Magnetic Behavior of Cobalt in Garnets. I. Spin Resonance in Cobalt-Doped Yttrium Gallium Garnet

We have studied the spin resonance at 24 and 56 kMc/sec of cobalt in single crystals of yttrium gallium garnet (YGaG). We find resonances associated with four different combinations of site and charge state. Tetrahedral Co²⁺ has the ⁴A₂ ground term with the tetragonal splitting parameter 2D=-36 cm^-1. Octahedral Co²⁺ has the ⁴T₁ ground term with the trigonal splitting Δₜ=+650 cm^-1. It is seen in otherwise pure YGaG, as well as in crystals counterdoped with Si⁴⁺. Tetrahedral Co³⁺ has the ⁵E ground term, with D~14 cm^-1, $|a|=0.66$ cm^-1. This value of a, the fourth-order cubic field-splitting parameter, is exceptionally large and makes possible strong magnetic-dipole transitions between states with Sz=±2. Electric-dipole transitions are also observed between these states. Octahedral Co⁴⁺ has the strong-field ²T₂ ground term, with Δₜ=+1000 cm^-1; its spectrum is very weak relative to Co³⁺, even in crystals counterdoped with Ca²⁺. The absence of a resonance from octahedral Co³⁺ implies that this ion has the strong-field ¹A₁ ground term. Cobalt does not appear to enter the dodecahedral site in our crystals. Our results are complementary to, and on the whole consistent with, the optical data of Wood and Remeika. We review the information available on crystal fields at the octahedral and tetrahedral sites in garnet, and on the hyperfine structure of cobalt.