Hidden first-order phase transitions and large magnetocaloric effects in GdNi1−xCox

Abstract

We report a large magnetocaloric effect tunable between 70 and 115 K in a series of rare earth intermetallic compounds GdNi1−xCox with x varying from 0 to 0.15 that manifest rather unconventional second-order phase transitions with concurrent changes in both magnetic and crystallographic sublattices. While the Curie temperature, TC, linearly increases with x(Co), the maximum isothermal entropy change induced by magnetic field varying between 0 and 50 kOe slightly decreases from 14 J/kg K at 71 K when x(Co) = 0 to10 J/kg K at 115 K when x(Co) = 0.15. The temperature-averaged entropy change figures of merit calculated at temperature spans of 10 K for all examined compositions are comparable to those of the best known magnetocaloric materials that exhibit second-order phase transitions, including elemental Gd and La(Fe,Co,Si)13 alloys. Through a detailed analysis of the magnetic field-induced entropy changes along with assessment of critical exponents we explore the role magneto-elastic coupling plays in controlling magnetocaloric properties of GdNi1−xCox compounds. We also formulate a model based on the mean field theory approximation to describe both magnetic and magnetocaloric properties of the title materials in qualitative agreement with experimental results.