From ordered antiferromagnet to spin glass: A new phase Mn4Ir7−xMnxGe6

A new ternary phase, Mn₄Ir_7−xMn_xGe₆ (0?x?1.3), was studied by X-ray and neutron powder diffraction and SQUID magnetometry. The crystal structure is cubic, of the U₄Re₇Si₆ type, space group , Z=2, with the lattice parameter at 295 K. Within the limited range of homogeneity small variations of the composition yield dramatic changes of the magnetic structure. For x=0 long-range antiferromagnetic order is formed below the transition temperature 228 K, with large magnetic moments on Mn, 4.11(9) μ_B at 10 K, in a magnetic unit cell , c_M=2a_C. In contrast, for x=1.3 spin glass behavior is observed below 90 K. The Mn atoms form an ideal cubic framework, on which geometric frustration of competing nearest and next nearest neighbor antiferromagnetic interactions is suggested to explain the composition sensitive magnetic properties. A TiNiSi-type phase, IrMnGe, is found in samples of 1:1:1 composition quenched from the melt.     

Crystal structure and magnetic properties of the solid-solution phase Ca3Co2-vMnvO6

The homogeneity range of the Ca₃Co_2-vMn_vO₆ solid-solution phase covers the entire composition interval from v=0 to 1. A systematic powder X-ray and neutron diffraction, magnetic susceptibility, and magnetization study has been carried out to investigate effects of the Mn-for-Co substitution on structural and magnetic properties. The Mn substitution concerns primarily only the octahedral Co1 site of the Ca₃Co1Co2O₆ crystal structure, whereas the trigonal-prismatic Co2 site structurally is left essentially unaffected. The Ca₃Co_2-vMn_vO₆ crystal structure belongs to space group with unit-cell dimensions (in hexagonal setting) 9.084?a?9.134 Å and 10.448?c?10.583 Å. A cut through the magnetic phase diagram at 10 K shows a ferrimagnetic domain for 0?v<∼0.3 and an antiferromagnetic domain for ∼0.50<v<∼1. The magnetic ordering temperatures are quite low (<∼25/18 K), and even so further magnetic transitions appear to take place at still lower temperature. The legitimity and reliability of the different indicators used to establish the magnetic transitions, their individual accuracy, and mutual consistency are briefly discussed. Variable parameters of the crystal and magnetic structures of Ca₃Co1_1-vMn_vCo2O₆ are determined and their variation with v is briefly discussed in relation to chemical bonding. The magnetic structure in the ferrimagnetic region is essentially the same as that of the pristine v=0 phase, but since the moments at the Co2 site decrease and those at the (Co1,Mn) site increase with increasing v; characteristic traits of ferrimagnetism in magnetic susceptibility and magnetization gradually disappear. The magnetic arrangement in the antiferromagnetic region is characterized by differently sized moments at the (Co1,Mn) and Co2 sites, moments at adjacent sites in each of these sublattices being oppositely oriented along [001].     

Synthesis, structure, and magnetic properties of SrMn1−xGaxO3−δ (x=0-0.5) perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques derived from thermogravimetric measurements, we have extended the solubility limit of random substitution of Ga³⁺ for Mn in the cubic perovskite phase to x=0.5. In the cubic perovskite phase the maximum oxygen content is close to 3−x/2, which corresponds to 100% Mn⁴⁺. Maximally oxygenated solid solution compounds are found to order antiferromagnetically for x=0-0.4, with the transition temperature linearly decreasing as Ga content increases. Increasing the Ga content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.5Ga_0.5O_2.67(3) below 12 K. These properties are markedly different from the long-range antiferromagnetic order below 180 K observed for the layer-ordered compound Sr₂MnGaO_5.50 with nominally identical chemical composition.     

Crystal structures and magnetic properties of two-dimensional antiferromagnets Co1−xZnxTeMoO6

Crystal structures and magnetic properties of metal telluromolybdates Co_1−xZn_xTeMoO₆ (x=0.0, 0.1,…,0.9) are reported. All the compounds have an orthorhombic structure with space group P2₁2₁2 and a charge configuration of M²⁺Te⁴⁺Mo⁶⁺O₆. In this structure, M ions form a pseudo-two-dimensional lattice in the ab plane. Their magnetic susceptibility measurements have been performed in the temperature range between 1.8 and 300 K. The end member CoTeMoO₆ shows a magnetic transition at 24.4 K. The transition temperature for solid solutions rapidly decreases with increasing x and this transition disappears between x=0.4 and 0.5, which is corresponding to the percolation limit for the square-planer lattice. From the magnetization, specific heat, and powder neutron diffraction measurements, it is found that the magnetic transition observed in the CoTeMoO₆ is a canted antiferromagnetic ordering of Co²⁺ ions. The antiferromagnetic component of the ordered magnetic moment (3.12(3)μ_B at 10 K) is along the b-axis. In addition, there exists a small ferromagnetic component (0.28(3)μ_B) along the a-axis.     

Low temperature synthesis of Mn3O4 polyhedral nanocrystals and magnetic study

Manganese oxide (hausmannite) polyhedral nanocrystals were prepared by a microwave-assisted solution-based method using Mn(CH₃COO)₂ and (CH₂)₆N₄ at 80 °C. The as-prepared Mn₃O₄ nanocrystals were characterized by means of X-ray diffraction, field-emission transmission electron microscopy, field-emission scanning electron microscopy and Raman spectrum. Mn₃O₄ polyhedral nanocrystals prepared by microwave heating at 80 °C for 60 min were of cubic and rhombohedral shapes with the edge lengths in the range of 15-40 nm. Mn₃O₄ nanocrystals grew following the Ostwald ripening mechanism with increasing reaction time. High-resolution transmission electron microscopy and selected area electron diffraction confirm that the as-obtained polyhedral nanocrystals were single-crystalline. The magnetic behavior of Mn₃O₄ nanocrystals was studied. Mn₃O₄ nanocrystals show an obvious ferromagnetic behavior at low temperatures. The magnetic behavior of Mn₃O₄ nanocrystals was sensitive to crystal size. Ferromagnetic onset temperatures (T_c) of samples 1 and 3 are 40.6 and 41.1 K, respectively, lower than that observed for bulk Mn₃O₄ (42 K).     

Verwey-type transition within the Pb3Rh7−xMnxO15 solid solution

A complete solid solution was found between isostructural Pb₃Mn₇O₁₅ and Pb₃Rh₇O₁₅. Single-crystals of two members of the solid solution Pb₃Rh_7−xMn_xO₁₅ (x=1.07 and 2.26) were grown and their crystal structures were determined. The Verwey-type transition for Pb₃Rh₇O₁₅ at 185 K remains with a 3% substitution of Mn for Rh but disappears with 4% substitution of Mn for Rh. The magnetic ordering temperature found for Pb₃Mn₇O₁₅ at about 70 K is maintained at a 43% substitution of Rh for Mn but has disappeared for 57% substitution of Rh for Mn. The unit cell volume of this layered structure contracts with increasing x for Pb₃Rh_7−xMn_xO₁₅ phases, but the structure actually expands in the direction perpendicular to the layers due to increased separation between the layers.     

Structural and physical properties evolution in the 6H BaRu1−xMnxO3 synthesized under high pressure

The 6H BaRu_1−xMn_xO₃ with the hexagonal BaTiO₃ structure was synthesized using high-pressure sintering method. It is found that the lattice parameter deviates from Vegard's law at x=0.3 for the solid solutions due to the charge transfer effects at B-site. The substitution of Mn for Ru cations gives rise to the short-range magnetic ordering, due to the disordered arrangement of Ru and Mn cations. The compounds are weak ferromagnetic in the x range 0.05-0.40, with the maximal Curie temperature T_c 175.2 K at x=0.10. They are of spin-glass-like magnetism at lower temperature at x?0.1. With Mn doping, the 6H BaRuO₃ transforms to a semiconductor from the primal metal at x=0.30. The resistance as a function of temperature below about 70 K follows the two-dimensional variable-range hopping conduction mechanism in BaRu_0.50Mn_0.50O₃.     

Structural and physical properties evolution of BaIr1−xMnxO3 solid solutions synthesized by high-pressure sintering

The BaIr_1−xMn_xO₃ (0.0≤x≤1.0) solid solutions were synthesized by using the solid-state chemical method and high pressure sintering in the pressure range 0-5 GPa. According to the pressure-composition “phase diagram” at 1000 °C, the 9M BaIr_1−xMn_xO₃ transforms to the 6M form at 5 GPa and x≤1/6. In the x range 0.5-1.0, it transforms to the 9R form in a large pressure range. For the 9M BaIrO₃, the Mn ions substitution for Ir ions enhances the semiconducting property, and reduces the weak ferromagnetism. When x is larger than 1/3, the 9M/9R BaIr_1−xMn_xO₃ behave spin-glass-like state at low temperature, with the glass transition temperature T_g about 60 K. For the 6M BaIrO₃, the Mn ions doping results in that it transforms to insulator and spin-glass-like magnetism from the initial paramagnetic metal.     

Synthesis, electronic transport and magnetic properties of Zr1−xYxNiSn, (x=0-0.25) solid solutions

The crystal structure of the Zr_1−xY_xNiSn half-Heusler solid solutions is synthesized and their crystal structure is determined. Electrical resistivity and thermoelectric Seebeck coefficient are measured in the 80-380 K temperature range, whereas magnetic susceptibility is measured at 290 K. It is established that substitution of Zr host atoms by Y in the ZrNiSn intermetallic semiconductor is equivalent to doping by acceptor impurities. Self-consistent ab initio calculations, based on the full potential local orbital (FPLO) minimum basis method, are performed to investigate the electronic and thermoelectric properties of these alloys. Spin polarized within the framework of the coherent potential approximation (CPA) are included.     

Effects of magnesium substitution on the magnetic properties of Nd0.7Sr0.3MnO3

Effects of magnesium substitution on the magnetic properties of Nd_0.7Sr_0.3MnO₃ have been investigated by neutron powder diffraction and magnetization measurements on polycrystalline samples of composition Nd_0.7Sr_0.3MnO₃, Nd_0.6Mg_0.1Sr_0.3MnO₃, Nd_0.6Mg_0.1Sr_0.3Mn_0.9Mg_0.1O₃, and Nd_0.6Mg_0.1Sr_0.3Mn_0.8Mg_0.2O₃. The pristine compound Nd_0.7Sr_0.3MnO₃ is ferromagnetic with a transition temperature occurring at about 210 K. Increasing the Mg-substitution causes weakened ferromagnetic interaction and a great reduction in the magnetic moment of Mn. The Rietveld analyses of the neutron powder diffraction (NPD) data at 1.5 K for the samples with Mg concentration, y=0.0 and 0.1, show ferromagnetic Mn moments of 3.44(4) and 3.14(4) μ_B, respectively, which order along the [001] direction. Below 20 K the Mn moments of these samples become canted giving an antiferromagnetic component along the [010] direction of about 0.4 μ_B at 1.5 K. The analyses also show ferromagnetic polarization along [001] of the Nd moments below 50 K, with a magnitude of almost 1 μ_B at 1.5 K for both samples. In the samples with Mg substitution of 0.2 and 0.3 only short range magnetic order occurs and the magnitude of the ferromagnetic Mn moments is about 1.6 μ_B at 1.5 K for both samples. Furthermore, the low-temperature NPD patterns show an additional very broad and diffuse feature resulting from short range antiferromagnetic ordering of the Nd moments.     

Mn5Si3-type host-interstitial boron rare-earth metal silicide compounds RE5Si3: Crystal structures, physical properties and theoretical considerations

A series of binary rare-earth metal silicides RE₅Si₃ and ternary boron-interstitial phases RE₅Si₃B_x (RE=Gd, Dy, Ho, Lu, and Y) adopting the Mn₅Si₃-type structure, have been prepared from the elemental components by arc melting. Boron “stuffed” phases were subsequently heated at 1750 K within a high-frequency furnace. Crystal structures were determined for both binary and ternary series of compounds from single-crystal X-ray data: hexagonal symmetry, space group P6₃/mcm, Z=2. Boron insertion in the host binary silicides results in a very small decrease of the unit cell parameters with respect to those of the binaries. According to X-ray data, partial or nearly full boron occupancy of the interstitial octahedral sites in the range 0.6-1 is found. The magnetic properties of these compounds were characterized by the onset of magnetic ordering below 100 K. Boron insertion induces a modification of the transition temperature and θ_p values in most of the antiferromagnetic binary silicides, with the exception of the ternary phase Er₅Si₃B_x which was found to undergo a ferromagnetic transition at 14 K. The electrical resistivities for all binary silicides and ternary boron-interstitial phases resemble the temperature dependence of metals, with characteristic changes of slope in the resistivity curves due to the reduced electron scattering in the magnetically ordered states. Zintl-Klemm concept would predict a limiting composition RE₅Si₃B_0.6 for a valence compound and should then preclude the stoichiometric formula RE₅Si₃B. Density functional theory calculations carried out on some RE₅Si₃Z_x systems for different interstitial heteroatoms Z and different x contents from 0 to 1 give some support to this statement.     

Neutron diffraction and magnetic study of the Nd0.7Pb0.3Mn1−xFexO3 (0?x?0.1) perovskites

The effect of Fe doping on the ferromagnetic Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0, 0.025, 0.05, 0.075, 0.1) phases has been studied in order to analyze the double-exchange interaction. The structural and magnetic study has been carried out by neutron powder diffraction and susceptibility measurements between 1.7 and 300 K. The substitution of Fe at the Mn site results in reductions in both the Curie temperature T_c and the magnetic moment per Mn ion without appreciable differences in the crystal structures. All the compounds crystallize in Pnma space group. The thermal evolution of the lattice parameters of the Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0.025, 0.05, 0.075) compounds shows discontinuities in volume and lattice parameters close to the magnetic transition temperature. Increasing amounts of Fe³⁺ reduces the double exchange interactions and no magnetic contribution for x=0.1 is observed. The magnetic structures of Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0, 0.025, 0.05, 0.075) compounds show that the Nd and Mn ions are ferromagnetically ordered.     

Electron doping effect on structural and magnetic phase transitions in Sr2−xNdxFeMoO6 double perovskites

Polycrystalline Sr_2−xNd_xFeMoO₆ (x=0.0, 0.1, 0.2, 0.4) materials have been synthesized by a citrate co-precipitation method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature-dependent NPD data shows that the compounds (x=0.0, 0.1, 0.2) crystallize in the tetragonal symmetry in the range 10-400 K and converts to cubic symmetry above 450 K. The unit cell volume increases with increasing Nd³⁺ concentration, which is an electronic effect in order to change the valence state of the B-site cations. Antisite defects at the Fe-Mo sublattice increases with the Nd³⁺ doping. The Curie temperature was increased from 430 K for x=0 to 443 K for x=0.4. The magnetic moment of the Fe-site decreases while the Mo-site moment increases with electron doping. The antiferromagnetic arrangement causes the system to show a net ferrimagnetic moment.     

Selective substitution of vanadium for molybdenum in Sr2(Fe1−xVx)MoO6 double perovskites

The crystal and magnetic structures of Sr₂(Fe_1−xV_x)MoO₆ (0.03?x?0.1) compounds are refined by alternately using X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) data collected at room temperature. The refinement results reveal that the V atoms selectively occupy the Mo sites instead of the Fe sites for x?0.1. The 3d/4d cation ordering decreases with the increase of the V content. Slight distortions in the lattice and metal octahedra are shown at 300 K, and the distortions increase at 4 K. The magnetic structure at 4 K can be modeled equally well with the moments aligning along [001], [110] or [111] directions. The total moments derived from the NPD data for the [110] and [111] direction models agree well with the magnetic measurements, whereas the [001] model leads to a smaller total moment. Bond valence analysis indicates that Sr ions are properly located in the structure and Mo ions are compatible with both the Fe sites and the Mo sites. The electronic effects are suggested to be responsible for the selective occupation of the V on the Mo sites due to the different distortions of the FeO₆ and MoO₆ octahedra.     

Subsolidus phase equilibria and properties in the system Bi2O3:Mn2O3±x:Nb2O5

Subsolidus phase relations have been determined for the Bi-Mn-Nb-O system in air (750-900 °C). Phases containing Mn²⁺, Mn³⁺, and Mn⁴⁺ were all observed. Ternary compound formation was limited to pyrochlore (A₂B₂O₆O′), which formed a substantial solid solution region at Bi-deficient stoichiometries (relative to Bi₂(Mn,Nb)₂O₇) suggesting that ≈14-30% of the A-sites are occupied by Mn (likely Mn²⁺). X-ray powder diffraction data confirmed that all Bi-Mn-Nb-O pyrochlores form with structural displacements, as found for the analogous pyrochlores with Mn replaced by Zn, Fe, or Co. A structural refinement of the pyrochlore 0.4000:0.3000:0.3000 Bi₂O₃:Mn₂O_3±x:Nb₂O₅ using neutron powder diffraction data is reported with the A and O′ atoms displaced (0.36 and 0.33 Å, respectively) from ideal positions to 96g sites, and with Mn²⁺ on A-sites and Mn³⁺ on B-sites (Bi_1.6Mn²⁺_0.4(Mn³⁺_0.8Nb_1.2)O₇, (?227), a=10.478(1) Å); evidence of A or O′ vacancies was not found. The displacive disorder is crystallographically analogous to that reported for Bi_1.5Zn_0.92Nb_1.5O_6.92, which has a similar concentration of small B-type ions on the A-sites. EELS spectra for this pyrochlore were consistent with an Mn oxidation between 2+ and 3+. Bi-Mn-Nb-O pyrochlores exhibited overall paramagnetic behavior with negative Curie-Weiss temperature intercepts, slight superparamagnetic effects, and depressed observed moments compared to high-spin, spin-only values. At 300 K and 1 MHz the relative dielectric permittivity of Bi_1.600Mn_1.200Nb_1.200O₇ was ≈128 with tan δ=0.05; however, at lower frequencies the sample was conductive which is consistent with the presence of mixed-valent Mn. Low-temperature dielectric relaxation such as that observed for Bi_1.5Zn_0.92Nb_1.5O_6.92 and other bismuth-based pyrochlores was not observed. Bi-Mn-Nb-O pyrochlores were readily obtained as single crystals and also as textured thin films using pulsed laser deposition.     

Neutron diffraction studies of RSn1+xGe1−x (R=Tb-Er) compounds

The magnetic structures of RSn_1+xGe_1−x (R=Tb, Dy, Ho and Er, x≈0.1) compounds have been determined by neutron diffraction studies on polycrystalline samples. The data recorded in a paramagnetic state confirmed the orthorhombic crystal structure described by the space group Cmcm. These compounds are antiferromagnets at low temperatures. The magnetic ordering in TbSn_1.12Ge_0.88 is sine-modulated described by the propagation vector k=(0.4257(2), 0, 0.5880(3)). Tb magnetic moment equals 9.0(1) μ_B at 1.62 K. It lies in the b-c plane and form an angle θ=17.4(2)° with the c-axis. This structure is stable up to the Nèel temperature equal to 31 K. The magnetic structures of RSn_1+xGe_1−x, where R are Dy, Ho and Er at low temperatures are described by the propagation vector k=(1/2, 1/2, 0) with the sequence (++−+) of magnetic moments in the crystal unit cell. In DySn_1.09Ge_0.91 and HoSn_1.1Ge_0.9 magnetic moments equal 7.25(15) and 8.60(6) μ_B at 1.55 K, respectively. The moments are parallel to the c-axis. For Ho-compound this ordering is stable up to T_N=10.7 K. For ErSn_1.08Ge_0.92, the Er magnetic moment equals 7.76(7) μ_B at T=1.5 K and it is parallel to the b-axis. At T_t=3.5 K it tunes into the modulated structure described by the k=(0.496(1), 0.446(4), 0). With the increase of temperature there is a slow decrease of k_x component and a quick decrease of k_y component. The Er magnetic moment is parallel to the b-axis up to 3.9 K while at 4 K and above it lies in the b-c plane and form an angle 48(3)° with the c-axis. In compounds with R=Tb, Ho and Er the magnetostriction effect at the Nèel temperature is observed.     

Thermochemistry of Li1+xMn2−xO4 (0?x?1/3) spinel

Lithium substituted Li_1+xMn_2−xO₄ spinel samples in the entire solid solution range (0?x?1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x?0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na₂O·4MoO₃ at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x?0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn³⁺ to Mn⁴⁺ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn³⁺ to Mn⁴⁺ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction.     

Non-stoichiometric FexWN2: Leaching of Fe from layer-structured FeWN2

Non-stoichiometric Fe_xWN₂ (x∼0.72) was synthesized via leaching of Fe from layer-structured stoichiometric FeWN₂ by soaking in sulfuric acid at ca. 50 °C. The synthesized products were characterized by powder X-ray diffraction (pXRD), secondary electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and magnetic measurements. Non-stoichiometric Fe_xWN₂ has the same symmetry unit cell as stoichiometric FeWN₂ (P6₃/mmc), but the lattice parameters change: the a-axis expands by 0.16% while the c-axis decreases by 1.5%. Polycrystalline powder of Fe_xWN₂ showed similar morphologies as those of FeWN₂. The calculated electronic structure of stoichiometric FeWN₂ shows a more ionic-bonding character between Fe and N than that between W and N, which presumably allows for the partial Fe leaching from between the W-N prismatic layers. The magnetic susceptibility of Fe_xWN₂ smoothly decreases with increasing temperature from 3 to 300 K, unlike the broad maximum seen near 27 K in stoichiometric FeWN₂.     

Correlation between structure and magnetic properties of Cd-substituted La0.7(Ca0.3−xCdx)MnO3 CMR manganites

Structural, electrical and magnetic properties of Cd-doped La_0.7(Ca_0.3−xCd_x)MnO₃ (0?x?0.3) manganites are presented. All compositions were indexed in the orthorhombic (Pnma) space group, except the Cd_0.3 sample, indexed as a combination of trigonal and orthorhombic (Pnma) space groups. Substitution of Ca by Cd has a strong influence on the magnetic and magnetoresistive properties of these compounds, continuously decreasing both the magnetic moment and the Curie temperature (from 3.5 μ_B and 270 K for the x=0 composition to 1.59 μ_B and 90 K for the fully doped x=0.3 one). Samples corresponding to x=0 and 0.1 show a semiconductor-metal transition at temperatures close to the Curie ones. The measured magnetoresistance change is about 49% at 270 K and 95% at 165 K for those samples, respectively. However, the x=0.2 and 0.3 compositions show insulating behaviour in the whole temperature range studied, with values of the magnetoresistance about 85% at 105 K and 74% at 90 K, respectively. The observed weakening of the double-exchange mechanism as the Cd doping level in these samples increases is discussed in terms of structural properties, cationic disorder and Mn³⁺/Mn⁴⁺content ratio.     

Neutron powder diffraction study of the magnetic and crystal structures of SrFe2(PO4)2

The crystal and magnetic structures of SrFe²⁺₂(PO₄)₂ have been determined by neutron powder diffraction data at low temperatures (space group P2₁/c (no. 14); Z=4; a=9.35417(13) Å, b=6.83808(10) Å, c=10.51899(15) Å, and β=109.5147(7)° at 15 K). Two magnetic phase transitions were found at T₁=7.4 K (first-order phase transition) and T₂=11.4 K (second-order phase transition). The transition at T₂ was hardly detectable by dc and ac magnetization measurements, and a small anomaly was observed by specific heat measurements. At T₁, strong anomalies were found by dc and ac magnetization and specific heat. The structure of SrFe₂(PO₄)₂ consists of linear four-spin cluster units, Fe2-Fe1-Fe1-Fe2. Below T₁, the propagation vector of the magnetic structure is k=[0,0,0]. The magnetic moments of the inner Fe1-Fe1 atoms of the four-spin cluster unit are ferromagnetically coupled. The magnetic moment of the outer Fe2 atom is also ferromagnetically coupled with that of the Fe1 atom but with spin canting. The four-spin cluster units form ferromagnetic layers parallel to the [−101] plane, while these layers are stacked antiferromagnetically in the [−101] direction. Spin canting of the outer Fe2 atoms provides a weak ferromagnetic moment of about 1 μ_B along the b-axis. The refined magnetic moments at 3.5 K are 4.09 μ_B for Fe1 and 4.07 μ_B for Fe2. Between T₁ and T₂, a few weak magnetic reflections were observed probably due to incommensurate magnetic order.