Study On The Structure And Magnetic Related Properties Of Double Rare Earth Laves Phase Intermetallics

The compounds formed by the atom ratio of R:T=1:2?R=Rare earth,T=Transition metal?are all of Laves phase.And the RCo₂Laves phase have evoked a far reaching interest since 1960s due to the rich magnetic characteristics.This work focuses on three tasks.1.Through the replacement of light rare earth,it is hope to that the critical ratio of light rare earth and heavy rare earth in the compound is found.Study on the type of phase transition and the magnetocaloric effect.2.The hysteresis phenomenon of compounds at low temperature was studied when the light rare earth and heavy rare earth are mixed into the RCo₂compound.3.The structure and magnetic properties of Pr_1-xDy_xNi₂compounds were studied,and comparative analysis of magnetic properties with Pr_1-xDy_xCo₂.The study on the(Pr_1-xHR_x)Co₂?HR=Gd,Tb,Dy;x=0.0-1.0?series compounds found that the substitution of heavy rare earth on the Pr Co₂does not change the Mg Cu₂-type cubic crystal structure?space group is Fd-3m?at room temperature.With the incorporation of heavy rare earth,the unit cell parameters of Pr Co₂(a=7.3090?;V=388.5061?³)was been reduced,hence the main diffraction peaks of the X-ray diffraction shifts to high angle.Because of the competition between the exchange coupling of light rare earth with Co and the exchange coupling of heavy rare earth with Co in compounds.The magnetic moment of the compound in the ferromagnetic state at the same temperature decreases with the incorporation of a small amount of heavy rare earth,but increases when the heavy rare earth reaches a critical quantity.Just at the x?0.4 the Pr_1-xDy_xCo₂compounds show first-order transition.In the other two systems(Pr_1-xGd_xCo₂,Pr_1-xTb_xCo₂),all of the compounds show the second-order phase transition.The composition is very important to the magnetic entropy change value,and the incorporation of heavy rare earth will seriously reduce the relative cooling capacity of the compounds.This is because the magnetic moment of the compounds wildly decreased after the heavy rare earth was incorporated,and the temperature change does not cause a large change in the magnetic moment.During the magnetic measurement in low temperature?10 K?,it was found that the compound of(Pr_1-xHR_x)Co₂?HR=Gd,Tb,Dy;0.0<x<1.0?showed magnetic hysteresis.The maximum value of the coercivity(Pr_0.6Gd_0.4Co₂H_C=0.97 T,Pr_0.5Tb_0.5Co₂H_C=0.49 T,Pr_0.6Dy_0.4Co₂H_C=2.8 T)was found near the critical point?the critical ratio value of heavy rare earth to light rare earth?.The competition of the magnetic exchange coupling between rare earth and the cobalt is the main reason for the hysteresis.Whether the increase or decrease of the unit cell volume,it will change the distance between the atoms in the lattice,which will cause the crystalline electrical-field to change.The electron exchange is affected by the crystalline electrical-field to varying degrees,which makes the compound show coercivity at a specific composition.This experimental phenomenon provides new ideas for finding new permanent magnetic materials.The study on the Pr_1-xDy_xNi₂?x=0.0-1.0?compounds found the crystal structure is Mg Cu₂-type and the space group is Fd-3m at room temperature.With the incorporation of Dy atoms,the lattice parameter decreases from 7.2592??Pr Ni₂?to 7.1610??Dy Ni₂?,and the unit cell volume decreases from 387.0163?³?Pr Ni₂?to 367.2087?³?Dy Ni₂?.The distance between atoms in the unit cell decreases,and magnetic moment of the compound increase with the Dy induces the magnetic moment of Ni atoms.In addition,the doping of Dy atoms enhances the magnetic moment of the highly localized 4f electron layer of the rare earth element,which increases the Curie temperature from 15 K?x=0.4?to 25 K?x=1.0?.Magnetic measurements show that the compounds are all second-order phase transitions.The change of magnetic entropy gradually increases with the incorporation of Dy.