News and posts

Time: Tuesday, October 23rd, 17:00
Speaker: Carl DAVIES, Nijmegen

Ever-increasing demand for faster and more energy-efficient information processing and storage has fuelled intense fundamental studies on the magnetization dynamics displayed by ferroics.  Here, we experimentally and numerically explore how magnetization dynamics within the dielectric bismuth-substituted yttrium iron garnet (Bi:YIG) can be controlled using the thermal load associated with a laser pulse.  In particular, we focus on how a laser-induced diminishment of the growth-induced crystalline anisotropy can affect magnetization reversal and domain-wall motion, thus revealing a new degree of freedom for the optical control of magnetism.
In the first set of experiments and calculations, we show that non-linear precessional magnetization dynamics can be triggered in Bi:YIG via a laser-induced modification of the effective magnetic field. Indeed, the amplitude of the resulting precession can be large enough to allow for magnetic recording at sub-nanosecond timescales, within just one-half of a precessional period. Surprisingly, we observe that the magnetic damping becomes anomalously large during the switching process, rendering the switching route robust.
In the second set of experiments, we explore how the velocity of moving domain walls (DWs) can be affected by a laser pulse.  Using the technique of double photography, we show that the impact of light on the DW velocity is a function of the optical intensity and the velocity itself. The results are explained in terms of interplay between photo-induced localization of Bloch lines, due to local intensity-dependent change of the magnetic anisotropy, and the velocity-dependent Magnus force which inhibits localization of Bloch-lines.

Time: Tuesday, October 23rd, 16:10
Speaker: Sangeeta SHARMA, Berlin

The type of magnetic coupling between the constituent atoms of a solid, i.e. ferromagnetic, anti-ferromagnetic or non-collinear, is one of the most fundamental properties of any magnetic material. This magnetic order is governed by the exchange interaction, which is associated with a characteristic time scale at which spin-flip scattering processes occur and change the intrinsic magnetic structure. These time scales can be determined from the exchange parameters and are of the order of 40-400 fs for transition metal atoms. It is a challenge to manipulate this magnetic order at sub-exchange time scales.
We demonstrate[1,2,3,4,5] that spin transfer driven by inter-site spin-selective charge transfer is one of the key mechanisms that underpins spin manipulation at sub-exchange time scales. This charge flow is induced by optical excitations and represents both the fastest possible response of an electronic system to a laser pulse, as well as a response highly sensitive to pulse intensity and structure. By investigating a wide range of interfaces and multi-sub-lattice magnetic materials we demonstrate a rich phenomena of sub-exchange spin manipulation, including even changing the magnetic order of a material from AFM to FM on femtosecond time scales. We furthermore are able to formulate three simple rules that predict the early time qualitative dynamics of magnetization for ferromagnetic, anti-ferromagnetic and ferri-magnetic materials.

 

[1]   V. Shokeen et al., Phys. Rev. Lett. 119, 107203 (2017)
[2]   K. Krieger et al. J. Phys. Condens. Matter 29, 224001 (2017)
[3]   P. Elliott et al., Scientific Reports 6, 38911 (2016)
[4]   J. K. Dewhurst et al., Computer Phys. Comm. 209, 92 (2016)
[5]   K. Krieger et al., J. Chem. Theory Comput. 11, 4870 (2015)

Time: Tuesday, October 23rd, 15:00
Speaker: Peter OPPENEER, Uppsala

Essential spin operations such as spin-polarized current-induced switching or all-optical helicity-dependent spin switching have already been shown to be realizable, however, the underlying fundamental mechanisms are still poorly understood. In this tutorial I aim to address theoretical advancements in three areas, 1) ultrafast laser-induced demagnetization, 2) all-optical helicity-dependent magnetization switching (AOS), and 3) spin-polarized current or electric-field induced spin switching. Ultrafast laser-induced demagnetization was discovered two decades ago [1], however, the underlying fundamental processes of the fast magnetization decay continue to be debated. I shall discuss various proposed microscopic mechanisms [2-4] as well as ab initiocalculations that might lead to identification of the responsible mechanism(s) and compare to recent experiments.
All-optical switching is a very different, interesting process in which the magnetization of a material is manipulated with ultrashort circularly polarized laser pulses [5,6]. Its origin is still not well understood. In order to progress in this technologically relevant area, ab initiomaterials’ specific theory is needed to know precisely how much spin and orbital magnetization can be induced by a circularly polarized pulse by the inverse Faraday effect in absorbing metals [7]. To understand the process of AOS, multiscale modeling is required, which combines the effects of thermal laser heating with laser-induced magnetization to reach a predictive description of AOS in real materials [8].
Switching the magnetization of a thin layer by a spin-polarized current is the underlying principle of STT-MRAM technology. This process has often been modeled in micromagnetics by adding ad hoca spin-transfer torque (STT) to the phenomenological Landau-Lifshitz-Gilbert (LLG) of magnetization dynamics. The LLG equation has been extended further in recent years, by adding other terms, such as spin-orbit torques (SOTs) [9,10]. It is our aim is to achieve a unified theoretical framework to describe the ultrafast spin dynamics in magnetic systems and quantify the spin dynamics’ quantities on the basis of materials’ specific abinitiocalculations. To this end I discuss recent theory that rigorously shows that the LLG equation can be derived from the fundamental Dirac equation [11], and in addition, leads to the nonrelativistic adiabatic and nonadiabatic STT terms as well as relativistic SOT terms.
The presented theory work has been done together with R. Mondal, M. Berritta, A.K. Nandy, L. Salemi, K. Carva, U. Ritzmann, P. Maldonado, and J. Hurst, and with D. Hinzke and U. Nowak.

[1] E. Beaurepaire, J.-C. Merle, A. Daunois, and J.-Y. Bigot, Phys. Rev. Lett. 76, 4250 (1996).
[2] B. Koopmans et al, Nat. Mater. 9, 259 (2010).
[3] E. Carpene et al, Phys. Rev. B 78, 174422 (2008).
[4] M. Battiato, K. Carva, and P.M. Oppeneer, Phys. Rev. Lett. 105, 027203 (2010).
[5] C.D. Stanciu et al, Phys. Rev. Lett. 99, 047601 (2007).
[6] C.-H. Lambert, S. Mangin et al, Science 345, 1337 (2014).
[7] M. Berritta, R. Mondal, K. Carva, and P.M. Oppeneer, Phys. Rev. Lett. 117, 137203 (2016).
[8] R. John et al, Sci. Rep. 7, 4114 (2017).
[9] I.M. Miron et al. Nature 476, 189 (2011)
[10] H. Kurebayashi et al, Nat. Nanotechn. 9, 211 (2014).
[11] R. Mondal, M. Berritta, and P.M. Oppeneer, Phys. Rev. B 94, 144419 (2016).

Time: Tuesday, October 23rd, 12:00
Speaker: Anna POGREBNA, Nijmegen

The idea to change properties of media with the help of light has been intriguing people for a long time. Recently it was demonstrated that the laser induced ultrafast spin dynamics in ferrimagnetic GdFeCo can be dramatically influenced by the application of high magnetic fields. It was observed that across a critical field called the spin-flop field, the material shows distinct spin dynamics.  The underlying mechanism was attributed to the spin dynamics associated with the two-different magnetic sublattices. However, these measurements were sensitive only to the iron magnetic sublattice. It has already been shown that, one can element specifically probe in the visible spectral range, the laser triggered spin dynamics in the ferrimagnetic TbFeCo. In this work we therefore consider TbFeCo to study the ultrafast spin dynamics associated with the individual magnetic sublattices across the spin-flop transition.
In our experiment, TbFeCo with out-of-plane magnetic anisotropy was studied in an out-of-plane magnetic field up to 28 T. Our study reveals anomalous hysteresis loops and strong field-dependence of the laser-induced dynamics in the vicinity of the compensation point in this ferrimagnet. In particular, the experiment shows that applying fields weaker than the exchange interaction between Tb and Fe, both sublattices show fast spin dynamics (≈ 200 fs), associated with laser-induced demagnetization. If the field is strong enough to trigger a spin-flop transition (≈ 13 T), both the sublattices show a dramatic slowing down of the spin dynamics. Here we explain these observations as manifestations of the first and second order phase transitions in this ferrimagnet.

Time: Tuesday, October 23rd, 11:20
Speaker: Kihiro YAMADA, Nijmegen

Ultrafast magnetization switching with an ultra-short laser pulse attracts much attention for future applications for magnetic recording, operating with low energy consumption and at high speed. Since the pioneering work in ferrimagnetic GdFeCo [1], various systems have been tested, including metals [2], semiconductors [3], and insulators [4].  Recently, it has been demonstrated that also ferromagnetic multilayers display all-optical helicity-dependent switching (AO-HDS) [5], i.e., the magnetization can be controlled using circularly-polarized laser pulses. However, to observe switching, many pulses were required [6]. Here, we demonstrate that the number of pulses required for AO-HDS can be dramatically reduced by using a dual-pulse method. In the experiment, we used a single stack of Pt (3 nm)/Co (0.6 nm)/Pt (3 nm) with perpendicular magnetic anisotropy, which is very widely used for static as well as ultra-fast experiments in spintronics. In order to fully flip the magnetization, we used two pulses fulfilling different roles: the first short (70 fs) linearly-polarized laser pulse for demagnetizing and the second longer (3 ps) circularly-polarized laser pulse for switching. Five such pairs of pulses fully flip the magnetization for a time separation between the two laser pulses of 2 ps, and the fluence of the first and second laser pulse of 2.29 and 1.94 mJ/cm2, respectively. In stark contrast, using only long circularly polarized laser pulses, 100-150 pulses are still necessary. The results suggest that the dual-pulse method is a potential route towards realizing efficient AO-HDS in ferromagnetic metals on an ultrashort time scale.

 

[1] C. D. Stanciu et al., Phys. Rev. Lett. 99, 047601(2007).

[2] S. Mangin et al., Nat. Mater. 13, 286-292 (2014).

[3] A. J. Ramsay et. al., Phys. Rev. Lett. 114, 067202 (2015).

[4] A Stupakiewicz et. al., Nature 542, 71 (2017)

[5] C-H. Lambert et al., Science 345, 1337-1340 (2014).

[6] R. Medapalli et al., Phys. Rev. B 96, 224421 (2017).

Time: Tuesday, October 23rd, 10:30
Speaker: Markus MÜNZENBERG, Greifswald

Based on many observations in a large number of experiments by now we have the different puzzle pieces to understand ultrafast magnetism and to develop new frontiers. THz spintronics and all-optical spin manipulation are becoming more and more feasible. We can build on first applications. Nevertheless the disentangling of different ultrafast processes is still a challenge. Differences in the theoretical models arise from the localized and delocalized nature of ferromagnetism. Transport effects are intrinsically non-local in spintronic devices and at interfaces an delocalized, laser induced magnetization is more a local effect. But, in dependence of the sample this may be very different. I will go though some recent examples from our group to demonstrate the subtle interplay.

Time: Tuesday, October 23rd, 9:20
Speaker: Bert KOOPMANS, Eindhoven

 

Novel schemes for controlling the ferromagnetic state at femtosecond time scales by pulsed laser excitation have received great interest. Driving systems into the strongly non-equilibrium regime, it has been shown possible not only to quench magnetic order, but also to switch the magnetization by single laser pulses – so-called all-optical switching (AOS). More recently, it has been proposed that pulsed laser excitation can also induce spin currents over several to tens of nanometers, which can act as an additional source of sub-picosecond magnetization dynamics. Thereby, an interesting link between the fields of ‘femtomagnetism’ and spintronic transport physics has emerged. In this presentation, further integration of all-optical magnetic control and spintronic will be explored in an attempt to develop hybrid spintronic-photonic devices.
After a general introduction into laser-induced magnetization dynamics, different processes that give rise to laser-induced spin currents will be distinguished. In particular I will address recent experiments that have demonstrated laser-induced spin transfer torqueon a free magnetic layer, using a collinear multilayer configuration consisting of a free in-plane layer on top of a PMA injection layer and separated by a nonmagnetic spacer [1]. As it will be shown, these non-collinear fs spin currents are absorbed within a few nanometers, and thereby provide ideal conditions for exciting THz spin waves. This allowed us to map out the dispersion of the frequency, , and the Gilbert damping,  , of thin Co(B) layers [2,3].
In the second part, AOS in spintronically relevant structureswill be discussed. In particular, focus will be on highly efficient AOS and current-induced domain wall motion in the very same system: Pt/Co/Gd trilayers, displaying strong spin-orbit torques and Dzyaloshinskii-Moriya interaction. It will be shown that the magnetization of this synthetic ferrimagnetic thin film system can be reversed fully deterministically using single fs pulses. Threshold fluences are determined as a function of Co thickness and record-low efficiencies corresponding to below 50 fJ needed to switch a 50x50 nm²are found [3]. Simple modelling using a multi-layer version of the microscopic three-temperature model will be discussed. Moreover we quantitatively determined spin orbit torques and analyzed the coherent current-driven motion of opposite (up-down and down-up) domain walls. Finally, optically writing magnetic information ‘on-the-fly’ will be demonstrated. In these experiments, AOS is established while a constant electrical current is driving the magnetic information along a magnetic racetrack [4]. Such a scenario is envisioned to provide a route towards on-chip spintronic-photonic memories.

 

1. J. Schellekens, K.C. Kuiper, R.R.J.C. de Wit, and B. Koopmans, Nature Comm. 5, 4333 (2014).
2. L.M. Lalieu, P.L.J. Helgers, and B. Koopmans, Phys. Rev. B 96, 014417 (2017).
3. L.M. Lalieu et al., to be published.
4. L.M. Lalieu, M.J.G. Peeters, S.R.R. Haenen, R. Lavrijsen, and B. Koopmans, Phys. Rev. B 96, 220411(R) (2017).
5. L.M. Lalieu et al., to be published.