Exchange Bias in Ni-Mn-Sn Heusler Alloy Films

We report a relatively large exchange bias effect observed for the first time in Ni-Mn-Sn thin films with different microstructure and composition: a $Ni_{50}Mn_{36}Sn_{14}$ epitaxial film (A), a $Ni_{50}Mn_{43}Sn_7$ film which is phase decomposed (B), and a $NiMn//Ni_{50}Mn_{25}Sn_{25}$ bilayer (C). Despite the samples differ markedly in both microstructure and composition $H_{EB}$ does not substantially differs at 5 K. Exchange bias decreases with increasing T approximately as $H_{EB}$ (T) ∝ $H_{EB}$ (5K)/T with $H_{EB}$ (5K) of 180 Oe and 60 Oe for sample B and C, respectively and almost linearly for sample A with $H_{EB}$ (5K) = 65 Oe. Blocking temperature where the exchange bias vanishes is 40, 50 and 80 K for sample A, C and B, respectively. The results suggest that the role of AFM/FM interfaces is not substantial in formation of exchange bias in Ni-Mn-Sn Heusler alloy films and exchange bias is rather related to AFM/FM interactions in nanoscale.