Introduction

In recent years, two dimensional compounds with K2NiF4-type structure (a type of tetragonal structures) have generated great interest after the discovery of superconductivity, magnetoresistance (MR), spin/charge stripes in nickelates and manganites1,2,3,4,5,6,7,8,9,10,11,12,13. Two-dimensional layer structured perovskite compound Sr2CoO4 is one of K2NiF4-type structured materials with space group I4/mmm1. The structure of Sr2CoO4 consists of corner sharing CoO6 octahedra with two-dimension CoO2 planes separated by insulating rock-salt layers of SrO. In the past reports, both Sr2CoO4 single-crystalline thin films and polycrystalline bulks were reported as a metallic ferromagnets with a fairly high Curie temperature (TC) of 255 K1,2,3. The susceptibility data above TC of Sr2CoO4 can be well fitted to the Curie-Weiss law χ = C/(T + Θ). The observed value of the effective magnetic moment per Co ion μeff (μB/Co) is 4.11, which can approximately coincide with that expected for spin only moment of the intermediate-spin (IS) state (t2g4eg1, S = 3/2) Co4+ (3.87 μB/Co) and be quite different from the values of the low-spin (LS) state (t2g5eg0, S = 1/2) Co4+ (1.73 μB/Co) and high-spin (HS) state (t2g3eg2, S = 5/2) Co4+ (5.92 μB/Co)1,4. Below TC, a cluster-glass state exists in Sr2CoO4 system5. It has been observed that Sr2CoO4 reveals large magnetic anisotropy where the c-axis is the magnetic easy axis6. The coercivity (HC) of Sr2CoO4 is approximately 2.2~2.5 T at 5 K from polycrystalline sample1 and single-crystalline film2. It suggests great potential of Sr2CoO4 for high quality memory applications7. The half-metallicity of Sr2CoO4 has been predicted6. Different relationships of the electrical resistivity (ρ) versus temperature were observed in polycrystalline Sr2CoO4 and single-crystalline Sr2CoO4 film2,4. The temperature dependence of ρ in polycrystalline Sr2CoO4 exhibits semiconducting characteristics4. The ρ above TC for the polycrystalline Sr2CoO4 can be well fitted by the variable range hopping (VRH) model ρ = ρ0exp(T0/T)1/4. By comparison, in single-crystalline Sr2CoO4 film, the inter-CoO2-plane ρ of c-axis shows a sharp peak at TC, with a metallic behavior below TC and a semiconducting behavior above TC2. In contrast, the intra-CoO2-plane ρ of b-axis shows metallic characteristics2. At low temperature, large negative MR was observed in Sr2CoO41. The MR reaches a maximum at HC. Therefore, the magnetic and electrical properties of Sr2CoO4 at low temperature, especially below 5 K, will be rather significant to study. Moreover, certain exceptional and unusual physical phenomenons were observed frequently at low temperature, such as superconductivity, magnetic jump and quantum tunneling14,15,16,17,18,19,20.

An interesting phenomenon, a staircaselike behavior which is analogous to resonant quantum tunneling of magnetization, was indeed observed in our Sr2CoO4 polycrystalline sample below 2.8 K. So far, to our knowledge, this is the first time that the staircaselike behavior was observed in Sr2CoO4. It may suggest the new application potentials of Sr2CoO4 in magnetic materials and devices. Thus, systematical experiments are urgently needed to study the exceptional phenomenon of Sr2CoO4 at low temperature and explore the possibility of the staircaselike behavior for various practical applications. In this work, the magnetic and electrical properties of polycrystalline Sr2CoO4 were studied below 5 K. The staircaselike behavior was observed in a series of magnetic and electrical curves, such as magnetization versus field (M-H) and resistivity versus field (ρ-H) curves. Based on the reported researches and explanations on the staircaselike behaviors observed in other materials, the mechanism of the staircaselike behavior in Sr2CoO4 was discussed in detail.

Results

Figure 1 shows the powder X-ray diffraction (XRD) pattern of polycrystalline Sr2CoO4 measured at room temperature. The main diffraction peaks of the sample can be fitted well with the XRD profile of Sr2CoO4 and indexed using the lattice parameters for a tetragonal structure with a = 3.8372 Å and c = 12.1935 Å. A few additional peaks (marked by #) corresponding to nonmagnetic impurity SrO2 can be observed in the pattern. However, this SrO2 impurity phase is present in a small amount from the weak intensity of the peaks and has no effect on the magnetic properties of our sample. The inset of Fig. 1 shows the scanning electron microscope (SEM) photograph of Sr2CoO4. The grains of the sample, with the average size approximately 20 μm, are dense and distribute uniformly.

Figure 1
figure 1

The XRD pattern of polycrystalline Sr2CoO4 sample.

The inset shows the SEM image of polycrystalline Sr2CoO4 sample.

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The M-H curve of Sr2CoO4 measured at 1.8 K with a field sweep rate of 25 Oe/s is displayed in Fig. 2(a). The saturation magnetization is 1.02μB/Co and the HC is approximately 1.9 T. The large HC is caused by high anisotropy in Sr2CoO48. Most interestingly, unlike the general hysteresis loops, a staircaselike behavior can be observed from the M-H loop in Fig. 2(a). The steps on both sides of the hysteresis loop are central symmetry. The span (ΔM) of the four stairs on one side of the loop decreases with the increasing of the applied field (see Fig. 2(a)). The dM/dH versus field curve (Fig. 2(b)) clearly shows that the four jumps on one side occur at −1.84 T (1), −2.56 T (2), −3.20 T (3), −3.70 T (4), respectively. Moreover, two almost invisible jumps are reflected (see the # in Fig. 2(b)). The inset of Fig. 2(a) shows the three measurement runs from the same piece of sample measured at 1.8 K with a field sweep rate of 25 Oe/s. It can be seen that the steps on these curves of the same piece of sample are obviously misaligned for different measurement runs under the same measurement condition. This result suggests the randomness of the staircaselike behavior in different measurement runs.

Figure 2
figure 2

(a) Magnetization versus field (M-H) curve at 1.8 K with a field sweep rate of 25 Oe/s for the Sr2CoO4. (b) dM/dH-H curve at 1.8 K for the Sr2CoO4. The inset of (a) shows the three measurement runs of the same piece of sample obtained under the same measurement condition.

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Figure 3 shows the M-H curves of Sr2CoO4 measured at different temperatures. It can be observed that with increasing of the temperature, the quantity of the steps decreases and the positions of the corresponding steps move towards the direction of high field. At 2.8 K, the staircaselike behavior disappears completely. These results suggest that the staircaselike behavior is sensitive excessively to the slight temperature variation. The inset of Fig. 3 shows the M-H curves of Sr2CoO4 measured at 2 K with different magnetic field sweep rates. With the increasing of the sweep rate, the quantity of the steps increases gradually and the positions of the corresponding steps move towards the low field (see the arrow in the inset of Fig. 3). It can be deemed that the staircaselike behavior in Sr2CoO4 is dependent on the magnetic field sweep rate.

Figure 3
figure 3

Magnetization versus field (M-H) curves for the Sr2CoO4 sample at different temperatures with the same field sweep rate of 25 Oe/s.

The inset shows M-H curves for the Sr2CoO4 at 2 K with different magnetic field sweep rates.

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Figure 4 shows the ρ-H curve of Sr2CoO4 measured at 2 K. The resistivity reaches a maximum at HC, which is consistent with the previous reports1,2. This phenomenon can be considered as tunneling MR at domain boundaries. It is attributed to the field suppression of the spin-dependent scattering at domain boundaries8. The staircaselike behavior can be also observed from the ρ-H curve. The insets (a) and (b) of Fig. 4 show the ρ-H curves of Sr2CoO4 measured at different temperatures and magnetic field sweep rates, respectively. The steps in the inset (a) disappear gradually with the increasing of the temperature. The three ρ-H curves in the inset (b) of Fig. 4 are misaligned for different field sweep rates. The positions of the corresponding steps on the ρ-H curves also move towards the low field with the increasing of the field sweep rate (see the arrow in the inset (b) of Fig. 4). The steps on the ρ-H curves of the same piece of sample are also misaligned for different measurement runs under the same measurement condition (figure not shown). These phenomena are consistent with the above magnetic results of Sr2CoO4 (see the M-H curves of Fig. 3).

Figure 4
figure 4

Resistivity versus field (ρ-H) curve for the Sr2CoO4 at 2 K with a field sweep rate of 25 Oe/s.

The inset (a) shows ρ-H curves for the Sr2CoO4 sample at different temperatures with the same field sweep rate of 25 Oe/s. The inset (b) shows ρ-H curves for the Sr2CoO4 sample at 2 K with different magnetic field sweep rates.

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Discussion

Three main characteristics of the staircaselike behavior in Sr2CoO4 are concluded from the measured results: (i) the positions of the steps are varying in different measurement runs, (ii) the steps only appear at low temperature (T < 2.8 K) and disappear with a slight increase of the temperature, (iii) the steps are dependent on the temperature and field sweep rate. The possible mechanism of the staircaselike behavior will be systematically discussed below.

The similar staircaselike behaviors in hysteresis loops have been also reported in many types of materials, such as Ca3Co2O614,15, [Mn4]2 dimer16, FexMg1-xCl221, PrVO322, UGe223,24 and amorphous Dy-Cu25. Simultaneously, different theories have been presented to explain the staircaselike behaviors. The main three theories are resonant quantum tunneling14,15,16,17,18,19,20, random field21,22,23,24,25,26,27,28,29,30,31,32,33,34,35 and intrinsic pinning of magnetic domain walls36,37,38.

Resonant quantum tunneling has been applied to systems involving a large number of identical high-spin materials14,15,16,17,18,19,20, as in the case of Ca3Co2O614,15 and Mn12 acetate20. Ca3Co2O6 is a type of perovskite material with K4CdCl6-type structure (an infinite chain-type structure). The analogous steps can be observed from the M-H curves of Ca3Co2O6 at low temperature14,15. The steps are resulted from the transformation and change of the percentage of different magnetism in the materials caused by the applied field at different temperatures14. The chain-type structure is the key factor to the staircaselike behavior. The intrachain coupling is ferromagnetic and the interchain coupling is antiferromagnetic. However, Sr2CoO4 is one type of two-dimensional layer structured compound. Obviously, no chain-type structure exists in Sr2CoO4. On the other hand, the most important characteristic of the staircaselike behavior in quantum-effect system is that the positions of the steps are temperature-independent below a critical temperature17,18. The results from the Fig. 3 of Sr2CoO4 show that the steps in the six M-H curves exhibit no similar characteristic of temperature independence. This result indicates that the staircaselike behavior in Sr2CoO4 is incompatible with resonant quantum tunneling.

The presence of random fields is another explanation that can lead to staircaselike behavior. Under this mechanism, a given domain is flipped by an external field, thus reversing the magnetization of the neighboring domains and finally resulting in an avalanche of flipping domains21,22,23,24,25,26,27,28,29 considering the random field Ising model (RFIM)31,32,33,34. Each jump in one curve corresponds to an avalanche process where the spins (of one or more clusters in the polycrystalline Sr2CoO4) align with the applied magnetic field26. The noteworthy characteristic of the steps in this theory is the randomness. The positions of the steps are varying in different measurement runs. Meanwhile, the steps can be only observed at low temperature. The ferromagnetic clusters in Sr2CoO4 sample play a crucial role for this phenomenon13,26,27,28. Below the critical temperature at which the steps are vanished, the ferromagnetic cluster-sizes in the sample increase and the cluster percolation process yields an increase in the ferromagnetic correlation length with lowering the temperature26. The larger cluster-size can result in the bigger avalanche, which gives rise to the distinct jumps. Above the critical temperature, the thermal activation is dominating26 and the cluster-size is so small, which can only cause small avalanche. As a consequence, the jumps become sightless and the hysteresis loop becomes smooth. This type of staircaselike behaviors is dependent on temperature, but independent on field sweep rate. Such an explanation has been proposed in site-diluted metamagnet FexMg1−xCl221, single crystal antiferromagnet PrVO322, single crystalline UGe223,24, disordered systems such as the amorphous Dy-Cu25, polycrystalline CeNi1−xCux26,27,28,29 and liquid quenched R3Co alloys30. All the features of the steps in Sr2CoO4 are similar to the characteristics of staircaselike behaviors in PrVO322, UGe223,24 and CeNi1−xCux26,27,28,29. The other features of the steps, except the dependence of magnetic field sweep rate, can be well explained by the random field theory. The dependence of magnetic field sweep rate may result from the magnetocaloric effect22,35. The positions of the corresponding steps move to higher field with the decreasing of the sweep rate. It suggests the existence of adiabaticity in Sr2CoO4. In the adiabatic state, the energy released in the spin reversal process dissipates tardily35. With the increasing of the sweep rate, the energy accumulates rapidly and facilitates the reversal of neighboring spins. It results in the sweep rate dependence of the steps. From this point of view, the fundamental reason of the staircaselike behavior in Sr2CoO4 may be ascribed to an avalanche of flipping domains in terms of the random field theory.

The intrinsic pinning of magnetic domain walls is compatible with the magnetization jumps observed in alloy samples36,37,38. The domain walls motioning inside the ferromagnetic domains depend on the pinning effect introduced by foreign elements and the local crystal fields. The pinning effect can result in the creation of energetic barriers, which influence the magnetization process at low temperature36. In the case of EuBaCo1.92M0.08O5.5−δ (M = Zn, Cu)37, Zn2+ and Cu2+ are the origin of the pinning of the narrow domain walls. When the magnetic field becomes high enough to overcome the pinning effect, the domain walls tend to disappear and the spins of the ferromagnetic domains are all aligned. This type of the staircaselike behaviors strongly depends on the external magnetic field sweep rate. When the magnetic field changes slowly enough, the M-H curve becomes normal with no jump38. This phenomenon was similar to the result from the inset of Fig. 3 in Sr2CoO4. In the perfect Sr2CoO4 crystals, no substituted defect results in the effective pinning. However, here, the saturated moment of the Sr2CoO4 sample (1.02μB/Co) is lower than the calculated value (1.97 μB/Co)2. Meanwhile, the μeff of Co ion (4.11 μB/Co) in Sr2CoO4 is also different from the spin only moments of LS Co4+ (1.73 μB/Co), IS Co4+ (3.87 μB/Co) and HS Co4+ (5.92 μB/Co)1,4. These results suggest that multiple spin states may exist in our Sr2CoO4 sample. The interactions between the neighboring IS or HS Co ions (Co(IS or HS)-O-Co(IS or HS)) are antiferromagnetic39,40, though the ground state of Sr2CoO4 is ferromagnetic6. It means that antiferromagnetism and ferromagnetism are coexistent in Sr2CoO4, which can lead to multiple magnetic phases. The multiple magnetic phases may result in the intrinsic pinning of magnetic domain walls36,41,42 and further contribute to the magnetization and magneto-transport staircaselike behavior in the Sr2CoO4.

In summary, layered perovskite compound Sr2CoO4 polycrystalline sample was synthesized by high temperature and high pressure method. The magnetic and magneto-transport properties of Sr2CoO4 were studied at low temperature. A staircaselike behavior on M-H and ρ-H curves was observed in polycrystalline Sr2CoO4 below 2.8 K. The steps appear with a certain degree of randomness in different measurement runs. The staircaselike behavior is dependent on the temperature and the magnetic field sweep rate. The fundamental reason of the staircaselike behavior can be considered as the presence of random fields, leading to an avalanche of flipping domains. The multiple magnetic phases which can result in the intrinsic pinning of magnetic domain walls, may contribute to the magnetization and magneto-transport staircaselike behavior in the Sr2CoO4.

Methods

Polycrystalline sample of composition Sr2CoO4 was synthesized under high pressure at high temperature. Starting materials of SrO2 and Co were well mixed in a molar ratio of SrO2 : Co = 2 : 1. The mixture was sealed into a gold capsule. The capsule was first compressed at 6 GPa in a high pressure apparatus (flat-belt-type-high-pressure apparatus, 1500 ton), then heated to 1200 °C for 30 minutes and finally quenched to room temperature followed by releasing of pressure. The crystal structure of the polycrystalline sample was identified by the powder X-ray diffraction (XRD, Rigaku Smartlab3), using Cu-Kα radiation (λ = 1.54184 Å). The morphology of the sample was observed using a scanning electron microscope (SEM). The dc magnetic measurements were investigated using a vibrating sample magnetometer (VSM) integrated in a physical property measurement system (PPMS-9, Quantum Design). The electrical resistivity of the sample was measured with a Quantum Design PPMS-9 system using the standard four-probe ac method.

Additional Information

How to cite this article: Li, Q. et al. Magnetization and magneto-transport staircaselike behavior in layered perovskite Sr2CoO4 at low temperature. Sci. Rep. 6, 27712; doi: 10.1038/srep27712 (2016).