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Register a new userMagnetomechanical coupling and ferromagnetic resonance in magnetic nanoparticles
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We address the theory of the coupled lattice and magnetization dynamics of freely suspended single-domain nanoparticles. Magnetic anisotropy generates low-frequency satellite peaks in the microwave absorption spectrum and a blueshift of the ferromagnetic resonance (FMR) frequency. The low-frequency resonances are very sharp with maxima exceeding that of the FMR, because their magnetic and mechanical precessions are locked, thereby suppressing Gilbert damping. Magnetic nanoparticles can operate as nearly ideal motors that convert electromagnetic into mechanical energy. The Barnett/Einstein-de Haas effect is significant even in the absence of a net rotation.
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We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have similar magnitudes, and thus that both torques are equally important for understanding high-frequency voltage-driven magnetization dynamics in MTJs. As an example, we show that VCMA can increase the sensitivity of an MTJ-based microwave signal detector to the sensitivity level of semiconductor Schottky diodes.
We study the effect of surface anisotropy on the spectrum of spin-wave excitations in a magnetic nanocluster and compute the corresponding absorbed power. For this, we develop a general numerical method based on the (undamped) Landau-Lifshitz equation, either linearized around the equilibrium state leading to an eigenvalue problem or solved using a symplectic technique. For box-shaped clusters, the numerical results are favorably compared to those of the finite-size linear spin-wave theory. Our numerical method allows us to disentangle the contributions of the core and surface spins to the spectral weight and absorbed power. In regard to the recent developments in synthesis and characterization of assemblies of well defined nano-elements, we study the effects of free boundaries and surface anisotropy on the spin-wave spectrum in iron nanocubes and give orders of magnitude of the expected spin-wave resonances. For an 8 nm iron nanocube, we show that the absorbed power spectrum should exhibit a low-energy peak around 10 GHz, typical of the uniform mode, followed by other low-energy features that couple to the uniform mode but with a stronger contribution from the surface. There are also high-frequency exchange-mode peaks around 60 GHz.
The dynamic magnetic susceptibility of magnetic materials near ferromagnetic resonance (FMR) is very important in interpreting dc-voltage in electrical detection of FMR. Based on the causality principle and the assumption that the usual microwave absorption lineshape around FMR is Lorentzian, general forms of dynamic susceptibility of an arbitrary sample and the corresponding dc-voltage lineshape are obtained. Our main findings are: 1) The dynamic susceptibility is not a Polder tensor for material with arbitrary anisotropy. Two off-diagonal elements are not in general opposite to each other. However, the linear response coefficient of magnetization to total rf field is a Polder tensor. This may explain why two off-diagonal elements are always assumed to be opposite to each other in analyses. 2) The frequency dependence of dynamic susceptibility near FMR is fully characterized by six numbers while its field dependence is fully characterized by seven numbers. 3) A recipe of how to determine these numbers by standard microwave absorption measurements for an arbitrary sample is proposed. Our results allow one to unambiguously separate the contribution of the anisotropic magnetoresistance to dc-voltage from that of the anomalous Hall effect. With these results, one can reliably extract the information of spin pumping and the inverse spin Hall effect, and determine the spin-Hall angle. 4) The field-dependence of susceptibility matrix at a fixed frequency may have several peaks when the effective field is not monotonic of the applied field. In contrast, the frequency-dependence of susceptibility matrix at a fixed field has only one peak. Furthermore, in the case that resonance frequency is not sensitive to the applied field, the field dependence of susceptibility matrix, as well as dc-voltage, may have another non-resonance broad peak. Thus, one should be careful in interpreting observed peaks.
We present a study by ferromagnetic resonance at microwave Q band of two sheets of cobalt nanoparticles obtained by annealing SiO2 layers implanted with cobalt ions. This ex- perimental study is performed as a function of the applied magnetic field orientation, tempera- ture, and dose of implanted cobalt ions. We demonstrate that each of those magnetic sheet of cobalt nanoparticles can be well modelled by a nearly two dimensional ferromagnetic sheet hav- ing a reduced effective saturation magnetization, compared to a regular thin film of cobalt. The nanoparticles are found superparamagnetic above around 210 K and ferromagnetic below this blocking temperature. Magnetostatic calculations show that a strong magnetic field gradient of around 0.1 G/nm could be produced by a ferromagnetic nanostripe patterned in such magnetic sheet of cobalt nanoparticles. Such a strong magnetic field gradient combined with electron para- magnetic resonance may be relevant for implementing an intermediate scale quantum computer based on arrays of coupled electron spins, as previously reported (Eur. Phys. J. B (2014) 87, 183). However, this new approach only works if no additional spin decoherence is introduced by the spin waves exitations of the ferromagnetic nanostructure. We thus suggest theoretically some possible magnetic anisotropy engineering of cobalt nanoparticles that could allow to suppress the spin qubit decoherence induced by the unwanted collective excitation of their spins.
Biosensing with ferromagnet-based magnetoresistive devices has been dominated by electrical detection of particle-induced changes to the devices static magnetic configuration. There are however potential advantages to be gained from using field dependent, high frequency magnetization dynamics for magnetic particle detection. Here we demonstrate the use of nano-confined ferromagnetic resonances in periodically patterned magnetic films for the detection of adsorbed magnetic particles with diameters ranging from 6 nm to 4 $mu$m. The nanopatterned films contain arrays of holes which can act as preferential adsorption sites for small particles. Hole-localized particles act in unison to shift the resonant frequencies of the various modes of the patterned layer with shift polarities determined by the localization of each mode within the nanopatterns repeating unit cell. The same polarity shifts are observed for a large range of coverages, even when hole-localized particles are covered by quasi-continuous particle sheets. For large particles however, preferential adsorption no longer occurs, leading to resonance shifts with polarities which are independent of the mode localization. Analogous shifts are seen in continuous layers where, for small particles, the shift of the layers fundamental mode is typically about 10 times less than in patterned systems and induced by relatively weak fields emanating beyond the particle in the direction of the static applied field. This highlights the importance of having confined modes consistently positioned with respect to nearby particles.
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