Amorphous Gd-Co Films Research Articles

Experimental Evidence of Chiral Ferrimagnetism in Amorphous GdCo Films.

Inversion symmetry breaking has become a vital research area in modern magnetism with phenomena including the Rashba effect, spin Hall effect, and the Dzyaloshinskii-Moriya interaction (DMI)-a vector spin exchange. The latter one may stabilize chiral spin textures with topologically nontrivial properties, such as Skyrmions. So far, chiral spin textures have mainly been studied in helimagnets and thin ferromagnets with heavy-element capping. Here, the concept of chirality driven by interfacial DMI is generalized to complex multicomponent systems and demonstrated on the example of chiral ferrimagnetism in amorphous GdCo films. Utilizing Lorentz microscopy and X-ray magnetic circular dichroism spectroscopy, and tailoring thickness, capping, and rare-earth composition, reveal that 2 nm thick GdCo films preserve ferrimagnetism and stabilize chiral domain walls. The type of chiral domain walls depends on the rare-earth composition/saturation magnetization, enabling a possible temperature control of the intrinsic properties of ferrimagnetic domain walls.

Amorphous gadolinium-cobalt films as sensitive media for the topographic mapping of nonuniform temperature fields

The results of an investigation of the response of the domain structure of Gd-Co films to stationary, spatially nonuniform planar thermal fields created by one or two nominal point heat sources are presented. Several simple techniques for the topographic mapping of these fields by observing and measuring the parameters of the domain structure of amorphous Gd-Co films are formulated.

Amorphous gadolinium-cobalt films with in-plane anisotropy for magnetoresistive sandwiches

This paper reports an investigation into the possibility of developing in-plane magnetization in amorphous Gd-Co films, and of using such films in magnetoresistive sandwiches.

Generic source of perpendicular anisotropy in amorphous rare-earth-transition-metal films.

We show that the magnetic dipolar energy of antiparallel dipoles in the surface layers contributes to the intrinsic uniaxial perpendicular anisotropy in amorphous rare-earth-transition-metal films. The volume anisotropy energy density K u of this contribution depends inversely on the thickness of the film. For an amorphous GdCo film with a thickness of 1000 A the calculated K u is on the order of 10 3 -10 4 ergs/cm 3 . Dependence of K u on temperature and atomic concentration is also calculated

Exchange anisotropy in amorphous Gd-Co films

It was previously reported that multilayer samples of Co/GdCo2 annealed at 500 °C exhibited exchange anisotropy. One observes a B-H hysteresis loop centered at a finite field, the ‘‘exchange’’ field. To identify the ‘‘hard’’ magnetic phase(s) responsible for the shifted loops we have studied thin films of GdCox as well as multilayers of Co/GdCox, where x varies from 2 to 8.5. We saw shifted loops in two distinct groups of samples, neither of which is multilayered. The first group was GdCox in the composition range between GdCo3 and GdCo5. The loop displacement at room temperature is about 5 Oe. The coercivities of these samples are about 10 Oe. The second group of samples had a composition close to GdCo2. For these samples most of the B-H loop is centered at zero field with a coercivity of 1–2 Oe. However, at ±80 Oe there are small loops which account for about 20% of the total magnetization. X-ray measurements indicate that both groups of samples are microcrystalline or amorphous. The second set of samples discussed above exhibit a maximum in diffracted intensity at 2θ=36°, where GdCo2 has a strong (220) peak. The first group of samples have a broad maximum at 38°, where neither GdCo3 nor GdCo5 has peaks. Gd2Co7 has a very weak peak at this angle and Gd2Co17 has strong peaks but would have to be metastable.

Amorphous GdCo: Effect of surface segregation on reading of the magnetization by spin-polarized photoemission

Spin-polarized photoemission can be used to read the surface magnetization direction of perpendicularly magnetized amorphous GdCo films. Effects specific of the photoemission process are observed: (1) surface depolarization by disordered Gd moments in the outermost segregated layers of the film, (2) change of the polarization upon varying the probing depth of the photoelectrons. Both effects are consequences of the inhomogeneous surface composition of amorphous GdCo due to segregation.

Selective resputtering-induced anisotropy in amorphous films

Resputtering effect on the uniaxial magnetic anisotropy in amorphous GdCo films has been investigated. A sputtering apparatus has been constructed which is able to change the energy and the accommodation rate of the resputtering Ar ions independently of each other. The energy of Ar ions bombarding the substrate electrode or the film has been measured by means of a Faraday cup and has been found to be smaller than the bias voltage applied to the substrate electrode. The deposition rate of the film has been found to decrease linearly with an increase in the accommodation rate of the resputtering Ar ions. The product of the uniaxial anisotropy constant and the deposition rate has been found to be proportional to the accommodation rate of Ar ions with constant energy. This result agrees with the selective resputtering-induced anisotropy model.

Anomalous aging characteristics of amorphous GdCo films

Amorphous GdCo films with perpendicular anisotropy have been prepared by dc diode getter sputtering under argon or neon pressure. To study the aging behavior, the films with no protecting layer were used and exposed to air at room temperature. We have observed that the perpendicular anisotropy in these uncoated GdCo films significantly increases with time. Furthermore, the negative (in-plane) anisotropy which existed at the initial stage during aging actually decreases with time and finally disappears.

Compositionally modulated Gd-Co amorphous films by bias sputtering

The new sputtering technique was developed in which film deposition and ion bombardments were performed alternately and independently. By the technique, amorphous Gd-Co films were prepared with various thickness of the periodically deposited layers (t d ). It was confirmed that ion bombardments by applying the bias of -100 \sim-200 V change the composition within only about two atomic layers near the surface. The films with the compositional modulation of amplitude of a few at.% were synthesized in the range of t d larger than about 0.6 nm. A large anisotropy perpendicular to the film plane appeared abruptly at t d larger than 2.0 nm and the anisotropy decreased as t d increased. The results obtained may be interpreted qualitatively by the improved pair-ordering model.

Perpendicular anisotropy in rare earth-transition metal amorphous films prepared by dual ion beam sputtering

The effect of resputtering on perpendicular magnetic anisotropy has been investigated for amorphous Gd-Co, Gd-Fe, Tb-Co and Tb-Fe films using a dual ion beam sputtering system. In Tb-Co and Tb-Fe films, perpendicular anisotropy increases remarkably owing to resputtering, while in Gd-Co and Gd-Fe films, it changes only through compositional change caused by resputtering. It is likely that the different behaviors between two cases are due to the fact that Tb is anisotropic and Gd is isotropic atom. However, its mechanism is not clear.

Hall effect in amorphous GdFe, GdCo and GdCoMo films

The Hall tangent of amorphous GdCo, GdFe and GdCoMo films has been investigated. The main conclusion is that both sublattices take part in the Hall scattering as suggested by McGuire model. A good agreement with experiment was obtained only after accounting for the charge transfer from Gd to Fe and Co atoms. The absolute spontaneous Hall coefficients for Gd,Fe and Co in amorphous state differ among each other.

Dc getter sputtered amorphous GdCo films: Magnetic anisotropy and in-depth chemical composition

dc getter sputtering was used to deposit amorphous GdCo films. No bias voltage was applied to the substrates during the deposition. A uniform chemical composition throughout the depth of the film was observed using Rutherford backscattering spectrometry. These GdCo films show perpendicular magnetic anisotropy. Furthermore, the magnetic anisotropy can be well described by a uniaxial film whose easy axes are distributed within a range of 15° around the film plane normal.

Transport properties of Gd-Co amorphous films in the vicinity of the compensation composition

An attempt has been made to explain the physical phenomena underlying the spontaneous Hall effect (SHE) in amorphous Gd-Co films. The experimental investigations were performed for films with compositions in the vicinity of the compensation point (for cobalt concentration from 68 up to 82 at%). The results obtained show that the SHE in the films investigated can be governed by the side-jump mechanism, though these results seem to imply that skew-scattering is the origin of the SHE.

Temperature Dependence of Anomalous Loop in Double-Layered Amorphous GdCo Film

The magnetic properties of double-layered amorphous GdCo film consisting of films with different compensation temperatures were studied. The theoretical hysteresis loops and their temperature dependence were deduced using the double-layer model. It was confirmed that the magnetic properties of the double-layered amorphous GdCo film agreed well with the prediction by the double-layer model. From the temperature dependence of the double-layered film, it is shown that an apparent compensation temperature exists between the original compensation temperatures in each layer. The appearance of the inverted loop is explained using the double-layer model; this was confirmed by experiments in the double-layered amorphous GdCo film.

Composition and temperature dependence of resistivity in amorphous Gd-Co films

The composition and temperature dependence of electrical resistivity were studied for a series of amorphous Gd-Co films prepared by an rf-sputtering method. With increasing Gd concentration the electrical resistivity at 4.2 K increased from 180 μΩ cm to 350 μΩ cm and the temperature coefficient of resistivity at 288 K decreased and became negative above 220 μΩ cm.

FMR study of the oxidation of amorphous GdCo films

The FMR method ang g-factor determination in particular have been shown to be useful tools for detecting the oxidation process in some amorphous GdCo films. Typical results prove that the Gd oxidation process takes place over the whole volume of the film on annealing in air at temperatures up to 170°C. The air annealing does not lead to a marked inhomogenization of the film.

Measurement of Magneto-Optical Kerr Effect Using Piezo-Birefringent Modulator

An advanced magneto-optical technique is described, which allows simultaneous determination of two mutually-conjugate parameters–Kerr rotation and reflectance magneto-circular dichroism–using a piezo-birefringent modulator. This technique provides a sensitivity of 0.002° for Kerr rotation at λ=500 nm with 12 nm resolution, using a halogen lamp (150 W) as the light source. The wavelength region covered by the method can be extended from 300–2500 nm by suitable selection of gratings and light detectors. The new technique was used to measure magneto-optical hysteresis loops for several wavelengths and magneto-optical spectra on an rf-sputtered amorphous Gd-Co film, and the results are reported here.

Transport properties of amorphous Gd-Co films

The electrical resistivity and the galvanomagnetic effect in Gd-Co sputtered films were studied as a function of temperature between 1.5 and 390 K. The logarithmic temperature dependence of electrical resistivity was dominant in the region from 1.5 to 35 K and disappeared after crystallization. The quadratic temperature dependence of electrical resistivity was observed from 50 to 390 K together with the resistivity minimum. The temperature dependence of transverse magnetoresistivity was measured to investigate the effect of magnetic domain walls to the temperature dependence of electrical resistivity. The temperature dependence of transverse magnetoresistivity was about one hundredth as large as that of electrical resistivity.

Increase in Uniformity of Amorphous GdCo Films by Control of Oxygen Quantity

The spatial uniformity of compositions and of the magnetic property of rf-sputtered amorphous GdCo thin films was investigated by Auger electron spectroscopy, electron probe microanalyzer, and polar Kerr hysteresis loops. It was found that by applying a negative dc bias to the substrate during sputtering, the quantity of oxygen incorporated into the film decreases and considerable uniformity of the oxygen and of magnetic property is obtained, though the magnetic property shows a slight increase in compensation temperature toward the edge. This slight change was found to come from a change in composition of the metals. A further increase in uniformity was achieved by intentionally introducing a small amount of oxygen during the deposition process.

The influence of strong bias voltage on the electrical and magnetic properties of the amorphous Gd-Co films

Some electrical and magnetic properties of Gd-Co films obtained by dc sputtering with bias voltage V b are discussed. We observed a decrease in the compensation temperature and in the anomalous Hall resistivity with increasing bias voltage while the specific resistivity increases. The anisotropy constant, however, increases up to V b = -200 V, where it reaches a maximum and then it decreases. Electrical conductivity of the films prepared with V b greater than -100 V showed tunneling character. We conclude that not only resputtering of Gd atoms but also oxidation of Gd takes place during deposition when bias voltage is applied.