Program: NATIONAL PROGRAM OF RESEARCH AND DEVELOPMENT II

Contract No. 71-032/2007 ; Subprogram: S7; Project type: CP

Project code: VALS

Period: 36 months (Sept.2007-Sept.2010)

Project Title: Spin Valves: from combinatorial processing towards enhanced properties

Project Director: Dr. Kuncser Victor- Eugen

Coordinator: National Institute for Materials Physics (INCDFM), Bucuresti-Magurele, -CO

Parteners:

	National Institute of Lasers, Radiation and Plasma (INCDFLPR), Bucuresti-Magurele (Dr. Ion Mustata) –P1
	Polytechnic University of Bucharest (UPB), Center of Biomaterials (Dr. Florin Miculescu) –P2

The magnetization of a ferromagnetic material can be positive or negative along some convenient axis, at room temperature. These stable states can be correlated with binary “0” and “1” in digital applications. Further on, the magnetization states can be recorded in special magnetic media and then communicated to the external word via spin dependent conduction mechanisms, specific to magnetorezistive sensors. Such a sensor requires two ferromagnetic thin layers (the spin polarizer and the spin analyzer), interfacing a very thin conductive or dielectric layer. The electrical resistance of the conductive/dielectric layer changes via the modification of the magnetic configuration of the two neighborhood ferromagnetic layers, which influences the propagation of the spin polarized electronic waves. Controlling the electronic conduction of a nanostructure through the electronic spin gave rise to a new field of electronics, called spintronics. The involved nanostructures are called spin valves. The main conduction mechanisms in such structures are based on quantum mechanics phenomena and, therefore, at least one dimension of the system has to be in the nanometer range. The two involved quantum mechanics phenomena are: the electron spin scattering mechanism (in case of the conductive layer) and (ii) the spin dependent tunneling process (in case of the dielectric layer). The proposed project corresponds to the international trend towards optimization and improvements of spin valve structures, with important applications in nowadays top technologies, like as: nonvolatile random access memory, reading heads, magnetic field sensors, magnetorezitive biochip for bio-molecular recognition, etc.). A new concept for the efficient study of such systems is proposed. It is based on the simultaneous processing of a large number of nanostructures with different parameters ranging in a well controlled way over a convenient range of values. Among such combinatorial processing methods there is the thermo-ionic vacuum arc allowing spatial distributed (and matrix-like indexed) substrates for sample preparation. The system parameters will depend on the substrate location (and/or temperature) with respect to the evaporation sources. Alternatively, similar nanostructures will be prepared by rf sputtering and thermal evaporation, for a detailed comparison among samples obtained via different methods. Fe based ferromagnetic films will be considered. The structure and morphology of the systems will be observed by X-ray diffraction and Electron Microscopy. The elemental composition and its depth profile will be studied by Energy Dispersive X-ray and Rutherford backscattering, respectively. The Fe phase distribution and local spin configuration will be analyzed at the ferromagnetic/conductor and ferromagnetic/dielectric interface as well as at different depths away from the interface via Conversion Electron Mossbauer Spectroscopy, within the ⁵⁷Fe tracer layer technique. The magnetization reversal and the magnetic relaxation phenomena in the ferromagnetic layers will be observed at low temperature via high sensitivity magnetometry and at room temperature via Magneto-Optic Kerr effect. The magnetoconduction properties will be studied over a large range of temperatures and applied fields. The detailed characterization of the combinatorially processed nanostructures would lead to: (i) the understanding of the relationships between structure-morphology-magnetism and magnetoconduction in spin valves, (ii) optimization of magnetoconduction properties and (iii) designed of new spin valve systems with enhanced properties.

Main objective: Study of magnetic phenomena in spin valve structures

Proposed targets/dead lines

Designing and implementation of the specific technology for the combinatorial processing of thin films. Preliminary depositions and characterizations of thin films	17.12. 2007
Combinatorial processings and complex characterization of different components of spin valves	20.09.2008
Combinatorial processing and magnetic, structure and morphologic characterization of GMR spin valves	15.09.2009
Processing of spin valve structures by rf sputtering and their complex characterization	12.12.2009
Combinatorial processing and structural and morphological characterization of TMR type spin valves. Optimization of different spin valve structures.	30.10.2010

Output: combinatorial preparations and optimization of spin valve structures;

Dissemination: scientific publications, conferences, website

Fulfilled targets:

There were fulfilled all the five objectives of the project.

Publications:

v Exchange bias and spin valve systems with Fe-Mn antiferromagnetic pinning layers, obtained by the thermo-ionic vacuum arc method, V. Kuncser, M.Valeanua, G. Schinteiea, I.Mustata, C.P.Lungu, A.Anghel,H.Chiriacc, R.Vladoiu and J.Bartolome, Journal of Mag. and Mag. Mat., 320  (14) E226-E230 (2008)

v Direct measurement of depth-dependent Fe spin structure during magnetization reversal in Fe/MnF2 exchange-coupled bilayers, W.A.A Macedo, B.Sahoo, J.Eisenmenger, M.D.Martins, W./Keune, V.Kuncser, R.Rohlsberger, O.Leupold, J.Nogues, Kai Liu, K.Schlage, I.Felner, I.K.Schuller and R.Ruffer,    Physical Rev B 78, 224401 (2008)

v Spin configurations and interfacial diffusion in exchange bias and spin valve systems with Ir-Mn antiferromagnetic pinning layers, V Kuncser, G.Schinteie, P.Palade, I.Mustata, C.P.Lungu, N.Stefan, H.Chiriac, R.Vladoiu and G.Filoti, Hyp. Int., 191 (2009) 135-141

v Interlayer magnetic coupling and interfacial atomic diffusion in AF/Fe/Cu/Fe (AF=FeMn and IrMn) multilayer systems: V. Kuncser, W.Keune, U von Hoersten, G.Schinteie, Thin Solid Films, 518 (2010) 5981

v Magnetic properties of Fe-Co ferromagnetic layers and Fe-Mn/Fe-Co bilayers obtained by thermo-ionic vacuum arc, V.Kuncser, G.Schinteie, P.Palade, I.Jepu, O.Pompilian, I.Mustata, C.P.Lungu, F.Minculescu and G.Filoti, Journal of Alloys and Comp. 499 (2010) 23

v Inter layer magnetic coupling in exchange bias and spin valve structures with Fe-Mn and Ir-Mn antiferromagnetic layers, V.Kuncser, W.Keune, U von Hoersten and G. Schinteie, Journal of Opt. and Adv. Mat. 12, 6 (2010) 1385

v Magnetic configuration and relaxation in iron based nano-particles:A Mossbauer approach, V.Kuncser, G.Schinteie, R.Alexandrescu, I.Morjan, L.Vekas and G.Filoti, A.Aldea, V.Barsan (eds.), Trends in Nanophysics, , Springer-Verlang Berlin Heidelberg 2010

v Determination of the step-shape angular spin distribution in layered systems by ⁵⁷Fe Mossbauer spectroscopy: A general treatment., V Kuncser, W.Keune, sent to Nuclear Instruments and Methods in Phys. Res. B

v Magnetic nanophases: from exchange coupled multilayers to nanopowders and nanocomposites (rev. paper), V.Kuncser, O.Crisan, G.Schinteie, F.Tolea, P.Palade, M.Valeanu and G. Filoti, care urmeaza sa fie publicata in volumul omagial editat de IUCN Dubna, Modern Trends in Nanoscience

v The non-collinear Fe vspin structure in (Sm-Co)/Fe exchange-spring bilayers: layer resolved Mossbauer spectroscopy and electronic structure calculations, V.M.Uzdin, A.Vega, A.Khrenov, W.Keune, V.E.Kuncser, J.S.Jiang and S.D.Bader, trimisa la Phys. Rev. B.

Conferences:

v Spin Configurations and Interfacial Diffusion in Exchange Bias and Spin valve Systems with Mn-Fe and Mn-Ir Antifferomagnetic Pinning Layers, V Kuncser, G.Schinteie, P.Palade, I.Mustata, C.P.Lungu, N.Stefan, H.Chiriac, R.Vladoiu and G.Filoti, The International Symposium on the Applications of the Mossbauer Effect (ISIAME 2008), Budapest

v Insight of temperature dependent distributions of hyperfine parameters in Mossbauer spectroscopy, V.Kuncser, P.Palade, G.Schinteie, G.Filoti, Int. Conf. on the Application of the Mossbauer Effect (ICAME) 2009, 19-24.07. July 2009, Wien, Austria

v Exchange Spring Effects in FePt/FeCo multylayers, O.Crisan, V.Raghavendra Reddy, V.Kuncser and A.Gupta, Int. Conf. on the Application of the Mossbauer Effect (ICAME) 2009, 19-24.07. July 2009, Wien, Austria

v Fe K-edge EXAFS and ⁵⁷Fe-Mossbauer Spectroscopy effect investigation of antiferromagnetic FeF2 powder, B.Sahoo, W.Keune,C.Borca, M.Janousch, V.Kuncser and R.Rohlsberger, Int. Conf. on the Application of the Mossbauer Effect (ICAME) 2009, 19-24.07. July 2009, Wien, Austria

v Magnetic configuration and magnetic relaxation of nanoparticles: a Mossbauer approach, Victor Kuncser, Workshp on Trends in nanoscience: theory, experiment, technology, 23-30 August 2009, Sibiu, Romania

v Easy axis distribution and spin configuration, Victor Kuncser, Workshp on Trends in nanoscience: theory, experiment, technology, 23-30 August 2009, Sibiu, Romania

v Unexpected magnetic properies of arrays of Ni-Cu nanowires, V.Kuncser, E.Matei, I.Enculescu, G.Schinteie and G.Filoti, Fourth Seeheim Conference on Magnetism (SCM) Aprilie 2010, Frankfurt, Germania, prezentare orala

v Spin valve like magnetic behavior of Fe_xCo_1-x/Cu/Fe₅₀Co₅₀ trilayer structures without antiferromagnetic layer, G.Schinteie, V.Kuncser, I.Mustata, C.P.Lungu, F.Miculescu and G. Filoti, Fourth Seeheim Conference on Magnetism (SCM) Aprilie 2010, Frankfurt, Germania, prezentare poster

v Structural and magnetic study of FePt/Fe(Co) exchange spring magnets, V.Raghavendra Reddy, A.Gupta, O.Crisan and V.Kuncser, Fourth Seeheim Conference on Magnetism (SCM) Aprilie 2010, Frankfurt, Germania, prezentare poster

v Effect of oxygen vacancies on magnetic impurity (M-M) exchange in anatase M:TiO2 (M= Mn, Fe or Co), N. Plugaru and R. Plugaru, Psi-k, Septembrie 2010, Berlin, Germania, prezentare poster

v Granular magnetoresistive films preparation and characterization, C. P. Lungu, I. Jepu, I. Mustata, A. M. Lungu, C. Ticos, C. Porosnicu, A. Anghel, P. Chiru, Twelfth International Conference on Plasma Surface Engineering”, September 13 - 17, 2010, Garmisch-Partenkirchen, Germany

Infrastructure and brief results

- Processing by rf sputtering: A system with only one target is used. The films are obtained via an rf discharge in low pressure Ar atmosphere (4.10^-2 mbarr). The Ar ions are accelerated via the rf field towards the metallic target. The atoms from the target are sputtered by the ion bombardment and deposited on the substrate which is placed in front of the target at less than 100 mm away. The base pressure is generally less than 3.10-6 mbar, for more than 2 hours, for a suitable degassing of the target. The discharge power during the preparation of the spin valve systems (and corresponding components) is 100 W.

Direct exchange bias systems with F=Fe-Co , AF=Ir-Mn, Fe-Ni-Cr buffer, rf sputtered on Si substrate.

- Combinatorial depositions via TVA: TVA is a very interesting and powerful deposition method which consists in the electron bombardment of a material placed at the anode (via an electron gun) and the initiation of a plasma discharge in the vapours of the evaporated material. The procedure allows the simultaneous evaporation from a couple of crucible (placed at many anodes) with the condition of using a corresponding number of electron guns (bi and tri-component thin films can be obtained in such a case). If the sample holder is provided with different places for substrates, each substrate being uniquely indexed by a set of distances to the anodes, a set of samples with different compositions of the film (each composition corresponding to a set of distances to the anodes) can be obtained at one. The combinatorial deposition is based on using a sample holder with a matrix-like discrimination (4 lines x 9 columns) for the sample substrates. It was observed that the composition of the films when a multicomponent deposition is applied is not changing along the column length, but only along the line. That is, films of different elemental concentrations are obtained for samples indexed by a different number of columns. Therefore, the samples are going to be indexed in the followings, only by the column number.

Some characterization tools: SEM with EDX (P2). MOKE set-up and VSM device with facilities regarding magneto-conduction measurements (CO)

(a) (b)

Relative elemental content (at.%) in Fe-Mn films, combinatorial prepared from two sorces. The Fe content depends on both the substrate position and substrate temperature (e.g. 100 C (a) and 300 C (b))

GMR structures grown by RF Sputtering

There have been grown by rd sputtering 6 types of structures (3 types of inverse spin valve structures and 3 types of magnetic bilayers) resulting 14 magnetic multilayer structures. The inverse spin valve structures were of type Si(100)/buffer/F/Cu/F/AF whereas the bylayers of type Si(100)/buffer/AF/F. Ta (prepared by either TVA or rf sputtering) as well as Pt (111) were choosen as buffer. The ferromagnetic layer, F, (generally, 4 nm thick) consists of Fe₂₀Ni₈₀, Fe₅₀Co₅₀ or metallic Fe whereas the antiferromagnetic layer, AF, (between 7 and 40 nm thick) consists of Fe₅₀Mn₅₀ or Ir₂₀Mn₈₀. Structures involving different combinations of buffer, F films and AF films, with different thickness and deposited for different substrate temperatures and subsequently annealed in different conditions have been prepared. In some cases, the ferromagnetic components of the structures have been enriched in ⁵⁷Fe, in order to investigate the magnetic interactions in relation to the film phase composition via the powerfull tool of conversion electron Mossbauer spectroscopy (CEMS).

Examples:

The morphology and quality of the films were analized via SEM and the composition of the films by EDS (the interest was mainly focused on the composition of the AF film with main influence of the exchange coupling effect). It has been proven that excepting to the films prepared or subsequently annealed at 500C temperature of the substrate, the elemental composition of the films has respected the compositional expectation. Different components of the structures were structurally analysed via XRD and magnetically via MOKE and vibrational magnetometry. Local magnetic parameters and interaction were investigated by CEMS in correlation with the ferromagnetic film phase composition The magnetic characterization have pointed for a convenient behavior with respect to a spin valve structure only in Fe-Mn based multilayers. In such systems were evidenced two components in the hysteresys loops (one belonging to the coupled F layer and another one to the free magnetic F layers) both at low an dat room temperatue. The superposition of the two compenents depens drastically on the direction of the applied field with respect to the direction of [110] direction of the si substrate. It has been proven that the lack of the F/AF magnetic coupling in the Ir-Mn based systems is mainly due to an improper field cooling procedure, related to the high Neel temperature of the Ir-Mn layer.

A new concept for the efficient study of multilayer spin valve structures is proposed. It is based on the simultaneous processing of a large number of nanostructures with different parameters ranging in a well controlled way over a convenient range of values. As combinatorial processing method was proposed the thermo-ionic vacuum arc, allowing spatial distributed (and matrix-like indexed) substrates for sample preparation. Alternatively, similar nanostructures were prepared by rf sputtering, for a comparison among samples obtained via different methods. The structure and morphology of the systems were observed by X-ray diffraction and Electron Microscopy, whereas the elemental composition by Energy Dispersive X-ray spectroscopy. The Fe phase distribution and local spin configuration was analyzed at the ferromagnetic/conductor and ferromagnetic/dielectric interface as well as at different depths away from the interface via Conversion Electron Mossbauer Spectroscopy, within the ⁵⁷Fe tracer layer technique. The magnetization reversal in the ferromagnetic layers, observed via magnetometry and magneto-optic Kerr effect was correlated to the magnetoresistive effects. The theoretical and experimental study envisaged the optimization of such structures with respect to their magnetic and magneto-conduction behavior. According to such studies was found that: (i) the most conveninet buffer layers are the Cu films, (ii) by TVA processing the most convenient AF layers are Fe-Mn films, thicker than 20 nm and with more than 50% Fe content, (iii) optimal FM layers are Fe-Co films of aprox. 5 nm with less than 50% Fe content, (iv) optimal thickness of conductive Cu layer is 5 nm, (v) optimal isolating layers are of MgO, which texture can be modified by TVA deposition conditions (vi) optimal MgO films and GMR structures have been prepared by TVA, (vii) optimal TMR structures have been prepared by rf sputtering, with the MgO film grown by TVA, (viii) is essential to avoid the oxidation of the Fe-Co films.

MOKE loops (up) and magneto-resistance measurements – by two points technique - (down), obtained at room temperature simultaneously,

on optimized samples TVA4a (left) and TVA4b (right)