Hidden peculiar magnetic anisotropy at the interface in a ferromagnetic perovskite-oxide heterostructure

Understanding and controlling the interfacial magnetic properties of ferromagnetic thin films are crucial for spintronic device applications. However, using conventional magnetometry, it is difficult to detect them separately from the bulk properties. Here, by utilizing tunneling anisotropic magnetoresistance in a single-barrier heterostructure composed of La0.6Sr0.4MnO3 (LSMO)/LaAlO3 (LAO)/Nb-doped SrTiO3 (001), we reveal the presence of a peculiar strong two-fold magnetic anisotropy (MA) along the [110]c direction at the LSMO/LAO interface, which is not observed in bulk LSMO. This MA shows unknown behavior that the easy magnetization axis rotates by 90° at an energy of 0.2 eV below the Fermi level in LSMO. We attribute this phenomenon to the transition between the e g and t 2g bands at the LSMO interface. Our finding and approach to understanding the energy dependence of the MA demonstrate a new possibility of efficient control of the interfacial magnetic properties by controlling the band structures of oxide heterostructures.

however, suggest new ways for controlling the interfacial properties at an atomic level, which are not available in the bulk. To this end, the characterization of the interfacial magnetic properties is highly demanded, but it is difficult with conventional magnetometry because the interfacial properties are usually concealed by the dominant signals from the bulk. Here, by utilizing TAMR in an LSMO/LAO/Nb:STO junction, we obtain the carrier-energy dependence of MA of LSMO for the first time. We also reveal a peculiar strong two-fold symmetry component of MA at the LSMO/LAO interface, which is not observed in bulk LSMO. Moreover, this interfacial MA shows unknown behavior that the symmetry axis of this interface MA rotates by 90° at an energy of 0.2 eV below the Fermi level in LSMO. We attribute this phenomenon to the transition between the e g and t 2g bands at the LSMO interface. Our results suggest that controlling the band structure at interfaces will pave a new way for efficient control of the magnetization of FM thin films, which is essential for devices with low-power consumption.

Results
Sample preparation and characterizations. The heterostructure used in this study consists of LSMO (40 unit cell (u.c.) = 15.6 nm)/LaAlO 3 (LAO, 4 u.c. = 1.6 nm) grown on a TiO 2 -terminated Nb-doped SrTiO 3 (001) substrate (Nb:STO, Nb 0.5% wt.) by molecular beam epitaxy (MBE) (see Fig. 1a and Methods) 24,25 . The in-situ reflection high-energy electron diffraction (RHEED) patterns in the [100] direction of the 4-u.c. LAO and 40-u.c. LSMO layers show streaky patterns, and especially LSMO exhibits a bright pattern (Fig. 1b), indicating that the sample surface is atomically flat. In fact, the atomic force microscopy measurements show flat terraces and atomic steps with a height of ~0.4 nm, which is equal to one pseudocubic u.c. (Fig. 1c). In the x-ray reciprocal lattice map of the sample measured around the (204) c and (204) c reflections of the Nb:STO substrate at room temperature, we see two weaker peaks corresponding to the (260) o and (620) o reflections of the LSMO epilayer (we use the subscripts c and o for the pseudocubic and the orthorhombic crystal structures, respectively) (Fig. 1d). These results confirm that the LSMO layer is coherently grown with respect to the Nb:STO substrate. The (260) o and (620) o peaks of LSMO have nearly the same out-of-plane reciprocal lattice vector Q ⊥ , indicating that the (260) o and (620) o atomic plane spacings are equal. This is consistent with the common reports on LSMO thin films grown under tensile strain, indicating that the strain effect in LSMO is accommodated equally between the [100] c and [010] c directions 22 . For tunneling transport measurements, 600 × 700 μm 2 mesas were formed by standard photolithography and Ar ion milling. The bias polarity is defined so that the current flows from the LSMO layer to the Nb:STO substrate in the positive bias. Figure 2a shows the conduction band (CB) profiles of the LSMO/LAO/Nb:STO tunnel diode under positive and negative bias voltages V. The Fermi level E F is located at 10-20 meV above the CB bottom of STO due to the Nb doping (0.5% wt., the electron density n = 1 × 10 20 cm −3 ) 26 , while E F lies in the CB formed by the Mn 3d-e g states in LSMO. The LAO layer serves as a tunnel barrier with a height of ~2.4 eV for STO and ~2 eV for LSMO 27 . TAMR measurements were conducted as follows: dI/dV − V curves were measured at 4 K while applying a strong external magnetic field of 1 T, which aligned the magnetization direction parallel to the magnetic field, in various in-plane directions with an angle step of 10°. The change in dI/dV when rotating the external magnetic field is attributed to the change in the DOS at the LSMO/LAO interface or the LAO/STO interface. As illustrated in Fig. 2a, at positive (negative) V, electrons tunnel from Nb:STO to LSMO (from LSMO to Nb:STO), and thus dI/dV probes the DOS of unoccupied (occupied) states in LSMO. At each V and Φ, where Φ is the angle of the magnetization direction from the [100] c axis in the counter-clockwise direction in the film plane, we define ∆ ( )   Fig. 2c, where Δ(dI/dV) is plotted as a function of Φ at V ranging from −0.5 to 0.5 V. This plot shows a peculiar behavior that the symmetry axis changes at V ~ −0.2 V. We fit the data at each bias V using the following equation:

Discussions
We discuss the origins of these anisotropy components.  sin Φ cos Φ, where Φ is the angle between the magnetization and the current direction 32 . In Fig. 3, we show the PHR measured for the reference sample at various magnetic field directions, where θ is the angle between the magnetic field and the current flown in the [100] c axis. ΔR is defined as the PHR with respect to the one at a zero magnetic field. In contrast to the TAMR results, the PHRs of the reference sample measured when the magnetic field is parallel to the [110] c and [110] 33,34 , and that the OOR in the underlayer is transferred to the first 3-4 u.c. layers of LSMO 21,22 (Fig. 4a). Figure 4b shows four adjacent MnO 6 octahedra in a (001) c plane of LSMO near the LSMO/LAO interface. We see that under the OOR around the [111] c axis illustrated in Fig. 4a, the oxygen octahedra located along the [110] c direction rotate in the same direction (see the green oxygen spheres around Mn1 and Mn3). This rotation direction is the opposite to that of the adjacent rows of Mn atoms (see Mn2 and Mn4). When Fig. 4b is projected in the (110) c plane as shown in Fig. 4c  The most striking feature found in our study is the sign reversal of C 2[110] at V = −0.2 V (Fig. 2d). This behavior is likely related to the band structure of LSMO, as explained below. In Fig. 2d, one can see that C 4〈110〉 and C 2[100] show similar V dependence in all the V region. These results indicate that both originate from the same band located around E F of LSMO, i.e. the up-spin Mn 3d-e g band. The C 4〈110〉 and C 2[100] components disappear at ~V = −0.45 V, which means that E F is located at ~0.45 eV above the bottom of the e g band. This is consistent with the results of angle-resolved photoemission spectroscopy (ARPES) measurements for LSMO 35 . Therefore, the emergence of positive C 2[110] below V = −0.2 V is likely associated with the t 2g band, which is located below the e g band. Although the t 2g state is located at 0.5-1 eV below E F in bulk LSMO 35 , it is thought to be largely pushed up by the polar mismatch at the LSMO/LAO interface 36 . Thus, we attribute the sign change of C 2[110] to the transition from the e g band (V > −0.2 V) to the t 2g band (V < −0.2 V) at the LSMO interface. As mentioned above, due to the OOR at the LSMO/LAO interface, the DOSs of both the e g and t 2g bands in the [110] c direction are larger than those in the [110] c direction. However, the relationship between the DOS and the magnetization in these two band components is opposite: It is known that the electron transfer via the e g orbitals enhances the double exchange interaction and strengthens the ferromagnetism, while the one between the t 2g orbitals enhances the super-exchange interaction and weakens the ferromagnetism 37 . Therefore, the enhancement of DOS in the [110] c direction relative to that in the [110] c direction makes the [110] c axis the easy magnetization direction in the case of the e g orbitals, while hard magnetization direction in the case of the t 2g orbitals. The transition between these two bands at V = −0.2 V thus leads to the opposite magnetization direction-dependence of the DOS, as consequently observed by TAMR 5 .
In summary, using TAMR measurements, we have successfully obtained a high-resolution map of the MA spectrum of LSMO for the first time. In addition to the biaxial MA along 〈100〉 c and the uniaxial MA along [100] c , which originate from bulk LSMO, we found a peculiar uniaxial MA along the [110] c , which is attributed to the LSMO/LAO interface. The symmetry axis of this interface MA rotates by 90° at an energy of 0.2 eV below E F of LSMO, which is attributed to the transition from the e g band (>−0.2 eV) to the t 2g band (<−0.2 eV). These findings hint an efficient way to control the magnetization at the LSMO thin film interfaces, as well as confirm the rich of hidden properties at thin film interfaces that can be revealed only by interface-sensitive probes. This work also suggests the use of the TAMR measurement as a simple but highly sensitive method for characterizing interfacial magnetic properties of magnetic tunnel junctions, which is important for developing spintronic devices.

Methods
The heterostructure used in this study consists of LSMO (40 unit cell (u.c.) = 15.6 nm)/LaAlO 3 (LAO, 4 u.c. = 1.6 nm) grown on a TiO 2 -terminated Nb-doped SrTiO 3 (001) substrate (Nb:STO, Nb 0.5% wt.) by molecular beam epitaxy (MBE) with a shuttered growth technique 24,25 . The fluxes of La, Sr, Mn, and Al were supplied by Knudsen cells. The LAO and LSMO layers were grown at 730 °C with a background pressure of 2 × 10 −4 Pa of a mixture of oxygen (80%) and ozone (20%). After the growth, the sample was further annealed at 600 °C in ambient atmosphere for 1 hour to reduce the density of oxygen vacancies.
For tunneling transport measurements, a 50-nm-thick Au film was deposited on top of the sample, and 600 × 700 μm 2 mesas were then formed by standard photolithography and Ar ion milling. Au wires were bonded to the Au electrode and the backside of the Nb:STO substrate by indium.
The dI/dV-V characteristics were numerically obtained from the I-V data with a differential interval of 10 mV.
See Supplementary Information for more details on how to extract the ∆ ( ) dI dV data plotted in Fig. 2.

Data Availability.
The datasets of the current study are available from the corresponding author on reasonable request.