Abstract

The extreme sensitivity in the balance of competing magnetic interactions in geometrically frustrated magnets is illustrated clearly in Cr3+ spinels, ACr2O4. Antiferromagnetic (AFM) ordering in ACr2O4 occurs at a spin-Peierls transition. Both the low temperature magnetic and structural regimes are found to be highly sensitive to the A cation. In the case of ZnCr2O4 we find that very fine control of the reaction conditions is necessary to make stoichiometric ZnCr2O4, rather than hole doped Zn1+xCr2-xO4 (x ≤ 0.04). From analysis of magnetic measurements, specific heat and neutron diffraction we have probed the nature of the transitions at T_N. In light of these results how hole doping effects the low temperature properties and the role of the d2 Cr4+ cations on the isotropic d3 Cr3+ magnetic lattice will be discussed. The effect of hole doping in SCGO , SrCr8Ga4O19, a 2D analogue of the 3D magnetic lattice in spinels, will also be discussed.

Magnetic frustration in 1D systems arises due to competition between the nearest neighbour, J1 and next nearest neighbour, J2, interactions. Recently systems with ferromagnetic (FM) J1 and AFM J2 have received much attention due to the complex magnetic phase diagram and the presence of a FM quantum critical point (QCP) at alpha_c = J2/J1 = -0.25. In the second part of the talk a new 1D S=1/2 frustrated chain magnet, LiCuSbO4 will be introduced. The proximity of LiCuSbO4 to the FM critical point at alpha_c = -0.25 will be demonstrated by fitting of the thermomagnetic data to exact digitalization simulations. The absence of 3D ordering T > 100 mK will be discussed in relation to the arrangement of the spin-chains within the crystal structure.