Evidence of spin-phonon coupling in La₂NiMnO₆ double perovskite

Abstract

Herein, a correlation between B-site cation order and spin-phonon coupling in La₂NiMnO₆ double perovskite has been investigated. Raman spectra of La₂NiMnO₆ double perovskite annealed at 950 and 1400℃ have been measured in the 140-598 K range. A substantial softening of the phonon modes has been observed below the Curie temperature, which emphasized the presence of the spin-phonon coupling in the system. The spin-phonon coupling was found to be stronger in relatively more ordered La₂NiMnO₆ double perovskite. Thus, the magnitude of spin-phonon coupling was influenced by the Ni/Mn cation order.

1. Introduction

The La2NiMnO6 is a multifunctional material with different coexisting properties, such as ferromagnetic ordering [1], magnetoresistance [2], and magnetodielectric coupling [2,3]. Hence, it has a great prospect for spintronic applications. The rocksalt type ordering for B-site cations (Ni, Mn) was found in La₂NiMnO₆ double perovskite [1,4]. La₂NiMnO₆ double perovskite has a monoclinic P2₁/n structure with an alternate arrangement of corner shared NiO₆ and MnO₆ octahedra in the unit cell as illustrated in Fig. 1. The La ions occupy the voids between the two octahedra. In reality, synthesizing a fully ordered double perovskite with a perfect alternate distribution of Ni and Mn cations at B-site is impossible. Antisite disorder, i.e., partial interexchange of Ni and Mn ions is generally observed in La₂NiMnO₆ [5,6]. In La₂NiMnO₆, the ferromagnetic ordering with the Curie temperature, TC ~280 K appears due to superexchange interaction between alternatively arranged Ni²⁺ and Mn⁴⁺ ions [7-9]. For a perfect B-site ordering, the saturation magnetization (MS) of 5 µ_B/f.u. is expected. However, antisite disorder causes antiferromagnetic Ni²⁺-O^2--Ni²⁺ and Mn⁴⁺-O^2--Mn⁴⁺ superexchange interactions, thereby decreasing the saturation magnetization [5].

Fig. 1. The unit cell of La₂NiMnO₆ double perovskite having a monoclinic structure.

The long-range B-site cation ordering can be assessed by X-ray/neutron/electron diffraction, Raman spectroscopy, and magnetization measurements [1,2,4,10-12]. The superlattice reflections observed in X-ray diffraction patterns due to the doubling of lattice parameters can be used to evaluate the magnitude of B-site ordering in the material. The estimate of B-site ordering can be obtained more precisely from the value of saturation magnetization extracted from the dc field-dependent magnetization measurements. The B-site homogeneous configuration of Ni²⁺ and Mn⁴⁺ ions leads to the mono-clinic structure, while a random arrangement of the same results in an orthorhombic symmetry [6,13,14]. Therefore, polarized Raman spectroscopy can be utilized to characterize the B-site ordering in double perovskites. Polarized Raman spectroscopy has been previously performed on La₂NiMnO₆ and other related double perovskites [11,15-17]. Raman spectroscopy has also been widely employed to examine the spin-phonon coupling in many ferromagnetic double perovskites [11,17-19]. The effect of B-site ordering on the spin-phonon coupling was elucidated previously for La₂CoMnO₆ double perovskite thin films [17].

In this work, the possibility of spin-phonon coupling in La₂NiMnO₆ double perovskite is explored by temperature dependent Raman spectroscopy. Further, the role of cation order on the spin-phonon coupling is evaluated.

2. Experimental

Polycrystalline La₂NiMnO₆ double perovskite was synthesized using the sol-gel method as reported elsewhere [7]. Briefly, a homogenous solution is prepared from the stoichiometric amounts of high purity La₂O₃, Ni(NO₃)₂·6H₂O, and Mn(NO₃)₂ in deionized water. It is to be noted that La₂O₃ was first dissolved in the solution of deionized water and dilute HNO₃. The ethylene glycol and citric acid were added to the precursor solution in the 1:1 ratio. The final solution was heated at 80ºC, yielding a viscous gel, which was further dried at 200ºC. The as-prepared powder was ground to achieve a fine powder. The powder was calcinated at 500ºC for 12h followed by heating at 900ºC for 12h. Then the pellets were fabricated using a uniaxial press and annealed at 950 and 1400ºC for 12h. Raman spectroscopy measurements were performed on both La₂NiMnO₆ double perovskites annealed at 950 and 1400ºC using a micro-Raman spectrometer (Jobin-Yvon Horiba LABRAM-HR) in the temperature range 140~598 K range. The 633 nm line He-Ne laser was used to excite the vibrational modes of the samples.

3. Results and Discussion

In our previous report on La₂NiMnO₆ double perovskites, the role of annealing temperature on antisite disorder was explored [7]. Also, the influence of antisite disorder on saturation magnetization was elucidated. It was observed that antisite disorder increases as the annealing temperature was increased, thereby it resulted in the decrease of saturation magnetization. The sample annealed at 950ºC exhibited the highest saturation moment (Ms) with a mere 0.6 % antisite disorder. On the other hand, the lowest saturation magnetization was obtained for the sample annealed at 1400ºC with 15 % antisite disorder. Hence, La₂NiMnO₆ double perovskite annealed at 950ºC is highly ordered, while the one annealed at 1400ºC is relatively low ordered. However, all samples revealed the same paramagnetic to ferromagnetic transition, 280 K, irrespective of the annealing temperature.

Here, temperature-dependent Raman spectroscopy has been performed to evaluate the effect of Ni/Mn ordering on the spin-phonon coupling in both high and lowordered La₂NiMnO₆ double perovskites. Raman spectroscopy was performed in the temperature range, 140~598 K for both samples. Figure 2 shows the Raman spectra at different temperatures for La₂NiMnO₆ annealed at 950 and 1400ºC. The spectra exhibit two intense bands ~530 and 670 cm^-1 as observed previously [11]. The modes at ~530 and 670 cm^-1 are described with B_g and A_g symmetry, respectively [11]. The stretching vibrations associated with the (Ni, Mn)O₆ octahedra result in A_g mode [11,15,16]. While B_g mode appears due to both anti-stretching and bending vibrations. Both B_g and A_g modes get broadened with increasing temperature for both samples.

Fig. 2. Temperature-dependent Raman spectra of La₂NiMnO₆ double perovskite annealed at 950 and 1400ºC. A_g and B_g denote the modes at ~530 and 670 cm^-1, respectively.

Below the room temperature, the intensity of both modes increases for both the samples, while the intensity of Raman modes gradually decreases above room temperature because of the line broadening. It is also observed that the peak broadening in 1400ºC is larger than in 950ºC sample.

The full width at half maxima (FWHM) is related to the B-site cation ordering, hence, it is relatively higher for the low ordered sample. To examine the possibility of spin-phonon coupling, the position, \(\omega\)(T), of most intense mode ~670 cm^-1 as a function of temperature is analyzed. Figure 3 shows \(\omega\)(T) for both the samples. The Ag mode is modeled using an anharmonic model [16,19]:

\(\omega _{anh}(T)=\omega _{o}-C\left [ 1+2/\left ( e^{\frac{\displaystyleħ\omega _{o} }{kT }}-1 \right ) \right ]\)   (1)

where C is a constant, \(\omega\)_o temperature-independent line width and k is Boltzmann constant \(\omega\)(T) fit nicely for TC < T < 598 K as displayed in Fig. 3. However, it deviates from anharmonicity model in the ferromagnetic state, T < TC for both samples. In fact, A_g mode shows a clear softening in the ferromagnetic state.

Fig. 3. Temperature dependences of the position of breathing A_g mode for La₂NiMnO₆ double perovskite annealed at 950 and 1400ºC. Solid lines denote the temperature evolution of the anharmonic three-phonon models.

It is observed that softening is large in the ordered La₂NiMnO₆ double perovskite (annealed at 950ºC) and is less in the slightly disorder La₂NiMnO₆ double perovskite (annealed at 1400ºC). This means that cation ordering has a strong influence on the softening of A_g mode. The anomalous softening is the result of phonon renormalization because of the ferromagnetic ordering [16]. The phonon renormalization results in a coupling between the spin and phonon. This kind of behavior was also observed for ferromagnetic La₂NiMnO₆ films and La₂CoMnO₆ double perovskites [15,16]. The phonon renormalization \(\Delta \omega (T)=\omega (T)-\omega _{anh}(T)\) is related to the spin-spin correlation function between the ith and jth spin states for the nearest neighbor spin-spin interaction. The phonon renormalization function scales with the normalized magnetization \(M^{2}(T)/M_{Max}^{2}\) under the mean-filed approximation. Hence, the Ni/Mn ordering determines the degree of the spin-phonon coupling. A relatively lesser softening was noticed for the sample annealed at 1400ºC as compared to the 950ºC annealed sample. This signifies that the inhomogeneous arrangement of Ni and Mn ions prevents the softening of A_g mode, and thus, decreases the spin-phonon coupling.

4. Conclusion

Temperature dependent Raman spectroscopy was performed on La₂NiMnO₆ double perovskite from 140 to 548 K. An anomalous softening of the symmetric stretching vibrations associated with the (Ni, Mn)O₆ octahedra was found below the ferromagnetic ordering temperature, indicating the presence of spin-phonon coupling. The strength of phonon coupling was observed to be diminished as the degree of cation order in La₂NiMnO₆ double perovskite got reduced.