Study of AC Susceptibility of La0.8Sr0.2MnO3 at Different Frequencies

In the present work, the structural and ac magnetic susceptibility as a function of temperature at different frequencies of perovskite type La08Sr0.2MnO3 (LSMO) compound have been discussed. Conventional solid-state route was used to synthesize LSMO compound. Orthorhombic crystal structure with Pnma space group of the compound was witnessed from the analysis of X-ray diffractograms by Rietveld refinement. The real (χꞌ) and imaginary (χꞌꞌ) part of the ac susceptibility of the compound have been studied. The real part of magnetic susceptibility (χꞌ) of La0.8Sr0.2MnO3 sample increases as temperature decrease. This indicates that the paramagnetic-ferromagnetic phase transition occurs. At different frequencies, i.e. 100Hz and 500Hz the Curie temperature Tc are 326K and 336K respectively. Peaks observed in the imaginary part χꞌꞌ) of magnetic susceptibility plot indicate the presence of sample inhomogeneity.


Introduction
Lanthanum Manganite Oxides La1-xAxMnO3 (A= Sr 2+ , etc.) have gained a great deal of attention recently due to their wide range of electrical and magnetic characteristics [1].Due to the Magneto-Caloric effect (MCE), which is the temperature change of magnetic material associated with an variation in the external magnetic field in an adiabatic process, they have been thought to be possible next-generation Colossal Magneto Resistance (CMR) materials, magnetic refrigerators, and alternative catalysts for the oxidation of carbon monoxide and hydrocarbons [1,2].
The compound La1-xAxMnO3 exhibits multiple electronic, magnetic, and structural phase transitions at various temperatures depending on the composition x.It has been determined that significant coupling between spin, charge, orbital degree of freedom, and lattice vibrations are the cause of these phase transitions [3,4].
The parent compound LaMnO3 with perovskite structure is an antiferromagnetic Mott insulator.The cation and oxygen non-stochiometry in LaMnO3 (LMO) are primarily responsible for the aforementioned properties.Undoped LMO, which solely includes Mn 3+ ions, is an insulating substance that lacks the magnetoresistance effect [5].Whereas, the doped La1−xSrxMnO3 compound with varying composition is ferromagnetic with a metal-insulator transition (MI) at TC, exhibiting CMR.The primary characteristic of the MI can be qualitatively explained by a double exchange mechanism that involves the adjacent ions Mn 3+ and Mn 4+ and oxygen.The characteristics of these mixed valence oxides are also strongly influenced by the Jahn-Teller distortion and electron-phonon interactions [6].
However, according to Zener's hypothesized double-exchange Mn 3+ -O-Mn 4+ interactions, manganites exhibit ferromagnetism [7].In order to maintain charge neutrality, lanthanum ions are partially replaced in LMO with cations of a lower valence state (such as Sr 2+ ), changing the valence state of manganese from Mn 3+ to Mn 4+ (doping with holes) [8].Dopants such as Sr 2+ incorporation on the La site also affects lattice distortion by altering the Mn-O bond and Mn-O-Mn bond angle values.The degree of distortions is affected by external experimental circumstances like pressure, temperature, and environment in addition to the size of the cation [9].The electrical, magnetic, and transport characteristics of manganites are also significantly influenced by factors including grain size and microstructure [10].
Ac Magnetic susceptibility (χ = χꞌ + χꞌꞌ) Measurements are an effective method for determining a material's magnetic state.The dynamics and magnetization of a material as a function of temperature, field amplitude, and frequency are reflected in the behavior of real and imaginary ac susceptibility components [11,12].
Moreira et al. study depicts the existence of inhomogeneities in CMR materials.It was demonstrated that in ferromagnetic materials, AC magnetic field induced Domain Walls Displacements (DWD) and Domain Magnetization Rotations (DMR) both of which are brought on by the supplied ac magnetic field, are mostly reflected below the TC value.It is necessary to have a better understanding of the fundamental variations in both and in various magnetic systems because the domain dynamics in ferromagnetic materials may alter the real and imaginary components of the ac susceptibility below TC [13].
In this study, we have studied structural and AC magnetic susceptibility (Both real and imaginary) of Sr doped LaMnO3at two different frequencies (100Hz and 500Hz).

Synthesis
The conventional solid-state route was used to synthesize LSMO ceramic.Lanthanum Oxide (La2O3), Strontium Oxide (SrO2) and Manganese Oxide (MnO2) precursors were taken in a stoichiometric ratio as follows: 2La2O3+4SrO2+4MnO2→4LaSrMnO3+5O2 These oxides were mixed and ground thoroughly in an agate mortar for 5-6 hours.Ethanol was also used to mix the precursors uniformly during grinding.The homogenous powder was calcined at 850˚C for 5 hours.For pelletization, the powder was mixed with polyvinylalcohol (PVA) and grinded it to get the fine mixture of samples.Further, a hydraulic press was used at a 2.5 tones per inch pressure with 10mm diameter and 1mm thickness.Finally, these pellets were sintered at 700˚C for 10 hours.

Characterization
To investigate the crystal structure acquired by the compound LaSrMnO3, the X-ray diffraction data was recorded using PROTO AXRD Benchtop Diffractometer with Cu-K1 radiation (λ=1.5406Å) at room temperature.For measurement of magnetic properties such as AC Susceptibility of the LSMO was carried out by using XPLORE Physical Quantities Measurement System (PQMS) made by Precision-Quazar Tech Pvt. Ltd.New Delhi, India.

X-ray Diffraction Analysis
The Rietveld refine X-ray Diffraction spectrum of LaSrMnO3 was illustrated in the Figure 1.Rietveld refinement of the XRD data was refined using FullProf software.The results reveal single phase crystal structure without impurity peaks.Also, the majority of the peaks are high and sharp, indicating that the sample crystallized correctly, as can be seen from Figure 1.

Real Part of Magnetic Susceptibility ()
Figure 2(a) and Figure 2(b) illustrates the variation of real part of magnetic susceptibility () of La0.8Sr0.2MnO3sample with temperature at different frequencies (100Hz and 500Hz).The magnetic susceptibility () of La0.8Sr0.2MnO3sample increases as the temperature decreases.This indicates that the paramagnetic-ferromagnetic phase transition occurs at the Curie temperature Tc at different frequencies (100Hz and 500Hz).
The Tc of La0.8Sr0.2MnO3sample is 326K and 336K at 100Hz and 500Hz respectively.This suggests that the freezing out of domain wall dynamics is linked to the frequency dependence of ac susceptibility [16].Inhomogeneities, defects, interstitials, and realignments of electron states within the domain walls may be the cause of frequency dependent mobility of the domain walls [13].
Generally, Arrhenius law is typically used to evaluate the dynamic behavior of domain walls for the relaxation time [17]: Where, U indicates the activation energy and f0 =1/τ0 indicates attempt frequency.

Imaginary Part of Magnetic Susceptibility ()
Figure 3 shows the imaginary part of magnetic susceptibility () of La0.8Sr0.2MnO3sample at various frequencies (100Hz and 500Hz).Peaks observed in the plot indicates that the presence of sample inhomogeneity [13].
The ac magnetic susceptibility technique is an efficient way to ambiguously detect inhomogeneities.Their findings are in good accord with observations of Ferromagnetic Resonance (FMR) measurements.Due to the existence of La-site vacancies in La0.8Sr0.2MnO3samples, the local structure may vary because the critical parameters affecting the characteristics, such as the Mn-O distance and Mn-O-Mn bond angles, would alter in the region of the vacancies [18].

Conclusion
In summary, perovskite type compound La0.8Sr0.2MnO3was successfully synthesized by Solid State Technique.The refined XRD data exhibited Orthorhombic crystal structure with Pnma space group.The average crystal size of LaSrMnO3 was 27.73 nm.Both the real and imaginary part of ac susceptibility of La0.8Sr0.2MnO3 at different frequencies (100Hz & 500Hz) was studied.The real part of susceptibility indicates the paramagnetic-ferromagnetic phase transition occurs at the Curie temperature (Tc) at different frequencies (100Hz and 500Hz).The Tc of La0.8Sr0.2MnO3sample is 326K and 336K at 100Hz and 500Hz respectively.Further, the peaks observed in the imaginary part of magnetic susceptibility () of La0.8Sr0.2MnO3sample at various frequencies (100Hz and 500Hz) indicates that the presence of sample inhomogeneity.