Bolometer detection of magnetic resonances in nanoscaled objects.

We report on a nanoscaled thermocouple (ThC) as a temperature sensor of a highly sensitive bolometer for probing the dissipative damping of spin dynamics in nanosized Permalloy (Py) stripes. The Au-Pd ThC based device is fabricated by standard electron beam lithography on a 200 nm silicon nitride membrane to minimize heat dissipation through the substrate. We show that this thermal sensor allows not only measurements of the temperature change on the order of a few mK due to the uniform resonant microwave (MW) absorption by the Py stripe but also detection of standing spin waves of different mode numbers. Using a 3D finite element method, we estimate the absorbed MW power by the stripe in resonance and prove the necessity of using substrates with an extremely low heat dissipation like a silicon nitride membrane for successful thermal detection. The voltage responsivity and the noise equivalent power for the ThC-based bolometer are equal to 15 V W(-1) and 3 nW Hz(-1/2), respectively. The ThC device offers a magnetic resonance response of 1 nV/(µ(B) W) corresponding to a sensitivity of 10(9) spins and a temperature resolution of 300 µK under vacuum conditions.

An Ultrasensitive Molecular Detector for Direct Sensing of Spin Currents at Room Temperature.

The experimental analysis of pure spin currents at interfaces is one major goal in the field of magnonics and spintronics. Complementary to the established Spin-Hall effect using the spin-to-charge conversion in heavy metals for information processing, we present a novel approach based on spin pumping detection by an interfacial, resonantly excited molecular paramagnet adsorbed to the surface of the spin current generating magnet. Here, we show that the sensitivity of this electron paramagnetic resonance (EPR) detector can be enhanced by orders of magnitude through intramolecular transfer of spin polarization at room temperature. Our proof-of-principle sample consists of octahedral-shaped ferrimagnetic Fe3O4 nanoparticles covered by oleic acid (OA) which has two paramagnetic centers, S1 and S2. S1 arises from the chemisorption of OA and is located directly at the interface to Fe3O4. S2 originates from radical formation at the center of the molecule close to the double bond of oleic acid and is not influenced by chemisorption. Using ferromagnetic resonance (FMR) excitation of the Fe3O4 nanoparticles to pump spins into S1, a population inversion of the spin-split levels of S2 is formed, vastly enhancing the detection sensitivity on the atomic scale.

Element-specific visualization of dynamic magnetic coupling in a Co/Py bilayer microstructure.

We present the element-specific and time resolved visualization of uniform ferromagnetic resonance excitations of a Permalloy (Py) disk-Cobalt (Co) stripe bilayer microstructure. The transverse high frequency component of the resonantly excited magnetization is sampled in the ps regime by a combination of ferromagnetic resonance (FMR) and scanning transmission X-ray microscopy (STXM-FMR) recording snapshots of the local magnetization precession of Py and Co with nanometer spatial resolution. The approach allows us to individually image the resonant dynamic response of each element, and we find that angular momentum is transferred from the Py disk to the Co stripe and vice versa at their respective resonances. The integral (cavity) FMR spectrum of our sample shows an unexpected additional third resonance. This resonance is observed in the STXM-FMR experiments as well. Our microscopic findings suggest that it is governed by magnetic exchange between Py and Co, showing for the Co stripe a difference in relative phase of the magnetization due to stray field influence.

Dynamic unidirectional anisotropy in cubic FeGe with antisymmetric spin-spin-coupling.

Strong unidirectional anisotropy in bulk polycrystalline B20 FeGe has been measured by ferromagnetic resonance spectroscopy. Such anisotropy is not present in static magnetometry measurements. B20 FeGe exhibits inherent Dzyaloshinskii-Moriya interaction, resulting in a nonreciprocal spin-wave dispersion. Bulk and micron sized samples were produced and characterized. By X-band ferromagnetic resonance spectroscopy at 276 K ± 1 K, near the Curie temperature, a distribution of resonance modes was observed in accordance with the cubic anisotropy of FeGe. This distribution exhibits a unidirectional anisotropy, i.e. shift of the resonance field under field inversion, of KUD = 960 J/m3 ± 10 J/m3, previously unknown in bulk ferromagnets. Additionally, more than 25 small amplitude standing spin wave modes were observed inside a micron sized FeGe wedge, measured at 293 K ± 2 K. These modes also exhibit unidirectional anisotropy. This effect, only dynamically measurable and not detectable in static magnetometry measurements, may open new possibilities for directed spin transport in chiral magnetic systems.

Biologically encoded magnonics.

Spin wave logic circuits using quantum oscillations of spins (magnons) as carriers of information have been proposed for next generation computing with reduced energy demands and the benefit of easy parallelization. Current realizations of magnonic devices have micrometer sized patterns. Here we demonstrate the feasibility of biogenic nanoparticle chains as the first step to truly nanoscale magnonics at room temperature. Our measurements on magnetosome chains (ca 12 magnetite crystals with 35 nm particle size each), combined with micromagnetic simulations, show that the topology of the magnon bands, namely anisotropy, band deformation, and band gaps are determined by local arrangement and orientation of particles, which in turn depends on the genotype of the bacteria. Our biomagnonic approach offers the exciting prospect of genetically engineering magnonic quantum states in nanoconfined geometries. By connecting mutants of magnetotactic bacteria with different arrangements of magnetite crystals, novel architectures for magnonic computing may be (self-) assembled.

Extracting the Dynamic Magnetic Contrast in Time-Resolved X-ray Transmission Microscopy.

Using a time-resolved detection scheme in scanning transmission X-ray microscopy (STXM), we measured element resolved ferromagnetic resonance (FMR) at microwave frequencies up to 10 GHz and a spatial resolution down to 20 nm at two different synchrotrons. We present different methods to separate the contribution of the background from the dynamic magnetic contrast based on the X-ray magnetic circular dichroism (XMCD) effect. The relative phase between the GHz microwave excitation and the X-ray pulses generated by the synchrotron, as well as the opening angle of the precession at FMR can be quantified. A detailed analysis for homogeneous and inhomogeneous magnetic excitations demonstrates that the dynamic contrast indeed behaves as the usual XMCD effect. The dynamic magnetic contrast in time-resolved STXM has the potential be a powerful tool to study the linear and nonlinear, magnetic excitations in magnetic micro- and nano-structures with unique spatial-temporal resolution in combination with element selectivity.

Invited Review Article: Microwave spectroscopy based on scanning thermal microscopy: resolution in the nanometer range.

Scanning thermal microscope-detected ferromagnetic resonance (SThM-FMR) combines a thermal near-field microscope with a FMR spectrometer and detects the thermal response due to resonant microwave absorption by measuring the resistivity change in the thermal nanoprobe. The advantage of this technique is to provide imaging capabilities at fixed resonance conditions as well as local microwave spectroscopy at the nanoscale. A technique that uses the same setup but detects the thermoelastic response of the sample is the scanning thermoelastic microscope-detected FMR (SThEM-FMR). This latter technique is advantageous when FMR spectra of single nanostructures have to be recorded at a fixed position. The experimental setups and the signal generation processes of SThM/SThEM-FMR are described in detail. With the SThM-FMR setups a temperature resolution of 1 mK and a local resolution of 30 nm are actually achieved. With SThEM-FMR the obtained local resolution is 10 nm. The detection limits of both techniques can be as low as 10(6) spins. To demonstrate the potential of these new techniques SThM/SThEM-FMR investigations of local magnetic anisotropies, magnetization dynamics of single nanodots and inhomogeneous FMR excitations due to finite size effects are presented. Simultaneously, information on the magnetic parameters, the topography, and the thermal properties is provided. To describe the further potential of this recently developed SThM-FMR technique, combined magnetoresistance and FMR investigations are presented and an outlook on possible future applications is given.

Microwave soft x-ray microscopy for nanoscale magnetization dynamics in the 5-10 GHz frequency range.

We present a scanning transmission x-ray microscopy setup combined with a novel microwave synchronization scheme for studying high frequency magnetization dynamics at synchrotron light sources. The sensitivity necessary to detect small changes in the magnetization on short time scales and nanometer spatial dimensions is achieved by combining the excitation mechanism with single photon counting electronics that is locked to the synchrotron operation frequency. Our instrument is capable of creating direct images of dynamical phenomena in the 5-10 GHz range, with high spatial resolution. When used together with circularly polarized x-rays, the above capabilities can be combined to study magnetic phenomena at microwave frequencies, such as ferromagnetic resonance (FMR) and spin waves. We demonstrate the capabilities of our technique by presenting phase resolved images of a ∼6 GHz nanoscale spin wave generated by a spin torque oscillator, as well as the uniform ferromagnetic precession with ∼0.1° amplitude at ∼9 GHz in a micrometer-sized cobalt strip.