Particle Size Effects on Charge Ordering and Exchange Bias in

Article pubs.acs.org/JPCC

Particle Size Effects on Charge Ordering and Exchange Bias in Nanosized Sm0.43Ca0.57MnO3 Vladimir Markovich,*,† Roman Puzniak,‡ Ivan Fita,§ Dmitrii Mogilyansky,∥ Andrzej Wisniewski,‡ Gad Gorodetsky,† and Grzegorz Jung†,‡ †

Department of Physics, Ben-Gurion University of the Negev, 84105 Beer-Sheva, Israel Institute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46, 02-668 Warsaw, Poland § Donetsk Institute for Physics & Technology, National Academy of Sciences, 83114 Donetsk, Ukraine ∥ The Analytical Research Services and Instrumentation Unit, Ben-Gurion University of the Negev, 84105 Beer-Sheva, Israel ‡

ABSTRACT: Size dependence of magnetic properties of Sm0.43Ca0.57MnO3 nanoparticles with average size of 15−60 nm has been investigated. Charge ordering, characteristic for the bulk form of this material, was found to be gradually suppressed with decreasing particle size and fully disappearing in 15 nm nanoparticles. Onsets of ferromagnetic contributions to the magnetization appear below 90 K, independently on the particle size, while spontaneous magnetization at 10 K increases with decreasing particle size. Magnetic hysteresis loops exhibit size dependent exchange bias effect manifested by horizontal HEB and vertical MEB shifts, appearing after field cooling. Variation of HEB and MEB shows opposite size dependences; HEB decreases while MEB increases with decreasing particle size. For large 60 nm particles, HEB decreases monotonously with increasing temperature, while for the smallest 15 nm particles it shows surprising nonmonotonic temperature dependence. Size and temperature dependences are discussed in terms of magnetic coupling between antiferromagnetic cores and shells containing ferromagnetic clusters in a frustrated spin configuration. to the direction of the magnetic field in which the sample was cooled.12 Experimental studies of NPs of electron-doped R1−xCaxMnO3 (x > 0.5) manganites confirmed that NP size decrease results in an appearance of a FM-like shell with width that progressively grows with decreasing size.4−8 Highly distorted perovskite GdFeO3-type structure of the narrow-bandwidth Sm1−xCaxMnO3 (SCMO) is favorable for charge localization but detrimental for double exchange (DE) interactions.13−15 Charge ordering appears in the doping range 0.4 < x < 0.85, and different CO configurations are stabilized depending on the Mn3+/Mn4+ content.13−15 Charge ordering in the bulk Sm0.43Ca0.57MnO3 appears at TCO ≈ 283 K, the highest TCO in the SCMO system, and is characterized by a huge difference between TCO and TN ≈ 135 K. In this work, we report on experimental investigations of the size dependence of magnetic properties of Sm0.43Ca0.57MnO3 NPs with average particle size ranging from 15 to 60 nm.

1. INTRODUCTION Magnetic nanoparticles (NPs) are in a focus of intensive investigations, mostly due to an appearance of many features that are not observed in their bulk counterparts.1−3 Of a special interest are complex transition metal oxides forming strongly correlated electron systems in which interplay between spins differently ordered, charge, orbital, and lattice subsystems, results in rich phase diagrams containing charge-, spin-, and orbital-ordered phases. It has been reported that by reducing the size of such systems to the nanoscale, one can significantly influence coupling between the subsystems, change the stability of ordered phases, and hence modify the physical properties of the system.4−8 Recently, particular attention has been paid to nanosized perovskite manganites with R1−xAxMnO3 formula, where R and A are rare earths and alkaline earths, respectively.4−11 Phenomenological models and Monte Carlo studies of antiferromagnetic (AFM) charge ordered (CO) nanomanganites predict that reduction of the system size to the nanoscale range leads to an enhancement of the surface charge density, a suppression of AFM/CO phase, and an emergence of ferromagnetic (FM) order with spin-glass (SG)-like behavior near the surface.11 As a consequence, natural AFM/FM interfaces and exchange bias (EB) effect appear in nanosized manganites. Generally, the EB effect shows up when a FM/AFM system is cooled in a static magnetic field below the Néel temperature, TN < TC.12 The induced unidirectional anisotropy shifts magnetization hysteresis loops, usually in the direction opposite © 2014 American Chemical Society

2. SAMPLE PREPARATION AND STRUCTURAL CHARACTERIZATION Nanocrystalline SCMO particles were prepared by glycine− nitrate method, similar to that developed for preparation of La0.7Ca0.3MnO3 nanopowders.16 The X-ray diffraction (XRD) data were collected using a Philips 1050/70 powder Received: February 10, 2014 Revised: March 2, 2014 Published: March 17, 2014 7721

dx.doi.org/10.1021/jp5014253 | J. Phys. Chem. C 2014, 118, 7721−7729

The Journal of Physical Chemistry C

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diffractometer with Kα radiation at λ = 1.541 Å. The XRD pattern of the as-prepared sample was found to be a mixture of calcite and amorphous phases, which transforms into almost pure orthorhombic phase with average size controlled by the annealing temperature. In the annealing process, the powders were heated at a rate of 5 °C/min to the desired temperature (600−850 °C) in a flow of 40% O2 and 60% Ar for 1 h, resulting in an average crystallite size of 15−60 nm. The XRD patterns of the samples calcined at various temperatures (600− 850 °C) are shown in Figure 1a. The diffraction peaks were

Table I. Crystalline Size, Lattice Parameters, and Unit Cell Volume of the Sm0.43Ca0.57MnO3 Samples Annealed at Various Temperatures lattice parameters (Å) calcination temp (°C) 600 660 750 790 850

crystalline size (nm)

a

b

c

vol (Å3)

± ± ± ± ±

5.377(2) 5.378(2) 5.388(1) 5.390(1) 5.401(1)

7.555(2) 7.550(2) 7.545(2) 7.546(1) 7.540(2)

5.371(2) 5.371(2) 5.362(1) 5.357(1) 5.350(1)

218.2(3) 218.1(3) 218.0(2) 218.0(2) 217.8(2)

15 17 25 33 60

2 2 2 3 5

progressive reduction of the unit cell with increasing of NPs size is observed. This behavior, opposite to the one reported recently by us for Sm0.27Ca0.73MnO3 NPs,21 may be related to slight variations of the oxygen content during the annealing. The evaluation of the grain size distribution was performed using the FW1/5/4/5 M method22 under an assumption that the size distributions follow the γ distribution. We have found that all samples are polydisperse nanopowders. The distribution functions for the extreme size samples, SCMO15 and SCMO60, are shown in the insets of Figure 1a. The average grain size with the size dispersion are ⟨D⟩ = 15 nm with σ = 8 nm for SCMO15 and ⟨D⟩ = 66 nm with σ = 18 nm for SCMO60, in good agreement with the values obtained independently by Debye−Scherrer method. The NPs sizes were additionally confirmed by transmission electron microscopy investigations. Figure 2a shows the high resolution

Figure 1. (a) XRD spectra (λ = 1.541 Å) of SCMO NPs samples, annealed at various temperatures (the parasitic phases SmMn2O5 and Mn3O4 are signed by full and empty triangles, respectively). Indexing is done in the orthorhombic setting of the Pnma space group. (b) Rietveld plot for SCMO60 sample. The experimental data points are indicated by open circles; the calculated and difference patterns are shown by solid lines. The Bragg positions of the reflections of the orthorhombic manganite are indicated by vertical lines below the pattern. Insets in panel a show the grain size distribution (GSD) functions for SCMO15 (left) and SCMO60 (right).

Figure 2. (a) TEM high resolution image of the SCMO15 sample (the dotted line shows the boundary between agglomerated grains); (b) EDS spectrum of the SCMO15 sample.

indexed in the orthorhombic setting of the Pnma space group. Small peaks of parasitic phases of SmMn2O5 (about 4 wt %) and of Mn3O4 (3−4 wt %) are indicated by triangles. Magnetic properties of the parasite phases are well-known;17−19 nevertheless, their influence on the magnetic properties of the samples investigated was found to be negligible. In particular, we did not observe any features in the measured magnetization or susceptibility characteristics, discussed in the next section, at temperatures at which phase transitions in the parasitic phases occur. In order to refine the lattice parameters and the crystallite size, we have performed the Rietveld analysis of the XRD spectra using the FullProf program.20 The Rietveld plot for the sample annealed at 850 °C is shown in Figure 1b. The average crystallite sizes, ⟨D⟩, calculated using the Debye−Scherrer equation, and lattice parameters are listed in Table I. Samples with an average particle size of 15, 17, ..., 60 nm will be hereafter referred to as SCMO15, SCMO17, ..., SCMO60. The lattice parameters for all samples are close to the parameters of ceramic polycrystalline Sm 0.43Ca0.57MnO3,15 and only a

micrograph of the SCMO15 sample. One may see that the average size of a SCMO15 single nanoparticle is indeed close to 15 ± 2 nm. The energy-dispersive X-ray spectroscopy (EDS) analysis confirmed the composition and the homogeneous distribution of the constituent elements; nominal atomic values Sm:Ca:Mn = 0.43:0.57:1.0. The approximate value of the oxygen content determined by EDS analysis is equal to 2.99 ± 0.04.

3. RESULTS AND DISCUSSION A. Magnetization. For magnetic measurements, SCMO particles were compacted, under a pressure of ∼5 kbar, in the form of cylinder-shaped samples, 2.4 mm in diameter and 3 mm in height. The measurements, using PAR (Model 4500) vibrating sample magnetometer (VSM), were carried out in the temperature range of 5−290 K and magnetic fields up to 15 kOe, applied perpendicularly to the rotation axis of the samples. The magnetization measurements in high magnetic fields, up to 7722



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90 kOe, were performed using the Physical Property Measurement System of Quantum Design with magnetic field applied along the rotation axis of the sample. Temperature dependences of the field cooled (FC) magnetization (MFC) and zero field cooled (ZFC) magnetization (MZFC) of various SCMO NPs, recorded at applied field of 1 kOe, are shown in Figure 3. One notices that the magnetization

Figure 3. Temperature dependence of zero field cooled, MZFC (open symbols), and field cooled, MFC (solid symbols), magnetization of SCMO NPs recorded in magnetic field H = 1 kOe.

of all samples changes only very slightly at T > 125 K. For all investigated NPs, MFC(T) and MZFC(T) start to increase with decreasing temperature below ∼125 K and then separate at the irreversibility temperature Tirr ∼ 55−60 K. The difference between MFC and MZFC increases strongly with further temperature decrease, and a pronounced peak in the temperature dependence of ZFC magnetization is observed. The temperature at which the peak appears can be associated with the blocking temperature, TB. For all investigated NPs, TB is contained in the range of 40−45 K. Notice that below 20 K, MFC of all NPs, with the exception of the largest SCMO60, strongly decreases with decreasing temperature. It is wellknown that at low temperatures MFC of super spin glasses (SSG) does not increase, and may even decrease, with decreasing temperature. In a marked difference to the SSG case, MFC of superparamagnets (SPM) always increases with decreasing temperature.23 TB of an ideal SSG is very close to Tirr. However, similarly to the behavior of isolated and weakly interacting NPs, the ZFC−FC irreversibility for SCMO NPs appears at temperatures well above TB.2,3 The temperature dependence of magnetization recorded in H = 15 kOe is shown in Figure 4. Temperature dependence M(T) of large SCMO60 NPs resembles that of the bulk Sm0.43Ca0.57MnO3 (see ref 14) and exhibits a charge ordering associated peak at TCO = 282 K (see Figure 4e). For temperatures below the peak, a significant temperature hysteresis of about 5 K is observed in ZFC−FC curves for SCMO60; see inset in Figure 4e. With decreasing particle size, the peak in magnetization widens and shifts toward lower temperatures, indicating a suppression of both the CO state and hysteretic effects. Moreover, all CO associated features, such as peaked magnetization and hysteresis, become invisible in smaller SCMO17 and SCMO15 NPs. The magnetic field of H = 15 kOe suppresses the irreversibility of the magnetization and the gap between MFC(T) and MZFC(T); nevertheless, the

Figure 4. (a−e) Temperature dependence of MZFC (blue closed squares) and MFC (red open stars) of SCMO samples recorded in H = 15 kOe. The dashed line points out the temperature of CO in the bulk. Inset in panel e demonstrates hysteresis between MZFC and MFC curves.

temperature Tirr remains practically unaffected. Temperature dependences MFC(T) and MZFC(T) separate at Tirr ≈ 55−60 K for all NPs, indicating the onset of a weak FM moment. Pissas et al.24 have suggested that the broad magnetization peak observed in the vicinity of CO transition may be associated with hopping of eg electrons, short-range FM correlated through the DE interactions. This leads to a positive value of the paramagnetic Curie temperature, Θ. However, at lower temperatures, eg electrons ″freeze out″ and FM interactions are replaced by superexchange ones, what leads to appearance of the AFM spin configuration. The H/MFC ratio, plotted in Figure 5 as an inverse of the high field susceptibility 1/χhf(T) = H/M, shows a marked minimum and a change in slope in the vicinity of the charge ordering temperature. The minimum in 1/χhf(T) progressively widens and shifts to lower temperatures with decreasing particle size. A pronounced change in the slope of the 1/χhf(T) is still visible for SCMO17 NPs indicating that the CO state still persists in these NPs. In contrast, 1/χhf(T) dependence for the smallest SCMO15 NPs exhibits a weak downturn. The dashed lines in Figure 5 are plotted according to the best fit of the Curie− Weiss law χhf(T) = C/(T − Θ) to the data, obtained within relatively narrow temperature intervals (300−330 K for SCMO25, SCMO33, and SCMO60; 270−330 K for SCMO17; and 200−330 K for SCMO15), where 7723



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Figure 5. Temperature dependence of H/M = 1/χhf measured at 15 kOe.

Figure 6. Temperature dependence of thermoremanent magnetization for SCMO samples.

C = Nμeff2/3kB, μeff is an effective Mn magnetic moment, kB is the Boltzmann constant, and N is the number of magnetic ions per unit volume. Since the temperature ranges where 1/χhf obeys Curie−Weiss law are relatively narrow, the fit gives us only a rough estimation of Θ. Nevertheless, the temperature evolution of 1/χhf has several interesting features. First, the value of Θ monotonously decreases with decreasing particle size from 143 K for SCMO60 to 65 K for SCMO15, indicating a progressive suppression, with decreasing particle size, of FM correlations in the PM phase. It appears that the strength of FM correlations in the PM phase and the value of the weak FM moment at low temperatures (Figures 3 and 4) behave in an opposite way upon decreasing particle size. Second, a sharp change in the slope of 1/χhf(T), from initially linear dependence at high temperatures to a CO characteristic upturn, shifts toward low temperatures and becomes more smeared with decreasing particle size. Third, for the smallest SCMO15 NPs, 1/χhf displays a linear behavior in relatively wide temperature range, 170−330 K, and undergoes only a slight downturn below 170 K, related to the appearance of FM clusters in the PM matrix. Such a behavior suggests that longrange CO correlations are completely destroyed in SCMO15 NPs. In order to obtain an additional insight into the temperature evolution of magnetic phases in SCMO nanoparticles, we have performed measurements of the thermoremanent magnetization, Mr. The thermoremanent magnetization (TRM) was recorded using the following procedure: the sample was cooled to T = 10 K in H = 15 kOe, then the field was switched off, and after a waiting time of 100 s the magnetization was recorded in a function of increasing temperature. The results show that TRM for all nanoparticles behaves in a similar way. Its value is very small at temperatures above 100 K and monotonously increases with decreasing temperature below 100 K; see Figure 6. The thermoremanent magnetization at low temperatures increases monotonously with decreasing particle size due to increasing contribution of FM components. The evolution of magnetic properties with decreasing particle size is also evidenced in the field dependence of magnetization M(H) recorded at 10 K after ZFC; see Figure 7a. The data show that SCMO NPs contain FM constituents whose relative volume increases with decreasing particle size. Figure 7b shows spontaneous magnetization, M0, evaluated by the linear extrapolation of the high field magnetization, from the range

Figure 7. (a) Magnetic field dependences of magnetization measured at T = 10 K after ZFC for SCMO nanoparticle samples. (b) Variation of spontaneous magnetization with particle size. The solid line is the result of fitting of the expression M0 = A/Dα to the experimental points. The parameters of the fitting are as follows: A = 1620 ± 420 emu nmα/g and α = 1.53 ± 0.15.

above 50 kOe to H = 0. The spontaneous magnetization apparently decreases monotonously with increasing particle size. The theoretical value of the saturation magnetization for fully ordered spins Mtheor is 3.4 μB/f.u. Since the evaluated M0 for SCMO15 at 10 K is 26.94 emu/g, i.e., 0.9 μB/f.u., one concludes that the FM phase occupies about 25% of the total volume of these particles. Moreover, SCMO15 magnetization at the maximum experimentally applied field of 90 kOe, M(H=90kOe) = 39.98 emu/g (1.34 μB/f.u.) is still much smaller than the expected theoretical value, pointing to a 7724



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significant role played by the AFM ordering. One may also notice that the estimated volume of the FM phase in the largest SCMO60 NPs occupies only about 4.5% of the total volume. One may look at the preceding results in the framework of the core−shell model and conclude that the core of all SCMO nanoparticles is AFM below TN while the shell contains a SGlike FM surface layer.4−8,11 Néel suggested that the magnetic moment of small AFM NPs should be attributed to incomplete magnetic compensation between spins in “up” and “down” sublattices.25 There are three general cases to be considered. In the first one, the uncompensated spins (US) occur at random in the NPs volume and, therefore, the number n of US becomes proportional to N1/2, where N is the total number of spins in a particle. In the second case, each particle consists of either an even or odd number of planes with parallel spins at alternating magnetization directions. In this case, n is proportional to N2/3. In the third case, Néel considered a random occupancy at the NPs surfaces for which n should be proportional to N1/3. Taking into account that N is proportional to D3, where D is the average diameter of the NP, the magnetic moment MUS associated with uncompensated spins should be equal to the ratio of n/N and therefore should be proportional to 1/D1.5, 1/ D, and 1/D2, for the three previously discussed cases, respectively. We have fitted the experimentally determined size dependence of the spontaneous magnetization in SCMO NPs with the expression M0 = A/Dα. The best fit is obtained for A = 1620 ± 420 emu nmα/g with α = 1.53 ± 0.15. Recently, we have fitted the experimentally determined size dependence of the spontaneous magnetization of Sm0.27Ca0.73MnO3 NPs with average particle size in the 20−50 nm range to the same expression.21 The best fit was obtained for A = 820 ± 330 emu nmα/g and α = 1.96 ± 0.24, pointing at a possible random occupancy of US at the NPs surfaces.21 The significant difference between the α parameter for Sm0.27Ca0.73MnO3 and that for Sm0.43Ca0.57MnO3 may be associated with a larger FM contribution in the last one. As a consequence, it is reasonable to suggest that in contrast to the case of Sm0.27Ca0.73MnO3 with random occupancy of US at the NPs surfaces, in Sm0.43Ca0.57MnO3 NPs random occupancy of US appears in the NPs volume. B. Magnetic Hysteresis. Magnetic hysteresis loops were measured at different temperatures after cooling in zero field and in applied field Hcool = 15 kOe. Figure 8a presents hysteresis loops recorded for SCMO samples at 10 K. The shifts along both magnetic field and magnetization axes are clearly seen in FC records while no shifts appeared in ZFC recordings (not shown). A difference between ZFC and FC magnetic hysteresis loops is a manifestation of the exchange bias phenomenon. Magnetic field induced shift of the hysteresis loop is generally defined as HEB = −(H1 + H2)/2, where H1 and H2 are the negative and the positive fields at which the magnetization equals zero.12,26 The vertical magnetization shift is defined as MEB = (M1 + M2)/2, where M1 and M2 are the magnetizations at the positive and negative points of intersection with the H = 0 axis. Figure 8b shows low field region of the magnetic hysteresis loops in an extended scale. It is clearly seen that EB effect in SCMO samples is size dependent. Figure 8c demonstrates hysteresis loops for SCMO60, recorded at various temperatures after ZFC and FC. One can see a decrease of the EB effect with increasing temperature, consistently with other observations.12,26 As the

Figure 8. (a) Field dependence of magnetization of SCMO samples at 10 K after FC in H = 15 kOe; (b) low field part of hysteresis loops of SCMO samples in the extended scale; (c) field dependence of magnetization of SCMO60 at various temperatures after ZFC and FC in H = 15 kOe.

temperature of SCMO60 approaches TN ∼ 70 K, the EB effect disappears. The increase of Hmax to 90 kOe and Hcool to 50 kOe results in a marked diminution of the EB effect for all investigated SCMO samples; see inset in Figure 9 and Figure 10. Generally, with increasing cooling field, Hcool, the alignment of FM cluster moments along a preferential direction is expected to be

Figure 9. Field dependence of magnetization of SCMO samples at 10 K after FC in Hcool = 50 kOe. Inset shows low field part of hysteresis loops of SCMO samples in the extended scale. 7725



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HEB = −

SAFMSFM μ0 t FMMFM

(1)

where J is the exchange integral per unit area across the FM/ AFM interface, SAFM and SFM are the order parameters of the AFM and FM layers, respectively, tFM is the thickness, and MFM is the magnetization of the FM layer. To reveal the variation of HEB as a function of the thickness of both FM (tFM) and AFM (tAFM) layers, it is convenient to rewrite eq 1 as HEB = ( −J /MFMt FM)(1 − 1/4R2)1/2

(2)

where parameter R ≡ KAFMtAFM/J determines the region of existence of EB in the system. The EB exists only for R ≥ 1; i.e., when the AFM anisotropy energy is large enough.33 In phase separated FM/AFM systems, tFM is generally associated with the size of FM clusters while tAFM with distances between them. When the volume fraction of the FM phase in the sample is large enough, SCMO15 case, tAFM is too small to provide enough AFM spins to restrain large FM moments. This results in a relatively small EB effect, and further decrease in tAFM may lead to complete disappearance of the EB.12 Thus, as suggested by Fita et al.,30 the origin of the strong HEB raised in CaMn0.9Ru0.1O3 under pressure is straightforwardly related to the decrease in the size of FM clusters embedded in the AFM matrix. Recent Monte Carlo simulations of the EB effect in FM/AFM bilayers and trilayers support this interpretation.32 Hu et al. have showed that the values of HEB are small and weakly depend on the layer thickness when tAFM/tFM < 1.0, while HEB increases steeply with increasing tAFM and a concurrent decrease of tFM. In particular, for tAFM/tFM > 1.0 the value of HEB is influenced by variations of both FM and AFM thickness as HEB = γ(tAFM/tFM), where γ is a parameter related to the FM saturated magnetization and FM/AFM interfacial coupling energy and has the dimension of magnetic field.32 Figure 11 confronts the temperature dependence of HEB and MEB for the smallest, SCMO15, and the largest, SCMO60, NPs. The temperature variation of HEB(T) and the vertical shift of MEB can be well-approximated34,35 by an exponential decay of the following form:

Figure 10. Size dependence of HEB and MEB for SCMO samples at 10 K after FC in H = 15 kOe measured in the range of ±15 kOe and after FC in H = 50 kOe measured in the range of ±90 kOe.

enhanced. Therefore, the effect of anisotropic averaging due to randomness should be reduced.27,28 Indeed, for various halfdoped L0.5Sr0.5MnO3 manganites (L = Y, Y0.5Sm0.5, and Y0.5La0.5), HEB increases sharply with increasing Hcool below 30 kOe. However, for Hcool above 30 kOe, HEB decreases due to growing of ferromagnetic clusters.27 This behavior has been explained in terms of interfacial exchange coupling, where spin configurations are strongly affected by the cooling field strength.27 Recent studies6,7,29 of the EB effect in manganite NPs have shown a significant impact of the particle size on HEB and MEB. Figure 10 presents experimental data for HEB and MEB obtained at 10 K in the modest, up to 15 kOe, and high, up to 90 kOe, magnetic fields. It is clearly seen that HEB and MEB exhibit opposite behavior; namely, MEB increases while HEB decreases with decreasing particle size. Our recent study of the EB effect in electron-doped La0.2Ca0.8MnO3 NPs with average particle size ranging from 15 to 37 nm revealed a monotonous increase of both HEB and MEB with decreasing particle size.6 One may point out that the volume of the FM phase in La0.2Ca0.8MnO3 NPs is much smaller than that in SCMO samples. Indeed, even if the relative volume of the FM phase in La0.2Ca0.8MnO3 NPs gradually increases with decreasing particle size, it reaches only about 5% of the total volume of the smallest 15 nm La0.2Ca0.8MnO3 NPs at 5 K.6 On the other hand, Huang et al.7 observed that HEB of La0.25Ca0.75MnO3 NPs follows a nonmonotonic size dependence and shows a maximum for particles with diameter around 80 nm at T = 5 K. Therefore, it has to be concluded that the maximal EB is observed in phase separated systems, including polycrystalline samples and NPs for some “optimal” ratio between the volumes of AFM and FM phases. Our recent study of the EB effect in CaMn0.9Ru0.1O3 has shown that the HEB is nearly inversely proportional to the saturated magnetization (MS) of the FM phase and a noticeable EB is prone to occur in phase separated system with small enough value of MS.30 Qualitatively it could be understood on the basis of the classic Meiklejohn−Bean (MB) model,31 which predicts for systems with FM/AFM interfaces that HEB is inversely proportional to the thickness of the FM layer, tFM, and depends on AFM anisotropy and thickness of the AFM layers, tAFM.12,32 The expression for HEB reads as follows:

HEB = HEB(0) exp( −T /T1)

(3)

MEB = MEB(0) exp( −T /T2)

(4)

where HEB(0) and MEB(0) are the values of HEB and MEB extrapolated to T = 0 K and T1 and T2 are constants. Expressions 3 and 4 successfully describe HEB(T) and MEB(T) behavior in various systems with frustrated interactions,27 thus including a possible contribution of the spin glass component to the EB effect. The best fits of eqs 3 and 4 to the experimental data for SCMO60 (Figure 11b) were obtained for HEB(0) = 930 ± 120 Oe, T1 = 10.8 ± 1.1 K, and MEB(0) = 0.380 ± 0.025 emu/g, T2 = 35 ± 3K. In contrast, HEB and MEB of the smallest SCMO15 NPs show a nonmonotonic temperature dependence, exhibiting a maximum and a minimum around 20 K for MEB(T) and HEB(T), respectively, and thus cannot be well described by eqs 3 and 4. Several studies of AFM manganites NPs6,7,11,21,29 considered a core−shell structure composed of AFM cores and shells with SG-like FM surface layers. It was previously proposed that the EB effect in AFM manganite NPs emerges due to appearance of interfaces between small FM clusters imbedded in a frustrated AFM phase.6,7,11 The effective thickness tFM in SCMO60 NPs is 7726



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Maity et al. have reported on an appearance of a spontaneous exchange bias (SEB) after ZFC and on its nonmonotonic temperature dependence, as well as on a conventional EB effect after FC, in nanocomposites of BiFeO3 (94%)−Bi2Fe4O9 (6%).45 They claimed that the EB behavior in this system originates from a specific superinteraction bias coupling between the FM core of finer Bi2Fe4O9 (19 nm) NPs and the AFM moment in the bigger (112 nm) BiFeO3 particles via SSG moments at the interface.45 This is similar to the case of basically AFM manganite NPs with size of 10−20 nm in which the overwhelming part of the volume constitutes AFM cores, while interaction between small FM clusters in frustrated configuration at the NPs surfaces leads to the formation of the SSG state.46 The “conventional EB” term used previously refers to the EB effect appearing after field cooling through the Néel temperature, TN, while the term “spontaneous EB” refers to nonconventional FC, e.g., to the application of magnetic field only at T < TN, followed by FC to low temperatures.47 It is worth emphasizing that spontaneous EB has previously been observed by us in 15 nm La0.2Ca0.8MnO3 AFM NPs after ZFC to T < TN ≈ 140 K and subsequent FC in 15 kOe to 10 K.48 Further studies are definitely needed to clarify the nature of the remarkable nonmonotonic variation of the temperature dependence of the HEB in small manganite NPs. Magnetic measurements of nanosized SCMO have revealed that the long-range CO in 60 nm NPs is very similar to CO in the bulk system. Charge order becomes progressively suppressed with decreasing particle size and disappears completely when the average particle size is reduced to 15 nm. Suppression of a robust CO with decreasing size is accompanied by a progressive increase of a weak FM moment. This observation is found to be in good agreement with the recent reports on various nanosized CO manganites.4−8,11 However, unlike the magnetization (Figure 4), the inverse of high field susceptibility 1/χhf(T) shows similar temperature dependences for the nanosized SCMO having different particle sizes. Namely, CO associated change in the slope of 1/χhf(T) in Figure 5 is clearly visible even for 17 nm NPs. One should remember, however, that the distributions of particle sizes in SCMO17 and SCMO25 are relatively wide, which may account for the deviation from the Curie−Weiss law. We recall that we have observed CO disappearance in an ensemble of SMCO NPs with average particle size of 15 nm in which the distribution of particles does not extend beyond 30 nm; see the inset to Figure 1a. Therefore, taking into account the actual size distributions, one may only claim that CO is indeed completely suppressed in SCMO NPs with sizes below 30 nm.

Figure 11. Temperature dependence of HEB and MEB for SCMO15 (a) and SCMO60 (b) after FC in 15 kOe. The lines in panel a are guides for the eyes. Lines in panel b are the best fits with eqs 3 and 4.

relatively small, which leads to a significant HEB. Our experimental observations presented in Figure 10 are in line with the preceding scenario. Recently, nonmonotonic temperature dependent HEB has been observed in various exchange-biased systems.36−38 Jammalamadaka et al.36 explained oscillations in the temperature dependence of HEB(T) in 10 nm Pr0.5Ca0.5MnO3 NPs by propagation of the incommensurate charge density wave (ICDW) with a strongly temperature dependent wavelength in the AFM core. Such an origin of the oscillatory exchange bias has been originally suggested for FM/AFM layers.37,38 An incommensurate charge density wave scenario was also evoked39−41 for some typical charge-ordered polycrystalline manganites in which an AFM/CO phase coexists with the FM phase. Even in principle feasible, the possibility of low energy excitations in the form of ICDW in AFM CO manganite films and NPs has been recently questioned.42,43 Fisher et al.42 have studied electrical resistivity in an (001)-oriented epitaxial Pr0.48Ca0.52MnO3 film with distinct superlattice reflections associated with CO. They found that the electrical resistivity shows no sign of the CO transition at TCO, and consequently, no evidence for CDW sliding was found. An alternative explanation of nonmonotonic temperature dependence of HEB in SCMO15 NPs may be based on possible changes in the magnetic structure of the AFM core, such as, for example, canting of manganese moments. Recent study of the EB effect in manganite bilayers of collinear FM La0.7Sr0.3MnO3 and noncollinear multiferroic TbMnO3 has revealed that the temperature dependence of HEB is sensitive to the long-range order of Tb3+ ions and to possible canting of Mn moments.44

4. CONCLUSION In summary, we have investigated magnetic properties of compacted Sm0.43Ca0.57MnO3 NPs with average particle size ranging from 15 to 60 nm, prepared by the glycine−nitrate method. With decreasing particle size, the relative volume of the ferromagnetic phase increases monotonously, while the charge-ordered phase progressively weakens and disappears completely in NPs of an average size of 15 nm, consistently with recent literature reports on various nanosized electrondoped manganites.4,6,10,21,49 The spontaneous FM moment at T = 10 K increases with decreasing particle size from ≈4.6 emu/g for 60 nm to ≈27 emu/g for 15 nm particles. Magnetic hysteresis loops exhibit size dependent exchange bias effect, with HEB decreasing and MEB increasing with decreasing particle size. The HEB decreases monotonously with increasing 7727



Article

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temperature for 60 nm particles, while the HEB of smaller 15 nm particles exhibits a surprising nonmonotonic variation with temperature. The temperature dependence of the HEB and variation of spontaneous magnetic moment with decreasing particle size were discussed in terms of magnetic coupling between the AFM core and FM-like shell.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (+ 972 8 6477127). Fax: (+ 972 8 6472903). E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Polish Ministry of Science and Higher Education under Research Project No. N 202 1037 36, by the Polish NCN Grant 2012/05/B/ST3/ 03157, and by the Israeli Science Foundation administered by the Israel Academy of Sciences and Humanities Grant 754/09.

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