Single-crystalP31NMR studies of the frustrated square-lattice compoundPb2(

Abstract

The static and dynamic properties of V 4+ spins ͑S =1/ 2͒ in the frustrated square-lattice compound Pb 2 ͑VO͒͑PO 4 ͒ 2 were investigated by means of magnetic susceptibility and 31 P nuclear magnetic resonance ͑NMR͒ shift ͑K͒ and 31 P nuclear spin-lattice relaxation rate 1 / T 1 measurements on a single crystal. This compound exhibits long-range antiferromagnetic order below T N Ӎ 3.65 K. NMR spectra above T N show two distinct lines corresponding to two inequivalent P sites present in the crystal structure. The observed asymmetry in hyperfine coupling constant for the in-plane ͑P1͒ P site directly points toward a distortion in the square lattice at the microscopic level, consistent with the monoclinic crystal structure. The nearest-and next-nearestneighbor exchange couplings were estimated by fitting K versus temperature T by a high-temperature series expansion for the spin susceptibility of the frustrated square lattice to be J 1 / k B = ͑−5.4Ϯ 0.5͒ K ͑ferromag-netic͒ and J 2 / k B = ͑9.3Ϯ 0.6͒ K ͑antiferromagnetic͒, respectively. 1 / ͑T 1 T͒ is almost T independent at high temperatures due to random fluctuation of spin moments. Below 20 K, the compound shows an enhancement of 1 / ͑T 1 T͒ which arises from a growth of antiferromagnetic spin correlations above T N . Below T N and for the field applied along the c axis, the NMR spectrum for the P1 site splits into two satellites and the spacing between them increases monotonically with decreasing T which is a direct evidence of a columnar antiferromagnetic ordering with spins lying in the ab plane. This type of magnetic ordering is consistent with expectation from the J 2 / J 1 Ӎ −1.72 ratio. The critical exponent ␤ = 0.25Ϯ 0.02 estimated from the temperature dependence of the sublattice magnetization as measured by 31 P NMR at 11.13 MHz is close to the value ͑0.231͒ predicted for the two-dimensional XY model.

Effective One-Dimensional Coupling in the Highly Frustrated Square-Lattice Itinerant Magnet CaCo2−yAs2

“…There are many examples of materials in which geometry of the lattice leads to frustration, such as pyrochlore, spinel, or Kagomé systems [1,7]. However, in the case of a square lattice system, the frustration can arise from competing nearest-neighbor (NN) and nextnearest-neighbor (NNN) interactions [8,9].…”

“…There are many examples of materials in which geometry of the lattice leads to frustration, such as pyrochlore, spinel, or Kagomé systems [1,7]. However, in the case of a square lattice system, the frustration can arise from competing nearest-neighbor (NN) and nextnearest-neighbor (NNN) interactions [8,9].Compounds with the chemical formula ATM 2 As 2 , (with A = Ca, Sr, Ba and TM = Mn, Fe, Co, Ni), form a large class of quasi-two-dimensional (quasi-2D) materials containing layers of T M ions on a square lattice, which are stacked along c. Despite the crystal structure being three dimensional, they are considered quasi-2D for magnetism, as the interactions between layers are much smaller than those within the layers. Much of the recent motivation for the study of these materials is due to the proximity of antiferromagnetic (AFM) order and hightemperature superconductivity in the doped variants of TM = Fe compounds [10-13].…”

Frustrated couplings between alternating spin-12chains in AgVOAsO4

“…For example, V +4 phosphates are known as a playground for studying spin-1 2 frustrated square lattices. [6][7][8][9][10] Further on, symmetry restrictions on magnetic orbitals induce a variety of one-dimensional (1D) spin lattices in vanadium compounds with 2D 11,12 or even three-dimensional (3D) [13][14][15] crystal structures. In the following, we present an interesting implementation of the quasi-1D spin lattice in the 3D crystal structure of AgVOAsO 4 .…”

Field Dependence of XY-Behavior in Frustrated S = 1/2 Square Lattices

“…Furthermore the exchange interaction is frustrated, because of a ferromagnetic (FM) exchange J 1 along the sides of the squares and an antiferromagnetic (AFM) exchange J 2 along the diagonal. For Pb 2 VO(PO 4 ) 2 J 1 = -3.2 K (FM) and J 2 = 7.7 K (AFM) [5,6] whereas for SrZnVO(PO 4 ) 2 J 1 = -7.5 K and J 2 = 8.6 K [7]. In both compounds the interlayer exchange induces a columnar antiferromagnetic order at T N = 3.5 K for Pb 2 VO(PO 4 ) 2 [3] and T N = 2.7 K for SrZnVO(PO 4 ) 2 .…”