S. N. Kaul

Magnetic properties of the weak itinerant-electron ferromagnet Ni75Al25: II. The effect of compositional disorder

The results of a detailed study of the magnetic properties of well-characterized polycrystalline NipAl100?p () alloys are presented and discussed in the light of the existing theories. Extreme care has been exercised in the sample preparation to ensure that the site disorder (invariably present in any alloy system) does not interfere with the compositional disorder brought about by the reduction in the concentration of the magnetic (Ni) atoms. Thus, the observed variation in the magnetic properties with Ni concentration (p) is solely controlled by the compositional disorder. Like site disorder, compositional disorder smears out the sharp features in the density of states (DOS) curve near the Fermi level, EF, and reduces the DOS at EF, N(EF), and thereby causes a fall (an enhancement) in the values (value) of the spontaneous magnetization at 0?K, M0, the spin-wave stiffness at 0?K, D0, and the Curie temperature, TC (zero-field differential susceptibility at 0?K, ?0). However, compositional disorder, unlike site disorder, gives rise to smooth variations in N(EF), the inverse Stoner enhancement factor , M0, D0, TC, D0/TC and ?0 with p. These variations in the case of M0(p), D0(p) and TC(p) are very well described by the power laws , and with p>pc (pc = ?the percolation threshold for the appearance of long-range ferromagnetic order) predicted by the percolation theories for these quantities on a regular three-dimensional (d = 3) percolating network. The alloys in question exhibit a crossover in the spin dynamics from the hydrodynamic (magnon) to critical (fracton) regime at a well-defined temperature Tco*(p). An elaborate analysis of the magnetization data in terms of the percolation models permits a reasonably accurate determination of the magnon-to-fracton crossover line in the magnetic phase diagram, the percolation-to-thermal crossover exponent, fractal dimension, fracton dimensionality, the percolation critical exponents for spontaneous magnetization, spin-wave stiffness, correlation length and conductivity. The results of this analysis also vindicate the Alexander?Orbach conjecture and the Golden inequality for d = 3 percolating ferromagnetic networks.

Effect of site disorder on the asymptotic critical behaviour of Ni75Al25

Extensive bulk magnetization and ac susceptibility measurements have been performed over a wide temperature range on well characterized polycrystalline Ni75Al25 samples `prepared in different states of site disorder. A detailed data analysis unambiguously establishes (i) the existence of multiplicative logarithmic corrections to the mean-field (MF) power laws in the asymptotic critical region near the ferromagnetic-paramagnetic phase transition and (ii) a gradual crossover to the Gaussian fixed point at temperatures outside the critical regime, irrespective of the degree of site disorder present. The latter crossover is followed by yet another crossover from Gaussian to pure MF regime in all the samples. Accurate determination of the universal amplitude ratio Rχ = DBδ-1Γ, the asymptotic critical exponents β, γ and δ and the logarithmic correction exponents x-, x+ and x0 for spontaneous magnetization, initial susceptibility and the magnetization versus field isotherm at the Curie temperature TC, coupled with the observations made on the same system previously, not only rules out completely the possibility of isotropic short-range Heisenberg or isotropic long-range dipolar or uniaxial dipolar asymptotic critical behaviour in Ni75Al25 but also indicates strongly that, in the asymptotic critical region, the weak itinerant-electron ferromagnet Ni75Al25 behaves as an isotropic d = 3, n = 3 ferromagnet in which the attractive interactions between magnetic moments decay with intermoment distance (r) as J(r)~1/r(3/2)d, and that site disorder is irrelevant in the renormalization group sense.

Effect of off-stoichiometry and site disorder on the properties of Ni3Al: II. Magnetics

A detailed comparison between the magnetic behaviours of the as-prepared ap-NixAl100−x alloys with x = 74.3, 74.8, 75.1 and 76.1 at.% (that have both compositional disorder and site disorder) and annealed counterparts (that have only compositional disorder) over a wide range of temperatures and magnetic fields (H) permits us to draw the following conclusions about the role of disorder. Regardless of the type of disorder, Curie temperature, TC, and spontaneous magnetization at 0 K, M0, decrease in accordance with the power laws TC(x) = tx(x−xc)τ and M0(x) = mx(x−xc)ψ as (the threshold Ni concentration below which the long-range ferromagnetic order ceases to exist). Site disorder lowers the value of xc by nearly 1 at.% Ni, enhances TC for a given composition (more so as ) by increasing the number of Ni nearest neighbours for a given Ni atom, and leaves M0 essentially unaltered because site disorder has essentially no effect on the density of states, N(EF), at the Fermi level, EF, and the shape of the density-of-states curve near EF (except for x≈xc, where site disorder tends to primarily enhance N(EF) and thereby stabilize long-range ferromagnetic order for Ni concentrations below the threshold concentration, at.%, dictated by compositional disorder). At low and intermediate temperatures, spontaneous magnetization, M(T,H = 0), as well as the in-field magnetization, M(T,H), exhibit non-Fermi liquid behaviour in the samples ap-Ni74.3 and ap-Ni74.8. As xc is approached from above, i.e. as the compositional disorder increases, stronger deviations from the Fermi liquid behaviour occur and the temperature range over which the non-Fermi liquid behaviour persists widens. In contrast, the ap-Ni75.1 and ap-Ni76.1 alloys follow the behaviour that the self-consistent spin-fluctuation theory predicts for a weak itinerant-electron ferromagnet with no disorder. Both compositional disorder and site disorder have no effect on the critical behaviour of the alloys near the ferromagnetic-to-paramagnetic phase transition.

Site occupancy of Fe in ternary Ni75−xFex+yAl25−yalloys

The results of a detailed structural and magnetic study clearly indicate that regardless of the thermal history of the samples, Fe has a strong preference for the Ni sites in Ni-poor (nonstoichiometric) Ni75Al25 alloys. Fe substitution has a profound effect on the nature of magnetism in Ni25Al25

Magnetic relaxation in a three-dimensional ferromagnet with weak quenched random-exchange disorder

AbstractIsothermal remanent magnetization decay,Mr(t), and ‘in-field’ growth of zero-field-cooled magnetization,MZFC(t), with time have been measured over four decades in time at temperatures ranging from 0.25Tc to 1.25Tc (whereTc is the Curie temperature, determined previously for the same sample from static critical phenomena measurements) for a nearly ordered intermetallic compound Ni3Al, which is an experimental realization of a three-dimensional (d = 3) ferromagnet with weak quenched random-exchange disorder. None of the functional forms ofMr(t) predicted by the existing phenomenological models of relaxation dynamics in spin systems with quenched randomness, but only the expressionsn

Effect of Site Disorder on Martensitic Transformation in Ferromagnetic Ni55Fe20Al25 Alloy as Inferred from Magnetic and Magneto-Transport Measurements

M_r (t) = M_0 [M_1 exp ( - t/tau _1 ) + (t/tau _2 )^{ - alpha } ]

Experimental observation of quantum corrections to electrical resistivity in nanocrystalline soft magnetic alloys

n andn