Magnetization measurements of a Gd 0.5 Ba 0.5 CoO 3 perovskitelike compound have revealed an anomalous behavior at T i 240 K and T C 277 K that corresponds to appearing and disappearing spontaneous magnetization. The transition at T i is accompanied by the jump of conductivity and a giant magnetoresistance. Below T i , Gd 0.5 Ba 0.5 CoO 3 exhibits metamagnetic behavior. At about T M 370 K, a transition from a semiconductive to a quasimetallic state has been observed. It is supposed that a ferromagnetic low-spin cobalt state occurs in the temperature range between T i and T C .[S0031-9007(98)05789-5] 72.15.Gd, 75.70.Pa La 12x A x CoO 3 (A Sr, Ba) cobaltites are of considerable interest because of the peculiar way their magnetic and transport properties change with composition and temperature [1][2][3][4][5]. While LaCoO 3 shows a high resistivity and antiferromagnetic exchange interaction, the La 12x Sr x CoO 3 solid solutions evolve toward a ferromagnetic intermediate-spin state with itinerant 3d electrons as x increases [4][5][6][7]. The composition with x 0.5 is a metallic ferromagnet with a moment approximately 1.5m B per formula unit and the Curie temperature of 220 K. The size of the Ln ion is well known to influence strongly the magnetic and transport properties of the compounds with the perovskite structure. However, the data on cobaltites of rare earth elements doped by Ba or Sr are quite limited. The studies of Ln 12x Sr x CoO 3 (Ln Pr, Nd, Sm) have shown these materials to be similar to La 12x Sr x CoO 3 with T C increasing with x as well as with the size of the rare earth ion [3].This paper reports the discovery of a new family of magnetic semiconductors exhibiting both antiferromagnetferromagnet and metal-insulator first-order phase transitions. The nature of these transitions and properties of Gd 0.5 Ba 0.5 CoO 3 compound differs from those observed in manganites as well as other related materials studied earlier. We have examined structural, magnetic, and transport properties of the Gd 12x Ba x CoO 3 perovskites in order to investigate the cobaltite properties as a function of lanthanide ionic radii and a variation of the spin state of the cobalt ions with temperature. We have found that Gd 0.5 Ba 0.5 CoO 3 exhibits a first-order phase transition accompanied by dramatic changes of magnetic properties and an anomalous behavior of electrical transport at about T i 240 K.The studied ceramic samples were prepared by a solid state reaction. Mixtures of Gd 2 O 3 , BaCO 3 , Co 3 O 4 were pressed into pellets, sintered at 1473 K for 5 h in air and then cooled to room temperature at a rate of 100 K͞h. This process was repeated in order to obtain a homogeneous solid solution. According to the powder x-ray diffraction (XRD) patterns, the specimens were single phase. The XRD data at room temperature were indexed on the basis of a distorted perovskite-type structure with orthorhombic symmetry. For Gd 0.5 Ba 0.5 CoO 3 the lattice parameters were calculated to be a 3.909 Å, b 3.876 Å, and c 3.768 Å. As oxygen ...

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A study of crystal structure, elastic, and magnetic properties of low-doped Nd 1−x Ca x MnO 3 (x 0.15) perovskites has been carried out. The ferromagnetic component is shown to increase under hole doping and, simultaneously, the temperature of the orbital order-disorder phase transition decreases. The mechanism of the concentrational transition from a weak ferromagnetic state (x = 0) to a ferromagnetic one (x > 0.15) is discussed using a two-phase model, according to which the samples consist of weak ferromagnetic and ferromagnetic phases exchange coupled at their boundary. It is found that interaction between different magnetic phases leads to spin reorientation which takes place for 0.06 x 0.1 compounds around T eff ∼ 9 K. In the temperature range from 5 to 20 K, metamagnetic behaviour is revealed for the Nd 0.92 Ca 0.08 MnO 2.98 sample. H versus T as well as T versus x magnetic phase diagrams, which are characterized by the missing of a canted phase, are proposed. The appearance of orientational transitions is explained on the basis of a magnetic analogue of the Jahn-Teller effect taking into account that the magnetic moments of Nd ions are ordered parallel to the moments of Mn ions in the ferromagnetic phase, and opposite to the direction of the weak ferromagnetic vector at T > T eff in the weak ferromagnetic phase.

The magnetic properties of the site ordered multicomponent vanadate Mg 3 Fe 4 ͑VO 4 ͒ 6 are studied using dc magnetization and electron paramagnetic resonance ͑EPR͒ measurements. The static susceptibility shows antiferromagnetic interactions between Fe 3+ spins with a Curie-Weiss temperature ⌰ = −111͑1͒ K, while a transition to a spin-glass-like state is observed at T Ϸ 8.5 K, indicating appreciable spin frustration. EPR measurements corroborate the presence of antiferromagnetically coupled Fe 3+ spins from high temperatures, while a distinct change in the temperature variation of the EPR parameters is observed at T Ͻ 80 K, complying with the mean-field energy scale provided by the Curie-Weiss temperature of the system. The resulting magnetic inhomogeneity that persists despite the absence of cation disorder between magnetic and diamagnetic metal ions is attributed to the presence of small amounts of oxygen deficiency that modifies the connectivity and superexchange coupling between Fe 3+ spins and leads to a disordered magnetic ground state.

Inelastic neutron scattering measurements were performed on single crystal Co3V2O8 wherein magnetic cobalt ions reside on distinct spine and cross-tie sites within kagomé staircase planes. This system displays a rich magnetic phase diagram which culminates in a ferromagnetic ground state below TC ∼ 6 K. We have studied the low-lying magnetic excitations in this phase within the kagomé plane. Despite the complexity of the system at higher temperatures, linear spin-wave theory describes most of the quantitative detail of the inelastic neutron measurements. Our results show two spin-wave branches, the higher energy of which displays finite spin-wave lifetimes well below TC, and negligible magnetic exchange coupling between Co moments on the spine sites. PACS numbers: 75.30.Ds, 75.50.Dd, 75.10.Dg Magnetic materials in which the constituent magnetic moments reside on networks of triangles and tetrahedra have been of great interest due to their propensity for exotic ground states, a consequence of geometrical frustration [1]. While ferromagnetically-coupled moments on such lattices generally do not result in such ground states, ferromagnets, and materials which display both ferromagnetic (FM) and antiferromagnetic (AFM) interactions, on such lattices remain of great interest, in part due to the relative scarcity of well-studied examples, and in part due to intriguing spin ice [2] and multiferroic phenomena [3] which characterize some of these ground states. The kagomé lattice is comprised of a two-dimensional network of corner-sharing triangles. Several realizations of magnetic moments on stacked kagomé lattices with varying degrees of crystalline order have been extensively studied. Recently studied examples include jarosites, such as KFe 3 (OH) 6 (SO 4 ) 2 [4] and herbertsmithite ZnCu 3 (OH) 6 Cl 2 [5], both of which show evidence of strong magnetic frustration. The stacked kagomé staircase materials M 3 V 2 O 8 (M=Ni,Co) display orthorhombic crystal structures with space group Cmce [6]. Their kagomé layers are buckled and composed of edge-sharing M 2+ O 6 octahedra. These layers are separated by non-magnetic V 5+ O 4 tetrahedra. The buckled kagomé layers are perpendicular to the orthorhombic b-axis and form what is known as a stacked kagomé staircase structure. Figure 1 shows the projection of the kagomé staircase onto the a-c plane. The two inequivalent M sites are known as spines (M1) and crossties (M2). The superexchange interaction between spine and cross-tie sites and between two adjacent spine sites : Co (1) : Co (2) a c R+δ1 R-δ2 R-δ1 R+δ2 R JSS JSC JSC (a) (b) FIG. 1: [color online] (a) A schematic diagram of the kagomé staircase structure as reduced to 2D in the a-c plane. The cobalt ions are represented by open and solid circles for spine (M1) and cross-tie sites (M2), respectively. Chains of spine sites running parallel to the a-direction are alternatively above (⊙) and below the plane (⊗). (b) The basis used in the linear spin-wave theory calculation.are denoted by J sc and J ss , respectively.One member...

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