The Characteristics And Mechanism Of Shock-Induced Demagnetizationin Nd-Fe-B Ferromagnet

When magnetized material is subject to a shock wave, a reduction in magnetization isobserved, which is called shock demagnetization effect. The physical effect on shock–induceddemagnetization of ferromagnet can be utilized for designing explosive–driven pulsed powergenerator. A great many experimental results are direct evidence of the completedemagnetization of the Nd–Fe–B ferromagnet under shock wave compression, yet themechanism of the shock wave demagnetization has not been understood clearly. Therefore, itis greatly significant to investigate the magnetism of Nd–Fe–B under shock wave andestablish the simulation method of the shock demagnetization discharge processe, which isimportant to improve the performance of explosive–driven ferromagnetic generator.In thispaper,the compression of unmagnetized Nd–Fe–B permanent magnets is executedby using shock waves with differentpressures in a one–stage light gas gun system.Themanganin pressure gauge is used to measure the pressure–time signal of the Nd–Fe–Bferromagnet. The shock Hugoniot relation is described by the linear relation in the pressurerange from3.3to7.2GPa, by which the parameters in the Grüneisen type equation of stateare determined.The simulation model is established according to the planar impact experiment,and the numerical simulation is performed by the nonlinear dynamics method.In order to analyzing the magnetism of shock demagnetization, the microstructure,crystal structure, and magnetic propertiesof the magnets are examined with scanningelectronic microscopy, x–ray diffraction, hysteresis loop instruments,and a vibrating samplemagnetometer, respectively. There are three reason for the shock demagnetizationphenomenon. Firstly, there is a strong interactionthrough the main phase within intergranularfracture. Intergranular fracture weakenedexchange couplinginteraction of the main phasesbetween grain boundaries. Secondly, there are large number of micro–cracks and holes in thegrain boundary phases, around which low anisotropy region occur and nucleation fieldweaken. Last, the orientation of the main phase and the grain boundary phase are changedbecause of the slippage and fracture inthe grain boundary phase.In order to analyzethe mechanism of shock demagnetization of Nd2Fe14B underhighapplied pressure, animprovedtwo–sublatticemodelbased on themolecularfieldtheory. As to the ferromagnetic compounds, the equivalent pressure field is introduced to calculate the pressuredependence of magnetostriction coefficient, susceptibility and magnetization.Thermomagnetic curvesof Nd2Fe14Bcompound under various levels of pressure arecalculated.The criterion of the ferromagnetic–paramagnetic phase transition occurred inNd2Fe14B at different temperature and pressure is obtained.The results indicate that the Curietemperature of Nd2Fe14B decreases against pressure which is the main reason of shockdemagnetization.In order to analyze the magnetic properties of Nd2Fe14B compound under high pressure,we introduced the equivalent pressure field to Hamiltonian of three–dimensional Ising model.The Monte–Carlo simulation is used to calculate pressure dependence of specific heatcapacity, susceptibility and magnetization. The results show that Curie temperature ofNd2Fe14Bgradually shifts to the low temperature region with the increasing pressure, so thatferromagnetic–paramagnetic phase occurs more easily. Curie temperature of theNd2Fe14Bshifts toward the high temperature region with the increasing magnetic field, whichimpedes the ferromagnetic–paramagnetic phase transition occurs.The shock–induced demagnetization discharge experiments were conducted by explosiveplanar shock wave loading Nd–Fe–B hard ferromagnetic and the output voltage and currentcurves were acquired. The magnetic field of permanent magnet column was calculatedaccording to magnetization circle current. The induced electromotive force variation withtime was derived from the theoretical calculation model. Equivalent circuit model of theexperiments was established to derive output current and the voltage profile. The computingresults show that induced EMF depends on the shock wave velocity in magnet, remanence,coercive force, magnet radius and coil turns instead of the length and maximum magneticenergy product of the magnet. However, the length of the magnet affects impulse width ofoutput voltage or current pulse.