The temperature dependence of the spontaneous magnetization of nickel and iron

An explanation of the apparent T² dependence of the magnetization of Fe and Ni and some ferromagnetic alloys at low temperatures is given in terms of an energy gap in the spin-wave spectrum. It is suggested that the use of only the first term in the series expansion of the Bose—Einstein integral function, rather than the Robinson expansion, gives a more accurate representation of the experimental data.     

Low temperature pressure dependence of the muon hyperfine fields in iron,cobalt and nickel

The pressure dependence of the μ⁺ local magnetic fields in polycrystalline Fe and Ni and a Co single crystal has been measured at 77 K, up to 0.7 GPa, using a He gas high pressure setup. The pressure derivatives dlnB_μ/dP in units of mT/GPa are +4.4±1.0 (Fe), -0.7±1.1 (Co) and +0.63±0.10 (Ni). From these values the hyperfine field volume derivatives are deduced. Using these values together with previously determined room temperature derivatives the thermal expansion part of the temperature dependence of the hyperfine field can be calculated. The remaining explicit temperature dependence below 300 K, which deviates markedly from the temperature dependence of the bulk magnetization, is discussed.     

The temperature dependence of magnetization of Gd-Tb alloys

It is shown that a mean field theory can describe very closely the variation of magnetization with temperature found in a series of Gd-Tb alloys. The proportionality of the exchange constants to the density of the states at the Fermi energy has been confirmed. The values for the densities of states for Gd and Tb required for satisfactory fitting of the experimental results are in agreement with values calculated from electronic specific heat measurements.     

The temperature dependence of the magnetization of Fe-P, Co-P and Ni-P alloys

Foils of Fe_{100 ? x}, Co_{100 ? x}P_x and Ni_{100 ? ×P×} and were prepares by Eletro-deposition with 0 < x < 28. Materials with x is purely microcrystalline, material with x > 15 is purely amorphous. So the effects of composition and structural order on the magnetic properties of these alloys could be separated. The decrease of the magnetization with increasing temperature was measured by means of a vibrating sample magnetometer. The results are discussed on the basis of the spin wave concept. To describe the deviations of the experimental results from exact T^{$\text{3}\text{2}$}-laws temperature dependent spin wave stiffnesses are favored. The ratios spin wave stiffness to Curie temperature proved to be characteristic constants for each of the three alloy systems. The magnetic properties of a series are mainly determined by the transition metal component. The series differ from each other in the same sense as the pure metals. The change of the properties with the phosphorus content is first of all an effect of composition and far less an effect of structural order or disorder.     

Local spin excitations and Curie temperature of iron

The expression for the Curie temperature of ferromagnetic metals in the mean field approximations is obtained for arbitrary types of exchange interactions. In the framework of local spin density functional approach an exact formula for the effective exchange parameter J₀ is derived. The numerical calculations for ferromagnetic iron illustrate the possibilities of the method.     

Oxidation of iron,cobalt and nickel at the Curie temperature

The large differences in the logarithmic oxidation kinetics reported as a change in activation energy with the oxidation rate decreasing from ferromagnetic to paramagnetic states are discussed for iron, cobalt and nickel. The observed anomalies at the Curie point are explained by a mechanism which takes into account both surface-state charge at the metal/oxide interface and space charge in the growing oxide.     

Universal temperature dependence of the magnetization of gapped spin chains

A Haldane chain under applied field is analyzed numerically, and a clear minimum of magnetization is observed as a function of temperature. We elucidate its origin using the effective theory near the critical field and propose a simple method to estimate the gap from the magnetization at finite temperatures. We also demonstrate that there exists a relation between the temperature dependence of the magnetization and the field dependence of the spin-wave velocity. Our arguments are universal for general axially symmetric one-dimensional spin systems.     

Elementary excitations at magnetic surfaces and their spin dependence

The elementary surface excitations are studied by spin-polarized electron energy loss spectroscopy on a prototype oxide surface [an oxygen passivated Fe(001)-p(1×1) surface], where the various excitations coexist. For the first time, the surface phonons and magnons are measured simultaneously and are distinguished based on their different spin nature. The dispersion relation of all excitations is probed over the entire Brillouin zone. The different phonon modes observed in our experiment are described by means of ab initio calculations.     

Magnetoresistivity of single-crystal films of iron,cobalt, and nickel binary alloys

Magnetoresistivity measurements of binary systems of alloy films Fe-Co, Fe-Ni, and Co-Ni in the [100] direction for the alloy systems Fe-Co and Fe-Ni up to 25 wt. % Co and Ni, respectively, and in the [100] direction for the remaining alloys are presented. A test of Berger's theory on the influence of spin-orbit interaction on transition processes in ferromagnetic alloys of nickel is carried out. The results obtained are compared with the results derived from bulk polycrystalline samples [1, 2].     

Observations of magnetic structure in films of iron,cobalt, and their alloys

A method of observing domain structure with an ÉM-7 electron microscope and a method of obtaining films with 180° boundaries are described. We consider the domain structure in the alloys 45% Co, 10% Mn, 45% Fe, and 45% Co, 10% Cr, 45% Fe, and also in films of Fe and Co. The width of the boundaries of polycrystalline films of Co was measured.     

Low-frequency electrical resistance of iron,cobalt, and nickel in the vicinity of their Curie temperatures

Alternating-current electrical resistance measurements between 17 Hz and 100 kHz were made on high purity Fe, Co, and Ni wires in the vicinity of their Curie temperatures (T _c). The electrical resistance was independent of frequency for temperatures (T) aboveT _c. As the temperature was lowered, however, there was an abrupt jump in the electrical resistance atT _c followed by a gradual decrease toward its dc value. The magnitude of the electrical resistance jump atT _c increased as the square root of the frequency. The enhancement of the electrical resistance forT≦T _c is produced by an abrupt decrease of the skin depth atT _c which, in turn, is due to the sudden increase in the initial magnetic permeability atT _c. Measurements of the ac electrical resistance in the vicinity of the Curie temperature of certain ferromagnetic metals can be utilized to 1) accurately determine the Curie temperature using frequencies as low as 17 Hz, and 2) quantitatively determine the initial magnetic permeability as a function of temperature and heat treatment.     

The temperature dependence of the resistance and the spin density wave energy gap of antiferromagnetic chromium

The anisotropic electrical resistance of antiferromagnetic chromium was measured in the temperature range of 10 K – 200 K. The two-band model was used to analyze the temperature dependence of the resistance, considering the effective number of conduction electrons associated with the SDW energy gap.     

Temperature dependence of the magnetization of canted spin structures

Numerous studies of the low-temperature saturation magnetization of ferrimagnetic nanoparticles and diamagnetically substituted ferrites have shown an anomalous temperature dependence. It has been suggested that this is related to freezing of canted magnetic structures. We present models for the temperature dependence of the magnetization of a simple canted spin structure in which relaxation can take place at finite temperatures between spin configurations with different canting angles. We show that the saturation magnetization may either decrease or increase with decreasing temperature, depending on the ratio of the exchange coupling constants. This is in agreement with experimental observations.     

Time dependence of the magnetization process of RE-TM alloys

We have investigated the time dependence of the magnetization process of RE-TM alloys. We show from magnetic measurements as well as by direct in-situ observations, using the Kerr effect, that the magnetization reversal takes place by nucleation of a few isolated domains which expand with time (GdFe) but also, for Tb (Co) based alloys, by nucleation only. For low fields (H < H_c) both mechanisms are thermally activated. All the results will be quantitatively analysed using the Fatuzzo theory. The origin and magnitude of the activation energies of these processes will be discussed.     

Infrared spectra of the matrix-isolated chlorides of iron,cobalt, and nickel

Iron, cobalt, and nickel metal cathodes were sputtered with various mixtures of ³⁷Cl₂/³⁵Cl₂/Ar to produce Ar matrices at 14 K that contained the mono-, di-, and trichlorides of the corresponding metal. The measured infrared absorption spectra of these matrices allowed the identification and characterization of FeCl₂, FeCl₃, CoCl, CoCl₂, CoCl₃, and NiCl₂. The derived vibrational constants of the electronic ground state of ⁵⁹Co³⁵Cl are ω_e = 457.8 ± 3.0 cm^?1 and ω_eχ_e = 2.0 ± 1.5 cm^?1. The antisymmetric stretching mode frequencies of six isotopomers of FeCl₂ and three isotopomers of CoCl₂ were identified and measured. The dichlorides of iron, cobalt, and nickel were all determined, in contrast with previous work, to be nonlinear with bond angles of 161, 157, and 161°, respectively. The estimated uncertainty is 5°. For both iron and cobalt trichlorides, the measured stretching mode frequencies were used to derive a ClMCl angle in excess of 120°, as would be expected for planar molecules with somewhat anharmonic vibrations. Observed adsorption peaks could be assigned to the ν₃(E) modes of planar (D_3h) Fe and Co trichlorides and the corresponding modes of the isotopomers. These observations strongly suggest that recent data supporting a pyramidal geometry for FeCl₃ should be reexamined. The geometry of CoCl₃ has not previously been determined.     

The effect of aluminum overlayers on the magnetization of iron and iron-nickel alloys

Iron and iron-nickel films 2 to 10 nm thick were prepared in a high vacuum magnetometer. The magnetic moment of each iron or iron nickel film was measured in situ, then an aluminum overlayer (2–30 nm thick) was deposited on the ferromagnetic film, and the magnetic moment was again measured in situ. The aluminum overlayer decreases the magnetic moment as if the iron film or iron-nickel film thickness decreases by about 0.3 to 0.5 nm.