Nuclear Magnetic Resonance Detection of a Spin-Wave Gap in Fe2B

Measurements of the 11B frequency vs temperature in the ferromagnetic intermetallic Fe2B for 4.2°≤T≤373°K give evidence of a spin-wave gap even in zero applied field. Above about 100°K, the reduced frequency is approximately proportional to T3/2 (with a small admixture of a T5/2 term). However at lower temperatures, a significant deviation of the reduced frequency from the T3/2 dependence is apparent. The deviation can be understood in terms of a spin-wave gap, Tg, which is present because of the relatively large magnetocrystalline anisotropy and magnetization.1 Assuming proportionality between reduced frequency and bulk magnetization, we have extracted1 the constants of proportionality in the spin-wave approximation, C (the coefficient of the T3/2 term) and D (the coefficient of the T5/2 term). We find that D/C=(1.0±0.4)×10−3. Our data set an approximate upper limit of about 1°K on Tg. These and other NMR and Mössbauer results on hyperfine magnetic fields and quadrupole interactions in both Fe2B and Fe2Zr will appear elsewhere.2 Anisotropy found in the Fe2B Mössbauer spectra can be interpreted as a super-position of hyperfine patterns arising from an anisotropic magnetic field and electric field gradient in combination with either two magnetically inequivalent Fe sites (such as was observed in Fe2Zr) or a two dimensional distribution of magnetization directions. The measured pressure variations ∂v/∂P of the 11B and 91Zr NMR frequencies in Fe2B and Fe2Zr, respectively, are (−21±2) and (31±2) Hz·bar−1.