Investigation of the mechanism of the anomalous Hall effects in Cr2Te3/(BiSb)2(TeSe)3 heterostructure.

The interplay between ferromagnetism and the non-trivial topology has unveiled intriguing phases in the transport of charges and spins. For example, it is consistently observed the so-called topological Hall effect (THE) featuring a hump structure in the curve of the Hall resistance (Rxy) vs. a magnetic field (H) of a heterostructure consisting of a ferromagnet (FM) and a topological insulator (TI). The origin of the hump structure is still controversial between the topological Hall effect model and the multi-component anomalous Hall effect (AHE) model. In this work, we have investigated a heterostructure consisting of BixSb2-xTeySe3-y (BSTS) and Cr2Te3 (CT), which are well-known TI and two-dimensional FM, respectively. By using the so-called "minor-loop measurement", we have found that the hump structure observed in the CT/BSTS is more likely to originate from two AHE channels. Moreover, by analyzing the scaling behavior of each amplitude of two AHE with the longitudinal resistivities of CT and BSTS, we have found that one AHE is attributed to the extrinsic contribution of CT while the other is due to the intrinsic contribution of BSTS. It implies that the proximity-induced ferromagnetic layer inside BSTS serves as a source of the intrinsic AHE, resulting in the hump structure explained by the two AHE model.

Observation of spin-glass-like characteristics in ferrimagnetic TbCo through energy-level-selective approach.

Rare earth (RE)-transition metal (TM) ferrimagnetic alloys are gaining increasing attention because of their potential use in the field of antiferromagnetic spintronics. The moment from RE sub-lattice primarily originates from the 4f-electrons located far below the Fermi level (EF), and the moment from TM sub-lattice arises from the 3d-electrons across the EF. Therefore, the individual magnetic moment configurations at different energy levels must be explored to clarify the microscopic mechanism of antiferromagnetic spin dynamics. Considering these issues, here we investigate the energy-level-selective magnetic moment configuration in ferrimagnetic TbCo alloy. We reveal that magnetic moments at deeper energy levels are more easily altered by the external magnetic field than those near the EF. More importantly, we find that the magnetic moments at deeper energy levels exhibit a spin-glass-like characteristics such as slow dynamics and magnetic moment freezing whereas those at EF do not. These unique energy-level-dependent characteristics of RE-TM ferrimagnet may provide a better understanding of ferrimagnet, which could be useful in spintronic applications as well as in spin-glass studies.

Temperature dependence of intrinsic and extrinsic contributions to anisotropic magnetoresistance.

Electrical conduction in magnetic materials depends on their magnetization configuration, resulting in various magnetoresistances (MRs). The microscopic mechanisms of MR have so far been attributed to either an intrinsic or extrinsic origin, yet the contribution and temperature dependence of either origin has remained elusive due to experimental limitations. In this study, we independently probed the intrinsic and extrinsic contributions to the anisotropic MR (AMR) of a permalloy film at varying temperatures using temperature-variable terahertz time-domain spectroscopy. The AMR induced by the scattering-independent intrinsic origin was observed to be approximately 1.5% at T = 16 K and is virtually independent of temperature. In contrast, the AMR induced by the scattering-dependent extrinsic contribution was approximately 3% at T = 16 K but decreased to 1.5% at T = 155 K, which is the maximum temperature at which the AMR can be resolved using THz measurements. Our results experimentally quantify the temperature-dependent intrinsic and extrinsic contributions to AMR, which can stimulate further theoretical research to aid the fundamental understanding of AMR.

A scalable molecule-based magnetic thin film for spin-thermoelectric energy conversion.

Spin thermoelectrics, an emerging thermoelectric technology, offers energy harvesting from waste heat with potential advantages of scalability and energy conversion efficiency, thanks to orthogonal paths for heat and charge flow. However, magnetic insulators previously used for spin thermoelectrics pose challenges for scale-up due to high temperature processing and difficulty in large-area deposition. Here, we introduce a molecule-based magnetic film for spin thermoelectric applications because it entails versatile synthetic routes in addition to weak spin-lattice interaction and low thermal conductivity. Thin films of CrII[CrIII(CN)6], Prussian blue analogue, electrochemically deposited on Cr electrodes at room temperature show effective spin thermoelectricity. Moreover, the ferromagnetic resonance studies exhibit an extremely low Gilbert damping constant ~(2.4 ± 0.67) × 10-4, indicating low loss of heat-generated magnons. The demonstrated STE applications of a new class of magnet will pave the way for versatile recycling of ubiquitous waste heat.

A multicaloric material as a link between electrocaloric and magnetocaloric refrigeration.

The existence and feasibility of the multicaloric, polycrystalline material 0.8Pb(Fe1/2Nb1/2)O3-0.2Pb(Mg1/2W1/2)O3, exhibiting magnetocaloric and electrocaloric properties, are demonstrated. Both the electrocaloric and magnetocaloric effects are observed over a broad temperature range below room temperature. The maximum magnetocaloric temperature change of ~0.26 K is obtained with a magnetic-field amplitude of 70 kOe at a temperature of 5 K, while the maximum electrocaloric temperature change of ~0.25 K is obtained with an electric-field amplitude of 60 kV/cm at a temperature of 180 K. The material allows a multicaloric cooling mode or a separate caloric-modes operation depending on the origin of the external field and the temperature at which the field is applied.

Large spin Hall magnetoresistance and its correlation to the spin-orbit torque in W/CoFeB/MgO structures.

The phenomena based on spin-orbit interaction in heavy metal/ferromagnet/oxide structures have been investigated extensively due to their applicability to the manipulation of the magnetization direction via the in-plane current. This implies the existence of an inverse effect, in which the conductivity in such structures should depend on the magnetization orientation. In this work, we report a systematic study of the magnetoresistance (MR) of W/CoFeB/MgO structures and its correlation with the current-induced torque to the magnetization. We observe that the MR is independent of the angle between the magnetization and current direction but is determined by the relative magnetization orientation with respect to the spin direction accumulated by the spin Hall effect, for which the symmetry is identical to that of so-called the spin Hall magnetoresistance. The MR of ~1% in W/CoFeB/MgO samples is considerably larger than those in other structures of Ta/CoFeB/MgO or Pt/Co/AlOx, which indicates a larger spin Hall angle of W. Moreover, the similar W thickness dependence of the MR and the current-induced magnetization switching efficiency demonstrates that MR in a non-magnet/ferromagnet structure can be utilized to understand other closely correlated spin-orbit coupling effects such as the inverse spin Hall effect or the spin-orbit spin transfer torques.

Thermal spin injection and accumulation in CoFe/MgO/n-type Ge contacts.

Understanding the interplay between spin and heat is a fundamental and intriguing subject. Here we report thermal spin injection and accumulation in CoFe/MgO/n-type Ge contacts with an asymmetry of tunnel spin polarization. Using local heating of electrodes by laser beam or electrical current, the thermally-induced spin accumulation is observed for both polarities of the temperature gradient across the tunnel contact. We observe that the magnitude of thermally injected spin signal scales linearly with the power of local heating of electrodes, and its sign is reversed as we invert the temperature gradient. A large Hanle magnetothermopower (HMTP) of about 7.0% and the Seebeck spin tunneling coefficient of larger than 0.74 meV K(-1) are obtained at room temperature.

Single crystalline ß-Ag2Te nanowire as a new topological insulator.

A recent theoretical study suggested that Ag(2)Te is a topological insulator with a highly anisotropic Dirac cone. Novel physics in the topological insulators with an anisotropic Dirac cone is anticipated due to the violation of rotational invariance. From magnetoresistance (MR) measurements of Ag(2)Te nanowires (NWs), we have observed Aharanov-Bohm (AB) oscillation, which is attributed to the quantum interference of electron phase around the perimeter of the NW. Angle and temperature dependences of the AB oscillation indicate the existence of conducting surface states in the NWs, confirming that Ag(2)Te is a topological insulator. For Ag(2)Te nanoplates (NPLs), we have observed high carrier mobility exceeding 22,000 cm(2)/(V s) and pronounced Shubnikov-de Haas (SdH) oscillation. From the SdH oscillation, we have obtained Fermi state parameters of the Ag(2)Te NPLs, which can provide valuable information on Ag(2)Te. Understanding the basic physics of the topological insulator with an anisotropic Dirac cone could lead to new applications in nanoelectronics and spintronics.

New crystal structure: synthesis and characterization of hexagonal wurtzite MnO.

Most transition metal oxides have a cubic rocksalt crystal structure, but ZnO and CoO are the only stable transition metal oxides known to possess a hexagonal structure. Unprecedented hexagonal wurtzite MnO has been prepared by thermal decomposition of Mn(acac)(2) on a carbon template. Structural characterization has been carried out by TEM, SAED, and a Rietveld analysis using XRD. The experimental and theoretical magnetic results indicate magnetic ordering of the hexagonal wurtzite MnO. Density functional calculations have been performed in order to understand the electronic and piezoelectric properties of the newly synthesized hexagonal wurtzite MnO.

Interface effect of magnetic properties in Ni nanoparticles with a hcp core and fcc shell structure.

We have fabricated hexagonal close-packed (hcp) Ni nanoparticles covered by a face-centered cubic (fcc) Ni surface layer by polyol method. The magnetic properties have been investigated as a function of temperature and applied magnetic field. The magnetic behavior reveals that the system should be divided magnetically into three distinct phases with different origins. The fcc Ni phase on the shell contributes to the superparamagnetism through a wide temperature range up to 360 K. The hcp Ni phase at the core is associated with antiferromagnetic nature below 12 K. These observations are in good agreement with the X-ray absorption spectroscopy and magnetic circular dichroism measurements. In our particular case, the unique hcp core and fcc shell structure gives rise to an additional anomaly at 20 K in the zero-field-cooled magnetization curve. Its position is barely affected by the magnetic field but its structure disappears above 30 kOe, showing a metamagnetic transition in the magnetization versus magnetic field curve. This new phase originates from the magnetic exchange at the interface between the hcp and fcc Ni sublattices.

Structure-induced ferromagnetic stabilization in free-standing hexagonal Fe(1.3)Ge nanowires.

Single-crystalline free-standing hexagonal Fe(1.3)Ge nanowires (NWs) are synthesized for the first time using a chemical vapor transport process without using any catalyst. Interestingly, Fe(1.3)Ge NWs are found to be ferromagnetic at room temperature, while bulk Fe(1.3)Ge has the lower critical temperature of 200 K. We perform first-principles density functional calculations and suggest that the observed strong ferromagnetism is attributed to the reduced distances between Fe atoms, increased number of Fe-Fe bonds, and the enhanced Fe magnetic moments. Both experimental and theoretical studies show that the magnetic moments are enhanced in the NWs, as compared to bulk Fe(1.3)Ge. We also modulate the composition ratio of as-grown iron germanide NWs by adjusting experimental conditions. It is shown that uniaxial strain on the hexagonal plane also enhances the ferromagnetic stability.

Diffusion-driven crystal structure transformation: synthesis of Heusler alloy Fe3Si nanowires.

We report fabrication of Heusler alloy Fe(3)Si nanowires by a diffusion-driven crystal structure transformation method from paramagnetic FeSi nanowires. Magnetic measurements of the Fe(3)Si nanowire ensemble show high-temperature ferromagnetic properties with T(c) >> 370 K. This methodology is also successfully applied to Co(2)Si nanowires in order to obtain metal-rich nanowires (Co) as another evidence of the structural transformation process. Our newly developed nanowire crystal transformation method would be valuable as a general method to fabricate metal-rich silicide nanowires that are otherwise difficult to synthesize.

Itinerant helimagnetic single-crystalline MnSi nanowires.

We report the synthesis of free-standing MnSi nanowires via a vapor transport method with no catalyst and measurements of their electrical and magnetic properties for the first time. The single-crystalline MnSi nanowire ensemble with a simple cubic (B20) crystal structure shows itinerant helimagnetic properties with a T(c) of about 30 K. A single MnSi nanowire device was fabricated by a new method using photolithography and a nanomanipulator that produces good ohmic contacts. The single-nanowire device measurements provide large (20%) negative magnetoresistance and very low electrical resistivity of 544 microOmegacm for the MnSi nanowire.

Composition-tuned Co(n)Si nanowires: location-selective simultaneous growth along temperature gradient.

We report the simultaneous and selective synthesis of single-crystalline Co(n)Si NWs (n = 1-3) and their corresponding crystal structures--simple cubic (CoSi), orthorhombic (Co(2)Si), and face-centered cubic (Co(3)Si)--following a composition change. Co(n)Si NWs were synthesized by placing the sapphire substrates along a temperature gradient. The synthetic process is a successful demonstration of tuning the chemical composition in Co(n)Si NWs. The synthesis and detailed crystal structure of single-crystalline Co(2)Si and Co(3)Si are reported for the first time including the bulk and the nanostructure phases. The electrical and magnetic properties of Co(2)Si NWs are investigated and compared with those of CoSi NWs.

Ferromagnetic signal in nanosized Ag particles.

A new technique using an inductively coupled plasma reactor equipped with a liquid-nitrogen cooling system was developed to prepare Ag nanoparticles. The magnetic signal from these Ag particles with diameters of 4 nm showed, surprisingly, a signal with combined ferromagnetic and diamagnetic components, in contrast to the signal with only one diamagnetic component from bulk Ag. The same technique was used to prepare the Ag/Cu nanoparticles, which are Ag nanoparticles coated with a Cu layer. Compared to the Ag nanoparticles, these showed a greatly enhanced superparamagnetic signal in addition to the same value of the ferromagnetism. The comparison between the Ag and the Ag/Cu nanoparticles indicated that the ferromagnetic components are a common feature of Ag nanoparticles while the greatly enhanced paramagnetic component of Ag/Cu, which dominates over the background diamagnetic component from the Ag core, is from the outer Cu shell.

Kinetics of monodisperse iron oxide nanocrystal formation by "heating-up" process.

We studied the kinetics of the formation of iron oxide nanocrystals obtained from the solution-phase thermal decomposition of iron-oleate complex via the "heating-up" process. To obtain detailed information on the thermal decomposition process and the formation of iron oxide nanocrystals in the solution, we performed a thermogravimetric-mass spectrometric analysis (TG-MS) and in-situ magnetic measurements using SQUID. The TG-MS results showed that iron-oleate complex was decomposed at around 320 degrees C. The in-situ SQUID data revealed that the thermal decomposition of iron-oleate complex generates intermediate species, which seem to act as monomers for the iron oxide nanocrystals. Extensive studies on the nucleation and growth process using size exclusion chromatography, the crystallization yield data, and TEM showed that the sudden increase in the number concentration of the nanocrystals (burst of nucleation) is followed by the rapid narrowing of the size distribution (size focusing). We constructed a theoretical model to describe the "heating-up" process and performed a numerical simulation. The simulation results matched well with the experimental data, and furthermore they are well fitted to the well-known LaMer model that is characterized by the burst of nucleation and the separation of nucleation and growth under continuous monomer supply condition. Through this theoretical work, we showed that the "heating-up" and "hot injection" processes could be understood within the same theoretical framework in which they share the characteristics of nucleation and growth stages.

Magnetic properties of single-crystalline CoSi nanowires.

We have observed unusual ferromagnetic properties in single-crystalline CoSi nanowire ensemble, in marked contrast to the diamagnetic CoSi in bulk. High-density freestanding CoSi nanowires with B20 crystal structure are synthesized by a vapor-transport-based method. The reaction of cobalt chloride precursor with a Si substrate produces high-aspect-ratio CoSi nanowires. The high-resolution transmission electron microscopy and electron diffraction studies reveal superlattice structure in CoSi nanowires with twice the lattice parameter of simple cubic CoSi lattice. The zero-field-cooled and field-cooled (ZFC-FC) measurements from the nanowire ensemble show freezing of the disordered surface spins at low temperatures. The magnetoresistance (MR) measurements of single nanowire devices show a negative MR whose magnitude gets larger at lower temperatures.

Analysis of H(c2)(θ,T) for Mg(B(1-x)C(x))(2) single crystals by using the dirty two-gap model.

To understand the effect of carbon doping on the superconductivity in MgB(2), we obtained the angle- and temperature-dependent upper critical fields [H(c2)(θ) and H(c2)(T)] for Mg(B(1-x)C(x))(2) single crystals (x = 0.06 and 0.1) from resistivity measurements while varying the temperature, the field, and the direction of the field. The detailed values of the diffusivity for two different directions for each σ-band and π-band were obtained to explain both the temperature- and the angle-dependent H(c2) by using the dirty-limit two-gap model. The induced impurity scattering of the σ-band and the π-band for both the ab-plane and the c-direction is studied.