For the (Y1-xUx)B4 system, long-range ferromagnetism only occurs for 0.0

Crow, JE

Fisk, Z

Wallash, A

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitleSaturation magnetization in the anomalous ferromagnet, (Y, U)B4Permalinkhttps://escholarship.org/uc/item/47p1d4pgJournalJournal of Magnetism and Magnetic Materials, 54(PART 1)ISSN0304-8853AuthorsWallash, ACrow, JEFisk, ZPublication Date1986-02-01DOI10.1016/0304-8853(86)90703-1Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, availalbe at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaJournal of Magnetism and Magnetic Materials 54-57 (1986) 547-548 547 SATURATION MAGNETIZATION IN THE ANOMALOUS FERROMAGNET, (Y, U)B, + A. WALLASH, J.E. CROW and Z. FISK ‘) Physm Department, Temple Uniuersity, Philadelphia, PA 19122, USA For the (Y,_,U,)B, system, long-range ferromagnetism only occurs for 0.05 < x i 0.55. The anomalous magnetic phase diagram has been attributed to a delocalization of the U Sf-electrons due to increasing f-f overlap as the average U-U separation is varied. Measurements of the saturation magnetic moment versus x in the ferromagnetic region and measurements of the lattice constants versus x are presented. The magnetic to nonmagnetic transition seen in most U-based alloys and intermetallic compounds is a result of the delocalization of the f-electrons due to f-f over- lap and/or f-spd hybridization. H.H. Hill established that f-f overlap significantly contributes to this delo- calization for U-U separations less than 3.4-3.6 A, whereas f-spd hybridization tends to dominate for larger U-separations [l]. The U-U separation in UB, is 3.7 A which is slightly larger than Hill’s critical separation and UBd is weakly paramagnetic, presumably due to the delocalization of the f-electrons caused by f-f overlap. Upon dilution of UB, by YB,, an anomalous magnetic phase diagram is obained. Previously, it was reported that the (Y, ~,U,)B, system was paramagnetic for x > 0.6, ferromagnetic for 0.1 < x < 0.6 and paramagnetic for x < 0.1 [2]. Also, it has been shown that the varia- tions of the lattice constants [3], hyperfine field [4], and paramagnetic susceptibility [5] versus x are consistent with a two-site model. This model assumes that the 5f electrons associated with U ions having 4 or less U nearest neighbours (nn) become localized and develop a local magnetic moment, whereas those with more than 4 nn remain weakly paramagnetic. We have measured the lattice constants versus x and the saturation magnetiza- tion versus x and T for (Yt_,U,)B,. The variation of the lattice constants with x is consistent with those previously published [3] and the saturation magnetiza- tion dependence on x mirrors the variation of the Curie temperature, T,, with x. The samples were prepared in a conventional inert atmosphere arc furnace. Appropriate amounts of Y and LJ were added to compensate for the slight evaporation of these more volatile constituents which occured during melting. The lattice constants were measured using a Siemens 219/o diffractometer and the magnetization was measured using a commercial vibrating sample magne- tometer. Both YB4 and UB, crystallize in the tetragonal ThB, structure [2]. Shown in fig. 1 is the variation of the Work supported by NSF DMR8219782 and US DOE. ‘) Los Alamos National Laboratory, Los Alamos, NM 87545, USA. r;2 a 7”‘c‘------.. 4’02 7.09- 7,071 I I I I I I I 1 1 Y% 0.2 0.4 0.6 0.8 13.96 "84 X Fig. 1. Lattice constants versus x for the tetragonal system. (Y, -xU,)Bzt. lattice constants, a and c, versus x. These results are very similar to those previously reported by Hill et al. [3]. Note the clear departure in the vicinity of x = 0.45 from a linear Vegard’s law for both a and c. This departure from the initial linear dependence of a and c for x > 0.45 has been attributed to a delocalization of the 5f electrons due to increasing f-f overlap as the average U-U separation is reduced with increasing x. Such behavior is commonly seen in Ce-based alloys and intermetallic compounds [6] and was also reported for W, Y)Sb ]71. YB. 0.2 0.4 -80 X Fig. 2. Curie temperature and saturation magnet moment versus x for (Yt -,U,)B., 0304-8853/86/$03.50 0 Elsevier Science Publishers B.V. 54x A. Wullush et ul. / Anonmlous ferromugnet f Y. U/B4 Shown in fig. 2 by the dashed curve is the U-con- centration dependence of r,, the ferromagnetic Curie temperature. The T, versus x behavior shown in fig. 2 was determined from an Arrott plot analysis of the field and temperature dependence of the magnetization. The q. versus x behavior shown in fig. 2 is similar to the behavior previously reported by Giorgi et al. [2] with the exception that our ferromagnetic/paramagnetic phase boundary is shifted slightly to lower x-values. The maxi- mum T, of 14.5 K is consistent with the previous measurements. For x > 0.3 the rapid depression of 7; with increasing x has been attributed to a quenching of the local moments due to the delocalization of the 5f electrons caused by increasing f-f overlap. Note that this rapid depression of T, with increasing x occurs in the region where the delocalization as seen in the lattice constants becomes apparent. Shown in fig. 3 is an Arrott plot [S] of the field and temperature dependence of the magnetization for x = 0.25. From such a plot both the temperature depen- dence of the saturation magnetization and T, can be determined. Shown in fig. 2 by the solid curve is the zero temperature saturation moment/U-ion, pLo, versus x, as determined from the extrapolation of the tempera- ture dependence of the saturation magnetization. Note. the U-concentration dependence of p,, qualitatively re- sembles that seen for r, versus x. For a local moment model without crystalline electric field (CEF) effects, pLo should be nearly independent of x. Qualitatively. the observed dependence of p,, and T, can be explained with a local moment model assuming CEF effects with a E 13.0 K F 14.0 K G 16.0 K Ii 18.0 K 1 20.0 K 0 25 50 15 100 125 H/M (kOe/emu) Fig. 3. M2 versus H/M (Arrott Plot) at various temperatures for 0’0.75%25V% (J = 4) 5f configuration and a nomagnetic singlet ground state. Using a two site model and assuming the ex- change and CEF parameters are independent of x, then an appropriate set of parameters can be selected such that the mean field 7; goes to zero at x = 0.1 and s = 0.8 with the maximum occurring near .Y = 0.5. Such behavior only qualitatively reproduces the observed be- havior of r, versus x. An alternate explanation of these results may be available in an itinerant model with the variation in the lattice constants reflecting the delocalization of the f- electrons in much the same way as occurs in the -y-u transition in Ce [9]. As shown by Pickett et al.. a slight increase of the f-f overlap can account for the isostruct- ural transitions and lattice collapse in Ce. Similarly. the increase of f-f overlap and lattice pressure with increas- ing U-concentration could result in a localized-itinerant transition in (Y. U)B4. With an itinerant model. the r, versus x could be qualitatively accounted for using a Stoner model [lo]. Furthermore, the approximate scal- ing of p0 with T, and the reduced size of p,, as compared to that expected for a well localized magnetic system can be easily obtained with an itinerant theory of magnetism. Measurements of the magnetization versus tempera- ture and magnetic field up to 9 T. along with measure- ments of the pressure dependence of 7; and pLo are presently being pursued with the hope of establishing which model is more appropriate. (11 PI [31 [41 [51 [61 [71 PI [91 PO1 H.H. Hill. Plutonium 1970 and other Actinides ed. W.N. Miner (The Metallurgical Society of the AIME. New York, 1970) p. 2. A.L. Giorgi, E.G. Szklarz. R.W. White and H.H. H11l. J. Less-Common Metals 34 (1974) 348. H.H. Hill, A.L. Giorgi, E.G. Szklarz and J.L. Smith. J. Less-Common Metals38 (1978) 239. E. Fukushima. V.O. Struebing and H.H. Hill, J. Phw. Sot. Japan 39 (1980) 921. A. Wallash, J.E. Crow and Z. Fisk. J. Appl. Phys. 57 (1985) 3143. See. for example, Valence Instabilities, eds. P. Wachter and H. Boppart (North-Holland. Amsterdam, 1982) J. Magn. Magn. Mat. 47&48 (1985). B. Frick. J. Schoenes, F. Hulliger and 0. Vogt, Solid State Commun. 49 (1984) 1133. A. Arrott. Phys. Rev. 108 (1957) 1394. W.E. Pickett, A.J. Freeman and D. Koelling. Phys. Rev. 823 (1981) 1266. See for example: M. Shimizu. Rep. Prog. Phys. 44 (1981) 21. 

Saturation magnetization in the anomalous ferromagnet, (Y, U)B4

Abstract

Similar works

Full text

Available Versions

eScholarship - University of California