Magnetic and cryogenic magnetocaloric properties of NaGdF4 nanocrystals

We studied the magnetic and magnetocaloric properties of NaGdF4 nanocrystals (NCs), which are crystallized in the P63/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 ≤ 3 K and magnetic fields H = 1–40 kOe. Particularly, at temperatures T = 2–25 K, NaGdF4 NCs show a large magnetocaloric effect (MCE). For a field change ΔH = 60 kOe, the maximum magnetic entropy change (|ΔSmax|) at ∼3.2 K and the relative cooling power (RCP) are about 50.5 J/kg K and 410 J/kg, respectively. These values are larger than those obtained from conventional cryogenic materials reported previously. Having considered the MCE vs the internal field (Hi), we have found that |ΔSmax(Hi)| values increase about 8%–34% compared with the |ΔSmax(H)| values. Magnetic-field dependences of both |ΔSmax| and RCP obey the power law y=axn. The assessments of a magnetic-order exponent n and the analyses of M2(H/M) and N(T,H) data proved NaGdF4 NCs exhibit a short-range magnetic order and undergo a second-order phase transition.