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Register a new userStructural phase diagram and magnetic properties of Sc-substituted rare earth ferrites R1-xScxFeO3 (R=Lu, Yb, Er, and Ho)
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We have studied the structural stability of Sc-substituted rare earth (R) ferrites R1-xScxFeO3, and constructed a structural phase diagram for different R and x. While RFeO3 and ScFeO3 adopt the orthorhombic and the bixbyite structure respectively, the substituted compound R1-xScxFeO3 may be stable in a different structure. Specifically, for R0.5Sc0.5FeO3, the hexagonal structure can be stable for small R, such as Lu and Yb, while the garnet structure is stable for larger R, such as Er and Ho. The formation of garnet structure of the R0.5Sc0.5FeO3 compounds which requires that Sc occupies both the rare earth and the Fe sites, is corroborated by their magnetic properties.
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We report on optical studies of the thin films of multiferroic hexagonal (P.G. 6mm) rare-earth orthoferrites RFeO3 (R=Ho, Er, Lu) grown epitaxially on a (111)-surface of ZrO2(Y2O3) substrate. The optical absorption study in the range of 0.6-5.6 eV shows that the films are transparent below 1.9 eV; above this energy four broad intense absorption bands are distinguished. The absorption spectra are analyzed taking into account the unusual fivefold coordination of the Fe(3+) ion. Temperature dependence of the optical absorption at 4.9 eV shows anomaly at 124 K, which we attribute to magnetic ordering of iron sublattices.
The magnetic phase diagrams of RMnO3 (R = Er, Yb, Tm, Ho) are investigated up to 14 Tesla via magnetic and dielectric measurements. The stability range of the AFM order below the Neel temperature of the studied RMnO3 extends to far higher magnetic fields than previously assumed. Magnetic irreversibility indicating the presence of a spontaneous magnetic moment is found near 50 K for R=Er, Yb, and Tm. At very low temperatures and low magnetic fields the phase boundary defined by the ordering of the rare earth moments is resolved. The sizable dielectric anomalies observed along all phase boundaries are evidence for strong spin-lattice coupling in the hexagonal RMnO3. In HoMnO3 the strong magnetoelastic distortions are investigated in more detail via magnetostriction experiments up to 14 Tesla. The results are discussed based on existing data on magnetic symmetries and the interactions between the Mn-spins, the rare earth moments, and the lattice.
Spin reorientation and magnetisation reversal are two important features of the rare-earth orthorhombic provskites ($RM$O$_{3}$s) that have attracted a lot of attention, though their exact microscopic origin has eluded researchers. Here, using density functional theory and classical atomistic spin dynamics we build a general Heisenberg magnetic model that allows to explore the whole phase diagram of the chromite and ferrite compounds and to scrutinize the microscopic mechanism responsible for spin reorientations and magnetisation reversals. We show that the occurrence of a magnetization reversal transition depends on the relative strength and sign of two interactions between rare-earth and transition-metal atoms: superexchange and Dzyaloshinsky-Moriya. We also conclude that the presence of a smooth spin reorientation transition between the so-called $Gamma_4$ and the $Gamma_2$ phases through a coexisting region, and the temperature range in which it occurs, depends on subtle balance of metal--metal (superexchange and Dzyaloshinsky-Moriya) and metal--rare-earth (Dzyaloshinsky-Moriya) couplings. In particular, we show that the intermediate coexistence region occurs because the spin sublattices rotate at different rates.
The time-differential perturbed-angular-correlation (TDPAC) technique was applied to the study of the internal electric-field gradient (EFG) in Eu- and Ho-sesquioxides in their cubic bixbyite phases. The results, as well as previous characterizations of the EFG at $^{181}$Ta sites in oxides with the bixbyite structure, were compared to those obtained in experiments using $^{111}$Cd as probe, and to point-charge model and {it ab initio} results calculations for the EFG tensor at impurity sites in binary oxides. These studies provide quantitative information about electronic processes and the structural relaxations induced by the presence of impurity probes in the host lattices, and confirm the existence of nonionic contributions to the EFG in these systems. Our FP-LAPW calculations show that this nonionic contribution to the EFG is the dominating one, and that it is originated in the population of {it p} states (5{it p} in the case of Cd, 6{it p} for Ta).
The lanthanide orthoborates, $Ln$BO$_3$, $Ln$ = Gd, Tb, Dy, Ho, Er, Yb crystallise in a monoclinic structure with the magnetic $Ln^{3+}$ forming an edge-sharing triangular lattice. The triangles are scalene, however all deviations from the ideal equilateral geometry are less than 1.5%. The bulk magnetic properties are studied using magnetic susceptibility, specific heat and isothermal magnetisation measurements. Heat capacity measurements show ordering features at $T leq$ 2 K for $Ln$ = Gd, Tb, Dy, Er. No ordering is observed for YbBO$_3$ at $T geq$ 0.4 K and HoBO$_3$ is proposed to have a non-magnetic singlet state. Isothermal magnetisation measurements indicate isotropic Gd$^{3+}$ spins and strong single-ion anisotropy for the other $Ln^{3+}$. The change in magnetic entropy has been evaluated to determine the magnetocaloric effect in these materials. GdBO$_3$ and DyBO$_3$ are found to be competitive magnetocaloric materials in the liquid helium temperature regime.
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