Observation of spin textures in La_1−xSr_xMnO₃ (x = 0.175)

Topological spin textures such as magnetic bubbles and magnetic skyrmion have been attracted attention in the field of magnetic materials because of their potential practical applications to new spin devices and quantum computing applications.^1–5 The magnetic skyrmion is characterized as a specific type of bubbles with whirling magnetic spins.¹ These complex topological spin textures are relevant to nontrivial physical phenomena such as the quantum and topological Hall effects in some chiral magnets with some non-centrosymmetric space groups.^6–8 Recently the existence of spin textures has been revealed in some chiral magnets such as MnSi and FeGe by neutron scattering experiments in the reciprocal space and Lorentz TEM observation in real space.^9–12 In addition, type-II magnetic bubbles and magnetic biskyrmion were found in the ferromagnetic phases of La_0.875Sr_0.125MnO₃ with perovskite-type structure and La_1.37Sr_1.63Mn₂O₇ with layered perovskite-type structure, which belong to the crystal structures with the centrosymmetric space group.^13,14 Recently complex spin textures at room temperature were reported in amorphous ferromagnetic Fe/Gd films with perpendicular magnetic anisotropy.¹⁵ So, it is anticipated that spin textures such as type-I and type-II magnetic bubbles should appear in the ferromagnetic phase with perpendicular magnetic anisotropy.

Perovskite-type manganites with the chemical formula A_1−xB_xMnO₃ (where A is a rare-earth and B is an alkali-earth atom) exhibit a wide diversity of ground states and some anomalous phase transitions. One of the most dramatic phenomena is the magnetic-field-induced insulator-to-metal transition, which is referred to as the colossal magnetoresistance effect. La_1−xSr_xMnO₃ shows a variety of ground states, which depend strongly on the Sr concentration (x).^16,17 In particular, the ground state in x > 0.15 is identified as the ferromagnetic metallic phase with the centrosymmetric orthorhombic structure (space group: Pbnm). Note that magnetic easy axes in the ferromagnetic phase of La_1−xSr_xMnO₃ for x > 0.15 are parallel to the [111] direction in the high-temperature rhombohedral structure and, on the other hand, to the [001] direction in the low-temperature orthorhombic structure.^18,19 In our previous work, Lorentz TEM observation in the ferromagnetic metallic phase of La_0.825Sr_0.175MnO₃ revealed that magnetic stripe domains with the Bloch-type magnetic domain wall appeared in the orthorhombic structure without external magnetic field (B = 0 T).²⁰ However, magnetic response of the magnetic stripe domains by applying external magnetic field has not been investigated so far.

In this work we have investigated topological spin textures and their responses by applying external magnetic field perpendicular to the thin plate of La_1−xSr_xMnO₃ for x = 0.175 by small-angle electron diffraction (SmAED) and low-temperature Lorentz transmission electron microscopic (TEM) experiments.

Single crystals of La_1−xSr_xMnO₃ (x = 0.175) were grown by the Floating Zone method.¹⁶ The crystal structure and the crystal orientation in the obtained single crystals were examined by powder x-ray diffraction (XRD) with Cu Kα radiation and by back-plate Laue-type x-ray diffraction techniques. The temperature variation of the magnetization was measured in a temperature range between 298 K and 2 K by using vibrating sample magnetometer (VSM) equipped with Quantum Design Physical Property Measurement System (PPMS). A thin plate for the Lorentz TEM observation was obtained by grinding with alumina powder and subsequently Ar⁺ ion sputtering at room temperature. An in-situ Lorentz TEM observation was conducted in order to clarify magnetic microstructures and magnetic responses of spin texture by applying magnetic field perpendicular to the thin plate. Note that external magnetic field was applied to the thin plate by exciting objective lens of the TEM. The SmAED experiments were carried out with the angular resolution of 10⁻⁵ ∼ 10⁻⁶ rad.²⁰ 

Figure 1(a) shows a powder x-ray diffraction profile obtained in the rhombohedral phase of La_0.825Sr_0.175MnO₃ at room temperature. All the diffraction peaks in Fig. 1(a) can be indexed based on the rhombohedral structure with the $R 3 ̄ c$ space group. This implies that the obtained single crystal sample is single phase without any impurities. Figure 1(b) shows change of the magnetization (M) as a function of temperature (T) in La_0.825Sr_0.175MnO₃. Note that the magnetization curves were acquired on heating from low temperature after cooling in zero magnetic field (ZFC) and applied fields (FC) of 4000 Oe. The magnetization curve on heating shows a drastic decrease from M = 3.4 μ_B around 270 K. This means that the ferromagnetic transition temperature should be T_C = 270 K. In addition, a cusp around 190 K with the thermal hysteresis appears due to the structural phase transition from the rhombohedral (⁠$R 3 ̄ c$⁠) to the orthorhombic (Pbnm) structures at T_s = 190 K, as shown in the inset of Fig. 2(b). These results are consistent with the experimental results in the previous work¹⁵.

Figure 2(a) displays a typical magnetic microstructure without external magnetic field (B ∼ 0 T) at 100 K, showing the presence of magnetic stripe domains consisting of characteristic meandering lines with paring of bright and dark contrast. Thus, in order to elucidate spatial distribution of magnetic moments in each domains, SmAED experiments with the angular resolution of 10⁻⁶ rad were carried out at 100 K. Note that the SmAED pattern was shown in the inset of Fig. 2(a). A spot at the 000 position in the reciprocal space appears as indicated by an arrow, in addition to some split spots due to magnetic deflection by magnetic moment in the sample. The appearance of the spot at the 000 position implies that there exists magnetic moment perpendicular to the thin plate in the magnetic stripe domains. Based on these experimental results, spatial distribution of the magnetic moments in the magnetic striped domain structure was illustrated in Fig. 2(b). The regions (α) between paired bright and dark contrast has in-plane magnetic moments parallel to the thin plate and other regions (β) have out-of-plane magnetic moments perpendicular to the thin plate.

We investigated magnetic response of magnetic stripe domains by changing the strength of perpendicular magnetic field in the range of B = 0 ∼ 1 T at 100 K. Note that external magnetic field was applied perpendicular to the (001) plane in the orthorhombic structure. As shown in Fig. 3(a), meandering magnetic stripe domains with paired bright and dark contrasts can be seen clearly in B = 0 T. Note that red arrows in Fig. 3(a) represent the directions of the magnetic moments. As shown in Fig. 3(b), when the magnetic field was applied perpendicular to the thin plate, the magnetic stripe domains were pinched off in a weak magnetic field of 185 mT, giving rise to dumbbell-shaped textures with bubble–like closed end. On increasing the strength of the magnetic field furthermore, type-I magnetic bubbles with the clockwise and counterclockwise curl of the in-plane magnetization appeared, as shown in Fig. 3(c). In addition, type-II magnetic bubbles are also distributed in a random manner. Namely, three distinct types of magnetic bubbles can be observed, as shown in three dotted rectangular of Fig. 3(c). Figure 3(d) shows illustrated description of these three distinct types of magnetic bubbles; (1) type-I magnetic bubbles with clockwise spin helicity, (2) type-I magnetic bubbles with counterclockwise spin helicity and (3) type-II magnetic bubbles. It is revealed that the complex spin textures of type-I and type-II magnetic bubbles appear by applying vertical magnetic field, which is in contrast to the case of the spatial regular configuration of the skyrmion lattice in the chiral magnets. These features of spin textures in La_0.825Sr_0.175MnO₃ are consistent with those of spin textures found in Sc-doped M-type barium ferrite.²¹ 

Figure 4 shows changes of type-I magnetic bubbles with respect to the spin configuration by tilting the thin plate. Note that the relationship between magnetic easy axis of the [001] direction and the direction of the applied magnetic field was depicted schematically in the inset of Fig. 4(b). It is found that type-I magnetic bubbles, which were obtained by applying magnetic field parallel to the magnetic easy axis, were transformed into type-II magnetic bubbles by tilting slightly the thin plate by θ = 2.0° around the [001] direction, as shown in Fig. 4(c). In some cases, type-I magnetic bubbles were transformed into the stripe domains by tilting the thin plate in the opposite direction by θ = − 2.7°, as shown in Fig. 4(b). These results indicated that the spin configuration of the magnetic bubbles depend strongly on the relationship between the magnetic easy axis and the orientation of applied magnetic field. These experimental results imply that type-I magnetic bubbles in the orthorhombic structure with the centrosymmetric crystal structure are stabilized by long-range dipole-dipole interactions, as opposed to the Dzyaloshinskii-Moriya (DM) interaction responsible for stabilizing the skyrmion phase in the non-centrosymmetric crystal structures such as MnSi and FeGe.^10,11

We have investigated magnetic spin textures in the ferromagnetic metallic phase of La_0.825Sr_0.175MnO₃ by SmAED technique and low-temperature Lorentz TEM. The magnetic stripe domains were transformed into magnetic bubbles with the type-I and type-II spin configurations by applying perpendicular magnetic field. When the magnetic field was applied parallel to the magnetic easy axis along the [001] direction, type-I magnetic bubbles appeared predominantly. When the thin plate was slightly tilted around the [001] axis, type-I magnetic bubbles were transformed into type-II magnetic bubbles. These results suggested that magnetic bubbles in La_0.825Sr_0.175MnO₃ are stabilized by long-range dipole-dipole interactions. It is anticipated that topological spin textures such as magnetic bubbles would be discovered in a large number of ferromagnetic materials with perpendicular magnetic anisotropy.

This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).