Effects of Tb³⁺dopants in the La_1−xSr_xMnO₃bulk and nanoparticle ferromagnets

Abstract

Three forms of La,Sr-manganites are synthesized and the role of Tb doping is investigated. First two systems are sol-gel nanoparticles and sintered ceramics of the composition La0.56Tb0.07Sr0.37MnO3, whereas the third system is formed by comparable nanoparticles La0.51Tb0.06Sr0.43MnO3 synthesized in molten salt. The samples show pseudocubic perovskite structure with only small tilts of MnO6 that point to Ibmm symmetry in the bulk and [Formula: see text] symmetry in nanoparticles. SQUID magnetometry and neutron diffraction reveal a complete FM order of Mn spins in bulk, a reduced order in nanoparticles, and non-zero moments at A sites. Detailed analysis suggests that the dodecahedral coordination of A sites adapts to small terbium size, and the resulting crystal field splitting of Tb(3+) yields a singlet ground state. The response to exchange and external fields is characterized as a giant Van Vleck paramagnetism in contrast to the Curie-type behaviour of Tb-based orthoaluminates and orthocobaltites with the quasi-doublet ground state.

Magnetic properties of rare-earth-doped La0.7Sr0.3MnO3

“…In contrast to these expectations, the Curie-type tail is absent or largely suppressed in the temperature dependence of net magnetization in figure 5. Similar observation was reported earlier for the strontium richer compound La 1−x Sr x MnO 3 (x ≈ 0.4) [7], where similar absence of Curie-type tail was attributed to the singlet ground state of Tb 3+ ions due to the trigonal distortion of the local coordination polyhedra. The issue of the quasi-doublet or singlet ground states of Tb 3+ ions in perovskites is discussed in more details in sections 3.4 and 4 below.…”

“…A different behaviour has been revealed upon Tb 3+ doping into La 1−x Sr x MnO 3 systems of higher strontium content (x ≈ 0.4) that show commonly the rhombohedral R c 3 perovskite structures. Here, the A-site distortion is described by the trigonal point group D 3 and, assuming the same distortion also around Tb dopants, the calculations are suggesting a singlet ground state, followed by a doublet at excitation energy of about 1.5 meV [7]. Instead of Ising moments and Curie-type paramagnetism in the quasi-doublet case, the singlet ground state of non-Kramers Tb 3+ in the R c 3 perovskite manganite is characterized by giant Van Vleck paramagnetism (χ VV ≈ 0.4 emu mol −1 ).…”

Effect of Tb3+doping in mixed-valence manganites and cobaltites

“…Similarly large Tb 3+ moments have been detected for the Tb-doped cobaltite La 1−x Ca x CoO 3 having a similar strongly distorted orthoperovskite structure [17]. On the contrary, there are very recent reports on the Tb 3+ doping in La 1−x Sr x MnO 3 manganites of the rhombohedral R3c perovskite structure, which point to a significant splitting of Tb 3+ quasi-doublet, being of 1 meV order [18,19]. In such a case, the Tb 3+ ions contribute at the lowest temperatures by giant Van Vleck paramagnetism-they carry no magnetic moment (no first order Zeeman splitting) unless local magnetic fields of external or molecular origin are present.…”

“…for La 0.7 Tb 0.1 Sr 0.2 MnO 3 at 2 K, is apparently spread over large temperature range instead of forming a common Curie tail. Such behaviour is a signature for existence of giant Van Vleck paramagnetism due to Tb 3+ ions in singlet ground state [18,19]. The data for cobaltite systems in right panel of figure 4 illustrate a completely different behaviour.…”