Study On Negative Magnetization And Magnetocaloric Effects In Transition Metal Oxides

Magnetic materials exhibit abundant physical properties due to the complex interactions of spin-spin and spin-orbit,which makes them become functional materials of general concern in recent years.This dissertation is based on transition metal oxides,negative magnetization and magnetocaloric effects have been researched which are the two kinds of effects that have both fundamental research significance and important application.The main work and innovations of the dissertation are summarized as follows:?1?Mn-doped Y-Cr based transition metal oxide materials with perovskite structure of nominal composition of YCr_1-xMn_xO₃ were prepared by sol-gel method and annealed properly.The results show that Mn doping with appropriate content can achieve obvious negative magnetization effect below compensation temperature,and the greatest negative magnetization effect appears in Mn doped samples of x=0.3.Based on the comprehensive analysis of the results,the causes of negative magnetization effect caused by Mn doping are discussed.It is considered that there are two kinds of magnetization in Mn-doped samples,one is the positive magnetization contributed by Cr³⁺-O^2--Cr³⁺and Mn³⁺-O^2--Mn³⁺,and the other is the reverse magnetization which is contributed by Cr³⁺-O^2--Mn³⁺.The observed negative magnetization effect is due to the absolute value of reverse magnetization which is greater than the positive magnetization.?2?Fe-doped Eu-Cr based transition metal oxide materials with perovskite structure of nominal composition of EuCr_1-x-x Fe_x O₃ were prepared by sol-gel method and annealed properly.The results show that Fe doping can effectively inhibit the interaction between Cr-Cr,destroy the canted antiferromagnetic structure and weaken the original weak ferromagnetism.The sample shows obvious negative magnetization effect when the amount of Fe doping reaches x=0.55,and the magnetic compensation temperature T_comp up to 200 K is obtained,which is much higher than the previously reported magnetic compensation temperature.For x=0.55 samples,the magnetization measurements under different magnetic fields at given temperatures show that it is easy to switch between positive and negative magnetization states by changing the magnitude of the magnetic field,and such switching can be achieved at much higher temperatures than liquid nitrogen.Based on the comprehensive analysis of the experimental results,the changes in magnetic behavior caused by Fe doping and the causes of negative magnetization effect are discussed.It is considered that there are two kinds of magnetization in Fe-doped samples,one is positive magnetization contributed by Eu³⁺paramagnetism,weak ferromagnetism caused by Cr³⁺-Cr³⁺and Fe³⁺-Fe³⁺canted magnetic structure,and the other is reverse magnetization contributed by Cr³⁺-Fe³⁺.The observed magnetism decreases with the increase of Fe doping because of the increasing importance of reverse magnetization.The negative magnetization observed in x=0.55 sample is due to the fact that the absolute value of reverse magnetization is greater than the positive magnetization caused by various possible reasons.?3?Mn-doped Co-Cr spinel structure transition metal oxides with nominal composition of Co(Cr_1-xMn_x)₂O₄ were prepared by sol-gel method and annealed properly,and their magnetic properties were systematically studied.The results show that the magnetism below T_C temperature decreases with the doping of Mn at Cr site,and the samples show an obvious negative magnetization effect in the range of x=0.3-0.5.The magnetized state stability experiments of the samples with negative magnetization effect are carried out.The results show that as long as the temperature and magnetic field remain unchanged,both positive and negative magnetized states show stable magnetized states which hardly change with time.By changing the temperature or magnetic field intensity,the good and stable negative/positive magnetized states can be switched,which can be realized thermal/magnetic-assisted switching effect.On the basis of comprehensive analysis,the causes of magnetic change and negative magnetization effect caused by Mn doping are discussed.It is considered that Mn doping introduces magnetic interaction between Cr³⁺-Mn³⁺ions and this interaction will lead to additional reverse magnetic moment.With the increase of Mn doping amount,the absolute value of reverse magnetic moment increases,which weakens the magnetism of the samples.While in the x=0.3-0.5doping range,because of the contribution of the magnetic interaction between Cr³⁺-Mn³⁺ions,the absolute value of the reverse magnetic moment is larger than that of the positive magnetic moment,which results in negative magnetization effect.?4?Mn-doped Ni-Cr spinel structure transition metal oxides with nominal composition of Ni(Cr_1-xMn_x)₂O₄ were prepared by sol-gel method and annealed properly,and their magnetic properties were systematically studied.The results show that Mn doping not only greatly inhibits ferromagnetism below T_C temperature,but also significantly changes the magnetic properties of the samples.Three distinct characteristic temperatures,T_comp1,T_SR and T_comp2,were observed experimentally for samples doped between 0.15 and 0.3.In the temperature range of T_comp2<T<T_comp1,the sample x=0.15-0.3 shows obvious negative magnetization effect,and the degree of negative magnetization effect reaches the maximum at T_SR temperature.For materials with negative magnetization effect,our results show that switching between stable positive and negative magnetized states can be achieved by changing the magnitude of magnetic field of the lower magnetic field at liquid nitrogen temperature,which shows a more practical application prospect.?5?The magnetocaloric effects of YCr_1-xMn_x O₃,EuCr_1-xFe_x O₃,Co(Cr_1-xMn_x)₂O₄and Ni(Cr_1-xMn_x)₂O₄ magnetic materials with negative magnetization effect and R₂CoMnO₆?R=Sm,Dy?double perovskite-type materials were studied.The results show that all samples exhibit positive magnetocaloric effect related to paramagnetic-ferromagnetic transition near T_C temperature.In addition,it is more important to observe the reverse magnetocaloric effect in negative magnetization materials and another positive magnetocaloric effect with large magnetic entropy change in Dy₂CoMnO₆ double perovskite materials.Based on the comprehensive analysis of the experimental results,it is considered that the reverse magnetocaloric effect observed in negative magnetization materials is related to the existence of two different orientation magnetic moments,positive and negative.Due to the strong magnetocrystalline anisotropy and complex magnetic interaction in the material,the magnetic field is not enough to make the reverse magnetic moments flip along the direction of the magnetic field,resulting in an increase in the magnetic entropy of the system,and therefore exhibits the reverse magnetocaloric effect.Another positive magnetocaloric effect observed at low temperature in Dy₂CoMnO₆ system is related to the coexistence of ferromagnetic and antiferromagnetic phases in the system caused by a larger ion radius of Dy³⁺ions.Competition between the two effects,the magnetic field drives the steering of magnetic moment in antiferromagnetic phase and different temperature dependence of the interaction between ferromagnetic and antiferromagnetic phases leads to another positive magnetocaloric effect with large magnetic entropy change at low temperature.