Electronic and magnetic phase diagram of SmNi 1

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I. INTRODUCTION
Perovskite-type RMO 3 oxides, where R 3ϩ is a trivalent rare-earth ion and M 3ϩ is a trivalent 3d transition-metal ion, show a huge variety of different physical properties.Nowadays, because of their potential technological applications, there is a renewed interest in the study of mixed-valence manganese oxides.Other RMO 3 also display new and unusual behavior open to study, such as orbital and charge ordering phenomena as well as large changes in the electrical, magnetic, and structural properties, driven by small variations of temperature, composition, or magnetic field. 1 Many of these properties are still not understood.
Here, we present a study of RNi 1Ϫx Co x O 3 series with R ϭSm 3ϩ .[3][4][5][6][7][8][9][10] RCoO 3 oxides show temperature-dependent electronic and spin-state transitions.The ground state of LaCoO 3 is a nonmagnetic insulator whose Co 3ϩ configuration is of low spin ͑LS͒, t 2g 6 e g 0 (Sϭ0).The competition between Co 3ϩ crystalfield splitting and intra-atomic Hund coupling leads to a temperature-driven transition from LS to either thermally excited high spin ͑HS͒, t 2g 4 e g 2 (Sϭ2), or intermediate spin ͑IS͒, t 2g 5 e g 1 (Sϭ1), configuration. 2 In addition, RCoO 3 oxides show a crossover from a semiconducting to a metallic regime above 500 K.The reported metal-insulator transition temperature (T MI ) for SmCoO 3 is 2 600 K. On the other hand, LaNiO 3 is one of the few RMO 3 oxides which shows metallic conduction in the whole temperature range studied.In the other members of this series (RϭPr 3ϩ , Nd 3ϩ , Sm 3ϩ , Eu 3ϩ ), either strong electronic correlations or antiferromagnetic polaronic effects bring about a temperature-driven metalinsulator ͑MI͒ transition. 3RNiO 3 shows a complex phase diagram which consists of paramagnetic metal, paramagnetic insulator and antiferromagnetic insulator regions whose boundaries change with the rare-earth size.Moreover, several kinds of orbital ordering have been proposed to explain the complex low-temperature antiferromagnetic ͑AF͒ arrangement in the series. 4The magnetic ordering of the Ni 3ϩ ions can be described by a propagation vector kϭ( 1 2 , 0, 1 2 ) relative to the orthorhombic unit cell.The proposed magnetic ground state implies the symmetrical coexistence of ferroand antiferromagnetic couplings along the three pseudocubic axes. 4In SmNiO 3 , the MI transition takes place at 400 K and an AF ordering 3 at 220 K.
RNi 1Ϫx Co x O 3 mixed oxides show ferromagnetic interactions in the Ni/Co sublattice for the intermediate x range, 5,6 0.3рxр0.8.Some of these compounds exhibit giant magnetoresistive effects in the low-temperature spin-glass-like regime. 7,8In addition, different temperature-driven and composition-driven MI transitions have been found in the transport properties.
We report the electrical and magnetic SmNi 1Ϫx Co x O 3 phase diagram.X-ray powder diffraction, ac initial magnetic susceptibility, dc magnetization, electrical resistivity, and magnetoresistance measurements have been performed in a wide temperature range and in fields of up 12 T.We discuss diverse conduction behavior found in SmNi 1Ϫx Co x O 3 , compare it with results obtained for series with large rare-earth ions (R 3ϩ ϭLa 3ϩ , Nd 3ϩ ) and, finally, look for possible relations between structural, transport, and magnetic properties in these compounds.
The complex phase diagram obtained experimentally for SmNi 1Ϫx Co x O 3 series suggests two mechanisms of electronic localization.Electronic and magnetic correlations localize the current carriers for xр0.2 as it occurs in the undoped 3,4 SmNiO 3 .The semiconducting behavior found for xу0.3 can be likely explained by disorder effects, i.e., the Anderson localization.

II. EXPERIMENT
SmNi 1Ϫx Co x O 3 samples (xϭ0, 0.03, 0.075, 0.1, 0.15, 0.2, 0.3, 0.5, 0.7, and 0.9͒ were prepared under high oxygen pressure ͑200 bar͒ at 1000 °C, from a precursor obtained by means of a citrate sol-gel route.A stoichiometric mixture of Sm 2 O 3 , Co, and 2NiCO 3 3Ni͑OH͒ 2 4H 2 O was first dissolved in a nitric acid solution, and then appropriate citric acid and ethylene glycol amounts were added.Such solutions were heated until green-red gels were formed.These gels were dried by heating at 300 °C.The remaining brown powders were calcined in alumina crucibles overnight with a flow of oxygen at 500 °C in order to obtain the precursors.These precursors were pressed to 5 Kbar and sintered in the highpressure system for 12 h.SmCoO 3 and SmScO 3 oxides were prepared by standard solid-state ceramic procedures at 1200 °C in air, with some intermediate regrindings. The oxygen content for SmNi 1Ϫx Co x O 3Ϫ␦ samples was determined from thermogravimetric analysis.A Texas-Instrument TGA system was used in a reducing N 2 /H 2 ͑95:5͒ flow, working at a heating rate of 10 °C/min.A small oxygen deficiency was detected in SmNiO 3Ϫ␦ (␦ϭ0.04), while all other SmNi 1Ϫx Co x O 3Ϫ␦ oxides show a nearly perfect stoichiometric composition, with ␦ϭ0.00Ϯ0.02.
Step-scanned x-ray-diffraction ͑XRD͒ patterns were collected at room temperature using a D-max Rigaku system with a rotating anode.A graphite monochromator selected the Cu-K ␣1,2 radiation.
Electrical conductivity and magnetoresistance measurements were performed by a standard six-probe method on 1.5ϫ1.5ϫ10mm sintered bar-shaped samples, in 4рT р700 K temperature range.A superconducting coil was used to produce steady magnetic fields of up to 12 T. Magnetic measurements ͑ac susceptibility and dc magnetization͒ were made between 4 and 400 K with a commercial Quantum Design ͑superconducting quantum interference device͒ magnetometer, equipped with an ac susceptibility attachment, and with a vibrating sample magnetometer in the 350рT р700 K temperature range.Magnetization was measured in external magnetic fields of up to 5 T.

III. RESULTS
Profile analysis of x-ray-diffraction patterns indicates single-phase compounds for all SmNi 1Ϫx Co x O 3 , 0рxр1, samples.The diffractograms were indexed in the orthorhombic Pbnm spatial group, which is one of the most common distorted structures derived from the Pm3 ¯m cubic perovskite.Both SmNiO 3 and SmCoO 3 structures agree with the ones reported earlier. 9When nickel atoms are substituted by cobalt atoms, only slight differences in lattice parameters, distances, and angles between the atoms are found.The average transition-metal-oxygen distance (d M -O ) and the superexchange angle (͗M -O-M ͘) change from 1.955 Å and 152.4°forSmNiO 3 to 1.926 Å and 154.6°forSmCoO 3 , respectively.Lattice parameters are listed in Table I.

A. Magnetic properties
In Fig. 1 temperature dependence of dc magnetic susceptibility dc , measured under an external magnetic field of 1 T, is shown for xр0.20 samples.The main magnetic contribution to dc arises from paramagnetic Sm 3ϩ ions.SmNiO 3 susceptibility exhibits a small kink at 220 K that corresponds to the onset of the antiferromagnetic ͑AF͒ arrangement of the low spin Ni 3ϩ ions, in agreement with neutron-diffraction data 4 and heat-capacity measurements. 10When nickel sublattice is doped with cobalt, a similar feature is observed at 205, 190, and 170 K for xϭ0.03, 0.075, and 0.1, respectively.It has been interpreted as a signature of AF ordering.Therefore the Ne ´el temperature T N decreases with increasing Co content at a rate of about 500 K/x in this region.For xϭ0. 15    shows dc vs T for SmScO 3 , SmCoO 3 , and SmNiO 3 .The perovskite oxide SmScO 3 is isomorphous to SmNiO 3 and the Sc 3ϩ is nonmagnetic ion (3d 0 ).Consequently, only paramagnetic Sm 3ϩ ions contribute to its susceptibility.A comparison of SmCoO 3 and SmScO 3 magnetic data shows clearly that a majority of cobalt ions in SmCoO 3 is in the S ϭ0 Co 3ϩ electronic configuration, for 4рTр275 K. Above 275 K, the susceptibility of SmCoO 3 is larger than that of SmScO 3 ͑see inset of Fig. 1͒.It may be explained by an excitation of Co 3ϩ ions to states with S 0 as has been reported elsewhere. 11he most striking feature of magnetic properties of the SmNi 1Ϫx Co x O 3 series is the appearance of short-range ferromagnetic interactions for 0.3рxр0.8.These are evidenced by a spin-glass-like region observed at low temperature for these compounds.Figure 2 shows how ac susceptibility of SmNi 1Ϫx Co x O 3 , 0.2рxр1.0,samples varies with temperature in the 4рTр80 K temperature range.SmNi 1Ϫx Co x O 3 ac susceptibility curves peak sharply at 16, 19, 18, 14, and 6 K for xϭ0.3, 0.5, 0.6, 0.7, and 0.8, respectively.Zero-field cooling dc magnetic-susceptibility data, measured with external fields of 1 T, are plotted in the inset of Fig. 2 for the 0.3рxр0.9samples.Cobalt doping in SmNiO 3 and nickel doping in SmCoO 3 give rise to ferromagnetic interactions between the 3d ions and induce a spin-glass-like magnetic regime.A similar behavior was found earlier 6 for intermediate x values in RNi 1Ϫx Co x O 3 (RϭLa 3ϩ , Nd 3ϩ ).
Figure 3 shows how ac susceptibility varies with temperature for different oscillating field frequencies in SmNi 0.3 Co 0.7 O 3 .The peak at 14 K decreases in magnitude and shifts to higher temperatures as frequency increases.This dependence is found only at temperatures below the freezing temperature T f (T f corresponds to the temperature of the susceptibility cusp͒.A measure of the frequency shift is obtained from ⌬T f /T f (log ), which equals 0.023.This value is close to the reported ones for other spin-glass systems. 12In Fig. 3, dc susceptibility curves, measured under external magnetic fields of 0.1 and 1 T, are also plotted.The ac susceptibility cusps are smeared out to broad maxima and T f shifts to lower temperatures as the magnetic-field strength increases.Magnetization versus external magnetic field curves M (H) are presented in the inset of Fig. 3 for x ϭ0.7.At Tϭ50 K, the typical paramagnetic linear behavior, far from a saturation, is found.On the contrary, at temperatures below T f , e.g., Tϭ5 K, magnetization drifts upwards, suggesting ferromagnetic interactions.The curve at Tϭ5 K is also far from saturation.Extrapolating high-field magnetization values to 0 Hϭ0, we obtain an effective ferromagnetic moment of S eff ϭ0.04.The inset of Fig. 3 shows both zero-field cooled ͑ZFC͒ and field cooled ͑FC͒ magnetization curves at 5 K.In the low-field region, FC values are larger than the ZFC ones, showing an irreversibility in the magnetization measurements related to the freezing procedure.All of these magnetic properties are consistent with the proposed spin-glass-like behavior at low temperatures for 0.3рx р0.8 SmNi 1Ϫx Co x O 3 oxides.SmScO 3 susceptibility data give us Sm 3ϩ contribution ( Sm 3ϩ) to the total susceptibility; therefore the signal associated with the transition-metal sublattice ( Ni/Co ), can be obtained: Ni/Co (T)ϭ(T)Ϫ Sm 3ϩ( T), where (T) is the experimental SmNi 1Ϫx Co x O 3 susceptibility.All Ni/Co (T) data were fitted to the relation 13 Ni/Co (T)ϭ 0 ϩC/(T Ϫ⌰).Here, C is the Curie constant, eff the effective magnetic moment, ⌰ the Curie-Weiss temperature, and 0 the temperature-independent term of susceptibility.The best-fit parameters obtained from our measurements are listed in Table I.These parameters allow us to construct a magnetic phase diagram of SmNi 1Ϫx Co x O 3 .For xр0.2, ⌰ is negative and eff is close to the expected effective moment for the LS Ni 3ϩ and LS Co 3ϩ paramagnetic Ni/Co sublattice.This suggests the existence of AF interactions and a lowtemperature AF ordering.For 0.3Ͻxр0.8,crossovers of both ⌰ and eff are observed; ⌰ changes from negative to positive and eff shows larger values than the predicted ones for LS Ni 3ϩ /LS Co 3ϩ .It seems that ferromagnetic interactions show up, in agreement with the low-temperature spinglass-like regime.In the third region, for xϭ0.9, the Curie-Weiss temperature is nearly zero.This corresponds to a paramagnetic behavior of magnetically isolated Ni and Co ions.The temperature independent term 0 in SmNi 1Ϫx Co x O 3 series is large ( 0 Ͼ10 Ϫ4 emu/mol Oe); the maximum value of 0 coincides with a composition-driven MI transition (x ϭ0.3).The large values of 0 are usually related to strong electronic correlations, and a 0 maximum could be a signature of a composition-driven Mott transition. 14

B. Transport properties
Electrical resistivity of SmNi 1Ϫx Co x O 3 changes drastically with both the relative nickel/cobalt composition and the temperature.Figure 4 shows the temperature dependence of the electrical resistivity for xϭ0, 0.03, 0.075, 0.1, 0.15, and 0.2.Undoped SmNiO 3 shows a temperature-driven MI transition.As was reported earlier, 10,15 the resistivity (T) jumps by more than one order of magnitude in a temperature interval of 20 K.It shows a hysteretic behavior around T MI .Above T MI , SmNiO 3 exhibits a metallic behavior; resistivity varies linearly with temperature: (T)ϭ 0 ϩA e-ph T. The linear (T) dependence indicates a conduction dominated by electron-phonon scattering.Here, 0 is residual resistivity and A e-ph gives the strength of electron-phonon interaction.Below T MI , there is a change to a semiconductinglike resistivity behavior, but resistivity does not follow a simple energy activation model.
When the transition-metal sublattice is doped lightly with cobalt, T MI decreases.It shifts to 315, 252, and 163 K for xϭ0.03, 0.075, and 0.1, respectively.With increasing x the magnitude of the resistivity jump at MI transition decreases and the transition itself becomes broader.Moreover, the thermal hysteresis found in SmNiO 3 is not observed for 0.03 рxр0.1 for the same measurement conditions ͑see Fig. 4͒.MI transitions are also found for xϭ0.15 and xϭ0.2.Here, they occur at 73 K for xϭ0.15 and at 25 K for xϭ0.2, with a jump of resistivity of two orders of magnitude.In addition, the thermal hysteresis is large for these samples ͑about 50 K͒.In samples with 0.15рxр0.2, the onset of the magnetic arrangement T N coincides with T MI , while for xр0.10 oxides, T MI ϾT N .The data for 0.15рxр0.2are quite similar to the reported ones 16 for NdNiO 3 and NdNi 1Ϫx Cu x O 3 series, where T MI ϭT N .
In the inset of Fig. 4, ln vs 1/T plots are presented for 0рxр0.2samples.A continuous decrease in the slope of ln (1/T) is observed, denoting a decrease in the activation energy, E a ϭd ln /d(1/T), with the temperature.Resistivity data cannot be fitted either to the Arrhenious law, ln(/ 0 ) ϭE a /K B T, or to other simple activation models of form (T)ϭ 0 exp(⌬/K B T) p with pϭ 1 2 or pϭ 1 4 .Here, K B is the Boltzman constant, 0 is the value of resistivity in T→ϱ limit, while E a or ⌬ are activation energies.
In an earlier paper 10 we have pointed out a small anomaly in the SmNiO 3 resistivity data which occurs at 220 K in the semiconducting regime.It coincides with the T N for this compound.This anomaly is clearly visible in the derivative d(ln )/dT vs T plots.In Fig. 5  tion range.(T) changes one order of magnitude for x ϭ0.3 and more than eight orders of magnitude for xϭ0.9 in the temperature range studied.Several conductions regimes can be seen depending on the temperature.A constant activation energy regime is observed between 100 and 250 K.The resistivity data for 100рTр250 K can be fitted to the Arrhenious law yielding E a values of 5, 19, 40, and 67 meV for xϭ0.3, 0.5, 0.7, and 0.9, respectively.In the inset of Fig. 6, ln vs 1/T plots are shown for xу0.3.Below 100 K, E a decreases with decreasing temperature.For Tр50 K, resistivity data are fitted to (T)ϭ 0 exp͓(⌬/K B T) p ͔.The best fit yields pϷ0.25 which indicates a variable range hopping ͑VRH͒ conduction mechanism.Thus we observe a change from pϭ1 to pϭ 1 4 between 100 and 50 K.Such continuous change from thermal carrier activation or nearest-neighbor hopping (pϭ1) at high temperatures to VRH (pϭ 1 4 ) at low temperatures often occurs in disordered systems. 17Above 250 K, E a increases with increasing temperature and reaches a maximum at about 500-600 K. E a decreases with increasing temperature thereafter.This behavior can be related to the changes in the spin state of the Co 3ϩ ions ͑see inset of Fig. 1͒.Therefore high-temperature resistivity (300рT р700 K) for 0.3рxр0.9shows a crossover from activated conduction to the so-called degenerated metallic regime 18 at about 600 K.It shows a MI transition quite similar to that reported by Yamaguchi, Okimoto, and Tokura 18 for SmCoO 3 and other RCoO 3 oxides.
Samples with xϭ0.5 and xϭ0.2 were chosen for magnetoresistance ͑MR͒ studies in SmNi 1Ϫx Co x O 3 series.Figure 7 shows resistivity versus temperature under external magnetic fields of 0 and 5 T for xϭ0.5.MR ratio, defined as MR ϭ100͓( 0 H)Ϫ(0)͔/(0), is also plotted.The two curves ͑0͒ and ( 0 Hϭ5 T) are nearly identical for T Ͼ25 K. Negative MR is observed only at low temperatures, below the freezing temperature T f .Therefore magnetoresistance effects are coupled to the spin-glass-like behavior.Magnetoresistance as a function of the external magnetic field at different temperatures is shown in the inset of Fig. 7.At Tϭ15 K, SmNi 0.5 Co 0.5 O 3 shows an approximately linear, nonsaturated MR up to 12 T, with a MR ratio 8 of about 45% at highest applied fields.Similar behavior of MR have been observed in mixed-valence RM O 3 perovskites, such as cobaltates 19 La 1Ϫx Sr x CoO 3 , manganites 20 Tb 1/3 La 1/3 Ca 1/3 MnO 3 or La 2/3 Ca 1/3 MnO 3 doped 21 with Al and In.
Figure 8 shows the temperature dependence of resistivity under magnetic fields of 0 and 10 T for xϭ0.2.Negative MR is found below the onset of the magnetic arrangement at T N Ϸ48 K.For this sample, the temperature-driven MI transition and AF ordering take place at the same temperature, T MI ϭT N .In the inset of Fig. 8 isotherms of MR, up to 12 T, are presented.The MR behavior at Tϭ40 K is the most striking one.The values of MR at this temperature depend on the thermal history of sample and show temperature and field hysteresis.In spite of the AF arrangement, suggested by the magnetic susceptibility behavior ͑see Fig. 1 and Table I͒, the negative MR effect and its hysteresis seem to indicate that magnetic interactions for xϭ0.2 are more complex, likely with ferromagnetic contributions.These data support the idea that cobalt doping induces short-range ferromagnetic correlations in the Ni/Co sublattice.These correlations are progressively enhanced as x increases, and lead to a change from AF to a spin-glass-like behavior at xϷ0.3.

C. Magnetic and transport phase diagram
Figure 9 shows a phase diagram for the SmNi 1Ϫx Co x O 3 series which has been constructed using our results of the magnetic and transport measurements.Here, the temperature-driven MI transitions have been represented by solid circles.The ground state (T→0) for all of the compositions is semiconducting.However, there is a sharp change at xϷ0.2 which divides the phase diagram in two principal regions with different transport behavior.For xр0.2, cobalt doping favors the metallic state.T MI values decrease linearly with x at approximately a constant rate of 2000 K/x.Resistivity data in semiconducting regime cannot be fitted to a simple activation law.Such features resemble the properties of RNi 1Ϫx Cu x O 3 series. 16At xϷ0.3, resistivity data suggest a crossover to another regime with different transport mechanisms.For 0.3рxр1.0and below 500 K, the conduction is thermally activated.A constant activation energy is well defined between 100 and 250 K and varies from 5 meV for x ϭ0.3 up to 67 meV for xϭ0.9.We will propose two simple models for both regions.The insulating states for xр0.2 likely come out from strong electronic and antiferromagnetic correlations, as it has been proposed for the RNiO 3 series. 10herefore the corresponding MI transition is Mott-like.On the other hand, the structural and magnetic disorder could be responsible for the xу0.3 insulator state ͑Anderson transi-tion͒.
The onset of the magnetic transitions, either the Ne ´el temperatures T N or the freezing temperatures T f , are represented by crosses.Our magnetic data lead to distinguish the following three magnetic regions at low temperatures: ͑a͒ For xр0.2, AF interactions between the Ni 3ϩ ions are predominant and the SmNiO 3 magnetic structure remains up to xϭ0.2.Nevertheless, cobalt doping induces progressively an increase in the ferromagnetic correlations.Neutrondiffraction experiments in this region would provide the necessary evidence to support this conclusion.
͑b͒ For 0.3рxр0.8, the randomness and the competition between the ferromagnetic and AF interactions bring about a spin-glass-like state.It shows spin-blocking characteristic features and a spontaneous magnetization at low temperatures.
͑c͒ For 0.9рxр1, no sign of magnetic ordering above Tϭ4 K has been observed.The ferromagnetic correlations, which are evident at intermediate x concentrations, have disappeared.A paramagnetic behavior with predominant LS Co 3ϩ ions, t 2g 6 e g 0 , follows.

IV. DISCUSSION
The electronic configurations of the nickel and cobalt ions in RNiO 3 and RCoO 3 compounds are LS Ni 3ϩ (t 2g 6 e g 1 , Sϭ Electronegativity of Ni 3ϩ creates a highly covalent nickel ground state in the RNiO 3 series through Ni3d and O2p hybridization. 3New electronic states out of the formally LS 3ϩ configurations can be put forward in the SmNi 1Ϫx Co x O 3 series.Ni 3ϩ electronegativity could lead either to a charge transfer Ni 3ϩ ϩCo 3ϩ Ni 2ϩ ϩCo 4ϩ or to a stabilization of the high spin Co 3ϩ configuration (Sϭ1 or Sϭ2) as a result of a small local lattice distortion. 22These new stabilized Ni 2ϩ , Co 3ϩ , or Co 4ϩ configurations are not a majority in the M ϭNi/Co sublattice but they permit formation of small clusters with magnetic interactions embedded in the paramagnetic matrix.A kind of dynamic mechanism ͑hopping͒ with spin memory may bring about ferromagnetic interactions within these clusters.As the spin of the carrier is coupled to the atomic internal magnetic moment following the Hund rule, the maximum hopping probability occurs when the two nearest sites are ferromagnetically aligned.An external magnetic field enhances this alignment and negative magnetoresistance effect appears.A similar carrier hopping mechanism, induced by double exchange ͑DE͒ interactions, is commonly used to explain the magnetoresistance in mixedvalence manganites and cobaltates. 19,23Therefore these ideas allow us to propose an exchange mechanism similar to DE but taking place between different transition-metal ions as a plausible origin for the spin-glass-like state and the coupled magnetoresistance for intermediate x concentration in the SmNi 1Ϫx Co x O 3 series.However, the DE between Co ions with different valence can also contribute to the observed behavior.In both cases, the presence of Ni is necessary to induce changes in the electronic state of the Co ions.
The insulator ground state and the temperature-driven MI transition for the RNiO 3 have been explained by a competition between two main factors. 3,10,24On the one hand, hybridization between the Ni3d and O2 p orbitals which gives the conduction bandwidth W favors metallic behavior.On the other hand, the strong electronic and antiferromagnetic correlations lead to a carrier localization.The presence of spin polarons with large effective mass and strong local AF correlations has been suggested for these systems. 10Therefore all these properties place RNiO 3 on the borderline between the models proposed for Mott insulators ͑in this case, charge-transfer type͒ and for strong correlated metallic systems. 25hen RNiO 3 oxides are doped with transition metals ͑RNi 1Ϫx M x O 3 series with M 3ϩ ϭCu 3ϩ , Co 3ϩ , Fe 3ϩ ), T MI decreases drastically.Qualitatively, the decrease is not related to the M 3ϩ ionic configuration and is independent of the nature of the doping element. 26The temperature-driven MI transition is progressively suppressed by either electron (Cu 3ϩ ) or hole (Co 3ϩ , Fe 3ϩ ) injection in the conduction band.We propose that a weakening of the AF correlations can explain these results.A similar T MI decrease and the related stabilization of the metallic conduction by doping was extensively studied for the RNi 1Ϫx Cu x O 3 series, where metallic phases have been obtained upon doping 16 with Cu.SmNi 1Ϫx Co x O 3 oxides with xр0.2 show the same behavior.A Mott insulator ͑charge-transfer type͒ regime is proposed as the origin for the thermally activated conduction at low temperatures in this concentration range.
The above arguments suggest that metallic ground state should exist for xу0.3.On the contrary, a new semiconducting behavior appears.In order to understand the possible origin for these states we propose a simple model based on the Anderson localization.In SmNi 1Ϫx Co x O 3 series, the conduction band associated with the e g electrons is made up of two different overlapping 3d-ion orbitals as Co and Ni are located at random.Therefore the tail of conduction band with localized states is formed. 27There exists a critical energy in the conduction band, the mobility edge (E ), which separates localized and extended energy levels.In this model, the appearance of metallic or nonmetallic conduction depends on the relative position of the Fermi energy level E F and the mobility edge E .By changing the concentration of the carriers, for instance by electron doping, E F can cross the mobility edge and an the Anderson transition is expected.An analogous situation would occur for hole doping.
We propose, for highly doped SmNi 1Ϫx Co x O 3 oxides, x у0.3, the existence of the Anderson localization.First, in these compounds, the electronic and AF correlations, which are responsible for the MI transition in the RNiO 3 series, have been practically removed by doping.Then, the disorder effects give rise to a semiconducting ground state.When Co replaces Ni atoms, the carrier concentration is reduced.The Fermi level likely moves below the mobility edge, and a thermally activated hopping conduction occurs.Activation energy is proportional to E ϪE F .The activation energy dependence found in the 100рTр250 K range for the 0.3 рxϽ0.9SmNi 1Ϫx Co x O 3 oxides supports this model.

V. CONCLUSION
The SmNi 1Ϫx Co x O 3 series have been extensively studied by means of XRD, ac and dc magnetic susceptibility, dc resistance, and magnetoresistance in order to establish the phase diagram from 4 K up to 700 K. Below xр0.2, the temperatures of MI transition and of the AF ordering, which have been observed in SmNiO 3 , decrease with increasing x.This leads to three regions in the phase diagram: paramagnetic metal, paramagnetic insulator, and antiferromagnetic insulator.Between xϭ0.2 and xϭ0.3, a sharp change in the magnetic and transport properties occurs.A new semiconducting ground state appears and short-range ferromagnetic interactions are evident.These give rise to new paramagnetic-metal, paramagnetic-insulator, and spin-glassinsulator region for 0.3рxр1.
The origin of the spin-glass state and of the negative magnetoresistance found for intermediate x values have been discussed in terms of the high Ni 3ϩ electronegativity.New electronic configurations for the nickel and cobalt ions, out of the low spin ϩ3 valence states, as well as short-range ferromagnetic interactions induced by a dynamic mechanism with spin memory can account for such features.Finally, different models have been proposed in order to explain the transport behavior along the series.Spin polarons with highly effective local AF correlations, which explain the properties of SmNi 1Ϫx Co x O 3 for 0рxр0.2,are progressively removed by Co doping.The new semiconducting regime for 0.3рx р0.9 is mainly determined by disorder effects.
and xϭ0.2, the susceptibility curves peak at 95 and 48 K, respectively, which coincide with the MI transition reported below.The T N values obtained for xϭ0.15 and xϭ0.20 samples are far away from the expected values for T N predicted by the T N (x) dependence found for xϽ0.10.The inset of Fig. 1

FIG. 7 .
FIG. 7. Temperature dependence of magnetoresistance and electrical resistivity in zero field and in a magnetic field of 5 T for SmNi 0.5 Co 0.5 O 3 .The inset shows in detail the magnetoresistance curves ͓(H)Ϫ(0)͔/(0) vs H at the indicated temperatures.Solid lines are guides to the eyes.

FIG. 9 .
FIG. 9. Schematic phase diagram of SmNi 1Ϫx Co x O 3 series.Cobalt concentration ͑x͒ is shown along the X axis and temperature along the Y axis.Solid circles correspond to T MI temperatures, crosses to T N and T f , and open circles to the kink in the resistivity data ͑see text͒.AF represents an antiferromagnetic regime, SG spinglass-like, P paramagnetism, M metallic, and I the two semiconducting regimes described in the text.Solid lines are the proposed boundary regions.

TABLE I .
Lattice parameters and magnetic constants for SmNi 1Ϫx Co x O 3 series.Magnetic constants were obtained by fitting Ni/Co , Ni/Co (T)ϭ(T)Ϫ Sm 3ϩ, to the equation Ni/Co (T)ϭ o ϩC/(TϪ⌰).The temperature range was 100-250 K except for xϭ0.15 ͑125-250 K͒ and for xϭ0 ͑280-400 K͒.These limits are imposed by both the MI transition of some of the samples and the spin transition of Co ions.The expected paramagnetic effective moments are: teo ϭ͓(1Ϫx)( LS Ni 3ϩ) 2 ͔ 1/2 ¯(*).The data of SmNiO 3 have been fitted to a Curie-Weiss law, the addition of the 0 parameter does not improve the fit significantly.
8ϭ0), respectively.Nevertheless, a recent study8of the paramagnetic susceptibility of the isostructural RNi 0.3 Co 0.7 O 3 , LaGa 0.3 Co 0.7 O 3 , and LaNi 0.3 Ga 0.7 O 3 compounds, where Ga 3ϩ is a 3d nonmagnetic (Sϭ0) ion, as well as the magnetic properties of the mixed RNi 1Ϫx Co x O 3 oxides with intermediate x values point out that new electronic configurations for nickel and cobalt ions are needed to bring about ferromagnetic interactions in the RNi 1Ϫx Co x O 3 series.