Effect of Ni2+ (S = 1) and Cu2+ (S = ½) substitution on the antiferromagnetic ordered phase Co2(OH)PO4 with spin glass behaviour

Abstract

The hydroxyphosphates with formula Co_1.7M_0.3(OH)PO₄ (M = Ni, Cu) have been prepared from hydrothermal synthesis. The compounds have been characterized by X-ray powder diffraction and spectroscopic measurements. Diffuse reflectance data of both phases show bands belonging to the two chromophores, octahedral and trigonal bipyramidal, of the Co(II) ions together with others associated to the chromophores of nickel(II) and copper(II) ions. Magnetization measurements of Co_1.7Ni_0.3(OH)PO₄ show the presence of two maxima at ca. 62 and 5 K, respectively. The first peak was attributed to a three-dimensional antiferromagnetic ordering and the second one reveals the existence of a spin glass-like state. This behaviour was confirmed from the ac measurements obtained at different frequencies and applied fields. The Co_1.7Cu_0.3(OH)PO₄ phase exhibits only one maximum at 65 K in both χ_m and ac measurements associated to the three-dimensional antiferromagnetic interactions; however, the existence of spin glass behaviour is not observed. ZFC-FC curves for the cobalt–nickel and cobalt–copper compounds show irreversibility just below T_N indicating the existence of higher spin decompensations than in the Co₂(OH)PO₄ phase. This difference and its influence in the magnetic anomalies observed at low temperatures is the more important effect in the magnetic measurements of the ordered Co₂(OH)PO₄ phase substituted at about 15% of the Co²⁺ (S = ) by Ni²⁺ (S = 1) or Cu²⁺ (S = ½) ions. The origin of the frustration, which is necessary to achieve a disordered ground state, is investigated in these kinds of spin glass compounds. Taking into account the possible magnetic exchange pathways, which are essential to install competition and ensure cooperativeness of the freezing process, correlations between structural and magnetic properties have been performed.