The study of interacting spin systems is of fundamental importance for modern condensed matter physics. On frustrated lattices, magnetic exchange interactions cannot be simultaneously satisfied, and often give rise to competing exotic ground states 1 . The frustrated 2D ShastrySutherland lattice 2 realized by SrCu 2 (BO 3 ) 2 3 is an important test to our understanding of quantum magnetism. It was constructed to have an exactly solvable 2-spin dimer singlet ground state within a certain range of exchange parameters and frustration. While the exact dimer state and the antiferromagnetic order at both ends of the phase diagram are well known, the ground state and spin correlations in the intermediate frustration range have been widely debated 2-12 . We report here the first experimental identification of the conjectured plaquette singlet intermediate phase in SrCu 2 (BO 3 ) 2 . It is observed by inelastic neutron scattering after pressure tuning at 21.5 kbar. This gapped plaquette singlet state with strong 4-spin correlations leads to a transition to an ordered Néel state above 40 kbar, which can realize a deconfined quantum critical point.In the field of quantum magnetism, geometrically frustrated lattices generally imply major difficulties in analytical and numerical studies. For very few particular topologies however, it has been shown that the ground state, at least, can be calculated exactly as for the Majumdar-Gosh model 13 that solves the J 1 -J 2 zigzag chain when J 1 = 2J 2 . In 2D, the Shastry-Sutherland model 2 consisting of an orthogonal dimer network of spin S=1/2 was developed in order to be exactly solvable. For an inter-dimer J to intra-dimer J exchange ratio α ≡ J /J ≤ 0.5 the ground state is a product of singlets on the strong bond J. Numerical calculations have further shown that this remains valid up to α ≤∼ 0.7 and for small values of 3D couplings J between dimer layers. At the other end, for ∼ 0.9 ≤ α ≤ ∞ the system approaches the well known 2D square lattice, which is antiferromagnetically (AFM) ordered, albeit with significant quantum fluctuations that are believed to include resonating singlet correlations resulting in fractional excitations 14 . The phase diagram of the Shastry-Sutherland model, both with and without applied magnetic field, has been intensively studied by numerous theoretical and numerical approaches 3 . In the presence of magnetic field, magnetization plateaus at fractional values of the saturation magnetization corresponding to Mott insulator phases of dimer states, as well as possible superfluid and supersolid phases have been extensively studied 6,15,16 . At zero field, the main unsolved issue is the existence and nature arXiv:1603.02039v1 [cond-mat.str-el]

Interfacial exchange coupling is known to improve the permanent magnetic performance (i.e., maximal energy product) in composites of magnetically hard and soft particles. The prevailing strategy, employed in a plethora of compositions, consists in maximizing the coupling between the hard and soft phases and optimizing material parameters such as particle size or phase composition. In CoFe2O4–FeCo nanocomposites, it is experimentally shown that interparticle uncoupling in combination with the sizes of the soft phase grains below the single‐domain threshold leads to enhanced magnetic properties at room temperature, while maximizing exchange coupling implies a collapse in coercivity and hence in the maximal energy product. The results are corroborated by micromagnetic calculations and the origin of the exchange‐induced softening is discussed. It is emphasized that engineering interfaces in order to optimize, rather than maximize, the degree of exchange coupling are a necessary requirement to improve the energy product in nanocomposite magnets and to successfully develop advanced rare‐earth‐free permanent magnets.

The influence of synthesis and compaction parameters is investigated with regards to formation of high performance SrFe12O19 bulk magnets. The produced magnets consist of highly aligned, single-magnetic domain nanoplatelets of SrFe12O19. The relationship between the magnetic performance of the samples and their structural features is established through systematic characterization by Vibrating Sample Magnetometry (VSM) and Rietveld refinement of powder X-ray diffraction data (PXRD). The analysis is supported by complementary techniques including Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM) and X-ray pole figure measurements. SrFe12O19 hexagonal nanoplatelets with various sizes are synthesized by a supercritical hydrothermal flow method. The crystallite sizes are tuned by varying the Fe/Sr ratio in the precursor solution. Compaction of SrFe12O19 nanoplatelets into bulk magnets is performed by Spark Plasma Sintering (SPS). Rietveld refinement of the pressed pellets and texture analysis of pole figure measurements reveal that SPS pressing produces a high degree of alignment of the nanoplatelets, achieved without applying any magnetic field prior or during compaction. The highly aligned nanocrystallites combined with crystal growth during SPS give rise to an enormous enhancement of the magnetic properties compared to the as-synthesized powders, leading to high performance bulk magnets with energy products of 26 kJ m(-3).

Neutron powder diffraction and transport measurements have been used to investigate the PrBaCo 2 O 5.48 compound between room temperature and 820 K. A structural phase transition, involving a rearrangement of oxygen vacancies, was found at T OD = 776 K. Across the transition the perovskite structure loses its vacancy ordering, and the crystal symmetry changes from orthorhombic Pmmm to tetragonal P4/mmm. The resisitivity measurements for temperatures above ϳ350 K yield high values of , indicating that the compound is rather semiconducting than metallic as usually accepted. A model in terms of thermally activated hole ͑polaronic͒ hopping is proposed.