Free-Layer Dynamics of a Synthetic Spin Valve With Antiparallel Pinning Evidence of Strong Damping and Shortcomings of the Macrospin Picture
F. Wegelin, A. Krasyuk, D. Valdaitsev, S. A. Nepijko, H. J. Elmers,and G. Schönhense
Applying a biasing magnetic field on a thin micron-sized permalloy layer leads to suppression of domain wall creation and thus to the suppression of Landau-Liftschitz flux-closure pattern formation. The magnetization dynamics of such a magnetically pinned and uniformly magnetized platelet differs from an unbiased particle because the pinning field acts like an additional local bias field.
Such an intrinsic biasing field generating a uniform magnetic ground state can be realized in a spin valve by depositing a multilayer stack of an antiferromagnetic layer (AF) and several ferromagnetic layers (FM) with varying coercivity which are separated by non-magnetic spacers (NM) such as Cu or Ru. An advanced spin valve structure, designed to maximally exploit the GMR (giant magnetoresistive) effect, is depicted in Fig. 1a.The elements have been fabricated by NAOMI-Sensitec GmbH, Mainz according to industrial standards. The in-plane resistance and ΔR/R in dependence of an applied field is shown in Fig. 1b. At the interface between the bottommost CoFe (FM) and the PtMn layer (AF) a unidirectional exchange anisotropy is established by cooling down the stack from above Neél temperature while simultaneously applying a field of 1 Tesla. A second CoFe layer of equal thickness is separated from the first by a Ru layer, whose thickness has been adjusted to evoke a strong antiferromagnetic coupling between the two CoFe layers. This way, their magnetic moments cancel out. Due to the strong antiferromagnetic coupling the CoFe/Ru/CoFe sub-stack is magnetically inert against the field magnitudes applied in this experiment ("artificial ferromagnet"). The magnetically soft CoFe/NiFe free layer is separated from the CoFe/Ru/CoFe sub-stack by a Cu layer providing a weak antiferromagnetic coupling (in its first anti-parallel maximum).
Fig. 1: GMR spin valve multilayer stack (left) and measurement of the MR-effect (ΔR/R = 14%).
The experiment has been performed using stroboscopic XMCD-PEEM in the geometry of Fig. 1c at BESSY II, Berlin. Field pulses Hp of several mT were generating exploiting the Oerstedt field in a coplanar waveguide on which the magnetic platelets have been deposited and shaped using FIB. Ultra-short soft X-ray pulses at the Ni L3 absorption edge were used for probing while imaging the XMCD-contrast with PEEM at sub-100 nm lateral resolution. A digital electronic delay allowed stepwise shifting of the time between pump and probe pulse. The time resolution of 15 ps is not limited by the X-ray pulse length (3 ps) but by the electronic jitter of the trigger pulse (about 12 ps).
Magnetic platelets with dimensions 15 x 10 µm (upper structure) and 10 x 5 µm (lower structure in Fig. 2a) on a microstripline of 20 µm width have been periodically pumped with pulses of few 100 ps width. Both rectangular platelets exhibit an essentially uniform magnetization state with a weak buckling structure but without Landau-type features. A densely packed system of interacting low-angle Neél walls which stabilizes itself is responsible for the buckling state.
The pulse field Hp causes an in-plane rotation of the magnetization whose ground state orientation is initially parallel to the strip line due to the exchange anisotropy field Hexch (Fig. 1c). As the magnetic field pulse propagates through the stripline, the magnetization rotates almost coherently out of its initial ground state orientation and falls back into it after the pulse has passed, as visible in the snapshot series shown in Fig. 3. This critically damped oscillation avoids magnetic ringing and thus is advantageous for fast magnetic switching. The situation is substantially different from the Permalloy case (link zu Krasyuk 4_2).
Fig. 2: Selected images (time in ps) of a sequence of period 2 ns measured with time increment of 15 ps. Bottom: Field pulse profile and magnetic response in terms of the rotation angle Φ of the magnetization.
For particles of the same layer structure but with different shape we observed clear indications of inhomogeneous magnetization precession due to magnetic modes, see Fig.3. Hence the damping does not quench such eigenmodes completely.
Fig. 3: Sequence of selected snapshots of the XMCD contrast of a quadratic 5x5 µm2 and elliptical 6x3 µm2 spin-valve element acquired simultaneously at the indicated time delay. The external field amplitude μ0H=1 mT with time dependence according to (a) is applied along the horizontal x-axis. The easy magnetization direction points along the perpendicular y-axis. The gray level indicates the magnetization component along the x axis. For some delay times, the magnetization vector in the centre of the square particle is indicated by arrows.
This work was supported by the DFG (SPP 1133) and BMBF 03N6500. Thanks are due to the BESSY staff and, in particular, to D. Schmitz HMI, Berlin for excellent support.
Results were published in:
Magnetization dynamics in microscopic spin-valve elements: Shortcomings of the macrospin picture
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