Y. Q. Zhang

Large magnetoresistance over an entire region from 5 to 380 K in double helical CoMnSi compound

A large magnetoresistance (MR) is observed in a double helical CoMnSi compound over the entire temperature region from 5 K to the maximum measuring temperature of 380 K, with the largest MR ratio of -18.3% at 245 K and the smallest MR ratio of -5.5% at 85 K at 5 T. This phenomenon is ascribed to two different mechanisms in different temperature regions. The suppressed spin fluctuations of the double helical structure are responsible for the MR below 110 K. However, in consideration of the natural multilayer superstructure of CoMnSi, the larger MR above 110 K is ascribed to the decrease in K-space restrictions when the change in magnetic structure from double helical order to fan order occurs.

Defect-induced charge-order melting in thin films of Pr0.5Ca0.5MnO3

We have investigated the relation between defect structure and charge order melting in thin films of epitaxial Pr0.5Ca0.5MnO3 (PCMO), grown under strain on SrTiO3. We compared the behavior of an 80 nm film grown in one deposition step at 840 degrees C with the behavior of a film grown in two steps. In the two-step case, a thin PCMO layer of 10 nm was deposited at 120 degrees C, followed by 70 nm deposited at 840 degrees C. The increase of the growth temperature leads to complete crystallization of the first layer and the lattice constants of the two-step grown film indicate that tensile strain is still present. On the other hand, a magnetic field of only 5 T is required to melt the charge-order state in the two-step grown film, which is a much lower than the value for the normally grown film. This appears to be connected to a larger amount of threading dislocations present in the first (recrystallized) layer. (c) 2007 American Institute of Physics.

Giant magnetoresistance in Mn2−xCuxSb (x⩽0.4) compounds

Magnetic and magnetotransport properties of Mn2-xCuxSb (x=0, 0.1, 0.2, and 0.4) compounds have been studied. A phase transition from a ferrimagnetic to an antiferromagnetic state occurs in the Cu-substituted compounds as the temperature is reduced. A magnetic field-driven transition takes place in the Cu-containing compounds, changing the spin configuration from antiferromagnetic to ferromagnetic. The giant magnetoresistance effect is observed in these compounds, accompanying the transition. The largest magnetoresistance ratio of -40% is observed for Mn1.9Cu0.1Sb at 170 K in a magnetic field of 5 T

Dislocations in charge-ordered Pr0.5Ca0.5MnO3 epitaxial thin films prepared by a two-step growth technique

Charge-ordered Pr0.5Ca0.5MnO3 (PCMO) thin films epitaxially grown on SrTiO3 (100) substrates were prepared by a two-step growth technique which resulted in a 10 nm thick first layer and a 70 nm thick main layer. The dislocations in the as-received films were investigated using conventional and high-resolution transmission electron microscopy. Pure-edge misfit dislocations with Burgers vectors and line directions were found to be the major interfacial defects responsible for the full misfit relief in the PCMO films. These dislocations constitute a square grid of dislocation lines parallel to the PCMO/SrTiO3 interface. In contrast, two types of dislocations were identified within the first layer. One is of edge type with Burgers vectors and line directions ; the other, of screw type with Burgers vectors and line directions . Cross-slip of the latter may contribute to the multiplication of misfit dislocations necessary for a total misfit relaxation. Few threading dislocations were observed in the main layer. The dislocation configurations in the films are discussed in detail.

Giant magnetoresistance associated with a first-order transition between two ferrimagnetic states in Mn 2−x Zn x Sb (x < 0.3) compounds

A giant magnetoresistance (GMR) is observed in the Mn2−xZnxSb ( x< 0.3) system, with the largest MR ratio of −37.6% in a field of 5 T at 120 K for the Mn1.9Zn0.1Sb compound. Different from other Mn2Sb-based compounds, the GMR in Mn2−xZnxSb is closely correlated with a field-induced transition from a weak ferrimagnetic (WFI) state to a ferrimagnetic (FI) state. It is understood that the influences of both super-zone gap and spin-dependent scattering are responsible for GMR in the present system. Magnetic hysteresis and phase coexistence of the WFI and the FI phases suggest that this WFI–FI transition is of first order. The different mechanisms responsible for butterfly loops of magnetization/resistivity curves in different magnetic states are discussed.

Microstructure of the potentially multiferroic Fe/BaTiO3 epitaxial interface

The Fe/BaTiO3 thin-film layered structure is a prototype of charge-mediated composite multiferroics, which is a promising but challenging route to achieve a sizable magnetoelectric effect. The real structure of the interface between the ferromagnetic Fe and ferroelectric BaTiO3 layers is crucial. In this paper, epitaxial Fe layers were successfully grown on top of BaTiO3 layers by carefully controlling the pulsed laser deposition and magnetron sputtering procedures. A detailed study of interfacial structure and defects at the Fe/BaTiO3 interface was carried out by transmission electron microscopy (TEM). Electron diffraction patterns and diffraction contrast images reveal a definite epitaxial relationship between Fe and BaTiO3 (001) films and a semi-coherent interface with nearly periodic interfacial dislocations. Based on high-resolution TEM images from both [010] and [110] direction observations, the interfacial dislocations were found to be partial with Burgers vectors and line directions of ⟨010⟩. By employing high-resolution Z-contrast imaging, the positions of individual atoms columns were resolved. The formation mechanism of interfacial dislocations was proposed in terms of geometrical models of the interface structure. On the basis of the remaining strain analysis in each layer, the effects of both BaTiO3 thickness and the SrTiO3 substrates on the density of the interface defects were discussed.

Magnetic, electronic transport and magneto-transport behaviours of (Co1−xMnx)2P compounds

Magnetic, electronic transport and magneto-transport behaviours of (Co1-xMnx)(2)P (0.55 <= x <= 0.675) compounds have been systematically investigated. A typical metallic-conductivity behaviour is observed in the ferromagnetic compound (Co0.45Mn0.55)(2)P. The increase in the Mn concentration gives rise to dramatic changes in magnetic, electronic transport and magneto-transport behaviours. With increasing temperature, a first-order phase transition from antiferromagnetism to ferromagnetism takes place at about 145 K, 185K and 240K for x = 0.60, 0.625 and 0.65, respectively. (Co0.4Mn0.6)(2)P and (Co0.375Mn0.625)(2)P compounds experience a metal-insulator transition (Anderson transition) with decreasing temperature. An external magnetic field of 5 T strongly influences the Anderson transition, lowering the transition temperature from 80 to 55K for (Co0.4Mn0.6)(2)P and from 115 to 70K for (Co0.375Mn0.625)(2)P. In contrast with this metal-insulator transition, an insulating behaviour appears in the temperature range from 10 to 300K for (Co0.35Mn0.65)(2)P and (Co0.325Mn0.675)(2)P compounds. Below the antiferromagnetic-ferromagnetic transition temperature TAF-F, a metamagnetic transition can be induced by an external magnetic field. The metamagnetic transition is accompanied by a maximum magnetoresistance ratio of -7%, -6.3% or -3.7% at 5 T in the (Co0.4Mn0.6)(2)P, (Co0.375Mn0.625)(2)P or (Co0.35Mn0.65)(2)P compound at 10 K. The mechanisms of magnetoresistive behaviours are discussed in terms of the formation of a super-zone gap in the antiferromagnetic state.

Large negative magnetoresistance in the Mn2Sb0.88Ge0.12 compound

Magnetic, magneto-transport properties and electronic specific heat coefficient of the Mn2Sb0.88Ge0.12 compound have been studied. A metamagnetic transition from antiferromagnetic to ferrimagnetic, which is induced by an external field, is accompanied with a large negative magnetoresistance (MR) effect. The MR ratio at room temperature is ?11% at a magnetic field of 5?T. The field dependence of the MR ratio and the magnetization of the Mn2Sb0.88Ge0.12 compound, together with the electronic specific heat coefficients, confirm that the negative MR effect is originated from the reconstruction of Fermi surface, due to the collapse of the super-zone gap after the metamagnetic transition.