Search

The crystal structure, cryogenic magnetic properties, and magnetocaloric performance of double perovskite Eu2NiMnO6 (ENMO), Gd2NiMnO6 (GNMO), and Tb2NiMnO6 (TNMO) ceramic powder samples synthesized by solid-state method have been investigated. X-ray diffraction structural investigation reveal that all compounds crystallize in the monoclinic structure with a P21/n space group. A ferromagnetic to paramagnetic (FM-PM) second-order phase transition occurred in ENMO, GNMO, and TNMO at 143, 130, and 112 K, respectively. Maximum magnetic entropy changes and relative cooling power with a 5 T applied magnetic field are determined to be 3.2, 3.8, 3.5 J/kgK and 150, 182, 176 J/kg for the investigated samples, respectively. The change in structural, magnetic, and magnetocaloric effect attributed to the superexchange mechanism of Ni2+–O–Mn3+ and Ni2+–O–Mn4+. The various atomic sizes of Eu, Gd, and Tb affect the ratio of Mn4+/Mn3+, which is responsible for the considerable change in properties of double perovskite.

Nanoscale-layered ferromagnets have demonstrated fascinating two-dimensional magnetism down to atomic layers, providing a peculiar playground of spin orders for investigating fundamental physics an...

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.

International audience; A systematic study is conducted on an electrically controlled magnetization in tri-layers containing two layers of ferromagnetic (FM) SrRuO3, with antiferromagnetic (AFM) insulating manganites sandwiched in between. Magneto-resistance curves exhibit that a huge weak anti-localization or localization shapes around zero magnetic field and the localization shapes and coercive fields change simultaneously by currents. This results propose that interface-driven magnetic inhomogeneity such as the FM clusters embedded in the AFM matrix, and corresponding localizations play a role as a bridge between the current and magnetism. The results suggest that interfacial magnetism can cause new functions in a heterostructure, leading to an expansion of research subjects of current-controlled magnetism.

Double perovskite Eu2NiMnO6 (ENMO) Gd2NiMnO6 (GNMO) and Tb2NiMnO6 (TNMO) ceramic powder have been synthesized by solid-state reaction and their crystal structure, microstructure, cryogenic magnetic properties, and magnetocaloric performance have been investigated. Structural studies by using X-ray diffraction shows that all compounds crystallize in the monoclinic structure with a P21/n space group. A ferromagnetic to paramagnetic (FM-PM) second-order phase transition occurred in ENMO, GNMO, and TNMO around 143, 130, and 112 K, respectively. The values of maximum magnetic entropy change and relative cooling power at an applied field of 5 T are found to be 3.2, 3.8, 3.5 J/kgK and 150, 182, 176 J/kg respectively, for the studied sample. The change in structural, magnetic, and magnetocaloric effect ascribed to the superexchange mechanism of Ni2+ – O – Mn3+ and Ni2+ – O – Mn4+. Due to different atomic size of Eu, Gd, Tb changes the ratio of Mn4+/Mn3+ which is responsible for the variation of properties significantly in double perovskite.

We investigate the magnetoresistance of ferrimagnetic GdFeCo across the magnetization compensation temperature ${T}_{\mathrm{M}}$. The magnetic field dependence of longitudinal resistivity (${\ensuremath{\rho}}_{\mathrm{xx}}$) shows opposite trends below and above ${T}_{\mathrm{M}}$, and the variation of ${\ensuremath{\rho}}_{\mathrm{xx}}$ with $B$ becomes more significant as the temperature decreases. The observed unconventional magnetoresistance is attributed to the sperimagnetism of GdFeCo. Further investigations on the transverse resistivity (${\ensuremath{\rho}}_{\mathrm{xy}}$) of GdFeCo unveils that, contrary to the recent reports that the transition metal dominates transport of rare-earth transition-metal ferrimagnets, the Gd contribution to magnetoresistance is comparable to the FeCo contribution, showing that the transport of GdFeCo is antiferromagnetic. Our results therefore show that ferrimagnets are a convenient platform for studying antiferromagnetic spin transport and also are potential materials that can enable antiferromagnetic spin devices.

We report our systematic investigation of structural, magnetic, and magnetocaloric properties of double perovskite compounds Ho2FeMnO6, Ho2CoMnO6, and Ho2NiMnO6, which crystallize into monoclinic crystal structure with P21/n space group. The magnetic phase transition temperatures are determined to be 7.5 K for Ho2FeMnO6, 69 K for Ho2CoMnO6, and 71.5 K for Ho2NiMnO6, where Ho2FeMnO6 exhibits an antiferromagnetic to paramagnetic phase transition, while Ho2CoMnO6 and Ho2NiMnO6 exhibits ferromagnetic to paramagnetic phase transitions. The magnetic entropy change (−ΔSM), which are found to be 10.2, 3.4 and 4.7 J/kgK at ΔH = 7 T for Ho2FeMnO6, Ho2CoMnO6, and Ho2NiMnO6 samples, respectively. The magnetic and magnetocaloric characteristics of double perovskite compounds are well explained based on structural and electronic properties. For future applications, our quantitative method provides a guideline for magnetic property engineering of double perovskite systems containing ferromagnetic 3d transition metals.

Spin orbit torque (SOT) is essential to magnetization modulation in various ferromagnet/non-magnet bilayers. In this study, we demonstrated that SOT can be enhanced in a hybrid system composed of a perovskite oxide NdNiO3 (NNO) and a Ni81Fe19/Pt bilayer. We also find that the SOT enhancement might be attributed to spin absorption at the interface between the NNO and Ni81Fe19 layers. Our findings suggest that metal-oxide hybrid structures can be promising systems for the development of efficient spin–orbitronic devices.

The non-equilibrium state between magnons and phonons is the key to understand the spin-caloric phenomena. We developed a unique optical reflectometry technique to spatially resolve Kerr angle ( θK) and optical reflectance ( R) in a magnetic insulator—thulium iron garnet (TmIG). The TmIG was subjected to a thermal gradient to estimate populations of thermally excited magnons and phonons through the variation of θK and R. The results showed that the spatial gradient of θK is different from that of R, indicating the non-equilibrium state between magnons and phonons. Particularly, the characteristic decay length of θK was significantly influenced by the heating power and the magnetic field, suggesting non-linear magnon scattering in a high magnon density regime. Our work not only provides a scheme to investigate the spatial profiles of magnons and phonons but also reveals the magnon–phonon non-equilibrium in TmIG. Hence, this report will stimulate further studies based on magnon–phonon non-equilibrium such as a transverse spin Seebeck effect and Bose–Einstein condensation.

Straightforward growth of nanostructured low-bandgap materials is a key issue in mass production for electronic device applications. We report here facile nanowall growth of MoS2-MoSX using sputter deposition and investigate the electronic properties of the nanowalls. MoS2-MoSX nanowalls become gradually thicker and taller, with primarily (100)-plane growth directions, with increasing deposition time. Nanowalls combine with nearby walls when a rapid thermal annealing (RTA, 200 °C–500 °C) process is applied. All samples have conventional low-bandgap semiconductor behavior with exponential resistance increase as measurement temperature decreases. The 750 nm-thick MoS2-MoSX nanowalls have a sheet carrier mobility of up to 2 cm2·V−1·s−1 and bulk carrier concentration of ~1017–1019 cm−3 range depending on RTA temperature. Furthermore, perpendicular field-dependent magnetoresistance at 300 K shows negative magnetoresistance behavior, which displays resistance decay by applying a magnetic field (MR ratio in the −1 % range at 5 T). Interestingly, 400 °C RTA treated samples show a resistance upturn when applying an external magnetic field of more than 3 T. Our research suggests tuneability of MoS2 nanowall size and mesoscopic electronic transport properties.

Spin–orbit coupling (SOC) offers an alternative technique for generating pure spin currents in non-magnetic materials and controlling spin precessions for spin-field effect transistors. In addition, introducing SOC into graphene causes pristine graphene to evolve into a new condensed matter phase, such as the topological insulator state (quantum spin Hall state). Thus, the control of SOC in graphene is essential for its functional spin-orbitronic applications. Here, we report the nonvolatile tuning of SOC in graphene through the proximity effect from a ferroelectric substrate, Pb(Zr52,Ti48)O3 (PZT). Ferroelectric poling by applying a gate voltage induces a change in the SOC strength in addition to shifting the charge neutral point in graphene. The variations in SOC were extracted from weak localization within the quantum interference theory of graphene. Our analyses show that the dipole moments from the PZT polarization significantly enhance the z → −z asymmetric and symmetric SOCs of graphene. Unlike the impurity doping and/or gating, our methodology leads to the nonvolatile electrical control of SOC, thereby paving the way for versatile spin-orbitronic applications of graphene.

Spin pumping and the spin transfer torque in the normal metal (NM)/ferromagnetic materials (FM) heterostructure strongly depend on the interfacial spin memory loss. Therefore, the interface engineering of the NM/FM bilayer is important for enhancing spin pumping and spin-orbit torque efficiency for spin-orbitronic devices. In this study, we investigate the influence of the graphene interlayer between Pt and yttrium iron garnet (YIG) on spin pumping. Measurements of both the coplanar wave guide ferromagnetic resonance and inverse spin Hall effect with ferromagnetic resonance are performed to study the role of graphene in the transmission of the spin current at the interface. Results indicate a decreased Gilbert damping constant with the insertion of graphene interlayer, which causes the spin mixing conductance and inverse spin Hall voltage to be significantly reduced. The observed decrease in damping constant and spin injection is attributed to the presence of magnetic proximity effect and interfacial spin-orbit scattering, which causes graphene to serve as a spin-sink layer.

In type-I multiferroics, where ferroelectricity and magnetism arise from different origins, the explicit measurement of magnetoelectric effect is commonly prohibited due to the weak magnetoelectric coupling. From our investigation of magnetic and dielectric properties in a perovskite PbFe0.5Ti0.25W0.25O3, we have directly demonstrated magnetic-field-driven change of dielectric constant in a highly non-linear fashion. Our result offers a potential utilization of magnetoelectric functionality in type-I multiferroics.

Highly oriented single-crystalline ferromagnetic Co nanowire (NW) arrays were synthesized on sapphire substrates via a single-step chemical vapor deposition (CVD) method. On an m-cut sapphire substrate, Co NWs were vertically grown in epitaxial relationship with the substrate without using any catalysts or templates. On an r-cut sapphire substrate, Co NWs were horizontally grown in two perpendicular directions. Furthermore, we report that the Co NWs were transformed into Co3O4 nanotubes by thermal annealing under dilute O2 conditions. Such formation of hollow structures is ascribed to favored outward diffusion of Co ions. The present vertically aligned arrays of single-crystalline Co NWs could be utilized for advanced magnetic memory applications owing to their uniform orientations.

We found new two-dimensional (2D) quantum (S=1/2) antiferromagnetic systems: CuRE2Ge2O8 (RE=Y and La). According to our analysis of high-resolution X-ray and neutron diffraction experiments, the Cu-network of CuRE2Ge2O8 (RE=Y and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular b-axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a long-range order at 0.51(1) and 1.09(4) K for the Y and La systems, respectively. Interestingly, they also exhibit field induced phase transitions. For theoretical understanding, we carried out the density functional theory (DFT) band calculations to find that they are typical charge-transfer-type insulators with a gap of Eg = 2 eV. Taken together, our observations make CuRE2Ge2O8 (RE=Y and La) additional examples of low-dimensional quantum spin triangular antiferromagnets with the low-temperature magnetic ordering.

The magnetic resonance of Fe nanoparticles has been observed in single-crystal Fe/MgO/Fe magnetic tunnel junctions (MTJs). In the MTJs, one magnetic layer is a continuous Fe film, which acts as a magnetic reference layer, and the other is made up of noncontinuous nanoparticles of Fe, which act as a free layer. The magnetic resonance is electrically excited by the spin torque and is detected as the homodyne detection voltage, which is called the spin-torque diode effect. From the magnetoresistance characteristics, the average diameter of the Fe nanoparticles is estimated to be 6.8 nm; these nanoparticles exhibit superparamagnetic behavior at room temperature. This spin-torque magnetic resonance in magnetic nanoparticles enables us to study the magnetization dynamics in small spin systems.

Spin wave dispersion relations in a ferromagnetic nanowire with and without a domain wall are investigated by means of micromagnetic simulations. We confirm that the spin wave dispersion relation is shifted due to the spin wave Doppler effects, when the spin-transfer torque is applied. We also investigate the position-dependent ferromagnetic resonance spectra in the nanowire and find strong local spin wave interferences because of the formation of the standing wave due to reflections from a domain wall. It is observed that the spacing between the local spin wave constructive interferences is uniform, and it is revealed that the spacing is related with the spin wave Doppler shift.

Hybrid nanoparticles of LiFePO4 with carbon and lithium phosphates were synthesized through organic–inorganic co-assembly procedure using a triblock copolymer (F108 or P123). We found that the triblock copolymers play a critical role in controlling size of hybrid particle and the degree of crystallinity of the inorganic nanostructures. The hybrid using P-123 had more graphitic carbon which resulted in fast electron mobility. Also, magic angle spinning nuclear magnetic resonance (MAS-NMR) revealed that the crystallinity of the hybrid using P123 is higher than that using F108 which is not measurable in X-ray diffraction. Electrochemical performance of the hybrid using P123 as a cathode material in Li-ion batteries showed superior rate capability at 20 C of charging rate and 2 C of discharging rate without capacity loss, in which discharge capacity was 102 mAh/g.

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.

We have investigated the effect of insertion of a Ag layer in ITO film as well as electron beam irradiation to the multilayer films on the electrical and optical properties of the ITO-based multilayer deposited by magnetron sputtering method at room temperature. Inserting a very thin Ag layer between ITO layers resulted in a significant decrease in sheet resistance and increased the optical band gap of the ITO/Ag/ITO multilayer to 4.35 eV, followed by a high transparency of approximately 80% at a wavelength of 375 nm. We have also fabricated ultraviolet light-emitting diodes (LED) by using the ITO/Ag/ITO p-type electrode with/without electron beam irradiation. The results show that the UV-LEDs having ITO/Ag/ITO p-electrode with electron beam irradiation produced 19% higher optical output power due to the low absorption of light in the p-type electrode.

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.

Spin-transfer torque allows the magnetization of nanopillar devices to be switched electrically. Incorporating asymmetries into the design of such a device generates a linear out-of-plane torque component that could help prevent the unwanted spontaneous reversal of the nanopillar’s magnetization. Spin-transfer torque1,2 (STT) allows the electrical control of magnetic states in nanostructures3,4,5. The STT in magnetic tunnel junctions (MTJs) is of particular importance owing to its potential for device applications6,7. It has been demonstrated8,9,10,11 that the MTJ has a sizable perpendicular STT ( , field-like torque), which substantially affects STT-driven magnetization dynamics. In contrast to symmetric MTJs where the bias dependence of is quadratic8,9,10,12,13, it is theoretically predicted that the symmetry breaking of the system causes an extra linear bias dependence11. Here, we report experimental results that are consistent with the predicted linear bias dependence in asymmetric MTJs. The linear contribution is quite significant and its sign changes from positive to negative as the asymmetry is modified. This result opens a way to design the bias dependence of the field-like term, which is useful for device applications by allowing, in particular, the suppression of the abnormal switching-back phenomena.

Among the common ferromagnetic metals, cobalt has distinct magnetic properties, so incorporation of cobalt nanoparticles in the nickel nanofibers, which reveals better magnetic properties compared with the bulk, might have considerable impact. In this study, we are introducing cobalt-doped nickel nanofibers prepared by electrospinning. Electrospinning of a colloidal solution rather than a sol-gel (which is widely utilized in the conventional electrospinning technique) has been invoked as a novel strategy to prepare cobalt nanoparticles/ nickel acetate/poly(vinyl alcohol) nanofiber mats. Physiochemical characterizations indicated that calcination of the dried electrospun mats in argon atmosphere at 700 °C for 5 h leads to produce cobalt-doped nickel nanofibers. Overall, magnetic properties studied pointed to improvement of the magnetic parameters of the synthesized cobalt-doped nickel nanofibers compared with the pristine ones.

Using conventional methods to synthesize magnetic nanoparticles (NPs) with uniform size is a challenging task. Moreover, the degradation of magnetic N Ps is an obstacle to practical applications. The fabrication of silica-shielded magnetite NPs on carbon nitride nanotubes (CNNTs) provides a possible route to overcome these problems. While the nitrogen atoms of CN NTs provide selective nucleation sites for NPs of a particular size, the silica layer protects the NPs from oxidation. The morphology and crystal structure of NP―CNNT hybrid material is investigated by transmission electron microscopy (TEM) and X-ray diffraction. In addition, the atomic nature of the N atoms in the NP―CNNT system is studied by near-edge X-ray absorption fine structure spectroscopy (nitrogen K-edge) and calculations of the partial density of states based on first principles. The structure of the silica-shielded NP―CNNT system is analyzed by TEM and energy dispersive X-ray spectroscopy mapping, and their magnetism is measured by vibrating sample and superconducting quantum interference device magnetometers. The silica shielding helps maintain the superparamagnetism of the NPs; without the silica layer, the magnetic properties of NP―CNNT materials significantly degrade over time.

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.

We have investigated electrical and magnetic properties of single-crystalline Fe5Si3 nanowires. The nanowire ensemble shows ferromagnetic properties with a high Tc of 380 K, small coercivity, and n...

ZnS nanowires and nanobelts doped with Mn and Fe were synthesized by a chemical vapor transport method. The Mn/Fe-doped and co-doped ZnS nanostructures were grown on Au-coated Si substrates. The key to this synthetic process of co-doping lies in the use of metal chloride as a metal carrier. Crystal structure and chemical compositional analyses by transmission electron microscopy (TEM) indicate that the nanowires and nanobelts are single-crystalline and uniformly doped with dopants. Strong emission bands were found from photoluminescence (PL) spectra of Mn/Fe-doped and co-doped ZnS nanowires. The magnetic property measurements from the nanostructure ensemble show that the Curie temperature is above room temperature. The synthesized Mn/Fe-doped and co-doped ZnS nanostructures can be employed in the fabrication of nanoscale magnetic and optical devices.

Ga1−xMnxAs nanowires were synthesized with finely controlled Mn contents (x = 0, 0.01, 0.02, 0.03, and 0.05) by the vapor transport method. They consisted of single-crystalline GaAs nanocrystals (avg. diameter = 60 nm) grown along the [111] direction. The Mn doping decreases the lattice constant, most significantly at x ≈ 0.03. X-ray photoelectron spectroscopy revealed that as the Mn content increases, the binding energy of Ga 2p shifts to a higher energy, which can be correlated with the hybridization between the Mn2+ ions and the holes. X-ray absorption spectroscopy and X-ray magnetic circular dichroism confirmed that the Mn2+ ions substitute into the tetrahedrally coordinated Ga sites and that the magnetic moment is maximized at x = 0.03, where the lattice constant is minimized and the binding energy of Ga 2p is maximized. The magnetization measurement revealed that all of these nanowires exhibited room-temperature ferromagnetic behavior, which is also observed most significantly for x ≈ 0.03.

Work at Korea Advanced Institute of Science and Technology (KAIST) was supported through the Creative Research Institute Project and the Cavendish-KAIST Cooperation Project of the Korean Ministry of Science and Technology and Works at SungKyunKwan University were supported by he BK21 program, the 21st Century Frontier R&D Pro- gram for Hydrogen Energy and the Basic Atomic Energy Research Institute program.

We report the synthesis of ternary Fe1-xCoxSi nanowires by a vapor transport method. Structural characterization of the nanowires shows a single-crystalline nature with B20 crystal structure. Spatially resolved elemental analysis revealed the homogeneous distribution of all elemental components along the nanowire. The nanowire ensemble shows ferromagnetic properties with a sudden change in magnetization at applied magnetic field near zero. Magnetoresistance measurements on a single-nanowire device show positive magnetoresistance, which does not saturate up to the highest applied field. These ferromagnetic nanowires could find application as spin filters in the new field of nanospintronics.