🔬 materials science

Laser induced ultrafast Co 3d and Ho 4f spin dynamics in CoHo ferrimagnetic alloys

This study experimentally confirms that in ferrimagnetic Co $_{80}$ Ho $_{20}$ alloys, the Holmium 4f spins demagnetize significantly slower (0.87 ps) than the Cobalt 3d spins (0.22 ps), thereby validating the proposed conditions for domain-wall-mediated all-optical toggle switching.

Original authors: Deeksha Gupta, Boonthum Kunyangyuen, Matthias Riepp, Karsten Holldack, Radu-Marius Abrudan, Torsten Kachel, Niko Pontius, Christian Schüßler-Langeheine, Mircea Vomir, Michel Hehn, Nicolas Bergeard

Published 2026-08-28

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CC BY 4.0

Original authors: Deeksha Gupta, Boonthum Kunyangyuen, Matthias Riepp, Karsten Holldack, Radu-Marius Abrudan, Torsten Kachel, Niko Pontius, Christian Schüßler-Langeheine, Mircea Vomir, Michel Hehn, Nicolas Bergeard

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Magnetism is often thought of as a static property, a permanent quality of materials like iron that allows them to stick to a refrigerator door. However, when scientists shine an incredibly fast pulse of laser light on certain magnetic materials, they can disrupt this order in a fraction of a second. This field, known as femtomagnetism, explores how magnetic spins—the tiny internal compasses of atoms—react to sudden bursts of energy. The behavior becomes even more complex in alloys made from two different types of magnetic elements: transition metals and rare-earth metals. In these materials, the two types of atoms are coupled together but point in opposite directions, creating a delicate balance. Researchers have long been trying to understand how to flip the magnetic direction of these materials using a single, ultrafast laser pulse, a process that could revolutionize how data is stored in computers. The key to this flipping mechanism appears to lie in the timing: if one type of atom loses its magnetic strength much faster than the other, it creates a window of opportunity for the entire material to switch its orientation.

A team of researchers set out to test a specific theory about this timing using an alloy made of cobalt and holmium. While previous studies had looked at similar materials containing dysprosium, the ultrafast behavior of holmium had remained a mystery. The scientists needed to know if holmium reacted slowly enough to allow the proposed switching mechanism to work. To find out, they created thin films of the cobalt-holmium alloy and subjected them to intense, femtosecond laser pulses. They then used two different high-speed measurement techniques to watch what happened to the magnetic spins of the cobalt and the holmium atoms separately. One method tracked the cobalt atoms, while the other, more specialized technique allowed them to observe the holmium atoms directly.

The results revealed a stark difference in how the two elements responded to the laser. The cobalt atoms reacted almost immediately, losing their magnetic order in about 0.22 picoseconds. This is an incredibly short duration, a trillionth of a second, but it was fast enough for the cobalt to essentially "forget" its magnetic direction almost instantly. In contrast, the holmium atoms held on to their magnetic alignment much longer, taking roughly 0.87 picoseconds to demagnetize. This means that for a brief moment after the laser hit, the cobalt was completely disordered while the holmium was still trying to maintain its magnetic structure. This time gap is exactly what the researchers were looking for. It supports the idea that the slower holmium atoms can transfer their angular momentum to the faster, disordered cobalt atoms, effectively pushing the entire material to flip its magnetic pole.

The study also confirmed that this behavior is consistent with what has been observed in other alloys, such as those containing dysprosium, but distinct from alloys containing terbium. In the case of terbium, the atoms react too quickly, closing the time window needed for this specific type of switching to occur. By measuring the precise speeds of the cobalt and holmium spins, the researchers provided the missing experimental evidence needed to validate the model for all-optical switching in these materials. Their work shows that the unique speed difference between the two types of atoms is not just a curiosity, but a fundamental requirement for a new way to control magnetism with light. This finding helps narrow down which materials are suitable for future high-speed data storage technologies, moving the field closer to understanding exactly how to engineer magnetic alloys that can be switched with a single flash of light.

Technical Summary: Laser Induced Ultrafast Co 3d and Ho 4f Spin Dynamics in CoHo Ferrimagnetic Alloys

Problem and Context
Transition metal (TM) / rare-earth (RE) ferrimagnetic alloys are critical model systems for understanding all-optical helicity-independent switching (AO-HIS) driven by femtosecond laser pulses. Recent studies have identified a specific toggle switching mechanism in CoDy and CoHo alloys, as well as Co/Gd ultrathin bilayers, which relies on domain-wall motion and coalescence. This mechanism is hypothesized to occur when the Co sublattice is fully demagnetized while the characteristic demagnetization time of the RE sublattice ( $\tau_{RE}$ ) is significantly longer than that of the Co sublattice ( $\tau_{Co}$ ). This temporal separation allows for angular momentum transfer from the RE to the Co sublattices while minimizing angular momentum loss from the Co.

While element-resolved studies have established $\tau_{Co} \approx 0.2$ ps and $\tau_{Dy} \approx 0.6$ ps in CoDy alloys, and theoretical predictions suggest Ho should exhibit a similarly slow response, the ultrafast 4f spin dynamics of Ho in CoHo alloys had remained experimentally unexplored. Furthermore, the absence of AO-HIS in CoTb alloys has been attributed to a potentially shorter $\tau_{Tb}$ , yet direct comparative data for CoHo was lacking to validate the proposed reversal model.

Methodology
The authors investigated a ferrimagnetic $\text{Co}_{80}\text{Ho}_{20}$ amorphous alloy deposited on a $\text{Si}_3\text{N}_4$ membrane with a multilayer structure designed to act as a heat sink and prevent oxidation. The study employed two complementary techniques at room temperature (effective working temperature $\approx 300$ K):

Time-Resolved X-ray Magnetic Circular Dichroism (TR-XMCD): Performed at the BESSY II synchrotron facility, this technique provided element-specific resolution. The team monitored the transmission of 100 fs circularly polarized X-ray pulses tuned to the Co $L_3$ and Ho $M_5$ absorption edges as a function of pump-probe delay. The sample was maintained at a cryostat temperature of 200 K to compensate for laser-induced heating, ensuring the effective temperature remained near room temperature.
Time-Resolved Magneto-Optical Kerr Effect (TR-MOKE): Conducted in ambient air using a 1030 nm pump and 515 nm probe, this method primarily probes the Co sublattice dynamics, serving as a cross-validation for the Co-specific TR-XMCD data.

Key Results
The study successfully extracted characteristic demagnetization times for both sublattices:

Co Sublattice: Both TR-MOKE and TR-XMCD at the Co $L_3$ edge revealed a fast quenching of magnetization with a characteristic time of $\tau_{Co} = 0.22 \pm 0.01$ ps. The demagnetization amplitude was approximately 25%, followed by a rapid recovery.
Ho Sublattice: TR-XMCD at the Ho $M_5$ edge showed a significantly slower response. The Ho magnetization exhibited a maximum demagnetization amplitude of roughly 60% reached over several picoseconds, with a characteristic demagnetization time of $\tau_{Ho} = 0.87 \pm 0.15$ ps.
Dynamics Comparison: The data confirms a distinct temporal separation where the Co sublattice has almost fully recovered by 3 ps, while the Ho sublattice is still in the process of demagnetizing. This behavior mirrors observations in CoDy alloys but contrasts with the faster dynamics reported for Tb in CoTb alloys.

Significance and Claims
The primary contribution of this work is the first experimental verification of the ultrafast spin dynamics in CoHo alloys. The results demonstrate that $\tau_{Ho}$ is substantially longer than $\tau_{Co}$ , satisfying the specific condition proposed by Kunyangyuen et al. for the occurrence of domain-wall-mediated AO-HIS.

The paper claims that these findings support the hypothesis that distinct spin dynamics between the TM and RE sublattices are a prerequisite for this specific switching mechanism. The observed slower dynamics of Ho compared to Tb provides a plausible explanation for why AO-HIS is observed in CoHo and CoDy but not in CoTb alloys. The work adds to the limited experimental database regarding element-resolved ultrafast dynamics in Co-RE alloys, confirming that the demagnetization time of the Co sublattice remains relatively constant across different rare-earth elements, while the RE dynamics vary significantly.

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