The Magnetic Phase Transition and Spin Fluctuations in the Geometrically Frustrated Antiferromagnetic Spinel CdCr2O4: An Experiment Using the SPINS Triple-Axis Spectrometer

CdCr2O4 is a magnetic compound that crystallizes into what is known as a cubic spinel structure, and the magnetic properties stem from the Cr3+ ions, which form a network of corner-sharing tetrahedra. Despite the presence of relatively strong antiferromagnetic, nearest-neighbor interactions between these ions, the peculiar spatial arrangement of the Cr atoms within the spinel structure actually serves to suppress magnetic order. In fact, true long-range, elastic magnetic order is established only after cooling to the Néel temperature TN = 7.8K, which is one order of magnitude smaller than the Curie-Weiss temperature |ΘCW | = 88K, the temperature at which magnetic order is expected. In addition, a structural transition, in which the dimensions of the cubic unit cell distort tetragonally such that c > a = b, occurs at the same temperature as the onset of long-range magnetic order at TN . Below TN , CdCr2O4 exhibits normal spin wave excitations, which are inelastic features characteristic of ordered magnetic phases. These excitations transform into quasielastic spin fluctuations above TN , the scattering from which is broad in both momentum Q and energy h̄ω, and persist up to T ≈ |ΘCW |. This is consistent with the presence of short-range ordered (small) clusters of spins having short-lived (dynamic) magnetic correlations in the paramagnetic (disordered) phase. By comparing the neutron inelastic scattering intensity to model calculations, we find that these clusters organize into hexagonally shaped loops of antiferromagnetically ordered