Charge ordering transition in GdBaCo2O5: evidence of reentrant behavior
M. Allieta, M. Scavini, L. Lopresti, M. Coduri, L. Loconte, S. Cappelli, C. Oliva, P. Ghigna, P. Pattison, V. Scagnoli
1 Charge ordering transition in GdBaCo O : evidence of reentrant behavior M. Allieta , M. Scavini , L. Lopresti , M. Coduri , L. Loconte , S. Cappelli , C. Oliva , P. Ghigna , P. Pattison , V. Scagnoli Università degli studi di Milano, Dipartimento di Chimica; CNR-ISTM, and INSTM, Unit Milan, Via C. Golgi 19, I-20133 Milan, Italy; Centre for Materials Crystallography, Århus University, Langelandsgade 140, DK-8000 Århus C., Denmark; Università degli Studi di Pavia, Dipartimento di Chimica, Viale Taramelli 16, 27100 Pavia, Italy; Swiss-Norwegian Beamlines, European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, 38043 Grenoble Cedex 9, France Laboratory of Crystallography, BSP, Ecole polytechnique fédérale de Lausanne, CH-1015 Lausanne, Switzerland European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, 38043 Grenoble Cedex 9, France; Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland. Abstract
We present a detailed study on the charge ordering (CO) transition in GdBaCo O system by combining high resolution synchrotron powder/single crystal diffraction with electron paramagnetic resonance (EPR) experiments as a function of temperature. We found a second order structural phase transition at T CO =247 K ( Pmmm to Pmma ) associated with the onset of long range CO. At T min ≈ T CO, the EPR linewidth rapidly broadens providing evidence of spin fluctuations due to magnetic interactions between Gd ions and antiferromagnetic couplings of Co /Co sublattices. This likely indicates that, analogously to manganites, the long-range antiferromagnetic order in GdBaCo O sets in at ≈ T CO . Pair distribution function (PDF) analysis of diffraction data revealed signatures of structural inhomogeneities at low temperature. By comparing the average and local bond valences, we found that above T CO the local structure is consistent with a fully random occupation of Co and Co in a 1:1 ratio and with a complete charge ordering below T CO . Below T ≈
100 K the charge localization is partially melted at the local scale, suggesting a reentrant behavior of CO. This result is supported by the weakening of superstructure reflections and the temperature evolution of EPR linewidth that is consistent with paramagnetic (PM) reentrant behavior reported in the GdBaCo O parent compound. *Corresponding Authors: e-mail: [email protected], [email protected] INTRODUCTION
Charge ordering (CO) observed in mixed valence perovskites is an intriguing phenomenon which has attracted a lot of interest in the last two decades.
The stabilization of CO depends on the commensurability of the charge carrier density with respect to the lattice periodicity and is driven by the competition of the inter-site Coulomb and kinetic energies of electrons. The onset of CO phase triggered, e.g., by temperature ( T ), gives rise to an abrupt change in the transport and/or magnetic properties reflecting an intricate interplay between charge, spin, orbital and lattice degrees of freedom. One of the most remarkable examples of perovskitic oxides showing CO is the colossal magnetoresistive (CMR) R A MnO (R: trivalent ion, A: Ca, Sr) manganites. In these systems the charge carriers localization due to the occurrence of Mn /Mn CO pattern is accompanied by a sudden increase of the electrical resistivity and by the onset of antiferromagnetic (AFM) phase at T N ≤ T CO . Similar effects have been observed for the layered perovskite-like compounds Ln BaCo O δ ( Ln : Lanthanide or Y). These systems display a larger CMR effect than Mn-based perovskites and a variety of magnetic and transport properties, depending on oxygen concentration δ . In particular, δ affects the mixed valence state of the cobalt ions, resulting in different Co /Co and Co /Co ratios with cobalt species stabilizing in several spin states. In the case of Ln BaCo O system ( Ln = Y , Tb , Dy , Ho ), where a Co /Co ratio equal to 1, neutron powder diffraction (NPD) shows the insurgence of both charge and magnetic ordering associated with structural phase transitions. In the high- T paramagnetic (PM) state, these compounds display a tetragonal structure P mmm and with a × a ×2 a cell metric, where a is the primitive cubic perovskite lattice parameter (Fig 1 ( a )). At T N ≈
350 K, these systems undergo a magnetic transition leading to G -type AFM phase, associated to an orthorhombic-distorted lattice with Pmmm and the same a × a ×2 a cell metric (Fig 1 ( b )). Despite the symmetry lowering, a unique Co site in square pyramidal environment is still present in this phase. Below T ≈
210 K, the NPD measurements suggested the occurrence of 4 another structural phase transition leading to a change of both cell metric and crystal symmetry. In this new low- T phase with Pmma and a ×2 a ×2 a cell metric (Fig 1 ( c )), two distinct Co sites are present and the transition can be interpreted in terms of CO. The AFM structure in the charge ordered state is qualitatively explained by Goodenough-Kanamori (GK) rules for superexchange and ≈ µ B difference between the refined magnetic moments of the non-equivalent Co sites at saturation is fully consistent with a complete Co /Co separation. However, the bond valence sums (BVS) calculated for the Co-O distances are far from the ideal +2 and +3 values, suggesting that only partial charge redistribution arises at T CO . In addition, the occurrence of AFM ordering at higher T than for long range CO ( T N > T CO ) is opposite of what is observed in Mn-based compounds. This is the signature of a more complex scenario, indicating that magnetic, electronic and structural properties of Ln BaCo O systems still remain to be reconciled in a self-consistent picture. In GdBaCo O (GBCO), the occurrence of a CO transition below T ≈
250 K has been suggested by heat capacity and internal friction measurements. However, the existence of a CO-driven structural phase transition in this compound still remains unproven to date. In a previous work, we assigned the correct space group of GBCO at room temperature and we carefully studied the P4/mmm to Pmmm phase transition across T N ≈
350 K. Here, we focus on the CO transition by combining X-ray powder diffraction (XRPD), synchrotron single crystal X-ray diffraction (SCD) and EPR spectroscopy on GBCO. These techniques are sensitive to either to CO or to the magnetic degree of freedom allowing us to explore the transition both from the structural and spin relaxation point of view. As a result, we deduced the presence of a second order structural phase transition associated with CO at T =247K. By cooling down to T ≈
100 K, we observed a local structural response consistent with a reentrant behavior of CO. The EPR measurements suggest that the origin of CO as well as reentrant behavior is driven by an intimate interplay of electron localization and magnetic spin (dis)ordering. 5
EXPERIMENTAL
The same single crystal and powdered GBCO samples employed in our previous study were used for the present diffraction measurements with synchrotron X-rays. SCD experiments were performed at the BM1A station of the Swiss-Norwegian beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). Data acquisitions were recorded by double-crystal Si(111) monochromated X-ray radiation ( λ = 0.70826(2) Å) on a Kuma KM6-CH six-circle kappa goniometer equipped with an Oxford Diffraction CCD area detector and an Oxford Cryosystems N gas blower. The sample was mounted on the top of a glass capillary fibre and SCD data collections were performed at 9 different temperatures between 100 K and 380 K by three ω -scans (132, 136 and 216 deg wide) with ∆ω = 1 deg steps at fixed 2 ϑ , κ and ϕ axes (in degrees: 30, 30, 0; 30, –45, 0; 0, 0, 180). Overall, this scheme provided, on average, a ≈
97% complete dataset within the Cu-sphere resolution (sin ϑ/λ = 0.652 Å -1 ). Moreover, the temperature evolution of the intensity of the Pmma (116) reflection, extinct in the high- T phases, was monitored by repeated ϕ scans. CrysAlysPro was employed to perform the data collection and reduction procedures. The unit cell parameters at each temperature (see Table S1 in the SI ) were determined from least-square fitting of the orientation matrix against the observed peak positions of, on average, ≈
800 intense reflections. Empirical absorption correction based on multiple measures of equivalent reflections was applied to all the data sets according to the SCALE3 ABSPACK algorithm The crystal structure at each temperature was refined with SHELX. Assessment of the sample quality can be found in the SI . XRPD experiments were performed at the ID31 beamline of the ESRF by selecting λ = 0.39620(5)Å. Twelve patterns in the 0 ≤ ϑ ≤ range data were collected for 1 hour counting time between 5K and 298K. The sample was cooled down to 80 K by using a N gas blower (Oxford Cryosystems), whereas a liquid-helium-cooled cryostat was employed to achieve the lowest temperature. Moreover, some patterns were collected at λ = 0.35422(5) Å in the 80 ≤ T ≤
400 K 6 range. In particular, at selected temperatures ( T =80, T =180K and T =298 K) several scans were summed up for 7 hours counting time ( Q max ≈
27 Å -1 ) to achieve statistical significance for Pair distribution function (PDF) analysis. Data were analyzed using the Rietveld method as implemented in the GSAS software suite. Absorption correction was performed through the Lobanov empirical formula implemented for the Debye-Scherrer geometry. Line profiles were fitted using a modified pseudo-Voigt function accounting for asymmetry correction. In the last refinement cycles, scale factor(s), cell parameters, positional coordinates and isotropic thermal parameters were allowed to vary as well as background and line profile parameters. EPR measurements were performed at a Bruker ELEXSYS spectrometer equipped with an ER4102ST standard rectangular cavity at X band (9.4 GHz) frequency every 5 K in the temperature range 115-450 K. The derivative d P /d H of power P absorbed was recorded as a function of the static magnetic field H. RESULTS I. SCD and XRPD across the CO transition
To probe the occurrence of low T structural phase transitions associated with possible CO effects, the reciprocal lattice as determined from SCD data was carefully screened to detect possible superstructure reflections. Actually, some commensurate superstructure diffraction spots with h /2 indices clearly appeared below 250 K, implying the low temperature doubling of the cell axis. Conversely, no recognizable superstructure peaks were detected above T ≈
250 K. SCD structural models as a function of T were obtained within the spherical atom approximation, with the thermal motion of heavy metal atoms modelled as anisotropic. The Pmmm room-temperature structure was employed as a suitable starting point for refining the positional and thermal motion parameters down to 250 K. Below T = 250 K, we adopted the structural Pmma et al. as a starting point for the least-square refinement procedure. Table S1 of the SI summarizes the final statistics of the structural refinements, together with relevant details of the SCD experiments. As reported in a similar structural study, the transition from Pmmm to Pmma with a ×2 a ×2 a unit cell can be followed by the temperature dependence of ( hkl ) reflections with h =2 n +1 and l ≠
0. Figure 2 ( a ) shows the temperature dependence of the intensity of the Pmma (116) superstructure reflection, that is extinct in the
Pmmm phase. On the basis of these results, T CO =247 K can be assumed as a reasonable estimate for the CO transition temperature, as at this temperature the (116) superstructure reflection begins to be clearly significant ( ≈ I ( T )= I (0)( T c - T / T c ) β with T c = T CO = 247 K, the least-square estimate of the critical exponent β came out as large as 0.483(5) (inset of Fig.2( a )), quite close to the ideal value for a second-order transition ( β = 0.5). It is worth noting that, for T < ≈
120 K, the temperature dependence of (116) shows a slightly positive slope. Clear signature of the CO transition was also found by analyzing the XRPD patterns through, e.g., the appearance of the
Pmma (102) reflection across T CO (Fig.2( b ), inset). An accurate investigation of XRPD patterns does not reveal further peaks splitting as well as superstructure reflections corresponding to a modulation of a ×2 a ×2 a periodicity down to 5 K. The structural parameters obtained from SCD data were used as starting values to perform Rietveld refinements against XRPD data as a function of temperature. Figure 3 shows an example of the Rietveld refinement performed at 5 K. From the quality of the fit it can be seen that XRPD patterns in the CO state can be well reproduced down to the lowest temperature by the Pmma superstructure. In Table S2 of the SI , selected structural data and agreement factors obtained for all patterns collected at different temperatures are listed. In Pmmm and
Pmma structural models, we constrained isotropic thermal parameters related to oxygen positions to be the same. In Fig. 4, we present the refined lattice parameters and cell volume as a function of T obtained by SCD and XRPD. A reduced orthorhombic cell metric was used for comparison purposed between 8 the Pmmm and
Pmma structures. As previously reported, the structure is tetragonal at T > T N and with decreasing T the orthorhombic distortion turns out to be appreciable in terms of the lattice metrics. In particular, below T N the distortion becomes more evident and the difference between a and b cell edges increases toward the CO transition (see Fig.4) The a -axis suddenly increases while the b -axis steeply decreases at T < T CO and this anisotropic thermal expansion of the unit cell parameters is maintained down to T ≈
100 K. Below this temperature, both the axes reach a plateau by approaching constant values down to T = 5 K. This complex behaviour suggests an intricate interplay between CO and the atomic interactions on the ab -plane of GBCO. It worth noting that, as shown in Fig.4 (a), (c), there are some differences between the results obtained from the SCD and XRPD techniques. By approaching the T CO from above, a and b axes obtained from the two techniques are in fairly good agreement up to 298 K, but for T < T CO the anisotropic thermal expansion of the unit cell parameters seems to be different. In particular, in SCD results also the b lattice parameter exhibits a small lengthening across transition, resulting in an overall increase of the unit cell volume at T CO (Fig.4 (c), full circles). This behavior was not detected by XRPD that, on the contrary, found only a weak variation of the slope of the temperature dependence of V across T CO . However, looking at the general trends of the structural parameters across the CO transition (Fig. 4), we can say that – at least qualitatively – SCD and XRPD gave similar results. In the following discussion, we will focus on the Co valence states and their interplay with the corresponding average Co coordination geometries. Selected Co-O distances ( d Co-O ) as a function of T are reported in Fig.5( a ), ( b ). The presence of two distinct Co crystallographic sites in the Pmma
CO phase gives rise to an anisotropic thermal expansion of d Co-O . In particular, by decreasing T below T CO , the d Co2-O1/O2 expands while d Co1-O1/O2 suddenly shrinks. Both distances approach constant values down to 5K. The same behavior is shown by the mean Co-O distances (< d Co-O >) related to the Co2 and Co1 sites (Fig.5(c)). The evolution of d Co-O across T CO provides evidence of a smaller volume of the Co1O with respect to 9 the Co2O square pyramid. According to tabulated Shannon ionic radii, this is consistent with Co1 and Co2 sites occupied by Co and Co , respectively. To evaluate ionic charges of Co from the experimental d Co-O, we calculated BVS by using the tabulated parameters. The results for all the cations of GBCO are shown in Fig. 6. BVS calculations for Gd and Ba cations gave reliable results yielding to valences of ≈ ≈ T CO , BVS related to Co provides evidence that the sites are occupied by cations in mixed valence +2.3, a value slightly lower than that expected for a 1:1 ratio of Co /Co . Below T CO , BVS found valences as large as ≈ ≈ al. , the Pmma structure can be described by an ordered alternation of Co O and Co O pyramids stacked along the a and c axes, while both Co O and Co O pyramids are allowed to run along the [010] direction (Fig. 1). It should be noted that the estimated valence of the Co /Co and Co1 sites are markedly lower than +2.5 and +3 expected values, respectively. Such deviations from formal reference values are rather common in perovskite compounds and the observed valence depletion can be possibly due to a charge disorder affecting the Co sites. Moreover, this behavior is consistent with data reported for Ho and Tb parent compounds, for which the calculated BVS suggest partial charge redistribution at both Co1 and Co2 sites. II. PDF across the charge ordering transition
PDF analysis of the XRPD data collected at T = 80 K, 180 K, 298 K was carried out using the formalism of G ( r ) functions. G ( r ) is obtained via the sine Fourier Transform (FT) according to : ( ) QQrQSQrG Q )dsin(1]-)([2 max ∫ = π (1) 10 where S ( Q ) is the total scattering structure function, Q = sin θ/λ and r is defined in the space of interatomic distances . S ( Q ) is calculated from the experimental scattering intensity I coh ( Q ) containing both Bragg and diffuse scattering contributions. To consistently evaluate I coh ( Q ), the raw diffracted intensity profile I ( Q ) collected at each temperature was corrected for background scattering, attenuation in the sample, multiple and Compton scattering. The reduction process was done using the PDFGetX2 software and full-structure profile refinements were carried out on the observed G ( r ) using the PDFgui program. The program assesses the degree of accuracy of the refinement by the agreement factor R w defined as: )( )( −= ∑∑ ii calciiiW Gw GGwR (2) Figure 7 (a) shows the experimental PDF curves obtained in the 1.3Å ≤ r ≤
10Å range. Each positive peak in G ( r ) function is proportional to the probability of finding two atoms separated by a distance r averaged over all pairs of atoms in the sample. The main temperature induced fluctuations in G( r ) curves are given by the peak sharpening observed upon cooling. This is consistent with Debye law and therefore it cannot be considered as a response of the PDF to CO transition. The experimental profiles were fitted in the 1.8 ≤ r ≤
30 Å range using the same structural models employed for interpreting the SCD and XRPD patterns. In general, they gave a good description of the PDF in a wide range of r , as testified by the good R w values obtained: 0.082 ( T =80 K), 0.083 ( T =180 K) and 0.079 ( T =298 K). Focusing on the very short r range (1.8 ≤ r ≤ ≈ T dependence in agreement with the reciprocal space analysis (see Fig.S1 in the SI). On the other hand, the short range Co-O next neighbor distances (in the 1.9 ≤ r ≤ T . At 298 K PDF displays just a single peak near 2 Ǻ besides the termination ripple at r ≈ a )). At 180 K a bimodal distribution of the same PDF peak is found in agreement with the CO Pmma structure (Fig.8( b )). Indeed, below T CO the originally single
100 K (Fig. 2(b)). Below this temperature, its intensity begins to slightly decrease. At T ≈
5 K it roughly shows the same 12 value as that measured at ≈
200 K, which is ≈
60% of its maximum value. The observed weakening of the (102) intensity is then consistent with a reduction of the CO state strength and agrees with the PDF outcome at T =80K. III EPR across the charge ordering transition
Figure 9 shows the EPR spectra as a function of temperature for GBCO sample. As found in GdBaCo O δ compounds with δ ≥ resonance, that in turn is caused by exchange interaction between localized 4 f electron spin and the spins of Co atoms. Looking at the temperature dependence of the EPR spectra, it is clear that the signals markedly change across the structural transitions. In general, the available analytical functions well describe the shape of EPR spectra at each temperature but the fits performed gave meaningless parameters. We decided to extract the peak-to-peak linewidth ( ∆ H pp ) as a direct observation of EPR data as depicted in the inset of Fig.9. Moreover, since in our spectra the baseline is not well defined, we cannot directly determine the peak position. Hence, we will consider only the ∆ H pp parameter throughout. The temperature dependence of ∆ H pp is shown in Fig.10. In high- T PM regime regime (370 K ≤ T ≤
450 K), ∆ H pp decreases linearly by following the Korringa-type relation ∆ H pp = ∆ H pp0 + bT with ∆ H pp0 = 2858(8) G and b = 2.27(2) G/K. By further cooling, ∆ H pp shows a departure from the linear narrowing. On approaching T CO upon cooling, ∆ H pp shows a weak drop at T ≈
365 K close to T N and it goes through a minimum at T min ≈
300 K. Below T min. the ∆ H pp has a complex thermal behavior. It displays a fast broadening reaching a maximum value at T ≈ T CO and then rapidly decreases below T CO . Interestingly, below 160K the ∆ H pp seems to follow the linear temperature dependence observed at high T in the PM phase. 13 DISCUSSION
GBCO undergoes two structural phase transitions upon cooling. In our previous investigation we found the first crystallographic transformation associated with a second-order phase transition. In particular, at T N ≈ P mmm ) to orthorhombic ( Pmmm ), inducing the loss of the C axis along c . The breaking of the tetragonal symmetry perturbs the coordination environment around Gd and Co, and one half of oxygen atoms on the CoO plane become symmetry-independent at T < T N . Below T CO =247 K, a second structural phase transition takes place associated with the CO of the Co /Co ions. The space group changes from Pmmm to Pmma , with the simultaneous doubling of a axis. The analysis of the temperature dependence of the intensity of the Pmma (116) superstructure reflection across this transition showed a critical exponent close to the ideal value for a second-order transition. However, Fauth et al. studied the CO structural phase transition in the Tb, Dy and Ho parent compounds and some differences have to be noted. First, the transition temperature is significantly higher for the GdBaCo O system ( T CO = 247 K vs T CO = 205-215 K) suggesting that T CO increases with the size of the rare earth ion. It should be noted that such an increase is at least three times higher on passing from Tb to Gd ( ≈
30 K) than from Ho to Tb ( ≈
10 K). Secondly, Fauth et al . interpreted the CO phase transition as first order on the basis of their DSC measurements on the HoBaCo O sample. All of this evidence suggest that the effect of Gd on the transport and magnetic properties of the GBCO structure is critical and it implies a significant increase of the range of existence of the low- T CO Pmma phase. By further cooling, we found the signatures of a new structural anomaly. The depletion of the
Pmma (102) superstructure reflection below T ≈
100 K indicates that the CO state emerges at T CO but weakens below ≈
100 K. Accordingly, at T = 80 K the outcomes of our PDF analysis suggest that GBCO seems to recover a charge-disordered phase at the local scale. Due to the large width of the peak, the distance distribution of the Co-O first coordination shell is still consistent with the Pmma structural model. However, the corresponding local BVS value suggests a partial melting of 14 the charge localization. This structural difference between the local and the long range scales is well known to occur in many strongly correlated perovskite-like oxides showing properties such as magnetodieletric or superconductivity . Reentrant CO behavior has been reported on LaSr Mn O layered perovskite and its origin has been linked to the occurrence of a ferromagnetic-metallic state upon cooling. As for the present case, possible reasons of this behavior can be investigated by considering the temperature evolution of spin relaxation as probed by EPR. Different relaxation mechanisms are known to result in a linear temperature dependence of ∆ H pp above the CO transition. The decreasing ∆ H pp on cooling can be mediated by the exchange scattering between the local moments (4f Gd localized states) and itinerant charge carriers of Co ions. In the absence of bottleneck and dynamic effects, the b in Korringa-type relation is given by: )( JENgkb
FBB µπ= (3) where the term J fs is the exchange coupling constant between the conduction electrons and localized spin and N ( E F ) stands for the density states at the Fermi energy. From the linear fit of our EPR data in 360 ≤ T ≤
450 K range we found a low value of b ≈ N ( E F ) value. This is consistent with the fairly low resistivity reported for GdBaCo O δ ( δ →
0) systems ( ≈ Ω cm at T = 370 K ), which implies a non-negligible delocalization for conduction electron in GBCO. To account for the Korringa-type behavior in charge disordered PM phase, we consider that the electron delocalization of conduction electrons can be due to the motion of an extra electron from e g level of Co ion in high spin (HS) configurations to an empty e g orbital of Co ion in intermediate spin (IS) state. Note that these simple models imply a double exchange (DE) interactions between Co ions in different spin states, and they have been widely used to explain the insulator-to-metal transition observed in
15 GdBaCo O with δ ≈ The charge disordered GBCO phase is characterized by a random distribution of Co /Co and e g electron hopping from an HSCo to next ISCo can give rise to electron delocalization in all the spatial directions. These isotropic DE paths between two pyramidal sites, defined as A and A can be sketched as: gg et -HSCo + gg et -ISCo → gg et -ISCo + gg et -HSCo (4) It is worth to note that these exchange channels are reminiscent of the well known DE between Mn and Mn invoked to correlate magnetic and electronic properties in mixed valence CO manganites. While the PM contribution of Gd dominates the magnetization of GBCO, the temperature dependence of magnetization of YBaCo O – the only parent compound with non magnetic ions – shows two drops at T N and T CO . ∆ H pp data observed on GBCO show a weak upturn at T ≈
365 K, which possibly mimics the drop observed on YBaCo O magnetization around T N . This temperature above T N perfectly matches with the onset of the tetragonal (200) peak broadening precursor of the P mmm to Pmmm transition. This provides a clear evidence which links the onset of spin ordering to the structural phase transitions in GBCO. Basically, the emergence of AFM can be discussed by assuming that the structural transition is accompanied by a crossover from DE to super-exchange Co-O-Co interactions. According to GK rules the relevant super-exchange couplings which hold for the AFM interactions in the Pmmm phase are between HS-Co -ISCo along the [001] direction and HSCo -HSCo , IS-Co -ISCo along the [010] direction. However, these magnetic interactions can be considered dominant only if a preliminary Co /Co ordering is assumed at T N . Actually, incomplete or short range charge ordering has been invoked to explain the resistivity measurements in the T CO ≤ T ≤ T N range of LnBaCo O parent compound. However, we did not observe any signature of the CO phase in the short range PDF of GBCO at RT. 16 AFM interactions between Co ions and partial conduction electron delocalization can generate short-range AFM correlation between Gd through the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction. In the T CO ≤ T ≤ T N range, the conduction electrons are mobile to some extent and they can transfer the Co AFM spin fluctuations to the Gd sites via the RKKY mechanism. This gives rise to an effective alternating internal magnetic field which relaxes the Gd spins. In principle, upon cooling one could expect the strengthening of AFM interaction between Co ions and an increase of the spin relaxation rate of Gd , which produces the broadening of the EPR signal. This is exactly what is observed in our EPR data below T min ≈
300 K (see Fig.10). The EPR line broadening while decreasing T has been interpreted as a precursor behavior of AFM transitions. According to available theories, the critical increase of ∆ H pp due to AFM spin fluctuations can be described by the relation ∆ H pp = a [( T - T N )/ T N ] - m where a and m are constants. In our data, ∆ H pp starts to increase at T min. ~ 1.2 T CO and rapidly broadens as T CO is approached from above. In the inset of Fig.10 the log-log plot of ∆ H pp as a function of [( T - T CO )/ T CO ] is shown. A linear correlation is evident just above T CO , analogue to that expected near T N for AFM spin fluctuations. We determined m = - 0.12 in agreement with that found in other layered compounds where the Gd spin dynamics is affected by 3d metal spins fluctuations.
26, 27
The CO transition in GBCO seems to be related to spin fluctuations involving Gd ions. This implies that T N ≈
250 K ≈ T CO should be set as the actual Néel temperature. When T is further lowered, long-range AFM order is driven by a long range CO. On the other hand, we speculate that the transition at 350K is more related to the onset of incomplete CO giving rise to short range AFM interaction, as also previously suggested. This is consistent with the magnetization of YBaCo O which shows a more marked drop at T CO than T N . The emergence of long-range CO is therefore associated with a strong localization of the conduction electrons that weakens the RKKY coupling between Co and Gd below T CO . Since the super-exchange interactions are stronger and not frustrated within the 3D CO network, the overall AFM coupling increases with CO distortion. The internal field owing to spin fluctuations gradually disappears and the EPR signal is dominated by the PM contribution of Gd spins, which narrows 17 ∆ H pp on cooling. Below 160 K, ∆ H pp seems to follow the same linear temperature dependence observed at high- T . Looking at our data in the low- T region, the temperature dependence of ∆ H pp is indeed similar to that observed in the PM phase. Reentrant PM behavior has been also observed at T = 75 K in GdBaCo O and the transition has been associated to a spin state transition (SST) from IS to low spin (LS) gg et state at Co site. SST involving the same Co spin states was also reported for LaCoO at T ≈
100 K. Moreover, the switch from IS to LS of Co introduces non-magnetic ions within the super-exchange framework of the CO phase of GBCO. As a result, super-exchange couplings involving Co disappears, disrupting the AFM order that gradually weakens on cooling. Since in the CO phase the electron localization is mainly driven by AFM couplings between ordered Co /Co , the emergence of PM phase at low- T well matches the partial melting of charges localization observed from structural analysis. To explain the interplay between the structural and magnetic ground state evolution as a function of T in GBCO we propose the following mechanism. According to DE between the randomly distributed HSCo and ISCo , the conduction electrons in the high- T PM phase are delocalized to some extent. On going through T N ≈
350 K, the Co /Co ions start to partially order, leading to the coexistence of both localized and delocalized charge carriers. Because of incomplete CO, the crossover from DE to super-exchange interactions gives rise to RKKY couplings between short-range AFM and Gd spins. At T min ≈ T CO the emergence of AFM spin fluctuations are a precursor effect of the AFM transition associated with the onset of CO. Once the long range CO is initiated the density of delocalized conduction electron rapidly decreases. As a result, ∆ H pp decreases on cooling and below T ≈
160 K it gradually approaches the linear T -dependence observed in the high- T PM phase. Below this point, we observed the mismatch between the local and the average BVS and the decrease of superstructure reflection intensity suggesting a reentrant behaviour of CO. These latter observations, together with T N ≈ T CO , leads us to believe that CO layered cobaltite shows strong similarity with manganites. CONCLUSION
In the present study we have clearly revealed the intimate interplay of electron localization and magnetic spin ordering in GBCO. The second order
Pmmm to Pmma structural phase transition associated with CO at T CO =247 K was directly observed by SCD and XRPD techniques. In particular, the emerging of the CO phase is supported by (i) the behavior of the superstructure reflections, (ii) the temperature dependence of the Co-O distances and (iii) the computed BVS. EPR measurements gave important insights into the CO transition holding the emergence of spin fluctuations at T min ≈ T CO.
As reported for other layered compounds, the spin fluctuations can be considered a precursor effect of AFM transition associated to the onset of CO in GBCO system. Hence, in agreement with the YBaCo O magnetization curve, which shows a more marked drop at T CO than T N , we suggest that the long range AFM order takes place at ≈ T CO in GBCO. PDF analysis of diffraction data shows new structural features. At 298K and 180K the local structure is consistent with a fully random occupation of Co and Co in a 1:1 ratio and with a complete CO. On the other hand, at T =80K the local range PDF seems to be consistent with a melting of charges localization. In turn, this suggests that a reentrant CO transition occurs in GBCO. This behaviour, analogous to that found in CMR manganites, is supported by the temperature dependence of ∆ H pp below 160K and is consistent with the PM reentrant behavior found for the GdBaCo O parent compound. Acknowledgements
The authors gratefully acknowledge the European Synchrotron Radiation Facility for provision of beam time and Dr. Adrian Hill for assistance in using the ID31 beamline. The authors would like to thank Dr D. Chernyshov for useful discussions. The Danish National Research Foundation through the Center for Materials Crystallography (CMC) has also been also very much appreciated . FIGURES
FIG.1 (Colour online) Packing and atom numbering schemes for each of the three phases of GdBaCo O this work: P mmm (a), Pmmm (b) and
Pmma (c). For each symmetry-independent metal ion, its next-neighbour Oxygen coordination environment is highlighted by black bonds. 20 FIG.2 (Colour online) ( a ) Temperature dependence of intensity of the Pmma (116) superstructure reflection (circles). Inset: ln-ln plot of the intensity versus ( T c - T )/ T c . ( b ) The temperature evolution of Pmma (102) reflection as collected at ID31 is reported as example (empty circles). The arrow indicates the onset of the intensity weakening. The inset shows XRPD profiles related to (102) reflection across the CO transition. Note that the intensity of superstructure reflections is much higher than the error bars given by the background. 21 FIG. 3 Measured (dots), calculated (line) powder diffraction patterns and residuals (bottom line) for GBCO at T =5K and at λ = 0.39620(5) Å. The inset shows a magnified view of the low angle diffraction peaks where the CO superlattice reflection is clearly shown. 22 FIG.4 Temperature dependence of the (a) a, b axes (b) c -axis and (c) unit cell volume. Full and empty circles are data from SCD; full and empty squares and triangles are data from XRPD. 23 FIG.5. Temperature dependence of (a), (b) selected Co-O and (c) average
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