Magnetic and cryogenic magnetocaloric properties of NaGdF4 nanocrystals

We studied the magnetic and magnetocaloric properties of NaGdF4 nanocrystals (NCs), which are crystallized in the P6(3)/m hexagonal structure. Analyzing M(T, H) data, we found an existence of ferromagnetic and antiferromagnetic interactions in NCs. This gives rise to a minor hysteresis loop at temperatures T <= 3K and magnetic fields H=1-40kOe. Particularly, at temperatures T=2-25K, NaGdF4 NCs show a large magnetocaloric effect (MCE). For a field change Delta H=60kOe, the maximum magnetic entropy change (|Delta S-max|) at similar to 3.2K and the relative cooling power (RCP) are about 50.5J/kgK and 410J/kg, respectively. These values are larger than those obtained from conventional cryogenic materials reported previously. Having considered the MCE vs the internal field (H-i), we have found that |Delta S-max(H-i)| values increase about 8%-34% compared with the |Delta S-max(H)| values. Magnetic-field dependences of both |Delta S-max| and RCP obey the power law y = ax ''. The assessments of a magnetic-order exponent n and the analyses M-2(H/M) and N(T, H) data proved NaGdF4 NCs exhibit a short-range magnetic order and undergo a second-order phase transition. Published under license by AIP Publishing.