Effect of post-annealing temperature on the microstructure and magnetic properties of Ce:YIG thin films deposited on Si substrates

Amorphous Ce₁Y₂Fe₅O₁₂ (Ce:YIG) thin films deposited on single crystal Si(1 0 0) and thermally oxidized Si(1 0 0) substrates by pulsed laser deposition were annealed in the temperature range of 700-1000 °C in air. The annealing temperature dependence of microstructure and magnetic properties of Ce:YIG films was studied using X-ray diffraction combined with vibrating sample magnetometer. The results show that single phase of polycrystalline Ce:YIG thin films can be obtained by the post-annealing of as-deposited films at the temperature of 700 °C. However, two steps of phase segregation of Ce:YIG occur as the post-annealing temperature increases: at first, Ce:YIG is decomposed into YIG and non-magnetic CeO₂ when annealed at 800 °C; then YIG continues to be decomposed forming Fe₂O₃ when the temperature is increased up to 900 °C. Consequently, the saturation magnetization of Ce:YIG films decreases first and then increases with the post-annealing temperature going up, which indicates that the saturation magnetization of Ce:YIG films is mainly related to the phase composition of the films. Meanwhile, the presence of SiO₂ buffer layer can significantly enhance the saturation magnetization of Ce:YIG films.     

Study on the Gilbert damping of polycrystalline YIG films with different capping layers

This paper describes the effect of 5-nm thick platinum (Pt), aluminum (Al) and silicon oxide (SiO_x) capping layers on the static and dynamic magnetic properties of 400-nm thick polycrystalline YIG films deposited on a Pt buffer layer. Both static and dynamic magnetic properties of Pt capped YIG film are totally different among all YIG films. Namely, the squareness of the magnetization curve for Pt capped YIG film increases, indicating that Pt capped YIG film is magnetically softer than other YIG films. Interestingly, the effective Gilbert damping parameter of Pt capped YIG films is about four times as large as those of other YIG films, and its value is approximately 9.52 × 10⁻⁴. However, the value of Gilbert damping is 2.55 × 10⁻⁴, 3.46 × 10⁻⁴ and 3.85 × 10⁻⁴ respectively for no capping, SiOx capping and Al capping samples respectively. This huge change in Gilbert damping parameter is mainly originating from the spin pumping effect, which arises at the interface of a material having strong spin orbit interaction such as Pt. Moreover, the enourmous increase in the value of effective anisotropic field and decrese in effective saturation magnetization indicates interface anisotropy is induced in Pt capped sample. These results suggest that the static and dynamic magnetic properties of YIG film can be controlled by selecting an appropriate capping layer.     

Synthesis and laser patterning of Bi-doped Y3Fe5O12 crystals in germanosilicate glasses

New germanosilicate glasses giving the crystallization of yttrium iron garnet Y₃Fe₅O₁₂ (YIG) and Bi-doped YIG, 23Na₂O-xBi₂O₃-(12−x)Y₂O₃-25Fe₂O₃-20SiO₂-20GeO₂ (mol%), are developed, and the laser-induced crystallization technique is applied to the glasses to pattern YIG and Bi-doped YIG crystals on the glass surface. It is clarified from the Mössbauer effect measurements that iron ions in the glasses are present mainly as Fe³⁺. It is suggested from the X-ray diffraction analyses and magnetization measurements that Si⁴⁺ ions are incorporated into YIG crystals formed in the crystallization of glasses. The irradiations (laser power: 32-60 mW and laser scanning speed: 7 μm/s) of continuous wave Yb:YVO₄ fiber laser (wavelength: 1080 nm) are found to induce YIG and Bi-doped YIG crystals, indicating that Fe³⁺ ions in the glasses act as suitable transition metal ions for the laser-induced crystallization. It is suggested that YIG and Bi-doped YIG crystals in the laser irradiated part might orient. The present study will be a first step for the patterning of magnetic crystals containing iron ions in glasses.     

Damage induced in garnets by heavy ion irradiations: a study by optical spectroscopies

The damage induced by heavy-ion irradiation has been studied in yttrium iron garnet (Y₃Fe₅O₁₂ or YIG) films, doped with Ca, Tb and Tm, grown by liquid-phase epitaxy on gadolinium gallium garnet (Gd₃Ga₅O₁₂ or GGG) substrates. Irradiations of doped-YIG epitaxial films and GGG substrates with 36-MeV ¹⁸³W and 12-MeV ¹⁹⁷Au ions were applied for fluences between 1 × 10¹³ and 3 × 10¹⁵ cm^–2 near room temperature. The radiation damage was monitored by micro-Raman spectroscopy and UV–visible optical absorption spectroscopy. Raman spectra revealed that amorphisation was achieved in YIG for both ions, whereas a high lattice disorder was induced in GGG without reaching amorphisation for the Au ion irradiation. Raman spectra also showed that a major damage of the tetrahedral sites was induced in GGG, as previously found for YIG. It is concluded that with such ions reaching the stopping power threshold of track formation in YIG and GGG the observed rate of amorphisation may result from a combination of electronic and nuclear energy losses as calculated using the unified thermal spike model.     

DCEMS of neon implantation in yttrium iron garnet

⁵⁷Fe-enriched, epitaxial Y₃Fe₅O₁₂ films have been implanted with 50 keV and 100 keV neon ions with a dose of 4·10¹⁴Ne⁺/cm². Depth-selective conversion electron Mössbauer spectroscopy has been performed at 300 K and 40 K. The results show that the 50 keV-implanted sample can be interpreted as an amorphous layer on top of an almost unperturbed YIG layer. In the 100 keV-implanted film a buried amorphous layer is observed.     

Variation of the saturation magnetization through a neon implanted YIG film

The saturation magnetization of a YIG film implanted with 5*10¹⁴ neon ions/cm² at an energy of 450 keV was studied. By removing very thin layers from the ion implanted part of the film the magnetization was found to change. As a result of the analysis of these changes in the saturation magnetization it was possible to establish a profile for the magnetization as a function of depth through the ion implanted part of the film. The profile is asymmetric and shows a decrease in the magnetization of up to 35 % of the initial value at a depth coinciding with the depth of maximum strain.     

Magnetic properties of erbium iron garnet in high magnetic fields up to 150kOe

The magnetic properties of single crystals of erbium iron garnet (ErIG) were studied in applied fields up to 150kOe between 1.4 and 300K. At low temperature, the macroscopic easy direction of the bulk magnetization is [100]; below the compensation temperature (80±2K), the magnetization presents non-linear field evolution. On the assumption of an isolated ground doublet, the anisotropy constantsK _i (i=1,2) of ErIG are given byK _i (Er)+K _i (YIG); theK _i are calculated as a function of theG andg tensor components. It is worthwhile noting that theK _i (Er) are strongly temperature dependent; so at low temperature the anisotropy of the garnet is determined by the rare earth ions, while in the 50 K regionK ₁(Er) becomes comparable toK ₁(YIG) with the opposite sign which results in a very weak anisotropy of the garnet. Above 50 K,K ₁(YIG) is predominant and the Fe³⁺ ions determine the garnet anisotropy.     

CEMS study of H2 annealed YIG films

We report on a CEMS investigation of the surface layer obtained in pure YIG film after an annealing of 20 h in H₂ at 480°C. We show that the surface layer has a garnet structure with the hyperfine fields coinciding with those of pure YIG. The magnetization in the surface layer is found to be parallel to that of the underlying film bulk. Furthermore we analyze the line widths of the Mössbauer peaks pertaining to such surface layer and compare the results with those obtained for a YIG film implanted with two different doses of Ne⁺, namely 7x10¹³ Ne⁺ cm^-2 at 50 keV and 2.7x10¹⁴ Ne⁺ cm^-2 at 50 keV. From this comparison it results that the peaks for the annealed film do not show any broadening while those for the implanted film are broadened. This clearly indicates that, within the experimental error, there is no damage in the surface layer obtained by annealing in H₂ and, in any case, the damage is much lower than found in the surface layers of YIG films implanted with doses of practical interest.     

Exchange coupling in ultrathin epitaxial yttrium iron garnet films

Magnetic exchange coupling has been observed for ultrathin films of yttrium iron garnet (Y₃Fe₅O₁₂ or YIG). Single-crystalline YIG films were prepared on yttrium aluminium garnet (Y₃Al₅O₁₂ or YAG) substrates by pulsed laser deposition. (111) and (110) oriented substrates were used. Film thicknesses were varied from 180 ? to 4600 ?. Epitaxial growth of YIG on YAG was obtained in spite of the lattice mismatch of 3%. Magnetic hysteresis loops recorded for ultrathin YIG films have a “bee-waist” shape and show a coupling between two different magnetic phases. The first phase is magnetically soft YIG. A composition study by secondary ion mass spectroscopy shows the second phase to be Y₃Fe_5-xAl_xO₁₂ due to the interdiffusion of Fe and Al at the film/substrate interface. This compound is known to be magnetically harder and to have weaker magnetization than YIG. The coupling of the two phases leads to a hysteresis loop displacement at low temperatures. This displacement varies differently with film thickness for two substrate orientations. Assuming an interfacial coupling, the maximal interaction energy is estimated to be about 0.17 erg/cm² at 5 K for (111) oriented sample. Received 3 June 2002 / Received in final form 7 October 2002 Published online 27 January 2003 RID="a" ID="a"Presently at LPM, Université H. Poincaré, BP 239, 54506 Vandœuvre-lès-Nancy e-mail: popova@lpm.u-nancy.fr     

Surface morphological changes and magnetic properties of Sc-substituted Y3Fe5O12 epitaxial films deposited on the GGG substrate

Sc-doped YIG films were grown on (1 1 1) oriented GGG crystalline substrate with disorientation angle within the range 0-25′. Sc³⁺ ion substitution was varied within the range 0.25-0.3 per formula unit. The films demonstrate different types of surface morphology versus film growth rate and substrate disorientation. Conditions for existence of these types of the surface morphology were defined. The field dependence of magnetic susceptibility at magnetization reversal in film plane and Faraday rotation at wavelength 633 nm for a magnetic field applied in perpendicular direction were measured to characterize the films grown. Films with “mirror-like” surface demonstrate a planar magnetization at room temperature.     

Indium-substitution and indium-less case effects on structural and magnetic properties of yttrium-iron garnet

The effect of indium (In) substitution in the dynamics of structure and ferrimagnetism of yttrium iron garnet (YIG) employing sintering temperature as a temporary agent of composition and structural changes was reported in this study. The nanoparticles of YIG powder samples with various In content (x=0.0–0.4) were prepared via the mechanical alloying (MA) technique. A brief, yet revealing characterization of the samples was carried out via transmission electron microscope, X-ray diffraction, Raman spectroscopy, B–H Hysteresisgraph, and LCR-metre. The X-ray diffraction analysis of the samples prepared via the MA indicates the formation of single phase YIG structure at much lower sintering temperature than that in the conventional ceramic technique. The lattice constant increases as In content increases which obeys Vegard's Law due to the larger In³⁺ ions replacing the smaller Fe³⁺ ions. The saturation induction increased reaching about 699.1 G for x=0.3 and decreased with further In substitution. Three stages of ordered magnetism formation were identified which attributed to development of crystallinity and larger grains for magnetic domain accommodation. The Curie temperature shows a decrement in their values with In content due to weakening of superexchange interactions. Raman shifts from 268.1 to 272.2 cm⁻¹ with increasing In content were observed due to stress developed in the YIG crystal structure.     

Optical characterization of thermal-stress induced by spin waves in thin-film ferrimagnetic structures

It is shown that intense spin-dipole waves (SDWs) excited in thin yttrium iron garnet (YIG) films induce an in-plane thermal stress (σ) of 1-2 MPa in a YIG/GGG structure (where GGG is gadolinium gallium garnet). In YIG/GGG with normal magnetization, σ shifts its ferromagnetic resonance frequency by ≈1 MHz, which is comparable to the linewidth of the absorption curve of YIG/GGG resonators. The effect was characterized by an optical technique that detects σ in the GGG substrate. It was also demonstrated that this effect can be used for the optical-microwave spectroscopy of spin waves in thin ferromagnetic films, by using thermal mapping of SDWs in the substrate. We have shown that this opens up the possibility of determining the contribution of the two-particle magneto-elastic interaction to the microwave heating of the sample.     

Room-temperature ferromagnetism of Cu ion-implanted Ga-doped ZnO

1 MeV Cu²⁺ ions have been implanted into un-doped ZnO and Ga-doped ZnO films with a dose of 1 × 10¹⁷ ions/cm² at room-temperature. Cu ion-implanted Ga-doped ZnO had ferromagnetism at room-temperature and the saturation magnetization of this sample was estimated to be 0.12 μ_B per Cu, while the Cu ion-implanted un-doped ZnO did not show ferromagnetic behavior. Near-edge X-ray fine structure (NEXAFS) spectroscopy revealed that a partial amount of implanted Cu ions existed as Cu²⁺ (d⁹) state in Ga-doped ZnO film. On the other hand, almost Cu atoms existed as Cu¹⁺ (d¹⁰) state in un-doped ZnO film. However, the subsequent annealing at temperature above 800 °C on this ferromagnetic sample induced the annihilation of ferromagnetism due to the formation of non-ferromagnetic Cu₂O phase.     

Magnetic properties of erbium iron garnet in high magnetic fields up to 150kOe

The magnetic properties of single crystals of erbium iron garnet (ErIG) were studied in applied fields up to 150kOe between 1.4 and 300K. At low temperature, the macroscopic easy direction of the bulk magnetization is [100]; below the compensation temperature (80±2K), the magnetization presents non-linear field evolution. On the assumption of an isolated ground doublet, the anisotropy constantsK _i (i=1,2) of ErIG are given byK _i (Er)+K _i (YIG); theK _i are calculated as a function of theG andg tensor components. It is worthwhile noting that theK _i (Er) are strongly temperature dependent; so at low temperature the anisotropy of the garnet is determined by the rare earth ions, while in the 50 K regionK ₁(Er) becomes comparable toK ₁(YIG) with the opposite sign which results in a very weak anisotropy of the garnet. Above 50 K,K ₁(YIG) is predominant and the Fe³⁺ ions determine the garnet anisotropy.     

Wavelength and temperature characteristics of BiYbIG film/YIG crystal composite structure for magneto-optical applications

We report the wavelength and temperature characteristics of novel Bi-substituted rare-earth iron garnet films grown on a YIG substrate by a modified liquid phase epitaxy (LPE) technique. The Faraday-rotation spectrum was measured by the magneto-optically modulated dual-frequency technique with the wavelength varied from 800 nm to 1700 nm. The resultant Bi_0.37Yb_2.63Fe₅O₁₂ (BiYbIG) LPE film/YIG crystal structure showed an increased Faraday-rotation coefficient due to Bi³⁺-ion doping on the dodecahedral sites of the iron garnet without increasing absorption loss; therefore, a good magneto-optical figure of merit, defined by the ratio of Faraday rotation and optical absorption loss, has been achieved (21.5 deg/dB and 30.2 deg/dB at 1300-nm and 1550-nm wavelengths, respectively, at room temperature). In addition, since the Yb³⁺ and Y³⁺ ions provide opposite contributions to the wavelength and temperature characteristics of the Faraday rotation, the resultant BiYbIG LPE film/YIG crystal structure showed a flat Faraday-rotation curve versus wavelength and temperature. The Faraday-rotation wavelength coefficient was reduced to 0.06 %/nm at 1550-nm wavelength. The Faraday-rotation temperature derivative was reduced to 0.006 deg/°C at 1300-nm wavelength and 0.007 deg/°C at 1550-nm wavelength, respectively. PACS 78.20.Ls; 81.15.Lm; 75.50.Gg     

Temperature and field dependences of photo-induced changes in the magnetization in yttrium iron garnet

The changes in the magnetization of yttrium iron garnet (YIG) when irradiated by a pulsed neodymium laser beam with wavelength λ=1.06 μm are investigated. Measurements are performed in the temperature range from 100 K to 600 K in various external magnetic fields. YIG single crystals grown along the crystallographic (100), (110), and (111) directions are chosen so that the external anisotropy of the indicated processes can be determined. Characteristic temperature intervals dominated by different mechanisms of variation of the magnetization under the influence of a laser pulse are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1263–1266 (July 1997)     

Improvement to the exchange field acting upon Tb ions in TbIG

The spin fluctuations of the magnetic ions play an important role on the magnetic properties of the crystals and lead to a new mechanism for the Curie-Weiss susceptibility. The exchange field H_exch acting on the rare-earth ions in Tb:YIG is improved based on the temperature dependence of the spin fluctuations, which is expressed as H_exch=n₀(1+γT+βT⁻²)M_YIG. By means of the improved exchange field, the magnetic and magneto-optical properties of Tb³⁺ ions in Tb:YIG are calculated. The calculated results are in good agreement with the measured data in the temperature range from 40 to 300 K.     

Photomagnetic effects and disaccommodation in Co-doped YIG

Y₃Fe₅O₁₂ (YIG) doped with ≦ 0.01 Co per molecule in combination with small dopes of V or (and) Mn show a decrease of the permeability in the dark (disaccommodation, DA) as well as when irradiated (photomagnetic effect, PE) with white light. All materials investigated exhibit DA and PE at 77 K, whereas in some cases effects occur at room temperature. The origin is attributed to domain wall pinning by magnetically anisotropic Co²⁺ ions that exchange electrons with Co³⁺ or other (V⁵⁺, Mn³⁺) ions. In view of the wide separation between the cobalt ions (?30 Å) the charge transport via iron ions is supposed to play an essential part.For a sample with higher cobalt dope the shape of the hysteresis loop at 77 K changes in the dark and the change can be hastened by irradiation. This phenomenon is attributed to the growth of a uniaxial anisotropy in the bulk of the material by Co²⁺, Co³⁺ ordering.From the electric resistivity of certain V and Si doped YIG materials it is concluded that V⁵⁺ ions oxidize Fe²⁺ ions according to V⁵⁺+Fe²⁺ → V⁴⁺+Fe³⁺.     

Pac study of the hyperfine interactions in154Gd3+ in rare-earth-garnets

The attenuation of the anisotropy of the γ-γ angular correlation for a rare-earth ion in magnetic iron and non-magnetic aluminum rare-earth garnets is governed by the static hyperfine electric field gradient (EFG) produced by the surrounding ions and by the static and fluctuating hyperfine magnetic fields produced by the 4f electrons of the ion. The latter effect depends upon the correlation time (τ_c) of the 4f electrons which, in turn, is determined by the interaction of the ionic spin with lattice vibrations and other magnetic ions. In an attempt to determine the significance of spin-spin relaxation on τ_c, perturbed angular correlation (PAC) measurements were performed on the S-state ion¹⁵⁴Gd³⁺ because spin-lattice interactions were expected to be small. The time-integral attenuation coefficients of the 1274/123 keV γ-γ cascade in¹⁵⁴Gd were measured in the temperature region 4.2–650 K for¹⁵⁴Gd³⁺ incorporated in GdAlG, GdIG, YAlG, and YIG. Rotation measurements were also made on the same cascade in the iron garnets at room temperature with an applied magnetic field in the range 0–15 kg. Employing independent measurements and calculated estimates of the static hyperfine magnetic field and EFG, the observed data could be interpreted in terms of reasonable values of τ_c. The correlation times were found to range from about 0.13 ns in YIG to about 1.30 ns in YAlG with GdAlG having a value of approximately 0.71 ns. In the case of GdIG, a self-consistent analysis of the data required a value of the lattice EFG which was larger than that estimated from the nearest neighbor point-ion model. The correlation time in GdIG was then found to be the same as in YIG.     

Origin of the growth induced anisotropy in (YBi)3Fe5O12 films

The single-ion anisotropy model together with the superposition model of the zero field splitting are used to analyse the growth-induced anisotropy of YIG films containing Bi³⁺ ions. Three different mechanisms are discussed. It is shown that the most probable source of anisotropy is the modification of the electron structure of Fe³⁺ ions by interactions in Fe³⁺O²;Bi³⁺ triads.