The BM5Se9 phases: Mössbauer studies of the superconductors and the ferromagnets

Syntheses performed in the A₃B/MSe₂ (A = Sb, Ga, Sn) (B = Nb, V, Ta) (M = V, Nb, Ta) systems lead to the formation of B(M/A)₅Se₉ phases which could be ferromagnets or superconductors. It has been shown that the presence of V modifies the superconducting exchanges and allows ferromagnetic couplings. Mössbauer spectroscopy of both ¹²¹Sb and ¹¹⁹Sn gives local electronic structure of the SbNb₅Se₉ superconductor and the Sn_0.94Ga_0.06V_0.30Nb_4.70Se₉ ferromagnet. For the latter compound, a complete study as a function of temperature is carried out showing that the Sn environment is not modified when the ferromagnetic transition occurs. The Debye temperature is calculated and compared to those obtained for other selenide compounds.     

单畴Ni球观测及NicoreNiOshell的铁磁/反铁磁耦合研究

 使用微波辅助聚合方法制备了单分散单畴Ni纳米球，由MFM发现，尺度分布在100～180 nm的Ni球的一个相关特征是条型磁畴结构。用XRD、TEM、XPS以及EDAX测量了由Ni球进一步制备的Ni_coreNiO_shell高度球型纳米结构。用VSM 和SQUID进一步讨论了其铁磁/反铁磁界面耦合效应，估算了交换耦合场与粒子尺寸的关系。     

Andreev Tunneling Through a Ferromagnet/Quantum-Dot/Superconductor System

We study Andreev tunneling through a ferromagnet/quantum-dot (QD)/superconductor system. By usingnonequilibrum Green function method, the averaged occupation of electrons in QD and the Andreev tunneling currentare studied. Comparing to the norma-metal/quantum-dot/superconductor, the system shows significant changes: (i)The averaged occupations of spin-up and spin-down electrons are not equal. (ii) With the increase of the polarizationof ferromagnetic lead, the Andreev reflection current decreases. (iii) However, even the ferromagnetic lead reaches fullpolarization, the averaged occupation of spin-down electrons is not zero. The physics of these changes is discussed.     

Half metallic Co2TiGe—a theoretical and experimental investigation

Using first principle calculations the electronic structure and magnetic properties of the half metallic Co₂TiGe was investigated. The electronic structure calculations were performed using two different parameterization schemes. The band gap obtained in the minority spin state using LDA scheme in TB-LMTO method was smaller than that obtained using GGA scheme in FP-LAPW method. To explore the half metallic property experimentally bulk Co₂TiGe was synthesized by arc melting under argon atmosphere. The magnetic moment per formula unit determined from saturation magnetization at 2 K was 1.9 μ_B, which was close to the theoretically calculated value of 2 μ_B. A detailed analysis of the low temperature transport and magnetotransport properties exhibit characteristic features of half metals such as T^9/2 dependence of low temperature resistivity, negligible temperature dependence of resistivity below 20 K and a cross over from positive to negative MR around 100 K.     

Exact and approximate solutions for the dilute Ising model

The ground state energy and entropy of the dilute mean field Ising model is computed exactly by a single order parameter as a function of the dilution coefficient. An analogous exact solution is obtained in the presence of a magnetic field with random locations. Results lead to a complete understanding of the geography of the associated random graph. In particular, we give the size of the giant component (continent) and the number of isolated clusters of connected spins of all given size (islands). We also compute the average number of bonds per spin in the continent and in the islands. Then, we tackle the problem of solving the dilute Ising model at strictly positive temperature. In order to obtain the free energy as a function of the dilution coefficient and the temperature, it is necessary to introduce a second order parameter. We are able to find out the exact solution in the paramagnetic region and exactly determine the phase transition line. In the ferromagnetic region we provide a solution in terms of an expansion with respect to the second parameter which can be made as accurate as necessary. All results are reached in the replica frame by a strategy which is not based on multi-overlaps.     

Measurement of electronic structure in van der Waals ferromagnet Fe5-xGeTe2

As a van der Waals ferromagnet with high Curie temperature, Fe_5-xGeTe₂ has attracted tremendous interests recently. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES), we systematically investigated the electronic structure of Fe_5-xGeTe₂ crystals and its temperature evolution. Our ARPES measurement reveals two types of band structures from two different terminations with slight k_z evolution. Interestingly, across the ferromagnetic transition, we observed the merging of two split bands above the Curie temperature, suggesting the band splitting due to the exchange interaction within the itinerant Stoner model. Our results provide important insights into the electronic and magnetic properties of Fe_5-xGeTe₂ and the understanding of magnetism in a two-dimensional ferromagnetic system.     

Magnon energy renormalization and low-temperature thermodynamics of O(3) Heisenberg ferromagnets

We present the perturbation theory for lattice magnon fields of the D$D$-dimensional O(3) Heisenberg ferromagnet. The effective Hamiltonian for the lattice magnon fields is obtained starting from the effective Lagrangian, with two dominant contributions that describe magnon–magnon interactions identified as a usual gradient term for the unit vector field and a part originating in the Wess–Zumino–Witten term of the effective Lagrangian. Feynman diagrams for lattice scalar fields with derivative couplings are introduced, on the basis of which we investigate the influence of magnon–magnon interactions on magnon self-energy and ferromagnet free energy. We also comment appearance of spurious terms in low-temperature series for the free energy by examining magnon–magnon interactions and internal symmetry of the effective Hamiltonian (Lagrangian).     

Exchange bias in ferromagnet/antiferromagnet bilayers

Since the exchange bias （EB） effect was discovered in the Co/CoO core-shell nanoparticles, it has been extensively studied in various ferromagnet （FM）/antiferromagnet （AFM） bilayers due to its crucial role in spintronics devices. In this article, we review the investigation of the EB in our research group. First, we outline basic features of the EB, including the effects of the constituent layer thickness, the microstructure and magnetization of the FM layers, and we also discuss asymmetric magnetization reversal process in wedged-FM/AFM bilayers. Secondly, we discuss the mechanisms of the positive EB and the perpendicular EB. Thirdly, we demonstrate the hysteretic behavior of the angular dependence of the EB and analyze the EB training effect. Finally, we discuss the roles of the rotatable anisotropy in the two phenomena.     

Influence of different band masses on ballistic charge transport in ferromagnet–superconductor–ferromagnet trilayers

We study transport properties of clean FISIF double-barrier junctions consisting of metallic or semiconducting ferromagnets (F), a superconductor (S), and insulating interfaces (I). We solve the scattering problem based on the Bogoliubov–de Gennes equation and calculate differential conductance for arbitrary interface transparency, different effective band masses and Fermi wave vectors in the conductors. We analyze size and coherence effects that characterize ballistic transport: subgap transmission and geometrical oscillations of the conductance. We find that different band masses, as well as different Fermi wave vectors, affect the transport properties in a way similar to interfaces of a finite transparency. In all these cases, charge transport is reduced to resonant tunneling through the quasi-bound states in the superconducting film.