Basic properties of superlattices constructed from ferromagnetic and antiferromagnetic films

805

Journal of Magnetism and Magnetic Materials 54-57 (1986) 805-806 BASIC PROPERTIES OF SUPERLATTICES AND ANTIFERROMAGNETIC FILMS Laura HINCHEY

CONSTRUCTED

FERROMAGNETIC

FROM

and D.L. MILLS

Department of Physics, Univeristy of California, Iruin, CA 92717, USA

We present theoretical studies of the properties of superlattices constructed from alternating layers of ferromagnetic and antiferromagnetic films. This paper confines its attention to superlattices in which the antiferromagnet consists of alternating sheets of spins parallel to the interfaces, and the spin alignment within each sheet is ferromagnetic in character. We use localized spin models of each material, and we present studies of the classical ground state of the system, as a function of magnetic field. For the case where the ferromagnetic films each contain an even number of atomic layers, we find a phase diagram which contains four distinctly different phases. We have calculated the spin wave spectrum of each phase, with particular attention directed toward the magnetic resonance spectrum of the superlattice structure. We find a diverse array of spectra, which differ strikingly as microscopic parameters which characterize the superlattice structure as a whole are varied.

In a previous p a p e r [1], we described the initial results of our theoretical study of superlattices formed from alternating layers of ferromagnetic a n d antiferrom a g n e t materials. In the model, the a n t i f e r r o m a g n e t consists of alternating sheets of spins within which ferromagnetic alignment occurs, each sheet parallel to the interfaces. If the a n t i f e r r o m a g n e t consists of a n even n u m b e r of atomic layers, in the ground state with no external field present, the magnetic m o m e n t s of the ferromagnetic spins alternate in sign as one moves d o w n the superlattice structure. Our earlier work [1] d e m o n strated that this g r o u n d state is u n s t a b l e if a Z e e m a n field H 0 is applied parallel to the 2 axis, along which the ferromagnetic m o m e n t s are initially aligned (with alternating sign). T h e critical field H~ 1) required to induce an instability decreases as 1 / N F, with N v the n u m b e r of layers in the ferromagnetic film. W e have completed studies of the ground state of this structrure as a function of magnetic field Ho, a n d we find a sequence of three phase transitions (each second order) between zero field, a n d fields sufficiently large that all spins align parallel to + 2 . F o r H~ 1) < H 0 < H(2)c, we have a low symmetry state in which spins in the ferromagnetic films with spins d o w n in zero field are twisted toward + 2 t h r o u g h angles larger t h a n the deviation away from + 2 induced in the ferromagnetic films with spins up in zero field. T h e n w h e n H~ 2) < H 0 < ,,(3) we have the " s u p e r l a t t i c e spin flop state", where the transverse m o m e n t in the ferromagnetic films alternate in sign as one moves d o w n the structure. This state has a glide p l a n e in its s y m m e t r y group. F o r H 0 > H~ 3), all spins are aligned along + 2. A superlattice with four layers in the ferromagnetic film, a n d four in the antiferromagnetic film has 16 layers in a basic unit cell, a n d there are sixteen b r a n c h e s of the spin wave dispersion curves, for p r o p a g a t i o n n o r m a l to the interfaces. F o r H~ 1) < H 0 < / 4 (2) a n d H (2) < H 0 < H~ 3), we show the lowest two b r a n c h e s in figs. l a , b, respectively. The lowest b r a n c h is always a G o l d n c

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stone mode, with linear slope with vanishing wave vector. The linear slope at the zone b o u n d a r y in fig. l b , a n d the vanishing gap there, is a consequence of the glide plane symmetry.

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L. Hinchey, D.L. Mills /Superlatticesfrom FM and AFMfilms

For H~ I ~ < H 0 < H ~ (3), the k = 0 second mode produces a strong feature in the calculated infrared absorption spectrum. This becomes a "soft mode" as H 0 is increased toward H~ 2). Fig. 2 is a plot of the dependence of the frequency of this mode on field, below and above the phase transition. These superlattices thus exhibit a rich behavior, as

the external magnetic field is varied. A full description of our work will appear elsewhere [2]. This research was supported by the A r m y Research Office, through Grant No. 485870-59766, 1985. [1] L. Hinchey and D.L. Mills, J. Appl. Phys. 57 (1985) 3687. [2] L. Hinchey and D.L. Mills, to be published.