Electrical detection of spin pumping due to the precessing magnetization of a single ferromagnet Costache, M. V.; Sladkov, M.; Watts, S. M.; van der Wal, C. H.; van Wees, B. J.

We demonstrate on-chip resonant driving of large cone-angle magnetization precession of an individual nanoscale permalloy element. Strong driving is realized by locating the element in close proximity to the shorted end of a coplanar strip waveguide, which generates a microwave magnetic field. We used a microwave frequency modulation method to accurately measure resonant changes of the dc anisotropic magnetoresistance. Precession cone angles up to 9 0 are determined with better than one degree of resolution. The resonance peak shape is well-described by the Landau-Lifshitz-Gilbert equation. PACS numbers:The microwave-frequency magnetization dynamics of nanoscale ferromagnetic elements is of critical importance to applications in spintronics. Precessional switching using ferromagnetic resonance (FMR) of magnetic memory elements 1 , and the interaction between spin currents and magnetization dynamics are examples 2 . For device applications, new methods are needed to reliably drive large angle magnetization precession and to electrically probe the precession angle in a straightforward way.We present here strong on-chip resonant driving of the uniform magnetization precession mode of an individual nanoscale permalloy (Py) strip. The precession cone angle is extracted via dc measurement of the anisotropic magnetoresistance (AMR), with angular resolution as low as one degree. An important conclusion from these results is that large precession cone angles (up to 9 0 in this study 3 ) can be achieved and detected, which is a key ingredient for further research on so-called spin-battery effects 4,5 . Moreover, measurements with an offset angle between the dc current and the equilibrium direction of the magnetization show dc voltage signals even in the absence of applied dc current, due to the rectification between induced ac currents in the strip and the timedependent AMR.Recently we have demonstrated the detection of FMR in an individual, nanoscale Py strip, located in close proximity to the shorted end of a coplanar strip waveguide (CSW), by measuring the induced microwave voltage across the strip in response to microwave power applied to the CSW 6 . However, detailed knowledge of the inductive coupling between the strip and the CSW is required for a full analysis of the FMR peak shape, and the precession cone angle could not be quantified. In other recent experiments, dc voltages have been measured in nanoscale, multilayer pillar structures that are related to the resonant precessional motion of one of the magnetic layers in the pillar 7,8 . In one case the dc voltage is generated by rectification between the microwave current applied through the structure and its time-dependent giant magnetoresistance (GMR) effect 8 . Similar voltages FIG.

We describe electrical detection of spin pumping in metallic nanostructures. In the spin pumping effect, a precessing ferromagnet attached to a normal metal acts as a pump of spin-polarized current, giving rise to a spin accumulation. The resulting spin accumulation induces a backflow of spin current into the ferromagnet and generates a dc voltage due to the spin dependent conductivities of the ferromagnet. The magnitude of such voltage is proportional to the spin-relaxation properties of the normal metal. By using platinum as a contact material we observe, in agreement with theory, that the voltage is significantly reduced as compared to the case when aluminum was used. Furthermore, the effects of rectification between the circulating rf currents and the magnetization precession of the ferromagnet are examined. Most significantly, we show that using an improved layout device geometry, these effects can be minimized.

The dynamics of non-equilibrium spin accumulation generated in metals or semiconductors by rf magnetic field pumping is treated within a diffusive picture. The dc spin accumulation produced in a uniform system by a rotating applied magnetic field or by a precessing magnetization of a weak ferromagnet is in general given by a (small) fraction of ω, where ω is the rotation or precession frequency. With the addition of a neighboring, field-free region and allowing for the diffusion of spins, the spin accumulation is dramatically enhanced at the interface, saturating at the universal value ω in the limit of long spin relaxation time. This effect can be maximized when the system dimensions are of the order of 2πD/ω, where D is the diffusion constant. We compare our results to the interface spin pumping theory of A. Brataas et al. [Phys. Rev. B 66, 060404(R) (2002)].

In response to an external, microwave-frequency magnetic field, a paramagnetic medium will absorb energy from the field that drives the magnetization dynamics. Here we describe a new process by which an external spin-injection source, when combined with the microwave field spin pumping, can drive the paramagnetic medium from one that absorbs microwave energy to one that emits microwave energy. We derive a simple condition for the crossover from absorptive to emissive behavior. Based on this process, we propose a solid-state, paramagnetic device in which microwave amplification by stimulated emission of radiation is driven by spin injection.