Magnetic Films Recent Highlights


Our vision is to address the grand challenges in condensed matter and materials physics via the exploration of the realm of nanomagnetism. Nanomagnetism is connected to fundamental questions of how the energy demands of future generations will be met via the utilization of wind turbines as a viable alternate energy source, and electric vehicles as alternatives to continued fossil-fuel consumption. Nanomagnetism is connected to the question of how the information technology revolution will be extended via the advent of spintronics and the possibilities of communication by means of pure spin currents. Nanomagnetism provides deep issues to explore in the realms of nanoscale confinement, physical proximity, far-from-equilibrium phenomena, and ultrafast and emergent behavior, and can even provide a window on the bio-realm via new therapeutic techniques and insights. While magnetism is regarded as the oldest field in all of science, nanomagnetism is fresh and vibrant and helped usher in the era of nanoscience and nanotechnology.

Recent Highlights:

Blowing Magnetic Skyrmion Bubbles

Magnetic skyrmion bubbles have been experimentally generated in a way very reminiscent of the process of blowing soap bubbles. This was accomplished at room temperature in readily accessible material systems. Magnetic skyrmions are topologically stable spin textures that exhibit many fascinating features. Amongst them, the efficient manipulation makes magnetic skyrmions as perfect information carriers for the low-power, nonvolatile, electrically reconfigurable data processing technologies. These aspects have been previously demonstrated at cryogenic temperatures in exotic material systems. Using an inhomogeneous in-plane current in a Ta/CoFeB/TaO trilayer, magnetic skyrmion bubbles were “blown” from a geometrical constriction. This can be understood to be the result of spatially divergent spin-orbit torques that give rise to instabilities of the magnetic domain structures. A current versus magnetic field phase diagram for skyrmion formation was determined and the efficient manipulation of these dynamically created skyrmions revealed, including depinning and motion. The topological distinction between magnetic skyrmions and topological trivial bubbles was also demonstrated. The results unambiguously show that spatially divergent current-induced spin-orbit torques can be an effective way for dynamically generating mobile magnetic skyrmions which, therefore, constitutes an important step towards skyrmion-based spintronics – skyrmionics. This dynamic approach for skyrmion generation could enable in the near future the demonstration of advanced skyrmionic device concepts, for example, a functional skyrmion racetrack memory.

Blowing magnetic skyrmion bubbles,
Wanjun Jiang, Pramey Upadhyaya, Wei Zhang, Guoqiang Yu, M. Benjamin Jungflesch, Frank Y. Fradin, John E. Pearson, Yaroslav Tserkovnyak, Kang L. Wang, Olle Heinonen, Suzanne G. E. te Velthuis, and Axel Hoffmann,
Science, 349, 283 (2015).

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Science, 349, 283 (2015).

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Paramagnetic Spin Seebeck Effect

When a thermal gradient is applied across a ferromagnetic material, it can drive a spin current via the spin Seebeck effect. A few years ago, it was shown that this effect also works in ferromagnetic insulators where the charge degree of freedom is strongly gapped. Here, a ‘pure’ spin current results from a thermal imbalance in the population of magnons. In this work, we have shown that the spin Seebeck effect can also occur in paramagnetic insulators that nominally do not support magnons. The result is surprising and also encouraging as it broadens the class of materials that may be used to generate spin currents, particularly at low temperatures. For example, in geometrically frustrated pyrochlore magnets that host spin ice and spin liquid ground states, the excitations can be quire unusual – such as magnetic monopoles and spinons. Coupling to these excitations and moving them around remains a challenge. Our results imply that we may be able create spin currents from these excitations using local thermal gradients, and detect them via the inverse spin Hall effect. The technique to carry out this measurement was developed in our group, where a patterned Au wire about 10 microns wide was used to deliver the heat locally to the surface of the paramagnet. This results in a large out-of-plane thermal gradient near the surface that drives a spin current out of the underlying paramagnet. The spin current is detected using the inverse spin Hall effect in W or Pt layers on the surface of the paramagnet. The signal is quite large and comparable to what is observed in the more conventional YIG/Pt structures. Note that the Au heater wire and the detector layer are electrically isolated from each other.

Paramagnetic Spin Seebeck Effect,
Stephen M. Wu, John E. Pearson, and Anand Bhattacharya,
Phys. Rev. Lett. 114, 186602 (2015).

APS Journal

Charge transfer from insulating LaMnO3 drives metallic LaNiO3 into an insulating state

We identified spectral signature of charge transfer from insulating LaMnO3 into metallic LaNiO3, which causes a metal-insulator transition in the LaNiO3 layers in LaMnO3/LaNiO3 superlattices. This interfacial-charge transfer is a technique for doping a material without introducing disorder. We expect that this better understanding how charge transfer at interfaces can be used to create novel electronic phases will lead to new materials design principles.

Spectral Weight Redistribution in (LaNiO3)n/(LaMnO3)2 Superlattices from Optical Spectroscopy,
P. Di Pietro, J. Hoffman, A. Bhattacharya, S. Lupi, and A. Perucchi,
Phys. Rev. Lett. 114, 156801 (2015).

APS Journal

Reduced Spin Hall effects from magnetic proximity

We demonstrated that for Pt and Pd increased induced magnetic moments are correlated with strongly reduced spin-Hall conductivities. In particular we see that spin Hall conductivities are reduced at low temperatures, but only if the Pd and Pt is in direct contact with a ferromagnetic permalloy layer. This observation finds an intuitive explanation in the development of a spin splitting of the chemical potential and the energy dependence of the intrinsic spin-Hall effect determined by first-principles calculations. Since for both paramagnetic Pd and Pt the energy dependence of the spin Hall conductivity is close to a maximum at the Fermi level, any increased magnetic moment, which is akin to an increase and decrease of the chemical potential for the majority and minority electrons, respectively, results in a decreased spin Hall conductivity for both spin channels. This work provides simple guidance towards the optimization of spin current efficiencies for devices based on spin-orbit coupling phenomena.

Reduced spin-Hall effects from magnetic proximity,
Wei Zhang, Matthias B. Jungfleisch, Wanjun Jiang, Yaohua Liu, John E. Pearson, Suzanne G.E. te Velthuis, Axel Hoffmann, Frank Freimuth, and Yuriy Mokrousov,
Phys. Rev. B 91, 115316 (2015)

APS Journal

Spin Hall Effects in Antiferromagnets

Antiferromagnet-based spintronics show unique advantages comparing to conventional spintronics. Their zero net magnetization and high excitation frequency avoid any mode-coupling with ferromagnets during ferromagnetic resonance. Furthermore, large anomalous Hall effect and spin Hall effect have been predicted for antiferro¬magnets. The physical origin of these effects, whether is from the heavy elements in the compounds or from the Berry phase of noncollinear spin textures requires experimental investigation. To address this problem, we studied the spin Hall effects in metallic antiferromagnet compounds, X50Mn50, where X = Fe, Pd, Ir, Pt (with increasing atomic number), using spin pumping and inverse spin Hall effect experiments. We measured the spin Hall angle of the antiferromagnets which follows the relationship PtMn > IrMn > PdMn > FeMn. This result highlights the important role of the spin orbit coupling of the heavy metals for the properties of the Mn based alloys through orbital hybridization. This was corroborated using first-principles calculations of the ordered alloys, which showed that the value of the spin Hall conductivity can vary significantly with crystal orientation and staggered magnetization. This work is a significant stepping-stone towards designing spintronics devices using antiferromagnets.

Spin Hall Effects in Metallic Antiferromagnets,
Wei Zhang, Matthias B. Jungfleisch, Wanjun Jiang, John E. Pearson, Axel Hoffmann, Frank Freimuth, and Yuriy Mokrousov,
Phys. Rev. Lett. 113, 196602 (2014)

APS Journal

Tailoring bond lengths with electrostatics

Strain is known to have profound influence on electronic and magnetic properties of complex oxides. However, epitaxial strain is limited in range, being able to create distortions in bond lengths and bond angles that are typically 10% of the equilibrium value via electrostatics. In bulk LaSrNiO4, the La and Sr cations occupy the A-site randomly, and the crystal possesses a center of inversion symmetry. We use oxide molecular beam epitaxy techniques to create an artificial polar analog of this material depositing monolayers of LaO, NiO2 and SrO in sequence, as shown in (a). An internal dipolar field is set up since the LaO carries a net (+1) charge and the NiO2 is nominally (-1). This artificial crystal is non-centrosymmetric. The cation layering can be determined by measuring crystal truncation rods and using an x-ray phase retrieval technique called ‘COBRA’, as shown in (b). LaSrNiO4 is known to be metallic. The internal dipolar electric fields in the polar analog are not entirely screened by the electrons at the Fermi level. In fact, the Ni-O bonds distort to screen these internal dipolar fields. These distortions can be larger than 10%. This in turn changes the Ni-O bond hybridization, which we have modeled using density functional theory, and measured with resonant x-ray spectroscopy.

Polar Cation Ordering: A Rout to Introducing >10% Bond Strain into Layered Oxide Films,
Brittany B. Nelson-Cheeseman, Hua Zhou, Prasanna V. Balachandran, Gilberto Fabbris, Jason Hoffman, Daniel Haskel, James M. Rondinelli, and Anand Bhattacharya,
Adv. Funct. Mater. 24, 6884 (2014)

Wiley Online Library

Spin Wave Multiplexer

The coherent transport of spin information is one of the great challenges in condensed matter physics and is of fundamental importance for the development of spintronic devices. Spin waves carry angular momentum and can be used to transport spin information over mesoscopic distances much larger than the spin diffusion length in metals. The energy dispersion of spin waves in thin films is highly anisotropic due to dipolar interactions. As a consequence spin waves typically only propagate along straight lines, which hinders the implementation of more complex two-dimensional device-structures. In order to overcome this problem, we use locally generated magnetic Oersted fields to alter the magnetization direction solely in designated regions of the spin wave conduits. This enabled us to tailor the dispersion relation in different parts of the structure for controlling the spin wave propagation. The feasibility of this concept is demonstrated by detecting the propagation of the spin wave with spatially resolved Brillouin light scattering spectroscopy. This work opens the door to more complex devices utilizing the unique properties of spin waves and thus is a significant stepping-stone towards designing functional spin wave structures.

Realization of a spin-wave multiplexer,
K. Vogt, F.Y. Fradin, J.E. Pearson, T. Sebastian, S. D. Bader, B. Hillebrands, H. Hoffmann, and H. Schultheiss,
Nature Comm. 5, 3727 (2014)


Understanding the energy-transfer mechanism in coupled spin vortices

Spin vortices have been the subject of significant scientific interest due to their relevance to the fundamental science and technological implications, such as logic devices for information storage, vortex-based magnonic crystals for information propagation, energy-efficient microwave oscillators and even in bio-medical investigations. The rotation direction for the dynamics of magnetic vortices is determined by the polarity of the vortex core, i.e., the magnetization direction at the center of a magnetic disk. Therefore, for two strongly coupled vortices, such as two touching disks, the frequency of the dynamics depends on whether the relative vortex core polarity is parallel or antiparallel. Unfortunately, exploring this behavior in an applied magnetic field has been elusive for a long time, since only a parallel core polarity can be established by magnetic field cycling. However, we have established recently that through resonant spin-ordering both the parallel and anti-parallel configuration can be reliably stabilized, which opened up the opportunity for a systematic field-dependent study of the magnetic dynamics for each configuration, which is reported in this work. For the parallel case an antisymmetric field dependence of the resonance frequency is observed, which depends on the relative orientation between the applied magnetic field and both core polarities. On the hand for the anti-parallel core polarities the frequency dependence is symmetric in applied field, and any magnetic field results in a softening of the dynamics. This is due to the fact that in the case of anti-parallel vortex polarities the energy relaxation for the energetically unfavorable (i.e., higher frequency) vortex core dynamic mode is faster and all the energy is transferred to the lower energy mode. This unusual one-way dissipation of energy stems from the complex interplay between magnetostatic coupling and dynamic interactions in the double-vortex system. This basic work is relevant to grand challenges in mesoscale materials science, dynamics of non-equilibrium systems, and in continuing the information technology revolution.

Dynamics of coupled vortices in perpendicular field,Shikha Jain, Valentyn Novosad Frank Y. Fradin, John E. Pearson, and Samuel D. Bader,
Appl. Phys. Lett. 104, 082409 (2014)

AIP Journal

Rational Design of the Nanocomposite Structure for High Performance Permanent Magnets

Permanent magnets are indispensible for conversion between mechanical and electrical energies. Nanocomposite permanent magnet materials based on the "exchange spring" mechanism, in which a soft magnetic phase is hardened via interfacial exchange coupling to a hard magnetic phase, offer the promise of a superior maximum energy product [(BH)max] and good high-temperature performance, and are a potential solution to the supply criticality in rare earth elements. However, for the past several decades, fabricating nanocomposite magnets with high performance has remained an unrealized technological goal. The development of the nanocomposite permanent magnet materials can benefit from a rational design of the structure. We have identified an optimal exchange-spring structure that balances property enhancement and the feasibility of scalable fabrication. By analyzing the magnetization processes in several model exchange-spring structures with both micromagnetic simulations and nucleation theory, we evaluated the nucleation field, which is the field at which magnetization reversal happens, and (BH)max as functions of the hard and soft phase dimensions for different exchange-spring geometries. We pointed out that the soft-cylinders-in-hard-matrix structure has the highest achievable (BH)max, and is amenable to scaled-up fabrication. On the other hand, the current practice of bottom-up syntheses starting with nanoparticles (both hard and soft) can only attain much lower values of (BH)max. Our work charts a new direction for the development of high-performance rare-earth-replacement magnet materials.

Rational design of the exchange-spring permanent magnet,
J. S. Jiang, and S.D. Bader,
J. Phys.: Cond. Matt., 26, 064214 (2014).

IOP Journal