Possible ferromagnetic interactions between IS Co³⁺ ions in Nb doped cobaltites

Abstract

In LnCoO 3 oxides (Ln-lanthanide) Co ions in the ground state are characterized by the low spin electronic configuration t ) with temperature enhancement [1][2][3][4][5]. For example, the spin state of cobalt ions in LaCoO 3 gradually changes from LS to IS with temperature increment in the temperature range of 40-100 K.The energy difference between LS and IS state of the Co 3+ ions is rather small (it is about 180 K in temperature scale) [6 -8]. Usually this energy difference (δE) increases with reduction of the CoO 6 octahedron volume, so it is possible to control δE, changing the mean ionic radius of Ln 3+. As a rule, the LS → IS transition temperature slightly increases in LnCoO 3 solutions with Ln 3+ ionic radius reduction [9 -11]. External pressure can also lead to low spin state stabilization [12]. The Sr 2+ ionic radius is significantly greater than that of the La 3+ ion, so it is possible to expect stabilization of the intermediate spin state of cobalt ions by substituting Sr 2+ ions for La 3+ ones. However, at such heterovalent substitution Co 4+ ions appear, leading to the ferromagnetic metallic ground state [13][14][15]. The origin of the ferromagnetic state in metallic cobaltites and manganites has been a subject of discussion for a long time [16].

Anomalous magnetic and spin glass behavior in Nb-substituted LaCo1−xNbxO3

“…Recently, Oygarden et al, reported the structural transition from rhombohedral to orthorhombic/monoclinic and reduction in the electrical conductivity with Nb substitution in LaCo 1−x Nb x O 3 , which are discussed in terms of spin state of Co ion [56] (0.545Å in LS, 0.56Å in IS and 0.61Å in HS) ions, which results increasing the average radius at the Co position and induce the lattice expansion [56]. Interestingly, the simultaneous substitution of Sr and Nb in LaCoO 3 prevent the creation of Co 4+ and it is reported that exchange interactions between IS/HS Co 3+ can induce ferromagnetism [57,58].…”

Anomalous magnetic and spin glass behavior in Nb substituted LaCo$_{1-x}$Nb$_x$O$_3$

“…Also, the ionic radius of Nb 5+ (0.64 Å )/Co 2+ [0.65 Å in LS (S = 0.5) and 0.745 Å in HS (S = 1.5)] is larger than that of the Co 3+ (0.545 Å in LS, 0.56 Å in IS and 0.61 Å in HS) ions, which results increasing the average radius at the Co position and induce the lattice expansion [56]. Interestingly, the simultaneous substitution of Sr and Nb in LaCoO 3 prevent the creation of Co 4+ and it is reported that exchange interactions between IS/HS Co 3+ can induce ferromagnetism [57,58].…”

Unraveling the magnetic interactions and spin state in insulating Sr$_{2-x}$La$_x$CoNbO$_6$

“…Also, the IS state found to be energetically close to the LS state when the hybridization between Co e g and O 2p orbitals has been considered [6]. In order to understand the temperature dependent magnetic susceptibility (χ-T) behavior of LaCoO 3 , it has been suggested that the presence of HS states favors the antiferromagnetic (AFM) exchange interactions, whereas the IS states of Co 3+ show the ferromagnetic (FM) interactions [8][9][10]. Hence, the presence of mixed states results in the competition between FM and AFM interactions, which depends on their relative population.…”