Journal of Superconductivity and Novel Magnetism

, Volume 26, Issue 3, pp 703–707

Gd Substitution Effects on the Magnetic Properties of the Pr1−xGdxCo4Si Compounds

Authors

    • Faculty of Arts and Sciences, Physics DepartmentNevşehir University
  • Selçuk Kervan
    • Faculty of Arts and Sciences, Physics DepartmentNevşehir University
  • Hüseyin Sözeri
    • National Metrology InstituteTUBİTAK-UME
Original Paper

DOI: 10.1007/s10948-012-1789-5

Cite this article as:
Kervan, N., Kervan, S. & Sözeri, H. J Supercond Nov Magn (2013) 26: 703. doi:10.1007/s10948-012-1789-5
  • 87 Views

Abstract

The crystal structure and magnetic properties of single phase Pr1−xGdxCo4Si compounds with x=0,0.2,0.4,0.6,0.8, and 1.0 have been investigated. X-ray analysis reveals that the compounds crystallize as a single phase having the hexagonal CaCu5-type structure with the space group P6/mmm. The substitution of Gd for Pr causes a linear decrease of the unit-cell parameters a and c, and the unit-cell volume V. Magnetic measurements indicate that all samples are ordered magnetically below the Curie temperature. The saturation magnetization at 4.2 K decreases upon the Gd substitution up to x=0.6, and then increases.

Keywords

Magnetic materialsX-ray diffractionMagnetic properties

1 Introduction

The study of magnetic properties of rare-earth (R)-3d transition metals (T) intermetallic compounds is at present a topic in the physics of materials both for the technological aspect related to the possible discovery of new permanent magnets and for the scientific interest connected with the presence of complex magnetic behavior of the 4f and 3d elements [15]. The intermetallic compounds with general formula RT5 and their substitutional derivatives exhibit a variety of interesting properties such as high coercivity, crystal field effects, magnetocaloric effect, spin fluctuation, magnetic anisotropy, etc. [69]. The partial substitution for Co in the RCo5 compounds by non-magnetic elements with an outer p-shell, such as B, Al, C, Ga or Si atoms produces remarkable effects on the crystallographic and magnetic properties of the host compounds [1012]. The RCo4Si compounds crystallize in the hexagonal CaCu5-type structure, with the space group P6/mmm. They are obtained by an ordered replacement of Co by Si in every second layer of the CaCu5 structure [13].

Previous investigations of the magnetic properties of RCo4Si compounds have evidenced that PrCo4Si is ferromagnetically ordered while in the case of GdCo4Si, a ferrimagnetic-type ordering was shown [1315]. The Curie temperature TC and the saturation magnetization at 4.2 K are 425 K and 4.61μB/f.u. for PrCo4Si, 470 K and 2.76μB/f.u. for GdCo4Si, respectively [14]. The compound GdCo4Si has also the compensation point Tcomp at 300 K [13].

In these compounds, the rare-earth sublattice couples either ferromagnetically or antiferromagnetically to the Mn sublattice in the case of light (Pr) or heavy rare earth (Gd), respectively. Therefore, it would be attractive to study the properties of the Pr1−xGdxCo4Si system to gain a deeper insight into magnetic interactions.

2 Experimental

Polycrystalline Pr1−xGdxCo4Si compounds with x=0.0,0.2,0.4,0.6,0.8, and 1.0 were prepared by the arc melting technique under an argon atmosphere of appropriate amounts of Pr (99.9 %), Gd (99.9 %), Co (99.5 %) and Si (99.9999 %) in a water-cooled copper boat. In order to achieve good homogeneity, the polycrystalline ingots were inverted and remelted several times. X-ray diffraction studies were carried out by using a Brucker D8 Advance diffractometer equipped with CuKαradiation. The magnetic properties of the Pr1−xGdxCo4Si compounds were studied by means of a SQUID magnetometer (Quantum Design) in the temperature range 4–400 K in magnetic fields up to 5 T. Magnetic measurements were conducted with powder samples fixed in the sample holder. The temperature dependence of dc magnetization was measured in the zero-field cooling (ZFC) mode in 1 T. The value of the Curie temperature (TC) has been estimated from the minimum in the temperature derivative of the magnetization (dM/dT) versus temperature curve. The compensation temperature (Tcomp) was defined by the minimum appearing in the MT curve.

3 Results and Discussion

It has been found from X-ray diffraction patterns at room temperature that

Pr1−xGdxCo4Si compounds are single phase and crystallize in a hexagonal structure of the CaCu5-type, having the space group P6/mmm. The lattice parameters a and c have been determined using the standard pattern matching method of the FULLPROF [16] program. Figure 1 presents the results of the profile fitting of the powder patterns for Pr0.6Gd0.4Co4Si. The lattice parameters a and c,c/a and the unit-cell volume V for the Pr1−xGdxCo4Si samples at room temperature as a function of Gd concentration x are shown in Fig. 2, while the refined unit-cell parameters a and c, unit-cell volume V and c/a are given in Table 1. It can be seen that substitution of Gd for Pr results in a linear decrease in the lattice constants a and c, and the unit-cell volume V. The lattice parameters obtained for x=0 and x=1 are in good agreement with previously reported values in the literature [1315]. The decrease of the unit-cell constants and the unit-cell volume may be associated with the smaller atomic radius of Gd atoms compared with Pr atoms. Also the unit-cell volume V of the Pr1−xGdxCo4Si samples at room temperature are slightly larger than that of the Nd1−xGdxCo4Si samples due to the smaller atomic radius of Nd atoms compared with Pr atoms [17].
https://static-content.springer.com/image/art%3A10.1007%2Fs10948-012-1789-5/MediaObjects/10948_2012_1789_Fig1_HTML.gif
Fig. 1

The profile fitting for the Pr0.6Gd0.4Co4Si compound

https://static-content.springer.com/image/art%3A10.1007%2Fs10948-012-1789-5/MediaObjects/10948_2012_1789_Fig2_HTML.gif
Fig. 2

Variation of the lattice constants a and c,c/a, and the unit-cell volume V with Gd concentration x at room temperature for the Pr1−xGdxCo4Si compounds

Table 1

The lattice constants a and c, the unit-cell volume V,c/a, the saturation magnetization MS, the magnetic moment of the Co atom MCo, the compensation temperature and the Curie temperature TC for the Pr1−xGdxCo4Si compounds

x

a (Å)

c (Å)

V3)

c/a

MS (μB/f.u.)

MCo (μB/Co atom)

Tcomp (K)

TC (K)

0.0

5.0115

3.9634

86.20

0.7908

4.91

0.43

 

425 [14]

0.2

5.0004

3.9542

85.63

0.7908

3.50

0.58

  

0.4

4.9946

3.9453

85.23

0.7899

1.53

0.60

  

0.6

4.9891

3.9363

84.85

0.7890

0.36

0.82

  

0.8

4.9831

3.9255

84.42

0.7878

0.93

1.01

152

 

1.0

4.9802

3.9138

84.07

0.7859

2.83

1.04

326

470 [14]

The temperature dependence of the magnetization of the Pr1−xGdxCo4Si compounds in the temperature range 4–400 K in an applied field of 1 Tesla is shown in Fig. 3. It can be seen that all samples order magnetically and there is no magnetic phase transition below room temperature (Table 1). The magnetization as a function of temperature behaves differently for the different compositions. The magnetization decreases for x=0.0 and 0.2 and increases for x=0.4 and 0.6 with increasing temperature. This behavior may be due to the complex competition between the sublattice magnetizations and different temperature dependences of the sublattice magnetizations. For the sample with x=0.4 and 0.6, the magnetic moment of the rare-earth sublattice becomes less than the magnetic moment of the Co sublattice and decreases more rapidly than the magnetic moment of the Co sublattice with increasing temperature, and thus the magnetization increases with increasing temperature at low temperatures. A similar behavior was observed in Nd1−xGdxCo4Si and Gd1−xCexMn2Ge2 compounds [17, 18]. The compensation temperature Tcomp is seen at 152 K for x=0.8 and 326 K for x=1. The observed compensation temperature for GdCo4Si compound is somewhat bigger than the value observed in [13]. Bearing in mind that the magnetic moment of the R sublattice decreases more rapidly with increasing temperature than the magnetic moment of the Co sublattice and taking into account that the total magnetic moment of R sublattice increases with increasing Gd content x, the Co sublattice moment can compensate the R sublattice moment at higher temperatures. Therefore, the compensation temperature increases from 152 K for x=0.8 to 326 K for x=1.
https://static-content.springer.com/image/art%3A10.1007%2Fs10948-012-1789-5/MediaObjects/10948_2012_1789_Fig3_HTML.gif
Fig. 3

Temperature dependence of magnetization measured in a field of 1 T for Pr1−xGdxCo4Si compounds

Figure 4 shows the magnetization of the Pr1−xGdxCo4Si compounds at 4.2 K as a function of applied magnetic field up to 5 Tesla. The values of saturation magnetization MS at 4.2 K as a function of Gd concentration, extracted by extrapolating the high field part of magnetization down to the zero field, are presented in Fig. 5 and also listed in Table 1. As shown by the graph of MS versus Gd content x, the saturation magnetization at 4.2 K first decreases in an approximately linear manner to a minimum at about x=0.6 with increasing x, before increasing with further increase in the Gd content. There is a change of magnetization direction at the compensating composition x=0.6, where the resultant magnetization direction, parallel to (Pr+Co) magnetization, is changed parallel to that of Gd magnetization at 4.2 K. The appearance of this minimum in the compositional dependence of MS can be explained in terms of a compensation concentration originating from the ferrimagnetic coupling between the Gd and the Co and Pr sublattice magnetizations. In R–T intermetallic compounds, it is well established that the exchange interaction between the 3d spins of the T elements and the 4f spins of the R elements exhibits ferromagnetic coupling for light R elements (less than half-full 4f shell), and antiferromagnetic or ferrimagnetic coupling for heavy R elements (more than half-full 4f shell) [1921]. Therefore, the competition between the total magnetic moment of the rare-earth sublattice and the magnetic moment of the Co sublattice occurs. At about x=0.6, the cobalt and rare-earth magnetizations in the Pr1−xGdxCo4Si compounds compensate at 4.2 K. Similarly, at low temperatures, the compensation has been observed in the Nd1−xGdxCo4B [22] and Pr1−xGdxCo4B [23] compounds at x=0.6. Assuming the free-ion magnetic moment value for Gd+3 (7μB) and Pr+3 (3.2μB), the cobalt magnetic moments MCo can be determined using a two sublattice ferrimagnetic model from the saturation magnetization values at 4.2 K (Table 1). These values are in good agreement with previously reported values [13, 14]. As Pr is replaced by Gd, the magnetic moment of the cobalt atom increases monotonically. This may be due to the larger Gd 4f localized moments.
https://static-content.springer.com/image/art%3A10.1007%2Fs10948-012-1789-5/MediaObjects/10948_2012_1789_Fig4_HTML.gif
Fig. 4

The magnetization of Pr1−xGdxCo4Si compounds as a function of applied field at 4.2 K

https://static-content.springer.com/image/art%3A10.1007%2Fs10948-012-1789-5/MediaObjects/10948_2012_1789_Fig5_HTML.gif
Fig. 5

The saturation magnetization MS and the magnetic moment of the cobalt atoms MCo at 4.2 K

4 Conclusions

The Pr1−xGdxCo4Si compounds with x=0,0.2,0.4,0.6,0.8, and 1.0 have been successfully synthesized. The crystalline structure has been refined from X-ray diffraction patterns using CuKα radiation. All compounds crystallize in the CaCu5-type structure. The substitution of Gd for Pr leads to linear decreases in the lattice constants and the unit-cell volume V. All compounds are ordered magnetically above room temperature. The compensation temperature Tcomp is seen at 152 K for x=0.8 and 326 K for x=1. The saturation magnetization at 4.2 K decreases upon the Gd substitution up to x=0.6, and then increases again.

Copyright information

© Springer Science+Business Media New York 2012