Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q8: Magnonics: Spin Wave Processes in Magnetic Materials |
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Sponsoring Units: GMAG Chair: Olle Heinonen, Seagate Technology Room: Portland Ballroom 255 |
Wednesday, March 17, 2010 11:15AM - 11:51AM |
Q8.00001: Photo-magnonics: excitation of magnonic materials by femtosecond laser pulses Invited Speaker: Analogue the photonic crystals, a periodic modification of a magnetic material is prepared by forming an anti-dot lattice for spin waves. The resulting bands are generally complex in the magnetic case because of different dispersions along different magnetization directions (backward volume and Damon-Eshbach mode). They depend on the variation strength of the periodic magnetostatic potential. All-optical femtosecond laser experiments allow the excitation of spin-waves with comparable amplitudes as field pulse and resonance techniques today. It is a promising valuable alternative method to study spin-waves and their relaxation paths in a magnonic material. Laser pulses with a duration of 60 fs from a Ti:Sapphire regenerative laser system are used for optical excitation (pump pulse) as well as for the observation of the subsequent magnetic relaxation (probe pulse). The initial local single spin-flip excitation is subsequently decaying into spin waves lower in energy within the pico- and nanosecond regime over a wide spectral range. In focus of our investigation is the propagation and localization of dipolar surface modes (Damon-Eshbach) in thin Nickel and (low damped) CoFeB film cubic and hexagonal lattice structures. Their mode dispersion is measured by applying different magnetic fields which shift the energy of the mode and allows identifying them. We find well defined modes in the condensed state with a specific pronounced k-value determining the properties of the propagating spin wave. One example for a distinct modification of the magnonic periodic structure is a line defect that can function as a wave guide inside the magnonic gap region. An increased intensity of the Damon Eshbach mode by a factor of two is found in the wave guide region. A study of these wave guides will allow to specifically design the material properties, making magnonic materials the material of choice for advanced spin computing devices. [Preview Abstract] |
Wednesday, March 17, 2010 11:51AM - 12:27PM |
Q8.00002: Magnetic excitations and ultrafast magnetisation reversal Invited Speaker: Ultrafast magnetic reversal stimulated by femtosecond lasers is an important area of research in terms of basic physics and potential applications. I will describe an atomistic model of thermally activated reversal in the picosecond and sub-picosecond regime based on the use of Langevin dynamics with an effective local field derived from the Heisenberg formalism. The model describes the sub-picosecond thermal demagnetisation and also longer timescales for recovery due to frustration effects. A novel linear reversal mechanism is also predicted in which rapid excitations lead to a non-precessional reversal of the macroscopic spin. This mechanism is suggested as being central to the mechanism of optically induced magnetisation reversal, which is shown to exhibit reversal times of the order of hundreds of femtoseconds. [Preview Abstract] |
Wednesday, March 17, 2010 12:27PM - 1:03PM |
Q8.00003: Magnon gases and condensates Invited Speaker: A magnon gas is an excellent model for the investigation of interacting bosonic particles. Its potential is due to the wide controllability of the magnon density as well as of the spectral properties influencing the magnon-magnon interaction. The recent observation of Bose-Einstein condensation of magnons at room temperature demonstrates this clearly. The most effective mechanism to inject magnons into the gas is parametric pumping which creates a condensate of photon-coupled magnon pairs, referred to as a p-magnon condensate. The role of the p-magnon condensate formed at half of the pumping frequency is manifold: it serves both as an energy source and as a strong disturbing factor for the entire spin-wave system. Formation, thermalization and disintegration of the p-magnon condensate as well as its interaction with the Bose-Einstein condensate (BEC) of magnons constitute a hot topic of research. To investigate the evolution of these two condensates we use time- and wavevector-sensitive Brillouin light scattering spectroscopy in combination with conventional microwave techniques. The talk focuses in particular on the behavior of the parametrically driven magnetic medium after the pump source is switched off. This defines the important problem of the pump-free evolution of a non-equilibrium magnon system. I report on the experimental discovery of the direct disruptive influence of the p-magnon condensate on the BEC of magnons. The sharp increase in the intensity of the BEC simultaneously with the fast decay of the p-magnon condensate caused by the shutdown of the pump field is a manifestation of this phenomenon. Furthermore, the application of a second pump pulse, while the BEC is freely relaxing, results in the re-population of the p-magnon condensate and in a subsequent decrease of the BEC density. The thermalization of the additionally injected portion of p-magnons restores the equilibrium BEC density, which jumps up again after the end of the second pump pulse. The presented experiments establish the first observation of the interaction between two physically different condensates of Bose particles. [Preview Abstract] |
Wednesday, March 17, 2010 1:03PM - 1:39PM |
Q8.00004: Modification of Spin Wave Propagation by Current Injection Invited Speaker: We studied the effect of an electric current on the spin wave propagation in magnetic wires, and found the following two effects. (i) Current injection changes the velocity of spin wave; the velocity is increased or decreased depending on the current polarity. (ii) Current injection modifies the attenuation length of spin wave; the attenuation length of spin wave can increase when the spin waves and electrons move in the same direction. The first finding can be interpreted as the time-domain observation of the spin-wave Doppler shift by current injection [1]. The second effect is thought to be affected by the nonadiabaticity of the spin transfer torque and thus can be used to estimate the nonadiabaticity [2]. \\[4pt] [1] V. Vlaminck and M. Bailleul, Science 322, (2008) 410. \\[0pt] [2] S. M. Seo, K. J. Lee, H. Yang, and T. Ono, Phys. Rev. Lett. 102, (2009) 147202. [Preview Abstract] |
Wednesday, March 17, 2010 1:39PM - 2:15PM |
Q8.00005: Current-induced spin wave Doppler shift Invited Speaker: In metal ferromagnets -namely Fe, Co and Ni and their alloys- magnetism and electrical transport are strongly entangled (itinerant magnetism). This results in a number of properties such as the tunnel and giant magnetoresistance (i.e. the dependence of the electrical resistance on the magnetic state) and the more recently addressed spin transfer (i.e. the ability to manipulate the magnetic state with the help of an electrical current). The spin waves, being the low-energy elementary excitations of any ferromagnet, also exist in itinerant magnets, but they are expected to exhibit some peculiar properties due the itinerant character of the carriers. Accessing these specific properties experimentally could shed a new light on the microscopic mechanism governing itinerant magnetism, which -in turn- could help in optimizing material properties for spintronics applications. As a simple example of these specific properties, it was predicted theoretically that forcing a DC current through a ferromagnetic metal should induce a shift of the frequency of the spin waves [1,2]. This shift can be identified to a Doppler shift undergone by the electron system when it is put in motion by the electrical current. We will show how detailed spin wave measurements allow one to access this current-induced Doppler shift [3]. From an experimental point of view, we will discuss the peculiarities of propagating spin wave spectroscopy experiments carried out at a sub-micrometer length-scale and with MHz frequency resolution. Then, we will discuss the measured value of the Doppler shift in the context of both the old two-current model of spin-polarized transport and the more recent model of adiabatic spin transfer torque. \\[4pt] [1] P.Lederer and D.L. Mills, Phys.Rev. 148, 542 (1966).\\[0pt] [2] J. Fernandez-Rossier et al., Phys. Rev. B 69, 174412 (2004)\\[0pt] [3] V. Vlaminck and M. Bailleul, Science 322, 410 (2008). [Preview Abstract] |
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