Temperature dependence of magnetoresistance in theNi3Al system

Abstract

The transverse magnetoresistance of stoichiometric and off-stoichiometric polycrystalline samples of the Ni3A1 system has been measured over the temperature range 1.2 & T & 90 K in magnetic fields up to 23 kOe. Negative magnetoresistance was observed in all cases. In samples with Ni concentrations of 75 at. % or greater the magnitude of the magnetoresistance reached a maximum at T = Tc. The data are in qualitative agreement with the theoretical predictions of Chang et al. based on a model of the magnetoresistance of an itinerant ferromagnet, but the observed magnetoresistance is considerably larger than predicted.

Effect of off-stoichiometry and site disorder on the properties of Ni3Al: I. Electrical and magneto-transport

“…(II) However, so far as the MR is concerned, the agreement between the experimental observations and the theoretical predictions, based on a two-(s-d) band model [11], is remarkably good [11,12] over wide ranges of temperature and field in the strongly exchange-enhanced paramagnetic samples with x < x c ; a similar agreement between theory and experiment is observed [11,12] in the weakly ferromagnetic compositions (x > x c ) only at low temperatures T < 30 K and high fields H > 100 kOe, where the negative MR (arising from the field-induced change in the density of states at the Fermi level of the d sub-bands) is masked by the 'conventional' positive MR due to the orbital motion of the conduction (s) electrons in a magnetic field (the socalled Lorentz force contribution). Subsequent studies [13][14][15] of electrical resistivity and magnetoresistance in this alloy system yielded results that are not only in conflict with one another but also with the previous ones [10][11][12]. For instance, Sasakura et al [13] observe a crossover from the T 2 variation of ρ(T ) in the range 1.2 K T 2.5 K to the T 5/3 variation for temperatures 10 K T T C , in polycrystalline ordered Ni 3 Al.…”

“…The ordered Ni 3 Al phase has a homogeneity range that extends from 73 at.% Ni to 77 at.% Ni and, as the Ni concentration (x) increases from 73 at.%, long-range ferromagnetic order sets in at the critical concentration (boundary) x c ≈ 74.5 at.%. Early investigations of electrical resistivity [10], ρ(T ), and magnetoresistance (MR) [11,12] in the polycrystalline samples of Ni x Al 100−x alloys with x ranging from 73.5 to 76.0 at.% revealed the following. (I) In the temperature interval 1.2 K T 4.2 K, ρ(T ), for all the compositions in the range specified above, is described by the expression [10] ρ(T, x) = ρ(0, x) + A(x)T α (x) .…”

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

“…According to the phase diagram [1] of binary Ni p Al 100− p alloys, the intermetallic compound Ni 75 Al 25 has a homogeneity range which extends from p l = 73.5 at.% to p u = 76.5 at.%. The investigations [2][3][4][5][6][7][8][9] of magnetic properties in the composition range p l p p u have revealed that these properties are extremely sensitive to Ni concentration ( p)-so much so that the long-range weak itinerant-electron ferromagnetism breaks down completely (i.e., the Curie temperature, T C , drops to zero) if p falls below p c ≈ 74.5 at.% [2][3][4][5][6][7] and paves the way to exchange-enhanced paramagnetism for p just below p c . By contrast, one such investigation due to Dhar et al [8] places this critical Ni concentration ( p c ) at 75.1 ± 0.2 at.%.…”

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

“…A recent appraisal [1] of the previously reported [2][3][4][5][6][7][8][9] results on the magnetic behaviour of binary Ni x Al 100−x alloys with 73.5 at.% x 76.5 at.% highlighted the controversies that surround the nature of magnetism in this weakly ferromagnetic alloy system. Attributing the disagreement between the results of earlier investigations to a complex interplay between the compositional disorder and site disorder, and to a complete 1 Author to whom any correspondence should be addressed.…”

“…neglect of the spin-wave contribution to magnetization at low temperatures, Kaul and Semwal made an attempt to unravel the individual roles of compositional disorder [1] and site disorder [10,11]. To this end, the study of magnetization in the 'as-prepared' Ni x Al 100−x alloys [1] with 74 at.% x 76 at.% (the Ni concentration range that includes the critical concentration [2][3][4][5][6][7] x c ≈ 74.5 at.% below which long-range ferromagnetic order ceases to exist) and in the alloy of stoichiometric composition, Ni 3 Al, 'prepared' in different states of site disorder [10,11], revealed the following. (i) Regardless of the degree of site disorder present, spin waves at low temperatures (0.09T C T 0.28T C ), zero-point and thermally excited local spindensity fluctuations at intermediate temperatures (0.32T C T 0.62T C ), and non-propagating thermally excited spin fluctuations at temperatures close to the Curie point, T C (0.65T C T 0.95T C ), completely account for [10,11] the thermal demagnetization of spontaneous magnetization, M(T, 0), and 'in-field' magnetization, M(T, H ).…”