Klaus Witter

Magnetic and magneto-optical properties of rare-earth transition-metal alloys containing Dy, Ho, Fe, Co

Amorphous rare‐earth transition‐metal alloys of composition REl−xTMx with RE=Dy, Ho; TM=Fe,Co and 0<x<1 and Dyl−x(Fe,Co)x were prepared by evaporation. The saturation magnetization, uniaxial anisotropy, coercivity, and Faraday rotation were investigated as a function of composition and temperature. Also, the spectral variation of the Kerr rotation was measured. The magnetization data indicate a strong dispersion of the RE moments due to randomly oriented local crystal field axes. The strong turndown of the Curie temperature for the Fe‐rich alloys suggests that an additional dispersion is present in the Fe subnetwork. The mean field theory was used to analyze the temperature variation of the magnetization yielding smaller TM spin values and exchange coupling constants as compared with those of the Gd and Tb analogs. The uniaxial anisotropy constant Ku for Dy‐Co based alloys was found to vary with the square of the Dy subnetwork magnetization as predicted by the random single‐ion theory. The anisotropy of D...

Magnetic and magneto‐optic properties of bismuth‐ and aluminum‐substituted iron garnet films

The saturation magnetization Ms, the uniaxial anisotropy Ku, the optical absorption α, the Faraday rotation θF, and the Faraday ellipticity ψF of epitaxial garnet films of composition Y3−xBixFe5−yAlyO12 have been investigated for x≤1.34 and y≤2.22. The temperature dependence of Ms and θF at λ=633 nm have been measured in the range 4.2 K≤T≤TC. Ku, TC, θF, and ψF reveal a linear dependence on the bismuth content. The dependence of θF and ψF on the aluminum concentration shows that both sublattices are involved and indicate that the magneto‐optical effects have to be ascribed to iron pair transitions. The magnitude of the growth‐induced anisotropy is essentially controlled by the supercooling ΔTs and increases linearly with ΔTs. The obtainable magnetic and magneto‐optical data for magneto‐optical applications are discussed and compared with those of garnet films exhibiting a compensation temperature.

Magnetic and magneto‐optic properties of praseodymium‐ and bismuth‐substituted yttrium iron garnet films

Garnet films of composition R3−x−yPrxBiyFe5O12 with R=Lu, Y, and 0≤x≤1.8, 0≤y≤1.7 have been grown from different melt compositions and under various growth conditions onto substituted gadolinium gallium garnet substrates with (100), (110), (111), and (211) orientation. The dependence on composition and temperature of the saturation magnetization Ms, the uniaxial anisotropy constant Ku, the Faraday rotation θF, and the Faraday ellipticity ψF, have been investigated. The change of Ms as compared to that of Y3Fe5O12 is essentially associated with the alignment of the Pr moment parallel to the net iron moment and the increase of the Curie temperature originating mainly from the bismuth. However, both contributions are negligible in the room‐temperature range. The uniaxial anisotropy is strongly affected by both Pr and Bi. The anisotropy contributions ΔKu /x 0 turn out to be approximately additive. Both Pr and Bi give rise to a pronounced enhancement of the magneto‐optical effects. At λ=633 nm θF i...

Growth‐induced uniaxial anisotropy of bismuth‐substituted iron‐garnet films

The temperature dependence of the growth‐induced uniaxial anisotropy constant Kgu of epitaxially grown bismuth‐substituted garnet films of composition R3−xBixFe5O12 with R=Y, Lu has been measured in the temperature range 4.2 K≤T≤TC. Films with (100), (110), (112), and (111) orientation have ben investigated and exhibit a very different temperature variation of Kgu. The experimental data are compared with theoretical results based on a recently reported single‐ion theory applying to bismuth‐induced uniaxial anisotropies. Kgu is calculated in terms of the actual sublattice magnetizations inferred from the fit of the molecular field theory to the measured saturation magnetization.

Pinning of 180° Bloch walls at etched nuclear tracks in LPE‐grown iron garnet films

For increasing the magnetic‐wall coercivity Hwc in liquid‐phase epitaxial (LPE) ferrimagnetic garnet films of composition (Gd,Bi)3(Fe,Al,Ga)5O12, magnetic‐wall ’’traps’’ are formed via bombardment by xenon ions with 180‐MeV/ion energy and doses between 106 and 108 cm−2. For efficient wall pinning, the width of the nuclear damage tracks associated with the ion trajectories in the film have been enlarged to about the wall width by using a selective (chemical) etchant that makes use of the drastically increased etching rate in the damaged track volume. Therefore, channels of cylindrical or prismatic cross section are created having a width of a few 102 to about 103 A and a length of more than 10 μm at the given etching conditions. The pinning capability of such channels can be further enhanced in films that are grown under planar compressive or tensile misfit strain. Then, strain relaxation occurs in the vicinity of these channels which results in steep gradients of the magnetic‐wall energy via magnetostrict...

MOPS, a magneto‐optic‐photoconductor sandwich for optical information storage

A magneto‐optic photoconductor sandwich, called MOPS, has been assessed as a memory medium for optical information storage. The present version of MOPS is composed of an epitaxial (Gd,Bi)3 (Fe,Ga)5O12 film and a Cu‐doped CdS photoconductor film between coplanar In2O3/SnO2 electrodes. The chip contains 1.4×105 bits, requires 1 μW of optical power at 6328 A for switching a memory cell in a magnetic field of 100 Oe and electric field of 1.2×104 V/cm, exhibits a read‐out contrast of 21 : 1 at a read‐out efficiency of 0.1%, and has a cycle time of ∼100 μs. MOPS has not been degraded by 107 switching cycles of arbitrary memory cells. No cross talk between neighboring bit sites has been observed.

Thermomagnetic switching on rare‐earth transition‐metal alloy magneto‐optic disks

The thermomagnetic switching behavior of magneto‐optic disks with amorphous TbFeCo and GdTbFe films was investigated on a test recorder. Domain length, write and erase fields, and erase laser power required for total domain erasure was studied for various film compositions, which differ in compensation temperature Tcomp and Curie temperature TC. With increasing TC the domain length at fixed writing laser pulse energy is reduced and the power required to erase domains with a certain size is increased. The optimum external field, for minimal write noise, decreases with increasing TC−Tcomp. The experimental dependencies agree with calculated results obtained from the force balance controlling the domain formation.

Magnetoelastic and optoelastic coupling in (111)‐ and (110)‐oriented bismuth‐iron garnet films prepared by sputter epitaxy

High‐quality iron‐garnet films of composition Gd3−vBivFe5−w−x−yGdw GaxAlyO12−zArz with 1.02≤v≤1.45, 0.01≤w≤0.36, 0.22≤x≤0.24, 0.26≤y≤0.38, and 0.002≤z≤0.014 have been grown epitaxially on (111)‐ and (110)‐oriented gallium garnet substrates with the lattice constant in the range 1.2475≤a0(nm)≤1.2553 by rf magnetron sputtering in argon plasma. For the study of the optoelastic and magnetoelastic properties of these sputter‐epitaxial films the lattice mismatch Δa⊥=as−a⊥f has been varied in the range −1.7<Δa⊥(10−2 nm)<0.5 by using substrates with various lattice constants. The near‐infrared optical anisotropy normal to (Δnxy) and in the plane (Δnyz) of (110)‐oriented films turn out to depend linearly on Δa⊥ over this large range of lattice misfits. No growth‐induced contribution has been observed. The same behavior is found for the induced uniaxial (K⊥) and in‐plane (K∥) magnetic anisotropy constants at room temperature. The optoelastic coefficients P44 and ΔP, as well as the magnetostriction constants λ111 an...

Growth‐induced anisotropy and Faraday rotation of bismuth‐substituted europium‐iron‐garnet films

The saturation magnetization Ms, the growth‐induced uniaxial anisotropy constant Kgu, and the Faraday rotation θF, of [111]‐oriented epitaxial iron‐garnet films of composition Eu3−xBixFe5O12 with x≤1.3, have been investigated as a function of composition and temperature, and, have been compared with bismuth‐free films of composition Eu3−xLuxFe5O12 and Eu3−uPbuFe5O12 with u≤0.124. The results indicate a strong dependence of Kgu on growth direction, lead and bismuth content, and growth parameters, which are associated with the noncubic occupancy of the Bi3+, Pb2+, and Eu3+ ions. A maximum Kgu value of 8×104 J m−3 occurs for x=1.3 and T=4.2 K. The temperature dependence of Kgu for bismuth‐substituted films is interpreted in terms of the calculated Eu3+ and the Bi3+‐Fe3+ contributions. At λ=633 nm and T=295 K, the lead‐ and bismuth‐induced Faraday rotations ΔθF/u and ΔθF/x are −1.1×106 and −2.06×106 deg m−1, respectively, in good agreement with results reported for other rare‐earth garnets. These data confirm...

Influence of nuclear tracks on the coercivity of single crystalline (Gd, Bi)3(Fe, Ga)5O12 LPE garnet films☆

Abstract The domain wall coercivity in single-crystalline liquid-phase epitaxial (LPE) garnet films is much too low for high-density information storage application. The defect density has to be increased by roughly 5 orders of magnitude to achieve storage densities of up 10 6 bit/cm 2 . A sensitive control of the coercivity can be attained with nuclear tracks from high-energy (1.4 MeV/nucleon) Xe or U ions with densities ranging from 10 7 to 10 10 cm -2 .