Possible Exotic Magnetism in the Anti-Perovskite Nitride Cr$_{3}$PtN

Samples of the anti-Perovskite nitride Cr$_{3}$PtN were synthesized for bulk magnetic studies. X-ray diffraction confirmed the structure and revealed no secondary phases within instrumental sensitivity ($\sim $2-4 vol. {\%}). Bulk magnetic properties were studied by SQUID magnetometry at $T$ = 5-300 K in magnetic fields $H$ up to 6.5 T. Highly hysteretic ferromagnetism was found, with a Curie temperature $T_{c}\approx $ 110 K. (Prior to nitriding, the Cr$_{3}$Pt starting material was paramagnetic.) At 5 K, the coercive field $H_{c}$ is $\sim $2.3 T. The curious and possibly exotic feature is that the saturation magnetic moment is small, 0.2 G-cm$^{3}$/gram: if the signal arises from bulk Cr$_{3}$PtN, the corresponding moment is only 0.1 $\mu _{B}$ per formula unit, which is quite small for a 100 K ferromagnetic. The saturation magnetization varies as $M_{sat}\sim $ (1-$T$/$T_{c})^{\beta }$ with critical exponent $\beta $=0.40. In isostructural Pd-based Cr$_{3}$PdN (not single phase), no ferromagnetism was found above 5 K. DFT calculations of the band structure for the ideal anti-Perovskite compounds revealed a high electronic density of states $N(E_{F})$ for Cr$_{3}$PtN and a somewhat lower value for Cr$_{3}$PdN.