Experimental evidence of the ferrimagnetic ground state of ... - CiteSeerX

EUROPHYSICS LETTERS

15 November 2002

Europhys. Lett., 60 (4), pp. 608–614 (2002)

Experimental evidence of the ferrimagnetic ground state of Sr2 FeMoO6 probed by X-ray magnetic circular dichroism ´le ´my 1 , H. Jaffre `s 1 , J. Vogel 2 , F. Petroff 1 , M. Besse 1 , V. Cros 1 , A. Barthe 3 3 3 A. Mirone , A. Tagliaferri , P. Bencok , P. Decorse 4 , P. Berthet 4 , Z. Szotek 5 , W. M. Temmerman 5 , S. S. Dhesi 3 , N. B. Brookes 3 , A. Rogalev 3 and A. Fert 1 1

Unité mixte de Physique CNRS/THALES Domaine de Corbeville, 91404 Orsay Cedex, France and Université Paris Sud - 91405 Orsay Cedex, France 2 Laboratoire Louis Néel - B.P. 166, 38042 Grenoble Cedex, France 3 European Synchrotron Radiation Facility (ESRF) B.P. 220, 38043 Grenoble Cedex, France 4 Laboratoire de Physico-Chimie de l’Etat Solide Bˆ atiment 410, 91405 Orsay Cedex, France 5 Daresbury Laboratory - Daresbury, Warrington WA4 4AD, Cheshire, UK (received 15 July 2002; accepted in final form 5 September 2002) PACS. 75.70.Cn – Interfacial magnetic properties (multilayers). PACS. 68.37.-d – Microscopy of surfaces, interfaces, and thin films. PACS. 75.70.Rf – Surface magnetism.

Abstract. – We have investigated the magnetic structure of a Sr2 FeMoO6 single crystal by X-ray magnetic circular dichroism at the L2,3 edges of Fe and Mo sites. The spin magnetic moments we find on Fe (3.05µB ) and Mo (−0.32µB ) give, for the first time, a direct confirmation of their ferrimagnetic ordering. The presence of a finite spin moment on Mo together with only very small orbital moments on both Fe and Mo confirms that the predicted half-metallicity of the Sr2 FeMoO6 compound is due to a configuration with five localized d electrons forming a high-spin moment on Fe and one s antiparallel delocalized electron shared between the Mo and the other sites.

In the field of spin electronics, the search for new half-metallic materials with a high Curie temperature (TC ) represents a very important challenge. Such materials whose conductivity is fully spin-polarized are highly desirable for utilization as electrodes for spin valves, magnetic tunnel junctions and logic devices. It has been recently reported that double perovskites Sr2 FeMoO6 (SFMO) is a possible suitable candidate. Its half-metallic nature has been previously suggested by band structure calculations [1] with a TC above room temperature (RT) around 400 K [2, 3] and is supported by a sharp low-field magnetoresistance (MR) attributed to intergrain tunneling in their polycrystalline phase well above RT [4]. In this compound, Fe and Mo ions are expected to alternate on the B and B sites of the A2 BB O6 double perovskite lattice. Despite an apparent similarity of its structure compared to the manganites, the high TC given the large distance between the Fe sites, has led to a c EDP Sciences

M. Besse et al.: Experimental evidence of the ferrimagnetic etc.

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questioning about the mechanism explaining the electronic and magnetic properties resulting in a half-metallic ground state of SFMO. In all the proposed models, its electronic structure is composed of localized up-spins borne by the Fe3+ (S = 5/2) ions and a conduction band partially occupied by the single itinerant down-spin electron provided by the Mo5+ ions. In order to explain the strong antiferromagnetic coupling between the localized Fe3+ spin and the delocalized electron, giving rise to a significant TC , standard [1] or extended double exchange [5, 12] has been invoked. Both should result in a ferrimagnetic arrangement with a total spin moment close to 4µB /f.u. In fact, the basic requirements for half-metallicity are the existence of a finite magnetic polarization of the non-magnetic Mo ions aligned oppositely to the Fe moment and also the high-spin (S = 5/2) arrangement of the five other d electrons localized on the Fe site, that is a strong intra-atomic ferromagnetic exchange compared to the crystal field separating the t2g and eg energy levels in octahedral sites. Any deviation from these conditions could induce a Fe low-spin phase [6] destroying the half-metallicity. Therefore, it is clear that the access to the on-site specific magnetism (on Fe, Mo and O) gives an important insight to the innermost conduction mechanism. Nevertheless, on the experimental side, serious controversies remain. One of them is the value or even the existence of the spin magnetic moment at the Mo site obtained by different techniques. While previous X-ray magnetic circular dichroism (XMCD) data [7] do not yield any observable spin magnetic moment on Mo in agreement with neutron diffraction [8], the same neutron technique [9] revealed a moment close to 1µB . Another discrepancy concerns the valency of Fe: on the one hand, by X-ray absorption spectroscopy (XAS) with linear polarized light, Ray et al. [7] conclude that Fe is in the 3+ state. On the other hand, M¨ ossbauer spectroscopy data either agree with this value [13] or establish that Fe is in a Fe2.5+ state [14, 22]. Consequently, these contradictory observations question the mechanism responsible for both the coupling and the conduction and thus indirectly the reality of the half-metallicity of SFMO. The use of the chemical selectivity of XMCD gives the opportunity to access, through the use of the magneto-optical sum rules [16, 17], the individual spin and/or orbital moments carried by each type of atom as well as their relative orientation. This has deeply motivated the study by XMCD of a high-quality SFMO single crystal supporting, in fine, the picture of a localized moment on the Fe sites and a conduction process mediated by the delocalized electron of Mo which is the cornerstone for observing half-metallicity in this compound [1, 5, 12]. This constitutes the central point of the present work. The preparation of the Sr2 FeMoO6 single crystal is described in the following. First, a SFMO polycrystalline sample was prepared by solid-state reaction. Stoichiometric mixtures of SrCO3 , Fe2 O3 and MoO3 were well ground in an agate mortar. Powder calcination was performed at 1280 ◦ C for 20 h (200 ◦ C/h) in a flow of H2 (5%)/argon. The obtained powder was then reground and pressed into a rod 100 mm in length and 8 mm in diameter. The formed rod was then sintered three times (after regrinding and shaping) for 20 h in the same conditions as used for powder synthesis. X-ray diffraction patterns indicated that the obtained feed rod was clearly single-phased. The Sr2 FeMoO6 crystal growth was carried out in a floating-zone furnace equipped with double hemi-ellipsoidal mirrors coated with gold (NEC, SC-N15HD) in which two halogen lamps were used as heating source. The crystal was grown in argon flow at a rate of 10 mm/h with rotating seed and feed rods in opposite directions. The powder diffraction pattern of a pulverized part of the melt-grown rod was consistent with the tetragonal I 4/m m m symmetry and a full pattern matching refinement with the FULLPROF program gave a = b = 5.577 ˚ A and c = 7.880 ˚ A. The formation of a single crystal was then confirmed by the Laue reflection method. Using X-ray diffraction (XRD), the intensity ratio of 2.4% for peaks (101) and (200) gave a disorder corresponding to around 15% [22]. The saturated magnetization measured with a SQUID magnetometer at T = 5 K is 3.2 ± 0.1µB

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per formula unit. The difference with the expected 4µB value corresponds, within the model of Ogale [18], to 10% Fe-Mo antisites to be compared to 15% found by XRD. The Curie temperature is measured to be around 400 K. XAS and XMCD experiments were performed at the L2,3 edges of Mo and Fe. The 2p → nd transitions are the most suited absorption edges for the studies of the d band magnetism of transition metals [15]. XMCD spectra were recorded at 10 K under a 5 T magnetic field applied perpendicularly to the film (either parallel or antiparallel to the X-ray beam), large enough to saturate the magnetization along the field. The Fe L2,3 edges, recorded in the total electron yield detection mode, have been performed at the ESRF ID-8 beamline that delivers 100% circularly polarized light. For the Mo L2,3 edges, recorded in fluorescence mode, the experiments were carried out at the ID12A beamline, on which the circular polarization rate drops from 97% before the double crystal Si(111) monochromator down to about 14% at Mo L3 (2524 eV) and 5.4% at Mo L2 (2628 eV). In a complex transition metal ionic compound such as SFMO, the L2,3 absorption edges present a very rich fine structure that can be used as fingerprints of the ground state. This peak structure is very sensitive to the valency and to the spin state of the probed atom, both closely linked to the crystal field and the exchange splitting. In fig. 1, we display a Fe L2,3 absorption spectrum with an energy resolution of 0.25 eV performed on the SFMO single crystal (using the average of left and right circularly polarized light). At both absorption edges, the spectrum exhibits a weak lower-energy shoulder together with a doublet structure at the white line position in which both peaks have almost the same magnitude. Such a complex fine structure is not compatible with a pure Fe3+ valency state (nor with Fe2+ ) as found by Ray et al. [7] in an ordered SFMO powder. To go beyond, its interpretation demands the mixing of both valencies for which the Fe2+ L2,3 threshold is at lower energy than the Fe3+ one [19]. In order to provide a quantitative description of the spectral features, we have carried out calculations in the frame of the ligand-field atomic formalism applying a 70% reduction of Slater integrals and for a 10Dq equal to 1.9 eV close to the value calculated by Kobayashi et al. [1]. Although hybridization effects are not fully taken into account, one can expect to reproduce the shape of the experimental spectrum by weighing up the contributions of Fe2+ and Fe3+ sites in an octahedral symmetry, irrespective of the detailed fine structure [20]. The best agreement


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Fig. 2 – X-ray absorption measured with left (σ + ) and right circularly polarized light (σ − ) and dichroism at the L2,3 Fe edge at T = 10 K and H = 5 T.

between the experimental data and the calculation corresponds to a linear combination of 66% Fe2+ (d6 ) and 34% Fe3+ (d5 ). In XAS experiments, the time scale for the measurement is defined by the time of the photon absorption, which is much smaller than the time scale of the electron hopping between two different sites. This enables one to obtain a snapshot of the electronic configuration of this compound and consequently to estimate the Fe2+ and Fe3+ contributions. We point out that the valency thus found is in good agreement with 2.5+ deduced from M¨ ossbauer spectroscopy [14, 22]. Finally, recent bulk-sensitive photoemission spectroscopy experiments also conclude the mixed-valent Fe2+ -Fe3+ configuration [21]. First-principles local spin density (LSD) calculations with self-interaction correction (SIC) provide also a useful tool for studying valencies of ions in solids [23]. Our LSD-SIC calculations have identified the 3d5 (Fe3+ ) configuration to have the lowest total energy [24]. In this case, we have found Sr2 FeMoO6 to be half-metallic with an Fe magnetic moment of 3.71µB , aligned oppositely to the Mo moment of −0.43µB . The magnetic moments of oxygen and Sr atoms are rather small, 0.11 and 0.02µB , respectively. The 3d4 configuration is less favorable, resulting in a metallic solution with an Fe moment of 3.47µB , and a Mo moment of −0.41µB . Interestingly, the 3d6 configuration has also been less favorable, with Sr2 FeMoO6 becoming insulating, exhibiting a small energy gap, and Fe having a magnetic moment of 3.46µB , aligned in parallel with the Mo moment of 0.08µB . However, since all the configurations lie close in energy, it is very likely that the true ground state is a combination of them. Unlike in ref. [1], in the LSDSIC calculations it was not necessary to theoretically optimize the oxygen positions to obtain the half-metallic ground state. However, implementing such an optimization has resulted in a reduction of both Fe and Mo magnetic moments to 3.64 and −0.35µB , respectively. We now turn to the results of dichroism experiments which give an insight to a site-specific magnetic description. In fig. 2, we report the absorption spectra with left and right circularly polarized light and their difference (XMCD spectra) at the Fe L2,3 edges. We have used a standard procedure for the data analysis removing a simple two-step–like function from the XAS spectra to take only into account the 2p-3d transitions [25]. From the multiplet calculations, we can deduce that the mean number of d holes on the Fe sites is about 4.34. Using this value in the sum rules and assuming the magnetic dipole moment to be negligible, we finally obtain the spin magnetic moment of Fe to be (+3.05 ± 0.2)µB . This value is smaller than the 3.71µB per Fe site we obtained in our LSD-SIC calculations [24]. This

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Fig. 3 – X-ray absorption (sum of the spectra obtained with two X-rays polarizations) and dichroism at the L2,3 Mo edge at T = 10 K and H = 5 T. The inset shows both Mo M2,3 edge and Fe L2,3 edge which were measured on the same beam line ID8 at the ESRF.

difference might be attributed to the existence of site disorder, which results in a reduction of the effective magnetic moment of Fe [18, 26]. Within the model of Ogale et al. [18], taking into account antiferromagnetic interactions in the Fe-O-Fe sequence, we have evaluated the reduced Fe moment (for 10% of Fe-Mo antisites) at 2.9µB . We cannot completely exclude that the magnetic properties could be slightly different at the surface since the total electron yield mode is surface sensitive. Nevertheless, we emphasize that similar XMCD measurements performed during the same experimental runs on an ordered SFMO powder containing a larger proportion of grain boundaries gave quantitatively the same result. One of the main results of the present work is the first direct experimental observation of a magnetic moment at the Mo site. In fig. 3, we show the total absorption and the XMCD spectrum at the Mo L2,3 edges. Due to the large energy separation between the two peaks, they were measured separately and then normalized to a ratio 2 : 1 at the step height at an energy much larger than that of the white line (i.e. in an energy region where the XAS intensity is equal for the two helicities) corresponding to the proportion of initial states 2p3/2 and 2p1/2 taking part to the electronic transitions. Furthermore, XMCD spectra have been corrected for the incomplete circular polarization. At first glance and without any assumption, it is clear that Mo atoms bear a non-negligible magnetic moment, since the dichroic signal is well above the experimental uncertainties. In order to evaluate it, as for the Fe case, the number of Mo d holes was taken equal to the value deduced from our multiplet calculations, i.e. nh = 9.66. Finally, we find mspin = −0.32 ± 0.05µB on Mo. This evidences that the conduction is ensured by the delocalized Mo single spin having a density probability of about 30% at the Mo site (0.32µB per Mo atom and not 1µB as expected from a simple ionic picture Fe3+ -Mo5+ ). It also corroborates the ground state determined from the fits between experimental data and atomic multiplet calculations. This result is in fair agreement with the band structure calculations performed by Kobayashi et al.: −0.29µB [1] and those we have performed in the LSD-SIC: −0.43µB [24]. As we discussed in the beginning, it should be pointed out that other values of the Mo magnetic moment, spanning from 0 to 0.9µB , have already been deduced indirectly from neutron diffraction experiments [8,9]. The large discrepancy between these data can orig-


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inate from the fitting procedure used which probably fails to deduce such a small magnetic moment with a good accuracy. Surprisingly enough, our result is in clear contradiction with the recent XMCD experiments at the Mo M2,3 edges performed by Ray et al. [7], who claimed a negligible Mo magnetic moment. In order to explain the conduction properties of SFMO with a non-significant spin density at the Mo sites, these authors proposed that the itinerant electron could be predominantly transferred via hybridization effects from Mo to all others Fe and the six O sites. In that case, one expects that O atoms would bear a magnetic moment of the order of 0.1µB . Our measurements performed at the O K edge on the SFMO single crystal do not show an appreciable dichroic signal (less than 1% of the XAS intensity at the K edge). At this edge, one probes only the orbital part of the magnetic moment. However, Pellegrin et al. have measured 1.6% dichroism at the O K edge in manganite La0.6 Sr0.3 MnO3 [10], in which band structure calculation predicts a hybridization-induced moment of 0.08µB per O atom [11]. By comparison, we estimate that in SFMO the magnetic moment per O atom is definitively smaller than 0.05µB . Analyzing identically the two sets of data at the L2,3 edges of Fe and Mo with the same procedure, the relative orientation of the two magnetic moments can be checked directly by comparing the sign of their evaluated values. Their opposite signs clearly show that Fe and Mo magnetic moments are antiparallel to each other. Additional confirmation for this result was obtained by measuring the Fe L2,3 and Mo M2,3 edges on the same beamline (ID8). The resulting XMCD spectra are presented as an inset in fig. 3. Even if the signal-to-noise ratio is rather poor in the energy range of Mo M2,3 edges, it is nevertheless clear that the shape as well as the XMCD sign for Mo are equal to those obtained at the L2,3 edges. It confirms the antiparallel orientation between Fe and Mo magnetic moments. A further argument in favor of the electronic configuration with five localized majority electrons (high-spin state) on Fe and one delocalized electron shared between Mo and the other sites is provided by the experimental determination of the orbital moment. Indeed, the orbital moments measured [27] on either Fe and Mo sites is negligibly small, +0.02µB for Fe and −0.05µB for Mo, with an error bar of the same order of magnitude. Their cancellation is expected from a localized high-spin Fe3+ ion bearing no orbital moment according to standard Hund’s rule together with an itinerant Mo electron for which the orbital contribution is almost completely quenched. This observation tends to rule out the presence of a possible low-spin phase on the Fe site for which a zero orbital moment would be very fortuitous. In summary, spin moments of +3.05 and −0.32µB for Fe and Mo, respectively, Sr2 FeMoO6 single crystal and no significant orbital moment were obtained from our XMCD measurements at the L2,3 edges of Fe and Mo sites. A total magnetic moment of 2.85µB per Fe-Mo pair, deduced from dichroism measurements, is fairly close to the macroscopic magnetization of 3.2µB obtained by SQUID magnetometry. For the first time, a direct proof of the antiferromagnetic interaction between both sublattices is given by the respective signs of the dichroic signal. Also, the calculated proportion of 66% of Fe2+ and 34% of Fe3+ is in good agreement with the spin moment measured on Mo sites. All this evidence seems to confirm, consistently with our LSD-SIC calculations, that a localized spin carried on Fe sites and a delocalized electron of Mo are responsible for the half-metallicity of this compound. ∗∗∗ The authors thank A. Hamzic for his critical reading and also F. Pailloux and M. Sacchi for fruitful discussions. This work was supported by the EU through the project SCG “AMORE” and RTN “Computational Magnetoelectronics” (HPRN-CT-2000-00143).

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