Magnetostrictive Research On Light Rare-earth Pr-based Alloys At Low Temperature

In1972, Clark et. al found that REFe2(RE=rare earth) alloys with the cubic Laves phase structure had giant magnetostriction property. As one kind of intelligent material, REFe2(RE=rare earth) alloys have been researched for many years due to their potential application. Especially for Terfenol-D (Tbo.27Dyo.73Fe2) alloy, as the core materials used for producing underwater acoustic transducer, precision actuator, intelligent sensor and the like devices, it has been used in high-tech fields such as military equipment, ultrasonic detection and precision automation etc.. However, Terfenol-D mainly contains expensive heavy rare earth Tb and Dy, which limits the widely useing of it. According single ion model, PrFe2alloy possess large magnetostriction constant at low temperature. Because of this and its low price, much attention has been paid on magnetostrictive alloys with high-Pr concentration in recent years. In this paper, the synthesis, magnetic properties and magnetostriction of Pr-based Laves phase magnetostrictive alloys, which contains light rare earth Ce, Nd and heavy rare earth Tb, are investigated. We will discuss as below.1. The crystal structure and magnetostriction property of PrxCe1-xFe1.9(0.5≤x≤1.0) alloysWe have synthesized PrxCe1-xFe1.9(0.5≤x≤1.0) magnetostrictive alloys with cubic Laves phase by a high-pressure synthesis method, and all the alloys exhibit cubic Laves phase with MgCu2-type structure. It can be concluded that high pressure makes great effect on the formation of the Laves phase. The variation of the lattice parameter indicates that degree of localization of4f electrons of Ce is enhanced. The Curie temperature increases with the increase of Pr concentration, it is mainly because of the strong hybridization of Ce4f and Fe3d electrons. The composition dependence of the ratio λ/K1for PrxCe1-xFe1.9alloys at room temperature reaches a maximum value at x=0.8, indicating that Pr0.gCe0.2Fe1.9possesses a good magnetostrictive property at room temperature. The magnetostriction in the magnetic field direction (λ‖)) of PrFe1.9alloy at5K reaches up to3700ppm at the biggest field of50kOe, the magnetostriction (λ‖)) vs T curve indicates that the delocalization of Ce appears in the whole temperature range, which decreases the magnetostrictive property of the alloys.2. The huge magnetostriction and magnetic transition of PrFe1.9alloy at low temperatureWe have synthesized PrFe1.9magnetostrictive alloy by a high-pressure synthesis method, and the alloy exhibits single cubic Laves phase with MgCu2-type structure. Magnetostriction and structure of PrFe1.9compound at low temperature was then investigated. The M-T curve of the alloy at5kOe magnetic field reveals that there is a magnetic phase transition of PrFe1.9alloy in temperature range of10K-100K, from the magnetic phase of the low temperature (easy magnetization direction along<100>) to the magnetic phase of the high temperature (easy magnetization direction along <111>), this indicates that the crystal structure should be changed in this temperature. A high-precision XRD step scanning at low temperature was investigated, and the profiles of the reflections indicate that the crystal structure of the alloy indeed changes at this temperature range. According to the Morphotropic Phase Boundary theory in Ferromagnets, we deduce that the alloy is composed of coexisting R and T phases at the transition temperature, at which the alloy should have a lower K1. The calculation results show that the alloy has a lower K1at about50K, and the magnetostriction (λ‖) vs T curve of PrFe1.9at the field of5kOe shows that λ‖has a significant increase with the increase of temperature, it indicates that the change of K1is also significant with the increase of temperature. The magnetostriction (λ‖)-λ(?)) of PrFe1.9reaches up to7670ppm at the field of90kOe at the transition temperature (about50K), and the spontaneous magnetostriction in<111> direction which is calculated from the XRD reaches up to11000ppm at this temperatrue. The calculation result from the profiles of the high-precision XRD reflections indicates that the PrFe1.9alloy also has a huge magnetostriction value in the<100> direction, which can not be fit by a simple single-ion function and it is necessary for the further research to explain this phenomenon.3.The low temperatue magnetostriction of PrxNd1-xFe1.9and PrxTb1-xFe1.9alloysThe crystal structure, magnetic and magnetostriction properties of Laves phase compounds PrxTb1-xFe1.9(0≤x≤1.0) and PrxTb1-xFe1.9(0≤x≤1.0), which were synthesized at high pressure, were studied. The alloys exhibit cubic Laves phase with MgCu2-type structure. The initial magnetization curve of PrxTb1-xFe1.9at5K indicates that the magnetocrystalline anisotropy constant (K1) of PrFe1.9alloy is bigger than KI of NdFe1.9alloy, and Pr0.2Nd0.8Fe1.9alloy shows a minimum value of KI, which indicates that the substitution of Nd indeed decreases KI of the alloys at low temperature. The magnetostriction curve at5K shows that Pro.2Ndo.8Fe1.9alloy has a better low field magnetostrictive property. This curve also indicates that the magnetostriction of the PrxNd1-xFe1.9alloys at high magnetic field mainly attributes to Pr. Temperature dependence of the magnetostriction (λ‖) of PrxTb1-xFe1.9alloys at the field of5kOe shows that the substitution of Nd reduces the KI of the alloys remarkably, the magnetostriction of Pr0.2Nd0.8Fe1.9and Pr0.8Nd0.2Fe1.9alloys is twice as much as PrFe1.9alloy in the low temperature. The magnetostriction (λ‖) of Pr0.8Nd0.2Fe1.9alloy reach up to1082ppm at the temperature of100K, which makes it a potential candidate for application at this temperature. The rearch of the magnetostriction of PrxTb1-xFe1.9alloys in low temperature reveals that the substitution of Tb does not improve the magnetostriction property of the alloy in the low temperature.