Magnetic Glassy Behavior of Pr0.6Ca0.4MnO3 Nanoparticles: Effect of

Article pubs.acs.org/JPCC

Magnetic Glassy Behavior of Pr0.6Ca0.4MnO3 Nanoparticles: Effect of Intra and Interparticle Magnetic Interactions on Magnetodielectric Property K. Devi Chandrasekhar,*,†,‡ A. K. Das,† and A. Venimadhav§ †

Department of Physics, Indian Institute of Technology, Kharagpur 721302, India Department of Physics and Center for Nanoscience and Nanotechnology, National Sun Yat Sen University, Kaohsiung 804, Taiwan § Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur 721302, India ‡

S Supporting Information *

ABSTRACT: Studies of the dc and ac magnetization on Pr0.6Ca0.4MnO3 (PCMO) nanoparticles indicate the metastable magnetic behavior with random ferromagnetic and antiferromagnetic interactions that led to spin glass (SG) behavior below 90 K. Magnetodielectric (MD) effect has been investigated by dispersing the SG PCMO nanoparticles in a polyvinylidene fluoride (PVDF) matrix. It is found that for low volume fraction of PCMO the MD varies as M2, which indicates the intrinsic magnetoelectric (ME) coupling in PCMO nanoparticles. For higher volume fractions (above the percolation threshold) MD goes with the M4. The magnetization along with microstructural investigations suggested the significant role of inter/intraclusters magnetic interactions on MD property.

1. INTRODUCTION Ferroelectromagnets that possess simultaneous magnetic and electric ordering have recently stimulated much scientific interest due to the prediction of novel application in spintronics and memory technology.1,2 The coexistence of multiple order parameters in the single/multiphase multiferroic materials often exhibits intriguing cross coupling between the magnetic and the electric polarization order parameters.3 To characterize the multiferroic systems, researchers often measure the linear ME coupling given by Pi = αijHj; where Pi is the electrical polarization vector, Hj is the magnetic polarization vector, and αij is the first order linear ME coupling term.4 Recently the second order ME coupling, i.e., MD effect has been shown as an indirect evidence for the multiferroic nature of the materials.5,6 According to Ginzburg−Landau theory, for ferroelectromagnet system the change in dielectric permittivity under the magnetic field should obey the linear variation with the magnetization, i.e., Δε α M2. Indeed MD depends on the spin pair correlation function across the paramagnetic (PM) to ferro/antiferromagnetic (FM/AFM) ordering i.e. Δε α ⟨Si· Sj⟩.5−7 Several systems such as single phase multiferroics (such as BiMnO35 and Z-type hexaferrite8) and composite systems (such as Er2O3/SiO2,9 and BaTiO3/LaMnO310) proved to be multiferroic by MD measurement. In fact, various nonpolar systems such as SeCuO3/TeCuO3 follow the Δε α M2 relation across the PM to FM/AFM transition.7 The spin pair correlation length in a different spin structures (i.e., AFM or FM) plays a crucial role on behavior and sign of MD effect.6,7 Apart from the spin−spin correlation function, until-to-date © 2014 American Chemical Society

several other mechanisms are also explored to understand the dielectric anomalies in different systems such as spin reorientation transition,11 magnetoexchange striction,12 magnetostructural transition,13 etc. Higher order ME coupling is predicted to be more pronounced in nanodimensional systems.3,4 Apart from the higher order ME effect, a higher order MD effect is often found in several systems. Recently, the Δε α M4 variation has been noticed in nanodimensional systems such as La2NiMnO6/Bi2NiMnO6 thin films14 and PCMO/PVDF nanocomposites;15 however, the mechanism to observe such higher order MD effect is not clear. Apart from intrinsic MD behavior, spurious effects such as Maxwell−Wagner (MW) interfacial polarization, magnetoresistance (MR) can also give large extrinsic MD values.16 In order to estimate the genuine MD nature of the system, one should look to the material properties in the intrinsic region (where MW interfacial polarization is entirely absent). In this study, to shed light on the origin of higher order MD and eliminate the parasitic MD contributions, an extensive study of temperature and field dependent dielectric properties with respective to PCMO volume fraction are preformed. In this present study, we have taken the semiconducting Pr1−xCaxMnO3 nanoparticles as the filler particles. In the Pr1−xCaxMnO3 system, the hole doping level in the range of 0.3 ≤ x ≤ 0.5 exhibits rich physics due to the interplay of different Received: September 3, 2014 Revised: November 6, 2014 Published: November 10, 2014 27728

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kinds of ordering.17 The high temperature charge ordering (CO) phenomena compete with the low temperature ferromagnetic clusters which form the magnetoelectric phase separation phenomena.18 We have taken the Pr0.6Ca0.4MnO3 composition, where the existence of bond and site center CO has been predicted to show the multiferroic property.18 The results are organized in three sections: (I) establish the SG magnetic behavior of PCMO nanoparticles; (II) distinguish the different relaxation mechanism across the percolation threshold (Supporting Information); and (III) study the role of microstructure, magnetic inter/intraparticles magnetic interaction on low temperature magnetodielectric properties. For clarity and comparison, some of the published data points for the sample of PCMO/PVDF having 0.22 volume fraction (refs 15 and 19) have been included.

3. RESULTS AND DISCUSSION 3.1. PCMO Microstructure. The TEM micrograph of nanoparticles is shown in Figure 1, where the particles are agglomerated to form irregular clusters with individual particle having the rectangular shape of width ∼50 nm and length varied between ∼50−130 nm.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Pr 0.6 Ca 0.4 MnO 3 Nanoparticles. Pr0.6Ca0.4MnO3(PCMO) nanoparticles were prepared by sol− gel method. We have taken Pr6O11 (Loba Chemie, India), Ca(NO2)36H2O (Loba Chemie, India) and MnCl44H2O (Loba Chemie, India) as precursor materials. At first stoichiometric amount of Pr6O11 was dissolved in HNO3 to make the praseodymium nitrate. Aqueous solution of Ca(NO2)36H2O and MnCl44H2O were prepared in deionized water. A homogeneous metal nitrates solution was obtained by mixing all of the above chemicals in a beaker. To this mixture volume, 1.5 times volume of ethylene glycol was added drop by drop with continuous stirring. The resulting emerald color solution was heated at 80 °C for 1 h. This solution was turned into a thick gel after heating at 150 °C for 10 h. Subsequent heating at 250 °C initiate the self-burning of gel and black fluffy powder was obtained. The resulting powder was heat-treated at 700 °C for 2h. 2.2. Preparation of PCMO/PVDF Nanocomposites. Nanocomposites with different volume fraction of PCMO nanoparticles in PVDF matrix were prepared by sonication mixing method. Polyvinylidene fluoride powders were purchased from Sigma Aldrich, USA. A known amount of PCMO nanoparticles and PVDF powers were dispersed in 50 mL of methanol separately and sonicated for 30 min. These solutions were mixed to get the homogeneous dispersion of PCMO nanoparticles in PVDF matrix, and we continued the sonication process for another 2 h. The solution was dried overnight at 80 °C to obtain the homogeneous powder. The pellet was prepared under 10 MPa of pressure at room temperature and annealed at 200 °C mainly to reduce porosity. 2.3. Characterizations. Surface morphology of the nanoparticles and nanocomposites were carried out using Highresolution transmission electron microscopy (HRTEM) and field emission electron microscopy (FESEM), respectively. Temperature and magnetic field dependent dielectric properties were performed using the Hioki 3532−32 LCR meter in cryofree superconducting magnetic system (Janis) with Lakeshore temperature controller (332). Measurements were done with an ac excitation voltage of 3 V and frequency range 500 Hz to 100 kHz. For dielectric measurements, sample was applied to the wet silver paste and dried at 80 °C for several hours to make parallel plate capacitor geometry. The data were collected using Labview 8.5 software. Temperature and field dependent magnetic properties were measured using Quantum design MPMS-VSM system.

Figure 1. TEM image of PCMO nanoparticles.

3.2. Temperature Dependent Ac and Dc Magnetization. To establish the magnetic nature of the PCMO nanoparticles, ac and dc magnetization measurements with respect to the magnetic field (M−H), temperature (M−T) and time (M−t) were performed. The temperature dependent zero field cooled (ZFC), and field cooled warming (FCW) magnetization curves are shown in Figure 2a; a paramagnetic to ferromagnetic like transition around 114 K can be seen, and interestingly no CO anomaly ∼240 K is observed. For the bulk case, the CO peak has been noticed in the Pr1−xCaxMnO3 (0.3 ≤ x ≤ 0.5) system, however the strength of CO ordering decreases with the particle size irrespective of the calcium doping level.17,20,21 The absence of CO in the nanosized manganites indicated to the surface disorder that effetely destroys the stabilization of high temperature CO pattern.20,21 Huge irreversibility (TIRR) between the ZFC−FCW magnetization curves has been found. The ZFC curve shows a sharp peak (TP) like anomaly around 90 K whereas the FCW curve shows a continuous increase of magnetization. The thermomagnetic reversibility and shifting of Tirr and TP to low temperatures with increase of field are common features of some of the metastable magnetic systems such as SG/cluster glass (CG) and superparamagnetic particles (SPM).21−23 In the present case, the high aspect ratio of nanoparticles along with the competing interaction having dominant FM interaction can result in interacting SPM nature/SG nature. Consequently, the hump at 90 K can be considered as the blocking of the FM cluster. The appearance of Tirr well above the TP and increase of separation between Tirr and TP by increasing the applied magnetic field indicated a cluster-like SG behavior of the system. Further down in temperature, another magnetic field independent week magnetic anomaly around ∼41 K is found in both FCW−ZFC curves and is very much similar to that of bulk canted antiferromagnetic (CAF) transition.17 The upturn of magnetic transition below 42 K has been observed several authors and can be assigned to the reentrance of spin glass (RSG) nature in half doped PCMO nanoparticles; this was found to depend on the particle size.21,24 However, in the present study the rigid nature of this magnetic transition with respect to applied DC field, in fact, hints at the CAF transition, the ground state of the PCMO bulk system.17 To elucidate the 27729

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Figure 2. (a) ZFC−FC magnetization of PCMO nanoparticles with different applied fields; Inset shows the H−T phase diagram. (b) M−time curves at different waiting time at 10 K. (c) Time dependent S(t) curves for different waiting times. (d) Temperature variation of χ′(T) for reference and the waiting curve; inset shows the temperature dependent memory effect (Δχ′(T)).

In the H−T plane, the SG system exhibits critical lines across the FM and SG behavior. The existence of such lines was predicted from the mean-field theoretical model.26 Under the magnetic field, the critical lines are predicted on the H−T plane, which marks the phase transition and several parameters such as anisotropy, dipole−dipole interaction and volume fraction of disordered spins.26,28 This model assumes phase transition in SG and considers that the large field will destroy the frozen spin states. From the mean field theory, the critical lines in H−T plane can be described as26

magnetic nature of PCMO nanoparticles, we have performed a more detailed magnetization studies. Magnetization as a function of time was conducted to understand the existence of the glassy nature of PCMO nanoparticles. Time dependent magnetic relaxation behavior under ZFC protocol for different waiting time and magnetic fields were performed. In this protocol, the sample was first cooled in zero field from well above the glass transition temperature (Tg) to a temperature below Tg (10 K). The system was kept for aging with a definite waiting time (tw) then magnetic field is imposed, and magnetization data was recorded with respect to time. Figure 2b shows the aging effect at 10 K for an applied field of 100 Oe for three different waiting times (tw). In the case of classical SG and RSG systems the signature of the aging effect with the waiting time is reflected as an inflection point in the magnetic viscosity curve S(t) defined as S(t) = (1/h)((∂M)/(∂ln t)); where, h is the applied field.21 A maximum in S(t) occurs at the time known as the relaxation time (τs) and it is approximately equal to waiting time tw, implying an aging effect in accordance with the droplet model.23 It is clear from Figure 2c that the temperature at which the maximum relaxation occurred increases with increasing the waiting time, but the maximum does not exactly coincide with the waiting time; instead it appears at a higher time as observed in other manganite systems such as Pr0.5Ca0.5MnO3, and La0.7‑xYxCa0.3MnO3.21,25 The strong dipolar interaction significantly influences the evolution of glassy dynamics. To further elucidate these intriguing features in the nanoparticles assembly, we have performed AC susceptibility single stop, wait memory effect in ZFC mode at 60 K as shown in Figure 2d. The difference between ZFC curves of AC susceptibility gives a cusp at the waiting temperature (inset to Figure 2d), indicating the memory effect at both the temperatures. The cusp in the difference curve originates from the clustering nature of PCMO nanoparticles, where the FM clusters are strongly interacting below blocking temperature and such kind of behavior commonly appears for the nanoparticle assembly.26,27

p ⎛ T⎞ H = H 0 ⎜1 − ⎟ TB ⎠ ⎝

(1)

where TB is the blocking of interacting FM clusters, and H0 is the magnetic field. For an isotropic Ising SG system, de Almeida−Thouless (AT) estimated the p values as 3/2, whereas the isotropic Heisenberg case p takes the value of 1 /2 and is called the Gabay−Toulouse (GT) phase line.26,28 In other words, the spin disorder at the surface of SG layer obeys the H2/3dependence and is called as surface SG, while for volume SG systems, TB is expected to follow the H1/2 dependence. Recently, the phase transitions in H−T critical line from AT to GT behavior have been observed in Pr0.5Sr0.5MnO3 nanoparticles, where such a transition was assigned to the dipolar interaction among the SPM clusters.29 In the present case, the plot of H vs Tp is shown in the inset to Figure 2a; the variation of Tp at higher field is smaller compared to that of lower magnetic fields, where a steady variation is seen. However, a close observation shows that, the variation of Tp at high and low fields coincides with the TCAF of the ZFC−FC magnetization curve (Figure 2a).The obtained fitted parameters in the low field region (p = 2.22 ± 0.22, H0 = 1852 + 20 Oe, and TB = 121.5 + 1.2 K) is close to AT critical line that indicates a volume SG response. On the other hand, at high fields the determined parameter (p = 0.72 ± 0.12, H0 = 9520 ± 50 Oe, and TB = 21.5 ± 1.2 K) signifies the surface SG response and the crossover from AT to the GT line occurs at 40 K, and 27730

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Figure 3. Temperature dependent (a) εeff and (b) tan δ for 100 kHz and (c) variation of tan δ with 0 and 5 T magnetic field for different volume fraction samples.

the figure, irrespective of the composition the applied magnetic field forces the dielectric relaxation to the low temperatures; importantly the magnitude of loss tangent decreases with increasing the magnetic field. The separation between the 0 and 5 T loss peak position nominally increases from 12 to 13 K with the increase of the volume fraction from 0.038 to 0.225, which indicates that the dynamics of relaxation with respect to the volume fraction is almost similar under 0 and 5 T magnetic fields. Theerthan et al. distinguishes interfacial polaronic relaxation in the Fe-doped BaTiO3 system; where the loss tangent under magnetic field changes its magnitude instead of peak shift.31 Based on this, the present magnetic tunability of dielectric relaxation can be assigned to intrinsic PCMO relaxation rather than interfacial polarization. With the increase of volume fraction of PCMO nanoparticles, the real part of the dielectric permittivity displays a broad step like enhancement. The polaronic relaxation of PCMO nanoparticle might be a possible origin for this behavior. It is expected that, for a specific composite, the microstructure of the composite would remain invariant with temperature; however, with variation of PCMO volume fraction, the microstructure changes and the fact that near the percolation threshold the PCMO nanoparticles are getting closer thereby increase the polaron transport and correspondingly the dielectric response. 3.3. Effect of Volume Fraction on Dielectric, Magnetic and MD behavior of PCMO/PVDF Nanocomposites. The variation of εeff with the volume fraction of the PCMO nanoparticle for 20 K is shown in the inset to Figure 3a. As shown in figure the εeff increases linearly with the increase of PCMO volume fraction up to 0.11%, their by a small enhancement in slope can be observed and interestingly the point of slope change matches with the percolation threshold found at room temperature.19 At high temperature, the blocked interfacial charge carriers are activated with the thermal energy and exhibits percolation transport with volume fraction. At low

this matches with the TCAF in ZFC−FC magnetization data. At temperatures below 100 K, 3D SG is evident due to the phase separation added to the nanometric size. Below 41 K, as the core gets ordered to CAF structure, the spin disorder at the surface due to nanometric size give rise to the 2D SG nature. At low temperatures, particles can be considered to have an AFM core with SG shell. 3.2. Low Temperature Dielectric Relaxation Dynamics of the PCMO/PVDF Composites. The dielectric properties of the magnetic glassy nanoparticles have been studied by dispersing them in the PVDF matrix. Temperature dependent dielectric study has revealed three relaxations in the temperature window of 10−300 K as shown in the Supporting Information. The high temperature relaxations at 200 and 270 K show different dynamics for different volume fraction samples [Supporting Information] while the low temperature relaxation ∼70 K is common for all the volume fractions and is related to the PCMO nanoparticles. The temperature dependent dielectric permittivity and loss tangent of the low temperature relaxation are shown in Figure 3a,b. As shown in Figure 3, the pure PVDF does not exhibit any relaxation; however, with the addition of PCMO nanoparticle (0.038%), the loss tangent exhibits dielectric relaxation peak. Further increasing the volume fraction of PCMO nanoparticle, the magnitude of the peak gets enhanced with a slight shift of the peak position to the high temperature, which indicates the enhancement of strength of polaronic relaxation with the PCMO volume fraction. The dynamics of this polarons relaxation can be controlled under magnetic field.15,30 In fact, the low temperature tuning of polaronic relaxation has been observed in many of the multiferroic perovskite oxide systems such as Fe-doped BaTiO3 and Pr0.7Ca0.3MnO3.30,31 The variation of loss tangent with and without magnetic field for different volume fraction samples is shown in Figure 3c. For clarity, data are shifted vertically along the Y-axis. As shown in 27731

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Figure 4. (a) Isothermal magnetization at 70 K for different volume fraction samples and (b) variation of MD (%) and magnetization at 4.6 T with respect to the volume fraction; inset shows the tan δ variation with volume fractions. Normalized plots of (c) MD (%) vs M2 and (d) MD (%) vs M4 for all samples.

Figure 5. (a) Comparison magnetic memory effect for pure PCMO, 0.015 and 0.225 volume fractions. FESEM micrographs of 0.015 and 0.225 volume fraction samples, respectively. (b and c) Model diagram for magnetic interaction of ferromagnetic clusters for 0.015 and 0.22 volume fraction samples, respectively.

the volume fraction of PCMO nanoparticles; similar to the dielectric properties (inset to Figure 2a) a finite slope change was noticed near the 0.11 volume fraction sample. Magnetization also follows the similar variation as that of MD. A one to one correspondence of magnetization and MD suggest that the MD is closely related to the magnetization of PCMO nanoparticles. Inset of Figure 4b shows the variation of tan δ with respect to the volume fraction at 5 T magnetic fields. As shown in the figure, tan δ increases with the volume fraction of PCMO nanoparticles and reaches a maximum value of 0.18 for 0.22 volume fraction sample. Interesting remark is that tan δ remains similar even for a 5 T magnetic field, indicating a minimum loss in the composite system.

temperatures, the percolation process due to MW interfacial polarization may not be meaningful as the MW effect is insignificant below 120 K (Supporting Information figure). However, the microstructure for volume fractions at 0.11% and above have PCMO nanoparticles in contact with each other and that enhances the polaron conductivity as a whole, correspondingly, it appears as a small jump in εeff. The M−H loop of the samples with varying PCMO volume fraction is shown in Figure 4a. As expected the magnetization enhances with volume fractions and it exhibits the nonsaturation tendency up to 5 T. Figure 4b shows the isothermal magnetization at 5 T and MD behavior with 100 kHz frequency for different volume fractions at 70 K. The MD increases with 27732

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for the ϕPCMO = 0.015. In fact, for the ϕPCMO = 0.015 sample, interaction is expected to be negligible, but this is restricted by the PCMO clustering while the sample preparation and nanoparticle clusters of 200−400 nm can be easily picked up by highly sensitive ac susceptibility. Though the interparticle interactions are evident with the increase of volume fractions of PCMO, the exact relation to higher-order MD is intriguing. Considering each PCMO cluster as a giant magnet and the interactions among such giant magnetic clusters can give rise to an additional M2 term. On the other hand, interconnection of PCMO particles can also give rise to MR α M2 as an additional contribution.35 However, present composites exhibit very high resistivity (ρ = 107 ohm-cm) at room temperature and beyond the measurable range of the available electrometer (Keithley 5714) at low temperatures, which precludes the MR measurements.

From the Ginzburg−Landau phenomenological theory, the fluctuation of the spin-pair correlations play a crucial role to understand the MD effect in different spin structures (FM/ AFM) and is given as7 ε=

ε0 1 + 2ε0I(T )

I (T ) =

∑ g(q) q

and MqM −q (T ), i.e., MD ∝ M2 (2)

where q denotes the wave vector, and g(q) indicates the coupling between polarization and spin correlations, and ⟨MqM‑q⟩ denotes the thermal average of instantaneous spin− spin correlation. As from eq 2 the observed dielectric properties depend on the g(q) and ⟨MqM‑q⟩; accordingly, in FM materials the spin fluctuations are maximum near the magnetic ordering and hence an MD peak can be expected near TC.7 In fact, a similar kind of relation is valid for birelaxor material; that is in the absence of long-range ordering the magnetostriction and electrostriction between electric and magnetic nano polar regions leads to occurrence of large quadratic magnetoelectric effect (M2).32 In order to validate a relation between the spin− spin correlations on MD with respective volume fraction of PCMO nanoparticles, we have plotted the normalized MD vs M2 at 70 K in Figure 3c for different volume fraction samples. At lower volume fractions, in high magnetic field (above 0.28 T), a linear relation with M2 can be found, and this might be related intrinsic multiferroic behavior of PCMO nanoparticles. In fact, local probe techniques such as TEM and PFM are utilized to probe the local ferroelectric properties of PCMO system.33,34 However, the higher volume samples do not obey M2 relation; instead they exhibit M4 relation as shown in Figure 4d. In order to understand the higher order MD behavior in high loaded PCMO nanoparticles, we have compared the microstructure, magnetic and MD behavior for low volume fraction (ϕPCMO = 0.015) sample and high volume fraction (ϕPCMO = 0.22). The role of magnetic interaction with respect to the volume fraction we have performed memory experiments (in AC susceptibility). For comparison, we have maintained the same experimental protocol as described above and are shown in Figure 5a. In this typical protocol, first the system was cooled down to low temperature and the reference curve (χref ′ ) was recorded while the heating mode was from 5 to 300 K. In the next run during the cooling system was halted below the glass transition temperature for a waiting time of 5000 s. After this aging process, system is cooled to low temperature and the magnetization was recorded during the heating mode and is called as χ′halt. For a nonequilibrium canonical SG and system of interacting magnetic clusters, the system attains the several metastable states during the aging process, and that reflected as the dip in the difference curve of δ′χ = χ′halt − χ′ref.22,26 For comparison, memory dip for PCMO nanoparticles is also shown (same as Figure 5a), and this shows the highest interparticle interactions. The dip in the experiment, scales with the PCMO volume fraction or the strength of interparticle interaction and it is smaller for the ϕPCMO = 0.015 sample compared to that of the ϕPCMO = 0.22 sample. Magnetic memory effect can be nicely correlated with the microstructure of the composites in Figure 5i,ii, where the distribution of PCMO is denser with extensive clustering in ϕPCMO = 0.22 sample; this indicates that the interparticle magnetic interactions are much stronger in ϕPCMO = 0.22 and nominal

4. CONCLUSIONS In summary, temperature dependent dielectric properties with respect to the volume fraction indicate several interesting dynamics of charge carriers across the percolation threshold. A detailed study of volume fraction dependent MD property indicates that active interparticle magnetic interactions play a crucial role on the MD.

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ASSOCIATED CONTENT

* Supporting Information S

Temperature dependent dielectric relaxations are presented for different volume fraction of PCMO in PCMO/PVDF nanocomposites. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(K.D.C.) Telephone: +886 988873428. E-mail: sekhar. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the DST fast track project. The authors also acknowledge the use of a SQUID VSM facility in IITKGP.

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REFERENCES

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dx.doi.org/10.1021/jp508918u | J. Phys. Chem. C 2014, 118, 27728−27734