Preparation of magnetic tips for spin-polarized scanning tunneling microscopy on ${\mathrm{Fe}}_{1+y}\mathrm{Te}$

The interplay of electronic nematic modulations, magnetic order, superconductivity, and structural distortions in strongly correlated electron materials calls for methods which allow characterizing them simultaneously, to allow establishing directly the relationship between these different phenomena. Spin-polarized STM enables studying both electronic excitations as well as magnetic structure in the same measurement at the atomic scale. Here we demonstrate preparation of magnetic tips, both ferromagnetic and antiferromagnetic, on single crystals of FeTe. This opens up preparation of spin-polarized tips without the need for sophisticated ultrahigh-vacuum preparation.

Figure 1

Crystal structure of FeTe. (a) Crystal structure of $\mathrm{Fe}\mathrm{Te}$ showing four unit cells; Te atoms are shown in different red tones, Fe atoms in yellow. (b) Surface termination when cleaving between FeTe layers; the surface has a Te termination. The known magnetic structure obtained from neutron scattering is indicated by red and blue arrows [7, 12].

Figure 2

Preparation of magnetic tips on FeTe. (a) Schematic illustration of the process of picking up excess iron atoms on ${\mathrm{Fe}}_{1+y}\mathrm{Te}$ to prepare a ferromagnetic tip. (b) Topographic STM image showing how a tip without magnetic contrast turns into one which exhibits magnetic contrast due to a tip change $\left(V_b=-80\mathrm{mV},I_t=1.5\mathrm{nA}\right)$. Atoms are frequently moved or picked up by scanning with a rather large tunneling current of up to a few nA. (c) STM image $\left(V_b=90\mathrm{mV},I_t=0.2\mathrm{nA}\right)$ of hole left behind due to a tip indentation. The line profile across the hole is shown. The hole indicates that an $\mathrm{FeTe}$ cluster was picked up. (d) Fourier transform $\tilde{z}\left(\mathbf{q}\right)$ of a topography which exhibits magnetic contrast showing peaks corresponding to the square lattice of $\mathrm{Te}$ atoms $({\mathbf{q}}_{\mathrm{Te}}^a$ and ${\mathbf{q}}_{\mathrm{Te}}^b)$ and the antiferromagnetic order $\left({\mathbf{q}}_{\mathrm{AFM}}\right)$.

Figure 3

Magnetic field dependence of images obtained with an antiferromagnetic tip. (a), (b) STM topographies obtained at magnetic fields of $+7\mathrm{T}$ (a) and $-7\mathrm{T}$ (b), both taken in the same location. The stripes due to the magnetic contrast maintain the same phase with respect to defects on the surface $\left(V_b=80\mathrm{mV},I_t=100\mathrm{pA}\right)$. (c) Line cuts extracted by averaging in a direction normal to ${\mathbf{q}}_{\mathrm{AFM}}$ at selected fields from a series of images at the location marked by a white dashed rectangle in (a) and (b). (d) Field dependence of the magnetic contrast: the amplitudes of the Fourier components of the magnetic modulation $|\tilde{z}\left({\mathbf{q}}_{\mathrm{AFM}}\right)|$ as well as for the atomic lattice vectors $|\tilde{z}\left({\mathbf{q}}_{\mathrm{Te}}^a\right)|$ and $|\tilde{z}\left({\mathbf{q}}_{\mathrm{Te}}^b\right)|)$ are shown. (e) Phase $\phi \left(\mathbf{q}\right)=\arg \left(\tilde{z}\left(\mathbf{q}\right)\right)$ of the Fourier components at ${\mathbf{q}}_{\mathrm{Te}}^a$ $\left(={\mathbf{q}}_{\mathrm{CDW}}\right)$, ${\mathbf{q}}_{\mathrm{Te}}^b$, and ${\mathbf{q}}_{\mathrm{AFM}}$ as a function of field.

Figure 4

Magnetic field dependence of contrast obtained with a ferromagnetic tip. (a)–(d) Topographic images acquired at different magnetic fields, (a) and (b) have been obtained while ramping the field from positive to negative field and (c) and (d) during ramping in the opposite direction $\left(V_b=60\mathrm{mV},I_t=200\mathrm{pA}\right)$. The images shown are selected from a series, showing the images right before and after the magnetization of the tip has switched. (e) Field dependence of the magnetic contrast: the amplitudes of the Fourier components of the magnetic modulation $|\tilde{z}\left({\mathbf{q}}_{\mathrm{AFM}}\right)|$ as well as for the atomic lattice vectors $|\tilde{z}\left({\mathbf{q}}_{\mathrm{Te}}^a\right)|$ and $|\tilde{z}\left({\mathbf{q}}_{\mathrm{Te}}^b\right)|$ are shown. The amplitudes at the atomic lattice vectors remain almost constant, whereas the one at ${\mathbf{q}}_{\mathrm{AFM}}$ shows a change by up to $\sim 60\%$. (f) Phase $\phi \left(\mathbf{q}\right)=\arg \left(\tilde{z}\left(\mathbf{q}\right)\right)$ of the Fourier components at ${\mathbf{q}}_{\mathrm{Te}}^a$ $\left(={\mathbf{q}}_{\mathrm{CDW}}\right)$, ${\mathbf{q}}_{\mathrm{Te}}^b$, and ${\mathbf{q}}_{\mathrm{AFM}}$ as a function of field. As for the amplitude, the phase for ${\mathbf{q}}_{\mathrm{Te}}^a$ and ${\mathbf{q}}_{\mathrm{Te}}^b$ stays almost constant, while the one at ${\mathbf{q}}_{\mathrm{AFM}}$ reveals a hysteresis due to a ferromagnetic cluster at the apex of the tip. Data were taken while sweeping the field first down and then up again; green arrows indicate the direction of field sweep.

Figure 5

Intensity of CDW peak. (a) STM image of a twin boundary (taken at field $B=0\mathrm{T})$, the stripes are mainly seen on the right side of the boundary. Letters b and c in panel (a) mark areas where topographies shown in (b) and (c) have been cut out, (d) and (e) show their Fourier transforms. It can be seen that the peaks of the atomic lattice (marked by blue ellipses) are more intense in the direction of the SDW wave vector (marked by a red ellipse). Since both are taken with the same tip, this asymmetry has to be related to a modulation of the charge density near the Fermi level $\left(V_b=80\mathrm{mV},I_t=200\mathrm{pA}\right)$.

Figure 6

Magnetization dynamics of a ferromagnetic tip. (a) Amplitudes $|\tilde{z}\left({\mathbf{q}}_{\mathrm{Te}}^{\mathrm{a}}\right)|,|\tilde{z}\left({\mathbf{q}}_{\mathrm{Te}}^b\right)|$, and $|\tilde{z}\left({\mathbf{q}}_{\mathrm{AFM}}\right)|$ and (b) phase $\phi \left({\mathbf{q}}_{\mathrm{Te}}^{\mathrm{a}}\right),\phi \left({\mathbf{q}}_{\mathrm{Te}}^b\right)$, and $\phi \left({\mathbf{q}}_{\mathrm{AFM}}\right)$ as a function of magnetic field for a magnetic tip which shows a phase shift at fields of $-0.4\mathrm{T}$ and $-0.2\mathrm{T}$. (c) Tunneling current as a function of time measured at $-0.2\mathrm{T}$, right at the field where the phase shift occurs, obtained at $V_b=-80\mathrm{mV}$ and with open feedback loop. It shows spontaneous transitions between two magnetizations of the tip; the noise also appears in topographic images taken at the same field as shown the current does not exhibit the transitions, shown here for a magnetic field of $-0.8\mathrm{T}$; the noise also disappears in topographic images.

Figure 7

Characterization of in-plane component of magnetization. (a) STM image of a twin boundary (taken at field $B=0\mathrm{T})$, the stripes are only seen on the right side of the boundary; (b) scheme explaining the symbol used in panels (c)–(e) to indicate the magnetization direction of the tip and the angle $\alpha$. (c)–(e) STM topographies cut out from images taken in the same location as (a) at magnetic fields of +5 T, 0 T, and $-5$ T, all with the same tip $\left(V_b=80\mathrm{mV},I_t=100\mathrm{pA}\right)$. The in-plane magnetization direction of the tip extracted from the topographies is shown by a double arrow. Regions I and II shown in panels (c)–(e) are indicated by solid lines in (a); panel (d) is cut out from the image shown in (a).