Unusual ferromagnetic critical behavior owing to short-range antiferromagnetic correlations in antiperovskite Cu1-xNMn3+x (0.1 ≤ x ≤ 0.4)

For ferromagnets, varying from simple metals to strongly correlated oxides, the critical behaviors near the Curie temperature (T-C) can be grouped into several universal classes. In this paper, we report an unusual critical behavior in manganese nitrides Cu1-xNMn3+x (0.1 <= x <= 0.4). Although the critical behavior below T-C can be well described by mean field (MF) theory, robust critical fluctuations beyond the expectations of any universal classes are observed above T-C in x = 0.1. The critical fluctuations become weaker when x increases, and the MF-like critical behavior is finally restored at x = 0.4. In addition, the paramagnetic susceptibility of all the samples deviates from the Curie-Weiss (CW) law just above T-C. This deviation is gradually smeared as x increases. The short-range antiferromagnetic ordering above T-C revealed by our electron spin resonance measurement explains both the unusual critical behavior and the breakdown of the CW law.