Journal of Magnetism and Magnetic Materials 262 (2003) 479–484 Spin reorientation in Er1ÀxYxCo10Mo2 and ErCo10ÀyNiyMo2 compounds N.H Luonga,*, N Chaua, N.D Dunga, M Kurisub, G Nakamotob a Center for Materials Science, Faculty of Physics, College of Science, Viet Nam National University, Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam b School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsumokuchi, Ishikawa 923-1292, Japan Abstract Er1ÀxYxCo10Mo2 (x ¼ 0; 0.2, 0.4, 0.6, and 0.8) and ErCo10ÀyNiyMo2 (y ¼ 0; 1, 2, and 3) compounds have been studied in magnetization and susceptibility measurements Interesting magnetization curves at selected temperatures have been observed The results show that both these systems undergo a spin reorientation of the axis-to-plane type (referred to decreasing temperature) The spin-reorientation temperature, TSR ; is found to decrease not only with increasing Y content in the Er1ÀxYxCo10Mo2 compounds but also with increasing Ni concentration in the ErCo10ÀyNiyMo2 compounds The causes of the observed concentration dependence of TSR are discussed r 2003 Elsevier Science B.V All rights reserved PACS: 75.25.+z; 75.30.Gw Keywords: Hard magnetic materials; Rare earth—transition metal compounds; Spin reorientation; Anisotropy Introduction The intermetallic compounds of the type R(T,M)12 (R=rare earth, T=Fe, Co, Ni and M=Ti, V, Mo, Cr) have been regarded as potential starting materials for permanent magnet applications These compounds crystallize in the tetragonal ThMn12 structure with space group I4/ mmm It can be derived from the magnetic measurements on the RCo10Mo2 compounds performed by Xu and Shaheen [1] that in these compounds the moments of the light rare earth couple parallel to the Co moments and that the moments of heavy rare earths couple antiparallel to the Co moments, as is usually observed [2] *Corresponding author Fax: +84-4-85-89496 E-mail address: luongnh@vnu.edu.vn (N.H Luong) Zeng et al [3] and Tang Ning [4] have shown that in these compounds the Co sublattice displays easy c-axis magnetization These authors also reported that an axis-to-plane spin reorientation occurs at the spin-reorientation temperature, TSR ; (referred to decreasing temperature) in ErCo10Mo2 The present investigation has been undertaken in order to study the spin-reorientation phenomena in the Er1ÀxYxCo10-Mo2 and ErCo10ÀyNiyMo2 compounds Magnetization isotherms in several of these compounds have also been studied Experimental The Er1ÀxYxCo10Mo2 (x ¼ 0; 0.2, 0.4, 0.6, and 0.8) and ErCo10ÀyNiyMo2 (y ¼ 0; 1, 2, and 3) compounds were prepared by arc-melting 0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V All rights reserved doi:10.1016/S0304-8853(03)00081-7 N.H Luong et al / Journal of Magnetism and Magnetic Materials 262 (2003) 479–484 stoichiometric amounts of high purity elements (99.9% for Er and Y, 99.99% for Co, Ni and Mo) Subsequently, the samples were annealed in vacuum at 980 C for days X-ray diffraction and thermomagnetic analysis were used to check the quality of the samples All the samples were of single phase Magnetic measurements were carried out in applied magnetic fields up to 50 kOe and in the temperature range from 1.8 to 300 K by using a SQUID Magnetization (M) as a function of temperature from 100 to 700 K in a magnetic field of kOe has also been measured in a vibrating sample magnetometer The AC susceptibility (wac ) of the samples has been measured as a function of temperature from 20 to 200 K The spin-reorientation temperature TSR is determined as the temperature where in the magnetically ordered state the maximum in the wac ðTÞ curve occurs 6.0 ErCo10Mo2 T= 150 K Magnetization (µB/f.u.) 480 4.0 T= 50 K 2.0 T= 1.8 K 0.0 10 20 30 40 50 Magnetic field (kOe) Fig Magnetization curves at selected temperatures for ErCo10Mo2 Results and discussion Results of magnetization measurements in applied magnetic fields up to 50 kOe at selected temperatures for ErCo10Mo2 are shown in Fig The observation that the magnetic isotherm at 1.8 K passes through the origin means that in this compound the Er-sublattice magnetization MEr and the Co-sublattice magnetization MCo are equal to each other The value for MEr and MCo is mB/f.u This value for MEr and MCo has been also reported by Zeng et al [3] and Tang Ning [4] As one can see from Fig 1, magnetization of the compound increases with increasing temperature As the temperature increases, the two sublattice magnetizations bend towards each other, giving rise to the total magnetization We note that the highest lying magnetization curve in Fig is measured at 150 K, i.e at a temperature higher than TSR (see below) Fig presents the temperature dependence of the magnetization for the Er1ÀxYxCo10Mo2 compounds in a magnetic field of kOe The MðTÞ curves exhibit clear peaks which are associated with the spin reorientation In order to determine precisely the values of TSR for these compounds, we have measured the AC susceptibility as a Fig Temperature dependence of the magnetization for the Er1ÀxYxCo10Mo2 compounds function of temperature from 20 to 200 K The results are shown in Fig Sharp peaks have been observed in the wac ðTÞ curves for the Er1ÀxYxCo10Mo2 compounds investigated Apart from these peaks at TSR ; there is an anomaly in the wac ðTÞ curves for ErCo10Mo2 and Er0.8Y0.2Co10Mo2 The TSR values for the Er1ÀxYxCo10Mo2 compounds are presented in Table As one can see from this table, the spin-reorientation temperature is found to decrease with increasing Y content Our values N.H Luong et al / Journal of Magnetism and Magnetic Materials 262 (2003) 479–484 x = 0.8 Er1-xYxCo 10 M o x = 0.6 x = 0.4 x = 0.2 χac (a.u.) x = 0.0 20 60 100 140 Temperature (K) 180 Fig Temperature dependence of the AC susceptibility for the Er1ÀxYxCo10Mo2 compounds Table Spin-reorientation temperature TSR in the Er1ÀxYxCo10Mo2 compounds x TSR (K) 0.0 0.2 0.4 0.6 0.8 132 127 102 89 60 for TSR for the compounds with x ¼ and 0.2 are in good agreement with the corresponding values of 125 and 115 K reported by Zeng et al [3] and Tang Ning [4] The sharp cusp observed in the MðTÞ and wac ðTÞ curves are taken as evidence of spin reorientation of the axis-to-plan type with decreasing temperature The spin reorientation in the Er1ÀxYxCo10Mo2 is explained by the competition between Co-sublattice anisotropy and Er-sublattice anisotropy When erbium atoms are replaced by non-magnetic yttrium, the planar anisotropy of the erbium sublattice decreases, causing a reduction of the spin-reorientation temperature We note that the spin reorientation in the Er1ÀxYxCo10Mo2 compounds persists up to high Y concentrations (x ¼ 0:8) In the Nd1ÀxYxFe11Ti compounds, the spin reorientation is not observed when the Y content is as high as x ¼ 0:8 [5], whereas in the Tb1ÀxYxFe11Ti compounds the 481 spin reorientation is observed only in the samples with x ¼ 0; 0.2 and 0.4 [6] However, the above observation for the Er1ÀxYxCo10Mo2 compounds is similar to that for the Dy1ÀxYxFe11Ti compounds in which spin reorientation exists in the whole series of samples (x ¼ 0; 0.2, 0.4, 0.6 and 0.8) [7] On the basis of this analysis we could expect that the TSR would increase with the reduction of the Co-sublattice anisotropy Therefore, it is interesting to investigate the system ErCo10ÀyNiyMo2 in which Co atoms are partly replaced by Ni Very interesting magnetization isotherms were obtained in the ErCo10ÀyNiyMo2 compounds Fig shows magnetization curves for these compounds with x ¼ 1; and at selected temperatures As it is seen in this figure, metamagnetism is observed at 1.8 K for all substituted samples The magnetization behaviour strongly depends on bending process under influence of the external magnetic field and temperature The temperature dependence of the magnetization of the ErCo9NiMo2 sample in a magnetic field of kOe and in the temperature range from 100 to 700 K is presented in Fig as an example Similar MðTÞ curves were obtained for ErCo10ÀyNiyMo2 with x ¼ 0; and Values for the Curie temperature, TC ; were determined from MðTÞ curves by plotting M versus T and extrapolating the steep part to M ¼ 0: These values are presented in Table As one can see in this table, the Curie temperature strongly decreases with increasing Ni content It is well known that in the 4f–3d compounds the Curie temperature is mainly determined by 3d–3d interaction Substitution of nickel for cobalt reduces mainly this interaction, leading to the decrease of TC : As we can see in Fig 5, the MðTÞ curve exhibits at the low-temperature part a clear peak, which is associated with spin reorientation As in the case of the Er1ÀxYxCo10Mo2 compounds, in order to determine precisely the values of the spin-reorientation temperature, we have measured the AC susceptibility in the ErCo10ÀyNiyMo2 samples as a function of temperature The results are shown in Fig Sharp peaks have been observed in the wac ðTÞ curves for all the samples investigated N.H Luong et al / Journal of Magnetism and Magnetic Materials 262 (2003) 479–484 482 20 6.0 Er Co9 Ni Mo Magnetization (emu/g) Magnetization (µB/f.u.) ErCo 9NiMo T= 150 K 4.0 T= 50 K 2.0 T= 1.8 K 15 10 100 0.0 H = 1kOe 200 300 400 500 600 700 Temperature (K) 10 (a) 20 30 Magnetic field (kOe) 40 50 Fig Temperature dependence of the magnetization for ErCo9NiMo2 in a magnetic field of kOe 6.0 Magnetization (µB/f.u.) ErCo 8Ni 2Mo Table Curie temperature, TC ; and spin-reorientation temperature, TSR ; for the ErCo10ÀyNiyMo2 compounds The values for TSR obtained in Ref [8] for the ErCo11ÀyNiyTi compounds are also shown for comparison 4.0 T= 150 K y T= 1.8 K 2.0 T= 50 K 0.0 10 (b) 20 30 Magnetic field (kOe) 40 Magnetization (µB/f.u.) 4.0 T= 1.8 K T= 150 K T= 50 K (c) 10 20 30 40 Magnetic field (kOe) TC (K) TSR (K) TSR (K) [8] 482 416 332 250 132 127 109 108 145 155 160 165 Fig presents the temperature dependence of the susceptibility measured by SQUID for the ErCo10ÀyNiyMo2 compounds The curves in this figure exhibit also sharp peaks at the temperatures where peaks in the wac ðTÞ curves occur, i.e at the spin-reorientation temperature The values for TSR determined from the curves in Fig are collected in Table As can be seen in this table, spinreorientation temperatures decrease with increasing Ni content In the wac Tị curve for the sample with x ẳ there is a less pronounced second peak, the origin of which is still unclear ErCo7Ni3Mo2 0.0 ErCo11ÀyNiyTi 50 6.0 2.0 ErCo10ÀyNiyMo2 50 Fig Magnetization curves at selected temperatures for the ErCo10ÀyNiyMo2 compounds (a) x ¼ 1; (b) x ¼ 2; (c) x ¼ 3: N.H Luong et al / Journal of Magnetism and Magnetic Materials 262 (2003) 479–484 500 ErCo10-yNiyMo2 y=0 ErCo 10-y Ni yMo y=1 400 Temperature (K) χac (a.u.) y=2 y=3 80 100 120 140 160 180 Temperature (K) Fig Temperature dependence of the AC susceptibility for the ErCo10ÀyNiyMo2 compounds ErCo10–y Niy Mo2 H = 50 Oe Susceptibility (emu/g) 0.02 y=0 y = 1.0 y = 2.0 0.01 y = 3.0 0.00 483 100 200 Temperature (K) 300 Fig Temperature dependence of the susceptibility for the ErCo10ÀyNiyMo2 compounds As mentioned above, the spin reorientation in the ErCo10Mo2 compound is due to competition between uniaxial Co-sublattice anisotropy and planar Er-sublattice anisotropy Substitution of nickel for cobalt reduces the uniaxial Co-sublattice anisotropy In this case, if the behaviour of the Er-sublattice anisotropy remains the same, we would expect an increase of the spin-reorientation temperature as the planar anisotropy of the 300 TC 200 TSR 100 0 Ni concentration y Fig Dependence of the spin-reorientation temperature TSR and of the Curie temperature TC on the Ni concentration in the ErCo10ÀyNiyMo2 system Er-sublattice dominates in a wider temperature range An increase of the spin-reorientation temperature with increasing Ni content indeed has been observed in the ErCo11ÀyNiyTi compounds [8], as can be seen in Table However, as mentioned above, the situation in the ErCo10ÀyNiyMo2 compounds is different When the Ni content increases, the value of the Curie temperature decreases rapidly In the compounds with lower values of TC ; the temperature dependence of the Er-sublattice anisotropy probably changes, causing the reduction of spin-reorientation temperature Fig presents the dependence of the spin-reorientation temperature and of the Curie temperature on the Ni concentration in the ErCo10ÀyNiyMo2 system Apparently the Er-sublattice anisotropy in both Er1ÀxYxCo10Mo2 and ErCo10ÀyNiyMo2 systems favour easy-plane anisotropy (second-order anisotropy constant K1R o0) The anisotropy of the rareearth sublattice is crystal-field (CF) induced In lowest-order approximation, it can be given by K1R ẳ 3=2ịaJ /r2 S/O02 SA02 ; ð1Þ where aJ is the second-order Stevens factor, /r2 S the mean value of the second power of the 4f radius, /O02 S the thermal average of the secondorder Stevens operator O02 ; and A02 the secondorder CF coefficient Since aJ for Er is positive, we can conclude on the basis of Eq (1) that A02 > for 484 N.H Luong et al / Journal of Magnetism and Magnetic Materials 262 (2003) 479–484 the Er1ÀxYxCo10Mo2 and ErCo10ÀyNiyMo2 compounds Tang Ning [4] and Tang et al [9] have also found that A02 > for the RCo10Mo2 and RCo11Ti series Only DyCo10Mo2 and DyCo11Ti deviate from the expected behaviour, which is probably due to the neglect of higher-order CF terms in this simple approach The generally accepted view is that the crystal field should be roughly the same in isostructural compounds containing chemically similar elements, like Co and Fe However, investigation on the isostructural series RFe10Mo2 [10] and RFe11Ti [5–7,11] compounds, exhibiting a similar competition between the rare-earth sublattice anisotropy and the 3d-sublattice anisotropy, have shown that A02 is negative Thus sign reversal of the rare-earth anisotropy in Fe-rich versus Co-rich intermetallic compounds with ThMn2 structure has been found This puzzling fact is confirmed by density functional calculations [12,13] According to Kuz’min et al [12], the slightly different charge distributions around the Fe and Co atoms situated on the 8j sites neighbouring the rare-earth are chiefly responsible for the difference in the sign of A02 : It is interesting to investigate the R(T,M)12 compounds in which Co is partly replaced by Fe We have performed an experimental study of the effect of substituting Fe for Co on the spin reorientation in ErCo10Mo2 [14] Acknowledgements The authors are grateful to Prof Nguyen Xuan Phuc of the Institute of Materials Science (IMS), NCST for collaboration, Dr Vu Van Hong, Dr Dao Nguyen Hoai Nam and Dr Nguyen Huy Dan of the IMS for assistance in preparation of some samples and in measurements of the AC susceptibility This work was supported by the National Natural Science Council of Vietnam and National Basic Research Program KT-420101 References [1] X Xu, S.A Shaheen, J Magn Magn Mater 118 (1993) 16 [2] J.J.M Franse, R.J Radwanski, in: K.H.J Buschow (Ed.), Magnetic Materials, Vol 7, Elsevier Science, Amsterdam, 1993, p 308 [3] D.C Zeng, N Tang, T Zhao, Z.G Zhao, K.H.J Buschow, F.R de Boer, J Appl Phys 76 (1994) 6837 [4] T Ning, Ph.D Thesis, University of Amsterdam, 1998 [5] N.H Luong, N.P Thuy, J.J.M Franse, J Magn Magn Mater 104–107 (1992) 1301 [6] P.H Quang, N.H Luong, N.P Thuy, T.D Hien, J.J.M Franse, IEEE Trans Magn 30 (1994) 893 [7] P.H Quan, N.H Luong, N.P Thuy, T.D Hien, J.J.M Franse, J Magn Magn Mater 128 (1993) 67 [8] P.H Quan, N.H Luong, N.P Thuy, N.L Quang, Proceedings of the 2nd International Workshop on Materials Science, IWOMS’99, Hanoi, October 1995, p 145 [9] N Tang, D.C Zang, J.H.V Brabers, F.R de Boer, K.H.J Buschow, J Magn Magn Mater 150 (1995) 241 [10] X.C Kou, R Grossinger, G Wiesinger, J.P Liu, F.R de Boer, I Kleirschorth, H Kronmuler, Phys Rev B 51 (1995) 8254 [11] X.C Kou, T.S Zhao, R Grossinger, H.R Kirchmayer, X Li, F.R de Boer, Phys Rev B 47 (1993) 3231 [12] M.D Kuz’min, M Richter, K.H.J Buschow, Solid State Commun 113 (2000) 47 [13] M.D Kuz’min, M Richter, H Eschrig, K.H.J Buschow, J Magn Magn Mater 226–230 (2001) 1118 [14] N.H Luong, N Chau, N.D Dung, B.N.Q Trinh, in press ... evidence of spin reorientation of the axis-to-plan type with decreasing temperature The spin reorientation in the Er1ÀxYxCo10Mo2 is explained by the competition between Co-sublattice anisotropy and Er-sublattice... temperature We note that the spin reorientation in the Er1ÀxYxCo10Mo2 compounds persists up to high Y concentrations (x ¼ 0:8) In the Nd1ÀxYxFe11Ti compounds, the spin reorientation is not observed... concentration y Fig Dependence of the spin- reorientation temperature TSR and of the Curie temperature TC on the Ni concentration in the ErCo10ÀyNiyMo2 system Er-sublattice dominates in a wider