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Magnetic structure of HCP Ho-light rare earth alloys
185
Journal of Magnetism and Magnetic Materials 31-34 (1983) 185-186 M A G N E T I C S T R U C T U R E OF HCP H a - L I G H T R A R E E A R T H A L L O Y S S. K A W A N O A N D N. A C H I W A Research Reactor Institute, Kyoto University, Kumatori, Sennan, Osaka 590-04, Japan
Neutron diffraction measurements were performed on single crystals of hcp Ho-La, Ho-Ce and Ho-Pr alloys. The magnetic phase diagram for these alloys was clearly determined. N6el temperatures and turn angles showed different behaviors from the case of Ha-heavy rare earth alloys. The nature of an exchange interaction in Ha-light rare earth alloys was discussed. 1. Introduction Ha,fight rare earth alloys form completely disordered alloys with close packed structures such as hexagonal, samarium-type, double hexagonal and face centered cubic, with increasing light rare earth metals [1]. Though magnetic properties of each phase are very interesting, they have been not well established, because of difficulty of single crystal growth. Previously authors reported brief results of neutron diffraction on Ha-light rare earth alloys, where the nature of an exchange interaction is significantly different from that in Ha-heavy rare earth alloys [2-4]. In the present investigation, the main purpose is to determine clearly the magnetic phase relation and turn angles of a helical structure in detail, and to clarify the effects of an introduction of light rare earth metals into pure Ha on magnetic properties, with neutron diffraction of single crystals.
2. Experimental and results Single crystals used in this experiment were prepared with a strain anneal method. Neutron diffraction measurements were performed in the a * - c * reciprocal plane over the temperature range from 4.2 K to room temperature by a stepwise linear scan along the c*-axis with a double axis neutron diffractometer installed at Kyoto University Reactor (KUR). The magnetic phase diagram for hcp Ha-light rare earth alloys is given in fig. 1. As temperature is lowered, alloys of more than 84 at% Ha concentration exhibit a helical structure (helix), where magnetic moments are confined to the basal plane, at N+el temperature TN and a conical structure (cone), where moments along the c-axis order ferromagnetically and basal plane moments retain the helical structure, at a Curie temperature Tc. On the other hahd, in alloys of less than 80 at% Ha concentration the conical structure vanishes and only the helical structure occurs down to low temperatures. Temperature dependence of turn angles of the helical structure for H o - L a alloys is shown in fig. 2. Almost same behaviours of turn angles are obtained for H o - C e and H o - P r alloys. Turn angles decrease with nearly linear dependence on temperature and at about 40 K
150 0 : Ho-Lo
( :
o : Ho-Ce
/
Zl : Ho-Pr Poramog:eti~//~/n
I00
-~F -o ® 8 32 5 0 ~, o zE
o--i,
Ha Helix
~0
-(6O
Cone
--
.
70
,
,
80 90 I00 Ha concentration (at % )
Fig. I. The magnetic phase diagram for hop Ha-light rare earth alloys. Data for pure Ha are cited from ref. [5]. reach constant values. Strictly in H o - P r alloys values of turn angles at low temperatures rather increase with decreasing Ha concentration.
3. Discussion In pure Ha the helix-cone transition occurs with decreasing temperature [5]. In the present alloys, although Ha-rich alloys show the similar magnetic struc-
"~ a
50
e "~ 40
o : Ho95La.oa a : HO.gLo.I O : Hoeka. 2 A: H°75La.25 oO...o-gg"° V: HosLo4 .~..d~dp.OOo o I
1-
30
o~
40
4b
6'0
go
,60
,~'o
Temperature (K) Fig. 2. Temperature dependence of turn angles of the helical structure for hcp H o - k a alloys.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
S. Kawano, N. Achiwa / Magnetic structure of H o - L a , Ce, Pr alloys
186 150
6O
o : Ho-La
,-IN
o : Ho-Ce
¢0
zx : H o - P r
>, _o
,i ,111s"2
E ~ IOC
IsI /~Q .j.1 /
eq) /" o o o_ /
/
/
/
/
"1"t
t.0~
-~
,A,
ta LUf
o r-
I i /
/°°
Ho
I-
o : Ho-La
//
g 5c
//
0
/
/
2o
/
o : Ho-Ce
Ho
z~ : H o - P r
// /
.
//
3
l-c
4
5
deGennes foctor, G
Fig. 3. Variation of magnetic ordering temperatures against the de Gennes factor G. Broken curves represent the case of Ho-Y alloys [6]. ture change to pure Ho, T N decreases rapidly and the helical structure is intabilized by an ititroduction of light rare earth metals, while the conical structure is relatively retained. Magnetic ordering temperatures and turn angles of rare earth metals and their alloys are conventionally arranged as a function of de Gennes factor G. In fig. 3, magnetic ordering temperatures for H o - h g h t rare earth alloys are plotted against the de Gennes factor G, together with the case of H o - Y alloys [6]. In heavy rare earth metals and their alloys Tr~ varies with the 2 / 3 power of G [7]. The broken curve of T~ for H o - Y alloys in fig. 3 obeys this law satisfactorily. In the present alloys, T N does not follow the 2 / 3 power law, but is reduced very rapidly with decreasing the de Gennes factor. The variation of an initial turn angle toi, which is obtained at T~, and final one tot, which is obtained at 4.2 K, for H o - l i g h t rare earth alloys with the de Gennes factor G is shown in fig. 4. In H o - Y
0
0
i
I
I
2
|
3
4
5
deGennes factor,G
Fig. 4. Change of turn angles with the de Oennes factor G. Broken curves show the case of Ho-Y alloys [6]. alloys both toi and tot approach the constant value of 50 ° with decreasing the de Gennes factor, as shown in fig. 4. On the contrary in H o - h g h t rare earth alloys toi decreases to reach tof, which is almost constant. The behaviors of T~ and toi suggest that the mean free path of conduction electrons becomes shorter and the exchange interaction has a smaller range in H o - l i g h t rare earth alloys than in H o - h e a v y rare earth alloys.
References [1] C.C. Koch, J. Less-Common Metals 22 (1970) 149. [2] S. Kawano and N. Achiwa, J. Phys. Soc. Japan 37 (1974) 569. [3] S. Kawano and N. Achiwa, J. Phys. Soc. Japan 38 (1975) 285. [4] S. Kawano and N. Achiwa, Ann. Rep. Res. React. Inst. Kyoto Univ. 13 (1980) 171. [5] W.C. Koehler, J.W. Cable, M.K. Wilkinson and E.O. Wollan, Phys. Rev. 151 (1966) 414. [6] H.R. Child, AEC-Report, ORNL-TM-1063 (1965) 48. [7] W.C. Koehler, J. Appl. Phys. 36 (1965) 1082.
Journal of Magnetism and Magnetic Materials 31-34 (1983) 185-186 M A G N E T I C S T R U C T U R E OF HCP H a - L I G H T R A R E E A R T H A L L O Y S S. K A W A N O A N D N. A C H I W A Research Reactor Institute, Kyoto University, Kumatori, Sennan, Osaka 590-04, Japan
Neutron diffraction measurements were performed on single crystals of hcp Ho-La, Ho-Ce and Ho-Pr alloys. The magnetic phase diagram for these alloys was clearly determined. N6el temperatures and turn angles showed different behaviors from the case of Ha-heavy rare earth alloys. The nature of an exchange interaction in Ha-light rare earth alloys was discussed. 1. Introduction Ha,fight rare earth alloys form completely disordered alloys with close packed structures such as hexagonal, samarium-type, double hexagonal and face centered cubic, with increasing light rare earth metals [1]. Though magnetic properties of each phase are very interesting, they have been not well established, because of difficulty of single crystal growth. Previously authors reported brief results of neutron diffraction on Ha-light rare earth alloys, where the nature of an exchange interaction is significantly different from that in Ha-heavy rare earth alloys [2-4]. In the present investigation, the main purpose is to determine clearly the magnetic phase relation and turn angles of a helical structure in detail, and to clarify the effects of an introduction of light rare earth metals into pure Ha on magnetic properties, with neutron diffraction of single crystals.
2. Experimental and results Single crystals used in this experiment were prepared with a strain anneal method. Neutron diffraction measurements were performed in the a * - c * reciprocal plane over the temperature range from 4.2 K to room temperature by a stepwise linear scan along the c*-axis with a double axis neutron diffractometer installed at Kyoto University Reactor (KUR). The magnetic phase diagram for hcp Ha-light rare earth alloys is given in fig. 1. As temperature is lowered, alloys of more than 84 at% Ha concentration exhibit a helical structure (helix), where magnetic moments are confined to the basal plane, at N+el temperature TN and a conical structure (cone), where moments along the c-axis order ferromagnetically and basal plane moments retain the helical structure, at a Curie temperature Tc. On the other hahd, in alloys of less than 80 at% Ha concentration the conical structure vanishes and only the helical structure occurs down to low temperatures. Temperature dependence of turn angles of the helical structure for H o - L a alloys is shown in fig. 2. Almost same behaviours of turn angles are obtained for H o - C e and H o - P r alloys. Turn angles decrease with nearly linear dependence on temperature and at about 40 K
150 0 : Ho-Lo
( :
o : Ho-Ce
/
Zl : Ho-Pr Poramog:eti~//~/n
I00
-~F -o ® 8 32 5 0 ~, o zE
o--i,
Ha Helix
~0
-(6O
Cone
--
.
70
,
,
80 90 I00 Ha concentration (at % )
Fig. I. The magnetic phase diagram for hop Ha-light rare earth alloys. Data for pure Ha are cited from ref. [5]. reach constant values. Strictly in H o - P r alloys values of turn angles at low temperatures rather increase with decreasing Ha concentration.
3. Discussion In pure Ha the helix-cone transition occurs with decreasing temperature [5]. In the present alloys, although Ha-rich alloys show the similar magnetic struc-
"~ a
50
e "~ 40
o : Ho95La.oa a : HO.gLo.I O : Hoeka. 2 A: H°75La.25 oO...o-gg"° V: HosLo4 .~..d~dp.OOo o I
1-
30
o~
40
4b
6'0
go
,60
,~'o
Temperature (K) Fig. 2. Temperature dependence of turn angles of the helical structure for hcp H o - k a alloys.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
S. Kawano, N. Achiwa / Magnetic structure of H o - L a , Ce, Pr alloys
186 150
6O
o : Ho-La
,-IN
o : Ho-Ce
¢0
zx : H o - P r
>, _o
,i ,111s"2
E ~ IOC
IsI /~Q .j.1 /
eq) /" o o o_ /
/
/
/
/
"1"t
t.0~
-~
,A,
ta LUf
o r-
I i /
/°°
Ho
I-
o : Ho-La
//
g 5c
//
0
/
/
2o
/
o : Ho-Ce
Ho
z~ : H o - P r
// /
.
//
3
l-c
4
5
deGennes foctor, G
Fig. 3. Variation of magnetic ordering temperatures against the de Gennes factor G. Broken curves represent the case of Ho-Y alloys [6]. ture change to pure Ho, T N decreases rapidly and the helical structure is intabilized by an ititroduction of light rare earth metals, while the conical structure is relatively retained. Magnetic ordering temperatures and turn angles of rare earth metals and their alloys are conventionally arranged as a function of de Gennes factor G. In fig. 3, magnetic ordering temperatures for H o - h g h t rare earth alloys are plotted against the de Gennes factor G, together with the case of H o - Y alloys [6]. In heavy rare earth metals and their alloys Tr~ varies with the 2 / 3 power of G [7]. The broken curve of T~ for H o - Y alloys in fig. 3 obeys this law satisfactorily. In the present alloys, T N does not follow the 2 / 3 power law, but is reduced very rapidly with decreasing the de Gennes factor. The variation of an initial turn angle toi, which is obtained at T~, and final one tot, which is obtained at 4.2 K, for H o - l i g h t rare earth alloys with the de Gennes factor G is shown in fig. 4. In H o - Y
0
0
i
I
I
2
|
3
4
5
deGennes factor,G
Fig. 4. Change of turn angles with the de Oennes factor G. Broken curves show the case of Ho-Y alloys [6]. alloys both toi and tot approach the constant value of 50 ° with decreasing the de Gennes factor, as shown in fig. 4. On the contrary in H o - h g h t rare earth alloys toi decreases to reach tof, which is almost constant. The behaviors of T~ and toi suggest that the mean free path of conduction electrons becomes shorter and the exchange interaction has a smaller range in H o - l i g h t rare earth alloys than in H o - h e a v y rare earth alloys.
References [1] C.C. Koch, J. Less-Common Metals 22 (1970) 149. [2] S. Kawano and N. Achiwa, J. Phys. Soc. Japan 37 (1974) 569. [3] S. Kawano and N. Achiwa, J. Phys. Soc. Japan 38 (1975) 285. [4] S. Kawano and N. Achiwa, Ann. Rep. Res. React. Inst. Kyoto Univ. 13 (1980) 171. [5] W.C. Koehler, J.W. Cable, M.K. Wilkinson and E.O. Wollan, Phys. Rev. 151 (1966) 414. [6] H.R. Child, AEC-Report, ORNL-TM-1063 (1965) 48. [7] W.C. Koehler, J. Appl. Phys. 36 (1965) 1082.