Magnetic and transport properties of the mixed 3d-5d-4fdouble perovskite Sm2CoIrO6.

Iridium-based double perovskites having mixed 3d-5d-4fmagnetic sub-lattices are expected to exhibit exotic magnetic phenomenon. In this paper, we report a study of structural, magnetic and transport properties of the mixed 3d-5d-4fdouble perovskite Sm2CoIrO6(SMCO), which crystallizes in monoclinic structure with space groupP21/nand the crystal symmetry remains same throughout the measured temperature down to 15 K. High resolution synchrotron x-ray diffraction reveals an isostructural phase transition around 104 K. Magnetization measurements on polycrystalline samples indicate that SMCO orders ferrimagnetically atTFiM= 104 K; while, a second transition is observed below 10 K due to the rare-earth (Sm3+) ordering. The ferrimagnetic transition is well-understood by Néel's two-sublattice model, which is primarily ascribed to antiferromagnetic coupling between Co2+and Ir4+sub-lattices. Electronic transport measurement shows the insulting behaviour of SMCO, which follows Mott variable-range hopping conduction mechanism. However, dielectric measurements as a function of temperature rules out the presence of magneto dielectric coupling in this compound.

Observations of ferromagnetic cluster glass and exchange bias behavior in the double perovskite compound La2Cu0.9Cr0.1IrO6.

La2CuIrO6 is a spin-orbit coupled Mott insulator, and shows a transition to noncollinear antiferromagnetic state from paramagnetic state below 74 K, and further into a weak ferromagnetic state below 54 K. Despite having two different magnetic phases, the La2CuIrO6 compound does not exhibit exchange bias phenomenon. In this present work, we report an experimental investigation on the structural and magnetic properties of the double perovskite compound La2Cu0.9Cr0.1IrO6 through high-resolution synchrotron x-ray diffraction, x-ray absorption near edge structure (XANES), and temperature and field-dependent magnetization measurements. Powder x-ray diffraction analysis reveals that the sample crystallizes in triclinic structure (space group P [Formula: see text]) alike parent La2CuIrO6 compound, while XANES measurements rule out the possibility of valence state alteration between constituting elements in this sample. Interestingly, La2Cu0.9Cr0.1IrO6 compound is found to exhibit ferromagnetic cluster glass behavior, where field-cooled magnetization undergoes two ferromagnetic transitions. A significant enhancement of ferromagnetic component is also evident from hysteresis loop study, which is likely associated with the electron hopping between J eff = 1/2 pseudospin state of Ir4+ ions and empty eg-orbital of Cr3+ ions. Exclusively, this Cr-doped compound exhibits exchange bias effect, which is related to the complex interfacial exchange coupling between the ferromagnetic clusters and the host antiferromagnetic matrix.

Optical and magnetic properties of Gd1-x Sr x CrO3 (0 ⩽ x ⩽ 0.15).

Optical and magnetic properties of Gd1-x Sr x CrO3 (0 ⩽ x ⩽ 0.15) polycrystalline samples have been investigated to identify the role of Sr2+ ion in GdCrO3 compound. Quantitative structural analysis using Rietveld refinement suggests the formation of single orthorhombic phase up to x = 0.1 composition, and in addition, a secondary monazite-type structure of SrCrO4 arises for x = 0.15 composition. Raman and Fourier transform infrared spectra also confirm the presence of SrCrO4 component for higher concentration of Sr2+ ion. Mixed valence state of Cr such as Cr3+ and Cr4+ is obtained from x-ray photoelectron spectroscopy. The presence of Cr4+ ion induces a reduction of optical band gap with increase of Sr content. The unoccupied states originate from Cr4+ modify d-d transitions. Larger cell volume and breaking of magnetic exchange pathways through nonmagnetic element (Sr) weaken the strength of interaction and consequently lower Néel and spin reorientation temperatures.

Spectroscopic and magnetic investigations of a spin-frustrated Mn-doped CoAl2O4 spinel.

Single-phase polycrystalline spin-frustrated spinel oxides Co1-xMnxAl2O4 (0 ≤ x ≤ 0.3) have been prepared to investigate the optical and magnetic properties. Linear variation of the lattice parameter along with the characteristic hyperfine electron paramagnetic resonance (EPR) signal establish the fact that the Mn2+ ions are incorporated at Co2+ sites of the CoAl2O4 lattice. Optical absorption spectra reveal three absorption features in wavelength regions: 250-400 nm, 500-700 nm and 1000-1700 nm. The optical band gap associated with the d-d transition increases from 1.84 eV to 1.88 eV with 30% Mn substitution. Temperature dependent magnetization measurements indicate a clear transformation of the magnetic ground state from the collinear antiferromagnetic state (for x = 0) to the spin-glass-like state (for x = 0.1) to the cluster-glass-like state (for x = 0.2 and 0.3) with the increase of Mn concentration. In addition, our time dependent isothermal remanent magnetization (IRM) study further fortifies the above transformation of the magnetic ground state. The value of the magnetic frustration parameter moderately decreases with Mn substitution, but the compositional variation is not monotonous.

Exchange bias effect in a finite site disordered canted antiferromagnet.

We report the temperature and magnetic field dependent magnetic properties of the single-phase polycrystalline La0.8Sr0.2Cr0.7Ru0.3O3 sample to explore the intrinsic magnetic phases of the sample. Our combined temperature and field dependent magnetization studies reveal the formation of ferromagnetic (FM) cluster-glass in the antiferromagnetic (AFM) matrix of host LaCrO3. Interestingly, the as-studied sample exhibits both zero-field-cooled (horizontal shift) and field-cooled (vertical shift) exchange bias effects and, in both cases, magnitude of exchange bias field continuously increases with the decrease of temperature. Our successive hysteresis loop measurements completely ruled out the effect of any minor hysteresis loop and thus, establishes this vertical shift as conventional field-cooled exchange bias (CEB) effect, originating from the uncompensated spins of randomly substituted canted AFM spin structure. A significantly larger value of CEB field (7.5 kOe) at 5 K is achieved for a cooling field of 50 kOe, not usually observed in conventional FM/AFM interfacial exchange-bias systems.

Evolution of magnetic properties and exchange interactions in Ru doped YbCrO3.

Magnetic properties of YbCr1-x Ru x O3 as a function of temperature and magnetic field have been investigated to explore the intriguing magnetic phenomena in rare-earth orthochromites. A quantitative analysis of x-ray photoelectron spectroscopy confirms the mixed valence state (Yb(3+) and Yb(2+)) of Yb ions for the highest doped sample. Field-cooled magnetization reveals a broad peak around 75 K and then becomes zero at about 20-24 K, due to the antiparallel coupling between Cr(3+) and Yb(3+) moments. An increase of the Ru(4+) ion concentration leads to a slight increase of compensation temperature T comp from 20 to 24 K, but the Néel temperature remains constant. A larger value of the magnetic moment of Yb ions gives rise to negative magnetization at low temperature. An external magnetic field significantly modifies the temperature dependent magnetization. Simulation of temperature dependent magnetization data, below T N, based on the three (two) magnetic sub-lattice model predicts stronger intra-sublattice exchange interaction than that of inter-sublattice. Thermal hysteresis and Arrot plots suggest first order magnetic phase transition. Random substitution of Ru(4+) ion reduces the magnetic relaxation time. Weak ferromagnetic component in canted antiferromagnetic system and negative internal magnetic field cause zero-field-cooled exchange bias effect. Large magnetocrystalline anisotropy associated with Ru creates high coercivity in the Ru doped sample. A maximum value of magnetocaloric effect is found around the antiferromagnetic ordering of Yb(3+) ions. Antiferromagnetic transition at about 120 K and temperature induced magnetization reversal lead to normal and inverse magnetocaloric effects in the same material.

Correlation among disorder, electronic and magnetic phases of SrRuO3.

Electric and magnetic properties of Sr1-xBaxRu1-xTixO3 (0 â©½ x â©½ 0.8) have been investigated to find the interrelationship between metallicity and ferromagnetism in SrRuO3 (SRO). The simultaneous doping of Sr and Ru with Ba and Ti results in single phase SRO at x = 0.1 and mixed phase of SRO and hexagonal BaTiO3 (h-BTO) at x â©¾ 0.2. Co-doping at Sr and Ru sites gives rise to oxygen vacancy and mixed valency of Ru (Ru(3+) and Ru(4+)). Room temperature resistivity increases due to modification of p(O)-d(Ru) hybridization and phase segregation. Temperature dependent resistivity reveals metal-insulator transition around 232 K at x = 0.1 and insulator down to 2 K at x â©¾ 0.2. The insulating state (x = 0.1) at low temperature is well described by weak localization and electron-electron interaction. Temperature dependence of resistivity (x â©¾ 0.2) follows Mott's three dimensional variable range hopping model. Localization length and average hopping distance decrease with the increase of x, indicating the presence of more disorder. Ferromagnetic transition temperature decreases to 149 K at x = 0.1 and remains constant up to x = 0.5. The Curie-Wiess (CW) temperature (ΘCW) decreases monotonically and becomes negative at x = 0.5. The effective magnetic moment estimated from CW law is smaller than that of pure SRO due to the formation of Ru(3+) ions. The saturation magnetization diminishes, suggesting the demagnetization factor owing to diamagnetic h-BTO. The coercivity increases from 6700 Oe (x = 0) to 12 500 Oe (x = 0.4) and then decreases to 3700 Oe (x = 0.5). Ferromagnetic cluster comprising of doped SRO gives rise to the formation of a Griffith-like phase. The co-occurrence of high jump in resistivity ratio and disappearance of ferromagnetism suggests an interplay between transport process and magnetism at low temperature.