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Pulsed NMR of chlorine in the triangular XY-like antiferromagnet RbFeCl3
Journal of Magnetism and Magnetic Materials 31-34 (1983) 721-722
721
PULSED NMR OF C H L O R I N E I N THE TRIANGULAR X Y - L I K E A N T I F E R R O M A G N E T RbFeCi 3 N. W A D A
Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060, Japan K. A M A Y A , S. T O K U D A
a n d T. T O M I K A W A Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan
Dynamics of the triangular XY-like antiferromagnet RbFeCI 3 is studied by the pulsed NMR of 35C1. An unusual intense echo signal is observed at zero field in the 120°-structure phase after a demagnetization procedure. The signal shows a time dependent decay and a sensitive field dependence.
In RbFeC13, with a hexagonal crystal structure, the specific heat and susceptibility [1] and neutron diffraction [2] studies show successive phase transitions characteristic of the triangular XY-like antiferromagnetic system• Below TN3 = 1.95 K, the spins lying in the c-plane (easy plane) have ferromagnetic arrangement along the c-axis and a 120°-structure in the triangular lattice of the c-plane. Whereas, because the actual anisotropy in the easy plane is expected to be fairly small in this case, e.g. the dipole-dipole interaction does not contribute to the anisotropy, the 120°-structure may easily be changed by the external field a n d / o r by some small perturbation. We studied this by means of the pulsed N M R of chlorine. In zero field at T = 1.5 K, the echo signal is observed at 17, 11 and 6 MHz whose line width is 0.5 MHz and the relaxation time T2 is 31 #s. Comparing these with the energy levels of 35C1 which is under the electric field gradient potential [3], we estimate that one-third of the 4~
T
T O0 o
C
OO
v 2
z" = / . O w e < w 1 = w2, • - 5 psec
,1
rep. rate = 50 HZ H0= 6 kOe
o,
-
Fig. 1. The rf-field H i dependence of the unusual echo intensity at T = 1.3 K. 0304-8853/83/0000-0000/$03.00
0~0 0 •
"L'= 40 psec "~2 • J~ "-"
.
Wl=W2 --" 5 psec rep. rote • =50Hz
•
H1 =15Oe
I
C J = -"P . . . . . . 2 4 H o 6 ( k O e 8)
10
Fig. 2. The echo intensity versus the starting field H o of demagnetization, at T = 1.3 K. 35C1 nuclei are under the transferred hyperfine field H, of 27 kOe acting parallel to the principal z-axis of the electric field gradient potential. This means that the magnetic moments lie along the a-axis. Besides the normal echoes mentioned above, an unusual echo is found in zero field at almost the same frequencies after the demagnetization of the external field H, in which H and the if-pulse field HI must be applied parallel to the c-plane. The signal intensity 1 versus H I and I versus the starting field H 0 of H dependences are measured at T = 1.3 K and are shown in figs. 1 and 2, respectively. The double pulse duration ~', widths of the double pulses w I and w2, and the repetition rate are also given. As shown in these figures, the unusual signal is strongly enhanced by the demagnetization process starting from H o > 2 kOe and by the strong rf-field H n >_ 13 Oe. Thus, the normal echo is negligibly small compared with this unusual signal. Just after the demagnetization, the unusual signal appears and grows exponentially in time and saturates in several seconds. (The saturated intensity is plotted in
© 1983 N o r t h - H o l l a n d
722
N. Wada et aL / N M R in the triangular antiferromagnet RbFeCI 3
•
~.
W1 =W 2
i
=5psec ]
"~ rep.rate ~4I =500Hz~ _ H1=24o ' ~L 10 t
~
a)
(sel~ ~0
Fig. 3. The logarithmic time decay of the echo intensity at T = 1.3 K.
Fig. 4. (a) The very intense echo signal after the demagnetization; (b) the enduring echo when the above double pulse is changed to the single pulse.
figs. 1 and 2.) Then, the signal begins to decrease slowly. This is shown in fig. 3 for the fast repetition rate of 500 Hz, which shows the logarithmic decay. The extraordinary echo signal is strongest just at H = 0 0 e , and decreases rapidly and disappears already when H is applied to ± 100 Oe. We note, further, the following observation. After the saturation of the signal intensity, as shown in fig. 4a, even if the repetition of the double pulse is changed to the single pulse repetition, one still finds the signal at the time • after each rf-pulse, as shown in fig. 4b. Though this signal decays slowly in time at zero field, it disappears whenever H 0 is applied and demagnetized again. This enduring signal may be explained as a kind of stimulated echo due to the long relaxation time 7"1. Thus, we observed extremely enhanced echo signal. This shows fairly weak anisotropy energy in the easy plane. The domain boundary effect may also be important. The step-like increase of the signal at HI = 13 Oe in fig. 1 may show that the perturbation of the rf-field in the easy plane easily influences the arrangement of the magnetic moments, for instance the crea-
tion of domains with H l after the single domain formation with H 0. The logarithmic time decay of the echo signal is common to the nonlinear appearances, e.g. decays of the remanent magnetization of a spin glass and the flux pinning in a superconductor. In the long range ordered state below T m having high symmetry, we observed the spin dynamics induced by the external magnetic fields Hj and H 0 in the easy plane. Detailed studies including magnetization and susceptibility measurements are now going on. The authors acknowledge Prof. T. Haseda for valuable discussions and Prof. K. Hirakawa and Mr. K. Ubukoshi for supplying the good single crystal. References
[1] T. Haseda, N. Wada, M. Hata and K. Amaya, Physica 108B (1981) 841. [2] N. Wada, K. Ubukoshi and K. Hirakawa, J. Phys. Soc. Japan 51 (1982) 2833. [3] W.B. Euler, C. Long, W.G. Moulton and B.B. Garret, J. Magn. Res. 32 (1978) 23.
721
PULSED NMR OF C H L O R I N E I N THE TRIANGULAR X Y - L I K E A N T I F E R R O M A G N E T RbFeCi 3 N. W A D A
Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060, Japan K. A M A Y A , S. T O K U D A
a n d T. T O M I K A W A Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan
Dynamics of the triangular XY-like antiferromagnet RbFeCI 3 is studied by the pulsed NMR of 35C1. An unusual intense echo signal is observed at zero field in the 120°-structure phase after a demagnetization procedure. The signal shows a time dependent decay and a sensitive field dependence.
In RbFeC13, with a hexagonal crystal structure, the specific heat and susceptibility [1] and neutron diffraction [2] studies show successive phase transitions characteristic of the triangular XY-like antiferromagnetic system• Below TN3 = 1.95 K, the spins lying in the c-plane (easy plane) have ferromagnetic arrangement along the c-axis and a 120°-structure in the triangular lattice of the c-plane. Whereas, because the actual anisotropy in the easy plane is expected to be fairly small in this case, e.g. the dipole-dipole interaction does not contribute to the anisotropy, the 120°-structure may easily be changed by the external field a n d / o r by some small perturbation. We studied this by means of the pulsed N M R of chlorine. In zero field at T = 1.5 K, the echo signal is observed at 17, 11 and 6 MHz whose line width is 0.5 MHz and the relaxation time T2 is 31 #s. Comparing these with the energy levels of 35C1 which is under the electric field gradient potential [3], we estimate that one-third of the 4~
T
T O0 o
C
OO
v 2
z" = / . O w e < w 1 = w2, • - 5 psec
,1
rep. rate = 50 HZ H0= 6 kOe
o,
-
Fig. 1. The rf-field H i dependence of the unusual echo intensity at T = 1.3 K. 0304-8853/83/0000-0000/$03.00
0~0 0 •
"L'= 40 psec "~2 • J~ "-"
.
Wl=W2 --" 5 psec rep. rote • =50Hz
•
H1 =15Oe
I
C J = -"P . . . . . . 2 4 H o 6 ( k O e 8)
10
Fig. 2. The echo intensity versus the starting field H o of demagnetization, at T = 1.3 K. 35C1 nuclei are under the transferred hyperfine field H, of 27 kOe acting parallel to the principal z-axis of the electric field gradient potential. This means that the magnetic moments lie along the a-axis. Besides the normal echoes mentioned above, an unusual echo is found in zero field at almost the same frequencies after the demagnetization of the external field H, in which H and the if-pulse field HI must be applied parallel to the c-plane. The signal intensity 1 versus H I and I versus the starting field H 0 of H dependences are measured at T = 1.3 K and are shown in figs. 1 and 2, respectively. The double pulse duration ~', widths of the double pulses w I and w2, and the repetition rate are also given. As shown in these figures, the unusual signal is strongly enhanced by the demagnetization process starting from H o > 2 kOe and by the strong rf-field H n >_ 13 Oe. Thus, the normal echo is negligibly small compared with this unusual signal. Just after the demagnetization, the unusual signal appears and grows exponentially in time and saturates in several seconds. (The saturated intensity is plotted in
© 1983 N o r t h - H o l l a n d
722
N. Wada et aL / N M R in the triangular antiferromagnet RbFeCI 3
•
~.
W1 =W 2
i
=5psec ]
"~ rep.rate ~4I =500Hz~ _ H1=24o ' ~L 10 t
~
a)
(sel~ ~0
Fig. 3. The logarithmic time decay of the echo intensity at T = 1.3 K.
Fig. 4. (a) The very intense echo signal after the demagnetization; (b) the enduring echo when the above double pulse is changed to the single pulse.
figs. 1 and 2.) Then, the signal begins to decrease slowly. This is shown in fig. 3 for the fast repetition rate of 500 Hz, which shows the logarithmic decay. The extraordinary echo signal is strongest just at H = 0 0 e , and decreases rapidly and disappears already when H is applied to ± 100 Oe. We note, further, the following observation. After the saturation of the signal intensity, as shown in fig. 4a, even if the repetition of the double pulse is changed to the single pulse repetition, one still finds the signal at the time • after each rf-pulse, as shown in fig. 4b. Though this signal decays slowly in time at zero field, it disappears whenever H 0 is applied and demagnetized again. This enduring signal may be explained as a kind of stimulated echo due to the long relaxation time 7"1. Thus, we observed extremely enhanced echo signal. This shows fairly weak anisotropy energy in the easy plane. The domain boundary effect may also be important. The step-like increase of the signal at HI = 13 Oe in fig. 1 may show that the perturbation of the rf-field in the easy plane easily influences the arrangement of the magnetic moments, for instance the crea-
tion of domains with H l after the single domain formation with H 0. The logarithmic time decay of the echo signal is common to the nonlinear appearances, e.g. decays of the remanent magnetization of a spin glass and the flux pinning in a superconductor. In the long range ordered state below T m having high symmetry, we observed the spin dynamics induced by the external magnetic fields Hj and H 0 in the easy plane. Detailed studies including magnetization and susceptibility measurements are now going on. The authors acknowledge Prof. T. Haseda for valuable discussions and Prof. K. Hirakawa and Mr. K. Ubukoshi for supplying the good single crystal. References
[1] T. Haseda, N. Wada, M. Hata and K. Amaya, Physica 108B (1981) 841. [2] N. Wada, K. Ubukoshi and K. Hirakawa, J. Phys. Soc. Japan 51 (1982) 2833. [3] W.B. Euler, C. Long, W.G. Moulton and B.B. Garret, J. Magn. Res. 32 (1978) 23.