Unexpected ferromagnetic ordering enhancement ...

Unexpected ferromagnetic ordering enhancement with crystallite size growth observed in La0.5Ca0.5MnO3 nanoparticles G. Iniama, P. de la Presa, J. M. Alonso, M. Multigner, B. I. Ita, R. Cortés-Gil, M. L. Ruiz-González, A. Hernando, and J. M. Gonzalez-Calbet Citation: Journal of Applied Physics 116, 113901 (2014); doi: 10.1063/1.4895707 View online: http://dx.doi.org/10.1063/1.4895707 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Coexistence of considerable inter-particle interactions and spin-glass behavior in La0.7Ca0.3MnO3 nanoparticles J. Appl. Phys. 115, 17B504 (2014); 10.1063/1.4862522 Enhanced exchange bias effect in size modulated Sm0.5Ca0.5MnO3 phase separated manganite J. Appl. Phys. 115, 093906 (2014); 10.1063/1.4867523 Impact of reduced dimensionality on the magnetic and magnetocaloric response of La0.7Ca0.3MnO3 Appl. Phys. Lett. 102, 062414 (2013); 10.1063/1.4792239 Size effect on the structural, magnetic, and magnetotransport properties of electron doped manganite La0.15Ca0.85MnO3 J. Appl. Phys. 111, 07D729 (2012); 10.1063/1.3680246 Magnetic properties and magnetocaloric effect of La0.8Ca0.2MnO3 nanoparticles tuned by particle size J. Appl. Phys. 111, 063922 (2012); 10.1063/1.3699037

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JOURNAL OF APPLIED PHYSICS 116, 113901 (2014)

Unexpected ferromagnetic ordering enhancement with crystallite size growth observed in La0.5Ca0.5MnO3 nanoparticles s-Gil,5 G. Iniama,1 P. de la Presa,2,3,a) J. M. Alonso,2,4 M. Multigner,2 B. I. Ita,1 R. Corte lez,5 A. Hernando,2,3 and J. M. Gonzalez-Calbet2,5 M. L. Ruiz-Gonza 1

Department of Pure and Applied Chemistry, University of Calabar, Calabar, Nigeria Instituto de Magnetismo Aplicado, UCM-ADIF-CSIC, 28230 Las Rozas, Spain 3 Fac. CC Fısicas, Dpto. Fısica de Materiales, Univ. Complutense de Madrid, 28040 Madrid, Spain 4 Instituto de Ciencia de Materiales, CSIC, 28049-Madrid, Spain 5 Fac. CC Quımicas, Dpto. Quımica Inorg anica, Univ. Complutense de Madrid, 28040 Madrid, Spain 2

(Received 22 July 2014; accepted 3 September 2014; published online 15 September 2014) In this paper, the physical properties of half-doped manganite La0.5Ca0.5MnO3 with crystallite sizes ranging from 15 to 40 nm are investigated. As expected, ferromagnetic order strengthens at expense of antiferromagnetic one as crystallite size is reduced to 15 nm. However, contrary to previously reported works, an enhancement of saturation magnetization is observed as crystallite size increases from 15 to 22 nm. This unexpected behavior is accompanied by an unusual cell volume variation that seems to induce ferromagnetic-like behavior at expense of antiferromagnetic one. Besides, field cooled hysteresis loops show exchange bias field and coercivity enhancement for increasing cooling fields, which suggest a kind of core-shell structure with AFM-FM coupling for crystallite sizes as small as 15 nm. It is expected that inner core orders antiferromagnetically, whereas uncompensated surface spins behave as spin glass with ferromagnetic-like ordering. C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4895707] V I. INTRODUCTION

Particular interest is focused on investigation of half doped manganite perovskites since discovery of colossal magneto-resistance (CMR) in systems with general formula Ln1xAxMnO3 (Ln ¼ lanthanide, A ¼ divalent metal). Half doped manganites show very interesting properties such as charge and orbital order, phase segregation and high CMR values. Moreover, this system is, in general, on the borderline to show metallic ferromagnetic behavior (FM) or insulating antiferromagnetic one (AFM).1 Consequently, the properties of these materials can be externally modified by application of magnetic or electric fields, pressure, irradiation, etc.2,3 Moreover, slight variations in carrier density by varying either cationic or anionic composition can also lead to significant variations in the electronic properties of halfdoped manganites.4,5 The physical properties of manganite perovskites with mixed valence Mn3þ/Mn4þ depend, in addition to other things, on whether the outermost electrons are localized on individual transition-metal sites or delocalized throughout the solid in addition to other configurations. In the case of La0.5Ca0.5MnO3, the delocalized electrons interact ferromagnetically bellow Tc. However, as temperature further decreases, electron charges become to localize around Mn3þ/Mn4þ ions, giving place to a strong lattice-orbital coupling, which finally develops in a long-range ordered pattern. This phenomenon is referred as charge ordering (CO).6 The onset of CO is accompanied with a change in magnetic interactions from FM to AFM, manifested by a a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]

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drop of magnetization below CO temperature TCO ¼ 225 K. At still lower temperatures, a long-range AFM arrangement of CE type is formed, with a Neel temperature TN ¼ 155 K, that implies the presence of CO according to the structure proposed by Goodenough.7 Electron microscopy studies by Mori et al.8 show that, between 135 and 95 K, La0.5Ca0.5MnO3 is a mixture of microdomains of an AFM CO phase and a FM charge-disordered phase; this mixture changes chiefly to AFM phase below 95 K. However, there exists always around 15% of FM delocalized charge regions dispersed in the AFM CO matrix, even at very low temperature.4,8,9 Since CO and the subsequent AFM arrangement are long-range order phenomena, the reduction of crystallite size dimension induces inevitably changes in the magnetic or electronic ordering. It has been reported that size reduction to the nanometer scale partially suppresses this mixed ordering and induces ferromagnetic-like states.10–21 Indeed, several works show that the nanoestructured La1xAxMnO3 have two common features that are not present in bulk materials: (i) loss of charge and orbital order; (ii) strengthening of FM order.3,20–22 Since the onset of charge and orbital order is accompanied by the development of a modulation with q ¼ 12 e; 0; 0 ,6,15 which requires particle sizes of at least 100 nm,11 it is then expected that order becomes frustrated for particle sizes bellow 100 nm. This is experimentally observed by several authors, even for larger particle sizes.10–12 However, there is no consensus on the origin of FM order. In general, FM order is attributed to the disorder caused by reduction of particle size resulting in a competition between the double exchange and superexchange interactions which are normally present in these systems.2,3 This fact, together with the loss of CO associated to a CE type

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AFM ordering, favors FM interactions which otherwise are present only at high temperatures. Several authors propose a “core-shell” model to explain the magnetic behavior of these materials at nanometer size.18,19,22 Thus, whereas core retains the bulk AFM properties, a large spins disorder at surface favors the onset of FM ordering.14,16 This model can explain the experimental observation of gradual loss of FM interactions with increasing particle size as a result of relative surface decrease (FM ordering) as volume increases (AFM ordering). The interaction between the AFM and FM fractions can be clearly assessed by measuring exchange bias field, which is a fingerprint of AFM regions in intimate contact with FM ones.23 On the other hand, some authors ascribe the onset of the magnetic behavior changes to changes in crystal structure due to the structural stresses caused by particle size reduction.11,17 Particle size reduction causes tensions at surface that lead to an effective hydrostatic pressure on nanoparticle volume. This hydrostatic pressure reduces the unit cell favoring the onset of FM ordering. Therefore, in half doped Mn perovskite, particle size reduction gives place to a cell volume contraction, which is not usual in such systems. On the contrary, particle size reduction is normally accompanied by a net cell volume increase in most of the Ln1xAxMnO3.16–18 The discrepancies in the experimental results previously shown are also present in the theoretical models. Montecarlo simulation of the surface of either bulk or nanosized manganites shows that CO is suppressed and a weak FM ordering appears at surface.24 In addition, calculations by means of density-functional theory and dynamical mean-field theory, carried out on a defect free isolated nanoclusters, establish that size reduction induces structural changes that finally lead to a CO weakening, making FM state energetically favourable compared to AFM one.13 In this paper, we systematically investigate the physical properties of nanocrystals of half-doped manganite La0.5Ca0.5MnO3 with crystallite sizes ranging from 15 to 40 nm. Our results show a noticeable increase of FM like behavior for 15 nm nanoparticles regarding bulk material. However, contrary to previously reported works for these nanoparticles, saturation magnetization enhances as crystallite size increases from to 15 to 22 nm, suggesting FM ordering enhancement. Finally, saturation magnetization decreases again with crystallite size increase, as expected. Besides, magnetic coupling between FM and AFM phases is investigated by measuring exchange bias field and coercivity enhancement for different cooling fields, which confirms a “core-shell” like structure with an AFM core interacting with a FM shell.

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CaCO3 (12.5 mM) and 10 ml of ethylene glycol (Eg) are successively incorporated. The resultant solution is heated for several hours at 90 C until it evaporates, leading to formation of a resin (poliesther) containing a random distribution of cations. Finally, the dark brown powder obtained is milled in agate mortar and then heated at 600 C during 24 h for calcination of the organic materials. Four different samples are prepared by subsequent thermal treatments for 2 h at annealing temperatures, Ta, 700 C (LC700), 800 C (LC800), 900 C (LC900), and 1000 C (LC1000). B. Structural and morphological characterization

Structural characterization and average crystallite size are measured by X-ray diffraction (XRD) using Cu-Ka radiation in a PANalytical X’Pert Pro MPD diffractometer. The XRD patterns can be indexed to a perovskite structure Ln0.5A0.5MnO3 like.25,26 The mean crystallite sizes hDi are calculated from Scherrer’s formula. Particles size and shape are determined by scanning electron microscopy (SEM) by using a JEOL JSM 6335F scanning electron microscope operated at 30 keV. In addition, an elemental analysis by energy dispersive spectroscopy (EDS) is performed in each sample by means of JEOL JSM 6400. The anionic composition is determined by means of thermogravimetric analysis in a Cahn D-200 electrobalance. Particle morphology is determined by transmission electron microscopy (TEM) in a JEOL JEM 2100 electron microscope. C. Magnetic characterization

Static magnetic measurements are carried out on powder samples by means of a Quantum Design vibrating sample magnetometer. Hysteresis loops are measured up to 5 T at 5 K and 300 K. Zero field-cooled and field-cooled curves (ZFC-FC) are obtained from 5 to 350 K at 50 and 1000 Oe applied magnetic field. Magnetization at ZFC is measured up to 350 K at 1 T for samples LC700 and LC900. Hysteresis loops in sample LC800 are measured at ZFC-FC in a Quantum Design SQUID. Before measuring ZFC hysteresis loop at 5 K and 5 T, a demagnetizing field of 3 T is applied at room temperature. The measurements for FC hysteresis loops are performed as follow: 3 T is applied to the sample at room temperature; then the field is turned off in oscillating mode in order to demagnetize the sample. A field of 0.5 T is applied, and the sample is cooled down to 5 K; then, hysteresis loop is measured at 5 T. The sample is warmed up to room temperature and procedure is repeated for different applied fields: 1, 2, 3, 4, and 5 T.

II. EXPERIMENTAL PROCEDURE

III. RESULTS AND DISCUSSION

A. Synthesis

A. Structure and morphology

The synthesis of La0.5Ca0.5MnO3 is achieved from a solgel method by adding 6.25 mM of La2O3 to 250 ml of acidified water (5 ml nitric acid) heated at 50 C for 5 min. A clear solution is formed under vigorous stirring, and then 10 g of citric acid is added. MnCO3 (25 mM) is added under vigorously stirring until a transparent solution is obtained.

X-ray diffraction (XRD) patterns (Figure 1) at room temperature of thermal treated samples show that they are single phase and can be indexed on the basis of an orthorhombic cell with space group Pnma.25 Additionally, EDS analyses in sample LC1000 show an average composition La0.49(4)Ca0.48(4)Mn1.02(4)O3, which, taking into account the


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FIG. 2. (a) Cell volume as a function of annealing temperature. (b) Mean crystallite size calculated by Scherrer’s formula (errors are smaller than 1 nm). The lines are drawn for guide the eyes.

FIG. 1. XRD pattern of samples La0.5Ca0.5MnO3 nanoparticles annealed at different temperatures. The bars indicate the diffraction pattern corresponding to bulk.

experimental errors, fits pretty well to the nominal one. Since all the samples are made starting from same nanoparticles batch, they all have same stoichiometry and carrier concentration irrespective of size. In this point, it is worth noting that oxygen content of all samples corresponds to full anionic sublattices in all the cases, as confirmed by thermogravimetric analysis. Thus, final composition of the samples is La0.49(4)Ca0.48(4)Mn1.02(4)O3.00(1). The fact that nanoparticles have the same cationic and anionic composition allows us to assure that the changes observed on the magnetic properties must be related to particle size reduction. As it can be observed from Fig. 1, all the diffraction peaks in the nanoparticle samples are broad. In addition, diffraction reflections slightly differ from bulk and vary with Ta. This variation reflects small changes in cell parameters with the corresponding influence on cell volume, as shown in Fig. 2. It is worth stressing that cell volume first decreases with Ta up to reaching a minimum at Ta ¼ 900 C; then, it increases up to reaching a maximum for bulk samples. Figure 2(a) shows cell volume behavior as a function of Ta. Surprisingly, cell volume in these nanoparticles is always smaller than cell volume of bulk, a behavior which is unusual in most of the Ln1xAxMnO3 compounds.16–18 Sarkar et al.11 report also a smaller cell volume for La0.5Ca0.5MnO3 with 15 nm particle size with a continuous cell volume increase with annealing, up to reaching a bulk value for particle size at micrometer range.

The diffraction peak widths decrease as Ta increases, indicating an increase of the average crystallite sizes hDi. By using the Scherrer’s formula, hDi is calculated to be around 15 nm for LC700, then slightly increases to 16 nm for LC800 and finally reaches 22 nm for LC900 (see Fig. 2(b)). By heating the sample at 1000 C, the crystallite size increases markedly to 40 nm, suggesting the beginning of a sintering process. By comparing Figs. 2(a) and 2(b), it is observed that there is no correlation between cell volume and crystallite size Ta dependence: whereas the last increases exponentially with Ta, the former has a minimum at Ta ¼ 900 C. We would like to point out that there are no contradictions between our results and those reported by Sarkar et al.11 regarding cell volume dependence with crystallite size. In our work, cell volume decreases between 15 and 22 nm crystallite size, a size range which has not been investigated by Sakar et al.11 On the other hand, both works match up two relevant results: (1) Cell volume of 15 nm nanoparticles is smaller than in bulk, (2) As crystallite size increases from 15 to 40 nm, cell volume also increases. Figures 3(a), 3(b), and 3(c) show particle size dependence with Ta measured by SEM. At Ta ¼ 700 C, the particles have almost rounded shape with size around 50 nm. As Ta increases, the particle size slightly increases and the particles tend to agglomerate. However, as Ta ¼ 1000 C, an abrupt change in particle size could indicate the beginning of sintering process [see Fig. 3(d)]. Similar particle size values are observed by TEM. The comparison of particles and crystallites size suggests that the particles are composed majority by several crystallites of similar sizes as determined by XRD (see Fig. 4).


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FIG. 3. SEM images of (a) LC700, (b) LC800, (c) LC900, and (d) LC1000 at a magnification of 50 000. The bars represent 100 nm. The vertical bars in (a) are separated by 50 nm, indicating the agglomerate size.

B. Evidence of core-shell structure with AFM-FM behavior

Core-shell structure brings about interesting physical and chemical properties of nanostructured materials that normally differ from their individual single-component counterpart.27 From the hysteresis loops measured at 5 K for LC700, LC800, LC900, LC1000, and bulk (Ta ¼ 1400 C), it is possible to observe two magnetic behaviors which are clearly distinguishably. At low field, a coercive field around 700 Oe is observed in the nanoparticles, contrasting with the almost null coercive field measured in bulk samples.26,28 At high field, FM phase should saturate; however, magnetization increases linearly with applied field suggesting that a second interaction is present. Due to the AFM nature of bulk, it is assumed that this second contribution is AFM. In order to

FIG. 4. TEM image of sample LS700 at different magnifications: (a) at 100000X showing nanoparticles around 50 nm, (b) at 600000X showing different crystallites in the nanoparticle.

disentangle both, FM and AFM contributions, a lineal fit is performed at high fields (between 3 and 5 T); interception with M axis is the value of saturation magnetization of FM phase (MsFM). Inset Fig. 5(a) shows MsFM for each sample, it is observed that FM saturation magnetization first increases and then decreases for Ta ¼ 1000 C. In bulk material, there exists a field induced AFM to FM transition above 10 T, i.e., the magnetic moments are fully ferromagnetically aligned at high field and low temperature, with a saturation magnetization Ms ¼ 110 emu/g.28,29 Considering the ratio MsFM/Ms, the percentage of magnetic moments that aligns ferromagnetically is estimated as: 25, 28, 30, 18, and 14% for LC700, LC800, LC900, LC1000, and bulk samples, respectively. The magnetization increase suggests that FM ordering is strengthening at expense of AFM one as crystallite size becomes larger. This is an unexpected result because most authors report an enhancement of AFM ordering as crystalline size rises.10–12,14,30 For sample annealed at 1000 C, the magnetization decreases but remains still higher than in bulk material. Figure 5(b) shows hysteresis loops measured at 5, 150, 200, and 300 K for sample LC700. The hysteresis loops of bulk samples shows negligible coercive field, whereas the nanoparticles show a coercive field of 650 Oe for all samples. This coercivity enhancement could be associated to spin glass with FM like behavior at low temperature: the frozen magnetic moments at the grain surfaces give place to a rising in surface anisotropy increasing coercive field.31 Spin-glass like transition is confirmed by a strong coercivity decrease as temperature increases from 5 to 150 K, supporting the hypothesis that there is a change on the anisotropy due to the canted frozen spins at the surface. The ZFC-FC measurements at H ¼ 1000 Oe show that all samples undergo a transition from paramagnetic to ferromagnetic state at around 230–250 K, where TC is identified


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typical behavior for spin glass transitions. Spin-glass like transition, where a change from a state of disordered spins undergoes to a magnetic state with ordered magnetic spins, fits in a multidomain nanoparticles scenario in which crystallite boundaries with FM like behavior surrounds very crystalline AFM cores; this gives place to a kind of “core-shell” structure.18,27 Several studies suggest that the surface effects, like different composition at grain boundaries or low crystallinity degrees, bear on different ways to the interaction between magnetic moments of neighboring grains or between magnetic moments inside grains and surface spins.32,33 We define spin glass like transition temperature, TSG, as the temperature at which ZFC curve reaches a maximum. As can be seen from Fig. 6, TSG slightly increases as Ta increases from 700 to 800 C, whereas net magnetization increases markedly in the whole temperature range. As Ta increases to 900 C, the magnetization at ZFC of LC900 decreases regarding LC800, whereas magnetization at FC remains close to the LC800 one. Finally, magnetization of LC1000 is distinctly much smaller than the others. ZFC magnetization measured at different applied fields can enlighten about the dominant processes involved in La0.5Ca0.5MnO3 nanoparticles (see Fig. 7). At low applied field (50 Oe), the ZFC curves of LC700 and LC900 show near negligible magnetization up to 50 K. As temperature increases, magnetization in sample LC700 increases faster than in LC900 sample, up to reaching a maximum at

FIG. 5. Hysteresis cycles (a) at 5 K for samples LC700, LC800, LC900, LC1000, and bulk. Inset shows MsFM for each sample; (b) for LC700 sample measured at 5, 150, 200, and 300 K. Inset shows hysteresis cycle at low field.

by a minimum in the dM/dT curves (see Fig. 6). At low temperature, ZFC and FC curves differ and finally converge at a different temperature for each sample, well below TC, a

FIG. 6. ZFC-FC curves measured at H ¼ 1000 Oe for LC700 (circles), LC800 (squares), LC900 (up triangles), LC1000 (down triangles), and bulk (stars) samples. The inset shows the Curie temperatures for each sample.

FIG. 7. ZFC curves measured at (a) 50 Oe, (b) 1000 Oe, and (c) 1 T for LC700 (squares) and LC900 (circles).


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TSG ¼ 185 K. The ZFC magnetization of LC900 remains smaller than in LC700 up to TSG ¼ 225 K, which is close to TC [see Fig. 7(a)]. This behavior can be explained with a kind of core/shell structure, in which the core, with a relative good crystallinity degree, shows AFM ordering while the shell is made up by spin glass with FM like behavior. At low applied field, AFM ordering of the core is the dominant process in the largest particles (LC900) whereas spin glass surface makes a minor contribution to FM like ordering. The contrary occurs for the smaller particles (LC700), where the spin glass shell makes a relative significant contribution to the net magnetization. Therefore, net magnetization for sample LC900 remains smaller than in sample LC700 up to TSG [see Fig. 7(a)]. A similar behavior is observed in ZFC curves with applied field 1000 Oe [see Fig. 7(b)]. As applied field increases, more frozen spins at surface align with external magnetic field contributing to net magnetization enhancement in both samples; TSG decreases for both samples, as expected when higher magnetic field is applied in a spinglass structure. However, very different behavior is observed in ZFC curves measured at 1 T: the magnetization in sample LC900 is higher than in LC700 [Fig. 7(c)] in the whole temperature range. This behavior cannot be explained with a simple core-shell structure with AFM-FM behavior, but further discussion is needed. We come back to this point later. All these results suggest that crystal size reduction favors FM behavior, at expense of AFM one, probably due to the large numbers of cells at grain boundaries which order FM like. The lacking of saturation magnetization at low temperature and its reduced value compared to the fully ferromagnetically aligned moments (Ms ¼ 110 emu/g) indicate that there exists an AFM phase even in crystallite sizes as small as 15 nm. Therefore, AFM ordering could take place inside the grains, whereas it is frustrated at surface due to the lacking of magnetic compensation. A simple estimation of surface atoms contribution to the whole magnetization can be done by calculating the ratio rc of cells at surface to cells inside volume of hDi and comparing it to the ratio rM of MsFM in LC700 to fully ferromagnetic alignment moments in bulk, Ms. The cell parameters of orthorhombic La0.5Ca0.5MnO3 in ˚ , b ¼ 7.687 A ˚ , and c ¼ 5.419, with pseubulk are a ¼ 5.439 A ˚ . The diagonal of a pseudodocubic cell parameter ac ¼ 3.83 A ˚ cubic unit cell is d ¼ 6.65 A ¼ 0.665 nm. The size of minimum coherent diffraction domain, i.e., mean crystallite size hDi, is around 15 nm for LC700 and, consequently, there are 15/0.665 ¼ 22 cells inside volume cell. Assuming a cubic particle shape, cells’ number inside coherent diffraction volume is 22 22 22 ¼ 10648, and the number of cells at surface is 22 22 6 ¼ 2904. Therefore, the relationship surface to volume cells is close to rc ¼ 0.27. On the other hand, the ratio MsFM in sample LC700 (27 emu/g) to Ms of bulk (110 emu/g) is rM ¼ 0.25. Therefore, rM rc suggest that FM behavior of the nanoparticles come mainly from the surface atoms. C. Exchange bias phenomenology

As previously mentioned, TEM, SEM, and DRX results show that the particles are conformed by several crystallites.

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Whereas hysteresis loops at low temperatures can be describe with FM and AFM contributions, the ZFC curves show that magnetic behavior at low temperature is better related to a spin glass with FM like behavior which order fully FM above TSG. As deduced from the previous calculation, a possible configuration of the nanocrystals is a “kind” of core/shell structure, in which the core is very crystalline and orders antiferromagnetically whereas the shell is related to the boundaries between crystallites and shows FM like behavior. The interaction between the AFM and FM fractions can be clearly assessed by measuring exchange bias field, which is a fingerprint of AFM regions in intimate contact with FM ones. A simple intuitive model of exchange bias phenomena can be described as an alignment of the AFM spins parallel to the FM ones occurring during FC procedure. Depending on AFM anisotropy strength, two limiting cases can be predicted: (1) A shift of hysteresis loops associated to a large AFM anisotropy; (2) Coercivity enhancement (without any loop shift) for weak AFM anisotropy. For nanostructured systems with core/shell structure, both effects can be observed simultaneously, due to, for example, structural defects or grain size distribution, which bring about local variations of the AFM anisotropy (see Ref. 23 and references therein) In order to confirm this configuration, hysteresis loops are measured in FC under different fields. As it can be observed in Fig. 8(a), ZFC hysteresis loop is symmetrical and centered in origin with a coercive field about 650 Oe. However, the FC magnetization curves exhibit a shift in the hysteresis loop toward negative magnetic fields in addition to a coercivity increase; which are characteristic properties related to exchange bias phenomenology in core/shell nanoparticles.23,34 Therefore, negative shift of the hysteresis loops together with coercivity enhancement confirm that there exits an exchange coupling between spin glass shell with FM like behavior and AFM core of the nanocrystallites, as already reported for other manganites nanoparticles.18,19,35 The exchange bias field is defined as HEB ¼ ðHcl þ Hcr Þ=2, where Hcl and Hcr are left and right coercive field, respectively. As it can be seen from Fig. 8(a), for FC hysteresis loop under 0.5 T cooling field, HEB is negative and about 175 Oe; furthermore, it becomes less negative as cooling field increases, i.e., it decreases in absolute value. On the other hand, as HEB decreases, Hc increases with cooling field [see Fig. 8(b)]. This behavior is related to AFM anisotropy strength. For large AFM anisotropy, magnetic field reversal makes FM spins to rotate, whereas AFM spins remain fixed. Consequently, due to the interface coupling, they exert a microscopic torque to the spins in the FM layer, trying to keep them in their original positions. Thus, the magnetic field required to completely reverse the magnetization in the FM phase is higher than in the case of lacking AFM-FM coupling. The net effect is a shift of the hysteresis loop along the magnetic field axis, HEB. As cooling field increases, more AFM cores reverse with magnetic field, and both, the FM and the AFM spins, rotate together. The extra energy associated with the creation of an irreversible twist in the AFM structure translates into an enhanced coercive field, Hc.


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FIG. 8. (a) ZFC-FC hysteresis loops for sample LC800 at different cooling field and (b) cooling field dependence of absolute values of exchange bias field (red points) and coercive field (blue circles). Lines are drawn to guide the eyes.

Therefore, we can assume that the spins at boundaries between crystallites are contributing to FM like ordering, whereas AFM ordering takes place inside of the crystallites, forming a sort of core-shell structure with AFM-FM coupling. However, AFM ordering inside crystals cannot be CE-type because particle size of at least 100 nm is required for the onset of this AFM order.11 Therefore, AFM order inside crystals must be a simpler type, as described by Goodenough et al. or Schiffer et al. for this system.1,7

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pressure boosts FM ordering.29 Sarkar et al. proposed that surface pressure makes the nanocrystals behave as material under high hydrostatic pressure; they estimated that induced hydrostatic pressure is about 6 GPa for equiatomic manganite nanoparticles with 15 nm particle size and can lead to cell volume reduction about 4%; hydrostatic pressure falls below 1 GPa for particles larger than 100 nm.11 On the other hand, Dash et al. report that, even for hydrostatic pressure as tiny as 0.1 GPa, saturation magnetization increases with pressure in bulk samples, suggesting that hydrostatic pressure reinforces FM behavior.29 As shown in Fig. 2, cell volume for LC700 (hDi 15 nm) is smaller than in bulk, and it is further reduced as crystallite size grows to 22 nm. This is an unexpected behavior of cell volume with crystallite size growth that cannot be attributed only to surface pressure because this should decrease as crystallite size increases. Therefore, another mechanism should exist which increases the hydrostatic pressure reducing the cell volume with increasing crystallite size. A possible mechanism could be a competition between particle and crystallite size increase: If particle size does not increase as fast as crystallite size, this could induce an effective hydrostatic pressure on crystallite volume that bears on cell volume behavior. As FM ordering enhancement depends on induced hydrostatic pressure and external applied field,29 ZFC magnetization as a function of temperature (Fig. 7) fits also this assumption: for low applied field, AFM ordering is dominant in the largest particles (LC900); however, for higher applied field, the higher hydrostatic pressure inside nanocrystals of sample LC900 promotes FM ordering at expense of AFM one. Consequently, temperature dependence of net magnetization in LC900 is higher than LC700 for high applied fields [see Fig. 7(c)]. Figure 9 shows cell volume and saturation magnetization MsFM for nanocrystals and bulk; it is evident that MsFM enhancement is related to cell volume decrease and both can be explained by an increase of hydrostatic pressure in the nanocrystals. Therefore, an effective pressure could cause crystal structure to deviate from bulk structure and favors FM ordering at expense of AFM one. In conclusion, we

D. Cell volume decrease enhances FM ordering

As shown Sec. III B, FM contribution of sample LC700 (d ¼ 15 nm) can be related to disorder existing in the boundaries between crystallites. Therefore, it is expected that the AFM phase increases at expense of the FM one as grain size increases with Ta, as reported by several authors.10–12,14,30 However, unlike previous reported works, saturation magnetization increases with crystallite size growth [see inset Fig. 5(a)], indicating that another mechanism is inhibiting AFM phase and strengthening FM ordering. It is known that application of hydrostatic pressure of about 15 GPa leads to reduction of 6% in cell volume in La0.5Ca0.5MnO3.36 Additionally, a recent study of the effects of hydrostatic pressure in bulk samples shows that saturation magnetization and FM to AFM transition temperature (but not Curie temperature) are affected by pressure; all these anomalies are enhanced with field and suggest that hydrostatic

FIG. 9. Cell volume (squares) and saturation magnetization MsFM (circles) for samples LC700, LC900, LC1000, and Bulk. Lines are drawn to guide the eyes.


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propose that FM ordering comes mainly from atoms at crystallite boundaries but there exists another FM contribution originated from hydrostatic pressure inside the crystallites that enhances FM ordering. IV. CONCLUSIONS

Nanoparticles of La0.5Ca05MnO3 with mean crystallite size around 15 to 40 nm are obtained by sol-gel method by annealing at 700, 800, 900, and 1000 C. Morphological and structural characterizations performed by TEM, SEM, and DRX show that the particles are conformed by several crystallites with a kind of core/shell structure, with a very crystalline core and crystallite boundaries associated to a shell. The core/shell structure consists in AFM core in intimate contact with spin glass surface which behaves FM like. This is confirmed by exchange bias phenomenology measured at different cooling fields: exchange bias field and coercive field increase are characteristic properties for AFM/FM coupling in nanostructured systems. It is well known that size reduction of bulk La0.5Ca05 MnO3 below bellow 100 nm promotes FM ordering at expense of AFM one, this FM contribution normally decreases as particle size increases.10–12 Contrary to reported by other authors, we observe an enhancement of FM ordering in a crystallite size range from 15 to 22 nm. This behavior is observed for the first time and can be attributed to the sum of two contributions: (1) a FM like behavior of spin glass at crystallite boundaries and (2) an effective induced hydrostatic pressure that promotes FM behavior inside AFM cores. ACKNOWLEDGMENTS

This work was supported by grants from the Spanish Ministry of Science and Innovation (Nos. MAT2009-14741C02-00 and MAT2011-23068) and the Madrid regional government CM (No. S009/MAT-1726). 1

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