Iron Garnet Crystals Research Articles

Effect of stress from the glue on single-crystal X-ray intensities at high or low temperatures

For high- or low-temperature single-crystal X-ray data collection, the glues used to bind crystals to silica rods may induce strong modifications of the intensities. When an yttrium iron garnet (YIG) crystal was cooled, reflections which were equivalent at room temperature became different. This is explained by strong stresses of the glue on the crystal which lead to an anisotropic modification of the secondary extinction. At low temperatures, `glues' such as beeswax or paraffin do not exert stresses.

Fiber optic tachometer based on the Faraday effect

A fiber optic tachometer is described that uses the Faraday effect in a ferrite to modulate light from a multimode fiber. A yttrium iron garnet (YIG) crystal is used as the magnetooptic modulator. A permanent magnet is placed near the crystal, such that a nearby rotating steel toothed wheel changes the magnetic field inside the crystal, causing modulation of the intensity of the 1.3-microm light. Modulation depths up to 50% have been achieved with signal-to-noise ratios of over 15 dB. A single multimode fiber connects the remote sensor head to the LED light source and detector.

Anisotropy of the linear magnetic birefringence in erbium iron garnet

Linear magnetic birefringence (LMB or the Cotton–Mouton effect) measurements have been performed on single crystals of erbium iron garnet (ErIG) in the 4.2–300-K temperature range at 1.15 μm wavelength in magnetic fields up to 20 kOe, applied along the three cubic crystallographic directions. Light propagation was parallel to the [1̄10] direction. Except for the [110] direction at low temperatures the LMB is proportional to the internal field, and a strong anisotropy is observed in the whole temperature range. At the compensation temperature (80 K) the ‘‘LMB susceptibilities’’ change sign and are found to be proportional to the inverse of the temperature square in a wide range of temperatures. A macroscopic analysis is presented in terms of electric dipole contributions: Thesecond-order magneto-optical coefficient of the Er3+ is deduced in the spontaneous state.

Magnetostatic resonance of holes

Existence of a simple magnetostatic mode localized around a hole drilled in an infinite ferromagnetic medium was recently verified by the present authors. In this experiment, the conditions of an infinitely long hole were reproduced by placing a drilled ferromagnetic plate (an yttrium iron garnet crystal) between two silver coated soft iron blocks which acted as ‘‘magnetic mirrors.’’ In the present work, using similar arrangements, we report further results on hole magnetostatic resonance: By measuring the unloaded linewidth as a function for frequency on various samples, we show that for hole aspect ratio near unity the dominant loss of the resonator still comes from the ferromagnetic material and not from the mirrors. We also report observation of hole resonance in low-loss ceramic samples, and as a tentative illustration we briefly demonstrate application of hole resonance to single-pole transmission filters.

LINEAR BIREFRINGENCE IN Bi-SUBSTITUTED IRON GARNET CRYSTALS

Linear birefringence of iron garnet bulk crystals were investigated at a wavelength λ=1.15μm and at room temperature. In Bi-substituted iron garnet crystals, the linear birefringence was composed of the magnetic linear birefringence (MLB) and some metastable birefringence. The metastable birefringence was suppressed by annealing, and was attributed to the growth-mduced birefringence. The notable growth-induced birefringence was not observed In Bi-free iron garnet crystals. These results indicate that Bi ion plays an. important role to induce large growth-induced birefringence. Effect of the Bi-substitution on the MLB effect was also investigated. The anisotropy parameter of the MLB effect increased with amount of Bi.

PHASE DIAGRAM IN THE SYSTEM (Gd2O3+Y2O3)-Fe2O3 AND CRYSTAL GROWTH OF (GdY)3Fe5O12

Single crystals of gadolinium yttrium iron garnet (GYIG) have been grown by the travelling-solvent floating-zone (TSFZ) method with Fe2O3 as the solvent. The zone materials consist of 80% Fe2O3 and 20% Gd2O3-Y2O3 mixture. The crystal of approximately 7 x 40 mm in size is obtained.

APPLICATION OF BISMUTH-SUBSTITUTED IRON GARNET FILMS TO MAGNETIC FIELD SENSORS

Sensitivity constants and their temperature dependence, corresponding to Verdet constant, of Bi subsutituted iron garnet crystals grown by LPE method are studied. By measuring the sensitivity constants with temperature, it was found that Bi1.3(YLa)1.7Fe5O12 crystals have the large sensitivity constant of 6.1 °/cm Oe and show the temperature dependence of +3.7% between 300 and 400 K. Temperature dependence of sensitivity constants was explained by a simple theoretical calculation.

Growth of single-crystal Nd 3Fe 5O 12 and Pr 3Fe 5O 12 Garnets

Bulk single crystals of neodymium iron garnet and praseodymium iron garnet were prepared for the first time by low temperature liquid phase epitaxy on Sm 3(ScGa) 5O 12 seeds with nearly matching lattice parameters. The measured properties of NdIG were: lattice parameter = 12.596 Å, Curie temperature = 294°C, and 4π M s = 2080 Oe. PrIG has a lattice parameter of 12.646 Å, a Curie temperature of 296°C, and 4π M s = 2065 Oe.

Study of radiation damage in an ion implanted rare-earth iron garnet crystal

The potential of a combination of Rutherford backscattering and channeling with the 16O(α, α) 16O nuclear resonance at 3.045 MeV to study radiation damage in all sublattices of oxide crystals is demonstrated. As a model, (Y, Sm, Lu, Ca) 3,(Ge, Fe) 5O 12 epitaxial layers on 〈111〉 gadolinium-gallium garnets were subjected to 80 keV Ne + ion bombardment. It is shown that the lattice damage is different for different constituents. Heavy ions were found more off-site than oxygen. A 350°C anneal, showing optimum magnetic characteristics in bubble memory applications, leads to complete recrystallization of all sublattices down to a depth of 100 nm, but leaves a disordered layer between 100 and 300 nm depth. Supposedly, this layered structure causes enough strain to suppress hard bubbles, but it is still low enough not to influence other bubble properties.

Y3Fe5O12 single-crystal growth by top seeded solution growth method

Single crystals of yttrium iron garnet have been grown by the top seeded solution growth (TSSG) method with Fe2O3 as the solvent. The starting materials consist of 79%–82% Fe2O3 and 21%–18% Y2O3. Crystals of approximately 38×35×8 mm in size and about 12 g in weight were obtained. The ferromagnetic resonance, magnetization, Néel temperature, and Mössbauer measurements, and the etch-pattern observation have been performed on crystals growth by the TSSG method and are compared to similar measurements on crystals grown by the slow cooling method from a PbO flux.

Magneto-optic properties of rare earth iron garnet crystals in the wavelength range 1.1-1.7μm and their use in device fabrication

The wavelength and temperature dependence of the Faraday rotation together with the optical loss spectra of yttrium, gadolinium, terbium, dysprosium, holmium, erbium and ytterbium iron garnets are reported for wavelengths in the range 1.1-1.7 mu m. It is demonstrated that the Faraday rotation of all the iron garnets decreases with increasing wavelength with the exception of ytterbium iron garnet which, for wavelengths in excess of approximately 1.3 mu m, possesses a wavelength independent Faraday rotation of 5.7+or-0.3 degrees mm-1. The performance of magneto-optic switches, isolators, modulators and deflectors constructed using each of these materials is discussed with particular emphasis on their operation at 1.3 mu m and 1.55 mu m, the wavelengths of interest in current optical fibre systems. It is concluded that whilst yttrium iron garnet or holmium iron garnet are best suited to the fabrication of devices operating at single wavelength, for broadband operation ytterbium iron garnet is to be preferred.

Doped yttrium iron garnet for thermistor-bolometers

Yttrium Iron Garnet (YIG) crystals doped with divalent or tetravalent ions are shown to be promising as basic material for thermistor-bolometers. This is the first proposed device application of the electrical properties of magnetic garnets. To illustrate the material performances, the static current-voltage characteristic and the voltage response versus current and frequency under a.c. laser irradiation are reported and discussed. The most attractive properties of doped YIG for thermistor-bolometer applications result to be the high responsivity, the speed of the response, and the high intrinsic absorbance in the ultraviolet, in the visible and in the near infrared.

Experimental verification of spin-wave theory in high fields (invited)

The magnetic moment of single crystals of nickel, iron, cobalt, and yttrium iron garnet have been measured between 1.6 and 288 K, in magnetic fields up to 180 000 Oe, along the different crystallographic axes with an accuracy of 0.01%. From these measurements are verified: (i) The H−1/2i Holstein-Primakoff test on the (∂M/∂Hi,Hi) curves, where Hi is the internal field. (ii) The isothermal magnetization M(Hi,T) is represented by the equation M(Hi,T)=Ms(T)+A(T) ⋅ H1/2i+B(T) ⋅ Hi. (iii) The spontaneous magnetization Ms(T) is determined by fitting the experimental points M(Hi,T) with the above equation. (iv) From the Ms(T) variation we determine the absolute saturation moment M=0.619 μB/atom for Ni, 2.226 μB/atom for Fe, 1.729 μB/atom for Co (Hi∥c axis) and 1.72 μB/atom (H⊥c axis), 10.062 μB/mole 5Fe2O3⋅3Y2O3 for YIG. (v) Writing a priori Ms(T)=M0(1−anTn−an′Tn′−an″Tn″) the variation of the spontaneous magnetization with the temperature, and plotting the variation of log [M0−Ms(T)]/M0 as a function of log T, one proves n=3/2, n′=5/2, n″=0 for nickel and iron; for cobalt n=3/2 from points above 100 K; for YIG, n appears to be 5/2. (vi) From the a3/2 values are determined the stiffness coefficients D of iron, cobalt, and nickel which are in reasonable agreement with those determined by neutron diffraction. (vii) By subtracting the theoretical contribution of the spin waves as a function of magnetic field and temperature from the M(Hi,T) data, we obtain an additional superimposed susceptibility for iron, cobalt, and nickel as a function of the temperature, due to their metallic nature.

Magnetic properties on holmium iron garnet in high magnetic field. Optical absorption spectra in the infra-red region

On single crystals of holmium iron garnet (HoIG), magnetic properties have been studied in magnetic field up to 150kOe applied parallel to the main crystallographic directions in the 4.2–300K temperature range. Above 130 K, the magnetization is isotropic and linear magnetic field dependent as previously found in polycrystals and predicted by Neel's ferrimagnetic theory. Nevertheless the paramagnetic Curie temperature is much higher than the polycrystal value. Below 130K, due to the onset of the umbrella structure, the ferrite magnetization presents a non linear field variation with [111] as easy direction. The field evolutions of the anisotropy constants (K1 andK2) were calculated. Optical absorption measurements of both 58→5I6 and 5I8→5I7 transitions are reported and compared to the results of the literature in terms of inequivalent magnetic sites.

Spin-waves in nickel, iron, and yttrium-iron garnet

The magnetostatic properties of single crystals of nickel, iron, and yttrium-iron garnet have been measured along the three crystallographic axes, in magnetic fields up to 180 000 Oe and at different temperatures between 4.21 and 286 K. The relative accuracy is 0.01%. It is found that the magnetization values as a function of the internal magnetic field satisfy the Holtstein-Primakoff test. A method to determine the spontaneous magnetization is given. The magnetization curves are interpreted in terms of the spin-wave theory; a superimposed susceptibility is determined for nickel and iron.

Lattice distortions caused by growth striations near the surface of garnet crystals

Local lattice distortions due to growth striations near the surface of flux-grown (PbO/PbF 2) (111) yttrium iron garnet crystals (YIG) and Ga-substituted yttrium iron garnet crystals (Ga-YIG) were studied by means of double-crystal X-ray reflection topography. Topographic contrast of growth striations could be separated into those caused by local variations of lattice spacing and those due to local misorientations of the lattice planes at the boundaries. The results are in good agreement with calculated lattice constant profiles and a relaxation model of bulk stresses at the crystal surface. Assuming equivalent amounts of the main impurities Pb and F the lattice constant profiles were calculated taking into account the local concentrations of Pb or Pb and Ga, respectively, which were determined by step scan measurements with an electron probe microanalyzer (EPMA).

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 regionK1(Er) becomes comparable toK1(YIG) with the opposite sign which results in a very weak anisotropy of the garnet. Above 50 K,K1(YIG) is predominant and the Fe3+ ions determine the garnet anisotropy.

Site occupancy of ferrous ions in iron garnets

Fe 2+ site occupancy in iron garnets is investigated by means of optical absorption and magnetic circular dichroism. In particular measurements are carried out on bulk single crystals of yttrium iron garnet (YIG) doped with Si 4+, Nb 5+, Sn 4+, Zr 4+, Ti 4+ and Th 4+. The results are discussed in the light of the recent discovery of tetrahedral Fe 2+ in Nb-doped YIG. We find that, in general, Fe 2+ occupies both octahedral and tetrahedral sites, the actual distribution being related to site occupancy of the donor centers. This correlation may be relaxed by photoinduced charge transfer between iron sites, at low temperatures. Contrary to recent attributions, no dodecahedral component is found in the infrared spectra above 1 μm wavelength.

Using lead sulphate instead of lead oxide in flux growth of yttrium iron garnet(YIG) crystals

Crystal growth of YIG from fluxes containing lead sulphate in place of lead oxide in the usual lead oxide-lead fluoride-boron oxide flux system has been tried. Lead sulphate decomposes during crystal growth giving lead oxide and sulphur trioxide. Due to the influence of sulphur trioxide in the system the yield of crystals almost doubles. There is no change either in the morphology of the crystals or their lattice parameter. It is possible that solubility of YIG is different in the new flux and the changed solubility causes the increase in yield of crystals.

Magnetic anisotropy of europium in iron garnet

Magnetization measurements have been made of single crystals of europium iron garnet (EuIG), Ga-doped EuIG and Al-doped EuIG. Cubic anisotropy constants K 1 and K 2 have been evaluated from the magnetization curves. Temperature dependence and concentration dependence of the anisotropy are compared with the theory by Foglio and van Vleck.