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Suppression of the long-range magnetic order in Pb3(Mn1

Suppression of the long-range magnetic order in Pb3(Mn1?x Fe x)7O15

upon substitution of Fe for Mn

N.V.Volkov a,E.V.Eremin a,n,O.A.Bayukov a,K.A.Sablina a,L.A.Solov’ev b,D.A.Velikanov a,

N.V.Mikhashenok a,E.I.Osetrov a,J.Schefer c,L.Keller c,M.Boehm d

a Kirensky Institute of Physics,Russian Academy of Sciences,Siberian Branch,Krasnoyarsk660036,Russia

b Institute of Chemistry and Chemical Technology,Russian Academy of Sciences,Siberian Branch,Krasnoyarsk660049,Russia

c Laboratory for Neutron Scattering,ETH Zurich an

d Paul Scherrer Institut,CH-5232Villigen PSI,Switzerland

d Institut Laue-Langevin,6ru

e Jules Horowitz,BP156,38042Grenoble,Cedex9,France

a r t i c l e i n f o

Article history:

Received9October2012

Received in revised form

12April2013

Available online25April2013

Keywords:

Crystal growth

Ferrimagnetism

Layered magnetic compounds

a b s t r a c t

Structure and magnetic properties of Pb3(Mn1?x Fe x)7O15single crystals withх?0–0.2grown by

spontaneous crystallization from solution in melt have been investigated.All the crystals belong to the

hexagonal space group P63/mcm.The magnetic properties appeared to be strongly dependent on the iron

doping level.At small(х?0.05)dopant concentrations,the value of magnetization and Neel temperature

T N decrease insigni?cantly(T N?70K).With increasingх,the three-dimensional magnetic ordering does

not occur and temperature dependences of magnetization atх≥0.1exhibit spin-glass-like features in the

low-temperature region.

1.Introduction

Manganites with mixed-valence of manganese are oxide com-

pounds that have been attractive objects of investigation for the

last few decades.Rich variety of their physical properties caused

by the interplay of charge,spin,and orbital degrees of freedom

and possibility of controlling these properties make these materi-

als interesting for both fundamental research and application[1].

The most systematically studied compounds are the manganites

with the perovskite structure R1?x A x MnO3(R is the rare-earth

element andАisСа,Sr,Ba,Pb,etc.).In the perovskite structure,

Mn3+and Mn4+cations are localized in octahedra joint vertices.

This circumstance plays a key role in the picture of the exchange

interactions.Mixed-valence manganese oxides with the structures

different from perovskite one remain understudied;however,a

number of recent studies have been devoted to the materials(for

instance,Pb3Mn5V2O16[2]and BaMn3O6[3])where oxygen

octahedra have,as a rule,common edges and form single layers.

Being still not clearly understood,various intriguing physical

phenomena observed in the doped perovskite-like manganites

stimulate the search for other oxide families containing mixed-

valence manganese ions with the structure different from the

perovskite one.

Of particular interest is the natural mineral zenzenite with the

chemical formula Pb3(Fe3+Mn3+)4Mn4+3O15where Mn3+ions are

partially replaced by Fe3+ions.Despite the arti?cial zenzenite was

grown long ago[4],its physical properties have not been studied.

There is a known work of Bush et al.[5]devoted to the investiga-

tion of magnetic and electrical properties of the crystal with the

chemical formula Pb3Mn6O13.In our previous studies,we reported

data on the magnetic[6],dielectric[7],and calorimetric[8]

properties of the Pb3Mn7O15single crystals with mixed-valence

manganese ions.We found anomalies on the temperature depen-

dence of magnetization atТ1?160K,Т2?70K,andТ3?25K,

which were consistent with the anomalies on the temperature

dependence of speci?c heat.The temperature dependences ofε′

andε″also exhibit anomalies in the temperature range150–210K

that strongly depend on frequency.The most important questions

unanswered by now are where the magnetic phase transitions

originate from and how they transform the magnetic structure.

There still has been a lack of uni?ed interpretation of anomalies on

the temperature dependence of complex permittivity.It is unclear

whether there are charge ordering and small-radius polarons

and,if there are,whether they are evoked by the stereoactivity

of Pb2+ions.

The crystal structure of Pb3Mn7O15was described?rst by

Darriet et al.[9]on the basis of the orthorhombic Cmc21space

https://www.wendangku.net/doc/6e15004843.html,ter,Marsh and Herbstein.[10]reconsidered the structure

Contents lists available at SciVerse ScienceDirect

journal homepage:https://www.wendangku.net/doc/6e15004843.html,/locate/jmmm

Journal of Magnetism and Magnetic Materials

https://www.wendangku.net/doc/6e15004843.html,/10.1016/j.jmmm.2013.04.054

n Correspondence to:Kirensky Institute of Physics,SB RAS,Akademgorodok50,

Building38,Krasnoyarsk660036,Russia.Tel.:+73912432635;

fax:+73912438923.

E-mail address:eev@iph.krasn.ru(E.V.Eremin).

Journal of Magnetism and Magnetic Materials342(2013)100–107

in the space group Cmcm using the single-crystal data of Darriet et al.[9].Then,Le Page and Calvert[11]proposed the hexagonal unit cell with the space group P63/mcm.In Holstman's et al.work [4],zenzenite Pb3(Fe3+Mn3+)4Mn4+3O15also has the hexagonal P63/ mcm symmetry.According to the data of our preliminary X-ray diffraction(XRD)measurements on powders prepared by grinding of Pb3Mn7O15single crystals,most of the observed XRD peaks can be satisfactorily indexed in the hexagonal space group P63/mcm at room temperature[6].Recently,we carried out additional struc-tural studies on a high-resolution synchrotron in the temperature range15–295K[12].The results appeared surprising:the obtained orthorhombic structure with the space group Pnma was not found in the previous studies on Pb3Mn7O15[4,6,9–11].The thermogravi-metric analysis allowed us to determine the oxygen content x?14.9370.05in the samples[12].No structural phase transi-tions were observed within the investigated temperature range 15–300K[12].As we discovered later,upon heating Pb3Mn7O15, the room-temperature orthorhombic Pnma structure transformed ?rst(atТ1?400K)to a spatially modulated structure and then(at Т2?560K),to the hexagonal P63/mcm structure[13].

These contradictory structural data might originate from an enhanced sensitivity of the Pb3Mn7O15structure to the crystal growth conditions or deviations in the synthesis parameters, almost unavoidable at repeatable synthesis.Another possible explanation might be the in?uence of impurity traces in initial chemical reagents.We grew the crystals with different dopants(Li, Ga,Ge,Ru,etc.)in small concentrations(～5at%)and found that small amounts of impurities embedded in Pb3Mn7O15did not affect its crystal structure.In some samples with impurities,the Neel temperature dropped from70to65K,which was apparently related simply to the diamagnetic dilution[13].Doping of Pb3Mn7O15with3-d and4-d ions in amounts of～10–30at%led to the noticeable changes in the structure and magnetic proper-ties,as was shown for Pb3Mn5.5Ni1.5O15[14].

In this study,we systematically investigate the effect of substitu-tion of iron for manganese on the magnetic and structural properties of the Pb3(Mn1?x Fe x)7O15single crystals.The choice of iron as a substitute was imposed by the following circumstances.According to the results of the recent studies on the effect of Fe substitution on the magnetic and transport properties of manganites with the general formula La(Sr,Pb)(Mn1?x Fe x)O3[15,16],in these compounds iron ions are always in the high-spin3t2g2e g state.The authors of the mentioned works followed variations in the magnetic properties of the crystals with the change in the Mn3+/Mn4+ratio.We assume that embedding of Fe3+in our compound can also change the Mn3 +/Mn4+ratio.In order to identify the positions,valences,and electron states of iron ions,we performed XRD and Mossbauer studies on our samples with embedded Fe57.Despite the ionic radii of Mn3+and Fe3+are close,the octahedra occupied by the Jan–Teller Mn3+ions upon their replacement by Fe3+will become less distorted.Will this affect the structural and magnetic properties of Pb3Mn7O15?Here,we attempt to answer this question.

2.Experimental details

2.1.Sample preparation

Single crystals of Pb3(Mn1?x Fe x)7O15manganites withх?0, 0.05,0.1,0.15,and0.2were grown by a?ux method.As a?ux, PbO was chosen,known as an effective solvent for many oxide compounds and preventing incorporation of foreign ions into the lattice.The synthesis was started with heating the mixture of appropriate amounts of high purity PbO,Mn2O3,and Fe2O3in a platinum crucible at10001C for4h.Then,the crucible was slowly cooled down to9001with the rate v?2–51/h and,?nally,the furnace was cooled to room temperature.Single crystals of a plate-hexagonal shape with black shiny facets were found at the solidi?ed liquid surface.The plates were up to40mm in“dia-meter”.The grown crystals were mechanically extracted from the ?ux.The magnetic measurements reported in this study were performed on well-polished plate-like samples of the required dimension that were cut from the resulting single-crystal plates. The samples were oriented by the back-Laue method.

2.2.Measurements

Powder X-ray diffraction(PXRD)data were collected on a PANalytical X’Pert PRO diffractometer equipped with a solid state detector PIXcel and a secondary graphite monochromator.To ensure the reproducibility of the analysis,two powder samples of the material were prepared.The samples were ground in an agate mortar with octane and further annealed at1073K to reduce microstrains.The full-pro?le crystal structure analysis was made by applying the Rietveld formalism[17]and the derivative difference minimization(DDM)[18]re?nement method.In the full-pro?le re?nement,the effects of preferred orientation,aniso-tropic broadening,and surface roughness were taken into account.

Mossbauer measurements were performed with anЕМ1104Мсspectrometer at room temperature with a Co57(Cr)source on powders obtained from single crystals doped with iron enriched with a Fe57isotope by86%.The doping levels of Pb3(Mn1?x Fe x)7O15 wereх?0.05andх?0.15.The X-ray and Mossbauer measurements con?rmed that iron ions are almost completely embedded in the crystal matrix.The Mossbauer spectra were identi?ed in two stages.At the?rst stage,the probability distributions of quadru-pole splittings P(OS)in the experimental spectrum were deter-mined with?tting of the isomeric chemical shift common for the entire doublet group.The features in the P(OS)indicate possibly nonequivalent positions.This information was used in construct-ing the model spectrum.At the second identi?cation stage,this spectrum was?t to the experimental spectrum by varying the entire set of super?ne parameters.During the?tting,the exact Mossbauer parameters of nonequivalent positions were deter-mined.False features arising due to the use of the common chemical shift for the spectra set were nulli?ed.

The magnetic properties of the crystals were studied using dc magnetization measurements on a Physical Property Measure-ment System(PPMS,Quantum Design)at temperatures from2to 300K in magnetic?elds up to90kOe.

3.Result and discussion

3.1.Structural properties

Preliminary analysis of the PXRD data showed that all the Pb3(Mn1?x Fe x)7O15crystals withх?0.05,0.1,0.15,and0.2were isostructural to zenzenite[12]and the high-temperature phase of Pb3Mn7O15[14].The structure of the crystals is characterized by the hexagonal P63/mcm unit cell with four sites of Mn atoms:Mn1 (12i),Mn2(8h),Mn3(6f),and Mn4(2d).

To analyze the distribution of Fe over Mn-related sites,we performed the PXRD measurements for Pb3(Mn1?x Fe x)7O15with х?0.15using both Cu Kαand Co Kαradiations,for which Mn and Fe atoms have noticeably different anomalous dispersion coef?-cients.The structure re?nement for the two prepared samples at two different wavelengths was performed?rst with a model containing only Mn atoms in the corresponding sites.The distinc-tion in the anomalous dispersion properties of Fe and Mn manifested itself in systematic differences of the re?ned isotropic displacement parameters(U iso)of the Mn-sites obtained from Cu

N.V.Volkov et al./Journal of Magnetism and Magnetic Materials342(2013)100–107101

Kαand Co Kαdata.The values of these differences normalized to their estimated standard uncertainties(e.s.u.)are plotted for all the10atoms of the structure in Fig.1.The largest positive differences were reproducibly observed for sites Mn2(8h)and Mn3(6f),suggesting that the Fe atoms were localized mainly in these sites.

To?nally re?ne the structure,we merged the PXRD patterns for the two samples and included Fe atoms in the model.The re?nement was made for both Cu Kαand Co Kαdata.The occupancy fractions of Fe atoms were chosen such that the differences in the re?ned U iso parameters for the Mn-related sites were minimum.The results are summarized in Table1.The method described above yielded an estimated total Fe content in the structure of16.5at%,which is close to15at%introduced in the synthesis.

3.2.Mossbauer properties

Mossbauer spectra of Pb3(Mn1?x Fe x)7O15with x?0.05and0.15 represent nonsymmetrical quadrupole doublets(Fig.2).Proces-sing of the spectra by two singlets shows that the left lines of the spectra are noticeably wider than the right ones:0.38mm/s against0.33mm/s for the sample with x?0.05and0.43mm/s against0.39mm/s for the sample with x?0.15.The difference in the doublet linewidths indicates that the nonsymmetrical char-acter of the doublet is related not to the Gol'danskii–Karyagin effect but to the presence of several nonequivalent positions of iron.To determine a number of possible nonequivalent positions in the material,the distribution of probability of quadrupole splittings(QS)in the experimental spectrum P(QS)was built (Fig.3).In determining P(QS),the isomer chemical shift(IS) common for the entire doublet group was?t.

It can be seen from Fig.3that the P(QS)distribution has two features for the composition withх?0.05and three features for the composition withх?0.15.They are indicated by arrows in the Fig.3.The number of the features points out the number of possible nonequivalent positions of iron in the material.Based on these data,we built model spectra and?t them to those

obtained

Fig. 1.Values of e.s.u.-normalized differenceΔU iso/s(ΔU iso).between isotropic

displacement parameters obtained from structure re?nement on Cu Kαand Co

Kαdata for two Pb3(Mn0.85Fe0.15)7O15samples.

Table1

Experimental conditions and structure re?nement results for Cu Kαand Co Kαdata.

Structural formula Pb3(Mn0.835Fe0.165)7O15

Space group P63/mcm

Radiation Cu KαCo Kα

2Θrange9–110111–1441

Cell parameters a?10.0229(2)A a?10.0227(2)A c?13.6079(2)A c?13.6080(2)A

Cell volume1183.88(5)A31183.84(5)A3 Z4

R-DDM 5.93% 5.65%

R Bragg 2.80%

2.43%Fig.2.Mossbauer spectra of Pb3(Mn1?x Fe x)7O15withх?0.05andх?0.15at room

temperature.

Fig.3.Distribution of quadrupole splittings(observed—circle,calculated—solid line)in the spectrum of Pb3(Mn1?x Fe x)7O15withх?0.05andх?0.15.

Table2

Mossbauer parameters of Pb3(Mn1?x Fe x)7O15withх?0.05andх?0.15.Column IS is isomer chemical shift,column QS is quadrupole splitting,column W is Mossbauer absorption linewidth,columns A(M.E.)andА(R)are populations of the none-quivalent positions estimated using the Mossbauer technique and determined by the X-ray method.

IS,mm/s

70.005

QS,mm/s

70.01

W,mm/s

70.01

A(M.E.)

70.02

(R)

Position

x?0.050.3550.610.210.06Fe2(8h)

0.3710.420.240.15Fe3(6f)

x?0.150.3780.220.290.060.06Fe1

(12i)

0.3600.680.370.260.28Fe2(8h)

0.3640.430.310.250.28Fe3(6f)

N.V.Volkov et al./Journal of Magnetism and Magnetic Materials342(2013)100–107 102

experimentally,varying the entire set of the parameters of the super ?ne structure.The ?tting results are given in Table 2,where the isomer chemical shift is indicated relative to α-Fe and W is the Mossbauer absorption linewidth.

The values of the IS indicate that we deal with Fe 3+cations occupying sextantal (octahedral)coordination by oxygen.Popula-tions А(M.E.)of the nonequivalent positions for the composition with х?0.15estimated using the Mossbauer technique coincide well with the A(R)populations determined by the X-ray method.This correlation allows identi ?cation of the quadrupole doublets observed using the Mossbauer effect.

As can be seen from Table 2,at a low doping level,iron occupies two crystallographic positions,Fe2(8h)and Fe3(6f),preferring the Fe3(6f)positions.With increasing doping level (х?0.15),iron starts occupying the third,Fe1(12i)position,preferring the Fe2(8h)and Fe3(6f)positions populated by iron ions already with equal prob-abilities.Broadening of the quadrupole splitting distribution (Fig.3)with increasing doping level indicates inhomogeneity of local surroundings of iron positions.This is con ?rmed by the doublet linewidths (Table 2).An increase in the quadrupole splittings with doping level implies an increase in lattice distortions.3.3.Magnetic properties

First,let us clarify the terms.The “along-axis ”direction implies the direction along the аaxis for the orthorhombic Pnma structure or the direction along the сaxis of the hexagonal P63/mcm structure (Fig.4);the “in-plane ”direction is the direction in the b –c plane for Pnma or in the a –b plane for P63/mcm .

Fig.5shows temperature dependences of magnetization mea-sured on the Pb 3(Mn 1?x Fe x )7O 15single crystals in the “in-plane ”magnetic ?eld.For the samples with х?0,the low-temperature behavior of magnetization and the isothermal curves up to 80kOe at different directions of the magnetic ?eld relative to the crystal-lographic axes were described in detail in our previous work [6].With decreasing temperature,near Т1?160K,we observe a low,strongly broadened peak (the inset in Fig.5(a))on the temperature dependence of magnetization.The nature of this anomaly is not quite clear;it is related,most likely,to the occurrence of cluster ordering.With a further decrease in temperature,at Т2?70K,the long-range magnetic order occurs with a weak spontaneous ferromagnetic moment lying in the crystal plane and related apparently to the noncollinear character of magnetic sublattices.At Т3?25K,there is,probably,a spin-reorientation transition.Doping of Pb 3Mn 7O 15by Fe ions in small concentrations (х?0.05)leads to the situation when the value of magnetization and Neel temperature T N decrease insigni ?cantly and the broad peak at Т1?160Kspread more (the inset in Fig.5(b)).With a further increase in x ,the form of the magnetization curves drastically changes (Fig.5(с–e)):the broad peak at Т1?160K vanishes,the long-range magnetic order does not occur,and the temperature dependences of magnetization for ≥0.1reveal the spin-glass-like features at low temperatures,with typical divergence of magnetization at different regimes (with and without magnetic ?eld)of sample cooling.

Fig.6demonstrates the temperature dependence of inverse magnetic susceptibility χ?1for all the samples.It can be seen that above Т 200K the dependence is linear,i.e.,is described by the Curie –Weiss law with paramagnetic Curie temperature θрexp depending on iron content in the samples.The inset in Fig.6shows the dependence of θрexp on x concentration of Fe 3+ions.The general trend to a decrease in the absolute value of θwith increasing iron content is observed.In addition,with increasing х,the linear portion of the dependence of χ?1?nishes at lower

temperatures.

Fig.4.Unit cells for the orthorhombic Pnma [12](a)and hexagonal P63/mcm [6](b)

symmetries.

Fig.5.Temperature dependences of magnetization of Pb 3(Mn 1?x Fe x )7O 15in-plane applied ?eld under the zero-?eld-cooled (open circle)and ?eld-cooled (close circle)conditions (H ?500Oe).Inset shows the same curves enlarged.(a)x =0,(b)x =0.05(c)x =0.1,(d)x =0.15and (e)x =

0.2.

Fig. 6.Temperature dependences of inverse magnetic susceptibility χ?1for Pb 3(Mn 1?x Fe x )7O 15.The inset shows the concentration dependence of the para-magnetic Curie temperature.

N.V.Volkov et al./Journal of Magnetism and Magnetic Materials 342(2013)100–107103

Fig.7.Field dependences of magnetization for Pb 3Mn 7O 15in the “in-plane ”orientation of a magnetic ?

eld.

Fig.8.Field dependences of magnetization for Pb 3(Mn 1?x Fe x )7O 15with х?0.05in the “in-plane ”orientation of a magnetic ?eld.

N.V.Volkov et al./Journal of Magnetism and Magnetic Materials 342(2013)100–107

104

We measured ?eld dependences of magnetization at different temperatures for all values of х.At x ?0,the isothermal magneti-zation curves for Pb 3Mn 7O 15obtained in the “in-plane ”and “along-axis ”magnetic ?elds below T N show that the direction of the magnetic moment lies in the b –c plane (Fig.7).With decreasing temperature,the value of this moment grows attaining 0.6emu/g in an “in-plane ”?eld of 500Oe (Fig.5а).In Fig.7,one can see the hysteresis curves for the “in-plane ”direction of a magnetic ?eld with a coercive force of ～20kOe at Т?4.2K.The hysteresis vanishes at Т?70K.Up to 80kOe,no trend to saturation is observed at any temperatures.For the “along-axis ”direction of a magnetic ?eld,there is no hysteresis and the ?eld dependence of magnetization is linear.

The samples with х?0.05reveal very similar hysteresis loops (Fig.8);the Neel temperature slightly decreases (T N ?68K)as compared to that of the samples with х?0.We can say that small doping had the same effect as diamagnetic dilution accompanied,as a rule,by reduction of the transition temperature and the magne-tization value.At Т?4.2K and Н?500Oe,the maximum magneti-zation values are s ?0.6emu/g at х?0and s ?0.5emu/g at x ?0.05.

In the samples with х≥0.1,there is no hysteresis in the form observed in the samples with х?0and х?0.05.Fig.9shows ?eld dependences of magnetization for the sample with х?0.1obtained at different temperatures.The remainder of the hysteresis is observed up to Т?25K.Magnetization measured in the ?eld H ?500Oe decreases by more than an order of magnitude as compared to that of the nominally pure Pb 3Mn 7O 15samples.At Т?40K,the hysteresis is not revealed at all;the ?eld dependence of magnetization is linear,which is typical of paramagnets and antiferromagnets.In the samples with х?0.15and х?0.2,the considered hyster-esis vanishes at all temperatures.Fig.10shows the isotherms for the sample with х?0.15obtained at different temperatures from Т?2K to Т?40K.All the dependences have the same slope and the same magnetization value;at low temperatures,the depen-dences measured in increasing and decreasing magnetic ?eld do not coincide.It is noteworthy that the magnetization curves for х?0.15and х?0.2are nearly identical at appropriate temperatures.

Note that,while in the pure sample and in the samples with х?0and х?0.05the hysteresis phenomena are observed only in the in-plane magnetic ?eld,in the samples with х≥0.1the ?eld dependences have the same form regardless of direction of the applied magnetic ?eld,i.e.,the magnetic characteristics of these samples are isotropic.

We attempted to explain such a surprising result,when substitution of a magnetic ion for a magnetic ion leads to the break of the long-range magnetic order in the system,by the following suggestions.As was established,the Pb 3Mn 7O 15crystal has the pronounced layered structure;all manganese ions are in the octahedral surrounding of oxygen ions and nominally pure Pb 3Mn 7O 15has nine nonequivalent positions of Mn ions at temperatures below 400K.Taking into account that the paramag-netic Curie temperature of Pb 3Mn 7O 15is θ??590K and the temperature of the three-dimensional magnetic ordering is T N ?72K,it is obvious that the magnetic structure is susceptible to strong geometrical frustrations [19].In view of this,the magnetic structure is very complex and described neither as a classical two-sublattice antiferromagnet nor as a classical

two-

Fig.9.Field dependences of magnetization of Pb 3(Mn 1?x Fe x )7O 15with х?0.1in the “in-plane ”and “along-axis ”orientation of magnetic ?eld.

N.V.Volkov et al./Journal of Magnetism and Magnetic Materials 342(2013)100–107105

sublattice ferrimagnet.Most likely,there is the complex (possibly,triangular Kagome-type)magnetic ordering in the b –c (P nma )planes.The magnetically ordered planes are coupled by the magnetic interaction implemented via columns consisting of two manganese ions Mn 3+–Mn 3+.In study [20],it was established using an empirical bond-valence-sum method that the positions in the columns are fully occupied by Mn 3+ions,which is quite natural,since these ions are Jahn –Teller and tend to strong distortions of the octahedral surrounding.It follows from the X-ray data that the oxygen octahedron is distorted the most just in the columns.

Upon substitution of iron ions for manganese ones in Pb 3(Mn 1

?x Fe x )7O 15,a part of Fe

ions pass to the Fe2(8h)positions (in the hexagonal arrangement,P63/mcm ).As a result,due to the compe-tition of the exchange interactions,the geometry of the interplanar exchange implemented via the Fe 3+–Mn 3+columns can change.At the doping level х40.1,the three-dimensional ordering is appar-ently completely broken and the magnetic behavior of the system acquires the spin-glass-like character.

4.Conclusions

The Pb 3(Mn 1-x Fe x )7O 15single crystals with х?0,0.05,0.1,0.15,and 0.2were grown by spontaneous crystallization from solution in melt.The XRD investigations have shown that all the Pb 3Mn 7O 15single crystals doped with iron ions belong to the hexagonal space group P 63/mcm .The XRD structure analysis and the Mossbauer data consistently indicate that iron ions enter the

compound in the trivalent Fe 3+state and that the Mn2(8h)and Mn3(6f)sites in the structure are predominantly substituted by Fe.The magnetic properties of Pb 3(Mn 1?x Fe x )7O 15strongly depend on the doping level.At the low doping level (x ?0.05),the temperature dependence of magnetization does not qualitatively change.There are only a minor decrease in magnetization,as compared to Pb 3Mn 7O 15,at the same external magnetic ?eld and reduction of the Neel temperature T N .The form of the ?eld dependences of magnetization is nearly invariable.The situation becomes absolutely different starting from х?0.1.The broad peak at Т1?160K vanishes,the long-range magnetic order does not occur,and the temperature dependences of magnetization at х≥0.1exhibit the spin-glass-like features at low temperatures,with characteristic divergence of magnetization at different regimes of sample cooling (with and without magnetic ?eld).In the samples with х≥0.1,the hysteresis in the form observed at х?0and х?0.05is absent.Unlike the samples with х?0and х?0.05,in which the hysteresis phenomena are observed only for the magnetic ?eld directed in the plane,in the samples with х≥0.1the ?eld depen-dences have the same form regardless of direction of the applied magnetic ?eld;in other words,in these samples,the magnetic characteristics are isotropic.

Acknowledgments

The study was supported by the Ministry of Education and Science of Russian Federation,Project no.8365by the

Siberian

Fig.10.Field dependences of magnetization of Pb 3(Mn 1?x Fe x )7O 15with х?0.15in the “in-plane ”and “along-axis ”orientation of magnetic ?eld.

N.V.Volkov et al./Journal of Magnetism and Magnetic Materials 342(2013)100–107

106

Branch of the Russian Academy of Sciences,integration Project nos.29and2.5.2.

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