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Journal of Magnetism and Magnetic Materials 285 (2005) 379–385

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ESR study of new layered semiconductor Ga2Cr1.33Se5

D. Skrzypeka,�, I. Okonska-Kozlowskab, K. Szamockab, E. Malickab

aA.Chelkowski Institute of Physics, University of Silesia, Uniwersytecka 4 , 40-007 Katowice, PolandbInstitute of Chemistry, University of Silesia, Bankowa 14, 40-007 Katowice, Poland

Received 12 March 2004; received in revised form 14 July 2004

Available online 27 August 2004

Abstract

The Ga2Cr1.33Se5 compound is a layered semiconductor and it can be visualised as layers of chromium ions with

nearest-neighbour distances of 0.378 nm within layers and interlayer distances are alternately of 0.626 and 0.923 nm.

The ESR measurements of a single crystal as a function of temperature were performed; a single Lorentzian-type

spectrum was observed for T420K. The observed angular anisotropy of the linewidth has been satisfactorily explainedon the basis of the anisotropic exchange model. The peculiarities of ESR parameters (intensity, DB, Br) below T ffi 20K

can be interpreted taking into account two mechanisms: (i) presence ferromagnetic, exchange interactions between in-

plane chromium ions and antiferromagnetic interplane coupling which means magnetic dimensionality crossover from

2- to 3-d system; (ii) appearance of a spin disorder.

r 2004 Elsevier B.V. All rights reserved.

PACS: 75.50; 76.30; 68.65

Keywords: Electron spin resonance; Low-dimensional magnetic systems; Single crystals; Exchange interactions; Layered structure

compound

1. Introduction

Ternary selenide spinels, exhibiting interestingstructural, magnetic and electrical transport prop-erties have been the subject of numerous studies[1–9]. It was shown that replacement of the di- ortrivalent cation by a third metal resulted in newchemical compounds with essentially changed

- see front matter r 2004 Elsevier B.V. All rights reserve

/j.jmmm.2004.07.056

onding author.

ddress: dskrzyp@us.edu.pl (D. Skrzypek).

properties interesting for both basic research andtechnological aspects [10–15]. Quaternary sele-nides, crystallising with the spinel-type structure,can usually be obtained in a whole concentrationrange of the dopant [16–21], while those with anon-spinel structure show a limited solubility andoften the end-member of the system remainsunknown [21–23]. In the course of our studies onthe Cd, Zn–Ga–Cr–Se solid solutions it wasinteresting to prepare the end-member of thesystem, i.e., Ga2/3Cr2Se4. However, our attempts

d.

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D. Skrzypek et al. / Journal of Magnetism and Magnetic Materials 285 (2005) 379–385380

to grow this compound, using chemical transportreactions established that with high Ga concentra-tion the spinel structure did not form. The newsingle crystals, without spinel structure, wereobtained and described with the formulaGa2Cr1.33Se5 in our previous paper [24].The structure of this compound was described

as a close packed hexagonal arrangement ofselenium ions, forming layers perpendicular tothe c-axis. The cations occupy tetrahedral (Ga)and octahedral (Cr) voids between the layers.Along the c-axis there is a repetition c/3 with fivesymmetrically different layers. The sequence of thelayers results from the metal vacancies also haveimpact on the properties of the crystal. Thetemperature dependence of the magnetic suscepti-bilitity obeys the Curie–Weiss law with positiveyC–W=137.7K, pointing to ferromagnetic interac-tions. Nevertheless, no evident magnetic transitionwas found down to 4.2K, but a small perturbationin the monotonic curve below T ¼ 20K wasobserved. There was no saturation of magnetisa-tion at 4.2K in a stationary field of 15T; themoment has scarcely reached the value m ¼

2:0mB=Cr3þ: The electrical transport measure-

ments showed the compound to be the p-typesemiconductor.The present work is a continuation of our

studies on the influence of the Ga3+ ions on theproperties Ga–Cr–Se systems. The ESR spectro-scopic characterisation of the single crystal ofGa2Cr1.33Se5 is reported.

2. Experimental

Single crystals were grown by chemical vapourtransport in closed quartz ampoules with anhy-drous chromium chloride, CrCl3, as a transportingagent and with the selenide Ga2Se3 as a solidphase. These ampoules were heated in a horizontalzone furnace for 7 days (the temperature gradientwas between 25 and 38K). The temperature of thesolution and crystallisation zone was 1100 and1067K, respectively. After 7 days the furnace wascooled to room temperature with a rate of 20K/h.Single crystals of hexagonal plate-like shape withedges of about 0.1–5mm have been obtained. The

crystal of dimensions 0.33mm� 0.31mm� 0.11mmselected for standardless energy dispersive absorp-tion analysis (EDAX) showed the presence of Cr,Ga and Se. The formula Ga2Cr1.33Se5 wasobtained from X-ray diffraction measurementswith a four-circle Kuma KM-4 diffractometer [24].The electron spin resonance spectra were

recorded with a standard EPR spectrometeroperating at X-band (9GHz) frequency, using100 kHz field modulation. The microwave fre-quency was measured using Hewlett-Packard 534microwave frequency counter. The temperaturedependence measurements were performed in thetemperature range from 4 to 300K with an OxfordInstruments ESR-900 continuous-flow heliumcryostat.The crystals grow in the thin plates with well-

defined reflection plane, simplifying the orienta-tion of the sample. The sample of dimensions0.2mm� 0.2mm� 0.1mm was glued to an L-shaped sample holder and the applied magneticfield was rotated in the two planes: ab-planeperpendicular to the c-axis and ac-plane whichcontains the c-axis (c-axis is perpendicular to thereflection plane of the crystal).The measurements for two selected single

crystals (about similar dimensions) were con-ducted. For each sample the low-temperaturemeasurements were repeated. In all cases theresults were reproducible. The values of the ESRparameters: DB-linewidth and Br-resonance field,at T440K were obtained on the basis of the bestfit for the simulated Lorentzian profile in compar-ison with the experimentally observed spectra. AtTo40K, data results of several temperaturecycles, at that: Br means crossing point and DB

means peak-to-peak linewidth.

3. Results and discussion

The room temperature ESR spectrum ofGa2Cr1.33Se5 consists of a strong, symmetrical linewith g ¼ 1:99; this value is very close to that foundfor isolated Cr3+ ions in various compounds. Theline-shape is strictly Lorentzian and this spectrumcan be attributed to the large clusters of chromiumions coupled by exchange. The temperature

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evolution of the ESR spectra is displayed in Fig. 1.The dramatic modifications of the spectrum at lowtemperatures are clearly observed (i) deviationfrom Lorentzian line-shape, (ii) shift of theresonance field (Br), and (iii) broadening of thelinewidth (DB).The position and the linewidth of the ESR line

were measured at 100 intervals in the two principalplanes (ab and ac) of the crystal at roomtemperature. In both planes the g-factor isisotropic and also in the ab-plane the angularvariation of the linewidth was not observed. Theline-shape is a Lorentzian in whole angle range.The ESR theory of a two-dimensional systemdeveloped by Richards and Salamon [25] treats theline-shape by assuming diffusive motion for thelong-time dependence of the time correlationfunctions. This analysis leads to a more or lesspronounced deviation from the Lorentzian line-shape and it depends on orientation of a sample.When the expected anomalies are less pronouncedthis mean that the influence of diffusive modesdiminishes with increasing dimensionality of thespin lattice. In our investigations, the room-temperature line-shape is Lorentzian and angulardeviation is not observed. This result means thatinterlayer interactions are not negligible.The linewidth, when the applied magnetic field

was rotated in the ac-plane, showed angulardependence (see Fig. 2). It is known that the

0.0 0.2 0.4 0.6 0.8 1.0

Inte

nsity

[arb

. u.]

T = 5,3 KT = 9 KT = 16 KT = 35 KT = 67 K

T = 108 K

T = 217 K

T = 319 K

magnetic field [T]

Fig. 1. The temperature evolution of the ESR spectrum of

Ga2Cr1.33Se5 crystal.

long-time behaviour of the spin dynamic isgoverned by spin diffusion. In particular, forlayered two-dimensional systems this long-timespin-diffusive behaviour gives a contributionproportional to (3cos2 y�1)2 in the description ofESR linewidth [25]. In the case where thespin–orbit interaction is present, additional aniso-tropic and antisymmetric exchange interactionsare important and produce broadening of the ESRline described by cos2 y angular dependence [26].The experimental results, which are shown in

Fig. 2, we analyzed quantitatively take intoaccount both contributions just the same as theanisotropy exchange model [25,26]:

DB ¼ A þ B cos2yþ C ð3 cos2y� 1Þ2; (1)

where y is the angle between the direction of themagnetic field and c-axis—the normal to thechromium layers in Ga2Cr1.33Se5.The fitting parameters are: A ¼ 36:0� 0:1mT;

B ¼ 5:0� 0:1mT; C ¼ 0:2� 0:1mT:We can see that the main contribution to the

linewidth is an isotropic one and the observedangular anisotropy of the linewidth has beensatisfactorily explained on the basis of theanisotropy exchange model.The plots of the temperature dependence of the

ESR linewidth, resonance field and intensity in therange 100–300K are shown in Fig. 3. The appliedmagnetic field was perpendicular to the reflection

Fig. 2. The angular variation of ESR linewidth of Ga2Cr1.33Se5at a room temperature. B was rotated in an ac-plane. The solid

line is the fit of the theory to the data.

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100 150 200 250 300 350

0.324

0.326

0.328

0.330

32

36

40

44

(a)

linew

idth

[mT

]in

tens

ity [a

rb.u

.]

(b)

(c)reso

nanc

e fie

ld [T

]

temperature [K]

Fig. 3. The temperature dependence of ESR parameters at a

high temperature range. BJc-axis.

0 20 40 60 80 100 120 140

0

5

10

15

20

25

0

5

10

15

20

rela

tive

inte

nsity

temperature [K]

90-ZFC 90-FC

0-ZFC 0-FC

Fig. 4. The relative ESR susceptibility versus temperature.

D. Skrzypek et al. / Journal of Magnetism and Magnetic Materials 285 (2005) 379–385382

plane of the crystal and parallel to the c-axis (O-option; BJc-axis). The values of the ESR para-meters (DB and Br) were obtained as the fittingparameters.Starting from the room temperature the line-

width values monotonically (almost linearly)decreased as the temperature reduced until140K. A linear temperature dependence of theESR linewidth is reported in paramagnetic state ofvarious low-dimensional systems. This behaviouris generated by relaxation mechanism such as thephonon modulation of anisotropic exchange inter-action [27].The values for the intensity obtained from the

second integration of field derivative absorptioncurve increase monotonically with decreasingtemperature and it may be inferred that exchangeamong chromium ions is of a ferromagnetic type.As the temperature is reduced from the roomtemperature the shift of the resonance field isobserved—it means the existence of short rangeorder.

The values of the intensity, linewidth andresonance field 4.2–140K range are plotted againsttemperature in Figs. 4–6, respectively. In all cases,the most dramatic changes in ESR parameters andthe lineshape behaviour are observed below 30K.We show the results of ESR measurements madeon the crystal under field-cooled (FC-1T) andzero-field-cooled (ZFC) conditions and for twodirections of the external static field: B isperpendicular to the reflection plane of crystal(O-option ) and B is an in-reflection plane ofcrystal (90-option). The sample was mounted on aquartz holder which was rigidly affixed to agoniometer and its movement was impossible.Fig. 4 shows the relative ESR susceptibility

(calculated as double integration of the spectrum)versus temperature. When the magnetic field isapplied in-plane the temperature dependence issimilar for ZFC and FC options up to 30K. Next,FC susceptibility increases monotonically up toliquid helium temperature, whereas ZFC suscept-ibility exhibits peculiarities: its values decrease in

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0 20 40 60 80 100 120 140

0.02

0.04

0.06

0.08

0.10

0.04

0.08

0.12

0.16

0.20

90-ZFC 90-FClin

ewid

th [T

]

temperature [K]

0-ZFC 0-FC

Fig. 5. The ESR linewidth as a function of temperature—the

comparison for the different conditions of measurements.

0 20 40 8060 100 120 140

0.16

0.20

0.24

0.28

0.32

0.32

0.36

0.40

0.44

reso

nanc

e fie

ld [T

]

temperature [K]

90-ZFC 90-FC

0-ZFC 0-FC

Fig. 6. The resonance field as a function of temperature—the

comparison for the different conditions of measurements.

D. Skrzypek et al. / Journal of Magnetism and Magnetic Materials 285 (2005) 379–385 383

10KoTo15K range, and increase in 5KoTo9K range and again decreases at 4.2K. ForESR measurements with the external field perpen-dicular to the plane and in FC option the intensityof the spectrum shows an almost continuousincrease until 4.2K. At the same time, 0-ZFCintensity suddenly jumps to a lower value at T ¼

10K and next it increases when temperature islowered. The reduction of the spin susceptibilitymay be explained taking into account twomechanisms: one is the presence of antiferromag-netic exchange interactions and second is asso-ciated with the presence of a spin disorder.The changes of ESR linewidth as a function of

temperature in 4.2–140K range are reported inFig. 5. The line-shape analysis shows that down toT ffi 20K the ESR line remains Lorentzian andbelow this temperature a deviation towards anasymmetrical line occurs. With decreasing tem-perature the ESR line shows a continuous broad-ening until T ffi 20K; at the same time the

increase of DB becomes more drastic below 40K.Next, for 0-ZFC, 90-ZFC and 0-FC options thelinewidth passes through a maximum and againdecreases. In addition, the big angular anisotropyof DB at low temperatures is observed.The comparison of the rate of linewidth changes

versus temperature d(DB)/dT shows a significantdifference: d(DB)/dT E10�4(T/K) for 40KoTo120K and d(DB)/dTE10�3(T/K) for To40K.In a review paper by Benner et al. [28] cited a

number very interesting ESR studies on different2-d magnetic systems. Both theoretical and experi-mental investigations concentrate on analysis ofthe temperature dependence of the linewidth andthe angular variation of the linewidth and line-shape were also studied. The results obtained by usare similar to those recorded by other authors intheir studies of 2-d magnetic systems [28 andreferences cited therein]. The quantitative explana-tion has not yet been offered in the literature butaccording to suggestions in papers Benner et al.

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we believe that low-temperature behaviour oflinewidth is connected with antiferromagneticinterplanar coupling. However, the investigatedmaterial does not show any long-range order up toT ¼ 4K: As the critical behaviour is sensitivelyaffected by even a very small anisotropy in thespin–spin interactions or by weak interplanecoupling, depending on the strength of suchperturbations in real layered compounds it ispossible to observe dimensionality crossover froma 2-d to 3-d system. As a consequence, the criticalbehaviour of ESR linewidth is rather complicatedand diverse.We found that the resonance field dependence

on temperature is very complex (see Fig. 6). Whenthe temperature is lowered in 30KoTo140Krange, the shift resonance field towards highervalues for BJc and towards lower values for BJa isobserved. This means the anisotropy of internalfields in the crystal. The temperature dependenceof Br below T ffi 30K exhibits some peculiarities.For 0-ZFC, 0-FC and 90-ZFC options theresonance field suddenly jumps to a lower valueand next it is jumped up again and below 10K it isdecreased by decreasing the temperature. Thisbehaviour is the indication of spin-disorder in thecrystal.

4. Conclusions

The ESR study leads to the following conclu-sions:

The two-dimensional behaviour of Ga2Cr1.33Se5is supported by data of the crystal structure ofthis compound. The crystal can be visualised as layers ofchromium ions with nearest-neighbour dis-tances of 0.378 nm within layers and interlayerdistances are alternately of 0.626 and 0.923 nm. The observed angular anisotropy of the line-width has been satisfactorily explained on thebasis of the anisotropic exchange model. The fact that the observed line-shape is aLorentzian curve tells us that interlayer interac-tions are not negligible. The temperature dependence of ESR para-meters (DB and Br ) in temperature range

100–300K is very weak in comparison withdramatic changes which are observed belowT ffi 40K:

The peculiarities of ESR parameters (intensity,DB, Br ) below T ffi 20K can be interpretedtaking into account two mechanisms:(i) presence of ferromagnetic, exchange inter-actions between in-plane chromium ions andantiferromagnetic interplane coupling whichmeans magnetic dimensionality crossoverfrom a 2-d to 3-d system.

(ii) appearance of a spin disorder.

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