Single-Crystal and Powder Neutron Diffraction Study of FeXMn1-XS Solid Solutions
Galina Abramova, Juerg Schefer, Nadir Aliouane, Martin Boehm, German Petrakovskiy, Alexandr Vorotynov, Mikhail Gorev, Vladimir Sokolov
SSingle-Crystal and Powder Neutron Diffraction Study of Fe X Mn S Solid Solutions
Galina Abramova , Juerg Schefer , Nadir Aliouane , Martin Boehm , German Petrakovskiy ,Alexandr Vorotynov , Mikhail Gorev , and Vladimir Sokolov Kirensky Institute of Physics, Russian Academy of Sciences, Siberian Branch,Krasnoyarsk, 660036 Russia Paul Scherrer Institute, Laboratory for Neutron Scattering (LNS),CH-5232 Villigen PSI, Switzerland Institut Max von Laue - Paul Langevin, Grenoble Cedex 9, France Nikolaev Institute of Inorganic Chemistry, Siberian Branch,Russian Academy of Sciences, Novosibirsk, 630090 Russia Institute of Engineering Physics and Radio Electronics, Siberian Federal University,prosp. Svobodny 79, Krasnoyarsk, 660041 Russia Fe X Mn S (0 PACS number_s_: 75.25._z, 61.05._a, 75.80._q, 64.70.Nd I. INTRODUCTION α-MnS belongs to MnO-type substances with the strongelectron correlations and the rock salt structure 1, 2 .Hydrostatic pressure applied to such systems causes anumber of phenomena, including an insulator-to-metaltransition, reduction of a magnetic moment (high-spin-to-low-spin transition), and a change in crystal symmetry (ref.in 3, 4 ). Below Neel temperature T N , the MnO-type oxidesand MnS exhibit the FCC II (AFM-II) antiferromagneticorder . Under ambient conditions, they are magneticsemiconductors. Under a hydrostatic pressure of 50-100GPa at room temperature, the MnO-type compoundsundergo the insulator-to-metal transition . The effect of thepressure on the magnetic and crystal structures of thecompounds still remains understudied .In this study, we used the chemical pressure to mimic thechanges in physical properties observed by the hydrostaticone in MnS by substituting Mn (an ionic radius of 0.97Å, the high-spin octahedral state) by Fe (an ionic radiusof 0.92 Å, the high-spin octahedral state). We synthesizednew Mott-type compounds with a chemical formulaFe X Mn S (0 X < 0.3) and the rock salt structure. Wereport the results of single-crystal and powder neutrondiffraction for the Fe X Mn S (0 < X < 0.3) samples andthe electrical and structural properties of the latter. II. EXPERIMENTAL Previously, we reported the details of synthesis of Fe X Mn S single crystals and X-ray diffraction data for thesematerials . Powder neutron diffraction studies of theFe X Mn S samples with x = 0.05, 0.18, and 0.27 werecarried out at the Institut Laue-Langevin (ILL) using a D1A thermal diffractometer with = 1.91°A attemperatures of 2-300 K. Single-crystal neutron diffractionstudies of the samples with x = 0.25 and 0.29 were carriedout at = 1.178 Å in a four-circle Eulerian cradle attemperatures of 2-300 K using a TriCS thermalinstrument and a SINQ neutron spallation source at thePaul Scherrer Institute (PSI). Simultaneously, wemeasured the integral intensity and (hkl) position of themagnetic and nuclear peaks for the Fe X Mn S samples.The neutron and X-ray diffraction data obtained wererefined using a Fullprof program based on the Rietveldmethod. Resistance and lattice strain were measured at theKirensky Institute of Physics. The electrical propertieswere investigated with the use of a PPMS facility(Quantum Design) at temperatures of 2-300 K. Thermallattice expansion measurements were performed in thetemperature range 120–300 K with a heating rate of 3 Kmin -1 using a NETZSCH DIL-402C push-rod dilatometerwith a fused silica sample holder. The results werecalibrated by taking SiO as a reference to eliminate theinfluence of thermal expansion of the system.Parallelepiped samples were cut from large single crystalsso as to have parallel surfaces perpendicular to the (100)crystallographic directions; distance L between the MnSsurfaces was 4.915 mm. These samples were used inelectrical and thermal lattice expansion measurements.III. RESULTS AND DISCUSSIONA. Neel temperature of Fe X Mn S X-ray study showed that, at room temperature, theFe X Mn S single crystals, similar to the α-MnS phase, arecharacterized by the face-centered cubic (FCC) structure of2a NaCl type (Fm-3m space group). Substitution of ironions Fe for manganese ions Mn in Fe X Mn S leads toa decrease in the lattice parameter from 5.224 (x = 0) to5.165 Å (x = 0.29). The obtained parameter of a FCC unitcell at 300 K (Fig.1) for the Fe X Mn S samples with x =0.29 (under the action of chemical pressure x ) is close tothe value observed in MnS under a hydrostatic pressure of3–4 GPa . A decrease in the cubic lattice parameterunder the chemical pressure in Fe X Mn S at roomtemperature is accompanied by reduction of electricalresistance by six orders of magnitude. We found that, at 2-300 K, conductivity of the Fe X Mn S samples changesfrom the semiconductor type for x < 0.25 to metal one forx X Mn S and the interatomicdistance of the magnetic ions. FIG.1. The change of relative parameter of the cubic NaCl latticeunder hydrostatic pressure (P) for MnS [ ] and upon Fesubstitution (x in Fe X Mn S) at room temperature. Single-crystal and powder neutron diffraction data taken atdifferent temperatures on a D1A (ILL), TriCS, and HRPT(PSI) showed that the structure of the Fe X Mn S samplesis similar to that of MnS. The single-crystal and powderneutron diffraction patterns above the Néel transitioncontain only odd (111, 113, 331, and 511) Braggreflections corresponding to the nuclear reflections, whichare typical of the MnS paramagnetic state (Fm-3m spacegroup). Substitution of Fe ions in the Fe X Mn S samplesby Mn ions is accompanied by a decrease in the integralintensity of the nuclear Bragg reflections from the oddplanes and an increase in the integral intensity of thenuclear Bragg reflections from the even planes, becausemanganese (-0.37) and iron (+0.96) ions have negative andpositive neutron scattering amplitudes (in the units of 10 -12 cm ). Ions of Fe and S have positive neutron scatteringamplitudes. For this reason, the neutron pattern of FeOdiffers from the patterns of MnS and MnO by the presenceof the even planes 5, 6, 16 . Our Fe X Mn S compoundsexhibit the varying integral intensity of the reflections onthe patterns upon replacement of Mn ions (S = 5/2) by Fe ions (S = 2). These data also confirm the formation ofthe Fe X Mn S solid solutions.Below Neel temperature T N , MnO, FeO, and MnS exhibitthe FCC II (AFM II) antiferromagnetic order withferromagnetic (FM) sheets of the (111) planes that areantiferromagnetically stacked along the [111] direction . The lattices of MnO, FeO, and NiO are trigonalydistorted along the [111] direction; thus, the latticesymmetry is consistent with the symmetry of the magneticstructure . The magnetic reflexes caused by the AFM-IIorder appear at the (h/2 k/2 l /2) reciprocal lattice points,where h, k, and l are the odd numbers. FIG.2. Powder neutron diffraction patterns of MnS at 20 K.Insert: direction of the magnetic moments of Mn ions in the NaCllattice. Figure 2 presents powder neutron diffraction patterns ofMnS that were taken on a HRPT ( = 1.886 A) at 20 K.The magnetic structure of the MnS sample is similar tothat reported in . The direction of the magnetic momentsof Mn ions in the MnS sample was obtained tocorresponds to <110> (insert in Fig.2); the magneticmoments are ferromagnetically ordered on the (111) planesand antiferromagnetically ordered between these planeswith the spin propagation vector (1/2,1/2,1/2). Figure 3presents the powder neutron diffraction patterns of theFe X Mn S samples taken on a D1A (ILL) = 1.911 A) at2 K. The magnetic structure of Fe X Mn S at x . The magnetic structure of Fe X Mn S at x . The observed magnetic Braggreflections are of the odd type only. The intensity of allodd magnetic peaks with spin propagation vector(1/2,1/2,1/2) vanishes at T N .Figure 4a shows temperature dependences of the relativeintegral intensity of the magnetic Bragg reflections I(T)/I(2K) observed for the Fe X Mn S samples. The integralintensity of one peak on the pattern is given by I = ( Imax/FWHM ), where FWHM is the total width at a halfmaximum of the peak. Magnetization M of the magneticsublattice is proportional to the square root of themeasured neutron intensity (I). In Fe X Mn S, we found ashift of the Neel temperature from 150 (x = 0) to 200 X ). This indicates the enhancement of the super-exchange3interaction constant with decreasing distance betweenmagnetic atoms upon substitution Mn Fe. The estimatedvalues of the exchange integral of the superexchangeinteraction along the <100> direction for the (Mn,Fe)- S -(Mn,Fe) bonds are J = - 4.15 K for MnS and J = - 7.27K for Fe X Mn S (x = 0.29). Thus, substitution Mn Fecauses a decrease in cubic lattice parameter a and anincrease in the Neel temperature. The Neel temperatureshifts occurring in MnS upon this substitution for our caseand under hydrostatic pressure ( ) are compared in Fig. 5.A linear increase of the Neel temperature with increasingX in Fe X Mn S is observed only in the case ofsemiconductor substances (x < 0.25). FIG.3. Powder neutron diffraction patterns of Fe X Mn 1- X S with x= 0 (1);=0.05(2);=0.18(3);=0.27(4) at 2 K. FIG. 4. Temperature dependences of the relative integralintensity of the magnetic reflections (a) and thermalexpansion coefficient (b) for Fe X Mn S samples, b:MnS (1); x=0.1 (2); x=0.29 (3); without the anomalouslattice behavior (4). FIG.5. Comparison of the Neel temperature shift in MnS underhydrostatic pressure [13] and upon Fe substitution (ourinvestigation). B. Lattice strain in Fe X Mn S Small rhombohedral distortion of the cubic NaCl lattice ofMnS was found by B. Morosin at a Neel temperature of150 K and by H. van der Heide et al. at about 200 K . Wefound no substantial evidences for the change of symmetrylattice of Fe X Mn S with 0 < x < 0.25 at low temperatures(2–7 K). However, as can be seen in Fig. 4b, there is ananomaly of thermal expansion coefficient of thesesamples at the Neel temperature ( is measured along the[100] lattice direction, i. e., for the (100) planes). Thisfigure shows temperature dependences of the thermallattice expansion coefficient = (1/L)dL/dT of theFe X Mn S single crystals. The anomalous contribution to is observed in a wide temperature range (for example,100-260 K for MnS), being maximum at the Neeltemperature.The temperature dependence of magnetization M rel (inrelative units) obtained by Imax(T)*FWHM (T = 100K)/Imax(T = 100 K)*FWHM(T) for Fe X Mn S (x = 0.05,T N = 165 K) is shown in Fig. 6 . For comparison, in thesame figure the temperature dependence of relativeanomalous contribution S rel to the lattice strain of MnS (T N = 150 K) is shown. The relative value of the anomalouslattice strain is obtained by S rel = dL/L m (T)/dL/L m (T=100K), S rel < 0 (not shown in the figure). The temperaturedependences of the structural and magnetic properties areconsistent, which allows us to conclude that the anomalouscontribution dL/L m < 0 to the lattice strain near the Neeltemperature is due to the magnetic transition (the negativemagnetoelastic effect along the [100] lattice direction forMnS). Magnetoelastic constants B1 along the [100]direction and B2 along [111] direction of the FCC latticecan be estimated from the anomalous contribution to thelattice strain (dL/L) m<100> = - (2/3) B1 /(C -C ) and(dL/L) m<111> = - (2/3) B2 /C in the magnetically orderedstate, C , C , and C are the elastic modules for MnS.4The observed value of (dL/L) m along [100] is -0.0008 at106 K for MnS, (C -C ) = 0.69∙10 +12 din/cm . Theestimated value B1 = 83 J/cm for MnS at 106 K is typicalof the spinel compounds of 3d-elements. FIG. 6. Comparison of relative magnetization M rel of Fe X Mn Swith x =0.05 (curve 1) and relative anomalous lattice strain S rel ofMnS (curve 2, negative anomalous lattice strain). C. Properties of the single crystals Fe X Mn S with0.25 x The step-wise semiconductor-to-metal electron transitionin pure MnS is observed at room temperature under apressure of about 26 GPa . Our investigation of theelectrical resistance shows that MnS is the semiconductorin the temperature range of 77-300 K and it undergo thechange of the conductivity type under the chemicalpressure. The temperature dependence of electricalresistance of the Fe X Mn S at x = 0.25 is shown in Fig.7a, for the illustration. The presented data clearly indicatemetal conductivity; the resistance decreases withtemperature below 240-250 K. The magnetic properties ofsingle crystals of Fe X Mn S at x = 0.25 and 0.29 wereinvestigated by a TriCS method at the PSI. Thetemperature dependences of the integral intensity of themagnetic peaks (Fig. 7c) are similar to those of theFe X Mn S samples with x < 0.25. The temperaturehysteresis of magnetization of about 10 o at the Neeltemperature is found upon heating and cooling the sampleswith x = 0.25 and 0.29. This fact indicates the occurrenceof the first-order magnetic transition. Small residualintensity of the magnetic peaks is observed at T > T N inzero magnetic field and may indicate the occurrence of theshort-range order up to 220 K. We have found the Fe X Mn S single crystals with x at 3.3 GPa. Such behavior the (111)reflection can be explain by splitting of this reflection intotwo reflections (003 and 101) due to the structuraltransition from the cubic Fm-3m to rhombohedral R3m –like phase. The geometry of our neutron experiment alloyto us the observation only the transformation of the (111)reflection of Fm-3m state to (003) reflection of R3mphase. FIG. Temperature dependences of electrical resistance (a),lattice parameter (b), and integral intensity of the magnetic peak(1/2;3/2;1/2) (c) for x = 0.25. It can be seen from Fig.7b that the structural transition isobserved at about 240 K and precedes the magnetictransition T N = 192 K. This structural transition isaccompanied by the change in the conductivity type fromthe high-temperature semimetal state to the low-temperature metal one (Fig. 7a) upon cooling the sample.Since the critical temperatures of the structuraltransformation and of the magnetic transition in Fe X Mn S are different, the mechanism of the observed structuretransition may be of nonmagnetic origin. The structuraltransition and the change in the conductivity type break alinear increase in the Neel temperature of the Fe X Mn Ssolid solutions (Fig. 5a). At the same time, the temperaturerange of the local short-range magnetic order in theFe X Mn S samples 0.25 < x<0.29 extends up to 300 K,especially, in the applied magnetic field. The more detaileddata of the Fe X Mn S samples 0.25 < x<0.29 will bepresented in next manuscript.5 IV. CONCLUSIONS We studied the effect of the chemical pressure (Fe cationsubstitution) on the magnetic, electrical, and structuralproperties of α-MnS belonging to the MnO-typesubstances at temperatures of 2-300 K. We found that thechemical pressure leads to a decrease in the cubic latticeparameter and in electrical resistance and causes theinsulator-to-metal transition. A linear increase in the Neeltemperature is observed by the neutron diffraction for thesemiconductor substances Fe X Mn S (0< x<0.25) withincreasing Fe cation substitution. The magnetic structureof Fe X Mn S solid solutions at x X Mn S at x X Mn S (0.25 < x<0.29), the changein the nuclear and magnetic lattice symmetry was found. V. 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