U. Ebels

Electrochemical control and selection of the structural and magnetic properties of cobalt nanowires

In this letter we present a convenient way of controlling the direction of the uniaxial magnetocrystalline anisotropy in arrays of electrodeposited hcp Co nanowires. Combining electron microscopy and ferromagnetic resonance measurements, it is shown that using an appropriate pH of the electrolytic solution, the hcp c axis can be oriented parallel or perpendicular to the wires axes simply by changing the deposition current density or deposition rate. This reorientation of the c axis leads to a drastic change in overall magnetic anisotropy as the crystal anisotropy either competes for perpendicular oriented c axis or adds to the shape anisotropy for parallel oriented c axis

Magnetic micropatterning of FeNi/FeMn exchange bias bilayers by ion irradiation

Ion irradiation is an excellent tool to modify magnetic properties on the submicrometer scale, without modification of the sample topography. We utilize this effect to magnetically pattern exchange bias double layers using resist masks patterned by electron-beam lithography. Ion irradiation through the masks leads to a lateral modification of the magnetization reversal behavior and allows one to study the magnetization reversal as a function of the exchange bias field strength on a single sample. Results are presented on the macroscopic and microscopic magnetization reversal using the magneto-optic Kerr effect and magnetic force microscopy, respectively.

100 ps precessional spin-transfer switching of a planar magnetic random access memory cell with perpendicular spin polarizer

Ultrafast spin-transfer precessional switching between two stable states of a magnetic random access memory device is demonstrated in structures comprising a perpendicularly magnetized polarizing layer (PL⊥), an in-plane magnetized free layer (FL), and an in-plane magnetized analyzing layer (AL) in a PL⊥/spacer/FL/spacer/AL stack. Back and forth switching can be achieved with sub-ns current pulses of the same polarity. The spin-torque influence from the analyzer leads to an asymmetric dependence of the switching properties as a function of the current sign and initial state.

Precessional spin-transfer switching in a magnetic tunnel junction with a synthetic antiferromagnetic perpendicular polarizer

This paper reports sub-nanosecond precessional spin-transfer switching in elliptical magnetic tunnel junction nanopillars. This result is obtained in samples integrating a synthetic antiferromagnetic perpendicular polarizer and a tunnel junction with in-plane magnetized electrodes. The out-of-plane precession of the free layer magnetization results in oscillations of the switching probability as a function of the pulse width. At 9.25 MA/cm2 current density, these oscillations have a period of 1 ns with a high degree of coherence.

Ferromagnetic resonance studies of nickel and permalloy nanowire arrays

Arrays of nickel and permalloy nanowires have been studied by ferromagnetic resonance for frequencies ranging from 10 MHz up to 40GHz using a micro-strip transmission line. Measurements of the transmission coefficient show intense resonance peaks which exhibit a pronounced dependence on the frequency-field values. The interplay between the angular-field resonance frequency and the material parameters has been studied in detail. It is shown that by changing the wire packing density, the dispersion relation can change from strongly dependent to quasi-independent behavior as a function of the applied field direction. This effect is explained in terms of the dipolar coupling between wires and its competition with the wire shape anisotropy

Injection locking of tunnel junction oscillators to a microwave current

Injection locking of a spin transfer nano-oscillator, based on an in-plane magnetized magnetic tunnel junction and generating the frequency f0, to an external signal of varying frequency fe is studied experimentally and with macrospin simulations. It is shown, that if the driving signal has the form of a microwave current, the locking effect is well-pronounced near fe≅2f0, but is almost completely absent near fe≅f0, confirming predictions of analytical theory. It is also shown that noise plays an important role in the locking process, causing the linewidth of the locked oscillation to substantially exceed that of the driving signal.

Spin pumping damping and magnetic proximity effect in Pd and Pt spin-sink layers

Ultrathin films of the heavy elements Pd and Pt are widely used in spintronics as converters between charge and spin currents and vice versa. One open question has been whether the strongly paramagnetic nature of these elements influences the interconversion process. The authors show, using a combination of ferromagnetic resonance and x-ray magnetic circular dichroism measurements, that a related spin current absorption process (spin pumping damping) responds to the presence of ferromagnetic order in the polarizable Pd and Pt layers. They observe a change in the thickness dependence of spin current absorption when the Pd and Pt layers are in direct contact with a ferromagnet, compared with when they are in indirect contact. This change correlates with the presence of induced magnetic moments in Pd and Pt as revealed by element-specific x-ray magnetic circular dichroism magnetometry. The results show that the decoherence process for spin current in such layers is not independent of geometry, but rather is influenced by the presence of induced ferromagnetic order.

Modulation bandwidth of spin torque oscillators under current modulation

For practical applications of spin torque nano-oscillators (STNO), one of the most critical characteristics is the speed at which an STNO responds to variations of external control parameters, such as current or/and field. Theory predicts that this speed is limited by the amplitude relaxation rate Γp that determines the timescale over which the amplitude fluctuations are damped out. In this study, this limit is verified experimentally by analyzing the amplitude and frequency noise spectra of the output voltage signal when modulating an STNO by a microwave current. In particular, it is shown that due to the non-isochronous nature of the STNO the amplitude relaxation rate Γp determines not only the bandwidth of an amplitude modulation, but also the bandwidth of a frequency modulation. The presented experimental technique will be important for the optimisation of the STNO characteristics for applications in telecommunications or/and data storage and is applicable even in the case when the STNO output signal is only several times higher than noise.

Linewidth reduction in a spin-torque nano-oscillator caused by non-conservative current-induced coupling between magnetic layers

We demonstrate by macrospin simulations that in a spin-torque nano-oscillator having synthetic antiferromagnet fixed layer, the non-conservative dynamic coupling between the free and fixed layers caused by spin-torque effect leads to a substantial reduction of the linewidth of the current-induced spin wave mode, involving oscillations in all three magnetic layers. By analysing the phase and amplitude noise extracted from the simulated signal, we prove that the obtained linewidth reduction is related to the reduction of the dimensionless non-linear amplitude-phase coupling parameter ν.

Spin torque driven excitations in a synthetic antiferromagnet

Spin polarized current induced self oscillations have been investigated in both the free layer and the synthetic antiferromagnetic (SAF) pinned layer of spin valve nanopillars. Compared with free layer excitations, the acoustic type SAF excitations are characterized by high emitted power and much narrower linewidth. Furthermore, in contrast to free layer excitations, the SAF in-plane precession mode exhibits an unexpected crossover from redshift (df/dI 0) in frequency f versus current I as the in-plane magnetic field is increased. From simulations we identify this crossover as a signature of large amplitude spin current induced precessional dynamics in the SAF.