Spin torque oscillators (STOs) often exhibit multiple modes, leading to complex behavior. One example is mode hopping between different eigenmodes of a magnetic tunnel junction (MTJ) STO. This mode hopping is a strong function of current and angle between the magnetization in the free and fixed layers, and away from anti-parallel configuration, mode hopping can be the dominant decoherence process. Another example is the linewidth of a nanocontact STO that can be a complex non-monotonic function of temperature in regions where two or more modes are excited by the oscillators. These phenomena require a generalization of the single-mode nonlinear STO theory to include mode coupling. We derive equations describing the slow time evolution of the coupled system and show they describe a dynamically driven system, similar to other systems that exhibit mode hopping in the presence of thermal fluctuations. In our description, mode coupling also leads to additional coupling between power and phase fluctuations, which can in certain limited cases lead to longer relaxation times for power fluctuations, and consequently to larger linewidths through the nonlinear frequency shift.

O. Heinonen, P. Muduli, E. Iacocca, and Johan Åkerman, Decoherence, Mode Hopping, and Mode Coupling in Spin Torque Oscillators, IEEE Trans. Magn. 49, 4398 (2013).

We have observed distinct temperature-dependent magnetization reversal modes in a perpendicular (Co/Pd)4/Co/Cu/(Co/Ni)4/Co pseudo-spin-valve, which are correlated with spin-transport properties. At 300 K, magnetization reversal occurs by vertically correlated domains. Below 200 K the hysteresis loop becomes bifurcated due to laterally correlated reversal of the individual stacks. The magnetic configuration change also leads to higher spin disorders and a significant increase in the giant magnetoresistance effect. First order reversal curve measurements reveal that the coupled state can be re-established through field cycling and allow direct determination of the interlayer coupling strength as a function of temperature.

J. E. Davies, D. A. Gilbert, S. M. Mohseni, R. K. Dumas, J. Åkerman, and Kai Liu, APL 103, 022409 (2013).

Theory predicts that a strongly injection-locked spin-torque oscillator (STO) should show a characteristic ringing frequency both on its approach to the locked state and under the influence of thermal noise. While experiments have so far failed to detect such ringing, we here show numerically and analytically how current modulation of injection-locked STOs can excite the ringing frequency in a resonant manner, and hence increase the experimental sensitivity. The complexity of such dynamics leads to a nonlinear resonance, which can even unlock the STO as a function of the modulation strength. The results presented here offer a plausible method for experimentally measuring the ringing frequency of STOs. Moreover, the onset of unlocking also provides a measure for the maximum modulation strength that can be applied to phase-locked STOs.

Ezio Iacocca and Johan Åkerman, Resonant excitation of injection-locked spin-torque oscillators, PRB 87, 214428 (2013).

It has been argued that if multiple spin wave modes are competing for the same centrally located energy source, as in a nanocontact spin torque oscillator, that only one mode should survive in the steady state. Here, the experimental conditions necessary for mode coexistence are explored. Mode coexistence is facilitated by the local field asymmetries induced by the spatially inhomogeneous Oersted field, which leads to a physical separation of the modes, and is further promoted by spin wave localization at reduced applied field angles. Finally, both simulation and experiment reveal a low frequency signal consistent with the intermodulation of two coexistent modes.

Randy K. Dumas, E. Iacocca, S. Bonetti, S. R. Sani, S. M. Mohseni, A. Eklund, J. Persson, O. Heinonen, and Johan Åkerman, Spin-Wave-Mode Coexistence on the Nanoscale: A Consequence of the Oersted-Field-Induced Asymmetric Energy Landscape, PRL 110, 257202 (2013).

A truly unique experience! It was a great honor and pleasure to be invited to EINC 2013 and speak about our recent results on Magnetic Droplet Solitons. The Ester Island is a fascinating place with a dramatic and exciting history which can be highly recommended. Although it's essentially on the opposite side of the planet, the travel is surprisingly manageable, with e.g. Air France flying directly from Paris to Santiago and then Lan flying to Easter Island. I want to particular highlight the evening plenary talks given by Nobel Laureates Dan Shechtman (Nobel Prize 2011) and Claude Cohen-Tannoudji (Nobel Prize 1997), who gave very inspiring presentations on “Quasi- Periodic Materials – Crystal Redifined”, and “Manipulating Atoms with Photons and Photons with Atoms”.

MML (International Symposium on Metallic Multilayers) is one of the best thin film magnetism conferences out there. The 8th version of it was no exception. Randy Dumas and Johan Åkerman attended and presented two posters [1,2] and an invited talk [3]. The next MML 2016 will be held in Uppsala, Sweden! Expect to see the entire Åkerman Group to be attending, if allowed.



[1]  "A non-volatile spintronic memory element with a continuum of resistance states", Y. Fang, R. Dumas, T. Anh Nguyen, S. Mohseni, S. Chung, C. Miller, J. Åkerman, Poster P-71
[2] "Spin wave mode coexistence in nanocontact spin torque oscillators", R. Dumas, E. Iacocca, S. Bonetti, S. Sani, M. Mohseni, A. Eklund, J. Persson, O. Heinonen, J. Åkerman, Poster P-124
[3] "Magnetic droplet solitons in nano-contact spin-torque oscillators with perpendicular magnetic anisotropy", J. Åkerman, M. Mohseni, S. Sani, J. Persson, A. Nguyen, S. Chung, Y. Pogoryelov, P. Muduli, E. Iacocca, A. Eklund, R. Dumas, S. Bonetti, A. Deac, M. Hoefer, Invited Talk We-1

PhD student Yeyu Fang is one of twelve Chinese graduate students in Sweden awarded the Chinese Government Scholarship 2012 for outstanding self-financed students studying abroad.

Yeyu received a diploma at a ceremony at the Chinese Embassy in Stockholm on Friday, May 17. Prof. Johan Åkerman held a short speech as representative of the supervisors of the 12 students.

Big congrats from everybody in the group!

Anh Nguyen, Randy Dumas and Johan Åkerman have published a paper in Phys. Rev. B on Brillouin Light Scattering measurements of fundamental spin wave modes in tilted exchange springs [1]. These results are part of an ongoing collaboration with the University of Perugia, Italy, and also involved collaborators from Florence, Italy, and NIST Gaithersburg, USA.

[1] S. Tacchi, T. N. Anh Nguyen, G. Carlotti, G. Gubbiotti, M. Madami, R. K. Dumas, J. W. Lau, Johan Åkerman, A. Rettori, and M. G. Pini, "Spin wave excitations in exchange-coupled [Co/Pd]-NiFe films with tunable tilting of the magnetization", Phys. Rev. B 87, 144426 (2013)