MAGNETIC ORDERING IN DILUTE YTb AND YEr ALLOYS

Dilute YEr alloys show the existence of sinusoidaily modulated antiferromagnetism down to the lowest impurity concentrations studied. Extrapolation of the Ndel temperatures for both YEr and YTb suggests a critical concentration is 0.8 % Tb, Er. Ordering in such dilute alloys may result from exchange enhancement in the yttrium host. Following our earlier study of YTb alloys [I], we der contamination in the incident beam is removed by have investigated the magnetic phase diagram of YEr a cooled beryllium filter. The sample to detector disalloys with Er concentrations between3 % and 10 % , tance was fixed at 0.6 m, allowing a range of scattering using a combination of heat capacity measurements angle of f 3 ' to be covered for one setting of the multiand neutron diffraction. Heat capacity measurements detector. The variation in detector cell efficiency was were made on arc-melted polycrystalline samples, uscalibrated using the high level of isotropic scattering ing a modified adiabatic heating technique, described produced at low scattering angles by a perspex cylinelsewhere [Z]. Figure 1 shows the magnetic contribuder. Overall the cell efficiencies varied by less than tion to the specific heat of four alloys, obtained by 10 % , except for a few lines of cells near the very edge subtracting the specific heat of pure Y. The data show of the detector, which were masked out in the data clearly a cooperative anomaly due to a magnetic phase processing. The samples were mounted in a variable transition superimposed on a Schottky anomaly aristemperature cryostat which allowed measurements in ing from the repopulation of crystal field levels. A the range 1.7 K to 150 K. For the Y(6 % )Er crystal detailed analysis of these data, using a mean field apscans were made at a fine mesh of points along the (001) proach and the known crystal field level scheme [3] will and (101) lines in reciprocal space at a temperature of be presented in a forthcoming paper. 4.3 K. There was no indication of magnetic diffuse or For the neutron diffraction experiments single crysB r a g scattering along the (001) direction, but intense tals were grown by annealing polycrystalline alloys peaks were found at the (lOQ) and (1,0,2-Q) under argon at temperatures between 1200 OC and p0sitions,'with Q = 0.28(2r/c). This value of Q is 1350 OC. ~h~ measurements were carried out with identical to that found in YTb alloys [I], which disthe D~~ diffractometer at ILL, ~ ~ ~ ~ ~ b l ~ . hi^ has play helical antiferromagnetism with a 50' interlayer a monochromatic beam of 4.52 PI and a turn angle. In the present case however, the absence two dimensional rnultideteCtor with 64 x 16 cells. orof bra!% peaks at the (00Q) positions shows that the magnetic moments in the YEr alloys are aligned along .the c-axis direction and are longitudinally modulated 1.5 with wavevector Q. This corresponds to a modulation . . . . .< : wavelength of 20 A, or 7 atomic layers. This mag‘. C ......... ) " ;-..,... . .-37..:. ....... netic structure is consistent with the known single ion ..... . . ' ' . I ....... :,. ..-.. anisotropy for Er in Y: susceptibility measurements on . ..... ; I I , . : .:. . ........ ...... ... . . . very dilute YEr alloys [3] show large anisotropy with u, -...&:>G> ; ., .; . :. ,. . :. {.' 2 . . .........:......:. . 2. ; , .: _ . A . . . <.I. . . X I I very much greater than XI. Each of the four YEr x . .;.. c rn / .f ........... <,., .". ............ u 7 . . . .. . .:. :. :. crystals studied, containing 3, 4, 6 and 8 % Er, were 1 :i hh<*:. , .;* d .*., .: ~.-,~,~:..~.:.:.;:-. . . . . . ': . . rn . . . . . ., . ._. ' ...... . . . , .: .,.:.: .,,.: found to display Bragg scattering at the (10Q) posi5 . 5 . . . . . . ., . tion at low temperatures. The widths of the Bragg . , . . . . .: . . . . . , : ;. . : : x ..-K?>.:5;2, ... >' :... . . . . . u .....; ..:.. .: :.: . . . . ...... . . . peaks were resolution limited, showing that the mod. . . . . . . . _ . . . . . . . . :I . . . . .. . . . . . :.. . ulated order is established over very large distances. . 8 . The temperature variation of the integrated intensio ties under the Bragg peaks is shown in figure 2. The 0 5 10 15 20 25 30 35 40 45 50 TEMP. [ K I Ndel temperatures deduced from these data and the Fig. 1. Magnetic component of specific heat for YEr heat capacity anomalies vary linearly with Er concenalloys with 3,5,7 and 10 % Er versw temperature obtained tration. Extrapolation would suggest that the critical by subtracting the specific heat of a pure Y sample. concentration for long range order is 5 0.8 % Er. This l~ns t i tu te Laue Langevin, BP 156X, 38042 Grenoble Cedex, France. Present address: ICI Research Laboratories, Runcorn, G.B. 2~irection des Matkriaux, ONERA, 92320 Chatillon, France. present address: Physics Dept., Reading University, RG6 2AF, G.B. 4 ~ r e s e n t address: Physics Dept., University of Mosul, Iraq. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888159 C8 356 JOURNAL DE PHYSIQUE Fig. 2. Integrated intensities of (1,0,Q) satellite bragg peak for YEr alloys a s a function of temperature. value agrees well with that deduced from our earlier measurements on YTb alloys. Discussion and conclusion The persistance of long range magnetic order to such low concentrations of rare earth impurity suggests that the indirect exchange between rare earth impurities in yttrium has extremely long range. Independent s u p port for this conclusion is provided by the spin wave measurements of Wakabayashi and Nicklow [4] on alloys of Y(10 % Tb) and Y(10 % Ho). They found that the fourier transform of the interplanar exchange couplings J (q) displays a very large peak at the modulation wavevector (O,O,Q). There is a large curvature in J(q) in the vicinity of the maximum, which can be related to the range of the exchange couplings in a simple way: expansion of J (q) of q near (0, 0, Q) gives where P' = T' J (T) / J (T)