Anisotropy In Garnets Research Articles

Laser-induced excitation and decay of coherent optical phonon modes in an iron garnet

The excitation of optical phonon modes at resonance has been recently identified as offering a promising energy-efficient pathway for the ultrafast switching of magnetization. Here, using time-resolved pump–probe measurements, we explore the characteristics of different optical phonon modes in ferrimagnetic cobalt-doped yttrium iron garnet. By rotating the linear polarization of both the pump and probe, we identify a large spectrum of optical phonons within the frequency band of 3–11 THz. We determine the lifetimes of the different phonons using band filtration, revealing that the phonon mode with the eigenfrequency 3.4 THz survives for up to 13 ps. Its long lifetime, comparable with the relaxation time of photo-magnetic anisotropy in garnets, renders this phonon mode the most promising candidate for ultrafast phononic switching of magnetization.

Anisotropy and phase states of garnet ferrite films with misoriented surfaces

Anisotropy of garnet ferrite films is investigated in the framework of the two-parametric model. It is shown that a garnet ferrite film with an arbitrarily oriented surface is characterized by biaxial anisotropy. The directions of the easy, intermediate, and hard magnetization axes are determined as functions of the misorientation angle and weak cubic anisotropy. It is demonstrated that the region of existence of homogeneous states in a magnetic field is bounded by a slant astroid. The magnetic susceptibility tensor and the ferromagnetic resonance frequency are calculated, and the dispersion law of spin waves is determined.

Magnetic and optical anisotropy in garnets induced by spatially modulated elliptically polarized light

We consider the formation of the optical and magnetic anisotropy in non-magnetic at room temperature, and magnetic garnets induced by spatially modulated elliptically polarized light. Experimentally, such conditions are realized by illuminating the sample with two linearly polarized coherent light beams with mutually orthogonal polarization states during the recording of holographic gratings. We develop a theoretical model by calculating the occupancies’ kinetics of anisotropic impurity centers, localized in different garnet sublattices. This model is applied to the CaMnGe garnet and is compared with the experimental results, which are fairly well described. A conclusion about the possibility to record magnetic gratings in the magnetic garnet YIG : Co, is drawn by calculating the spatial profile of the photoinduced magnetic anisotropy.

BIREFRINGENCE IMAGES OF SCREW DISLOCATIONS VIEWED END-ON IN GGG AND YAG CRYSTALS

Birefringence images of screw dislocations in Gadolinium Gallium Garnet (GGG) and Yttrium Aluminum Garnet YAG) crystals are studied in detail. Birefringence images of screw dislocations viewed end-on were observed for the first tiane. The one-to-one correspondence between birefringence images of screw dislocations viewed end-on and that viewed from the side was demonstrated. The contour of equal intensity of birefringence image of screw dislocation viewed end-on in GGG and YAG crystals was derived by considering the photo-elastieal anisotropy of garnet, and the result yields good agreement Avith experimental observations.

Bubble properties of garnet films prepared from molybdate fluxes

Magnetic bubble garnet films have been prepared from a flux composed of lithium and rare earth molybdates using liquid phase epitaxial techniques. In general, the quality and magnetic properties of 〈111〉 films prepared on GGG substrates compare favorably with the properties of similar garnets prepared from PbO based fluxes. A notable exception, however, is a persistent, unusually low value of the growth induced anisotropy (Ku). Typically, Y3−xEuxFe5−yGayO12 films grown at ∼950 °C containing as much as Eu=0.6 and Ga=0.9 have a moment (4πMs) ?400 G and Ku?103 ergs/cm3 when grown from the molybdate flux. Films prepared on 〈100〉 substrates exhibit comparably low Ku values and K1 values of ∼−2.3×103 ergs/cm3. As these films are virtually contaminant free with regard to the flux components, and PbO and B2O3 additions have essentially no effect on Ku, our present understanding of the nonrandom rare earth ion distribution responsible for growth induced anisotropy in garnets may not be adequate to explain these data. Other anomalies which have been observed in these films and will be discussed are the occurrence of triangular bubble domains and square lattice bubble arrays.

Growth rate dependence of the growth-induced anisotropy in garnets. A simple theoretical model

A simple theoretical model is proposed to describe the dependence of the growth-induced anisotropy in garnets on the growth conditions. In particular, the influence of the growth rate and the composition of the liquid phase is discussed in the case of {111} garnet films. A comparison is made with experimental results. Es wird ein einfaches theoretisches Modell vorgeschlagen, um die Abhängigkeit der Wachstums-anisotropie in Granaten von den Wachstumsbedingungen zu beschreiben. Im Besonderen wird der Einfluß der Wachstumsrate und der Zusammensetzung der flüssigen Phase für den Fall von {111}-Granatschichten diskutiert. Ein Vergleich mit experimentellen Ergebnissen wird durch-geführt.

Growth-induced anisotropy in garnets with mixed diamagnetic rare-earth ions: Y3−xLuxFe5O12

It is shown that the observations of Gyorgy, Sturge, and Van Uitert, on the anisotropy of Y3−xLuxFe5O12, are in excellent agreement with the mechanism suggested by Akselrad and Callen. The data are consistent with a tetrahedral iron mechanism, although they do not exclude an octahedral iron mechanism. The relative contributions of the two sublattices and the relevant physical considerations are discussed. The mechanism of preferential site ordering is also discussed further, with particular reference to relative site sizes.

Growth-Induced Noncubic Anisotropy Arising from the Tetrahedral Sites in Garnets

We have observed noncubic growth-induced anisotropy in garnets prepared to preclude the mechanism based on rare-earth ions, such as Eu3Fe5−xGaxO12 with a single rare-earth ion, or such as Y3−2xCa2xVxFe5−x O12 with only diamagnetic ions on the dodecahedral sites; all samples contain a mixture of constituents on tetrahedral sites. We show that the noncubic anisotropy can be explained by distortion of the local symmetry around tetrahedral ions caused by preferential site ordering among their tetrahedral neighbors. A similar distortion of tetrahedral symmetry by ordering of dodecahedral ions should contribute in mixed rare-earth garnets.

Rare earth ion properties and the growth-induced magnetic anisotropy in garnets

The recently discovered growth-induced magnetic anisotropy in rare earth garnets is expressed in terms of functions involving the relative size of the rare earth ions, and in terms of the anisotropic exchange or dipole-dipole fields acting on these ions.

Rare earth-yttrium iron-gallium garnet epitaxial films for magnetic bubble domain applications

(RE,Y) 3(Fe,Ga) 5O 12 garnets, where RE=Eu or Gd, have been developed for use as magnetic bubble domain materials. Magnetic and crystallographic studies were used to find suitable compositions for epitaxial thin films on nonmagnetic garnet substrate crystals. Properties of films grown by liquid phase epitaxy on Czochralski Gd-Ga garnet (111) substrates are discussed. Uniaxial anisotropy in the films can be interpreted as being the result of a lattice mismatch stress mechanism. The Gd-Y films with uniaxial anisotropy are in tension while the Eu-Y films are in compression as shown by lattice spacing differences between films and their substrates. Uncracked, high stability Eu-Y films with very high compressive stresses were grown. Annealing experiments reveal significant changes in anisotropy which correlate with large changes in film lattice spacing. Thus, it is possible to obtain high uniaxial anisotropy in garnets containing only one kind of rare earth ion together with a nonmagnetic ion such as yttrium. These films support domains with very high wall velocities at typical device drive fields.

Modification of Magnetic Anisotropy in Garnets by Ion Implantation

Ion implantation into epitaxial films of certain magnetic garnets grown on nonmagnetic substrates has produced layers with uniaxial anisotropy suitable for magnetic ``bubble'' formation and propagation. Implantation of hydrogen or lithium ions causes the garnet lattice to expand and the constraint of the unimplanted material places the implanted layer in lateral compression. In garnets with positive magnetostriction, this stress results in an induced easy axis of magnetization perpendicular to the surface. Helium-ion implantation does not cause lattice expansion or induce magnetic anisotropy. This suggests that the phenomenon is due to a chemical effect rather than lattice damage.

Radio-Frequency Determination of New Growth-Induced Anisotropy in Garnets for Bubble Devices

A new type of magnetocrystalline anisotropy has been observed under {110} faces in several fluxgrown mixed rare-earth iron garnets. This anisotropy is different in direction and magnitude from that previously reported in similar garnets under {211} faces. In Eu2ErGa0.6Fe4.4O12 with 4πM near 350 G at 300°K, microwave resonance measurements have shown that {110} face material has a large effective uniaxial anisotropy. The [100] in the growth plane is the easy axis, [011] normal to this plane is medium, and [011̄] parallel to this plane is hard. Using a resonance equation appropriate to the orthorhombic magnetic symmetry and neglecting the relatively small cubic terms, the effective uniaxial anisotropy field is ∼1300 Oe. This anisotropy leads to a new cutting procedure (type 3 cut) for bubble devices: {100} platelets are cut normal to the natural {110} faces. Using 15-μ-thick platelets of this type, bubbles ∼8μ in diameter were propagated. The domain wall mobility is ∼170 cm/sec/Oe. These properties are comparable to those of {111} platelets cut under {211} surfaces of the same crystals.