The RNiO 3 perovskites are known to order antiferromagnetically below a material-dependent Néel temperature T N . We report experimental evidence indicating the existence of a second magnetically-ordered phase in TlNiO 3 above T N = 104 K, obtained using nuclear magnetic resonance and muon spin rotation spectroscopy. The new phase, which persists up to a temperature T * N = 202 K, is suppressed by the application of an external magnetic field of approximately 1 T. It is not yet known if such a phase also exists in other perovskite nickelates.Although first synthesized already in 1970 [1], the rare-earth nickelates RNiO 3 have been the subject of intense research efforts during the last decade due to their peculiar metal-insulator transition at T MI and subsequent antiferromagnetic (AFM) order between Ni spins below a Néel temperature T N [2, 3]. By substituting rare-earth ions R 3+ with different radii, the perovskite lattice can be distorted continuously. This distortion affects the magnetic couplings of Ni spins by modifying the Ni-O-Ni superexchange angle. A phase diagram of RNiO 3 , mostly based on previously published results, is shown in Fig. 1. Despite recent theoretical [4][5][6] and experimental [7] progress, the nature of the paramagnetic (PM) insulating phase is still unclear. The AFM order below T N is characterized by a propagation vector k pc = ( 1 /4, 1 /4, 1 /4) with respect to the (pseudo-)cubic unit cell of the ideal perovskite structure. Thus, a period of the magnetic structure comprises four Ni sites along each pseudocubic crystal axis. Previous experimental reports suggested a collinear "up-up-down-down" structure [8-10], while others argued for a non-collinear spiral spin configuration [11][12][13][14]. The AFM phase is predicted to be ferroelectric [15,16], but this has not yet been confirmed experimentally. Since the magnitude of the ferroelectric polarization is very different for the two candidate spin arrangements [16], investigations of the magnetism of RNiO 3 are crucial for the understanding of their possible multiferroicity. Nuclear magnetic resonance (NMR) is a powerful technique to locally probe magnetic behavior. However, to our knowledge, the only previous NMR work involving a nickelate perovskite was a 139 La-NMR study of LaNiO 3 [17], which is a PM metal at all temperatures.In the present work, we present the first NMR investigation of an insulating member of the RNiO 3 family, in the form of a combined NMR and muon-spin rotation (µSR) study of the magnetic properties of TlNiO 3 . This material is known to have the same qualitative behavior as the analogous rare-earth nickelates [18]. From an NMR perspective, TlNiO 3 is unique among the RNiO 3 compounds because both 203 Tl and 205 Tl nuclei are excellent for NMR. In addition to the previously known AFM phase below T N = 104 K, our measurements reveal a previously unknown magnetically-ordered phase between T N and T * N = 202 K. An applied magnetic fields of 1 T is sufficient to suppress the static magnetic order between T N an...

We report a study of the normal-and superconducting-state electronic properties of the centrosymmetric compound SrPt 3 P via 31 P nuclear-magnetic-resonance (NMR) and magnetometry investigations. Essential features such as a sharp drop of the Knight shift at T < T c and an exponential decrease of the NMR spin-lattice relaxation ratio 1/(T 1 T ) below T c are consistent with an s-wave electron pairing in SrPt 3 P, although a direct confirmation in the form of a Hebel-Slichter-type peak is lacking. Normal-state NMR data at T < 50 K indicate conventional features of the conduction electrons, typical of simple metals such as lithium or silver. Our data are finally compared with available NMR results for the noncentrosymmetric superconductors LaPt 3 Si and CePt 3 Si, which adopt similar crystal structures.

Cu(pz) 2 (ClO 4 ) 2 (with pz denoting pyrazine, C 4 H 4 N 2 ) is among the best realizations of a two-dimensional spin-1 /2 square-lattice antiferromagnet. Below T N = 4.21 K, its weak interlayer couplings induce a 3D magnetic order, strongly influenced by external magnetic fields and/or hydrostatic pressure. Previous work, focusing on the [H, T ] phase diagram, identified a spin-flop transition, resulting in a field-tunable bicritical point. However, the influence of external pressure has not been investigated yet. Here we explore the extended [p, H, T ] phase diagram of Cu(pz) 2 (ClO 4 ) 2 under pressures up to 12 kbar and magnetic fields up to 7.1 T, via magnetometry and 35 Cl nuclear magnetic resonance (NMR) measurements. The application of magnetic fields enhances T X Y , the crossover temperature from the Heisenberg to the X Y model, thus pointing to an enhancement of the effective anisotropy. The applied pressure has an opposite effect [dT N /dp = −0.050(8) K/kbar], as it modifies marginally the interlayer couplings, but likely changes more significantly the orbital reorientation and the square-lattice deformation. This results in a remodeling of the effective Hamiltonian, whereby the field and pressure effects compensate each other. Finally, by comparing the experimental data with numerical simulations we estimate T BKT , the temperature of the Berezinskii-Kosterlitz-Thouless topological transition and argue why it is inaccessible in our case.

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