Papers

AVS2004 Session MI-WeA: Exchange Coupling, Surfaces, and Interfaces

Wednesday, November 17, 2004 2:00 PM in Room 304A

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		MI-WeA-1 Spin-Dependent Quantum Size Effects in Ultrathin Co Single Crystals R. Zdyb, E. Bauer (Arizona State University) We have prepared micron-sized ultrathin (0001)-oriented Co single crystals with thicknesses varying between 1 and 10 monolayers by epitaxy on W(110) and studied the spin-dependent reflectivity of electrons with energies up to about 20 eV in spin-polarized low energy electron microscope. Similar to our previous study of ultrathin (110)-oriented Fe single crystals¹ , quantum size effects allow the determination of the exchange splitting of the sp band above the vacuum level. We will also report the results of our attempts to understand the apparent oscillatory spin reorientation transition in ultrathin Co films². ¹ ¹ R. Zdyb and E. Bauer, Phys. Rev. Lett. 9 (2002) 1485. ² T. Duden and E. Bauer, Phys. Rev. Lett. 77 (1996) 2308.
2:20 PM		MI-WeA-2 Electronic Resonances of Isolated Mn and Interacting Mn-Mn Complexes on GaAs (110) Surfaces¹ A. Richardella, D. Kitchen, A. Yazdani (University of Illinois at Urbana-Champaign) Using low temperature scanning tunneling microscopy (STM) Mn on GaAs (110) surfaces has been studied. We present results for isolated Mn adatoms evaporated at low temperature on in situ cleaved n-type and p-type GaAs substrates. Localized modifications of the density of states of the substrates due to Mn are shown. Isolated Mn adatoms can exhibit two equivalent stable bonding states which STM atomic manipulation can induce transitions between. Additionally, certain tunneling parameters lead to increased mobility of Mn on the surface. It is shown isolated Mn's display a strong preference to pair along certain lattice directions. This pairing presents a unique opportunity for studying the interaction of magnetic impurities mediated through the underlying semiconductor states. Resonances due to these Mn-Mn interactions are presented using local density of states (LDOS) spectra and energy resolved spatial maps. In particular it is shown that similarly spaced Mn-Mn pairs can exhibit a number of distinct localized electronic resonances. Studies are ongoing into whether these varied localized states result from the relative spin orientations of the impurities with respect to each other and the surface. ¹ This work was supported by ARO MURI DAAD19-01-1-0541.
2:40 PM		MI-WeA-3 Non Linear Aspects of Ultrathin Film Magnetism D. Pescia, O. Portmann (ETH Zurich, Switzerland); M. Buess (University of Regensburg, Switzerland); A. Vaterlaus (ETH Zurich, Switzerland); C.H. Back (University of Regensburg, Switzerland) We report on two experiments. In the first one, the stripe phase of perpendicularly magnetized ultrathin films is shown to form a Mermin-like 2D solid with algebraic correlations. The second one consists in exciting the spin motion in a vortex-like spin configuration by an ultrashort magnetic field pulse and imaging the local spin dynamics with pico-second time resolution. Both experiments reveal new non-linear aspects of ultrathin film magnetism.
3:20 PM		MI-WeA-5 Overcoming Thermal Fluctuations in Ferromagnetic Nanostructures using Exchange Bias J. Nogués, V. Skumryev (Inst. Catalana de Recerca i Estudis Avancats, Spain); S. Stoyanov, Y. Zhang, G. Hadjipanayis (U. of Delaware); D. Givord (CNRS-Grenoble, France); K. Liu (U. of California-Davis); C. Leighton (U. of Minnesota); H. Masuda, K. Nishio (Tokyo Metro. U., Japan); I.V. Roshchin, I.K. Schuller (UCSD); J. Eisenmenger (U. Ulm, Germany); J. Sort, J.S. Muñoz, S. Suriñach, M.D. Baró (U. Autònoma de Barcelona, Spain) Today's interest in nanoparticle magnetism is stimulated by a variety of potential applications, ranging from ultra-high density information storage to medicine. Most applications rely on the magnetic order of the nanoparticles being stable with time. However, in small particles, thermal fluctuations may affect the magnetization stability and possibly lead to superparamagnetism. In this study, we will demonstrate that the exchange coupling between ferromagnetic (FM) nanostructures and antiferromagnetic (AFM) hosts can lead to the magnetic stabilization, i.e. enhancement of coercivity (H_C), increase of remanence (M_R) and ultimately improvement of the superparamagnetic blocking temperature (T_B). Three different cases will be discussed: (i) Co particles ball milled with NiO¹, where a small increase of the coercivity and the remanence is observed. (ii) Fe nanostructures deposited on FeF₂ layers², where a clear enhancement of the remanence of the hysteresis loop is seen and appears to be linked with its coercivity enhancement. (iii) Co nanoparticles embedded in CoO³, where the blocking temperature is substantially improved with the concomitant increase of H_C and M_R. In particular, 4nm-Co particles, embedded in a CoO matrix, remain ferromagnetic up to the Néel temperature of CoO. This corresponds to almost 30-fold increase in the blocking temperature compared to the uncoupled nanoparticles. The AFM-FM coupling can be viewed as providing an extra source of anisotropy, thus leading to magnetization stability. ^* Work supported by EU, CICYT and DGR (UAB), NSF, Seagate (UDEL) and US-DOE (UCSD) ¹ J. Sort et al., Appl. Phys Lett. 75, 3177 (1999) ² K. Liu et al., Appl. Phys Lett. 81, 4434 (2002) ³ V. Skumryev et al. Nature 423, 850 (2003).
4:00 PM		MI-WeA-7 Growth and Study of Fe/Mn₃N₂(010)Bilayers for Use in Spintronics Applications R. Yang, M.B. Haider, H.A. Al-Brithen, A.R. Smith (Ohio University) Exchange biasing(EB)systems have drawn much attention in recent years, prompted by the intriguing physics and its prominent role in magnetic sensing devices.^1,2,3 Yet, the detailed mechanism, which includes the spins at the ferromagnetic(FM)/antiferromagnetic(aFM)interface, is not fully understood. In this paper, we investigate the growth of an EB system using Fe/Mn₃N₂ (010) bilayers. The Mn3N2 (010) surface aFM magnetic structure is well understood by means of recent spin-polarized scanning tunneling microscope (SP-STM) studies.⁴ Mn₃N₂ has a face-centered tetragonal(fct)rocksalt-type structure. The magnetic moments of the Mn atoms are FM within (001) planes, and are layerwise aFM along [001]. The Néel temperature of Mn3N2 is 925K.^4,5 The next step is to study the initial stages of Fe growth on this surface. In this paper, we try to investigate the atomistic changes to the surface which occur at the initial stages of the growth. The growth begins with a 440 nm thick Mn₃N₂ aFM layer and substrate is MgO(001). After that, a small coverage (0-10ML) of Fe is deposited in different growth temperatures in the range 350-550°C. The growth is monitored by reflection high-energy electron diffraction (RHEED). After growth, samples are transferred under ultra high vacuum (UHV) directly to STM analysis chamber . STM images reveal a stepped surface with terrace width about 50 Å . The dependence of the film properties on growth parameters such as growth temperature, film thickness and annealing time will be discussed. ¹W. H. Meiklejohn, et al., Phys. Rev. 102,1413 (1956). ² C. L. Chien, et al., Phys. Rev. B 68, 014418 (2003). ³ L. Ritchie, et al., J. Mag. Mag. Mater. 247, 187 (2002). ⁴ H.Q. Yang, et al., Phys. Rev. Lett. 89, 226101 (2002). ⁵ G. Kreiner, et al., J. Alloys Compd. 183, 345 (1992).
4:20 PM		MI-WeA-8 Magnetic and Structural Properties of Lattice Matched Epitaxial Antiferromagnetic/Ferromagnetic Bilayers P. Mani, V.V. Krishnamurthy (The University of Alabama); S. Maat (Hitachi Global Storage Technologies); A. Kellock (IBM Research Division); G.J. Mankey (The University of Alabama) The antiferromagnetism of FePt₃ films grown on Al₂O₃(11-20) and MgO(110) has been confirmed by neutron scattering. Fe_xPt_1-x (0.2 < x < 0.3) exhibits antiferromagnetic ordering below 160K depending on the film growth temperature, substrate symmetry and composition of the alloy.¹ These FePt₃ films offer a fascinating route to understand the relationship between structure and magnetism in exchange bias systems since Fe_xPt_1-x grows as an ordered antiferromagnet when deposited at 750°C and as a disordered ferromagnet when deposited at 150°C. In addition, ferromagnetic films of CoPt₃ have the same lattice constant as FePt₃. Thus these film systems provide a pathway to create strain-free interfaces to test the current models of exchange bias. Both positive and negative exchange bias were observed in strained Fe/FePt₃ bilayers with different in-plane cooling field directions.² In the present study, Fe_xPt_1-x films are grown on MgO(111)and a-axis sapphire by co-sputtering Fe and Pt. Epitaxy and alloy ordering have been confirmed by x-ray diffraction. Rutherford backscattering spectrometry and energy dispersive analysis of x-rays were used to characterize the composition of these alloys, which confirms that stoichiometry can be controlled within a tolerance of 1% in our ultra clean sputtering system. Detailed measurements of the structural and magnetic properties of lattice matched FePt₃/Fe_0.25Pt_0.75 and FePt₃/CoPt₃ and their relation to current models of exchange bias behavior will be discussed. ¹ S. Maat, O. Hellwig, G. Zeltzer et al., Phys. Rev. B, 63, 134426 (2001). ² R.L. Compton, M.J. Pechan, S. Maat, and Eric. E. Fullerton, Phys. Rev. B, 66 (5), 054411 (2002).
4:40 PM		MI-WeA-9 Momentum Transfer in Exchange Bias Systems of the Type F/af/aaf R. Mattheis, K. Steenbeck (IPHT, Germany) In F/AF exchange bias systems (F ferromagnet, AF antiferromagnet) momentum is transferred from the F film to the AF lattice by coupling via the AF interface net moment and the AF anisotropy. To study the dynamics of the AF lattice during rotation of the F layer magnetisation we sandwiched the F/AF system at the free AF side by a completely symmetric artificial antiferromagnet (AAF) of the kind CoFe/0.8 nm Ru/CoFe. At not too high magnetic field strength this AAF acts, due to its own coercivity and vanishing magnetic net moment, as a fixed spin system coupled at the opposite side of the AF. By means of torquemetry at 10K and 300K the rotational loss and exchange bias strength was determined for different AF thickness t. We found: a) The AAF can fix the AF spin system also at very low thickness t where K is zero. At this thickness exchange bias is found in F/AF system coupled to an AAF but not in the pure F/AF system. b) Fixing of the AF spins by coupling an AAF leads to a drastically reduction of the rotational losses in the AF and rotational anisotropy at a thickness where both where maximum without an AAF. The results show that overcoming the limited anisotropy in very thin AF films is possible by their coupling to an AAF. From the thickness dependence of the observed effects a deeper inside of the exchange bias phenomena is obtained and will be discussed in more detail.
5:00 PM		MI-WeA-10 Training in Exchange Bias Systems: The Role of Anisotropy A. Hoffmann (Argonne National Laboratory) The coupling between a ferromagnet and an antiferromagnet can give rise to a directional anisotropy called exchange bias. In order to establish the direction of the exchange bias, the coupled ferro-/antiferromagnetic system is generally cooled in the presence of an external magnetic field through the ordering temperature of the antiferromagnet. In many systems the magnitude of the exchange bias is reduced upon subsequent field cycling after the initial field cooling. These field-training effects are suspected to be due to irreversible changes in the magnetic microstructure of the antiferromagnet, but a comprehensive theoretical understanding is still missing. I will present numerical simulations based on a simple coherent rotation model, which suggest that the symmetry of the anisotropy in the antiferromagnet plays a crucial role for the understanding of these training effects. Namely, the existence of more than one antiferromagnetic easy anisotropy axes can initially stabilize a non-collinear arrangement of the antiferromagnetic spins, which relaxes into a collinear arrangement after the first magnetization reversal of the ferromagnet. This explains quite naturally why training effects are only observed for exchange bias systems with high symmetry antiferromagnets, while they are absent for antiferromagnets with uniaxial anisotropy. Furthermore, this simple and universal model reproduces many of the experimentally observed training effects. The model gives rise to a rotation of the effective easy axis for the ferromagnet after the first field reversal, such that the first magnetization reversal shows a large sudden jump, while all subsequent reversals are more gradual. I will compare in detail the calculated hysteresis loops with experimentally measured ones on the prototypical Co/CoO exchange bias system. This work was supported by the Department of Energy, Basic Energy Sciences under contract No. W-31-109-ENG-38.