Ferromagnetic cluster-glass phase in Ca(Co1−xIrx)2−yAs2

Abstract

Single crystals of Ca(Co1−xIrx)2−yAs2 with 0≤x≤0.35 and 0.10≤y≤0.14 have been grown using the selfflux technique and characterized by single-crystal x-ray diffraction (XRD), energy-dispersive x-ray spectroscopy, magnetization M, and magnetic susceptibility χ measurements versus temperature T, magnetic field H, and time t, and heat-capacity Cp(H,T) measurements. The XRD refinements reveal that all the Ir-substituted crystals crystallize in a collapsed-tetragonal structure as does the parent CaCo2−yAs2 compound. A small 3.3% Ir substitution for Co in CaCo1.86As2 drastically lowers the A-type antiferromagnetic (AFM) transition temperature TN from 52 to 23 K with a significant enhancement of the Sommerfeld electronic heat-capacity coefficient. The A-type AFM structure consists of ab-plane layers of spins ferromagnetically aligned along the c axis with AFM alignment of the spins in adjacent layers along this axis. The positive Weiss temperatures obtained from Curie-Weiss fits to the χ(T>TN) data indicate that the dominant magnetic interactions are ferromagnetic (FM) for all x. A magnetic phase boundary is inferred to be present between x=0.14 and x=0.17 from a discontinuity in the x dependencies of the effective moment and Weiss temperature in the Curie-Weiss fits. FM fluctuations that strongly increase with increasing x are also revealed from the χ(T) data. The magnetic ground state for x≥0.17 is a spin glass as indicated by hysteresis in χ(T) between field-cooled and zero-field-cooled measurements and from the relaxation of M in a small field that exhibits a stretched-exponential time dependence. The spin glass has a small FM component to the ordering and is hence inferred to be comprised of small FM clusters. The competing AFM and FM interactions along with crystallographic disorder associated with Ir substitution are inferred to be responsible for the development of a FM cluster-glass phase. A logarithmic T dependence of Cp at low T for x=0.14 is consistent with the presence of significant FM quantum fluctuations. This composition is near the T=0 boundary at x≈0.16 between the A-type AFM phase containing ferromagnetically-aligned layers of spins and the FM cluster-glass phase.

Short-range ferromagnetic order due to Ir substitutions in single-crystalline Ba(Co1− x Ir x )2As2 (0 ⩽ x ⩽ 0.25)

“…This result contrasts with the calorimetric properties of Ca(Co 1−x Ir x ) 2−y As 2 crystals where γ significantly increases with increasing Ir concentration [56].…”

Carrier Tuning of Stoner Ferromagnetism in ThCr$_{\mathbf{2}}$Si$_{\mathbf{2}}$-Structure Cobalt Arsenides

“…Iron and cobalt pnictide metals harbor weak to moderate magnetism driven by features in their electronic band structure lying close to the Fermi energy E F . Tuning the chemical composition of such materials has resulted in intriguing properties related to the underlying magnetism including non-Fermi liquid behavior [1], magnetic glassiness [2], electronic nematicity [3], as well as unconventional superconductivity [4][5][6]. While often discussed using a local-moment description [7][8][9], it is clear that the itinerant nature of the magnetism in these compounds is essential for facilitating the tunability of these phenomena.…”

Suppression of antiferromagnetic order and strong ferromagnetic spin fluctuations in Ni-doped CaCo2As2 single crystals

“…In a magnetic system with FM quantum fluctuations a C/T ∼ ln T contribution is observed [27,30,[66][67][68][69][70][71]. We found that the zero-field C p (T ) at low T of the x = 0.11, 0.16, 0.21, 0.31, 0.41, and 0.52 crystals can also be well described by…”

“…The fitted parameters are listed in Table V. The FM spin fluctuations are observed over a wide composition range in Ca(Co 1−x Ni x ) 2−y As 2 in contrast to the ranges observed in isostructural Sr(Co 1−x Ni x ) 2 As 2 [27] and Ca(Co 1−x Ir x ) 2−y As 2 [30] crystals where the spin fluctuations were limited to narrow composition ranges. The Sommerfeld coefficient γ in Table V increases by nearly a factor of two with only 3-5% Ni substitutions, indicating a corresponding rapid increase in the electronic density of state at the Fermi energy D(E F ).…”

“…In contrast to 122-type iron-arsenide compounds, none of the parent cobalt arsenides exhibit superconductivity even with doping but are still found to be rich in physics associated with itinerant magnetism of the conduction electrons [14][15][16][17][18][19][20][21][22][23][24]. Interesting physical properties are observed including non-Fermi-liquid behavior associated with a quantum critical point, spin-glass behavior, and helical magnetic ordering in electron-or hole-doped 122-type CoAs-materials [25][26][27][28][29][30][31][32]. For example, metallic SrCo 2 As 2 is a Stoner-enhanced paramagnet and does not exhibit any magnetic ordering down to a temperature T of 0.05 K; however, stripe-type AFM fluctuations were revealed via inelastic neutron-scattering (INS) measurements [16][17][18].…”