Magnetic and transport behaviors of non-centrosymmetric Nd7Ni2Pd

Crystallographic, magnetic, electrical transport and thermodynamic properties of pseudo-binary Nd7Ni2Pd compound have been studied using temperature-dependent x-ray powder diffraction and physical property measurements. Compared to the ferromagnetic parent Nd7Pd3, the ground state of Nd7Ni2Pd is antiferromagnetic, and it exhibits strong metamagnetism. The measurements indicate two antiferromagnetic transitions in fields less than 8 kOe: a second-order paramagnetic to antiferromagnetic at TN2 = 29 K and a weak first-order antiferromagnetic to antiferromagnetic transition at TN1 = 24.5 K. The compound becomes ferromagnetic in fields of 8 kOe and higher with TC = 30 K. Temperature dependence of lattice parameters is anomalous, most prominently in the basal plane at ∼30 K; however, there is no detectable structural distortion or clear volume discontinuity around 25 K, suggesting a significant weakening of the first-order transition when compared to the binary Nd7Pd3. Disciplines Materials Science and Engineering This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/ameslab_manuscripts/ 578


INTRODUCTION
Rare earth (R) intermetallic compounds are central to the condensed matter physics, solid-state chemistry, and materials science communities because of peculiar physical properties arising from the interplay between the localized and delocalized electronic states, which can be manipulated by tailoring their chemical and crystallographic makeups. [1][2][3][4][5] Among a broader class of rare-earth-based materials, compounds lacking the center of inversion are rather rare but they often exhibit physical behaviors of both fundamental and practical significance. [6][7][8] In particular, R7Pd 3 compounds with R = La-Nd, Sm, Gd crystallize in the hexagonal Th7Fe 3 structure type described by the non-centrosymmetric space group symmetry P6 3 mc. In this type of crystal structure, the Pd atoms reside on a single 6c site, while the R atoms occupy three different crystallographic sites (2×6c and 2b), resulting in three different magnetic sub-lattices formed by the lanthanide atoms. [9][10][11] The magnetic and electrical properties of Nd7Pd 3 12-15 show that upon cooling the compound undergoes at least two phase transitions: a second-order paramagnetic (PM) -antiferromagnetic (AFM) ordering at T N ∼ 37 K, which is followed by an anhysteretic first-order phase transition from the AFM to a ferromagnetic (FM) state at T C ∼ 33 K. Recently it was demonstrated that the first-order transition at T C is a symmetry-driven magneto-structural transformation between the high-temperature hexagonal and the low-temperature orthorhombic (space group Cmc2 1 ) crystal structures. 16 The structural distortion in Nd7Pd 3 occurs because the noncentrosymmetric P6 3 mc adopted by both PM and AFM parents does not support a stable FM state; on the other hand, Cmc2 1 , which is a subgroup of P6 3 mc, makes a collinear arrangement of the three independent Nd sublattices in the FM state possible. It is worth noting, however, that even though the first-order character of the AFM-FM transition in Nd7Pd 3 is clearly established, the distortion itself is subtle: it could not be detected with laboratory x-ray diffraction ARTICLE scitation.org/journal/adv or with high-resolution neutron diffraction and was only observed using high-resolution synchrotron data. 16 Even though physical properties of R7Pd 3 with different R have been studied in the past, 9,11,12 the influence of partial substitution of the 4d Pd with a 3d element on the magnetism and transport has been not, leaving a wealth of potentially interesting magnetism unexplored. Since the 3d electrons of, e.g. Ni, are less localized, than 4d electrons of Pd 17 and, especially, than 4f electrons of Nd, one may expect varying competition of localized and itinerant magnetism, and even more complex magnetic structures in Nd7NixPd 3-x than in the binary Nd7Pd 3 . With this in mind, we report crystallographic, magnetic, transport, and calorimetric investigation of pseudo-binary Nd7Ni 2 Pd compound, where 2/3 of Pd atoms are randomly substituted by chemically similar Ni atoms.

EXPERIMENTAL TECHNIQUES
Polycrystalline Nd7Ni 2 Pd was prepared by arc melting stoichiometric amounts of the constituent elements in an argon atmosphere, followed by annealing at 750 ○ C for one week and slow cooling to room temperature. Room-temperature crystal structure and phase purity were determined using x-ray powder diffraction (XRPD) using a PANalytical X'Pert diffractometer. XRPD measurements as a function of temperature at H = 0 and 20kOe were carried out on a Rigaku TTRAX rotating anode powder diffractometer as described in Ref. 18. Both ac and dc magnetization measurements were performed in a superconducting quantum interference device magnetometer (Quantum Design) and a physical property measurement system (PPMS, Quantum Design). Heat capacity

ARTICLE
scitation.org/journal/adv (Cp(T)) and ac electrical transport measurements were performed using PPMS.

RESULTS AND DISCUSSION
The XRPD data at T = 300K show that Nd7Ni 2 Pd is practically single phase with hexagonal Th7Fe 3 -type structure (hP20, P6 3 mc, No. 186). The only Bragg reflection not matching the Nd7Ni 2 Pd phase corresponds to a very weak peak at 2θ = 32.4 ○ (Fig. 1a), which may be attributed to a trace amount of NdNi 1−x Pdx. The obtained values of lattice parameters are a = 9.9716(2) Å, and c = 6.3231(2) Å, lower than those of the Nd7Pd 3 . They, as well as the unit cell volume, lie on straight lines connecting the corresponding values of Nd7Pd 3 and Nd7Ni 3 , as expected for a Ni/Pd composition ratio of 2/1 (Fig. 1b).
The zero-field cooled (ZFC), field-cooled cooling (FCC), and field-cooled warming (FCW) magnetization M(T) data measured at H = 0.01, and 10 kOe are shown in Fig. 2. The substitution of Ni for Pd in Nd7Pd 3 leads to an AFM ground state with AFM transitions at T N1 = 24.5K and T N2 = 29K. The transition at T N1 (see Cp(T) below) is likely weakly first-order transformation. With increasing magnetic field, M(T) anomaly at T N1 becomes more pronounced than the one at T N2 , and both peaks disappear at H ≥10kOe when the compound becomes FM (Fig. 2b). The Curie temperature, T C , obtained from M(T) data at H = 10kOe and defined as the minimum value of dM/dT, is 30K. The inverse dc magnetic susceptibility

FIG. 3. M(H) measured at (a) T = 5 K and (b) T = 13 K.
The magnetization was measured up to ±90 kOe but only the low field data are shown for clarity. The hysteresis loops were recorded after cooling the sample in a zero magnetic field followed by changing the magnetic field from 0 to 90 kOe (1 st cycle), then 90 → -90 kOe (2 nd , and 3 rd cycle), then -90 → +90 kOe (4th, and 5 th cycle), and finally +90 → 0 kOe (6 th cycle).

ARTICLE
scitation.org/journal/adv (χ -1 = H/M) follows the Curie-Weiss law. The effective magnetic moments, p eff , and Weiss temperature, θp, are 3.58 μ B /Nd and 30 K, respectively, The former is in a very good agreement with the g[J(J+1)] 1/2 = 3.62 μ B value of Nd 3+ while the positive θp value that is identical to T C points to the dominance of FM interactions in the compound. Ac magnetic susceptibility measured in an ac field of 1 Oe with zero bias dc magnetic field at different frequencies shown in Fig. 2d. Consistent with M(T), a real part of ac susceptibility shows two AFM transitions T N1 = 24.5 K and T N2 = 29 K and an additional peak at 20.5 K, which is different from that observed in Nd7Pd 3 , where it occurs at 16 K and is due to the NdPd impurity. 19 Fillion et al. 20 reported that NdNi orders FM at 28K, suggesting the peak at 20.5K observed in Nd7Ni 2 Pd is due to a pseudo-binary NdNi 1-x Pdx phase, also detected in the XRPD data. M(H), at T = 5 and 13 K measured on a ZFC sample is shown in Fig. 3. Consistent with low-field M(T) data, ZFC M(H) clearly shows the AFM ground state both at T = 5 and 13 K. When the magnetic field increases, M(H) at 5 K indicates a robust meta-magnetic behavior with a critical field ∼±4 kOe, and the compound becomes FM with magnetization approaching saturation when H reaches and exceeds ±8 kOe. The metamagnetic behavior is also observed at 13 K (Fig. 3b) and the critical field decreases to about ±3 kOe. The saturation magnetization is 2.03 μ B /Nd 3+ , which is slightly higher compared to that of Nd7Pd 3 (1.8 μ B /Nd 3+ ). 12,16 Heat capacity, Cp(T), measured at various external magnetic fields (illustrated as Cp/T in Fig. 4) clearly shows two peaks corresponding to T N1 and T N2 observed in M(T) data. The relatively sharp and rather narrow peak at T N1 suggests a first-order phase transition at T N1 . Both peaks at T N1 and T N2 remain practically unchanged up to 5 kOe, but the two anomalies merge into a single broad maximum when the applied field is H = 10 kOe, which is typical for second-order phase transitions. The absence of Cp/T anomaly at T = 16 K confirms that the corresponding peak seen in the ac magnetic susceptibility data of Fig. 2d is likely from a minor impurity.
The electrical resistivity, ρ(T), is nearly temperature independent at T ≤ 7 K (Fig. 5), increasing afterward, indicating metallic behavior. The anomaly is observed near T N1 = 29K and a minor FIG. 5. Electrical resistivity of Nd 7 Ni 2 Ni measured as a function of temperature at zero magnetic field. Inset shows magnetoresistance measured as a function of magnetic field at T=2K. peak is observed at T N2 = 23K. ρ(H) shows a sharp change when the sample transforms from the AFM to FM state and it saturates when the compound is firmly in the FM state for H ≥ 10kOe. The magnetoresistance (Fig. 5, inset) reaches -12%. Incomplete reversibility of magnetoresistance is related to similar irreversibilities seen in M(H) data at 5 K, Fig. 3a.
The temperature dependent XRPD at H = 0 and 20 kOe show that the compound crystallizes in the Th7Fe 3 structure type in the whole temperature region. However, considering that the structural distortion was not observed with laboratory XRPD in Nd7Pd 3 16 we cannot rule out that a similar weak P6 3 mc → Cmc2 1 distortion also occurs in Nd7Ni 2 Pd. If fact, given that FM state is not supported by the hexagonal P6 3 mc, it is nearly certain that the structural distortion is indeed present in Nd7Ni 2 Pd; high-resolution synchrotron XRPD

ARTICLE
scitation.org/journal/adv measurements are likely required to demonstrate this symmetry altering transition. At the same time, partial substitution of Pd with Ni in Nd7Ni 2 Pd practically removes the discontinuous behavior of lattice parameters, both at 0 and 20 kOe (Fig. 6), which explains the very weak first-order character of the low-temperature transition. However, the contraction along c axis as well as the expansion along the a-axis, and the resulting decrease of the c/a ratio and the increase of the unit cell volume in the magnetically ordered state are observed in both the parent Nd7Pd 3 and in the Ni substituted Nd7Ni 2 Pd. The application of the magnetic field has a negligible effect on the lattice parameters.

SUMMARY
We show that partial substitution of Pd with Ni changes the ground state from FM in Nd7Pd 3 to AFM in Nd7Ni 2 Pd. The AFM transition that occurs upon cooling at T N1 = 24.5 K is weakly first order, and it is preceded by a second-order AFM ordering at T N2 = 29 K. The ρ(T,H) show metallic behavior with magnetoresistance reaching -12% at H = 30 kOe and T = 2 K. Temperature-dependent XRPD reveals anomaly in the lattice parameter a at ∼30 K, however, there is no detectable structural distortion or volume discontinuity around 25 K, suggesting a significant weakening of the first-order transition compared to the binary Nd7Pd 3 . Compared to Nd7Pd 3 , Nd7Ni 2 Pd exhibits strong metamagnetic behavior with the application of an external magnetic field.