Ferromagnetism Of Nickel Research Articles

Pressure-induced collapse of ferromagnetism in nickel

Transition metals Fe, Co, and Ni are the canonical systems for studying the effect of external perturbations on ferromagnetism. Among these, Ni stands out as it undergoes no structural phase transition under pressure. Here we have investigated the long-debated issue of the pressure-induced magnetization drop in Ni from first principles. Our calculations confirm an abrupt quenching of magnetization at high pressures not associated with any structural phase transition. We find that the pressure substantially enhances the crystal-field splitting of Ni-$3d$ orbitals, driving the system toward a new metallic phase violating the Stoner Criterion for ferromagnetic (FM) ordering. Analyzing the charge populations in each spin channel, we show that the next-nearest-neighbor interactions play a crucial role in quenching FM ordering in Ni and materials alike.

Room temperature ferromagnetism and half metallicity in nickel doped ZnS: Experimental and DFT studies

The nickel doped nanocrystalline ZnS thin films were deposited onto glass substrates by chemical bath deposition (CBD). Also ZnS:Ni nanoparticles were synthesized by CBD/co-precipitation method. Powder X-ray diffraction (p-XRD) studies demonstrate that both thin films and nanoparticles correspond to sphalerite (cubic) phase of ZnS with slight shift towards higher 2θ values due to incorporation of nickel in the ZnS lattice. The crystallite sizes estimated by Scherrer equation were 4 and 2.6 nm for ZnNiS thin films and nanoparticles, respectively. Scanning Electron Microscopy (SEM) images reveal that the morphology of thin films is based on quasi-spherical particles with nano scale dimensions. Energy Dispersive X-ray (EDX) spectroscopy confirms that the as-deposited thin films have a stoichiometry consistent with the nickel doped ZnS. Full-potential linearized augmented plane wave (FP-L/APW) method based on spin-polarized density functional theory (DFT) was employed to investigate the electronic and magnetic properties of ZnNiS for the doping concentration. Exchange-correlation functional was studied using generalized gradient approximation (GGA + U) method. Electronic band structures and density of states (DOS) demonstrate 100% spin polarization (half metallicity) with ferromagnetic exchange interactions. Superconducting quantum interference device (SQUID) analysis confirms the theoretical observation of ferromagnetism in nickel doped ZnS. These ZnS based half metallic ferromagnets seem to have virtuous applications in future spintronic devices.

The effects of applied current on one-dimensional interdiffusion between copper and nickel in spark plasma sintering

Spark plasma sintering (SPS) is a powder metallurgy technique that employs the use of fast sintering kinetics to produce final consolidated components in a matter of minutes. In order to use blended powders in SPS to obtain fully alloyed parts, diffusion during sintering must be understood. An investigation into the effects of current on the diffusion of copper and nickel was performed using SPS. Bulk specimens were used to generate diffusion couples in SPS in alternating orientations with respect to the direction of the current. Control samples were produced using a horizontal insertion vacuum furnace. Experiments were performed at temperatures between 850 °C and 1000 °C for 3 h. Concentration profiles were obtained by the use of both energy-dispersive spectroscopy and a Monte Carlo simulated correction curve. Diffusion coefficients and activation energies were calculated for samples produced by SPS and annealing without current. It was shown that, at temperatures near 0.9 Tm, the application of current in SPS inhibits diffusion between copper and nickel due to the re-orientation of electrons caused by the loss of ferromagnetism in nickel. Activation energy for diffusion is, however, decreased due to the temperature gradients arising from the difference in resistivity between the two species.

Metamagnetism and weak ferromagnetism in nickel (II) oxalate crystals

Microcrystals of orthorhombic nickel (II) oxalate dihydrate were synthesized through a precipitation reaction of aqueous solutions of nickel chloride and oxalic acid. Magnetic susceptibility exhibits a sharp peak at 3.3 K and a broad rounded maximum near 43 K. We associated the lower maximum with a metamagnetic transition that occurs when the magnetic field is about ≥ 3.5 T. The maximum at 43 K is typical of 1D antiferromagnets, whereas weak ferromagnetism behavior was observed in the range of 3.3–43 K.

Magnetic properties of Ni doped gallium nitride with vacancy induced defect

Investigations have been carried out to study the ferromagnetic properties of transition metal (TM) doped wurtzite GaN from first principle calculations using tight binding linear muffin-tin orbital (TBLMTO) method within the density functional theory. The present calculation reveals ferromagnetism in nickel doped GaN with a magnetic moment of 1.13 μ B for 6.25% of Ni doping and 1.32 μ B for 12.5% of nickel doping, there is a decrease of magnetic moment when two Ni atoms are bonded via nitrogen atom. The Ga vacancy ( V Ga) induced defect shows ferromagnetic state. Here the magnetic moment arises due to the tetrahedral bonding of three N atoms with the vacancy which is at a distance of 3.689 Å and the other N atom which is at a distance of 3.678 Å .On the other hand the defect induced by N vacancy ( V N) has no effect on magnetic moment and the system shows metallic character. When Ni is introduced into a Ga vacancy ( V Ga) site, charge transfer occur from the Ni ‘d’ like band to acceptor level of V Ga and formed a strong Ni–N bond. In this Ni– V Ga complex with an Ni ion and a Ga defect, the magnetic moment due to N atom is 0.299 μ B .In case of Ni substitution in Ga site with N vacancy, the system is ferromagnetic with a magnetic moment of 1 μ B.

High-temperature magnetic-field-induced activation of room-temperature ferromagnetism in Ce1−xNixO2

We report room-temperature ferromagnetism in nickel doped ceria (Ce1−xNixO2) powders prepared using the sol-gel process. Magnetometry studies on as-prepared samples reveal a weak ferromagnetic (FM) behavior in the range 0<x<0.04 (8.6×10−4μB∕Ni ion for 4%). For x>0.04, the FM magnetization steadily decreases for increased dopant concentration. The weak FM behavior of the samples improved dramatically after undergoing high-temperature (⩽500°C) activation in the presence of an external magnetic field (H⩽1T). The magnitude of the improvement, which was very reproducible, was strongest for 10% Ni doped CeO2 with saturation magnetization Ms increasing by 600 times. Curie temperatures of the activated samples varied in the 597–646K range depending on the Ni concentration and preparation conditions. X-ray diffraction studies did not reveal any noticeable structural changes as a result of the activation process.

Gutzwiller wave functions for correlated electrons: theory and applications

This work covers three lectures on Gutzwiller wave functions for correlated electron systems. In the first lecture, some basic ideas on model Hamiltonians and their motivation are outlined, Gutzwiller's variational approach to Landau Fermi-liquid theory is sketched, and, as a first application, the theory is applied to the one-band Hubbard model where the Brinkman–Rice metal-to-insulator transition is found. The second lecture is devoted to the perturbative calculation of expectation values. In this rather technical part the variational analog of Wick's theorem, Feynman diagrams, and the linked-cluster theorem are introduced. The important differences between the variational approach and the standard Green function technique are the absence of a time-dependence and the flexibility to select the expansion parameter of the variational perturbation theory. When this flexibility is used appropriately, not a single self-energy diagram must be calculated in the limit of large dimensions, i.e. in the limit of large lattice coordination number the expectation values can be evaluated exactly for general Hubbard-type models for all interaction strengths and band fillings. The third lecture covers the application of the theory to ferromagnetism of nickel. After a discussion of the nickel problem, multi-band Hubbard models and Gutzwiller wave functions are introduced and parameterized. The favourable comparison between the results from Gutzwiller theory and a large number of experiments shows that correlations play an important role in itinerant ferromagnets.

Theory of ferromagnetism for metals of cubic symmetry

The conditions for the occurrence of ferromagnetism in 3d metals of cubic symmetry are obtained based on the concept of strong interaction in a single unit cell. The Hubbard model with infinitely strong repulsion is invoked for a quantitative description. The calculations are carried out in the zero-loop and one-loop approximations. The reasons for the occurrence of ferromagnetism in nickel and a-iron and its absence in Pd, Pt, and γ-iron are analyzed.

Spin-resolved inverse photoemission study on the temperature behavior of a ‘magnetic’ band in nickel

The unoccupied Z 2-d-band in nickel has been studied as a function of temperature employing spin-resolved inverse photoemission of Ni(110). This uppermost d-band has only the majority spin states occupied at T = 0 and contributes therefore to the ferromagnetism in nickel. Our data indicate a collapsing band behavior for spin-up and spin-down state upon approaching the Curie temperature T c. The results are compared with predictions of different theoretical models of ferromagnetism.

Magnetism in iron and nickel

The fluctuating local band theory of itinerant electron ferromagnetism in nickel and iron is investigated with the use of first-principles numerical calculations. In this theory the excitations predominantly responsible for the phase transition are fluctuations in the direction of local magnetization. The free energy in the presence of a fluctuation is evaluated numerically in the approximation that this direction changes in time and space slowly enough to justify the use of the static approximation and second-order perturbation theory. The energies and wave functions used to incorporate the band and wavevector dependence of the relevant interaction matrix elements were obtained by Slater-Koster fits to earlier ab initio self-consistent energy bands. Results for nickel and iron are obtained in terms of an effective classical Heisenberg exchange. This is compared with other theoretical calculations and available experimental data. From the numerical results, it is concluded that both quantum effects (the time dependence of the exchange field) and local-field effects are important to account for the transition temperature ${T}_{C}$.

Transition from Pauli paramagnetism to band ferromagnetism in nickel, few atomic layers thick

Ni films of a few atomic layers are condensed in UHV at 10 K on metallic substrates of amorphous Pb75Bi25 and crystalline Pb, Cu and Bi. Anomalous Hall effect measurements are used to determine the magnetization and the susceptibility of the films. Ni films with a thickness smaller than two atomic layers possess no magnetic moment but show an enhanced susceptibility. Between two and three atomic layers of Ni the susceptibility diverges and thicker films possess a magnetic moment. The experimental results can be well described with a band model of magnetism. A thin Ni film condensed onto a non-magnetic metal represents a system in which the density of states is changed with thickness, causing a transition from Pauli paramagnetism to band ferromagnetism. In contrast to Ni the ferromagnetic metal Fe possess already for a coverage of 1/6 atomic layer a magnetic moment and shows a paramagnetic behavior.

Correlation theory of ferromagnetism in a narrow energy band

A unified theory of ferromagnetism is developed for a system containing a single electronic energy band with delta-function interactions between electrons. Using a Green function technique an approximate solution of the correlation problem is obtained, the decoupling scheme being uniquely determined from the random phase approximation. Spin waves are thus taken into account. Conditions are derived for the existence of saturated solutions to the self-consistency equations, and these appear to be satisfied in nickel. The mechanism for the existence of ferromagnetism in nickel is then simply explained on the basis of the theory. An analytic expression for the low-temperature magnetization of a saturated ferromagnet is calculated.

The effects of hydrogen and oxygen chemisorption on the ferromagnetic anisotropy of nickel-on-silica particles

After a consideration of the results so far obtained on the effects which oxygen and hydrogen chemisorption have on the ferromagnetism of nickel, it is shown that investigation of the effect of adsorption on the ferromagnetic anisotropy of nickel can be a considerable aid in forming an understanding of the chemisorptive bond on nickel. Then follows a brief discussion on the origin of ferromagnetic anisotropy and on the setup of experiments furnishing information about the effect of chemisorption on the ferromagnetic anisotropy. Experiments are presented in which the effect of hydrogen chemisorption on the magnetization of nickel-on-silica particles is determined by measurements in fields alternating with frequencies of 1 to 20 kc/sec and at temperatures from 77° to 373°K. It is demonstrated that the experimental results can be accounted for to a high degree. To permit a reliable comparison to be made between the effects of hydrogen and oxygen, the distribution of oxygen over the nickel particles in the samples must be equal to that of hydrogen. This has been achieved by deriving the oxygen from nitrous oxide. It appears that hydrogen chemisorption decreases the anisotropy of nickel particles up to about 50 A in diameter much more than does oxygen chemisorption. This suggests that the anisotropy is affected by a surface layer which is modified differently by hydrogen adsorption than by oxygen adsorption. Measurements on larger nickel particles (diameters about 70 A) demonstrate that, in a relative sense, hydrogen adsorption has a much smaller effect on the anisotropy of these particles. The cause of this difference, which is related to the size of the nickel particles, is discussed. A detailed analysis of the variation of the effect of oxygen adsorption with the frequency of the magnetizing field shows that oxygen adsorption decreases the anisotropy of larger nickel particles at low coverages only. At higher coverage, no further decrease in anisotropy is observed. Finally, an explanation is developed for the difference between the effects of hydrogen and oxygen adsorption on the anisotropy, as well as for the absence of a further decreasse in anisotropy energy on oxygen adsorption at higher coverages on larger nickel particles.

ON THE INTERNAL FRICTION PEAK ASSOCIATED WITH THE PRESENCE OF CARBON IN NICKEL

It is well-known that the stress-induced rotation of the magnetic domains in nickel gives rise to an internal friction peak (when internal friction is plotted as a function of temperature of measurement). An internal friction peak associated with the presence of carbon in nickel was recently observed in our laboratory. In this paper are described further experiments which demonstrate conclusively that this new internal friction peak is not connected with ferromagnetism of nickel but depends upon the amount of carbon in solid solution in nickel. More accurate determinations of the activation energy associated with this internal friction peak show that this activation energy is indeed very close to the activation energy for the diffusion of carbon in nickel. These experiments thus show that the new internal friction peak is associated with the stress-induced micro-diffusion of carbon in nickel.A brief discussion on the mechanism of this internal friction peak in terms of the existence of holes in the crystal lattice of nickel is given.

The investigation of the magnetic properties of non-homogeneous systems: part II, the magnetic properties of nickel amalgams

The apparatus described in Part I is used to measure the magnetic susceptibilities of a series of nickel amalgams containing 0.01 to 2.68 per cent of nickel by weight over the temperature range 15° to 300° c. All freshly prepared amalgams which have not previously been heated are diamagnetic, the nickel exhibiting a paramagnetic mass-susceptibility which is approximately independent of temperature. When an amalgam is heated to 225° c. it becomes ferromagnetic, and the ferromagnetism persists when the amalgam is cooled. The results are explained in terms of some new views on the ferromagnetism of nickel recently put forward by Neel.

Non-Magnetic Films of Nickel

Properties of sputtered nickel films.---(1) Magnetic. These films, prepared by sputtering in hydrogen on to a cooled surface, with a 1000 volt d.c. generator, are initially non-magnetic, showing neither magneto-optic rotation nor tractive effects in a magnetic field. After heating to 300\ifmmode^\circ\else\textdegree\fi{} or 400\ifmmode^\circ\else\textdegree\fi{}C, however, they become strongly magentic and exhibit both these effects. Films much thicker than 120m\ensuremath{\mu}, or those sputtered with an induction coil, are likely to be magnetic from the start. (2) Crystal structure. X-ray spectrograms by the powder method show for the heat-treated (magnetic) film the face-centered cubic lattice as for ordinary nickel, but prove that the original non-magnetic film is amorphous. These facts suggest that ferro-magnetism in nickel, at any rate, is not a property of the individual atom as customarily supposed, but of the crystalline aggregate. A number of possible objections to this point of view are taken up and discussed. (3) Electric. The gain in magnetic properties on heat-treatment is accompanied by a decrease of resistance to a fraction (a fifth in one case) of its initial value. The original film also gives only about 1 percent of the Hall effect that it does after heat-treatment. (4) Color. Some films, particularly those deposited on a surface at liquid-air temperatures, display remarkable colors. On examination of the reflected light with a spectroscope, only a narrow spectral region is seen to be absent. If the nickel films are thin, covering with an optically dense liquid completely destroys the colors.