Study Of Magnetic Ordering And Cation Distributions In Spinel Ferrites M_xMn_1-xFe₂O₄?M=Zn,Mg,Al?

Spinel ferrites have received much attention because their promising applications in various technologies,such as magnetic refrigerator,microwave device,color image,high density storage devices and magnetic fluid,etc.In each spinel ferrites unit cell with a general molecular formula?A?[B]2O4,the bigger oxygen anions have a face-centered-cubic close-packed structure,while the smaller metal cations occupied the interstitial positions of oxygen ions.There are two types of interstitial sites occupied by cations: 8 of the tetrahedral?A?sites,and 16 of the octahedral [B] sites.The tetrahedral?A?sites are smaller than the octahedral [B] sites.In traditional theory,the magnetic ordering of spinel ferrites was explained using the super exchange interaction and double exchange interaction.A lot of researchers have studied the spinel ferrites M_xMn_1-xFe₂O₄?M=Zn,Mg,Al?,but there is still some debates for the relationship between cation distributions and magnetic structure.We prepared three series of samples,ZnxMn_1-xFe₂O₄?0.0?x?1.0?,Mgx Mn_1-xFe₂O₄?0.0?x?1.0?and AlxMn_1-xFe₂O₄?0.0?x?0.5?using chemical coprecipitation method.The crystal structures of the samples were analyzed using X-ray diffraction patterns at the room temperature.The magnetic properties of the samples were measured using a Physical Properties Measurement System.We obtained the data of the cation distribution for the samples by fitting the average molecular magnetic moments,using the O2 p itinerant electron model and Quantum mechanical equivalent potential barrier model proposed by our group.We therefore have the results as follows:?1?All the three samples M_xMn_1-xFe₂O₄?M=Zn,Mg,Al?have a single-phase cubic spinel structure with space group Fd3 m,the crystal lattice constant,a,are decreasing with the doping x increasing,and the volume averaged crystallite sizes of all samples were estimated to be larger than or close to 100 nm,so that surface effects are expected to be very weak in all the samples.?2?The magnetic hysteresis loops of these three samples were measured at 10 K and 300 K.We calculated the magnetic moments per formula?mexp?of the samples at10 K,and found the different mexpvariation tendencies for the three series samples.Then we found that mexphad different variation tendency.The values of mexpfor the Znx Mn1-x Fe₂O₄?0.0?x?1.0?increased to a maximum when x=0.4,then it decreased quickly with increasing doping level x.However,for the MgxMn1-x Fe₂O₄?0.0?x?1.0?and Alx Mn1-x Fe₂O₄?0.0?x?0.5?samples,the values of mexpshowed a monotonicallydecreased tendency with increasing x.?3?There are six factors to affect the cation distribution in the samples: First,the ionization energy and the distance between cations and anions.Second,the Pauli repulsive energy of electron cloud.Third,the balance tendency of charge density.Fourth,ionicity,the absolute values of average valence of oxygen is smaller than two,while the average valence of cation is also smaller than their traditional values.Fifth,in Mn_1-xFe₂O₄,the magnetic moments of Mn3+?3d4?is antiparallel to Mn2+?3d5?,Fe3+?3d5?and Fe2+?3d6?.The last,the angles between the cation?Fe,Mn?magnetic moments increasing from zero as the doping level of the Zn,Mg and Al cations increases.?4?For all three series of samples,the portion of Fe?Fe2+ and Fe3+?and Mn2+cations occupying the [B] sites lies more than 55%.This results in the magnetic moment direction of the samples being the same as that of the [B] sublattice.?5?For ZnxMn_1-xFe₂O₄?0.0?x?1.0?samples,when x<0.4,the Zn2+ content at the?A?sites increases rapidly,resulting in the magnetic moment of the?A?sublattice decreasing rapidly and the total magnetic moment of the samples increasing.When x>0.4,the Zn2+ content at the [B] sites increases rapidly with increasing x,resulting in the magnetic moment of the [B] sublattice decreasing rapidly and the total magnetic moment of the samples also decreasing rapidly.?6?For MgxMn_1-xFe₂O₄?0.0?x?1.0?and AlxMn_1-xFe₂O₄?0.0?x?0.5?samples,the contents of Mg2+ and Al?including Al2+ and Al3+?cation increases approximately linearly at both the?A?and [B] sites,while the increased content at the [B] sites is more than that at the?A?sites for every doping level.This results in the total magnetic moments decrease approximate linearly with increasing x.