Y. Y. Wu

Effective method to identify the vacancies in crystalline GeTe

X-ray diffraction and photoelectron spectroscopy of different Co-doped concentration GeTe have been made to identify the vacancies in rock-salt GeTe. The results show that Co occupies germanium vacancy and forms Co–Te bond, but the alloy retains a rock-salt structure when the concentration of doped Co is less than the vacancy ratio in crystalline GeTe. If we construct 8% germanium vacancy and relative distortion in rock-salt GeTe, the magnetism calculation based on spin-polarized density functional theory of Co-doped GeTe agrees well with superconducting quantum interference device magnetometer result. From experiment and calculation results, we can confirm the existence of vacancies in rock-salt GeTe and the ratio is 8%.

Giant in-plane anisotropy in manganite thin films driven by strain-engineered double exchange interaction and electronic phase separation

We investigate epitaxial Pr0.65(Ca0.7Sr0.3)0.35MnO3 thin film grown on orthorhombic (110) NdGaO3 substrate which breaks the lattice symmetry and affects the phase separated ground state. As a result of the anisotropic substrate strain, giant in-plane magnetic and magnetotransport anisotropy are observed, which is related to the anisotropic coupling and competition between the double-exchange interaction and the Jahn-Teller distortion. Furthermore, the in-plane anisotropy shows a distinct enhancement near the metal-insulator transition, implying a significant contribution from the phase separation to the anisotropic transport behaviors. V C 2011 American Institute of Physics. [doi:10.1063/1.3643442] Since the discovery of “colossal magnetoresistance” (CMR), 1‐3 perovskite manganites have drawn significant attention due to both the fundamental science and the potential applications. In these materials, the complex and strong coupling between spin, charge, orbital, and lattice degrees of freedom gives rise to multiple competing phases with essentially distinct physical properties. Moreover, these phases are quite “soft” in the sense that their free energies are close to each another, thus forming a delicate balance which is very sensitive to internal/external perturbations, such as electric field (current), 4,5 light, 6 phonon, 7 and strain, 8‐13 in addition to magnetic field. Among these tuning parameters, strain stands out as it is ubiquitous in thin films and devices, and it has been shown that strain also can significantly affect the charge, spin, and orbital orders in manganite thin films. 14‐18

Size-dependent scaling of exchange bias in NiFe2O4/NiO nanogranular systems synthesized by a phase separation method

Exchange bias (EB) effect has been studied in a series of nanogranular systems of ferrimagnetic (FiM) NiFe2O4 nanoparticles embedded into antiferromagnetic (AFM) NiO matrix, synthesized by a phase pprecipitation method from diluted Ni(1−x)FexO3 (x = 0.09) oxides. For these systems, the crystalline size (DNFO) of NiFe2O4 ranging from ∼3 nm to ∼55 nm has been obtained with thermal treated at different temperatures from 550 °C to 1000 °C. Magnetization measurement shows that both exchange bias field (HEB) and vertical magnetization shifts (MShift) can be exhibited below 250 K after field cooling procedure. The HEB and MShift decrease monotonically with crystalline size, and their behavior strongly depend on the crystalline size of NiFe2O4 nanoparticles. Linear relationship between HEB and MShift is observed for systems with smaller sizes (DNFO ≤ 8 nm), reveals a straightforward correlation between them. This phenomenon is ascribed to the interfacial exchange coupling between FiM NiFe2O4 clusters and spin-gla...

Topological phase transitions driven by magnetic phase transitions in Fe(x)Bi2Te3 (0≤x≤0.1) single crystals.

We propose a phase diagram for Fe(x)Bi2Te3 (0≤x≤0.1) single crystals, which belong to a class of magnetically bulk-doped topological insulators. The evolution of magnetic correlations from ferromagnetic to antiferromagnetic gives rise to topological phase transitions, where the paramagnetic topological insulator of Bi2Te3 turns into a band insulator with ferromagnetic-cluster glassy behavior around x∼0.025, and it further evolves to a topological insulator with valence-bond glassy behavior, which spans over the region from x∼0.03 up to x∼0.1. This phase diagram is verified by measuring magnetization, magnetotransport, and angle-resolved photoemission spectra with theoretical discussions.

Anomalies of inverse direct current susceptibility in spin-chain compounds Ca3Co2-xMnxO6

Various types of anomalies of inverse dc susceptibility (H/M) have been observed in spin-chain compounds Ca3Co2-xMnxO6. For x = 1.0, the H/M curve exhibits a field-dependent downturn from the paramagnetic Curie-Weiss behavior, signaling the presence of a Griffiths-like cluster phase. A slight decrease of x leads to a rapid suppression of the Griffiths-like singularity, followed by an appearance of field-independent, non-Griffiths-like behavior at x = 0.92. This feature persists until x = 0.33, at and below which the H/M curve presents an unambiguous, field-independent upturn from the Curie-Weiss law. These Mn-doping-dependent anomalies of H/M curves are associated with complex competing magnetic interactions.

Spin reorientation and spin-flop transition in multiferroic manganites Y1–xTbxMnO3 (x = 0, 0.1, 0.2) single crystals

We investigated the structure and magnetic properties of the multiferroic hexagonal manganite Y1−xTbxMnO3 (x = 0, 0.1, 0.2) single crystals. At 23 K, a Mn spin reorientation transition, which is not reported in the parent compound YMnO3, is observed in Y0.8Tb0.2MnO3. At a lower temperature, another new transition is observed in the doping system, which is attributed to the formation of long range antiferromagntic order of the doped Tb3+ moments. Based on the experimental results, we suggest that the effect of Tb doping is to bring about the increase of the Mn-O-Mn bond angle and the relief of the magnetic frustration. With increasing the doping level, for x = 0.2, when a magnetic field is applied parallel to the c axis, the field induced spin-flop transition is appeared, which indicates the reorientation of the Mn3+ moments along with the field-induced ferromagnetic ordering of the Tb3+ moments. These results suggest that the possibility of the Tb doping can change the magnetic structure and ferroelectric...

High magnetic-field-induced phase transitions and refined magnetic phase diagram of Gd5Ge4

We have performed magnetization measurements of Gd5Ge4 with little or no ferromagnetic (FM) impurities under a pulsed magnetic field up to 40 T. The results reveal previously undetected transitions between 85 and 105 K, that is, a distinct second-order antiferromagnetic (AFM)-paramagnetic (PM) transition, followed by a first-order PM-ferromagnetic (FM) transition. The distinct variation of the slope of the M(H) curves is observed around the AFM-PM transition. Combining our high-field data and those reported earlier, a new magnetic field-temperature phase diagram, in which the AFM, PM, and FM states are well separated, is constructed.

Magnetic field-induced metamagnetic transitions of Pr0.5Ca0.5Mn0.97Ga0.03O3

Measurements of isothermal magnetization and electrical transport on Pr0.5Ca0.5Mn0.97Ga0.03O3 have been performed in a magnetic field up to 15 T. Two distinct metamagnetic transitions, which may relate to the collapse of the pseudo-CE-type and CE-type antiferromagnetic phases (CE, a composite antiferromagnetic structure composed of a chequeboard of alternating C and E type), have been observed in the temperature region of 2.5–140 K. Different from the traditional magnetization steps, the two metamagnetic transitions are not dependent on the field sweep rate. These peculiar features are sensitive to the microstructure of the sample. A temperature-field phase diagram has been constructed and found to exhibit a minimum critical field, similar to other phase-separated systems.

Avalanche transition with super-bandwidth magnetic field response in (La0.73Bi0.27)0.67Ca0.33MnO3

The magnetic and electrical behaviors of an optimizing doping (La0.73Bi0.27)0.67Ca0.33MnO3 were investigated. An avalanche transition with a super-bandwidth of magnetic field response, in which the magnetic field sweep rate ranging from 10 Oe/s to 4886.8 T/s, was observed in the manganite. The insensitivity of the critical field of the avalanche transition to the magnetic field weep rates could not be described well within the framework of martensitic transition scenario. Based on the hybridization between the Bi3+-6s lone pair electrons and O2–2p electrons, we assume that the observed super-bandwidth avalanche transition is an intrinsic behavior resulting from the s-p hybridization. The robust transition makes the material extremely attractive for potential applications in super-bandwidth magnetic field response sensor. The simple structure of the current system also provides an ideal platform for understanding the physics underlying the avalanche transition.

Hole density of ferromagnetic semiconductor (Ga, Mn)As studied via pulsed high magnetic field

Transport measurements of as-grown and annealed (Ga,Mn)As samples were conducted under pulsed high magnetic field up to 40T. Assuming that the magnetization follows modified Brillouin function, we obtained the hole densities of the (Ga,Mn)As samples at various temperatures, which generally increase with temperature and become two times larger after annealing process. The hole density determined by electrical transport measurement at low magnetic field up to 1.5T is found to be underestimated.