Evidence of ferromagnetically coupled Nd3+ ion pairs in weakly doped Nd : LiYF4 and Nd : YVO4 crystals as revealed by high-resolution optical and EPRspectroscopies

Electron paramagnetic resonance (EPR) and high-resolution fluorescence spec tra of Nd3+ ions in weakly doped LiYF4 and YVO4 crystals are analyzed. A si mple model, based on an effective spin-Hamiltonian approach is proposed to explain the general features of the EPR and optical neodymium spectra in th ese matrices. Pairs of satellite lines whose intensities grow with neodymiu m content an observed on each side of the main EPR signals of isolated Nd3 ions. These satellites are assigned to Nd3+-Nd3+ ion pairs coupled by magn etic dipolar interaction. The calculated Nd-Nd distances are found to be in good agreement with the Y3+-Y3+ distances in LiYF4 and YTO4 hosts. The con centration-dependent satellites accompanying the neodymium F-4(3/2) --> I-4 (9/2) transition are quantitatively explained as being due to several types of ferromagnetically coupled pairs of Nd3+ ions, each ion of a pair being located at the regular Y3+ site with S-4 and D-2d point symmetry in LiYF4 a nd YVO4, respectively. The exchange coupling values J are found in the rang e +0.8 to +4.9 cm(-1). From literature data. it appears that such ferromagn etically coupled Nd3+ pairs with J = +3 cm(-1) also quantitatively explain the optical satellite structure in Nd:Y3Al5O12 (YAG).