Magnetic properties of NdFeB-based alloy under high-pressure torsion

Abstract

Abstract When a multicomponent NdFeB-based magnetic alloy is deformed using high-pressure torsion (HPT), a quasi-stationary state is reached after 2.5 anvil revolutions, which corresponds to an equivalent strain of ∼40 at the sample mid-radius. In this state, torque self-oscillations are observed with a period of about 1.5 s and an amplitude of ∼10 % around the average value of 550 N m−1. Such self-oscillations are accompanied by strong acoustic emission. Before HPT, the alloy under study has an almost rectangular hysteresis loop with saturation magnetization J s = 135 emu g−1 and coercivity H c = 34.8 kOe. HPT deformation at initial stages transforms this alloy to the class of soft magnets: H c drops to 1.35 × 10−4 kOe, while J s practically does not change. An increase in strain leads to a gradual increase in H c to 9.61 kOe and a decrease in J s to ∼100 emu g−1 at the number of anvil revolutions n = 7. This is explained by HPT modification of the regular grain-boundary network of neodymium-rich paramagnetic phase layers. These layers provide magnetic isolation between grains of the Nd2Fe14B ferromagnetic phase. Periodic changes in torque and J s with increasing torsion angle are caused by transitions from the amorphous phase to the crystalline one and vice versa.