Weak magnetic doping effect on the magnetic transition, magnetocaloric properties, and super-invar behavior in TbCo2V system

In this work, the V-doped effect on the magnetic transition and magnetocaloric properties have been studied systematically for TbCo2Vx (0 ≤ x ≤ 0.2) series. All samples were characterized by powder X-ray diffraction (XRD) as cubic structures (Fd-3 m) at room temperature. The Curie temperatures TC of TbCo2Vx decrease monotonically from 234 K (x = 0) to 214 K (x = 0.2) corresponding to a monotonically increasing value of lattice a. The slightly diminished magnetic saturation moment MS from 6.35 μB/f.u to 6.12 μB/f.u with the x value increasing indicates weak magnetic doping in this system, which can also be reflected in the slight decrease of the maximum values of magnetic entropy change (-ΔSM) from 6.23 J kg-1K−1 to 5.27 J kg-1K−1 (0–5 T). X-ray photoelectron spectrum (XPS) measurements confirm that the weak magnetic doping originates from the multivalences of V2+ and V4+, compared with other normally nonmagnetic V-doped cases reported before. Both the Banerjee criteria and rescaled magnetic entropy change lines deny the first-order transition in parent compound TbCo2 as reported recently. Almost zero thermal expansion near TC in TbCo2 with thermal expansion coefficient αl = -1.43 × 10-6 K−1 indicates weak magnetostriction, which is another experimental hint for second-order transition. The deviation of the critical exponents to Mean-field theory in TbCo2 implies some crystal defects and inhomogeneous polycrystalline distribution inside, which should also be a key point for the divergence of first-order and second-order transition in different samples of TbCo2 as reported before. Super-invar behavior with αl = 4.4 × 10-7 K−1 was found in the x = 0.2 component by macro-thermal expansion measurement.