B. N. Sahu

Temperature dependence of FMR and magnetization in nanocrystalline zinc ferrite thin films

Single phase nano-crystalline zinc ferrite thin films were deposited by RF-magnetron sputtering on quartz substrate at room temperature (RT) in pure Argon environment and annealed (in air) at different temperatures. Temperature dependence of magnetization was studied on these films using both VSM and by observing FMR (in X band). Value of exchange stiffness constant (D) was obtained by fitting Bloch’s law to the low temperature magnetization data. The value of D decreased monotonously with the annealing temperature (TA) of the samples. A film annealed at TA = 523 K, exhibited the highest magnetization value. The FMR line width of the films decreased with increase in measurement temperature. At RT (∼293 K), the lowest value of line width (ΔH) was 15 kA/m and 13 kA/m in parallel and perpendicular configuration respectively for the sample annealed at TA = 623 K.

A Study of FMR Linewidth and Magnetic Order in Nanocrystalline ZnFe 2 O 4 Thin Films

Zinc ferrite films were deposited on fused quartz substrate at different substrate temperatures using a pulsed laser deposition technique. The deposited samples were ex situ annealed at different temperatures up to 650 °C in air for 2 h. It was found that the spontaneous magnetization of the films depends both on the grain size and on the substrate temperatures. The largest values of spontaneous magnetization for our sample were 3000 G at 300 K and 6600 G at 10 K, for the film deposited at substrate temperature of 450 °C. The ferromagnetic resonance linewidths of the films were found to depend both on substrate and annealing temperatures. A relatively low linewidth of 195 Oe was observed in parallel configuration for the film deposited at 250 °C and annealed at 350 °C.

Preparation of Low Microwave Loss YIG Thin Films by Pulsed Laser Deposition

Y₃Fe₅O₁₂ thin films with thickness 10 nm ≤ t ≤ 1440 nm were grown on Gd₃Ga₅O₁₂ (111) substrates by pulsed laser deposition. The X-ray diffraction experiments confirmed the films are pure yttrium iron garnet (YIG) phase with preferred (111) orientation. The magnetic and microwave properties were studied as a function of film thickness by the dc magnetization and ferromagnetic resonance (FMR) measurements. The FMR linewidth (ΔH) was found to decrease with the increase in film thickness (10 nm ≤ t ≤ 45 nm), attaining a minimum value of ΔH_⊥ = 5 Oe and ΔH_|| = 6 Oe, for perpendicular and parallel resonance, and then rising with further increase in thickness. Acid etching experiments were performed to understand the mechanism contributing to ΔH. The increase in ΔH with thickness (t > 45 nm) may be explained in terms of extrinsic mechanisms, such as inhomogeneities present at the surface of the films. However, the decrease in ΔH with thickness (t <; 45 nm) is believed to be due to the surface anisotropy effect. The films showed low coercivity values in the range of ~1.5-7 Oe, which is an indicator of good quality YIG films.

Effect of thickness on magnetic and microwave properties of RF-sputtered Zn-ferrite thin films

Zinc ferrite thin films of varying thickness were deposited at ambient temperature using RF-magnetron sputtering. The films were annealed at temperatures in the range 250 °C to 650 °C in air for 2 hrs. The magnetization of the film was observed to depend on the average grain size and also on thickness of the film. It was found that thermal annealing reduces the peak to peak ferromagnetic resonance (FMR) line width. A low in-plane line width of 195 Oe and a line width of 170 Oe in perpendicular configuration was observed for a 240 nm thickness film annealed at TA=450 °C.

Microwave properties of RF- sputtered ZnFe2O4 thin films

In this work, RF- magnetron sputtering technique has been employed to deposit nanocrystalline ZnFe2O4 thin films at room temperature. The as grown films were ex-situ annealed in air for 2 h at temperatures from 150°C to 650°C. X-ray diffraction, vibrating sample magnetometer and ferromagnetic resonance were used to analyze the phase formation, magnetic properties and microwave properties respectively. From the hysteresis loops and ferromagnetic resonance spectra taken at room temperature, a systematic study on the effect of O2 plasma on microwave properties with respect to processing temperature has been carried out.

Effect of substrate temperature on magnetic properties of MnFe2O4 thin films

MnFe2O4 thin films were pulsed laser deposited on to quartz substrate from room temperature (RT) to 650 °C in a pure argon environment. Temperature dependence of spontaneous magnetization (4πMS) was measured on these films from 10 K to 350 K using a vibrating sample magnetometer. Ferromagnetic resonance (FMR) study was also carried out at 300 K. The exchange stiffness constant (D) values were obtained by fitting the 4πMS data to the Bloch’s equation. The D values of the films thus found decreases while the 4πMS value increases, though non-monotonically, with the increase in TS and tend to reach bulk values at TS = 650 °C. The variation in D and 4πMS values of the films are explained based on the degree of inversion and oxidation state of cations in thin films.

Temperature and field dependent magnetization studies on nano-crystalline ZnFe2O4 thin films

Single phase nano-crystalline zinc ferrite (ZnFe2O4) thin films were deposited on fused quartz substrate using the pulsed laser deposition technique. The films were deposited at different substrate temperatures. The field dependence of magnetization at 10 K shows hysteresis loops for all the samples. Temperature dependence of the field cooled (FC) and zero field cooled (ZFC) magnetization indicated irreversible behavior between the FC and ZFC data, and the irreversibility depends on the measuring magnetic field. The thermo-magnetic irreversibility in the magnetization data is correlated with the magnitude of the applied field and the coercivity (HC) obtained from the M-H loops.Single phase nano-crystalline zinc ferrite (ZnFe2O4) thin films were deposited on fused quartz substrate using the pulsed laser deposition technique. The films were deposited at different substrate temperatures. The field dependence of magnetization at 10 K shows hysteresis loops for all the samples. Temperature dependence of the field cooled (FC) and zero field cooled (ZFC) magnetization indicated irreversible behavior between the FC and ZFC data, and the irreversibility depends on the measuring magnetic field. The thermo-magnetic irreversibility in the magnetization data is correlated with the magnitude of the applied field and the coercivity (HC) obtained from the M-H loops.

Synthesis and magnetic studies of Ni/NiO core/shell nanoparticles

Ni/NiO core/shell nanoparticles were synthesized by chemical reduction method followed by oxidation by two different methods; (a) in air and (b) in microwave oven. Structural studies showed that the thickness of NiO shell on Ni core is less in air oxidized sample than the microwave oxidized samples which were supported by the magnetic studies. The samples prepared by air oxidation showed positive exchange biasing where as the samples prepared by microwave oxidation showed negative exchange biasing. Our study also showed that the thickness of the antiferromagnetic NiO is responsible for the different types of magnetic interactions at the interfaces between antiferromagnetic NiO and ferromagnetic Ni in Ni/NiO core/shell nanoparticles.

Exchange coupled Co-ferrite/Zn-ferrite bilayer

Nanocrystalline Co-ferrite, Zn-ferrite single layers and Co-ferrite/Zn-ferrite bilayer were deposited by the pulsed laser deposition technique on amorphous fused quartz substrate at substrate temperature of 450°C. The magnetic properties of the bilayer and of the single layer films were studied at 300 K and at 10 K. Magnetic measurements at 10 K clearly showed a two stepped magnetic hysteresis loop corresponding to the switching of the magnetic moments of the soft Zn-ferrite and the hard Co-ferrite layers. Magnetic properties of the bilayer are found to be different from that expected, if there is no coupling between the two individual layers. The observed magnetic properties are attributed to the exchange coupling between the two layers at the interface.