RU2777497C1 - Method for excitation of standing spin waves in nanostructured epitaxial ferrite garnet films using femtosecond laser pulses - Google Patents

Abstract

FIELD: nanotechnology.

SUBSTANCE: invention relates to the field of spintronics and ultrahigh frequency technology and, in particular, to the creation of optically controlled filters, modulators and routers of microwave radiation, logic elements and signal converters based on epitaxial films of ferrite garnets. The method for local, limited by the diffraction limit, excitation of standing spin waves in a nanostructured epitaxial ferrite garnet film obtained on a gadolinium gallium garnet substrate using femtosecond pulsed coherent laser linearly polarized radiation includes the following operations. The profile of the epitaxial ferrite garnet film is made in the form of a one-dimensional lattice with a rectangular meander cross section by means of nanolithography and the said substrate with a ferrite garnet film is placed in an external permanent magnetic field with a strength of 5-2000 E. The surface of an epitaxial nanostructured ferrite garnet film of the specified profile is irradiated with femtosecond pulsed coherent linearly polarized laser radiation, providing excitation of standing spin waves. The wavelength of the exciting radiation is 400-1500 nm, the pulse duration is 50-500 fs and the repetition frequency is up to 0.2 GHz. The efficiency and speed of dynamic optomagnetic spin dynamics control for excitation of standing spin waves in magnetic epitaxial films of ferrite garnets are improved, the frequency and order of excited CERS can be adjusted when the magnitude of the external magnetic field and the parameters of the exciting radiation change, as well as the fundamental possibility of excitation and registration of CERS in an ultra-small region of the sample comparable to the diameter of the laser focus spot while the size of the excitation region is physically limited by the diffraction limit.

EFFECT: expansion of the range of methods for excitation of standing spin waves in nanostructured epitaxial ferrite garnet films.

1 cl, 4 dwg

Description

The invention relates to the field of spintronics and microwave technology, and, in particular, to the creation of optically controlled filters, modulators and microwave radiation routers, logic elements, signal converters, etc. based on epitaxial films of ferrite garnets (EPFG). This invention makes it possible, using femtosecond laser pulses, to excite spin dynamics in the form of standing spin waves (SSW) in EPSG samples in a localized region comparable to the diameter of the laser beam focusing spot, while the size of the excitation region is physically limited only by the diffraction limit. The invention can be used in the field of spintronics, magnonics and microwave technology, in the design of quantum computing devices, etc., as well as in complex laboratory research.

It is known that in magneto-dielectric epitaxial films of ferrite garnets under the action of femtosecond pulses of coherent laser radiation, an effective magnetic field may arise, which leads to a dynamic change in magnetization as a result of the inverse magneto-optical Faraday effect when excited by circularly polarized radiation [Kovalenko V., Nagaev E .L. Photoinduced magnetism // Uspekhi Fizicheskikh Nauk.-1986.-T. 148.-S. 561-602. Kozhaev M.A., Chernov A.I., Savochkin I.V., Kuzmichev A.N., Zvezdin A.K., Belotelov V.I. Peculiarities of the inverse Faraday effect arising in ferrite garnet films exposed to femtosecond laser pulses // JETP Letters. - 2016. - T. 104. - S. 851-855. Van der Ziel J., Pershan P., Malmstrom L. Optically-induced magnetization resulting from the inverse Faraday effect // Physical Review Letters. - 1965. - Vol. 15, no. 5. - P. 190. Hansteen F., Kimel A., Kirilyuk A., Rasing T. Femtosecond photomagnetic switching of spins in ferrimagnetic garnet films // Physical review letters. - 2005. - Vol. 95, no. 4. - P. 047402.], or as a result of the inverse magneto-optical Kerr effect, the inverse Cotton-Moutton effect, as well as photoinduced anisotropy when excited by linearly polarized radiation [Hansteen F., Kimel A., Kirilyuk A., Rasing T. Femtosecond photomagnetic switching of spins in ferrimagnetic garnet films // Physical review letters.-2005.-Vol. 95, no. 4.- P. 047402. V.I. Belotelov and A.K. Zvezdin, Inverse transverse magneto-optical Kerr effect, Phys. Rev. B 86, 155133 (2012). Yoshimine, I.; Satoh, T.; Iida, R.; Stupakiewicz, A.; Maziewski, A.; Shimura, T. Phase-Controllable Spin Wave Generation in Iron Garnet by Linearly Polarized Light Pulses. J. Appl. Phys. 2014, 116(4), 043907. Shen, L.Q.; Zhou, L.F.; Shi, J.Y.; Tang, M; Zheng, Z.; Wu, D.; Zhou, S.M; Chen, L. Y.; Zhao, H.B. Dominant Role of Inverse Cotton-Mouton Effect in Ultrafast Stimulation of Magnetization Precession in Undoped Yttrium Iron Garnet Films by 400-nm Laser Pulses. Phys. Rev. B: Condensation. Matter Mater. Phys. 2018, 97 (22), 224430]. Such pulsed excitation of the spin magnetic subsystem of the crystal in a small limited area of space leads to the emergence of propagating spin waves [Kimel A.V., Zvezdin A.K. Dynamics of magnetization induced by femtosecond light pulses // Physics of Low Temperatures. - 2015. - T. 41. - S. 878-886. Savochkin I.V., Kozhaev M.A., Chernov A.I., Kuzmichev A.N., Zvezdin A.K., Belotelov V.I. Dynamics of magnetization induced by femtosecond optical pulses in epitaxial ferrite-garnet films near the edge of the absorption zone // Solid State Physics. - 2017. - T. 59. - S. 883-887. Chernov A., Kozhaev M., Vetoshko P. et al. Local probing of magnetic films using optical excitation of magnetostatic waves // Solid State Physics. - 2016. - V. 58, No. 6. - S. 1093-1098.]. If such a magnetic excitation turns out to be closed in a spatially limited resonator, then under certain conditions the appearance of standing spin vibrational modes is possible.

A similar spatial limitation of spin-wave dynamics can be implemented by micro- and nanostructuring of the EPPG surface in the form of one- and two-dimensional gratings [Chernov A., Kozhaev M., Vetoshko P. et al. Local probing of magnetic films using optical excitation of magnetostatic waves // Solid state physics. - 2016. - V. 58, No. 6. - S. 1093-1098. A.V. Chumak, A.A. Serga, S. Wolff, B. Hillebrands, M.R. Kostylev. Design and optimization of one-dimensional ferrite-film based magnonic crystals // Journal of Applied Physics 105, 083906 (2009); https://doi.org/10.1063/1.3098258 . L. Halageka, M. Vanwolleghem, F. Vaurette, J. Ben Youssef, K. Postava, et al.. Magnetoplasmonic nanograting geometry enables optical nonreciprocity sign control. Optics Express, Optical Society of America-OSA Publishing, 2018, 26 (24), pp. 31554.] by means of lithography followed by etching. Thus, by selecting a different period, duty cycle, and grating etching depth, it is possible to create conditions for the effective excitation of standing spin modes of different frequencies and different orders (with different spatial profiles). In addition, the amplitude of standing spin waves can be controlled by changing the parameters of the external magnetic field (direction and magnitude) and the optical excitation source (polarization, angle of incidence, and wavelength).

A fairly large number of methods for excitation of standing spin waves in epitaxial films of ferrite garnets and devices based on them that implement these methods are known. Such methods of excitation of the SCB include strip antennas and strip resonators, waveguide resonators, and coaxial cells. Microstructuring is used to modulate signals, as well as to isolate narrow resonant frequencies.

A frequency filter of a microwave signal on magnetostatic waves is known (Patent RU 2666968 C1 IPC H01R 1/20 (2006.01), publ. 13.09.2018, Bull. No. 26), containing a substrate, a magnetic element placed on the substrate, made of a film of yttrium iron garnet, piezoelectric element with metal electrodes formed on the surface of the magnetic element, input and output converters of magnetostatic waves, characterized in that the magnetic element, which is a magnonic crystal, has the shape of an extended rectangle with ends pointed along the longitudinal axis and periodic geometric inhomogeneities in the form of triangular elements placed on opposite sides of the rectangle, and the period of the triangular elements is selected from the condition of formation of the Bragg band gap in the range of wave numbers from 100 cm ^-1 to 300 cm ^-1 ; the piezoelectric element has a length less than the length of the magnetic element, and the input and output transducers of magnetostatic waves are placed on the free surface of the magnetic element from the side of the pointed ends, while the outer electrode of the piezoelectric element is solid, and the electrode adjacent to the surface of the magnetic element has the form of an interdigital transducer with a period T selected from the condition T=2P, where P is the period of the triangular elements.

In the present invention, magnetostatic waves (standing spin waves) are excited along the entire magnetic element using microstrip transducers.

Common feature with the claimed solution is the possibility of excitation of standing spin waves in a film of ferrite garnet.

The disadvantage of the technical solution is the fundamental impossibility of local excitation of SCR in a small area of the sample, since microstrip converters (antennas) excite the dynamics of the spin precession of the magnetic subsystem throughout the sample.

A functional element on magnetostatic spin waves is known (Patent RU 2617143 C1 IPC H01R 1/215 (2006.01), publ. 04.21.2017, Bull. No. 12), containing a ferromagnetic film placed on a substrate, microstrip converters for excitation and reception of magnetostatic spin waves (MSV) in the base ferromagnetic film, located along the edges of the film, an external source of magnetic field, characterized in that the element has two pairs of microstrip transducers, which form two parallel linear channels of MSV propagation, spaced from each other by a distance that ensures placement between these channels of the MSW resonator interacting with linear channels, each linear MSW propagation channel is made in the form of a system of single cylindrical inclusions made of ferromagnetic material formed in the base ferromagnetic film and located uniformly along the length of the channel, and the MSW resonator is a system of single cylindrical inclusions made of ferromagnetic material formed in the base ferromagnetic film and evenly spaced around the circumference, and the inclusions of the ferromagnetic material have a higher magnetization than the base ferromagnetic film.

In the present invention, magnetostatic (standing) spin waves are excited in a ferromagnetic film using microstrip transducers, which form two parallel linear spin wave propagation channels.

A common feature with the claimed solution is the possibility of excitation of standing spin waves not in the entire sample of the ferrite garnet film, but in some selected areas.

The disadvantage of the technical solution is the fundamental impossibility of local excitation of SSW in micro- and nano-domains, since microstrip converters (antennas) excite the dynamics of the spin precession of the magnetic subsystem along the entire spin wave propagation channel, which occupies almost half of the entire film.

An optically controlled switch on magnetostatic waves is known (Patent RU 2727293 C1 IPC H01R 1/11 (2006.01), H01R 5/00 (2006.01), publ. the fact that the waveguide structure is made of a film of yttrium iron garnet located on a gallium-gadolinium garnet substrate and has antennas, while the film of the waveguide structure is made horseshoe-shaped with an inner rounding radius equal to its width, and the control light source is located in such a way that the radiation direction was oriented perpendicular to the film rounding region, the antennas were located on the surface of the yttrium iron garnet film near its edges, and the antenna length was a multiple of the film width.

In this invention, magnetostatic (standing spin) waves are excited in a ferrite garnet film using strip antennas, and the spin wave propagation is controlled by irradiating the waveguide with light from a source (laser).

A common feature with the claimed solution is the possibility of excitation of standing spin waves in a ferrite garnet film, as well as the use of a laser as a control element.

The disadvantage of the technical solution is the fundamental impossibility of local excitation of the SWR in a small area of the sample, since strip antennas excite the dynamics of the spin precession of the magnetic subsystem throughout the sample.

A functional component of magnonics on a multilayer ferromagnetic structure is known (Patent RU 2702915 C1 IPC H01R 1/218 (2006.01), publ. 10/14/2019, Bull. No. 29), containing a substrate of a non-magnetic dielectric, ferromagnetic layers of yttrium iron garnet (YIG), microstrip transducers for excitation and reception of magnetostatic spin waves (MSWs), a magnetic field source, characterized in that it is made in the form of a multilayer 3D structure, including external and internal ferromagnetic layers separated from each other by a layer of non-magnetic substance and located one above the other, while the surface of the substrate in cross section has the shape of a meander formed by a set of periodic grooves, the longitudinal axis of which is perpendicular to the direction of MSW propagation, and the outer and inner ferromagnetic layers have a period coinciding with the period of the protrusions, side faces and grooves formed by the grooves on the substrate surface, and the magnetic field of the magnetic source landmark fields but perpendicular to the plane of the substrate with the possibility of excitation in both ferromagnetic layers of bulk MSWs.

In the present invention, magnetostatic (standing) spin waves are excited in a ferrite garnet film using microstrip transducers.

Common feature with the claimed solution is the possibility of excitation of standing spin waves in a film of ferrite garnet.

The disadvantage of the technical solution is the fundamental impossibility of local excitation of the SCR in a small area of the sample, since microstrip converters excite the dynamics of the spin precession of the magnetic subsystem throughout the sample.

As a prototype, an electric field-controlled functional element of magnonics (Patent RU 2745541 C1 IPC H01R 1/22 (2006.01), publ. meander, the longitudinal axis of which is perpendicular to the direction of propagation of magnetostatic waves (MSWs), covered with a ferromagnetic film of yttrium iron garnet, and microstrip transducers for excitation and reception of MSWs in a ferromagnetic film, a magnetic field source, characterized in that the substrate is made of a piezoceramic material and has an extended straight section in the form of a bar and a V-shaped extension at the end, while the transducer for excitation of the MCB is located on the side of the rectilinear section, and two transducers for receiving the MCB are located on the extended section with the possibility of branching and multiplexing the input signal, moreover, the grooves in the form of a meander are placed on a straight line On the side of the transducer for excitation of the MSW, the magnetic field of the magnetic field source is oriented parallel to the aforementioned longitudinal axes of the grooves, and the electrodes for applying the control electric field to the substrate are placed on the side faces of the bar with the possibility of piezomagnetic interaction in the structure to change the position and create an additional band gap in the spectrum MSV.

In the present invention, magnetostatic (standing) spin waves are excited in a ferrite garnet film using microstrip transducers.

The disadvantage of the technical solution is the fundamental impossibility of local excitation of magnetostatic waves (standing spin waves) in a small area of the sample, since microstrip converters excite the dynamics of the spin precession of the magnetic subsystem throughout the sample.

The technical result of the claimed invention is a significant increase in the efficiency and speed of dynamic optomagnetic control of spin dynamics for the excitation of standing spin waves in magnetic epitaxial films of ferrite garnets, the possibility of tuning the frequency and order of the excited CERs when changing the magnitude of the external magnetic field and the parameters of the exciting radiation, as well as the fundamental possibility excitation and registration of SWV in an ultra-small region of the sample, comparable to the diameter of the focusing spot of the laser beam, while the size of the excitation region is physically limited only by the diffraction limit.

The method for excitation of standing spin waves in nanostructured epitaxial ferrite garnet films using femtosecond laser pulses includes performing nanotexturing on the surface of the EPPG using nanolithography in the form of a spatial one-dimensional grating having a cross section profile in the form of a meander, placing the sample in an external constant magnetic field, irradiating the sample surface with femtosecond pulses of coherent linearly polarized laser radiation, while the period, duty cycle and depth of etching of the one-dimensional grating are selected depending on the required frequency of the SCR, and the thickness of the EPPG is selected so that etching does not occur up to the substrate, and part of the film remains at the bottom of the etching grooves, the period of the one-dimensional grating can vary within 400-1000 nm, grating duty cycle can vary within 200-500 nm, etching depth can vary within 200-500 nm, excitation wavelength can vary within 400-1500 nm, duration The pulse width can vary within 50–500 fs, the pulse repetition rate can vary within 0–0.2 GHz, and the external constant magnetic field can vary within 5–2000 Oe. laser pulses having a fixed time shift relative to the exciting radiation pulses, while the wavelength of the recording radiation can vary within 400-1500 nm, and the pulse repetition rate is exactly equal to the repetition frequency of the exciting radiation pulses.

Common features of the prototype with the claimed solution are the use of a ferrite garnet magnetic film, the outer profile of which is made in the form of a one-dimensional lattice (meander), placing the film in an external magnetic field, excitation of spin dynamics in the form of standing spin (magnetostatic) waves. In this case, in the prototype, the excitation and registration of standing spin (magnetostatic) waves is carried out using microstrip transducers.

The distinguishing features of the invention are:

- performing texturing directly on the magnetic film, and not on the substrate;

- texturing depth is less than the thickness of the magnetic film

excitation of standing spin waves is carried out using femtosecond pulses of coherent linearly polarized laser radiation;

- the possibility of tuning the frequency and order of the excited standing spin wave when changing the wavelength of the exciting laser radiation

- registration of standing spin waves is carried out using femtosecond pulses of coherent plane polarized laser radiation, shifted in time relative to the pulses of exciting radiation.

The combination of distinctive and restrictive features provides the inventive step of the claimed technical solution.

The claimed method uses the principle of excitation of spin dynamics in the form of standing spin waves in a one-dimensional grating, made by nanotexturing an epitaxial film of ferrite garnet, by creating local magnetization in an ultra-small region of the sample when it is irradiated with femtosecond pulses of coherent linearly polarized laser radiation.

The claimed invention is based on the possibility of excitation of spin dynamics as a result of a short-term change in magnetization in an ultra-small region of the sample due to the inverse magneto-optical Cotton-Mouton effect when this region is irradiated with femtosecond pulses of coherent linearly polarized laser radiation. The developed method for excitation of standing spin waves in epitaxial ferrite garnet films using femtosecond laser pulses makes it possible to create optically controlled filters, modulators and routers of microwave waves, logic elements, signal converters based on single-crystal epitaxial films of ferrite garnets, and can be used in the field of spintronics. , magnonics and microwave technology, in the design of quantum computing devices, etc.

The invention is illustrated by figures, where:

Fig. 1 is a general diagram of the design of the device and a diagram of the method of excitation of the CER;

Fig. 2 is a drawing of a cross section of a one-dimensional lattice in the form of a rectangular meander;

Fig. 3 - snapshot of a one-dimensional grating on the surface of the EPPG in the form of a rectangular meander (scanning electron microscope);

Fig. 4 - CER registration schedule The positions in the drawings indicate:

1 - substrate from a single crystal of gadolinium-gallium garnet (GGG);

2 - epitaxial film of ferrite garnet (EPFG);

The method is implemented as follows (Fig. 1). A single-crystal epitaxial film of ferrite garnet (EPG) 2 is grown on substrate 1 from non-magnetic gadolinium-gallium garnet (GGG) by liquid-phase epitaxy. is not carried out over the entire depth of film 2. A beam of monochromatic linearly polarized laser radiation is directed to film 2 at an angle θ in the form of successive femtosecond pulses (exciting radiation), and another beam of monochromatic linearly polarized laser radiation is in the form of successive femtosecond pulses (recording radiation), while the pulses of the recording radiation are shifted in time by a fixed value relative to the pulses of the exciting radiation. The excitation of spin dynamics in the form of SSW is carried out due to the inverse Cotton-Mouton effect, and the registration of SSW is carried out due to the direct Faraday effect; for this, the angle of rotation of the polarization plane of the radiation that has passed through the sample at the registration point is analyzed.

On FIG. Figure 2 shows a section of a one-dimensional lattice on the surface of an EPPG in the form of a rectangular meander. Here, the thickness of the EPPG is denoted by h, and the thickness of the layer at the bottom of the etching grooves is h ₀ , then the etching depth of the grooves is hh ₀ . The grating period is denoted by d, and the duty cycle is d ₀ , respectively.

On FIG. 3 shows a snapshot of a one-dimensional grating on the surface of the EPPG in the form of a rectangular meander, made in accordance with the drawing in Fig. 2. The picture was taken using a scanning electron microscope; the picture shows a scale bar. A single-crystal epitaxial film of cation-substituted ferrite garnet with the nominal composition (BiLu) ₃ (FeGa) ₅ O ₁₂ was grown by liquid-phase epitaxy from a solution melt on a gadolinium gallium garnet substrate 500 μm thick with the surface oriented in the (111) crystallographic plane. In this case, the thickness of the EPPG film was h=300 nm, the thickness of the layer at the bottom of the etching grooves h ₀ =75 nm, and the depth hh ₀ =225 nm. The grating period was d=450 nm, and the duty cycle d ₀ =225 nm. The grating in the form of a rectangular meander was made using electron lithography followed by ion etching. The lithography area on the sample surface was 0.5 × 0.5 mm.

An EPPG sample with a one-dimensional grating was placed in an external constant magnetic field with a strength of 870 Oe, which was directed along the x-axis (Fig. 1) in the film plane, perpendicular to the grating. The grating surface was irradiated with coherent laser radiation (wavelength λ=885 nm) with linear polarization (exciting radiation). The radiation was a sequence of pulses with a duration of 250 fs and a repetition rate of 80 MHz. Radiation power 15 mW, angle of incidence θ=5°. Registration of SWV was carried out using coherent laser radiation (wavelength λ=427.5 nm) with linear polarization (recording radiation). The radiation was a sequence of pulses with a duration of 250 fs and a repetition rate of 80 MHz, while the pulses of the recording radiation were shifted in time by values from -0.2 ns to 2.5 ns with respect to the pulses of the exciting radiation using an optical delay line. On FIG. Figure 4 shows a graph of the registration of SWR using the direct Faraday effect; for this, the angle of rotation of the polarization plane of the recording radiation that passed through the sample at the point of study was analyzed. The time delay was provided by the phase shift of the pulses of the exciting and recording radiation. For this configuration, simultaneous excitation of the zero-order SWR with a frequency of 2.3 GHz, the first order with a frequency of 3.1 GHz, and the second order with a frequency of 3.9 GHz is implemented.

This method has several advantages:

- the possibility of ultrafast excitation of spin dynamics in the form of SCR during the duration of one femtosecond pulse;

- the possibility of excitation of SWV in a very small area of the sample, physically limited only by the diffraction limit;

- the ability to control the amplitude of the SWR by changing the parameters of the external magnetic field (direction and magnitude) and the optical source of excitation (polarization, angle of incidence and wavelength);

- the possibility of excitation of SWS of different frequencies and wavelengths (of different orders) by varying the parameters of the grating meander (period, duty cycle, etching depth) and the optical excitation source (angle of incidence and wavelength);

- the possibility of recording and studying the temporal dynamics, profile and spatial distribution of CER.

Claims (1)

A method for local, limited by the diffraction limit, excitation of standing spin waves in a nanostructured ferrite garnet epitaxial film obtained on a gadolinium gallium garnet substrate using femtosecond pulsed coherent laser linearly polarized radiation, which includes making the profile of the ferrite garnet epitaxial film in the form of a one-dimensional gratings with a cross section in the form of a rectangular meander and placing said substrate with a film of ferrite garnet in an external constant magnetic field, characterized in that the implementation of the specified profile of the epitaxial ferrite garnet film is carried out by nanolithography, while the one-dimensional grating period is 400-1000 nm, and the etching depth of the grating grooves, which is 200-500 nm, is less than the film thickness, the mentioned substrate with a film of ferrite garnet is placed in an external constant magnetic field with a strength of 5-2000 Oe and the surface of the epitaxial nanostructured surface is irradiated. th film of ferrite garnet of the specified profile by femtosecond pulsed coherent linearly polarized laser radiation, which provides excitation of standing spin waves, while the wavelength of the exciting radiation is 400-1500 nm, the pulse duration is 50-500 fs and the repetition rate is up to 0.2 GHz.

Images