Anisotropy Of The Exchange Interaction And Magnetocrystalline Anisotropy In Rare-earth-Fe Compounds And Their Hydrides

A rare-earth-transition metal intermetllic compoumd is a candidate for excellent materials when it fufills the following conditions:it must has a large spontaneous magnetization ,a strong uniaxial magnetocrystalline anisotropy and a high magntic ording temperature. In this thesis ,the magnetic properties of several R-T compounds and their corresponding interstial compounds are studied by comparision of calculations with a series of experiment. The experiments include the magnetization curves along the crystal axes at a series of temperature.For Rare-earth-transation metal intermetallic compound ,the R-Co(Fe) exchange interaction has been assumed to be isotropic .For Yco5,Y2Co7,La2Co14B and Y2Fe17 compound ,the Co and Fe sublattices exhibit a large magnetic anisotropy compared with that for pure metallic Co. The large residual orbital angular moments and so not very small anisotropy of the R-Co exchange interaction as well as the anisotropy of the magnetic moments of the Co ions . The apparent anisotropy of the magnetic moment of the Co atoms, indeed, has been observed for YCo5,Rfe11Ti and Y2Fe17. The observation of anisotropy of the exchange interaction is not so straightforward . The Gd3+ ion being at the S state with negligibly small magnetocrystalline anisotropy, the MCA of a Gd-Co compound expected to be nearly the same with those of the corresponding Rn-Co(Rn: Y,Lu,La) compound. In this connection,we analyzed the magnetization curves at 4.2K for Gd2Fe17 and Gd2Fe17H3 single crystals by assuming that the Gd-Fe exchange interaction is isotropic and found that the fitted value of the magnetic anisotropy constand (MAC) at 4.2 K is strikingly different from those of Lu2Fe17 and Lu2Fe17H3. For example, the fitted value of MAC for Gd2Fe17 K1(4.2 K)=-62K/F.U. is strikingly different from –42K/F.U.of Lu2Fe17.For Gd2Fe17,the magnetocrystalline anisotropy of which is much smaller than the above compounds ,the value of the MAC at 4.2K was measured to be K1+K2=-7.6K/F.U.which differs little from –8.6K/F.U.of that for Y2Co17.In this work we deduce general phenomenological expressions of the R-Co(Fe) exchange interaction and analyze the magnetic properties of Gd2Fe17 and Gd2Fe17H3 on the basis of the single-ion model.The results show that the neglection of the anisotropy of the R-Co(Fe) exchange interaction causes the unreasonably large differences between the fitted MAC constants K1(4.2 K) and K2(4.2 K) of Gd2Fe17 , Gd2Fe17H and Lu2Fe17 , Lu2Fe17H3. It is concluded that the anisotropy of the Gd-Fe exchange interaction in the large MAC Gd-Fe compounds is unnegligible.The magnetization curves of single crystals for Rfe11Ti and RFe11TiH(R=Tb,Ho) at 4.2 K and higher temperature are accordant with the experimental curves. It will be found that the insertion of H significantly change the values of crystalline -electric-field (CEF) parameters Anm ,which show that the insertion of H significantly change the anisotropy of R ions . It is analyzed that the crystalline-electric-field (CEF) parameters Anm,s changing trend accords with that of the magnetization curves. This will more approve the reliability of our the values of the crystalline-electric-field(CEF) parameters Anm. This will help us understand these compounds thoroughly.