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MMM 2020

Advanced Static and Dynamic Spin Depth Profiling

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

We were reluctant to give up the benefits of meeting face-to-face, and know that shifting to a virtual conference is a significant change for this community. Nevertheless, we are first and foremost concerned about the health and safety of all our Conference participants and their loved ones on returning home after travelling.

The MMM 2020 Steering Committee was working exceptionally hard, along with our sponsors, the IEEE Magnetics Society and AIP Publishing, to make this event serve the needs of our participants, who live in time zones all around the globe, in the best possible way. 

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symposium

Temperature dependence of electrical resistivity as well as neutron diffraction measurements of (Cr84Re16)100−xVx alloys with x = 0, 4.2, 5.5 and 6.2 are reported here. Previous studies probing the quantum critical behaviour (QCB) in the (Cr84Re16)100−xVx alloy system revealed prominent anomalies in physical properties close to the previously defined critical concentration of xc ≈ 10.5 [1]. One of the key indicators of critical behaviour is the Sommerfeld coefficient, γ [2,3]. γ is obtained by fitting the low temperature specific heat (Cp) data to the equation Cp = γT + βT3 [3], where the first and second terms are the electronic and lattice contributions to specific heat respectively [4]. For the (Cr84Re16)100−xVx alloy system, γ(x) showed a broad deep minimum at x ≈ 4.4 and a continuous decrease through x = 10.5 [1]. The latter anomaly was associated with the onset of QCB, but the anomaly at x ≈ 4.4 had to be probed further. It was proposed that the minimum corresponds to the transition from the commensurate spin-density-wave (CSDW) phase to the incommensurate (I) SDW phase upon increasing x. There is a larger gap in the electronic spectrum in the CSDW phase than in the ISDW phase [5]. In the CSDW phase, there is perfect nesting of the electron and hole Fermi sheets which leads to stronger reduction of density of states at the Fermi level. This hypothesis has now been further investigated with neutron powder diffraction (NPD) that provides information directly from the reciprocal lattice to facilitate identification of the magnetic order. Results indicate that the base alloy with x = 0 is in the CSDW state as expected and the x = 4.2, 5.5 and 6.2 alloys display ISDW trends. An additional magnetic peak associated with the Cr2O3 phase was also identified in the (Cr84Re16)100−xVx alloys using NPD. Quantification analyses revealed that the Cr2O3 content was less than 2 wt.% in all of the samples.References: 1. B.S. Jacobs, A.R.E. Prinsloo, C.J. Sheppard and A.M. Strydom, J. Appl. Phys., Vol. 113, p.17E126 (2013) 2. L. Reddy, H.L. Alberts, A.M. Strydom, A.R.E. Prinsloo and A.M. Venter, J. Appl. Phys., Vol. 103, p.07C903 (2008) 3. J. Takeuchi, H. Sasakura and Y. Masuda, J. Phys. Soc. Jpn., Vol. 49, p.508 (1980) 4. T. Mamiya and Y. Masuda, J. Phys. Soc. Jpn., Vol. 40, p.390 (1976) 5. E. Fawcett, H.L. Alberts, V.Y. Galkin, D.R. Noakes and J.V. Yakhmi, Rev. Mod. Phys., Vol. 66, p.25 (1994)

Neutron Diffraction Studies of the (Cr84Re16)100−xVx Alloy System

Magnetic Nanoparticles for Biomedical Diagnostics and Imaging: Recent Advances and Perspectives

 Antiferromagnetic Sprintronics: Transport and Dynamics in Metals, Insulators, and Magnetic Tunnel Junctions

Physics and Applications in Transmission and Control of Spin-Orbit Torques

Next Generation Artificial Spin Ice

New Approaches for Information Processing Coupling Spintronics and Magnonics

Novel Approaches to the Excitation and Control of Nano-Scale Propagating Spin Waves 

Modern spintronic based devices, such as magnetic random-access memories and magnetic read head sensors have been designed on the principles of exchange bias effect [1]. A unidirectional interfacial exchange coupling between a ferromagnet (FM) and an antiferromagnet (AFM) is the primary reason for the shift of field-cooled hysteresis loop leading to the exchange bias effect [2]. Other prominent signatures of exchange bias are enhancement of coercivity and observation of training effect. This phenomenon is generally interfacial in origin, however sometimes spins from the ‘bulk’ part of the AFM can also contribute to exchange bias [3]. Besides, the interface can be ferromagnetic, antiferromagnetic or spin-glass in nature. Interestingly, all the above features are system dependent and can vary with the thickness of the films.
 
We have investigated the role of ‘bulk’ spins of the AFM and also explored the magnetic nature of interface in exchange coupled Fe/IrMn bilayers by varying the thickness of the IrMn layer. In contrast with the conventional FM/AFM exchange bias systems [4], the exchange bias field (HEB) was found to decrease with increasing cooling field (HFC) due to random interface effect [2, 5]. The analysis of the training effect data indicates the presence of frozen and rotatable interfacial spins that contributed to the observed loop shift and coercivity enhancement, respectively.

 
References: [1] K. O’Grady, S. J. Greaves, S. M. Thompson, J. Magn. Magn. Mater., 322, 883 (2010)

[2] K. D. Usadel, U. Nowak, Phys. Rev. B, 80, 014418 (2009)

[3] A. C. Basaran, T. Saerbeck, J. de la Venta, H. Huckfeldt, A. Ehresmann, Ivan K. Schuller Appl. Phys. Lett., 105, 072403 (2014)

[4] L. D. Bianco, F.Spizzo, M.Tamisari, S.Laureti, Solid State Commun., 151, 351 (2011)

[5] S. Nayak, P. K. Manna, V. Thiruvengadam, B. B. Singh, S. Bedanta, J. Magn. Magn. Mater., 499, 166267 (2020)

Exchange bias in Fe/Ir20Mn80 bilayers: role of spin-glass like interface and ‘bulk’ antiferromagnet spins

Topological spin textures, such as skyrmions, have attracted intense interest in recent years. Magnetic skyrmions have been studied almost exclusively in bulk ferromagnets (FM) or FM multilayers. Antiferromagnets (AF) have also been predicted to harbor topological spin textures such as AF skyrmions. Topological Hall effect has been a primary indicator of spin textures in magnetic materials. We observe unambiguous evidence of topological Hall effect in Pt/Cr2O3 bilayers grown on Al2O3(0001) and Al2O3(11-20), where Cr2O3 epitaxial film is an antiferromagnetic insulator[1]. Although Cr2O3 is insulating, the topological magnetic textures can be detected through a novel phenomenon: “spin-Hall topological Hall effect” (SH-THE), similar to what recently observed topological Hall effect in Pt/Tm3Fe5O12 bilayers[2]. The Pt/Cr2O3 bilayers exhibit clear topological Hall resistivity for Cr2O3 thicknesses below 6 nm near and above room temperature, which is above the Néel temperature of Cr2O3, revealing the key role of thermal fluctuations in the formation of spin textures[3]. The similarity of topological Hall signals in (0001) and (11-20)-oriented Cr2O3 films indicates that the emergence of spin textures is insensitive to crystalline orientation. This first observational evidence of topological Hall effect in HM/AFI bilayers significantly expands our materials base to include the large family of AF insulators for the exploration of AF-based skyrmion technology.References: [1]. Yang Cheng, Sisheng Yu, Menglin Zhu, Jinwoo Hwang and Fengyuan Yang, “Evidence of Topological Hall Effect in Pt/Antiferromagnetic-Insulator Bilayers,” Phys. Rev. Lett. 123, 237206. [2]. A. S. Ahmed, A. J. Lee, N. Bagués, B. A. McCullian, A. M. A. Thabt, A. Perrine, P.-K. Wu, J. R. Rowland, M. Randeria, P. C. Hammel, D. W. McComb, and F.. Y. Yang, “Spin-Hall topological Hall effect in highly tunable Pt/ferrimagnetic-insulator bilayers,” Nano Lett., 19, 5683 (2019). [3]. Wenbo Wang, Matthew W Daniels, Zhaoliang Liao, Yifan Zhao, Jun Wang, Gertjan Koster, Guus Rijnders, Cui-Zu Chang, Di Xiao and Weida Wu, "Spin chirality fluctuation in two-dimensional ferromagnets with perpendicular magnetic anisotropy" Nat. Mater. 2019, 18, 1054.

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Neutron Diffraction Studies of the (Cr84Re16)100−xVx Alloy System

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Terahertz Emission From an Exchange-Coupled Synthetic Antiferromagnet

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