ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 272–276 (2004) e417–e418 Spin reorientation in tetragonal ErxYxCo10Mo2 and ErCo10ÀxNixMo2 compounds N.H Haia, D Fruchartb, D Gignouxc,*, N.H Luonga, N Chaua b a Center for Materials Science, Vietnam National University, 334 Nguyen Trai, Hanoi, Viet Nam Laboratoire de Cristallographie, CNRS, 25 ave des Martyrs, BP 166, 38042 Grenoble Cedex 9, France c Laboratoire Louis N!eel, CNRS, 25 ave des Martyrs, BP 166, 38042 Grenoble Cedex 9, France Abstract The thermal dependence of the easy magnetisation direction of tetragonal Er1ÀxYxCo10Mo2 and ErCo10ÀyNiyMo2 compounds were studied from the angular dependence of the magnetisation vector measured on oriented powders Whereas one of the studied materials exhibits rather usual first-order transitions at TSR ; the other materials present quite complex and unusual spin reorientation which are discussed by considering the different contributions to the anisotropy r 2003 Published by Elsevier B.V PACS: 75.25.+z; 75.30.Gw; 75.30.Kz Keywords: (ErY)(CoNiMo)12 tetragonal compounds; Spin reorientation; Magnetocrystalline anisotropy Tetragonal Er1ÀxYxCo10Mo2 and ErCo10ÀyNiyMo2 compounds belong to the family of tetragonal ThMn12 type materials which are mainly studied in the context of their potentiality as permanent magnet materials Many of these materials, in particular, those of the type RFe12ÀxMx (R=rare earth, M=Mo,Ti,Cr,V,Nb,Ta,y) which have been widely studied, exhibit spin reorientations SR These reorientations were first detected from anomalies in the thermal variations of the AC or even DC-susceptibility They were clearly evidenced and in some cases quantitatively studied from magnetic measurements on oriented powders and single crystals It is worth noticing that it has been observed that susceptibility anomalies are not systematically associated with SR It is then necessary, in order to identify such SR, to perform magnetisation measurements on oriented powders if no single crystals are available For such a purpose the best measurements consist in measuring the angular dependence of the magnetisation component ðM> Þ perpendicular to the applied field *Corresponding author Tel.: +3-347-688-7916; fax: +3347-688-1191 E-mail address: gignoux@grenoble.cnrs.fr (D Gignoux) 0304-8853/$ - see front matter r 2003 Published by Elsevier B.V doi:10.1016/j.jmmm.2003.12.754 In ErCo10Mo2, anomalies of susceptibility at 143 and 350 K were previously ascribed to SR whereas, from X-ray on oriented powder the easy magnetisation direction EMD at 300 K has been deduced to be intermediate between the c-axis and the basal plane [1] In order to have a better insight of such properties we have performed experiments of the angular dependence of the magnetisation vector in oriented powders of this compound and of those obtained by substituting Er for nonmagnetic Y, and Co for Ni, the latter being likely nonmagnetic This technique allows an accurate determination of the angle that makes the EMD with the c-axis For all the studied samples, X-rays show that the EMD at 300 K is the c-axis and the thermal dependences of the susceptibility exhibit well pronounced anomalies Moreover angular dependence of M> have shown that c remains the easy axis of any temperature below 300 K in ErCo10Mo2 For each of the other compounds we have observed three types of different and unusual (for two of them) behaviours The angular dependences of M> for Er0.6Y0.4Co10Mo2 at different characteristic temperatures are shown in Fig 1, where y is the angle between the applied field ARTICLE IN PRESS e418 N.H Hai et al / Journal of Magnetism and Magnetic Materials 272–276 (2004) e417–e418 Fig Angular dependence of Mx at different temperatures in Er0.6Y0.4Co10Mo2 and the c-axis The value of y for which the M> passes by zero with a positive derivative corresponds to the angle ye that makes the EMD with c: Moreover the larger the amplitude of the maximum of M> the larger the anisotropy It is clear, as shown in Fig 2, that the easy direction rapidly passes between 110 and 90 K from the cÀaxis to the basal plane, whereas at 100 K the easy direction is intermediate and the anisotropy very small As in other compounds such as TbFe10.5Mo1.5 [2], this transition is likely of first order The observed width of the transition probably comes from non homogeneity of the sample as it has been shown in DyFe11Ti [3] The two other types of temperature dependences of the magnetisation directions are quite original As shown in Fig 2, in Er0.4Y0.6Co10Mo2 magnetisation is along c below 100 K and above 200 K whereas between these temperatures it tilts away from c: In ErCo8Ni2Mo2 the behaviour is still more complex as shown in Fig Whereas above 100 K magnetisation is along c; it becomes intermediate below this temperature with ye passing by zero around 40 K It is worth noticing that in all these compounds, there is no link between the temperatures of the susceptibility anomalies and the different characteristic temperatures of the thermal dependence of ye : The origin of susceptibility anomalies is then probably elsewhere They could be due to macroscopic effects such as domain walls as it has already been suggested [4] Fig Thermal dependence of the angle that makes the magnetization with the c-axis In these compounds exchange interactions are generally assumed to be large enough so that magnetic moments can be considered as collinear with Er and Co sublattices antiferromagnetically coupled The complex dependences of EMD then arise from the interplay of the different contributions to the anisotropy On the one hand the magneto-crystalline anisotropy of rare earth ions in the 1–12 compounds is characterised by secondorder crystal field parameters especially small for uniaxe compounds It is on the same order of magnitude as the fourth-order parameter The balance between these two terms can be at the origin of SR, as in ErFe10.5Mo1.5 [5], which are not the result, as in other compounds, of the competition between rare earth and 3d metal anisotropies On the other hand Co atoms occupy three different crystallographic sites, with second-order anisotropy constants K, of different signes and magnitudes References [1] X Xu, S.A Shaheen, J Magn Magn Mater 118 (1993) L6 [2] R Vert, D Fruchart, D Gignoux, J Magn Magn Mater 242 (2002) 820 [3] M.D Kuz’min, J Appl Phys 88 (2000) 7217 [4] L.M Garcia, J Bartolom!e, F.J Lazaro, C de Francisco, J.M Munoz, Phys Rev B 54 (1996) 15238 [5] B Garcia-Landa, D Gignoux, R Vert, D Fruchart, R Skolozdra, J Phys.: Condens Matter 10 (1998) 1403 ... passing by zero around 40 K It is worth noticing that in all these compounds, there is no link between the temperatures of the susceptibility anomalies and the different characteristic temperatures... in DyFe11Ti [3] The two other types of temperature dependences of the magnetisation directions are quite original As shown in Fig 2, in Er0.4Y0.6Co10Mo2 magnetisation is along c below 100 K and. .. that makes the magnetization with the c-axis In these compounds exchange interactions are generally assumed to be large enough so that magnetic moments can be considered as collinear with Er and