Magnetostructural coupling, magnetic ordering and cobalt spin reorientation in metallic Pr0.50Sr0.50CoO3 cobaltite

In half-doped Pr0.50A0.50CoO3 metallic perovskites, the spin-lattice coupling brings about distinct magnetostructural transitions for A=Ca and A=Sr at temperatures close to 100 K. However, the ground magnetic properties of Pr0.50Sr0.50CoO3 (PSCO) strongly differ from Pr0.50Ca0.50CoO3 ones, where a partial Pr3+ to Pr4+ valence shift and Co spin transition makes the system insulating below the transition. This work investigates and describes the relationship between the Imma-to-I4/mcm symmetry change [Padilla-Pantoja et al, Inorg. Chem. 53, 12297 (2014)] and the original magnetic behavior of PSCO versus temperature and external magnetic fields. The FM1 and FM2 ferromagnetic phases, above and below the magnetostructural transition (TS1=120 K) have been investigated. The FM2 phase of PSCO is composed of [100] FM domains, with magnetic symmetry Im'm'a (mz=0). The magnetic space group of the FM1 phase is Fm'm'm (with mx=my). Neutron data analyses in combination with magnetometry and earlier reports results agrees with a reorientation of the magnetization axis by 45 deg within the a-b plane across the transition, in which the system retains its metallic character. The presence below TS1 of conjugated magnetic domains, both of Fm'm'm symmetry but having perpendicular spin orientations along the diagonals in the xy-plane of the tetragonal unit cell, is at the origin of the anomalies observed in the macroscopic magnetization. A relatively small field m0H>30 mT is able to reorient the magnetization within the a-b plane, whereas a higher field (m0H,[H//z]>1.2 T at 2K) is necessary to align the Co moments perpendicular to a-b plane. Such a spin reorientation, in which the orbital and spin components of the Co moment rotate joined by 45deg, was not observed with other lanthanides different to praseodymium.


I. INTRODUCTION
Following the extensive investigations on the nature of the spin-state (SS) changes in undoped LnCoO 3 compounds, the SS of trivalent cobalt is being examined in a variety of cobaltites because of its proved ability to condition the transport, magnetic and electronic properties of compounds like the (Ln 1-y Ln' y ) 1-x A x CoO 3 (Ln,Ln': lanthanides, A: alkalineearth) perovskites. [1][2][3][4] In this context, the physical properties of half-doped Pr-based Pr 0.50 A 0.50 CoO 3 specimens are attracting the interest due to the observation of nonconventional phase transitions and distinct unexpected properties with A=Ca 4-11 and Sr. [12][13][14][15][16][17][18] In this way, Ca doped Pr 0.50 Ca 0.50 CoO 3 [and other related (Pr,Ln) 1-x Ca x CoO 3 cobaltites near half-doping (x~1/2)], exhibits an exotic metal-insulator transition 1 produced by two concurrent phenomena: (i) an abrupt Co 3+ spin-state change 10 and (ii) a partial Pr 3+ to Pr 4+ valence shift. [7][8][9] Pr 0.50 Ca 0.50 CoO 3 (PCCO) is orthorhombic (Pnma) and metallic but it becomes insulating at T MI ~ 80 K. An electron is transferred from some Pr atoms to Co sites 6-10 and a concomitant SS crossover promotes the stabilization of the Co 3+ low spin (LS) state. 10 Remarkably, PCCO exhibits exceptional photoresponse capabilities of potential interest for ultrafast optical switching devices. 11 The generation of metallic domains in the sample after photoirradiation in the non-conducting state occurs thanks to the strong connection between volume expansion, electron mobility and excited spin-states. 10 The structural, magnetic and electronic properties of Pr 0.50 Sr 0.50 CoO 3 (PSCO) apparently differ from PCCO (without magnetic order due to the LS state stabilization in the trivalent Co sites). PSCO is ferromagnetic (FM) below T C ∼ 230 K and metallic in all the temperature range. Mahendiran et al 12 initially reported unexpected magnetic anomalies at T S1 ∼ 120 K.
The discovery of a second magnetic transition and intriguing step-like behavior of the magnetization, which decreases or increases depending on the magnitude of the applied field, [12][13][14][15][16] was followed by the detection of structural anomalies at the same temperature by Troyanchuk et al . 15 The lack of consensus on the structural properties of PSCO led to different structural descriptions, used to justify visible changes in diffraction data at low temperatures. 13,[15][16][17] On the other hand, some works attributed the magnetostructural transition to a phase separation at T<120 K, proposing a two-phase magnetic state at low temperature. 16 Finally, a reliable description of the crystal structure evolution across T S1 was reported in 2014 by Padilla et al. in ref. 18. From the high temperature cubic phase, upon decreasing temperature PSCO follows the Pm-3m →R-3c→Imma→I4/mcm transformations. Hence the symmetry change at the magnetostructural transition at about T S1 implies an orthorhombictetragonal conversion (O-T). 18 The absence of this transition in other half-doped cobaltites without Pr ions and the spontaneous Pr valence shift reported in PCCO and other (Pr,Ln) 1-x Ca x CoO 3 cobaltites motivated to investigate the possible importance of the Pr 4f -O 2p hybridization for the structural changes in PSCO. 13,16,18 Unlike PCCO, a Pr 3+ to Pr 4+ oxidation process was ruled out in PSCO by means of x-ray absorption spectroscopy (XAS) studies at Pr M 4,5 and Pr L 3 edges and charge-transfer multiplet calculations. 19 Similarly, XAS measurements of the temperature evolution of the Co L 2,3 edges showed that the spin state of Co ions remains nearly unaltered across the anomalous transition. 19 Evidences of the interplay between the magnetic and crystal structures were obtained from transverse susceptibility and magnetostriction measurements which point to likely changes in the magnetocrystalline anisotropy at T S1 . [12][13][14]20 Lorentz Transmission Electron Microscopy (LTEM) images reported by Uchida et al. also suggested a reorientation of the magnetization axis by 45º when studying the evolution of the magnetic domain structure under electronbeam. 21 The importance of the spin-lattice coupling has been also confirmed by means of xray magnetic circular dichroism (XMCD) experiments at the Co L 2,3 edges. 22 They reveal a sizeable orbital momentum in Co atoms that evolves in like manner as the atomic spin moment (or the macroscopic magnetization) across the two successive magnetic transitions, pointing to a coupling between the ordered electronic spins and the orbital states of 3d electrons.
We present a neutron diffraction investigation of the singular magnetic properties of Pr 0.50 Sr 0.50 CoO 3 that clarifies the temperature and field evolution of the magnetic symmetry in this system. The relevance of the structural symmetry changes on its magnetic behavior has been elucidated.

II. EXPERIMENTAL DETAILS
Polycrystalline ceramic samples of PSCO were prepared by the conventional solid-state reaction method under an oxygen atmosphere as reported in Ref. 18. High-purity Co 3 O 4 and Pr 6 O 11 oxides and SrCO 3 were used as precursors. The last two annealings were performed at 1100 ºC (for 12h) and 1170 ºC (for 24h) under O 2 , making a slow cooling. Powder samples and compacted pellets were used for the measurements. Samples quality was checked by xray diffraction patterns collected at room temperature using a Siemens D-5000 diffractometer and Cu Kα radiation. They were single-phase and free from impurities. The magnetic response to dc and ac magnetic fields was measured using a Superconducting Quantum Interferometer Device (SQUID) and Physical Properties Measuring System (PPMS) from Quantum Design. The latter was also used for electrical transport measurements using the four-probe method and silver paste.
Neutron powder diffraction (NPD) measurements in the D20 diffractometer of the ILL were performed using a high take-off angle of 118° for the Ge(115) monochromator, and a radial oscillating collimator that precedes a microstrip PSD detector, covering an angular range of 150°. In this range high resolution data as a function of temperature was obtained warming the sample in a cryofurnace from 15 K up to 443 K. In ramp mode the temperature shift for individual scans was smaller than 5 K. Additional NPD patterns were also recorded at fixed selected temperatures. A cryomagnet was used on D1B to apply magnetic fields up to 5 T.
All the structural and magnetic Rietveld refinements were made using the Fullprof program. 23 Crystallographic tools from the Bilbao Crystallographic server were also used. 24

III. RESULTS AND DISCUSSION
The ac susceptibility was measured under a dc field of 75 Oe, superimposed to an ac field of 10 Oe. As illustrated in Figure 1(a), the magnetic transitions produce on cooling pronounced upturns in the real component of the ac susceptibility (χ'), forming two separated peaks with maxima at 92 and 225 K respectively. Regarding the onset of the two peaks in χ'(T), the first one starts to develop at ~245 K and the second at ~120 K. Magnetization (M) was also measured as a function of temperature and applied magnetic field using a commercial SQUID. A comparison of the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations under 1 KOe plotted in Fig. 1

(b) shows a similar hump in both M(T) curves at about
T S1~1 20K. A small splitting in the curves is detected below T C ≈230 K which increases below T S1 . Moreover, it was reported in earlier works that the sharp decrease in M(T) when cooling under small fields is accompanied by thermal hysteresis. 12,13 The metallic resistivity is depicted in the inset of Fig. 1(b). The temperature evolution of the coercive field (H c ) was determined from the hysteresis loops recorded at diverse temperatures. The obtained H c (T) curve is represented in Fig. 1(c). Overall, H c increases with decreasing the temperature but a clear anomaly (peak shape) is observed at T S1 . In the inset, the field dependence of the magnetization is plotted at two temperatures representative of the two ferromagnetic phases. From now on we will name FM1 to the distinctive ferromagnetic state of the I4/mcm phase (T< T S1 ), and FM2 to the ferromagnetic state of the Imma cell below the Curie temperature (T S1 <T< T C ). The maximum value of the magnetization M(H) below T S1 (at 5 K and 7 T applied field) was very close to 2 µ B /f.u. (M=1.96µ B /f.u.). 22 To avoid misunderstandings, hereafter we will label the ferromagnetic models F x,y or z always referred to the unit cell setting of the tetragonal phase, where z denotes the coordinate along the longest axis 2a 0 . Therefore, under this definition, F z along this paper always means that spins are pointing parallel to the longest (vertical) 2a 0 -axis, which is the c axis in I4/mcm but it is the b-one in Imma phase.
In an effort to obtain some insight on the anisotropy of the magnetization in the orthorhombic phase, we performed careful refinements of the D20 neutron diffraction pattern at 140 K using different moment orientations. The differences using different magnetic anisotropies were small. Nevertheless the best fit was obtained for the F x configuration. F y or F z models generate wrong ferromagnetic intensities (in comparison to the experimental ones) in well resolved peaks like (123) and (321), being experimentally higher the magnetic contribution to the last. Moreover, these two models also produce an excess of (103) magnetic intensity, whereas the F x configuration nicely reproduces all magnetic intensities.
In addition, several possible magnetic or Shubnikov space groups (MSGs) compatible with the Imma symmetry and k=0 were considered. Among them the magnetic subgroups Im'm'a (allowing F x G z , expressed in the tetragonal setting) or Imm'a' (compatible with F y and G x F z , referred to the tetragonal setting). 26 Table I). The Rietveld refinement at 140 K (above but very close to the O-T phase transition) using the Im'm'a MSG symmetry converges to the F x model (and m z =0, tetragonal setting) and it is plotted in Figure 3,

III-b-1 Discerning between out-of-plane and in-plane ordering
The tetragonal magnetic symmetry I4/mc'm' (allowing only m z components, F z model) was immediately discarded as it clearly generates wrong magnetic intensities. It is shown in Fig

III-b-2 Making compatible neutron and LTEM results
As the best solution for the magnetic order in the tetragonal phase we found the Fm'm'm (#69.524) magnetic SG [transformation to standard setting: (-c, a-b, -a- Table I). The refinement of the 15 K pattern with this symmetry is perfectly satisfactory, yielding a FM ordered moment As shown in Table I, the Fm'm'm magnetic symmetry permits two independent components (m x and m y , expressed in the tetragonal setting) perpendicular to the vertical tetragonal axis.
If |m x |≠|m y | the diagonal ferromagnetic FM[110] order is splitted into two non-collinear sublattices that deviate from the diagonal line. Calling δ to the angle formed by the Co moments at (0,0,0 | m x ,m y ,0) and (1/2,1/2,0 |m y ,m x ,0) tetragonal sites, it is important to emphasize that the intensity of (101)/(110) reflections would be proportional to δ if the moments were not collinear. Collinearity is here experimentally confirmed because these reflections have null intensity at 2θ=24.53º in Fig. 5, leading to the diagonal m x =m y model (F xy ).

III-c Temperature evolution of the ordered FM moment (zero field NPD)
The amplitude of the ordered FM moment per cobalt atom was refined as a function of temperature using the set of neutron patterns collected below T c . Its evolution is shown in The samples here presented were previously investigated by x-ray magnetic circular dichroism (XMCD) measurements at the Co L 2,3 edges, 22 which revealed an unquenched orbital angular momentum in Co atoms. In Fig. 6

III-d Neutron powder diffraction under magnetic field at 2K
Neutron diffraction experiments were extended to measurements under magnetic field in the low temperature FM1 phase. For that a cryomagnet was placed on the D1B powder difractometer. A sintered cylindrical bar of PSCO was ZFC down to 2K. Then isothermal NPD measurements under field were carried out while increasing the vertically applied magnetic field up to a maximum value of 5T. NPD patterns were collected under constant fields within the 0→1 T (µ 0 ∆H= 0.1 T step) and 1→5 T (0.5 T step) intervals. .

IV. CONCLUDING REMARKS
In the precedent sections we have investigated the puzzling magnetic properties of the half-doped PSCO cobaltite. Even though the structural evolution with temperature was previously and extensively described in ref. 18, its magnetic properties and the nature of the magnetostructural transition were not well understood. An orbital contribution to the magnetization (around 1/3 of the spin component) was revealed by XMCD, and the spinorbit coupling term in this cobaltite promotes a parallel alignment of spin and orbital magnetic moments at both sides of the magnetostructural transition. 22 Very likely a reorientation of the orbital moment at T S1 is triggered by the Imma → I4/mcm phase transition and, through the spin-orbit coupling, the ordered Co spins rotate between the  Together with previous LTEM results, the analysis of the magnetic structures using neutron diffraction confirms a spin reorientation accompanying the higher symmetry of the tetragonal I4/mcm cell. We have demonstrated that above T S1 the FM2 phase of PSCO is composed of [100] FM domains (F x ), with magnetic symmetry Im'm'a (m x ≠0, m z =0).
For clarity reasons, the coordinates x-y-z are always referred to the setting of the tetragonal cell (√2a 0 x √2a 0 x 2a 0 ). Below T S1 there is a change in the magnetocrystalline axis. The coupled orbital and spin components of the moment rotate by 45º and the easy axis aligns parallel to the diagonal of the tetragonal unit cell (F xy ). The magnetic space group of the low temperature phase is Fm'm'm (with m x =m y ). The appearance of the 4-fold tetragonal axis brings on degeneration and two equivalent magnetic easy-axes, which are associated to the two conjugated magnetic domains with spin orientations [110] and [1][2][3][4][5][6][7][8][9][10]. Therefore, the loss of magnetization (negative step) under low fields (H< H cr ) is produced by the presence of conjugated [110] and [1][2][3][4][5][6][7][8][9][10] ferromagnetic domains after the Imma → I4/mcm transition. A schematic view of magnetic ordering and magnetic domains above and below T S1 is shown in Figure 9. For clarity reasons, time-reversal type domains are not shown in this figure. The coexistence of different types of domains at low temperatures depends on the sample history and the external applied field. On the other hand, the origin of the sudden positive jump in the magnetization under moderate fields could be ascribed to a larger J/K a ratio, with J and K a being the double-exchange term and the magnetocrystalline energy, respectively. In the tetragonal phase the Co-O-Co angle parallel to c becomes completely flat and could favor the ferromagnetic doubleexchange. 18 Recapitulating, the evolution of neutron diffraction data through the two successive magnetic transitions in Pr 0.50 Sr 0.50 CoO 3 agrees with a reorientation of the easy-axis of cobalt atoms favored by the Imma → I4/mcm symmetry change at T S1 =120 K. The magnetic symmetry Im'm'a (m x ≠0, m z =0) in the orthorhombic phase below T C generates [100] type ferromagnetic domains (F x ). Neutron data analyses in combination with earlier reports agrees with a reorientation of the magnetization axis by 45º within the a-b plane in the low temperature phase (F xy ). The presence below T S1 of conjugated magnetic domains of Fm'm'm symmetry with spin orientations [110] and [1][2][3][4][5][6][7][8][9][10] (|m x |=|m y |) is at the origin of the anomalies observed in the macroscopic magnetization. A relatively small field of µ 0 H cr [⊥z] ≈ 30 mT is able to reorient the magnetization within the a-b plane, whereas a higher field (µ 0 H cr, [//z] ~1.2 T at 2K) would be necessary to align the Co moments perpendicular to this plane.