A Datta

Efficient control of the structural, magnetic and electrical properties of Rare-earth (RE) based perovskites (ABO₃) is crucial for advanced spintronic applications and can be achieved by means of site-specific substitution. In this comprehensive study, we explore the role of (Nd)_A-site and (Mn)_B-site co-substitution on the physical properties of pristine LaCoO₃ perovskite. The resulting compound La_0.5Nd_0.5Co_0.5Mn_0.5O₃ (LNCMO) with an orthorhombic (Pbnm) crystal structure (a =5.4811(4) Å, b = 5.5074(4) Å and c =7.7491(5) Å) exhibits a significantly reduced Jahn-Teller distortion (JT) compared to pristine LaCoO₃ with a monoclinic (I2/a) structure. The ac-magnetic susceptibility χ(T, f, H_dc ) measurements and wait time dependence of the isothermal magnetization M(t) provide clear evidence for the re-entrant spin-glass-like behavior with freezing temperature T_SG =133 K below the ferrimagnetic Curie temperature (T_FiM ∼137.4 K). Additionally, the asymmetric response of magnetic relaxation in the system to positive and negative temperature cycling well below the freezing temperature has been explained by means of the hierarchical model. Furthermore, a giant coercivity (H_C ∼14 kOe) and remanence (M_R ∼ 6440 emu mol⁻¹ ) with weak loop-asymmetry indicates the presence of large magnetic anisotropy in LNCMO, evidenced by a high magneto-crystalline anisotropy field (H_K ∼80 kOe) and anisotropy constant (K₁ ∼ 1.45 × 10⁷ erg cm⁻³). The unique electronic structure of trivalent Nd (5f ³) and mixed valent Mn (3d ⁴/3d ³) with spin-orbit coupling energies 4 eV and 11.43 eV/10.51 eV, respectively and competing exchange interaction (∼0.67 meV) between Mn and Co cations in different pathways results in enhanced magnetic-order parameters. Moreover, the co-substitution results to a pseudo-first order like state close to the spin-state transition H^* of Co (S = 0 → 2) which has been verified by the modified Arrott plot analysis yielding critical exponents β = 0.67, γ = 1.44 and δ = 3.13. A field-induced metamagnetic transition H_T emerges in the range 30 K ⩽T⩽ 130 K which has been mapped along with other parameters resulting in a H-T phase diagram which clearly distinguishes various magnetic phases and sharp crossover between the different states providing a clear and vivid picture of the overall magnetic structure of LNCMO.

We report a detailed study on the composition (x) dependence of structural, electronic, magnetic, and optical studies of nickel chromate spinel (NiCr₂O₄) at various levels of Mn substitution at B sites. No significant structural distortion from cubic symmetry Fd-3m was noticed for all the compositions in the range 0 ⩽ x⩽ 1 of Ni(Cr_1−x Mn _x )₂O₄. However, there is significant alteration in the bond angles ∠B–O–B (90.51°-93.86°) and ∠A–O–B (122.48°–124.90°) (both of which follow completely opposite trend with increasing x) and bond lengths A–O (1.82–1.94 Å) and B–O (2.02–2.08 Å). The corresponding lattice parameter (a) follows Vegard’s law (8.32 ± 0.001 Å ⩽ a ⩽ 8.45 ± 0.001Å). The electronic structure determined from the x-ray photoelectron spectroscopy reveals the divalent nature of Ni (with spin–orbit splitting energy Δ ∼ 17.62 eV). While the Cr and Mn are stable with trivalent electronic states having Δ =8 and 11.7 eV, respectively. These results are in consonance with the cationic distribution (Ni)_A[(Cr_1−x Mn _x )₂]_BO₄ obtained from the Rietveld refinement analysis. Interestingly, the current series shows a direct bandgap (E _G) semiconducting nature in which E _G varies from 1.16 to 2.40 eV within the range of x = 0.85–0. Such variation of E _G (x) is consistent with the compositional variation of the crystal structure data with anomalous change between x = 0.25 and 0.6. Beyond this range, the E _g mode (140 cm⁻¹) in Raman spectra arising from Mn–O octahedral decreases continuously and vanishes at higher Mn concentrations. Our analysis shows that all the investigated compounds show long-range ferrimagnetic ordering below the Néel temperature, T _FN due to the unequal magnetic moments of the cations. However, both the ordering temperature T _FN and saturation magnetization (M _S) increases progressively from 73.3 K (1500 emu mol⁻¹) to 116 K (3600 emu mol⁻¹) with increasing the Mn content from 0 to 1, yet the maximum anisotropy (H_K ~4.5 kOe, K ₁~2.5 × 10⁴ erg cc⁻¹) shows an opposite trend with x. Such variation is ascribed to the altered magnetic superexchange interactions between the cations located at A and B sites following the trend J _BB > J _AB > J _AA, (J _BB/k _B =13.36 K).

Here, we report the influence of Jahn–Teller active Cu substitution on the charge-ordering (CO) characteristics of one of the well-known manganite Pr_0.45Sr_0.55MnO₃ (S55) with a distorted tetragonal structure. Magnetization studies unveil a complex magnetic phase diagram for S55, showing distinct temperature ranges corresponding to various magnetic phases: a ferromagnetic phase dominated by the Double Exchange interaction with T _C ∼ 220.5 K, an antiferromagnetic phase below T _N ∼ 207.6 K induced by CO with a transition temperature of T _CO ∼ 210 K consistent with the specific heat C _P(T) data, and a mixed phase in the range T _N < T < T _CO due to the competitive interplay of these two interactions. Dilute substitution of Cu at the Mn B-sites disrupts the robust charge-ordered state, leading to enhanced ferrimagnetic order with T _FN ∼273 K and significant magnetocrystalline anisotropy as confirmed by ferromagnetic resonance (FMR) studies. The Cu-substituted system displays a distinct cationic distribution compared to the pristine S55, contributing to its diverse magnetic structure. Our findings also reveal the irreversible metamagnetic transition (H _T-Max ∼ 8.85 kOe at 180 K) associated with the CO phenomena in S55 and the first-order nature of the phase transition across T _CO. The magnetic heat capacity critical analysis (C _Mag = A (T − T _N)^−α ) yields the exponent, α = 0.097 (0.154) in the region T > T _N (T < T _N) consistent with the magnetic structure. The temperature dependence of FMR resonance field ΔH _Res(T), peak-to-peak width H _PP(T), and Gilbert damping factor α _G(T) show clear anomalies across the magnetic transitions signifying the important role of admixtured (3+/4+) electronic state of Mn. Additionally, a strong correlation between the FMR α _G(T) and switchable magnetic entropy change (ΔS _Max ∼ −8/+ 3 J kg⁻¹ K⁻¹ for ΔH = 90 kOe) has also been established in S55.

We report the mixed valence and intermediate spin-state (IS) transitions in Pr substituted LaCoO₃ perovskites in the form of bulk and nanostructures. Various compositions (x) of La_1−x Pr _x CoO₃ (0 ⩽ x⩽ 0.9) were synthesized using the sol–gel process under moderate heat treatment conditions (600 °C). The structural analysis of these compounds reveals a phase crossover from the monoclinic phase (space group, s.g.: I2/a) to an orthorhombic one (s.g.: Pbnm), and a rhombohedral phase (s.g.: R-3c) to an orthorhombic one (s.g.: Pnma) in the bulk and nanostructures, respectively, for the composition range 0 ⩽ x⩽ 0.6. Such a structural transformation remarkably reduces the Jahn–Teller distortion factor Δ_JT: 0.374 → 0.0016 signifying the dominant role of the IS state (S _Avg = 1) of trivalent Co ions in the investigated system. Magnetization measurements reveal the ferromagnetic (FM) nature of bulk LaCoO₃ along with a weak antiferromagnetic (AFM) component coexisting with an FM component. This coexistence results in a weak loop-asymmetry (zero-field exchange-bias effect ∼134 Oe) at low temperatures. Here the FM ordering occurs due to the double-exchange interaction (J _EX /k _B∼ 11.25 K) between the tetravalent and trivalent Co ions. Significant decrease in the ordering temperatures was noticed in the nanostructures (T _C ∼ 50 K) as compared to the bulk counterpart (∼90 K) due to the finite size/surface effects in the pristine compound. However, the incorporation of Pr leads to the development of a strong AFM component (J _EX/k _B ∼ 18.2 K) and enhances the ordering temperatures (∼145 K for x = 0.9) with negligible FM correlations in both bulk and nanostructures of LaPrCoO₃ due to the dominant super-exchange interaction: Co^3+/4+‒O‒Co^3+/4+. Further evidence of the incoherent mixture of low-spin (LS) and high-spin (HS) states comes from the M–H measurements which yields a saturation magnetization of M _S ∼ 275 emu mol⁻¹ (under the limit of 1/H → 0) consistent with the theoretical value of 279 emu mol⁻¹ corresponding to the spin admixture: 65% LS + 10% IS of trivalent Co along with 25% of LS Co⁴⁺ in the bulk pristine compound. A similar analysis yields: Co³⁺ [30% LS + 20% IS] + Co⁴⁺ [50% of LS] for the nanostructures of LaCoO_3, yet the Pr substitution decreases the spin admixture configuration. The Kubelka–Munk analysis of the optical absorbance results in a significant decrease in the optical energy band gap (E _g:1.86 → 1.80 eV) with the incorporation of Pr in LaCoO₃ which corroborates the above results.