Papers

AVS2001 Session MI+TF-ThA: Magnetic Thin Films and Surfaces I

Thursday, November 1, 2001 2:00 PM in Room 110

Thursday Afternoon

Start	Invited?	Item
2:00 PM		MI+TF-ThA-1 John Thornton Award Lecture - Magnetic Multilayers: Past, Present and Future ¹ S.D. Bader (Argonne National Laboratory) Highlights of magnetic multilayer research at Argonne are presented. The most recent past can be taken as the era of giant magnetoresistance multilayers. From there we move to the present where we are addressing issues associated with magnetic pinning across diverse interfaces. Illustrative examples include the coupling between ferromagnets and antiferromagnets, as well as between "hard" and "soft" ferromagnets. The former is of importance in understanding the design of spin valves and magnetic random access memory (MRAM). The later provides a possible nanotech route to the creation of a new generation of ultra-strong permanent magnets. In the future the expectation is that lateral patterning, self-assembly and spintronics will open new vistas. ¹ ¹ This work was supported by the U. S. Department of Energy, Office of Science, under contract number W-31-109-ENG-38.
2:40 PM		MI+TF-ThA-3 Occupied and Unoccupied Metallic Quantum Well States in the Cu/fccM/Cu(100) [M=Ni, Fe] System A.G. Danese, R.A. Bartynski (Rutgers University); D.A. Arena, M. Hochstrasser, J.G. Tobin (Lawrence Livermore National Lab & Lawrence Berkeley Lab) Multilayers of alternating magnetic (FM) and non magnetic (NM) layers have attracted a great deal of attention due to their technological importance. We have studied the Metallic Quantum Well (MQW) electronic structure of the prototypical NM/FM/NM systems, Cu/fccM/Cu(100) [M=Ni,Fe], using both angle resolved photoemission (PE) and inverse photoemission (IPE) along the Gamma-bar X-bar direction. We have also used a phase accumulation model (PAM) to calculate the dispersions of MQW electronic states along this axis. The PAM predicts that MQW states will have a high effective mass when they lie in the energy and momentum region of the projected spin polarized band gap of the underlying FM material. PE of the Cu/fccNi/Cu(100) system shows one high effective mass state inside the Ni band gap and another near the gap edge while IPE shows one just above the Ni gap, in good qualitative agreement with the PAM. Numerous MQW states were seen using IPE on Cu/fccFe/Cu(100) but no pronounced high-effective-mass state was seen in the Fe band gap. We believe this can be explained if the Fe film is actually NM which will move the location of the high effective mass states from where they are expected. The PAM also predicts that MQW states will increase in energy as a function of increasing Cu thickness. Although we observed this in our IPE results for MQW states in Cu/fccFe/Cu(100) and PE of Cu/fccNi/Cu(100), our IPE data for Cu/fccNi/Cu(100) show MQW states decreasing in energy with increasing Cu thickness. This same result was observed for Cu films on a Ni(100) single crystal and attributed to lattice mismatch between Cu and Ni, but we have shown that strain cannot account for the behavior of these MQW states and are currently studying how the interface roughness between the Cu and Ni film may provide an explanation. We will discuss our results in the context of the PAM and address the origins of discrepancies between the PAM's predictions and our measurements.
3:00 PM		MI+TF-ThA-4 The Magnetic Properties of Fe₅₀Mn₅₀/Cu Multilayers G.J. Mankey, S. Maat, L. Shen, S.C. Byeon, E. Ada (University of Alabama); J.L. Robertson, M.L. Crow, T.C. Schulthess, W.H. Butler (Oak Ridge National Laboratory) The temperature dependent magnetic properties of 50-period multilayers of Fe₅₀Mn₅₀/ Cu were investigated by squid magnetometry and neutron scattering. Squid magnetometry of a polycrystalline multilayer revealed that field cooling of the multilayer from above the Néel temperature aligns the uncompensated spins, resulting in ferromagnetic ordering of a small fraction of the sample. This ferromagnetic ordering may contribute to the exchange-bias effect observed in Fe₅₀Mn₅₀/ferromagnet layers. For neutron diffraction measurements, epitaxial multilayers with a fcc (111) surface orientation were produced by magnetron sputtering on H-terminated Si(110). The neutron diffraction measurements reveal a wider mosaic spread in the magnetic lattice relative to the chemical lattice and that only a portion of the Fe₅₀Mn₅₀ alloy was antiferromagnetically ordered. These observations suggest the domain walls occupy a significant fraction of the Fe₅₀Mn₅₀ volume. The critical behavior of the antiferromagnetic ordering was determined by measuring the temperature dependence of the magnetic diffraction peak with neutron diffraction. For the first heating cycle, a Néel temperature of 510K and critical exponent of 0.357 are found, consistent with bulk 3D Heisenberg behavior. However, measurements during subsequent heating cycles showed that annealing to 480 K irreversibly changes the microstructure of the multilayer, resulting in a reduction in the magnetization, a reduction of the critical exponent, and an increase of the Néel temperature. XPS depth profiling of the multilayer before and after annealing shows that the interface widths increase due to intermixing of the Fe₅₀Mn₅₀ and Cu layers. The intermixing is the cause of the changes in magnetic properties. Sponsored by ARO DAAH-04-96-1-0316, NSF MRSEC DMR-9809423, and DOE DMR DE-AC05-96OR22464.
3:20 PM		MI+TF-ThA-5 Magnetic Reversal of Exchange-coupled Co/Pt Multilayers Probed by Resonant Soft X-ray Scattering O. Hellwig, S. Maat, E.E. Fullerton (IBM Almaden Research Center); J.B. Kortright (Lawrence Berkeley National Laboratory) The balance between exchange, anisotropy and dipolar energies determines the domain structure in ferromagnetic films. For systems with perpendicular magnetic anisotropy such as Co/Pt multilayers, this often results in stripe domain patterns. In this study we modify the energy balance in Co/Pt multilayers by selectively replacing Pt layers in the structure by CoO or Ru layers. Adding antiferromagnetic CoO layers leads to a perpendicular exchange bias below the Neel temperature of the CoO. The addition of 0.9-nm Ru layers antiferromagnetically couples adjacent Co/Pt blocks and changes the characteristics of the magnetic reversal behavior dramatically. We use resonant soft X-ray small-angle scattering in addition to more conventional methods such as magnetometry and magnetic force microscopy to investigate the domain structure as well as the magnetic reversal process. By measuring both the angle and field dependence of the magnetic scattering we quantify the domain formation during reversal. For the CoO interlayers we find that the domain nucleation process in the biased samples is asymmetric to positive and negative field sweeps but once nucleated the domain patterns are symmetric to the field sweep directions. Zero field cooling in an aligned stripe domain pattern after in-plane demagnetization results in a periodic biasing of the system. At low temperature this leads to a memory effect in the domain pattern that forms during field reversal. The multilayers with Ru interlayers exhibit two distinct reversal modes that reflect the competition between the dipolar and the interlayer exchange energy. Such systems highlight the opportunity to tune the magnetic domain structure and reversal in perpendicular multilayers. Olav Hellwig was partially supported by the Deutsche Forschungsgemeinschaft via a Forschungsstipendium under the contract number HE 3286/2-1.
4:00 PM		MI+TF-ThA-7 Magnetic Phases of Fe Monolayers on Ni/Cu(001) M. Farle (Technische Universitaet Braunschweig, Germany); H. Poppa (Lawrence Berkeley National Laboratory) The spin reorientation transition (SRT) of Fe on 4 to 9 ML Ni on Cu(001) is studied by spin-polarized low-energy electron microscopy (SPLEEM) in situ at 295 K. The formation of magnetic domains is monitored during the growth of the Fe monolayers with video rates at a resolution of about 100 nm. The x,y and z components of the magnetization vector are determined. On 8.2 ML Ni/Cu(001) we find three different magnetic phases as a function of Fe thickness. a) 0.2 to 2.8 ML Fe : large out-of-plane magnetic domains, b) 2.8 to 6 ML Fe : large in-plane magnetic domains, c) > 6 ML Fe no magnetic contrast. At the transition from a) to b) which starts at 2.6 ML and ends at 3.0 ML narrow stripe domains appear with a tilted orientation with respect to the film plane. At the transition from b) to c) the size, shape and direction of the in-plane magnetic domains does not change, only the magnetic contrast is lost. This indicates a transition from a ferromagnetic to the paramagnetic state of 6 ML Fe on 8 ML Ni ! Our domain observations are disussed in terms of current concepts of spin-reorientation transitions.
4:20 PM		MI+TF-ThA-8 Spin-resolved Electronic Structure Studies of Ultrathin Films of Fe on GaAs M. Spangenberg, E.A. Seddon (CLRC Daresbury Laboratory, UK); E.M. McCash (University of York, UK); T. Shen (University of Salford, UK); S.A. Morton, G.D. Waddill (University of Missouri-Rolla); J.G. Tobin (Lawrence Livermore National Laboratory) Fe thin films of up to 5.5nm were deposited on singular and vicinal GaAs substrates and their magnetic and structural properties investigated by spin polarized photoelectron spectroscopy, magnetic linear dichroism in photoemission, magnetization measurements and X-ray diffraction. On both types of substrate the Fe grows predominantly as delta Fe. In agreement with literature results, the magnetization measurements and the magnetic linear dichroism results indicate very similar magnetic properties for the Fe films grown on the two substrate types. However, comparison of the spin polarized valence bands of the Fe films on the singular and the vicinal substrates reveal very significant differences. The possible origins of these observations will be presented. Comparisons and rationalisations will also be made (were possible) between our observations on Fe on GaAs and literature reports for other Fe thin film systems. For example, we have found that the spin polarized valence bands of Fe deposited on the vicinal GaAs exhibit similar features to those reported in the literature for 10nm Fe(100) films on Cu3Au(100) . In contrast, the spin polarized valence bands of Fe films on the singular GaAs are very different to all Fe thin film literature reports, with the differences concentrated in the minority spin channel.
5:00 PM		MI+TF-ThA-10 Growth Mode Dependence of Magneto Optical Signal Evolution in an Ultrathin Film: Layer by Layer vs. 3D Growth J.L. Menendez, G. Armelles, A. Cebollada, J.L. Costa-Kramer (Instituto de Microelectronica de Madrid (CNM-CSIC), Spain) In this work the magneto-optical properties of the first stadium of the initial growth of a ferromagnetic material deposited on top of a substrate will we analyzed. Two different growth modes will be discussed: layer by layer and three dimensional growth mode. A comparison of the magneto-optical signal for the two growth modes will be presented for three different configurations of the applied magnetic field: magnetic field applied perpendicular to the layer (polar) and in the plane of the layer (transverse and longitudinal). The main results are: In the layer by layer growth mode, the intensity of the magneto-optical Kerr signal is a linear function of the thickness of the deposit layer and the magnetic moment of the layer (i.e., the intensity is a linear function the amount of deposit material for the three configurations). Therefore, the intensity of the magneto optical signal can be used to analyze the evolution of the magnetic moment of the layer as we increase the thickness. In the case of three dimensional growth mode, the intensity of the magneto-optical Kerr signal does not only depend on the magnetic moment of the layer, but also on the shape and amount of islands present in the layer. Contrary to the layer by layer growth mode the intensity of the magneto-optical properties is not a linear function of the amount of the deposited material and therefore can not be directly correlated with the magnetic moment.