JOURNAL OF APPLIED PHYSICS 108, 063918 共2010兲
Size-driven magnetic transitions in La1/3Ca2/3MnO3 nanoparticles V. Markovich,1,a兲 I. Fita,2,3 A. Wisniewski,2 D. Mogilyansky,4 R. Puzniak,2 L. Titelman,4 and G. Gorodetsky1 1
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 3 Donetsk Institute for Physics and Technology, National Academy of Sciences, 83114 Donetsk, Ukraine 4 Institute of Applied Research, Ben-Gurion University of the Negev, 84105 Beer-Sheva, Israel 2
共Received 28 June 2010; accepted 5 August 2010; published online 24 September 2010兲 Magnetic properties of electron-doped La1/3Ca2/3MnO3 manganite nanoparticles with average particle size ranging from 12 to 42 nm, prepared by the glycine-nitrate method, have been investigated in temperature range 5–300 K and in magnetic fields up to 90 kOe. Reduction in the particle size suppresses antiferromagnetism and decreases the Néel temperature. In contrast to bulk crystals, the charge ordering does not occur in all studied nanoparticles, while a weak ferromagnetism appears above 200 K. Low temperature magnetic hysteresis loops indicate upon exchange bias effect displayed by horizontal and vertical shifts in field cooled processes. The spontaneous and remanent magnetization at low temperature shows a relatively complex variation with particle size. The size-induced structural/magnetic disorder drives the La1/3Ca2/3MnO3 nanoparticles to a pronounced glassy behavior for the smallest 12 nm particles, as evidenced by large difference between zero field cooled and field cooled magnetization, frequency dependent ac-susceptibility, as well as characteristic slowing down in the spin dynamics. Time evolution of magnetization recorded in magnetic fields after field cooling to low temperatures exhibits pronounced relaxation and a very noisy behavior that may be caused by formation of some collective states. Magnetic properties of the nanoparticle samples are compared with those of La0.2Ca0.8MnO3 nanoparticles. These results shed some light on the coupling between charges and spin degrees of freedom in antiferromagnetic manganite nanoparticles. © 2010 American Institute of Physics. 关doi:10.1063/1.3488619兴 I. INTRODUCTION
Nanosized materials, such as nanoparticles 共NPs兲, nanowires, nanotubes, and nanocomposites are currently a focus of intense investigations, since finite-size effects induce a plethora of interesting phenomena.1–4 Recent studies of nanometer size effect in mixed valence manganites with a general formula R1−xAxMnO3, where R and A are rare- and alkaline-earth ions, respectively, have shown size dependent magnetic and orbital ordering 共OO兲 and various modes of coupling between spin subsystem and the lattice.5–12 Coexisting ferromagnetic 共FM兲 metallic domains and antiferromagnetic 共AFM兲 insulating regions were found in various manganites NPs, characterized by their unique core/shell spin configuration and magnetic interactions. Electron doped La1−xCaxMnO3 共x ⬎ 0.5兲 NPs are of special interest since their bulk counterparts exhibit a stable AFM ground state and charge ordering 共CO兲, that is, the real-space ordering of Mn3+ and Mn4+ ions. While the core of NPs may exhibit various magnetic phases and CO, occurring in the bulk, the shell being magnetically and structurally incommensurate with the core exhibits in general other FM and AFM ordering or paramagnetic 共PM兲 state. Incommensurate magnetization of the core and the shell and magnetic interactions between the particles may set off new magnetic a兲
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and electronic states in addition to those in the bulk. Surface magnetism existing in NPs may also drastically affect magnetic properties. Experimental studies of electron-doped La1−xCaxMnO3 共x ⬎ 0.5兲 NPs 共Refs. 5–12兲 have demonstrated relaxation effects of superexchange interaction in the surface layer and formation of FM-like shell, whose thickness progressively increases with decreasing particle size. It has been found for various electron-doped manganite particles that the reduction in particle size down to the nanometer scale results in suppression of AFM/CO state as the particle size is reduced to ⬃20 nm.5–12 Phenomenological model and Monte Carlo studies11 have shown an enhancement of surface charge density and confirmed a suppression of AFM/CO phase and an emergence of FM order near the surface. It is a common feature of AFM compounds that the uncompensated surface spins destroy the collinear AFM configuration when their particle size is reduced to nanoscale.13 In the case of electron-doped charge-ordered manganites it would impede the formation of the CO state.5–12 Similar destabilization of charge order and an onset of FM correlation was reported for nanosized charge-ordered Nd0.5Ca0.5MnO3, Pr0.65Ca0.35MnO3, and Pr0.5Ca0.5MnO3 manganites.6–10 It was reported that for La0.4Ca0.6MnO3 the reduction in the particle size to 20–60 nm results in full suppression of AFM/CO order and formation of FM-like ordering with relatively high spontaneous magnetization of ⬃1 B / f.u. corresponding to ⬃30% of FM fraction.7 However, Rozenberg et al.12 sug-
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gested that AFM/CO state in La0.4Ca0.6MnO3 NPs is relatively stable and only somewhat suppressed in ⬃17 nm particles in contradiction to the results of Lu et al.7 On the other hand, recent studies of magnetic properties by Zhou et al.14 have demonstrated that La1−xCaxMnO3 共x = 0.50, 0.67, 0.75兲 particles with the average particle size ⬃20 nm show lack of long-range CO and spontaneous magnetization at T = 4 K approaches a value of ⬃1.9, 0.5, and 0.13 B / f.u. for x = 0.50, 0.67, and 0.75, respectively. This suggests that the relative volume of the FM fraction approaches a value of about 50%, 15%, and 4% for x = 0.50, 0.67, and 0.75, respectively. It appears that samples of charge-ordered La1−xCaxMnO3 NPs exhibit diverse magnetic characteristics and additional studies definitely are needed to clarify the nature of ferromagnetism and magnetic properties of NPs of electron-doped manganites. Another interesting phenomenon often observed in AFM NPs, is exchange bias 共EB兲 effect, which generally occurs when the FM-AFM system is cooled in a static magnetic field through the Néel temperature 共TN兲 of the AFM phase. Manifestation of the EB effect was found in phase separated bulk Pr1/3Ca2/3MnO3 and Y0.2Ca0.8MnO3 manganites15,16 and in La1−xSrxCoO3 cobaltites17 due to intrinsic interface exchange coupling between the FM nanodroplets and surrounding AFM matrix15,16 or spin-glass 共SG兲 regions.17 In the case of nanosized AFM manganites, the variation in the superexchange interaction at the surface layer allows a formation of FM-like shells, resulting in a natural AFM/FM interfaces and EB effect.10,18–21 Detailed neutron powder diffraction, electron spin resonance, and magnetization studies of the phase diagram of La1−xCaxMnO3 in the range of electron doping have shown the existence of four phase boundaries x = 0.5, 2/3, 0.8–0.85, and 1 at which the compound forms a different crystallographic and magnetic phases.22,23 For distinct x = 2 / 3, below TCO ⬇ 270 K the room-temperature phase transforms into a charge-ordered low temperature orthorhombic phase with space group Pnma but with a tripled unit cell.24–26 In addition to the CO, below TN ⬇ 170 K, the x = 2 / 3 compound displays a noncollinear AFM structure with the a lattice parameter tripled and the c lattice parameter doubled with respect to the average crystallographic Pnma unit cell parameters.24–26 Our recent studies18 of basically AFM La0.2Ca0.8MnO3 NPs, with average particle size ranging from 15 to 37 nm, have shown that with decreasing particle size the OO transition shifts toward low temperatures while the spontaneous magnetization increases and approaches a value of 0.026 B / f.u. at T = 5 K for the smallest 15 nm particles. Moreover, upon field cooling, the particles display size dependent EB effect, most pronounced for smaller particles. The magnetization data reveal the presence of two FM components: first one appears at T ⬎ 200 K and may be attributed to surface magnetization and second one appears as a result of spin canting of AFM core or is developed at some interfaces inside NPs. It should be noted that the compound with doping level x = 0.8 displays OO but no CO. The AFM
ordering temperature and the structural transition 共a monoclinic distortion兲 temperature associated with OO 共TOO兲 coincide.22 It is worth noting that the diversity in results obtained for NPs of charge-ordered manganites by different research groups may be related to the various preparation methods. Even in the case of bulk manganites, extrinsic inhomogeneities may significantly modify their intrinsic physical properties.27 The features of manganite NPs may be much more sensitive to the preparation procedure than those of bulk samples and, possibly, this is one inevitable problem which leads to a diversity of results obtained. In this paper, we present magnetic studies of La1/3Ca2/3MnO3 共LCMO兲 particles, with average particle size ranging from 12 to 42 nm, performed in the temperature range 5–300 K and in magnetic field up to 90 kOe. The motivation for choosing La1/3Ca2/3MnO3 NPs as a subject of current studies is a need to compare the evolution of magnetic properties with decreasing particle size for distinct AFM charge-ordered manganite 共x = 2 / 3兲 with those observed for AFM manganite with no CO 共x = 0.8兲. The LCMO NPs studied here were prepared using the glycine-nitrate method, the same as previously used for preparation of the nanosized La0.2Ca0.8MnO3 samples.18 Similarly to the x = 0.8 particle case, we reveal that x = 2 / 3 NPs display the appearance of surface ferromagnetism at T ⬎ 200 K and the EB effect at low temperatures. However, a significant difference was found in the suppression of CO/OO state, which essentially completely disappears for all 共12–42 nm兲 x = 2 / 3 particles, though the indication of existence of OO was evidenced even for 15 nm, x = 0.8, particles.18
II. EXPERIMENTAL
Nanocrystalline LCMO particles have been prepared by the glycine-nitrate method, previously used for preparation of the nanosized La1−xCaxMnO3 关x = 0.2,28 0.3,29 and 0.8 共Ref. 18兲兴 powder. A stoichiometric amount of La共NO3兲3 · 6 H2O, Ca共NO3兲2 · 6 H2O, Mn共NO3兲2 · 4 H2O, and glycine 共C2H5NO2兲 were dissolved in water to form the precursor solution. A molar ratio of ⬃0.5 between glycine and nitrate was found appropriate for producing a singlephase perovskite compound. The precursor solution of each nitrate with glycine was mixed well by stirring during 4 h, then all solutions were merged together and resulting solution was mixed by stirring during 15 h, resulting in homogeneous mixture. This solution was heated using a hot plate up to ⬃100 ° C for 1.5–2 h to dehydrate. Afterwards, the solution became a transparent viscous gel. Subsequent heating of this gel to T ⬃ 300 ° C resulted in the autoignition with short combustion of few seconds, with formation of a black porous ash of La1/3Ca2/3MnO3 compound. Then, the powder in low layer was heated with a rate 5 ° C / min to desired temperatures 共650, 850, and 1000 ° C兲 in the flow of 40% O2 and 60% Ar for 1 h to get a series of LCMO nanocrystalline powders with various grain sizes. The x-ray diffraction 共XRD兲 data were collected on Philips 1050/70 powder diffractometer, with a graphite mono-
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TABLE I. Crystalline size and lattice parameters of the La1/3Ca2/3MnO3 samples annealed at various temperatures.
FIG. 1. 共Color online兲 共a兲 XRD spectra of LCMO12, LCMO25, and LCMO42 samples, annealed at 650 ° C, 850 ° C, and 1000 ° C, respectively. 共b兲 Rietveld plot for LCMO42 sample. The data points are indicated by open circles, the calculated and difference patterns are shown by solid lines.
chromator on diffracted beam providing K␣ radiation 共 = 1.541 Å兲 and operating at V = 40 kV and I = 30 mA. The XRD patterns were then analyzed by using the FULLPROF computer program in order to fit structure parameters and crystallite size. The NPs were also characterized by transmission electron microscopy 共TEM兲 equipped with energydispersive x-ray spectroscopy 共EDS兲 facilities. Cylindershape samples having a diameter of 2.4 mm and height of 3.0 mm prepared by compaction of La1/2Ca2/3MnO3 NP powder under pressure of ⬃5 kbar at room-temperature were used in our magnetic measurements. The measurements, using Princeton Applied Research 共model 4500兲 vibrating sample magnetometer 共VSM兲, were completed in the temperature range 5–290 K and magnetic fields up to 15 kOe, applied perpendicularly to the rotation axis of the samples. The measurements of ac-susceptibility in the temperature range 5–300 K, as well as the measurements of magnetization in high magnetic field up to 90 kOe, were performed using the magnetic option of the Physical Property Measurement System of Quantum Design.
Lattice parameters 共Å兲
Temperature of calcination 共°C兲
Crystalline size 共nm兲
Cell volume 共Å3兲
a
b
c
650 850 1000
12⫾ 1 25⫾ 1 42⫾ 2
217.9 218.4 218.1
5.369共2兲 5.369共1兲 5.365共1兲
7.558共2兲 7.567共1兲 7.568共1兲
5.370共2兲 5.376共1兲 5.371共1兲
similar to the ones of La1−xCaxMnO3 ceramics with x ⬇ 2 / 3.22 The samples with average crystallite size of 12, 25, and 42 nm will be denoted herein by: LCMO12, LCMO25, and LCMO42. The size of the NPs was confirmed by TEM and high resolution TEM. Figure 2 shows the bright field 共a兲 and high resolution 共b兲 images for LCMO12 sample. It was found that the size of LCMO12 single isolated NP, in limits of error, agrees with the data of x-ray measurements. The EDS analysis confirmed the composition and homogeneous distribution of the constituent elements with nominal atomic values La: Ca: Mn= 1 / 3 : 2 / 3 : 1.0. The approximate value of oxygen content determined by EDS analysis is equal to 2.99⫾ 0.04. It should be noted that some amount of amorphous phase still remains in the sample LCMO12 after calcinations at 650 ° C. This amorphous phase may be associated with the existence of parasitic phases, which may evade detection by XRD and by TEM due to the small crystallite size or due to a presence of poorly crystalline La-rich phases.30 Certainly, the occurrence of these phases may introduce compositional
III. RESULTS AND DISCUSSION
The XRD pattern of the as-prepared sample presents a mixture of perovskite and amorphous phase. After annealing at T ⬎ 650 ° C an almost pure orthorhombic perovskite phase was obtained. The XRD patterns of the samples calcined at various temperatures 共650, 850, and 1000 ° C兲 are shown in Fig. 1共a兲. The Rietveld plot for the sample annealed at 1000 ° C is shown in Fig. 1共b兲. The average crystallite sizes 具D典 were calculated using Debye–Scherrer equation and together with the lattice parameters are listed in Table I. It should be noted that the lattice parameters for all samples are
FIG. 2. 共a兲 TEM bright field and 共b兲 high resolution images of the LCMO12 sample.
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FIG. 3. 共Color online兲 Temperature dependence of ZFC M ZFC 共open symbols兲 and FC M FC 共solid symbols兲 magnetization of LCMO12 共a兲, LCMO25 共b兲, and LCMO42 共c兲 samples in magnetic field H = 10 kOe. Inset to 共b兲 shows M ZFC and M FC of LCMO25 sample measured in magnetic field H = 100 Oe.
disorder and may shift slightly the composition of main perovskite phase in the smallest LCMO12 NPs toward Ca-rich phase.30 Field cooled 共FC兲 magnetization 共M FC兲 and zero field cooled 共ZFC兲 共M ZFC兲 magnetization curves for LCMO12, LCMO25, and LCMO42 samples, recorded at an applied field H = 10 kOe, are shown in Figs. 3共a兲–3共c兲. It should be noted that the pronounced maximum in magnetization, well seen in charge-ordered La1−xCaxMnO3 共0.55–0.75兲 bulk samples, practically disappears in NPs and only for the largest LCMO42 NPs it may possibly persist with very little change around 250 K, see Fig. 3共c兲. Moreover, the tempera-
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ture dependence of the magnetization 关Figs. 3共a兲–3共c兲兴 presents some other noticeable features. As the temperature decreases below ⬃100 K, the magnetization ZFC and FC curves split off, possibly indicating the onset of weak FM moment at TC共on兲. Nevertheless, the splitting of both curves as well as the value of magnetization in magnetic field H = 10 kOe at low temperatures vary nonmonotonously with decreasing particle size. Namely, FC magnetization at T = 10 K and H = 10 kOe 共Fig. 3兲 is 2.106 emu/g 共0.066 B / f.u.兲, 2.754 emu/g 共0.087 B / f.u.兲, and 1.515 emu/g 共0.048 B / f.u.兲 for LCMO12, LCMO25, and LCMO42, respectively. Such behavior is surprising, since usually magnetization of AFM manganite NPs,6,10,18,20 and in general of all AFM NPs,13 increases with decreasing particle size due increasing number of uncompensated spins at the surface. The splitting between M FC and M ZFC magnetization as well as the irreversibility temperature Tirr at which the M ZFC共T兲 and M FC共T兲 diverge, depend strongly on applied magnetic field, see for example the magnetization of LCMO25 recorded in H = 100 Oe 关inset in Fig. 3共b兲兴. Such a behavior is reminiscent of SG, as with increasing field the magnetic energy becomes higher than the energy barrier between possible equilibrium orientations of magnetic moments.31 In order to get a better understanding of the magnetic behavior, we have measured the temperature dependence of ac-susceptibility of LCMO12, LCMO25, and LCMO42 samples 共Fig. 4兲. One may directly realize several noteworthy features. In distinct contrast to the behavior of acsusceptibility for La0.2Ca0.8MnO3 NPs, which exhibits two peak structure,18 the real part of the ac-susceptibility ⬘ of x = 2 / 3 particles displays only one wide and frequency dependent peak, see Fig. 4. Since the susceptibility of paramagnets increases monotonously with decreasing temperature, while antiferromagnets show a decrease in susceptibility with a decrease in temperature, below the transition temperature from PM to AFM state, we associate this peak with the Néel temperature
FIG. 4. 共Color online兲 关共a兲, 共b兲, and 共c兲兴 Temperature dependence of real component of ac-susceptibility ⬘ of LCMO12, LCMO25, and LCMO42 samples measured during heating at different frequencies of 10, 100, 1000, and 10000 Hz and in magnetic ac field of 10 Oe. 关共d兲, 共e兲, 共f兲兴 ⬙共T兲 curves for LCMO12, LCMO25, and LCMO42 samples, registered during heating, at 10, 100, 1000, and 10000 Hz in magnetic field of 10 Oe. Inset to 共a兲 shows log共兲 vs 关log共T f − Tg兲 − Tg兴 dependence, describing critical slowing down of the spin dynamics.
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TN, see Fig. 3. It appears that the TN of larger particles: 25 nm 共⬇158 K兲 and 42 nm 共⬇161 K兲 is only slightly reduced in comparison with the Néel temperature of bulk samples 共⬇170 K兲, while for LCMO12 NPs, TN ⬇ 106 K is much smaller than that of the bulk samples. It should be also noted that both ⬘ and ⬙ exhibit a well pronounced frequency dependence for all particles 关see Figs. 4共a兲–4共f兲兴. As seen in Figs. 4共b兲 and 4共c兲, the temperature at which the peak occurs for LCMO25 and LCMO42 does not depend on the frequency, whereas it shifts to higher temperatures with increasing frequency for smaller LCMO12 particles from 106 K for f = 10 Hz to 110.3 K for f = 10 kHz 关see Fig. 4共a兲兴. The frequency dependence of the peak position temperature is a direct indication of slow spin dynamics leading to associate this peak also with the freezing temperature of FM clusters, Tf. Indeed, such a frequency dependent temperature-shift is a reminiscence of a spin/cluster glasslike behavior and it can be characterized by frequency shift per decade K = ⌬Tf / Tf⌬共log 兲, where Tf refers to the temperature of the maximum of ⬘ and ⌬Tf is the temperature-shift at a given frequency. The calculated K factor for LCMO12 is equal to 0.013 and falls in the range of the values typical for spin glasses.32 This behavior is quite conventional for spin glasses and can be explained as the critical slowing down of the spin dynamics32 described by the expression: = 0关共Tf − Tg兲 / Tg兴−z, where is the relaxation time, 0 is the microscopic time constant, and z is a dynamic critical exponent. For NPs, 0 can be assigned to the superparamagnetic relaxation time of a single particle of an average size. A dynamic scaling analysis according to critical slowing down was performed with the fitting of three variable parameters Tg, 0, and z of the linear dependence log共兲 versus log关共Tf − Tg兲 / Tg兴 to the experimental data Tf共f兲 关inset in Fig. 4共a兲兴. In fact, inset in Fig. 4共a兲 shows a good agreement of the power law for slowing down of spin dynamics with Tf共f兲 data. The best fit gives the following values: Tg ⬇ 103.0 K, 0 ⬇ 3.6⫻ 10−12 s, and z ⬇ 6.8. The obtained magnitudes of 0 and z are found in the range of admissible values for spin glasses.31,32 It appears that only smaller LCMO12 particles exhibit necessary features of magnetic SG. In the frame of the core-shell model, individual AFM NPs consist of a AFM ordered core and FM-like disordered shell, which exhibits SG-like features.6,10–12 It should be noted that the nature of the surface shell contribution remains unclear, what is well reflected in the variety of terms used to describe its properties, such as: “disordered surface state,” “uncoupled spins,” or “SG-like behavior.”33 In general, the thickness of the surface layer, which has magnetic structure that is incommensurate with the core, increases with decreasing particle size.34–36 Therefore, SG-like properties are enhanced for smaller particles. The evolution of magnetic properties with decreasing particle size is also evidenced by the magnetization M versus H dependences, recorded at 5 K after ZFC, see Fig. 5共a兲. It is worth noting that the magnetization curves indicate upon the existence of FM constituents imbedded in the NPs whose relative volume increases with decreasing particle size, namely, an enhancement of FM regions at the expense of AFM phase. It was found that values of spontaneous magne-
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FIG. 5. 共Color online兲 共a兲 Magnetic field dependences of magnetization measured at T = 5 K for LCMO NP samples. 共b兲 Resulting magnetization M ⴱ vs H curve, obtained after substracting the contribution of AFM core determined at T = 5 K for ZFC.
tization M 0, evaluated by linear extrapolation of the high field 共ⱖ40 kOe兲 magnetization to zero field, at 5 K are: 2.35 emu/g 共0.074 B / f.u.兲, 1.98 emu/g 共0.062 B / f.u.兲, 1.01 emu/g 共0.032 B / f.u.兲, for LCMO12, LCMO25, and LCMO42, respectively. In order to determine the magnetic contribution of the AFM component, the high field susceptibility of LCMO samples hf was evaluated by linear extrapolation of ZFC M共H兲 dependence 共Fig. 5兲 in the field range of 40–90 kOe. The obtained values of hf are equal to 1.04 ⫻ 10−4, 8.02⫻ 10−5, and 5.02⫻ 10−5 emu/ g Oe for LCMO12, LCMO25, and LCMO42, respectively. Monotonous increase in hf with decreasing particle size indicates the weakening of AFM phase in the core of NPs. Since the theoretical value of the magnetization for the fully ordered spins M theor is 3.34 B / f.u., we may conclude that the volume of the FM phase in LCMO12 is only of about 2.2% at 5 K. Moreover, magnetization value of M = 11.65 emu/ g 共0.37 B / f.u.兲 observed for smaller LCMO12 particles at H = 90 kOe is also much smaller than the M theor. Such a behavior manifests the dominant role of the AFM phase and is in agreement with recent results for La0.25Ca0.75MnO3 NPs,20 and with our observations for La0.2Ca0.8MnO3 NPs.18 Recently, the core-shell formation was also proposed by Dong et al.11 to describe the magnetic structure of AFM manganite particles. In a compliance with previous studies, we suggest that the core of all LCMO NPs is AFM below TN, while the shell may embody a SG-like FM surface layers.5–12,18,20 Though both M 0 and hf increase with decreasing particle size 关Fig. 5共a兲兴, the magnetization curve M共H兲 of the smallest LCMO12 particles exhibits a complex behavior. In particular, in the magnetic field range 1.5⬍ H ⬍ 15 kOe magnetization curve M共H兲 关Fig. 5共a兲兴 of LCMO12 lies below the corresponding dependence for
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FIG. 6. 共Color online兲 共a兲 Field dependence of magnetization of LCMO25 sample at 10 K measurerd after ZFC and FC in H = 15 kOe. 共b兲 The hysteresis loop measured at 10 K after FC with indication of EB parameters. M r denotes the remanent magnetization. 共c兲 Field dependence of magnetization at various temperatures measured after FC.
LCMO25. In principle, ZFC M共H兲 curves can be described by the equation M共H兲 = M FM共H兲 + hfH; here hf is the high field AFM susceptibility of the core and M FM is ascribed to the FM component, particularly due to uncompensated surface spins. The development of FM component for LCMO25 and LCMO42 samples becomes evident in Fig. 5共b兲, where we show the resulting magnetization M ⴱ共H兲 obtained after subtracting the linear background M AFM = hfH from measured M共H兲 dependence. Resulting curves display magnetic saturation in magnetic field of a few kOe, thus exhibiting the behavior of FM-like magnetization. On the other hand, the M ⴱ of LCMO12 sample does not saturate up to H ⬃ 60 kOe and for H ⬎ 60 kOe it even slightly decreases with increasing magnetic field. There are few possible reasons for such complex field dependence of M ⴱ. First, it is a presence of SG-like phase or alternatively, field induced transformation between different magnetic phases. One should note that very similar behavior was previously observed for bulk Pr1/3Ca2/3MnO3 sample15 and it was related to intrinsic coexistence of different magnetic phases. Second, such a behavior may be attributed to the effect of enhanced surface anisotropy. Nevertheless, it cannot be excluded that the liner background contribution to the measured magnetization was overestimated. Figure 6共a兲 presents hysteresis loops recorded for LCMO25 sample at 10 K after ZFC and FC at Hcool = 15 kOe. The horizontal and vertical shifts along both magnetic field and magnetization axes are clearly seen for FC but
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FIG. 7. 共Color online兲 共a兲 Temperature dependence of spontaneous magnetization M 0 and remanent magnetization M r for LCMO12 and LCMO42 samples, extracted from hysteresis loops recorded at various temperatures. Temperature dependence of M EB 共b兲 and HEB and HC 共c兲 for LCMO12 and LCMO42 samples after FC in 15 kOe.
absent in ZFC process. The above difference between ZFC and FC magnetic hysteresis loops manifests the phenomenon of EB. In the absence of the vertical shift, the EB and coercive fields are usually defined as HEB = 共HC1 + HC2兲 / 2 and HC = 共HC2 − HC1兲 / 2, where HC1 and HC2 are coercive fields for decreasing and increasing magnetic field.37 Since a significant vertical shift takes place, we have defined HEB and HC using the above relations, but with HC1 and HC2 obtained at intercepts of the loop with horizontal line, which characterizes the vertical loop offset M EB,38 see Fig. 6共b兲. The measurements of magnetization loops at various temperatures have shown that a spontaneous magnetization tentatively attributed to a presence of the FM phase is still observed up to 230 K. Only above this temperature M共H兲 hysteresis loops do not display spontaneous magnetization, see Fig. 6共c兲. Figures 7共a兲–7共c兲 summarize the results of the determination of M 0, M r, M EB, and HEB for the smallest LCMO12 and the largest LCMO42 particles. As the temperature increases, both, M 0 and M r, for 12 nm particles decrease rapidly and become very small already at T ⬃ 80 K, though very small spontaneous magnetization is observed up to 230 K, see Fig. 7共a兲. In contrast, M 0 and M r for LCMO42 slowly decrease with increasing temperature and become unobservable at much higher temperatures 共at ⬇150 K for M r and at ⬇240 K for M 0兲. Similarly, vertical shift M EB gradually decreases with increasing temperature and fully disappears at TN 共at T ⬎ 80 K for LCMO12 and at T ⬎ 150 K for LCMO42兲, see Fig. 7共b兲. For both samples, the EB field HEB decays with temperature and turns out to be unobservable at
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FIG. 8. 共Color online兲 Temperature dependence of thermoremanent magnetization for LCMO12, LCMO25, and LCMO42 samples.
FIG. 9. 共Color online兲 Time variation in magnetization after subsequent cooling to 10 K in magnetic field of 5 kOe for LCMO12 sample.
T ⬎ 60 K, while HEB of LCMO12 exhibits also marked peak at T = 20 K. The temperature dependence of coercive field HC 关Fig. 7共c兲兴 is similar to the temperature variation of M r 关Fig. 7共a兲兴 and it tends to zero at T ⬃ 80 K for LCMO12 and at T ⬃ 150 K for LCMO42. In order to get additional view on the temperature evolution of magnetic phases, we have performed measurements of thermoremanent magnetization 共TRM兲 M TRM in the following way: the sample was cooled down to T = 10 K in magnetic filed H = 15 kOe, then the magnetic field was switched off and after a wait of 100 s the magnetization was recorded. It was found that M TRM for all NPs increases monotonously with decreasing temperature, see Fig. 8. Nevertheless, M TRM of the smallest particles 共12 nm兲 becomes very small at T ⬎ 100 K, while for larger 共25 and 42 nm兲 particles it tends to zero only at T ⬃ 170 K, see Fig. 8. It appears that for all LCMO NPs M TRM practically diminishes at ⬃TN. This observation is found to be in an agreement with results extracted from hysteresis loops, see Fig. 7共a兲 for remanent magnetization. Similarly to behavior of magnetization of La0.2Ca0.8MnO3 NPs, the magnetization of La1/3Ca2/3MnO3 particles exhibits unstable behavior while performing the measurements with a VSM in magnetic fields of few kOe and at a temperature range 10–100 K.18 In order to get a better understanding of this instability, we have recorded three subsequent time evolution of FC magnetization at time intervals of 6 s and elapsed time of about 2000 s, see Fig. 9. Our data clearly show that the magnetization first displays a strong relaxation process with enhanced level of the noise. After about 1000 s, the magnetization approaches some saturated value with much lower noise, though it still remains considerably noisy. Moreover, the training effect characterized by the reduction of the low temperature magnetization is clearly seen in Fig. 9. Though, we noticed a somewhat similar noisy magnetization in La0.2Ca0.8MnO3 NPs,18 observed training effect 共Fig. 9兲 and a tendency to reach some saturated value after relaxation are new interesting features, which were not observed for La0.2Ca0.8MnO3 NPs. Previously, training effect and thermal cycling effect were found in low Cr doped Pr0.5Ca0.5Mn1−xCrxO3 polycrystalline La0.5Ca0.5MnO3, and samples 共0.01ⱕ x ⱕ 0.025兲,39 La0.325Pr0.3Ca0.375MnO3.40 It was proposed that thermal cy-
cling effect accounts for the enlargement of the interfacial elastic energy,39,40 thereby impedes the growth of the FM phase and leads to a significant decrease in magnetization. In bulk samples, the interfacial elastic energy arises due to the coexistence of FM and charge-ordered domains with distinct lattice parameters and different surface/volume ratio. Let us discuss the evolution of magnetic phases in La1/3Ca2/3MnO3 nanosize samples upon cooling from PM state. It was previously suggested,22 that for bulk La1−xCaxMnO3 共0.6⬍ x ⬍ 0.69兲 the broad magnetization peak occurring at high temperature is related to a structural transition due to CO and not to the appearance of FM interactions, while the small anomaly in the M共T兲 curve marks the AFM ordering 共TN兲. In La1/3Ca2/3MnO3 NPs 共Figs. 3 and 4兲, both magnetization and ac-susceptibility do not exhibit a peak characteristic for CO transition. Therefore, we suggest that a CO state in La1/3Ca2/3MnO3 nanosized samples is highly suppressed in an agreement with the results published recently.6,8,10 At the same time, our recent results for La0.2Ca0.8MnO3 NPs 共Ref. 18兲 show a different behavior. In particular, a gradual shift toward a lower temperature with decreasing particle size, associated with OO, was observed in M ZFC共T兲 and M FC共T兲 and in ac-susceptibility. Furthermore, even magnetization of smaller 15 nm particles exhibited a peak characteristic for OO. It has been recently suggested10 that the stability of AFM/CO state may play a crucial role in the evolution of magnetic properties with a decrease in NPs size. The C-type AFM structure observed in bulk La0.2Ca0.8MnO3 manganite41 is much more stable than the CE-type AFM structure characteristic for bulk La0.4Ca0.6MnO3 and Pr0.5Ca0.5MnO3 manganites.10,42 For this reason, the OO state does not disappear even in 15 nm La0.2Ca0.8MnO3 particles, whereas in La1/3Ca2/3MnO3 NPs the CO state is already completely suppressed for average particle size ⱕ42 nm. In AFM NPs, the number of spins deviating from the AFM arrangement usually increases with reducing the particle size,13 implying increasing ratio of the uncompensated spins, which have a key role in EB effect.37 Then, the uncompensated surface spins favor the FM coupling, leading to the formation of FM clusters and their growth upon reducing particle size.6,11,18 Indeed, we recently observed monotonous increase in both spontaneous magnetization at low tempera-
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tures and remanent magnetization with decreasing particle size in La0.2Ca0.8MnO3 NPs having average size ranging from 15 to 37 nm. Magnetic properties of LCMO25 and LCMO42 samples exhibit similar variation with size while LCMO12 exhibits a more complex behavior 共see Figs. 5 and 8兲. As pointed out already, the composition of the main perovskite phase in LCMO12 may differ slightly from this of larger LCMO25 and LCMO42 particles due to presence of amorphous phase. A thorough neutron diffraction and magnetization studies of bulk electron-doped La1−xCaxMnO3 共0.5ⱕ x ⱕ 0.9兲 have shown that the phase diagram can be described with four coexisting stable distinct phases 共at x = 1 / 2, 2/3, 4/5, and 1兲 and a metastable 共x = 3 / 4兲 phase.22 In all these compositions, La1−xCaxMnO3 system forms a distinct crystallographic and magnetic phases. Pissas and Kallias22 have proposed that in the range 2 / 3 ⱕ x ⱕ 4 / 5 there is a coexistence of three phases due to fluctuations which tend to stabilize x = 3 / 4 charge-ordered state. It appears that a slight shift in composition may modify the coexisting phases, as evidenced by unusual behavior of M共H兲 curve 共Fig. 5兲, the temperature dependence of TRM 共Fig. 8兲 and the amazing time variation in magnetic field 共Fig. 9兲. Slow relaxations of magnetization and resistivity were observed in various phase separated perovskites,43–45 resembling that of the SG. It is worth noting that a description of the phase separated state as comprising two different phases is highly oversimplified.46 Numerous studies clearly indicate that the phase separated state is not a state of thermodynamic equilibrium and strong competition between different phases may be reflected by the dynamical behavior. The most pronounced relaxation and noisy dynamics are observed in the smallest LCMO12 NPs, see Fig. 9. As already mentioned LCMO12 NPs may exhibit small compositional deviation from x = 2 / 3 and a coexistence of three different phases in the NP core. The core-shell model in which FM-like disordered shells and AFM ordered cores form FM cluster glass was suggested for electron-doped LCMO nanocrystals.5–12,18,20 It appears that at least four different magnetic phases may coexist and compete in the smallest LCMO12 NPs, resulting in unusual time dependent variation in magnetization with high level of the noise.18 IV. CONCLUSIONS
In summary, for La1/3Ca2/3MnO3 NPs with average particle size ranging from 12 to 42 nm, the FM moment attributed to the shell of the NPs appears even at temperatures T ⬎ 200 K. The FM moment at low temperatures increases with decreasing particle size. The volume of the FM phase for the smallest 12 nm particles is only of about 2.2% and spontaneous magnetization is equal to 0.074 B / f.u. at 5 K, basically signifying the AFM phase. All La1/3Ca2/3MnO3 NPs at low temperatures display glasslike features, such as a gap between ZFC and FC magnetization and significant frequency dependence of acsusceptibility. Nevertheless, a characteristic frequency shift in the peak temperature in ac-susceptibility was found only for the smallest 12 nm particles. For these NPs, the dependence of the SG freezing temperature, Tf, on f satisfies the
conventional critical slowing down law: = 0关共Tf − Tg兲 / Tg兴−z with = 1 / f, 0 ⬇ 3.6⫻ 10−12 s, z ⬇ 6.8, and a glass temperature, Tg ⬇ 103.0 K. The time dependence of magnetization of LCMO12 particles, recorded in magnetic fields ⱖ5 kOe after cooling in the same field, exhibits a clear relaxation and a very noisy character at low temperatures. Furthermore, upon field cooling, the particles display EB effect. The temperature variation in EB field, remanent magnetization, and spontaneous magnetic moment for the particles with different size are dependent on magnetic coupling between the AFM core and the FM-like shell. Comparison of the results obtained for AFM La1/3Ca2/3MnO3 NPs with recent observations18 made in AFM La0.2Ca0.8MnO3 NPs reveals some common features as well as significant differences. In particular, NPs of both x = 2 / 3 and x = 0.8 composition, exhibit a monotonous enhancement of weak ferromagnetism linked to the reduction in the particle size, which is found at T ⬎ 200 K. Moreover, magnetic hysteresis loops indicate size dependent EB effect displayed by horizontal and vertical shifts in FC processes. It seems that above features are typical for AFM manganite NPs of different composition, while the conflicting literature data on magnetic properties of AFM La1−xCaxMnO3 共x ⬎ 0.5兲 may be rather attributed to the extrinsic effects of inhomogeneities. The temperature dependencies of magnetization of La0.2Ca0.8MnO3 particles with average particle size 15–37 nm show size dependent peak 共at TOO = 153 K for smaller 15 nm particles and TOO = 201 K for larger 37 nm particles兲 and two peak structure of ac-susceptibility, where high temperature peak is associated with establishment of orbital ordered AFM state.18 Such features are absent in La1/3Ca2/3MnO3 NPs and for all particles with average particle size between 12 and 42 nm only one wide peak, associated with transition to AFM state, shows up. This fact indicates that due to reduction of the particle size 共under 42 nm兲 the surface effects dominate and suppress the CO. We suggest that the C-type AFM structure observed in bulk La0.2Ca0.8MnO3 manganite is much more stable than the 2/3-type one in La1/3Ca2/3MnO3, resulting in surviving of OO even in 15 nm La0.2Ca0.8MnO3 particles and leading to an disappearance of any CO state in La1/3Ca2/3MnO3 at particle size ⱕ42 nm. These results shed light on the evolution of spin configuration in electron-doped manganite NPs. ACKNOWLEDGMENTS
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