We present77 Se-NMR measurements on single-crystalline FeSe under pressures up to 2 GPa. Based on the observation of the splitting and broadening of the NMR spectrum due to structural twin domains, we discovered that static, local nematic ordering exists well above the bulk nematic ordering temperature, Ts. The static, local nematic order and the low-energy stripe-type antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation rate measurements, are both insensitive to pressure application. These NMR results provide clear evidence for the microscopic cooperation between magnetism and local nematicity in FeSe. PACS numbers:Much attention in recent research on iron-based superconductivity (SC) has been paid to understanding the nature of the electronic nematic phase, which breaks rotational symmetry while preserving time-reversal symmetry [1,2]. In the archetypical "122" compounds AFe 2 As 2 (A=Ca, Sr, Ba) [3,4], the nematic phase is closely tied to the stripe-type antiferromagnetic (AFM) phase in the phase diagram, suggesting a magnetic origin for the nematic state. Among the Fe-based SCs, FeSe is known to be an exception. At ambient pressure, FeSe undergoes a transition to the nematic phase at a bulk structural phase transition temperature T s ∼ 90 K, as well as to SC below T c ∼ 8 K, but has no stripe-type AFM ordered phase. Under pressure (p), T s is suppressed to ∼20 K at p ∼1.7 GPa [5][6][7] and an AFM ordered state emerges above ∼0.8 GPa [8][9][10][11]. In addition, T c is enhanced from 8 K at ambient pressure to ∼37 K at p ∼ 6 GPa [12]. The decrease of T s (p) and increase of T N (p) under pressure suggests competition between nematic and magnetic orders. Furthermore, NMR measurements [13,14] showed Korringa behavior above T s , consistent with an uncorrelated Fermi liquid, while AFM spin fluctuations (SFs) were found to be strongly enhanced only below T s . These observations suggested that SFs are not the driver for nematic order and therefore pointed to an orbital mechanism for the nematicity [14]. An orbital mechanism was also suggested by Raman spectroscopy [15].In contrast, several recent studies have suggested cooperation between nematicity and magnetism in FeSe. High-energy x-ray diffraction measurements [7] found that the orthorhombic distortion is enhanced in the magnetic state at p = 1.5 GPa. Furthermore, above 1.7 GPa T s (p) and T N (p) were found to coincide as a simultaneous first-order magneto-structural transition. These observations are consistent with a spin-driven mechanism for nematic order in FeSe. Similarly, inelastic neutron scattering (INS) measurements at ambient pressure [16,17] showed that commensurate stripe-type AFM SFs are in fact present well above T s , which could possibly drive the nematic transition. These SFs were not seen by NMR [13,14] due to a spin gap above ∼ 90 K. In addition, 77 Se-NMR data under pressure [18] revealed a first-order transition to a stripe-type magnetic ordered state, and suggested a magnetic driven nematicity. Therefore, the o...

We have investigated the magnetic and magnetocaloric properties of antiferromagnetic Ising spin chain CoV 2 O 6 by magnetization and heat capacity measurements. Both monoclinic α-CoV 2 O 6 and triclinic γ-CoV 2 O 6 exhibit field-induced metamagnetic transition from antiferromagnetic to ferromagnetic state via an intermediate ferrimagnetic state with 1/3 magnetization plateau. Due to this field-induced metamagnetic transition, these systems show large conventional as well as inverse magnetocaloric effects. In α-CoV 2 O 6 , we observe field-induced complex magnetic phases and multiple magnetization plateaux at low temperature when the field is applied along c axis. Several critical temperatures and fields have been identified from the temperature and field dependence of magnetization, magnetic entropy change and heat capacity to construct the H-T phase diagram. As compared to α-CoV 2 O 6 , γ-CoV 2 O 6 displays a relatively simple magnetic phase diagram. Due to the large magnetic entropy change and adiabatic temperature change at low or moderate applied magnetic field, γ-CoV 2 O 6 may be considered as a magnetic refrigerant in the low-temperature region.

We have investigated the temperature and magnetic field dependence of magnetization, specific heat (C p ), and relative sample length change (∆L/L 0 ) for understanding the fieldinduced spin-structural change in quasi-one-dimensional spin chain α-CoV 2 O 6 which undergoes antiferromagnetic (AFM) transition below T N =15 K. Analysis of C p (T ) shows that an effective S=1/2 Ising state is realized below 20 K, though the magnetic fluctuations persist well above T N . C p and the coefficient of linear thermal expansion (α) exhibit strong H dependence in the AFM state. We also observe a huge positive magnetostriction [∆L(H)/L 0 ] below 20 K which does not show any tendency of saturation up to 9 T. With increasing field, a sharp and symmetric peak emerges below T N in both C p (T ) and α(T ) due to field-induced first order ferrimagnetic/ferromagnetic-paramagnetic transitions. The large value of magnetostriction below T N suggests strong spin-lattice coupling in α-CoV 2 O 6 .

We report synthesis and magnetic properties of quasi-one-dimensional spin-12 Heisenberg antiferromagnetic chain compound BaNa2Cu(VO4)2. This orthovanadate has a centrosymmetric crystal structure, C2/c, where the magnetic Cu2+ ions form spin chains. These chains are arranged in layers, with the chain direction changing by 62∘ between the two successive layers. Alternatively, the spin lattice can be viewed as anisotropic triangular layers upon taking the interchain interactions into consideration. Despite this potential structural complexity, temperature-dependent magnetic susceptibility, heat capacity, electron spin resonance intensity, and nuclear magnetic resonance (NMR) shift agree well with the uniform spin-1/2 Heisenberg chain model with an intrachain coupling of J/kB≃5.6 K. The saturation field obtained from the magnetic isotherm measurement consistently reproduces the value of J/kB. Further, the 51V NMR spin-lattice relaxation rate mimics the one-dimensional character in the intermediate temperature range, whereas magnetic long-range order sets in below TN≃0.25 K. The effective interchain coupling is estimated to be J⊥/kB≃0.1 K. The theoretical estimation of exchange couplings using bandstructure calculations reciprocate our experimental findings and unambiguously establish the onedimensional character of the compound. Finally, the spin lattice of BaNa2Cu(VO4)2 is compared with the chemically similar but not isostructural compound BaAg2Cu(VO4)2.