Lattice, Magnetic, and Electronic Transport Properties in Antiperovskite M3AX CompoundsView this Special Issue
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Chemical Phase Separation of Superconductive and Ferromagnetic Domains in
Various ZnNyNi3−xCox compounds with differing Co content, x, were synthesized, and their magnetic properties were investigated. Uniform solid solutions could not be obtained at low Co content (); instead micrometer-scaled ferromagnetic ZnNyNi0.6Co2.4 domains formed embedded within a superconductive ZnNNi3 bulk, showing chemical phase separation of superconductive ZnNNi3 and ferromagnetic ZnNyNi0.6Co2.4. At intermediate levels of Co concentration (), this two-phase separation might persist, and the superconductive behavior was strongly suppressed in this composition region. Only at high Co concentration () the uniform ferromagnetic solid solution ZnNyNi3−xCox (with most likely ) formed. The phase separation behavior is intrinsic to the system, reflecting the existence of a miscibility gap in ZnNyNi3−xCox for the samples with , and was shown not to be attributable to incomplete synthesis. In the two-phased samples, high-quality granular contact between the superconductor and ferromagnet has been realized, suggesting that the production of useful devices requiring high-quality contacts between superconductors and ferromagnets may be possible by making use of this two-phase situation.
He et al. discovered a new antiperovskite superconductor MgCNi3 that has a superconducting transition temperature K . This compound has attracted attention in the context of the relationship between superconductivity and ferromagnetism, because the material includes large amounts of ferromagnetic Ni and has structural similarities with f.c.c. elemental Ni. Some researchers have supposed that the ferromagnetic correlation is associated with the superconductivity of MgCNi3. A theoretical calculation has pointed out that this compound is located near a ferromagnetic state and that the emergence of ferromagnetism may be induced by hole doping .
In order to reveal the superconducting gap symmetry and to clarify the microscopic origin of the superconductivity in MgCNi3, various types of experiments have been carried out [3–14]. However, a rigid consensus has not been obtained yet about the origin of superconductivity in MgCNi3. Stimulated by the discovery of MgCNi3, several new antiperovskite compounds have been synthesized including two new superconductors, CdCNi3 and ZnNNi3, and complementary theoretical studies have been performed, especially for these new superconductors [15–33].
In this study we synthesized and investigated the physical properties of the system composed of superconductive ZnNNi3 and ferromagnetic . ZnNNi3 is a superconductor with K that has the same antiperovskite structure as MgCNi3 [34, 35]. is a ferromagnet with a Curie temperature above room temperature. It should be mentioned that the nitrogen content of is about half of that in ZnNNi3 (), which seems to be the only stable nitrogen content of this material . The nitrogen content of has been confirmed by measuring the weight change before and after sintering.
These two compounds have the same antiperovskite structure and almost the same lattice constant (3.756 and 3.758 Å for ZnNNi3 and , resp.), which make them likely to form a solid solution with a whole value of . However the chemical phase separation of superconductive ZnNNi3 and ferromagnetic domains has been observed. In this paper, we report the synthesis and the two-phase separation of superconductivity and ferromagnetism in the system in detail.
The samples were prepared from elemental Zn, Ni, and Co powders. The powders were weighed and mixed to a nominal composition of and were then pressed into pellets. Extra Zn powder was added to compensate for loss due to vaporization. The pellets were sintered in NH3 gas in the following temperature sequence: (1) 400°C for 3 h, (2) 520°C for 5 h, and (3) 550–600°C for 5 h several times with intermediate grinding steps. The NH3 gas decomposes to chemically active hydrogen and nitrogen at high temperatures, and the active nitrogen penetrates into the sample to nitrify the sample. This has been shown to be an effective method for forming 3d-transition metal nitrides [37–39].
X-ray diffraction patterns were obtained using Cu Kα radiation. The magnetization measurements were performed using a Quantum Design SQUID magnetometer. Magnetization was measured with zero-field cooling (ZFC). In order to investigate the homogeneity of the sample, an electron probe microanalyzer (EPMA) was used.
3. Results and Discussion
Figure 1 shows the powder X-ray diffraction patterns obtained for various samples. All of the diffraction patterns indicate a cubic structure with Pm3m space group. No impurity peaks were detected, showing single-phased samples. The lattice parameters were determined to be a nearly constant value of 3.756 Å for all samples, and systematic changes in the lattice constant were not observed as the Ni : Co ratio was varied.
Figure 2 shows the temperature dependence of the magnetic susceptibility for with , 0.25, 0.5, and 0.75 samples. All samples show superconductive behavior. The onset of was seen to slightly decreased as the Co content () was increased. Though the superconducting volume fraction (SVF) decreases as increases, the SVF values are large enough for bulk superconductivity up to (SVF = 12% estimated from magnetization value at 1.8 K). The bulk superconductivity disappears in samples with above 0.75 (data with not shown).
Figure 3 shows the field dependence of magnetization curves at 1.8 K and 3.5 K with (a) and (b) . Below (1.8 K), the magnetization curves show superconducting character but overlap with ferromagnetic character for both samples. The ferromagnetic character becomes more obvious for the sample with larger Co content (compare data in Figures 3(a) and 3(b) at 1.8 K). At the lower temperature, superconductivity seems to coexist with ferromagnetism, but above (3.5 K), the superconductive character disappears and only the ferromagnetism survives (see 3.5 K data of Figures 3(a) and 3(b)). In order to clarify the origin of this coexistence, we analyzed the samples using EPMA.
Figures 4(a) and 4(b) show the elemental mapping analysis for Ni (Figure 4(a)) and Co (Figure 4(b)) over a μm2 area of the sample obtained using an acceleration voltage of 15 kV and probe diameter of 1 μm. In Figure 4(a), darker blue colors indicate areas deficient in Ni content. From this figure, it can be seen that there are some blue islands that are tens of micrometers in size and have much less Ni content than the surrounding areas. On the other hand, in Figure 4(b), areas with brighter red colors indicate that the Co content is enhanced in those regions. Comparing these two figures, it is seen that within the islands with very small Ni content seen in Figure 4(a), the Co content is very large. The chemical composition of these islands was revealed to be and the volume fraction of the islands can be estimated from image mapping to be about 5%. Except for these islands, the overall chemical composition was found to be nearly pure superconductive ZnNNi3. In order to clarify the magnetic property of , we synthesized a sample and measured its field-dependent magnetization at 1.8 K (Figure 5(a)). As clearly seen in Figure 5(a), is ferromagnetic. In Figure 5(b), magnetization data is superimposed with the 1.8 K data shown in Figure 3(a); the magnetization data was scaled by 0.05, corresponding to the volume fraction of (5%) estimated from the EPMA mapping data. It is clear that the ferromagnetic character seen in the sample is well explained by the 5% reduced magnetization behavior of . It appears that the origin of the coexistence of superconductive and ferromagnetic behavior arises from a chemical phase separation where ferromagnetic regions are embedded within the superconductive ZnNNi3 background. It should be noted that the overall average composition of this 95%-ZnNNi3/5%- sample is , which corresponds to only half the Co content of the nominal composition of ZnNNi2.75Co0.25. We suspect that the discrepancy may be explained by the existence of small or thin portions in the ZnNNi3 grain boundary areas, which we failed to adequately detect by mapping analysis. These small portions may lose their long range ferromagnetic coherence because of their nonbulk morphology. In order to clarify this point, more detailed chemical analysis is needed.
There remains a question of whether the phase separation comes from the intrinsic nature of this compound or from incomplete sample preparation. In Figure 6, the temperature-dependent magnetic susceptibility of samples prepared after different numbers of sintering cycles in process (3) 550–600°C for 5 h (see Section 2) is shown. From this figure, it can be seen that the magnetization value at 1.8 K increases with increasing number of sintering cycles and almost saturates for the sample after 17 sintering cycles. This indicates that the chemical reaction has gone to completion and that the sample has reached a thermodynamic equilibrium state. Therefore, in this system, more than 17 sintering cycles are enough to achieve complete chemical reaction. The samples used in this study were synthesized with more than 17 sintering cycles; therefore, the two-phase separation cannot be attributed to incomplete synthesis but instead must be intrinsic to the system. It seems that Co ions cannot be substituted into Ni sites in the and 0.5 samples, instead, two-phase separation of ZnNNi3 and arises. In other words, a miscibility gap exists in systems for at least and 0.5. In an Mn-doped system synthesized by the same recipe used in the present study, the superconductivity completely disappeared with a tiny amount () of doping, which indicated the formation of a uniform solid solution, even with small doping concentrations . This experimental result also reinforces the peculiar character of the Co-doping system and supports the existence of a miscibility gap. Why does not a uniform solid solution of form between the ZnNNi3 and which have nearly the same crystal structures? As already mentioned, it has been recognized that, in the preset synthesis conditions, the nitrogen content of must be about 0.5  unlike (). Strictly speaking, the crystal structure of ZnNNi3 is different than from the viewpoint of nitrogen content. Therefore, it is reasonable to imagine that ZnN0.5Co3 cannot dissolve into ZnNNi3, even though the overall crystal structure and lattice parameters are almost the same. In contrast, for high values, homogeneous solid solutions of may be realized because the islands of observed by EPMA mapping seem to be homogeneous within their islands. The nitrogen content of is inferred to be 0.5 due to the compositional proximity to ZnN0.5Co3. A lower nitrogen content ZnN0.5Ni3 phase can be synthesized under 50%-H2+50%-NH3 conditions (For synthesizing ZnN0.5Ni3, the concentration of NH3 gas has to be diluted down to 50% by H2 gas, while for the case of ZnNNi3, 100%-NH3 gas is needed.) and may exist as a pseudostable phase under the present synthesis conditions using 100%-NH3 gas. Therefore, it can be supposed that small amounts of ZnN0.5Ni3 could dissolve into ZnN0.5Co3 to form the solid solution at high concentrations, with a most likely value of . If appropriate synthesis conditions were found that allowed the N content to be 1 for , the formation of uniform solid solutions at all values could be possible. For example, this may be accomplished by using NH3 gas at more than 1 atm.
In Figure 7, SVF, , and magnetization values obtained in a 1 field at 1.8 K () are shown as a function of the Co content, . The SVF value decreases linearly as Co content increases up to about 0.5. This behavior is consistent with a two-phase situation. With linearly increasing , the relative ratio of the superconducting region linearly decreases. is nearly constant and suddenly disappears at . increases linearly up to about 2 and strongly increases above . This implies that the two-phase situation extends up to , and at the uniform solid solution forms and shows ferromagnetism. However this hypothesis contradicts the experiment because the superconductivity disappears below . This discrepancy might be explained by the influence of the magnetic field made by ferromagnetic regions adjacent to the superconductive ZnNNi3 region under the two-phase situation, which may strongly suppress or wholly destroy the superconductive behavior. In order to clear this point, further investigations employing NMR or μSR experiments are needed.
Finally, it should be mentioned that in the two-phase situation a prototype of a ferromagnet-superconductor granular contact device is naturally realized. The nature of the ferromagnet-superconductor grain boundary is expected to be good because the ferromagnetic and superconductive ZnNNi3 have almost the same crystal structure and lattice constant. This indicates the possibility for use in π-junction quantum bit and magnetoresistance devices and similar applications by tuning the morphological characteristics of the contact boundary, such as contact strength, shape of the boundary, and each domain size. These parameters may be controllable within conventional solid state reaction techniques by optimizing synthesis conditions such as temperature, sintering time, and nitrogen partial pressure, without the special equipment used in producing thin film devices.
It has been revealed that, in systems with , instead of forming uniform solid solutions, micrometric scale ferromagnetic domains embed within a superconductive ZnNNi3 bulk, showing chemical phase separation of superconductive ZnNNi3 and ferromagnetic . Our results suggest that, for , two-phase separation persists, but the superconducting region is strongly suppressed or almost destroyed possibly by the magnetic field produced by surrounding ferromagnetic regions. Above , the uniform solid solution (with most likely ) forms, and in this compositional region the system shows long range ferromagnetism. The two-phase separation nature is intrinsic to the system, reflecting the existence of a miscibility gap in with and suggestively with . The origin of this unexpected chemical phase separation is probably due to the differences in stable nitrogen content between () and (). By taking advantage of this two-phase situation, useful devices requiring high-quality granular contacts between superconductors and ferromagnets could be produced.
This work was partly supported by Research Institute and Instrumental Analysis Center of Yokohama National University.
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