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

Abstract

We report the structural, magnetic, transport and electronic properties of Nb substituted LaCo1−xNbxO3 (x = 0-0.2). The Rietveld analysis of x-ray diffraction data demonstrate structural phase transitions from rhombohedral to orthorhombic and further to monoclinic with increasing the Nb concentration up to x ≥ 0.2. Interestingly, we observed dramatic changes in the magnetization (M) with increasing the Nb concentration, as the M sharply increases below 10 K even at 2.5% substitution. Furthermore, ac susceptibility data show the spin-glass behavior in x = 0.1 sample. We find that the density of states near the Fermi level decreases and the activation energy increases, which results in the decreasing conductivity with higher Nb concentration. A significant shift in the peak position of A2g phonon mode has been observed using Raman spectroscopy, which indicates the change in the coupling due to the structural distortion with Nb substitution. The core-level photoemission study confirms that the Nb is present in 5+ valence state. Our study reveals that the nonmagnetic Nb 5+ (d 0 ) substitution converts Co 3+ ions to Co 2+ and stabilize both in the high-spin state. Our results suggest that structural and spin-state transitions as well as the difference in the ionic radii between Nb 5+ and Co 3+ are playing an important role in tuning the physical properties.

Spin dynamics and unconventional magnetism in insulating La$_{(1-2x)}$Sr$_{2x}$Co$_{(1-x)}$Nb$_{x}$O$_3$

“…It is well reported that regardless of having larger ionic radii, Sr 2+ does not induce any structural transformation in the system up to 50% concentration [36]. Whereas, our recent study reveals that the Nb 5+ substitution at Co site induces structural transformation of lower symmetry from the rhombohedral to orthorhombic, and then monoclinic at higher concentration [7].…”

“…We used strontium carbonate (SrCO 3 ), cobalt oxide (Co 3 O 4 ) and niobium oxide (Nb 2 O 5 ) as purchased, whereas purchased lanthanum oxide (La 2 O 3 ) powder was dried at 700 0 C for 12 hrs prior to use. Stoichiometric amount of starting materials (all ≥99.9% from Sigma/Alfa) were thoroughly mixed with the help of mortar-pestle and reacted at 1100 0 C for 12 hrs in air, pellets of calcined powder were cold pressed at 2000 psi and sintered in air at 1475 0 C for 6 -10 hrs [7,37].…”

“…The spin-state transition present in the LaCoO 3 is an open question for the researchers even after enormous theoretical as well as experimental efforts since many decades [1][2][3][4][5][6][7]. In LaCoO 3 , Co is in Co 3+ (3d 6 ) state and their ground state is characterized by the low spin electronic configuration [LS; S=0; t 6 2g e 0 g ], whereas other excited states are defined as intermediate spin state (IS; S = 1; t 5 2g e 1 g ) and high spin state (HS; S = 2; t 4 2g e 2 g ).…”

“…The majority of researchers intended that ferromagnetism in cobaltites attribute to the phenomenon of double exchange interaction (between Co 3+ -Co 4+ ), as comprehended in manganites [34,35]. Other interesting case is that the hole doping (Ca, Sr, Ba at La site) in LaCoO 3 drives the system in metallic regime whereas the substitution of Nb at Co site impels towards the insulating regime [7]. Therefore, the co-substitution of both cations on their respective sites in LaCoO 3 would alter its transport behavior.…”

“…Here, we have simultaneously substituted Sr and Nb in place of La and Co in a ratio of 2:1, respectively. The substitution of Sr 2+ at La 3+ site changes the Co ion from Co 3+ to Co 4+ , and the substitution of one Nb 5+ at Co site converts two Co 3+ into the Co 2+ ions [7,30]. Therefore with co-substitution of Sr and Nb in 2:1 ratio, the valence state of Co ion can be maintained in 3+.…”