Barium Hexaferrite Thin Films Research Articles

In-plane hard magnetic BaFe12O19 thin films grown on a-plane Al2O3 112¯0 substrates

Ferrimagnetic barium hexaferrite (BaFe12O19) thin films of varying thicknesses were grown on a-plane Al2O3 112̅0 substrates at elevated temperatures using pulsed laser deposition. X-ray diffraction measurements, Raman spectroscopy, and atomic force microscopy analyses were used to investigate the BaFe12O19 film growth and structure. The results indicate that the formation of an α-Fe2O3 ‘bridge layer’ promotes the growth of a BaFe12O19 phase showing an in-plane uniaxial magnetic anisotropy with an easy-axis of magnetization along the [ 0001 ] hexagonal c-axis direction. The crystallographic relationship between the existing phases is given by BaFe12O19 101̅0 //α-Fe2O3 112̅0 //Al2O3 112̅0. As the thickness increases, the fraction of the BaFe12O19 phase increases further. This is accompanied by a change in grain morphology turning from a columnar to a more elongated shape, where the short axis of the BaFe12O19 grains points along the [ 0001 ] easy axis direction.

Characterisation of barium hexaferrite thin films in microwave frequency band

The characterization of barium hexaferrite thin-films at microwave frequencies is important for determining their electromagnetic properties by measuring the elements of the permeability tensor. Layers of 15 µm were deposited by RF sputtering on a coplanar line. As a result of this deposition and annealing technique, the magnetic moments are mainly oriented in the plane of the layer. By measuring the S parameters under magnetic field conditions, the µ and κ elements of the permeability tensor were extracted and their variations as a function of the applied field were highlighted. Possible applications to absorbent layers are being considered.

Effect of Annealing Atmosphere on Microstructure, Magnetic, and Microwave Properties of Barium Hexaferrite Thin Films

Effect of Annealing Atmosphere on Microstructure, Magnetic, and Microwave Properties of Barium Hexaferrite Thin Films

Thickness effect on structural properties of post annealed barium hexaferrite films deposited by ion beam sputtering

Barium hexaferrite thin films were prepared by ion beam deposition on sapphire (102) single crystal substrates. The structural properties of the films were analyzed by means of X-ray diffraction, atomic force microscopy, magnetic force microscopy and scanning electron microscopy. The obtained results were interpreted as a presence of layered structure with different types of texture and its formation mechanism is proposed. The significant influence of BaFe12O19 thickness on its microstructure was observed. Films obtained under certain conditions are potentially fit for further growing of oriented hexaferrite with out-of-plane orientation of c-axis.

Magnetron Sputtered Perpendicular Barium Hexaferrite Thin Films Produced by the Multilayered Method

We report on the synthesis and characterization of thin M-type barium hexaferrite (BaFe12O19 or BaM) films on sapphire (0001) substrates by RF magnetron sputtering. Single layers of 150 nm, 200 nm (S200), 400 nm (S400), 600 nm, and 700 nm were fabricated. Multilayered technique with inner-layer annealing was employed to fabricate the 400 nm (M400) BaM films. The thickness dependence of the magnetic, structural, elemental analysis, and surface morphological properties of the BaM films has been investigated using VSM, XRD, EDX, and SEM, respectively. For the M400 BaM films, excellent magnetic properties with high values of out-of-plane coercivity (Hc┴ = 3.1 KOe) and squareness (S┴ = 0.9) were obtained. The surface morphology of the M400 shows a platelet-like grains, which have their c-axis orientation in the out-of-plane direction. The magnetic properties and XRD data of the S200 films and M400 were about the same. For the single S400, S600, and S700 films, the magnetic properties have been reduced due to the excess oxygen concentration and the exchange interactions, the stress relaxation of the lattice constant, and the formation of the acicular grains. The acicular grains of the S400 films favor their c-axis aligned in the in-plane direction.

Micromagnetic Study on the Magnetization Reversal of Barium Hexaferrite (BaFe12O19) Thin Film

This study investigates a magnetization reversal mechanism based on the hysteresis curve of Barium Hexaferrite (BFO) thin film by micromagnetic simulation through parallel and perpendicular magnetization directions along the axes. The hexagonal shape of the BFO film was modeled with thicknesses of 5, 10, and 15 nm and a diameter size ranging from 50 to 100 nm. It was found that the coercivity field H C and the saturation field H S of the BFO film decreased as the diameter size increased and thickness decreased. It was observed that the nucleation field H N increased as the diameter size increased. An analysis of energies showed that the demagnetization energy was dominantly influenced by the diameter and thickness in comparison to the anisotropic energy. From the hysteresis curve, the switching time was also investigated. Interestingly, the switching time was faster for the thinner BFOs with a diameter under 70 nm. For particles larger than 70 nm in diameter, the switching time showed fluctuation irrespective of the BFO thickness. Based on these results, a diameter size of 70 nm is proposed as the critical size for producing the equal time for switching domain polarity.

Influence of ERTA on magnetocaloric properties of Sr doped BaFe12O19 thin films

The effect of electron beam rapid thermal annealing (ERTA) on the crystallographic, structural, magnetic and magnetocaloric properties of Sr doped M-type barium hexaferrite thin films (Ba1−xSrxFe12O19) deposited with set optimized parameters on silicon (001) substrates were investigated. Amorphous to crystalline phase transitions occurred when the as-deposited thin films were annealed, however crystallinity reduced as a function of Sr concentration. The as-deposited thin films showed uniform grain size and distribution with reduction in grain size as a function of Sr concentration, whereas annealed thin films showed increase in agglomerated structures as a function of Sr concentration. As-deposited thin films with x = 0, 0.05 and 0.1 showed weak ferromagnetic response. Magnetic hysteresis loops and magnetic force microscopy images indicate improved c-axis in-plane magnetization with reduced coercivity for Sr doped thin films and saturation magnetization for thin films subjected to ERTA. The Arrott plots in positive quadrant suggested a typical M-type ferrite behaviour with second-order ferromagnetic phase transitions around the Curie temperature. The entropy changes associated with magnetic phase transitions were determined using magnetization measurements performed in the range of 50–105 K for as-deposited and 65–125 K for ERTA samples under different magnetic fields. The calculated maximum relative cooling power (RCP) of the as-deposited and ERTA thin films are 1941 mJ/cc and 3065 mJ/cc for 50 kOe magnetic fields. This suggests that Ba1−xSrxFe12O19 with x = 1 is a promising candidate for micro and nano magnetic refrigeration systems.

Fabrication and micromagnetic modeling of barium hexaferrite thin films by RF magnetron sputtering

Abstract The synthesis and characterization of thin M-type barium hexaferrite (BaFe12O19 or BaM) films on silicon are reported. Multilayer in situ technique was employed to anneal the films at 850–900 °C for 10 min. The thickness dependence of the magnetic properties of the BaM films has been investigated using VSM. For the BaM 150 nm thickness film, acicular BaM grains were present having their c-axis randomly oriented. For the BaM films thicker than 150 nm, lattice relaxation favors the c-axis to be aligned in the film plane. The micromagnetic simulation was used to model the out-of-plane and the in-plane hysteresis loops. We have achieved good matching between the experimental data and the model. Using the micromagnetic model, we have estimated the deflection angle of c-axis from the normal plane θ = 25° for the 150 nm thick film.

Effect of Al on the microstructure, magnetic and millimeter-wave properties of high oriented barium hexaferrite thin films

The microstructure, magnetic and millimeter-wave properties of high oriented barium hexaferrite (BaAlxFe12−xO19) thin films with Al doping level x from 0 to 2 are reported. The films were grown on Pt/TiO2/SiO2/Si substrate by Sol-gel method. It is found that with increasing x from 0 to 2 the hexagonal grain disappear, together with Curie temperature dropped from 449°C to 332°C and saturated magnetization (4πMs) decreased from 3.8kG to 1.9kG, it is attributed to the fact that the Fe ions were substituted by non-magnetic Al ions, leading to the Fe3+-O-Fe3+ super-exchange interaction became weak. The ferromagnetic resonance (FMR) measurement showed that the FMR linewidths is as low as 113Oe @ 58GHz, and the FMR frequency shifted to higher frequency range when increasing Al doping level. These result offer the potential application of barium ferrite thin films in tunable millimeter wave devices such as filter, circulator and isolator.

Magnetic properties and millimeter wave loss of highly oriented scandium substituted barium hexaferrite thin films for millimeter wave applications

Scandium substituted barium hexaferrite (BaFe12−xScxO19, BaM) were grown on platinum-coated Si substrates using sol–gel method. It is found that scandium pay an important role in the structure, magnetic and millimeter wave properties of BaM system. The number of hexagonal grains decreased with increasing the substituted contents x of Sc. XRD results confirmed that BaM films have crystal texture of c-axis grains perpendicular to film plane, and the degree of texture for all the films are as high as 92%. The anisotropy field decreased from 16.2 kOe (x = 0) to 2.5 kOe (x = 2), meanwhile, the saturated magnetization decreased from 4.1 to 1.8 kG. The millimeter wave loss was studied by ferromagnetic resonance (FMR) system from 35 to 60 GHz, it is found that it has lowest FMR linewith of 129 Oe at 45 GHz for x = 0 sample, and the FMR field shift to higher for substituted thin films, together with the FMR linewith became broader. These results mean that the anisotropy field and FMR of BaM thin films were tunable and these thin films can be used in millimeter wave devices.

Fabrication and characterization of magnetically tunable metal-semiconductor schottky diode using barium hexaferrite thin film on gold

Barium Hexaferrite (BaM) is an extensively studied magnetic material due to its potential device application. In this paper, we study Schottky junction diodes fabricated using gold and BaM and demonstrate the function of a spintronic device. Gold (50 nm)/silicon substrate was used to grow the BaM thin films (100-150 nm) using pulsed laser deposition. I-V characteristics were measured on the Au/BaM structure sweeping the voltage from ±5 volts. The forward and reverse bias current-voltage curves show diode like rectifying characteristics. The threshold voltage decreases while the output current increases with increase in the applied external magnetic field showing that the I-V characteristics of the BaM based Schottky junction diodes can be tuned by external magnetic field. It is also demonstrated that, the fabricated Schottky diode can be used as a half-wave rectifier, which could operate at high frequencies in the range of 1 MHz compared to the regular p-n junction diodes, which rectify below 10 kHz. In addition, it is found that above 1 MHz, Au/BaM diode can work as a rectifier as well as a capacitor filter, making the average (dc) voltage much larger.

Magnetic property and stress study of barium hexaferrite thin films with different structures

M-type barium hexaferrite (BaM) thin films with different structures (ds100: single-layer 100 nm thick film; ds200: single-layer 200 nm thick film; db200: bilayer 200 nm thick film, where the thickness of each layer is 100 nm and the total film thickness is 200 nm) were deposited on (001) sapphire (Al2O3) substrate by radio frequency (RF) magnetron sputtering. Effects of different structures on the crystallographic, morphological, magnetic properties and stress were investigated in details. For the structure of ds100, BaM film processes platelet-like grains, which shows excellent magnetocrystalline anisotropy and large squareness ratio S⊥ (0.62). For the structure of ds200, there are a lot of acicular grains. Thus, the c-axis in-plane oriented and/or randomly oriented grains induce poor magnetocrystalline anisotropy. For the structure of db200, the platelet-like grains dominate the microstructure. Both squareness ratio S⊥ (0.67) and coercivity Hc⊥ (98.6 kA/m) of the db200 show the highest values among the three kinds of the film structures. In addition, with the increase of film thickness from 100 nm to 200 nm, the stress in film decreases gradually. Compared with the other structures, the bilayer 200 nm thick film (db200) obtains the smallest stress.

The effect of sol–gel preparation conditions on structural characteristics and magnetic properties of M-type barium hexaferrite thin films

We have shown the possibility to obtain M-type barium hexaferrite thin films with thickness of ~200–450 nm on the surface of dielectric α-Al2O3 substrates with low microwave dielectric loss (tan δ ~ 10−4 GHz) by a sol–gel method. For the production of high-dense homogeneous thin films of M-type barium hexaferrite (BaFe12O19, BHF) with nanorod-like grains and a uniform distribution of iron and barium ions, we have studied the synthesis conditions for thermally stable film-forming solutions with high concentrations of barium ions. Films with a c-axis magnetic texture were obtained by spin-coating the former solutions on α-Al2O3 substrates and annealing at temperatures between 473 and 1073 K. The resulting textured M-type BHF films have demonstrated the following magnetic parameters: H c⊥ = 334 kA/m, H c|| = 167 kA/m; M s⊥ = 0.005 emu, M s|| = 0.003 emu for the films’ thickness of ~200 nm, and H c⊥ = 360 kA/m, H c|| = 338 kA/m; M s⊥ = 0.009 emu, M s|| = 0.007 emu for the films’ thickness of ~450 nm. These M-type BHF thin films can serve as a promising basis for further development of multilayer microwave resonant elements.

Magnetic properties of in-plane oriented barium hexaferrite thin films prepared by direct current magnetron sputtering

In-plane c-axis oriented Ba-hexaferrite (BaM) thin films were prepared on a-plane (112¯0) sapphire (Al2O3) substrates by DC magnetron sputtering followed by ex-situ annealing. The DC magnetron sputtering was demonstrated to have obvious advantages over the traditionally used RF magnetron sputtering in sputtering rate and operation simplicity. The sputtering power had a remarkable influence on the Ba/Fe ratio, the hematite secondary phase, and the grain morphology of the as-prepared BaM films. Under 80 W of sputtering power, in-plane c-axis highly oriented BaM films were obtained. These films had strong magnetic anisotropy with high hysteresis loop squareness (Mr/Ms of 0.96) along the in-plane easy axis and low Mr/Ms of 0.03 along the in-plane hard axis. X-ray diffraction patterns and pole figures revealed that the oriented BaM films grew via an epitaxy-like growth process with the crystallographic relationship BaM (101¯0)//α-Fe2O3(112¯0)//Al2O3(112¯0).

Quasi-epitaxial barium hexaferrite thin films prepared by a topotactic reactive diffusion process

Quasi-epitaxial barium hexaferrite thin films (BaM) with crystallographic c-axis parallel to film normal were prepared through a topotactic reactive diffusion process using two-step solution deposition on c-plane sapphire. The two-step spin coating process involves preparing an epitaxial hematite film, coating the film with barium precursor solution and thermal annealing. The crystal orientation and magnetic anisotropy of BaM thin films were investigated by X-ray diffraction analysis, SEM observation and magnetic measurements. Hysteresis loops showed good magnetic anisotropy and high remanence ratio (RR) Mr/Ms=0.97. The films fabricated by two-step spin coating process displayed wider rocking curve width but better magnetic anisotropy than one-step spin coating. The possible mechanism of this discrepancy is discussed in this paper.

Effects of the oxygen partial pressure during deposition on the material characteristics and magnetic properties of BaM thin films

Barium hexaferrite thin films were deposited on Si (100) substrates using a radio frequency magnetron sputtering system. The effects of oxygen partial pressure during deposition process on the material characteristics and magnetic properties of BaM thin films were investigated. X-ray diffraction data indicated that with the oxygen partial pressure ratio increasing from 0% to 8%, the diffraction peaks shift slightly towards the larger angles and the corresponding lattice parameters (a and c) decrease. The saturation magnetization (Ms) ranges from 200 to 358kA/m as the oxygen partial pressure ratio is varied from 0% to 8%. These changes are attributed to the different oxidation states of Fe ions in the films with oxygen excess or vacancies. When deposited with 1% oxygen partial pressure, the film possesses the highest saturation magnetization (358kA/m) and exhibits better perpendicular c-axis orientation in comparison with those deposited without oxygen partial pressure or with higher oxygen partial pressure.

Al doped Ba hexaferrite (BaAlxFe12-xO19) thin films on Pt using metallo-organic decomposition

We grew a series of aluminum-substituted M-type barium hexaferrite (BaAlxFe12-xO19) thin films on a Pt (111) template and Si wafer using metallo-organic decomposition technique. We varied the composition from x = 0 to x = 2 with 0.25 step increments. X-ray diffraction patterns confirm highly textured c-axis polycrystalline films while atomic force microscope measurements allow us to estimate the lateral grain sizes which range from 0.2-1 micron depending on Al content. The microwave properties of these films were studied using a broadband ferromagnetic resonance spectrometer from 35 to 70 GHz. The measured out of plane effective anisotropy field increases in a nearly linear fashion with increasing Al concentration, between 12.8 kOe for x = 0 and 25 kOe for x = 2. The measured ferromagnetic resonance linewidths were relatively low, on the order of 150–300 Oe for compositions below x = 1, increasing significantly up to 800 Oe for x = 2. The easy axis magnetic hysteresis loops exhibit high squareness.

Influence of Magnetron Effect on Barium Hexaferrite Thin Layers

In this paper, we study the effects of a magnet, located in the cathode, on barium hexaferrite thin films deposited by RF magnetron sputtering technique. During the process, these effects can modify thickness, roughness and stress of coatings. The characteristics of the deposited layers depend on the substrate position that is located opposite of magnetron cathode. In the "magnetron area", one can observe that the high stress can produce cracks or detachment of layers and the increasing of both depositing rate and surface roughness. After sputtering elaboration, barium hexaferrite films are in a compressive stress mode. But, after the post-deposition heat treatment these films are in a tensile stress mode. To improve the quality of BaM films, the subsrtate has to be set outside the magnetron area.

Self-Biased Microstrip Junction Circulator Based on Barium Ferrite Thin Films for Monolithic Microwave Integrated Circuits

Barium hexaferrite thin films with thickness of 8 μm had been deposited on a sapphire substrate by radio-frequency magnetron sputtering. The results show that the grain size is about 2 μm in diameter. The coercivity is about 2.2 kOe and the saturation magnetization is about 4000 Gs with remanence ratio of 0.58. Based on the sputtered thin films, a self-biased microstrip junction circulator had been developed. The measured S-parameters indicate that a nonreciprocal effect greater than 10 dB occurs at about 26 GHz. The maximum isolation is 44 dB and the minimum insertion loss is 27 dB. The origins of the large insertion losses have been discussed.

Properties of barium hexa-ferrite thin films dependent on sputtering pressure

M-type barium ferrite thin films were deposited onto sapphire (00l) substrates by radio frequency magnetron sputtering. An ultra-thin layer about 20nm was deposited and annealed before continuous deposition of the films up to 500nm under different sputtering pressures: 0.2, 0.5, 0.8 and 1.0Pa, respectively. It was found that the atomic ratios of Fe to Ba increased from 9.3 to 15.0 with the increase of the pressure. The films sputtered at all pressures have c-axis normal to the film plane by a four circle X-ray diffractometer, which is an improvement of the films directly sputtered on the substrate. Needle-like grains were formed on the surface of the films under higher sputter pressure with bubble domains, which is originated from high magnetocrystalline anisotropy of the film. Magnetic hysteresis loops recorded by vibrating sample magnetometer agree with them, where in-plane and out-of-plane loops of the samples prepared under high sputtering pressures are quite different, while they are almost identical of the samples under low pressures. The influence of the sputtering pressure was understood by that with the increase of the pressure, resputtering of the films was increased. Nucleation with c-axis normal to the film plane was deteriorated. Thus samples prepared under high pressure have more needle-like crystallites which have c-axis parallel to the film plane.