Study Of Exchange Bias Effect In Composite Materials Fe₃O₄/FeO And CoFe₂O₄/Sr₂FeMoO₆

In recent years,it has been found that the exchange coupling effects exist in the interface of magnetic nanomaterials or thin film materials which have ferromagnetic/anti-ferromagnetic structure.Exchange bias,as the main form of exchange coupling,refers to the fact that when the external magnetic field is cooled above the antiferromagnetic Neel temperature,the hysteresis loop after the material is magnetically cooled will have a significant horizontal offset.As the main form of the exchange coupling,exchange bias refers to the fact that when the external magnetic field is cooled above the antiferromagnetic Neel temperature,the hysteresis loop after magnetical cooling will have a significant horizontal offset.Due to the wide application prospects in the spintronics devices and spin valves fields,this exchange coupling effect has attracted widespread interest of researchers.In the application research of the effect,the extent of offset is different among the various materials.However,due to its wide accessible,the simple preparation and good stability of the ferromagnetic and ceramic materials,they are favored by researchers.In this paper,a series of studies in the magnetic property and influencing factors are studied by two exchange bias systems core/shell magnetic nanoparticles Fe₃O₄/FeO and grain boundary doped ceramic materials CoFe₂O₄/Sr₂FeMoO₆.Thereby,further understanding for exchange bias can we get.In this thesis,most of the previous studies,they used chemical methods to synthesize samples and the preparation process was relatively complicated.In this experiment,we prepare samples by a conventional solid-state reaction method.Core/shell magnetic nanomaterials Fe₃O₄/FeO and ceramic materials CoFe₂O₄/Sr₂FeMoO₆ were all prepared by a conventional solid-state reaction method.In the above systems,the anti-ferromagnetic material FeO and the hard magnetic material CoFe₂O₄ is served as anti-ferromagnetic component in their systems,respectively.Due to the interface coupling effect,the pinning fields of different strengths to the neighboring ferromagnetic materials will be produced.The pinning field H_E is used as a measurement of exchange bias effect magnitude,it is the amount of abscissa of the center of the hysteresis loop away from the origin.The main research work and results are shown as below:(?)For the core/shell magnetic nanomaterials Fe₃O₄/FeO,the study found that the system exchange bias has a large dependence on temperature,which is specifically manifested by the value of H_E increasing gradually with the decreasing temperature.In order to further study the effect of temperature on the exchange bias of the system,we performed different cold field tests on the core/shell nanomaterials Fe₃O₄/FeO at different temperatures and found that the lower test temperature,the material's exchange bias effect more obvious.This phenomenon is attributed to the ferromagnetic coupling effect at the core/shell material interface.It is worth noting that the pinning field of the material near 200 K tends to zero,which also shows that the Neel temperature of the anti-ferromagnetic material has a decisive effect on the material exchange bias effect.(?)For the grain boundary modified ceramic material CoFe₂O₄/Sr₂FeMoO₆,we characterized it in the same way.The measurements indicated that the hysteresis loop and the tunneling reluctance intersection point of the material after magnetic cooling are significantly shifted and change with the content of the doped phase CoFe₂O₄.The measurements indicated that the system exchange bias effect is changed along with the CoFe₂O₄ content.The results show that the system exchange bias field is significantly reduced when the content of CoFe₂O₄ in the ceramic material is insufficient or excessive.This is due to that doping appropriate amount of CoFe₂O₄ into the grain boundaries of Sr₂FeMoO₆ promotes the ordering of ceramic grains and increases the atomic magnetic moment at the interface of material.By changing the content of CoFe₂O₄ in the system,we achieved the regulation of the tunneling magnetoresistance intersection and hysteresis loop offset.