Structural, magnetic and dielectric properties of rare earth based double perovskites RE2NiMnO6 (RE=La, pr, Sm, Tb)

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Structural, magnetic and dielectric properties of rare earth based double perovskites RE2NiMnO6 (RE ¼ La, pr, Sm, Tb) P. Neenu Lekshmi a, M. Vasundhara a, Manoj Raama Varma a,n, K.G. Suresh b, M. Valant c a

Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology, CSIR, Trivandrum 695019, India Department of Physics, Indian Institute of Technology Bombay, 400076 Mumbai, India c Materials Research Laboratory, University of Nova Gorica 5000, Slovenia b

art ic l e i nf o

Keywords: Double perovskites Ferromagnetic Dielectric-relaxor Spintronics

a b s t r a c t The structural, temperature variation of magnetic and dielectric properties of RE2NiMnO6 (RE¼ La, Pr, Sm, and Tb) double perovskites have been investigated. RE2NiMnO6 have crystallized into pure monoclinic bulk phase with space group P21/n, shows ferromagnetism and relaxor dielectric behavior in intermediate temperatures. Further, magnetic and dielectric properties show a correlation with the radius of RE ions and thereby with the average Ni–O–Mn bond angle in the monoclinic double perovskite structure. These investigations suggest the occurrence of a spin-lattice coupling in RE2NiMnO6 which makes them attractive materials in the field of spintronics. & 2014 Elsevier B.V. All rights reserved.

1. Introduction Perovskites have been studied extensively not only due to the versatile nature of their structure but more importantly due to their multifunctional properties. Perovskites exhibit magnetic (e.g., La1 xM2 þ xMnO3), dielectric (BaTiO3), high temperature superconducting (BaPb1  xBixO3), piezoelectric (PbZr1 xTixO3), and ferroelectric properties (KNbO3) [1–4]. Double perovskite oxides, A2B0 B″ O6, (A- alkaline earth metals such as Ca, Ba, Sr, etc., or rare earth ions of larger ionic radii, B0 and B″- transition metal cations or lanthanides with smaller ionic radii) have received paramount interest since they provide unique opportunities to induce and control the multiferroic behavior in oxide systems. They display interesting properties such as, metallic/half-metallic ferromagnetic and magnetoresistive properties as observed in Sr2FeMoO6 (halfmetallic ferromagnet) [5], Sr2FeReO6 (half-metallic ferrimagnet) [6] and Sr2FeWO6 (antiferromagnetic insulator) [7], together with a number of interesting phenomena, including room temperature magnetoresistance, magneto-capacitance and magnetostriction. In recent years, many studies on perovskite ceramics have been carried out with the objective of finding materials which possess coupled magnetic and electric properties for application in magnetoelectronics as non-volatile memories. The presence of mutually coupled magnetic and dielectric properties, the so-called magnetodielectric (MD) or magneto-capacitance (MC) effect, are the current n Correspondence to: Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology [NIIST], CSIR, Trivandrum 695 019, Kerala, India. Tel.: þ 91 471 2515377; fax: þ 91 471 2491712. E-mail address: [email protected] (M.R. Varma).

focuses of interest since they provide possibilities for developing materials having vast scientific and novel technological impact [8,9]. Multiferroics or magnetoelectrics have recently received considerable interest for both technological and fundamental importance. The existence of ferromagnetism and ferroelectricity simultaneously in a single phase with potential spin-phonon and spin-polar couplings, offer attractive opportunity to design various unconventional devices, such as multiple-state memory elements, electric-field controlled magnetic sensors etc. The coexistence of various electronic order parameters and their simultaneous interactions in a single phase material also hold new challenges in the field of basic research and in technology as well [9,10]. It has been reported that certain double perovskites show interesting spin glass behavior depending on the type, charge and size of the cation present in B-site. Spin glasses are magnetic systems which exhibit a freezing transition to a state of new kind of order in which the spins are aligned in random directions. Re-entrant spin glass (RSG) transition is a well-known phenomenon of spin glasses emerging from either long-range ferromagnetic (FM) or anti-ferromagnetic (AFM) ordering above the spin glass transition temperature. [11–16] The Rare Earth (RE) based double perovskite La2NiMnO6, is a single material platform with multiple functions and is expected to have unprecedented device applications [17]. These complex physical properties arise from competing interactions between magnetic (spins), structural (phonons) and polarization (charges) order parameters. La2NiMnO6 is a ferromagnetic insulator with a positive superexchange interaction between Ni and Mn cations [18–22]. Such behavior is significant in spintronic materials as it helps in achieving MD properties. It has been observed that properties of La2NiMnO6 will generally change noticeably with the substitution of La ion by other RE

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elements with smaller ionic radius. Moreover, the entire series of these oxides (RE2NiMnO6) have been reported to be ferromagnetic with a decrease in their Curie temperatures, TC, or magnetic exchange interactions, JNi–Mn, due to the larger deviation of Ni–O–Mn bond angle, from 1801 with the substitution of other RE elements. RE2NiMnO6 exhibit several interesting properties, such as large magneto-capacitance, magneto-resistance and relaxor ferroelectricity, however, research works on these materials are relatively sparse as compared to La2NiMnO6 and have drawn much attention recently [23–26]. Therefore, in the present investigation, we present a soft chemical method for the synthesis of RE2NiMnO6 (RE¼La, Pr, Sm and Tb) double perovskites and report their structural, magnetic and dielectric characterisations.

group. The lattice parameters and volume obtained from refinement using monoclinic P21/n space group are found similar to the reported values of monoclinic RE2NiMnO6 double perovskites [24,26]. After the complete refinement, we achieved a good agreement between the observed and calculated XRD patterns at 300 K, for La2NiMnO6 and Pr2NiMnO6 as shown in Fig. 1(a) and (b), respectively as a representative of the series. Table 1 summarizes the lattice parameter, unit cell volume, bond angle and reliability factors after the Rietveld refinement for RE2NiMnO6. It is observed that there is a systematic decrease in lattice parameter, unit cell volume and bond angles of RE2NiMnO6 with decrease in ionic radii of RE ion.

3.2. Magnetic characterization 2. Materials and methods Single-phase polycrystalline RE2NiMnO6 samples were prepared from a soft chemistry procedure [27]. Powder X-ray diffraction pattern obtained with a PANalytical X’pert Pro diffractometer in Bragg–Brentano geometry with Cu-Kα radiation in 2θ range of 101– 901 with a step size of 0.0171. Rietveld refinement of the diffraction pattern was carried out using the GSAS software. Magnetic characterisations were performed in a SQUID VSM magnetometer (Quantum Design). Dielectric measurements have been performed with LCR meter (Agilent E4980A) that has been compensated for the coaxial cable length and open and short circuit calibrated.

3. Results and discussion 3.1. Structural characterization The Rietveld refinement of X-ray diffraction patterns of the polycrystalline RE2NiMnO6 (RE ¼La, Pr, Sm and Tb)] confirmed the presence of a single phase monoclinic structure with P21/n space

The thermo-magnetization curves, M (T), under zero fieldcooled (ZFC) and field-cooled (FC) cycles for all the compounds of RE2NiMnO6 have been studied in the temperature range from 10 to 300 K and are shown in Fig. 2 along with their derivatives in the insets. All samples have been found to undergo magnetic transition at higher temperature ( 4100 K). Further these magnetic transitions are confirmed as paramagnetic (PM) to ferromagnetic (FM) transition, TC, from the Curie–Weiss law fit, χ ¼ C=ðTΘÞ, to the M (T) curves (Fig. 2). It is obvious that the fitting lines cut the positive x-axis, indicating the dominant role of the ferromagnetic interactions in the compounds. The 1/χ (T) curve of Tb2NiMnO6 shows a peculiar behavior which could be due to the presence of Griffiths phase (GP), [28] which needs detailed investigation. The effective paramagnetic moment meff ¼2.828 √C, where C is the Curie constant, is obtained from the least-squares fit of the data in the linear region above TC. The fitted values of C and Θ along with μeff are listed in Table 2. In case of La2NiMnO6, double transitions are observed in the M (T) curve under 500 Oe, the derivative of ZFC clearly measures double transitions at  160 K and  240 K, respectively. As per previous

Fig. 1. Observed, calculated and difference XRD patterns corresponding to the Rietveld refinement of RE2NiMnO6 at room temperature. Inset: Crystallographic structure. (a) La2NiMnO6 and (b) Pr2NiMnO6.

Table 1 Lattice parameter, unit cell volume, bond angle and reliability factors after the Rietveld refinement for RE2NiMnO6. RE

a (Å)

b (Å)

c (Å)

β (deg)

Unit cell volume (Å3)

o Ni–O–Mn 4 (deg)

Rwp (%)

La Pr Sm Tb

5.4802(1) 5.4455(1) 5.3459(1) 5.2719(1)

5.5224(1) 5.4692(1) 5.5073(1) 5.5471(1)

7.7632(1) 7.6964(2) 7.6032(1) 7.5275(1)

89.94(1) 90.01(1) 90.06(1) 89.79(1)

234.95(1) 229.22(1) 223.84(2) 220.12(1)

164.3 155.6 152.6 149.5

7.9 4.6 4.9 3.3

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Fig. 2. Temperature dependence of field-cooled (FC) and zero-field-cooled (ZFC) dc magnetization of RE2NiMnO6. Insets: dM/dT versus T. Table 2 Parameters obtained from Curie–Weiss fit and calculated μeff for RE2NiMnO6. RE

C (emu K mol  1 Oe  1)

La Pr Sm Tb

3.44 70.002 5.2470.001 4.15 70.02 9.1116 70.01

Θ (K)

μeff (μB/F.U.)

248.74 70.65 222.95 70.03 172.5 71.06 108.95 71.35

5.25 70.002 6.48 70.001 5.76 70.02 8.53 70.01

reports, highly ordered La2NiMnO6 shows a ferromagnetic transition at about 270 K and there are no indications of a subsequent magnetic transition at lower temperatures [17]. The magnetic interactions in La2NiMnO6 are usually governed by 1801 superexchange process arising from Ni–O–Mn bond. The electronic configurations of Ni and Mn play a crucial role in determining the strength of super-exchange coupling and the different possible oxidation states includes: Ni2 þ (d8: t62g eg2), Mn4 þ (d3: t32geg0), low-spin Ni3 þ (d7: t62g eg1) and high spin Mn3 þ (d4: t32geg1). In an ordered La2NiMnO6 phase due to the strong superexchange interaction in Ni2 þ –O–Mn4 þ bonds, a large value of TC is usually observed. Moreover, various past studies on La2NiMnO6 [10,21] have clearly shown that the ordered phase includes only Ni2 þ and Mn4 þ oxidation states, while the disordered phase is due to Ni3 þ and Mn3 þ ions. Thus in the disordered phase, a low value of TC ( 150 K) is observed due to the superexchange interaction between Ni3 þ –O–Mn3 þ bonds. However, the exact nature of the two magnetic transitions in La2NiMnO6 is still unclear. Some reports attributed the two magnetic transitions to the presence of two phases while some others establish these two magnetic features as intrinsic parts of a homogeneous system arising due to antisite defects [10,21,22]. In order to understand such controversial system detailed structural and magnetic characterisations are essential. But in the present study, due to such experimental limitations, we summarise that La2NiMnO6 is undergoing a second magnetic

transition, at 160 K (TC2), below TC1 (240 K). However, Pr2NiMnO6 clearly shows a single magnetic transition indicating the presence of a much ordered Ni/Mn ions. The M (T) curves for Sm2NiMnO6 and Tb2NiMnO6 (measured under 100 Oe fields) show a ferromagnetic transition at higher temperatures around 160 K and 110 K, respectively, followed by a low temperature re-entrant magnetic transitions at around 20 K and 12 K, respectively. One plausible cause of the observed low temperature peak in the compound is due to the presence of an inherent coupling of Ni–Mn network with RE3 þ ion [24,26], which induces competing magnetic exchange interactions and results in a re-entrant behavior, the same in the case of Sm2NiMnO6 has been proved and published recently [27]. 3.3. Dielectric behavior The temperature dependence of the real part of dielectric permittivity, ε0 (Τ), at different frequencies and temperature variations of dielectric loss and tan δ (T) at different frequencies for all the compounds of RE2NiMnO6 have been investigated. As a representative of the series ε0 (Τ) and tan δ (T) plots for La2NiMnO6 and Pr2NiMnO6 are shown in Fig. 3. A relaxor like dielectric behavior is clear from the frequency dependent peak in ε0 (Τ) and tan δ (T). Tmax is the temperature at which tan δ attains its maximum and is found to be frequency-dependent and shifts to higher temperatures with increase in the measuring frequency. The Tmax for tan δ at representative frequencies for RE2NiMnO6 are shown in Table 3. In the present results, instead of a single relaxation, double relaxations are observed in ε0 (Τ) plots. The experimental data can be very well explained on the basis of simple Koops-like model, which assumes that RE2NiMnO6 samples act as a multilayer capacitor [29]. Considering two R||C circuits in series; subscript 1: grain and 2: grain boundary. With electrode surface area S and layer thickness d1, R1 ¼ ρ1 d1/S; R2 analogously; as usual C1 ¼ Cgeoε1d/d1. Here d is the sample

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Fig. 3. Real part of the dielectric permittivity (ε0 ) of RE2NiMnO6 as a function of temperature at several frequencies. Inset: tan δ as a function of temperature at several frequencies (a) La2NiMnO6 and (b) Pr2NiMnO6.

Table 3 Tmax of tan δ (T) for different frequencies and the obtained activation energy (Ea1) for RE2NiMnO6. RE2NiMnO6

Frequency (kHz)

Tmax from tan δ (T) (K)

Ea1 (eV)

La2NiMnO6

1 5 10 50 100 1 5 10 50 100 1 5 10 50 100 1 5 10 50 100

77 86 106 114 131 158 177 189 215 233 176 203 217 251 270 206 231 246 280 296

0.10

Pr2NiMnO6

Sm2NiMnO6

Tb2NiMnO6

0.17

0.20

0.24

thickness and Cgeo, the ‘geometric capacitance’ (or ‘empty cell capacitance’), given by Cgeo ¼ ε0 S/d. ε1 is the relative permittivity of layer 1. Assuming that an activation process governs the conductivity and thus we have: ρ1 ¼ ρ1inf expðEa1 =kB TÞ½R1 ¼ R1inf expðEa1 =kB TÞ. Here Ea1 is the activation energy; ρ2 and so analogously. According to this model, low frequencies conductivity is due to the grain boundaries, while the higher frequencies dispersion is due to the conducting grains. At low temperatures the resistivities tend to infinity, the permittivities of the layers are independent of temperature and frequency showing a rather low, constant value. During warming up from very low temperature to relatively higher ones, the resistivities decrease. First the grains attain the ‘conduction relaxation’ state while the grain boundary resistance is still very large. The sample circuit is now the grain circuit R1 jjC 1 in series with the capacitance C2. Hence, Z ¼ f1 þ jug=fjC 2 ωð1 þ jωR1 C 1 Þg, with u ¼ ωR1 ðC 1 þ C 2 Þ. Further, n


ε¼

C2 C geo



ð1 þ jωR1 C 1 Þ

f1 þ jug

o

 ¼

 C 2 f1 þ x2  jωR1 C 2 g C geo f1 þ u2 g

with x2 ¼ ω2 R1 2 C 1 ðC 1 þ C 2 Þ Hence, tan δ ¼ 

(

)

ε″ ωR1 C2 C2 ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ ε0 ð1 þ x2 Þ ðC1 ðC1 þ C2 ÞÞ

 x : ð1 þ x2 Þ

So we find that tan p δ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exhibits ffi a relaxation like peak with peak height of about ð1=2Þ ðC 2 =C 1 Þ, obviously independent of the measuring frequency. In this ‘apparent’ relaxation phenomenon the parameters are a mixture of the parameters of the individual layers. Dielectric relaxation arising from the grains is due to the electron hopping between Ni /Mn ions. At still higher temperatures the grain boundaries attain the ‘conduction relaxation’ state too. This may result in a ‘colossal dielectric constant’ effect. Since for a higher frequency a shorter relaxation time is necessary, the concurrent increase of ε‘ will occur at a higher temperature. However, at still higher temperatures the colossal dielectric constant is governed by the ratio ρ2/ρ1. This ratio is decreasing with increasing temperature in case the activation energy Ea2 is larger than Ea1. But the colossal dielectric constant effect depends also on the ratio of the resistances R1 and R2 which may lead to an opposite trend. The increasing dc contribution causes an upturn of tan δ with increasing temperature. This may be interpreted as a shift to higher temperatures, as expected for an activated process: the ‘apparent’ activation is again some combination of the actual ones Ea1 and Ea2. Further study is necessary to see how the high-temperature part can be described better. The activation energies (Ea1) associated with the lower relaxation of tan δ (T) for RE2NiMnO6 are calculated based on the Arrhenius law, τ ¼ τ0 eðEa1 =kB T max Þ . The obtained values are given in Table 3. 3.4. Structural correlation of properties X-ray diffractograms of RE2NiMnO6 showed that the crystal structures of the compounds were found to be in monoclinic structure with space group P21/n at room temperature and the detailed structural analysis by Rietveld refinement indicates a systematic variation in lattice parameter. The variation of lattice parameter, unit cell volume and Ni–O–Mn bond angle with change in ionic radii of RE ion is shown in Fig. 4(a). The thermo-magnetic analysis of ZFC and FC indicates presence of double magnetic transitions in La2NiMnO6. The transition at 240 K is confirmed as FM–PM transition from the CW fit and detailed analysis is required

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a soft chemical method. Detailed structural analysis by Rietveld refinement confirms the monoclinic structure of RE2NiMnO6 with P21/n space group at room temperature and shows a variation in lattice parameters and Ni–O–Mn bond angle with the radius of RE ions. The thermo-magnetic analysis and dielectric characterization shows that the magnetic and dielectric properties of RE2NiMnO6 are of structural origin which inturn indicates a possibility for spin-lattice coupling. From these observations we can conclude that RE2NiMnO6 double perovskites show a signature of magnetodielectric coupling which makes them of prime importance as attractive spintronic materials.

Acknowledgments The authors thank Prof. Peter E. Brommer, Editor, Physica B for the valuable discussions on dielectric relaxation. P. Neenu Lekshmi is thankful to Council of Scientific and Industrial Research (CSIR), India for granting the Senior Research Fellowship. Dr. Manoj Raama Varma and Dr. M Vasundhara acknowledges CSIR, India granted Project SURE (No. CSC0132) and Govt. Of India DST project (No. SR/S2/CMP-0012/2009) on double perovskites for the financial support.

References

Fig. 4. (a) Variation of the unit-cell parameter, cell volume and bond angle with RE ionic radii and (b) Variation of Tmax of tan δ (T) and Ea1 with RE ionic radii. Inset: Variation of magnetic, TC with RE ionic radii.

for the transition at 160 K. In addition, Sm2NiMnO6 and Tb2NiMnO6 show a low temperature, spin glass transition, Tf due to coupling of Ni–Mn network with RE3 þ ion. Interestingly, there is a systematic decrease in values of TC with ionic radii of RE ion as shown in the inset of Fig. 4(b) which corroborates the structural analysis. The increase in Tmax and Ea1 with decreasing rare earth ion radius (REradii) clearly indicates that the electron hopping between Ni and Mn is more suppressed with decreasing REradii [24,26]. The systematic variation of Tmax and Ea1 with the RE ionic radii are shown in Fig. 4 (b). Thus it is confirmed that magnetic property and dielectric behavior of RE2NiMnO6 have structural origin. Ni–O–Mn bond angle gradually decreases with decrease in ionic radii of RE ion, as a result the electron hopping between Ni and Mn is more suppressed which in turn reduces the magnetic TC and Tmax with increase in Ea1 for dielectric relaxation. This tendency implies some coupling between the magnetic and dielectric properties of RE2NiMnO6 which indicates a spin-lattice coupling (connection between spin orientations on the two magnetic sublattices, Ni and Mn) which is of a capital importance in exploring spintronic materials. 4. Conclusions Single-phase polycrystalline powder samples of double perovskite oxide RE2NiMnO6 (RE ¼La, Pr, Sm, Tb) were synthesized by

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