Intricacies of the Co$^{3+}$ spin state in Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$: an x-ray absorption and magnetic circular dichroism study

TL;DR

This study uses x-ray absorption and magnetic circular dichroism to reveal that Co$^{3+}$ ions in Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ are in a high spin state with an unquenched orbital moment, challenging expectations based on octahedral elongation.

Contribution

It provides direct spectroscopic evidence of the high spin state of Co$^{3+}$ in a specific oxide, and uses model calculations to analyze its stability against other spin states.

Findings

Co$^{3+}$ ions are in a high spin state with $S=2$

Significant unquenched orbital moment observed

High spin state remains stable despite octahedral elongation

Abstract

We report on a combined soft x-ray absorption and magnetic circular dichroism (XMCD) study at the Co-$L_{3,2}$ on the hybrid 3$d$/5$d$ solid state oxide Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. Our data indicate unambiguously a pure high spin state $(S=2)$ for the Co$^{3+}$ (3$d^6$) ions with a significant unquenched orbital moment $L_z/2S_z=0.25$ despite the sizeable elongation of the CoO$_6$ octahedra. Using quantitative model calculations based on parameters consistent with our spectra, we have investigated the stability of this high spin state with respect to the competing low spin and intermediate spin states.

Equations

$$\frac{L_z}{2S_z+7T_z}=\frac{2}{3}\cdot \frac{{\int }_{L_{2,3}}({\sigma }^{+}-{\sigma }^{-})dE}{{\int }_{L_3}({\sigma }^{+}-{\sigma }^{-})dE-2{\int }_{L_2}({\sigma }^{+}-{\sigma }^{-})dE},$$

Full text

Cobalt compounds have aroused a great deal of attention in the scientific community because of the complex and large diversity of physical phenomena displayed, including metal-insulating transitions  raccah67a ; Martin97 ; Imada98 , superconductivity  takada03a , large magneto-resistance  perez97a and high thermoelectric power  terasaki97a . This richness of electronic and magnetic properties is closely related not only to the possibility of stabilizing cobalt in different valence states but also to its ability to present different spin states, the so-called spin-state degree of freedom  Sugano ; Goodenough71 . For example, in an octahedral coordination, Co3+ ions, which have the $d^{6}$ configuration, can exist in three possible spin states: a high spin (HS) state ($S=2$, $t_{2g}^{4}e_{g}^{2}$), a low spin (LS) state ($S=0$, $t_{2g}^{6}e_{g}^{0}$) and even an intermediate spin (IS) state ($S=1$, $t_{2g}^{5}e_{g}^{1}$)  Sugano ; Goodenough71b ; Haverkort06 .

This spin state degree of freedom is evident in LaCoO3 where the Co3+ ions have a non-magnetic LS ground state and undergo a gradual transition with increasing temperature to a magnetic spin state  Heikes64 ; Blasse65 ; Naiman65 . The nature of the magnetic spin state (IS or HS) was heavily disputed in literature for over four decades, till an XMCD study clearly demonstrated it to be HS Haverkort06 . Calculations of the Co3+ energy level diagram show that the LS (HS) state can be stabilized by a large (small) value of the crystal field 10$Dq$. The IS is always higher in energy for CoO6 octahedra close to regular, like in LaCoO3  Haverkort06 .

However, the IS state, with one electron in the $e_{g}$ states, is Jahn-Teller active and, hence, can gain energy and become the ground state in the presence of a sufficiently large distortion of the local structure  Maris03 . For this reason the spin state of the Co3+ ions in layered cobaltates, where the elongated distortion of the CoO6 octahedra favors and may stabilize the IS state, have been subject of intense debate. In the case of layered La2-xSrxCoO4, contradicting scenarios for the Co3+ ions were considered to interpret the complex structural, magnetic and transport properties of the system as a function of Sr doping: LS Co3+, IS Co3+  Zaliznyak00 ; Zaliznyak01 ; Chichev06 , HS-to-IS transition Moritomo97 , and mixing of HS/IS  Horigane07 ; Horigane08 . Only recently the spin state of Co3+ in layered La2-xSrxCoO4 was demonstrated by X-ray absorption spectroscopy (XAS) studies to be LS  Chang09 ; Merz11 for x = 0.5 and a mixture of LS/HS for x $\geq 1$ Merz11 ; Guo16 ; Li16 . Band formation has been proposed to possibly provide another route for the stabilization of IS Co3+  Korotin96 ; Ou16 .

In this context, the recently reported synthesis of the layered Sr2Co0.5Ir0.5O4, where the cobalt ions have been suggested to have the 3+ valence, is very interesting Mikhailova17 . In fact, on one hand, the introduction of Co3+ ions in a compound with relatively large lattice parameters, like Sr2IrO4, should lead to a reduced crystal field making the LS state energetically less favourable with respect to the HS state. On the other hand, the elongation of the CoO6 octahedra, revealed by Co-$K$ EXAFS Mikhailova17 , should favor a Jahn-Teller active IS state. Moreover, the hybridization of the Co 3$d$ orbitals with the spatially extended Ir 5$d$ orbitals could enhance the importance of band formation. Hence, the layered Sr2Co0.5Ir0.5O4 provides the opportunity to investigate whether the combined effect of CoO6 elongation and band formation can stabilize the IS state as the ground state. Experimentally, Sr2Co0.5Ir0.5O4 is an insulator with antiferromagnetic interactions with an effective moment of $\mu_{eff}=3.2(1)~{}\mu_{B}$/f.u. Mikhailova17 . Similar to the parent compound Sr2IrO4 Kim2008 ; Kim2009 , also in Sr2Co0.5Ir0.5O4 the large spin-orbit coupling is expected to be the leading energy scale in determining the ground state of the Ir5+ ions. If these expectations are correct, then the Ir5+ ions should have a singlet $J=0$ non-magnetic ground state and provide only a Van Vleck contribution to the magnetism of the system. The measured value of $\mu_{eff}$ is lower than the value $3.46~{}\mu_{B}$/f.u. expected for a spin-only HS Co3+ ($3.6~{}\mu_{B}$/f.u., if the orbital moment $L=1$ is also considered) and significantly larger than the value $2.00~{}\mu_{B}$/f.u. expected for a spin-only IS Co3+. Co-$K$ XAS of Sr2Co0.5Ir0.5O4 indicated a spin state of the Co3+ ion higher than LS  Mikhailova17 . Yet, as pointed out by Vanko et al. Vanko06 , the Co-$K$ XAS cannot distinguish unequivocally between IS and HS scenarios due to the relatively small differences in the calculated lineshapes of the pre-edge features. Hence, on the base of the experimental data available in literature both scenarios of a pure HS or a mixture IS/HS are equally possible for the Co3+ ions in Sr2Co0.5Ir0.5O4. The fact that Sr2Co0.5Ir0.5O4 was synthesized as a single phase and stoichiometric without oxygen deficiency is also important, as oxygen vacancies, which are often present in other cobalt oxides, can cause the formation of CoO5 pyramids (or CoO4 tetrahedra as in Sr2Co1.2Ga0.8O5  Istomin15 ) which may stabilize the HS state Hu04 ; Belik06 or provide the space for stabilizing the HS in a neighbour octahedral Co3+ ion  Li11 ; Hu12 ; Chen14 ; Istomin15 .

Here we report on an investigation of the local magnetism of the Co3+ ions in Sr2Co0.5Ir0.5O4 by employing soft XAS and XMCD spectroscopies, two powerful techniques that are element selective and, through the lineshapes of the spectra, extremely sensitive to the valence, spin, and orbital or crystal field state of the ion under study. Furthermore the application of sum rules to the XMCD data allowed us to have quantitative information on the orbital moment with respect to the spin moment on the selected ion.

Synthesis of the layered Sr2Co0.5Ir0.5O4 was carried out from stoichiometric powder mixtures of home-made Co3O4 with IrO2 (Umicore) and SrCO3 (Alfa Aesar, 99.99%) at 1200 ∘C in air for 80 h. Co3O4 was obtained by thermal decomposition of Co(NO3)26H2O at 700 ∘C in an oxygen flow. In order to obtain fully oxidized Sr2Co0.5Ir0.5*O4 samples for spectroscopic studies, post-annealing in steel autoclaves at 400 *∘*C and 5000 bar O2 pressure was performed for five days. The phase analysis and the determination of the unit cell parameters were performed using x-ray powder diffraction Mikhailova17 . Transition metal cations Co and Ir are in edge-sharing oxygen octahedra, that are elongated along the c-axis. Reference compound NdCaCoO4 was prepared by a solid state reaction. Starting materials of Nd2O3, CaCO3 and Co3O4 were mixed in a stoichiometric ratio and ground thoroughly in an agate mortar, and then sintered in air at 1150 *∘*C for about 7 days with several intermediate grindings. The sample was post-annealed in 5000 bar O2 at 450 *∘*C for 1 day.

Co-$L_{3,2}$ x-ray absorption spectra (XAS) of Sr2Co0.5Ir0.5O4 were recorded at the 08 beamline of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. CoO and NdCaCoO4 samples were also measured as Co2+ and Co3+ reference compounds, respectively. All Co-$L_{3,2}$ spectra were collected at room temperature using total electron yield mode (by measuring the sample drain current) with an energy resolution of about 0.25 eV.

XMCD spectra at the Co-$L_{3,2}$ edges of Sr2Co0.5Ir0.5O4 were collected at the DEIMOS beamline Ohresser14 of SOLEIL in Paris (France) with a photon-energy resolution of 0.4 eV and a degree of circular polarization close to 100%. The sample was measured at $T$ = 20 K and in a magnetic field of 6 Tesla. The spectra were recorded using the total electron yield method. The sample was cleaved $in~{}situ$ in order to obtain a clean surface for total electron yield measurements. The pressure during the measurements was below 510-10* mbar.

In order to evaluate directly the local electronic structure of the Co ion in Sr2Co0.5Ir0.5O4, we also collected the Co-$L_{3,2}$ spectrum of NdCaCoO4 as a reference compound for LS Co3+ with the same K2NiF4 structureLi16 , and of CoO as a reference for HS Co2+, see Fig. 1. The ”center of gravity” of the $L_{3}$ white line of Sr2Co0.5Ir0.5O4 lies about 2 eV higher in energy than that of CoO, and only 0.1 eV higher than that of NdCaCoO4, which confirms the Co3+ valence in the Ir-compound. An Ir5+/Co3+ scenario was already suggested by our previous hard XAS study Mikhailova17 , although the Co $K$-edge XAS used there cannot give accurate Co valence estimation and is not very sensitive to the presence of Co2+ impurities. The lack of a shoulder in the Co-$L_{3,2}$ spectrum at 777.8 eV, which is a fingerprint of HS Co2+ with an octahedral local symmetry, clearly indicates the absence of any Co2+ impurities in the Sr2Co0.5Ir0.5O4 sample.

Having ascertained the purity of the sample, we now focus on the determination of the Co3+ spin state. In this context, it is important to note that the multiplet structure of the x-ray absorption spectrum depends strongly on the valence, the orbital and spin state. Hence, the spectral line shape can be used as fingerprint to determine the spin state. Despite having the same Co3+ valence, the line shape of the Co $L$-edge spectrum of Sr2Co0.5Ir0.5O4 is very different from that of NdCaCoO4, which suggests the two materials have different spin states. In order to understand this difference in spectral features we have performed a quantitative analysis of the spectrum by using the well proven configuration interaction cluster model that includes the full atomic multiplet theory Haverkort06 . The calculations were performed using the XTLS codeTanaka94 . We used a set of parameters close to that of LaCoO3 Haverkort06 , but now considering also the tetragonal distortion effect on the crystal field calc_par . The Co $3d-$O 2$p$ hybridization strengths were estimated according to Harrison’s prescriptionHarrison . The resulting energy level diagram is given in Fig. 2: NdCaCoO4 in the left (a) part of the figure and Sr2Co0.5Ir0.5O4 in the right (b) part. The spin state of the energy levels is indicated by the colours: LS (red), IS (green), HS state (blue), and a mixture of LS/IS (orange). The calculated x-ray absorption spectra are plotted in Fig. 1 as red curves.

The energy level diagram of NdCaCoO4 has the LS state as the lowest state, see Fig. 2(a), and the corresponding calculated Co-$L_{2,3}$ spectrum is displayed in Fig. 1 (bottom red curve). We can clearly observe that the experimental spectrum of NdCaCoO4 is nicely reproduced: all important features have very similar energy positions and intensities. For Sr2Co0.5Ir0.5O4 we have instead the HS state as the ground state, see Fig. 2(b). We have calculated and plotted the corresponding spectrum in Fig. 1 (top red curve). Also here we can find an excellent match between the calculated and measured spectrum for Sr2Co0.5Ir0.5O4. We can therefore conclude that the Co ion in NdCaCoO4 is in the LS configuration while the Co ion in Sr2Co0.5Ir0.5O4 is in the HS configuration. We would like to note that the measured and calculated spectra of NdCaCoO4 are very different from those of Sr2Co0.5Ir0.5O4, and yet, for each compound we can find an excellent match between experiment and simulation. This can be taken as an indication that this spin state assignment is quite robust.

We will now look at the IS scenario and investigate whether we can exclude it for Sr2Co0.5Ir0.5O4 on the basis of the spectra collected. As a start, we look at Fig. 2(b) and we can see that there are many different IS states possible. At the same time, we can notice that the IS states are very much higher in energy (by more than 0.5 eV) than the HS states. This means that the local distortions are by far not large enough to stabilize the IS state in the real crystal structure of Sr2Co0.5Ir0.5O4. If the IS state were to be stabilized, then it should be done by band formation as mentioned above Korotin96 ; Ou16 . We then infer that in such a case, the band width must be so large that it overcomes the 0.5 eV energy difference, thereby also making the relatively small Co $3d$ spin-orbit interaction energy no longer to be an important quantity. We therefore propose to carry out the calculations for the IS states with the spin-orbit coupling (SOC) switched off. The corresponding energy level diagram is also displayed in Fig. 2(b) and the calculated spectra are presented in Fig. 1 (middle red curves) for the three lowest IS states, namely of the type $d_{z^{2}}\underline{d_{xz}/d_{yz}}$, $d_{x^{2}-y^{2}}\underline{d_{xy}}$, and $d_{z^{2}}\underline{d_{xy}}$ in ascending energy order. Here, the underline denotes a hole, the ”/” symbol indicates a hole or an electron shared by two orbitals, and $z^{2}$ is an abbreviation for $3z^{2}-r^{2}$. The notation is the same as that employed by Haverkort $et~{}al.$Haverkort06 and considers the full $t_{2g}$ shell of LS as a starting point from which the other orbital configurations are generated. We would like to note that the IS state studied in Merz11 is quite different because the authors used there an extremely large $e_{g}$ splitting that does not apply to the material studied here.

Comparing the measured Sr2Co0.5Ir0.5O4 spectrum to the three IS simulations we can quickly reject the $d_{z^{2}}\underline{d_{xz}/d_{yz}}$ and $d_{z^{2}}\underline{d_{xy}}$ scenarios: the experimental features at the $L_{3}$ edge are quite dissimilar from that in the $d_{z^{2}}\underline{d_{xz}/d_{yz}}$ simulation, while the spectral structures at both the $L_{2}$ and $L_{3}$ edges of the experiment are very different from those of the $d_{z^{2}}\underline{d_{xy}}$ simulation. Only the $d_{x^{2}-y^{2}}\underline{d_{xy}}$ simulation seems to reproduce the measured spectrum quite well. It is interesting to note that this $d_{x^{2}-y^{2}}\underline{d_{xy}}$ type of state is in fact the IS state that was proposed in the original work of Korotin $\it{et~{}al.}$ Korotin96 . In order to resolve whether the Sr2Co0.5Ir0.5O4 has really the HS Co3+ state or rather the $d_{x^{2}-y^{2}}\underline{d_{xy}}$ IS state, we can make use of an important characteristic of this particular Korotin IS state, namely that it is a real-space orbital state and thus will not carry any orbital momentum, while the HS state, instead, will have a large orbital momentHaverkort06 . XMCD is a powerful technique to investigate this.

We have performed XMCD measurements at the Co-$L_{3,2}$ on Sr2Co0.5Ir0.5O4 and presented the results in Fig. 3. The x-ray absorption spectra were taken using circular polarized light with the photon spin parallel and antiparallel aligned to the magnetic field. The difference spectrum, called XMCD, and the sum spectrum, called XAS, are reported as blue and red curves, respectively. In Fig. 3 we have displayed also the theoretical Co-$L_{3,2}$ XAS and XMCD spectra for the Co3+ in the HS configuration as obtained from our full-multiplet configuration-interaction calculations. Since we are dealing with a polycrystalline sample, we simulated the experimental data by summing two calculated spectra: one for light with the Poynting vector in the xy plane and one with the Poynting vector along the z-axis, with a weighting ratio 2:1. We can observe that the measured XMCD spectrum can be excellently reproduced by the HS simulation. This provides further evidence for the HS state of the Co ions in Sr2Co0.5Ir0.5O4. We would like to note that the XMCD spectrum of HS Co3+ in octahedral symmetry has been rarely reported in literature: only in LaCoO3 single crystal Haverkort06 and thin films Merz10 ; Mehta09 .

The large difference in intensity of the measured dichroic signal between the $L_{3}$ and $L_{2}$ edges shown in Fig. 3 is a clear sign that the Co ions have a significant unquenched orbital moment Thole92 . This then establishes directly that the Co ion in Sr2Co0.5Ir0.5O4 cannot be in the real-space IS state as discussed above. Instead we have a HS state that does carry orbital momentum despite the tetragonal distortion. To be quantitative, we include in Fig. 3 the integral of the XMCD signal over energy (green lines) of the measured and simulated spectra. We can clearly observe that the integrals converge to a finite non-zero value. We now apply the sum rules for XMCD developed by Thole $et~{}al.$ Thole92 and Carra $et~{}al.$ Carra93 , which provide the ratio:

[TABLE]

where $T_{z}$ denotes the intra-atomic magnetic dipole moment. For 3$d$ transition metal ions in an octahedral symmetry this term $T_{z}$ is a small number Teramura96 and is expected to be a little increased by the slight tetragonal distortion existing in the present compound. Indeed, from our configuration-interaction full-multiplet calculations we found the magnetic dipole moment for Co3+ to be relatively small compared to the large spin moment of Co3+ HS: $T_{x}/S_{x}=T_{z}/S_{z}=0.05$. In other words, the important quantum number of $L_{z}/2S_{z}$ can be obtained directly from the experimental Co $L_{3,2}$ XMCD spectrum without requirement of theoretical simulations. From the application of the sum rules we obtained the ratio $L_{z}/2S_{z}$ = 0.25 sum_rules . This value is similar to that of thermally populated HS Co3+ ions observed in LaCoO3 Haverkort06 . The unquenched orbital moment of the HS Co3+ ions in Sr2Co0.5Ir0.5O4 is significant and might be estimated as $m(orb)=0.76~{}\mu_{B}$ if we take the Co3+ spin moment $m(spin)=3.06~{}\mu_{B}$, as calculated in a previous theoretical work Ou14 .

It is worthy to note that the calculated XMCD signal for the HS Co3+ in an applied field of 6 Tesla is 4.5 times larger than the measured one. A reduced XMCD signal could be explained by the presence of an antiferromagnetic arrangement of the Co3+ moments, where the signal is given only by the canting of the moments induced by the applied field. Neutron diffraction measurements on our sample did not reveal the presence of a long-range magnetic order till the lowest temperatureMikhailova17 . However, the negative Weiss constant ($W=-27$ K) reveals AFM interactions between the Co ions, and a rise in the magnetic susceptibility with a maximum at $T^{*}=23$ K suggests the possible onset of a short range antiferromagnetic order of the Co ions in Sr2Co0.5Ir0.5O4 below $T^{*}$ Mikhailova17 . Following the hypothesis of a short range antiferromagnetic order, we introduced in the Hamiltonian an exchange field applied along the c-axis (the easy axis for an elongation distortion of the CoO6 octahedron) and obtained the XMCD (reported in Fig. 3) as the average of the spectra calculated for opposite directions of the exchange field. An exchange field of about 2 meV is needed in order to reproduce the size of the experimental XMCD spectrum.

Pure HS Co3+ is known to exist in tetrahedral CoO4 Hollmann09 and square pyramidal CoO5 coordination Belik06 ; Hu04 . However, for Co3+ in octahedral local symmetry a pure HS state is rarely documented Ehrenberg09 , while the LS state Chang09 ; Hu04 ; Burnus06 or a mix of HS and LS states are usually found Haverkort06 ; Hu12 . One might wonder why Co3+ is in a pure HS state in Sr2Co0.5Ir0.5O4, while NdCaCoO4 shows Co3+ in pure LS state, despite having exactly the same 214 layered structure. Previous cluster calculations showed that the ground state of Co3+ is in either HS or LS depending on the bond length, and a crossover LS-HS occurs at a Co-O distance of about 1.93 Å Chen14 . In the NdCaCoO4 reference material, the short average Co-O bond length of 1.91 Å corresponds to the LS state of Co3+. The quite long average Co-O bond length (1.97 Å from EXAFS measurementsMikhailova17 ) of Sr2Co0.5Ir0.5O4 puts this material well in the region where the Co3+ HS state is stable. Sr2CoRuO6-d with even a longer average Co-O distance of 1.98 Å, also provides a rare example of octahedrally coordinated HS Co3+ ions Chen14 . Yet, this compound is not stoichiometric in oxygen as demonstrated by the presence (7.5%) of Co2+ impurities. One, therefore, might still question whether oxygen vacancies stabilize the HS state in Sr2CoRuO6-d like in many other Co3+ materials with oxygen vacancies Hu12 ; Li11 . The lack of Co2+ impurities shown by our XAS data, on the other hand, confirms that in the case of Sr2Co0.5Ir0.5O4 the HS state is not induced by oxygen vacancies.

The next question we would like to answer is what structural parameters might give rise to an IS ground state. To this aim we have calculated the energy level diagram of the Co3+ ion (reported in Fig. 4) as a function of the apical Co-O distance for different values of the in-plane Co-O distance. In the calculations the ionic crystal field parameters were calculated using a point charge model, while the hybridization strengths were estimated according to Harrison’s prescriptionHarrison . We considered only the case of an elongated distortion, as a compressed distortion would induce a HS, not an IS state. Fig. 4(a) shows that for a relatively long in-plane Co-O distance, like 1.95 Å, as in Sr2Co0.5Ir0.5O4, the HS is always the ground state no matter how large is the tetragonal elongation of the CoO6 octahedra. In fact the effective energy splitting between the $t_{2g}$ and $e_{g}$ orbitals is too small to win against Hund exchange interaction, which favours a single occupation of the orbitals with parallel spin alignment. If the in-plane Co-O distance is reduced to 1.90 Å, then the $d_{z^{2}}\underline{d_{xy}}$ IS state can become the lowest level in energy, as shown in Fig.4(b), but only when the apical Co-O distance is longer than 2.35 Å. The $d_{z^{2}}\underline{d_{xy}}$ state is a real-space IS state and, thus, the orbital moment is zero. The apical distance required for stabilizing the $d_{z^{2}}\underline{d_{xy}}$ IS state is reduced if the in-plane distance is further decreased to 1.85 Å, as shown in Fig.4(c). Therefore, to stabilize an IS state, an elongated Jahn-Teller distortion is not enough, but also a short in-plane Co-O distance (shorter than 1.91 Å) is needed. At low temperature one of the two Co3+ sites of the TlSr2CoO5 compound has the Co-O bond lengths Co-O${}_{in-plane}=1.79$ Å and Co-O${}_{apical}=2.19$ Å Doumerc99 ; Doumerc01 . It would be very interesting to perform an XMCD investigation of this compound to see whether the IS state is stabilized for one of the two Co3+ sites.

To summarize, soft XAS and XMCD measurements at the Co-$L_{3,2}$ edge reveals a pure Co3+ with high spin state in the hybrid 3$d$-5$d$ solid-state oxide Sr2Co0.5Ir0.5O4 with a layered K2NiF4 structure type. We attribute the stability of the Co3+ HS state to the long average Co-O bond length in Sr2Co0.5Ir0.5O4. Our calculations predict that besides an elongated Jahn-Teller distortion, a short in-plane Co-O distance of less than 1.91 Å is needed in order to have the Co3+ in an IS ground state. The application of sum rules on the Co-$L_{3,2}$ XMCD data of Sr2Co0.5Ir0.5O4 gives nearly the same $L_{z}/2S_{z}$ ratio of about 0.25 as observed in LaCoO3. Our XMCD study demonstrates, thus, the existence of a significant unquenched orbital moment of the Co3+ ions despite the CoO6 octahedra are sizeable elongated in Sr2Co0.5Ir0.5O4.

The research in Dresden was partially supported by the Deutsche Forschungsgemeinschaft through SFB 1143 and by the BMBF, project grant number 03SF0477B (DESIREE). K. Chen benefited from support of the German funding agency DFG (Project 600575).