The Investigation On The Hard Magnetic Properties And Coercivity Mechanism Of Non Rare Earth Co-Zr Based Permanent Magnetic Alloys

In this paper, we have investigated the magnetic properties, phase evolution, andmicrostructure of Co-Zr based nanocomposite non rare earth permanent magnets tosolve the mechanism of magnetization and magnetization reversal in nanocrystallinecomposite magnets, clarify the relationship between the strength of exchangecoupling effect and size of grain size, and develop an efficient way to design the highperformance non rare earth permanent magnets, which is of great significance to thedevelopment of the magnetism and magnetic materials.Firstly, it is found that the small substitution of Ti for Zr in the Co80Zr18B2melt-spun ribbons resulted in a significant improvement of magnetic properties. Theoptimal magnetic properties of coercivityiHc=4.5kOe, and maximum energyproduct (BH)max=5.0MGOe were obtained in the Co80Zr15Ti3B2ribbons produced ata wheel speed of30m/s. X-ray diffraction (XRD) and thermomagnetic analysis (TMA)revealed that the Co-Zr-Ti-B ribbons mainly consisted of Co11Zr2phase, and the Tiatoms have entered into the lattice of Co11Zr2phase and Co23Zr6phase resulting in thedecrease of their Curie temperature. That can be attributed to the entry of Ti into thelattice of Co11Zr2phase and Co23Zr6phase, and results in the band change of them.The investigation of microstructure indicated that the grain size decreasedsignificantly with the increase of Ti content. A suitable grain size of Co11Zr2phasewas believed to be the main reason for the drastic increase in coercivity. On the otherhand, the appropriate refinement of grain led to a significant enhancement ofintergrain exchange interaction in the Co80Zr15Ti3B2, which is the reason for the improvement of (BH)max.Secondly, we found that the origin of the coercivity enhancement in theCo77Zr18Cr3B2magnet. The effects of a partial substitution of Cr for Co on themagnetic properties of Co80Zr18B2alloy are investigated in this paper. Themagnetization and remanence decrease with the increase of Cr content, whereasiHcfirstly increases correspondingly reaching a maximum value of5.1kOe for x=3andthen decrease with further increasing the Cr content. In order to obtain a highercoercivity, the melt-spun ribbons are annealed at the temperature range of500℃-700℃spaced50℃apart and the maximumiHcof7.0kOe is obtained in themelt-spun Co77Zr18Cr3B2after annealed at550℃. XRDand TMA are employed todetermine the phase composition. It is found that the sample is comprised of thesingle Co11Zr2which is responsible for the magnetic hardness. The Cr atoms enterinto the lattice of Co11Zr2, which results in a decline in Tcbut an increase in themagnetocrystalline anisotropy field Ha. SEM investigations show a microstructureconsisting of equiaxed grains whose average size is about300-350nm. The origin ofcoercivity enhancement in Co77Zr18Cr3B2is ascribed to the increase in Ha. For thesample with x=3, the maximum coercivity is obtained after annealing at550℃. Inthe case of as-spun sample, the coercivity is much lower. That can be explained by the“random anisotropy model” developed by Herzer. Based on the SEM results, thecritical grain size of highest coercivity is considered to be about300-350nm. In thecase of the as-spun sample, the grain size (about30-50nm) is much smaller than Lex.According to Herzer, when the grain size is below Lex, the effectivemagneto-crystalline anisotropy decreases with respect to the decrease of grain size. Itis supposed that the magneto-crystalline anisotropy is averaged out by thenanocrystalline structure. Generally speaking, if the effective magneto-crystallineanisotropy increases, it is favorable for the improvement of coercivity. Oppositely, ifthe effective magneto-crystalline anisotropy decreases, it is unfavorable for theimprovement. Therefore, we deduce that the decrease of coercivity in the as-spun sample is due to the reduction in the effective magnetocrystalline anisotropy.Thirdly, we have produced the Co80Zr16W2B2melt-spun ribbons with highcoercivity and maximum energy product. The effects of a partial substitution of W forZr on the magnetic properties, phase evolution, and microstructure of Co80Zr18-xWxB2(x=0,1,2and3) melt-spun ribbons are investigated in this paper. For the as-spunsamples, the optimal magnetic properties ofiHc=6.6kOe and (BH)max=4.5MGOeare achieved in the Co80Zr16W2B2. TMA and XRD results suggest that the as-spunCo80Zr16W2B2mainly comprises of hard magnetic phase. Scanning electronmicroscope (SEM) results indicate that W addition refines the microstructure, whichresults in an increase in the intergrain exchange interaction. The enhancement inremanence and squareness of demagnetization curve can be attributed to the increaseof the strength of intergrain exchange coupling. Moreover, Haof hard magnetic phasein the sample with x=0and2are measured and the value in the sample with x=3was improved significantly resulting in a higher coercivity. Therefore, it is concludedthat the addition of W leads to a much finer microstructure and higher Haandimproves both the coercivity and maximum energy product drastically.Finally, we have produced the high performance nanocomposite Co82Zr13V5magnet with soft and hard phases coupling. Generally speaking, the magneticproperties of quench magnets are affected by the microstructure significantly, whereasthe microstructure of the materials produced by melt-spinning is usuallyinhomogeneous. Annealing the amorphous melt-spun ribbons can solve the issue andresult in a quite homogenous microstructure. In this paper, the amorphous Co82Zr13V5melt-spun ribbons are produced at a wheel speed of40m/s, and a high maximumenergy product (BH)max=5.0MGOe with a relatively high coercivityiHc=4.0kOe isobtained after annealing the sample at560℃. The x-ray diffraction andthermomagnetic analysis suggest the sample is composied of hard magnetic Co11Zr2phase and soft magnetic Co23Zr6phase. Moreover, it was found that Tcof Co11Zr2andCo23Zr6decreases with the increase of V content, suggesting that some of the V atoms have entered into the lattice of the two phases and leads to their band structure change.Besides, microstructure investigation indicates that its grain size is within the range of50–100nm. Based on the results, it is concluded that the high performance Co-Zr-Vnanocomposite magnets with soft and hard coupling phases can be achieved by theamorphous ribbons after a suitable annealing temperature and time.