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Surface magnetization dynamics at the nanosecond time scale
Journal of Magnetism and Magnetic Materials 226}230 (2001) 463}464
Surface magnetization dynamics at the nanosecond time scale F. Sirotti , P. Torelli , A. Mirone , S. Girlando, L. Floreano, G. Panaccione *, G. Rossi Laboratoire LURE, CNRS-CEA-MESR, F-91405 Orsay, France INFM, Dip. di Fisica, Universita% di Modena e Reggio Emilia, I-41100 Modena, Italy INFM, TASC Laboratory, S.S. 14, Km 163.5, 34012 Basovizza, Trieste, Italy Laboratorium fu( r Festko( rperphysik, ETH- Zu( rich, CH-8093 Zurich, Switzerland
Abstract The surface magnetization dynamics at the nanosecond time scale is measured via spin polarization of the total electron yield obtained from synchrotron radiation pulses in 100 kV Mott scattering experiments. Di!erent reversal dynamics are observed for surface and bulk in an amorphous soft-ferromagnetic ribbon, indicating a weak coupling of the surface in the ns- range. Measurements vs. pulse duration of the reversal "eld reveal the presence of spring coupling behaviour if the surface is well or poorly exchange-coupled to the bulk. 2001 Elsevier Science B.V. All rights reserved. Keywords: Domain dynamics; Surface magnetism; Synchrotron radiation
Surface magnetic properties present strong di!erences with respect to the bulk ones, due to the reduced coordination represented by the solid termination. The magnetization curve of the surface of a ferromagnet re#ects then the intrinsic properties of the surface magnetic moment and anisotropy, as well as the exchange coupling to the bulk substrate [1]. Experiments on the magnetization reversal dynamics of surfaces can be done by measuring the surface magnetization, for example via the spin polarization (SP) of photoelectrons in a time resolved mode. The excitation of the photoemission by Synchrotron Radiation pulses of 500 ps duration at 120 ns intervals allows to follow in **real time++ magnetization curves in the time scale of microseconds, and to reach, in pumpprobe experiments, the nanosecond time resolution. Such experiments have shown the strong dependence of the coercive "eld upon the time scale of magnetization reversal for surfaces and bulk, and large di!erences between the two. The details of these experiments can be found in Ref. [2,3]. A special class of ferromagnets has been inves-
* Corresponding author. Tel.: #39-40-3758409; fax: #3940-3758400. E-mail address: [email protected] (G. Panaccione).
tigated insofar: the low-coercivity amorphous alloys based upon Fe, Co, Ni and B, of Co, Mn, Si, O (Vitrovac). These materials present the advantage of the low coercive "eld in standard reversal experiments, and lack of magnetocrystalline anisotropy. Furthermore, the growth on top of these substrates of iron "lms in amorphous rcp structure, or in "ne polycrystalline bcc structure provides magnetically distinct surfaces which are nevertheless exchange coupled with the substrate. Fig. 1 shows the magnetization reversal of an atomically clean Vitrovac sample and surface as a function of time in the presence of a raising applied magnetic "eld. The dashed line represents the current injected in the electromagnet which provides the applied "eld. It reaches a constant value within 80 ns. The solid curve is an inductive measurement of the bulk magnetization reversal, obtained with a search coil. The bulk magnetization starts within 15$5 ns to vary, reaches the demagnetized state (half of the domains are reversed) within 80 ns, and completes the reversal within about 300 ns. The surface measurements (SP) are represented by the dots. A delay time before the onset of reversal is observed (about 30$10 ns), followed by a very fast reversal with a surface demagnetization time of about 40 ns and a full reversal time (saturation) at about 100 ns. This result indicates how independent is the dynamics of the bulk and the surface magnetization
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 8 9 5 - 7
464
F. Sirotti et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 463}464
Fig. 1. Bulk (continuous line) vs. Surface (points) dynamics for an amorphous soft-ferromagnetic ribbon (Vitrovac), by applying the same current pulse with a risetime of 70 ns (dashed line). The surface magnetization reversal advances of about 70 ns the bulk one.
Fig. 2. Surface magnetization reversal curves obtained for negative pulse duration ranging between 0.48 and 3.72 s. The magnetic "eld values are !32H before t and #12H after t .The continuous line represents bulk magnetization reversal curve for the transition to !32H .
reversal: the di!erent delay of the onset of reversal, and the fact that the surface is saturated in the new magnetization direction at a time when only half of the bulk domains are reversed imply a weak exchange coupling between surface and bulk. The di!erence in magnetization dynamics of bulk and surface can be further studied by inducing a sequence of
magnetization reversal events with di!erent time structure. The data of Fig. 2 represent the results of a sequence of two reversals: the "rst one was induced by the application of a "eld of !32H (in units of the standard coercive "eld H ), the second one, by a smaller "eld of #12H after an interval ranging between 0.2 and 5 s. We can observe two limiting cases: if the interval between the application of the two reversal "elds is too short, the surface magnetization cannot reach saturation in the direction of the "rst "eld before the new opposite "eld is applied. We therefore, see a cusp-like deviation of the surface magnetization from and back of the original magnetization state. For long intervals, i.e. longer then 3.72 s the surface and bulk magnetization (solid line) have reached saturation in the direction of the "rst applied "eld pulse, and all the following reversal curves collapse on the same curve which therefore, represent, the switching dynamics of the surface of a system which was magnetically in equilibrium before the application of the second reversal "eld. For all the intermediate time intervals the situation is such that the system is away from equilibrium before the second reversal "eld is applied. The surface reaches saturation after the "rst reversal within 0.5 s, i.e. much ahead of time then the bulk. At the time of the second reversal the system is therefore likewise a loaded spring with the bulk still mostly oriented in the original magnetization direction, i.e. the same as the new applied "eld. The surface magnetization in this situation springs back to saturation in a very short time. This spring-loaded behaviour is observed for all time intervals, between the two consecutive reversals, up to 3.72 s. Again one observes that for short intervals, of the order of 1 s, the surface magnetization is almost antiparallel to the bulk magnetization, i.e. very much away from the equilibrium condition dictated by the surface to bulk ferromagnetic exchange coupling. We have shown that it is possible to measure the dynamical response of surfaces and bulk of a ferromagnet in realistic systems. The time scale of these experiments is longer than the one for magnetization rotation of single-domain nanometer size particles, but it is signi"cant one for surface magnetism, i.e. for extended systems where the distinction of surface and bulk is physically well de"ned.
References [1] H.C. Siegmann, J. Phys.: Condens. Matter. 4 (1992) 8935. [2] F. Sirotti et al., Phys. Rev. B 61 (2000) R9221. [3] F. Sirotti et al., J. Appl. Phys. 83 (1998) 1563.
Surface magnetization dynamics at the nanosecond time scale F. Sirotti , P. Torelli , A. Mirone , S. Girlando, L. Floreano, G. Panaccione *, G. Rossi Laboratoire LURE, CNRS-CEA-MESR, F-91405 Orsay, France INFM, Dip. di Fisica, Universita% di Modena e Reggio Emilia, I-41100 Modena, Italy INFM, TASC Laboratory, S.S. 14, Km 163.5, 34012 Basovizza, Trieste, Italy Laboratorium fu( r Festko( rperphysik, ETH- Zu( rich, CH-8093 Zurich, Switzerland
Abstract The surface magnetization dynamics at the nanosecond time scale is measured via spin polarization of the total electron yield obtained from synchrotron radiation pulses in 100 kV Mott scattering experiments. Di!erent reversal dynamics are observed for surface and bulk in an amorphous soft-ferromagnetic ribbon, indicating a weak coupling of the surface in the ns- range. Measurements vs. pulse duration of the reversal "eld reveal the presence of spring coupling behaviour if the surface is well or poorly exchange-coupled to the bulk. 2001 Elsevier Science B.V. All rights reserved. Keywords: Domain dynamics; Surface magnetism; Synchrotron radiation
Surface magnetic properties present strong di!erences with respect to the bulk ones, due to the reduced coordination represented by the solid termination. The magnetization curve of the surface of a ferromagnet re#ects then the intrinsic properties of the surface magnetic moment and anisotropy, as well as the exchange coupling to the bulk substrate [1]. Experiments on the magnetization reversal dynamics of surfaces can be done by measuring the surface magnetization, for example via the spin polarization (SP) of photoelectrons in a time resolved mode. The excitation of the photoemission by Synchrotron Radiation pulses of 500 ps duration at 120 ns intervals allows to follow in **real time++ magnetization curves in the time scale of microseconds, and to reach, in pumpprobe experiments, the nanosecond time resolution. Such experiments have shown the strong dependence of the coercive "eld upon the time scale of magnetization reversal for surfaces and bulk, and large di!erences between the two. The details of these experiments can be found in Ref. [2,3]. A special class of ferromagnets has been inves-
* Corresponding author. Tel.: #39-40-3758409; fax: #3940-3758400. E-mail address: [email protected] (G. Panaccione).
tigated insofar: the low-coercivity amorphous alloys based upon Fe, Co, Ni and B, of Co, Mn, Si, O (Vitrovac). These materials present the advantage of the low coercive "eld in standard reversal experiments, and lack of magnetocrystalline anisotropy. Furthermore, the growth on top of these substrates of iron "lms in amorphous rcp structure, or in "ne polycrystalline bcc structure provides magnetically distinct surfaces which are nevertheless exchange coupled with the substrate. Fig. 1 shows the magnetization reversal of an atomically clean Vitrovac sample and surface as a function of time in the presence of a raising applied magnetic "eld. The dashed line represents the current injected in the electromagnet which provides the applied "eld. It reaches a constant value within 80 ns. The solid curve is an inductive measurement of the bulk magnetization reversal, obtained with a search coil. The bulk magnetization starts within 15$5 ns to vary, reaches the demagnetized state (half of the domains are reversed) within 80 ns, and completes the reversal within about 300 ns. The surface measurements (SP) are represented by the dots. A delay time before the onset of reversal is observed (about 30$10 ns), followed by a very fast reversal with a surface demagnetization time of about 40 ns and a full reversal time (saturation) at about 100 ns. This result indicates how independent is the dynamics of the bulk and the surface magnetization
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 8 9 5 - 7
464
F. Sirotti et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 463}464
Fig. 1. Bulk (continuous line) vs. Surface (points) dynamics for an amorphous soft-ferromagnetic ribbon (Vitrovac), by applying the same current pulse with a risetime of 70 ns (dashed line). The surface magnetization reversal advances of about 70 ns the bulk one.
Fig. 2. Surface magnetization reversal curves obtained for negative pulse duration ranging between 0.48 and 3.72 s. The magnetic "eld values are !32H before t and #12H after t .The continuous line represents bulk magnetization reversal curve for the transition to !32H .
reversal: the di!erent delay of the onset of reversal, and the fact that the surface is saturated in the new magnetization direction at a time when only half of the bulk domains are reversed imply a weak exchange coupling between surface and bulk. The di!erence in magnetization dynamics of bulk and surface can be further studied by inducing a sequence of
magnetization reversal events with di!erent time structure. The data of Fig. 2 represent the results of a sequence of two reversals: the "rst one was induced by the application of a "eld of !32H (in units of the standard coercive "eld H ), the second one, by a smaller "eld of #12H after an interval ranging between 0.2 and 5 s. We can observe two limiting cases: if the interval between the application of the two reversal "elds is too short, the surface magnetization cannot reach saturation in the direction of the "rst "eld before the new opposite "eld is applied. We therefore, see a cusp-like deviation of the surface magnetization from and back of the original magnetization state. For long intervals, i.e. longer then 3.72 s the surface and bulk magnetization (solid line) have reached saturation in the direction of the "rst applied "eld pulse, and all the following reversal curves collapse on the same curve which therefore, represent, the switching dynamics of the surface of a system which was magnetically in equilibrium before the application of the second reversal "eld. For all the intermediate time intervals the situation is such that the system is away from equilibrium before the second reversal "eld is applied. The surface reaches saturation after the "rst reversal within 0.5 s, i.e. much ahead of time then the bulk. At the time of the second reversal the system is therefore likewise a loaded spring with the bulk still mostly oriented in the original magnetization direction, i.e. the same as the new applied "eld. The surface magnetization in this situation springs back to saturation in a very short time. This spring-loaded behaviour is observed for all time intervals, between the two consecutive reversals, up to 3.72 s. Again one observes that for short intervals, of the order of 1 s, the surface magnetization is almost antiparallel to the bulk magnetization, i.e. very much away from the equilibrium condition dictated by the surface to bulk ferromagnetic exchange coupling. We have shown that it is possible to measure the dynamical response of surfaces and bulk of a ferromagnet in realistic systems. The time scale of these experiments is longer than the one for magnetization rotation of single-domain nanometer size particles, but it is signi"cant one for surface magnetism, i.e. for extended systems where the distinction of surface and bulk is physically well de"ned.
References [1] H.C. Siegmann, J. Phys.: Condens. Matter. 4 (1992) 8935. [2] F. Sirotti et al., Phys. Rev. B 61 (2000) R9221. [3] F. Sirotti et al., J. Appl. Phys. 83 (1998) 1563.