- Email: [email protected]
Nonreciprocity of spin waves in magnonic crystals created by surface acoustic waves in structures with yttrium iron garnet
Journal of Magnetism and Magnetic Materials 395 (2015) 180–184
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Nonreciprocity of spin waves in magnonic crystals created by surface acoustic waves in structures with yttrium iron garnet R.G. Kryshtal, A.V. Medved n Fryazino Branch of Kotel’nikov Institute of Radioengineering and Electronics of Russian Academy of Sciences, Vvedensky sq., 1, Fryazino, Moscow region 141190, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 15 May 2015 Received in revised form 15 July 2015 Accepted 25 July 2015 Available online 28 July 2015
Experimental results of investigations of nonreciprocity for surface magnetostatic spin waves (SMSW) in the magnonic crystal created by surface acoustic waves (SAW) in yttrium iron garnet films on a gallium gadolinium garnet substrate as without metallization and with aluminum films with different electrical conductivities (thicknesses) are presented. In structures without metallization, the frequency of magnonic gaps is dependent on mutual directions of propagation of the SAW and SMSW, showing nonreciprocal properties for SMSW in SAW – magnonic crystals even with the symmetrical dispersion characteristic. In metalized SAW – magnonic crystals the shift of the magnonic band gaps frequencies at the inversion of the biasing magnetic field was observed. The frequencies of magnonic band gaps as functions of SAW frequency are presented. Measured dependencies, showing the decrease of magnonic gaps frequency and the expansion of the magnonic band gap width with the decreasing of the metal film conductivity are given. Such nonreciprocal properties of the SAW – magnonic crystals are promising for signal processing in the GHz range. & 2015 Elsevier B.V. All rights reserved.
Keywords: Magnonic crystal Spin wave Surface acoustic wave Yttrium iron garnet Nonreciprocity Metal film Finite conductivity
1. Introduction Magnonic crystals represent a magnetic media with artificially created spatially periodic variations of some of their parameters sensitive for the spin waves [1,2]. A magnonic crystal exhibits magnonic band gaps in which the propagation of magnetic (spin or magnetostatic) waves is forbidden. Magnonic band gaps are due to the artificial periodicity of the magnetic properties that acts as a Bragg reflection grating for spin waves with the proper wavelength. Different configurations of magnonic crystals possessing unique properties are invented and are under wide investigations during the last fifteen years (see, for example [3–14]). In [9] it was shown that the periodic variations of magnetic properties may be created in magnetics by a surface acoustic wave (SAW) due to magnetostriction. The depth ( 25 dB) of the corresponding magnonic band gaps at reasonable SAW intensity is comparable with that in “usual” static magnonic crystals (see, for example, [6]). A certain attention is paid at the present time to investigations of magnonic crystals with nonreciprocal properties for spin waves [13–16] promising for signal processing applications. Nonreciprocity of waves usually means the change of parameters of n
Corresponding author. E-mail address: [email protected] (A.V. Medved).
http://dx.doi.org/10.1016/j.jmmm.2015.07.086 0304-8853/& 2015 Elsevier B.V. All rights reserved.
the wave at the reverse of the direction of its propagation. Nonreciprocity in magnonic crystals may appear as the change of magnonic band gaps at the reverse of the direction of spin wave propagation. These properties of magnonic crystals are usually achieved due to an asymmetry of the forward and backward branches of spin wave dispersion curves in metalized magnetic structures [17]. The use of metal layers in such magnonic crystals, however, is needed to be cleared more. The influence of metal films with finite conductivity on spin wave nonreciprocity in magnonic crystals based on magnetic-metal structures was studied theoretically in [16]. As far as we know only one work [18] on experimental investigations of the metal thickness dependence of spin wave propagation in magnonic crystals was published at the present time. No publications are recognized at the present time on discussion of nonreciprocity in SAW – magnonic crystals. In the present work we describe results on studying of manifestations of7 nonreciprocal properties of surface magnetostatic spin waves (SMSW) in the dynamic magnonic crystals arising at propagation of surface acoustic waves (SAW) in structures containing yttrium iron garnet (YIG) layers, including metalized YIG layers. The influence of metalizing films conductivity on the properties of SAW – magnonic crystals (frequency positions and the breadth of the magnonic band gaps) are experimentally investigated.
R.G. Kryshtal, A.V. Medved / Journal of Magnetism and Magnetic Materials 395 (2015) 180–184
2. Method and experimental setup As in static magnonic crystals spin waves in dynamic crystals cannot propagate at the frequencies within the magnonic band gap and are effectively reflected at those frequencies. In the case of SAW – dynamic magnonic crystals the frequency of the reflected SMSW, fr, are shifted (up or down) by the frequency of SAW, F, with respect to the frequency of the incident SMSW, fi, in accordance with laws of inelastic scattering of SMSW on SAW [19– 21]: fr ¼fi 7F, /kr/ þ/ki/ ¼ /q/, where kr and ki – wave vectors of reflected and incident SMSW, respectively, and q – wave vector of SAW, sign þin the equation corresponds to the case of counterpropagation of SAW and spin wave. The frequencies at which these reflected waves reach their maximum levels equal to the frequencies of magnonic gaps. It is convenient to study magnonic crystals using these reflected waves. The reasons for this are the following. The reflected waves and the incident wave have different frequencies spaced at the frequency of the SAW [19,20]. They do not interfere with each other when using a selective receiver that permits to measure reflected signals at relatively low levels of the input waves in comparison with the direct magnonic band gap measurements demanding much more powerful SAW [9]. In the present work we had been using this “reflected wave method”. Configuration of our experimental arrangement is schematically depicted in Fig. 1. We used the reflected waves approach described here above and so called “bridge method” for SAW excitation in YIG–GGG samples [9], when SAW excited in the piezoelectric plate transfer to YIG–GGG structure through an acoustic contact especially created between the adjacent surfaces of the plate and the structure. YIG films of 5-μm-thick grown on 500μm-thick GGG substrate of (111) crystallographic plane were used in our experiments. The YIG–GGG samples were cut out in the shape of 18 5 mm2 rectangular. Aluminum films through a mask were deposited on the YIG surface as shown in Fig. 1 by the thermal evaporation method in vacuum (without substrate heating). The length of Al films was 6 mm, the width was 5 mm. It should be noted, that the experimental results, as our experiments showed, were practically independent of the length of the film of Al, provided that it was longer than 3 mm. SAW was excited by an interdigital transducer (IDT) with 23 MHz center frequency fabricated by photolithography on the surface of the Y-Z LiNbO3 base plate. SAW passed to the GGG–YIG sample through a special acoustic contact between the base plate and YIG–GGG sample as shown in Fig. 1. Insertion losses at SAW
Fig. 1. Top and side views of the configuration of experimental samples and connections to measurement circuits.
181
excitation in GGG–YIG structures in 50-Ω circuit with matching were estimated in our experiments as 10 dB. The power of SAW at frequency 20 MHz was equaled to 0.1 mW. SMSW were excited and detected by means of an aluminum strip-antenna of 20-μmwidth and 0.5-μm-thick deposited onto the surface of the dielectric plate (alumina) and placed on the surface of YIG film as shown in Fig. 1. The microwave power supplied to the antenna for the excitation of SMSW was equal to 1 μW. Natural microscopic irregularities of the adjacent surfaces of the YIG–GGG structure and the antenna on the dielectric plate did not allow the SAW to leak into the dielectric plate, in contrast with the surfaces of the LiNbO3 base plate and YIG–GGG structure where a special liquid lubricant created acoustic contact. The dielectric plate with the antenna positioned on the surface of the YIG–GGG brings, as experiments showed, to addition attenuation of the SAW less than 1 dB. The apertures of IDT and the antenna were 5 mm. The edges of YIG–GGG structures were subjected to a special treatment (tapered by a diamond needle file) to minimize SAW and SMSW reflections from the structure's edges. External magnetic field HO ¼640 Oe was applied parallel to the antenna.
3. Experimental results and discussion First of all, the structures without metallization and then the structures with relatively thick aluminum films of 3 μm (greater than the depth of the skin layer) were investigated. Microwave signals of frequency f in the range from 3.5 GHz to 3.9 GHz were applied to the antenna through a circulator. The antenna could excite SMSW in both directions and could also be used to detect the SMSW coming to it from both directions. The efficiency of excitation of the relatively long SMSW used in our experiments was practically equal in both directions due to rather thin YIG films (without metallization) [22]. Output signals from the antenna were measured by a selective receiver tuned to frequencies f þF or f F, where F is the frequency of SAW. When the receiver was tuned to f F, the signal corresponding to the reflected SMSW arising in area B (see Fig. 1) where SAW and SMSW of frequency f propagate in the same direction is measured. When the receiver is tuned to fþ F the reflected SMSW arising in area A is measured. In this case, the reflected signal occurs at counterpropagation of the SAW and SMSW (see Fig. 1). When measuring samples with metal films the distance between the metal film and the antenna along the wave propagation must be no longer than 0.5 mm. Our experiments showed, in this case, this distance practically did not influence the measurement results (the change of the levels of measured signals did not exceed 1 dB). Relative levels, A, of reflected SMSW measured as a function of frequency f of incident SMSW at F¼20 MHz and HO ¼640 Oe are presented in Fig. 2 for different experimental situations: a) pure YIG–GGG structure without metallization; b) metalized YIG–GGG with Al film of 3-mm – thick, for mutually opposite directions of magnetic field. The maximum of the curve 1 in Fig. 2a (SAW and SMSW propagates in the same direction – area B in Fig. 1) is achieved at f1 ¼3.6403 GHz and that of the curve 2 in Fig. 2a (counter-propagation – area A in Fig. 1) is achieved at f2 ¼3.6205 GHz, in agreement with the theory of inelastic scattering of SMSW by SAW [19,20] (corresponding explanatory chart of scattering is given in Fig. 3a). This means that the magnonic band gap frequencies depend on the direction of SMSW propagation relative to the direction of SAW propagation. Thus, SAW – dynamic magnonic crystals have nonreciprocity properties for SMSW even with the symmetrical dispersion characteristic (without any metal covering) and the reference direction here is set by the direction of SAW propagation rather than by the direction of the biasing magnetic field.
182
R.G. Kryshtal, A.V. Medved / Journal of Magnetism and Magnetic Materials 395 (2015) 180–184
Fig. 2. Relative levels, A, of reflected SMSW measured as a function of frequency, f, of the external microwave signal applied to the antenna at F ¼20 MHz and HO ¼ 640 Oe, YIG film: thickness 5 μm, 2ΔH¼ 0.5 Oe, 4π Mo ¼ 1760 G. (a) YIG–GGG structure without metallization. Curve 1 – SAW and SMSW propagate in the same direction (area B in Fig. 1); curve 2 – counter-propagation of SAW and SMSW (area A in Fig. 1). (b) Metalized YIG–GGG with Al film of 3-mm – thick. Curves 3 and 4 – SAW and SMSW propagate in the same direction at two mutually opposite directions of the magnetic field; the curves 5 and 6-oncoming propagation SAW and SMSW in the region of the sample covered with the aluminum film (area A in Fig. 1) for two opposite directions of the magnetic field.
The inversion of the magnetic field does not change significantly the curves 1 and 2 at least the resonance frequencies f1 and f2 remain unchanged within the error of measurement (70.5 MHz). This behavior was expected because with a relatively small thickness of the YIG film in the structure without a metal layer the dispersions of SMSW propagating in opposite directions are almost the same. The curves 3 and 4 in Fig. 2b correspond to the situation of the SAW and SMSW propagating in the same direction for two opposite directions of the biasing magnetic field in the area B where the surface of YIG is without metal, while the curves 5 and 6 represent the reflected SMSW at oncoming propagation SAW and SMSW in the region of the sample covered with the aluminum film (area A in Fig. 1) for two opposite directions of the magnetic field. The curve 3 is almost identical with the curve 1 and f1 ¼f3, which was to be expected, as they correspond to the same experimental situation – waves propagate in the same direction in the region without metal (area B in Fig. 1). When the SAW and SMSW are propagating towards each other in the metalized region (area A) the maximum of the reflection curve 5 is at higher frequencies f5 ¼ 3.721 GHz. At the inversion of the magnetic field, the
curve 6 is shifted from curve 5 reaching the maximum at f6 ¼3.734 GHz. All these measured dependencies of the reflected SMSW on frequency can be explained by the laws of inelastic scattering of spin waves by SAW. Explanatory charts of the conditions imposed on the wave vectors and the frequencies of the interacting waves, at which magnonic band gaps arise in the corresponding experimental situations is shown in Fig. 3. The resonance frequencies of reflected SMSWs are the frequencies of corresponding magnonic band gaps for SMSW in the SAW dynamic magnonic crystal and their shift at the inversion of the magnetic field demonstrates the nonreciprocity in such a magnonic crystal with metallization. Fig. 4 represents measured dependencies of the frequencies of magnonic gaps on the SAW frequency for two mutually opposite directions of the magnetic field in the GGG–YIG–Al structure with the thickness of Al film greater than the skin layer depth. This result shows the additional possibilities of magnonic crystal tuning by changing the SAW frequency, that may be useful in the creation of new devices for information processing. We used our experimental technique to investigate nonreciprocal structures GGG–YIG–aluminum film with different thicknesses of Al films, even less than the depth of the skin layer. It was possible to use one and the same YIG–GGG sample with metal films of different thicknesses. The metal film of the previous thickness had been etched out (in aqueous alkali KOH solution) and then a film with another thickness was evaporated on the YIG–GGG structure. Taking into account that the specific conductivity of fabricated films may differ from the tabulated value of ideal aluminum all our experimental results are given depending on sheet conductivity rather than on the thickness of fabricated films. Conductivity of deposited aluminum films was determined by measuring the resistance of a so called “bystander” (another YIG–GGG structure with golden electrodes fabricated on the surface of YIG) placed side by side with our sample during the deposition process, that provided the deposition of aluminum film similar to that deposited on our sample. Measured relative levels of signals corresponding to the reflected SMSW at counterpropagating with the SAW of 20 MHz (see area A in Fig. 1) as a function of the input signal frequency are given in Fig. 5 for three values of sheet conductivities of the aluminum film (curves 1, 2, 3) and for “pure” YIG–GGG (curve 4) at co-propagating SMSW and SAW (area B in Fig. 1). The curves 1, 2 and 3 were obtained at the receiver tuning to fþ F frequencies and curve 4 – at tuning to f F frequencies. The curve 4 served as a reference curve for maintaining the constant level of SAW power when changing samples with films of different conductivity. From Fig. 5 it is seen that the decrease in conductivity leads to a decrease of the resonance frequency of the reflected SMSW and to the broadening of the resonance curve. This behavior is consistent with the concept of scattering of SMSW on SAW, since the decrease in the conductivity of the metal film changes the slope of the dispersion curves and, therefore, must be a decrease in resonant frequency and with decreasing film thickness less than the depth of the skin layer increases the propagation loss of the waves, that leads to the broadening of the resonance curve of the reflected SMSW [20]. By the other words, the frequency of the magnonic gaps is decreasing and the widths of the magnonic gaps are broadening with the decrease of conductivity. At the inversion of the magnetic field all curves in Fig. 5, except the curve 4, were shifted along the frequency axis as it follows from nonreciprocal properties of the structures. The results of measurements of the dependencies of the resonance frequency of the reflected SMSW (frequency of the magnonic band gaps) and the width of the resonance curve (the width of the magnonic band gaps) on conductivity of the metal
R.G. Kryshtal, A.V. Medved / Journal of Magnetism and Magnetic Materials 395 (2015) 180–184
183
Fig. 3. Explanatory charts to the results in Fig. 2 – conditions imposed on the wave vectors and the frequencies of the interacting waves, at which magnonic band gaps arise in the corresponding experimental situations. fFMR – frequency of ferromagnetic resonance at a given external magnetic field. (a) “Pure” YIG–GGG structure without metallization. The solid line curves–branches of the dispersion f(k) of SMSW propagating in mutually opposite directions. kr, ki, q – wave numbers of reflected and incident SMSW and SAW, respectively. (b) Metalized YIG–GGG structure. Solid and dotted line curves, denoted as D↓and D↑, – dispersion curves f(k) of SMSW at mutually opposite directions of the magnetic field. k↑r , k↑i , k↓r , k↓I – wave numbers of reflected and incident SMSW at two mutually opposite directions of the magnetic field, respectively. The arrows at the letters D and k symbolize the direction of the magnetic field.
Fig. 4. Measured dependencies of magnonic gap frequencies, fog, on SAW frequency, F, in metalized YIG–GGG structure (curves 1 and 2) and in “pure” YIG–GGG (curve 3) at two mutually opposite directions of the magnetic field. The arrows at the letters Ho symbolize the direction of the magnetic field.
Fig. 6. Measured resonance frequency, fg of the reflected SMSW (magnonic gap frequency) and the width of the resonance curve (width of the magnonic gap), Δfg as functions of conductivity, s, of the metal film in the GGG–YIG–Al structure. Counter-propagation of SMSW and SAW. F¼20 MHz.
film in the GGG–YIG–Al structure are presented in Fig. 6. These results show that the metal film conductivity, less than 80 S, has a strong influence on the propagation of SMSW in the magnonic crystal based on the metalized YIG. The conductivity of the film of ideal aluminum of the thickness equal to the thickness of the skin layer, δ, at 3.7 GHz (δ ¼1.35 μm) would be 50 S. This means, that the thickness of aluminum films of the order of 1.5 δ is required to avoid excessive losses of SMSW in such magnonic crystals. As it was indicated earlier the SAW power used at our measurement was 0.1 mW. At higher powers the width of the resonance curves begins to depend on SAW power [20].
4. Conclusion
Fig. 5. Relative levels, A, of reflected SMSW measured as a function of frequency, f, of the external microwave signal applied to the antenna at F¼ 20 MHz in metalized YIG–GGG structure for three values of sheet conductivities of the aluminum film curves 1 – 110 S, curve 2 – 20 S, curve 3 – 12 S, – counter-propagation SMSW and SAW and for “pure” area of the YIG surface (curve 4) – co-propagation SMSW and SAW. YIG film: thickness 5.2 μm, 2ΔH¼ 0.5 Oe, 4π Mo ¼ 1760 G.
Thus we have experimentally shown peculiarities of nonreciprocity manifestations for surface magnetostatic waves in dynamic magnonic crystals created by SAW propagating in structures with YIG layers. Such dynamic magnonic crystals have nonreciprocity for spin waves even in structures with symmetrical with respect to the axis of frequency dispersion characteristics. The reference direction is set by the direction of SAW propagation rather than by the direction of the magnetic field. As in static magnonic crystals in SAW – dynamic magnonic crystals based on
184
R.G. Kryshtal, A.V. Medved / Journal of Magnetism and Magnetic Materials 395 (2015) 180–184
GGG–YIG–metal structures nonreciprocity for SMSW was observed also at the inversion of the direction of the magnetic field. So in SAW – dynamic magnonic crystals based on the GGG–YIG– metal structure the nonreciprocity managed both as by the direction of the biasing magnetic field and by the direction of SAW propagation takes place. The influence of the aluminum film conductivity on magnonic gaps frequency and the gap's width in SAW – magnonic crystals at the base of metalized YIG–GGG structures were investigated. The conductivity of the aluminum film higher than 80 S is required to avoid excessive losses of SMSW in such magnonic crystals. In addition, in these crystals it is possible to adjust the frequency of the magnonic band gaps and their depth by changing the frequency and power of SAW. These nonreciprocal properties investigated in our work may be helpful for designing new nonreciprocal devices of signal processing in GHz frequency range. In this our work, we used, for simplicity, so-called “bridge method” for excitation of SAW, at designing practical devices piezoelectric films deposited onto YIG surface should be used for SAW excitation.
References [1] J.O. Vasseur, L. Dobrzynski, B. Djafari-Rouhani, Phys. Rev. B 54 (1996) 1043. [2] Yu. V. Gulyaev, S.A. Nikitov, L.V. Zivotovskii, A.A. Klimov, Ph Tailhades, L. Presmanes, C. Bonningue, T.S. Tsai, S.L. Vysotskii, Y.A. Filimonov, JETP Lett. 77 (2003) 567.
[3] A.V. Chumak, A.A. Serga, B. Hillebrands, M.P. Kostylev, Appl. Phys. Lett. 94 (2009) 172511. [4] A.V. Chumak, T. Neumann, A.A. Serga, B. Hillebrands, M.P. Kostylev, J. Phys. D: Appl. Phys. 42 (2009) 205005. [5] Kai H. Chi, Yun Zhu, Rong W. Mao, James P. Dolas, Chen S. Tsai, J. Appl. Phys. 109 (2011) 07D320. [6] Mitsuteru Inoue, Alexander Baryshev, Hiroyuki Takagi, Pang Boey Lim, Kohei Hatafuku, Josho Noda, Kenji Togo, Appl. Phys. Lett. 98 (2011) 132511. [7] F.S. Ma, H.S. Lim, Z.K. Wang, S.N. Piramanayagam, S.C. Ng, M.H. Kuok, Appl. Phys. Lett. 98 (2011) 153107. [8] Bai Lihui, Kohda Makoto, Nitta Junsaku, Appl. Phys. Lett. 98 (2011) 172508. [9] R.G. Kryshtal, A.V. Medved, Appl. Phys. Lett. 100 (2012) 192410. [10] M. Krawczyk, D. Grundler, Condens. Matter 26 (2014) 123202. [11] X.G. Wang, G.H. Guo, Z.X. Li, D.W. Wang, Y.Z. Nie, W. Tang, Europhys. Lett. 109 (3) (2015) 37008. [12] S.L. Vysotskii, S.A. Nikitov, E.S. Pavlov, Yu. A. Filimonov, J. Commun. Technol. Electron. 58 (4) (2013) 347–352. [13] M. Mruczkiewicz, M. Krawczyk, G. Gubbiotti, S. Tacchi, Yu.A. Filimonov, D. V. Kalyabin, I.V. Lisenkov, S.A. Nikitov, New J. Phys. 15 (2013) 113023. [14] R. Huber, M. Krawczyk, T. Schwarze, H. Yu, G. Duerr, S. Albert, D. Grundler, Appl. Phys. Lett. 102 (2013) 012403. [15] M. Mruczkiewicz, E.S. Pavlov, S.L. Vysotsky, M. Krawczyk, Yu. A. Filimonov, S. A. Nikitov, Phys. Rev. B 90 (2014) 174416. [16] M. Mruczkiewicz, M. Krawczyk, J. Appl. Phys. 115 (11) (2014) 113909. [17] S.R. Seshadri, Proc. IEEE 58 (3) (1970) 506–507, http://dx.doi.org/10.1109/ PROC.1970.7680. [18] N. Kanazawa, T. Goto, J.W. Hoong, A. Buyandalai, H. Takagi, M. Inoue, J. Appl. Phys. 117 (17) (2015) 17E510. [19] A.F. Popkov, Fiz. Met. Metalloved. 59 (1985) 463. [20] R.G. Kryshtal, A.V. Medved, A.F. Popkov, J. Commun. Technol. Electron. 39 (1994) 132. [21] V. Chumak, P. Dhagat, A. Jander, A.A. Serga, B. Hillebrands., Phys. Rev. B 81 (2010) 140404(R). [22] W.S. Ishak, Proc. IEEE 76 (1988) 171.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Nonreciprocity of spin waves in magnonic crystals created by surface acoustic waves in structures with yttrium iron garnet R.G. Kryshtal, A.V. Medved n Fryazino Branch of Kotel’nikov Institute of Radioengineering and Electronics of Russian Academy of Sciences, Vvedensky sq., 1, Fryazino, Moscow region 141190, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 15 May 2015 Received in revised form 15 July 2015 Accepted 25 July 2015 Available online 28 July 2015
Experimental results of investigations of nonreciprocity for surface magnetostatic spin waves (SMSW) in the magnonic crystal created by surface acoustic waves (SAW) in yttrium iron garnet films on a gallium gadolinium garnet substrate as without metallization and with aluminum films with different electrical conductivities (thicknesses) are presented. In structures without metallization, the frequency of magnonic gaps is dependent on mutual directions of propagation of the SAW and SMSW, showing nonreciprocal properties for SMSW in SAW – magnonic crystals even with the symmetrical dispersion characteristic. In metalized SAW – magnonic crystals the shift of the magnonic band gaps frequencies at the inversion of the biasing magnetic field was observed. The frequencies of magnonic band gaps as functions of SAW frequency are presented. Measured dependencies, showing the decrease of magnonic gaps frequency and the expansion of the magnonic band gap width with the decreasing of the metal film conductivity are given. Such nonreciprocal properties of the SAW – magnonic crystals are promising for signal processing in the GHz range. & 2015 Elsevier B.V. All rights reserved.
Keywords: Magnonic crystal Spin wave Surface acoustic wave Yttrium iron garnet Nonreciprocity Metal film Finite conductivity
1. Introduction Magnonic crystals represent a magnetic media with artificially created spatially periodic variations of some of their parameters sensitive for the spin waves [1,2]. A magnonic crystal exhibits magnonic band gaps in which the propagation of magnetic (spin or magnetostatic) waves is forbidden. Magnonic band gaps are due to the artificial periodicity of the magnetic properties that acts as a Bragg reflection grating for spin waves with the proper wavelength. Different configurations of magnonic crystals possessing unique properties are invented and are under wide investigations during the last fifteen years (see, for example [3–14]). In [9] it was shown that the periodic variations of magnetic properties may be created in magnetics by a surface acoustic wave (SAW) due to magnetostriction. The depth ( 25 dB) of the corresponding magnonic band gaps at reasonable SAW intensity is comparable with that in “usual” static magnonic crystals (see, for example, [6]). A certain attention is paid at the present time to investigations of magnonic crystals with nonreciprocal properties for spin waves [13–16] promising for signal processing applications. Nonreciprocity of waves usually means the change of parameters of n
Corresponding author. E-mail address: [email protected] (A.V. Medved).
http://dx.doi.org/10.1016/j.jmmm.2015.07.086 0304-8853/& 2015 Elsevier B.V. All rights reserved.
the wave at the reverse of the direction of its propagation. Nonreciprocity in magnonic crystals may appear as the change of magnonic band gaps at the reverse of the direction of spin wave propagation. These properties of magnonic crystals are usually achieved due to an asymmetry of the forward and backward branches of spin wave dispersion curves in metalized magnetic structures [17]. The use of metal layers in such magnonic crystals, however, is needed to be cleared more. The influence of metal films with finite conductivity on spin wave nonreciprocity in magnonic crystals based on magnetic-metal structures was studied theoretically in [16]. As far as we know only one work [18] on experimental investigations of the metal thickness dependence of spin wave propagation in magnonic crystals was published at the present time. No publications are recognized at the present time on discussion of nonreciprocity in SAW – magnonic crystals. In the present work we describe results on studying of manifestations of7 nonreciprocal properties of surface magnetostatic spin waves (SMSW) in the dynamic magnonic crystals arising at propagation of surface acoustic waves (SAW) in structures containing yttrium iron garnet (YIG) layers, including metalized YIG layers. The influence of metalizing films conductivity on the properties of SAW – magnonic crystals (frequency positions and the breadth of the magnonic band gaps) are experimentally investigated.
R.G. Kryshtal, A.V. Medved / Journal of Magnetism and Magnetic Materials 395 (2015) 180–184
2. Method and experimental setup As in static magnonic crystals spin waves in dynamic crystals cannot propagate at the frequencies within the magnonic band gap and are effectively reflected at those frequencies. In the case of SAW – dynamic magnonic crystals the frequency of the reflected SMSW, fr, are shifted (up or down) by the frequency of SAW, F, with respect to the frequency of the incident SMSW, fi, in accordance with laws of inelastic scattering of SMSW on SAW [19– 21]: fr ¼fi 7F, /kr/ þ/ki/ ¼ /q/, where kr and ki – wave vectors of reflected and incident SMSW, respectively, and q – wave vector of SAW, sign þin the equation corresponds to the case of counterpropagation of SAW and spin wave. The frequencies at which these reflected waves reach their maximum levels equal to the frequencies of magnonic gaps. It is convenient to study magnonic crystals using these reflected waves. The reasons for this are the following. The reflected waves and the incident wave have different frequencies spaced at the frequency of the SAW [19,20]. They do not interfere with each other when using a selective receiver that permits to measure reflected signals at relatively low levels of the input waves in comparison with the direct magnonic band gap measurements demanding much more powerful SAW [9]. In the present work we had been using this “reflected wave method”. Configuration of our experimental arrangement is schematically depicted in Fig. 1. We used the reflected waves approach described here above and so called “bridge method” for SAW excitation in YIG–GGG samples [9], when SAW excited in the piezoelectric plate transfer to YIG–GGG structure through an acoustic contact especially created between the adjacent surfaces of the plate and the structure. YIG films of 5-μm-thick grown on 500μm-thick GGG substrate of (111) crystallographic plane were used in our experiments. The YIG–GGG samples were cut out in the shape of 18 5 mm2 rectangular. Aluminum films through a mask were deposited on the YIG surface as shown in Fig. 1 by the thermal evaporation method in vacuum (without substrate heating). The length of Al films was 6 mm, the width was 5 mm. It should be noted, that the experimental results, as our experiments showed, were practically independent of the length of the film of Al, provided that it was longer than 3 mm. SAW was excited by an interdigital transducer (IDT) with 23 MHz center frequency fabricated by photolithography on the surface of the Y-Z LiNbO3 base plate. SAW passed to the GGG–YIG sample through a special acoustic contact between the base plate and YIG–GGG sample as shown in Fig. 1. Insertion losses at SAW
Fig. 1. Top and side views of the configuration of experimental samples and connections to measurement circuits.
181
excitation in GGG–YIG structures in 50-Ω circuit with matching were estimated in our experiments as 10 dB. The power of SAW at frequency 20 MHz was equaled to 0.1 mW. SMSW were excited and detected by means of an aluminum strip-antenna of 20-μmwidth and 0.5-μm-thick deposited onto the surface of the dielectric plate (alumina) and placed on the surface of YIG film as shown in Fig. 1. The microwave power supplied to the antenna for the excitation of SMSW was equal to 1 μW. Natural microscopic irregularities of the adjacent surfaces of the YIG–GGG structure and the antenna on the dielectric plate did not allow the SAW to leak into the dielectric plate, in contrast with the surfaces of the LiNbO3 base plate and YIG–GGG structure where a special liquid lubricant created acoustic contact. The dielectric plate with the antenna positioned on the surface of the YIG–GGG brings, as experiments showed, to addition attenuation of the SAW less than 1 dB. The apertures of IDT and the antenna were 5 mm. The edges of YIG–GGG structures were subjected to a special treatment (tapered by a diamond needle file) to minimize SAW and SMSW reflections from the structure's edges. External magnetic field HO ¼640 Oe was applied parallel to the antenna.
3. Experimental results and discussion First of all, the structures without metallization and then the structures with relatively thick aluminum films of 3 μm (greater than the depth of the skin layer) were investigated. Microwave signals of frequency f in the range from 3.5 GHz to 3.9 GHz were applied to the antenna through a circulator. The antenna could excite SMSW in both directions and could also be used to detect the SMSW coming to it from both directions. The efficiency of excitation of the relatively long SMSW used in our experiments was practically equal in both directions due to rather thin YIG films (without metallization) [22]. Output signals from the antenna were measured by a selective receiver tuned to frequencies f þF or f F, where F is the frequency of SAW. When the receiver was tuned to f F, the signal corresponding to the reflected SMSW arising in area B (see Fig. 1) where SAW and SMSW of frequency f propagate in the same direction is measured. When the receiver is tuned to fþ F the reflected SMSW arising in area A is measured. In this case, the reflected signal occurs at counterpropagation of the SAW and SMSW (see Fig. 1). When measuring samples with metal films the distance between the metal film and the antenna along the wave propagation must be no longer than 0.5 mm. Our experiments showed, in this case, this distance practically did not influence the measurement results (the change of the levels of measured signals did not exceed 1 dB). Relative levels, A, of reflected SMSW measured as a function of frequency f of incident SMSW at F¼20 MHz and HO ¼640 Oe are presented in Fig. 2 for different experimental situations: a) pure YIG–GGG structure without metallization; b) metalized YIG–GGG with Al film of 3-mm – thick, for mutually opposite directions of magnetic field. The maximum of the curve 1 in Fig. 2a (SAW and SMSW propagates in the same direction – area B in Fig. 1) is achieved at f1 ¼3.6403 GHz and that of the curve 2 in Fig. 2a (counter-propagation – area A in Fig. 1) is achieved at f2 ¼3.6205 GHz, in agreement with the theory of inelastic scattering of SMSW by SAW [19,20] (corresponding explanatory chart of scattering is given in Fig. 3a). This means that the magnonic band gap frequencies depend on the direction of SMSW propagation relative to the direction of SAW propagation. Thus, SAW – dynamic magnonic crystals have nonreciprocity properties for SMSW even with the symmetrical dispersion characteristic (without any metal covering) and the reference direction here is set by the direction of SAW propagation rather than by the direction of the biasing magnetic field.
182
R.G. Kryshtal, A.V. Medved / Journal of Magnetism and Magnetic Materials 395 (2015) 180–184
Fig. 2. Relative levels, A, of reflected SMSW measured as a function of frequency, f, of the external microwave signal applied to the antenna at F ¼20 MHz and HO ¼ 640 Oe, YIG film: thickness 5 μm, 2ΔH¼ 0.5 Oe, 4π Mo ¼ 1760 G. (a) YIG–GGG structure without metallization. Curve 1 – SAW and SMSW propagate in the same direction (area B in Fig. 1); curve 2 – counter-propagation of SAW and SMSW (area A in Fig. 1). (b) Metalized YIG–GGG with Al film of 3-mm – thick. Curves 3 and 4 – SAW and SMSW propagate in the same direction at two mutually opposite directions of the magnetic field; the curves 5 and 6-oncoming propagation SAW and SMSW in the region of the sample covered with the aluminum film (area A in Fig. 1) for two opposite directions of the magnetic field.
The inversion of the magnetic field does not change significantly the curves 1 and 2 at least the resonance frequencies f1 and f2 remain unchanged within the error of measurement (70.5 MHz). This behavior was expected because with a relatively small thickness of the YIG film in the structure without a metal layer the dispersions of SMSW propagating in opposite directions are almost the same. The curves 3 and 4 in Fig. 2b correspond to the situation of the SAW and SMSW propagating in the same direction for two opposite directions of the biasing magnetic field in the area B where the surface of YIG is without metal, while the curves 5 and 6 represent the reflected SMSW at oncoming propagation SAW and SMSW in the region of the sample covered with the aluminum film (area A in Fig. 1) for two opposite directions of the magnetic field. The curve 3 is almost identical with the curve 1 and f1 ¼f3, which was to be expected, as they correspond to the same experimental situation – waves propagate in the same direction in the region without metal (area B in Fig. 1). When the SAW and SMSW are propagating towards each other in the metalized region (area A) the maximum of the reflection curve 5 is at higher frequencies f5 ¼ 3.721 GHz. At the inversion of the magnetic field, the
curve 6 is shifted from curve 5 reaching the maximum at f6 ¼3.734 GHz. All these measured dependencies of the reflected SMSW on frequency can be explained by the laws of inelastic scattering of spin waves by SAW. Explanatory charts of the conditions imposed on the wave vectors and the frequencies of the interacting waves, at which magnonic band gaps arise in the corresponding experimental situations is shown in Fig. 3. The resonance frequencies of reflected SMSWs are the frequencies of corresponding magnonic band gaps for SMSW in the SAW dynamic magnonic crystal and their shift at the inversion of the magnetic field demonstrates the nonreciprocity in such a magnonic crystal with metallization. Fig. 4 represents measured dependencies of the frequencies of magnonic gaps on the SAW frequency for two mutually opposite directions of the magnetic field in the GGG–YIG–Al structure with the thickness of Al film greater than the skin layer depth. This result shows the additional possibilities of magnonic crystal tuning by changing the SAW frequency, that may be useful in the creation of new devices for information processing. We used our experimental technique to investigate nonreciprocal structures GGG–YIG–aluminum film with different thicknesses of Al films, even less than the depth of the skin layer. It was possible to use one and the same YIG–GGG sample with metal films of different thicknesses. The metal film of the previous thickness had been etched out (in aqueous alkali KOH solution) and then a film with another thickness was evaporated on the YIG–GGG structure. Taking into account that the specific conductivity of fabricated films may differ from the tabulated value of ideal aluminum all our experimental results are given depending on sheet conductivity rather than on the thickness of fabricated films. Conductivity of deposited aluminum films was determined by measuring the resistance of a so called “bystander” (another YIG–GGG structure with golden electrodes fabricated on the surface of YIG) placed side by side with our sample during the deposition process, that provided the deposition of aluminum film similar to that deposited on our sample. Measured relative levels of signals corresponding to the reflected SMSW at counterpropagating with the SAW of 20 MHz (see area A in Fig. 1) as a function of the input signal frequency are given in Fig. 5 for three values of sheet conductivities of the aluminum film (curves 1, 2, 3) and for “pure” YIG–GGG (curve 4) at co-propagating SMSW and SAW (area B in Fig. 1). The curves 1, 2 and 3 were obtained at the receiver tuning to fþ F frequencies and curve 4 – at tuning to f F frequencies. The curve 4 served as a reference curve for maintaining the constant level of SAW power when changing samples with films of different conductivity. From Fig. 5 it is seen that the decrease in conductivity leads to a decrease of the resonance frequency of the reflected SMSW and to the broadening of the resonance curve. This behavior is consistent with the concept of scattering of SMSW on SAW, since the decrease in the conductivity of the metal film changes the slope of the dispersion curves and, therefore, must be a decrease in resonant frequency and with decreasing film thickness less than the depth of the skin layer increases the propagation loss of the waves, that leads to the broadening of the resonance curve of the reflected SMSW [20]. By the other words, the frequency of the magnonic gaps is decreasing and the widths of the magnonic gaps are broadening with the decrease of conductivity. At the inversion of the magnetic field all curves in Fig. 5, except the curve 4, were shifted along the frequency axis as it follows from nonreciprocal properties of the structures. The results of measurements of the dependencies of the resonance frequency of the reflected SMSW (frequency of the magnonic band gaps) and the width of the resonance curve (the width of the magnonic band gaps) on conductivity of the metal
R.G. Kryshtal, A.V. Medved / Journal of Magnetism and Magnetic Materials 395 (2015) 180–184
183
Fig. 3. Explanatory charts to the results in Fig. 2 – conditions imposed on the wave vectors and the frequencies of the interacting waves, at which magnonic band gaps arise in the corresponding experimental situations. fFMR – frequency of ferromagnetic resonance at a given external magnetic field. (a) “Pure” YIG–GGG structure without metallization. The solid line curves–branches of the dispersion f(k) of SMSW propagating in mutually opposite directions. kr, ki, q – wave numbers of reflected and incident SMSW and SAW, respectively. (b) Metalized YIG–GGG structure. Solid and dotted line curves, denoted as D↓and D↑, – dispersion curves f(k) of SMSW at mutually opposite directions of the magnetic field. k↑r , k↑i , k↓r , k↓I – wave numbers of reflected and incident SMSW at two mutually opposite directions of the magnetic field, respectively. The arrows at the letters D and k symbolize the direction of the magnetic field.
Fig. 4. Measured dependencies of magnonic gap frequencies, fog, on SAW frequency, F, in metalized YIG–GGG structure (curves 1 and 2) and in “pure” YIG–GGG (curve 3) at two mutually opposite directions of the magnetic field. The arrows at the letters Ho symbolize the direction of the magnetic field.
Fig. 6. Measured resonance frequency, fg of the reflected SMSW (magnonic gap frequency) and the width of the resonance curve (width of the magnonic gap), Δfg as functions of conductivity, s, of the metal film in the GGG–YIG–Al structure. Counter-propagation of SMSW and SAW. F¼20 MHz.
film in the GGG–YIG–Al structure are presented in Fig. 6. These results show that the metal film conductivity, less than 80 S, has a strong influence on the propagation of SMSW in the magnonic crystal based on the metalized YIG. The conductivity of the film of ideal aluminum of the thickness equal to the thickness of the skin layer, δ, at 3.7 GHz (δ ¼1.35 μm) would be 50 S. This means, that the thickness of aluminum films of the order of 1.5 δ is required to avoid excessive losses of SMSW in such magnonic crystals. As it was indicated earlier the SAW power used at our measurement was 0.1 mW. At higher powers the width of the resonance curves begins to depend on SAW power [20].
4. Conclusion
Fig. 5. Relative levels, A, of reflected SMSW measured as a function of frequency, f, of the external microwave signal applied to the antenna at F¼ 20 MHz in metalized YIG–GGG structure for three values of sheet conductivities of the aluminum film curves 1 – 110 S, curve 2 – 20 S, curve 3 – 12 S, – counter-propagation SMSW and SAW and for “pure” area of the YIG surface (curve 4) – co-propagation SMSW and SAW. YIG film: thickness 5.2 μm, 2ΔH¼ 0.5 Oe, 4π Mo ¼ 1760 G.
Thus we have experimentally shown peculiarities of nonreciprocity manifestations for surface magnetostatic waves in dynamic magnonic crystals created by SAW propagating in structures with YIG layers. Such dynamic magnonic crystals have nonreciprocity for spin waves even in structures with symmetrical with respect to the axis of frequency dispersion characteristics. The reference direction is set by the direction of SAW propagation rather than by the direction of the magnetic field. As in static magnonic crystals in SAW – dynamic magnonic crystals based on
184
R.G. Kryshtal, A.V. Medved / Journal of Magnetism and Magnetic Materials 395 (2015) 180–184
GGG–YIG–metal structures nonreciprocity for SMSW was observed also at the inversion of the direction of the magnetic field. So in SAW – dynamic magnonic crystals based on the GGG–YIG– metal structure the nonreciprocity managed both as by the direction of the biasing magnetic field and by the direction of SAW propagation takes place. The influence of the aluminum film conductivity on magnonic gaps frequency and the gap's width in SAW – magnonic crystals at the base of metalized YIG–GGG structures were investigated. The conductivity of the aluminum film higher than 80 S is required to avoid excessive losses of SMSW in such magnonic crystals. In addition, in these crystals it is possible to adjust the frequency of the magnonic band gaps and their depth by changing the frequency and power of SAW. These nonreciprocal properties investigated in our work may be helpful for designing new nonreciprocal devices of signal processing in GHz frequency range. In this our work, we used, for simplicity, so-called “bridge method” for excitation of SAW, at designing practical devices piezoelectric films deposited onto YIG surface should be used for SAW excitation.
References [1] J.O. Vasseur, L. Dobrzynski, B. Djafari-Rouhani, Phys. Rev. B 54 (1996) 1043. [2] Yu. V. Gulyaev, S.A. Nikitov, L.V. Zivotovskii, A.A. Klimov, Ph Tailhades, L. Presmanes, C. Bonningue, T.S. Tsai, S.L. Vysotskii, Y.A. Filimonov, JETP Lett. 77 (2003) 567.
[3] A.V. Chumak, A.A. Serga, B. Hillebrands, M.P. Kostylev, Appl. Phys. Lett. 94 (2009) 172511. [4] A.V. Chumak, T. Neumann, A.A. Serga, B. Hillebrands, M.P. Kostylev, J. Phys. D: Appl. Phys. 42 (2009) 205005. [5] Kai H. Chi, Yun Zhu, Rong W. Mao, James P. Dolas, Chen S. Tsai, J. Appl. Phys. 109 (2011) 07D320. [6] Mitsuteru Inoue, Alexander Baryshev, Hiroyuki Takagi, Pang Boey Lim, Kohei Hatafuku, Josho Noda, Kenji Togo, Appl. Phys. Lett. 98 (2011) 132511. [7] F.S. Ma, H.S. Lim, Z.K. Wang, S.N. Piramanayagam, S.C. Ng, M.H. Kuok, Appl. Phys. Lett. 98 (2011) 153107. [8] Bai Lihui, Kohda Makoto, Nitta Junsaku, Appl. Phys. Lett. 98 (2011) 172508. [9] R.G. Kryshtal, A.V. Medved, Appl. Phys. Lett. 100 (2012) 192410. [10] M. Krawczyk, D. Grundler, Condens. Matter 26 (2014) 123202. [11] X.G. Wang, G.H. Guo, Z.X. Li, D.W. Wang, Y.Z. Nie, W. Tang, Europhys. Lett. 109 (3) (2015) 37008. [12] S.L. Vysotskii, S.A. Nikitov, E.S. Pavlov, Yu. A. Filimonov, J. Commun. Technol. Electron. 58 (4) (2013) 347–352. [13] M. Mruczkiewicz, M. Krawczyk, G. Gubbiotti, S. Tacchi, Yu.A. Filimonov, D. V. Kalyabin, I.V. Lisenkov, S.A. Nikitov, New J. Phys. 15 (2013) 113023. [14] R. Huber, M. Krawczyk, T. Schwarze, H. Yu, G. Duerr, S. Albert, D. Grundler, Appl. Phys. Lett. 102 (2013) 012403. [15] M. Mruczkiewicz, E.S. Pavlov, S.L. Vysotsky, M. Krawczyk, Yu. A. Filimonov, S. A. Nikitov, Phys. Rev. B 90 (2014) 174416. [16] M. Mruczkiewicz, M. Krawczyk, J. Appl. Phys. 115 (11) (2014) 113909. [17] S.R. Seshadri, Proc. IEEE 58 (3) (1970) 506–507, http://dx.doi.org/10.1109/ PROC.1970.7680. [18] N. Kanazawa, T. Goto, J.W. Hoong, A. Buyandalai, H. Takagi, M. Inoue, J. Appl. Phys. 117 (17) (2015) 17E510. [19] A.F. Popkov, Fiz. Met. Metalloved. 59 (1985) 463. [20] R.G. Kryshtal, A.V. Medved, A.F. Popkov, J. Commun. Technol. Electron. 39 (1994) 132. [21] V. Chumak, P. Dhagat, A. Jander, A.A. Serga, B. Hillebrands., Phys. Rev. B 81 (2010) 140404(R). [22] W.S. Ishak, Proc. IEEE 76 (1988) 171.