Magnetic States and Excitations of Complex Spin Systems

The interplay between spin, lattice, charge and orbital degree of freedom has been extensively studied in various transition-metal oxides. Upon cooling, in a typical magnetic system, long-range order appears below the Curie-Weiss temperature. However, the long-range order of spins, either fully or partially, can be suppressed due to the effects like the geometrical arrangement of spins, substitution of different ions, and low dimensionality. The correlation between different degrees of freedom sometimes drives the system to novel phases through a phase transition.

Neutron scattering is a very powerful technique to study the magnetic properties of such materials and the interplay between the different degrees of freedom. In this work, intriguing phases of two vanadates with exotic complex spin systems are studied by using elastic and inelastic neutron scattering techniques.

When the orbital degree of freedom is degenerate, the Jahn-Teller distortion can occur inducing a phase transition into an orbitally ordered state. The two vanadates which are the subjects of this study, BaV10O15 and CoV2O4, both contain V^{3+} (t_{2g}^2) ions which are Jahn-Teller active.

BaV10O15 for instance is a highly frustrated magnet that shows fascinating physics with possible mixed valance ions with a V^{3+}:V^{2+} ratio of 4:1. Although it is a frustrated complex spin system with 80 magnetic ions in the unit cell, prior studies suggest a trimer formation by 60% of V ions that is accompanied by a structural phase transition at 123 K and followed by long range order below 43 K. However, the magnetic ground state of the low temperature phase is unknown. To understand the nature of the magnetic ground state and the interplay between lattice, spin, charge and orbital degrees of freedom, we performed elastic and inelastic neutron scattering measurements on single crystal and polycrystalline samples of BaV10O15. The neutron scattering data show that the best model to explain the data is when the low temperature long range ordered phase contains only two ordered V ions (V1 and V2) which form pseudo-squares connected in the $bc$ plane while the other three ions remain disordered in a spin singlet state, which agrees with the previously reported trimer formation. We determined the Hamiltonian that can describe the magnetic excitation observed in this system and discussed the possible orbital ordering that can lead to this magnetic ground state. Evidence for singlet to triplet excitations from the trimer is also presented as higher energy excitations around 33 meV. The important results presented here for this system is a prime example how a complex spin system achieve a simple magnetic state through orbital ordering by relieving spin frustration.

The other vanadate system, spinel CoV2O4, with V^{3+} ions located at vertices of a network of corner-sharing tetrahedra, also provides a good playground to investigate the effects of different degrees of freedom. Among the family of spinel vanadates, CoV2O4 lies near the boundary of itinerant electron limit making it difficult to probe the effects of spin, lattice and orbital degrees of freedom. Our polarized and unpolarized elastic neutron scattering measurements indicate that the Co and V ions order ferrimagnetically at 169 K and V spins start canting at T=90 K. Also, the strain measurements performed by a collaboration study shows a second order type structural change in the order of 10^{-4} with a maximum distortion at ~ 100 K. These important results of combining the elastic neutron scattering and the strain data indicate that the system undergoes two phase transitions, first from paramagnetic to a collinear ferrimagnetic state and secondly to a noncollinear ferrimagnetic state and an orbital glassy state of V t_{2g} orbitals at ~ 90 K. The magnetic excitations of a single crystal CoV2O4 in the ordered phase is also reported and a spin Hamiltonian to describe the inelastic neutron scattering data is provided.

These neutron scattering studies on the magnetic states and the excitations of BaV10O15 and CoV2O4 provide important results to understand the interplay between different degrees of freedom in the two systems and similar materials.