Ultrafast demagnetization at high temperatures

Abstract

Time-resolved pump-probe measurements were made at variable heat accumulation in Co/Pd superlattices. Heat accumulation increases the baseline temperature and decreases the equilibrium magnetization. Transient ultrafast demagnetization first develops with higher fluence in parallel with strong equilibrium thermal spin fluctuations. The ultrafast demagnetization is then gradually removed as the equilibrium temperature approaches the Curie temperature. The transient magnetization time-dependence is well fit with the spin-flip scattering model.

Heat diffusion in magnetic superlattices on glass substrates

“…The combination of large light absorption in the metallic film and small glass thermal conductivity can raise T acc above that of a thick metallic film. Heat accumulation and large temperature gradients in our samples explain the observed forces on magnetic domain walls during all-optical switching [26] and how a final demagnetized state can be obtained from both heat accumulation T acc and transient T tr ultrafast demagnetization [31].…”

“…Part of the signal is proportional to the polarization rotation θ M = − πd λ nQ z for a beam propagating along the z−axis due to a magnetization M z ∝ Q z , where Q z is the off-diagonal magneto-optical coefficient in the susceptibility matrix [30]. This part is anti-symmetric in B, can be removed in a combination S = 1 2 S(+B) + S(−B) , is relatively small [31] and is neglected here. The part of the signal symmetric in B is a thermal modulation of the transmittance T, due to the temperature coefficient of refractive index dn dT .…”

Magnetic entropy dynamics in ultrafast demagnetization

“…From this measurement it can be inferred that, when the modulated pump beam is added, the pre-factor of the last term in equation ( 2) is ∂ 2 F ∂P∂λ ∝ ∂M ∂P ∝ ∂M ∂T acc . In the last step above, the pump beam power P modulation has been replaced with a heat accumulation temperature T acc modulation that can be obtained by solving the heat diffusion equation [10][11][12]. The modulation of baseline sample temperature at frequency f c with a chopped pump beam may be compared to variations of temperature with heating or cooling stages [13,14].…”