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MAGNETIC SYSTEMS 2009Effect of the M/Co substitution on magnetocrystalline anisotropy andmagnetization in SmCo 5−x M x compounds (M=Ga; Al)The RCo 5 compounds have been discovered a few decadesago but are still attracting much interest in order to understandtheir exceptional intrinsic magnetic properties. Theinfluence of substitution of p elements (Al, B, Ga...) forCo on the magnetic properties of the RCo 5 type phases isalso an active field of research nowadays. The samples formagnetization measurements were processed from powderswith a particle size smaller than 25 µm. The powder wasoriented at room temperature using an orientation field oftypically 10 kOe and fixed in slow setting epoxy resin. Themagnetic field was applied either parallel or perpendicularto the alignment direction. According to the X-ray analysis,the CaCu 5 type structure of the RCo 5 is preserved upon substitutionof Ga and Al for Co in the SmCo 4 Al and SmCo 4 Gacompounds. According to the neutron diffraction resultson these isotype compounds, the Al and Ga atoms havea pronounced preference for the Co 3g atomic position inthe hexagonal CaCu 5 structure. For both SmCo 4 Al andSmCo 4 Ga, at room temperature, the easy magnetization directioncoincides with the c-axis of the hexagonal lattice.Ga and Al for Co substitution induce a large reduction ofthe Curie temperature from typically 900 K to 1000 K forthe RCo 5 to around 500 K only for the RCo 4 Al or RCo 4 Gaphases. The RCo 4 M phases containing M=Ga and Al havesimilar Curie temperature.As can be seen from the isothermal curves in figures 121and 122, the use of high magnetic field is required dueto the large magnetocrystalline anisotropy of the samples.The spontaneous magnetization has been determined froma linear extrapolation of the M(H) curve to zero appliedfields. The saturation magnetization has been obtainedfrom extrapolation of the M(H) curve with H parallel tothe alignment direction and larger than 20 T according toM(H) = M sat + a/H2. The replacement of one Co atomby one Ga or Al atom induces a strong decrease of the saturationmagnetization from 8.46 µB/formula unit at 4 K toonly about 4 µB/f.u. Such dramatic decrease of the Co magnetizationis consistent with earlier reported results on isotypecompounds. SmCo 5 is well known for it exceptionallylarge magnetocrystalline anisotropy µ 0 H a ∼ 52 T at 4 K.The anisotropy field can be estimated by the field at whichthe two magnetization curves will merge for the two differentorientations. The anisotropy is found to be largerfor the Ga than for Al containing compounds; 89 T and83 T respectively. An effective magnetocrystalline coefficienthas been derived from the anisotropy field accordingto the relation H a = 2K e f f /M s . The values found for theSmCo 4 Al and SmCo 4 Ga are 16.3 and 19.3 MJ/m 3 respectively,that is to say of the same order of magnitude thanfor the SmCo 5 . Consequently, the huge anisotropy field observedfor the SmCo 4 Al and SmCo 4 Ga result from largeanisotropy constant and a strong decrease of the saturationmagnetization respectively. The presence of Al or Ga hasbeen reported to significantly reduce the Co sublattice contributionto the magnetocrystaline anisotropy in the YCo 4 Alor YCo 4 Ga isotype compounds. Thus we conclude thatthe huge anisotropy field reported here for SmCo 4 Al andSmCo 4 Ga results from a reinforcement of the Sm contributionto the magnetocrystalline anisotropy in comparison tothe SmCo 5 phase. Further studies are in progress to investigatethe thermal dependence of the magnetic features oftheses phases.Figure 121: Isothermal (4.2K) high field magnetization curve ofSmCo 4 Al recorded parallel to the easy magnetization direction inorder to obtain the spontaneous magnetization M spon . Inset; determinationof the saturation magnetization M sat .Figure 122: Isothermal hysteresis cycle recorded at 4 K on orientedSmCo 4 Al when applying the magnetic field perpendicularto the alignment direction (c axis).M. GuillotA. Laslo, C.V. Colin, O. Isnard (Institut Néel, CNRS and Université Joseph Fourier)88