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

Abstract

Inelastic neutron scattering measurements on the itinerant antiferromagnet (AFM) CaCo2−yAs2 at a temperature of 8 K reveal two orthogonal planes of scattering perpendicular to the Co square lattice in reciprocal space, demonstrating the presence of effective one-dimensional spin interactions. These results are shown to arise from near-perfect bond frustration within the J1-J2 Heisenberg model on a square lattice with ferromagnetic J1, and hence indicate that the extensive previous experimental and theoretical study of the J1-J2 Heisenberg model on local-moment square spin lattices should be expanded to include itinerant spin systems.Magnetic frustration arises when competing interactions between magnetic moments (spins) cannot be mutually satisfied. It suppresses the development of longrange magnetic order, and often creates enhanced spin fluctuations, which can lead to a variety of novel phases including quantum spin liquids [1, 2], spin and electronic nematic phases and unconventional superconductivity [3][4][5][6]. 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]. The ATM 2 As 2 materials adopt several different magnetic structures, including Néel-(or checkerboard)type AFM (e.g., BaMn 2 As 2 [14]), stripe-type AFM (e.g., AFe 2 As 2 [13]), and A-type (e.g., CaCo 2−y As 2 [15]). The AFM order in the TM = Fe and Co variants is itinerant in nature, possessing an ordered moment of µ 1µ B /TM.Despite their itinerant nature, the magnetic structures and spin fluctuations can be minimally described by considering NN (J 1 ) and NNN (J 2 ) magnetic exchange interactions between magnetic ions on a square lattice [16]. In general, the magnetic ground state is determined by the relative strengths of J 1 and J 2 , with Néel-type, stripe-type AFM, and FM/A-type ordering occurring for [8,13]. The system becomes frustrated and ordering in any of these magnetic structures is suppressed when |J 1 | ≈ 2J 2 and J 2 > 0 (AFM). A frustration parameter, η = J 1 /2J 2 , quantifies the level of the magnetic frustration. Maximum frustration occurs when η = 1 (−1), and the stripe-type and Néel-type stripe-type and ferromagnetic (FM) ground states compete [8,13].The fr...

Structural and magnetic studies on SrCo₂As₂ and Ca(Co_1-xFe_x)_yAs₂ compounds using x-ray and neutron scattering

“…Similarly, the T N and the ordered moment also decreased and T N ∼ 0 for x ∼ 0.12 [124]. And the inelastic neutron studies of CaCo 1.86 As 2 show that there is a maximum magnetic frustration which arise from nearest and next-nearest neighbor magnetic ion interactions, so that the stripe-type and FM ground states compete in CaCo 1.86 As 2 [16]. Therefore, to obtain a better understanding exploring the spin fluctuations in compounds with values of x corresponding to the point at which the A-type AFM order suppressed is the optimum choice.…”

“…This is due to the strong interlayer As-As bond formation [13,14,15]. The recent inelastic neutron studies reveal that there is a maximum magnetic frustration which arise from nearest and next-nearest neighbor magnetic ion interactions in CaCo 1.86 As 2 [16]. So that stripe-type and FM ground states compete.…”

“…16 shows the phase diagram for Ca(Co 1−x Fe x ) y As 2 determined from this work and from data given in Refs [34]. and[35].…”

Studies of spin dynamics in 122 transition metal arsenides using inelastic neutron scattering technique

“…Spin liquid is a state, arising due to magnetic frustration, where long-range magnetic order is absent down to T = 0 K. A system in the spin liquid state is expected to show exotic phenomena [54,55]. However, there are no examples of square-lattice system near this region of maximum frustration [50]. Therefore, the J 1 -J 2 phase diagram in Fig.…”

Pressure-tuned Frustration of Magnetic Coupling in Elemental Europium

“…Such a J 1 − J 2 model near frustration, with spins damped by conduction electrons, was proposed by Wu et al 38,39 to account for the quantum critical point versus isoelectronic As→P doping in BaFe 2 As 2 . Recently Sapkota et al 37 reported near-perfect J 1 −J 2 frustration in an itinerant metallic system, square lattice CaCo 1.86 As 2 tuned (naturally) by Co vacancies. Frustration in Eu is in a stoichiometric lattice with local moments and RKKY interactions, so the mechanism of frustration -volume evolution of many exchange constants -is distinct.…”