Pis’ma v ZhETF, vol. 109, iss. 3, pp. 176 - 177

February 10

Spin wave effects in transport between a ferromagnet and a Weyl

semimetal surface

A. Kononov, O. O. Shvetsov, A. V. Timonina, N. N. Kolesnikov, E. V. Deviatov¹⁾

Institute of Solid State Physics of the Russian Academy of Sciences, 142432 Chernogolovka, Russia

Submitted 27 November 2018

Resubmitted 27 November 2018

Accepted 27

November 2018

DOI: 10.1134/S0370274X19030081

Similarly to topological insulators, Weyl semimetals

as tunneling through a potential barrier. On the other

have topologically protected Fermi arc surface states,

hand, an overall symmetric increase in dV/dI is a famil-

which are connecting projections of Weyl nodes on the

iar effect for electron scattering by emission of phonons

surface Brillouin zone [1]. WTe₂ is one of the realizations

and magnons [10].

of Weyl semimetal [2]. Spin- and angle-resolved pho-

For any transparency of Ni-WTe₂ junctions, we ob-

toemission spectroscopy data indeed demonstrate spin-

serve complex dV/dI peaks or dips structures at high

polarized surface Fermi arcs, and spin polarized Fermi

currents. These dV/dI features are well reproducible in

pockets in bulk spectrum [3, 4].

different cooling cycles (see also Fig. 1 below). They are

Intriguing spin properties of Weyl semimetals make

symmetric with respect to the current sign. There is no

it attractive material for spin investigations. The gener-

noticeable hysteresis with the current sweep direction

ation of both out-of-plane and in-plane spin-torque has

for experimental dV/dI(I) curves.

been demonstrated recently in few layers WTe₂ at room

The observed dV/dI(I) non-linearity as well as

temperature with ST-FMR and second harmonic Hall

dV/dI peaks or dips structures are sensitive to the mag-

measurements [5]. On the other hand, current-induced

netic field and temperature. The effect of temperature

excitation of spin waves, or magnons, is possible at large

is weak below 0.5 K. At higher temperatures, dV/dI(I)

electrical current densities for normal-ferromagnet junc-

non-linearity is diminishing. Above 1 K, the differential

tions [6-9]. Thus, it is reasonable to study spin-polarized

resistance is almost constant, so dV/dI(I)s are of stan-

transport between a ferromagnet and a Weyl semimetal

dard Ohmic behavior.

surface.

Figure 1 demonstrates evolution of dV/dI(I) curves

WTe₂ compound was synthesized from elements by

with magnetic field, which is applied along a, b and

reaction of metal with tellurium vapor in the sealed

c WTe₂ crystal axes, respectively. The effect of mag-

silica ampule. The WTe₂ crystal (with dimensions

netic field is sophisticated: in high fields, the zero-bias

500 µm × 100 µm × 0.5 µm) is transferred on top of

nonlinearity is suppressed, while the level of dV/dI(I)

the 50 nm thick ferromagnetic nickel leads with ≈ 10 ×

high-current saturation is unchanged, so that dV/dI(I)

10 µm² overlap and weakly pressed to form planar Ni-

curve is of clear Ohmic behavior above some magnetic

WTe₂ junctions.

field. In lower fields, the positions of dV/dI peaks are

We investigate transport properties of single Ni-

shifting to smaller currents.

WTe₂

junction by a three-point technique. From

We should connect the obtained results with spin-

dV/dI(I) independence on the particular choice of

dependent transport between a ferromagnetic Ni lead

current and voltage probes to the WTe₂ crystal, we

and WTe₂ surface states:

verify that the Ni-WTe₂ junction resistance dominates

(i) A ferromagnetic lead is essential, since neither

in the obtained dV/dI(I) curves.

dV/dI peaks nor an overall symmetric increase in dV/dI

For the transparent interface with low Ni-WTe₂

can be observed for normal or superconducting leads to

junction resistance, dV/dI is rising at low biases with

a single WTe₂ crystal for different junction transparen-

saturation at higher ones, see Fig.1. This behavior is

cies.

inconsistent with trivial impurity or roughness scatter-

(ii) Both dV/dI peaks and overall dV/dI(I) behav-

ing at the interface, which can generally be described

ior can be controlled by magnetic field, see Fig. 1.

(iii) Strong temperature dependence in the 30 mK-

¹⁾e-mail: dev@issp.ac.ru

1.2 K range can only originate from WTe₂ surface state,

176

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2019

Spin wave effects in transport between a ferromagnet and a Weyl semimetal surface

177

Fig. 1. (Color online) Evolution of dV /dI(I) curves with magnetic field, which is applied along a, b and c WTe2 crystal axes,

respectively. Qualitative effect is similar: the level of dV /dI(I) high-current saturation is constant; the zero-bias nonlinearity

is suppressed; the positions of dV /dI peaks are shifting to smaller currents. The effect is stronger in normal field, while there

is no difference for two in-plane orientations. Color scale on the left reflects differential resistance levels in (a), color scale

on the right refers to (b) and (c). The curves are obtained at 30 mK for the transparent Ni-WTe2 junction. The gradual

evolution of peaks’ positions also proves excellent reproducibility of these dV /dI features

since transport properties of Ni layer and well compen-

positions are shifted [7] to lower currents, see Fig.1, be-

sated WTe₂ bulk carriers are invariant in this tempera-

cause an external field simplifies spin-wave excitation in

ture range.

the WTe₂ surface state.

(iv) Fermi arc surface states contribution can be re-

Full text of the paper is published in JETP Letters

liably identified in charge transport between WTe₂ sur-

journal. DOI: 10.1134/S0021364019030020

face and a single non-magnetic contact.

Spin effects can be anticipated in WTe₂ surface

1. As a recent review see N. P. Armitage, E. J. Mele, and

states due to the presence of spin textures in the WTe₂

A. Vishwanath, Rev. Mod. Phys. 90, 015001 (2018).

Fermi arcs. Inelastic transport with magnon emission [9]

2. P. Li, Y. Wen, X. He, Q. Zhang, C. Xia, Z.-M. Yu,

is the most realistic variant, since the switchings are gov-

S. A. Yang, Z. Zhu, H. N. Alshareef, and X.-X. Zhang,

erned [7] by magnetic field in Fig.1.

Nature Comm. 8, 2150 (2017).

The crucial point is that the low-temperature zero-

3. P. K. Das, D. D. Sante, I. Vobornik et al. (Collabora-

bias resistance is smaller than the value, obtained at

tion), Nature Comm. 7, 10847 (2016).

high biases, temperatures, or magnetic fields, see Fig. 1.

4. B. Feng, Y.-H. Chan, Y. Feng et al. (Collaboration),

At zero bias, one can expect that spin polarization of

Phys. Rev. B 94, 195134 (2016).

some carriers at the WTe₂ surface is aligned parallel

5. D. MacNeill, G. M. Stiehl, M. H. D. Guimaraes,

to one in the ferromagnet due to the complicated spin

R. A. Buhrman, J. Park, and D. C. Ralph, Nature Phys.

13, 300 (2017).

texture of the topological Fermi arc surface state. This

6. M. Tsoi, A. G. M. Jansen, J. Bass, W.-C. Chiang,

allows a direct transport channel even for spin-polarized

M. Seck, V. Tsoi, and P. Wyder, Phys. Rev. Lett. 80,

carriers, which is reflected in low junction resistance at

4281 (1998).

zero bias. When increasing the current through the sur-

7. M. Tsoi, A. G. M. Jansen, J. Bass, W.-C. Chiang,

face state, spin-momentum locking produces preferable

V. Tsoi, and P. Wyder, Nature 406, 46 (2000).

spin polarization. It suppresses transport due to the re-

8. O. P. Balkashin, V. V. Fisun, I. K. Yanson, L. Yu. Tri-

quirement on spin rotation in transport events, which is

puten, A. Konovalenko, and V. Korenivski, Phys. Rev.

reflected as the overall dV/dI increase for both signs of

B 79, 092419 (2009).

the current.

9. T. Balashov, A. F. Takács, M. Däne, A. Ernst, P. Bruno,

Similarly to the transparent metallic junctions [6, 7],

and W. Wulfhekel, Phys. Rev. B 78, 174404 (2008).

the onset of the current-driven magnon excitations ap-

10. E. B. Myers, D. C. Ralph, J. A. Katine, R. N. Louie, and

pears as dV/dI peaks. In low magnetic fields, the peaks

R. A. Buhrman, Science 285, 867 (1999).

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