Magnetic phase diagram ofLa2−xSrxCoO4revised using muon-spin relaxation

Abstract

We report the results of a muon-spin relaxation (μSR) investigation of La 2−x Sr x CoO 4 , an antiferromagnetic insulating series which has been shown to support charge ordered and magnetic stripe phases and an hourglass magnetic excitation spectrum. We present a revised magnetic phase diagram, which shows that the suppression of the magnetic ordering temperature is highly sensitive to small concentrations of holes. Distinct behavior within an intermediate x range (0.2 x 0.6) suggests that the putative stripe ordered phase extends to lower x than previously thought. Further charge doping (0.67 x 0.9) prevents magnetic ordering for T 1.5 K.DOI: 10.1103/PhysRevB.93.140406Thirty years after the discovery of high-T c superconductivity, the role of multiple order parameters in the cuprates continues to be a topic of intense research, with the interplay between correlations of charge order (CO), antiferromagnetic spin order (SO), and superconductivity the topic of much investigation [1][2][3][4][5]. Although a detailed comparison of specific systems has been found to be problematical, in order to elucidate the details of the physics at play in a different context it is illuminating to study isostructural nonsuperconducting 3d transition metal oxides [6] such as the antiferromagnetic series La 2−x Sr x CoO 4 (LSCO). Controlled doping of holes into the two-dimensional CoO 2 layers is achieved via substitution of Sr for La but, in contrast to the cuprates, a spin-blockade mechanism means the series remains an insulator for x 1 [7]. The previously reported phase diagram of LSCO, determined by neutron scattering [8], shows a region of nearest-neighbor antiferromagnetism (nnAFM) for low dopant concentrations (x 0.3) and incommensurate (IC) magnetism consistent with stripelike CO and SOs for an intermediate doping range 0.3 x 0.6. Recently, muon-spin rotation (μSR) and NMR measurements made on the stripe ordered x = 1/3 compound [9] revealed the importance of a range of time scales in the magnetic ordering of the material, with μSR showing both the onset of static magnetic order and the freezing of dynamical processes at temperatures significantly lower than the ordering temperature previously identified using neutrons. This is because μSR is sensitive to fluctuations on the microsecond time scale (set by the muon gyromagnetic ratio γ μ = 2π × 135.5 MHz T −1 ). Such slow fluctuations appear static in neutron scattering measurements where the energy resolution E ≈ 1 meV constrains the sensitivity of the technique to fluctuations with a much faster time scale

Suppression of the outwards-dispersing branches in hour-glass magnetic spectra induced by nanoscale phase separation in La2−xSrxCoO4

“…Figure 6 shows the Fourier transformation of the measured time spectra at zero field for the x = 0 sample at various temperatures. Distinct from the previous μSR results in the literature [31], spontaneous muon spin precession is already observed below T N ∼ 275 K, which is consistent with our neutron measurements and signals the high quality of the magnetic samples. Two distinct internal fields can be observed below T N which is most likely the consequence of two inequivalent muon sites within the magnetic structure.…”

“…However, we obtain a different magnetic phase diagram with distinctly higher ordering temperatures than reported in Ref. [31]. Figure 6 shows the Fourier transformation of the measured time spectra at zero field for the x = 0 sample at various temperatures.…”

“…Since these values were obtained by neutron scattering which is probing a different timescale, we also reexamined the magnetic phase diagram by means of μSR measurements similar to Ref. [31]. However, we obtain a different magnetic phase diagram with distinctly higher ordering temperatures than reported in Ref.…”

Incommensurate spin correlations in highly oxidized cobaltates La2−xSrxCoO4

“…Although an incommensurate magnetic regime was found in the related cobaltate material Pr 2−x Ca x CoO 4 , 24 only a paramagnetic regime has been reported in La 2−x Sr x CoO 4 for x > 0.6. 22,25,26 Here we observed a continuation of incommensurate magnetism across the half-doping and incommensurate magnetic peaks in the entire hole-doped regime of La 2−x Sr x CoO 4 above the half-doping level (1/2 < x ≤ 0.9). Thus, we were able to measure the previously unknown spin excitations of highly hole-doped cobaltates and to observe the differences between the low and high-doping regimes, which consequently allowed us to identify the strong exchange interactions in the undoped regimes as the fundamental elements of the presence of the high-energy portion of the hourglass spectra.…”

“…During the review we noticed that a µSR study was carried out on the magnetic phase diagram of La 2−x Sr x CoO 4 . 26 The phase diagram there is different due to the fact that there is oxygen non-stoichiometry in the samples used in the µSR study. 26 Nevertheless, for the high Sr regime the µSR study 26 observes that the spin fluctuations slow down upon cooling, not inconsistent with our neutron observations of quasi-static spin correlations regarding the different time scales probed in neutron (few ps) and µSR (∼ µs) measurements 26 (or also in NMR measurements 25 ).…”