Stabilization of ferromagnetic ordering in cobaltite double perovskites of La2CoIrO6 and La2CoPtO6

We investigated the local electronic structure and magnetic properties of the cobaltite double perovskites La2CoIrO6 and La2CoPtO6 using Co L2,3-edge x-ray absorption spectroscopy and x-ray magnetic circular dichroism. Despite similarity in the local electronic structure (Co2+ high-spin states) as well as in the crystal structure (P21/n), only La2CoIrO6 exhibits substantial orbital and spin magnetic moments of Co2+, whereas they are much weaker in the case of La2CoPtO6. This composition dependence is consistent with the results of magnetization measurements. The details of the mechanism of ferromagnetic ordering in the Co2+ sublattice in La2CoIrO6 and the lack thereof in La2CoPtO6 are explained in terms of the orbital hybridization of the Co minority-spin t2g state and the Ir/Pt jeff = 1/2 state.

a maximum number of spin-up electrons, a low-spin (LS) state with a minimum number of spin-up electrons, or an intermediate-spin state. In particular, the energies of the HS and LS states in Co 3+ are very close, so the occupation numbers, and consequently, the net spin moments, can vary considerably with temperature (T) [7,8]. Hence, in principle, the cobaltites require more careful inspection in terms of the chemistry and the spin states of Co ions than any other 3d transition metal oxides.
Here, we examine the local moments of Co ions and the total magnetization of La 2 CoMO 6 compounds, where M = Ir or Pt. The local electronic structure of Co was investigated using Co L 2,3 -edge x-ray absorption spectroscopy (XAS), and the magnetic properties were investigated using x-ray magnetic circular dichroism (XMCD). We found that the Co ions had an HS configuration (d 7 ) with a valence of +2 at moderate or low T 's for both samples. However, the magnetic properties differed significantly; La 2 CoIrO 6 was ferromagnetic at low T 's, and La 2 CoPtO 6 was almost paramagnetic. We analyzed the orbital and spin moments in detail and demonstrated that such dependence on composition can be explained in terms of hybridization, as in other double perovskites.
An La 2 CoIrO 6 (La 2 CoPtO 6 ) polycrystalline sample was prepared via solid-state synthesis from a stoichiometric mixture of La 2 O 3 , IrO 2 (PtO 2 ), and Co 3 O 4 . The La 2 O 3 was heated to 700 °C for two days, and the resulting La 2 CoMIrO 6 (M = Ir or M = Pt) phases were sintered at T 's in the range 900-1150 °C for several days with intermediate grinding. Crystallographic data for each specimen were obtained using powder x-ray diffraction (XRD; Rigaku Miniflex2, Cu Kα radiation) with θ − 2θ scans. The magnetic properties were characterized using a Quantum Design superconducting quantum interference device (SQUID) in the range 2 K ⩽ T ⩽ 300 K, and with magnetic fields in the range 0-5 T. XAS and XMCD measurements were performed with circularly polarized x-rays generated by an elliptically polarizing undulator at the 2A beamline in the Pohang Light Source. Data were collected in total electron yield mode. For the XMCD measurements, we used x-rays with fixed helicity and alternated the direction of the external magnetic fields (H = 8 kOe) to ensure a constant beam profile. Figure 1 shows powder XRD patterns of the La 2 CoIrO 6 and La 2 CoPtO 6 obtained at room temperature. The two patterns are similar, which suggests that the crystal structures were almost identical with similar lattice constants. This is most probably due to the similar ionic radii of Ir 4+ and Pt 4+ (0.625 Å) [9]. Each of the reflection peaks was assigned according to a monoclinic crystal symmetry (P2 1 /n) [10][11][12] with lattice constants of a ~ 5.58 Å, b ~ 5.67 Å, and c ~ 7.92 Å ( [10]). The crystal structure is shown in the inset of figure 1. For both systems, the unit cell consisted of La 2 CoMO 6 (where M = Ir or M = Pt), in which the Co or M ions share the same cation site in the perovskite structure, leading to an alternating CoO 6 and MO 6 octahedra [13]. Rotation of the octahedral oxygen coordination and buckling of octahedra are expected [5,13]. Recent studies of double perovskites have shown that a significant charge reconstruction occurs among the cations [6]. As will be shown later (see figure 3), we confirm that the valence states of the cations in cobaltite double perovskites are La 3+ , Co 2+ , and M 4+ . It follows that inter-site interactions between the Co 2+ (d 7 ) and M 4+ (d 5 for Ir and d 6 for Pt) ions can be expected.
Despite the similarity in the crystal structure, La 2 CoIrO 6 and La 2 CoPtO 6 exhibited significantly different magnetic properties. The T-dependence of the magnetization was obtained under zero-field cooling (ZFC) and field cooling (FC), over a range of 2 ⩽ T ⩽ 300 K with H = 5 kOe, as shown in figure 2(a). La 2 CoIrO 6 exhibited a strong signature for ferromagnetic moment at low T, with a large contrast between the FC and ZFC data, whereas La 2 CoPtO 6 did not. This suggests that La 2 CoIrO 6 is ferromagnetic but La 2 CoPtO 6 is almost paramagnetic with a very small ferromagnetic moment.
The inset shows the inverse of the magnetic susceptibility χ M as a function of T, which can be used to determine T C . The values of T C 's estimated by extrapolating the maximum gradient to the abscissa, were T C ~94 K for La 2 CoIrO 6 , whereas T C < 50 K for La 2 CoPtO 6 . In the case of La 2 CoIrO 6 , a clear step-like feature near T = 100 K validates the extrapolation method. However, in the case of La 2 CoPtO 6 , it is hard to choose an extrapolation point reliably, because the slope near T = 50 K varies slowly. Therefore, we could obtain only an upper bound of T C ~ 50 K for La 2 CoPtO 6 .
The lower T C of La 2 CoPtO 6 , as well as the weaker ferromagnetic signature, is indicative of strong suppression of the magnetic moment. This contrast is highlighted by the magnetization curve as a function of H, shown in figure 2(b). The magnetization measured at T = 43 K exhibits strong FM in La 2 CoIrO 6 ; however, the FM was significantly weaker for La 2 CoPtO 6 (by a factor of approximately 7). Therefore, we may conclude that the FM moment was suppressed in La 2 CoPtO 6 .
The measured magnetization at T = 43 K was ~0.8 μ B per formula unit (f.u.) in La 2 CoIrO 6 (see figure 2). In theory, however, the nominal saturated spin moment should be 2 μ B /f.u., assuming antiferromagnetic (AFM) coupling of the high-spin Co 2+ (S = 3/2) and Ir 4+ (J eff = 1/2), or 4 μ B /f.u., assuming FM coupling. This suggests that the measured magnetization  was less than half the nominal value for AFM coupling. Furthermore, as shown in figure 2(b), the magnetization in both samples increased linearly as a function of H, even for magnetic fields significantly larger than the coercive field. These phenomena were reported by Narayanan et al [5], who attributed the persistent increase in the magnetization at fields up to 5 Tesla to the non-collinear magnetism; i.e. incomplete field alignments of the canted Co 2+ magnetic moments with relatively weak H. Nevertheless, we may conclude that the FM in La 2 CoPtO 6 was significantly weaker than the FM in La 2 CoIrO 6 .
To investigate the details of the mechanism of FM ordering in La 2 CoIrO 6 , here, we examine the local electronic structure and local magnetic moment of the Co ions using Co L 2,3edge XAS and XMCD. Figure 3(a) shows the Co L 2,3 -edge XAS spectra of La 2 CoIrO 6 , and figure 3(b) shows that of La 2 CoPtO 6 . We used linearly polarized x-rays to identify the local chemistry of the Co ions regardless of the magnetism. For comparison, the spectrum of CoO taken from [14] is also shown in both figures. The spectra acquired at T = 43 K (below T C for La 2 CoIrO 6 ) were similar to each other as well as to the spectrum of CoO. This suggests that for both samples, the chemistry and the spin state of the Co ions were the same as those of CoO, i.e. an HS state of Co 2+ with seven d electrons (i.e. ↑ ↓ ↑ t t e 2g 3 2g 2 g 2 ). At T = 300 K (significantly higher than T C ), the spectra were similar to the low-T spectra, except for a slight broadening of the features partly due to thermal vibrational motion of atoms. In fact, the detailed T evolution of the features would also reflect the local structural changes in accordance with the spin-lattice coupling in cobalt oxides (see e.g. [15] for the case of CoO thin film). However, the overall features were maintained suggesting almost invariant high spin state in Co 2+ over this T range for both samples. This finding excludes the possibility of the involvement of another many-body state in Co 2+ .
In contrast, the spin moment varied significantly with T and with composition. Figures 3(c) and (d) show the T-dependence of the XMCD spectra for the two samples. We used circular polarization and measured the difference in the absorption intensity between H = +8 kOe and H = −8 kOe. For La 2 CoIrO 6 , the XMCD was clearly observed at T = 43 K (below T C ) whereas it vanished almost completely at T =

43K
Iinear polarization 300 K (above T C ). It suggests that the Co 2+ spin alignment indeed contributes to the magnetization. For La 2 CoPtO 6 , the XMCD at T = 43 K was approximately 5 times weaker than for La 2 CoIrO 6 (and it was reduced further at T = 300 K), reflecting a weaker local spin moment, which is consistent with the magnetization data shown in figure 2.
The lineshapes of the low-T XMCD spectra in figure 3(d) were similar to those shown in figure 3(c), despite the reduced amplitude. This suggests that the weak magnetism in La 2 CoPtO 6 does not result from an evolution of the electronic structure; rather, the HS state dominates for both La 2 CoIrO 6 and La 2 CoPtO 6 , but the spin orientations among different Co 2+ ions were less coherent for La 2 CoPtO 6 . Therefore, intersite spin interactions, rather than competition between local many-body states, are significant in determining the magnetic properties of this system.
We obtained the orbital and spin moments in Co 2+ using the XMCD sum rules [16,17] and the procedures described in [18][19][20]. The value of the orbital moment m orb was deduced directly from the sum rule, while that of the spin moment m spin was done by multiplying a theoretical correction factor of 1.1 for Co 2+ ( [20]). In the procedures, the m orb 's and m spin 's are calculated from the L 3 -and L 2 -edge peak areas measured after compensating the saturation effect and subtracting the weak incoherent higher-energy backgrounds. This process minimizes the possible experimental artifact caused by the saturation of signals from the thick powder specimens. The measured m spin and m orb (and consequently, m tot ) may contain errors from several experimental and theoretical origins as shown in the table. The experimental origins include incomplete circular polarization of x-rays, magnetocrystalline anisotropy, or inaccuracy in determining the integrated intensity [19]. We estimate these could contribute the errors of up to ~10%. The theoretical origins are from the uncertainty in determining the number of d electrons and in determining the correction factor to obtain the m spin 's. The number of d electrons can differ by ~10% depending on charge transfer, and the correction factor can by 5-10% of the central values in the case of Co 2+ HS [20]. Therefore, we took the maximal value (20%) to estimate the total error to be up to ~30%.
The m orb 's and m spin 's at T = 43 K are listed in table 1, together with the sum (m tot ) and the ratio (m orb /m spin ). Overall, the moments for La 2 CoPtO 6 were small, which suggests very weak FM ordering, whereas those for La 2 CoIrO 6 were appreciable. This contrast is consistent with that in the magnetization data (figure 2). The ratio m orb /m spin highlights the significance of the orbital angular momentum. We found m orb /m spin ~0.58 for both systems, which is larger than, for examples, that of Fe 2+ in Fe 3 O 4 (0.18; [21]) and Mn 2+ in (Ga,Mn)As (~0.037; [22]). This clearly indicates a considerable unquenched orbital moment, leading to the possibility of a substantial distortion of the octahedral symmetry of the oxygen coordination [23].
It should be noted that even in La 2 CoIrO 6 , the measured m tot for Co 2+ 3d (~0.48 μ B ) is much smaller than the nominal value for the spin moment in the HS Co 2+ (3 μ B ). As discussed above, this may be the result of partially aligned Co spins due to non-collinear magnetism [5,6] with the moderate magnetic fields (H < 10 kOe). The net magnetic moment in La 2 CoIrO 6 measured by SQUID is approximately ~0.8 μ B at T = 43 K (see figure 2(a)). According to Kolchinskaya et al [6], the 5d magnetic moment in the Ir 4+ ion is readily saturated to −0.38 μ B even with the moderate magnetic field. Provided that the total magnetic moment (including paramagnetic one) in Ir 4+ is mostly contributed by the 5d orbital state and the magnetism in La 3+ or O 2ions is negligible, the AFM coupling [2,5,6] between the Co 2+ and Ir 4+ (pseudo)spins should result in a magnetic moment in Co 2+ of approximately +1.2 μ B /f.u. Therefore, the m tot for Co 2+ 3d measured by XMCD is only ~40% of the total magnetic moment in Co 2+ . This implies that the magnetic contribution from Co sp orbital states should be also substantial, as is observed in the Co K-edge XMCD measurement [6].
As with other double perovskites, spin-spin interactions between neighboring Co 2+ and M 4+ (M = Ir or M = Pt) can be described by a hybridization mechanism [4,24], which dictates that orbital hybridization or virtual electron hopping between the two cations lowers the hybridized electron state so as to stabilize a given spin state. Figure 4(a) shows an energy level diagram for the orbital hybridization assuming FM coupling, and figure 4(b) shows a similar energy level diagram assuming AFM coupling. In the HS state of Co 2+ with an octahedral crystal field, two spin-down electrons occupy the t 2g level below the Fermi energy, whereas five spin-up electrons occupy the t 2g and e g levels far below the Fermi energy (not shown), resulting in a total S = 3/2 spin-up state. For Ir 4+ , single electrons occupy the j eff = 1/2 level beneath the Fermi energy, and the other four electrons occupy the j eff = 3/2 states (not shown), resulting in a total J eff = 1/2 (pseudo) spinup state. In Pt 4+ , two electrons occupy both j eff = 1/2 levels, resulting in a J eff = 0 state.
For FM coupling (see figure 4(a)), only the spin-down states are unoccupied in Co 2+ , so hybridization can occur only for spin-down electronic states. For this reason, the j eff = 1/2 state in Ir 4+ cannot hybridize with the Co t 2g state, whereas the (pseudo) spin-down state of Pt 4+ can in the case of the FM coupling. Conversely, with AFM coupling (see figure 4(b)), only the spin-up states are unoccupied in Co 2+ , so hybridization The magnetic spin moment m spin , orbital moment m orb , and the total magnetic moment m tot = m spin + m orb of Co 2+ ions in La 2 CoIrO 6 and La 2 CoPtO 6 , as determined using the XMCD sum rules. The error bars account for the uncertainty in determining the theoretical corrections as well as in estimating the peak areas in the experimental spectra. We used data taken at T = 43 K with an external magnetic field of H = 8 kOe after correcting for the saturation effect. can occur for spin-up electronic states. Thus, both the Ir 4+ and Pt 4+ j eff states can hybridize with the Co t 2g state in the case of the AFM coupling. Therefore, it follows that in La 2 CoIrO 6 , only AFM coupling is allowed to stabilize the ferromagnetic ordering, whereas in La 2 CoPtO 6 , both couplings are allowed with equal probability, so a net zero moment is expected. This suggests that the Ir 4+ ions, which intervene in the network of Co 2+ ions, can induce FM order within the Co 2+ ion sublattice. According to the model described above, strong inter-site orbital hybridization is essential for stabilization of the FM ordering in the sublattice. Because the hybridization strength should depend on the dispersion and symmetry of the orbitals, mainly electrons occupying shallow levels will determine the magnetic coupling. It is well known that, although shallow, the j eff state is localized due to strong spin-orbit coupling. This will result in weak hybridization and consequently weak control over the spin orientations of nearby Co 2+ ions. This may explain the small magnetic moment of Co and the persistent increase with an increasing external magnetic field.
In conclusion, control over the magnetism of La 2 CoMO 6 by replacing M = Ir 4+ with M = Pt 4+ was evidenced by the magnetization and XMCD measurements. This was explained in terms of orbital hybridization of the unoccupied minorityspin state in Co 2+ and the occupied j eff state in Ir 4+ /Pt 4+ . FM ordering in the Co 2+ sublattice in La 2 CoIrO 6 was stabilized owing to the AFM-coupled Ir j eff = 1/2 ⊕ Co t 2g state, whereas such magnetic ordering was absent in La 2 CoPtO 6 because of the absence of a preferential spin orientation in the hybridized state. The dominance of the Co 2+ HS state, even at 300 K, suggests that inter-site spin interactions are more important than many-body on-site interactions in determining the magnetic properties of La 2 CoIrO 6 .