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Microwave ultrasonic measurement of nuclear spin diffusion
Journal of Magnetism North-Holland
and Magnetic
Microwave
Materials
104-107
ultrasonic
(1992) 957-958
measurement
of nuclear
spin diffusion
J.F. Gregg and J.S. Lord Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK A novel magnetic resonance technique which uses spatially described. In addition we present relaxation measurements refinements of the same technique. Nuclear magnetic resonance may be observed ultrasonically by feeding in radiofrequency power at the Larmor frequency in the form of an acoustic wave and observing the phonon-nuclear coupling by its effect on the ultrasonic attenuation. The sensitivity of acoustic NMR increases as the fourth power of the frequency, so, to demonstrate this technique we have chosen the nucleus lb5Ho in holmium vanadate (HoVO,). This is a so-called enhanced nucleus and resonates in the low microwave frequency regime for easily achievable laboratory magnetic fields. This system has been extensively studied [l]. Since the experiments are conducted in the microwave frequency range in fields of order 1 T, we use thin film ultrasonic transducers to generate and detect the acoustic waves. These are grown in situ on the samples studied. The samples are insulating single crystals with good mechanical properties and they are cut and polished to give two optically flat parallel faces onto one of which the transducer is sited. The tran%ducers are grown by depositing firstly a 1000-2000 A layer of fast evaporated gold (with a chrome flash underlayer to improve adhesion), followed by a layer of zinc oxide. The latter varies in thickness depending on the desired transducer operating frequency but is generally of order 0.5 pm thick for operation between 1 and 2 GHz. The zinc oxide film is grown by rf sputtering in a 50/50% argon/oxygen atmosphere at a pressure of lo-’ Torr and is piezoelectrically active. The transducer position is defined by a gold topdot of diameter 300 km evaporated on top of the zinc oxide film. This transducer launches a longitudinal ultrasonic pulse into the crystal which reflects off the opposite polished face and returns to the transducer where part of the acoustic energy backconverts to electrical signal and the remainder performs another round trip across the crystal. The system used is tetragonal and the ultrasonics are injected parallel to the crystalline u-axis. When the holmium nuclei are resonant with the ultrasonic pulse, acoustic energy is absorbed by the spins and this is detected by observing the corresponding increase in the ultrasonic attenuation. Fig. 1 shows ultrasonic echo height as a function of magnetic field 0312-8853/92/$05.00
0 1992 - El sevier Science
Publishers
selective spin excitation to detect nuclear and ultrasonic holeburning data which
spin diffusion is are afforded by
for a concentrated sample at 0.52 K. The resonance consists of seven unresolved hyperfine lines, the structure being just discernible on the high field side of the resonance. This series of experiments was performed using two successive pulses, a saturating pulse and a probe pulse of much lower power which may be of different frequency. Fig. 2 shows a typical relaxation curve for the concentrated spin system which was obtained by saturating and probing at the same frequency. Fig. 3 shows the variation with temperature of the relaxation time. Measurement by saturating and measuring at different points in the resonance lineshape afforded the same values of T1. It is noticable that the temperature dependence of the relaxation follows an unusual powerlaw which is close to T-“. The exponential fit to the relaxation curves is highly satisfactory over most of the curve as seen in fig. 2. However, closer inspection of the initial relaxation behaviour shows a rapid fall in signal on a timescale much faster than the measured relaxation time. It is interpreted as being indicative of spin diffusion. When the ultrasonic excitation pulse hits the opposite face of the crystal and turns back on itself, a standing wave pattern is generated, and within this region the magnetic spins are progressively less saturated as one moves
.3
.35
.4
.45
.5
Flald
/
.ss
.6
.65
tssla
Fig. 1. Ulkasonic echo height as a function of magnetic field for a 100 longitudinal acoustic wave in HoVO,. The temperature is 1.38 K, frequency 800 MHz.
B.V. All rights
reserved
J. F. Gregg. J.S. Lord / hkasuremmt
958
I
0
Fig. tion and The
r--m -‘Ins
2. Recovery of the ultrasonic absorption following saturaof the transition. The later decay is exponentially fitted corresponds to a spin lattice relaxation time of 108 mS. initial transient is attributed to spin diffusion. T = 0.87 K, frequency 810 MHz, B = 0.622 T parallel to (110).
from pending relation
the
ultrasonic
of’nuclear spin diffusion
antinodes
to the
upon the ultrasonic power to the peak power required
nodes.
Thus,
employed
and
:.45
1.50 Frequency (GHZ)
1.55
/.
Fig. 4. The hyperfine multiplet in dilute HoVO, before and after burning an acoustic hole at line-centre. Note the appearance of two antiholes either side corresponding to the two adjacent hyperfine transitions.
deits
for saturation, the spin polarisation is incompletely reduced to zero and the system consists of alternate regions of saturated and unsaturated nuclei whose separation is half the ultrasonic wavelength. This inhomogeneous polarisation pattern collapses on a timescale determined by the spatial frequency of the pattern and the spin diffusion rate. Experimentally this decay time is of the order of 4 mS in good agreement with the calculated value. To verify further this interpretation, similar experiments were performed on a specimen of dilute (2%) HoVO,. As expected the spin diffusion phenomenon was absent since the diffusion rate is now very much smaller. The reduced spin-spin interaction also had
i
Fig. 5. The progressive pulse reshaping which is observed in dilute HoVO, for increasing acoustic input power. The frequency used was 1525 MHz and the temperature was 0.52 K.
the consequence that the hyperfinc multiplet took longer to reach a common temperature after saturation at a single frequency and it was found possible to measure the widths of the hyperfine components by holeburning as shown in fig. 4. Moreover, as seen in fig. 5 there is considerable evidence of acoustic pulse reshaping in the dilute material, but unfortunately we were unable to detect any sign of pulse doubling and hence were unable to demonstrate unambiguously the occurrence of self-induced transparency as opposed to dispersion. The authors wish to thank Erwin Hahn, Maurice Goldman, Jacques-Francois Jacquinot and Claude Fermon for many helpful discussions. Reference Fig. 3. The variation of nuclear spin-lattice relaxation time with temperature. The curve follows a power law Tm4.7*0.3.
(11 Bleaney
et al. Proc. R. Sot. London
Ser. A 416 (1988) 93.
and Magnetic
Microwave
Materials
104-107
ultrasonic
(1992) 957-958
measurement
of nuclear
spin diffusion
J.F. Gregg and J.S. Lord Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK A novel magnetic resonance technique which uses spatially described. In addition we present relaxation measurements refinements of the same technique. Nuclear magnetic resonance may be observed ultrasonically by feeding in radiofrequency power at the Larmor frequency in the form of an acoustic wave and observing the phonon-nuclear coupling by its effect on the ultrasonic attenuation. The sensitivity of acoustic NMR increases as the fourth power of the frequency, so, to demonstrate this technique we have chosen the nucleus lb5Ho in holmium vanadate (HoVO,). This is a so-called enhanced nucleus and resonates in the low microwave frequency regime for easily achievable laboratory magnetic fields. This system has been extensively studied [l]. Since the experiments are conducted in the microwave frequency range in fields of order 1 T, we use thin film ultrasonic transducers to generate and detect the acoustic waves. These are grown in situ on the samples studied. The samples are insulating single crystals with good mechanical properties and they are cut and polished to give two optically flat parallel faces onto one of which the transducer is sited. The tran%ducers are grown by depositing firstly a 1000-2000 A layer of fast evaporated gold (with a chrome flash underlayer to improve adhesion), followed by a layer of zinc oxide. The latter varies in thickness depending on the desired transducer operating frequency but is generally of order 0.5 pm thick for operation between 1 and 2 GHz. The zinc oxide film is grown by rf sputtering in a 50/50% argon/oxygen atmosphere at a pressure of lo-’ Torr and is piezoelectrically active. The transducer position is defined by a gold topdot of diameter 300 km evaporated on top of the zinc oxide film. This transducer launches a longitudinal ultrasonic pulse into the crystal which reflects off the opposite polished face and returns to the transducer where part of the acoustic energy backconverts to electrical signal and the remainder performs another round trip across the crystal. The system used is tetragonal and the ultrasonics are injected parallel to the crystalline u-axis. When the holmium nuclei are resonant with the ultrasonic pulse, acoustic energy is absorbed by the spins and this is detected by observing the corresponding increase in the ultrasonic attenuation. Fig. 1 shows ultrasonic echo height as a function of magnetic field 0312-8853/92/$05.00
0 1992 - El sevier Science
Publishers
selective spin excitation to detect nuclear and ultrasonic holeburning data which
spin diffusion is are afforded by
for a concentrated sample at 0.52 K. The resonance consists of seven unresolved hyperfine lines, the structure being just discernible on the high field side of the resonance. This series of experiments was performed using two successive pulses, a saturating pulse and a probe pulse of much lower power which may be of different frequency. Fig. 2 shows a typical relaxation curve for the concentrated spin system which was obtained by saturating and probing at the same frequency. Fig. 3 shows the variation with temperature of the relaxation time. Measurement by saturating and measuring at different points in the resonance lineshape afforded the same values of T1. It is noticable that the temperature dependence of the relaxation follows an unusual powerlaw which is close to T-“. The exponential fit to the relaxation curves is highly satisfactory over most of the curve as seen in fig. 2. However, closer inspection of the initial relaxation behaviour shows a rapid fall in signal on a timescale much faster than the measured relaxation time. It is interpreted as being indicative of spin diffusion. When the ultrasonic excitation pulse hits the opposite face of the crystal and turns back on itself, a standing wave pattern is generated, and within this region the magnetic spins are progressively less saturated as one moves
.3
.35
.4
.45
.5
Flald
/
.ss
.6
.65
tssla
Fig. 1. Ulkasonic echo height as a function of magnetic field for a 100 longitudinal acoustic wave in HoVO,. The temperature is 1.38 K, frequency 800 MHz.
B.V. All rights
reserved
J. F. Gregg. J.S. Lord / hkasuremmt
958
I
0
Fig. tion and The
r--m -‘Ins
2. Recovery of the ultrasonic absorption following saturaof the transition. The later decay is exponentially fitted corresponds to a spin lattice relaxation time of 108 mS. initial transient is attributed to spin diffusion. T = 0.87 K, frequency 810 MHz, B = 0.622 T parallel to (110).
from pending relation
the
ultrasonic
of’nuclear spin diffusion
antinodes
to the
upon the ultrasonic power to the peak power required
nodes.
Thus,
employed
and
:.45
1.50 Frequency (GHZ)
1.55
/.
Fig. 4. The hyperfine multiplet in dilute HoVO, before and after burning an acoustic hole at line-centre. Note the appearance of two antiholes either side corresponding to the two adjacent hyperfine transitions.
deits
for saturation, the spin polarisation is incompletely reduced to zero and the system consists of alternate regions of saturated and unsaturated nuclei whose separation is half the ultrasonic wavelength. This inhomogeneous polarisation pattern collapses on a timescale determined by the spatial frequency of the pattern and the spin diffusion rate. Experimentally this decay time is of the order of 4 mS in good agreement with the calculated value. To verify further this interpretation, similar experiments were performed on a specimen of dilute (2%) HoVO,. As expected the spin diffusion phenomenon was absent since the diffusion rate is now very much smaller. The reduced spin-spin interaction also had
i
Fig. 5. The progressive pulse reshaping which is observed in dilute HoVO, for increasing acoustic input power. The frequency used was 1525 MHz and the temperature was 0.52 K.
the consequence that the hyperfinc multiplet took longer to reach a common temperature after saturation at a single frequency and it was found possible to measure the widths of the hyperfine components by holeburning as shown in fig. 4. Moreover, as seen in fig. 5 there is considerable evidence of acoustic pulse reshaping in the dilute material, but unfortunately we were unable to detect any sign of pulse doubling and hence were unable to demonstrate unambiguously the occurrence of self-induced transparency as opposed to dispersion. The authors wish to thank Erwin Hahn, Maurice Goldman, Jacques-Francois Jacquinot and Claude Fermon for many helpful discussions. Reference Fig. 3. The variation of nuclear spin-lattice relaxation time with temperature. The curve follows a power law Tm4.7*0.3.
(11 Bleaney
et al. Proc. R. Sot. London
Ser. A 416 (1988) 93.