Precessional dynamics in microarrays of nanomagnets

Copyright © 2005 American Institute of PhysicsTime resolved scanning Kerr microscopy has been used to study the response of square Ni88Fe12/Co80Fe20 bilayer elements to a pulsed magnetic field. Measurements were performed upon a square element of 6000 nm size and upon 64, 120, 220, 425, and 630 nm square elements that formed square arrays of about 4000 nm total size. While the frequency of precession of the magnetization of the 6000 nm element could be described with a macrospin model, the frequencies observed in the arrays of submicron size elements differed from the macrospin prediction. This observation may be understood in terms of the increasing nonuniformity of the demagnetizing field as the element aspect ratio is decreased

Time resolved studies of edge modes in magnetic nanoelements (invited)

Copyright © 2006 American Institute of PhysicsMicromagnetic simulations have been performed to investigate the frequencies and relative amplitudes of resonant magnetic modes within nanomagnetic elements of varying size that have been previously studied by time resolved Kerr magnetometry. The magnetic response of a nanoscale element generally consists of the edge and center localized modes. For 2.5 nm thick elements, a crossover from center to edge mode excitation occurs as the element size is reduced to less than 220 nm. Additional modes appear in the spin wave spectrum as the thickness of the element is increased. The frequency of the edge mode is particularly sensitive to the strength of the exchange interaction, dipolar interactions with nearest neighbor elements, and rounding of the corners of the element. Simulations with in-plane pulsed fields show that the edge mode becomes dominant in elements of somewhat larger size, emphasizing the importance of the edge mode in technological applications. (C) 2006 American Institute of Physics

Imaging collective magnonic modes in 2D arrays of magnetic nanoelements

Copyright © 2010 The American Physical SocietyWe have used time resolved scanning Kerr microscopy to image collective spin wave modes within a 2D array of magnetic nanoelements. Long wavelength spin waves are confined within the array as if it was a continuous element of the same size but with effective material properties determined by the structure of the array and its constituent nanoelements. The array is an example of a magnonic metamaterial, the demonstration of which provides new opportunities within the emerging field of magnonics

Large amplitude magnetization dynamics and the suppression of edge modes in a single nanomagnet

Copyright © 2011 American Institute of PhysicsLarge amplitude magnetization dynamics of a single square nanomagnet have been studied by time-resolved Kerr microscopy. Experimental spectra revealed that only a single mode was excited for all bias field values. Micromagnetic simulations demonstrate that at larger pulsed field amplitudes the center mode dominates the dynamic response while the edge mode is almost completely suppressed. Controlled suppression of edge modes in a single nanomagnet has potential applications in the operation of nanoscale spin transfer torque oscillators and bistable switching devices for which the amplitude of the magnetization trajectory is often large and a more uniform dynamic response is desirable

Time- and vector-resolved magneto-optical Kerr effect measurements of large angle precessional reorientation in a 2×2 μ m2 ferromagnet

Copyright © 2009 American Institute of PhysicsThe precessional dynamics of a 2×2 μm2 CoFe/NiFe (4.6 nm) element stimulated by an in-plane pulsed magnetic field have been investigated using time- and vector-resolved Kerr microscopy measurements and micromagnetic simulations. The time-resolved signals were normalized to in-plane hysteresis loops obtained from the patterned material, and suggest that the magnetization reorients through an angle of 100°±10°. The simulations reveal that only the magnetization of the center region undergoes large angle reorientation, while the canted magnetization at the edges of the element remains pinned. An enhanced Gilbert damping parameter of 0.1 was required to reproduce the experimentally observed Kerr signals

Field Effect Magnetization Reversal in Ferromagnetic Semiconductor Quantum Wells

We predict that a novel bias-voltage assisted magnetization reversal process will occur in Mn doped II-VI semiconductor quantum wells or heterojunctions with carrier induced ferromagnetism. The effect is due to strong exchange-coupling induced subband mixing that leads to electrically tunable hysteresis loops. Our model calculations are based on the mean-field theory of carrier induced ferromagnetism in Mn-doped quantum wells and on a semi-phenomenological description of the host II-VI semiconductor valence bands.Comment: 5 pages, 4 figure

Thermally assisted magnetization reversal in the presence of a spin-transfer torque

We propose a generalized stochastic Landau-Lifshitz equation and its corresponding Fokker-Planck equation for the magnetization dynamics in the presence of spin transfer torques. Since the spin transfer torque can pump a magnetic energy into the magnetic system, the equilibrium temperature of the magnetic system is ill-defined. We introduce an effective temperature based on a stationary solution of the Fokker-Planck equation. In the limit of high energy barriers, the law of thermal agitation is derived. We find that the N\'{e}el-Brown relaxation formula remains valid as long as we replace the temperature by an effective one that is linearly dependent of the spin torque. We carry out the numerical integration of the stochastic Landau-Lifshitz equation to support our theory. Our results agree with existing experimental data.Comment: 5 figure

Observation of vortex dynamics in arrays of nanomagnets

Vortex dynamics within arrays of square ferromagnetic nanoelements have been studied by time-resolved scanningKerr microscopy (TRSKM),while x-ray photoemission electronmicroscopy has been used to investigate the equilibrium magnetic state of the arrays. An alternating field demagnetization process was found to initialize a distribution of equilibrium states within the individual elements of the array, including quasiuniform states and vortex states of different chirality and core polarization. Repeated initialization revealed some evidence of stochastic behavior during the formation of the equilibrium state. TRSKM with a spatial resolution of ∼300 nm was used to detect vortex gyration within arrays of square nanoelements of 250-nm lateral size. Two arrays were studied consisting of a 9 × 9 and 5 × 5 arrangement of nanoelements with 50- and 500-nm element edge-to-edge separation to encourage strong and negligible dipolar interactions, respectively. In the 5 × 5 element array, TRSKM images, acquired at a fixed phase of the driving microwave magnetic field, revealed differences in the gyrotropic phase within individual elements. While some phase variation is attributed to the dispersion in the size and shape of elements, the vortex chirality and core polarization are also shown to influence the phase. In the 9 × 9 array, strong magneto-optical response due to vortex gyration was observed across regions with length equal to either one or two elements. Micromagnetic simulations performed for 2 × 2 arrays of elements suggest that particular combinations of vortex chirality and polarization in neighboring elements are required to generate the observed magneto-optical contrast.Engineering and Physical Sciences Research Council (EPSRC

Current induced switching of magnetic domains to a perpendicular configuration

In a ferromagnet--normal-metal--ferromagnet trilayer, a current flowing perpendicularly to the layers creates a torque on the magnetic moments of the ferromagnets. When one of the contacts is superconducting, the torque not only favors parallel or antiparallel alignment of the magnetic moments, as is the case for two normal contacts, but can also favor a configuration where the two moments are perpendicular. In addition, whereas the conductance for parallel and antiparallel magnetic moments is the same, signalling the absence of giant magnetoresistance in the usual sense, the conductance is greater in the perpendicular configuration. Thus, a negative magnetoconductance is predicted, in contrast with the usual giant magnetoresistance.Comment: 4 pages, 3 figures, major rewriting of the technical par

Phase-resolved x-ray ferromagnetic resonance measurements in fluorescence yield

Copyright © 2011 American Institute of PhysicsPhase-resolved x-ray ferromagnetic resonance (XFMR) has been measured in fluorescence yield, extending the application of XFMR to opaque samples on opaque substrates. Magnetization dynamics were excited in a Co50Fe50(0.7)/Ni90Fe10(5) bilayer by means of a continuous wave microwave excitation, while x-ray magnetic circular dichroism (XMCD) spectra were measured stroboscopically at different points in the precession cycle. By tuning the x-ray energy to the L-3 edges of Ni and Fe, the dependence of the real and imaginary components of the element specific magnetic susceptibility on the strength of an externally applied static bias field was determined. First results from measurements on a Co50Fe50(0.7)/Ni90Fe10(5)/Dy(1) sample confirm that enhanced damping results from the addition of the Dy cap