A Gil - Crystal electric field effect in rtxx2 compounds - страница 1

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ВІСНИК ЛЬВІВ. УН-ТУ

Серія фізична. 2007. Вип.40. С.268-279

VISNYKLVIV UNIV. Ser.Physic. 2007. N40. P.268-279

PACS number(s): 71.70.Ch CRYSTAL ELECTRIC FIELD EFFECT IN RTxX2 COMPOUNDS

A. Gil

Faculty of Mathematical and Natural Sciences, JD University, Al. Armii Krajowej 13/15, 42-200 Czestochowa, Poland

The RTxX2 type metallic compounds where R is a rare-earth element, T is a d-electron element and X is a p-electron element (X = Si, Ge, Sn) crystallize in the orthorhombic crystal structure of the CeNiSi2 type.

The systems discussed exhibit complex magnetic behaviour. Their magnetism arises from the long range interaction of the magnetic moments localized on the rare-earth ions, probably those described by RKKY model. The magnetic ordering in these systems is not purely of the RKKY-type but is modified by the crystalline electric field effect, which can significantly influence the magnitude of the Neel temperature. The CEF effects are also responsible for the observed decrease in the rare-earth magnetic moments as compared with the free ion values.

The determination of the CEF parameters is important for understanding

the nature of magnetic properties of intermetallic compounds. The Bnm

parameters were calculated for RTX2 compounds using the so-called point-charge model. The quadrupole splitting observed in the Mossbauer spectra of GdTxSn2   compounds   (T=Fe, Co, Ni, Cu)   allow   to   determined the

experimental values of B20 parameters for RTX2 compounds.

Key words: crystal field, magnetic properties.

During the last few years the RTxX2 (x < 1) type intermetallic compounds (R is a rare-earth, T is a d-electron metal and X is a p-electron metalloid (X = Si, Ge, Sn)) have been a subject of intensive studies because of their intriguing physical properties, which have both fundamental and practical significance.

The RTxX2 compounds crystallize in different orthorhombic crystal structures [1]:

CeNiSi2-type (space group Cmcm);

TbFeSi2-type, which is very similar to the CeNiSi2, but R, T and Si atoms are located in alternating layers;

YIrGe2-type (space group Immm);

LuNiSn2-type (space group Pnma);

TiMnSi2 or ZrCrSi2-type (space group Pbam);

NdRuSi2-monoclinic type, which is described as a distorted variant of the CeNiSi2-type (space group P21/m).

© Gil A., 2007

All silicides crystallize in the stoichiometric structure (x=1), while germanides and stannides form mainly defected structures RTxX2 (x<1) with defects in d-electron metal sublattice.

Majority of these compounds crystallize in the orthorhombic CeNiSi2-type structure [2] (Fig. 1) and discussion in this work concerns compounds with this structure

type.

Magnetic measurements indicate that majority of compounds order antiferromagnetically at low temperatures. Above the Neel temperatures, the effective

magnetic moments are close to the free R3+ ion values (g^J(J +1)). Only for

compounds with T = Mn, a localized magnetic moment on Mn has been observed

(Fig. 2).

Fig. 1. Crystal structure of CeNiSi2-type

A. Gil

V

л

-4

 

' '-------

= = 4==* = = = = = = =

--г-т=

■ :

 

 

 

 

*■"*""■"----"*

 

Pr

 

Nd

 

 

Tb

 

Dy

 

 

Ho

 

4

Er

 

Fig. 2. Effective magnetic moments for RTxX2 compounds

The crystal structure has a distinct layer character. Rare-earth atoms occupy the layers perpendicular to the b-axis, and they are separated by layers of other atoms. In majority of compounds, the magnetic moments located in the same planes are coupled ferromagnetically, while the coupling between two moments in adjacent planes is antiferromagnetic.

Magnetic data for the RTxX2 ternary compounds are summarized in Table 1.

Table 1

Magnetic properties of RTxX2 compounds

Compound

TMO1

6p [K]

Meff [Mb]

Tc,n [K]

Mr [Mb]

MMD2

Ref.

CeMnSi2

F

420

3,52

398

0,23

II b

3,4

PrMnSi2

F

450

4,29

434,34

2,04

II b

3,4

NdMnSi2

F

460

4,41

441,40

1,8

II c

3,4

SmMnSi2

F

482

3,02

464

1,74

 

3

NdMn042Ge2

AF

-23,5

5,08

34

1,55

II c

5

SmMn04Ge2

 

-57,2

3,7

 

 

 

5

GdMn038Ge2

AF

-30,5

8,75

35

 

 

5

TbMn033Ge2

AF

-37,1

10,3

26

7,0

II c

5

DyMn0,32Ge2

AF

-17,8

11,1

30

 

 

5

HoMn033Ge2

AF

-12,8

11,2

7

7,3

II c

5

ErMn027Ge2

AF

-10,7

9,95

3,4

 

 

5

CeMnSn2

F

117

4,03

320

0,4

 

6

CeMn05Sn183

F

369

3,8

520

0,55

 

6

PrMn05Sn183

AF

-14

4,18

16

3,09

II c

7

CRYSTAL ELECTRIC FIELD EFFECT...

NdMn05Sn183

AF

-39

4,48

20

 

 

6

SmMn0,29Sn1,56

AF

-105

1,97

35

 

 

6

PrFeSi2

F

44

3,5

26

2,58

II b

3, 8

NdFeSi2

AF

41

3,6

6

3,17

II b

3, 8

CeFe0,63Ge2

NM Kondo

-115

2,8

 

 

 

9

CeCoSi2

PP

-240

1,6

 

 

 

10

NdCoSi2

AF

-28

3,0

2,15

2,18

 

11, 12

SmCoSi2

AF

 

 

4

 

 

11

GdCoSi2

AF

-8

7,5

7,5

 

 

11

TbCoSi2

AF

-18

9,8

16

9,16

II a

11, 13

DyCoSi2

AF

8

10,7

10

5,74

(a-c)

11, 14

HoCoSi2

AF

-6

10,5

6,3

6,72

II a

11, 14

ErCoSi2

AF

-17

9,6

2,6

4,63

(a-c)

11, 15

CeCo089Ge2

AF

Kondo

-90

2,55

3

 

 

9

CeCo0 48Sn2

 

-9

2,71

 

 

 

16

PrCo0,33Sn2

 

-2

3,52

 

 

 

16

NdCo0>33Sn2

 

-7

3,63

 

 

 

16

GdCo0 27Sn2

AF

 

 

16,5

 

 

17

TbCo0,25Sn2

AF

 

 

18,8

7,14

(a-c)

17

DyCo0,26Sn2

AF

 

 

7

 

 

17

HoCo0 23Sn2

AF

 

 

5,6

6,79

(a-c)

17

ErCoa24Sn2

AF

 

 

4,7

6,15

(a-c)

18

CeNiSi2

MV

-150

2,6

 

 

 

19

PrNiSi2

F

20

3,54

20

3,09

II c

20

NdNiSi2

F

8

3,57

9,5

2,38

II c

20

SmNiSi2

AF

 

0,88

9,8

 

 

21

GdNiSi2

AF

-33

9,4

19,3

5,8

 

21

TbNiSi2

AF

-17 -5

9,84 10,1

37,6 36,2

8,7

II c

21 22

DyNiSi2

AF

-12 21

11,1 10,6

25 24

7,5

II c

21

23

A. Gil

HoNiSi2

AF

3 3

10,6 11,05

10 10

8,13

II c

21 24

ErNiSi2

AF

4

20

9,56 10,11

3,1 3,4

7,6

II b

21 25

CeNiGe2

AF HF

-20,8

2,51

3,9, 3,2

 

 

19

PrNiGe2

F

13,8

3,26

15

 

 

21

NdNiGe2

F

7

3,31

15

 

 

21

 

 

6

3,37

6

2,32

II c

26

SmNiGe2

 

-13,6

0,97

 

 

 

21

GdNiGe2

AF

-33

8,6

24,5

 

 

21

TbNiGe2

AF

-31,5

10,57

42

8,8

II c

21, 27

TbNi0,4Ge2

AF

 

 

16

5,6

II c

28

TbNi0,6Ge2

AF

 

 

31

8,9

II c

28

TbNi08Ge2

AF

 

 

38

8,7

II c

28

DyNiGe2

AF

-12

10,96

22

 

 

21

HoNiGe2

AF

-5

10,71

7,6

 

 

21

HoNi0,64Ge2

AF

 

 

11

6,7

II c

27

ErNiGe2

AF

-2

9,69

2,5

 

 

21

ErNi0,65Ge2

AF

 

 

2,5

2,8

II a

 

CeNiSn2

AF

-6,9

2,56

4

 

 

19

CeNi0,84Sn2

AF

 

 

4, 3

2,0

II c

29

PrNiSn2

 

-2

3,69

 

 

 

16

NdNiSn2

 

-3

3,73

 

 

 

16

GdNi0,4Sn2

AF

 

 

27

 

 

30

TbNi0,26Sn2

AF

 

 

17,4

8,05

(a-c)

31

DyNi0,22Sn2

AF

 

 

8

9,2

(b-c)

32

HoNi0,16Sn2

AF

 

 

6,6

7,46

(a-c)

31

ErNi0,15Sn2

AF

 

 

4

9,16

II a

18

CeQi0,86Ge2

AF

7

 

15,6 16,5

1,85

II c

19

33

PrCu0,76Ge2

F

 

 

23

 

 

34

TbCu04Ge2

AF

-25

10,4

39

8,82

II c

35

HoCu033Ge2

AF

-8,0

10,52

8

8,18

II c

36

ErCu0,25Ge2

AF

4,1

9,03

4,5

7,89

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A Gil - Crystal electric field effect in rtxx2 compounds