Spin-glass behavior in single crystals of hetero-metallic magnetic warwickites MgFeBO4, Mg0.5Co0.5FeBO4, and CoFeBO4

Abstract

a b s t r a c tMagnetic properties of heterometallic warwickites MgFeBO 4, Mg 0.5 Co 0.5 FeBO 4, and CoFeBO 4 are presented, highlighting the effect of Co substitution on the magnetic properties of these compounds. The analysis of magnetization and heat capacity data has shown that these compounds exhibit a spin-glass transition below T SG ¼10, 20 and 22 K, respectively. Using zero field ac susceptibility as entanglement witness we find that the low dimensional magnetic behavior above T SG show quantum entanglement behavior χ(Τ)∝T À α(Τ) up to T E E 130 K. The α parameters have been deduced as a function of temperature and Co content, indicating the existence of random singlet phase in this temperature region. Above T E the paramagnetism is interpreted in terms of non-entangled spins giving rise to Curie-Weiss paramagnetism. The different intra-and inter-ribbon exchange interaction pathways have been calculated within a simple indirect coupling model. It is determined that the triangular motifs in the warwickite structure, together with the competing interactions, induce frustration. The spin-glass character is explained in terms of the substitutional disorder of the Mg, Fe and Co atoms at the two available crystallographic sites, and the frustration induced by the competing interactions. The Co substitution induces uniaxial anisotropy, increases the absolute magnetization and increases the spin-glass freezing temperature. The entanglement behavior is supported in the intermediate phase irrespective of the introduction of anisotropy by the Co substitution.

Crystal and local atomic structure of MgFeBO4 , Mg0.5 Co0.5 FeBO4 and CoFeBO4 : Effects of Co substitution

“…Indeed, in Ref. , we show that as Co is introduced first in the Mg 0.5 Co 0.5 FeBO 4 , the induced anisotropy is along the a crystal axis, while in the fully substituted CoFeBO 4 , it is along the b ‐axis. The striking magnetic anisotropy properties have been observed in the parent ludwigites .…”

“…These results indicate that the local distortion of the FeO 6 octahedra in MgFeBO 4 is almost temperature independent over the whole temperature range. By contrast, a rather pronounced temperature dependence of the DW factor of the average Fe–O distances was found for CoFeBO 4 below 27 K. In fact, in recent magnetization measurements, a spin‐glass magnetic transition has been found at 22 K . The freezing of the magnetic fluctuations of the Fe 3+ magnetic moments below the spin‐glass transition was observed in temperature‐dependent experiments .…”

“…In contrast, the electrical transport properties of the compounds do not show a significant dependence on Co content; i.e., in the three compounds with x = 0, 0.5 and 1.0, the electrical conduction was found to be governed by Mott variable range hopping at low temperatures (210–270 K) and by thermal activation at high temperatures (280–400 K) . Moreover, magnetization and susceptibility measurements as a function of temperature, field, and relaxation time indicate peculiar spin‐glass behavior and quantum entanglement . It is evident from these antecedents that the understanding of the structural details of these compounds, the chemical disorder as a function of Co substitution, and valence distribution are essential to interpret quantitatively the rich phenomenology of the physical properties of these series.…”

“…They offer the opportunity to study the effect of Co substitution on the structural, electric, and magnetic properties in a regular lattice that has chemical disorder. In contrast to MgFeBO 4 , which is magnetically isotropic , Co may induce anisotropy and modify the spin‐glass behavior. In a recent Mössbauer spectroscopic (MS) study as a function of temperature , we have found spin‐glass behavior in MgFeBO 4 and CoFeBO 4 , with different dynamics near T SG .…”

Antiferromagnetism of the cation-ordered warwickite system Mn2-Mg BO4 (x = 0.5, 0.6 and 0.7)

“…The numerous heterometallic warwickites are magnetically disordered systems. They undergo the transition to the spin-glass state with the intermediate temperature phase -random exchange Heisenberg antiferromagnetic chains [8,9,17,18] or demonstrate the properties of the random singlet phase [19]. The effects of the magnetic frustrations arising from the cation disorder and the crystal structure peculiarities are manifested in the extremely high parameter of magnetic frustration > 10 [9,17].…”