Unconventional picosecond strain pulses resulting from the saturation of magnetic stress within a photoexcited rare earth layer

Abstract

Optical excitation of spin-ordered rare earth metals triggers a complex response of the crystal lattice since expansive stresses from electron and phonon excitations compete with a contractive stress induced by spin disorder. Using ultrafast x-ray diffraction experiments, we study the layer specific strain response of a dysprosium film within a metallic heterostructure upon femtosecond laser-excitation. The elastic and diffusive transport of energy to an adjacent, non-excited detection layer clearly separates the contributions of strain pulses and thermal excitations in the time domain. We find that energy transfer processes to magnetic excitations significantly modify the observed conventional bipolar strain wave into a unipolar pulse. By modeling the spin system as a saturable energy reservoir that generates substantial contractive stress on ultrafast timescales, we can reproduce the observed strain response and estimate the time- and space dependent magnetic stress. The saturation of the magnetic stress contribution yields a non-monotonous total stress within the nanolayer, which leads to unconventional picosecond strain pulses.

Heat Transport without Heating?—An Ultrafast X‐Ray Perspective into a Metal Heterostructure

“…[ 25,48 ] The calculated spatio‐temporal electron and phonon temperature maps (Figure 2) were used as input to calculate the transient strain by integrating a linear masses‐and‐springs model [ 36 ] considering the geometrical limitation of thin films on ultrafast timescales. [ 27,37 ] The modeled strain was leveled to the 330 mW UXRD data by scaling the source term accordingly. The Grüneisen coefficient of Pt and Cu had to be adjusted by approximately ±20% to match the observed expansion between 300 ps to 1 ns, where the temperatures of all metal layers are in equilibrium.…”

“…After this time, even the expansive part of this bipolar strain pulse has completely entered the Cu layer. [ 26,27 ] The additional prolonged compression of Cu must originate from the leading part of a bipolar strain wave generated in the buried Ni layer that is almost four times thicker than the Pt. The combined experimental data of Figure 1 directly show that the Ni layer expands much faster and stronger than the Cu layer, through which the heat is conducted toward the Ni layer in the first place.…”

“…Femtosecond pump‐probe experiments using X‐rays can yield important additional insights, particularly when probing specific subsystems containing the heat energy. [ 22–27 ] Recently, ultrafast X‐ray diffraction (UXRD) experiments revealed the unconventional heat transport in a bilayer of 5 nm Au and 10 nm Ni. It does not matter if the optical absorption occurs in Au or Ni—the electronic heat energy is very rapidly (≲1 ps) captured by the electrons and phonons in Ni, whereas the Au lattice heats up surprisingly slowly on a 100 ps timescale.…”

“…Femtosecond pumpprobe experiments using X-rays can yield important additional insights, particularly when probing specific subsystems containing the heat energy. [22][23][24][25][26][27] Recently, ultrafast X-ray diffraction (UXRD) experiments revealed the unconventional heat transport in a bilayer of 5 nm Au and 10 nm Ni. It does not matter…”

The time-resolved hard X-ray diffraction endstation KMC-3 XPP at BESSY II

“…Rare earths metals like dysprosium are an interesting testbed of magnetic phenomena. The contributions of the spin system to the stress on the lattice show both contributions on a few picosecond timescale and on the nanosecond timescale (von Reppert et al, 2016aReppert et al, , 2020Koc et al, 2017b). In these systems also the thermal transport by the phonons and electrons is influenced by the scattering of the heat conducting quasiparticles from spin excitations: Dy exhibits a ferromagnetic low-temperature phase that is superseded by an antiferromagnetic phase at T C ' 90 K with a Neé l temperature of T N ' 179 K. In the antiferromagnetic phase a pronounced negative thermal expansion suggests a particularly strong spin-lattice interaction due to the magnetic properties of the Dy 4f electrons (Darnell & Moore, 1963).…”