Magnetic Phase Separation in the Oxypnictide Sr2Cr1.85Mn1.15As2O2.

Layered Sr2M3As2O2-type oxypnictides are composed of tetrahedral M2Pn2 and square planar MO2 layers, the building blocks of iron-based and cuprate superconductors. To further expand our understanding of the chemical and magnetic properties of the Sr2Cr3-xMnxAs2O2 solid solution, Sr2Cr2MnAs2O2 has been synthesized. The compound crystallizes in the I4/mmm tetragonal space group with a refined stoichiometry of Sr2Cr1.85Mn1.15As2O2. The M(2) site within the M2Pn2 slab is occupied by 42.7% Cr and 57.3% Mn, and the magnetic moments order antiferromagnetically below TN(M2) = 540 K with a C-type antiferromagnetic structure. The M(1) site within the MO2 layers is fully occupied by Cr, and antiferromagnetic order is observed below TN(M1) = 200 K. Along c, there are two possible interplanar arrangements: ferromagnetic with the (1/2, 1/2, 0) propagation vector and antiferromagnetic with the (1/2, 1/2, 1/2) propagation vector. Magnetic phase separation arises so that both propagation vectors are observed below 200 K. Such magnetic phase separation has not been previously observed in Sr2M3As2O2 phases (M = Cr, Mn) and shows that there are several competing magnetic structures present in these compounds.

Introduction

Since the discovery of high-temperature superconductivity in iron oxyarsenides in 2006, layered pnictide compounds have seen increased research focus. Their superconductivity originates from the iron arsenide layers,[1] which are present in multiple structure types such as LiFeAs,[2] SmFeAsO0.85,[3] K0.8Fe2Se2,[4] CaFe2As2,[5] α-FeSe,[6] and (Fe2As2)[Ca4(Sc,Ti)3O8].[7] It was soon discovered that upon substituting Fe for other transition metals, different phenomena emerge such as giant magnetoresistance in (La,Nd)MnAsO,[8] a spin reorientation transition in CeMnAsO,[9] colossal magnetoresistance in NdMnAsO1–F[10] persistent short-range order above TN in LaMnAsO,[11] p-type semiconductivity in LaCuChO (Ch = S, Se, Te),[12] and competing spin-density-wave magnetism and superconductivity in EuFe2As2.[13] Studying these materials could lead to advances in high-field applications and electronic data storage and open additional avenues to determine the exact mechanisms of unconventional superconductivity.

The oxyarsenides A2M2M’As2O2 (M and M’ represent different transition metals) are especially interesting, as they contain key structural elements of both the iron arsenide and cuprate superconductors. These materials crystallize in the tetragonal crystal system, space group I4/mmm, and are composed of square planar M(1)O2 layers and tetrahedral M(2)2As2 layers. These layers are stacked on top of each other perpendicular to the crystallographic c axis and are separated by layers of alkali-earth cations (A2+). Several oxyarsenides, specifically A2M3–M’As2O2 (A = Ba, Sr; M = Mn3,[14] Cr3,[15−17] Cr0.77Mn2.23,[18] MnZn2,[19,20] Mn2Cu,[19] Zn3[21]), have been synthesized and investigated. While Sr2Fe2CuAs2O2 should be the most interesting compound to study, being composed of the cuprate (CuO2) and iron-based (Fe2As2) superconductor superconducting layers, it was reported that syntheses did not yield a single-phase product.[19] However, it was discovered that CrAs becomes superconducting under pressure as the antiferromagnetic ordering is quenched[22] and that superconductivity could potentially be induced in Sr2Cr3As2O2, either with pressure or with chemical doping.[17] It was also determined that MnP becomes superconducting at ∼8 GPa.[23] This prompted our exploration of compounds within the series Sr2Cr3–MnAs2O2.

Very different magnetic structures have been reported for Sr2Mn3As2O2,[14,19] Sr2Cr3As2O2,[16] and Sr2Cr0.77Mn2.23As2O2.[18] Sr2Cr3As2O2 exhibits antiferromagnetic ordering of the M(2) sites below 590.3 K, with the propagation vector k = (1, 0, 0), and the moments aligned parallel to the c axis. Below 291 K, the M(1) sites order antiferromagnetically, with a K2NiF4-type antiferromagnetic structure and propagation vector k = (1/2, 1/2, 0) (moments aligned parallel to c), which causes a spin-flop of the M(2) spins onto the ab plane.[16] The Sr2Mn3As2O2 compound exhibits G-type antiferromagnetic ordering with the propagation vector k = 0 of the M(2) sites below 340 K with spins aligned parallel to the c axis and short-range two-dimensional magnetic correlations below 75 K so that the M(1) sublattice does not exhibit long-range magnetic order down to 4 K. Additional peaks were observed at 4 K but could not be satisfactorily modeled.[14,19] The mixed transition-metal compound Sr2Cr0.77Mn2.23As2O2 exhibits G-type antiferromagnetic ordering of the M(2) sites with k = 0 between 410 and 167 K (with moments along the c axis), and antiferromagnetic ordering of the M(1) sites below 167 K (moments along the c axis) with k = (1/2, 1/2, 0), which causes a reorientation of the M(2) moments to a C-type antiferromagnetic arrangement (moments along the c axis) with k = (1, 0, 0).[18]

The magnetic transition temperatures in these compounds are also affected by the transition-metal stoichiometry with TN decreasing as the Cr:Mn ratio decreases.[19] Xu et al.[15] have recently discussed the interlayer coupling of magnetic sublattices in Sr2Cr3As2O2, prompted by their observation of a spin-flop transition of the M(2) site magnetic moments upon the emergence of long-range order of the M(1) site moments, which was also reported by Liu et al.[16] A similar (field-induced) phenomenon has also recently been observed in EuMnBi2[24] and coupling of the magnetic sublattices has been observed in the 1111-type compounds such as SmFeAsO,[25] CeMnAsO,[26] and PrMnSbO.[27] However, in these examples, both magnetic sublattices have the same propagation vector. The coupling of magnetic lattices with different propagation vectors should not occur in principle; however, it seems that A2M2M’As2O2 oxypnictides exhibit this phenomenon.[15] The spin reorientation and magnetic coupling were attributed to an antisymmetric exchange (Dzyaloshinskii–Moriya interaction), however, no significant accompanying structural distortion could be determined.[15]

Since we have observed a similar spin-flip transition in Sr2Cr0.77Mn2.23As2O2,[18] we have started to explore the magnetic structures of other members of this solid solution series to better understand the interactions between the layers. We have now synthesized Sr2Cr2MnAs2O2, and here we report the crystal structure and changes in the magnetic structure with temperature.

Experimental Section

A polycrystalline sample of the nominal stoichiometry Sr2Cr2MnAs2O2 was prepared using a standard solid-state synthesis on a 1.2 g scale. The starting materials used were Mn (99.99%, Aldrich), Cr (≥99%, Aldrich), As (99.999%, Aldrich), and SrCO3 (99.9 + %, Aldrich). SrO was prepared by heating SrCO3 at 1250 °C for 12 h, followed by quenching and immediate use. Stoichiometric amounts of Mn, Cr, and As were mixed with a nonstoichiometric amount of SrO (2% deficiency). In agreement with the reported syntheses of Sr2Cr3As2O2[16,17] and Sr2Cr0.77Mn2.23As2O2,[18] we have observed that the deficiency increased phase purity. All starting materials were mixed and ground in an inert atmosphere (N2, UN1066 BOC) environment using an agate mortar and pestle. The mixture was pelleted and inserted into a Ta crucible (foil 0.05 mm, ≥99.9%, Aldrich), which was sealed in a quartz tube under vacuum. The pellets were heated at 1080 °C for 72 h twice, with intermediate regrinding.

Room-temperature powder X-ray diffraction was performed on a PANalytical Empyrean Powder diffractometer. Patterns were recorded using Cu Kα1 radiation, in the range of 5° < 2θ < 100°, with a step size of 0.01313°.

Variable temperature neutron diffraction experiments were performed on the D1B and D2B diffractometers at the Institut Laue-Langevin in Grenoble, France (ILL). Approximately, 1.5 g of the sample powder was inserted into a 9 mm vanadium can. Data were recorded on the D1B diffractometer with λ = 2.512 Å, with a temperature range of 1.5–420 K upon heating at the rate of 1 K/170 s and 375–673 K on heating at the rate of 1 K/200 s, separated into 600 s snapshots (3.5–3 K range, respectively). Further patterns were recorded on the D2B diffractometer with λ = 1.594 Å at 1.5, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, and 600 K. Data were collected for 2.5 h at each temperature step.

The temperature dependence of the magnetic susceptibility was performed with a Quantum Design Superconducting Quantum Interference Device (SQUID) magnetometer. Field-cooled (FC) and zero-field-cooled (ZFC) measurements were recorded in a field of 1000 Oe between 4 and 340 K. The temperature dependence of the electrical resistance was performed with a Quantum Design physical property measurement system (PPMS). Measurements were recorded on heating between 4 and 340 K.

Results and Discussion

Sr2Cr2MnAs2O2 crystallizes in the tetragonal crystal system with the space group I4/mmm. The unit cell of Sr2Cr2MnAs2O2 is composed of M(1)O2 planes and M(2)2As2 layers, separated by Sr2+ ions, as shown in Figure . Initial structural analysis was performed using Rietveld refinement on laboratory X-ray powder diffraction patterns. These were refined using a model of the Sr2Cr3As2O2 crystal cell,[15−17] modified to Sr2Cr2MnAs2O2 by replacing half of the M(2) site Cr occupancy with Mn. The patterns could be indexed well on the tetragonal unit cell, with cell parameters a = 4.0428(1) Å and c = 18.9578(6) Å. A small impurity phase of Sr2Cr2AsO3 is observed and fitted (0.7% by mass). This impurity has previously been identified by Xu et al. during the synthesis of Sr2Cr3As2O2.[15]

Figure 1

Crystal structure of Sr2Cr1.85Mn1.15As2O2 showing the alternating M(1) CuO2-type and M(2) FeAs-type layers along c.

The M(1) and M(2) transition-metal sites, located at Wyckoff positions 2a and 4d, respectively, have different local environments. The M(1) sites are in square planar or, arguably, distended octahedral coordination,[15] and the M(2) sites are in tetrahedral coordination. Powder neutron diffraction data were recorded to determine site occupancies and magnetic ordering of Sr2Cr2MnAs2O2. Rietveld refinement was first performed on the I4/mmm model with the GSAS/EXPGUI[28,29] software using the high-resolution diffraction pattern obtained at T = 600 K (Figure a), where no magnetic peaks were observed. The background was fitted using the Chebyshev polynomial function with 15 terms and the peaks were fitted using the pseudo-Voigt profile function. An excellent fit was obtained using the I4/mmm space group with a = 4.06066(4) Å and c = 19.1220(4) Å (χ2 = 2.91). The atomic displacement parameters were modeled isotopically with the Uisos for Cr and Mn constrained to the same value. Site occupancies were refined for all atoms and fixed to 1.0 where the refined values were within ±1% of the full occupancy. It was determined that the M(1) site is fully occupied by Cr, while the M(2) site is occupied by 42.7(4)% Cr and 57.3(4)% Mn. These values were used, with no further refinement, at all temperatures. This distribution was attributed to the effects of crystal field stabilization energy and Jahn–Teller distortion, as previously reported for Sr2Mn2.23Cr0.77As2O2.[18] The Cr2+ ion favors octahedral and square planar coordination[30] and fills completely the square planar M(1) site. In contrast, the Mn2+ ion displays no site selectivity and is therefore found at the M(2) site. This is also observed in A2MnZn2As2O2 (A = Sr, Ba), where the Mn2+ ions occupy the M(1) site and the Zn2+ ions occupy the M(2) site,[31] due to the tetrahedral site preference[30] of Zn2+. Additional Rietveld refinements against selected patterns are shown in Figures and S1, and the refined atomic parameters, cell parameters, and agreement factors from Rietveld fits are shown in Table S1.

Figure 2

Rietveld refinements against high-resolution neutron diffraction patterns of Sr2Cr1.85Mn1.15As2O2 at (a) T = 600 K, (b) 300 K, and (c) 1.5 K. The violet, cyan, brown, and black ticks represent the possible reflections of the nuclear, k1 = (1, 0, 0), k2 = (1/2, 1/2, 0), and k3 = (1/2, 1/2, 1/2) magnetic phases, respectively. The patterns were recorded on the D2B diffractometer with λ = 1.594 Å. The agreement factors from Rietveld fits are shown in Table S1.

Rietveld refinement shows that the stoichiometry is Sr2Cr1.85Mn1.15As2O2 so that, like Sr2Cr0.77Mn2.23As2O2,[18] the final synthesized compound contains less Cr than expected. The unit cell parameters exhibit normal thermal expansion (Figure S2). Due to the anisotropic shape of the unit cell, thermal expansion favors the c axis. The unit cell therefore elongates slightly with increasing temperature, and the c/a ratio changes from 4.68316(2) at 1.5 K to 4.70907(2) at 600 K (Table S1). The M(2)–As and Cr–O bond lengths are shown in Figure S3. Both bond lengths increase upon heating, while the As–M(2)–As angles do not change significantly (Table S2). There is also no evidence of magnetostriction at any temperature.

High-intensity neutron diffraction data collected on the D1B diffractometer during continuous sample heating were used to determine the onset of magnetic ordering. Additional peaks were observed below TN(M2) = 540 K in both the D2B data (Figure ) and D1B data (Figure ). These peaks could be indexed with the propagation vector k1 = (1, 0, 0), which corresponds to a C-type antiferromagnetic arrangement of the M(2) spins with spins aligned parallel to the c axis. This magnetic structure is the same as that previously reported for Sr2Cr0.77Mn2.23As2O2[18] at a low temperature (Figure a). Below 290 K, further additional broad peaks are observed (Figures and S4). Such peaks are reminiscent of short-range magnetic correlations, as observed in Sr2Mn3As2O2 below 75 K,[14] in Ba2Zn2MnAs2O2 below 30 K,[31] and in the pnictide layer of LaMnAsO from 360 (1) to 650 (10) K.[11] Upon cooling below 200 K (TN(M1)), sharper magnetic peaks are observed, which could be indexed by two propagation vectors. The magnetic peaks can be indexed by k2 = (1/2, 1/2, 0), which corresponds to a K2NiF4-type antiferromagnetic structure of the M(1) site magnetic moments with moments ordered parallel to the c axis, as shown in Figure b. The magnetic peaks can also be indexed with the propagation vector k3 = (1/2, 1/2, 1/2) (Figures and 4). The moments remain aligned parallel to the c axis, and the magnetic structure for this propagation vector is shown in Figure c. There is no change in the magnetic structure of the M(2)2As2 layer upon further cooling.

Figure 3

High-resolution neutron diffraction patterns at 1.5, 200, 300, 500, and 600 K (bottom to top) recorded on the D2B diffractometer with λ = 1.594 Å. The gradual appearance of magnetic diffraction peaks with a decreasing temperature is observed. The hkls are marked with x, +, *, and o, for the magnetic phases with the propagation vectors k1 = (1, 0, 0), k2 = (1/2, 1/2, 0), k3 = (1/2, 1/2, 1/2), and the nuclear phase, respectively.

Figure 4

Contour plot of high-intensity data recorded on the D1B diffractometer. Data were recorded on the D1B diffractometer with λ = 2.512 Å. The intensities are presented as √I. Broad magnetic peaks are observed between 200 and 290 K, as the ordering shifts from a long range to a short range before disappearing entirely. The peaks at 28 and 32° two thetas correspond to k3 = (1/2, 1/2, 1/2).

Figure 5

Magnetic structures of Sr2Cr1.85Mn1.15As2O2 below TN(M2) = 540 K (a) and below TN(M1) = 200 K (b, c). Below 200 K, all three magnetic structures are observed, with the M(1) magnetic structures depicted in (b, c) in competition. The k1 = (1, 0, 0), k2 = (1/2, 1/2, 0), and k3 = (1/2, 1/2, 1/2) magnetic moments are represented by violet, blue, and black arrows, respectively.

The moments on the M(1) site order with a K2NiF4-type magnetic structure. The intraplanar (ab plane) ordering of the magnetic moments is antiferromagnetic. Due to the body-centered tetragonal crystal structure of the compound, the coupling of the nearest neighboring MO2 layers’ magnetic moments is a net zero, as each atom has an equal number of ferro- and antiferromagnetically aligned neighbors. The interlayer coupling thus occurs between the next nearest neighboring (nnn) layers, which is favorable in these materials.[31] Along c, there are two possible nnn interplanar arrangements: ferromagnetic with a propagation vector of (1/2, 1/2, 0) (Figure b) and antiferromagnetic with a propagation vector of (1/2, 1/2, 1/2) (Figure c).

Below 200 K, a successful fit to the experimental data was achieved with a model containing 46(4)% k2 = (1/2, 1/2, 0) (Figure b) and 54(5)% k3 = (1/2, 1/2, 1/2) (Figure c), with the k3 and k2 moments’ magnitudes constrained so that √2|mk3| = |mk2|. Hence, magnetic phase segregation is observed below 200 K. The phase fraction distribution gradually changes to 29(2)% k2 = (1/2, 1/2, 0) and 71(4)% k3 = (1/2, 1/2, 1/2) at 168 K, with no further changes down to the lowest temperature measured. During refinement, the fractions of both phases were constrained with the assumption that the entire volume of the sample exhibits ordering of the M(1) site moments. Due to the unstable nature of refining both the magnitudes and the fractions of the separate phases concurrently, they were refined separately, iterating between refining moments and scales in several steps until sufficient convergence was achieved.

The variation of the M(1) and M(2) site magnetic moments of Sr2Cr1.85Mn1.15As2O2 with temperature is shown in Figure . Short-range magnetic order of the M(2) moments is detected below 290 K. At 200 K, the M(1) moments order antiferromagnetically and magnetic phase separation is observed. Magnetic phase separation occurs when there is competition between magnetic ground states. Examples include the colossal magnetoresistant manganite perovskites where chemical doping on the Mn site results in competition between a charge/orbital-ordered antiferromagnetic phase and a ferromagnetic metallic phase[32] and Ca3(Ru1–Ti)2O7 where two competing antiferromagnetic phases are observed in a narrow doping range (x = 0.02–0.05).[33] Such magnetic phase separation has not been previously observed in Sr2M3As2O2 phases (M = Cr, Mn). It has been established that the intraplanar coupling constant is several orders of magnitude greater than the interlayer coupling constant, due to the long interlayer distance in these compounds. As such, interlayer coupling occurs once the correlation length within the layers reaches a sufficient length.[34] The exchange energies for nnn layer ferromagnetic and antiferromagnetic alignment along c at the M(1) site would appear to be almost equivalent in this compound so that there are competing spin structures and two propagation vectors observed below TN(M1). Similar magnetic phase separation has previously been reported for RuSr2Y1.5Ce0.5Cu2O10−δ[35] and Rb2MnF4.[36]

Figure 6

Variation of the M(1) and M(2) site magnetic moments with temperature. The smaller open symbols represent high-intensity D1B data, while the larger closed symbols represent high-resolution D2B data. The red curve represents the M(2) moments below TN(M2) = 540 K. Below TN(M1) = 200 K, the blue curve represents the M(1) moments belonging to both the magnetic phases with k2 = (1/2, 1/2, 0) and k3 = (1/2, 1/2, 1/2). The moment magnitudes were constrained as √2|mk3| = |mk2|. The blue striped area (200 K < T < 290 K) is an approximation of the M(1) moments where short-range magnetic order is observed.

At 1.5 K, the saturated magnetic moments of the M(2) and M(1) sites are 2.86(2) and 3.57(5) μB, respectively. It has previously been shown that there is a correlation between TN at both the M(1) and M(2) sites with the Mn(1)–O and the Mn(2)–Mn(2) distances in A2Mn3Pn2O2 (A = Sr, Ba; Pn = P, As, Sb);[32] both magnetic ordering temperatures increase as the a cell parameter decreases as a result of increasing superexchange (M(1)–O–M(1) and M(2)–As–M(2)) and direct exchange (M(2)–M(2)) interactions in the M(1)–O2 and M(2)2As2 layers. Figure S5 shows that as x is increased in the Sr2Cr3-MnAs2O2 solid solution, there is a linear increase in the cell volume. As a result, there is a clear reduction in the antiferromagnetic ordering temperature at both the M(1) and M(2) sites as x increases (Figure ). This results in a lower TN in Sr2Cr1.85Mn1.15As2O2 compared to Sr2Cr3As2O2, despite both compounds having the same M(1)O2 layer composition (CrO2). While the TN in these compounds differs, the observed short-range magnetic order in Sr2Cr1.85Mn1.15As2O2 emerges at a similar temperature (∼290 K), as the long-range magnetic order in Sr2Cr3As2O2 (291 K). As the cell volume then decreases to 309.194(7) Å3 at 200 K, the correlation length in the CrO2 layers in Sr2Cr1.85Mn1.15As2O2 reaches a sufficient length for the long-range order to be established.

Figure 7

Variation of the magnetic moment magnitudes at 4 K (top) and ordering temperatures (bottom) with x in Sr2Cr3–MnAs2O2. The magnitudes of the moments at 4 K are represented by red (M(1) site) and black (M(2) site) circles. The ordering temperatures are represented by the red and black triangle marks for the long-range order of the M(1) and M(2) site magnetic moments, respectively. The blue triangle marks represent the temperatures below which the short-range order of the M(1) site magnetic moments is observed. The short-range order is observed between 290 and 200 K in Sr2Cr1.85Mn1.15As2O2 and below 75 K down to the lowest temperature measured in Sr2Mn3As2O2.

Due to the partially covalent nature of the M–As and M–O bonds, the magnitudes of the saturated magnetic moments are lower than the theoretical values for Cr2+ and Mn2+ (4 and 5 μB, respectively) in all members of the series. However, the magnitudes of the M(2) and M(1) site moments both rise as the Mn2+ occupancy increases. The magnitude of the M(1)- and M(2)-ordered moments increases almost linearly with x in Sr2Cr3–MnAs2O2, for 0 ≤ x ≤ 2.23 so that there does not seem to be any effect of Mn2+/Cr2+ disorder on the overall ordered moment. The results suggest that the M(2) site moment magnitude is directly dependent on the composition of the layer, as the magnitude increases in proportion to the concentration of Mn2+ in the M(2)2As2 layer. The M(1) moment also increases linearly with x, which is surprising given that there is no change in the M(1)O2 layer composition from x = 0–1.15. This increase in the M(1) moment could be a result of the reduction in covalency as the M(1)–O bond length increases across the series.

The magnetic structures reported so far for the Sr2Cr3–MnAs2O2 series are all different.[14,15,18] Sr2Cr3As2O2 and Sr2Cr1.85Mn1.15As2O2 both exhibit C-type antiferromagnetic order in the M(2)2As2 layer below TN(M2), whereas G-type antiferromagnetic order is observed for Sr2Mn2.23Cr0.77As2O2 and Sr2Mn3As2O2. This would suggest that the presence of Cr in the M(2)2As2 layer results in ferromagnetic interactions along c. As the order is established on the M(1) site below TN(M1), different magnetic structures are observed. In Sr2Cr3As2O2, the ordering of the M(1) site moments causes a spin-flop of the M(2) site moments.[15] In Sr2Cr0.77Mn2.23As2O2, the ordering of the M(1) moments is followed by a spin-flip of the M(2) moments, shifting the antiferromagnetic ordering from G-type to C-type. In Sr2Cr1.85Mn1.15As2O2, there is no evidence of a spin-flip or -flop so there does not appear to be any coupling of the M(1) and M(2) spins. This could be driven by the sizeable cation disorder in the M(2)2As2 layer, but further research is warranted. In contrast to other members of the series, magnetic phase segregation is observed in Sr2Cr1.85Mn1.15As2O2 below 200 K and the two phases remain in competition down to the lowest measured temperature.

The temperature variation of the magnetic susceptibility of Sr2Cr1.85Mn1.15As2O2 is presented in Figure . The reduction in susceptibility upon cooling is a signature of the antiferromagnetic transition in the M(2)2As2 layer at TN(M2) = 540 K. There is no evidence of any further magnetic transitions in the susceptibility. The low-temperature magnetic transition in Sr2Cr0.77Mn2.23As2O2 was also not evident in the magnetic susceptibility data.[18] There is no divergence between the FC and ZFC susceptibilities, and the upturn in the susceptibility below T ≈ 100 K is attributed to paramagnetic impurity spins.

Figure 8

Temperature variation of the field-cooled and zero-field-cooled (FC and ZFC) susceptibility of Sr2Cr1.85Mn1.15As2O2 (H = 1000 Oe).

The thermal variation of the electrical resistivity of Sr2Cr1.85Mn1.15As2O2 is presented in Figure and exhibits semiconducting behavior. The 290 K resistivity is 7.769(5) × 10–4 Ω cm, which is suggestive of a highly doped semiconductor verging on metallic behavior. The resistivity can be fit to the Arrhenius equation between 120 and 225 K giving a band gap of 2.85 meV. The electronic properties of the A2M2M’As2O2 compounds are dictated by the M(2)2As2 layers.[17,18] Cr2As2 layers exhibit metallic conductivity,[17] while Mn2As2 layers exhibit semiconducting behavior.[18] In the case of Sr2Cr1.85Mn1.15As2O2, the M(2)2As2 layers are composed of Mn1.15Cr0.85As2 and its resistivity exhibits a highly doped semiconductor-like temperature dependence. The conductivity of the Sr2Cr3–MnAs2O2 phases increases as the percentage of Cr on the M(2) site rises[17,18] so that the electronic properties can be tuned by varying the ratio of Cr:Mn on the M(2) site.

Figure 9

Temperature dependence of the electrical resistivity for Sr2Cr1.85Mn1.15As2O2. The inset shows the fit to the Arrhenius equation between 120 and 225 K.

Conclusions

We have synthesized and analyzed the crystal and magnetic structures of Sr2Cr1.85Mn1.15As2O2, which is the fourth member of the Sr2Cr3–MnAs2O2 solid solution. The magnetic properties are distinct from the other members of the series. In Sr2Cr1.85Mn1.15As2O2, below TN(M1), magnetic phase separation is observed so that the M(1) moments order with the propagation vectors k2 = (1/2, 1/2, 0) and k3 = (1/2, 1/2, 1/2). The two phases remain in competition down to the lowest measured temperature. The competing spin structures arise as the exchange energies for nnn layer ferromagnetic and antiferromagnetic alignment at the M(1) site along c are almost equivalent in this compound. The results show that it is possible to tune the magnetic transition temperatures and magnetic structures by doping on the Cr site in Sr2Cr3–MnAs2O2. Substitution of Sr2+ for Ca2+ and Ba2+ or a variable temperature neutron diffraction at high pressure could provide further insight into the effects of interlayer distances and cell size on magnetic ordering in the Sr2Cr3–MnAs2O2 compounds.