Magnetic, dielectric and magneto-dielectric behavior of half-doped LaSrCoMnO6

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Magnetic, dielectric and magneto-dielectric behavior of half-doped LaSrCoMnO6 P.R. Mandal, R. Sahoo, T.K. Nath n Department of Physics, Indian Institute of Technology Kharagpur, 721302 West Bengal, India

art ic l e i nf o

Keywords: Multiferroic Double perovskite Magneto-dielectric Magnetization Superexchange interaction Conductivity

a b s t r a c t A half-doped LaSrCoMnO6 (LSCMO) double perovskite synthesized by sol–gel method exhibits the coexistence of two perovskite phases, cubic (Fm-3m) and monoclinic (P21/n) crystal structures with volume ratio of 0.88:0.12. We have investigated the magnetic, dielectric, and magneto-dielectric properties of LSCMO. The magnetization measurement reveals that there exist two magnetic phase transitions at 266 K and 197 K corresponding to order and disorder magnetic phases, respectively. The dielectric and magneto-dielectric relaxations observed at 100 kHz around magnetic transition clearly provide an evidence of the correlation between dielectric and magnetic ordering. The relaxation process across the potential barriers leads to two anomalies before and after the magnetic phase transition temperature. The highest magneto-dielectric response is obtained to be 8.2% in the vicinity of the observed magnetic transition temperature. Modulus spectrum analysis reveals the intrinsic contribution to the magneto-dielectric behavior. & 2014 Elsevier B.V. All rights reserved.

1. Introduction Ferromagnetic insulators are of great research interest due to their rich physics and potential applications, because the ferroelectricity and ferromagnetism leading to magneto-dielectric effect simultaneously take place in these novel materials. In this respect, they are used as multiferroic materials. The ideal double perovskite with general formula AA0 BB0 O6 [A, A0 ¼ rare earth or alkali ions; B, B0 ¼ transition metal ions] not only plays an important role in multiferroic materials for superexchange interaction by making a ferromagnetic insulator, but also has possible technological applications due to colossal magneto-resistance, e.g. ferromagnetic metal Sr2FeMoO6 attains a huge magneto-resistance at high temperature and low magnetic field [1,2]. The magnetic and electronic transport properties can be tuned by varying the A and B site cationic order. As for example, in La2ACuO6 for A ¼Ni, Co, it shows antiferromagnetic behavior and for A ¼Mn, La2ACuO6 behaves like ferromagnetic material [3]. The half doped LaSrVMoO6 is half metallic antiferromagnet. La2NiMnO6 [4] and Sr2CrOsO6 [5] exhibit ferromagnetism as well as insulating properties. Interestingly, in addition La2NiMnO6 also exhibits good magneto-capacitance properties [6]. In La2NiMnO6, the oxygen octahedra around Ni and Mn induce strong superexchange

interaction between Ni2 þ (t62g e2g ) and Mn4 þ (t32g e0g ) following the insulating behavior and magneto-dielectric effect [7]. La2CoMnO6 shows well ordered monoclinic symmetry with good ferromagnetic and insulating behaviors [8,9]. Using first principle density functional calculations, it is stated that the insulating state in La2CoMnO6 is driven by the Coulomb-assisted spin-orbit coupling operative within the Co-d manifold [10]. La2CoMnO6 (LCMO) shows two ferromagnetic transitions at TC1  225 K and TC2  150 K [8]. Blasse et al. showed the presence of ferromagnetic Co2 þ –Mn4 þ superexchange interaction [11] in LCMO. Joy et al. presented the high-spin Mn3 þ and low-spin Co3 þ in rhombohedral phase and high spin Co2 þ and Mn4 þ in orthorhombic phase in La2CoMnO6 [12]. By performing X-ray absorption near-edge spectroscopy (XANES) experiments Joly et al. also suggested that the order phase involves Co2 þ and Mn4 þ ions and disorder state corresponds to Co3 þ and Mn3 þ ions in LaMn0.5Co0.5O3 [13]. In LCMO thin film Guo et al. suggested the existence of high spin state of Co2 þ and Mn4 þ in order state [14]. With 50% Sr doping at La site of La2CoMnO6 it is reported to be cubic phase with the space group of Fm-3m [15]. In the present work we have reported in details the structural, magnetic and electronic-transport properties of half-doped LaSrCoMnO6 double perovskite system.

2. Experimental details n

Corresponding author. E-mail address: [email protected] (T.K. Nath).

Polycrystalline sample of LaSrCoMnO6 (LSCMO) has been synthesized by chemical sol–gel technique [16]. The starting materials

http://dx.doi.org/10.1016/j.physb.2014.04.058 0921-4526/& 2014 Elsevier B.V. All rights reserved.

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are La2O3, Sr2(NO3)2, Co(NO3)2  6H2O, and Mn(CH3COOH)2. At first La2O3 is converted to lanthanum nitrate by dissolving it into appropriate amount of nitric acid. Stoichiometric amount of Sr2(NO3)2, Co(NO3)2  6H2O, and Mn(CH3COOH)2 are dissolved in distilled water to make a clear solution and then is mixed with lanthanum nitrate solution. The solution is then heated at 180 1C and stirred continuously. The finally obtained black floppy powder is grounded and sintered at 1000 1C in air for 4 h to produce the polycrystalline manganites. The as sintered powder has been pelletized (10 mm in diameter) and finally sintered at 1150 1C for 5 h. The crystalline phases of the annealed samples are identified by the high resolution X-ray diffraction technique (HRXRD, Pananalytic) using Cu Kα radiation (λ¼0.1542 nm) at room temperature. The structural characterization is also carried out by high resolution transmission electron microscopy (HRTEM) from JEOL Ltd., Japan. The static magnetic measurements are performed by Quantum design SQUID-VSM magnetometer. Resistivity is measured using a standard two probe technique. The dielectric constant and magneto-dielectric response have been measured with the help of a LCR meter (HIOKI Japan, 3532-50) employing a cryogen free closed cycle helium refrigeration variable temperature cryostat fitted in a superconducting magnet with a maximum magnetic field of 8 T (Cryogenics Ltd. U.K., 8TCFMVTI). To form a parallel capacitance geometry high quality of silver paste has been applied to both the faces of the pellet (diameter of 1 cm and thickness of 1 mm).

3. Results and discussions The room temperature HRXRD pattern of LSCMO is shown in Fig. 1. The HRXRD data reveals the single and well crystalline phase of LSCMO without any detectable impurity phases. X-ray diffraction pattern are refined by Rietveld analysis with the aid of the MAUD program. The XRD data discloses the coexistence of two perovskite phases, cubic (Fm-3m) and monoclinic (P21/n) crystal structures. The volume fraction ratio of cubic to monoclinic phases is 0.88:0.12. The inset of Fig. 1 shows the HRTEM image of LSCMO, where the spherical particle size is around 0.4–0.8 μm. Fig. 2(a) shows the temperature dependent dc magnetization under zero field cooled (ZFC) and field cooled (FC) conditions in

Fig. 2. (a) The field cooled and zero field cooled magnetization at an applied magnetic field of 100 Oe. The upper inset shows 1/χ vs. T plot and the straight line (red line) is the Curie–Weiss fit in the high temperature regime. (b) The M–H curves at different temperatures of 100 K, 200 K, and 300 K. The upper inset shows dM/dT curve indicating two magnetic transition temperatures. (c) The temperature variation of electrical resistivity behavior. Inset shows the VRH model fitting at paramagnetic region (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Fig. 1. (a) XRD patterns of LSCMO at room temperature and corresponding Rietveld refinement. Inset shows HRTEM micrograph of LSCMO.

the temperature range of 5–380 K with the applied magnetic field of 0.01 T. The thermo-magnetic curve shows two magnetic transitions which are consistent with previous reports [17]. The Curie temperatures have been determined by taking into account the first derivative of the temperature dependent magnetization (dM/dT) as shown in the inset of Fig. 2(b). The higher magnetic transition temperature is named as TC1 and the lower magnetic transition temperature is mentioned as TC2. The first magnetic transition (paramagnetic to ferromagnetic phase transition) is observed to be at TC1  266 K and it is followed by a second phase transition at TC2  197 K. TC1 is attributed to atomically ordered Co2 þ –O–Mn4 þ superexchange interaction. TC2 is assigned to the vibronic superexchange interaction of intermediate-spin Co3 þ –O– Mn3 þ (high spin) [17]. The vibronic superexchange interaction is

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less stable than the high temperature ferromagnetic superexchange interaction. In La2CoMnO6 system the ferromagnetic e2–O–e0 interaction arises due to the ordering of Mn4 þ and Co2 þ ions [18,19]. On the other hand Mn4 þ –O–Mn4 þ or Co2 þ – O–Co2 þ interaction may cause the antiferromagnetism created by point disorder. The substitution of Sr2 þ ions in place of La3 þ ions in half doped LSCMO will increase the Co3 þ concentration and introduce ferromagnetic ordering by double exchange (DE) interactions of Co2 þ –Co3 þ ions. The Curie–Weiss relation [χ ¼ C=T  θcw , where Curie constant C ¼ Np2ef f =2kB (with peff as effective paramagnetic moment, N as Avogadro's number, and kB as Boltzmann constant) and θcw is the paramagnetic Curie temperature] has been used to fit in the paramagnetic region in the temperature range of 270–353 K to investigate the magnetic ordering of LSCMO system. The upper inset of Fig. 2(a) shows 1/χ vs. temperature plot for LSCMO. The paramagnetic Curie temperature is obtained to be  27 K. The negative value of paramagnetic Curie temperature is an evident of dominant antiferromagnetic ordering. Fig. 2(b) shows the M–H hysteresis loops at the temperatures of 100, 200, and 300 K. At the temperature higher than TC (T 4278 K), the M–H behavior is linear which corresponds to a paramagnetic state. The temperature dependent dc resistivity measured up to 90 K as shown in Fig. 2(c) reveals the semiconductor like nature of LSCMO. It is well known that the hole doping and A-site disorder in manganites (ABO3) influence the magneto-transport properties. The combined effect of double exchange mechanism and strong electron–phonon coupling due to Jahn–Teller effect dominates the conduction mechanism in mixed-valence perovskite manganites [20,21]. In La2CoMnO6 the Jahn–Teller effect is significant on Co ions because of the unfilled t2g sub-shell of Co ions rather than the closed filled sub-shell of Mn ions. But substitution of La3 þ by the divalent cation Sr2 þ introduces a mixture of Mn3 þ and Mn4 þ ions in LSCMO. The Jahn–Teller distortion due to the localization of eg

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electrons in Mn3 þ ions leads to the formation of small polarons. The small polaron mechanism in paramagnetic region influences the electrical transport properties of LSCMO [22]. Mott pointed out that at low temperature the conductivity mechanism well sets with the variable range hopping (VRH) model. The VRH model can be described by ρVRH ¼ ρ0 expðT 0 =TÞ1=4 ; where ρ0 is the constant, T0 is the characteristic temperature with the relation T0 ¼ 24/(πkBN(EF)ξ3), where N(EF) is the density of localized state at Fermi level, kB is the Boltzmann constant, ξ is the decay length of the localized wave function. For LSCMO the VRH model fits well above 200 K. The T0 value obtained from the fit is 0.02  108 K and it is close to the reported value for LCMO [23]. The observed hopping energy (W¼ 0.25kBT0.25 T0.75) is 46 meV at 200 K 0 as shown in the inset of Fig. 2(c). Androulakis et al. also demonstrated that the dc resistivity of LSCMO follows the variable range hopping conductivity mechanism above 190 K [15]. The temperature dependent real (ε0 ) and imaginary (ε″) part of dielectric permittivity measured at the frequency range of 1–500 kHz are shown in Fig. 3(a) and (b), respectively. At low temperature below 60 K both the real and imaginary part of dielectric constant become frequency and temperature independent and designate the intrinsic dielectric properties of LSCMO. With increasing temperature a sharp rise of ε0 (as shown in Fig. 3(c)) corresponds to the observed relaxation peaks in ε″ as shown in Fig. 3(b). Above the temperature of 160 K ε0 reaches a plateau and at 100 kHz frequency possesses a colossal dielectric constant of 3420 which is attributed to the extrinsic effect on polarization due to Maxwell–Wagner interfacial polarization effect. With increasing frequency the shifting of the inflection point of temperature to higher temperature is attributed to the reversal polar region, which is responsible for high dielectric constant of LSCMO. The reversal of polar region, equivalent to

Fig. 3. (a) Temperature variation of real part of dielectric constant (ε0 ) in the frequency range of 1–500 kHz. (b) Temperature variation of imaginary part of dielectric constant (ε″) in the frequency range of 1–500 kHz. (c) Frequency variation of ε0 at different temperatures. Inset shows the ln τ vs. 1/T plot and solid line is the fit to the Arrhenius law. (d) Temperature variation of ε″ and tan δ at 100 kHz frequency.

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the hopping of electrons between Co2 þ and Mn4 þ ions, cannot follow the applied external electric field at higher frequencies leading to relaxor-like behavior [23]. The relaxor-like behavior instead of relaxor dielectrics can also be evidenced from the peak mismatch of temperature dependant tan δ and ε″. Fig. 3(d) shows the temperature dependant tan δ and ε″ at 100 kHz. The peak position in tan δ vs. temperature curve shifts by a large temperature separation to the high temperature region compared to ε″ vs. temperature curve. These representations facilitate to analyze the apparent polarization by inspection of the magnitude of mismatch between the peaks of the two parameters. The distinction in peak position rules out the existence of permanent dipole moment which certifies the absence of relaxor polarization in LSCMO. The Arrhenius law is adopted to analyze the thermally activated relaxation mechanism of LSCMO fitting the relaxation time τ vs. temperature T curve by τ ¼ τ0 expð 

Ea Þ kB T

where τ0 and Ea are the pre-exponential factor and activation energy required for relaxation process of LSCMO. From the Arrhenius plot as shown in the inset of Fig. 3(c) it is obvious that the fit does not follow a single activation mechanism. The obtained activation energy and relaxation times of LSCMO for the two temperature regions are 73 meV, 1.6  10  9 s (below 150 K) and 99 meV, 0.75  10  9 s (above 150 K), respectively. The obtained activation energy is much lower than the activation energy for ion jumping (0.2–1 eV). Thus the conductivity due to ion jumping is ruled out. The magnitude of the activation energy supports the polaronic charge carrier dependant relaxation mechanism [24,25]. From Fig. 3(d), the increase of dielectric loss at higher temperature is ascribed to the contribution of dc conductivity. The crossover of the activation energy takes place near TC1. The ac conductivity plays an important role to understand the origin of transport properties in the perovskite systems. Fig. 4(a) shows the frequency dependent ac conductivity at different temperatures. The conductivity plots of LSCMO shows the following characteristics: i) below 150 K, a step like behavior is observed at low frequency and with increasing frequency after the step a plateau is noticed, ii) the inflection point fc (at the temperature where the conductivity starts to increase sharply) shifts to higher frequency with increasing temperature, iii) above 150 K, the step like behavior vanishes and nearly frequency independent flat curves at low frequency region and frequency dependant conductivity at high frequency region are observed. In conductivity–frequency spectrum the increase of ac conductivity is attributed to the hopping process of charge carriers from one site to another site. The microscopic hopping process in most materials due to localized states can be described by a power law represented as “universal dielectric response” (UDR)

proposed by Jonscher

sac ¼ sdc þ A0 f n

ð1Þ

where sac is the total conductivity. sdc is dc conductivity observed at low frequency due to inability of the electric field to perturb the electron hopping at low frequency. The coefficient A0 and frequency independent exponent parameter n (0 on o1) reflect the intrinsic properties of the materials [26]. Fig. 4(b) shows the combined plot of M″ (M″ ¼ωC0Zʹ, where C0 ¼ ε0A/d, ε0 is the free space permittivity, A is the sample area and d is the sample thickness) and Z″ vs. frequency of LSCMO. M″ and Z″ vs. frequency curves show two distinct peaks at low and high frequency regions indicating two different thermal activation processes. The relaxation peak corresponding to high resistance and small capacitance at low frequency region is attributed to the grain boundary contribution because the smaller capacitance is dominating in modulus formalism. On the other hand the low resistance and high capacitance are assigned to the bulk contribution. The broad grain boundary and bulk relaxation peaks are correlated with a broad distribution of relaxation times. Both M″ and Z″ peak frequencies are marked by the small arrow as shown in Fig. 4(a). Interestingly, M″ and Z″ peak frequencies coincide with the crossover frequencies fc (marked arrow in Fig. 4(a) at 80 K) from low frequency dc conductivity plateau to dispersive conductivity. fc shifts to higher frequency with increase in temperature. So the three regions in conductivity vs. frequency plot are attributed to the dc conductivity at low regions, grain boundary conductivity at mid-frequency region and grain conductivity at high frequency region, which can also be confirmed from Fig. 4(b). The inset of Fig. 4(b) shows that at 200 K the modulus peak and Z″ peak coincide to each other. Fig. 5(a) shows the temperature dependant ε0 with (6 T) and without magnetic field measured at 100 kHz frequency. The high frequency has been taken to avoid the polarization due to external effect. The magneto-dielectric (MD) values at low and high temperature are nearly zero, whereas these values increase with increase in temperature from 70 K and attain maximum value of 8.2% near the dielectric step and magnetic transition temperature. The appearance of MD peak near magnetization temperature probably is an evidence of correlation between dielectric and magnetic ordering. Fig. 5(b) shows the isothermal frequency dependant MD effect at different magnetic fields. The nonshifting MD peak with frequency at different applied magnetic field rules out the existence of field induced dipolar relaxation process. The measured magneto-dielectic effect has both intrinsic and extrinsic origin. Maxwell–Wagner effect, polarization due to material–electrode interface, and magnetoresistance can give rise to the extrinsic origin in MD effect. The magnetodielectric effect is also investigated by varying the frequency and magnetic field. Fig. 5(b) shows the frequency

Fig. 4. (a) ac conductivity vs. frequency at different temperatures. (b) Z″ and M″ vs. frequency at 80 K. Inset shows Z″ and M″ vs. frequency at 200 K.

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plotted as shown in Fig. 5(b) at the same temperature. The MD peak undergoes the same frequency region of M″-peak which is associated with the bulk relaxation rather than grain boundary contribution.

4. Conclusions In summary, we have prepared successfully the single phase polycrystalline LSCMO with particle size of 0.4–0.8 μm. The structural, magnetic, dielectric, and magneto-dielectric properties have been studied extensively. The XRD data reveals that sol–gel synthesized LSCMO is well crystallized with the coexistence of two perovskite phases, cubic (Fm-3m) and monoclinic (P21/n) crystal structures without having any impurity phases. Two magnetic transitions have been observed corresponding to order and disorder magnetic ordering. The observed high dielectric constant value at high temperature is attributed to the Maxwell– Wagner type interfacial polarization. A reasonably large magnetodielectric response of  8% is observed in the vicinity to the magnetic transition temperature. ac conductivity measurements show two types of relaxation phenomena before and after 150 K. Finally, the modulus spectrum clearly reveals the intrinsic contribution to magneto-dielectric behavior in this LSCMO double perovskite system. References

Fig. 5. (a) The temperature dependent of ε0 at 0 and 6 T and the MD% for 100 kHz frequency. (b) MD% and M″ vs. frequency at 100 K. (c) MD% at 100 K measured at different applied magnetic fields.

dependent of MD% at 6 T magnetic fields. A nearly frequency independent MD% is observed at low frequency below 10 kHz. The frequency dependency of MD response starts above 10 kHz and is maximum nearly at the frequency of 100 kHz. The frequency dependent MD response is a signature of relaxation at 100 kHz frequency. To realize the origin of the magneto-dielectric relaxation the frequency dependent imaginary part of modulus (M″) is

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