Nuclear Magnetism of Submonolayer Solid 3He on Graphite

Submonolayer solid ³ He adsorbed on graphite has been studied with NMR measurement down to the order of 0.1 mK. The magnetization curves are well fitted to the Curie-Weiss law in the whole investigated density region (5.0 nm ^–2 - 8.5 nm ^–2 ). The obtained Weiss temperatures are found to be positive at a commensurate phase and an incommensurate one. However they are negative in the intermediate density region, which suggests the existence of an antiferromaynetic phase also in the Submonolayer. The change from an antiferro to a ferromagnetic behavior at around 7.3 nm ^–2 is similar to that of the second layer, and can be explained by a multiple spin exchange model. Therefore the ferromagnetic behavior in the second layer is not necessarily due to the liquid overlayer. While the ferromagnetic behavior at the phase is not reproduced by a simple density dependence of competing multiple exchange interactions and indicates the importance of the corrugation potential from the graphite substrate.     

Ferromagnetism of 3He Films in Very Low Applied Fields

³ He films on graphite provide a model system for 2D magnetism. At low temperatures, the thermodynamic properties of these films are dominated by exchange interactions. However more subtle interactions such as dipolar coupling and other anisotropies may eventually play a substantial role, especially in small magnetic fields. Our recent experiments use a SQUID system for both pulsed and continuous wave NMR at very low applied fields (B<1mT). The spin polarization can be estimated from both the amplitude and frequency shift. We find that at coverages above 20 atoms/nm ² there is a field independent (zero field) magnetization at finite temperatures. The temperature where this magnetization develops is significantly below J/k where J is the effective exchange energy. Although the apparent ordering occurs over a broad temperature range, there is a trend towards a narrowing of the transition region down to the lowest applied fields. Contrary to recent estimates of exchange parameters which assume a single phase, we find that below 24 atoms/nm ² , only a fraction of the spins contribute to the ferromagnetic behavior. This is more consistent with a mixed phase system in that coverage region.     

Nuclear Magnetism of Two Dimensional Solid 3He Adsorbed on Plated Graphite

Nuclear spin exchange in the low density two dimensional quantum solid formed by ³ He adsorbed on graphite plated by a bilayer of HD has been studied by heat capacity and magnetization measurements. The low densities result in a frustrated magnet with significantly larger effective exchange constants than observed in the antiferromagnetic second layer of ³ He on bare graphite. Promotion to a fluid overlayer occurs at 6.8 nm ^–2. A ferromagnetic anomaly is observed in magnetization measurements at 10 nm ^–2 with an effective exchange constant of 1.0 mK, about half the value observed at the corresponding anomaly on bare graphite.     

Evidence for a Self-bound Liquid State and the Commensurate–Incommensurate Coexistence in 2D 3He on Graphite

We made heat-capacity measurements of two dimensional (2D) ³He adsorbed on graphite preplated with monolayer ⁴He in a wide temperature range (0.1≤T≤80 mK) at densities higher than that for the 4/7 phase (=6.8 nm^?2). In the density range of 6.8≤ρ≤8.1 nm^?2, the 4/7 phase is stable against additional ³He atoms up to 20% and they are promoted into the third layer. We found evidence that such promoted atoms form a self-bound 2D Fermi liquid with an approximate density of 1 nm^?2 from the measured density dependence of the γ-coefficient of heat capacity. We also show evidence for the first-order transition between the commensurate 4/7 phase and the ferromagnetic incommensurate phase in the second layer in the density range of 8.1≤ρ≤9.5 nm^?2.     

Field dependence of the magnetization of adsorbed3He films at ultra low temperatures

We present measurements of the magnetization of pure³He films adsorbed on graphite at a density of p = 0.235 atoms/Ų, which corresponds to the 2D Heisenberg ferromagnetic regime. Different NMR frequencies (461.3kHz and 1.004 MHz) were used to study the magnetic field dependence of the nuclear magnetization. Measurements were performed on a Papyex sample to investigate the influence of the platelet size. The results are discussed in the context of theoretical models presented recently to describe these systems.     

Low Temperature Magnetization of Two Dimensional 3He on Three Layers of HD Preplated Graphite

We have measured the low temperature magnetization of submonolayer ³He on three layers of HD preplated graphite. NMR measurement has been performed by progressively adding ⁴He to the system, to prevent ³He atoms from being trapped in substrate heterogeneities and to change the areal density of ³He. The exchange constant (J) is found to have a similar density dependence to that for two layers of HD preplated one. The magnetization in the antiferromagnetic region increases gradually down to 100 K and shows no evidence corresponding to a spin gap.     

Magnetization measurement of 3He film adsorbed on sintered silver powder

The static magnetization of ³He film adsorbed on sintered silver powder has been measured for coverages from 0.10 to 0.54 atoms/»² in steps of 0.02 atoms/»² and at a coverage of 4.5 atoms/»². Below 0.20 atoms/»² the magnetization well obeys the Curie law, while above that coverage a ferromagnetic tendency appears. The coverage dependence of the magnetization greatly differs from that for graphite. At 0.23 atoms/»² the magnetization has apeak like that for graphite; however, the magnitude of the peak is not so large. Furthermore, a multipeak structure is found in the isotherm of the magnetization at 0.2 mK. It has two prominent peaks at 0.23 and 0.29 atoms/»², and two other peaks are found at 0.35 and 0.41 atoms/»². These appear periodically with an interval of 0.06 atoms/»², which is nearly equal to the coverage of one liquid layer.     

NMR Measurements of 2D 3He Adsorbed on ZYX Exfoliated Graphite

We describe first successful usage of ZYX exfoliated graphite to cool adsorbed two dimensional (2D) ³He to ugtra low temperatures (ULT). ZYX has much better characteristics as an adsorption substrate such as larger crystallite (platelet) size and smaller mosaic angle than Grafoil, a widely used exfoliated graphite in ULT experiments. In spite of the relatively small surface area of ZYX, we could perform a nuclear magnetization measurement of paramagnetic 2D solid ³He with continuous wave NMR below 10 mK. The data follow nicely the Curie law down to 200 µK. This indicates that ZYX can be used in experiments to study detailed density dependences of low temperature properties of 2D ³He including possible BCS transitions. We also show a preliminary resugt of pulsed NMR measurements of high density 2D fluid ³He adsorbed on ZYX preplated with a ⁴He monolayer.     

Mechanochemical processing and microstructural characterization of pure Fe2B nanocrystals

The results of current investigation demonstrate that mechanochemical processing can be used to synthesize high purity Fe₂B nanocrystals by selecting well-optimized milling conditions, reaction paths and proper starting materials. Microstructure, phase analyses, specific surface area, and magnetic properties of the synthesized nanocrystals were examined by using X-ray diffraction/spectroscopy, electron microscopy, nitrogen adsorption–desorption methods following Brunauer-Emmett-Teller equation and vibrating sample magnetometer techniques, respectively. Removal of MgO impurity phase by leaching the resulting powder in the acetic acid solution yielded single phase Fe₂B nanocrystals with the crystallite size and specific surface area of 12.5 nm and 29 m²/g, respectively. Magnetization results clearly indicated the ferromagnetic behavior of Fe₂B nanocrystals with saturation magnetization observed around 96.26 emu·g^?1. Electron microscope images revealed coaxial/spherical powder shape and morphology of the single-phase Fe₂B nanocrystals.     

On adsorption of aluminium and methyl groups on silica for TMA/H<Subscript>2</Subscript>O process in atomic layer deposition of aluminium oxide nano layers

A detailed chemisorption mechanism is proposed for the atomic layer deposition (ALD) of aluminium oxide nano layers using trimethyl aluminum (TMA) and water as precursors. Six possible chemisorption mechanisms, complete ligand exchange, partial ligand exchange, simple dissociation, complete dissociation via ligand exchange, complete dissociation and association, are proposed and related parameters like ligand to metal ratio (L/M), concentrations of metal atoms and methyl groups adsorbed are calculated and compared against reported values. The maximum number of methyl groups that can get attached on the surface is calculated in a different way which yields a more realistic value of 6·25 per nm² substrate area. The dependence of the number of metal atoms adsorbed on OH concentration is explained clearly. It is proposed that a combination of complete ligand exchange and complete dissociation is the most probable chemisorption mechanism taking place at various OH concentrations.     

NMR Characterization of 3He Nanoclusters Embedded in hcp 4He

The properties of ³He nanoclusters in phase-separated solid ³He-⁴He mixture were investigated using pulse NMR. Samples of pressure between 2.64 and 3.71 MPa and NMR frequencies, 62.5, 125, and 250 kHz were used. Magnetization was measured in the temperature range between 0.6 to 10 mK. The solid-like fraction was obtained from the relative magnitudes of magnetization and it showed a sharp increase with pressure around 2.94 MPa accompanying an abrupt change in Weiss temperature. For samples of pressure between 2.94 and 3.15 MPa, the magnetization could be separated into two components with different spin-spin relaxation time T ^* ₂'s. The short and long T ^* ₂ components showed ferromagnetic and antiferromagnetic tendencies, respectively. The long component showed an anomalous decrease of magnetization below 1.05 mK.     

Influence of hydrogen chemisorption on the magnetism of thin nickel films

Thin nickel films (thickness d = 13 × 10^-10 m) were prepared by evaporation on quartz substrates and were studied by means of ferromagnetic resonance spectroscopy under ultrahigh vacuum conditions (p < 2 × 10^-8 Pa).Since the ferromagnetic resonance data were found to change reversibly with hydrogen pressure and temperature, chemisorption isotherms could be derived which are in good agreement with well-known thermodynamic data.The results are discussed according to the ferromagnetic resonance theory of thin films. The magnetic properties are shown to be mainly influenced during chemisorption by changes in the magnetization. Each hydrogen atom adsorbed diminishes the magnetization of the sample by the contribution of one nickel atom.     

Structure,conduction mechanisms and multiferroic properties of (Sm,Cr) co-doped Bi0.89Sm0.11Fe0.97Cr0.03O3–NiFe2O4 composition thin films

(Sm, Cr) co-doped Bi_0.89Sm_0.11Fe_0.97 Cr_0.03O₃ (BSFC)–NiFe₂O₄ (NFO) composition thin films were successfully prepared on FTO/glass (SnO₂:F) substrates via a sol–gel method. The structure, surface morphology, leakage current, ferroelectricity and ferromagnetism of BSFC–NFO composition thin film have been investigated. X-ray diffraction analysis indicates that the thin film is polycrystalline and consisted of a rhombohedral perovskite (R3m space group) BiFeO₃ phase and a cubic (Fd-3m space group) inverse spinel NiFe₂O₄ phase. The BSFC–NFO composition thin film is promising in practical application because of its well saturated ferromagnetic (Ms = 19.45 emu/cm³) and ferroelectric (Pr = 39 µC/cm²) hysteresis loops with low order of leakage current density (J = 6.73 × 10^?6 A/cm², at an applied electric field of 100 kV/cm). Which suggest the ferroelectric and ferromagnetic properties can be improved by this composition thin film structure. Moreover, the various conduction mechanisms of BSFC–NFO composition thin film have also been studied.     

Coupling of the Nuclear Spin Alignment of Quadrupolar Nuclei to 3He at Interfaces

Measurements of the relaxation of nuclear spin alignments have been carried out using NQR spin echo studies of powdered antimony in contact with liquid ³ He. This system differs from previous studies of the relaxation and magnetization coupling of ³ He at surfaces because the magnetizations < S _z > 0. The spin alignments < S _z ² > are the relevant dynamical variables. The analysis of the experimental results shows that the coupling of the nuclear spin alignment of the antimony nuclei to the magnetization of the ³ He nuclei is determined by the spectral density of the fluctuations of the spin-spin dipolar couplings at the interface. This power density is attributed to the zero-point motions (atom-atom and atom-vacancy exchange) of the ³ He atoms on the surface of the metal particles.     

Ferromagnetic anomalies in two dimensional3He adsorbed on graphite

The second layer of solid³He adsorbed on graphite exhibits a pronounced exchange anomaly which is peaked sharply at a surface density of 0.075Å^–2. The much weaker exchange anomaly in the submonolayer solid film, first detected in heat capacity studies, has been investigated by measurements of the nuclear magnetic susceptibility to temperatures below 1mK and found to be ferromagnetic. We report the dependence of the effective exchange constant on surface density and propose a model in which both anomalies arise from similar two dimensional commensurate structures.     

Electronic excitation induced phase transformation in FSMA thin film

The influence of 120 MeV Ag ion irradiation on the structural and magnetic properties of Ni–Mn–Sn ferromagnetic shape memory alloy thin film is investigated. X-ray diffraction data confirms the phase transformation from martensite to austenite phase at a fluence of 1 × 10¹³ ions/cm², which is further supported by the change in surface morphology of the film with increasing fluence as evidenced by field emission scanning electron microscopy. Thermo-magnetic measurements reveal the increase in magnetization and decrease in phase transformation temperatures with increasing fluence. The maximum value of magnetization is ∼2.9 × 10⁵ Amp/meter for the film irradiated at a fluence of 1 × 10¹³ ions/cm². The results are explained on the basis of thermal spike model considering the core and halo regions of ion tracks in FSMA materials.     

Impact of Li<Superscript>3+</Superscript> ion irradiation on magnetic properties of Mn doped ZnO: correlation with defects and structural property

Present work aims to study the impact of 50 MeV Li³⁺ ion irradiation on magnetic properties and their correlation with the substitutional incorporation of Mn at Zn site (Mn_Zn) and defects in Mn:ZnO (Mn doped ZnO). 2 at% Mn:ZnO has been synthesized by solid-state reaction method and irradiated with fluences 10¹², 10¹³ and 10¹⁴ ions/cm². The concomitant changes have been probed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and magnetization measurements. XRD patterns indicate single-phase nature of the samples and Mn_Zn nicely varied with fluence of irradiation. Temperature dependent dc and ac magnetization (M-T) measurement reveals a minor presence of spin glass nature or superparamagnetism (SPM) though all of the samples are strongly intrinsically ferromagnetic and field dependent magnetization (M-H) measurement actually confirms this observation. It is noteworthy to observe that the ordering of spins and the magnetic parameters viz. saturation magnetization (M_S), remnant magnetization (M_R) and coercivity (H_C) can be tuned by fluences of irradiation. Finally, proportion of defect [particularly zinc vacancy (V_Zn)] and Mn_Zn seems to dictate the trend of variation of magnetization (M_S and M_R). Temperature dependent ac magnetization measurement and magnetic relaxation measurement actually detect a weak spin glass nature of the samples and eliminate the possibility of SPM.     

Magnetic properties of sputtered multilayers of Mo/Ni

We have studied the d.c. magnetization of Mo/Ni artificial superlattices in the temperature range 5–300 K and in magnetic fields of up to 10⁷/4πAm^-1 (10kG). The saturation magnetization and the Curie temperature behavior are consistent with expectations based on thin film effects. However, there are indications of ferromagnetic coupling across the normal metal and the coercive field shows an unexpected peak at a layer thickness of 25 Å.     

Magnetism in BiFe<Subscript>1−x</Subscript>Ni<Subscript>x</Subscript>O<Subscript>3</Subscript>: studied through electron spin resonance spectroscopy

The effect of Ni doping in BiFe_1?xNi_xO₃ (BFNO) multiferroics are studied by X-ray diffraction (XRD), Fourier transmission infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), hysteresis loop (M–H), temperature dependent magnetization (FC-ZFC) measurements and electron spin resonance (ESR) techniques. The XRD and FTIR studies indicate that the BFNO compounds remain in rhombohedral (R3c) phase without appearance of any structural transformation due to Ni doping. The XPS studies show the oxidation states of Fe ions as 3⁺, whereas Bi is found to be in a mixed valence state of 2⁺ and 3⁺ in BFNO. The Ni ion doping enhances the saturation magnetization from 0.179 emu/g (x?=?0.025) to 2.38 emu/g (x?=?0.20), which is higher than the reported values found in literature. The FC-ZFC magnetization studies suggest the presence of a magnetic phase transition from a weak ferromagnetic to a spin glass state at low temperature. The ESR studies confirm the ferromagnetic state of BFNO samples.