Study On The Magnetic Properties Of Hard CoFe₂o₄/soft CoFe₂ And CoFe₂/CoFe₂N Composites

Magnetic nanoparticles have rEceived much attenticon because they have vide application and rich physical properties.In the family of magnetic nanoparticles,spinel CoFe₂O₄ nanoparticles is widely used in magnetic recording medium,magnetic fluid,microwave devices,permanent magnet and biomedical fields because of its high coercivity?Hc?,large magnetic anisotropy,high Curie temperature,good chemical stability and low cost.CoFe₂O₄ is characteristic of the ferrimagnetism and has small saturation magnetization?Ms?and remanence ratio?Mr/Ms?.The device miniaturization requires the magnetic material to possess both the high Hc and large Ms as well as high Mr/Ms.Therefore,lots of researches devoted to improve Mr/Ms of CoFe₂O₄.CoFe₂ is a typical soft magnet and has the largest Ms value among all the binary alloys.In many previous reports,the hard CoFe₂O₄/soft CoFe₂ composite magnets were investigated in order to obtain the permonent magnet with both high Hc and large Ms through the exchange coupling between the hard and soft phases.Unfortunately,almost all the previous reports didnot improve the Mr/Ms ratio because the CoFe₂O₄/CoFe₂ composite nanoparticles agglomerate seriously,inevitably resulting in the strong interparticle dipolar interaction.So far,whether the strong interparticle dipolar interaction suppresses the exchange coupling and how it affects the magnetic properties have not been systematically investigated experimentally.Furthermore,for the CoFe₂O₄/CoFe,composite nanoparticles in previous reports,CoFe₂ lies at the surfacr of nanoparticle.It is well known that the distribution of hard and soft phases also play an important role in the magnetic properties.In this thesis,the CoFe₂?core?/CoFeO₄?shell?composite was prepared and investigated how the distribution affects the magnetic properties.This is one of the studying motivations of this thesis.Moreover,it has been reported that a-Fe₁₆N₂,prepared through the nitridation of soft a-Fe in the NH₃ atmosphere,is a kind of permanent magnet with high Hc and Ms.In this thesis,CoFe₂O₄ was reduced to CoFe₂ in the H₂ atmospher;Then CoFe₂ was nitrided in NH₃ to prepare Co-Fe-N,attempting to obtain the magnetic material with high Hc and Ms,which is the other motivation this thesis.In this thesis,CoFe₂O₄ nanoparticles were synthesized by the thermal decomposition of a metal-organic salt and the sol-gel methods,followed by the post-treatments to obtain the CoFe₂O₄-based composites.The main contents and aims are as follows:?1?The CoFe₂O₄/CoFe₂/SiO₂ sample was prepared through the the dilution by the non-magnetic SiO₂ and the reduction in the H₂,atmosphere.The interparticle distance and magnetic moment of particles were changed by tuning hte ratio of SiO₂ and COFe₂,and consequently the magnetic interaction in the nanoparticle system was changed,aiming to investigate the effects of magnetic interaction on the magnetic properties of the nanoparticle system.?2?The post-treatment to the CoFe₂ alloy in the oxygen atmosphere obtains CoFe₂/CoFe₂O₄,in order to investigate the novel magnetic properties in CoFe₂/CoFe₂04.?3?The CoFe₂O₄ nanoparticles were reduced in the H₂ ambient to prepare CoFe₂,and subsequently treated in NH₃ to prepare Co-Fe-N.The post-treatment in the NH₃ atmosphere is expected to improve the corrosion resistance and to observe the novel magnetic properties.This thesis is divided into six chapters,and the content of each chapter is as follows:In the first chapter,the research progress of CoFe₂O₄ nanoparticles and their composites is reviewed.In the second chapter,we introduce the methods of preparation and characterization of CoFe₂O₄ nanoparticles.In the third chapter,approximately single-domain-sized 9-,13-and 16-nm CoFe₂O₄ nanoparticles are synthesized using the thermal decomposition of a metal-organic salt.By means of dilution and reduction,we obtain the nanoparticle systems with different concentration,moment and anisotropy.These nanoparticles are used as the model systems to reveal the intrinsic correlation between the remanence ratio?Mr/Ms?and interparticle dipolar interaction,the strength of which is estimated by the maximum dipolar field Hdip.This correlation has never been systematically investigated experimentally.The results reveal that the correlation between Mr/Ms and Hdip roughly follows Mr/Ms?1/lgHdip independent of the size,concentration,moment and anisotropy of the magnetic nancparticles,and especially such the correlation is more accurate for the nanoparticle systems which have higher concentration or moment,i.e.stronger diipolar interaction.The deviation from Mr/Ms?1/lgHdip at low temperatures can be attributed to the effects of surface spins for the single phase CoFe₂O₄ nanoparticles and to the pinning effect of CoFe₂O₄ on CoFe₂ for the slightly reduced nanoparticles.In the fourth chapter,Well-dispersed uniform CoFe₂O₄ nanoparticles were synthesized and then some nanoparticles were reduced in a H₂/N2 gas mixture followed by oxidation in a pure O₂ ambient to prepare CoFe₂/oxide composite.The CoFe₂O₄ nanoparticles exhibited a high H,value of 15 731 Oe and an approximately theoretical Mr/Ms ratio of 0.83 at T = 10 K as a consequence of the strong surface anisotropy,which have seldom been obtained previously.The CoFe₂/oxide composites exhibited the novel phenomena in their temperature derivative curves of the difference between the FC and ZFC magnetization,which presented two peaks at a low temperature T1 and at a high temperature T2.The low-temperature region below T1 corresponds to the completely frozen state.In such completely frozen temperature regime,the virgin magnetization curves exhibited an anomalous behavior that the magnetization decreases with increasing the applied field in the high field region due to the enhanced pinning action of disordered moments in oxides on the moments of CoFe₂ alloy.These results revealed that the surface anisotropy and dipolar interaction as well as the pinning effect play a crucial rule in the magnetic properties of CoFe₂O₄ nanoparticles and CoFe₂/oxide composite.In the fifth chapter,the CoFe₂ alloy?CF?was prepared by reducing CoFe₂O₄ in the H₂ ambient.Subsequently the CF sample was nitrided in the NH₃ atmosphere to produce the composite of CoFe₂N and CoFe₂.The CF sample and the nitrided sample?CFN?at 1000 ? were investigated in detail,including their magnetostriction,thermal expansion,resistivity and corrosion resistance.The saturation magnetostriction coefficiency ?S and thermal expansion coefficient a at 300 K for the nitrided sample CFN were 50 ppm and 10 ppm/K,respectively,approximately equal to those for the CF sample.However,compared with the sample CF,the CFN sample pressents a de???case in temperature coefficient R??300K?of magnetostricition by?11%,the???maller resistivity and improved corrosion resistance in the H₂SO₄ sollution,which may expand the applications of the CoFe₂ in the fields needing lower resistivity or in the acidic environment.In the sixth chapter,we make the conclusion and outlook of this thesis.