B Swinnen

Interface and structure characterization of co/re multilayers using rbs channeling and hyperfine interactions

Abstract Cobalt/rhenium multilayers were prepared by rf and dc sputtering and studied using RBS and hyperfine interaction techniques. The layer thickness is determined with a precision of 3A, and the interface intermixing is less than 5Aup to 773 K. Solid state reactions occur for higher temperatures. The hyperfine field value confirms the hcp structure of the Co. A small shift of the hyperfine field measured as function of the cobalt layer thickness is explained by the effect of lattice stresses.

Magnetic anisotropy studied with PAC: magnetic hyperfine fields on 111Cd in bcc iron/cobalt multilayers

Abstract The technique of perturbed angular correlation (PAC) is introduced. It is shown how this technique can contribute to the of magnetic anisotropy in films and multilayers using its sensitivity to the absolute orientation of the hyperfine field. As an example, experiments are presented that demonstrate a [110] orientation of the magnetic hyperfine fields of bcc Fe and bcc Co in epitaxial Fe/Co multilayers.

A perturbed angular correlation study of Fe/Co multilayers on (110)-GaAs

Abstract Recent results on the magnetic hyperfine fields at the 111 Cd nuclear probe in MBE-grown (Fe x Co y ) 10 multilayers (with x = 10, 25 and 40 A ; y = 20 A ) reveal a sharp Co on Fe interface, while a diffuse interface Fe on Co is observed.

Magnetic and structural properties of Cr in Ag/Cr(001) multilayers

Abstract We show that it is possible to grow epitaxial Ag/Cr(001) multilayers on MgO(001) by molecular beam epitaxy (MBE) techniques. Channeling and XRD measurements reveal that the out-of-plane lattice parameter of Ag is strongly reduced, resulting in heavily strained Ag layers. Similar to results in Fe/Cr multilayers we observe in Ag/Cr multilayers an enhancement of the chromium Neel temperature. Using perturbed angular correlation (PAC) spectroscopy we see a spin density wave (SDW) out-of-plane magnetization in the Cr layers with thicknesses above 5.9 nm and definitely different spectra for thinner Cr layers.

Structure and magnetism of BCC-Fe/Co multilayers studied by means of perturbed angular correlation spectroscopy

Abstract We have used perturbed angular correlation spectroscopy to study Fe/Co superlattices grown on (1 – 10)-GaAs with a Co thickness around 20 A and Fe thicknesses between 10 and 40 A. We found that all hyperfine fields in these layers are along the [1 1 0]-axis in the plane of the multilayers. Measurements of the temperature dependence of the Cd hyperfine field at the center of the (1 – 10)-Co layers were interpreted in terms of gradually changing Co-structure. Below the growth temperature, thermal lattice expansion accounts for the anomalous temperature dependence of the hyperfine field. At higher temperature the monoclinic deformation of the Co lattice gradually relaxes until a nearly undeformed BCC-structure is achieved around 570 K. Further increasing the temperature causes a phase transition of the Co lattice that results in interdiffusion of Fe and Co. These results in accordance with theoretical calculations suggest that the BCC-like Co structure is stabilized by an interfacial energy contribution, particularly by lattice strain and that BCC-Co is not a thermodynamically metastable phase. We resolved satellite hyperfine fields that are unambiguously attributed to probe atoms in plateaus near a sharp interface. In addition, broad frequency distributions associated with a diffuse interface were obvious in the data. Therefore, we proposed a growth model for (1 – 10)-Fe/Co where interfaces of Co on top of Fe are sharp and those of Fe on top of Co are diffuse. Within the extended Stearns hyperfine-field model, the hyperfine-field data were interpreted in terms of magnetic moments. The moment profile derived for (1 – 10)-Fe/Co reveals a Friedel oscillation of both the Fe and Co near interface moments. The spatial variation of the moments is best described by an amplitude, decaying exponentially with the distance to the interface and modulated by a sine function of the same distance. Between 90 and 570 K, the amplitude, the decay constant and the wave vector of the modulation are constant within the values determined. The Friedel oscillation of the near interface moments is understood as due to spin waves traveling across the interface and interfering with those of the penetrated lattice. The experimental results suggest that the nodes of the spin waves should be pinned to the interface. The resulting oscillation is superimposed on the bulk moment. This picture evidently explains why the experimental moment profile at (1 – 10)-Fe/Co is symmetric about the interface.

Oscillation of the Fe and Co magnetic moments at the sharp (1–10) Fe/Co interface and temperature dependence of the near interface moments

By means of time differential perturbed angular correlation spectroscopy, we studied the 111Cd hyperfine fields in molecular beam epitaxy-grown Fe/Co multilayers on (1–10) GaAs as a function of temperature (90–570 K). Together with the known fields for Cd in bcc Fe and in bcc Co, we observed 4 sharply defined satellite fields. They have previously been associated with probe positions near a sharp Fe/Co interface. Whereas the Cd hyperfine field in Fe and its satellites decrease with temperature, the satellites on the Co field increase with temperature as the CdCo field does. We determine values for the near interface moments by using them as free parameters when fitting the experimental satellite fields within a semiempirical model for the hyperfine field. At all temperatures, the Fe and Co magnetic moments near the sharp interface between bcc Fe and the monoclinically deformed body centered Co oscillate as a function of the layer number over 2 layers in Co and 3 in Fe. The near interface magnetic moments ...

Spatial and temperature dependence of magnetic moment perturbations near the (1-10)Fe/Co interface

The spatial dependence of the magnetic moment perturbation near the (1-10)-Fe/Co interface was derived from hyperfine fields determined by perturbed angular correlation. The amplitude of the moment perturbation decays exponentially with the distance to the interface. The moment perturbation itself is modulated by a sine function of the distance d to the interface Δμ=A*exp(−p*|d|)*sin(−k*d). Between 90 K and 570 K, we observe that the amplitude and the penetration depth of the perturbation as well as the wave vector of the modulation are essentially constant within the accuracy of the values. In this temperature region the average values for the parameters determining the exact moment perturbation are A=1.03μB, p=2.25×109 m and k=1.45×1010 m−1 when d is defined positive at the Co side of the interface.

Impurity hyperfine fields in metastable body centered cubic Co

A new spectrum component is observed in Mossbauer spectra of thin body centered cubic Co layers doped with 57Co which have previously been shown to be in this metastable state with nuclear magnetic resonance. It is characterized with a large magnetic hyperfine field (31.2 T) and an isomer shift nearly equal to that of α‐Fe. The decrease of the isomer shift in bcc Co with respect to hcp Co is consistent with a smaller s–d charge transfer in bcc Co as compared to hcp Co. The influence of structure on the hyperfine field in a Co crystal is reflected in the decrease of the magnetic hyperfine field of Fe in bcc Co as compared to hcp Co and is similar to the volume dependence of the magnetic hyperfine fields in a Co crystal.

Mixed phase in cubic and hexagonal HoMn2 111Cd PAC and 119Sn, 57Fe Mössbauer studies

Abstract Hyperfine parameters on 57 Fe, 119 Sn and 111 Cd substituted into the Mn sublattice were measured by Mossbauer and PAC spectroscopies. From these results it is tentatively concluded that C15 and C14 HoMn 2 are mixed-phase compounds. In C14 HoMn 2 there is no (or small) moment on the 2a site.

Perturbed angular correlation study of 3d transition metal multilayers

Abstract The present paper aims to show that a perturbed angular correlation (PAC) investigation gives access to unique information on the (interface) magnetism in MBE-grown very thin magnetic multilayers. For the Fe/Cr(1 0 0) multilayer system we observe, below a critical Cr-thickness of 5.0 nm, a collapse of the spin-density-wave (SDW) ordening in chromium. While the magnetization in the Fe-layer is in-plane, the Cr-magnetisation in thicker layers is out-of-plane. In the Fe/Co (1–10) system, the observation of transferred magnetic hyperfine field satellites and their temperature dependence leads to the determination of an oscillating magnetic moment profile at the Fe/Co interface as well as a structure model for the superlattice.