Accessing the fundamentals of magnetotransport in metals with terahertz probes

Jin, Zuanming; Tkach, Alexander; Casper, F.; Spetter, Victor; Grimm, H.; Thomas, Andy; Kampfrath, Tobias; Bonn, Mischa; Kläui, Mathias; Turchinovich, Dmitry

doi:10.1038/nphys3384

Cited by 113 publications

(88 citation statements)

References 52 publications

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“…Moreover, this model predicts that the conduction is dominated by the majority-spin electrons, leading to a positive spin polarization of the current. This simple picture turns out to be quite accurate, as it has been very recently demonstrated using ultrafast terahertz spectroscopy [22]. However, as we shall show in this work, it severely fails to explain the transport properties in ferromagnetic atomic contacts.…”

Section: Introductionmentioning

confidence: 73%

Orbital origin of the electrical conduction in ferromagnetic atomic-size contacts: Insights from shot noise measurements and theoretical simulations

et al. 2016

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With the goal to elucidate the nature of spin-dependent electronic transport in ferromagnetic atomic contacts, we present here a combined experimental and theoretical study of the conductance and shot noise of metallic atomic contacts made of the 3d ferromagnetic materials Fe, Co, and Ni. For comparison, we also present the corresponding results for the noble metal Cu. Conductance and shot noise measurements, performed using a low-temperature break junction setup, show that in these ferromagnetic nanowires: (i) there is no conductance quantization of any kind, (ii) transport is dominated by several partially-open conduction channels, even in the case of single-atom contacts, and (iii) the Fano factor of large contacts saturates to values that clearly differs from those of monovalent (nonmagnetic) metals. We rationalize these observations with the help of a theoretical approach that combines molecular dynamics simulations to describe the junction formation with nonequilibrium Green's function techniques to compute the transport properties within the Landauer-Büttiker framework. Our theoretical approach successfully reproduces all the basic experimental results and it shows that all the observations can be traced back to the fact that the d bands of the minority-spin electrons play a fundamental role in the transport through ferromagnetic atomic-size contacts. These d bands give rise to partially open conduction channels for any contact size, which in turn lead naturally to the different observations described above. Thus, the transport picture for these nanoscale ferromagnetic wires that emerges from the ensemble of our results is clearly at variance with the well established conduction mechanism that governs the transport in macroscopic ferromagnetic wires, where the d bands are responsible for the magnetism but do not take part in the charge flow. These insights provide a fundamental framework for ferromagneticbased spintronics at the nanoscale.

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Section: Introductionmentioning

confidence: 73%

Orbital origin of the electrical conduction in ferromagnetic atomic-size contacts: Insights from shot noise measurements and theoretical simulations

et al. 2016

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“…Thus τ s /τ p is also a constant, which indicates that the spin relaxation in Pt is governed by Elliott-Yafet mechanism [12]. We also applied a THz technique [56] to directly measure momentum relaxation time and resistivity of Pt with 30 nm thickness, which gives τ p =(5±3) fs and ρ Pt =16 µΩcm at 300 K. Assuming that τ p is proportional to 1/ρ Pt , τ p in Pt/MgO/CFB is thus around 2.7 fs. Therefore the spin flip probability of each scattering τ p /τ s is around 7×10 −4 for Pt at 300 K. Our ρ Ta is about 342 µΩcm at 300 K, much larger than those reported for the resistivity of α-phase and even β-phase Ta [57,58], which might be due to oxidation of Ta after the top structure is etched.…”

Section: Ishe 1ωmentioning

confidence: 99%

Determination of spin relaxation times in heavy metals via second-harmonic spin injection magnetoresistance

et al. 2017

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In tunnel junctions between ferromagnets and heavy elements with strong spin orbit coupling the magnetoresistance is often dominated by tunneling anisotropic magnetoresistance (TAMR). This makes conventional DC spin injection techniques impractical for determining the spin relaxation time (τs). Here, we show that this obstacle for measurements of τs can be overcome by 2nd harmonic spin-injection-magnetoresistance (SIMR). In the 2nd harmonic signal the SIMR is comparable in magnitude to TAMR, thus enabling Hanle-induced SIMR as a powerful tool to directly determine τs. Using this approach we determined the spin relaxation time of Pt and Ta and their temperature dependences. The spin relaxation in Pt seems to be governed by Elliott-Yafet mechanism due to a constant resistivity×spin relaxation time product over a wide temperature range.PACS numbers: 72.25. Rb, 72.25.Ba, 73.50.Bk, 73.40.Rw The large applied potential of spin-orbit-torques for magnetic random access memory has stimulated intensive interest in investigating spin orbit coupling (SOC) in heavy metals such as Pt and Ta [1][2][3][4][5][6][7][8][9][10][11]. Their spin Hall angle (θ SH ), spin diffusion length (l s ) and spin relaxation time (τ s ), which influence switching efficiency are important parameters for determining their effectiveness, but especially the latter two are experimentally hard to assess. Accurate determination of τ s could also help to identify the spin relaxation mechanisms [12]. Though θ SH and l s have been measured by spin pumping [13][14][15][16][17] In fact, spin injection experiments in nonlocal spin valves [29][30][31][32][33][34][35] and 3-terminal geometries [36][37][38][39][40] are both powerful tools in measuring τ s in metals and semiconductors. In these experiments, ferromagnetic layer (FM)/tunnel barrier/nonmagnetic layer (NM) junctions are adopted to both inject a non-equilibrium spin accumulation and simultaneously determine their magnitude. These measurement were used to determine spin relaxation times in a wide variety of materials, e.g., τ s,Si =55 -285 ps for heavily doped silicon [40]; τ s,Graphene > 1 ns for graphene/BN [41]; τ s,Al =110 ps for aluminum [29], τ s,Cu =22 ps for copper [42] and τ s,Au =45 ps for gold [32].However, it is impractical to apply these spin injection experiments to measure τ s in heavy metals with strong SOC for at least two reasons. First, l s in this case is so short (about several nanometers) that the preparation of nonlocal spin valves with comparable dimensions is beyond current lithography capabilities. Second, the real contact resistance is r = r C + r SI , where r SI and r C are the contact resistance induced by spin injection (SI) and the original contact resistance without r SI , respectively. Here r SI equals to r N r C /(r N +r C ) and the spin resistance in the NM layer r N is defined as ρ N l sN . ρ N and l sN are the resistivity and spin relaxation length of NM, respectively. For this discussion we ignore the influence of spin resistance in FM on r SI due to...

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“…32,33 Indeed, it can be demonstrated that standard spinelectronic phenomena work at THz frequencies (Figure 4(a)). The GMR effect still operates at THz frequencies: Mott's two current model, with different relaxation channels on different spin channels, has been proven to be functional and different Drude relaxation times can be extracted for spin-up and spin-down electron transport 53 ( Figure 4(c)). The static GMR of 23% is comparable to the THz GMR of 25%.…”

Section: Novel Applications In the Thz Range A Thz Spintronicsmentioning

confidence: 99%

Perspective: Ultrafast magnetism and THz spintronics

Walowski

Münzenberg

2016

Journal of Applied Physics

336

205

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This year the discovery of femtosecond demagnetization by laser pulses is 20 years old. For the first time, this milestone work by Bigot and coworkers gave insight directly into the time scales of microscopic interactions that connect the spin and electron system. While intense discussions in the field were fueled by the complexity of the processes in the past, it now became evident that it is a puzzle of many different parts. Rather than providing an overview that has been presented in previous reviews on ultrafast processes in ferromagnets, this perspective will show that with our current depth of knowledge the first applications are developed: THz spintronics and all-optical spin manipulation are becoming more and more feasible. The aim of this perspective is to point out where we can connect the different puzzle pieces of understanding gathered over 20 years to develop novel applications. Based on many observations in a large number of experiments. Differences in the theoretical models arise from the localized and delocalized nature of ferromagnetism. Transport effects are intrinsically non-local in spintronic devices and at interfaces. We review the need for multiscale modeling to address the processes starting from electronic excitation of the spin system on the picometer length scale and sub-femtosecond time scale, to spin wave generation, and towards the modeling of ultrafast phase transitions that altogether determine the response time of the ferromagnetic system. Today, our current understanding gives rise to the first usage of ultrafast spin physics for ultrafast magnetism control: THz spintronic devices. This makes the field of ultrafast spin-dynamics an emerging topic open for many researchers right now. Published by AIP Publishing. [http://dx

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Accessing the fundamentals of magnetotransport in metals with terahertz probes

Cited by 113 publications

References 52 publications

Orbital origin of the electrical conduction in ferromagnetic atomic-size contacts: Insights from shot noise measurements and theoretical simulations

Orbital origin of the electrical conduction in ferromagnetic atomic-size contacts: Insights from shot noise measurements and theoretical simulations

Determination of spin relaxation times in heavy metals via second-harmonic spin injection magnetoresistance

Perspective: Ultrafast magnetism and THz spintronics

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