Structural, magnetic and critical behaviour across the magnetic transition in Co₂Ti_(1−x)Cr_(x)Al Heusler compounds

The present study reports the structural, magnetic and critical behaviour of as-cast polycrystalline Co₂Ti_(1−x)Cr_(x)Al (x = 0, 0.25, 0.5, 0.75, 1) compounds prepared using the arc-melting method. Room temperature X-ray diffraction (XRD) analysis revealed that a single-phase cubic structure is obtained for Co₂Ti_(1−x)Cr_(x)Al with x = 0, 0.25, and x = 1, however, a secondary monoclinic phase is obtained Co₂Ti_(1−x)Cr_(x)Al with x = 0.5, and x = 1. Moreover, the temperature-dependent magnetization, M(T), measurements reveal that all compounds undergo a second-order paramagnetic (PM) to ferromagnetic (FM) transition at the Curie temperature (T_C). The value of T_C increases from 119.0(1) K to 412.8(1) K when x increases from 0 to 0.75, followed by a decrease to T_C = 327.0(3) K for x = 1. Additionally, a Griffiths-like phase is observed, separating the PM phase and the FM phase for x = 0, x = 0.25, and x = 0.5. Magnetic field dependent magnetization, M(μ₀H), shows that the magnetic saturation moment (M_S) of the samples at 10 K is less than M_S calculated using the Slater-Pauling (SP) rule. The experimental value of M_S increases linearly up to x = 0.75 and then decreases for x = 1. The unusual behaviour of M_S is attributed to the presence of B2 structural disorder. The critical exponents β, γ, and δ associated with the magnetic behaviour of the compounds in the vicinity of T_C were calculated using the modified Arrott plots (MAP) and Kouvel-Fisher (KF) method for Co₂Ti_(1−x)Cr_(x)Al with x = 0, x = 0.75, and x = 1. The calculated values β are larger than those calculated using the mean-field theory (MFT), 3D Heisenberg model, 3D Ising model, and 3D XY model. The calculated value of γ for x = 0 is close to that obtained from the MFT. The values of the critical exponents do not correspond to any one of the various theoretical models, indicating complex magnetic interactions in these compounds.