Temperature and field dependence of the spin wave gap in NdCu2

NdCu2 shows a complex magnetic phase diagram below its Neel temperature of T-N = 6.5 K and for magnetic fields applied in the easy b direction. This complicated behavior is expected to be due to Ruderman-Kittel-Kasuya-Yoshida interaction of the 4f shells of the Nd ions in the presence of crystal field splitting. In order to obtain deeper insight into the mechanism leading to such different magnetic phases we started to study the spin wave spectra in a NdCu2 single crystal. In zero field as well as in fields applied in the easy b direction of magnetization, a pronounced minimum in the excitation spectrum has been observed and it forms an energy gap. The position of this spin wave gap does not coincide with any magnetic ordering wave vector in NdCu2, indicating anisotropic magnetic coupling. The temperature and field dependence of the spin wave gap have been studied. The position of the gap remains constant at q(gap) = (0.35, 0, 0), whereas the character of the excitations changes with temperature and the value of the energy gap changes with the magnetic field. Applying a field of 10 T in the hard c direction of magnetization changes the position of the minimum to q(gap) = (0.6, 0, 0) and gives proof of magnetic anisotropy. (C) 1999 American Institute of Physics. [S0021-8979(99)40808-4].