Topic Code: E

Magnon-induced interband spin-flip scattering contribution to resistivity and

magnetoresistance in a nanocrystalline itinerant-electron ferromagnet

Sharika Nandan Kaul

School of Physics, University of Hyderabad, Central University P.O., Hyderabad - 500046, Telangana, India

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ABSTRACT

In this talk, I will present the results of an exhaustive study of ‘zero-field’ electrical resistivity, (T), and transverse magnetoresistance, [Δ/](T), (in magnetic fields up to 90 kOe) over the temperature range extending from 1.8 K to 300 K in pulse electrodeposited nanocrystalline Ni sheets (of thickness 300 μm) with average crystallite size, d, ranging from 10 nm to 40 nm [1-3]. A quantitative comparison of the observed (T) and [Δ/] (T) with the predictions of the recent self-consistent calculations[4] not only permits an unambiguous identification of the scattering mechanisms responsible for (T) and [Δ/](T) in different temperature ranges but also a first accurate determination of their relative magnitudes. Like in bulk 3d transition metal ferromagnets,  varies with temperature as T2 at T ≲ 15 K. Contrary to the widely-held view that the T2 variation of  at low temperatures arises from the electron-magnon scattering, this contribution to (T) is shown to originate from the electron-electron scattering. In the temperature range 15 K ≤T ≤ 300 K, the phonon induced non-spin-flip intraband (s − s and d − d electron-phonon) scattering and magnon-induced spin-flip inter-band (s − d electron-magnon) scattering contributions to the intrinsic resistivity are concomitantly present and both have sizable magnitudes. The former contribution dominates over the latter at T > T* whereas the reverse is true at temperatures 15 K ≤ T < T*. T*, the temperature at which the electron-magnon and electron-phonon scattering contributions become equal, increases from ~ 30 K for d = 10 nm to ~ 65 K for bulk Ni. By contrast, the magnetoresistance in nanocrystalline Ni is solely due to the spin-flip s − d electron-magnon (e − m) scattering. The present work clearly brings out the importance of the thermal renormalization of the magnon mass [4] (caused mainly by the magnon-magnon interactions) over the temperature range 15 K ≤ T ≤ 300 K and demonstrates that the non-spin-flip (NSF) interband s − d electron-phonon (e − p) scattering makes a much weaker contribution to (T) than the NSF intraband e − p scattering over the entire temperature range 1.8 K ≤T ≤ 300 K. Increase in atomic disorder at the surfaces/interfaces and grain boundaries, as the surface area increases with shrinking average crystallite size, is basically responsible for the observed enhancement in e−p (T, H = 0) and reduction ine−m(T, H = 0).

Fig.1. The intrinsic resistivity, int(T ) (T ) − 0, where 0 is residual resistivity (open circles) in the temperature range 15 K ≤ T ≤ 300 K, along with the theoretical fits (continuous curves) for the nc-Ni sample with d = 10 nm. The theoretically calculated individual contributions from electron-phonon and electron-magnon scattering are also depicted.

Fig.2. Transverse magnetoresistance versus magnetic field isotherms at temperatures ranging from 2 K to 300 K for the nc-Ni sample with d = 10 nm.

REFERENCES

[1]. P. V. Prakash Madduri and S. N. Kaul, Physica B 448(2014) 147.

[2]. P. V. Prakash Madduri and S. N. Kaul, J. Alloy. Comp. 689 (2016) 533.

[3]. P. V. Prakash Madduri and S. N. Kaul, J. Magn. Magn.Mater. 418 (2016)143.

[4]. S. N. Kaul, J. Phys.: Condens. Matter 17 (2005) 5595.