Uniaxial Anistropy and Rotational Hysteresis in Thin Gadolinium FilmsB. R. Livesay, F. L. Grismore, and E. J. Scheibner Citation: Journal of Applied Physics 38, 1438 (1967); doi: 10.1063/1.1709655 View online: http://dx.doi.org/10.1063/1.1709655 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/38/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in On the scalability of doped hafnia thin films Appl. Phys. Lett. 104, 122906 (2014); 10.1063/1.4870075 Effects of preparation conditions on the magnetocaloric properties of Gd thin films J. Vac. Sci. Technol. A 31, 031506 (2013); 10.1116/1.4795817 Magnetization of 2.6T in gadolinium thin films Appl. Phys. Lett. 101, 142407 (2012); 10.1063/1.4757126 Impact of interfacial magnetism on magnetocaloric properties of thin film heterostructures J. Appl. Phys. 109, 063905 (2011); 10.1063/1.3555101 Tunable electrical conductivity in nanoscale Gd-doped ceria thin films Appl. Phys. Lett. 90, 263108 (2007); 10.1063/1.2752028

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1438 A. R. VON NElDA AND F. B. HAGEDORN

In conclusion, the flux-loss measurements obtained are most simply interpreted in terms of a classical diffusion process with an activation energy which is appreciably larger than that previously reported for thin film Cu-Permalloy diffusion. There is in addition, however, an initial flux-loss component (fast compared with the diffusion) which is believed due to oxidation

of a small fraction of the magnetic material. It is further worth noting that an extrapolation to temper­atures below 200°C leads to T'S so long that this initial process cannot be responsible for the aging mechanism in plated-wire memory elements.

We wish to thank W. H. Craft for supplying the plated wire.

JOURNAL OF APPLIED PHYSICS VOLUME 38, NUMBER 3 1 MARCH 1967

Uniaxial Anisotropy and Rotational Hysteresis in Thin Gadolinium Films * B. R. LIVESAY, F. L. GRISMORE, AND E. J. SCHEIBNER

Physical Sciences Division, Georgia Institute of Technology, Atlanta, Georgia

The planar anisotropy and rotational hysteresis of thin films of gadolinium have been studied using torque measurements. Films ranging from 300 to 700 A were prepared by evaporation at normal inci­dence onto heated quartz and glass substrates and overcoated with SiO. Electron- and x-ray diffraction patterns show a finite fraction of crystallites have a canted preferred orientation of the c axis. which probably accounts for the uniaxial anisotropy observed. The films were found to have a dominant sin 28-torque behavior for fields above 3 kOe and a sin 8-dependence below 300 Oe. The uniaxial anisot­ropy constants measured for these films are lOL I06 erg/cc. The films show a unidirectional rotational hysteresis. Peak loss occurs at field strengths between 0.5 and 1 kOe. and losses remain significant up to 10 kOe. A magnetic model based on a magnetization ripple structure is proposed to explain the ob­served data.

PLANAR torque measurements have been made on evaporated thin films of gadolinium. Previous

measurementsl of the temperature dependence of the magnetization of gadolinium showed strong anisotropic behavior. The magnetic anisotropy in the plane of the film influenced the measured magnetization to such a degree that it became desirable to look at the planar anisotropy in some detail.

Circular films of gadolinium having a diameter of 9 mm were prepared by evaporation at pressures less than 4X1o--7 Torr onto substrates of fused quartz and glass and overcoated with SiO before being removed to another chamber for magnetic measurements. Film thicknesses ranged between 300 and 700 A with evaporation rates about 100 A/min. Prior to deposition the substrates were baked at a temperature above 350°C for a day or more. Substrate temperatures during deposition were about 200°C and no applied magnetic field was used.

The anisotropy measurements were made with a torque magnetometer systeml at 10 kOe and below and at several temperatures. Only the 78°K measurements are reported here. Background torque measurements were made with a blank substrate oriented in the same manner as the film substrate.

Torque measurements showing the uniaxial be­havior of a 360-1 gadolinium film are presented in Fig. 1. The high-field torque data show a sin20 behavior as illustrated by the 5- and 10-kOe curves. The low-

* Work supported in part by the U.S. Atomic Energy Com­mission and National Aeronautics and Space Administration.

1 B. R. Livesay, L. K. Jordan, and E. J. Scheibner, J. Appl. Phys. 37, 1266 (1966).

field torque dependence is sinO as shown by the 60-0e curve. Intermediate-field measurements had large rotational hysteresis as seen in the SOO-Oe torque curve. The anisotropy constant, K", is about 2X104 erg/cc as determined from the high-field measurements. This gives a value of 21 Oe for Hk •

X-ray diffraction investigations of this film showed an 80%-90% fiber orientation with basal plane parallel to the substrate. The c axis of a major fraction of the remainder lies isotropically in the plane of the film. No planar anisotropy would result from these orienta­tions. However, a small fraction, of the order of a few percent as estimated by x-ray intensities, had a pre­ferred orientation with the c axis canted 35°-45° from the film normal, and towards the observed easy direc­tion. Since the minimum-crystalline energy configura­tion at 78°K is a cone2 of about 45° about the c axis, the canted fraction in conjunction with the film de­magnetizing factor produces a planar-uniaxial anisot­ropy. Based on bulk data the observed anisotropy of K,,=2X104 erg/cc would result from a 2% volume­fraction of canted material.

The measured torque curves are open at all angles, a condition generally attributed to irreversible switch­ing of randomly oriented uniaxial regions. However, the observed rotational hysteresis vs field data at high fields and the numerical value of the rotational hys­teresis integral are not consistent with such a model.

The plot of rotational hysteresis vs field, Fig. 2, shows a high peak at 500 Oe and a long tail of signifi­cant amplitude extending beyond 10 kOe. For a film

I C. D. Graham, J. Appl. Phys.34, 1341 (1963).

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THIN GADOLINIUM FILMS 1439

consisting of a distribution of uniaxial regions, either random or negatively oriented, the hysteresis goes to zero at an applied field equal to the maximum-anisot­ropy field component. From the measurements of crystalline anisotropy constants of Graham,2 this can be calculated as approximately 1 kOe for gadolinium at 77°K. Figure 2, however, shows a large observed hysteresis at 1 kOe.

The rotational hysteresis integra13

W = f WR/Md(l/H)

gives a value of W = 1.55 for this film. This is much larger than the values of O.38;.;iW;.;iO.415 predicted for variations of a Stoner-Wolfarth model.

_.20t:---------.04

.02

o 90· 180' ANGLE

FIG. 1. Planar torque curves for a 360-l\. gadolinium film.

3 M. Prutton, Thin Magnetic Films (Butterworths, Inc., Wash­ington, D.C., 1964), p. 116.

FIG. 2. Plot of ro­tational hysteresis as Wr

40

30

a function of applied Ku 20

field.

10

~·~+1~2~3~47-~5~6~7"8~~9-T,IO H (KILO- OERSTED)

The observed data can however be qualitatively explained by assuming a magnetization ripple struc­ture predominates in the film. Feldtkeller4,5 has shown that the effects of the ripple may be characterized by a uniaxial anisotropy Hkr and a threshold torque Lth governing irreversible rotation of the ripple-wall structure. In this model the film magnetization is influenced by the crystalline-uniaxial anisotropy fixed with reference to the film, and a second anisotropy axis of ripple-wall origin which is rotatable as the interacting torque between it and M exceeds Lth. Both Hkr and Lth go to zero as the ripple-wall angle approaches zero. Since the ripple-wall angle varies as 6..;:::::::.{Hk/(H+ Hk ) 100, the asymptotic characteristic of the observed rotational hysteresis curve is expected.

At low fields the angular excursion of M is small enough so the ripple structure remains fixed and a sinO torque curve results with low hysteresis. At some larger field the angular excursion of }.f becomes great enough so that the torque between M and the ripple structure equals the threshold torque. The ripple-wall structure is then dragged· along behind M causing high-rotational loss. At high fields the ripple structure essentially disappears yielding a sin28 torque due to Ku with low rotational hysteresis.

In addition to explaining the observed torque data, the model predicts the occurrence of a rotatable easy axis. This has been experimentally verified yielding 60-and SOO-Oe torque Curves shifted to the direction of an initially applied 10-kOe field. Also the model has been programmed on an analog computer with lIkr

and Lth as adjustable parameters. It is found that torque curves equivalent to those experimentally ob­served result, if both H kr and Lth are varied inversely proportional to the applied field.

The authors are indebted to Dr. E. A. Starke and Dr. R. J. Gerdes for diffraction analyses.

• E. Feldtkeller, J. Appl. Phys. 34, 2646 (1963). • E. Feldtkeller, Z. Physik 176, 510 (1963).

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