Magnetic and magnetocaloric properties of single crystal (Nd0.5Pr0.5)2Fe14B
Introduction
Since their discovery in 1984 [1,2], the R2Fe14B (R is a rare earth) type of compounds receive considerable scientific attention [[3], [4], [5], [6], [7], [8]]. Permanent magnets based on Nd2Fe14B show best hard magnetic properties [9]. In recent years, a significant progress has been achieved both in the material preparation techniques [[10], [11], [12], [13]] and in studying intrinsic properties of the Nd2Fe14B on successfully prepared single-crystalline specimens [[14], [15], [16]].
One of the drawbacks of the Nd-Fe-B magnets is a finite interval of operating temperatures. The largest hard magnetic properties are observed at temperatures ranging between the spontaneous spin-reorientation transition (SRT) temperature of 135 K [17,18] and the Curie temperature, TC of 585 K [3]. The former temperature limits the use of the Nd-Fe-B-type of magnets in superconducting devices, for which magnets with stable low-temperature characteristics are highly desirable [[19], [20], [21], [22]].
Fundamental magnetic properties (magnetic anisotropy, spin reorientations, etc.) of Nd2Fe14B can be readily controlled by various substitutions both in the rare-earth and iron sublattices [3]. For instance, partial substitution of Pr for Nd (the compounds form continuous solid solutions) in the (NdxPr1-x)2Fe14B alloys allows one to gradually decrease the SRT temperature [[23], [24], [25]] as the SRT does not exist in Pr2Fe14B. Therefore, the use of praseodymium allows us to prolong the application interval for the compound. Further, partial substitution of Pr for Nd increases coercivity of the compound and permits the adjustment of other magnetic parameters via the varied magnetic coupling between the 4f- and 3d-sublattices [26,27].
Some information on magnetic behavior of (NdxPr1-x)2Fe14B is available in literature. In particular, magnetization of several representatives of this family was studied at 4.2–250 K in magnetic fields up to 8 T [23] and 35 T [24]. Measurements of magnetocrystalline anisotropy constants were carried out in Ref. [28,29]. Calculations of the crystal field parameters and the temperature dependence of SRT were shown in Ref. [30]. To the best of our knowledge, peculiar behavior of magnetic properties in the vicinity of magnetic phase transitions [31] such as e.g. a magnetocaloric effect (MCE) in (NdxPr1-x)2Fe14 B at the SRT was not addressed by other researches up to now. (MCE is quantified as the adiabatic temperature change ΔТad or isothermal magnetic entropy change ΔSM of the sample in the varied magnetic field). Direct measurements of MCE in the R2Fe14B (R = Nd, Er) single crystals are presented in Ref. [32]. It was found that the largest MCE values ΔTad ≈ 0.8–0.9 K/T are observed at Curie temperatures while in the vicinity of SRTs the magnitude of MCE decreases markedly. Peculiar is the case of Er2Fe14B. This compound was actively investigated due to the changing sign of MCE at the SRT of 325 K [[33], [34], [35], [36]]. Here, the magnetocaloric effect can be achieved by changing the direction of the magnetic field with a constant amplitude instead of modulating its amplitude, thus giving a possible simplification of the refrigerator design [33,37]. Other applications of such materials are also possible, for example, in medicine [38]. The effect of interstitial atoms on MCE in Nd2Fe14B - H system was also studied in Ref. 32. The aim of this work was to study the magnetic and magnetocaloric properties of the Nd2Fe14B compound with partial substitution of Pr for Nd. For the (Nd0.5Pr0.5)2Fe14B single crystal we determine the values of MCA constants, magnetic phase transition temperatures, the sign and magnitude of MCE at the SRT. Universal entropy variation curve was obtained in magnetic fields up to 14 T. In addition, we used high magnetic fields up to 58 T to verify the presence of anisotropy of saturation magnetization characteristic of the parent Nd2Fe14B [39,40] in (Nd0.5Pr0.5)2Fe14B.
Section snippets
Experimental
Single crystal of (Nd0.5Pr0.5)2Fe14B was grown by a modified Czochralski method in a tri-arc furnace. Stoichiometric 8 g mixture of rare-earth metals (99.9% purity) and precursor Fe14B (prepared in induction furnace) was melted on a copper water-cooled crucible under an argon protective atmosphere. The ingot was turned several times and kept in molten state for about 1 h in order to ensure a good homogeneity. Then the crystal of 20 mm length and 4 mm diameter was pulled out at 10 mm/h pulling
Results and discussion
Our XRD data show that (Nd0.5Pr0.5)2Fe14B crystallizes in a tetragonal crystal structure (space group P42/mnm) of the Nd2Fe14B type. The lattice parameters are a = 0.8816 (1) nm and c = 1.2233 (3) nm, in good agreement with literature data [43]. Our analysis also suggests the presence of a small amount of impurities (<2%).
The temperature dependence of magnetization was measured at fixed external dc field. An example of thermomagnetic curve M(T) of the (Nd0.5Pr0.5)2Fe14B sample measured in a
Conclusions
We have investigated the magnetic and magnetocaloric properties of (Nd0.5Pr0.5)2Fe14B single crystal. By high-field magnetization measurements at 2 K it was found that the saturation magnetization anisotropy parameter p = 10−2 is 2.5 times smaller than that of Nd2Fe14B. The Curie temperature of (Nd0.5Pr0.5)2Fe14B comes to a value of 570 К which is slightly lower (by 15 K) than in ternary Nd2Fe14B. It was established that (Nd0.5Pr0.5)2Fe14B displays an easy cone – easy axis spin-reorientation
Acknowledgments
The work was performed within the scope of the state task of the FASO of Russia (theme no. 007-00129-18-00), in part was supported by the RFBR (project no. 16-33-60226 mol_a_dk), by the project ERA.Net RUS Plus: No146 - MAGNES″ financed by the EU 7th FP, grant no 609556 (K.R), by Materials Growth and Measurement Laboratory (https://mgml.eu) and by grant 16-03593S of Czech Science Foundation (A.A). We acknowledge the support of HLD at HZDR, member of the European Magnetic Field Laboratory (EMFL).
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