Controlling ion deposition to improve room-temperature the magnetoelectricity and microwave properties of M-type hexaferrite thin film

Abstract

Hexagonal ferrites have diverse applications in modern technologies because of their incredible magnetic properties. They have long been utilized as permanent magnets and these days they have a vast body of applications in microwave devices or in magnetic recording media and MRAMs. Tunable electromagnetic signal processing elements such as phase shifters, circulators, and filters are also utilizing ferrites. Magnetic tuning of ferrites in X band region and higher requires large electromagnets. Magnetoelectric ferrites have the possibility to control their magnetic parameters by applying electric field or voltage is a practical solution for miniaturizing tunable ferrite devices. Improved understanding of the magnetoelectric mechanism in hexagonal ferrites is very important to improve their ME properties. Magnetoelectric (ME) effect received lots of attention when it was first discovered in the 1960s. However, because of its general weakness especially at room temperature, limited knowledge about it, and a restricted number of compounds showing this effect, it was forgotten for decades. But these days, our knowledge about ME effect and its origin is much higher and a decent number of magnetoelectric materials are now discovered. Layered composites consisting of piezoelectric and magnetostrictive layers mechanically attached was considered as one possible solution to this issue. However, producing composites has its own difficulties and limits. On the other hand, some multiferroic materials such as some families of hexagonal ferrites (i.e. M, Y, and Z-type) are one of these materials which relatively show high magnetoelectric constant, alpha, at room temperature. Recently, SrCo2Ti2Fe8O19 was reported to be magnetoelectric at room temperature. M hexaferrite doped with Co--Ti has been investigated comprehensively for magnetic and microwave properties except for magnetoelectricity. We studied the effect of cobalt substitution on the magnetoelectricity of doped Sr-M hexaferrite. We showed that while the cobalt doping level plays an important role in magnetoelectricity of SrCo(2-x)Ti(2-0.5x)Fe8O19 but its effect is not linear and it has an optimum value. It is also shown that the magnetoelectric effect is not the same in every direction. Besides the doping level, occupancy of atomic sites and their interaction was also studied. It was known so far that in SrCo2Ti2Fe8O19, magnetic cations of Co2+ occupy octahedrally atomic sites in S block of the magnetoplumbite crystal structure. On the other hand, Ti4+ always occupies 12k atomic site between the S and R blocks and decouples the RS and RS rising spin spiral magnetic configuration which is the main mechanism of magnetoelectricity in hexaferrites. Using the alternate target laser ablation deposition (ATLAD) technique, we were able to force cobalt cations to move into the tetrahedral sites. This improves the magnetoelectric effect up to 50 times for the same level of doping. However, no significant changes in 4piMs and coercivity were observed. Improving the magnetoelectric coupling in Sr-M hexaferrite helped us to design electrically tunable microwave phase shifter. In general, tunability at high frequencies is a big issue in microwave ferrite devices. Utilizing the magnetoelectric effect brings us this opportunity to tune the magnetic property of the material inside of a device with the application of only a few volts. Preliminary results coming from HFSS simulator shows that 800 millidegree change can be achieved with the application of 5 V in a coplanar waveguide fabricated on SrCo2Ti2Fe8O19 in the frequency range between 10 kHz and 15 GHz.