Magnetic Structures in Reentrant Spin-Glasses Observed by Transmission Electron Microscopy

Direct observations of the magnetization structure in two reentrant spin-glass alloys [polycrystalline ${\mathrm{Ni}}_{1-x}{\mathrm{Mn}}_x$ and amorphous ${\left({\mathrm{Fe}}_{78}{\mathrm{Mn}}_{22}\right)}_{75}$ ${\mathrm{P}}_{16}$ ${\mathrm{B}}_6$ ${\mathrm{Al}}_3$] near liquid-helium temperature are reported. Magnetic domains several hundred μm long are seen. This structure hardly changes upon cooling through the reentrant transition ($T_f\simeq 35$ K). In situ observation of the domain-wall motions reveals a gradual crossover to a regime of strong irreversibilities. Reentrant spin-glass systems are also characterized by transverse fluctuations within the domain.