Sprecher
Beschreibung
Inelastic neutron scattering techniques and the study of spin dynamics in magnetic materials have long driven each other's advancements. Traditionally, studies on ferromagnets like iron and nickel were limited by coarse resolution, even with state-of-the-art instruments. However, employing the modern neutron spectroscopy method MIEZE [1], we probed nickel's spin wave dispersion and paramagnetic spin fluctuations with unprecedented detail at small momentum and energy transfers.
The MIEZE technique, implemented at the resonance spin-echo spectrometer RESEDA, uniquely enables the investigation of magnetic phenomena despite depolarizing samples and environmental conditions [2]. Its versatility allows for studying weak and low-energy magnetic dynamics with reasonable measurement times, thanks to its tolerance for a broad wavelength spectrum and large neutron flux. Recent upgrades to the instrument have improved background suppression, $q$ coverage, and energy resolution.
Analyzing the spin wave dispersion of isotropic ferromagnets using the Holstein-Primakoff theory (HPT) reveals insights into weak dipolar interactions [3]. The dispersion should be quadratic for a pure Heisenberg-like ferromagnetic system for small $q$ values $E_\mathrm{SW} \propto q^2$. In contrast, HPT predicts a linear $q$ dependence of the dispersion due to the long-range dipolar interactions between the magnetic moments. In addition to the dispersion, influences on the lifetime of critical fluctuations above $T_\mathrm{C}$ are also expected because the length scale $q_\mathrm{D}^{-1}$ should enter the dynamical scaling function as a second scaling variable and separate longitudinal from transversal modes [4].
[1] R. Gähler et al., Phys. Lett. A 1, 13 (1987)
[2] C. Franz et al., NIM A 939, 22-29 (2019)
[3] T. Holstein et al., Phys. Rev., 58, 1098 (1940)
[4] E. Frey et al., Phys. Lett. A 123, 1 (1987)
