Speaker
Description
Ferromagnetic resonance (FMR) is a unique technique used to determine fundamental properties of the studied magnetic material, such as magnetocrystalline anisotropy, $g$-factor, exchange constant, Gilbert damping, etc. in single crystals (SCs) or magnetic interactions between layers and components in multilayered nanostructured films and foils [1]. Ni-Mn-Ga Heusler alloys are a subclass of traditional shape memory materials that exhibit strains and shape change not only in response to the increase of temperature but also when exposed to relatively low ($<5$ kOe) external magnetic field. These alloys, called magnetic shape memory (MSM) alloys, are known for large reversible magnetic-field-induced strain (MFIS) of up to $12\%$ while staying in their martensitic phases ($10$M, $14$M and NM). FMR in epitaxially grown Ni-Mn-Ga thin films, where the twin structures are confined by the rigid substrate, has been thoroughly investigated, e.g. in [2]. On the contrary only a few papers exist on FMR in SC MSM alloys. The temperature and angular dependences of FMR were previously measured on bulk Ni-Mn-Ga SCs at X-band microwave frequency [3]. However, the derived anisotropy constants substantially differed from those determined from the magnetization measurements.
Here we present a broadband ferromagnetic resonance study in SC Ni$_{50}$Mn$_{28.1}$Ga$_{21.9}$ in the temperature range from $20$ °C to $140$ °C in which the martensite transformation occurs. Our results demonstrate that a large change (an order of magnitude) in the magnetocrystalline anisotropy at the martensitic phase transformation results in a sharp change of the resonance magnetic field. In a single variant martensite phase the resonance fields satisfy the Kittel’s resonance condition for a thin film with the gyromagnetic factor $g=2.0$. With the magnetic field parallel to the easy $c$-axis of the single variant martensite the resonance is observed only for frequencies larger than $22$ GHz. For the multivariant martensite case we considered the magnetic coupling between the twin variants for the satisfactory Kittel’s fit. We observe a week magnetocrystalline anisotropy in the austenite phase, comparable to the previous reports based on different magnetic measurements.
Acknowledgements
The authors acknowledge the support of OP JAK project No. CZ.02.01.01/00/22_008/0004591 and CSF grant No. 22-22063S.
References
[1] O. Yaln, Ed., “Ferromagnetic Resonance - Theory and Applications.” InTech, Jul. 31, 2013. https://doi.org/10.5772/50583
[2] P. V. Bondarenko et al., “Giant four-fold magnetic anisotropy in nanotwinned NiMnGa epitaxial films,” APL Materials, vol. 11, no. 12. AIP Publishing, Dec. 01, 2023. https://doi.org/10.1063/5.0162561
[3] V. G. Gavriljuk et al., “A study of the magnetic resonance in a single-crystal Ni50.47Mn28.17Ga21.36alloy,” Journal of Physics: Condensed Matter, vol. 18, no. 32. IOP Publishing, pp. 7613–7627, Jul. 31, 2006. https://doi.org/10.1088/0953-8984/18/32/010
