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It is well known that RKKY (Ruderman-­‐Kittel-­‐ Kasuya-­‐Yosida) interaction is susceptible to fine tuning. This tuning can be accomplished via change in J (coupling of magnetic moments to conduction electrons) or via change in kF (Fermi wave vector) of conducting electrons. Particularly interesting case is suppression of magnetic ground state by Kondo screening due to increase of the J. Additionally, frustration can bring a new layer of complexity into the system suppressing magnetic transition to lower temperatures. Co1/3NbS2 is such a layered compound where Co atoms are intercalated between metallic NbS2 host layers. Those Co atoms form triangular magnetic lattice exhibiting antiferromagnetic ordering below 26 K. The ordering mechanism is not yet fully understood, although RKKY and super-­‐exchange are most likely driving interactions. Pressure is known to be the major tool to influence and study coupling of magnetic moments to conducting electrons. We performed comprehensive experimental (electrical resistivity and thermopower under pressure, anisotropy of electrical resistivity under pressure, ac and dc magnetic susceptibility, elastic neutron scattering) and theoretical (ab-­‐initio calculations) study of Co1/3NbS2. Frustration, that suppress ordering to much lower temperature than Curie-­‐Weiss temperature, is clearly evidenced in magnetic susceptibility. This suppression of magnetic transition leads to deviation from Curie-­‐Weis law and to development of unusual convex curvature of electrical resistivity with temperature. Furthermore, ferromagnetic canting of antiferromagnetically ordered spins brings in additional interplay between first and second order phase transitions. Coupling of magnetic moments to conducting electrons manifests itself as a kink in the in plane electrical resistivity at the temperature of magnetic ordering. For electrical resistivity along the c-­‐axis, that shows upturn below the magnetic transition temperature, we propose spin blocked electronic transport. Hydrostatic pressure suppresses the magnetic ordering temperature to zero near 2 GPa.[1] Above this pressure new phase (metallic spin-­‐liquid candidate) emerges. And eventually at pressures above 3 GPa a minimum in the resistivity appears indicating Kondo screening. DFT calculations revealed rigid band approximation as reasonable approach, while preliminary results suggest that the pressure driven competition between ferro-­‐ and antiferromagnetic ordering results in the disappearance of magnetic order prior to emergence of Kondo screening.[2] 1. N. Barišić, et al., Phys. Rev. B 84 (2011) 075157. 2. P. Popčević, et al., in preparation.

We present the experimental phase diagram of LiHoxEr1-xF4, a dilution series of dipolar-coupled model magnets. The phase diagram was determined using a combination of ac susceptibility and neutron scattering. Three unique phases in addition to the Ising ferromagnet LiHoF4 and the XY antiferromagnet LiErF4 have been identified. Below x = 0.86, an embedded spin-glass phase is observed, where a spin glass exists within the ferromagnetic structure. Below x = 0.57, an Ising spin glass is observed consisting of frozen needlelike clusters. For x ∼ 0.3–0.1, an antiferromagnetically coupled spin glass occurs. A reduction of TC(x) for the ferromagnet is observed which disobeys the mean-field predictions that worked for LiHoxY1-xF4.

We study the strongly anisotropic quasi one dimensional S 1 quantum magnet NiCl2 amp; 8901;4SC NH2 2 using elastic and inelastic neutron scattering. We demonstrate that a magnetic field splits the excited doublet state and drives the lower doublet state to zero energy at a critical field Hc1. For Hc1 lt; H lt; Hc2, where Hc2 indicates the transition to a fully magnetized state, three dimensional magnetic order is established with the AF moment perpendicular to the magnetic field. We mapped the temperature magnetic field phase diagram, and we find that the total ordered magnetic moment reaches mtot 2.1 amp; 956;B at the field amp; 956;0H 6 T and is thus close to the saturation value of the fully ordered moment. We study the magnetic spin dynamics in the fully magnetized state for H gt; Hc2, and we demonstrate the presence of an AF interaction between Ni2 on the two interpenetrating sublattices. In the antiferromagnetically ordered phase, the spin waves that develop from the lower energy doublet are split into two modes. This is most likely the result of the presence of the AF interaction between the interpenetrating lattices

We present the experimental phase diagram of LiHoxEr1-xF4, a dilution series of dipolar-coupled model magnets. The phase diagram was determined using a combination of ac susceptibility and neutron scattering. Three unique phases in addition to the Ising ferromagnet LiHoF4 and the XY antiferromagnet LiErF4 have been identified. Below x=0.86, an embedded spin-glass phase is observed, where a spin glass exists within the ferromagnetic structure. Below x=0.57, an Ising spin glass is observed consisting of frozen needlelike clusters. For x∼0.3–0.1, an antiferromagnetically coupled spin glass occurs. A reduction of TC(x) for the ferromagnet is observed which disobeys the mean-field predictions that worked for LiHoxY1-xF4.

The spin-gap evolution upon Ca doping in Sr14-xCaxCu24O41 was systematically investigated using inelastic neutron scattering. We discover that the singlet-triplet spin-gap excitation survives in this series with x up to 13, indicating the singlet dimer ground state in these compounds. This observation corrects the previous speculation that the spin gap collapses at x similar to 13 by the NMR technique. The strong intensity modulation along Q(H) in x = 0 gradually evolves into a Q-independent feature in x > 11. This could be attributed to the localized Cu moment magnetism developing into an itinerant magnetism with increasing x. It is a surprise that the spin gap persists in the normal state of this spin-ladder system with metallic behavior, which evidences the possibility of magnetically mediated carrier pairing mechanism in a two-leg spin-ladder lattice.

We present the experimental phase diagram of LiHoxEr1-xF4, a dilution series of dipolar-coupled model magnets. The phase diagram was determined using a combination of ac susceptibility and neutron scattering. Three unique phases in addition to the Ising ferromagnet LiHoF4 and the XY antiferromagnet LiErF4 have been identified. Below x=0.86, an embedded spin-glass phase is observed, where a spin glass exists within the ferromagnetic structure. Below x=0.57, an Ising spin glass is observed consisting of frozen needlelike clusters. For x∼0.3–0.1, an antiferromagnetically coupled spin glass occurs. A reduction of TC(x) for the ferromagnet is observed which disobeys the mean-field predictions that worked for LiHoxY1-xF4.

We present an experimental study of the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)(2)(ClO4)(2) (pz denotes pyrazine-C4H4N2) using specific-heat measurements, neutron diffraction, and cold-neutron spectroscopy. The magnetic field dependence of the magnetic ordering temperature was determined from specific-heat measurements for fields perpendicular and parallel to the square-lattice planes, showing identical field-temperature phase diagrams. This suggest that spin anisotropies in Cu(pz)(2)(ClO4)(2) are small. The ordered antiferromagnetic structure is a collinear arrangement with the magnetic moments along either the crystallographic b or c axis. The estimated ordered magnetic moment at zero field is m(0)=0.47 (5)mu(B) and thus much smaller than the available single-ion magnetic moment. This is evidence for strong quantum fluctuations in the ordered magnetic phase of Cu(pz)(2)(ClO4)(2). Magnetic fields applied perpendicular to the square-lattice planes lead to an increase in the antiferromagnetically ordered moment to m(0)=0.93 (5)mu(B) at mu H-0= 13.5 T evidence that magnetic fields quench quantum fluctuations. Neutron spectroscopy reveals the presence of a gapped spin excitations at the antiferromagnetic zone center and it can be explained with a slightly anisotropic nearest-neighbor exchange coupling described by J(xy)(1)= 1.563 (13) meV and J(z)(1) = 0.9979(2)J(1)(xy).