PHYSICAL REVIEW APPLIED 20, 014068 (2023)

Single-Shot Helicity-Independent All-Optical Switching in Co/Ho Multilayers

Y. Peng,1 G. Malinowski,1,* J. Gorchon,1 J. Hohlfeld,1 D. Salomoni ,2 L.D. Buda-Prejbeanu,2
R.C. Sousa ,2 I.L. Prejbeanu,2 D. Lacour ,1 S. Mangin,1 and M. Hehn1,†
1
Université de Lorraine, CNRS, Institut Jean Lamour, F-54000 Nancy, France
2
Université Grenoble Alpes, CEA, CNRS, Grenoble INP, SPINTEC, 38000 Grenoble, France

(Received 28 March 2023; revised 26 May 2023; accepted 7 July 2023; published 31 July 2023)

Single-shot all-optical helicity-independent switching is established in Co/Ho multilayers extending

the number of material combinations showing this fascinating property. This ability is studied in wedge-
shaped Co/Ho multilayers by varying thicknesses and repetition numbers. Surprisingly, even though the
spin-orbit coupling is larger than in Tb and Dy, which should increase the dissipation of angular momen-
tum to the lattice, the pulse duration versus fluence state diagram is close to the one of the Gd-based
system.

DOI: 10.1103/PhysRevApplied.20.014068

I. INTRODUCTION found. AOHDS is quite general in various ferrimagnetic

and ferromagnetic materials [10–13]. In contrast, AOHIS
Controlling the local magnetic state in storage media
has been, for a long time, observed only in Gd-based RE-
simultaneously with higher-energy efficiency and at a
TM alloys [14,15] or multilayers [16–18]. Recently, it was
faster timescale is a challenge in modern technologies.
discovered that it can also be achieved in the RE-free
In conventional architectures, the magnetic moments are
Heusler alloy Mn2 Rux Ga [19–21] or multilayers including
manipulated using magnetic fields and spin currents and
Tb, Tb-Co, Dy-Co, Tb-Fe and a TM layer made of Co,
the reversal is obtained through a coherent spin precession
Fe, Co-Fe-B, and Py as well [22–24]. In this last class of
process at a subnanosecond timescale [1,2].
multilayers, the pulse duration and fluence state diagram
In the pioneering work by Beaurepaire et al. [3], fem-
and the reversal dynamics contrast with the one observed
tosecond (fs) laser pulse has been demonstrated to demag-
for Gd-based compounds suggesting that the reversal pro-
netize the Ni thin films within less than 1 ps opening
cess differs [24]. Indeed, the critical reversal fluence does
the field of ultrafast magnetization manipulation. Since
not depend on the laser pulse duration and the magneti-
then, considerable attention has been paid to the interac-
zation reversal occurs on a sub-ns timescale while critical
tions between fs laser pulse and spins due to the great
reversal fluence depends on the laser pulse duration and
potential applications in ultrafast spintronics. In particu-
reversal occurs on the ps timescale in the case of Gd-based
lar, all-optical switching (AOS) of magnetization has been
compounds. Since a clear view of the reversal process is
demonstrated experimentally in the rare earth-transition
still missing, additional information from the experiment is
metal (RE-TM) alloy Gdx Fey Co1−x−y using a fs laser
welcome. Here we report results substituting the RE with
pulse without the aid of magnetic field [4]. This intriguing
holmium. This RE has the largest total momentum [25]
experimental observation is highly significant for gener-
and a higher spin-orbit coupling than Tb and Dy [26]. We
ating smaller, faster, and less energy costly technological
therefore expect to strengthen the effects observed for Tb
implementations in information processing and recording
and Dy.
devices. Many works have been carried out in the area of
In this study, we experimentally demonstrate AOHIS
AOS, including its observation in a wide variety of ferro-
in [Co/Ho] multilayer stacks with perpendicular mag-
magnetic or ferrimagnetic materials [5,6], and the explo-
netic anisotropy. Surprisingly, even though the spin-
ration of the parameters for the achievements of AOS such
orbit coupling is larger than in Tb and Dy [26], which
as the fluence, helicity, and duration of laser pulses [7–
should increase the dissipation of angular momentum
9]. To date, both all-optical helicity-dependent (AOHDS)
to the lattice, the pulse duration and fluence state dia-
and independent (AOHIS) magnetization switching were
gram is closer to the one of the Gd-based system. The
study of this system could help to bridge the single-
pulse reversal processes observed, on one hand, in Gd
*
gregory.malinowski@univ-lorraine.fr based and, on the other hand, in the Tb- or Dy-based
†
michel.hehn@univ-lorraine.fr heterostructures.

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Y. PENG et al. PHYS. REV. APPLIED 20, 014068 (2023)

II. EXPERIMENTAL RESULTS coupling between Co and Ho at the Co/Ho interface. As

shown in Fig. 1(c), the MS versus T curve shows a compen-
Wedge-shaped glass/Ta(5)/Pt(5)/[Co(wedge, 0.5t1 –
sation temperature Tcomp at which the magnetic moments
1.5t1 )/Ho(wedge, 0.5t2 –1.5t2 )]N /Pt(1)/Ta(5) multilayers of the Co and Ho layers are equal, resulting in a zero net
(thickness in nm) were fabricated by dc magnetron sput- magnetization. Below Tcomp , the net magnetization is dom-
tering, where N indicates the repetition numbers of Co/Ho inated by Ho while it is dominated by Co above Tcomp . At
bilayers [schematic given in Fig. 1(a)]. Co and Ho layers Tcomp , HC diverges since MS equals zero.
are wedged in the same direction, resulting in a constant Ti:sapphire femtosecond-laser source and regenerative
thickness ratio (t1 : t2 ) all over the wedge. In the following, amplifier were used for the pump laser beam in AOS
the thickness of Co and Ho varies from 0.4 to 1.2 nm, and measurement. Wavelength and repetition rate of the fem-
0.45 to 1.35 nm, respectively, resulting in a 1:1.1 thickness tosecond laser are 800 nm and 5 kHz, respectively. Figure
ratio of the thickness of Co and Ho monolayer. The mag- 1(d) shows the result of the measurement performed on
netic hysteresis loops of the samples are characterized by the [Co(0.46)/Ho(0.52)]4 stack, in which the sample is
static magneto-optic Kerr effect (MOKE) at room tempera- exposed to four subsequent laser pulses with laser flu-
ture with a magnetic field applied perpendicular to the film ence F = 5.22 mJ/cm2 . We can clearly observe a switched
plane. All samples are perpendicular to film magnetiza- area after the first laser pulse. After the second pulse,
tion and exhibit high squareness of the hysteresis loops and the magnetic moments are the reverse to the initial state,
100% remanence [Fig. 1(b)]. The positive sign of the Kerr indicating a good sign of toggle magnetization switch-
rotation at high positive field shows that the multilayer is ing. Similar results are obtained in samples with slightly
dominated by the Co sublattice, although the thickness of thicker thicknesses [see [Co(0.6)/Ho(0.7)]4 ]. By further
the Ho layer is thicker than the Co layer. This is mainly increasing the thicknesses of Co and Ho, the magneti-
ascribed to the low Curie temperature of the Ho element, zation switching area after the first laser pulse becomes
which is around 19 K [27]. much less smooth [see [Co(0.81)/Ho(0.91)]4 ]. Moreover,
The saturation magnetization, MS , and coercive field, the subsequent laser pulses lead to the formation of a mul-
HC , have the characteristic variations as a function of tidomain structure, suggesting the loss of magnetization
temperature, T, shown by ferrimagnetic alloys. This toggle switching when the thickness of Co and Ho reaches
is expected from the antiferromagnetic (AF) exchange 0.8 and 0.9 nm, respectively. The shape of the domain after

(a) (b) (c)

Normalized Kerr rotation
(arb. units)

(d) (e) (f)

FIG. 1. (a) Schematic of the wedged [Co/Ho]N . The Co and Ho layers are wedged in the same direction with graded monolayer
thickness spanning 0.5t1 < tCo < 1.5t1 and 0.5t2 < tHo < 1.5t2 . The square brackets contain the bilayer structure that is repeated N times
within each sample. (b) Polar magneto-optical Kerr effect measurement performed at room temperature on [Co(0.46)/Ho(0.52)]4 ;
(c) saturation magnetization Ms and coercivity Hc as a function of temperature, which is performed on the same stack as (b). AOS
measurements performed on [Co/Ho]4 stacks with different Co and Ho thicknesses. (d) [Co(0.46)/Ho(0.52)]4 ; (e) [Co(0.6)/Ho(0.7)]4 ;
(f) [Co(0.81)/Ho(0.91)]4 (thickness in nm). The dotted circle regions correspond to the spots where the structure is excited by each
single laser pulse (fluence 5.22 mJ/cm2 , pulse duration 50 fs). Red and blue colors represent the net magnetization pointing up and
down.

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of Co (Ho) monolayer extends to 0.31 nm (0.34 nm). For

N = 4 and N = 5, the reversal occurs for all the thicknesses
in the wedge.
Increasing the laser fluence leads to the appearance of a
multidomain state at the center of the spot while an outer
Domain/circular domain

ring maintains the single-shot switching. As a result, we

can define Fswitch and Fmulti , the minimum fluence to get
the single-shot switching and to get the multidomain state,
respectively. We investigate the layer thickness depen-
dence of F switch and F multi for a fixed pulse duration of
50 fs. In Fig. 3(c), we can clearly see that F switch increases
almost linearly with the total thickness of the multilayer.
In the case of F multi , the variation with thickness is not

(a) (b)

FIG. 2. Analysis of the domain wall length and domain area

compared to a well-defined circular domain. Insets are the
domain patterns after the first pulse for different Co thickness
(the thickness ratio of Co and Ho is constant and equal to 1: 1.1).
An increase of the domain wall length is obviously associated to
a decrease of the domain area.

an odd number of pulses is not a 100% well-defined circle

and some small domains at the rim appear after an even
number of pulses.
We made a statistical analysis of the area of the reversed
domain and domain wall length divided by the val-
ues, which would be obtained for a well-defined circular
domain. The ratios as a function of Co thickness are
reported in Fig. 2. It exhibits that domain area ratio is (c)
below 1 and becomes smaller while domain wall length
ratio is above 1 and becomes larger with increasing the
thickness of Co. While the analysis of this result is beyond
the scope of this paper, we could attribute it to a change of
texture in the multilayer with layer thickness or change of
size of stable domains through the variation of saturation
magnetization, anisotropy, or effective exchange coupling.
We conclude that the thickness of Co and Ho layers are
critical to observe AOHIS and they have to be less than 0.8
and 0.9 nm, respectively, when repetition number N = 4.
Otherwise, the domain shape becomes dendritic and the
magnetization switching becomes randomized.
Based on the above results, we fixed the thicknesses of
Co layers ranging from 0.25 to 0.75 nm and Ho layers
ranging from 0.28 to 0.83 nm, while keeping the same ratio FIG. 3. (a) The range of Co and Ho layer thicknesses (shown
by light red color) where single-shot switching occurs in
fixed to 1:1.1 and deposited a series of samples with repeti- [Co/Ho]N multilayers, where the repetition number of the
tion of the bilayer varies from 2 to 5. As shown in Fig. 3, all Co/Ho bilayer N equals to 2, 3, 4, 5, respectively. (b) Single-
the samples show nice single-pulse switching. For N = 2, pulse reversal in V/Ta(5)Pt(5)[Co(0.5)/Ho(0.55)]N /Pt(1)Ta(5)
below a certain Co (Ho) single-layer thickness of approx- at 3.75 mJ/cm2 and pulse duration of 50 fs measured at location
imately 0.56 nm (0.62 nm), only the multidomain state is of blue and green dot. In each image, first line pulse 1 and 2,
observed. For larger Co and Ho thicknesses, a clear toggle second line pulse 3 and 4. (c) Variation of Fswitch and Fmulti as a
switching is observed. For N = 3, this minimum thickness function of total Co/Ho thickness for pulse duration of 50 fs.

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Y. PENG et al. PHYS. REV. APPLIED 20, 014068 (2023)

(a) (b)

Normalized Kerr rotation

Normalized Kerr rotation

Normalized Kerr rotation

Normalized Kerr rotation

(arb. units)
(arb. units)

(arb. units)

(arb. units)
Applied field (Oe) Applied field (Oe) Applied field (Oe) Applied field (Oe)

FIG. 4. Single-pulse reversal in (a) V/Ta(5)Pt(5)[Co(0.5)/Ho(0.55)]4 /Pt(1)Ta(5) and (b) V/Ta(5)Pt(5)[Co(0.5)/

Ho(0.55)]5 /Pt(1)Ta(5) after anneal. Top line: region of space showing different hysteresis loops: orange – square hysteresis,
blue – remanence equal to one with high field tails, green – hysteresis loop characteristic of stripe domains. Middle line: hysteresis
loop measured at location of blue and red dot, characteristic of the three regions of space. Bottom line: Kerr image after first and
second laser shot pulse with pulse duration of 50 fs (blue areas are magnetized down, red areas are magnetized up).

monotonic: while it increases when the total thickness is can explain the behavior observed by a decrease of the
below 4.5 nm, it saturates and can even decrease when exchange constant or of the magnetic anisotropy.
thickness increases. As a result, when thickness is too high, To gain further insight of the dependence of magneti-
F multi < F switch and only a demagnetized state is observed. zation switching on the laser pulse parameters, the state
We think that this trend is directly linked to the variation diagram describing the evolution of both Fswitch and Fmulti
of the stable domain size with thickness in thin film with as a function of laser pulse durations has been measured
PMA: in the range of thickness studied here, the size of (Fig. 5). In the state diagram, three regions including
domains decreases when thickness increases. As a result, AOHIS, multidomain state or no reversal, are categorized
even if a switch of magnetization occurs, a stabilization depending on the various magnetic states obtained after
of small domains with sizes less than the spot size occurs laser pulse irradiation. At the 35 fs, the laser fluence win-
during thermal cooling in the thickest film leading to a dow allowing for AOHIS is widest. By increasing the pulse
demagnetized state. As a result, the best thickness range length, Fswitch is increased, while Fmulti is almost constant.
for HIAOS to be observed is between 2 and 4.5 nm. Ideally, once Fswitch = Fmulti , the maximum pulse dura-
Another piece of evidence that the magnetic properties tion τmax for which AOHIS is reached. Here, the value
play a role on the single-pulse switching is reported in Fig. of τmax is around 1.1 ps above which only the demagne-
4. Samples measured in Fig. 3 with N = 4 and 5 have been tized multidomain structures can be achieved. This leads to
annealed at 200 °C during 1 h without any applied field. an approximately triangle shape of switching area, which
While PMA is conserved for all thicknesses, a large vari- is very similar with the one reported in Gdx Fey Co1−x−y
ety of hysteresis loops could be observed, ranging from alloys [14], even though the samples do not host Gd. In
square hysteresis to hysteresis loop characteristic of stripe contrast, the samples host a heavy RE with strong spin-
and bubble domains. In all cases, a Co-dominant response orbit coupling, similar to Tb and Dy but the state diagram
is kept, as before the annealing. Kerr images after the first of Co/Ho is significantly different with the one reported in
and second laser shot pulse with pulse duration of 50 fs multilayers including Tb, Tb-Co, Dy-Co, Tb-Fe and a TM
reveal that single-pulse switching no longer holds and is layer made of Co, Fe, Co-Fe-B and Py as well [22–24].
replaced by the formation of small domains and a demag- As a result, the current understanding of the HIAOS pro-
netized state. The effect of annealing on the interfaces cess in Gd that low spin-orbit and damping materials are
and crystalline structure has to be more deeply checked. required does not depict the behavior of Ho that needs to be
However, if interface alloying is considered, a decrease of understood. It suggests that other parameters like demag-
saturation magnetization is expected by the increase of Ho netization times, Curie temperature have to be explored to
momentum. So, only a decrease of the domain-wall energy gain more insights into the process.

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SINGLE-SHOT HELICITY. . . PHYS. REV. APPLIED 20, 014068 (2023)

(a) (b)

(c) (d)

FIG. 5. State diagram: switching fluence F switch (open black circle) and multidomain fluence F multi (full red sphere) as a func-
tion of the pulse duration for a single linearly polarized laser pulse. (a) [Co(1.21)/Ho(1.36)]2 ; (b) [Co(0.45)/Ho(0.49)]3 ; (c)
[Co(0.46)/Ho(0.52)]4 ; (d) [Co(0.39)/Ho(0.43)]5 .

III. CONCLUSIONS Est through its FRCR call (NanoTeraHertz and RaNGE
projects), by the impact project LUE-N4S part of the
In this study, we establish single-shot all-optical
French PIA project “Lorraine Université d’Excellence”,
helicity-independent switching is Co/Ho multilayers
reference ANR-15IDEX-04-LUE and by the “FEDER-
extending the number of material combinations showing
FSE Lorraine et Massif Vosges 2014-2020”, a European
this fascinating property. By varying the thicknesses of
Union Program. D.S. has received funding from the Euro-
Co and Ho, the number of repetitions of the bilayers, we
pean Union’s Horizon 2020 research and innovation pro-
show that Fswitch increases with the total thickness, while
gramme under Marie Skłodowska-Curie Grant Agreement
Fswitch − Fmulti , the windows of fluence for which AOHIS
No. 861300 (COMRAD).
occurs, has an optimum in total thickness ranging from 2
to 4.5 nm. The state diagram describing the evolution of
both Fswitch and Fmulti as a function of laser pulse duration
is very similar to that reported in Gdx Fey Co1−x−y alloys, [1] H. Liu, D. Bedau, D. Backes, J. A. Katine, J. Langer, and
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