Critical Behavior Of La0 Research Articles

Critical behavior in Sr-doped manganites La0.6Ca0.4−xSrxMnO3

The critical behavior of La0.6Ca0.4−xSrxMnO3 (x=0, 0.2 and 0.4) compounds has been investigated based on the data of static magnetic measurements in the vicinity of its critical temperature TC. Through various techniques such as modified Arrott plot, Kouvel–Fisher method and critical isotherm analysis, the estimated critical exponents are found to be (β=0.267, γ=0.992, δ=4.896 at TC=268.12), (β=0.501, γ=1.045, δ=2.992 at TC=344.37) and (β=0.386, γ=1.293, δ=4.88 at TC=371.02), for x=0, 0.2 and 0.4; close to tricritical mean field, mean field and 3D-Heisenberg model values, respectively. The reliability of the critical exponents values was confirmed by the Widom scaling relation and the universal scaling hypothesis. Following the Harris criterion, we deduced that the disorder in our case is relevant, which can be the cause of the change in the universality class. Moreover, the local exponent n was studied in terms of the field dependence of the magnetic entropy change. It has been noted that n evolves with field in the entire studied temperature range for all samples. The invalidity of the mean field model for x=0 and 0.4 contents was confirmed at the peak temperature. Far above the transition temperature, the Curie–Weiss law is well satisfied. The differences observed in the n values below the transition are mentioned. Furthermore, unlike the modified Arrott or Kouvel–Fischer methods, the performed analysis is shown to be useful to examine critical exponents and their effects in mixed phases materials.

Critical Behavior Near the Paramagnetic to Ferromagnetic Phase Transition Temperature in Polycrystalline La0.57Nd0.1Sr0.33Mn1−x Al x O3 (0.0≤x≤0.1)

The critical parameters provide important information concerning the interaction mechanisms near the paramagnetic to ferromagnetic transition. In this paper, we have investigated the effect of non-magnetic aluminum Al doping in Mn-site on the critical behavior in La0.57Nd0.1Sr0.33MnO3 compounds using magnetization methods near the Curie temperature TC through various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis. The values of critical exponents, derived from the magnetic data yield 0.326 ≤β≤0.366,1.265≤γ≤1.342 and 4.13 ≤δ≤5.07. The critical exponents found in this study obey the Widom scaling relation δ=1+γ/β, implying that the obtained values of β and γ are reliable. Experimental results have revealed that the samples exhibited the second-order magnetic phase transition with exponents which match well with those predicted for the three-dimensional Heisenberg (3D) around the critical temperature.

Critical behavior of La0.825Sr0.175MnO2.912 anion-deficient manganite in the magnetic phase transition region

For La 0.825 3+ Sr 0.175 2 +Mn3+O 2.912 2− anion-deficient manganite, the specific magnetization, the dynamic magnetic susceptibility, and the heat capacity are investigated. This material is found to be an inhomogeneous ferromagnet below the Curie point T C ≈ 122 K, which is much lower than the Curie point determined for the stoichiometric composition (T C ≈ 268 K). An increase in magnetic field by two orders of magnitude leads to an increase in the Curie temperature by ΔT ≈ 12 K. The presence of oxygen vacancies leads to the frustration of a part, namely, V fr ≈ 22%, of the indirect Mn3+-O-Mn3+ exchange interactions, but the spin glass state is not realized. The ferromagnetic matrix of the material under study is characterized by a scatter in the exchange interaction intensities. The heat capacity is found to exhibit an anomalous behavior. Based on the Banerjee magnetic criterion, it is established that the ferromagnet-paramagnet transition observed for La 0.825 3+ Sr 0.175 2+ Mn3+O 2.912 2− anion-deficient manganite is a second-order thermodynamic phase transition. The mechanism and origin of the critical behavior of the system under investigation are discussed.