Effect of Ising-type Tb^{3 +} ions on the low-temperature magnetism of La, Ca cobaltite

Abstract

Abstract. Crystal and magnetic structures of the x = 0.2 member of La 0.8−x Tb x Ca 0.2 CoO 3 perovskite series have been determined from the powder neutron diffraction. Enhancement of the diffraction peaks due to ferromagnetic or cluster glass ordering is observed below T C = 55 K. The moments evolve at first on Co sites, and ordering of Ising-type Tb 3+ moments is induced at lower temperatures by a molecular field due to Co ions. The final magnetic configuration is collinear F x for cobalt subsystem, while it is canted F x C y for terbium ions. The rare-earth moments align along local Ising axes within ab-plane of the orthorhombic P bnm structure. The behavior in external fields up to 70 − 90 kOe has been probed by the magnetization and heat capacity measurements. The dilute terbium ions contribute to significant coercivity and remanence that both steeply increase with decreasing temperature. A remarkable manifestation of the Tb 3+ Ising character is the observation of a low-temperature region of anomalously large linear term of heat capacity and its field dependence. Similar behaviours are detected also for other terbium dopings x = 0.1 and 0.3. PACS numbers: 61.05.fm;Effect of Ising-type Tb 3+ on the magnetism of La,Ca cobaltite 2

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

“…The resulting moments at Tb positions are thus canted (mode F x C y ), with moments fixed alternatively close to 1 1 0 and 1 1 0 directions for two crystallographic positions in Pbnm structure. Their saturated magnitude is 8.5 µ B that is in excellent agreement with the theoretical prediction for Tb 3+ quasidoublet (for more details see [17]), and observed canting is an inherent consequence of the Ising character of Tb 3+ pseudospins. Moreover, the FM components of Tb moments show a typical Curie or Curie-Weiss temperature dependence as seen in reciprocal plots in the insets of figure 7.…”

“…5 µ B /Tb. This is a strong that indication for a presence of relatively large Ising moments, carried by Tb 3+ ions in quasidoublet ground state [12,14,17].…”

“…Previous reports on the Tb 3+ -based aluminates [12,13] and cobaltites [14,15], all of the orthorhombic Pbnm perovskite structure, have shown that the Tb 3+ eigenstates are formally singlets, as expected for non-Kramers ions, but there is often an accidental degeneracy of the ground and the first excited state, which is responsible for Ising-like moments of Tb 3+ as high as 8.5 µ B [12,16]. 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].…”

“…This specific kind of accidental degeneracy is responsible for large Zeeman splitting, which is highly anisotropic and linear from relatively low fields H, so that the Tb 3+ ions can be viewed as Ising moments of 8.5 µ B magnitude (fictitious J′ = 1/2, g J′ ≈ 17). The local Ising axes are oriented within the ab plane, being inclined, for two non-equivalent A sites in Pbnm perovskites, to ±36° from the orthorhombic axis a [12,17]. In the Ibmm perovskites with all A sites equivalent, the Ising axes are oriented along the a axis in the structure [19].…”

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

“…The current paper investigates the behaviour of heavier RE 3+ ions that have been doped into the La 1−x Sr x MnO 3 (x = 0.30). Let us note that the above-mentioned reports on the RE-based aluminates [2,3], cobaltites [4,5], and also Tb-doped La 1−x Ca x CoO 3 [6], referred all to the orthoperovskite Pbnm structure, where the local neighbourhood of A sites is distorted to low symmetry given by the monoclinic point group C s . In that particular case, the ground and the first excited states of the non-Kramers Tb 3+ and Ho 3+ , though both in the singlet forms, are very close in energy (0.002-0.025 meV), representing thus a quasi-doublet of strong magnetic character [2,4].…”