Observation of magnetic excitons in LaCoO3

– An impurity-driven magnetic phase transition has been investigated in LaCoO3 at temperatures below that of the thermally induced spin state transition of the Co ion. We have discovered a saturating component of the magnetisation, which we attribute to previously unobserved interactions between magnetic excitons. These conclusions are confirmed by muon spin spectroscopy which indicates an ordering temperature of 50K in both the transverse and zero-field configurations. Low-energy muon measurements demonstrate that the magnetic behaviour is independent of implantation energy and hence a property of the bulk of the material. The magnetic exciton formation is attributed to the interaction between electrons bound at oxygen vacancies and neighbouring cobalt ions, and is proposed as the precursor to the magneto-electronic phase separation recently observed in doped lanthanum cobaltite. Introduction. – LaCoO3 is unique among the LaMO3 family (where M is a transition metal element) as it undergoes a thermally driven spin state transition. This occurs because the crystal-field splitting of the d-levels of the Co ions is only slightly greater than Hund’s exchange energy, leading to a situation where a small input of thermal energy can lead to the occupation of a higher spin state. Explicitly, a transition from a Co low spin (LS) S = 0, t2g eg , state to an intermediate spin (IS) S = 1, t2geg, state occurs at around 90K, although the exact nature of the transition is still a matter of some contention [1–9]. However, the low-temperature (T < 35K) magnetic properties of LaCoO3 are usually dominated by a Curie-like magnetic susceptibility, which is generally acknowledged to be related to defects. In particular, this “Curie tail” has been ascribed to paramagnetic impurities, ferromagnetic regions at the surface [10], and also to the formation of high spin (S = 16) magnetic polarons in LaCoO3 [1]. Nagaev et al. [11] have also proposed that the paramagnetic centres responsible for the Curie tail are magnetic polarons, although strictly they should be labeled “magnetic excitons”. In this model localized holes (electrons) bound to cobalt ions or oxygen interstitials c © EDP Sciences Article published by EDP Sciences and available at http://www.edpsciences.org/epl or http://dx.doi.org/10.1209/epl/i2004-10519-4 678 EUROPHYSICS LETTERS (vacancies) induce LS to high-spin (S = 2) transitions in neighboring Co ions leaving the remaining cobalt ions in the LS ground state. The radius of a magnetic exciton will extend over several unit cells, creating spin clusters of S = 10 to 15 which are dilute within the solid. The creation of the spin clusters is independent of the thermally excited LS-IS transition. Moreover, Nagaev et al. [11] have predicted that magnetic exciton formation is the precursor for magneto-electronic phase separation at high hole (electron) densities. Such a phase separation has been observed in LaCoO3 doped with divalent cations such as Sr [12, 13] and Ca [14]. The nature of a magnetic polaron or exciton implies that it may be most easily observed in a system displaying a diamagnetic ground state, making LaCoO3 an ideal candidate to study such an defect-driven magnetic state. In this letter we report the first measurements, to our knowledge, of the low-temperature muon-spin spectrum of LaCoO3. This technique is sensitive to local magnetism, and so can in principle probe localized entities such as magnetic polarons. We argue that the observed magnetic response from LaCoO3 is consistent with the formation of magnetic excitons. The existence of magnetic excitons in LaCoO3 could well be related to the formation of magnetic polarons in other families of oxide materials. Bulk Muon Spin Rotation or Relaxation (μSR) experiments were performed at the PSI in Switzerland upon the instrument Dolly. μSR is well suited to the exploration of local magnetic correlations as the spatial sensitivity of the probe decreases rapidly, giving an effective sampling radius of 20 Å. The energy of the incoming muons (4MeV) means a bulk implantation depth of approximately 0.5mm. We have also utilised the Low Energy Muon (LEM) beamline at the PSI [15], enabling depth profiling (from 20 nm to 160 nm) of the muon spin relaxation. Knowledge of the muon stopping site is required to fully understand the relaxation of the muons. We have made ab initio electronic structure calculations to obtain the position of the muon at the time of its decay, using the CASTEP density functional theory code [16] to examine the electron density. A detailed discussion of CASTEP as a tool to describe muon implantation will be given elsewhere. Complementary bulk magnetic measurements were performed using a Quantum Design MPMS SQUID magnetometer, all measurements were performed whilst warming from 2K–300K in fields up to 50 kOe. Measurements have been carried out on both polycrystalline and single-crystal samples, fabricated by a solid-state reaction [13] and floating zone furnace methods, respectively. Iodometric titration measurements on the polycrystal sample reveal a formula unit of LaCoO2.98±0.02, indicating a slight oxygen deficiency. Bulk μSR measurements. – To our knowledge, μSR has not been used to observe a pure LS (where S = 0) to IS transition, where the contribution to the muon relaxation is from one source: the valence electrons of the cobalt ions. Previous measurements have concentrated upon Spin-Peierls [17, 18] or Spin Crossover [19, 20] materials. For the present case of LaCoO3, our calculations predict that the fractional co-ordinates of the implanted muon within a rhombohedral 1 × 1 × 2 unit cell is [0.405, 0.463, 0.530]. This corresponds to a neutral position away from the atoms, meaning that any nuclear spin component will be dominated by the valence electron contribution from cobalt ions. Note that the muons will be influenced by the behaviour of more than one cobalt ion. Our bulk μSR measurements were performed on a single-crystal sample and initially made in a 100Oe Transverse Field (TF). The data were fitted over the entire temperature range using a relaxation function of the form Gx(t) = Atf exp [ − (λstf t)2 ] cos(2πνμt) +A f tf exp [ − ( σ tf t )2] , (1) where Atf and A f tf are the initial asymmetries of, respectively, the oscillatory and Gaussian components, where s and f are for slow and fast relaxing components. λtf and σ f tf are the depolarisation rates for the two separate decays and υμ is the muon precession frequency, Article published by EDP Sciences and available at http://www.edpsciences.org/epl or http://dx.doi.org/10.1209/epl/i2004-10519-4 S. R. Giblin et al.: Observation of magnetic excitons in LaCoO3 679 0 50 100 150 200 250 300 14 16 18 20 22 24 26 A Osc In it ia l A s y m m e tr y -A O s c ( % ) Temperature (K) 0 50 100 150 200 250 300 5.0x10 -4 1.0x10 -3 1.5x10 -3 2.0x10 -3 2.5x10 -3 3.0x10 -3 3.5x10 -3 4.0x10 -3 4.5x10 -3