Abstract-Broadband Terahertz Probes of Anisotropic Magnetoresistance Disentangle Extrinsic and Intrinsic Contributions

 

Lukáš Nádvorník, Martin Borchert, Liane Brandt, Richard Schlitz, Koen A. de Mare, Karel Výborný, Ingrid Mertig, Gerhard Jakob, Matthias Kläui, Sebastian T. B. Goennenwein, Martin Wolf, Georg Woltersdorf, Tobias Kampfrath 

https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.021030

Anisotropic magnetoresistance (AMR) is a ubiquitous and versatile probe of magnetic order in contemporary spintronics research. Its origins are usually ascribed to extrinsic effects (i.e., spin-dependent electron scattering), whereas intrinsic (i.e., scattering-independent) contributions are neglected. Here, we measure AMR of polycrystalline thin films of the standard ferromagnets Co, Ni, ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, and ${\mathrm{Ni}}_{50}{\mathrm{Fe}}_{50}$ over the frequency range from dc to 28 THz. The large bandwidth covers the regimes of both diffusive and ballistic intraband electron transport and, thus, allows us to separate extrinsic and intrinsic AMR components. Analysis of the THz response based on Boltzmann transport theory reveals that the AMR of the Ni, ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, and ${\mathrm{Ni}}_{50}{\mathrm{Fe}}_{50}$ samples is of predominantly extrinsic nature. However, the Co thin film exhibits a sizable intrinsic AMR contribution, which is constant up to 28 THz and amounts to more than $2/3$ of the dc AMR contrast of 1%. These features are attributed to the hexagonal structure of the Co crystallites. They are interesting for applications in terahertz spintronics and terahertz photonics. Our results show that broadband terahertz electromagnetic pulses provide new and contact-free insights into magnetotransport phenomena of standard magnetic thin films on ultrafast timescales.

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International team of scientists realizes a compact, efficient wideband source of terahertz radiation emitters

• 27 May 2016
 
• Source: Johannes Gutenberg University Mainz

Spintronics paves the way for new terahertz sources

http://www.research-in-germany.org/en/research-landscape/news/2016/05/2016-05-25-international-team-of-scientists-realizes-a-compact--efficient-wideband-source-of-terahertz-radiation-emitters.html

Terahertz waves have numerous advantages ranging from medical applications in imaging tissues to airport security systems. However, until now there was not a single unified source of terahertz emission which could provide usable terahertz radiation over a wide frequency range. Physicists from the Fritz Haber Institute, Berlin and Johannes Gutenberg-University Mainz (JGU) along with national and international partners have realized a new concept for the production of this electromagnetic radiation using spintronic emitters. These emitters are in the form of thin multi-layered metal films and make use of the spin property of the electron rather than the conventional semiconductor emitters, which use only the electron charge. "Using this property, we were able to show that it is possible to produce broadband emitters fully covering the 1 to 30 THz range which are also cost-efficient in terms of their industrial applications," said Professor Matthias Kläui of the JGU Institute of Physics. In recent years, this quantum property, i.e., the spin of the electron, has led to a new branch of spin-based electronics or spintronics. The new findings have recently been published in the scientific journal Nature Photonics.

Terahertz radiation is a part of the electromagnetic spectrum between microwave frequencies and infrared light, in the frequency range of 0.3 to 30 THz. Many materials absorb THz radiation in a characteristic manner, while textiles and plastics are largely transparent. Unlike X-rays, terahertz rays are harmless to biological structures. As a consequence, terahertz waves can be used for bioimaging – such as in body scanners at airports, for quality control of food, and for material identification.

One obstacle which prevents the wide-scale usage of these terahertz rays is the fact that current technologies require expensive and large apparatus to generate broadband THz. The spintronics-based emitters fabricated by researchers at the Fritz Haber Institute in Berlin and at Johannes Gutenberg University Mainz are scalable and can be used as table top emitters. Current semiconductor-based emitters can cover only a limited range of frequencies. However, using these novel spintronics emitters it is possible to cover the complete range of terahertz frequencies from 1 to 30 THz without gap. It is more energy-efficient, cheaper to manufacture, and easier to use than conventional sources.

Thin metal film at the heart of the emitter

"The new THz emitter resembles a photodiode or a solar cell: on illuminating the material with an ultrashort laser pulse, an ultrafast spin-current is generated. This spin current is then converted to a charge current via the Inverse Spin-Hall effect. Consequently, a transmitter antenna radiates an equivalent electromagnetic pulse with frequencies in the terahertz range," explained Samridh Jaiswal, co-author of the study and a member in Professor Mathias Kläui's group at Mainz University. In contrast to a solar cell, the metal films of the spintronics variant are only 5.8 nanometers thick, which ensures the impulse to be extremely short while at the same time preventing attenuation of the terahertz wave inside the emitter. The metal and layer thicknesses used were systematically optimized such that a relatively weak laser radiation is sufficient to produce the entire terahertz spectrum from 1 to 30 THz. “These results pave the way to novel sources of terahertz emitters,” said Jaiswal, who is a fellow in the Marie Curie Initial Training Network WALL "Controlling domain wall dynamics for functional devices," of which Mainz University is a partner. Samridh Jaiswal was in charge of the materials development.

To optimize the emitter performance, the scientists had to screen a large number of materials with varying material composition and geometry. The high-throughput Rotaris sputter deposition system installed at the Institute of Physics at Mainz University by Singulus Technologies was a crucial prerequisite to fabricate a large number of samples in a short time. The optimization procedure was further supported by calculations of theorists at Forschungszentrum Jülich. The topic of converting between spin and charge currents due to spin-orbit effects is part of the recently established Collaborative Research Center CRC/Transregio 173 "Spin+X: Spin in its collective environment," funded by the German Research Foundation (DFG).

Contact:

Professor Dr. Mathias Kläui
Condensed Matter Theory Group
Institute of Physics
Johannes Gutenberg University Mainz
55099 Mainz
Tel +49 6131 39-23633
Fax +49 6131 39-24076
E-Mail: klaeui[at]uni-mainz.de