Stair-like Metamagnetic Transition Induced by Controlled Introduction

Article pubs.acs.org/cm

Stair-like Metamagnetic Transition Induced by Controlled Introduction of Oxygen Deficiency in La0.5Ca0.5MnO3−δ R. Cortés-Gil,†,⊥ L. Ruiz-González,†,⊥ J. M. Alonso,‡,§,⊥ M. García-Hernández,§,⊥ A. Hernando,‡,∥,⊥ and J. M. González-Calbet*,†,⊥ †

Departamento de Química Inorgánica, UCM, Facultad de Químicas, Universidad Complutense CEI Moncloa, 28040 Madrid, Spain Instituto de Magnetismo Aplicado, UCM-CSIC-ADIF, Las Rozas, P.O. Box 155, 28230 Madrid, Spain § Instituto de Ciencia de Materiales, CSIC, Sor Juana Inés de la Cruz s/n, 28049 Madrid, Spain ∥ Departamento de Física de Materiales, Facultad de Físicas, Universidad Complutense, 28040 Madrid, Spain ‡

ABSTRACT: Stair-like metamagnetic transitions in the oxygen deficient La0.5Ca0.5MnO3−δ perovskite are here for first time reported. This experimental fact sheds light on the knowledge of metamagnetic transition in anion deficient phase segregations systems. Actually, it is shown that controlled reduction of the oxygen content constitutes an alternative pathway to the chemical doping at the A and B positions of the perovskite sublattice. In this way, point defects are created inducing the stair-like transition in a CE-type AFM (antiferromagnetic) manganite.

KEYWORDS: stair-like metamagnetism, La0.5Ca0.5MnO2.95, anionic vacancies, phase segregation, charge and orbital ordering

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INTRODUCTION Strongly correlated electron systems are on the cutting edge for the development of advanced materials. Among them, manganese perovskite related compounds, Ln1−xAExMnO3 (Ln = lanthanide, AE = alkaline-earth) have received a great deal of attention in the last years, owing to the strong interplay between lattice, spin, charge, and orbital degrees of freedom.1,2 These systems exhibit a broad phenomenology and for intermediate values of doping interesting features appear that are concomitant to phase transitions induced by temperature and magnetic field, commonly known as metamagnetic transitions. These phenomena are the basis of a plethora of technological applications. Among them, colossal magnetoresistance (CMR), colossal magnetostriction, and giant magnetocaloric (MC) effect, are the focus for the development of advanced sensors. For instance, the MC effect is an alternative to develop more efficient and environmentally friendly magnetic refrigeration devices.3−5 Near x = 0.5, it is recognized that the origin of the observed behavior is related to intrinsic phase-segregation (PS) due to the inhomogeneous coexistence of ferromagnetic metal (FMM) domains, with nanometric or submicrometric size, usually embedded in a charge and orbital ordered (CO-OO) antiferromagnetic (AFM) matrix.1,2 The equilibrium between these phases in the ground state can be dramatically disturbed by changing the temperature or by application of magnetic fields that weaken the CO-OO-AFM phase while promoting the development of the FMM state. © 2012 American Chemical Society

Metamagnetic transitions in Ln1−xAExMnO3 have been understood as related to the tolerance factor (t).1,6,7 This, in turn, can be modified by varying the chemical composition at both cationic and anionic sublattices. For instance, in half doped Ln0.5AE0.5MnO3, t decreases as the average size of the A cations of perovskite sublattice decreases, and higher magnetic fields are required to cancel the CO-OO-AFM state at low temperature.8−11 Furthermore, the CO-OO-AFM state can be destabilized if t increases or if disorder is induced at the A sites by introducing cations with different ionic radius. In these cases, the required field decreases and the metamagnetic transition is sometimes abrupt, giving rise to the so-call stair or step-like transitions.12−15 Such transitions can be also favored using other chemical strategies, for instance, substitutions at the Mn sublattice with magnetic16−19 or nonmagnetic cations.20−26 Thus, Mn-substitutions (below 5%) in Pr0.5Ca0.5MnO320,24 weaken the robust AFM ordering and favor the development of a phase separation between FM and AFM domains, making it susceptible to undergo a CO-OO-AFM/FM transition at low temperature under moderate magnetic fields. Several authors have interpreted this phenomenon on the basis of a martensitic transition due to prominent structural distortions associated to the different cell parameters between AFM and FM phases.13,22−24 Strains at the interface tend to Received: March 23, 2012 Revised: June 5, 2012 Published: June 13, 2012 2519

dx.doi.org/10.1021/cm300927w | Chem. Mater. 2012, 24, 2519−2526

Chemistry of Materials

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block the transformation, but when the applied field is large enough, the elastic energy is destabilized in an avalanche process leading to a magnetic plateau. Related staircase-like behaviors were also observed in resistivity and specific-heat measurements.10,16,18,19 However, it is worth recalling that some authors point out that such a mechanism would have difficulty accounting for the presence of multiple steps. On the basis of that, it has been proposed that the original ordering in CO domains could stabilize certain types of spin structures with increasing field leading to in such behavior.16 Nevertheless, the stair-like transition mechanism is still a matter of controversy because the absence of large strain, according to magnetostriction measurements, has been reported in other related manganites.12,27 This fact disagrees with the martensitic-like origin, suggesting that the magnetic stair transition can be understood on the basis of local defects, related to compositional variations at the A and B sites of the perovskite sublattice, which act promoting the transition in PS manganites.28,29 Moreover, these kinds of defects can also be introduced through a topotactic reduction process ABO3→ ABO3−δ.30 At this point, it is to be recorded that Komornicki et al.31 hypothesized that the vacancy ordering varies continually as a function of oxygen deficiency, being the random distribution of anionic vacancies attributed to low oxygen deficiency. For values of δ close to 0.15, anionic deficiency was thought to be ordered along rows of different length in a statistical fashion, preserving the perovskite symmetry, whereas vacancy rows of infinite length were expected for δ close to 0.20. When δ approaches to 0.25, vacancy rows order into planes, giving rise to local disordered intergrowths, which become ordered by increasing either the annealing times or the oxygen deficiency.32,33 This model is satisfactory for perovskiterelated ferrites and a similar ordering pathway was described by A. Reller et al.30 in perovskite-related manganites. In fact, starting from the ideal structure of a perovskite ABO3, the creation of one oxygen vacancy, that is, the removal of one oxygen, is accommodated by the reduction of two Mn4+ in octahedral coordination into two Mn3+ in square pyramidal coordination. Electron diffraction does not show superstructure spots in the 0 ≤ δ ≤ 0.12 range,30 suggesting that anionic vacancies are randomly distributed. Random arrangement of nonoccupied oxygen positions could also acts a source of local defects capable to induce metamagnetic transitions. In fact, the introduction of some anionic vacancies in La0.5Ca0.5MnO3 affects the 1:1 arrangement of octahedrally coordinated Mn3+ and Mn4+ and induces a rapid increase in the magnetization at low temperature.35 This fact is due to a partial breaking of the CO-OO state, which leads to a drastic increase in the FM fraction reaching the maximum value for y ∼ 2.95. However, when anionic vacancies content is increased, the FM interactions turn around into spin glass state and a minimum magnetization value is obtained for y ∼ 2.80.35−37 Thus, peculiar metamagnetic transitions can be triggered by oxygen deficiency in CO-OO half doped manganites. Although the effect of oxygen deficiency as driving force for metamagnetism has been previously reported in Ln1−xAExMnO3−δ (Ln = La, Pr, Nd and AE = Ca, Sr) manganites35−40 no stair-like magnetization behavior has been detected. In this sense, the aim of this paper is to shed some light on the stair-like transitions induced by the controlled introduction of anionic vacancies.

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EXPERIMENTAL SECTION

La0.5Ca0.5MnO3 (LCMO3) was synthesized according to the conventional ceramic method at 1400 °C during 110 h with intermediate grindings. This sample was used as starting material for the synthesis of reduced La0.5Ca0.5MnO2.95 (LCMO2.95) through a controlled topotactic La0.5Ca0.5MnO3 → La0.5Ca0.5MnO2.95 process in a Cahn D200 electrobalance under a reducing atmosphere of 200 mbar of H2 and 300 mbar of He. Under these conditions, the sample was heated at the rate of 6 °C/min to 400 °C. Once this oxygen loss is reached, the reduction process is shut down by exchanging the reducing atmosphere for an inert one, 500 mbar of He, and cooling the sample at room temperature. The cationic and anionic compositions of both samples were confirmed by means of X-ray energy dispersive spectroscopy and thermogravimetric analysis, respectively. Manganese oxidation states were determined by X-ray absorption near edge structure (XANES) spectroscopy, as reported elsewhere.37 The structural characterization was performed by means of X-ray diffraction (XRD). Measurements at room temperature were done in an X’Pert PRO Panalytical diffractometer, while a low temperature chamber, PheniX Cryostat, was used for the XRD studies under cooling conditions. Magnetic properties were measured in a SQUID magnetometer, in a temperature range from 2 to 300 K and magnetic fields up to 50 KOe and in a PPMS up to 140 kOe.

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RESULTS Structural Characterization. The XRD study, at room temperature, indicates that both samples LCMO3 and LCMO2.95 can be indexed on the basis of an orthorhombic perovskite cell (S.G. Pnma) whose lattice parameters are summarized in Table 1.41 Additional peaks characteristic of a superlattice have not been found either in XRD and Selected Area Electron Diffraction (SAED) patterns. Table 1. Lattice parameters Corresponding to LCMO3 and LCMO2.95 Samples a (Å) b (Å) c (Å) V (Å3)

La0.5Ca0.5MnO3

La0.5Ca0.5MnO2.95

5.397(1) 7.654(2) 5.398(1) 222.992

5.410(1) 7.670(1) 5.408(1) 224.315

On the other hand, it is also well-known that LCMO3 exhibits a CO-OO transition at 215 K involving a significant modification of the lattice parameters.41 In this sense, to analyze the effect of the introduction of 1.67% oxygen vacancies, LCMO2.95, the evolution of the lattice parameters of both samples, LCMO3 and LCMO2.95, was followed by means of XRD in the 50 K ≤ T ≤ 298 K range. The patterns reveal that orthorhombic symmetry is kept in both samples in this temperature range. Nevertheless, a remarkable increase of the orthorhombic distortion, associated to the CO-OO state, is observed for the LCMO3 sample (Figure 1a), whereas for the reduced LCMO2.95 one (Figure 1b), the lattice parameters are kept almost constant under the whole temperature range, which could be related to the disappearance of the CO-OO state in agreement with previous reported data.35,39,40,42 Actually, at room temperature, at which both samples are paramagnetic, the splitting of the (202) and (040) reflections is small in both cases (see Figure 2a). At T = 50 K (Figure 2b), a severe splitting happens for LCMO3, being assigned to the presence of CO-OO state.42 Because this separation does not appear for LCMO2.95, it can be inferred that the CO-OO state is missing, at least as a main component for this sample. Notwithstanding, it should be noticed that two satellite reflections are seen at 2θ 2520



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Figure 3. Temperature dependence of the magnetization under ZFC and FC conditions for (a) LCMO3 and (b) LCMO2.95 samples in an applied field of 1000 Oe.

Figure 1. Lattice parameters representation as a function of temperature for (a) LCMO3 and (b) LCMO2.95 samples.

LCMO3 shows a first transition from paramagnetic to ferromagnetic state at Tc ∼ 230 K. Magnetization increases from room temperature to 215 K, where it decreases as a consequence of the presence of a CO-OO state in agreement with XRD results. At T = 155 K, a second transition to a CEtype43 AFM state appears. The inset of Figure 3a shows M(H) curve recorded at 5 K. The extrapolation at H = 0 of the virgin curve in the range of 10−50 KOe leads to a magnetization value of 0.45 μB. Taking into account that the completely ordered state would exhibit a magnetization value of 3.5 μB,9 it can be assumed, in agreement with previous results,44 that LCMO3 exhibits, in spite being chemically homogeneous, a PS state because 13% of FM phase and 87% of AFM/CO-OO phase is present. Magnetization measurements, at the same conditions, were performed for LCMO2.95, as shown in Figure 3b. The magnetization increases gradually, and it reaches a value of about 0.41 μB at 5 K. This behavior corresponds to a FM material with Tc = 120 K, and there is no evidence that CO-OO seems to take place. However, the inset of Figure 3b shows a kink in the H/M (T) inverse susceptibility around 210 K, which is a signature of the onset of CO-OO.16 This fact is in agreement with XRD data, suggesting that the AFM/CO-OO state in LCMO3 partially turns into a FM phase, which coexists with a minority short-range CO-OO one, with the introduction of 1.67% of anionic vacancies. A similar behavior has been observed in other CO-OO systems in which doping is induced at the Mn-site.20,22 The magnetization measurements vs field of LCMO2.95 recorded at the 2 K ≤ T ≤ 10 K temperature range with a field spacing (fs) of 1000 Oe are displayed in Figure 4. The virgin magnetization curve corresponding to 5 K increases very fast, in the 0 Oe ≤ H < 10 kOe range, as a result of the alignment of the FM areas, which notably increase in respect to LCMO3. For the 10 kOe < H < 35 kOe range, the magnetization increases slightly and linearly with the applied magnetic field. The extrapolation of the virgin curve magnetization in this

Figure 2. Enlargement of the XRD patterns showing the (040) and (202) reflections of LCMO2.95 and LCMO3 samples at (a) 298 K and (b) 50 K.

= 47.1° and 2θ = 48.3°, respectively. Actually, they could be related to the (202) and (040) reflections of a CO-OO phase, suggesting that a small proportion of such state is kept for the reduced, LCMO2.95, sample. Magnetic Characterization. Figure 3a shows the temperature dependence of zero-field-cooled (ZFC) and field-cooled (FC) magnetization for LCMO3 in an applied field of 1000 Oe. 2521



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sharp transition from the AFM to the FM ordering as a consequence of the action of magnetic fields of around 35 kOe at T = 2 K. In this transition, the saturation is not reached. Notice that the measurements have been performed in the −50 kOe ≤ H ≤ 50 kOe range. In this sense, it could be expected that a wider field range could lead to more transitions. For that purpose, additional measurements in the field −140 kOe ≤ H ≤ 140 kOe range have been performed. Half hysteresis loops of LCMO2.95 in the 2 K < T < 5 K range up to 140 KOe are shown in Figure 5. The virgin curve

Figure 4. Field dependence of the magnetization measurements corresponding to LCMO2.95 recorded in the temperature 2 K < T < 10 K range with fs = 1000 Oe.

range leads to a magnetization value of M = 1.36 μB at H = 0. The nonsaturated magnetic state is attained because the expected magnetization for full polarization of the Mn spins (60% of Mn3+ and 40% of Mn4+) is 3.6 μB. At this point, it is worth recalling that the reduction pathway for the composition only involves the coexistence of both Mn4+ and Mn3+, as previously reported by XANES spectroscopy.37 Actually, the experimental magnetization value suggests 38% of the FM fraction, that is, three times higher than in LCMO3. The most remarkable variation appears at around 40 kOe where the virgin curve exhibits a large step where the magnetization increases up to 2.6 μB, involving a FM fraction of 65%. For fields larger than 40 kOe, the virgin curve exhibits a linear behavior, with 2.29 μB being the magnetization value around 50 kOe. Notice that the saturated magnetic state is never reached under this applied magnetic field. As the field decreases, the curve slows down linearly to about 10 kOe followed by a rapid drop of magnetization. The extrapolation to zero magnetic field, in the 50 KOe ≥ H ≥ 10 kOe field range, leads to a value of M = 2.1 μB, corresponding to 70% of the FM phase. Thus, at 40 KOe and 5 K, the magnetization undergoes an abrupt irreversible process that doubles the FM contribution. This fact is probably related to the percolation of the FM domains conforming a true thermodynamic phase. It is worth emphasizing that since the loop is not saturated, the system remains in a phase segregation scenario in which the introduction of 1.67% of anionic vacancies induces a drastic increase of the FM fraction in the manganite material. The virgin magnetization curves for the rest of temperatures have nearly the same initial slope suggesting a well reproducible behavior at the first stage. In contrast, a marked different evolution takes place as a function of temperature. Actually, the sharp step for temperatures 2 K ≤ T ≤ 5 K it is replaced, at higher temperatures, by a smooth tail. The characteristic field (Hs) associated with the step clearly shifts upward with the temperature. Although already reported in related materials,23,45 this behavior result is amazing because the temperature increasing should make easier the spin inversion diminishing the field value associated to the AFM/FM transition. A plausible explanation for this peculiar behavior could be related with some magnetic anisotropy, as it will be further discussed. From our data, it can be concluded that LCMO2.95 sample exhibits a PS state similar to the sample without anionic vacancies, LCMO3, but including a significant increase of the FM fraction as well as short-range CO-OO. The anionic vacancy creation destabilizes the CO-OO state favoring the

Figure 5. Half hysteresis loops of LCMO2.95 in the 2K < T < 5 K range with ΔH = 500 Oe up to 10 KOe and with ΔH = 3000 Oe up to 140 KOe. The inset shows the complete hysteresis at 2 K.

between 2 and 4 K evidences the presence of several steps whose magnitude (i.e., ΔM) decreases as the magnetic field increases suggesting that the system is driven to saturation. Nevertheless, for H = 140 kOe the magnetization value is 2.97 μB, far from the value, 3.6 μB, corresponding to the Mn total polarization. At 5 K, the number of steps decreases and the transitions become less sharp. This behavior suggests the disappearance of the step transition as the temperature increases, as observed for the 5 K < T < 10 K range. The inset of Figure 5 shows the complete hysteresis loop at 2 K. When the applied magnetic field is released from 140 kOe, the staircase-like effect does not appear. In this sense, the fielddecreasing branch of the loop exhibits a linear behavior with a slightly decreasing of magnetization to about 10 kOe, followed by a rapid drop of magnetization with a remnant value of about 0.08 μB. The curve between 0 and −140 kOe is similar to the one observed at the first quadrant and even more, some irreversibility is detected in both quadrants when the field is increased from −140 kOe to 140 kOe. This behavior could be consistent with the fact that LCMO2.95 is not in a saturated magnetic state within this field range because of the coexistence of two magnetic phases. A similar behavior has been observed in the 2 K ≤ T ≤ 5 K range. Figure 5 shows the magnetization curves vs H. Magnetization data were recorded every 500 Oe, up to 10 KOe, and every 3000 Oe between 10 KOe < H < 140 KOe. The influence of the fs, the field increment in the measurement, on the magnetization steps is well-known.18,19 It is worth emphasizing that the appearance or absence of these sharp steps strongly depends on the measuring protocols and it has been reported to play an important role.16,21,23 To perform a detailed study of the critical field of the steps and comparing to previous results a virgin curve was recorded at 2 K up to a maximum field of 140 Oe and with fs = 1000 Oe (Figure 6). The obtained results clearly show the presence of five huge steps, whose main 2522



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phase is a good candidate because it exhibits a CO-OO-AFM ground state. At this point, it is worth recalling that Granja et al.46 reported staircase-like metamagnetic transition in the half doped La0.5Ca0.5Mn1−xFexO3 in which the oxygen sublattice is completely filled. Following this strategy, it has been proved that anionic vacancies induce stepwise nucleation of a FM phase whose growth translates into the steps transitions. The introduction of anionic vacancies on LCMO3 modifies the ordered 1:1 arrangement of Mn4+ and Mn3+, since Mn3+ content increases. Thus, the magnetic behavior is shifted toward the FM region of LCMO3 magnetic phase diagram.47 Actually, the insertion of 1.67% anionic vacancies on LCMO2.95 leads to the same Mn4+/Mn3+ ratio as the FM La0.6Ca0.4MnO3, which does not show any CO-OO-AFM behavior. This composition reaches almost saturation under 5 kOe with 90% of FM phase, suggesting a markedly different behavior with respect to LCMO2.95. In fact, the nonsaturated magnetic state is attained for LCMO2.95 at those fields for which only 40% of FM saturation value is reached, signaling a majority AFM state. However, our XRD results and ZFC-FC curves suggest the existence of a minority CO-OO-AFM phase in LCMO2.95. In order to explain this controversy, it is necessary to suppose the coexistence of two kinds of AFM phases in which only one of them exhibits CO-OO. In fact, neutron diffraction studies carried out on Pr0.5Ca0.5Mn0.97Ga0.03O3 evidence two kinds of AFM phases: CE-type and pseudo-CE-type.48,49 Several authors assign the observed stair-like magnetization behavior to the discontinuous disappearance of the pseudo-CE-type AFM phase and the abrupt increase of the FM phase.48,49 The stair-like curve can be thus associated to the abrupt growth of the FM fraction at the expense of the vanishing of pseudo-CE-type AFM phase whereas the nonsaturated magnetic state must be related to the persistence of CE-type structure. A similar scenario could take place when some anionic vacancies are introduced in PS manganites showing CE-type AFM structure in the ground state such as LCMO3. At this point, it is worth recalling that when ordered anionic vacancies (δ = 0.25) are introduced in LaMnO3, showing A-type AFM structure, a metamagnetic transition appears for T ≤ 5 K.33 However, the different stairlike magnetic evolution with respect to La0.5Ca0.5MnO2.95 could be associated with the different A- or CE-type AFM structure adopted by parent manganite involved in the topotactic reduction. Microstructural features are of paramount importance for understanding the mechanism of the steṕs transitions. The loss of one O2− anion in a perovskite lattice leads to a 5-fold coordination where the most plausible situation is the formation of a square-pyramid.31,37 Even more, this pyramid can be different depending on the position from which the ion is removed, that is , equatorial or apical, as a consequence of the [MnO6] octahedral distortion.41,50 According to the experimental data, the anionic vacancies in LCMO2.95 should be randomly arranged providing the formation of local square pyramids, since no additional peaks characteristic of a superlattice have been found either in the XRD or SAED patterns. In this sense, anion vacancies could be either equatorial or apical and their different proportion, related to different Mn−O distances, could bring about local structural differences. As a matter of fact, theoretical calculations indicate that bond distances and structure factor are crucial parameters in order to determine the formation and migration energies for oxygen vacancies in perovskite related oxides.51−53

Figure 6. Field dependence of the magnetization at 2 K, for LCMO2.95 sample. The steps are clearly observed at the derivate representation, dM/dH.

features, Hs, Ms, and ΔMs, are displayed in Table 2. These sharp transitions denote the successive sudden collapse of CO-OOTable 2. Magnetization (Ms), Field (Hs), and Magnetization Increment (ΔMs) Corresponding to the Steps Shown in Figure 6 step

HS (kOe)

MS (μB)

ΔMS (μB)

I II III IV V

36 52 72 92 114

1.41 1.90 2.30 2.58 2.81

0.39 0.31 0.21 0.16 0.08

AFM interactions and the progressive enhancement of the FM fraction. Figure 6 indicates that ΔM linearly decreases (see inset) as the field increases, pointing out to a gradual decrease of the growth-rate of the FM fraction with the field. Nevertheless, for H = 140 KOe, the magnetization value is 2.97 μB, corresponding to a 84% of FM fraction. The steps are better observed in the dM/dH plot, where it can be seen that the plateaus between the large steps (I, II, III, IV, V) are disrupted by the occurrence of other small steps. These are clearly evidenced at the I, II, and III plateaus, diminishing their magnitude as the magnetic field increases (Table 2). The smaller steps follow a reproducible pattern that makes it difficult to explain in terms of experimental artifacts. The existence of such high number of steps in the virgin magnetization curve is a novel fact in manganites and must be related to the creation of oxygen vacancies.

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DISCUSSION Metamagnetic transitions have been described in several manganite systems involving doping at the A and B sites of the perovskite sublattice, while the origin of this behavior has been a controversial matter. Actually, a martensitic transition has been traditionally proposed as a plausible mechanism to explain the transition steps, while more recent approaches suggest different pathways based on the presence of compositional point defects related to the induced doping. Another pathway of inducing point defects without introducing different chemical elements but modifying the manganese oxidation state is reducing the oxygen content. For this purpose, the LCMO3 2523



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be assigned to spin flip processes in polyhedra that, presenting a particular kind of vacancy and helped by an already flipped next near neighbor Mn ion, can anticipate the metamagnetic transition at a lower field. As is shown in Figure 4, for 2 K < T < 5 K, the magnetic fields required for a metamagnetic transition increase as the temperature increases, which is consistent with a reinforcement of the FM state as the temperature is lowered. However, for T > 5 K, as the temperature increases the metamagnetic transition field decreases, pointing to a strengthening AFM state as the temperature decreases. This apparent contradiction does not come as a surprise because the interplay of anisotropy and AFM exchange at the nanoscale can give rise to anomalous thermal dependence of the magnetization processes.55,56 The increasing of HS vs T in the 2K < T < 5K range can be understood as the result of a thermal decrease of the anisotropy, if this occurs at a rate faster than that of the AFM exchange. The anisotropy tends to deviate the spin configuration from the ground AFM structure toward a canted configuration (Figure 7). This deviation might decrease the energy required for the transit toward the FM-like configuration. As the temperature rises, the anisotropy drops and, consequently, the spin configuration approaches to the AFM ground state giving rise to an increase of HS. Once the AFM configuration has been reached, further heating will only induce a decrease of the AFM coupling and a subsequent decrease of HS as illustrated in Figure 4.

Figure 7 shows the usual zigzag orbital ordering found in La0.5Ca0.5MnO3 superimposed to the charge order pattern that

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Figure 7. Schematic representation of the magnetic moment orientation for LCMO2.95.

CONCLUSIONS Stair-like metamagnetic transitions have been evidenced for first time in an oxygen deficient half-doped manganite. The magnetic and structural measuraments suggest that the origin of these transitions is related to the presence of point defects. According to the discussed data, it can be concluded that different magnetic phases coexist in La0.5Ca0.5MnO2.95 as a consequence of the 1.67% of oxygen vacancies. Actually, the Mn3+/ Mn4+ ratio increases, as the anionic vacancies are created, diminishing the charge and orbital ordering while the FM interactions through double exchange interactions are favored. Alternatively, the anionic vacancies would act as nucleation centers for the FM domains leading to different steps related to the different magnetic surroundings. Simultaneously, it should also be taken into account that the anionic vacancies involve the breakdown of the Mn−O−Mn bonds suppressing the double exchange mechanism, leading to an AFM superexchange interaction in which the CO state should not be present. Thus, a well differentiated response, under the presence of an external magnetic field, should be expected since the vacancy surrounding could be different.

alternates Mn3+ and Mn4+ ions. The observed metamagnetic transitions must correlate to the spin flip of half of the ions (i.e down spins) in the OO-unit cell, marked by the dashed line. Also, oxygen vacancies may occur in various unequivalent cell sites and these, in turn, promote the spin flip of Mn ions that, depending on their particular environment, may require higher or lower energies transitions. Hence, metamagnetic transitions appear in consecutive steps at various magnetic fields, as it is indeed observed. Let us focus on the unequivalent octahedra numbered A and B. Octahedra A corresponds to a Mn4+ ion, a non-Jahn−Teller (JT) ion, exhibits two Mn−O distances.54 Octahedra B corresponds to JT Mn3+ ion with three different Mn−O distances.41 Therefore, up to five nonequivalent oxygen vacancies can be created that would imply different energies for the spin flip process of the corresponding Mn ion. Up to 140 KOe, we observe precisely five jumps. This fact seems to be correlated to the dissimilar oxygen vacancies promoting the various metamagnetic transitions due to the five different oxygen environments. Consequently, each transition is triggered by different values of the applied field. The magnitude of each jump, ΔM, should be then related to the abundance of a particular kind of oxygen vacancy and may be further modulated by other effects since the basal vacancy formation energy is lower compared to the apical one in agreement with the Mn−O distances.51−53 Once the FM clusters associated to a specific type of vacancies are formed, that is, the applied magnetic field is high enough to operate the corresponding spin flip, the magnetization remains almost constant as the field increases, exhibiting plateaus between steps. As the field increases up to a value required for the nucleation of another FM cluster on a different type of vacancy, another jump is observed. The smaller steps appearing well within the plateaus point to spin flip processes for ions in statistically less probable environments. These could

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Fax: 34 91 394 43 52. Author Contributions ⊥

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partially supported by the Spanish Ministerio de Ciencia e Innovación grants: CSD2009-00013, 2524



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MAT2007-61954, MAT2011-23068 and MAT2011-27470C02-02. Research by R. Cortes-Gil has been also supported by a PICATA postdoctoral fellowship of the Moncloa Campus of International Excellence (UCM).

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ABBREVIATIONS Ln, lanthanide; AE, alkaline earth; CMR, colossal magnetoresistance; MC, giant magnetocaloric; PS, phase-segregation; FMM, ferromagnetic metal; CO-OO, charge and orbital ordered; AFM, antiferromagnetic; t, tolerance factor; LCMO3, La0.5Ca0.5MnO3; LCMO2.95, La0.5Ca0.5MnO2.95; XRD, X-ray diffraction; fs, field spacing; Hs, characteristic field; JT, Jahn−Teller

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