Role of higher-order effects in spin-misalignment small-angle neutron scattering of high-pressure torsion nickel

Abstract

Magnetic-field-dependent unpolarized small-angle neutron scattering (SANS) experiments demonstrate that high-pressure torsion (HPT) straining induces spin misalignments in pure Ni, which persist in magnetic fields up to 4 T. The spin-misalignment scattering patterns are elongated perpendicular to the applied magnetic field due to an unusual predominant longitudinal sin 2 -type angular anisotropy. Such a contribution cannot be explained by the conventional second order (in spin misalignment amplitude) micromagnetic SANS theory in the approach-to-saturation regime, nor can its magnitude relative to the other features of the cross sections by the third order micromagnetic SANS theory. This indicates that the high-density of crystal defects induced via HPT straining in Ni makes such higher-order effects in the micromagnetic SANS cross sections observable.

Small-angle neutron scattering by spatially inhomogeneous ferromagnets with a nonzero average uniaxial anisotropy

“…The anisotropy value turns out to be rather large, K 0 ¼ 0 M 2 0 Q=2 ffi 2.6 Â 10 4 J m À3 (using M 0 = 482 kA m À1 ). It was silently absorbed into the S H ðqÞ and S M ðqÞ response functions in the work of Oba et al (2021) but can be revealed using the present more sophisticated theory; and it agrees very well with the value of 1.8 Â 10 4 J m À3 estimated by Bersweiler et al (2021) using a correlation-function analysis on the same specimen.…”

“…Due to its inherent axial symmetry, high-pressure torsion (HPT) is another way to induce a uniaxial anisotropy in an originally isotropic sample. Oba et al (2021) report magnetic SANS cross section data (shown in Fig. 3) on an Ni sample subjected to such a treatment.…”

“…We have fitted the spin-misalignment perpendicular SANS cross section of HPT-Ni (at all q and H values besides 0 H = 0.1 T as will be explained later) from Oba et al (2021)…”

“…with the response functions from ( 27). The sin 2 term is introduced and discussed by Oba et al (2021) and is beyond the scope of the present work. Also, Oba et al (2021) assume S H ðqÞ and S M ðqÞ depend on the entire q vector, meaning that each angle can have its own fitted values of the scattering functions.…”

A Review of Ultrafine-Grained Magnetic Materials Prepared by Using High-Pressure Torsion Method

“…The HPT treatment of commercially pure nickel (99.6%) at liquid nitrogen temperature resulted in a nanostructure with an average grain size of 80 nm and a microhardness of up to 6200 MPa [ 30 ]. HPT strain was found to lead to spin misalignment in pure Ni by magnetic field-dependent unpolarized small-angle neutron scattering experiments and remains present in magnetic fields up to 4 T. The spin-misalignment scattering patterns are elongated perpendicular to the applied magnetic field due to the predominance of unusual longitudinal sin 2 θ-type angular anisotropy [ 31 ]. Different from Fe and Co, HPT has a more complicated effect on the magnetic properties of Ni.…”