Abstract
Although the content of Eu in the single crystal (La0.8Eu0.2)4/3Sr5/3Mn2O7 is very small, the effects of Eu ions on the magnetic and transport properties are quite evident at low temperature due to the large size difference between Eu and La ions, compared with the crystal’s parent compound La5/3Sr4/3Mn2O7. The sharp jumps in M(H) and ρ (H) curves suggest that a first-order Field-induced transition from magnetically disorder state to 3D long-range ferromagnetic metallic state takes place at 2 K. Both resistivity and magnetization dependents on the magnetic history and clearly demonstrate hysteretic behaviors. These results indicate that the system has a very fragile ground state and the local lattice distortions in bilayer manganites play an important role in the physical properties.
Introduction
Since the discovery of the colossal magnetoresistance (CMR) in manganese perovskite of R1-xA x MnO3 (R is a trivalent rare earth, A is divalent alkaline earth ion), a great deal of attention has been paid to understanding the properties of compounds [1]. These R1-xA x MnO3 manganites are the n =∞ members of the series of Ruddlesden-Poper (RP) phase A n +1B n O3n +1 [2]. More recently, bilayer manganites with n = 2 of the type La2-2xSr1+2xMn2O7 have been investigated and compared to the n =∞ case [3, 4]. Because the reduction of the structure dimension, the bilayer manganites are more attractive and interesting than the n =∞ series. They not only show high anisotropies in magnetic and transport properties, but also provide us with a qusi-2D environment for studying CMR effect. The physical properties of these perovskite manganites are mainly due to the interplay between charge, spin, orbital and lattice degrees of freedom. Understanding the origin of these interesting phenomena is a true challenge for solid condensed matter physics. The bilayer manganites La2-2xSr1+2xMn2O7 (x = 0.4) is one of the subjects studied frequently [3, 5–7]. Mn spins in neighboring layers within each bilayer of La1.2Sr1.8Mn2O7 are strongly canted at an angle dependent on both the magnetic field and the temperature above T c [6, 9]. More and more evidences suggest that distortions play an important role in the physical properties of La1.2Sr1.8Mn2O7 [10, 11]. In addition, non-La doping can also induce local lattice distortion, which may give rise to spin-glass-like behaviors or magnetization jumps. Two research group have respectively found magnetization jumps in (La0.4Pr0.6)1.2Sr1.8Mn2O7 and (La0.6Nd0.4)1.2Sr1.8Mn2O7 crystal [12, 13]. Both of the crystals have c-axis as the easy axis [13, 14]. For (La0.4Pr0.6)1.2Sr1.8Mn2O7, a magnetic field can realize a long-range ferromagnetic order and change the orbital occupancies of eg electrons [12, 15]. A decrease of resistance by a factor of one million is also observed in the crystal [14]. We note that magnetization (resistivity) jumps are also observed in some R1-xA x MnO3 compounds, such as Pr0.6Ca0.4Mn0.96Ga0.04O3 [16], Pr0.5Ca0.5Mn0.97Ga0.03O3 [17], Pr0.5Ba0.5MnO3 [18], Pr0.5Ca0.5Mn0.95Ga0.05O3 [19], and Nd0.5Sr0.5MnO3 [20]. Except Nd0.5Sr0.5MnO3, although the reasons reported for the jumps are a little different, most of the authors think that phase separation and inhomogeousness play an important role in the jumps. Because the critical magnetic field for the jump depends on the magnetic field sweep rate used for record the data, some researchers studying R1-xA x MnO3 argue that martensite transformation is the origin of the magnetization jump [16, 18, 19, 16, 18, 19].
In this work, we show that Eu-doped La5/3Sr4/3Mn2O7 is that a field-induced ferromagnetic transition, namely, a magnetization jump. Although only 4/45 A-site ions is substituted by Eu, the crystal’s properties are greatly different from its parent compound, suggesting a strong influence of the local lattice distortion induced by Eu. However, the crystal has many special characteristics of its own, unlike (La0.4Pr0.6)1.2Sr1.8Mn2O7 and (La0.6Nd0.4)1.2Sr1.8Mn2O7 crystals, indicating the sensitive A-site-ion dependence of bilayer mangnites.
Experimental procedures
Single crystals of (La0.8Eu0.2)4/3Sr5/3Mn2O7 were melt-grown in flowing air in a floating-zone optical image furnace. A large crystal with two shiny surfaces was cleaved from one of these crystals. X-ray diffraction (XRD) and back-reflection Laue XRD experiments were carried out to check the single crystallinity and determined the crystallographic direction. A piece of single crystal was cut and chosen to take magnetization and resistivity measurements in a commercial physical properties measurement system (Quantum Design, PPMS-14). The resistivity was measured by the standard four-point technique.
Results and discussion
Crystals from parts of the boule were crushed into powder, whose powder XRD patterns matched the patterns of the Sr3Ti2O7-type perovskite (space grounp I4/mmm) quite well. As shown in Fig. 1, some powder of the crystals is checked by inductively coupled plasma atomic emission spectroscopy (ICP). The ICP analysis reveals that the atomic ratio of La, Eu,Sr and Mn is 0.96:0.21:1.79:2.00, which is quite close to the expected ratio. The refinement of the patterns showed that a = 3.866 Å, c = 20.065 Å and c/a = 5.1897. For the single crystal La4/3Sr5/3Mn2O7 grown by us, the lattice constants were as follows: a = 3.876 Å, c = 20.090 Å and c/a = 5.1832. As anticipated after noting the contractions of 4f electrons in lanthanide ions, the lattice parameters of (La0.8Eu0.2)4/3Sr5/3Mn2O7 are slightly reduced as compared with the ones of La4/3Sr5/3Mn2O7. The ratio c/a is a scale of the lattice distortion. For (La0.8Eu0.2)4/3Sr5/3Mn2O7, it is 5.1897, larger than that of La4/3Sr5/3Mn2O7 5.1832.
XRD patterns for (La0.8Eu0.2)4/3Sr5/3Mn2O7 at room temperature. Upper curve: data and fit, with difference plot below. Bottom ticks indicate nuclear peaks corresponding to the I4/mmm space group.
So Eu-doping induces large local lattice distortion, especially, around Eu ion. The larger c/a favour 3d3z2- r2 orbital of eg electron. From this aspect, the crystal tends to have c-axis as easy axis and from 3D ferromagnetism more easily than LSMO (327) [8, 21–23]. However, magnetization measurements show that it is not the case. Therefore, here the local lattice induced by Eu-doping must plays a more important role. The changing tendency of the lattice parameters for our crystals is similar to the (La1-zNdz)1.2Sr1.8 . Mn2O7 series.
Figure 2 shows the temperature dependence of magnetization with H in the ab-plane and along c. The low-temperature FC magnetization anisotropy Mab/Mc is still larger than one, indicating that the easy axis is lying in the ab-plane. Nevertheless, Mab/Mc decreases more compared to the ratio of La1.2Sr1.8Mn2O7, implying the tendency of the easy axis tur-ning from ab-plane and c-axis. We note that the crystals containing more non-La rare earth ions, (La0.6Nd0.4)1.2Sr1.8Mn2O7 and (La0.4Pr0.6)1.2Sr1.8Mn2O7, have the c-axis as easy axis [13, 14]. Hence it can be concluded that the easy axis turns from ab-plane to c-axis. And the increasing of the non-La-doping also supports this conclusion [13].
M and T for (La0.8Eu0.2)4/3Sr5/3Mn2O7 taken at H = 5 KOe. ZFC and FC represent zero-field cooling and field cooling, respectively. When H is in the ab-plane, magnetization is represented by Mab; when H along c-axis, magnetization is represented by Mc.
For in-plane magnetization Mab, the ZFC and FC data exhibit a characteristic splitting below 30 K, which signifies the occurrence of a spin-glass-like transition [24]. Because Eu ion is very small than La or Sr, the Eu-doping must give rise to large local lattice distortions. The distortion traps hopes to carries and suppress the ferromagnetic double exchange interaction [25]. So within the regions of distortion the local magnetic order can be antiferromagnetic. Considering the coexistence of the FM (in La1.2Sr1.8Mn2O7 clusters) and AFM (around Eu) interactions at low temperatures, the magnetic irreversibility, the spin-glass-like behavior, are reasonable phenomenon, which is due to the magnetic frustration by the competing FM and AFM interactions [26]. Because the Eu content is very small, we argue that the spin-glass-like phase is just a transition zone between antiferromagnetic zone and ferromagnetic zone. Every zone’s fraction is dependent on magnetic field and temperature. Below 9 K, the sudden and sharp decrease of ZFC magnetization is attributed to the spin-glass-like phase and the burst-like increase of the fraction of the antiferromagnetic clusters.
We observe an interesting phenomenon that the MZFC(T) and MFC(T) curves almost overlap (Fig. 2). The out-of-plane magnetization does not show spin-glass-like behavior. This is a contrast to the in-plane magnetization. This result can be reasonably explained by considering the fact that magnetic moments prefer to lie in the ab-plane.
For La1.2Sr1.8Mn2O7, a magnetic field of 5KOe is enough for magnetization to become saturated, and realize 3D long-range ferromagnetic order at low temperature. For the crystal (La0.8Eu0.2)4/3Sr5/3Mn2O7, it does not show any long-range ferromagnetic order (Fig. 2). However, we still observe metal-insulator transition at 125K (Fig. 3). We deem percolation mostly possibly cause such a behavior [27]. The main reason causing the percolation is that Eu concentration is very low.
Temperature dependence of resistivity σab for (La0.8Eu0.2)4/3Sr5/3Mn2O7.
Therefore, our resistivity measurements prove that the crystal is in an inhomogeneous, namely phase separated. Under 70KOe, the magnetic field promotes the ferromagnetic phase, and results in the decrease of the resistivity. The isothermal (T = 2K) magnetic field dependent magnetization (Mab) curve of (La0.8Eu0.2)4/3Sr5/3Mn2O7 show a single jump of 3.8μB at 60KOe (Fig. 3). Mc(H) curve of (La0.8Eu0.2)4/3Sr5/3Mn2O7 show a jump of 3.8μB at 60KOe, too. After the occurrence of the jump, magnetization is saturated, indicating that the crystal takes on 3D long-range ferromagnetic order. Corresponding to the magnetization jump, the in-plane resistivity shows also a downward step at the same field as the M(H) data show upward step (Fig. 3b). These data indicate that the entire sample is involve in the step transition, rather than isolated regions.
Now we turn to the origin of the step-like behaviors. A scenario based on phase separation is proposed. Under zero-field, the ferromagnetic zones without Eu and the antiferromagnetic zones around Eu are separated by spin-glass-like regions [18–20].
However, the coupling between ferromagnetic regions is very weak. Hence the initial magnetization is very small. Because the antiferromagnetic regions have strong distortions induced by Eu, there must be high elastic energy associated with the strains at the antiferromagnetic/spin-glass-like interfaces. The elastic energy tends to block the ferromagnetic transition. Upon increasing the magnetic field, the magnetic moments in the ferromagnetic regions are aligned. On the other hand, the boundary of AFM/FM, spin-glass-like regions is shifting to the antiferromagnetic regions step by step. In other words, the ferromagnetic regions are enlarged at the cost of the antiferromagnetic component, and the driving force is the applied magnetic field. However, the elastic energy near the surface of the antiferromagnetic regions also increases. Finally, at the critical magnetic field, all the spins in the antiferromagnetic regions realize a sudden reorientation along the direction of applied field. This process generates a magnetization jump. It can be predicted that before the jump, all the magnetic moments in the ferromagnetic regions have directed to the external field. So after the spin reorientation of the antiferromagnetic regions, a fully long-range ferromagnetic state is reached. During the spin reorientation process, the local stress on the surface of the antiferromagnetic regions is released. After the spin reorientation, the Zeeman energy of the system is also lowered. So the full ferromagnetic state is energetically favorable. As the magnetic field decreasing, the long-range ferromagnetic metallic state is held.
Figure 4 shows, the crystal shows hysteretic behaviors. The magnetization Mab shows ferromagnetic characteristic. It remains high at strong magnetic field. When the field is low, it decrease rapidly. In contrast to Mab, the low-field magnetization Mc decrease slowly. These results also prove that the easy axis is in the ab-plane. For the in-plane resistivity ρab, it can not recover when H = 0 again. It remains very low. We believe the crystal is still in metallic state under zero field. The increase of the resistivity is just due to the scattering from magnetic domain walls.
H – dependent magnetization and at different temperature. For every loop, the magnetic field increases from 0 to 50 KOe first, and then decreases to 0.
After the crystal is submitted to a magnetic field annealing loop 0 ⟶ 100 KOe⟶0, we take M(T) and ρ (T) measurements at once again under 5 KOe and zero-field respectively. We observe ferromagnetic M (T) curve and metallic ρ (T) curve, which are very different from the original ones. After the warming process, the magnetically disordered state is recovered. In other word, our sample shows strong memory effects [28].
Figure 5(a) shows the magnetic field dependence of magnetization (M - H) curves of the (La0.8Eu0.2)4/3Sr5/3Mn2O7 at 2 K. Figure 5(b) shows magnetic field dependence of resistivity (R - H) curves. M - H curve reveals a jump of nearly 40 emu/g at about 7 T, and the in-plane resistivity shows also a downward step at the same field (Fig. 5(b)). These data indicate that the entire sample is involved in the step transition, rather than isolated regions. The step demonstrates that there exists a jerky growth of the ferromagnetic fractionin a phase-separation state during martensiticlike transformation.
(a) the magnetic field dependence of magnetization (M - H) curves of the (La0.8Eu0.2)4/3Sr5/3Mn2O7 at 2 K. (b) magnetic field dependence of resistivity (R - H) curves at 2 K with H//ab-plane.
In summary, we have investigated the low-temperature resistivity and magnetization of a single crystal of the bilayer manganite (La0.8Eu0.2)4/3Sr5/3Mn2O7. Small amount Eu ions bring about strong local lattice distortions, which result in the field-induced ferromagnetic transition and the memory effects. It is suggested that, in bilayer manganites, there is strong interplay between spin, charge, orbital and lattice. Therefore the system has fragile ground states. These possible ground states can be easily tipped by a magnetic field.
Footnotes
Acknowledgments
This work is supported by the National Natural Science Foundation of China under Grant Nos. 61565013, 51562032 and 11164019 and the Inner Mongolia Natural Science Foundation China under Grant Nos. 2015MS0109, NJZZ11166, NJZY16327and NJZY12202.