Surface-layer ferromagnetism and strong surface anisotropy in Ca1-xLaxB6 (x=0.005) evidenced by ferromagnetic resonance

In Ca1-xLaxB6 (x = 0.005) single crystals using 9.6 GHz microwave frequency at room temperature, we observed resonance lines which exhibit sample-shape dependence with respect to their resonance field, linewidth and numbers. A disk-shaped sample showed a single, symmetrical resonance line and a large lower-field shift. The resonance field of the disk sample was investigated in detail and exhibits cos(-1) theta dependence, where theta is the angle between the disk plane and the external magnetic field. This finding leads to the conclusion that the spins are strongly forced to stay in the disk plane by an anisotropy field. The symmetrical resonance line originates from surface spins within the microwave skin depth in a metallic sample. At a higher angle theta, the resonance line became antisymmetrical. At the microwave frequency of 24 GHz, the resonance line shape was also antisymmetrical even at theta = 0. We conclude that spins in Ca0.995La0.005B6 exist only within the surface layer of approximately 1.5 mum thick, and they feel strong surface magnetic anisotropy of an easy plane type.