Research On The Low Temperature Magnetic And Magnetostrictive Properties Of Pr-based Laves Phase Alloys

According to the Stevens’ equivalent operator method, the cubic Laves phase PrFe2and CeFe2alloys are potential candidates for giant magnetostrictive material at low temperatures with intrinsic magnetostriction λ111as large as5600ppm and6000ppm at0K, respectively. However, the magnetostriction of CeFe2observed in experiments is only60ppm because of the quenching of the orbital4f moment of Ce. So, considerable effort should be given on PrFe2alloy, which may exhibit the largest low temperature magnetostriction. In addition, the previous investigations on RFe2alloys indicate that the alloys exhibit large magnetocrystalline anisotropy at low temperatures, which makes the material too hard to be saturated. Therefore, it is very necessary to substitute Fe with other transition metal, such as Al and Co etc., or substitute Pr with other rare earth metals, which may greatly decrease the magnetocrystalline anisotropy and improve its low-magnetic field magnetostrictive properties at the same time. The substitution can offer these materials with more benefits for potential applications.In this paper, we investigate the magnetic and magnetostrictive properties of Pr-based alloys, the effect of Al and Co substitution for Fe, and the effect of substitute Pr with other rare earth metal in PrFe1.9alloys. Our work includes the following three contents:1. Temperature dependence of the magnetostriction in PrFe1.9and TbFe2polycrystalline alloysIn this section, we first investigate the temperature dependence of the magnetostriction λ111and the easy magnetization direction (EMD) in PrFe1.9and TbFe2alloys, by means of the step scanned high-precision XRD. A remarkable magnetostriction λ111as large as6700ppm is found at70K in PrFei9alloy. The EMD in PrFe1.9alloy changes from [111] to [100] as temperature decreases, and lies always along [111] between15K and300K in TbFe2alloy. The magnetostriction λ111in TbFe2alloy increases slowly from about2250ppm at300K to4040ppm at15K, but increases sharply from about1265ppm at300K to6700ppm at70K in PrFe1.9alloy. In the second part of this section, we investigate the temperature dependence of the magnetostriction in PrFe1.9and TbFe2alloys by the single-ion theory, and estimate the theoretical value of λ111in the two alloys at0K. The temperature dependence of λ111in PrFe1.9and TbFe2alloys follows well with the single-ion theory. For TbFe2, the extrapolated value at0K observed here is4200ppm, which is within the experimental error of the single crystal data reported by Clark. While for PrFe1.9, the extrapolated value is8000ppm, which is obviously larger than the result predicted by the Steven’s equivalent operator method (5600ppm). We think this is because that the rare earth sublattice moment increases sharply in PrFe1.9as temperature decreases.2. Effect of Al and Co substitution for Fe on magnetic and magnetostrictive propertiesin PrFe1.9alloysThe effects of Al and Co substitution for Fe on the EMD, the magnetic and magnetostrictive properties in PrFe1.9alloys are investigated. The substitution of Al for Fe decreases the Curie temperature and the value of λ111of PrFe1.9alloy. However, the substitution increases slightly the magnetostriction and decreases simultaneously the magnetocrystalline anisotropy of the alloy in low magnetic field. The rotation of the EMD from [111] to [100] between40and125K is observed in Pr(Al0.02Fe0.98)1.9alloy as temperature decreases by the step scanned XRD reflections. On the other hand, the substitution of Co for Fe decreases the Curie temperature Tc, and decreases the lattice constant and the spin reorientation temperature Tsr. Pr(Co0.2Fe0.8)1.9alloy exhibits minimum magnetocrystalline anisotropy and the maximum magnetostriction at300K simultaneously. Particularly, a small amount of Co substitution (x=0.2,0.4) is favorable to decrease the magnetocrystalline anisotropy, and increase the magnetostriction of the alloys at5K, which makes these materials more appropriate for practical applications. 3. Effect of Dy substitution for Pr on magnetic and magnetostrictive properties in PrFe1.9alloysIn the last section, we also investigate the effect of Dy substitution for Pr on magnetic and magnetostrictive properties in PrFe1.9alloys. It is found that Dy substitution decreases the magnetostriction in high-magnetic field (10≤H≤90kOe) at5K. However, a small amount Dy substitution (x=0.05) yields increased magnetostriction in low-magnetic field (H=5kOe) between10and50K. These features make the alloys a potential candidate for low temperature application. Furthermore, the magnetostriction of Dy-poor alloys (0≤x≤0.2) exhibits a maximum at some temperatures due to the change of the EMD in the alloys. While for Dy-rich alloys (0.45x≤1.0), abnormal magnetostriction temperature dependencies are observed due to the small and complex nature of X100of DyFe2. The magnetostriction of the alloy with Dy composition x=0.6exhibits minimum between250K and300K. This might be attributed to the magnetic moment compensation behavior of Dy3+and Pr3+ions in the alloys.