Er superlattices

PHYSICA ELSEVIER

Physica B 213&214 (1995) 242 244

Polarized neutron study of antiferromagnetic ordering in Dy/Er superlattices C. Dufour a, K. Dumesnil a, G. M a r c h a l a, Ph. M a n g i n a, M. H e n n i o n b' *, H. Kaiser c, J.J. R h y n e c a Laboratoire de Metallurgie Physique et Sciences des' Matbriaux-CNRS-URA 155, UniversitO de Nancy I - H. PoincarO, BP 239, 54506 Vandoeuvre los Nancy Cedex, France b Laboratoire LOon Brillouin, Centre d 'Etudes Nucleaires de Saclay, 91191 Gif-Sur-Yvette Cedex, France MURR, Universi~, q["Missouri Columbia, MO 6521 l, USA

Abstract

Dy/Er superlattices, grown on sapphire, exhibit a succession of magnetic phases. Below the ordering temperature of dysprosium, the helix of dysprosium propagates coherently through paramagnetic erbium. At low temperature, dysprosium becomes ferromagnetically ordered in each layer but the layers are antiferromagnetically stacked. A polarized neutron scattering study of the low-temperature phase is presented. It shows that the antiferromagnetic peaks are never purely spin-flip.

Polarized neutron scattering is a unique tool to provide separate information on the longitudinal and transverse components of the magnetization. In usual geometrical conditions, with a vertical field, the non-spin-flip scattering represents the nuclear structure and the longitudinal component of the projection of the magnetization in the reflection plane. The spin-flip scattering is related only to its transverse component. In the simple case where the magnetic moments lie in the reflection plane (x,z) and z is the quantization axis, the non-spin-flip scattering amplitude is proportional to (b + Pz) and the spin-flip scattering amplitude is proportional to Px- b is the nuclear scattering amplitude and p is the magnetic amplitude that is proportional to the magnetic moment of the atom. This technique was recently used to determine the magnetization in multilayers [1] and superlattices [2], * Corresponding author.

especially in systems where ferromagnetic layers are antiferromagnetically coupled through a non-magnetic spacer. For example, high-angle neutron scattering was performed on Gd/Y superlattices where, depending on the yttrium layer thickness, ferromagnetic gadolinium layers couple either antiferromagnetically or ferromagnetically through yttrium. It was shown that under a magnetic field, a small ferromagnetic component develops in the field direction and that the antiferromagnetic component aligns perpendicular to that field. This was established by showing that the ferromagnetic contribution was non spin flip and that the antiferromagnetic peaks were spin flip. Antiferromagnetic coupling between ferromagnetic Dy blocks has recently been observed in Dy/Lu [3] and Dy/Er [4] superlattices. In the Dy/Lu system, it was explained by dipolar coupling that competes with the interlayer exchange interaction. In order to better understand this coupling and its strength in Dy/Er super-

0921-4526/95/$09.50 .x" 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 1 0 0 1 1 8 - 2

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C. Du]bur et al. /Phvsica B 213&214 (1995) 242 244

120 80

1500

-

i +AF.

0 kOe

I++ 1000

AF.

500

40 h.

h

110 K

0 000

80 AF.

h

F.

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"

10 K

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"AF h

1.9

2

2.1

2.2

q

2.3 2,4 2.5 2.6

so ~

6.4 kOe 1000

(A ')

Fig. 1. Neutron diffraction patterns along the IO00 2j direction for a [Dy(34 A)/Er(23 A)] × 50 superlattice at 110 and 10 K. Y : Y buffer and coverage; n. : nuclear satellite: h. : magnetic peaks due to the Dy helix: Er:magnetic peaks due to the Er helix and AF.:peaks due to the antiferromagnetic structure of the Dy layers.

lattices, we performed polarized-neutron scattering to separate the two ferromagnetic and antiferromagnetic components of the magnetization and their evolution under field. Let us recall that bulk dysprosium is helical below TN = 176 K and undergoes a ferromagnetic transition driven by a magnetoelastic effect at Tc = 88 K. The magnetic moments always lie in the basal plane. Below TN ~ 84 K, bulk erbium exhibits an easy axis perpendicular to the basal plane. A magnetic modulation develops along the c-axis with a conical phase at low temperature. The superlattice samples were grown by molecular beam epitaxy on (1 1 2 0) sapphire substrates following the method proposed by Kwo et al. [5]. X-ray diffraction shows good crystal perfection with a mosaic width of 0.3 and coherence length of 700 ~,. The pattern exhibits several superlattice satellites. Unpolarized neutron scattering experiments were performed at M U R R and polarized neutron diffraction at LLB. Unpolarized neutron diffraction patterns along the (00 02) axis (the magnetic moments are in the reflecting plane) are shown in Fig, 1 for a [Dy(34 ~,)/Er (23 A)] × 50 superlattice. At 110 K, we observe a structure peak at the average lattice parameter of the Dy/Er superlattice at q = 2.23 ,g,- x, the yttrium buffer and cover contribution (Y) and a nuclear harmonic (n.) due to the chemical modulation. The position of this peak corresponds to

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1000

500

• ~10 o2.1 2.15 2.2 2.25 2.3 2,1Z ' 2.15 ~ _2.2 - ] 2.25 2.3 2.35 q (~. I) q (~-l)

Fig. 2. Polarized-neutron-diffraction patterns (I +÷ and I +-) along the (0 00 2) direction for a [Dy(34 A,)/Er(23 A,)] × 50 superlattice at 10 K under different magnetic fields applied along the a-axis: 0, 2.2, 6.4 and 0 kOe (after applying 6.4 kOe). n. :nuclear satellite; AF.: peaks due to the antiferromagnetic arrangement of the Dy layers; F. :peaks due to the ferromagnetic arrangement of the Dy layers.

a modulation wavelength of 57/k. The two pairs of peaks (h.) on the right and on the left sides are magnetic peaks arising from the dysprosium helix and their width confirms its coherence through the paramagnetic erbium. This behavior is similar to that observed in Dy/Y superlattices [6]. At low temperature (10 K), the peaks due to the Dy helix have almost disappeared, and the intensity of the average peak is unchanged but several new peaks have appeared (AF). Their positions relative to the average peak correspond to a doubling of the periodicity. This means that each dysprosium layer is ferromagnetic and that these layers are long-range coupled antiferromagnetically through erbium. The last peak (Er) corresponds to the inplane modulation of the conical phase

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C. Dujour et al./ Physica B 213&214 (1995) 242-244

of erbium. This behavior is similar to that observed in a Dy(60 A)/Er(60 A) superlattice [4]. Polarized-neutron spectra at 10K are presented in Fig. 2 for the [Dy(34/~)/'Er(23 ,g,)] × 50 superlattice. The polarization "flipping ratio" was 17 and the wavelength was 4.488 A. Two spin-dependent scattered intensities were measured: I + + and I + -. The two superscripts denote the initial and final neutron spin states. We restricted the measurements to the q range between 2.08 A i and 2.33 A 1. This q range covers the average nuclear peak (q = 2.23/~-1), a superlattice harmonic (n. at q = 2.12 A - 1) and two of the antiferromagnetic peaks (A F. at q = 2.18 and 2.28 A - l ) . As I ++ is larger than 1 + , no corrections were performed on I + +. I ÷ was corrected by subtracting 1 + +/17 to compensate for the finite flipping ratio. The magnetic field was applied along the a axis (Dy easy-magnetization direction in the basal plane). In very weak fields, the spin-flip scattering l + presents only two peaks at the antiferromagnetic positions and the non-spin-flip component 1 + + exhibits the average superlattice peak, the nuclear satellite and two antiferromagnetic peaks. The intensities of the AF peaks are the same for the + + and the + - diffraction indicating that the antiferromagnetic domains are randomly oriented, at least along the three equivalent easy axis. For a magnetic field of 2.2 kOe, the + + intensity of the average superlattice peak and of the superlattice harmonic have increased. This is the signature of a ferromagnetic contribution in the sample. At the same time the antiferromagnetic peaks have almost disappeared. Likewise, in the + - spectrum, the antiferromagnetic peaks have decreased and a small ferromagnetic contribution has appeared. For a field of 6.4 kOe, there is no evidence of any antiferromagnetic contribution either in the + + or + - scattering. The magnetic moments are fully aligned along the easy a axis (the small ferromagnetic component in I + can be due to incomplete correction of the polarization efficiency to which 1 + - is very sensitive).

After removing the field, no antiferromagnetic component reappears either in the + + pattern or in + - . A decrease of the main peak and of the nuclear satellite is observed in + + diffraction pattern and a ferromagnetic contribution reappears in + - . This means that ferromagnetic domains have replaced the antiferromagnetic ones of the initial configuration. Magnetization measurements exhibit a significant remanent magnetization, and thus the ferromagnetic domains are not randomly oriented. When the field is applied along the b-axis, the evolution is qualitatively the same but the antiferromagnetic state is slightly more stable. We never observed any situation where the antiferromagnetic contribution was only present in the spin flip scattering in contrast to the previous Gd/Y study [2]. When this contribution was present in the spin-flip scattering, a comparable one was systematically observed in the non-spin-flip scattering. This difference may result from the in-plane magnetic anisotropy that is absent in the G d system.

References [l] B. Rodmacq, Ph, Mangin and C. Vettier, Europhys. Lett. 15 (1993) 503. [2] C.F. Majkrzak, J.W. Cable, J. Kwo, M. Hong, D.B. McWhan, J.V. Waszczak and C. Vettier, Phys. Rev. Lett. 56 (1986) 2700. [3] R.S. Beach, J.A. Borchers, A. Matheny, R.W. Erwin, M.B. Salomon, B, Everitt, K. Petit, J.J. Rhyne and C.P. Flynn, Phys. Rev. Lett. 70 (1993) 3502. [4] K. Durnesnil, C. Dufour, M. Vergnat, G. Marchal, Ph. Mangin, M. Hennion, W.T. Lee, H. Kaiser and J.J. Rhyne, Phys. Rev. B 49 (1994) 12274. [5] J. Kwo, M. Hong and S. Nakahara, Appl. Phys. Lett. 49 (1986) 319. [6] R.W. Erwin, J.J. Rhyne, M.B. Salomon, J.A. Botchers, S. Shina, R. Du, J.E. Cunningham and C.P. Flynn, Phys. Rev. B 35 (1987) 6808.