Ultrafast behavior of induced and intrinsic magnetic moments in CoFeBPt bilayers probed by element-specic measurements in the extreme ultraviolet spectral range Clemens von Kor Schmising1Somnath Jana1Kelvin Yao1Martin Hennecke1Philippe Scheid1

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Ultrafast behavior of induced and intrinsic magnetic moments in CoFeB/Pt bilayers

probed by element-speciﬁc measurements in the extreme ultraviolet spectral range

Clemens von Korﬀ Schmising,1, ∗Somnath Jana,1Kelvin Yao,1Martin Hennecke,1Philippe Scheid,1

Sangeeta Sharma,1Michel Viret,2Jean-Yves Chauleau,2Daniel Schick,1and Stefan Eisebitt1, 3

1Max-Born-Institut f¨ur Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Straße 2A, 12489 Berlin, Germany

2SPEC, CEA, CNRS, Universit´e Paris-Saclay, CEA Saclay - 91191 Gif sur Yvette, France

3Technische Universit¨at Berlin, Institut f¨ur Optik und Atomare Physik, 10623 Berlin, Germany

(Dated: March 17, 2023)

The ultrafast and element-speciﬁc response of magnetic systems containing ferromagnetic 3dtran-

sition metals and 4d/5dheavy metals is of interest both from a fundamental as well as an applied

research perspective. However, to date no consensus about the main microscopic processes describ-

ing the interplay between intrinsic 3dand induced 4d/5dmagnetic moments upon femtosecond

laser excitation exist. In this work, we study the ultrafast response of CoFeB/Pt bilayers by prob-

ing element-speciﬁc, core-to-valence band transitions in the extreme ultraviolet spectral range using

high harmonic radiation. We show that the combination of magnetic scattering simulations and

analysis of the energy- and time-dependent magnetic asymmetries allows to accurately disentangle

the element-speciﬁc response in spite of overlapping Co and Fe M2,3as well as Pt O2,3and N7

resonances. We ﬁnd a considerably smaller demagnetization time constant as well as much larger

demagnetization amplitudes of the induced moment of Pt compared to the intrinsic moment of

CoFeB. Our results are in agreement with enhanced spin-ﬂip probabilities due to the high spin-

orbit coupling localized at the heavy metal Pt, as well as with the recently formulated hypothesis

that a laser generated, incoherent magnon population within the ferromagnetic ﬁlm leads to an

overproportional reduction of the induced magnetic moment of Pt.

I. INTRODUCTION

Combining ferromagnetic 3dtransition metals with

4d/5dheavy metals leads to magnetic systems with

new macroscopic functionalities. Co/Pt multilayers or

FePt nanoparticles, for example, exhibit very high mag-

netocrystalline anisotropies and are therefore important

model systems for data storage technology. Pt under-

layers can be exploited for spin-orbit torque induced

switching [1, 2] with potential for ultrafast applications

[3]. Light-driven processes allowing the manipulation and

control of ferromagnetic order are receiving renewed at-

tention due to the discovery of helicity-dependent, all-

optical switching in thin Co/Pt multilayers and FePt

based granular ﬁlms [4], with promising new develop-

ments based on tailored double-pulse excitation schemes

[5].

The ultrafast response of optically excited ferromag-

netic transition/heavy metal systems is characterized by

their very diﬀerent atomic spin-orbit coupling strength,

their distinct electronic structures and importantly by

the behavior of intrinsic 3dversus induced 4d/5dmag-

netic moments. The phenomenon of proximity-induced

magnetism is caused by hybridization of the 3dand

4d/5dbands and leads to parallel spin alignment be-

tween intrinsic and induced moment [6, 7]. The complex

interplay of the involved elements after optical excitation

has been studied in a growing number of element-speciﬁc

experiments based on resonant X-ray or extreme ultra-

∗korﬀ@mbi-berlin.de

violet spectroscopy, leading, however, to conﬂicting ob-

servations and corresponding competing theoretical ex-

planations. While experiments with Co/Pt bi- [8] or

multi- [9] layers as well as FePt alloys [10] suggest that

the induced Pt moment follows the dynamics of the fer-

romagnetic transition metal, later work found clear evi-

dence for a distinct, element-speciﬁc response: both, in

an ordered FePt compound [11] as well as in a Co/Pt

multilayer [12]. There, a signiﬁcantly slower dynamics

of Pt was found and rationalized with a higher mobil-

ity of Co or Fe compared to Pt majority electrons lead-

ing to an enhanced demagnetization rate of the transi-

tion metal due to superdiﬀusive spin currents. Addition-

ally, ground-state density-of-state calculations have pre-

dicted that a potential generation of incoherent magnons

would lead to an overproportional reduction of the in-

duced compared to the intrinsic magnetic moments: as

canting of the 3dspins changes the average spin align-

ment between neighboring atoms, the exchange interac-

tions on the 4d/5dheavy metal is reduced, leading to

a reduction of the induced moment. These calculations

showed that if the response of the transition metal is

dominated by Heisenberg-like, transversal excitation, the

induced moment behaves diﬀerently and exhibits a strong

reduction of its amplitude [11, 13]. The aforementioned

study investigating a CoPt alloy, qualitatively conﬁrmed

this hypothesis, revealing slightly larger demagnetization

amplitudes of Pt compared to Co [13]. An explana-

tion based on enhanced spin-orbit coupling of the heavy

metal Pd and a respective stronger spin-ﬂip probability

was invoked to rationalize an accelerated demagnetiza-

tion rate with increasing Pd concentrations in NiPd al-

loys [14]. Finally, we suggested a scenario where optical

arXiv:2210.11390v3 [cond-mat.mtrl-sci] 16 Mar 2023

intersite spin transfer (OISTR) [15], i.e., a laser driven

transfer of minority carriers from Pt to Co, competes

with spin-orbit driven spin-ﬂips locally enhanced on the

Pt atom, resulting in comparable magnetization dynam-

ics of Co and Pt. Experimentally, this hypothesis was

tested in an experiment investigating a CoPt alloy using a

combination of ultrafast magnetic circular dichroism and

helicity-dependent transient absorption spectroscopy in

the XUV spectral range [16]. Also based on OISTR, ab-

initio calculations of the ultrafast magnetization changes

of a FePd3compound predicted element-speciﬁc dynam-

ics with a qualitative dependence on the laser pulse in-

tensity [17].

However, when evaluating and comparing these results

in literature, it is important to acknowledge the diﬀerent

experimental techniques, diﬀerent excitation levels and

– most importantly – the diﬀerent studied sample ge-

ometries, ranging from alloys with diﬀerent stoichiome-

tries to bi- and multilayers with diﬀerent ﬁlm thicknesses

leading to very diﬀerent local environments. While in

reﬂection geometry with an enhanced surface sensitiv-

ity, super-diﬀusive spin currents may lead to a distinct

3dversus 4d/5dresponse [10–13], experiments in trans-

mission geometry yield signals averaged over the entire

sample volume and will be less aﬀected by intralayer spin

transport [8, 9, 16]. OISTR is conﬁned to spin transfer

among nearest neighbors [18, 19] and is hence expected to

be most pronounced in alloys with equal concentrations

of Co and Pt or ultrathin layered systems.

In this study, we report on distinct, layer-dependent

dynamics of a CoFeB(5.3 nm)/Pt(3.5 nm) bilayer ob-

served by element-speciﬁc, transverse magneto-optical

Kerr eﬀect (T-MOKE) measurements in the XUV spec-

tral range using radiation of a high-harmonic generation

(HHG) source. We demonstrate that the combination

of magnetic scattering simulations [20, 21] and an anal-

ysis of the ultrafast response at diﬀerent photon ener-

gies allows accurately separating the element-speciﬁc re-

sponse of Co or Fe and Pt in spite of strongly overlapping

resonances. The experimental results reveal a transient

magnetic state with a signiﬁcantly faster and more ef-

ﬁcient quenching of the induced Pt moments compared

to the response of the transition metal CoFeB alloy. A

systematic comparison with a CoFeB/MgO/Pt system,

exhibiting no induced magnetization of Pt, supports our

analysis.

II. EXPERIMENT

Si/Co40Fe40B20(5.3 nm)/MgO(1 nm)/Pt(3.5 nm) as

well as Si/Co40Fe40B20(5.3 nm)/Pt(3.5 nm) were de-

posited on Si substrates by magnetron sputtering.

Both samples exhibit an in-plane magnetization and

a square hysteresis loop with a small coercive ﬁeld

below 2 mT. The magnetic ﬁlms are protected against

oxidation by the Pt layer or MgO/Pt bilayer. The

thicknesses as well as interface roughnesses of below

� = 45°

XUV

Grating

CCD

Sample

Al filter

0 2 4 6 8 10 12

Depth (nm)

Absorption

(% nm-1)

Pt CoFeB Si

800 nm

CoFeB

Pt Si

MgO

I(B )

A = I(B )

I(B ) I(B )

I(B )

FIG. 1. Schematic of the T-MOKE geometry with an incident

angle θ= 45◦of the p-polarized XUV radiation. After reﬂec-

tion, the XUV pulses are spectrally dispersed by a ﬂat ﬁeld

concave grating and detected by a CCD camera. An external

magnetic ﬁeld, B, is aligned perpendicularly to the incoming

p-polarized XUV light. The sample is excited by laser pulses

at λ= 800 nm in a nearly collinear geometry. The inset shows

the depth-dependent absorption proﬁles of the CoFeB/Pt and

CoFeB/MgO/Pt samples.

0.5 nm have been conﬁrmed by independent X-ray

reﬂectivity measurements.

A schematic of the setup is shown in Fig.1. Intense

laser pulses at a repetition rate of 3 kHz, a central wave-

length of λ= 800 nm and a pulse length of 25 fs are

focused into a cell ﬁlled with neon gas, leading to higher

harmonic radiation. The emitted XUV spectrum consists

of discrete peaks with an approximate spectral width of

200 meV. Additionally, we generate continuous spectra

for the static characterization by averaging multiple ac-

quisitions with varied chirp and peak intensities of the

laser pulses as well as with varying gas pressures. The

p-polarized XUV radiation is incident on the sample at

an angle of θ= 45◦, reﬂected, and then spectrally dis-

persed by a ﬂatﬁeld concave grating and detected by a

charged coupled device (CCD). The magnetization of the

sample is set by applying an external magnetic ﬁeld per-

pendicular to the incident polarization, facilitating the

T-MOKE geometry [22]. The magnetic asymmetry, A,

is calculated as the normalized diﬀerence of two spec-

tra, I, recorded for opposite magnetization directions of

the CoFeB ﬁlm, cf. Fig.1. The time-resolved experi-

ments were performed in a pump-probe geometry using

pump pulses with a central wavelength of 800 nm and a

pulse duration of <30 fs as conﬁrmed by autocorrela-

tion measurements at the sample position. We estimate

the upper bound for the temporal resolution of the ex-

periment to be 35 fs. The absolute values of incident

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UltrafastbehaviorofinducedandintrinsicmagneticmomentsinCoFeB/Ptbilayersprobedbyelement-specicmeasurementsintheextremeultravioletspectralrangeClemensvonKorSchmising,1,SomnathJana,1KelvinYao,1MartinHennecke,1PhilippeScheid,1SangeetaSharma,1MichelViret,2Jean-YvesChauleau,2DanielSchick,1andStefanEise...

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Ultrafast behavior of induced and intrinsic magnetic moments in CoFeBPt bilayers probed by element-specic measurements in the extreme ultraviolet spectral range Clemens von Kor Schmising1Somnath Jana1Kelvin Yao1Martin Hennecke1Philippe Scheid1.pdf

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Ultrafast behavior of induced and intrinsic magnetic moments in CoFeBPt bilayers probed by element-specic measurements in the extreme ultraviolet spectral range Clemens von Kor Schmising1Somnath Jana1Kelvin Yao1Martin Hennecke1Philippe Scheid1

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