Zhaoming Tian

Exchange bias effect in a granular system of NiFe2O4 nanoparticles embedded in an antiferromagnetic NiO matrix

A granular system composed of ferrimagnetic NiFe2O4 nanoparticles, about 8 nm in size, embedded in an antiferromagnetic NiO matrix has been synthesized by a high-temperature phase precipitation method from Fe-doped NiO matrix. Both the exchange bias field and vertical magnetization shift can be observed in this system below 250 K after field cooling, above which the exchange bias disappears. Furthermore, the exchange bias field shows a linear dependence on the magnetization shift. This observed exchange bias effect is explained in terms of the exchange interaction between the ferrimagnetic phase and the spin-glass-like phase at the interface.

Polarization enhancement and ferroelectric switching enabled by interacting magnetic structures in DyMnO3 thin films

The mutual controls of ferroelectricity and magnetism are stepping towards practical applications proposed for quite a few promising devices in which multiferroic thin films are involved. Although ferroelectricity stemming from specific spiral spin ordering has been reported in highly distorted bulk perovskite manganites, the existence of magnetically induced ferroelectricity in the corresponding thin films remains an unresolved issue, which unfortunately halts this step. In this work, we report magnetically induced electric polarization and its remarkable response to magnetic field (an enhancement of ~800% upon a field of 2 Tesla at 2u2005K) in DyMnO3 thin films grown on Nb-SrTiO3 substrates. Accompanying with the large polarization enhancement, the ferroelectric coercivity corresponding to the magnetic chirality switching field is significantly increased. A picture based on coupled multicomponent magnetic structures is proposed to understand these features. Moreover, different magnetic anisotropy related to strain-suppressed GdFeO3-type distortion and Jahn-Teller effect is identified in the films.

Size-dependent training effect in exchange coupled NiFe2O4/NiO nanogranular systems

The particle size dependent training effect has been investigated on the exchange coupled NiFe2O4/NiO nanogranular systems, with average particle size (DNFO) of NiFe2O4 ranging from ∼3u2009nm to ∼55u2009nm. For all samples, analysis of the field cycles (n) dependence on exchange bias fields (HEB) suggests the existence of two distinct forms of training mechanism during training procedure. One is related to an athermal contribution leading to the abrupt single cycle training, the other is the conventional thermal activation mechanism responsible for the gradual reduction of HEB during the subsequent cycles. With the increase of particle size, the relative change of HEB and enhanced coercivity (△HC) after training display a nonmonotonic size-scaling behavior and reaches the maxima for DNFOu2009∼u200922u2009nm. In this system, this largest reduction reveals the weakest dynamic stability of the interfacial exchange coupling energy during field cycle process. Moreover, different decay rate of HEB and ΔHC with field cycles are obs...

Exchange bias effect in multiferroic CoCr2O4/Cr2O3 nanogranular system synthesized through a phase segregation route

A nanogranular system of multiferroic CoCr2O4 nanoparticles embedded in an antiferromagnetic Cr2O3 matrix has been synthesized through a high-temperature phase segregation route from a Co-doped Cr2O3 matrix. Magnetic studies show that exchange bias fields (HEB) accompanying vertical magnetization shifts (MShift) are observed at low temperatures after field cooled from 350 K. The corresponding exchange bias field can be as large as 1420 Oe, and the vertical magnetization shift reaches 0.116 emu/g at 10 K. The exchange bias field decreases with temperature increasing and disappears at T ≈ 70 K, while the coercive field (HC) initially increases with the temperature up to 40 K, and thereafter, it decreases to zero at 100 K. This exchange bias behavior is discussed in terms of the existence of exchange coupling between the ferrimagnetic CoCr2O4 core and spin glass-like phase at the interfaces.

Enhanced Ferromagnetic, Ferroelectric, and Dielectric Properties in BiFeO3-SrTiO3-Bi0.5Na0.5TiO3 Ceramics

Room-temperature multiferroic (1xa0−xa0x)BiFeO3-x(SrTiO3-Bi0.5Na0.5TiO3) (BFO-ST-BNT) (xxa0=xa00 to 0.5) solid solutions have been prepared by the modified Pechini method, and their structural, ferromagnetic, ferroelectric, and dielectric properties systematically studied. In addition, a gradual phase transformation from rhombohedral to pseudocubic structure was observed between the compositions with xxa0=xa00.2 and xxa0=xa00.3. Compared with pure BiFeO3, significantly enhanced remanent magnetization (Mrxa0=xa00.11xa0emu/g) was determined in the sample with xxa0=xa00.2 at room temperature. Although the ferroelectric properties exhibited overall enhancement with increasing x, relatively large remanent polarization Pr (1.14xa0μC/cm2 at 300xa0K) was achieved simultaneously at the optimal composition with xxa0=xa00.2. Moreover, we measured an outstanding relative dielectric constant of over 16,700 at frequency of 10xa0kHz for the sample with xxa0=xa00.2, which is attributed to polarization of defect dipoles. This comprehensive enhancement of the ferromagnetic, ferroelectric, and dielectric properties indicates that BFO-ST-BNT ceramics are promising candidates for use in multiferroic applications.

Magnetic memory effect at room temperature in exchange coupled NiFe2O4-NiO nanogranular system

Compared to the low temperature memory effect observed in magnetic nanoparticles (NPs), here we report a room temperature memory effect in a Ferrimagnetic (FiM)-Antiferromagnetic exchange coupled NiFe2O4-NiO nanogranular system, which is experimentally studied by different protocols of dc magnetization relaxation measurements below the blocking temperature TBu2009=u2009345u2009K. The interfacial exchange coupling between the FiM NiFe2O4 clusters and the spin-glassy like phase is proposed to provide an additional anisotropic energy, leading to the enhancement of the magnetic memory effect up to room temperature. The observed memory effect is discussed based on the multiple distribution of energy barriers for both the FiM NPs and interfacial magnetic exchange anisotropy.

Memory effect up to room-temperature in Ni/Ni2P core-shell structured nanoparticles

Memory effect has been studied in the system using magnetic nanoparticles with Ni nanocore encapsulated by non-magnetic and oxidation-resistant Ni2P nanoshell acquired through surface-phosphatizing Ni nanoparticles. The self-assembled array with interparticle spacing of about 6 nm shows memory effect up to 200 K below its average blocking temperature of 260 K. And reducing the interparticle spacing of the self-assembled array via annealing can further enlarge the temperature range of memory effect up to room-temperature. The memory effect can be understood based on the thermal relaxation theory of single-domain magnetic nanoparticles. Furthermore, the read-write magnetic coding is realized based on the temperature changes, using the memory effect up to room-temperature, which may be useful for future memory devices.

Anisotropic large magnetoresistance in TaTe4 single crystals

Strong anisotropic magnetotransport is reported in high-quality TaTe4 single crystals synthesized by flux methods. Large positive magnetoresistance (MR) and field-induced metal-semiconductor-like transition are observed at low temperatures with B perpendicular to c axis. The MR value reaches 3200% in 9u2009T at 2u2009K with B parallel to a axis, contrast to 79% for B along c axis. Angle dependent magnetoresistance with B rotated within ab plane displays eightfold symmetry and pronounced Shubnikov-de Haas (SdH) oscillations at low temperatures. The analysis of angle dependent resistivity, Hall effect and observed SdH oscillations suggest the high mobile electron and anisotropic Fermi surface responsible for the large anisotropic MR in TaTe4.

Evidence of decisive effect of crystal-field splitting in spin-state transition

Based on the first-principle calculations for 3D Hofmann-like spin-crossover (SCO) compound [Fe(C4H4N2){Pt(CN)4}], the discrepancy of transition mechanism is clarified with quantitatively distinguishable evidence of second order phase transition. It shows that the stretch around 0.2 Å of Fe-N bond length leads to the continuous structure expansion, as the energy splitting ΔEHL between low-spin and high-spin states reduces from 2.554 2 eV to −0.327 8 eV, and the crystal-field splitting (CFS) is reduced from 1.845 8 eV to 0.420 8 eV meanwhile. A physics image relating the calculations results with CFS in the frame of ligand-field theory is presented, which manifests that CFS is a necessary parameter to be introduced directly in the theory of spinstate transition.

Electrical transport and percolation in structural phase-separated manganites La1−xBaxMnO3

Microstructural studies show that manganites La1−xBaxMnO3 (0.33≤x≤0.95) begin structural phase separation into La0.67Ba0.33MnO3 and BaMnO3 for x>0.33. These composites form a cellularlike structure when the volume faction of La0.67Ba0.33MnO3 (fLBMO) is near the percolation threshold (fC). The percolation threshold (fC) for our composites is 0.18. This result is not consistent with the previous results, which prefer smaller percolation threshold value. This could be attributed to the contribution of grain boundaries. This grain-boundary contribution also induces the large low-temperature bump in electrical transport. The critical exponents t gained from the good fitting for the experimental data are 1.6 at 150 K and 1.7 at 300 K, which are in good agreement with the previous universal result: t=1.6–2.0 for the three dimensional space.