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Optical measurement of the dynamic process of a field-induced spin reorientation in ErCrO3
Solid State Communications, Vol. 20, Pp. 123—125, 1976.
Pergamon Press.
Printed in Great Britain
OPTICAL MEASUREMENT OF THE DYNAMIC PROCESS OF A FIELD-INDUCED SPIN REORIENTATION IN ErCrO3 T. Morishita, K. Aoyagi, K. Tsushima and T. Kigawa* Broadcasting Science Research Laboratories of Nippon HOsO Kyokai, Kinuta, Setagaya-ku, Tokyo 157, Japan (Received 5 June 1976 by Y. Toyozawa) 3 + excito~ imes.dynamiThe The field-induced spin reorientation in ErCrO3 has absorption been observed reorientation proceeds in approximately cally by optical measurements of the Cr exponential form with a timeconstant of 0.6 msec and is completed within a few milliseconds. A mixed phase of an antiferromagnetic and a weak-ferromagnetic states is shown to coexist at the boundary of the first order spin reorientation. THE SPIN REORIENTATION (SR) phase transitions in a series of rare-earth orthochromites (RCrO 3) have been extensively investigated optically as well as magnetically. However, very little is known about transient phenomena of the SR. The present paper is concerned with the first observation of the SRby an optical method. Below 133 K, ErCrO3 exhibits 1 I’4 weak-ferromagnetism (F describedby Bertaut’s notation 2), where F2 refers to a canting towards the crystal c-axis. The Cr~ions undergo the SR from r4(F2) for T> 10 Kto r1(0) for T< 10K in which they lose ferromagnetic component. The crystal in the I’~(0)phase is brought to the r4(F2) phase with an external2 field than spin the critical value (the stronger field-induced reorientation). alongItthe c-axis that a group of sharp absorption lines is known around 7300 A is the R-exciton lines associated with the transition from t~~4A~ to t~g2Eg of the single Cr3~ ion and that the selection rules for these excitons are determined by the type of the spin configuration.3 Therefore, it is expected that the observation of the
.7 K
i~ F
E II a±b HO
~
C
20 k0e
1.8 k0e
~
.6 k0e 1.0
k0e
72~5 7300
X (A)
Fig. 1. Changes in optical absorption spectra of the Cr51R-lines in ErCrO 3 as a function of the external absorption spectra of the R-exciton lines as a function magnetic field H0 at 1.7 K. The electric and magnetic of time provides information about the dynamic process vectors ively. of light are represented by i~ and ii respectof the SR. 3 The absorption spectra of the R-exciton lines at was afaces (110) platelet of 0.15 x 4 x 2mm withThe the sample 4 x 2 mm2 grown by the flux-method by 1.7 K are shown in Fig. 1 as a function of d.c. magnetic using PhO and PbF 4 the as magnet the flux.toThe sample was magplaced fields. In accordance withwith the published between poles of a 2d.c. apply external absorption lines observed a magneticdata, fields less netic fields parallel to the c-axis, and was immersed in than 1 kOe are assigned to the rX line in the I’~phase liquid He. A spex 3/4 m Czemy—Turner grating spectroand the two absorption lines with a magnetic field above meter was used to obtain the monochromatic beam with 2 kOe are assigned to the A and A~in the r 4 phase. It the 1.0 A bandwidth. The polarization, H II c, E a ±b, should be noted that the three absorption lines, r~, where H and E were the magnetic and the electric vectors A, andA are observed simultaneously in an intermedof light, respectively, was employed throughout this iate range between 1 and 2 kOe. This fact is interpreted work. as an evidence of the coexistence of both phases in the crystal and it suggests that the field-induced SR of the * On leave from Faculty of Science, Science University r1 (0) to T4(F2) type should be the first-order phase of Tokyo, Japan. transition. It also suggests the existence of domains •
.
,
123
124
FIELD-INDUCED SPIN REORIENTATION IN ErCrO3 1.0
Vol. 20, No.2
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4,
oAg
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UJ
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or’~ .7
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A
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K
1.7
I; Hb
II C iiii a±b 1.1 k0e
K
\
0
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IIIla±b
E
0
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~ _____________
~
0.1 ms -.
r~~i [~5O0e
Fig.the T~ lines 2. mixed 0Normalized as a function ofI theHo(koe) external intensities magnetic 2 of thefie!d A and H0 in phaseabsorption region. PULSE GENERATOR
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~.
PULSE COIL ~JCRYOSTAT
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Fig 4 Optical absorption spectra measured (a) without a pulsed magnetic field, and (b) and (c) with a pulsed field of lSOOe having a duration of 0.1 and LOmsec, respectively. A bias field Hb = 1.1 kOe is applied in all cases. Hd.C.(k0e)
1.7 K Hdc.(k0e) HIlc, Ellaib
BOXCAR INTEGRATOR X-Y RECORDER
I0
Fig. 3. Schematic diagram of the system used to observe dynamic processes. similar to the spin—flop domains in MnF2 In such a domain structure, spontaneous magnetizations differ from each other not only in the direction, but also in the absolute magnitude. Normalized intensities of the A and the r’: lines areFig. plotted as proportion a function of in 2. The of the F external magnetic field 1 (0) and F4(F~)in the crystal can be estimated from the normalized intensities; the intensities are determined from the height of the absorption peaks, since the half-width of the absorption lines appears unchanged over the range of the applied field. No correction for the demagnetizing field is made in Fig. 2. At the beginning of the SR, the demagnetizing field is negligible, therefore, we can directly determine the transition field from Fig. 2 to be 6 0.9 kOe at 1.7 K in agreement withfor an observations earlier work. of the dynamic proExperiments cess were based on techniques of time-resolved spectro.~
1SJ~
fL~2~Oe (a )
(b)
Fig. 5. Transient variations of the A~absorption intensity due to a The pulsed fieldfield of 400 Oeparallel having aand duration of 1.1 msec. pulsed is (a) (b) antiparallel to the bias fields. scopy and the magnetic field-modulation method. A schematic diagram of theturns) system is shown to in aFig. 3. The pulse forming coil (250 is attached sample bed, the direction of which agrees with that of the d.c. magnet. A trigger synchronized with the pulse was
Vol. 20, No.2
FIELD-INDUCED SPIN REORIENTATION IN ErCrO3
divided into two branches, one to a monitor for the pulse waveform and the other to a boxcar integrator, The dynamic processes were observed in two ways. First, the boxcar was operated with a fixed delay of the gate for a photomultiplier, and the spectrometer was scanned in wavelength. Some results of time-resolved absorption spectra are given in Fig. 4. These were obtamed (a) without a pulsed field, and (b) and (c) with the pulsed field of 150 Oe, the duration of which are 0.1 and l.Omsec, respectively. A d.c. magnetic field of 1.1 kOe was always superimposed on the pulse as a bias. In all cases, the gate was opened for 1 j.msec at the tail of the pulse. It is clear from Fig. 4(a) and (b) that the SR can not respond to the shorter pulsed field. Increasing the duration from 0.1 to 1.0 msec, we can see a growth in the intensity of the A line (c). Secondly, the spectrometer was fixed at the wavelength of the A; line and the delay of the gate (duration 10 ~sec) was scanned from 0 to 2 msec. The results give direct recordings of changes in the absorption intensities as a function of time. The SR was triggered with a pulse of 400 Oe (duration 1.1 msec, rise time 0.2 msec) superimposed on various bias fields of more than 1.0 kOe. The pulsed field was applied either parallel or antiparallel to the bias. The absorption vs time traces are shown in
125
Fig. 5. The ordinate means the change of the intensity of the A line. In the case of a parallel pulse (a), the traces have characteristic shape so long as the bias field is less than 1.6 kOe; that is, the increasing curve consists of two exponential parts with different time-constants. The former one, in which an initial growth of the F4(F2) nuclei may take place, continues for several tenths of millisecond with a time-constant about 0.6 msec. The latter, which may involve the fragmentation and the rearrangement to an equilibrium of F4(F2) domains, has a time-constant longer than 1 msec. We cannot find the former process, if we increase a bias field above 1.8 kOe. In the case of an antiparallel pulse (b), the reversed SR from F4(F2) to F1 (0) shows a simple exponential decrease with a time-constant similar to that of the first part in the parallel case. It is likely that the domains of the [‘4(F2) phase shrink homogeneously over the crystal. Further work is in progress to make a real-time observation on this crystal with an improved pulsed field. Acknowledgement — The authors thank Dr. Washimiya for reading the manuscript.
REFERENCES 1.
BERTAUT E.F., Magnetism III (Edited by RADO G.T. & SUHL H.), p. 149. Academic Press, New York (1963).
2.
EIBSCHUTZ M., HOLMES L, MAlTA J.P. & VAN UITERT LG., Solid State Commun. 8, 1815 (1970). SUGANO S., AOYAGI K. & TSUSHIMA K.,J. Phys. Soc. Japan 31, 706 (1971).
3. 4. 5.
MELTZER R.S., Phys. Rev. B2, 2398 (1970). KING A.R. & PAQUETTE D., Phys. Rev. Lett. 30, 662 (1973).
6.
COURTHS R., HUFNER S., PELZL J. & VAN UITERT L.G., Z. Phys. 249, 445 (1972).
Pergamon Press.
Printed in Great Britain
OPTICAL MEASUREMENT OF THE DYNAMIC PROCESS OF A FIELD-INDUCED SPIN REORIENTATION IN ErCrO3 T. Morishita, K. Aoyagi, K. Tsushima and T. Kigawa* Broadcasting Science Research Laboratories of Nippon HOsO Kyokai, Kinuta, Setagaya-ku, Tokyo 157, Japan (Received 5 June 1976 by Y. Toyozawa) 3 + excito~ imes.dynamiThe The field-induced spin reorientation in ErCrO3 has absorption been observed reorientation proceeds in approximately cally by optical measurements of the Cr exponential form with a timeconstant of 0.6 msec and is completed within a few milliseconds. A mixed phase of an antiferromagnetic and a weak-ferromagnetic states is shown to coexist at the boundary of the first order spin reorientation. THE SPIN REORIENTATION (SR) phase transitions in a series of rare-earth orthochromites (RCrO 3) have been extensively investigated optically as well as magnetically. However, very little is known about transient phenomena of the SR. The present paper is concerned with the first observation of the SRby an optical method. Below 133 K, ErCrO3 exhibits 1 I’4 weak-ferromagnetism (F describedby Bertaut’s notation 2), where F2 refers to a canting towards the crystal c-axis. The Cr~ions undergo the SR from r4(F2) for T> 10 Kto r1(0) for T< 10K in which they lose ferromagnetic component. The crystal in the I’~(0)phase is brought to the r4(F2) phase with an external2 field than spin the critical value (the stronger field-induced reorientation). alongItthe c-axis that a group of sharp absorption lines is known around 7300 A is the R-exciton lines associated with the transition from t~~4A~ to t~g2Eg of the single Cr3~ ion and that the selection rules for these excitons are determined by the type of the spin configuration.3 Therefore, it is expected that the observation of the
.7 K
i~ F
E II a±b HO
~
C
20 k0e
1.8 k0e
~
.6 k0e 1.0
k0e
72~5 7300
X (A)
Fig. 1. Changes in optical absorption spectra of the Cr51R-lines in ErCrO 3 as a function of the external absorption spectra of the R-exciton lines as a function magnetic field H0 at 1.7 K. The electric and magnetic of time provides information about the dynamic process vectors ively. of light are represented by i~ and ii respectof the SR. 3 The absorption spectra of the R-exciton lines at was afaces (110) platelet of 0.15 x 4 x 2mm withThe the sample 4 x 2 mm2 grown by the flux-method by 1.7 K are shown in Fig. 1 as a function of d.c. magnetic using PhO and PbF 4 the as magnet the flux.toThe sample was magplaced fields. In accordance withwith the published between poles of a 2d.c. apply external absorption lines observed a magneticdata, fields less netic fields parallel to the c-axis, and was immersed in than 1 kOe are assigned to the rX line in the I’~phase liquid He. A spex 3/4 m Czemy—Turner grating spectroand the two absorption lines with a magnetic field above meter was used to obtain the monochromatic beam with 2 kOe are assigned to the A and A~in the r 4 phase. It the 1.0 A bandwidth. The polarization, H II c, E a ±b, should be noted that the three absorption lines, r~, where H and E were the magnetic and the electric vectors A, andA are observed simultaneously in an intermedof light, respectively, was employed throughout this iate range between 1 and 2 kOe. This fact is interpreted work. as an evidence of the coexistence of both phases in the crystal and it suggests that the field-induced SR of the * On leave from Faculty of Science, Science University r1 (0) to T4(F2) type should be the first-order phase of Tokyo, Japan. transition. It also suggests the existence of domains •
.
,
123
124
FIELD-INDUCED SPIN REORIENTATION IN ErCrO3 1.0
Vol. 20, No.2
o~o—.o
>I-
4,
oAg
/
UJ
H zO.5
I
or’~ .7
D
A
(a)
K
1.7
I; Hb
II C iiii a±b 1.1 k0e
K
\
0
z
IIIla±b
E
0
Q.—
~ _____________
~
0.1 ms -.
r~~i [~5O0e
Fig.the T~ lines 2. mixed 0Normalized as a function ofI theHo(koe) external intensities magnetic 2 of thefie!d A and H0 in phaseabsorption region. PULSE GENERATOR
0 a-
Q—~E~~~TER
~_~~tjSAM
PLE
AMPLIFIER L
I ms us [us50 Oe I
7295
~.
PULSE COIL ~JCRYOSTAT
R-~OTO;LLOS~OPE~5QAMPLIF::LTI~IER
7300
Fig 4 Optical absorption spectra measured (a) without a pulsed magnetic field, and (b) and (c) with a pulsed field of lSOOe having a duration of 0.1 and LOmsec, respectively. A bias field Hb = 1.1 kOe is applied in all cases. Hd.C.(k0e)
1.7 K Hdc.(k0e) HIlc, Ellaib
BOXCAR INTEGRATOR X-Y RECORDER
I0
Fig. 3. Schematic diagram of the system used to observe dynamic processes. similar to the spin—flop domains in MnF2 In such a domain structure, spontaneous magnetizations differ from each other not only in the direction, but also in the absolute magnitude. Normalized intensities of the A and the r’: lines areFig. plotted as proportion a function of in 2. The of the F external magnetic field 1 (0) and F4(F~)in the crystal can be estimated from the normalized intensities; the intensities are determined from the height of the absorption peaks, since the half-width of the absorption lines appears unchanged over the range of the applied field. No correction for the demagnetizing field is made in Fig. 2. At the beginning of the SR, the demagnetizing field is negligible, therefore, we can directly determine the transition field from Fig. 2 to be 6 0.9 kOe at 1.7 K in agreement withfor an observations earlier work. of the dynamic proExperiments cess were based on techniques of time-resolved spectro.~
1SJ~
fL~2~Oe (a )
(b)
Fig. 5. Transient variations of the A~absorption intensity due to a The pulsed fieldfield of 400 Oeparallel having aand duration of 1.1 msec. pulsed is (a) (b) antiparallel to the bias fields. scopy and the magnetic field-modulation method. A schematic diagram of theturns) system is shown to in aFig. 3. The pulse forming coil (250 is attached sample bed, the direction of which agrees with that of the d.c. magnet. A trigger synchronized with the pulse was
Vol. 20, No.2
FIELD-INDUCED SPIN REORIENTATION IN ErCrO3
divided into two branches, one to a monitor for the pulse waveform and the other to a boxcar integrator, The dynamic processes were observed in two ways. First, the boxcar was operated with a fixed delay of the gate for a photomultiplier, and the spectrometer was scanned in wavelength. Some results of time-resolved absorption spectra are given in Fig. 4. These were obtamed (a) without a pulsed field, and (b) and (c) with the pulsed field of 150 Oe, the duration of which are 0.1 and l.Omsec, respectively. A d.c. magnetic field of 1.1 kOe was always superimposed on the pulse as a bias. In all cases, the gate was opened for 1 j.msec at the tail of the pulse. It is clear from Fig. 4(a) and (b) that the SR can not respond to the shorter pulsed field. Increasing the duration from 0.1 to 1.0 msec, we can see a growth in the intensity of the A line (c). Secondly, the spectrometer was fixed at the wavelength of the A; line and the delay of the gate (duration 10 ~sec) was scanned from 0 to 2 msec. The results give direct recordings of changes in the absorption intensities as a function of time. The SR was triggered with a pulse of 400 Oe (duration 1.1 msec, rise time 0.2 msec) superimposed on various bias fields of more than 1.0 kOe. The pulsed field was applied either parallel or antiparallel to the bias. The absorption vs time traces are shown in
125
Fig. 5. The ordinate means the change of the intensity of the A line. In the case of a parallel pulse (a), the traces have characteristic shape so long as the bias field is less than 1.6 kOe; that is, the increasing curve consists of two exponential parts with different time-constants. The former one, in which an initial growth of the F4(F2) nuclei may take place, continues for several tenths of millisecond with a time-constant about 0.6 msec. The latter, which may involve the fragmentation and the rearrangement to an equilibrium of F4(F2) domains, has a time-constant longer than 1 msec. We cannot find the former process, if we increase a bias field above 1.8 kOe. In the case of an antiparallel pulse (b), the reversed SR from F4(F2) to F1 (0) shows a simple exponential decrease with a time-constant similar to that of the first part in the parallel case. It is likely that the domains of the [‘4(F2) phase shrink homogeneously over the crystal. Further work is in progress to make a real-time observation on this crystal with an improved pulsed field. Acknowledgement — The authors thank Dr. Washimiya for reading the manuscript.
REFERENCES 1.
BERTAUT E.F., Magnetism III (Edited by RADO G.T. & SUHL H.), p. 149. Academic Press, New York (1963).
2.
EIBSCHUTZ M., HOLMES L, MAlTA J.P. & VAN UITERT LG., Solid State Commun. 8, 1815 (1970). SUGANO S., AOYAGI K. & TSUSHIMA K.,J. Phys. Soc. Japan 31, 706 (1971).
3. 4. 5.
MELTZER R.S., Phys. Rev. B2, 2398 (1970). KING A.R. & PAQUETTE D., Phys. Rev. Lett. 30, 662 (1973).
6.
COURTHS R., HUFNER S., PELZL J. & VAN UITERT L.G., Z. Phys. 249, 445 (1972).