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Vortex fluctuations and freezing of dipolar-coupled granular moments in thin ferromagnetic films

Jürgen Kötzler·Detlef Görlitz·Malte Kurfiß·Lars von Sawilski·Elena Y. Vedmedenko
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Abstract

Below the Curie temperature T_c of a Heusler-alloy film, consisting of densely packed, but exchange-decoupled nanograins, the spontaneous magnetization M_s(T) and static in-plane susceptibility χ_{||}(T) increase very slowly signalizing a suppression of magnetization fluctuations. The unpredicted variation χ_{||}(T) ~ G_d^2(T), where G_d ~ M_s^2 is the intergranular dipolar coupling, and also the magnetic freezing observed in the dynamic susceptibility at lower temperatures is quantitatively reproduced by Monte Carlo (MC) simulations with 10^4 dipolar-coupled moments on a disordered triangular lattice. At high temperatures, the MC spin configurations clearly reveal a dense gas of local vortex structures, which at low temperatures condense in regions with stronger disorder. This vortex depletion upon decreasing temperature seems to be responsible for the observed \textit{increase} of the magnetic relaxation time. For weak disorder, the temperature dependence of the MC vorticity and a singularity of the specific heat at T_v=1/2 G_d(T_v)/k_B indicate a thermal transition from a vortex gas to a state with a single vortex center plus linear vortex structures.

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Jürgen Kötzler, Detlef Görlitz, Malte Kurfiß, Lars von Sawilski, Elena Y. Vedmedenko. 2006-05-15. Vortex fluctuations and freezing of dipolar-coupled granular moments in thin ferromagnetic films. https://doi.org/10.1103/physrevb.73.224425

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