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Optically-excited magnetostrictive waves in ferromagnetic materials
Solid State Communications, Vol. 9, pp. 2 119—2121, 1971. Pergamon Press.
Printed in Great Britain
OPTICALLY-EXCITED MAGNETOSTRICTIVE WAVES IN FERROMAGNETIC MATERIALS K. Kubota Faculty of Engineering Science, Osaka University, Toyonaka, Japan (Received 16 September 1971 by Y. Toyozawa)
The radiation pressure of an optical beam excites elastic waves in various opaque materials, which causes a fractional change of magnetization in ferromagnetic substances through magnetostriction. Ferromagnets are divided broadly into two categories according to their magnetic behaviour shown in a pickup coil. This is considered due to magnetic anisotropy.
THIS PAPER reports the generation and the detection of magnetostrictive waves excited by an optional pulse. When a ferromagnetic material is irradiated by a laser beam, output voltage is obtamed by a pickup coil around the sample. This effect is related to a phenomenon that the radiation pressure of intense pulse light excites elastic waves in an opaque material.’ A dynamical property of magnetization is studied by this simple method.
—
A light beam (2mm in dia.) with a 50-nsec pulse half-width and a power of 6MW/cm2 is obtamed from a Q-switched Nd-glass laser. Samples studied are Ni, Fe, Co, Mumetal and ferromagnetic YIG. They are almost polycrystals except for one of the YIG samples, and are lisk- or rod-shaped about 3mm in diameter and 0.3 2mm length. The ends are polished optically flat or are left ground. The fractional rate of change of magnetization is detected by a pickup coil around the sample which has 6 20 turns and a coil length of 1 3 mm. Light is applied to one end of the sample with its propagation direction parallel to the coil axis. When studying an external-magnetic-field effect, the coil axis is perpendicular to a magnetic field. The output voltage from the coil is amplified and is displayed on a dual-trace oscilloscope. Other metals like Al, Cu, brass, and paramagn~icruby, Nd-glass do not give such a magnetic signal to the pickup coil,
FIG. 1. The upper beam shows the signal from the
pickup coil and the lower shows the laser light intensity. Sample; Ni (length 1.2mm). Sweep; 1O~sec/div. Obtained signals are classified into two separate categories. One of them is shown in Fig. 1. This is a typical trace obtained from Ni and YIG. It corresponds to the change of magnetization of about 10~G. The signal magnitude is proportional to the laser intensity. Reproducibility of the signal shape is quite good. It is observed even without an external magnetic field, and increases almost linearly with increasing magnetic field up to 700oe, which is the limit of the present experimental apparatus. When the direction of magnetic field is reversed, the signal shape is reversed exactly. When the Ni sample is heated, temperature dependence of the signal
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~“
‘-‘
2119
2120
MAGNETOSTRICTIVE WAVES IN FERROMAGNETIC MATERIALS
magnitude is similar to that of the spontaneous magnetization, and it vanishes at the Curie temperature (631°K).These show that the observed effect depends on the internal magnetiza tion of a ferromagnetic material. When Ni is shaped to a frame or a long rod and several pickup coils are wound to different parts of the sample, the signal starts slightly after the laser pulse according to the place of the coil. This delay time is almost proportional to the distance between the coil and the lightincident surface. The signal magnitude also decreases exponentially with increasing that distance. This means that the magnetic phenomenon propogates in the material. A signal velocity is defined by dividing the distance by the delay time. Considering that a pickup coil detects a magnetic change outside the coil, the signal velocity is nearly equal to the velocity of the longitudinal sound wave in a rod. Furthermore, the oscillation period of the signal shown in Fig. 1 is equal to the round-trip travel time of the longitudinal sound in a disk-type sample. They are 0.08, 0.2 and 0.5 p. sec for YIG of length 0.3, 0.8 and 1.85 mm, respectively, and about 0.6p.sec for Ni of length 1.2 and 1.4 mm. The observed magnetic phenomena seem to be due to the compressional sound wave in the sample. The radiation pressure of an optical pulse excites elastic waves in an opaque material on which it is incident. When a tension or a pressure is applied to a ferromagnetic material, the direction of internal magnetization changes through magnetostriction. When the compressional wave propagates in the material, it would be accompanied with the change of magnetization (magnetorestrictive wave). In the case of Ni, the decay time of the signal to one-half the maximum magnitude is nearly equal to that of the light-excited elastic waves detected by a piezoelectric crysyal (~ 30 p. sec). Al or rubber gives a large elastic signal to the piezoelectric detector, but causes no signal in the pickup coil, when it is irradiated by the optical pulse. When Al or rubber is bonded to one end of the Ni disk and the optical beam is applied to the former, the similar magnetic signal is obtained also from the coil around the Ni sample. Furthermore, this signal has a delay time from the laser pulse which corresponds to the travel of the sound wave in the transducer.
Vol. 9, No. 23
-______ FIG. 2. The upper beam shows the signal from
the pickup coil and the lower shows the laser light intensity. Sample; Fe (length 1.4 mm). Sweep 0.5 ~ sec/div.
The group of Fe, Co and Mumetal gives a quite-different signal shape from Ni and YIG. A typical trace is shown in Fig. 2. It is almost independent of the magnetic field up to 700 oe, and is not reversed for a reversal of the magneticfield direction. Propagation behaviour as was observed in Ni is not observed in the frame- or rod-shaped Fe sample. By sliding the sample inside the pickup coil, it appears that the signal is obtained almost from the sample surface only. When the laser intensity increases a little, there is a case where the signal of Fe also oscillates several times. Therefore there exists a clear distinction between Fe, Co on the one hand and Ni on the other hand. A physical quantity which describes a magnetic behaviour and is quite different between them is a magnetic anisotropy constant. The absolute value of Ni, 5 x i0~erg/cm3, is one order of magnitude smaller than that for Fe at room temperature. When the mechanism of magnetostriction depends on the rotation magnetization, the change of magnetization due to the change of the internal tension is proportional to the inverse of the magnetic anisotropy constant. At low temperatures below 200 °Kthe value for Ni increases to the same order of magnitude as that for Fe. When cooled to 150°Kin a vacuum dewar, the signal from Fe changes little, while the first peak of the signal from Ni becomes remarkable and the oscillation amplitude after that peak becomes very small. At low temperatures the signal shape of Ni tends to be similar to that of Fe.
Vol. 9, No. 23
MAGNETOSTRICTIVE WAVES IN FERROMAGNETIC MATERIALS
These effects are observed also by means of a ruby laser. Above-mentioned experimental results are general properties. In addition, each material shows its characteristic behaviour. For example, the signal of a single crystal depends on the directions of polarization and propagation of incident light to the crystallographic axis of the axis of easy magnetization. A semitransparent
material like YIG may include other mechanisms moreover (magnon Raman scattering, etc.) Detailed results about each material will be discussed elsewhere. Acknowledgements The author wishes to thank Professor J. Itoh for his helpful discussions. —
REFERENCE 1. KUBOTA K., to be published.
La pression de radiation d’un faisceau optique excite des ondes ~lastiques dans les matières opaques variées. De cela, un changement fractionné d’aimantation se produit en substances ferromagnétiques par Ia magn&ostriction. Elles se divisent généralement en deux categories suivant leur phénoméne magnétique qui se montre dans une bobine de pickup. On s’explique que cela se fonde sur l’ariisotropie magnétique.
2121
Printed in Great Britain
OPTICALLY-EXCITED MAGNETOSTRICTIVE WAVES IN FERROMAGNETIC MATERIALS K. Kubota Faculty of Engineering Science, Osaka University, Toyonaka, Japan (Received 16 September 1971 by Y. Toyozawa)
The radiation pressure of an optical beam excites elastic waves in various opaque materials, which causes a fractional change of magnetization in ferromagnetic substances through magnetostriction. Ferromagnets are divided broadly into two categories according to their magnetic behaviour shown in a pickup coil. This is considered due to magnetic anisotropy.
THIS PAPER reports the generation and the detection of magnetostrictive waves excited by an optional pulse. When a ferromagnetic material is irradiated by a laser beam, output voltage is obtamed by a pickup coil around the sample. This effect is related to a phenomenon that the radiation pressure of intense pulse light excites elastic waves in an opaque material.’ A dynamical property of magnetization is studied by this simple method.
—
A light beam (2mm in dia.) with a 50-nsec pulse half-width and a power of 6MW/cm2 is obtamed from a Q-switched Nd-glass laser. Samples studied are Ni, Fe, Co, Mumetal and ferromagnetic YIG. They are almost polycrystals except for one of the YIG samples, and are lisk- or rod-shaped about 3mm in diameter and 0.3 2mm length. The ends are polished optically flat or are left ground. The fractional rate of change of magnetization is detected by a pickup coil around the sample which has 6 20 turns and a coil length of 1 3 mm. Light is applied to one end of the sample with its propagation direction parallel to the coil axis. When studying an external-magnetic-field effect, the coil axis is perpendicular to a magnetic field. The output voltage from the coil is amplified and is displayed on a dual-trace oscilloscope. Other metals like Al, Cu, brass, and paramagn~icruby, Nd-glass do not give such a magnetic signal to the pickup coil,
FIG. 1. The upper beam shows the signal from the
pickup coil and the lower shows the laser light intensity. Sample; Ni (length 1.2mm). Sweep; 1O~sec/div. Obtained signals are classified into two separate categories. One of them is shown in Fig. 1. This is a typical trace obtained from Ni and YIG. It corresponds to the change of magnetization of about 10~G. The signal magnitude is proportional to the laser intensity. Reproducibility of the signal shape is quite good. It is observed even without an external magnetic field, and increases almost linearly with increasing magnetic field up to 700oe, which is the limit of the present experimental apparatus. When the direction of magnetic field is reversed, the signal shape is reversed exactly. When the Ni sample is heated, temperature dependence of the signal
“-‘
~“
‘-‘
2119
2120
MAGNETOSTRICTIVE WAVES IN FERROMAGNETIC MATERIALS
magnitude is similar to that of the spontaneous magnetization, and it vanishes at the Curie temperature (631°K).These show that the observed effect depends on the internal magnetiza tion of a ferromagnetic material. When Ni is shaped to a frame or a long rod and several pickup coils are wound to different parts of the sample, the signal starts slightly after the laser pulse according to the place of the coil. This delay time is almost proportional to the distance between the coil and the lightincident surface. The signal magnitude also decreases exponentially with increasing that distance. This means that the magnetic phenomenon propogates in the material. A signal velocity is defined by dividing the distance by the delay time. Considering that a pickup coil detects a magnetic change outside the coil, the signal velocity is nearly equal to the velocity of the longitudinal sound wave in a rod. Furthermore, the oscillation period of the signal shown in Fig. 1 is equal to the round-trip travel time of the longitudinal sound in a disk-type sample. They are 0.08, 0.2 and 0.5 p. sec for YIG of length 0.3, 0.8 and 1.85 mm, respectively, and about 0.6p.sec for Ni of length 1.2 and 1.4 mm. The observed magnetic phenomena seem to be due to the compressional sound wave in the sample. The radiation pressure of an optical pulse excites elastic waves in an opaque material on which it is incident. When a tension or a pressure is applied to a ferromagnetic material, the direction of internal magnetization changes through magnetostriction. When the compressional wave propagates in the material, it would be accompanied with the change of magnetization (magnetorestrictive wave). In the case of Ni, the decay time of the signal to one-half the maximum magnitude is nearly equal to that of the light-excited elastic waves detected by a piezoelectric crysyal (~ 30 p. sec). Al or rubber gives a large elastic signal to the piezoelectric detector, but causes no signal in the pickup coil, when it is irradiated by the optical pulse. When Al or rubber is bonded to one end of the Ni disk and the optical beam is applied to the former, the similar magnetic signal is obtained also from the coil around the Ni sample. Furthermore, this signal has a delay time from the laser pulse which corresponds to the travel of the sound wave in the transducer.
Vol. 9, No. 23
-______ FIG. 2. The upper beam shows the signal from
the pickup coil and the lower shows the laser light intensity. Sample; Fe (length 1.4 mm). Sweep 0.5 ~ sec/div.
The group of Fe, Co and Mumetal gives a quite-different signal shape from Ni and YIG. A typical trace is shown in Fig. 2. It is almost independent of the magnetic field up to 700 oe, and is not reversed for a reversal of the magneticfield direction. Propagation behaviour as was observed in Ni is not observed in the frame- or rod-shaped Fe sample. By sliding the sample inside the pickup coil, it appears that the signal is obtained almost from the sample surface only. When the laser intensity increases a little, there is a case where the signal of Fe also oscillates several times. Therefore there exists a clear distinction between Fe, Co on the one hand and Ni on the other hand. A physical quantity which describes a magnetic behaviour and is quite different between them is a magnetic anisotropy constant. The absolute value of Ni, 5 x i0~erg/cm3, is one order of magnitude smaller than that for Fe at room temperature. When the mechanism of magnetostriction depends on the rotation magnetization, the change of magnetization due to the change of the internal tension is proportional to the inverse of the magnetic anisotropy constant. At low temperatures below 200 °Kthe value for Ni increases to the same order of magnitude as that for Fe. When cooled to 150°Kin a vacuum dewar, the signal from Fe changes little, while the first peak of the signal from Ni becomes remarkable and the oscillation amplitude after that peak becomes very small. At low temperatures the signal shape of Ni tends to be similar to that of Fe.
Vol. 9, No. 23
MAGNETOSTRICTIVE WAVES IN FERROMAGNETIC MATERIALS
These effects are observed also by means of a ruby laser. Above-mentioned experimental results are general properties. In addition, each material shows its characteristic behaviour. For example, the signal of a single crystal depends on the directions of polarization and propagation of incident light to the crystallographic axis of the axis of easy magnetization. A semitransparent
material like YIG may include other mechanisms moreover (magnon Raman scattering, etc.) Detailed results about each material will be discussed elsewhere. Acknowledgements The author wishes to thank Professor J. Itoh for his helpful discussions. —
REFERENCE 1. KUBOTA K., to be published.
La pression de radiation d’un faisceau optique excite des ondes ~lastiques dans les matières opaques variées. De cela, un changement fractionné d’aimantation se produit en substances ferromagnétiques par Ia magn&ostriction. Elles se divisent généralement en deux categories suivant leur phénoméne magnétique qui se montre dans une bobine de pickup. On s’explique que cela se fonde sur l’ariisotropie magnétique.
2121