High‐Pressure Torsion for Giant Magnetoresistance and Better Magnetic Properties

Abstract

Processing through severe plastic deformation (SPD) leads to significant refinement of microstructures including the grain size and the dispersion of second phase particles. Among several SPD processes such as equal-channel angular pressing (ECAP), [1] high-pressure torsion (HPT), [2] accumulative roll bonding (ARB), [3] and multi-directional forging (MDF), [4] the process of HPT has advantages over the other SPD processes. Such advantages are that extremely large strain is introduced in the sample under very high pressure with an order of GPa, and therefore, finer microstructures are achieved in less ductile materials with neither breaking samples nor damaging tools (anvils and dies). Although there are many applications of the SPD processes for the enhancement of mechanical properties such as strength and ductility, the application is rather limited to functional properties related to magnetic and electric properties. [5][6][7][8] In this paper, we present an application of HPT to Cu alloys containing ferromagnetic Co and Fe particles. The importance underlining this approach is that regions of the second phase are fragmented to finely dispersed small particles and/or may be dissolved in the matrix to achieve supersaturation so that subsequent aging leads to a fine dispersion of precipitate particles. It is then shown that HPT is a potential process for producing giant magnetoresistance (GMR) in alloys prepared by conventional ingot metallurgy.The metallurgical requirement for a granular type of GMR is that ferromagnetic particles should be finely dispersed in the matrix of non-magnetic materials. This requirement is very similar to the one for strengthening materials by particle dispersion except that the particles should be ferromagnetic. To achieve such a fine dispersion for GMR, several processes have been attempted such as thin film deposition following sputtering, [9][10][11] meltspining from a molten alloy, [12,13] and mechanical milling of mixed powders. [14][15][16] For most cases, supersaturation was attained and therefore subsequent postaging gave rise to a fine dispersion of ferromagnetic particles in the Cu matrix. In this study, it is demonstrated that the HPT process should be an alternative for the production of GMR and this production is feasible using a bulk form of alloys prepared by conventional melting and casting procedures. ExperimentalAn ingot with a composition of Cu-10 wt%Co was prepared from high purity Cu (99.99%) and high purity Co (99.99%) using an arc-melting furnace in an argon atmosphere. Details were described in an earlier report [17] , but briefly, the ingot High-pressure torsion (HPT) was conducted on Cu alloys containing ferromagnetic Co and Fe particles. Electron probe microanalysis, X-ray diffraction analysis, and transmission electron microscopy confirmed that the particles were significantly refined through fragmentation and some fractions were dissolved into the Cu matrix with straining by HPT. Saturation magnetization decreases with straining and coercive force increa...

High Strength and Electrical Conductivity of Al‐Fe Alloys Produced by Synergistic Combination of High‐Pressure Torsion and Aging

“…High supersaturation of Fe in the Al matrix up to 0.67 wt% was also obtained in this condition . The supersaturation of alloying elements by HPT processing was reported in many alloying systems and the mechanisms particularly discussed . Figure a shows TEM bright‐ and dark‐field images with a corresponding SAED pattern from the sample processed through N = 20 revolutions and aged for 0.5 h (peak aged).…”

Microstructural Changes Influencing the Magnetoresistive Behavior of Bulk Nanocrystalline Materials

“…interfaces) is needed. SPD by HPT [17][18][19] is capable of providing these small particles, as it was already shown in [9][10][11]. Furthermore, it was presented in [11] that the HPT processing conditions and thus the evolving microstructure directly influence the magnitude of the drop in resistance.…”

“…Thus, the effect was labelled granular GMR. Materials processed by severe plastic deformation (SPD) using high-pressure torsion (HPT) show such a GMR effect [9][10][11] and this group of materials is in the focus of this investigation. As a definition, GMR can be distinguished from ordinary magnetoresistance (OMR) and anisotropic magnetoresistance (AMR) by its negativity in all directions of applied magnetic field with respect to the current flow direction [12] and furthermore by its isotropic behavior [5].…”

Fundamentals of Superior Properties in Bulk NanoSPD Materials

“…[102] The appearance of MR was also confirmed in a Cu-22wt%Fe alloy. [103] While the achieved magnitudes of the MR do not qualify as GMR, the results show that HPT is at least potentially promising for creating GMR in alloys.…”