2017 IEEE International Young Scientists Forum on Applied Physics and Engineering YSF-2017
Current Tunable Barium and Strontium Hexaferrite Millimeter Wave Resonators M. A. Popov Faculty of Radiophysics, Electronics and Computer Systems, Taras Shevchenko National University of Kyiv, Kyiv, Ukraine,
[email protected] magnetization and uniaxial anisotropy field. Thus, this ME effect belongs to current-induced one, yet the measured current densities are many orders of magnitude smaller than those, common for spin torque transfer.
Abstract— This paper reports the experimental results on resonance frequency tuning of a single crystal M-type hexaferrite resonators with an applied current. The tuning arises from nonlinear magnetoelectric interaction which changes both saturation magnetization and uniaxial anisotropy field. Frequency shifts up to ≈400 MHz for samples in the saturated state and ≈300 MHz in the multi-domain state were measured in the U-band. The tuning is found to be linear with electric power.
The specimens investigated in this work were pure strontium and barium hexaferrites (SrFe12O19 and BaFe12O19) with the M-type crystallographic structure, which are promising materials for use in low-loss millimeter-wave signal processing devices [7]. They have high magnetocrystalline anisotropy field which facilitates operation of ferrite-based components at mm-wave frequencies without the need for large external magnetic bias. Moreover a stable domain structure in hexaferrites potentially allows them to work in the absence of external magnetic fields (self-bias operation). That would drastically reduce the size and weight of such devices.
Keywords— hexaferrite resonator; millimeter wave band; magnetoelectric effect; current tuning
I.
INTRODUCTION
There have been a lot of researches in recent years on current (I)/electric field (E) control of magnetic parameters in magnetic thin films and composite structures [1-4]. Among them one major field of research is multiferroic materials, which combine magnetic and electric ordering either in the same substance or in an artificial composite metamaterial. When a coupling between magnetic and electric systems is provided, magnetic field H applied to such material induces electric polarization and, vice versa, electric field E affects saturation magnetization value. E-control of magnetization in single phase or composite multiferroics has been studied extensively [3, 4]. However, in the former one magnetoelectric (ME) effect is usually rather weak even at low temperatures. On the contrary, in composites comprising magnetostrictive and piezoelectric phases a strong strain-mediated ME coupling could be realized at room temperature. Device applications for the ME composites include magnetic sensors, gyrators and microwave signal processing devices.
Recently, there were a number of reports of a roomtemperature linear E-field ME effect in hexaferrites of various compositions, including M-type with Sc or Co-Ti substitutions [8-10]. Those compounds have a complex conical magnetic structure and thus their multiferroic properties are caused by nonuniform ME effect [11]. Yet, this is not the case for pure or weakly substituted M-type hexaferrites, which are known to have collinear arrangement of sublattices’ magnetizations and thus do not possess spontaneous electric polarization. Regardless, such hexaferrites may find application as a constituent part of strain-mediated ferrite-ferroelectric composites for use in tunable microwave devices. The E-field tuning of barium hexaferrite-PZT resonators was indeed reported previously [12, 13], yet the tuning range did not exceed 20 MHz even for close-to-breakdown electric field magnitudes. In this paper we will demonstrate that currentinduced frequency tuning in our case is one to two orders of magnitude higher than those of hexaferrite-piezoelectric artificial multiferroic composites.
Another approach to magnetic properties tuning by electric signals makes use of a well-investigated phenomena of spin torque transfer by a spin-polarized conduction electrons [1, 2]. This effect has been successfully utilized for current-induced domain wall movement, magnetization reorientation and microwave signal generation [5, 6]. However, magnetization control by spin-polarized current typically requires large current densities up to ≈107 A/cm2.
II.
The samples explored in this study were single-crystal BaFe12O19 (BaM) and SrFe12O19 (SrM) platelets with the hexagonal c-axis perpendicular to the plane, grown by floating zone technique. The resonators lateral dimensions and thicknesses are specified in caption to the Fig. 3. The conducting Pt/Ti electrodes with 1.5 µm thicknesses were deposited on top and bottom surfaces by magnetron sputtering to provide electrical contacts. The bottom surface was
This report describes a novel type of nonlinear ME effect in single crystal specimens of M-type hexagonal ferrites. Namely, it will be demonstrated, that when a DC current is applied along the hexagonal c-axis of a hexaferrite platelet it results in a formidable variations of resonance mode’s frequencies which was explained as a result of modifications of both saturation
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EXPERIMENTAL SETUP
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October 17-20, 2017 | Lviv, Ukraine
2017 IEEE International Young Scientists Forum on Applied Physics and Engineering YSF-2017 completely covered, whereas on top only a 0.75 mm wide conducting stripe was formed. The sample was soldered to a thick copper plate (that served both as a waveguide short and a heat sink) with a low-temperature alloy. Thus, when a voltage is applied between top and bottom electrodes, a DC current passes across the thickness of the ferrite sample resulting in a nonlinear ME effect which was registered using standard radiospectroscopic technique. The sketch of experimental setup is shown on Fig. 1.
III.
THEORETICAL BACKGROUND
In order to properly interpret experimental results one needs to establish a specific relation between magnetostatic modes’ frequencies and material’s magnetic parameters. That will allow us to extract the values of initial and ME-modified saturation magnetization and uniaxial anisotropy constant from collected data. Moreover, since measurements were conducted for both multidomain and saturated magnetic states of sample, such relation should be known for both these situations.
Microwave measurements were carried out using a precalibrated vector network analyzer (Agilent-E8361A) in a reflection measurements mode [14]. The sample cell was placed at the open end of a waveguide and the scattering matrix parameter S11 (return loss) vs. frequency dependencies were recorded and analyzed with and without electric current flowing through the sample.
A general approach to magnetostatic eigenmodes problem consists in representing magnetization vector M as a sum of static and dynamic parts and then solving the Landay-Lifshitz equation in linear approximation [15]. The effective magnetic field that enters this equation is found as a functional derivative from the total free energy density W according to H eff = −∂W/∂M . In turn, full energy W of ME material
During the experiment, this waveguide section was placed between the pole pieces of an electromagnet so that the bias field H0 was directed perpendicularly to the sample plane and, hence, parallel to the hexaferrite six-fold symmetry axis. Then a magnetic field magnitude was fixed at some specific value ranging from 0 to 10 kOe and frequency was swept through the waveguides operating range. The magnetostatic (MS) modes resonance frequencies at given Ho were then extracted from S11 vs. f curves. Two different cases were studied: (i) zero-biasfield resonances that take place when the sample is in multidomain state with stripe domain structure [15] and (ii) resonances in magnetically saturated (uniformly magnetized) state for bias magnetic field H0 >5 kOe.
comprises magnetic, electric and magneto-electric terms. Magnetic energy includes dipole-dipole, Zeeman and spinorbital contributions, and depends (among others) on saturation magnetization and anisotropy energy constant(s). It is the magneto-electric term that is responsible for microwave ME effect [16]. A phenomenological expression for this term, consistent with crystallographic symmetry of material is presented in [14]. After taking into consideration a standard electromagnetic boundary condition on the surface of the sample, dispersion equation then follows. The expressions for magnetostatic mode frequency in anisotropic ferrite in multi-domain and single domain states are well established [17]: (1) f r (H 0 = 0) = γH a ,
Next, the investigation the current-induced ME effect was conducted. Current pulses of a preset amplitude, rise/fall time
