Unusually large magnetic moment and tricritical behavior of the CMR compound NaCr₂O₄ revealed with high resolution neutron diffraction and ⁺ SR

The mixed valence Cr3+/Cr4+ compound NaCr2O4, hosts a plethora of unconventional electronic properties. In the present study, muon spin rotation/relaxation (mu(+) SR) and high-resolution time-of-flight neutron powder diffraction measurements were carried out on high-quality samples to clarify the complex magnetic ground state of this unique material. We identified a commensurate canted antiferromagnetic order (C-AFM) with a canting angle of the Cr spin axial vector equal to theta(c) = (8.8 +/- 0.5)degrees, and an estimated Cr moment mu(C)(Cr) similar to (4.30 +/- 0.01)mu(B). Such an unusually large value of mu(C)(Cr) is compatible with the existence of high-spin Cr sites created by the presence of an unconventional negative charge transfer state in NaCr2O4. In addition to the C-AFM structure, a novel magnetic supercell was also revealed. Such supercell display an incommensurate (IC)-AFM propagation vector (0 0 1/2 - delta), having a Cr moment mu(C)(Cr) = (2.20 +/- 0.03)mu(B). It is suggested that the C-AFM and IC-AFM modulations have two different electronic origins, being due to itinerant and localized contributions to the magnetic moment respectively. Finally, the direct measurement of the magnetic order parameter for the C-AFM structure provided a value of the critical exponent beta = 0.245 approximate to 1/4, suggesting a non conventional critical behavior for the magnetic phase transition in NaCr2O4.