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Europhysics News (2003) Vol. 34 No. 6 The extraordinry properties of magnetic oxides B. Raveau and A. Maignan One of the most famous classes of magnetic oxides concerns the manganites with the perovskite structure (Fig. 1a), which exhibit colossal magnetoresistance (CMR) properties. In these oxides of generic formula Ln1-xAxMnO3 (A = Ca, Sr, Ba) double exchange phenomena take place, which require a hopping of charge carriers from a Mn3+ to a Mn4+ ion through an oxygen atom. This effect generates a ferromagnetic metallic state at low temperature, and a metal-insulator transition versus temperatur, which coincides with a ferromagnetic to paramagnetic (or antiferromagnetic) transition (Fig. 1b). The latter are also coupled with a structural transition, implying the disappearance of the static Jahn-Teller distortion due to Mn3+, in the ferromagneticstate. As a consequence, at the vicinity of the Curie Temperature TC a large magnetoresistance can be obtained: the resistance of these materials can be decreased by several orders of magnitude by an external magnetic field of some teslas. A part of thesemanganites, those which contain smaller A cation (Ca2+), exhibit a more complex physical behavior, due to orbital and charge ordering phenomena. In these oxides, the manganese octahedra are occupied by Mn3+ and Mn4+ species in an ordered way, forming stries of Mn3+distorted octahedra due to Jahn-Teller effect, which alternate with stripes of quasi-regular Mn4+ octahedra. The physics of the latter compounds is then governed by the competition between ferromagnetism and charge/orbital ordering, i.e. the orbtal-charge ordered state which is antiferromagnetic and insulating transforms to a metallic ferromagnetic state on the application of a sufficiently high magnetic field, leading also to CMR effect. In these materials, orbital-charge ordering can be destroed by doping the manganese sites with magnetic cations such as chromium or ruthenium, which by magnetic coupling with adjacent manganese induce ferromagnetism and metallicity, so that spectacular modifications of the magnetic phase diagrams of these systes are obtained. In this way, insulator-metal transitions can be induced and CMR effects are enhanced under much lower magnetic fields.
In fact, the unusual behavior of the manganites is based on a new phenomeon, the electronic phase separation. It has indeed been observed that charge ordered insulating regions coexist at low temperature with small ferromagnetic regions at a submicrometer scale, in a coherent manner in the same matrix. Thus, the CMR effect forthe low TC manganites results from the percolative conduction through the small ferromagnetic domains embedded in the antiferromagnetic insulating matrix. Such an electronic phase segregation was also predicted from the large magnetostriction effects obseved in these materials, and described either as dynamic magnetic polarons, or as static phase segregation. Many other properties of these manganese perovskites are closely related to the peculiar double exchange phenomena, such as the photo induced metal-nsulator transition, as well as the electrical field induced magnetic transition that have been recently observed. The curious effect of thermal cycling upon the resistivity, and the step like behavior at low temperature of their magnetization, resistivit and specific heat versus magnetic field at low temperature, are also certainly related to the phase separation that exists in these compounds, and suggest a martensitic type mechanism.
Cobalt oxides form also a very important family with extraordinary magnetic properties. This is for instance the case of the compounds LnBaCo2O5.5 (Ln = lanthanide), which exhibit a metl-insulator (MI) transition coupled with a spin transition. These materials are ordered oxygen deficient perovskites, characterized by a layered ordering of the Ln3+ and Ba2+ cations, so that rows of CoO6 octahedra alternate with rows of CoO5 tetragonal pramids (Fig. 2a). As a result, there exist two sorts of trivalent cobalt in the structure at high temperature (T>TMI): the octahedral cobalt has a high spin (HS) t2g4eg2 configuration, whereas the pyramidal one has an intermediate spin (IS) t2g5eg1 configration. In contrast to manganites, the metal to insulator transition appears in these oxides at decreasing temperature (Fig. 2b) around 300 - 350K, and coincides with the transition in the paramagnetic susceptibility (Fig. 2c). This strong coupling betwee magnetism and transport properties is explained by a spin transition of octahedral Co3+ species from high spin (HS) t2g4eg2 to low spin (LS) t2g6, the t2g5eg1 (IS) pyramidal cobalt being unchanged at the transition. Recent thermoelectric power studies ofthese oxides, corroborate this viewpoint showing that S changes of sign at the transition from n-type in the metallic state to p-type in the insulating state. Moreover, these cobaltites also exhibit negative giant magnetoresistance at lower temperature. Fr instance, resistance ratio higher than 10 can be obtained under 7T (Fig. 2b), in the temperature range T < 200K, where a second transition from the paramagnetic to the antiferromagnetic state has been achieved. Note that negative magnetoresistance has aso been observed in several other cobaltites: the perovskite La1-xSrxCoO3 and the bismuth based cobaltites, (Bi, Cd)1Sr2CoO5 with a layered structure similar to the 1201-type cuprate.
Ruthenium oxides exhibit also an etremely rich physics
from the magnetism viewpoint. Several members of the Ruddlesden and
Popper series Srn+1RunO3n+1, with n
= 1, 2, 3, References For manganites: Ibarra et al., Magnetostriction in mixed valent magnetic oxides: in Modern Trends in Magnetostriction Study and Applications. Edit M.R.J. Gibbs - 2000 - Kluwer Acad. Publi. For cobaltites: J-C. Burley et al., J. Solid State Chem, accepted 2002. A.Maignan et al., Eur. Physic. J. B. 15, 657 (2000) M. Coutanceau et al., Solid State Comm. 96, 569 (1995). C. Frontera et al., J. Solid State Chem. accepted 2002; Phys. Rv. B 65, 180405 (2002). M. Respaud et al., Phys. Rev. B, 64, 214401 (2001). W.S. Kim et al., Solid State Com 116, 609 (2000). For ruthenates: S. Malo et al., Internal J. of Inorg. Mat. 2, 601 (2000). G. Co et al., Phys. Rev. B 56, R 5740 (1197); ibid 56, 321 (1997). O. Friedt et al., Phys. Rev. B 63, 174432 (2001). Copyright EPS and EDP Sciences, 2003 |
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chains of face sharing CoO6 octahedra and CoO6 trigonal prisms
forming a triangular array. (b) Magnetization half loop M(H) of
this phase registered at T = 2K.
,
whereas an antiferromagnetic intrachain coupling is obtained in the
(a, b) plane. Nevertheless the Co-Co intrachain distances are much larger
than the Co-Co interchain distances, so that the magnetism of Ca3Co2O6
can be described on he basis of a planar Ising triangular lattice where
each chain plays the role of one spin. Thus, the magnetic susceptibility
of this phase exhibits two transitions versus temperature at 24K and
12K, which correspond to the setting of the interchain antiferomagnetic
coupling and spin freezing (Tf) respectively. In this Ising
triangular ferromagnet, AC susceptibility measurements show large shift
of Tf from 12 to 16.5K, as the frequency increases by three
orders of magnitude. Remarkably, five plateaus can b observed at 2K
on the M(H) curve (Fig. 3b), which characteristic magnetic fields are
separated by ~ 1.2 T. This feature is reminiscent of the quantum tunneling
of mangnetization encountered in high-spin macromolecules. 
are metallic
ferromagnets with TC ranging from 105K to 165K, whereas the
n = 1 member Sr2RuO4, has often been consideed
as close to the ferromagnetic order but is in fact a superconductor
with a very low critical temperature of ~ 1K. Remarkably, the former
compounds, show a Fisher-Langer type anomaly of the conductivity versus
temperature at TC, which is suppressed unde a few T. In contrast,
the calcium homologous phases which are isostructural, exhibit very
different properties: the perovskite CaRuO3 (n = 1) is a paramagnetic
metal, whereas Ca2RuO4 (n = 2) is an antiferromagnetic
insulator. As a consequence, solid soluions also show complex magnetic
properties, as for example the oxides Ca2-xSrxRuO4,
for which a metal-insulator transition, associated with a structural
change and magnetic ordering is observed, for low strontium contents.
But the most fascinating propertes of ruthenium based oxides have been
recently obtained for the ruthenocuprates of the RuSr2GdCu2O8
family. The physical behavior of this oxide is unique in that, superconductivity
and ferromagnetism coexist within the same matrix. The structure of
this hase (Fig. 4a) derives from that of the 92K-superconductor YBa2Cu3O7,
by replacing the CuO4 groups by RuO6 octahedra. It can be
described as an oxygen deficient perovskite built up of single octahedral
perovskite ruthenium layers sandwiched between pyramial copper layers,
layers of Gd3+ and Sr2+ being stacked according
to a "1-2" order, between the CuO2 and RuO2 planes. The remarkable coexistence
of superconductivity with a Tc ~ 35K, and of ferromagnetism
with a TC ~ 133K is demonstrated from AC susceptiblity measurements
(Fig. 4b). Such a behavior shows the important role of the layered architecture
of the structure, in order to conciliate these two contradictory properties.
In conclusion, these few examples and those well known on iron oxides
show thattransition metal oxides represent a vast field of investigation
for the discovery of new extraordinary magnetic properties.