Study On The Characteristics And Mechanism Of Magnetic Reversal In Rare Earth Chromite Yb（Ca）CrO₃

The multiferroics have attracted great interest because of their technologicalpotential and profound theoretical research value. Recently, there is a renewedinterest in rare earth chromites due to the coexistence of magnetism andferroelecricity in these compounds. The investigation of magnetic properties of thesecompounds will help understand the physical mechanism of multiferroic properties.In this thesis, the crystal structure, surface morphology and magnetic prpertie ofYb（Ca）CrO₃were systematically observed by means of X-ray Diffraction （XRD）,Scanning Electron Microscope （SEM） and Physical Property Measurement System（PPMS）. In this system some magnetic anomalies including magnetic revesal,negative magnetization and cooling field dependence were investigated in detail. Themain contents are summarized as follow:In chapter one, the research progress of single-phase multiferroics was reviewedincluding a summary of crystal structure, magnetic structure, magnetic propertiesand multiferroic behaviors of the rare earth orthochromites.In chapter two, we introduced the main experimental methods, including thepreparation of samples, the structure analysis and subsequent physical propertymeasurements.In chapter three, the magnetic phase transition and magnetic reversal of YbCrO₃was systematically investigated. The results showed that a typical magnetictransition from paramagnetism to canted antiferromagnetism occured near the Néeltemperature TN¹18K where the Cr³⁺spins start to get ordered cantedantiferromagnetically. Due to the strong magnetic anisotropic exchange interactionsbetween Cr³⁺and Yb³⁺ions, YbCrO₃exhibits very interesting magnetic properties atlow temperature. In field cooling （FC） mode, accoding to molecular field theory, theweak ferromagnetic moment of the Cr³⁺spins produces an internal field HIto the Yb³⁺site below TN, whose spin direction is opposite to that of the net Cr³⁺moments.Hence, the Yb³⁺spins will get ordered due to the synergic interaction between themeasuremental field Hmeasand internal field HI. When Hmeasis smaller than HI, theYb³⁺spins will order along the direction of HIwhich is antiparallel to the directionof net moment of the Cr³⁺spins, and thus the magnetic reversal and negativemagnetization phenomena will be found in the temperature dependence ofmagnetization （M-T） curves. When Hmeasis larger than HI, the Yb³⁺spins will orderalong the direction of Hmeaswhich is parallel to the direction of net moment of theCr³⁺spins, thus the magnetic reversal and negative magnetization will be absent.Magnetic reversal and negative magnetization phenomena were also found in M-Tcurve under zero field cooling （ZFC） mode. we believe that there may exist a smalltrapped field （TF） resulting from trapped magnetic flux inside the superconductingmagnet of sample chamber of Physical Properties Measurement System （PPMS）before cooling to low temperature in ZFC mode. This means that the YbCrO₃samplemay be in fact cooled down to low temperature under a small trapped field beforemeasureming in ZFC mode. To investigate more details, we carried out two differentmeasurement protocols in ZFC mode. Experimental results showed that YbCrO₃hasa strong dependence on the cooling field which would significantly affects theconfiguration of magnetization. Our experiments turned out that the M-T curves ofYbCrO₃sample indeed showed opposite trend even though it was cooled undernegative trapped field （NTF） and positive trapped field （PTF） with the samemeasurement field Hmeas. Meanwhile, the M-T curves measured under the samemeasurement field Hmeaswith antiparallel directions almost had no differences bycooling in the same trapped field. These experimental results indicated that themagnetization configuration of YbCrO₃compound can memorizes the initialmagnetic state induced by the cooling field and behaves accordingly to it.In chapter four, to clarify the role of Yb³⁺and Cr³⁺/Cr⁴⁺in the magnetic reversalbehaviour, we systematically investigated the magnetic properties of bivalent alkaline-metal Ca doped YbCrO₃（Yb1-xCaxCrO₃, x=0.00-0.30）. We found that withthe Ca²⁺substitution of Yb³⁺, the sample range in color from green to dark brown.From the SEM photographs of the fracture surfaces of selected samples, we noticedthat with the increase of Ca concentration grain size decreased and connection ofgrains become denser. And this is consistent with the result we observed earlier thatsamples with small x become much harder than YbCrO₃. Crystal structure analysisshowed that all of the Ca-doped YbCrO₃samples crystallize in single phase and canbe indexed with an orthorhombic unit cell. The effects of Ca substitution on themagnetic properties of Yb1-xCaxCrO₃samples have been systematically characterized,which showed that M-T curves of the samples exhibit the same feature in FC modebesides a weakened magnetization M of Yb1-xCaxCrO₃samples with increasing Cadoping content x in the range of Tcompto TN. Meanwhile, there is no remarkablevariation for TNwith Ca doping. These results could be attributed to the reduction ofCr³⁺-O^2--Cr³⁺interactions due to the transformation of Cr³⁺to higher oxidized stateCr⁴⁺by Ca²⁺doping. In addition, other interactions between Cr³⁺, Cr⁴⁺and Yb³⁺would appear in Ca-doped YbCrO₃, which would also dilute the interactions ofCr³⁺-O^2--Cr³⁺and lead to a decrease of the slope of M-T curves near TN. What’s more,below Tmaxindicated as the temperature where M reaches its maximum magnitude, itis found that a noticeable hysteresis appears in Ca-doped YbCrO₃samples while theM-H curves of parent YbCrO₃sample is not hysteretic. Thus, we suggest that a newcomponent of ferromagnetic interaction could arise from the Cr³⁺-O^2--Cr⁴⁺exchangeinteraction, as is induced by the Ca²⁺doping.In chapter five, the main content of our work was summarized and relatedresearch prospects and challenges were illustrated.