Synthesis and characterization of electric and magnetic properties of the new intermetallic compounds M3(GaTe3)2 (M = Cr, Fe, Co)

ELSEVIER

InorganicaChimicaActa 268 (1998) 271-277

Synthesis and characterization of electric and magnetic properties of the new intermetallic compounds M 3 ( G a T e 3 ) 2 (M = Cr, Fe, Co) Jin-Seung Jung a, Hyun Hak Kim a, Seog Gu Kang a, Jong-Ho Jun b, Youngsook L. Buisson c, Lianwei Ren c, Charles J. O'Connor c,. a Department of Chemistry, Kangnung National UniversiO', Kangnung, 210-702, Korea h Department of Applied Chemisto', Kunkuk Universi~', Chung]u, Korea " Department of Chemistry, University of New Orleans, New Orleans, LA 70148, USA

Received 3 September 1996;revised 11 October 1996; accepted 3 June 1997

Abstract The new amorphous intermetallic materials, M 3(GaTe3)2 (M = Cr, Fe, Co) are prepared by using a rapid precipitation metathesis reaction between the Zintl phase material K3GaTe3 and the divalent transition metal halides in aqueous solution. The d.c. specific resistivity measurements of these materials exhibit metallic (M = Fe, Co) and semiconductor (M = Cr) behavior. The magnetic and electrical properties for these materials are examined as a function of temperature, Cr3(GaTe3)2 exhibits antiferromagnetic coupling between localized magnetic moments and highly temperature dependent semiconductivity. Two materials undergo a transition to a spin glass state at low temperature (for M =Fe, Tf= 22 K; for M =Co, Tf=2.2 K). Magnetization data are also reported as both thermal remanent magnetization (TRM) and isothermal remanent magnetization (IRM) as a function of magnetizing field and temperature. At a magnetization field of 1 kG and a temperature of 6 K, the Fe3(GaTe3)2 exhibit a photomagnetic response consistent with a disruption of the spin-glass state that results from a series of pulses of ultraviolet radiation. © 1998 Elsevier Science S.A. Keywords: Electricproperties; Magneticproperties;Intermetalliccompounds

1. Introduction Various methods are available for preparing solids and the method adopted depends to a certain extent on the form of the desired product. Probably the most widely used method for the preparation of solid state materials is the direct reaction of a mixture of solid starting materials at elevated temperatures. Therefore, both thermodynamic and kinetic factors are important in solid state reactions. One important aspect of the recent surge of interest in solid state chemistry has been the preparation of kinetically stabilized solids. These are prepared at relatively low temperatures and thus are not the thermodynamically stable products appearing on high-temperature phase diagrams. Recently, a simple solution phase technique has been proposed to yield a variety of binary and ternary intermetallic chalcogenides which cannot be prepared by other routes [ 1-3 ]. The reaction con si sts of electron transfer from precursor of Zintl polyanions with very high chemical reactivity to transition metal cations, which results in the rapid precipitation of the neutral solid product. The inter* Correspondingauthor.Tel.: + 1-504-2806311; fax: + 1-504-2806860. 0020-1693/98/$19.00 © 1998 ElsevierScienceS.A. All rights reserved PIlSO020- 1693(97)05757-5

metallic solids formed are often amorphous and metastable. We have recently synthesized a new ternary Zintl compound, K3GaTe3, from a direct combination of the elements [3b]. This new Zintl phase material allows subsequent metathesis reaction with MCI2 (M = Cr, Fe, Co) in aqueous solution to produce amorphous intermetallic materials. These materials exhibit some remarkable properties including specific resistivity that ranges from l04 to 10 3 f~ cm, a spin glass transition at temperatures between 2.2 and 22 K, and a photomagnetic effect in Fe3 (GaTe3) 2-

2. Experimental Due to the air sensitivity of the materials used in these reactions, all manipulations were carried out in an argonfilled glovebox containing less than 1 ppm of oxygen. 2.1. S y n t h e s e s 2.1.1. K3GaTe3

The ternary Zintl material K3GaTe 3 was prepared as described previously [3b].

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2.1.2. M 3 ( G a T g 3 ) 2 The transition metal compounds were prepared by the reaction of an aqueous solution of K3GaTe3 and MC12 (M = Cr, Fe, Co). In a typical preparation, a stoichiometric quantity of K3GaTe3 solution (50 ml, 0.03 M) was slowly added to a MCI2 solution (20 ml, 0.05 M) while stirring. A fine black precipitate was immediately formed, separated by suction filtration, washed with water and acetone, and dried overnight under vacuum. 2.2. Elemental analysis The presence of the three elements in each sample and the homogeneity of each sample were confirmed with an EDS equipped AMRAY model 1820 scanning electron microscope. The quantitative elemental analysis for these materials was carried out on a IL-S-12AA spectrometer. The atomic absorption standards were purchased from Johnson-Matthey. The experimental empirical formula determined for these materials gave the following chemical compositions: Fe2.ssGa2Te6.05; Cr2.90Ga2Te6.15; C03.3Ga2Te6. i. 2.3. Resistivity measurements Resistivity measurements for all materials were performed on pressed pellets over the temperature range 20-300 K using the four-prove van der Pauw technique [4]. The pressed pellets were made under a pressure of 16000 psi. The current was supplied by a Keithley model 224 programmable current source and the voltage drop across the sample measured with a Keithley model 181 digital nanovoltmeter. Electrical contacts to the samples were made using gold wires attached with silver paint. 2.4. Magnetic measurements The magnetic susceptibility and magnetization were measured on a SHE Corp. VTS-50 superconduction SQUID susceptometer that is interfaced to a IBM-AT computer system. Measurement and calibration techniques have been reported elsewhere [5]. Two types of experiments were conducted: magnetic susceptibility (M / H) as a function of temperature and magnetization as a function of field and temperature. In the magnetic susceptibility measurement, two different procedures were used: (i) zero field cooling, where the sample was slowly cooled in zero field to a temperature of 6 K at which time the measuring field of 1.0 kG was switched on and the magnetization was measured as a function of temperature. In the field cooling experiment (ii), the field ( 1.0 kG) was turned on at a temperature well above the spin glass freezing temperature before the sample was cooled. In the remanent magnetization measurement, both thermal remanent magnetization (TRM) and isothermal remanent magnetization (IRM) were obtained as a function of field and temperature. The TRM experiment involves slowly cooling the sample in an applied magnetic field to a measuring tern-

perature below the freezing temperature and then switching off the field and measuring the remanent magnetization after a specified elapsed time. On the other hand, IRM data are obtained by cooling the sample to the measurement temperature in zero field, applying a field for a certain time and then switching off the field and measuring the remanent magnetization after a specified elapsed time. 2.5. Photo-induced magnetic measurements The photo magnetic data were recorded using an experimental technique based on the STEPS technique that permits illumination of the sample with high intensity radiation while very precise magnetic data was being recorded on the SQUID susceptometer [6,7]. The radiation was supplied by a high pressure xenon lamp and the duration of the pulse of radiation was determined with a mechanical shutter of about 5 s. A high optical purity quartz supracil light pipe with cross section area of 16 mm 2 delivered the radiation to the specimen in the sample chamber of the superconducting susceptometer. Approximately 100 mg of Fe3(GaTe3)2 was packed into a small quartz tube (5 mm i.d.) that was attached to the end of the quartz light pipe. The photomagnetic response of the spin glass sample was measured at 6 K and zero field following cooling in a field of 1 kG.

3. Results and discussion

Amorphous intermetallics that contain magnetic transition metals are good candidates for showing spin glass behavior because they intrinsically posses order-disorder frustration in the orientations and interactions of the spins [8]. These intermetallic materials may be prepared because of the difference in the electronegativities of the elements that comprise the precursor Zintl phase material and the transition metal salt. The Zintl phase itself exhibits a very large distribution of charges that allows the Zintl material to exhibit salt like solubility properties in many solvents. It has been observed that there is a trend that correlates the solubility properties of ternary Zintl phases with their crystal structure. The Zintl phase materials which contain isolated Zintl anions in their solid are soluble in some polar solvents, for instance, K4SnTe4 is soluble in methanol and water [2], and K3SbTe3 is soluble in DMSO and DMF [3e]. On the other hand, the Zintl phase materials which have polymeric or layered structures such as K3Ga3As4 [9], K4In4X6 (X = As, Sb) [ 10] and LiSbTez [ 11 ], also have strong covalent bonding and are insoluble. The new intermetallics are prepared by using the same rapid precipitation metathesis procedure that we have employed to prepare other types of intermetailic solids. The X-ray powder diffraction patterns of these materials exhibit a lack of diffraction peaks and are consistent with a minimal amount of long-range crystalline order for the freshly prepared materials.

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The measured physical properties, for instance, magnetic spin glass state and amorphous semiconductivity are also consistent with those expected for an amorphous solid. The room temperature specific resistivity for M3 ( GaTe3 ) 2 varies considerably, depending on the metal, M. It has a value of 1 . 0 5 X 104 ~Q c m for Cr3(GaTe3)2, 4.36x 10 - 3 ~'~ c m for Fe3 (GaTe3) 2 and 1.08 × 10- 2 f~ cm for Co3 (GaTe3) 2- These resistivity values of Fe3(GaTe3)2 and Co3(GaTe3)2 indicate that the materials may be considered metallic conductors and are expected to have temperature independent behavior. The specific conductivity of Cr3(GaTe3)2 varies with a range of over five orders of magnitude in the temperature range of 100 to 300 K, indicating a semiconductor of a small energy gap. The specific conductivity of this material is highly temperature dependent as shown in the plot of the log of the specific conductivity (log ~r) versus the inverse of the temperature in Fig. 1. At high temperatures, a crystalline intrinsic semiconductor is expected to exhibit a linear plot of log o- as a function of inverse temperature. This plot is non-linear, but the higher temperature data may be subdivided and fit to two linear regions. A least square fit of the linear sections to the relation, o , = C exp(-Ea/kT), gives Ea=0.22 eV and C=0.47 f~ ' cm -~ in the temperature region 280-180 K and E, = 0.14 eV in the 180-120 K temperature range. Below 110 K, the specific conductivity is essentially temperature independent. The low temperature electronic behavior is consistent with that expected for a semiconductor with p- or n-type impurity centers. The onset of temperature independent behavior may then be attributed to a transition from intrinsic to extrinsic semiconductor behavior. Since this material is amorphous rather than crystalline, the fitted parameters may be viewed as an approximation of the semiconductor activation energy.

It was suggested by Mott and Davis that the conduction mechanism for a non-crystalline material having a valence of C of order 101~- J c m - ; or less is due either to cartier hopping between the localized states at a band edge or to phononassisted tunneling among localized states at the Fermi level (i.e., variable range hopping) [12]. In the latter case, a straight line in the plot of log o- versus 1/T is expected near room temperature since hopping occurs between nearest neighbors and at lower temperatures it becomes favorable for the carriers to tunnel to more distant sites and thus, the specific conductivity is expected to follow the Mott prediction of or = A e x p ( - B / T I / 4 ) . As shown in Fig. 2, the specific conductivity of C r 3 ( G a T e 3 ) 2 exhibits exp ( - B/T1/4) behavior. Another problem associated with amorphous materials is the grain boundary, which may be involved in the measurement. However, both small particle size and the fit of Mott's equation imply that there is no significant grain boundary present in this amorphous intermetallic pellet. In Fig. 3 the high temperature magnetic suseptibility data ( T > 50 K) shows that the compound exhibits Curie-Weiss paramagnetism with C = 5 . 2 0 e.m.u. K m o l - ' , 0= - 3 3 K. From the negative Weiss constant we can predict that there is a substantial amount of antiferromagnetic exchange in this complex. At lower temperatures the magnetic susceptibility begins to deviate from Curie-Weiss law, but there is no characteristic magnetic anomaly to allow a precise determination of the nature of the magnetic coupling. The measured magnetic data of Fe3(GaTe3)2 are illustrated in Fig. 4 as susceptibility and inverse susceptibility as a function of temperature. At temperatures above 120 K, the fit of the inverse magnetic susceptibility data of Fe3(GaTe3)2 to the Curie-Weiss law (X = C~ ( T - O) ) results in a Curie constant C of 5.70 e.m.u. K tool-~ and the Weiss 0 of 68.3 K, which indicates the

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opportunity to study the field sensitivity of the spin glass temperature. The spin glass freezing temperature of Fe3 (GaTe3) 2 is field dependent at low measuring magnetic field, as shown in Fig. 5. The freezing temperature increases from 22 to 24 K as the external magnetic field decreases. This is not surprising because the spins are frozen in a random orientation in a zero field cooling experiment. The spins try to maintain their random frozen alignment even when the field is turned on. This results in the spin glass freezing temperature moving to a relatively high temperature as the magnetic field is decreased, which is consistent with the manner in which the spin glass freezing temperature shifts in many other spin

J.-S. Jung et al. / lnorganica Chimica Acta 268 (1998) 271-277

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glass systems [ 13]. However, at very high magnetic field ( ~ 7 kG), the spin glass state is 'destroyed' and the freezing temperature no longer exists, as seen in the TRM and IRM experiments below• The most diagnostic experiment for the characterization of the spin glass state is the analysis of the field dependence of the isothermal remanent magnetization (1RM) and thermal remanent magnetization (TRM). The results of these experiments on Fe3 (GaTe3)2 are illustrated in Fig. 6. At weak fields the TRM measurements give large remanences relative to the IRM, while at strong fields the IRM and TRM converge to the same saturation remance. A hump which has been observed in many spin glasses does not occur in this TRM curve. The size and shape of the TRM maximum is time dependent and tends to shift to lower fields when

measured with longer time delay following magnetic field quench [ 14,15 ]. The TRM data for these measurements were recorded after 5 min elapsed time. We are in the process of examining the effect of the elapsed time on the shape of the magnetic remanence curves. The unusual magnetic properties of spin glasses make them excellent candidates for an experiment in which radiation is used to generate magnetic bubbles and holes on the surface of a material• The preliminary result of magnetic characterization of Fe3 (GaTe3)2 has been previously reported as a spin glass with a freezing temperature of 22 K [3b]. The most spectacular demonstration of the photoinduced magnetic effect appears on the TRM or IRM relaxation, MR~t), of the studied glasses in their spin glass phases [ 16]. A process for generating magnetic bubbles in a spin glass material of

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A plot of the SQUID magnetometer output as a function of time before, during and after the frozen spin glass is exposed to the radiation is illustrated in Fig. 8. Following the pulse, there is initially a rapid drop in the moment and then an increase in the magnetization to a new steady-state value that is substantially larger than the magnetization before the pulse.

4. Conclusions Fig. 7. A schematic diagram showing the process of magnetic bubble generation in Fe3(GaTe3)2. (a) The sample is frozen in zero applied magnetic field; (b) a magnetization field of 1 KG is applied to the sample; (c) the frozen spin glass is irradiated in a magnetic field; (d) a domain of magnetization is generated in the sample.

Fe3(GaTe3)2 is illustrated schematically in Fig. 7. The procedure for this experiment involves cooling the spin glass material to a temperature of 6 K, which is well below the spin glass freezing temperature (Tr=22 K) under zero applied magnetic field ( H = 0 kG), as shown in Fig. 7(a). This results in the freezing of a random orientation of the spins in the spin glass material. The magnetic field of 1 kG is then applied to the frozen spin glass (Fig. 7 ( b ) ) . Because of the nature of the spin glass state, the magnetization will show a slight increase to a value represented by the zero-field cooled magnetic susceptibility data. While the field is applied to the frozen spin glass specimen, the sample is exposed to the radiation supplied by the high pressure xenon lamp about 5 s illumination time, as shown in Fig. 7(c). The radiation is of sufficient intensity to cause local disruption of the spin glass state and a realignment of the spin glass spins to the direction of the applied magnetic field. The result of this radiation will be the generation of a spin glass moment within the domain walls determined by the boundary of the pulse, as shown in Fig. 7(d). The effect of this experiment on the magnetization is to cause an increase in the remanent magnetic moment of the sample.

The new amorphous metallic materials M3 (GaTe3) 2 have been prepared by using rapid precipitation metathesis reaction of GaTe3- 3 anion with divalent transition metal cations in solution phase. These materials undergo a transition to the spin glass state at Tr = 2.2 K for C03(GaTe3)2, Tr = 22 K for Fe3(GaTe3) 2 and also exhibit semiconducting conductivity for Cr 3(GaTe3) z at room temperature. The spin glass freezing temperature of Fe3(GaTe3)2 also shows the field sensitivity at low applied magnetic field. At temperatures well below the spin-glass freezing temperature, Fe3(GaTe3)2 has been shown to be an excellent host material for the generation of photoinduced magnetic bubbles.

Acknowledgements J.-S. Jung acknowledges support from KRF (International Cooperation Research with C.J. O'Connor).

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