磁性材料磁有序的分子场来源

Molecular field origin for magnetic ordering of magnetic materials

对于磁性材料磁有序能的来源,即外斯分子场来源,本文提出一个模型:在磁性金属和合金中的相邻离子实之间,以及磁性氧化物的相邻阴阳离子间,其外层轨道上高速运动的电子分别有一定概率形成三种不同的状态.1)具有一定寿命的自旋相反的电子对,称为外斯电子对(WEP);2)距离很近且自旋方向相同的电子,容易发生互相交换,交换前后电子的自旋方向保持不变;3)当一个离子外层轨道有2个电子,其相邻的离子外层轨道只有1个电子时,前者多出的电子可以跃迁到后者的轨道上,并且保持自旋方向不变.我们认为,WEP两个电子间的静磁吸引能是分子场(即磁有序能)的主要来源.进而,推导出WEP的能量表达式、两电子的平衡间距和最大间距,探讨了在几种钙钛矿结构锰氧化物中形成外斯电子对的概率,用以解释居里温度附近晶格常数随温度变化的特点.结果表明这个模型是合理的.

In 1907,Weiss proposed that there is a molecular field to explain the magnetic ordering of magnetic materials.However,it has not been clarified where the molecular field comes from so far.In recent decades,the magnetic ordering of metals and alloys were explained by using the direct exchange interaction of between electrons on neighboring atoms,while magnetic ordering of oxides were explained by using the super exchange interaction and double exchange interaction models.The intrinsic relation between those exchange interactions has not been well explained.This resulted in the fact that there are many puzzles for magnetic ordering of the magnetic materials.For example,what role the Cr cations play in spinel ferrite CrFe2O4;why the calculated molecular magnetic moment（3.85μB） for La0.85Sr0.15MnO3 by using double exchange interaction model is lower than its experimental value（4.20μB）;whether there is a relation between the average atom magnetic moment and their electrical resistivity for each of Fe,Co and Ni metals.These several puzzles have been explained recently by our group through using an O 2p itinerant electron model for magnetic oxides and a new itinerant electron model for magnetic metals.In this paper,a model for the molecular field origin is proposed.There are three states for the electrons rotating with high speed at the outer orbits of two adjacent ions of magnetic oxides or metals and alloys.1） There is a probability with which form the electron pairs with opposite spin directions and a certain life time,named Weiss electron pairs（WEP）;the static magnetic attraction energy between two electrons of WEP is the elementary origin of Weiss molecular field.2） There is a probability with which two electrons with the same spin direction exchange mutually.3） If there are two electrons at the outer orbit of an ion,then for its adjacent ion whose orbit has only one electron,the excess electron will itinerates between the ions.Furthermore,the energy equation of WEP,equilibrium distance,re0,and maximum distance,r_（em）,between electrons of WEP are derived.The probability with which WEP forms in each of several perovskite manganites is investigated.For perovskite manganites La0.8Ca0.2MnO3, La0.75Ca0.25MnO3, La0.70Sr0.30MnO3,the crystal cell constants increase linearly with temperature when the temperature is much lower than the Curie temperature,TC,while they show a rapid increase nonlinearly near TC.We then calculate the difference in Mn-O bond length at TC between the linear and the nonlinear variation,Δdobs.Obviously,when the distance between the two electrons of WEP,re,is larger than the rem,WEP and the magnetic ordering energy both disappear.Assuming △dobs=rem）-re0,the probabilities with which WEP appears in La0.8Ca0.2MnO3,La0.75Ca0.25MnO3,La0.70Sr0.30MnO3,are calculated to be 0.07%,0.31%and 3.13%,respectively.These results indicate that the WEP model for the magnetic ordering energy is qualitatively reasonable.