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Magnetization reorientation in DyIG at low temperature
Journal of Magnetism and Magnetic Materials 31-34 (1983) 811-812
811
M A G N E T I Z A T I O N R E O R I E N T A T I O N IN DyIG AT L O W T E M P E R A T U R E G. A U B E R T a n d B. M I C H E L U T T I Laboratoire Louis N~el, CNRS-USMG, B.P. 166X, 38042 Grenoble-C~dex, France
Torque measurements demonstrate that a change in the easy axis of magnetization occurs in DylG at about 16 K. Below 14 K three equivalent easy directions surrou:nd every (l I 1) direction (high temperature easy one). These results question the generally acknowledged magnetic structure of DylG at low temperature.
1. Experiment
[011]
Torque measurements were performed in the (110) plane of two different single crystal spheres of DylG between liquid helium and room temperature in applied magnetic fields up to 20 T. Above 80 K, the measurements were made with a conventional highly accurate torquemeter [1] in applied fields up to 2 T, high enough to make the sample single domain. We do no t give the detailed analysis of these
[111]
l
5d
/ • 35
~
[100]
z
~__2_----~ . j 5
10
T[K]
•
: I
i
i
,, 15
Fig. 2. Variation with temperature of the extra zeros of the
torque curveS.
B
14
c
15
o18
B,=I.gT
0
i
20 °
measurements following the methods of ref. [1] here, but simply mention that the first cubic anisotropy constant K~ is negative and that (111) is the easy direction of magnetization as is usual in ferrimagnetic garnets. At lower temperatures the magnetic anisotropy is larger and 2 T can no longer achieve the saturation of the magnetization of the sample. A new torquemeter [2] was specially designed to use a Bitter magnet (20 T) of the SNCI. Typical torque curves in the (011) plane are shown in fig. 1 where F is the torque measured along [011] and ~ the angle of the applied field B0 with the [100] direction. Below 20 K the torque is almost proportional to the applied field while the angular positions of the zeros are field independent. The transition between the low temperature behavior and the high temperature one takes place within a few kelvins and the evolution of the two extra zeros (on both sides of [ 111]) of the low temperatures torque curves is shown in fig, 2. The region between the dotted lines corresponds to a peculiar behavior with only one extra zero while the torque for [!11] is surprisingly no longer zero. 2. D i s c u s s i o n
Fi~. 1. Evolution with temperature of the torque curves in the (011) plane of a single crystal sphere of DylG.
Below 20 K, the dependence of the torque on the applied magnetic field clearly demonstrates that the
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
812
G. .4ubert, B. Michelutti / Magnetization reorientation in DylG
sample has a very large magnetic anisotropy and that it is never single domain (except for some specific directions of the applied field) in the considered range of applied fields ( < 20 T). Let 01 and 02 be the angular separations of the two zeros on both sides of [1 i 1] respectively. We notice that the relation tan 02 -- ½ tan 01
(1)
is fairly well obeyed up to 14 K where the peculiar behavior begins to appear. Relation (1) means that in every (110) plane there are two easy directions of magnetization located between the (111) and (100) axes of each (110) plane, 01 apart from (111). So, at low temperature, ~ 111) is not the easy direction of magnetization but an intermediate one, while there are three equivalent easy direction around ~111), 01 apart from it in the three (110) planes intersecting along that (111) axis. If the magnetic anisotropy energy is much larger than the Zeeman term, the sample is divided in magnetic domains, the magnetization vectors of which are along these easy directions and the torque curves result from the variation of the respective volumes of these domains with the direction of the applied field. When the applied field is along [111] the sample is equally divided into three types of domains with magnetization vectors along the three adjacent easy directions and the torque is zero. If we turn the field towards [100] in the (011) plane we get the zero torque at 01 from [111 ], and for this particular direction the sample is single domain. But if we turn the field towards [011] we do not
meet an easy axis and the torque becomes zero 02 apart from [l 1 l], direction which corresponds to the projection on the (011) plane of the two other directions, hence the relation (1). In this situation the sample is equally divided into two types of domains, the magnetization vectors of which are along these two easy directions. This analysis can of course be given a quantitative form which will be published elsewhere. The peculiar behavior near 15 K is not clearly understood for the moment. 3. Conclusions
These torque measurements dearly demonstrate that, below 14 K, the ~111~ direction is not the easy axis of magnetization for DyIG. Each ~ 111 ~ axis becomes an intermediate direction of magnetization surrounded by three equivalent new ones. These results question the generally acknowledged magnetic structure of D y I G at low temperature which was obtained from neutron diffraction experiments assuming a macroscopic magnetization along ~ 111 ~. Moreover they constitute a valuable background for developing a reliable microscopic model of the magnetic anisotropy of DyIG. References
[1] G. Aubert, J. Magn. Magn. Mat. 19 (1980) 396. [2] To be published.
811
M A G N E T I Z A T I O N R E O R I E N T A T I O N IN DyIG AT L O W T E M P E R A T U R E G. A U B E R T a n d B. M I C H E L U T T I Laboratoire Louis N~el, CNRS-USMG, B.P. 166X, 38042 Grenoble-C~dex, France
Torque measurements demonstrate that a change in the easy axis of magnetization occurs in DylG at about 16 K. Below 14 K three equivalent easy directions surrou:nd every (l I 1) direction (high temperature easy one). These results question the generally acknowledged magnetic structure of DylG at low temperature.
1. Experiment
[011]
Torque measurements were performed in the (110) plane of two different single crystal spheres of DylG between liquid helium and room temperature in applied magnetic fields up to 20 T. Above 80 K, the measurements were made with a conventional highly accurate torquemeter [1] in applied fields up to 2 T, high enough to make the sample single domain. We do no t give the detailed analysis of these
[111]
l
5d
/ • 35
~
[100]
z
~__2_----~ . j 5
10
T[K]
•
: I
i
i
,, 15
Fig. 2. Variation with temperature of the extra zeros of the
torque curveS.
B
14
c
15
o18
B,=I.gT
0
i
20 °
measurements following the methods of ref. [1] here, but simply mention that the first cubic anisotropy constant K~ is negative and that (111) is the easy direction of magnetization as is usual in ferrimagnetic garnets. At lower temperatures the magnetic anisotropy is larger and 2 T can no longer achieve the saturation of the magnetization of the sample. A new torquemeter [2] was specially designed to use a Bitter magnet (20 T) of the SNCI. Typical torque curves in the (011) plane are shown in fig. 1 where F is the torque measured along [011] and ~ the angle of the applied field B0 with the [100] direction. Below 20 K the torque is almost proportional to the applied field while the angular positions of the zeros are field independent. The transition between the low temperature behavior and the high temperature one takes place within a few kelvins and the evolution of the two extra zeros (on both sides of [ 111]) of the low temperatures torque curves is shown in fig, 2. The region between the dotted lines corresponds to a peculiar behavior with only one extra zero while the torque for [!11] is surprisingly no longer zero. 2. D i s c u s s i o n
Fi~. 1. Evolution with temperature of the torque curves in the (011) plane of a single crystal sphere of DylG.
Below 20 K, the dependence of the torque on the applied magnetic field clearly demonstrates that the
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
812
G. .4ubert, B. Michelutti / Magnetization reorientation in DylG
sample has a very large magnetic anisotropy and that it is never single domain (except for some specific directions of the applied field) in the considered range of applied fields ( < 20 T). Let 01 and 02 be the angular separations of the two zeros on both sides of [1 i 1] respectively. We notice that the relation tan 02 -- ½ tan 01
(1)
is fairly well obeyed up to 14 K where the peculiar behavior begins to appear. Relation (1) means that in every (110) plane there are two easy directions of magnetization located between the (111) and (100) axes of each (110) plane, 01 apart from (111). So, at low temperature, ~ 111) is not the easy direction of magnetization but an intermediate one, while there are three equivalent easy direction around ~111), 01 apart from it in the three (110) planes intersecting along that (111) axis. If the magnetic anisotropy energy is much larger than the Zeeman term, the sample is divided in magnetic domains, the magnetization vectors of which are along these easy directions and the torque curves result from the variation of the respective volumes of these domains with the direction of the applied field. When the applied field is along [111] the sample is equally divided into three types of domains with magnetization vectors along the three adjacent easy directions and the torque is zero. If we turn the field towards [100] in the (011) plane we get the zero torque at 01 from [111 ], and for this particular direction the sample is single domain. But if we turn the field towards [011] we do not
meet an easy axis and the torque becomes zero 02 apart from [l 1 l], direction which corresponds to the projection on the (011) plane of the two other directions, hence the relation (1). In this situation the sample is equally divided into two types of domains, the magnetization vectors of which are along these two easy directions. This analysis can of course be given a quantitative form which will be published elsewhere. The peculiar behavior near 15 K is not clearly understood for the moment. 3. Conclusions
These torque measurements dearly demonstrate that, below 14 K, the ~111~ direction is not the easy axis of magnetization for DyIG. Each ~ 111 ~ axis becomes an intermediate direction of magnetization surrounded by three equivalent new ones. These results question the generally acknowledged magnetic structure of D y I G at low temperature which was obtained from neutron diffraction experiments assuming a macroscopic magnetization along ~ 111 ~. Moreover they constitute a valuable background for developing a reliable microscopic model of the magnetic anisotropy of DyIG. References
[1] G. Aubert, J. Magn. Magn. Mat. 19 (1980) 396. [2] To be published.