the peak asymmetry is an appearance of a new
phase with structural parameters larger than those for the
parent phase. The diffraction patterns recorded at low tem-
peratures have uniquely confirmed the phase separation
scenario (Fig. 1). The structure refinement using two-phase
model with cubic unit cells for the diffraction patterns ob-
tained for T < 150 K, strongly improves the reliability fac-
tors. The refined structural data have confirmed gradual
extension of the new cubic phase with temperature de-
crease. The content of the phase with the smaller lattice
parameter calculated from the refinement of the NPD pat-
tern recorded at 2 K, is about 70% (the major phase). The
rough estimation of the oxygen content performed for the
NPD data taken at 2 K, assumes the oxygen content for the
phase with the larger lattice parameter to be smaller as
compared with the major structural phase. However, the
overlapping of the diffraction peaks hampers an accurate
determination of the oxygen content for the both structural
phases. The difference in the oxygen content assumes a
slightly different oxidation state and coordination for Co
ions in these phases. One can suggest that in the minor
phase Co ions have the oxidation state close to 3+ and are
located in the oxygen pyramids and octahedra whereas in
the major phase the cobalt ions are placed predominantly
in the oxygen octahedra and have a mixed 3+/4+ oxidation
state. It should be noted that stoichiometric
La0.5Ba0.5CoO3 is a single phase down to the liquid heli-
um temperature [10,11]. So, the oxygen deficit is the im-
portant factor in the phase separation.
The NPD data permit to clarify the magnetic structure
of the La0.5Ba0.5CoO2.88. Taking into account the SPD
data for La0.5Ba0.5CoO2.88 the additional peaks occurring
only on NPD patterns below 150 K can be referred to
magnetic neutron scattering. The NPD patterns were well
fitted assuming the coexistence of ferromagnetic and G-
type antiferromagnetic structures. An additional contribu-
tion to the intensities of the diffraction peaks (100), (110),
(210) testifies long range ferromagnetism within the com-
pound, whereas the new magnetic peaks indexed as (111),
(113), (313) in 2ap×2ap×2ap cubic metric assume antifer-
romagnetic ordering. Based on the structural data the fer-
romagnetic contribution is attributed to the major structural
phase, whereas G-type antiferromagnetic one is associated
with the minor phase with larger unit cell. The estimated
magnetic moments are approximately ~1.6 μB for F-type
phase and ~2 μB for G-type antiferromagnetic phase. The
Fig. 1. The synchrotron diffraction spectra for the
La0.5Ba0.5CoO3–δ compound at 4 K. The observed and calculated
profiles are noted by points and the line, respectively, the bottom
line represents their difference. The data are refined in 3Pm m
space group for the both structural phases. The inset shows the
magnified parts of the patterns at 300 K.
5 15 25 35 45
0
20
40
60
80
100
In
te
ns
ity
, a
rb
. u
ni
ts
In
te
ns
ity
, a
rb
. u
ni
ts
2 , degθ
2 , degθ
T = 4 K
31.0 31.5 32.0 32.5 33.0
300 K
T = 4 K
Antiferromagnet–ferromagnet transition in the cobaltites
Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 7 835
calculated moment for the ferromagnetic phase correlates
with the published data for the oxygen stoichiometric La
and Ba ordered LaBaCo2O6 (1.5 μB) and disordered
La0.5Ba0.5CoO3 (1.9 μB) ferromagnetic phases [10,11].
The La0.5Ba0.5CoO2.77 sample reveal exceptional be-
havior of the unit cell parameter with temperature change.
The unit cell parameter decreases with temperature reduc-
tion from room temperature down to ~170 K where pure
antiferromagnetic order develops. Whereas below the tem-
perature of antiferromagnetic ordering an unexpected
gradual increase of the lattice parameter is observed. The
unit cell volume estimated for the La0.5Ba0.5CoO2.77 com-
pound at 2 K is close to that observed at room temperature.
The phase separation phenomena as well as ferromagnetic
contribution into NPD pattern have not been observed.
We suggest that the most probable reason of the unusu-
al expansion of the unit cell as well as phase separation is
transition of the Co3+ ions to the high/low-spin state from
initially an intermediate spin state which is more stable at
high temperature. The structural phase separation can be
accompanied by redistribution of the electronic density
(electronic phase separation). The magnetization data seem
to be in agreement with this assumption. The oxidized
La0.5Ba0.5CoO3–δ being cooled slowly from 1200 °C
shows magnetic moment about 1.8 μB corresponding to the
intermediate spin state of the Co3+/Co4+ ions for pure fer-
romagnetic state. Decreasing of the oxygen content lead to
drop of the magnetic moment and anomalous magnetiza-
tion behavior thus indicating the spin-state transition
(Fig. 2). The pure antiferromagnetic compositions do not
show remnant magnetization. In accordance with [20] we
suggest that high-spin state is stabilized in pyramids
whereas low-spin state corresponds to octahedra.
A relatively large negative magnetoresistance is ob-
served in nearly stoichiometric oxidized La0.5Ba0.5CoO3–δ.
Figure 3 shows the temperature variations of electrical re-
sistivity measured at different external magnetic fields for
La0.5Sr0.5CoO3–δ (upper panel) and La0.5Ba0.5CoO3–δ (lo-
wer panel). Both the samples are pure ferromagnets. The
resistivity of La0.5Sr0.5CoO3–δ exhibits the metallic behav-
ior within the whole measured temperature range from 5 to
300 K. The nonpronounced anomaly in the ρ(T) depend-
ence was observed near the Curie point at the absence of
the external magnetic field. The magnetoresistance is not
revealed at low temperatures but appeared around TC, as is
shown in Fig. 3. The temperature variation of resistivity for
La0.5Ba0.5CoO3–δ is dramatically different. The metallic
behavior of the resistivity has been observed only in the
narrow temperature range near the Curie point TC ~ 170 K,
and it gradually changes to a semiconducting one below
150 K. An external magnetic field strongly extends the
metallic-like temperature range above the Curie point,
however at low temperatures the semiconductive behavior
does not essentially change. Figure 4 shows magnetoresis-
tance as a function of a field at various temperatures for
La0.5Ba0.5CoO3–δ. The magnetoresistance exhibits a local
maximum near the TC and increases gradually with further
cooling. At 5 K we obtained the MR value of about 20% in
the field of 14 T. The MR varies gradually with the field
and does not show any tendency to saturation with the
temperature decrease.
Fig. 2. The magnetization vs temperature dependencies of the
La0.5Ba0.5CoO3–δ quenched from 750 and 650 °C. The arrows
indicate temperature change direction.
0 50 100 150 200 250 3000.1
0.4
0.7
1.0
1.3
1.6
1.9
750 °C
H = 1 T
La Ba CoO0.5 0.5 3–δ
Tq = 650 °C
M
, e
m
u/
g
T, K
Fig. 3. The resistivity of ferromagnetic La0.5Sr0.5CoO3–δ (top
panel) and La0.5Ba0.5CoO3–δ (bottom panel) cooled at a rate of
100 °C/h vs temperature. The inset shows the resistivity behavior
near the Curie point.
0 50 100 150 200 250 300
9.0
7.5
6.0
4.5
3.0
2.5
2.0
1.5
1.0
0.5
T, K
T, K
120 140 160 180 200 220
10 T
5 T
3.4
3.5
3.6
3.7
3.8
0 T
14 T
H = 0
ρ
Ω
,
·c
m
10
–3 ρ
Ω
,
·c
m
10
–3
ρ
Ω
,
·c
m
10
–4
I.O. Troyanchuk, M.V. Bushinsky, D.V. Karpinsky, and V.A. Sirenko
836 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 7
There are different types of the large magnetoresistance
ratio observed in the ordinary cobaltites. The large magne-
toresistance was observed in the insulating spin-glass pha-
se of the lightly doped La1–xSrxCoO3 cobaltites [21,22]. It
is suggested that the interface effects associated with diffe-
rent magnetic and conductive states of ferromagnetic clus-
ters and paramagnetic matrix are responsible for nonsatu-
rated magnetoresistance observed in the large magnetic
field [21]. This type of magnetoresistance is the most pro-
nounced at low temperature. The second type of large
magnetoresistance was observed at the low temperature in
the insulating homogeneous ferromagnet La0.5Ba0.5CoO3
[10,11]. This type of magnetoresistance has similar tem-
perature and field dependences with magnetotransport pa-
rameters of the lightly doped cobaltites. The magnetoresis-
tance is not saturated in the large magnetic field and
strongly increases while temperature decreasing. In con-
trast to manganites there is no large magnetotransport
anomaly in the vicinity of the Curie point. The observation
of the homogeneous ferromagnetism in insulating
La0.5Ba0.5CoO3 in our opinion, confirms the hypothesis
that ferromagnetic properties of the hole-doped cobaltites
with the perovskite structure are governed by the
superexchange interactions similar to those in oxides of the
nonmixed valence type of magnetoactive ions (e.g., ferrites
with Fe3+ ions). The small magnetotransport and conduc-
tivity anomalies near the Curie point of the both insulating
and conductive cobaltites apparently do not support the
dominant role of the “double exchange” in defining of the
ferromagnetic ordering as it was suggested in a number of
works [6,10,11]. The insulating character of the
La0.5Ba0.5CoO3 ferromagnetic phase cannot be understood
by means of the “itinerant magnetism” model. Apparently
large magnetoresistance observed in homogeneous
La0.5Ba0.5CoO3 at low temperature is associated with de-
creasing of the insulating gap between eg and collective t2g
state upon external magnetic field.
2. Antiferromagnet–ferromagnet transition in iron
doped TbBaCo2O5.5 layered perovskite
The layered perovskites LnBaCo2O5.5 (Ln = lanthanide)
have attracted a great attention due to interplay between
magnetic and magnetotransport properties [23–26]. The
crystal structure of these compounds has an alternating
linkage of Ba2+ and Ln3+ layers along the c axis and layers
of CoO5 pyramids and CoO6 octahedrons along the a or b
axis [25]. As a result of ordering of Ln3+ and Ba2+ ions as
well of oxygen vacancies the crystal structure is described
with unit cell ap×2ap×2ap in the space group Pmmm.
The LnBaCo2O5.5 compounds exhibit following tran-
sitions: antiferromagnet–“ferromagnet” in the temperature
range 200 K ≤ T < 260 K, “ferromagnet”–paramagnet
(250 K < T < 300 K) and metal–insulator (300 K < T < 370 K)
depending on Ln ion size [23–25]. According to the neu-
tron diffraction studies the “ferromagnetic” phase consists
of the basic G-type antiferromagnetic (2ap×2ap×2ap) and
small ferromagnetic components, while the low-
temperature pure antiferromagnetic phase has magnetic
unit cell of 2ap×2ap×4ap-type [27–30]. The transition
antiferromagnet–“ferromagnet” leads to a drop of the resis-
tivity and a giant magnetoresistance effect [23–26]. The
reason for interplay between magnetic and
magnetotransport properties is not clear so far. In order to
understand magnetotransport properties the detailed de-
scription of the magnetic structure in the both phases is
necessary. Several studies of magnetic structure of the
LnBaCo2O5.5 compounds have been performed with di-
verging results [27–34]. According to [30,34] the magnetic
structure is noncollinear in both antiferromagnetic and fer-
romagnetic phases. The Co3+ ions in octahedrons adopt the
low-spin state [30,34], whereas in pyramids they are in the
high-spin state. In works [27,28] it was suggested that the
correct space group is Pmma and the crystal structure is
described with a 2ap×2ap×2ap supercell. In this model
Co3+ ions in both pyramidal and octahedral sublattices are
located in nonequivalent two sites and magnetic structure
can be described by a collinear model for both “ferromag-
netic” and antiferromagnetic phases. It worth to be noted
that symmetry analysis performed for both space groups
Pmmm and Pmma does not allow realization of the noncol-
linear magnetic structure with ferromagnetic component.
However, a synchrotron x-ray powder diffraction study of
YBaCo2O5.5 compound has revealed some additional
peaks associated with a 2ap×2ap×2ap crystal structure
supercell and small monoclinic distortion in the ferromag-
netic phase corresponding space group P112/a [35]. The
observed monoclinic distortion support the model of the
noncollinear magnetic structure for description of magnetic
and magnetotransport properties of LnBaCo2O5.5-type
compounds.
Very intricate properties have been revealed in
TbBaCo2O5.5–x/2 doped with Fe3+ ions [26,36]. In
Fig. 4. Field dependencies of the magnetoresistance ratio
MR = [ρ(H) – ρ(H = 0)]/ρ(H = 0)·100% for the ferromagnetic
La0.5Ba0.5CoO3–δ.
0 2 4 6 8 10 12 14–20
–15
–10
–5
0
H, T
100 K
170 K
50 K
5 K
15 K
M
R
, %
Antiferromagnet–ferromagnet transition in the cobaltites
Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 7 837
TbBaCo2–xFexO5.5 below x = 0.1 orthorhombic distortion
slightly increases with increasing Fe3+ content and disap-
pears above x = 0.15. The temperature of “ferromagnet”–pa-
ramagnet transition increases while the temperature of
“ferromagnet”–antiferromagnet transition decreases with
increasing Fe3+ content up to x = 0.1. Transition into te-
tragonal phase (x > 0.15) leads to a collapse of the ferro-
magnetic phase. The jump of conductivity as well as ther-
mal hysteresis associated with “ferromagnet”–antiferro-
magnet transition become much larger with Fe doping
within orthorhombic phase. The antiferromagnetic phase
becomes proof against external magnetic field. Mössbauer
study has revealed that Fe3+ ions predominantly occupy two
strongly distorted positions [36]. The results of measurements
of ZFC and FC magnetization of TbBaCo1.9Fe0.1O5.5 used
for neutron diffraction are shown in Fig. 5. Both ZFC and
FC magnetizations exhibit a sharp maxima at T = 300 K
which indicate the transition into a paramagnetic state. In
the temperature interval 200–170 K the field cooled mag-
netization decreases strongly with lowering temperature. In
this temperature range ZFC magnetization changes very
little thus indicating a huge magnetic anisotropy. It should
be noted that ZFC and FC magnetizations do not coincide
below 170 K down to liquid helium temperature due to a
presence of a small remnant magnetization. The good ag-
reement between calculated and observed profiles of NPD
patterns was reached using Pmmm space group with a
2ap×ap×2ap unit cell. The refinement in Pmma space
group with 2ap×2ap×2ap unit cell leads to the same factors
of reliability as refinement in terms of Pmmm model.
Analysis of the powder patterns collected at the temper-
ature 215 K shows that there is a set of the additional mag-
netic reflections which could be indexed using a
2ap×2ap×2ap magnetic unit cell. Furthermore the addition-
al contribution into (100) reflection has been clearly ob-
served. This contribution disappears at 130 K. The contri-
bution into reflection (100) corresponds to the ferromag-
netic component whereas the appearance of (111), (311),
(113), (331) and (313) reflections corresponds to the G-
type antiferromagnetic component. The contribution into
the G-type antiferromagnetic component increases with
decreasing temperature down to 130 K whereas the ferro-
magnetic contribution disappears. Such a type of behavior
of the antiferromagnetic component was observed first for
the LnBaCo2O5.5-type layered perovskites exhibiting
antiferromagnet–“ferromagnet” transition. The low-tempe-
rature antiferromagnetic phase of the undoped compounds
is described with a more complex 2ap×2ap×4ap magnetic
unit cell [27–32]. The refinement of the magnetic structure
at 215 K gives following values of magnetic moments for
the G-type antiferromagnetic component: 1.9 μB per Co
ion for the pyramidal sublattice and 0.9 μB for the octahe-
dral one. The ferromagnetic component is 1.0 μB per for-
mula unit and directed along the b axis, whereas the anti-
ferromagnetic component is along the a axis. All the
magnetic moments are placed within (a,b) plane. Decreas-
ing temperature down to 130 K leads to increasing magnet-
ic moment up to 2.6 μB for the pyramidal sublattice while
for octahedral one it practically does not change.
The magnetic moment value in the pyramidal sublattice
is 2.6 μB per ion at 130 K. This value is significantly more
than 2 μB associated with the intermediate spin state. It was
found that magnetic moments of the high-spin Co3+ ions in
Sr2Co2O5 and BiCoO3 are 3.3 and 3.4 μB, respectively
[37,38]. However, the magnetic moment of the Co3+ ion in
high-spin state (S = 2) should be about 4 μB. So, present data
can hardly be adjusted with a pure ionic model for Co ions
magnetic moments. In Refs. 2, 3 the Co3+ ions in LaCoO3
have been shown to have first excited state corresponding to
the high-spin one. Probably it is valid for LnBaCo2O5.5-type
compounds. Hence, the description of the magnetic state of
Co3+ ions in TbBaCo2O5.5 doped with iron could be done in
terms of a mixed low/high-spin magnetic state. The results
of NMR study of EuBaCo2O5.5 are in agreement with a
mixed low-high spin state model [39].
Both “ferromagnetic” and antiferromagnetic phases in
TbBaCo1.9Fe0.1O5.5 have the G-type magnetic structure
with a 2ap×2ap×2ap magnetic unit cell. In this magnetic
structure there are only two different magnetic positions
for both CoO5 and CoO6 layers. The transition into the
antiferromagnetic phase leads to a substantial increase of
magnetic moments in pyramidal sublattice, whereas mag-
netic moments in the octahedrons remain practically the
same. These results can be easily understood in noncolline-
ar model of the magnetic structure of the “ferromagnetic”
phase.
Apparently in the ferromagnetic phase an angle be-
tween magnetic moments of the Co3+ ions in pyramidal
sublattice in direction of the a axis is about 30°. In the anti-
ferromagnetic phase the magnetic structure seems to be
collinear and aligned along the b axis because the
2ap×2ap×2ap magnetic unit cell is preserved. Remind, that
in the undoped TbBaCo2O5.5 the low-temperature antifer-
Fig. 5. The temperature dependences of the magnetization for
TbBaCo1.9Fe0.1O5+γ measured in the FC and ZFC modes at
H = 0.05 T.
0 100 200 300
0
0.6
1.2
, KT
M
, e
m
u/
g
H = 0.05 T FC
ZFC
TbBaCo Fe O1.9 0.1 5+γ
I.O. Troyanchuk, M.V. Bushinsky, D.V. Karpinsky, and V.A. Sirenko
838 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 7
romagnetic phase remains noncollinear and the magnetic
unit cell contains four sites for magnetic ions which leads
to 2ap×2ap×4ap magnetic supercell [34].
In order to understand an origin of noncollinear mag-
netic structure it is necessary to consider exchange interac-
tions in the pyramidal sublattice. One can suppose ex-
change interactions in the pyramidal sublattice to be
negative in the case when nearest pyramids have common
oxygen ion and positive if not. In this case all the interac-
tions within (a,b) plane are negative whereas along с axis
negative and positive ones are alternated. This leads to a
frustration of magnetic interactions and the noncollinear
magnetic structure in the some temperature range could be
more favorable than the simple collinear one. The spin-
orbital interaction seems to be also very important factor
leading to canting of the magnetic moments. Really the
measurements of magnetic and magnetotransport proper-
ties performed on a single crystal have revealed a huge
magnetocrystalline anisotropy as well as giant anisotropic
magnetoresistance [31]. A number of noncollinear magnet-
ic structures in different magnetic materials have been un-
derstood in terms of itinerant magnetism approach with
relativistic interactions [40]. Doping with Fe3+ stabilizes
the collinear antiferromagnetic structure at low tempera-
tures because all the interactions Fe3+–Fe3+, Fe3+–Co2+,
Fe3+–Co3+ seem to be strongly antiferromagnetic. At high
Fe doping (x > 0.1) the orthorhombic symmetry transforms
to tetragonal one leading to ferromagnetic component dis-
appearing [26,36]. The ferromagnetic component collapse
is caused by random distribution of the octahedra and py-
ramids in tetragonal phase.
In the works [27,28] the noncollinear solution has been
overruled on the base of symmetry analysis. However the
noncollinear magnetic structures in LnBaCo2O5.5 com-
pounds seem to be a result of a very small monoclinic dis-
tortion whereas for symmetry analysis only orthorhombic
space groups were used. The noncollinear structure is sub-
tle balance between positive, negative exchange interac-
tions and magnetocrystalline anisotropy.
Using the conception of noncollinear magnetism one
can try to explain the peculiarities of magnetic and
transport properties of the layered cobaltites, associated
with “ferromagnet”–antiferromagnet transition. In contrast
to collinear model the noncollinear one provides pure fer-
romagnetic direction orthogonal to antiferromagnetic axis.
Hence the transition into noncollinear ferromagnetic phase
can produce strong changes in the vicinity of Fermi level.
Really in the noncollinear model antiferromagnetic com-
ponent is aligned along the a axis whereas along the b axis
the pure ferromagnetic component is realized thus leading
to a jump of conductivity at antiferromagnet–“ferromag-
net” transition.
3. Antiferromagnet–ferromagnet transition in layered
Sr3YCo4O10.5+δ-type cobaltites
Another class of anion-deficient layered cobaltites was
received quite recently. It has the chemical composition
Sr3LnCo4O10.5+δ (its reduced chemical formula is
Sr0.75Ln0.25CoO3–γ), where the rare earth ions can partially
substitute for strontium ions and vice versa [41–44]. Its
crystal structure is built by alternating anion-deficient
CoO4+δ layers and layers formed by CoO6 octahedra with
sharing vertices. This class of compounds is characterized
by high magnetic ordering temperature (up to 360 K) [45–47].
Spontaneous magnetization appears below 360 K, attains
its maximum value near room temperature, and then decre-
ases gradually down to liquid helium temperatures [45–49].
Several conjectures explaining the anomalous temperature
dependence of the magnetization have been put forward. In
Ref. 50, it was suggested that a certain fraction of Co3+
ions undergo the transition from the high-spin to low-spin
state with a decrease in temperature. However, the neutron
diffraction studies do not reveal any anomalous decrease in
the magnetic moment when the temperature is decreased
[48,49,51]. In the whole temperature range below the mag-
netic ordering point, the antiferromagnetic G-type structure
was observed, whereas it was impossible to reliably detect
the ferromagnetic contribution since it turned out to be
quite small. For this reason, in [48,49], the anomalous be-
havior of the magnetization was attributed to the phase
transition from the magnetic state with the spontaneous
magnetization to the purely antiferromagnetic state. It was
assumed that the transition was incomplete owing to the
chemical inhomogeneity of the samples. However, another
interpretation of the transition was proposed in Ref. 52,
where the ferromagnetic component was determined by the
neutron diffraction method. The model proposed in Ref. 52
attributes the anomalous behavior of the magnetization to
the presence of a weak magnetic sublattice within the
magnetic structure of the collinear ferrimagnet. In the
framework of this model, the spontaneous magnetization
results from the ordering of cobalt ions with different oxy-
gen coordination numbers in the CoO4+δ layer, whereas the
CoO6 layer is purely antiferromagnetic. According to
Ref. 52, the spontaneous magnetization appears owing to
the different temperature dependences of the magnetic
moments of cobalt in different sublattices of the collinear
ferrimagnet.
The temperature dependence of the magnetization for
the Sr3YCo4O10.6+δ sample obtained by fast cooling is
shown in Fig. 6. The measurements were performed upon
cooling and heating at different magnetic fields up to 14 T.
The cooling and heating rates were 0.5 K/min. Within the
temperature range of 210–240 K, a steep increase (de-
crease) in the magnetization with a temperature hysteresis
of 12 K has been observed. Such a temperature behavior of
the magnetization is characteristic of the first-order mag-
Antiferromagnet–ferromagnet transition in the cobaltites
Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 7 839
netic phase transition of the order–order type. In the high-
temperature phase, the magnetization is not saturated at
magnetic fields up to 14 T. Therefore, it is impossible to
determine accurately the spontaneous magnetization. Nev-
ertheless, its estimated value is no less than 0.25 μB per
cobalt ion. In the temperature range corresponding to the
magnetic phase transition, the magnetic field dependence
of the magnetization exhibits an anomalous growth of the
susceptibility at high fields and a pronounced field hystere-
sis of the magnetization. Such a behavior of the magnetiza-
tion is characteristic of metamagnets, i.e., materials where
the applied magnetic field induces another phase state with
a higher magnetization. The metamagnetic transition is
incomplete since a field of 14 T is insufficient to induce a
homogeneous high-temperature magnetic phase. At low
temperatures (T < 150 K), the magnetic field dependence
of the magnetization is almost linear, similar to that in anti-
ferromagnets, whereas the spontaneous magnetization is
practically absent. Both slightly reduced and oxidized sam-
ples show antiferromagnet–“ferromagnet” transition also.
The new results of the magnetic measurements suggest
that the behavior of the magnetization for the
Sr3YCo4O10.5+δ sample reported in Refs. 45–49 is associ-
ated with inhomogeneous yttrium distribution over the
sample. With an increase in the yttrium content, a purely
ferromagnetic behavior transforms to a purely antiferro-
magnetic behavior, passing through intermediate composi-
tions, which exhibit the first-order antiferromagnet–
“ferromagnet” phase transition (Fig. 7).
It worth to be noted that the behaviors of two different
classes of layered cobaltites, SrLnCo4O10.5 and
LnBaCo2O5.5, are very similar. Both classes of layered
cobaltites exhibit the antiferromagnet–ferromagnet transi-
tion and accurately prepared polycrystalline samples have
nearly the same spontaneous magnetization in the ferro-
magnetic phase. This is likely due to similar mechanisms
of ferromagnetism in both classes of compounds, where
the antiferromagnet–ferromagnet transition can occur in
the nominally G-type magnetic structure without the for-
mation of additional magnetic reflections in the low-
temperature antiferromagnetic phase [48,49,52,53]. This
behavior can be easily explained under the assumption that
the magnetic structure of the ferromagnetic phase is
noncollinear with the uncompensated magnetic moment as
in weak ferromagnets. The symmetry analysis forbidding
the existence of noncollinear ferromagnetism in
LnBaCo2O5.5 was performed within the framework of the
orthorhombic Pmma space group [27]. At the same time, a
careful x-ray structural analysis indicates that the true
symmetry is monoclinic (P112/a) [35]. It is necessary to
note that the true symmetry of Sr3YCo4O10.5+δ is also
monoclinic (possible space group A2/a) [45] rather than
orthorhombic, while the symmetry analysis in Ref. 52 was
performed on the basis of the latter symmetry.
4. Conclusions
The insulating La0.5Ba0.5CoO3 and metallic
Ln0.5Sr0.5CoO3 ordinary cobaltites are ferromagnetic due
to superexchange interactions via oxygen between cobalt
ions in the intermediate spin state. The insulating
La0.5Ba0.5CoO3 is only one known cobaltite exhibiting
cooperative static Jahn–Teller distortions of the CoO6
octahedra at low temperature. The oxygen content decreas-
ing leads to transformation from ferromagnetic to the G-
type antiferromagnetic structure through the phase separa-
ted state. The anomalous structural and magnetization be-
havior apparently is associated with a spin state transition.
The antiferromagnetic phase is characterized by high/low-
spin state of the Co3+ ions and lack of the ferromagnetic
contribution.
The ordering of the oxygen vacancies in the
LnBaCo2O5.5 and Sr3YCo4O10.5 layered cobaltites lead to
increases of the Neel point from 170 to 380 K and appear-
ance of the relatively small ferromagnetic component
which is identical for both type of the layered cobaltites.
Fig. 6. Temperature dependence of the magnetization for
Sr3YCo4O10.5+δ samples produced upon fast cooling from
1000 °C in different magnetic fields.
50 100 150 200 250 300
1
3
5
7
9
fast cooling
7 T
14 T
170 220 2700
0.5
1.0
0.03 T
, KT
, KT M
, e
m
u/
g M
, e
m
u/
g
YSr Co O3 4 10.5+δ
Fig. 7. Temperature dependences of the magnetization for
Sr3.2Y0.8Co4O10.5+δ (1), Sr3YCo4O10.5+δ (2), and
Sr2.6Y1.4Co4O10.5+δ (3) samples (H = 1 T).
0 50 100 150 200 250 300
0
2
4
6 H = 1 T
1
2
3
, KT
M
, e
m
u/
g
I.O. Troyanchuk, M.V. Bushinsky, D.V. Karpinsky, and V.A. Sirenko
840 Low Temperature Physics/Fizika Nizkikh Temperatur, 2012, v. 38, No. 7
The ferromagnetic component is associated with noncol-
linear magnetic structure. The noncollinear structure is
subtle balance between positive and negative exchange in-
teractions, magnetocrystalline anisotropy and accompany
by lowering of the crystal structure symmetry to satisfy the
symmetry criteria. The resistivity drop accompanying anti-
ferromagnet–ferromagnet transition in the LnBaCo2O5.5
seems to be result of a partial collapse of the insulating gap
in direction of the pure ferromagnetic component.
The authors would like to acknowledge the financial
support of the BRFFI (Grant Nos. T11D-003).
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