Laser-driven Spin Dynamics in Ferrimagnets: How does the Angular Momentum flow?

When exposed to intense laser pulses, the magnetization of a material can be manipulated very fast. Fundamentally, magnetization is connected to the angular momentum of the electrons in the material. A team of researchers led by scientists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) has now been able to follow the flow of angular momentum during ultrafast optical demagnetization in a ferrimagnetic iron-gadolinium alloy in great detail, in order to understand the fundamental processes and their speed limits. The results were published in Physical Review Letters.

When the magnetization of a ferromagnetic body is changed, it wants to start to rotate - this connection between the magnetization and the angular momentum was already observed in an experiment by Einstein and de Haas in 1915. The reason for this phenomenon is the fact that on a microscopic level, magnetization is intrinsically linked to the angular momentum of electrons. Unlike Einstein and de Haas at the time, physicists now know that both the orbital motion of the electron around the atomic nucleus as well as its spin - which is a purely quantum mechanical property which can to some extent be imagined as the rotation of the electron about its own axis - generate the magnetization. In fact, in a ferromagnetic solid the spin generates the lion’s share of the magnetization. When angular momentum is conserved, a change in magnetization must thus be accompanied with a change of other forms of angular momentum in the system - in the Einstein-de Haas Experiment, this was the resulting rotation of a suspended magnet after its magnetization had been changed. On a microscopic level, it is the corresponding motion of the atoms which constitutes the final reservoir of angular momentum.

Illumination with an ultrashort laser pulse is a means to demagnetize a material very fast - for the prototypical ferromagnets iron, cobalt and nickel, for example, the magnetization is extinguished within about one picosecond (10-12 s) after the laser pulse has hit the material. This has led to the question, through which channels the angular momentum associated with the magnetization is transferred to other reservoirs during the short time available. Researchers from MBI in Berlin together with scientists from Helmholtz Zentrum Berlin and Nihon University, Japan, have now been able to follow this flow of angular momentum in detail for an iron-gadolinium alloy. In this ferrimagnetic material, adjacent iron (Fe) and gadolinium (Gd) atoms have magnetization with opposite direction. The researchers have used ultrashort x-ray pulses to monitor the absorption of circularly polarized x-rays by the Fe and Gd atoms as a function of time after previous laser excitation. This approach is unique in that it allows tracking the magnetic moment during the ultrafast demagnetization at both types of atoms individually. Even more, it is possible to distinguish angular momentum stored in the orbital motion vs. in the spin of the electrons when the respective absorption spectra are analyzed.

With this detailed “x-ray vision”, the scientists found that the demagnetization process at the Gd atoms in the alloy is significantly faster than in pure Gd. This is, however, not due to an exchange of angular momentum between the different types of atoms, as one could have suspected based on their antiparallel alignment. “We understand the accelerated response of Gd as a consequence of the very high temperatures generated among the electrons within the alloy,” says Martin Hennecke, the first author of the study. Interestingly, a “reshuffling” of angular momentum between the spin and orbital motion of the electrons could also not been detected when following the laser-induced demagnetization with a temporal resolution of about 100 femtoseconds (10-13 s) - this is true locally at all the Fe and the Gd atoms. So how does the angular momentum flow? “Obviously, all angular momentum is fully transferred to the atomic lattice,” says Hennecke. “In line with recent theoretical predictions, the spin angular momentum is first transferred to the orbital motion at the same atom via the spin-orbit interaction, but we cannot see it accumulate there as it is directly moving on to the atomic lattice.” The latter process has recently been theoretically predicted to be as fast as 1 femtosecond, and the detailed experiments now confirm that this last transfer step is indeed not a bottleneck in the overall flow of angular momentum.

Given that short laser pulses can also be used to permanently switch magnetization and thus write bits for magnetic data recording, the insight in the dynamics of these fundamental mechanisms is of relevance to develop new approaches to write data to mass data storage media much faster than possible today.

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Publications since 2025

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A1-P-2025.01
Melting, bubblelike expansion, and explosion of superheated plasmonic nanoparticles

S. Dold, T. Reichenbach, A. Colombo, J. Jordan, I. Barke, P. Behrens, N. Bernhardt, J. Correa, S. Düsterer, B. Erk, T. Fennel, L. Hecht, A. Heilrath, R. Irsig, N. Iwe, P. Kolb, B. Kruse, B. Langbehn, B. Manschwetus, P. Marienhagen, F. Martinez, K.-H. Meiwes-Broer, K. Oldenburg, C. Passow, C. Peltz, M. Sauppe, F. Seel, R. M. P. Tanyag, R. Treusch, A. Ulmer, S. Walz, M. Moseler, T. Möller, D. Rupp, B. v. Issendorff

Physical review letters 134 (2025) 136101/1-7

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A3-P-2025.01
Second-harmonic generation in OP-GaAs0.75P0.25 heteroepitaxially grown from the vapor phase

L. Wang, S. R. Vangala, S. Popien, M. Beutler, J. M. Mann, V. L. Tassev, E. Büttner, V. Petrov

CrystEngComm 27 (2025) 1373-1376

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A3-P-2025.02
Diode-pumped Kerr-lens mode-locked Yb:MgWO4 laser

H.-Y. Nie, Z.-L. Lin, P. Loiko, H.-J. Zeng, L. Zhang, Z. Lin, G. Z. Elabedine, X. Mateos, V. Petrov, G. Zhang, W. Chen

Optics Letters 50 (2025) 1049-1052

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A3-P-2025.03
Growth, anisotropy, and spectroscopy of Tm3+ and Yb3+ doped MgWO4 crystals

G. Z. Elabedine, R. M. Solé, S. Slimi, M. Aguiló, F. Díaz, W. Chen, V. Petrov, X. Mateos

CrystEngComm 27 (2025) 1619-1631

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A3-P-2025.04
Growth, structure, spectroscopic, and laser properties of Ho-doped yttrium gallium garnet crystal

S. Slimi, H. Yu, H. Zhang, C. Kränkel, P. Loiko, R. M. Solé, M. Aguiló, F. Díaz, W. Chen, U. Griebner, V. Petrov, X. Mateos

Optics Express 33 (2025) 2529-2541

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A3-P-2025.05
Growth, spectroscopy and laser operation of disordered Tm,Ho:NaGd (MoO4)2 crystal

G. Z. Elabedine, Z. Pan, P. Loiko, H. Chu, D. Li, K. Eremeev, K. Subbotin, S. Pavlov, P. Camy, A. Braud, S. Slimi, R. M. Solé, M. Aguiló, F. Díaz, W. Chen, U. Griebner, V. Petrov, X. Mateos

Journal of Alloys and Compounds 1020 (2025) 179211/1-12

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A3-P-2025.06
Kerr-lens mode-locked, diode-pumped Yb,Gd:YAP laser generating 23 fs pulses

H.-Y. Nie, P. Zhang, P. Loiko, Z.-L. Lin, H.-J. Zeng, G. Zhang, Z. Li, X. Mateos, H.-C. Liang, V. Petrov, Z. Chen, W. Chen

Optics Express 33 (2025) 11793-11799

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A3-P-2025.07
Nanoindentation and laser-induced optical damage tests of CdSe nonlinear crystals

G. Exner, A. Carpenter, K. Cissner, A. Hildenbrand-Dhollande, S. Schmitt, A. Grigorov, M. Piotrowski, S. Guha, V. Petrov

Journal of the Optical Society of America B 42 (2025) A10-A14

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A3-P-2025.08
Phase-matching properties of AgGa(Se1-xTex)2 for SHG of a CO2 laser

K. Kato, V. Petrov, K. Miyata

Proceedings of SPIE 13347 (2025) 133470S/1-4

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A3-P-2025.09
Phase-matching properties of ZnSiAs2 in the mid-IR

T. Okamoto, N. Umemura, K. Kato, V. Petrov

Proceedings of SPIE 13347 (2025) 133470C/1-5

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A3-P-2025.10
Direct generation of 3.5 optical-cycle pulses from a rare-earth laser

N. Zhang, Y. Wang, H. Ding, F. Liang, Y. Zhao, J. Xu, H. Yu, H. Zhang, V. Petrov

Optics Letters 50 (2025) 3150-3153

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A3-P-2025.11
Power scaling of a non-resonant optical parametric oscillator based on periodically poled LiNbO3 with spectral narrowing

S. Das, T. Temel, G. Spindler, A. Schirrmacher, I. B. Divliansky, R. T. Murray, M. Piotrowski, L. Wang, W. Chen, O. Mhibik, V. Petrov

Optics Express 33 (2025) 5662-5669

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A3-P-2025.12
Sub-40-fs diode-pumped ytterbium-doped mixed rare-earth calcium oxoborate laser

H.-J. Zeng, Z.-L. Lin, H. Lin, P. Loiko, L. Zhang, Z. Lin, H.-C. Liang, X. Mateos, V. Petrov, G. Zhang, W. Chen

Optics Express 33 (2025) 17965-17975

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A3-P-2025.13
Spectroscopy and SESAM mode-locking of a disordered Yb:Gd2SrAl2O7 crystal

H.-J. Zeng, Z.-L. Lin, P. Loiko, F. Yuan, G. Zhang, Z. Lin, X. Mateos, V. Petrov, W. Chen

Optics Express 33 (2025) 15057-15066

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A3-P-2025.14
Watt-level, 1.6 ps χ(2)-lens mode-locking of an in-band pumped Nd:LuVO4 laser

H. Iliev, V. Aleksandrov, V. Petrov, L. S. Petrov, H. Zhang, H. Yu, I. Buchvarov

Optics Express 33 (2025) 17773-17781

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A3-P-2025.15
Refined phase-matching predictions for AgGa1-xInxS2 mixed chalcopyrite crystals

K. Kato, K. Miyata, V. Petrov

Journal of the Optical Society of America B 42 (2025) A6-A9

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A3-P-2025.16
35-fs diode-pumped mode-locked ytterbium-doped multi-component alkaline-earth fluoride laser

Z. Zhang, Z.-Q. Li, P. Loiko, H.-J. Zeng, G. Zhang, Z.-L. Lin, S. Normani, A. Braud, F. Ma, X. Mateos, H.-C. Liang, V. Petrov, D. Jiang, L. Su, W. Chen

Optics Letters 50 (2025) 1835-1838

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A3-P-2025.17
Diode-pumped few-optical-cycle laser based on an ytterbium-doped disordered strontium yttrium borate crystal

H. Zeng, Z. Lin, S. Sun, P. Loiko, H. Lin, G. Zhang, Z. Lin, C. Mou, X. Mateos, V. Petrov, W. Chen

Optics Letters 50 (2025) 2203-2206

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A3-P-2025.18
Refined Sellmeier and thermo-optic dispersion formulas for CdGeAs2

K. Kato, K. Miyata, V. Petrov

Journal of the Optical Society of America B 42 (2025) A24-A28

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A3-P-2025.19
Diode-pumped mode-locked Yb:Ca3La2(BO3)4 laser generating 35 fs pulses

H.-J. Zeng, Z.-L. Lin, G. Zhang, Z. Pan, P. Loiko, X. Mateos, V. Petrov, H. Lin, W. Chen

Optics Express 33 (2025) 22988-22996

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