Abstract
We report measurements on magnetization reversal in the Fe8 molecular magnet using fast pulsed magnetic fields of 1.5 kT s−1 and in the temperature range of 0.6–4.1 K. We observe and analyze the temperature dependence of the reversal process, which involves in some cases several resonances. Our experiments allow observation of resonant quantum tunneling of magnetization up to a temperature of ∼4 K. We also observe shifts in the maxima of the relaxation within each resonance field with temperature that suggest the emergence of a thermal instability—a combination of spin reversal and self-heating that may result in a magnetic deflagration process. The results are mainly understood in the framework of thermally-activated quantum tunneling transitions in combination with emergence of a thermal instability.
Highlights
Single molecule nanomagnets have been of great interest because of their quantum effects [1,2,3,4,5,6,7] and possible applications in quantum computers [8, 9] or magnetic refrigerants [10]
Dynamics of spin at low temperatures had been studied—including quantum tunneling magnetization (QTM)[2] and electron paramagnetic resonance (EPR) [11, 12]—and has been described through the so-called giant spin approximation [1] that assigns a single spin quantum number, S, to the ground-state spin levels
The measurements of spin dynamics are modeled with thermally-activated quantum tunneling transitions accounting for the self-heating that might result in a magnetic deflagration [19]
Summary
Single molecule nanomagnets have been of great interest because of their quantum effects [1,2,3,4,5,6,7] and possible applications in quantum computers [8, 9] or magnetic refrigerants [10]. An applied field in the direction of the easy axis, Hz, biases the potential well reducing the activation energy and increasing the relaxation rate, at specific resonant values of the magnetic field (Hz = nHR for integer n) corresponding to the crossings of energy levels with opposite spin projections (see, figure 1(b)). Transverse fields H⊥ reduce the activation energy and mix eigenstates of Sz increasing relaxation of magnetization by quantum tunneling. In this letter we investigate the temperature dependence of the magnetization reversal in the Fe8 molecular magnet under fast pulsed magnetic fields of 1.5 kT s−1 This fast sweep rate allows observation of quantum tunneling of the magnetic moment at high temperatures up to ∼ 4 K. The measurements of spin dynamics are modeled with thermally-activated quantum tunneling transitions accounting for the self-heating that might result in a magnetic deflagration [19]
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