Antiferromagnetic spintronics: materials, characterization, functionalities

Resume : Heusler compounds are a remarkable class of materials with more than 1,000 members and a wide range of extraordinary multifunctionalities including half-metallic high-temperature ferri- and ferromagnets, multiferroic shape memory alloys, and tunable topological insulators with a high potential for spintronics, energy technologies and magnetocaloric applications. Half Heusler and Heusler compounds can be designed antiferromagnetic, ferri- and ferromagnetic properties. Antiferromagnetic and compensated ferrimagnetic properties are intersting in the context of antiferromagnetic spintronics. Simple rules allows for a rational design of Heusler compounds [1]. Manganese and rare earth containing Half Heusler compounds are candidates for antiferromagnetic insulators, related structure type have to be considered [2]. Mn-rich Heusler compounds are a source for compensated ferrimangnets ranging from metals via bad metals to semiconductors [3-5]. [1] Tanja Graf, Stuart S. P. Parkin, and Claudia Felser, Progress in Solid State Chemistry 39 (2011) 1-50 [2] A. Beleanu, et a., Phys. Rev. B 88 (2013) 184429, arXiv:1307.6404 [3] Jürgen Winterlik, et. al., Adv. Mat. 24 6283 (2012). [4] A. K. Nayak, M. Nicklas, C. Shekhar, Y. Skourski, J. Winterlik, and C. Felser, Phys. Rev. Lett. 110 127204 (2013) [5] S. Ouardi, G. H. Fecher, J. Kübler, and C. Felser, Phys. Rev. Lett. 110 100401 (2013)

Resume : Half-metallic antiferromagnets (HMAFM) can generate spin-polarized current without exhibiting net magnetization and stray field, and thus are expected as the most ideal material for spintronics [1]. We have been looking for possible HMAFM starting from insulating ferrimagnetic cuprate [2,3] and antiferromagnetic poor metals [4] by element replacement. Recently we have developed a theory for antiferromagnetic topological insulator characterized by simultaneous non-zero charge and spin Chern numbers. This material carries zero-resistance edge current with full spin-polarization, which can be reversed by a gate electric field, while the sample shows zero net magnetization [5]. The works are supported by the WPI Initiative on Materials Nanoarchitectonics, MEXT of Japan. References: [1] X. Hu, Adv. Mater. 24, 294 (2012). [2] Y.-M. Nie and X. Hu, Phys. Rev. Lett. 100, 117203 (2008). [3] M. P. Ghimire, L.-H. Wu and X. Hu, submitted to J. Chem. Phys. Lett. (2014). [4] S.-J. Hu and X. Hu, J. Phys. Chem. C 114, 11614 (2010). [5] Q.-F. Liang, L.-H. Wu and X. Hu, New J. Phys. 15, 063031 (2013).

Resume : In a recent paper [1], it was demonstrated that the Mn2Au compound, known as a Pauli paramagnet, is actually an antiferromagnet, with an extrapolated Néel temperature, TN, around 1450 K (the Mn2Au phase decomposes at 953 K). The Mn moments are confined in the basal plane of the tetragonal structure, with weak associated basal plane anisotropy. AFM/FM Mn2Au/FeCo and Mn2Au/CuNi bilayers were prepared by ion sputtering. Large exchange-bias effects are observed at low temperature. The bias field decreases rapidly with temperature and vanishes around 100 K. This can be related to the fact that basal plane anisotropy is governed by 4th order anisotropy terms of which temperature dependence is much stronger than that of 2nd order terms. The bilayer anisotropic magnetoresistance (AMR) was measured between 4.2 K and 300 K. The respective contributions to the total AMR signal of the antiferromagnetic and ferromagnetic layers respectively will be separated out and analysed. The implication of the results for the possible use of Mn2Au in antiferromagnetic spintronics will be discussed. [1] V.M.T.S. Barthem, C.V. Colin, H. Mayaffre, M.-H. Julien and D. Givord, Nature Comm. DOI 10.1038/ncomms3892

Resume : Films of (Ga,Fe)N were grown by MOVPE and studied by transmission electron microscopy, synchrotron radiation based x-ray diffraction techniques, hard x-ray EXAFS and XANES. The magnetic response for these samples contains several contributions as determined by SQUID magnetometry [1]. Of technological importance is the characterization of the magnetism of self-assembled FexN nanocrystals with sizes up to 100 nm within the (Ga,Fe)N lattice. The XMCD, PEEM [2] as well as XMCD-PEEM and XLMD techniques in the soft x-ray range allow us to focus on the ferromagnetic and antiferromagnetic contributions of the Fe atoms to the overall magnetic response at room temperature. For some of the samples the XMCD-PEEM micrographs at the Fe L-edge indicate that only a percentage of the Fe rich nanocrystals exhibits ferromagnetic contrast. The XMCD-PEEM micrographs demonstrate that depending on their size, the ferromagnetic nanocrystals do not exhibit a single domain structure. We have been able to detect for several samples the weak Fe edge XLMD signal, which allows quantifying the antiferromagnetic contribution at room temperature. We discuss the XLMD results and the XMCD-PEEM micrographs aiming at identifying the chemical nature of the antiferromagnetic phase and ways of its stabilization. This work is supported by the Polish National Centre under Contract Nr. DEC-2011/03/D/ST3/02654. [1] A. Navarro-Quezada et al., PRB 81, 205206 (2010) [2] I.A. Kowalik et al., PRB 85, 184411 (2012)

Resume : The theoretical study of structural, electronic and magnetic properties of ordered dilute ferromagnetic semiconductors Al1-xMnxP with concentrations x = 0.0625, 0.125, and 0.25 in zinc blende (B3) phase, have been investigated using WIEN2k implementation of full potential linearized augmented plane wave (FP-LAPW) method within generalized gradient approximation (GGA) as exchange–correlation (XC) potential in order to seek out the possibility of new dilute magnetic semiconductor (DMS) compound for spintronic applications. The results show that Al0.9375Mn0.0625P, Al0.875Mn0.125P and Al0.75Mn0.25P compounds exhibit a half-metallic (HM) character, and their total magnetizations are an integer Bohr magneton of 4uB. Moreover, the analysis of the density of states of majority spin channel depict that large hybridization between 3p (P) and the 3d (Mn) partially filled states dominates the gap, which stabilizes the ferromagnetic state configuration associated with double-exchange mechanism.

Resume : A large body of research work has been devoted to the study of RMn2O5 magnetoelectrics, where R stands for Bi, Y or a rare earth ion. These compounds are characterized by complex crystal and magnetic structures, as well as a wealth of entangled magnetic and ferroelectric phase transitions [1,2]. In this study we present results of full relativistic GGA+U calculations performed on BiMn2O5, GdMn2O5 and ErMn2O5 compounds under applied pressure, aiming to reveal the changes in the spin and orbital magnetism and to relate them to the electronic structure evolution and local environment distortion. References [1] J. van den Brink and D. Khomskii, J. Phys.: Condens. Matter 20, 434217 (2008). [2] T. Kimura et al., Ferroelectrics, 354, 77 (2007).

Resume : It has been theoretically predicted that Fe2MnGa compound would display half metallic ferrimagnetic properties if embedded in a highly ordered Heusler structure ( L21 order) , consisting of four intertwined fcc sublattices. Meanwhile, a polycrystalline Fe2MnGa in the as quenched state was found to crystallize in tetragonal structure (I4/mmm space group, c/a=2.02) with Fe and Mn atoms randomly distributed over lattice sites [1]. Magnetization measurements indicate ferromagnetic (FM) order with Curie temperature of 750 K and a phase transition to antiferromagnetic (AFM) state below room temperature (RT). Typically for the first order phase transition , a phase separation and coexistence of AFM and FM phases is observed below RT, as evidenced by a strong EB effect. This separation has been linked to chemical disorder and random arrangement of atoms. To obtain more detailed information about the atomic order , 55Mn NMR and 57Fe Mossbauer studies have been undertaken in the bulk sample of Fe2MnGa as a function of temperature. At RT, in the FM state, both experiments revealed the existence of a broad distribution of hyperfine fields at the sites of the transition metal atoms, indicating a chemical and magnetic inhomogeneity. This was interpreted as the change of Mn–Mn interaction from FM to AFM with decreasing interatomic distance. Observation of zero field 55Mn NMR signal at 4.2K confirms the presence of regions with FM order. The observed very high value of the NMR restoring field may be indicative of zero field EB. Finally, the low temperature 55Mn NMR spectrum contains only a high frequency (~230 MHz) component, suggesting that Mn atoms contributing to the low frequency part of RT spectrum (~110 MHz) are involved in AF phase below the phase transition.

Resume : Gallium nitride GaN suitably doped with manganese centers has been anticipated for many promising applications as a dilute magnetic semiconductor DMS. The DMS materials are crucial in spintronics - spin based electronics which utilize both semiconducting and magnetic properties residing in one material. The known DMS materials are usually produced as thin films but highly dense sintered nanopowders can, potentially, be a new convenient form of the GaN/Mn materials for many challenging applications. This report describes the preparation, morphology, and structural and magnetic properties of sintered GaN/Mn nanopowders, a novel attractive form of such materials. GaN/Mn nanopowders were synthesized utilizing the anaerobic synthesis method modified to incorporate manganese centers into the GaN lattice. The metals precursors, Ga[N(CH3)]3 and Mn{N[Si(CH3)3]2}2 (Ga/Mn = 5, 10, 20 at.%), were mixed in a preset ratio in hexane and, after solvent evaporation, reacted under reflux with liquid NH3. Upon ammonia removal and evacuation of volatiles, the powders were pyrolyzed under an ammonia flow at temperatures in the range 500-900 ºC for 4 hours. The resulting nanopowders were sintered at 1000 ºC under the pressure of 7 GPa. Structural properties of the sintered materials (in the form of small discs) were investigated by XRD, morphology was observed using SEM/EDX, and magnetic properties were determined by SQUID. Acknowledgement: This study was supported by the Polish National Science Center NCN, Grant No. 2011/01/B/ST5/06592.

Resume : The understanding of the magnetoresistance (MR) responses as related to the way how the magnetization reverses in any kind of magnetic material is of great interest, not only for fundamental study, but importantly also for the improvement of the current spintronics technology. In this work, we demonstrate that by having direct experimental access to the magnetization vector during the reversal (by using a vectorial-Kerr magnetometry) we can predict the magnetoresistive signals. We demonstrate so in spin-valve [1] and in its basic constituents, i.e. single FM layer [2] and bilayer FM / AFM. We performed RT angular and field resolved M-H loops and the corresponding simultaneous resistance changes, and demonstrate that the R(H) curves depend exclusively dependence on the reversal processes (ultimately dictated by magnetic anisotropy of the system). Very importantly, from the vectorial-resolved M-H we can experimentally determine the relative angle between the magnetization vectors of the two FM layers in spin-valve and the magnetic torque and the angle enclosed between the magnetization vector and the applied current direction in a single FM layer. This allows for the simulation and prediction of the giant magnetoresistance (GMR), anisotropic magnetoresistance (AMR) and Planar Hall Effect (PHE) in the whole angular range and for any magnetic field values. [1] P. Perna et al. Phys. Rev. B 86, 024421 (2012) [2] P. Perna et al. Appl. Phys. Lett. 104, 202407 (2014)

Resume : Polyaromatic molecular materials are key components in organic electronics with applications including commercial OLEDs and organic photovoltaics. Phthalocyanines (Pcs) are of particular interest as they are able to incorporate a spin-bearing transition metal at their centre, which may lead to a combination of semiconducting and magnetic properties. In this talk, the magnetic properties of Pc films and nanostructures will be presented. The coupling depends strongly on the crystal phase, which can be tuned by the growth conditions [1, 2], or by functionalization of the molecular ring [3]. While the transition temperatures have traditionally been below 10 K, limiting the practical relevance of the materials, it will be shown that CoPc can be prepared as a one-dimensional antiferromagnet with a magnetic exchange energy up to 100 K, well above the boiling point of liquid nitrogen [4]. This behaviour is observed in flexible thin films and in nano-powders, but can be suppressed in the single crystal, leading to the possibility of magnetic switching. The unusually high and structure-dependent magnetic coupling is rationalised by high spin density on the dz2 orbital, and room temperature magnetism in co-facial structures has been predicted. [1] Heutz et al. Adv. Mater. 19 (2007) 3618. [2] Wang et al. ACS Nano 4 (2010) 3921. [3] Wu et al., J. Appl. Phys. 113 (2013) 013914. [4] Serri et al. Nat. Commun. 5 (2014) 3079.

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