Crystal structure and magnetic properties of the Nd_1-xGd_xCo_9.5V_2.5 compounds (x = 0-1) have been investigated. The compounds crystallize in the tetragonal ThMn₁₂ structure with space group I4/mmm. The lattice parameters a, c and the unit cell volume V decrease with the increase of Gd content. The Curie temperature of Nd_1-xGd_xCo_9.5V_2.5 increases monotonically upon the substitution of Gd for Nd. The critical field H_crit^L required to induce the magnetization jump decreases with increasing Gd content. The metamagnetic transition phenomenon becomes less obvious and even disappears with the substitution of Gd for Nd. The H_crit^H required for the metamagnetic transition increased with increasing Gd content at low substitution content. The saturation moment M_S of the compounds decreases first then increases with the increased Gd content.

1. Introduction

Ternary rare-earth compounds with the tetragonal ThMn₁₂ structure belong to the family derived from the RT₅ (CaCu₅-type structure, space group P6/mmm) and form an important class of materials that find numerous industrial applications in permanent magnets and magneto-optical recording, as well as in the aerospace domain [1, 2]. Among these intermetallic compounds, the R(Fe, M)₁₂ compounds have attracted considerable attention owning to their relatively high Curie temperature and saturation magnetization as well as large uniaxial magnetocrystalline anisotropy at room temperature. In order to understand the magnetism of the R(Fe, M)₁₂ compounds and improve their magnetic properties, a series of experiments concerning the substitution of Fe by Co have been undertaken [3, 4]. Jurczyk and Christyakov [5] showed that it is possible to prepare also Co rich compounds of the same structure. In general, the contribution of rare-earth metal to magnetic anisotropy energy has opposite signs in RFe_12-xM_x and RCo_12-xM_x [6]. Recently, we conducted a series of investigations on the ternary rare-earth compounds R(Fe, Co and V)₁₂ (R=Y, Nd) [7-10]. Some essential phenomena were observed: (i) A metamagnetization transition phenomenon occurred when a high field up to 140 KOe was applied, accompanied by an increase of the magnetization of about 3.1μ_B  per formula for NdCo_9.5V_2.5 [7]; (ii) Fe substitution for Co in NdCo_9.5V_2.5 could result in a shift of the critical field required for the metamagnetic transition, which may be understood by considering the combined effects of the exchange coupling parameter J_RT (between R and T atoms) and the T-sublattice moment μ_T  [9]; (iii) Nonmagnetic element Y substitution for Nd in NdCo_9.5V_2.5 decreases the critical field due to the weakening of the anisotropy field of R-sublattice. As a continuation of the systematic study on the metamagnetic transition of the compound NdCo_9.5V_2.5, in this paper we report the structure and magnetic properties of Nd_1-xGd_xCo_9.5V_2.5. Since Gd is a magnetic element and the 4f shell is half filled so that its second Steven factor α_J = 0, the substitution of Gd for Nd provides an opportunity for obtaining a deeper understanding of the magnetic properties of the compounds.                             

2. Experimental details

Samples of polycrystalline Nd_1-xGd_xCo_9.5V_2.5 (x=0, 0.1, 0.3, 0.6, 0.8, 0.9, 1) were prepared by arc melting the appropriate amounts of the constituent elements Nd, Gd, Co, and V with purity better than 99.9% under a high purity argon atmosphere. Appropriate excess amounts of Nd and Gd were added to compensate for the weight loss during arc melting and subsequent heat treatment. After arc melting, the polycrystalline specimens (ingots) were wrapped in Ta foil, sealed into evacuated quartz tube and annealed at 1373K for 1 week. To avoid possible phase transition during cooling, the samples were quenched into water. The crystal structure was determined by X-ray powder diffraction (XRD) technique. The XRD experiments were performed on a Rigaku D/max 2500 diffractometer with Cu Kα radiation in the 2θ range (20-120^o). A step-scan mode was adopted with a sampling time of 1-2 s and a step width of 2θ =0.02^o. The data were further analyzed via the Rietveld refinement technique by using the program Fullprof [11]. The temperature dependence of the ac susceptibility of the samples was measured by a mutual inductance method with a fixed frequency of 240 Hz. Curves of magnetization versus temperature (M-T) for the samples were measured by a SQUID magnetometer in a field of 500 Oe. The field dependence of the magnetization (M-H) at 5K of the fine powder was measured using a PPMS. 

3. Results and discussion

3.1 Crystallography

The structure of Nd_1-xGd_xCo_9.5V_2.5 (x = 0, 0.1, 0.3, 0.6, 0.8, 0.9, 1) was examined by X-ray diffraction and thermo-magnetic analysis. All the samples exhibit single-phase tetragonal ThMn₁₂ structure. The observed, calculated, and difference XRD patterns resulting from the Rietveld refinement [12, 13] of Nd_0.4Gd_0.6Co_9.5V_2.5 is shown in Fig. 1. The unit-cell parameters a, c and unit-cell volume V of the compounds are shown in Fig. 2 and listed in Table 1. Gd substitution in the Nd site leads to a decrease in the unit-cell volume from 333.1 ų for NdCo_9.5V_2.5 to 329.8 ų for GdCo_9.5V_2.5. The decrease in unit-cell volume with increasing Gd content consists with the fact that the atomic radius of Gd is smaller than that of Nd. The values of a, c and V for x = 0, 1 agree well with those reported in [10, 14]

3.2 The Curie temperature and exchange interaction

Temperature dependence of the magnetization measured in an applied field of 0.5 KOe is shown in Fig. 3. The Curie temperature T_C was derived at the temperature where dM/dT shows minimum on the high temperature region. The dependence of Curie temperature on the Gd concentration is shown in inset of Fig. 3 and listed in Table 1. Fig. 4 shows the temperature dependence of the ac susceptibility χ _ac of Nd_1-xGd_xCo_9.5V_2.5. The  exhibits a distinct susceptibility peak near the Curie temperature. According to Landau theory, the critical peak of χ _ac can be deliberated on the basis of the expression for the susceptibility where C₁ and C₃ are the coefficients in the expression for the free energy [14]. The Curie temperature can be obtained from the peak temperature where C₁(T_C') = 0 and listed in Table 1. The T_C derived from M-T curves coincidence well with that from the χ _ac peak. As listed in Table 1, the Curie temperature increases with increasing Gd content (shown in the inset of Fig. 3), reaching the value of 212 K for x = 1, which is in good accordance with the value reported in ref. [15].

In rare-earth compounds, three types of exchange interactions exist: R-R, R-T and T-T, and the R-R interaction is so weak that it usually can be neglected. The exchange interactions in the rare-earth intermetallics can be derived from the expression for the magnetic ordering temperature under the molecular field approximation [16]:

WhereT_Co,T_R and T_RCo represent the contributions to T_C owning to the Co-Co, R-R, and Co-R exchange interactions, respectively, and are given by

and

where

Where N_Co and N_R are the numbers of Co and R atoms per unit volume, and n_CoCo, n_RR and n_RCo are the molecular-field coefficients. Since the R-R interaction is very weak, n_RR may usually be neglected. It is obvious that the T_RCo is proportional to n_RCoand , where G(J) is the de Gennes factor. G(J) equals 1.84 and 15.75 for free Nd³⁺ ion and Gd³⁺ ion, respectively. The exchange coefficient n_RCo decreases continuously across the lanthanide series due to the contraction of the 4f shell and the smaller overlap between the 4f and 5d shells. It can be seen from the inset of Fig. 4 that the Curie temperature T_C increases almost linearly except for x=1 with the average square root of G(J). Thus, for the Nd_1-xGd_xCo_9.5V_2.5 compounds, the contribution of the de Gennes factor is dominant, leading to the monotonic increase of the Curie temperature with increasing Gd content.

3.3 The domain wall pinning effect and metamagnetic transition

At low temperature, a steep increase of the magnetization occurs as shown in Fig. 3. This typical behavior is characteristic of ferromagnetism when narrow domain walls are present, as found previously in other anisotropic rare-earth compounds. Fig. 5 (a)-(f) show the field dependence of the magnetization of Nd_1-xGd_xCo_9.5V_2.5 at 5 K from 0 to 140 kOe, which exhibits a field induced magnetic jump for all of the compounds below 10 kOe, as we observed previously. This behavior can be due to the narrow domain wall pinning effect. As shown in table 1, the critical field H_crit^L required to induce the magnetic jump decreases with increasing Gd content. H_crit^L is determined from the peak position in the differential of the magnetization with respected to the field (dM/dH-H). It is well known that the domain-wall pinning can result from a competing between the magnetocrystalline anisotropy of the compound and the exchange energy. With substitution of Gd for Nd, the required H_crit^L decreases monotonically due to the weakened rare-earth sublattice magnetocrystalline anisotropy. According to the law of approach to saturation, the saturation moment M_S of the compounds can be derived by extrapolating the high-field part of the M-1/H curve to 1/H=0. The derived M_S decreases for x = 0-0.8 and increases for x = 0.8-1.0 as shown in Table 1, which induced by the antiparallel of Gd moments and Co moments. With the substitution of Gd for Nd, the magnetization direction changed for x=0.8.  

In our recent investigation on NdCo_12-xV_x compounds, it is observed that when the applied field is higher than 15 kOe, the magnetization exhibits a plateaulike feature up to 60 kOe. When the applied magnetic field was up to 140 kOe, it was observed that a metamagnetic transition occurred at a critical field of about 60 kOe, as shown in Fig. 5(a). Substitution of Y for Nd in NdCo_9.5V_2.5 decreases the critical field H_crit^H required for the metamagnetic transition, and the metamagnetic transition becomes broad and eventually disappears with the increase of temperature, as shown in the inset of Fig. 5(g). Here, the magnetization curves were measured up to 140 kOe, it can be seen from Fig. 5(a)-(f) that at lower Gd substitution (x =0, 0.1, 0.3), the plateaulike feature remains, however, with increasing Gd content (x = 0.6, 0.8, 0.9, 1), the metamagnetic transition becomes broadened and disappears.  

Rao et al [8] revealed that when an external field applied perpendicular to the c axis, the Nd moments prefer to stay along the c axis while the Co moments will tend to realign in the a-b plane, leading to a tilting off the c axis of the magnetization vector corresponding to one of the two minima of the total energy. As the applied field increases, the energy gain from the Zeeman interaction will overcome the easy-c anisotropy of the Nd sublattice and the Nd moments will be pulled into the a-b plane, leading to the occurrence of the metamagnetic transition from the low magnetization state to the high magnetization state and producing a two-step behavior on the magnetization curve. For Nd_1-xGd_xCo_9.5V_2.5 compounds at lower Gd substitution, the critical field H_crit^H increases with increasing Gd content (H_crit^H is 62.5, 71.0, and 86.5 KOe corresponding to x = 0, 0.1, and 0.3, respectively). For the rare-earth sublattice, the effective field acting on it can be described as , the molecular field H_mol^R on each rare-earth atom is determined by the total number of nearest-neighbor magnetic Co atoms. In addition, the exchange-interaction field from the T sublattice acting on the R sublattice is described by

where Z_RT is the coordination number of T around R. J_RT  is the exchange-coupling parameter between R and T. Thus, the H_ex,R is closely associated with the exchange-coupling parameter J_RT and the T sublattice spins S_T. The substitution of Gd for Nd on one hand decreases the lattice parameters and leads to an enhanced J_TT and J_RT and increases the T- sublattice moment μ_T , on the other hand can also decrease the J_RT as mentioned by Liu et al [17] that an even stronger decrease of the radius of the 4f shell with increasing Z can lead to a smaller overlap of the 4f and 5d shells and a decrease of exchange field coefficients between rare-earth and Fe or Co by a factor of 2 or more from the light rare-earth to the heavy rare-earth elements. Combining the two effects, at low Gd substitution content, the decrease of J_RT may play an important role which leads to an increase of H_crit^H . Moreover, the metamagnetic transition, which is sharp on low Gd substitution, becomes broadened with increasing Gd content and disappears on high substitution. That can be explained by the substitution of Gd for Nd decreases the anisotropy of the R sublattice and therefore reduces the effect of magnetocrystalline anisotropy on the magnetization process.

4. Conclusion

The crystal structure and magnetic properties of Nd_1-xGd_xCo_9.5V_2.5 compounds with x = 0-1 have been investigated by means of X-ray diffraction and magnetic measurements. All the samples are single phase and crystallize in the tetragonal ThMn₁₂ structure with space group I4/mmm. The lattice parameters a, c and the cell volume V decrease with increasing Gd content. The Curie temperature increases monotonically upon substitution of Gd for Gd. A domain wall pinning phenomenon was observed in all of the compounds and the H_crit^L required to induce the magnetic jump decreases with increasing Gd content. The saturation moment M_S decreases for x = 0-0.8 and increases for x = 0.8-1.0. The metamagnetic transition phenomenon becomes less obvious even disappears with the continuous substitution of Gd for Nd and at low substitution the H_crit^H required for the metamagnetic transition increases with increasing Gd content.

Acknowledgements

This work is supported by the National Natural Science Foundation of China and the Natural Science Foundation of Tianjin. 

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