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Ferromagnetism in orthorhombic RAgAl3 (R = Ce and Pr) compounds

We present a detailed study on magnetic, thermodynamic and transport properties of polycrystalline RAgAl3(R = Ce and Pr) compounds. Both compounds crystallize in orthorhombic structure, which is distorted from the tetragonal BaAl4 structure with the space group Cmcm. Heat capacity measurement indicates the bulk magnetic ordering of the compounds. CeAgAl3 and PrAgAl3 order ferromagnetically at TC = 3.8 K and 5.8 K, respectively as it was confirmed from magnetic measurements. CeAgAl3 exhibits heavy Fermion behaviour. The Schottky behaviour in heat capacity data was observed in both compounds. The crystalline electric field (CEF) analysis of the magnetic parts of heat capacity of CeAgAl3 and PrAgAl3 yielded to a CEF level scheme with three doublets and nine singlets and with an overall splitting of 51 K and 180 K, respectively. Fit yielded a magnetic doublet state for CeAgAl3, whereas for PrAgAl3 a pseudo-doublet ground-state with an energy difference of 15 K has been obtained. The resistivity measurements display a low temperature drop at the magnetic ordering temperature of the compounds. Negative magnetoresistance (MR) due to the ferromagnetic ordering has been observed for both Ce and Pr compounds.


Introduction
Rare earth intermetallic compounds often show a variety of interesting physical properties due to the competition of electronic interactions between localized f-electrons and itinerant delectrons. The primary interaction occurs via the polarization of the conduction electrons, which is known as RKKY (Ruderman-Kittel-Kasuya-Yosida) interactions (indirect exchange) and the 4f electrons are strongly affected by their local environment. CEF is the factor contributing to large variety of magnetic, thermodynamic and electronic properties in rare earth intermetallics. Different centrosymmetric and non-centrosymmetric structures have been crystallized for various combinations of the RTX3 (R = rare earth, T = 3d/4d/5d -transition metal and X = p-block element)-type crystal structure. Recently reported several RTX3 compounds, crystallizing in BaNiSn3 type structure, exhibit a huge range of unusual magnetic or superconducting ground state properties. Of which, CePt3Si [1] and CeRhSi3 [2,3] have been receiving much attention due to the existence of spin orbit coupling induced by noninversion symmetry. Besides the tetragonal BaNiSn3 structure the existence of new ternary compounds RNixGa4-x with CePtGa3 structure have also been reported [4]. At the composition RPtGa3, within the homogeneous range of the BaAl4 type structure, the existence of the compound with an orthorhombic deviation from BaAl4 was observed [5,6]. In continuation of our studies on RTX3 compounds such as RCuGa3(R = Pr, Nd and Gd) [7] and RCoSi3 (R = Pr, Nd and Sm) [8], we have undertaken the systematic studies of polycrystalline RAgAl3 (R = La, Ce and Pr) samples possessing orthorhombic crystal structure. Initial magnetic and thermodynamic properties of ferromagnetic CeAgAl3 single crystals have been reported [9].
First report on single crystalline CeAgAl3 is found to exhibit ferromagnetic (TC = 3K) character suggesting BaNiSn3 type structure (space group = I4/mm) or PrNiGa3 structure (space group = I4/mmm) [9]. Subsequently Franz et. al solved the structural controversy of single crystalline CeAgAl3 and reported to be orthorhombic structure with the space group Cmcm (No. 63) [10].
Herein, we report the systematic investigations of structural, magnetic, thermodynamic, transport and magneto-transport properties of orthorhombic RAgAl3 (Ce and Pr) polycrystalline compounds for the first time.
The ingots were annealed at 700ºC under vacuum in a quartz ampoule for 20 days. To confirm the phase purity of the annealed sample, powder X-ray diffraction using Cu-Kα was carried out on the annealed sample. Stoichiometry and single phase nature were determined by energydispersive X-ray spectroscopy (EDAX) measurements. dc magnetization was measured as a function of temperature and magnetic field [isothermal magnetization] using the Quantum Design, Physical Property Measurement System (PPMS) from 2 K to 300 K up to the field of 50 kOe. In the zero field cooled (ZFC) process, the sample is cooled below TC in the absence of the magnetic field down to 2 K and then the magnetization was measured with increasing the temperature in constant applied magnetic field. In the field cooled (FC) process, the measurements were done in the applied magnetic field while the sample was cooling down to 2K. The ac susceptibility measurements were done as a function of temperature at various frequencies from 10 to 1488 Hz in a probing field of 2.5 Oe using the magnetic properties measurements systems (MPMS). Heat capacity was also measured using PPMS relaxation technique down to 1.8 K with external magnetic field up to 90 kOe. Also, four-probe resistivity and magnetoresistance (MR) measurements were carried out using PPMS from 1.8 to 300 K in zero field and magnetic fields up to 80 kOe. along with structural Rietveld refinement profile using GSAS software [11]. From the results obtained by Rietveld refinement of RAgAl3 (R = La, Ce and Pr), it is confirmed that the compounds adopt orthorhombic SrPdGa3 -type structure (space group Cmcm) revealing a distortion from the tetragonal BaAl4-type parent structure [12]. The crystallographic information related to lattice parameters and atomic positions are given in the Table 1 and   Table 2, respectively. Also, a small Ag/Al1 mixing occurs at the 4c sites, which are in good agreement with the recent report on single crystals of CeAgAl3 [10].     Oe.

Magnetic Susceptibility and Magnetization
In order to investigate the magnetic properties of CeAgAl3, the temperature dependence of magnetization was carried out in ZFC and FC modes at H = 100 Oe, as shown in Fig.3a. There is a slope change at ferromagnetic Curie temperature TC = 3.8 K. However, there is a difference between ZFC and FC curves indicating thermomagnetic irreversibility. This strong irreversibility of ZFC and FC magnetization curves below Tirr is expected to disappear as applied field is increased. This irreversibility behaviour is typically attributed either to domain wall pinning effect of ferromagnetic domains [13] or the presence of spin glass behaviour [14].

Heat Capacity
The temperature dependent specific heat of RAgAl3 (R = Ce, Pr) and its isostructural Where γ is the electronic coefficient above ordering temperature (also referred to paramagnetic Sommerfeld coefficient γP), β is Debye T 3 law lattice heat capacity coefficient and δ is higher order lattice terms. The estimated parameters (γ, β, δ) obtained from fitting of equation (1) in the paramagnetic region (T > TC) are given in Table 3 and the fitting is shown in Fig.5. Debye temperature (θD) has been calculated from β using the relation = ( 12 4

)
1/3 (2) where R is the molar gas constant and n is the number of atoms per formula unit. The calculated values are given in Table 3. Also, the Sommerfeld electronic coefficient γ determined by extrapolation of C/T up to lowest temperatures for PrAgAl3 gives the value of 15 mJ/molK 2 .
This indicates that PrAgAl3 is a normal metal. For better determination, it is necessary to perform measurements below 2 K, where this magnetic contribution diminishes. Table 3. The parameters obtained from fitting the heat capacity data using equation (1) above the ordering temperature (T > TC).
The estimated magnetic specific heat (C4f) exhibits the broad maximum around 10 K for Ce and two maxima at 10 K and 30 K for Pr respectively, as shown in Fig.6. This could be associated with the Schottky contribution arising from CEF splitting of ground state multiplet.
The solid lines shown in Fig.6 have been fitted to the magnetic heat capacity of CeAgAl3 and PrAgAl3 above ordering temperature based on the standard expression for Schottky heat capacity as given below, where R is a gas constant, Ei is the energy in units of temperature and gi is the degeneracy of the CEF split energy levels. The results of the fits are shown as schematic energy level diagram in Fig. 7. It is well-known that, both Ce and Pr compounds are in the trivalent state, as confirmed from the magnetic measurements. In CeAgAl3, the free ion Ce 3+ (J = 5/2) have six-fold degenerate ground state multiplets, which split into three Kramer's doublets possessing the energies of first and second excited states as Г1 = 15 K and Г2 = 51 K, respectively. Pr 3+ is a non-Kramer's ion with J = 4.
In orthorhombic symmetry, under the CEF effect, 2J+1 degenerate ground state splits into nine singlet levels. The C4f (T) of Pr 3+ displays two broad maxima around 10 K and 30 K due to the short range correlations. The Schottky fit equation (3) to the maximum at (10 K) in C4f (T) gives the excitation energies as 15, 33, 53, 70, 90, 100, 160 and 180 K. Thus the ground and first excited levels are separated by an energy gap of 15 K, which could be taken as pseudodoublet state that leads to magnetic ordering in PrAgAl3 compound [15 -17]. In CeAgAl3, the application of magnetic field of 20 kOe shifts the magnetic ordering peak towards high temperatures with the reduction in peak height, as shown in Fig.8. PrAgAl3 exhibits the similar behaviour of CeAgAl3 (Fig.8). An external field of H ≥ 2.5 kOe shifts the TC towards the higher temperature. For higher field of 90 kOe, the magnetic anomaly displays a clear broadening together with shifting to higher temperatures, which point to the ferromagnetic character of these compounds. The derived from fitting the equation (1) for CeAgAl3 in the paramagnetic state (T > TC) and magnetically ordered state (T < TC) show a mass enhancement of ≈ 221 and 352 mJ/mol K 2 respectively, as shown in Fig.5. These large values in both regions infer that CeAgAl3 is a heavy-Fermion compound. The application of magnetic field of H = 90 kOe suppresses the to a moderate value of 139 mJ/mol K 2 at 2 K (see Fig.8). Hence, a moderate enhancement in the density of states is seen even after the application of high magnetic field 90 kOe. In the case of PrAgAl3, the electronic coefficient = 15 mJ/mol K 2 is obtained in the paramagnetic state.
C4f(T) of PrAgAl3, shows a slight upturn at 3K for an applied field of 90 kOe revealing the nuclear Schottky effect Fig.9. shows the temperature dependence of resistivity of RAgAl3 (R = Ce and Pr), from 2 K to 300 K in zero field and in applied field of H = 90 kOe. The resistivity shows a metallic behaviour down to 1.8 K, which drops rapidly with a slope change around TC = 3.8 K and 5.5 K for CeAgAl3 and PrAgAl3, respectively. This is due to the gradual freezing of spin disorder scattering. The observed small hump at 250 K in PrAgAl3 may be due to the appearance of cracks in sample. At low temperature, the compounds do not follow the simple Fermi liquid (FL) behaviour due to the magnetic ordering. The residual resistivity ratios (RRR) for Ce and Pr have been determined from the relation ρ(300 K)/ρ0(2 K), where ρ0 is residual resistivity at 2 K. We have obtained rather good residual resistivity ratios of 5.6 and 5.8 for Ce and Pr, respectively, which reflect the quality of the sample. The application of magnetic field of 90 kOe in CeAgAl3 lowers the spin disorder scattering due to increase in the magnetic order, hence the slope tends to be smoothened around the ordering temperature. But in PrAgAl3, Fermi liquid (ρ T 2 ) behaviour is found in ρ (T) for 90 kOe at low temperatures. At low temperature (2 K) and high field, the magnitude of MR (H) is positive for both the compounds, which is associated with the Lorentz force effect as observed in metallic systems because the effect of the magnetic field is to increase the cyclotron frequency ( ) of the electron which can be defined as

Conclusion
We have investigated the structure, magnetic, thermodynamic and transport properties of polycrystalline RAgAl3 (R = La, Ce and Pr) compounds. The compounds of RAgAl3 series (R = La, Ce and Pr) crystallize in the orthorhombic structure, which is distorted from tetragonal BaAl4 structure. From different measurements, it has been concluded that Ce and Pr compounds undergo ferromagnetic ordering at low temperatures TC = 3.8 K and 5.5 K, respectively. The obtained effective magnetic moments were found to be close with trivalent Ce 3+ and Pr 3+ free-ion moments. ZFC and FC modes of dc magnetization along with the moderate frequency dependence in ac susceptibility of CeAgAl3 indicates the domain wall pinning effect. CEF levels have been deduced from magnetic heat capacity using Schottky specific heat for both the compounds. From the fit it is understood that the ground and first excited states of PrAgAl3 are separated by an energy difference of 15 K, which forms a pseudodoublet ground state that leads to magnetic ordering in this compound. The large value of Sommerfeld coefficient before and after application of magnetic field suggest that CeAgAl3 is a heavy Fermion compound. The electrical resistivity falls off rapidly below 3.8 K and 5.5 K for Ce and Pr, respectively around the ordering temperature, due to the loss in the spin-disorder scattering. MR study shows negative magnitudes at temperatures close to Tc and thereby displaying ferromagnetic ordering in both the compounds.