Anisotropic magnetoresistance in topological insulator Bi1.5Sb0.5Te1.8Se1.2/CoFe heterostructures

Topological insulator is composed of an insulating bulk state and time reversal symmetry protected two-dimensional surface states. One of the characteristics of the surface states is the locking between electron momentum and spin orientation. Here, we report a novel in-plane anisotropic magnetoresistance in topological insulator Bi1.5Sb0.5Te1.8Se1.2/CoFe heterostructures. To explain the novel effect, we propose that the Bi1.5Sb0.5Te1.8Se1.2/CoFe heterostructure forms a spin-valve or Giant magnetoresistance device due to spin-momentum locking. The novel in-plane anisotropic magnetoresistance can be explained as a Giant magnetoresistance effect of the Bi1.5Sb0.5Te1.8Se1.2/CoFe heterostructures.


INTRODUCTION
Recently, a new type of topological condensed matter system characterized by Z 2 topological invariant, which is different from the Chern number topological invariant in quantum Hall system, has been proposed by theorists and then confirmed quickly by experimental physicists [1][2][3][4][5] . This new phase is called topological insulator.
Topological insulator is composed of an insulating bulk state and time reversal symmetry protected two-dimensional spin helical surface states. Here, the spin-helicity means that the spin-up electrons propagate in one direction while spin-down electrons propagate in the opposite direction. The phenomenon of electron momentum determined spin orientation is called spin-momentum helical locking, which has been confirmed in recent angle-resolved photoemission (ARPES) experiments [6][7][8][9][10][11][12][13] . The protection against electron scattering was proved by scanning tunneling microscopy (STM) 14,15 . Various novel electrical transport behaviors of topological insulators have been theoretically predicted and experimentally reported in recent literatures  . Surface transport induced Aharonov-Bohm oscillation was observed in Bi 2 Se 3 and Bi 2 Te 3 nanowires 16,17 . Topological surface state induced Hall effect has been observed 23 . π Berry phase and strong spin-orbit interaction induced weak antilocalization on the exotic topological surface state has been thoroughly studied both theoretically and experimentally [24][25][26][27][28][29][30][31][32][33][34][35][36][37] . It is well known that probing helical surface transport in electrical transport experiments is challenging due to the large bulk contribution in prototype topological insulator systems, such as Bi 2 Se 3 , Bi 2 Te 3 , etc. Recently, new topological insulators Bi 2 Te 2 Se 38, 39 and Bi 2-x Sb x Te 3-y Se y 40, 41, 52 with huge bulk resistivity (1 -10 Ω cm) and dominated surface transport were synthesized. The topological surface states in the material systems have been confirmed by ARPES 41 . In our recent work, pronounced electrical field ambipolar 3 effect has been realized in gated devices fabricated by 100 -200 nm thick nanoflakes exfoliated from Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 crystals 52 . We also observed 2D weak antilocalization behavior in a 596 nm thick Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 single crystalline flake 52 . These results indicate that dominated surface transport can be realized in Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 single crystalline flakes with a thickness of several hundred nanometers. These materials offer a well characterized playground for studying the topological surface state.
Topological insulator/ferromagnet interface is a potentially very interesting system to investigate. Various novel devices based on ferromagnets and topological insulators have been proposed in recent years [42][43][44][45][46][47] . For example, spin and electron transport on topological insulators with ferromagnetic electrode has been studied theoretically 42 .
Inverse spin-galvanic effect has been predicted to occur at the interface between a topological insulator and a ferromagnet 43 . The thin film heterostructure based on topological insulators and ferromagnet was believed to generate giant magneto-optical Kerr effect and universal Faraday effect 45 . However, as far as we know, the transport experiment on topological insulator/ferromagnet has never been attempted so far.
In this work, we measured the magnetoresistance of a Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 /CoFe heterostructure. We observed unusual in-plane anisotropic magnetoresistance effect that is determined by the angle θ between the applied magnetic field and the current direction. With a model based on Giant magnetoresistance (GMR) effect due to the peculiar characteristic of the surface states of topological insulator, the spin-momentum locking, we successfully explain the novel in-plane anisotropic magnetoresistance. In this model, the two spin polarized electron transport channels are the spin polarized topological surface states and the ferromagnetic CoFe layer.

II. EXPERIMENTS
High quality single crystals of Bi heterostructures. Both the current and magnetic field were applied in the (001) plane.
The BSTS single crystals and BSTS/CoFe interface were examined by high resolution transmission microscopy (HRTEM).

A. The temperature dependence of resistivity
We have measured the R(T) curve of more than 15 pieces of 50-100 μm thick cleaved single crystals. All of them show similar R (T) behaviors. The resistance increases with decreasing temperature at high temperature regime, which indicates 5 that the Fermi level is in the band gap. Only the R (T) curves of three samples are plotted in Fig. 1. We name the samples as S1, S2 and S3. The resistivity at low temperature of every sample is higher than 1 Ω·cm. Based on the recent electric transport results, the topological surface conductance contribution can get to ~10% in a ~ 100 μm thick BSTS sample at 10 K 40,52 . As shown in the inset of Fig. 1, the temperature dependence of the magnetoresistance of the sample S3 changes from weak anti-localization type to normal parabolic type. The weak anti-localization type magnetoresistance is due to the π Berry phase of the helical surface state and the strong spin orbit interaction in the bulk 33,34,48 . The thickness dependence of weak anti-localization behavior indicate that the helical surface state contribute significantly to the weak anti-localization in a ~100 μm thick BSTS sample 52 .

B. In-plane anisotropic magnetoresistance measurements
To study the exotic helical surface of the topological insulator with significant surface transport, we performed in-plane anisotropic magnetoresistance measurements on BSTS/CoFe (5 nm) heterostructures. For every resistance vs. magnetic field measurement, the angle θ between the current and applied magnetic field was fixed. A series of the measurements were performed by changing θ from 0 o to 360 o with 15 o interval. In the measurements, the samples were cooled down to 10 K with zero field. After that, the magnetic field was set to 10 kOe to saturate the ferromagnetic moment in CoFe layer, then swept to -10 kOe and finally drove back to 10 kOe. When the magnitude of the magnetic field was larger than 800 Oe, the data was collected every 1 kOe. Between -800 Oe and 800 Oe, the resistivity was measured every 10 Oe. , in which the current is applied to C direction. Its anisotropic magnetoresistance shows the same characteristic as that of the BSTS(sample S1)/CoFe ( Fig. 2A). The current flow in BSTS (sample S3b)/CoFe is along direction A, which also shows similar anisotropic magnetoresistance behavior. From these measurements, we can obtain two important conclusions. Firstly, the novel square shape magnetoresistance can be confirmed in BSTS/CoFe structures fabricated from different BSTS crystals. Secondly, the current flowing along different crystalline direction shows negligible effect on the anisotropic magnetoresistance. We also studied the temperature dependence of the square shape magnetoresistance (sample S1) as shown in Fig. 3. All the measurements were carried out with θ = 30 o , because the 7 evolution of the square shape magnetoresistance is more easily to be studied. It is found that the square shape magnetoresistance disappears when the temperature is higher than 50 K. The high temperature magnetoresistance (T > 50 K) is composed of a positive magnetoresistance from BSTS single crystal and two very broad peaks (negative magnetoresistance) which may be originated from the CoFe layer. The square shape type anisotropic magnetoresistance and its temperature evolution are phenomena unforeseen in normal single layer ferromagnetic systems.
To study the exotic magnetoresistance behavior, we measured the anisotropic magnetoresistance of BSTS single crystal and 5 nm thick CoFe film on Si (111)

C. A model to explain the novel anisotropic magnetoresistance: spin-momentum locking induced Giant magnetoresistance
As shown in the cross section TEM of BSTS/CoFe heterostructure (Fig. 5), the interface between BSTS single crystal and CoFe thin film is clear although not very sharp. From the STEM picture, we can conclude that the element distribution between BSTS and CoFe is pretty sharp, which indicates that the chemical reaction between BSTS and CoFe is negligible. As we grew the CoFe thin film at room temperature, this result can be expected. Therefore, the effect of an unknown compound formed at the interface can be ruled out.
In order to explain the novel anisotropic magnetoresistance, we propose that, due to spin-momentum locking, the BSTS/CoFe heterostructure in fact form a spin valve or Giant magnetoresistance (GMR) device. The novel anisotropic magnetoresistance is a GMR effect.
As shown in Fig. 6(a) all the spins in the layer will flip at the coercive field, therefore the resistance shows a sudden jump at the coercive field (square shape R (H) curve). On the other hand, if the magnetic field is not along the easy axis, the spin flipping is a process of gradual evolution, therefore the resistance of the spin valve structure varies slowly with changing magnetic field. The applied current in a GMR device is usually in-plane.
The key point here is that the BSTS/CoFe system forms a spin-valve or GMR system with one fixed spin-polarized layer and one free spin-polarized layer. As shown in Fig. 6 we observed in our measurements (Fig. 2). It should be noted that the GMR effect is only determined by the relative orientation of the current flowing and magnetic field.
As the magnetic field is in-plane, the 180 o rotation of the sample is equivalent to the switching the current flowing direction to opposite direction, which will switch the spin orientation to opposite direction due to spin-momentum locking. As for the square shape magnetoresistance at 30 o , 210 o , 150 o , and 330 o , we believe that it is due to the magnetic anisotropy of the CoFe layer. As shown in Fig. 6(b), the BSTS/CoFe system has biaxial magnetic easy axes near θ = 30 o (210 o ) and θ = 150 o (330 o ). As discussed in the last paragraph, when the applied magnetic field is along the easy axis, all the spins in the free layer switches to the opposite direction simultaneously at the coercive field and therefore a square shape R (H) curve is obtained. The origin of the easy axis is still not clear yet. It may originate from the coupling between the helical surface state of Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 and CoFe, which diminishes at high temperature.
As the novel magnetoresistance behavior is due to the spin-momentum locking at the helical surface, it is also easy to explain the temperature dependence of the spin-valve type magnetoresistance as shown in Fig. 3. It should be noticed that the weak anti-localization behavior of BSTS (the inset of Fig. 1) and the GMR type magnetoresistance (Fig. 2) appear in the same temperature range and disappear with 12 increasing temperature. The interconnected results indicate that the disappearance of GMR magnetoresistance with increasing temperature may be due to the loss of significant helical surface transport at high temperature. The high temperature magnetoresistance (T > 50 K) is composed of a magnetoresistance from BSTS single crystal and an anisotropic magnetoresistance from the CoFe layer. It also should be noted in Fig. 3 that the magnetoresistance shows two downward sharp peaks at 20 K.
It might be due to the competing effect of thermal agitation energy and coupling between momentum-locked spin and magnetic moment of CoFe. It is well known that the GMR effect is only determined by the average spin polarization of electrons moving from high potential to low potential. A larger current density only means that the percentage of electrons moving to a certain direction becomes larger. The average spin polarization of the current will remain the same. As larger current density will not change the spin polarization of the current, the anisotropic magnetoresistance should remain the same with changing current density, which is also confirmed by our measurement as shown in Fig. 7.

IV. CONLUSION
In summary, novel in-plane anisotropic magnetoresistance has been observed in  Fig. 1. Temperature dependence of the resistivity of three BSTS single crystals (S1, S2 and S3). The inset shows the temperature dependence of the magnetoresistance of sample S3 at 10 K, 30 K, 50 K and 100 K respectively.