xixiang zhang - Academia.edu (original) (raw)

Papers by xixiang zhang

Applied Physics Letters, 2007

In this letter the authors present the observation of giant magnetothermal conductivity in NiMnIn... more In this letter the authors present the observation of giant magnetothermal conductivity in NiMnIn single crystals. Upon cooling, a martensitic transformation is accompanied by a ferromagnetic metal→ferrimagnetic poor-metal transition. Most strikingly, this transition can be shifted to lower temperature and even totally suppressed by a magnetic field. The magnetic field-induced phase transition leads to a large magnetoresistance and a large magnetothermal conductivity up to 70% and 120%, respectively. The specific heat measurements indicate that the large magnetotransport properties are due to the increasing the density of free electrons, suggesting existence of superzone gap in the low-temperature, ferrimagnetic martensite.

Applied Physics Letters, 2003

Magnetization and magnetotransport were measured in CoxAg1−x granular composites as a function of... more Magnetization and magnetotransport were measured in CoxAg1−x granular composites as a function of temperature and applied magnetic field. A transition from blocked to superparamagnetic behavior with increasing temperatures can be observed in magnetization, giant magnetoresistance and the extraordinary Hall effect measurements. However, the blocking temperature determined from magnetotransport measurements is systematically lower than the one estimated from magnetic measurements. This is due to the selective magnetic scattering, which is enhanced for smaller particles, while the magnetization probes the whole particle size distribution.

Journal of Applied Physics, 2000

A nanocomposite material has been characterized that contains nanometer size magnets that are fre... more A nanocomposite material has been characterized that contains nanometer size magnets that are free to rotate in response to an applied magnetic field. The composite consists of 5-10 nm crystals of ␥-Fe 2 O 3 dispersed in a solid methanol polymer matrix. The material was prepared by freezing a methanol-based ferrofluid of ␥-Fe 2 O 3 and subjecting it to a magnetic field applied in alternate directions to anneal the matrix. Before the field treatment, the solid displays magnetic behavior characteristic of an ordinary nanoscopic magnetic material. It is superparamagnetic above the blocking temperature ͑160 K͒ and hysteretic below, showing magnetic remanence and coercivity. After the field treatment to anneal the matrix, the same solid shows only Curie-Weiss behavior above and below the blocking temperature over the temperature range from 4.2 to 200 K and in response to applied magnetic fields as low as 1.59 kA/m. The data are consistent with a solid containing rotationally free, nanoscopic magnets encased in cavities of very small dimensions. The free rotation of the particles precludes the observation of magnetic relaxation phenomena that are characteristic of magnetic solids and ferrofluids. The present solid portends a class of magnetic materials with very little or no electrical and magnetic loss.

Chemistry of Materials, 2001

Magnetic data for a composite material consisting of nanocrystalline γ-Fe 2 O 3 dispersed in an a... more Magnetic data for a composite material consisting of nanocrystalline γ-Fe 2 O 3 dispersed in an alginate methanol matrix indicate the presence of rotationally free magnetic particles. The decoupling of the particles from the matrix was achieved by subjecting the sample to a magnetic field applied in alternating directions. Before field treatment, a magnetic blocking process is observed at low temperature, which is characteristic of nanoscale magnetic materials in the presence of barriers for the magnetic moment of the particles. After the field treatment, no magnetic hysteresis is observed and the magnetization follows a Curie-Weiss law. This behavior can only be interpreted as due to magnetic moments that follow the external magnetic field without delay, implying, therefore, the existence of free particles.

Nanotechnology, Jan 27, 2015

Transition metal oxides have attracted great interest as alternative anode materials for recharge... more Transition metal oxides have attracted great interest as alternative anode materials for rechargeable lithium-ion batteries. Among them, ruthenium dioxide is considered to be a prototype material that reacts with the Li ions in the conversion type. In situ transmission electron microscopy reveals a two-step process during the initial lithiation of the RuO2 nanowire anode at atomic resolution. The first step is characterized by the formation of the intermediate phase LixRuO2 due to the Li-ion intercalation. The following step is manifested by the solid-state amorphization reaction driven by advancing the reaction front. The crystalline/amorphous interface is consisted of {011} atomic terraces, revealing the orientation-dependent mobility. In the crystalline matrix, lattice disturbance and dislocation are identified to be two major stress-induced distortions. The latter can be effective diffusion channels, facilitating transportation of the Li ions inside the bulk RuO2 crystal and fur...

ACS Applied Materials & Interfaces, 2015

Anisotropic magnetoresistance (AMR) of the facing-target reactively sputtered epitaxial γ′-Fe 4 N... more Anisotropic magnetoresistance (AMR) of the facing-target reactively sputtered epitaxial γ′-Fe 4 N/CoN bilayers is investigated. The phase shift and rectangular-like AMR appears at low temperatures, which can be ascribed to the interfacial exchange coupling. The phase shift comes from the exchange bias (EB) that makes the magnetization lag behind a small field. When the γ′-Fe 4 N thickness increases, the rectangular-like AMR appears. The rectangular-like AMR should be from the combined contributions including the EB-induced unidirectional anisotropy, intrinsic AMR of γ′-Fe 4 N layer and interfacial spin scattering.

Physical Review B, 2002

A systematic study of Co(SiO 2) granular films by means of transmission electron microscopy ͑TEM͒... more A systematic study of Co(SiO 2) granular films by means of transmission electron microscopy ͑TEM͒, dc and ac initial magnetic susceptibility, and thermoremanent magnetization ͑TRM͒ is presented. The experimental results are compared with simulations of zero-field-cooled ͑ZFC͒ and field-cooled ͑FC͒ magnetization and TRM curves obtained using a simple model of noninteracting nanoparticles. The simulated ZFC/FC curves, using the actual parameters obtained from the TEM images, show a different behavior than the experimental magnetic data. The effect of the dipolar interaction among particles introduces a self-averaging effect over a correlation length ⌳, which results in a larger average ''magnetic'' size of the apparent particles together with a narrower size distribution. The analysis of the ZFC/FC curves in the framework of independent ''particle clusters'' of volume ⌳ 3 , involving about 25 real particles, explains very well the observed difference between the experimental data for the median blocking temperature ͗T B ͘ and their distribution width with respect to the ones expected from the structural observations by TEM. The experimental TRM curves also differ from those obtained from the theoretical model, starting to decrease at a lower temperature than expected from the model, also indicating the strong influence of dipole-dipole interactions.

Physical Review B, 2005

A dilute magnetic liquid with Co nanoparticles with an average diameter of 5 nm in hexane has bee... more A dilute magnetic liquid with Co nanoparticles with an average diameter of 5 nm in hexane has been studied systematically following the experimental approach proposed recently for observing memory effects in magnetic nanoparticles. All phenomena reported previously have been observed, which were earlier ascribed to the memory effect. However, the standard experiments for observing memory effects ͑low frequency as susceptibility and low field dc magnetization measurements͒ do not show the memory effect. To understand those observations, very detailed physical pictures, based on the relaxation of the individual particle, are proposed here.

The Journal of Physical Chemistry B, 2000

We have conducted a comprehensive study on the magnetic properties of five heavy rare-earth metal... more We have conducted a comprehensive study on the magnetic properties of five heavy rare-earth metallofullerenes M@C 82 (M) Gd, Tb, Dy, Ho, Er) in the temperature range of 1.8 and 300 K at a magnetic field up to 5 T. The isothermal magnetization curves of the endohedral metallofullerenes follow the Brillouin function only above certain temperatures. The fittings to the Curie-Wiess law and to the Brillouin function for the heavy rare-earth metallofullerenes both result in effective magnetic moments that are significantly smaller than those of the corresponding free M 3+ ions. The magnetic moment reduction and the imperfect paramagnetic behavior of M@C 82 were found to be related to the orbital angular momentum of entrapped M 3+ ions. The fullerene cage crystal field splitting, the partial hybridization of the orbitals of the entrapped metal atom and the carbon cage, and the interactions between the metal centers were proposed to account for the peculiar magnetic behaviors of the endohedral metallofullerenes.

Physical Review Letters, 2001

We report the first experimental observation of anisotropic magnetocaloric effect (MCE) in the Fe... more We report the first experimental observation of anisotropic magnetocaloric effect (MCE) in the Fe 8 clusters. It is found that the magnetic anisotropy plays a very important role in the determination of the magnetocaloric effect. The maximum and minimum MCE's are observed when the applied magnetic fields are parallel and perpendicular to the easy axis, respectively. The quantum spin Hamiltonian of a Fe 8 cluster is used to calculate the partition function and the magnetization in a range of temperature and magnetic field. Excellent quantitative agreement between the experimental data and calculation is observed.

Physical Review B, 2007

The memory effect was clearly observed in dc-sputtered metallic, magnetic Co-Ag granular films, w... more The memory effect was clearly observed in dc-sputtered metallic, magnetic Co-Ag granular films, which indicates the existence of a spin-glass-like phase at low temperatures. However, the memory effect diminished dramatically after the films were annealed at 300°C for 1 h. It was also found that the memory effect weakened gradually with increasing volume fraction of magnetic clusters. The experimental results indicate that the dipolar interaction is not the major origin for the formation of low-temperature spin-glass-like ͑SGL͒ phase in metallic, magnetic granular materials. On the other hand, a Ruderman-Kittel-Kasuya-Yosida-like exchange interaction between the nanoparticles may be responsible for the collective SGL dynamics and the resulting memory effect.

Physical Review B, 2004

Training effect of exchange bias in the ␥-Fe 2 O 3 coated Fe nanoparticles were studied. The expe... more Training effect of exchange bias in the ␥-Fe 2 O 3 coated Fe nanoparticles were studied. The experimental results indicate that the frozen spins in ␥-Fe 2 O 3 shells are responsible for both the horizontal and vertical shifts of the field-cooling hysteresis loops. To understand exchange bias and training effect in the nanoparticles, we modified the Stoner-Wohlfarth model by adding the unidirectional anisotropy energy term to the total energy. It is found that the exchange bias and training effect in the nanoparticles can be well interpreted within the modified model. Since the configuration of the frozen spins was gradually varying as the applied field cycled for the hysteresis loop measurements, the number of the spins frozen along the cooling-field direction decreased, consequently, the exchange bias field H E became smaller.

Physical Review B, 2010

We report superconducting resistive transition characteristics for array͑s͒ of coupled 4 Å single... more We report superconducting resistive transition characteristics for array͑s͒ of coupled 4 Å single wall carbon nanotubes embedded in aluminophosphate-five zeolite. The transition was observed to initiate at 15 K with a slow resistance decrease switching to a sharp, order of magnitude drop at ϳ7.5 K. The latter exhibits anisotropic magnetic field dependence, and the differential resistance versus current ͑voltage͒ measurements show a rich variety of details that are consistent with the establishment of coherence, in stages, as the temperature is lowered below 15 K. In particular, the resistance drop that starts at 7.5 K exhibits attributes that are consistent with the manifestations of a Berezinskii-Kosterlitz-Thouless transition that establishes quasi-long-range order in the plane transverse to the c axis of the aligned nanotubes.

Physical Review B, 2004

The extraordinary Hall effect (EHE) in ferromagnetic samples is generally attributed to scatterin... more The extraordinary Hall effect (EHE) in ferromagnetic samples is generally attributed to scatterings of iterant electrons in the presence of spin-orbit interactions. In this work, our study of the thickness dependence of the EHE in the ͑Ni 80 Fe 20 ͒ x ͑SiO 2 ͒ 1−x system showed the spontaneous Hall resistivity, xy S to be quite independent of the film thickness while the Hall coefficient, R S (ϵ xy S / M S , where M S is the saturated magnetization), increased monotonically owing to a depression in M S. We point out that the independence of xy S with reducing thickness could arise if the morphological structure of the sample becomes two dimensional with decreasing film thickness, which is expected from classical percolation theory. We also find in the ͑Ni 80 Fe 20 ͒ x ͑SiO 2 ͒ 1−x system (with varying x) that xy S ϰ xx ␥ where ␥ = 0.53, which disagrees with the value of 2 frequently attributed to the side jump effect, but which can be explained in terms of the more general form xy S = xx ⌬ ye / ⌳ SO , where ⌬ ye is the side jump displacement and ⌳ SO is the spin-orbit mean free path.

physica status solidi (b), 2010

We report superconducting resistive transition characteristics for array(s) of coupled 4-Angstrom... more We report superconducting resistive transition characteristics for array(s) of coupled 4-Angstrom single wall carbon nanotubes embedded in aluminophosphate-five (AFI) zeolite. The transition was observed to initiate at 15K with a slow resistance decrease switching to a sharp, order of magnitude drop between 7.5-6.0K. The transition has strong (anisotropic) magnetic field dependence. Differential resistance versus current (voltage) measurements indicate that the establishment of coherence proceeds in stages as the temperature is lowered below 15K. In particular, the sharp resistance drop and its attendant nonlinear IV characteristics are consistent with the manifestations of a Kosterlitz-Thouless (KT) transition that establishes quasi long range order in the plane transverse to the c-axis of the nanotubes, leading to an inhomogeneous system comprising 3D superconducting regions connected by weak links. Global coherence is established at below 5K with the appearance of a well-defined supercurrent gap at 2K.

Physica B: Condensed Matter, 2000

Resistance of samples prepared by compressing submicron graphite powders was measured in the temp... more Resistance of samples prepared by compressing submicron graphite powders was measured in the temperature range 1.8}300 K and magnetic "eld up to 4.5 T. The temperature dependence of resistance for some samples is satisfactorily described by a #uctuation-induced tunneling model RJexp[¹ /(¹#¹)]. In others resistance follows a power law RJ1/¹? with close to 1, and we propose a phonon-assisted bunch hopping (or tunneling) model for this case. The dominating mechanism is determined by the e!ective tunneling distance. Magnetoresistance (MR) is dominantly positive, with mesoscopic oscillations at low "elds, in the #uctuation-induced tunneling regime, and has a negative component in the phonon-assisted bunch hopping regime. Our results con"rm the existence of negative MR in systems with nearest-neighbor tunneling conduction and point the conditions for this e!ect. We interpret NMR as due to shifting of discrete electron levels towards the Fermi level in the magnetic "eld, which enhances the phonon-assisted bunch hopping.

New Journal of Physics, 2003

We review our experimental and theoretical studies on the ultrasmall single-walled carbon nanotub... more We review our experimental and theoretical studies on the ultrasmall single-walled carbon nanotubes (SWNTs) fabricated in the 1 nm channels of AlPO 4-5 (AFI) zeolite single crystals. The structure of the SWNT was characterized by transmission electron microscopy (TEM), diffuse x-ray diffraction, and micro-Raman measurements, all consistently indicating a diameter of 0.4 nm, at or close to the theoretical limit. The large curvature in the 0.4 nm SWNTs makes the nanotubes marginally stable. On the one hand, the free-standing 0.4 nm SWNTs can be thermally destroyed at a much lower temperature than larger sized SWNTs but, on the other hand, it introduces a variety of interesting material characteristics such as the large split of the G-like Raman modes, softening of the radial breathing modes, closing of the semiconducting gap so that the (5, 0) nanotubes are metallic, and the enhancement of the electronphonon coupling that makes these ultra-small nanotubes superconducting at a relatively high temperature (15 K). Band structure and dielectric function of the 0.4 nm SWNTs were calculated using the local-density-functional approach. The calculated dielectric functions yield predictions in very good agreement with the experimentally measured absorption spectra. The absorption bands can be identified as dipole transitions between states in the vicinity of the van Hove singularities. Further confirmation of these dipole-allowed transitions was obtained by the resonant Raman excitation spectrum. Electric transport measurements were conducted on the SWNT@AFI crystals. As the zeolite matrix is insulating, electric conduction can be ascribed to the nanotubes. It was shown that the conductivity of the 0.4 nm SWNTs is governed by a 1D electron hopping process at temperatures above 20 K. The measured magnetic and

Journal of the American Chemical Society, 2004

A) Synthesis N-tert-Butoxycarbonyl-3,4-dihydroxiphenylethylamine (1a). To a mix solvent of dioxan... more A) Synthesis N-tert-Butoxycarbonyl-3,4-dihydroxiphenylethylamine (1a). To a mix solvent of dioxane (5 mL) and H 2 O (2.5 mL), dopamine hydrochloride (0.48 g, 2.54 mmol) and NaOH (1 M, 2.54 mL, 2.54 mmol) were added. The solution was degassed for 3 times and stirred for 10 min, then tert-butyl dicarbonate (0.555 g, 2.54 mmol) in 5 mL of dioxane was dropped into the solution. After stirring for 20 hrs undre inert atmosphere, the solution was cooled in an ice bath. After adding 50 mL ethyl acetate, the mixture was acidified with dilute HCl solution (1 M) to pH = 3. The aqueous phase was washed with ethyl acetate (2 × 30 mL), and all organic fractions were combined. The organic phase was sequentially washed by H 2 O (3 × 30 mL), brine (3 × 30 ml) and dried over anhydrous Na 2 SO 4. After the organic phase was condensed to 10 mL, the product was purified by flash column (eluent: hexane/ethyl acetate/MeOH=3:1:0.5) to give 0.579 g of pure product (yield = 90%).

Journal of Physics: Condensed Matter, 2006

The field dependence of the peak temperature of the zero-field-cooled (ZFC) magnetization curve o... more The field dependence of the peak temperature of the zero-field-cooled (ZFC) magnetization curve of a magnetic nanoparticle system was studied using a diluted magnetic fluid composed of FePt nanoparticles. It is found that the peak temperature increases with increasing applied field below 3 kOe; it then decreases with further increasing the applied field. The non-monotonic field dependence of the peak temperature in magnetic particle systems is attributed to the anisotropic energy barrier distribution of the particles, and to the slowly decreasing magnetization (or to the non-Curie's law dependence of magnetization) above the blocking temperature. The non-Curie's law dependence of the magnetization is caused by large magnetic anisotropy and Zeeman energy of particles in high magnetic fields. Numerical simulation results, based on basic thermodynamics, and pure thermal relaxation and energy barrier distribution extracted from the low-field, experimental ZFC data show a good agreement with experimental results.

Journal of Physics: Condensed Matter, 2001

Co3(BO3)2/surfactant composites have been prepared by the controlled precipitation of aqueous cob... more Co3(BO3)2/surfactant composites have been prepared by the controlled precipitation of aqueous cobalt cations together with surfactant. The composite shows a layered structure as determined by x-ray diffraction, in which the Co3(BO3)2 layers of about 6 Å in thickness are in alternation with surfactant bilayers. The temperature-dependent magnetization measured in the zero-field-cooled (ZFC) and field-cooled (FC) processes and the AC-magnetic susceptibility

Applied Physics Letters, 2007

In this letter the authors present the observation of giant magnetothermal conductivity in NiMnIn... more In this letter the authors present the observation of giant magnetothermal conductivity in NiMnIn single crystals. Upon cooling, a martensitic transformation is accompanied by a ferromagnetic metal→ferrimagnetic poor-metal transition. Most strikingly, this transition can be shifted to lower temperature and even totally suppressed by a magnetic field. The magnetic field-induced phase transition leads to a large magnetoresistance and a large magnetothermal conductivity up to 70% and 120%, respectively. The specific heat measurements indicate that the large magnetotransport properties are due to the increasing the density of free electrons, suggesting existence of superzone gap in the low-temperature, ferrimagnetic martensite.

Applied Physics Letters, 2003

Magnetization and magnetotransport were measured in CoxAg1−x granular composites as a function of... more Magnetization and magnetotransport were measured in CoxAg1−x granular composites as a function of temperature and applied magnetic field. A transition from blocked to superparamagnetic behavior with increasing temperatures can be observed in magnetization, giant magnetoresistance and the extraordinary Hall effect measurements. However, the blocking temperature determined from magnetotransport measurements is systematically lower than the one estimated from magnetic measurements. This is due to the selective magnetic scattering, which is enhanced for smaller particles, while the magnetization probes the whole particle size distribution.

Journal of Applied Physics, 2000

A nanocomposite material has been characterized that contains nanometer size magnets that are fre... more A nanocomposite material has been characterized that contains nanometer size magnets that are free to rotate in response to an applied magnetic field. The composite consists of 5-10 nm crystals of ␥-Fe 2 O 3 dispersed in a solid methanol polymer matrix. The material was prepared by freezing a methanol-based ferrofluid of ␥-Fe 2 O 3 and subjecting it to a magnetic field applied in alternate directions to anneal the matrix. Before the field treatment, the solid displays magnetic behavior characteristic of an ordinary nanoscopic magnetic material. It is superparamagnetic above the blocking temperature ͑160 K͒ and hysteretic below, showing magnetic remanence and coercivity. After the field treatment to anneal the matrix, the same solid shows only Curie-Weiss behavior above and below the blocking temperature over the temperature range from 4.2 to 200 K and in response to applied magnetic fields as low as 1.59 kA/m. The data are consistent with a solid containing rotationally free, nanoscopic magnets encased in cavities of very small dimensions. The free rotation of the particles precludes the observation of magnetic relaxation phenomena that are characteristic of magnetic solids and ferrofluids. The present solid portends a class of magnetic materials with very little or no electrical and magnetic loss.

Chemistry of Materials, 2001

Magnetic data for a composite material consisting of nanocrystalline γ-Fe 2 O 3 dispersed in an a... more Magnetic data for a composite material consisting of nanocrystalline γ-Fe 2 O 3 dispersed in an alginate methanol matrix indicate the presence of rotationally free magnetic particles. The decoupling of the particles from the matrix was achieved by subjecting the sample to a magnetic field applied in alternating directions. Before field treatment, a magnetic blocking process is observed at low temperature, which is characteristic of nanoscale magnetic materials in the presence of barriers for the magnetic moment of the particles. After the field treatment, no magnetic hysteresis is observed and the magnetization follows a Curie-Weiss law. This behavior can only be interpreted as due to magnetic moments that follow the external magnetic field without delay, implying, therefore, the existence of free particles.

Nanotechnology, Jan 27, 2015

Transition metal oxides have attracted great interest as alternative anode materials for recharge... more Transition metal oxides have attracted great interest as alternative anode materials for rechargeable lithium-ion batteries. Among them, ruthenium dioxide is considered to be a prototype material that reacts with the Li ions in the conversion type. In situ transmission electron microscopy reveals a two-step process during the initial lithiation of the RuO2 nanowire anode at atomic resolution. The first step is characterized by the formation of the intermediate phase LixRuO2 due to the Li-ion intercalation. The following step is manifested by the solid-state amorphization reaction driven by advancing the reaction front. The crystalline/amorphous interface is consisted of {011} atomic terraces, revealing the orientation-dependent mobility. In the crystalline matrix, lattice disturbance and dislocation are identified to be two major stress-induced distortions. The latter can be effective diffusion channels, facilitating transportation of the Li ions inside the bulk RuO2 crystal and fur...

ACS Applied Materials & Interfaces, 2015

Anisotropic magnetoresistance (AMR) of the facing-target reactively sputtered epitaxial γ′-Fe 4 N... more Anisotropic magnetoresistance (AMR) of the facing-target reactively sputtered epitaxial γ′-Fe 4 N/CoN bilayers is investigated. The phase shift and rectangular-like AMR appears at low temperatures, which can be ascribed to the interfacial exchange coupling. The phase shift comes from the exchange bias (EB) that makes the magnetization lag behind a small field. When the γ′-Fe 4 N thickness increases, the rectangular-like AMR appears. The rectangular-like AMR should be from the combined contributions including the EB-induced unidirectional anisotropy, intrinsic AMR of γ′-Fe 4 N layer and interfacial spin scattering.

Physical Review B, 2002

A systematic study of Co(SiO 2) granular films by means of transmission electron microscopy ͑TEM͒... more A systematic study of Co(SiO 2) granular films by means of transmission electron microscopy ͑TEM͒, dc and ac initial magnetic susceptibility, and thermoremanent magnetization ͑TRM͒ is presented. The experimental results are compared with simulations of zero-field-cooled ͑ZFC͒ and field-cooled ͑FC͒ magnetization and TRM curves obtained using a simple model of noninteracting nanoparticles. The simulated ZFC/FC curves, using the actual parameters obtained from the TEM images, show a different behavior than the experimental magnetic data. The effect of the dipolar interaction among particles introduces a self-averaging effect over a correlation length ⌳, which results in a larger average ''magnetic'' size of the apparent particles together with a narrower size distribution. The analysis of the ZFC/FC curves in the framework of independent ''particle clusters'' of volume ⌳ 3 , involving about 25 real particles, explains very well the observed difference between the experimental data for the median blocking temperature ͗T B ͘ and their distribution width with respect to the ones expected from the structural observations by TEM. The experimental TRM curves also differ from those obtained from the theoretical model, starting to decrease at a lower temperature than expected from the model, also indicating the strong influence of dipole-dipole interactions.

Physical Review B, 2005

A dilute magnetic liquid with Co nanoparticles with an average diameter of 5 nm in hexane has bee... more A dilute magnetic liquid with Co nanoparticles with an average diameter of 5 nm in hexane has been studied systematically following the experimental approach proposed recently for observing memory effects in magnetic nanoparticles. All phenomena reported previously have been observed, which were earlier ascribed to the memory effect. However, the standard experiments for observing memory effects ͑low frequency as susceptibility and low field dc magnetization measurements͒ do not show the memory effect. To understand those observations, very detailed physical pictures, based on the relaxation of the individual particle, are proposed here.

The Journal of Physical Chemistry B, 2000

We have conducted a comprehensive study on the magnetic properties of five heavy rare-earth metal... more We have conducted a comprehensive study on the magnetic properties of five heavy rare-earth metallofullerenes M@C 82 (M) Gd, Tb, Dy, Ho, Er) in the temperature range of 1.8 and 300 K at a magnetic field up to 5 T. The isothermal magnetization curves of the endohedral metallofullerenes follow the Brillouin function only above certain temperatures. The fittings to the Curie-Wiess law and to the Brillouin function for the heavy rare-earth metallofullerenes both result in effective magnetic moments that are significantly smaller than those of the corresponding free M 3+ ions. The magnetic moment reduction and the imperfect paramagnetic behavior of M@C 82 were found to be related to the orbital angular momentum of entrapped M 3+ ions. The fullerene cage crystal field splitting, the partial hybridization of the orbitals of the entrapped metal atom and the carbon cage, and the interactions between the metal centers were proposed to account for the peculiar magnetic behaviors of the endohedral metallofullerenes.

Physical Review Letters, 2001

We report the first experimental observation of anisotropic magnetocaloric effect (MCE) in the Fe... more We report the first experimental observation of anisotropic magnetocaloric effect (MCE) in the Fe 8 clusters. It is found that the magnetic anisotropy plays a very important role in the determination of the magnetocaloric effect. The maximum and minimum MCE's are observed when the applied magnetic fields are parallel and perpendicular to the easy axis, respectively. The quantum spin Hamiltonian of a Fe 8 cluster is used to calculate the partition function and the magnetization in a range of temperature and magnetic field. Excellent quantitative agreement between the experimental data and calculation is observed.

Physical Review B, 2007

The memory effect was clearly observed in dc-sputtered metallic, magnetic Co-Ag granular films, w... more The memory effect was clearly observed in dc-sputtered metallic, magnetic Co-Ag granular films, which indicates the existence of a spin-glass-like phase at low temperatures. However, the memory effect diminished dramatically after the films were annealed at 300°C for 1 h. It was also found that the memory effect weakened gradually with increasing volume fraction of magnetic clusters. The experimental results indicate that the dipolar interaction is not the major origin for the formation of low-temperature spin-glass-like ͑SGL͒ phase in metallic, magnetic granular materials. On the other hand, a Ruderman-Kittel-Kasuya-Yosida-like exchange interaction between the nanoparticles may be responsible for the collective SGL dynamics and the resulting memory effect.

Physical Review B, 2004

Training effect of exchange bias in the ␥-Fe 2 O 3 coated Fe nanoparticles were studied. The expe... more Training effect of exchange bias in the ␥-Fe 2 O 3 coated Fe nanoparticles were studied. The experimental results indicate that the frozen spins in ␥-Fe 2 O 3 shells are responsible for both the horizontal and vertical shifts of the field-cooling hysteresis loops. To understand exchange bias and training effect in the nanoparticles, we modified the Stoner-Wohlfarth model by adding the unidirectional anisotropy energy term to the total energy. It is found that the exchange bias and training effect in the nanoparticles can be well interpreted within the modified model. Since the configuration of the frozen spins was gradually varying as the applied field cycled for the hysteresis loop measurements, the number of the spins frozen along the cooling-field direction decreased, consequently, the exchange bias field H E became smaller.

Physical Review B, 2010

We report superconducting resistive transition characteristics for array͑s͒ of coupled 4 Å single... more We report superconducting resistive transition characteristics for array͑s͒ of coupled 4 Å single wall carbon nanotubes embedded in aluminophosphate-five zeolite. The transition was observed to initiate at 15 K with a slow resistance decrease switching to a sharp, order of magnitude drop at ϳ7.5 K. The latter exhibits anisotropic magnetic field dependence, and the differential resistance versus current ͑voltage͒ measurements show a rich variety of details that are consistent with the establishment of coherence, in stages, as the temperature is lowered below 15 K. In particular, the resistance drop that starts at 7.5 K exhibits attributes that are consistent with the manifestations of a Berezinskii-Kosterlitz-Thouless transition that establishes quasi-long-range order in the plane transverse to the c axis of the aligned nanotubes.

Physical Review B, 2004

The extraordinary Hall effect (EHE) in ferromagnetic samples is generally attributed to scatterin... more The extraordinary Hall effect (EHE) in ferromagnetic samples is generally attributed to scatterings of iterant electrons in the presence of spin-orbit interactions. In this work, our study of the thickness dependence of the EHE in the ͑Ni 80 Fe 20 ͒ x ͑SiO 2 ͒ 1−x system showed the spontaneous Hall resistivity, xy S to be quite independent of the film thickness while the Hall coefficient, R S (ϵ xy S / M S , where M S is the saturated magnetization), increased monotonically owing to a depression in M S. We point out that the independence of xy S with reducing thickness could arise if the morphological structure of the sample becomes two dimensional with decreasing film thickness, which is expected from classical percolation theory. We also find in the ͑Ni 80 Fe 20 ͒ x ͑SiO 2 ͒ 1−x system (with varying x) that xy S ϰ xx ␥ where ␥ = 0.53, which disagrees with the value of 2 frequently attributed to the side jump effect, but which can be explained in terms of the more general form xy S = xx ⌬ ye / ⌳ SO , where ⌬ ye is the side jump displacement and ⌳ SO is the spin-orbit mean free path.

physica status solidi (b), 2010

We report superconducting resistive transition characteristics for array(s) of coupled 4-Angstrom... more We report superconducting resistive transition characteristics for array(s) of coupled 4-Angstrom single wall carbon nanotubes embedded in aluminophosphate-five (AFI) zeolite. The transition was observed to initiate at 15K with a slow resistance decrease switching to a sharp, order of magnitude drop between 7.5-6.0K. The transition has strong (anisotropic) magnetic field dependence. Differential resistance versus current (voltage) measurements indicate that the establishment of coherence proceeds in stages as the temperature is lowered below 15K. In particular, the sharp resistance drop and its attendant nonlinear IV characteristics are consistent with the manifestations of a Kosterlitz-Thouless (KT) transition that establishes quasi long range order in the plane transverse to the c-axis of the nanotubes, leading to an inhomogeneous system comprising 3D superconducting regions connected by weak links. Global coherence is established at below 5K with the appearance of a well-defined supercurrent gap at 2K.

Physica B: Condensed Matter, 2000

Resistance of samples prepared by compressing submicron graphite powders was measured in the temp... more Resistance of samples prepared by compressing submicron graphite powders was measured in the temperature range 1.8}300 K and magnetic "eld up to 4.5 T. The temperature dependence of resistance for some samples is satisfactorily described by a #uctuation-induced tunneling model RJexp[¹ /(¹#¹)]. In others resistance follows a power law RJ1/¹? with close to 1, and we propose a phonon-assisted bunch hopping (or tunneling) model for this case. The dominating mechanism is determined by the e!ective tunneling distance. Magnetoresistance (MR) is dominantly positive, with mesoscopic oscillations at low "elds, in the #uctuation-induced tunneling regime, and has a negative component in the phonon-assisted bunch hopping regime. Our results con"rm the existence of negative MR in systems with nearest-neighbor tunneling conduction and point the conditions for this e!ect. We interpret NMR as due to shifting of discrete electron levels towards the Fermi level in the magnetic "eld, which enhances the phonon-assisted bunch hopping.

New Journal of Physics, 2003

We review our experimental and theoretical studies on the ultrasmall single-walled carbon nanotub... more We review our experimental and theoretical studies on the ultrasmall single-walled carbon nanotubes (SWNTs) fabricated in the 1 nm channels of AlPO 4-5 (AFI) zeolite single crystals. The structure of the SWNT was characterized by transmission electron microscopy (TEM), diffuse x-ray diffraction, and micro-Raman measurements, all consistently indicating a diameter of 0.4 nm, at or close to the theoretical limit. The large curvature in the 0.4 nm SWNTs makes the nanotubes marginally stable. On the one hand, the free-standing 0.4 nm SWNTs can be thermally destroyed at a much lower temperature than larger sized SWNTs but, on the other hand, it introduces a variety of interesting material characteristics such as the large split of the G-like Raman modes, softening of the radial breathing modes, closing of the semiconducting gap so that the (5, 0) nanotubes are metallic, and the enhancement of the electronphonon coupling that makes these ultra-small nanotubes superconducting at a relatively high temperature (15 K). Band structure and dielectric function of the 0.4 nm SWNTs were calculated using the local-density-functional approach. The calculated dielectric functions yield predictions in very good agreement with the experimentally measured absorption spectra. The absorption bands can be identified as dipole transitions between states in the vicinity of the van Hove singularities. Further confirmation of these dipole-allowed transitions was obtained by the resonant Raman excitation spectrum. Electric transport measurements were conducted on the SWNT@AFI crystals. As the zeolite matrix is insulating, electric conduction can be ascribed to the nanotubes. It was shown that the conductivity of the 0.4 nm SWNTs is governed by a 1D electron hopping process at temperatures above 20 K. The measured magnetic and

Journal of the American Chemical Society, 2004

A) Synthesis N-tert-Butoxycarbonyl-3,4-dihydroxiphenylethylamine (1a). To a mix solvent of dioxan... more A) Synthesis N-tert-Butoxycarbonyl-3,4-dihydroxiphenylethylamine (1a). To a mix solvent of dioxane (5 mL) and H 2 O (2.5 mL), dopamine hydrochloride (0.48 g, 2.54 mmol) and NaOH (1 M, 2.54 mL, 2.54 mmol) were added. The solution was degassed for 3 times and stirred for 10 min, then tert-butyl dicarbonate (0.555 g, 2.54 mmol) in 5 mL of dioxane was dropped into the solution. After stirring for 20 hrs undre inert atmosphere, the solution was cooled in an ice bath. After adding 50 mL ethyl acetate, the mixture was acidified with dilute HCl solution (1 M) to pH = 3. The aqueous phase was washed with ethyl acetate (2 × 30 mL), and all organic fractions were combined. The organic phase was sequentially washed by H 2 O (3 × 30 mL), brine (3 × 30 ml) and dried over anhydrous Na 2 SO 4. After the organic phase was condensed to 10 mL, the product was purified by flash column (eluent: hexane/ethyl acetate/MeOH=3:1:0.5) to give 0.579 g of pure product (yield = 90%).

Journal of Physics: Condensed Matter, 2006

The field dependence of the peak temperature of the zero-field-cooled (ZFC) magnetization curve o... more The field dependence of the peak temperature of the zero-field-cooled (ZFC) magnetization curve of a magnetic nanoparticle system was studied using a diluted magnetic fluid composed of FePt nanoparticles. It is found that the peak temperature increases with increasing applied field below 3 kOe; it then decreases with further increasing the applied field. The non-monotonic field dependence of the peak temperature in magnetic particle systems is attributed to the anisotropic energy barrier distribution of the particles, and to the slowly decreasing magnetization (or to the non-Curie's law dependence of magnetization) above the blocking temperature. The non-Curie's law dependence of the magnetization is caused by large magnetic anisotropy and Zeeman energy of particles in high magnetic fields. Numerical simulation results, based on basic thermodynamics, and pure thermal relaxation and energy barrier distribution extracted from the low-field, experimental ZFC data show a good agreement with experimental results.

Journal of Physics: Condensed Matter, 2001

Co3(BO3)2/surfactant composites have been prepared by the controlled precipitation of aqueous cob... more Co3(BO3)2/surfactant composites have been prepared by the controlled precipitation of aqueous cobalt cations together with surfactant. The composite shows a layered structure as determined by x-ray diffraction, in which the Co3(BO3)2 layers of about 6 Å in thickness are in alternation with surfactant bilayers. The temperature-dependent magnetization measured in the zero-field-cooled (ZFC) and field-cooled (FC) processes and the AC-magnetic susceptibility