Double magnetic phase transitions and magnetotransport anomalies in a new compound Gd_\textbf{2}AgSi_\textbf{3}
DDouble magnetic phase transitions in a new compound Gd AgSi Baidyanath Sahu, a) R. Djoumessi Fobasso, b) and Andr´e M. Strydom Highly Correlated Matter Research Group, Physics Department, University of Johannesburg, PO Box 524,Auckland Park 2006, South Africa
Dc and ac–magnetic susceptibility ( χ ), specific heat ( C P ), electrical resistivity ( ρ ) and magnetoresistance mea-surements performed on the new polycrystalline compound Gd AgSi , crystallizing in the α –ThSi tetragonalstructure, are reported. Two magnetic phase transitions were observed in dc and ac susceptibility, specificheat and resistivity measurements at temperatures T N = 11 K and T N = 20 K, despite a single site oc-cupied by Gd atom, which is an indication of the complex magnetic behavior. This compound turns out beone of the rare Gd compound in which a minimum is observed in the temperature dependence of resistivityin the paramagnetic state and also negative magnetoresistance over a wide temperature range (above T N ),mimicking the behavior of exotic Gd PdSi , in this ternary family. The isothermal magnetic entropy andadiabatic temperature changes reach a value of 9.5 J/kg-K and 7.5 K respectively for the field change of 9 T.Keywords: Antiferromagnet, Superzone effect, Magnetoresistance, MagnetocaloricRare-earth (RE) based ternary intermetallic com-pounds have been receiving considerable attention due totheir interesting structural and physical properties suchas structural phase transition , heavy fermion, Kondobehaviour , intermediate valence , giant magnetoresis-tance , zero thermal expansion , spin glass behavior and magnetocaloric effect . Several studies have beendedicated to the RE TX series of compounds (RE=rare-earth elements, T = transition metals, X = p-blockelements), due to their structural aspects. The poly-crystalline RE TX compounds basically crystallize inthe ThSi type of structure which has two modifica-tions namely α -ThSi and β -ThSi phases. Most ofthe α -ThSi -type of tetragonal structure belong to aspace group of I /amd . However, compounds with β -ThSi mostly belong to space group P /mmm withAlB -type of hexagonal structure. In contrast, Tran has synthesized a series of U TGa (T = Ru, Rh, Ir,Pd, Au, Pt) compounds and found that they crystal-lize in the CeCu type of orthorhombic structure, whichbelongs to space group Imma . Based on different struc-tures, RE TX compounds show a variety of magneticground states, depending on indirect Ruderman-Kittel-Kasuya-Yosida exchange interaction between the ions.This magnetic interaction dictates the physical proper-ties of the 4f electrons giving rise to interesting and veryoften anomalous properties . Recently, Sarkar et. al have successfully synthesized a RE AgGe series of com-pounds and found for instance multiple magnetic phasetransitions in Nd AgGe .In this work, we report the successful synthesis of a newintermetallic compound Gd AgSi which forms as singlephase and we report on its magnetic, thermal and electri-cal transport properties. The magnetocaloric propertiesalso studied show that the compound exhibits a consid-erable magnetocaloric effect (MCE). a) Corresponding author b) Equal contributing author.
A. Synthesis and Experimental Details
A polycrystalline sample of Gd AgSi was synthesizedby arc-melting the constituent elements of ultra high pu-rity ( ≥ − K α radiation of wavelength 1.54 ˚A on aRigaku powder diffractometer. The obtained XRD pat-tern was analyzed by Rietveld refinement method usingthe FULLPROF software . Temperature dependenceof magnetization ( M ( T )) and field dependence of mag-netization (M( H )), were performed on a Physical Prop-erties Measurement System (PPMS), Dynacool attachedwith a Vibrating Sample Magnetometer (VSM), Quan-tum Design, USA. M ( T ) was recorded in the standardprocess of zero-field cooled (ZFC) and field cooled (FC)mode followed by measurements in field cooled warm-upmode (FW). In ZFC modes, the sample was first cooledto T = 2 K in zero field, then a field was applied andthereafter the data were recorded while warming. In FCmodes, the sample was first cooled to 2 K in the pres-ence of an applied field and the data were recorded duringwarming in the same field. The ac-susceptibility ( χ ac ( T ))measurement was carried out under different frequencywith a constant driven field of 3 Oe using same Dyna-cool system. The specific heat C P ( T ) measurement wascarried out in the same Dynacool system using a ther-mal relaxation method in temperature range of 2–300 K.The temperature dependent resistivity ρ ( T ) and isother-mal field dependence of resistivity ρ ( H ) were measuredby the standard four–probe ac–method using the sameapparatus. a r X i v : . [ c ond - m a t . m t r l - s c i ] F e b B. Results and Discussion1. Structural properties
FIG. 1. (a): X-ray powder diffraction data for Gd AgSi .Red symbols represent the experimental data and the blackline represents the calculated data. The difference betweenexperimental and calculated data is shown as blue line. Aset of vertical bars represents the Bragg peak positions of thetetragonal α -ThSi type structure. (b): The schematic rep-resentation of the tetragonal crystal structure of Gd AgSi .(c): Gd atoms arrangement in the crystal structure. Fig. 1 shows the Rietveld refinement of the powder x-ray diffraction (XRD) data obtained for Gd AgSi per-formed using the centrosymmetric I /amd space group.It is confirmed that the compound crystallizes in α -ThSi –type of tetragonal structure. The schematic repre-sentation of this structure is shown in the inset of Fig. 1.The crystallographic details obtained from the Rietveldrefinement fit are given in table I.The smallest Gd–Gd bond length in Gd AgSi is TABLE I. The lattice parameters and unit cell volumeGd AgSi compound obtained from the Rietveld refinementof XRD patterns for tetragonal phase along with the atomiccoordinate positions.a 4.094(2) ˚Ab 4.094(2) ˚Ac 14.178(2) ˚AV 183.461(3) ˚A R p (%) 17.7R wp (%) 11.2R exp (%) 5.69 χ (%) 4.74Atomic coordinates for Gd AgSi Atom Wyckoff x y z
Gd 4 a e e AgGe , 3.2311 ˚A for Pr AgGe and3.1996 ˚A for Nd AgGe ) have been reported for other2–1–3 compounds forming in the same α -ThSi crystalstructure with same space group. Each Gd atom is sur-rounded by 10 M nearest neighbours atoms.
2. Magnetic properties
Fig. 2 shows the temperature variation of field-cooled dc-magnetic susceptibility ( χ dc ( T ) = M ( T )/ H )of Gd AgSi measured under an applied field of H =1.0 T. Below 30 K, χ dc ( T ) exhibits two anomalies. Theexpanded low - temperature region of χ dc ( T ) is shownin the inset of Fig. 2 to highlight the double magnetictransition in Gd AgSi . This double transition will befurther investigated below.The inverse magnetic susceptibility ( χ − dc ( T )) of thecompound is presented on the right hand scale of Fig. 2.The χ − dc ( T ) data below 300 K and 70 K can be describedusing the Curie–Weiss law, χ dc = C/( T − θ P ), where Cis the Curie constant proportional to the square of theeffective magnetic moment ( µ eff ) and θ P is the param-agnetic Weiss temperature. The least–squares fit of theequation to the data leads to the values of µ eff = 8.12 µ B /Gd and θ P = -8 K. The obtained µ eff value is compa-rable to the theoretical free-ion value for Gd ion whichis ≈ µ B /Gd. The negative value of θ P indicates thatthe dominant interactions in the compound are antifer-romagnetic.Two anomalies are found at low temperature, one at FIG. 2. Left scale: temperature dependence of dc-magneticsusceptibility ( χ dc = M/H ) of Gd AgSi measured under amagnetic field of 1.0 T in FC mode. Right scale: inverse mag-netic susceptibility as function of temperature for the samedata. Inset: Expanded region at low temperatures of χ dc ( T )data showing two anomalies at T N = 11 K and T N = 20 K(see arrows).
20 K and another at 11 K. In order to get an insightinto the two magnetic transitions, temperature depen-dent magnetization of Gd AgSi in ZFC and FC modeswere measured at different applied magnetic fields andare represented in Fig. 3a – 3c. The two transitionsshow antiferromagnetic behavior with N´eel temperaturesT N = 11 K and T N = 20 K clearly visible in Fig. 3(a).One can also see from Fig. 3a – 3b that the M ( T ) showsan irreversibility behaviour between ZFC and FC mag-netization. However, the relative magnitude of the bifur-cation gradually decreases with increase in applied mag-netic field and the two branches overlap for field valuesabove 0.5 T. Another feature in M ( T ) is the variationof FC magnetization with fields. At low magnetic fields(H <
500 Oe), the FC magnetization starting from 2K decreases with increase in temperature, merges andfollows FC magnetization at T N . However, for field val-ues of H ≥ anisotropic exchange .Fig. 3(d) shows the real part of the χ ac ( T ) datarecorded at three different ac-field frequencies. The realcomponent χ (cid:48) ac ( T ) of ac-susceptibility data confirm thatthe sample exhibits two magnetic transitions. However,there is no signature of spin or cluster glass behavior inthe compound as the peaks in χ (cid:48) ac ( T ) data are frequencyindependent in the range of 500 to 9000 Hz. FIG. 3. (a)–(c):Temperature dependence of dc-magnetizationmeasured under ZFC and FC protocols at different appliedmagnetic fields. (d): Temperature variation of the real partof ac-susceptibility measured at different ac-field frequencieswith 3.0 Oe ac-driven field.
3. Specific heat
The temperature variation of specific heat ( C P ( T )) ofGd AgSi is presented in Fig. 4(a) along with its isostruc-tural non-magnetic analogue La AgSi (solid red line).Both compounds have C P values close to the Dulong-Petit value of 3nR ≈
147 J/mole-K, where n is the num-ber of atoms in the formula unit and R is the universalgas constant. The compound La AgSi exhibits a typicalbehavior for a non-magnetic metal between 2 K and 300K.The inset panel in Fig. 4(a) shows the expanded tem-perature region between 2 K and 30 K to highlight thedouble phase transition in Gd AgSi . One can notethat the low-temperature region of specific heat givesan evidence of two magnetic phase transitions with twonearby peaks at T N ≈
11 K and T N ≈
20 K, which isconsistent with the peaks observed in χ ( T ) data. The4f–magnetic contribution of specific heat ( C f ) was es-timated by subtracting the zero-field specific heat forisostructural La AgSi . The variation of C as a func-tion of temperature is shown in Fig. 4(a).The magnetic entropy S has been estimated usingthe term (cid:82) ( C /T ) dT . The variation of S as functionof temperature is shown in Fig. 4(b) and the expandedregion at low temperature is shown in inset of Fig. 4(b).The magnetic entropy at T N is S ≈ S ≈
15 J/mole-Gd-K at T N . Here, S showsonly 2/3 of Rln(2S+1) is released at T N , while the fulldoublet entropy is involved at T N . This results indi-cates that the Gd ion has a doublet ground state cor-responding to the higher transition temperature of T N . FIG. 4. (a): Zero-field specific heat ( C p ) of Gd AgSi in blacksymbols. The temperature dependence of zero-field specificheat of La AgSi is represented as a red line. The magneticcontribution ( C ) of specific heat is in blue symbols. In-set represents the expanded low-temperature region of C p ofGd AgSi , La AgSi and C of Gd AgSi . (b): Tempera-ture dependent magnetic entropy (S ). Inset represents theexpanded low temperature region of S . (c): The tempera-ture dependence of C p at different applied magnetic fields forGd AgSi . (d): Phase diagram for the variation of T N andT N with applied magnetic field. Magnetic entropy tends to saturate above 30 K.The temperature dependence of C p of Gd AgSi wasmeasured under different fields up to 5 T in the temper-ature range of 30 K to 2 K. Fig. 4(c) shows the C p vs.T plots for different values of magnetic field. One cannote from Fig. 4(c) that the peak at T N shifts to lowertemperatures with increase in applied magnetic fields.This behavior is expected for an antiferromagnetic tran-sition. However, the peak position at T N is not changingwith fields up to 3 T. At a field value of 5 T, only onepeak remains below T N which might shifts marginallylower at this field and this transition evidently requireshigher magnetic fields to be suppressed. The field vs. temperature phase diagram of Gd AgSi derived fromthe field dependent specific heat measurement is shownin Fig. 4(d). The specific heat result indicates that thecompound has a complex magnetic structure with mul-tiple order parameters.
4. Electrical resistivity
The zero-field electrical resistivity ρ ( T ) of Gd AgSi is shown in Fig. 5(a). The low-temperature region is ex-panded and shown in the inset of Fig. 5(a). The ρ ( T ) gradually increases with temperature above 40 K. Thisindicates that the compound shows ordinary metallicbehaviour in the paramagnetic region. However, ρ ( T )shows a broad minimum at T ≈
35 K and a maximumat T ≈ FIG. 5. (a): Temperature dependence of resistivity ( ρ ( T ))for Gd AgSi in zero applied magnetic field. Inset: ρ ( T )was measured under different magnetic field in low tempera-ture region. (b) Field dependence of the magnetoresistivityisotherms of Gd AgSi . Fig. 5a shows two kinks in ρ ( T ) at about 21 K and10 K (see arrows), marking the onset of magnetic order-ing. These transition temperatures are consistent withthe χ ( T ) and C p results. It is also observed that the ρ ( T ) does not drop immediately below either T N or T N ,which would be expected due to the loss of spin-disorderscattering . This behavior highlights the complexity ofthe magnetic structure in this compound which also re-flects from the formation of superzone boundary gapsin some portions of the Fermi surface . The inset ofFig. 5(a) shows the ρ ( T ) of Gd AgSi for different valuesof applied magnetic fields. As seen in the inset panel ofFig. 5a, both the transition temperatures T N and T N are getting suppressed with increase in magnetic field,which is commonly seen in antiferromagnetic materials.However, the superzone feature remains visible upto 5 T.It can also be noted that ρ ( T ) decreases with increasingmagnetic field which indicates a negative magnetoresis-tance (MR) feature .In order to obtain the magnetic field dependent magne-toresistance (MR), isothermal field dependent resistivitymeasurements were carried out at different temperaturein the range of 2 K to 30 K. MR was estimated by usingthe following formula:MR = ρ (H , T) − ρ (0 , T) ρ (0 , T) × , (1)where ρ (0 , T ) is the resistivity at zero magnetic field and ρ ( H, T ) is the resistivity at applied field ’H’. Fig. 5(b)shows the nature of field dependent MR. It is apparentfrom Fig. 5(b) that Gd AgSi exhibits negative MR overa wide range of temperature. Negative MR is also re-ported on other rare-earth based antiferromagnetic com-pound .
5. Magnetocaloric effect
In order to obtain the isothermal magnetic entropychanges for MCE study and for more confirmation aboutorder of magnetic phase transition, isothermal magneti-zation for different temperatures has been measured forGd AgSi . There was no hysteresis loop at 2 K (notshown), which confirmed that the compound shows softmagnetic behavior. Fig. 6(a) shows magnetization asfunction of field M ( H ) at different temperatures between2 K and 34 K with a step of 2 K for field cycle 0 T → AgSi at 2K for a field of 9 T is 5.6 µ B /Gd , which is less than thespontaneous magnetization of free Gd (gJ = 7 µ B ) mo-ments. One can see that isothermal M–H shows a typicalmetamagnetic behaviour below T N . The critical mag-netic field was determined from the maximum of d M /d H and found to be 0.9 T for 2 K. The Arrott–plots ( M vs. H/M ) was derived from the isothermal M ( H ) curves toinvestigate the nature of the phase transitions. Fig. 6(b)depicts the Arrott plot for Gd AgSi compound. Basedon the Banarjee criterion , the M vs. H/M plots shownegative slopes as a function of magnetic field, indicatinga first-order magnetic phase transition.The values of isothermal magnetic entropy change(∆ S m ) was calculated from magnetization isotherms us-ing the Maxwell thermodynamic relation :∆ S m ( T, H ) = H (cid:90) (cid:18) ∂M∂T (cid:19) dH. (2)Fig. 6(c) shows the temperature dependence of − ∆ S m for different values of magnetic field up to 9 T. There isno tendency to saturation for − ∆ S m values for field up FIG. 6. (a): Isothermal magnetization during magnetic fieldchange 0 T −→ AgSi at different temperaturesbetween 2 K and 34 K with a step of 2 K. (b) Arrott plots (M ) vs. H/M at different temperatures. (c) Temperature variationof magnetic entropy change (∆ S m ) for different fields. (d)Variation of adiabatic temperature change as a function oftemperature for different fields. to 9 T. The negative values of − ∆ S m at low temperaturefor small magnetic field change might be possible for themetamagnetic behavior. For relatively large magneticfield (H > − ∆ S m was observedfor the whole range of temperature. A maximum value of − ∆ S m = 9.5 J/kg-K is found around the transition tem-perature T N at a field of 9 T. This observed value is arelatively small value in comparison with other Gd TX MCE materials. The − ∆ S m vs. T shows only one peakat T = 20 K due to the presence of metamagnetic sig-nature in the MCE. The adiabatic temperature change(∆ T ad ) was evaluated from both − ∆ S m ( T , H ) and thezero-field specific heat data using the relation: ∆ T ad ( T, H ) = − T ∆ S m C p ( T, H ) . (3)The peak value of ∆ T ad is 7.5 K for a field change of9 T, as shown in Fig. 6(d). Meanwhile, it is also no-ticed that the − ∆ S m vs T graph shows a broadeningbehavior which might be an indication for a large refrig-erant capacity (RC). RC is a parameter to quantify theheat transferred from hot to cold sinks during an idealrefrigeration cycle and the value was estimated from thefollowing formula: RC = T (cid:90) T | − ∆ S m | dT, (4)where T and T are the temperatures corresponding tothe left and right sides of the half maximum − ∆ S m peak.The obtained values of RC are found to be as large as 300J/kg and 396 J/kg for field changes of 0–5 T and 0–9 T,respectively. C. Summary
In summary, we have experimentally studied the crys-tal structure, magnetic, transport, magnetoresistanceand magnetic cooling properties of a new stoichiomet-ric compound Gd AgSi . This polycrystalline samplesuccessfully formed as a single phase in a tetragonal α -ThSi -type crystal structure with space group I /amd .Magnetic susceptibility, electrical resistivity and specificheat measurements reveal that the compound exhibitstwo antiferromagnetic transitions at 11 K and 20 K.The superzone effect from resistivity is observed at be-low the transition temperature. From field dependentisotherm resistivity characterization, it is confirmed thatthe compound possess negative magnetoresistance in theboth antiferromagnetic ordered and paramagnetic region.The magnetic field induced first-order magnetic transi-tions in the magnetically ordered state is confirmed fromthe Arrott-plots. A considerable value of MCE, adia-batic temperature change and refrigeration capacity isobserved for this compound. These results contributetowards a better understanding of this class of materials. Acknowledgements
This work is supported by Global Excellence andStature (UJ-GES) fellowship, University of Johannes-burg, South Africa. DFR thanks OWSD and SIDA forthe fellowship towards PhD studies. AMS thanks theURC/FRC ( ) of UJ for assistance of financial sup-port.
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