Incommensurate spin correlations in highly oxidized cobaltates La2−xSrxCoO4

Abstract

We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La2−xSrxCoO4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommensurate magnetic peak positions but around the quarter-integer values that are typically exhibited by excitations in the checkerboard charge ordered phase. Our findings suggest that the strong inter-site exchange interactions in the undoped islands are critical for the emergence of hourglass spectra in the incommensurate magnetic phases of La2−xSrxCoO4.

Anomalous magnetic and spin glass behavior in Nb-substituted LaCo1−xNbxO3

“…Here, the double exchange interactions between Co 3+ and Co 4+ are playing an important role in controlling the magnetic/spin-state transitions. Also, the layered cobaltites La 2−x Sr x CoO 4 (containing both Co 2+ and Co 3+ ) are very interesting to understand the role of different spin as well as valence states of Co ions in controlling the magnetic, transport and electronic properties [35][36][37][38][39][40][41]. For example, it has been reported that the Co 2+ present mostly in HS state, whereas different spin-states are possible in Co 3+ [37,38].…”

Single Crystal Growth of Pure Co3+ Oxidation State Material LaSrCoO4

“…The Néel temperature is suppressed by hole doping x and, finally, incommensurate magnetic ordering appears for x > 1/3 [5]. At and below half-doping, the Co 3+ ions are in the nonmagnetic (S = 0) low-spin (LS) state [5,9,10], and a partial population of the (S = 2) high-spin (HS) state was observed for higher Sr-concentrations x [5].…”

“…The Néel temperature is suppressed by hole doping x and, finally, incommensurate magnetic ordering appears for x > 1/3 [5]. At and below half-doping, the Co 3+ ions are in the nonmagnetic (S = 0) low-spin (LS) state [5,9,10], and a partial population of the (S = 2) high-spin (HS) state was observed for higher Sr-concentrations x [5]. Additionally, for 1/2 < x ≤ 0.9, the hour-glass shaped magnetic excitations disappear [5], which was interpreted as an indication of the importance of nearest-neighbouring exchange interactions (within undoped Co 2+ clusters) for generating the upper part of the hour-glass spectrum [5].…”

“…Hard X-ray diffraction measurements for x = 1/3 [3] and neutron measurements for x = 0.4 [2], however, were not able to detect any charge stripes. Instead, checkerboard charge correlations were detected, and the hourglass spectrum could be explained in by a novel nano phase separation scenario based on La 2 CoO 4 -like undoped clusters and La 1.5 Sr 0.5 CoO 4 -like checkerboard charge ordered clusters [2][3][4][5][6]. Nevertheless, the discussion of the possible presence or absence of additional fractions of charge stripes in these cobaltates still continues [7].…”

Intricacies of the Co3+ spin state in Sr2Co0.5Ir0.5<

“…In the case of layered La 2−x Sr x CoO 4 , contradicting scenarios for the Co 3+ ions were considered to interpret the complex structural, magnetic and transport properties of the system as a function of Sr doping: LS Co 3+ , IS Co 3+ 15-17 , HS-to-IS transition 18 , and mixing of HS/IS 19,20 . Only recently the spin state of Co 3+ in layered La 2−x Sr x CoO 4 was demonstrated by X-ray absorption spectroscopy (XAS) studies to be LS 21,22 for x = 0.5 and a mixture of LS/HS for x ≥ 1 [22][23][24] . Band formation has been proposed to possibly provide another route for the stabilization of IS Co 3+ 25,26 .…”