Physica B 327 (2003) 262–265 Spin reorientation in ErCo10ÀxFexMo2 compounds Nguyen Hoang Luong*, Nguyen Chau, Nguyen Duc Dung, Bui Nguyen Quoc Trinh Center for Materials Science, Faculty of Physics, Vietnam National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam Abstract The spin reorientation in ErCo10ÀxFex Mo2 (x ¼ 0; 1, 2, and 4) compounds has been studied by measurements of the temperature dependence of the magnetization The spin reorientation in this system is due to the competing anisotropies of the erbium and the transition-metal sublattice A decrease of the transition-metal sublattice anisotropy is believed to be the reason of the observed increase of the spin-reorientation temperature with increasing Fe content r 2002 Elsevier Science B.V All rights reserved Keywords: Hard-magnetic materials; Rare-earth–transition-metal compounds; Spin reorientation; Anisotropy Introduction The intermetallic compounds of the type RT12ÀxMx (R=rare earth, T=Fe, Co and M=Ti, V, Cr, Mo) are promising candidates for the development of hard-magnetic materials These compounds have the ThMn12-type tetragonal structure In several systems of these 1:12 compounds a temperature-induced spin reorientation has been observed Zeng et al [1] and Tang Ning [2] have reported that in the ErCo10Mo2 compound a spin reorientation occurs from axial to planar magnetization upon cooling Earlier, we have studied spin-reorientation phenomena in the Er1ÀxYx Co10Mo2 and ErCo10ÀyNiy Mo2 compounds [3] In the present work, we report our study of the effect of substitution of Fe for Co on the spin *Corresponding author Tel./fax: +84-4-858-94-96 E-mail address: luongnh@vnu.edu.vn (N.H Luong) reorientation and also the magnetic-ordering temperature of ErCo10Mo2 Experimental ErCo10ÀxFex Mo2 (x ¼ 0; 1, 2, and 4) compounds were prepared by arc-melting a stoichiometric mixture of 3N-purity Er and 4N-purity Co, Fe and Mo After arc melting, the polycrystalline ingots were annealed in argon at 9801C for days The structure of the samples was examined in an D5005 X-ray diffractometer of Bruker, using Cu( Ka radiation with wavelength l ¼ 1:54056 A Magnetization (M) measurements were carried out in the temperature range between 100 and 660 K in a magnetic field of 200 Oe in a DMS 880 vibrating sample magnetometer of Digital Measurement Systems From the M2T curves the Curie temperature, TC ; and spin-reorientation temperature, TSR ; were derived 0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - ( ) - N.H Luong et al / Physica B 327 (2003) 262–265 Results and discussion Fig shows the X-ray-diffraction (XRD) pattern of ErCo9FeMo2, as an example Similar patterns are observed for the rest of the samples investigated XRD revealed the samples to have the ThMn12-type tetragonal structure with lattice constants as presented in Table One can see in Table that a and c increase with increasing Fe content substituted for Co In Fig the magnetization of ErCo10ÀxFex Mo2 samples, measured in the field of 200 Oe, is shown as a function of temperature The Curie temperatures TC determined from these curves are also Table Structural and magnetic properties of ErCo10Àx Fex Mo2 compounds x Lattice parameters ( a (A) ( c (A) 8.371 8.380 8.387 8.398 8.409 4.864 4.877 4.881 4.896 4.902 TC (K) TSR (K) 482 511 579 587 567 132 163 204 280 355 263 presented in Table TC reaches a maximum for x ¼ and decreases with further increasing x: The reason of this non-linear concentration dependence of TC could be the preferential occupation of Fe ions of different 3d sites and the different interactions between 3d atoms at different sites The clear peaks seen in Fig indicate that all ErCo10ÀxFex MO2 samples investigated possess a spin reorientation The spin-reorientation temperature TSR was determined as the temperature where the peak occurs in the MðTÞ curve The values of TSR are also presented in Table Zeng et al [1] and Tang Ning [2] have shown that in RCo10Mo2 the Co sublattice displays easy c-axis magnetization and that the Er moment couples antiparallel to the Co moment The sharp cusp seen in Fig is taken as evidence for a spin reorientation from axial to planar magnetization upon cooling below TSR : In Table 1, one can see that the spin-reorientation temperature in the ErCo10ÀxFex Mo2 system increases with increasing Fe concentration Our TSR value for ErCo10Mo2 is in good agreement with the value of 125 K reported by Zeng et al [1] and Tang Ning [2] The dependence of TSR and TC on the Fe concentration in the ErCo10ÀxFex Mo2 system is shown in Fig Fig X-ray diffraction pattern of ErCo9FeMo2 N.H Luong et al / Physica B 327 (2003) 262–265 264 14 H =200 Oe x=4 12 x=3 x=2 10 M (emu/g) x=1 x=0 100 200 300 400 T (K) 500 600 700 Fig Magnetization of the ErCo10ÀxFex Mo2 compounds as a function of temperature in magnetic field of 200 Oe 600 TC 500 T (K) 400 T SR 300 200 the T-sublattice decreases, leading to an increase of TSR : The anisotropy of the rare-earth sublattice is crystal-field (CF) induced In lowest-order approximation, it is given by K1R ẳ 32aJ r2 O02 A02 ; 1ị 2 where aJ is the second-order Stevens factor, r the mean value of the square of the 4f radius O02 is the thermal average of the second-order Stevens operator O02 ; and A02 the second-order CF coefficient Since aJ is positive for Er, we can conclude on the basis of Eq (1) that A02 > for the ErCo10ÀxFex Mo2 compounds Tang Ning [2] and Tang et al [4] have also found that A02 > for the RCo10Mo2 and RCo11Ti series Only the compounds DyCo10Mo2 and DyCo11Ti deviate from the expected behavior which is probably due to the neglect of higher-order CF terms in this simple approach The result that A02 > in the RCo10Mo2 and RCo11Ti series was quite surprising in view of the results reported for the isostructural series RFe10Mo2 [5] and RFe11Ti [6–9], in which the spin reorientation has the same nature, i.e it arise from the competition between the easy-axis T-sublattice anisotropy and the easy-plane R-sublattice anisotropy In the RFe10Mo2 and RFe11Ti series, A02 is negative This puzzling fact is confirmed by density functional calculations [10,11] According to Kuz’min et al [10], the slightly different charge distributions around the Fe and Co atoms situated on the 8j sites are mainly responsible for the difference in the sign of A02 : 100 x Fig The dependence of the spin-reorientation temperature, TSR ; and the Curie temperature, TC ; on the Fe concentration in ErCo10ÀxFex Mo2 compounds The spin reorientation in ErCo10ÀxFex Mo2 compounds can be explained by the competition between the T-sublattice anisotropy and the Ersublattice anisotropy When Co atoms are substituted by Fe atoms, the uniaxial anisotropy of Acknowledgements This work was supported by the National Natural Sciences Council of Vietnam and National Basic Research Program No KT-420101 References [1] D.C Zeng, N Tang, T Zhao, Z.G Zhao, K.H.J Buschow, F.R de Boer, J Appl Phys 76 (1994) 6837 [2] Tang Ning, Ph.D Thesis, University of Amsterdam, 1998 N.H Luong et al / Physica B 327 (2003) 262–265 [3] N.H Luong, N Chau, N.D Dung, M Kurisu, to be published [4] N Tang, D.C Zeng, J.H.V.J Brabers, F.R de Boer, K.H.J Buschow, J Magn Magn Mater 150 (1995) 241 [5] X.C Kou, R Grossinger, G Wiesinger, J.P Liu, F.R de Boer, I Kleirschroth, H Kronmuller, Phys Rev B 51 (1995) 8254 [6] N.H Luong, N.P Thuy, J.J.M Franse, J Magn Magn Mater 104–107 (1992) 1301 265 [7] P.H Quang, N.H Luong, N.P Thuy, T.D Hien, J.J.M Franse, IEEE Trans Magn 30 (1994) 893 [8] P.H Quang, N.H Luong, N.P Thuy, T.D Hien, J.J.M Franse, J Magn Magn Mater 128 (1993) 67 [9] X.C Kou, T.S Zhao, R Grossinger, H.R Kirchmayer, X Li, F.R de Boer, Phys Rev B 47 (1993) 3231 [10] M.D Kuz’min, M Richter, K.H.J Buschow, Solid State Commun 113 (2000) 47 [11] M.D Kuz’min, M Richter, H Eschrig, K.H.J Buschow, J Magn Magn Mater 226–230 (2001) 1118 ... interactions between 3d atoms at different sites The clear peaks seen in Fig indicate that all ErCo10ÀxFex MO2 samples investigated possess a spin reorientation The spin- reorientation temperature... below TSR : In Table 1, one can see that the spin- reorientation temperature in the ErCo10ÀxFex Mo2 system increases with increasing Fe concentration Our TSR value for ErCo10Mo2 is in good agreement... ErCo10ÀxFex Mo2 compounds The spin reorientation in ErCo10ÀxFex Mo2 compounds can be explained by the competition between the T-sublattice anisotropy and the Ersublattice anisotropy When Co atoms are