An over view of Cr substituted Fe-based nanocrystalline Fe73.5−xCrxCu1Nb3Si13.5B9 alloys

The paper focuses on the experimental investigations of the structural, crystallization and magnetic behavior of Fe_73.5-xCr_xCu₁Nb₃Si_13.5B₉ (x = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12.5, 15 & 17.5) alloys in the amorphous and annealed states. The samples are initially prepared in the amorphous state in the form of thin ribbons by rapid-quenching technique and subsequently annealed. The Characterization of the samples is performed by X-ray diffraction. The crystallization behavior has been studied by Differential Scanning Calorimetry (DSC). Magnetic properties have been measured using Superconducting Quantum Interference Device (SQUID) magnetometer, Vibrating Sample Magnetometer (VSM), Permagraph and LCR meter. Fe_73.5-xCr_xCu₁Nb₃Si_13.5B₉ amorphous alloys upon annealing undergoes phase transformation due to primary crystallization of bcc-Fe(Si) and secondary crystallization of Fe-B phases respectively revealed by DSC and confirmed by X-ray diffraction. The primary crystallization temperature shifts to higher temperature with Cr content implying the enhancement of thermal stability of the amorphous alloys against crystallization due to increasing amount of Cr. The average grain size of the Fe(Si) phase decreases under identical annealing condition as the Cr content is increased. The saturation magnetization and the Curie temperature of the amorphous alloys decrease linearly with the increase of Cr content for the entire composition range due to dilution of Fe magnetic moment and weakening of exchange interaction between magnetic atoms. The critical concentration for the onset of ferromagnetism of the studied system has been found to be 24+1 at % Cr by linear extrapolation of saturation magnetization and Curie temperature to zero as a function of Cr concentration. A decrease of magnetic moment of 0.055 μ_B and Curie temp- rature of 26° C per atomic percent of Cr substitution are determined. Higher Cr content amorphous alloys show magnetic hardening at low temperature with the manifestation of divergence between field-cooled and zero field-cooled magnetization, which increases with decreasing temperature as well as with the increase of Cr content. Temperature dependence of magnetization of the orphous alloys in the temperature range 0 ≤ T/T_c ≤ 0.8 , follow the Bloch´s spin-wave theory. The spin-wave stiffness constant, D decreases monotonically together with the Tc with the increase of Cr content having a D constant value of D/T_c ≈0.18, follow the Bloch´s spin-wave theory. The spin-wave stiffness constant, D decreases monotonically T. 0.18 in whole concentration range .The smaller value of D in the studied system compared with the crystalline ferromagnet indicates that the range of exchange interaction is shorter in these amorphous alloys as expected. Magnetization and Curie temperature of amorphous alloys increase with annealing temperature well below the crystallization temperature due to irreversible structural relaxation. However the depression of Tc is observed for the sample annealed at temperature close to the crystallization temperature. Initial permeability strongly depends on the annealing temperature, which sharply increases with the nanocrystallization of bcc-Fe(Si) phase. An enhancement of initial permeability by two orders of magnitude and a subsequent decrease of relative loss factor tanδ/μ´ have been observed lor the optimum annealed samples.