New Magnetic Materials
A typical magnetic device structure is an exchange-biased magnetic tunnel junction (MTJ) exhibiting tunnelling magnetoresistance (TMR). The structure includes an insulating barrier 1-3 nm thick sandwiched between two ferromagnetic layers, one of which is free to switch. The perpendicular resistance may change by several hundred percent when the magnetization of the two layers switches from antiparallel to parallel. For magnetic memory we need materials, which are thermally stable at ever-decreasing volumes, that switch rapidly under the influence of low spin-polarized switching currents (spin torque switching). There is a need for MTJs with new ferromagnetic materials with perpendicular anisotropy.
Thermal stability is determined by the ten-year data retention condition KuV > 1.5 eV, where V is the magnetic volume and Ku the uniaxial anisotropy constant. We need Ku > 1 MJm-3 to be able to reduce the volume of the free layer to 10 x 10 x 3 nm3 while retaining thermal stability. At this cell size, storage densities exceeding 5 Tb/inch2 can be achieved. The critical current density, Jc, required for spin torque switching is proportional to α Ms, where Ms is the saturation magnetisation and α is the Gilbert damping. We need to tune the saturation magnetisation and also decrease α as far as possible in order to reduce Jc. Commonly used transition-metal ferromagnets exhibit spin dynamics in the range up to a few GHz. High-anisotropy materials will extend this range, possibly as high as several hundred GHz. The challenge is to design new materials with a suitable combination of magnetic properties.
The family of cubic XYZ or X2YZ intermetallic compounds offers a wide range of interesting properties. Insulating, semiconducting and metallic materials can be readily obtained in bulk and thin film form, and both ferromagnetic and ferrimagnetic alloys, including half-metals such as Co2MnSi can be produced. Tetragonally-distorted manganese-based variants such as Mn3Ga or Mn3Ge show strong c-axis anisotropy, with potential for perpendicular MTJs. The elusive zero-moment fully-compensated ferromagnetic half-metal is especially interesting – We have found the first example: Mn2Ru0.5Ga.
Another very versatile family of materials are oxides with the perovskite structure, which includes Pauli paramagnets [CaRuO3], metallic [SrRuO3] and half-metalllic ferromagnets [(La0.7Sr0.)MnO3], weak ferromagnets [LaMnO3} and antiferromagnets [LaNiO3], as well as helical magnets [BiFeO3] ferroelectrics [BaTiO3] and insulators [LaAlO3, LaAlO3]. These oxides can be grown as thin films and combined in multifunctional stacks on an insulating substrate to generate a great variety of new properties. Especially remarkable is the appearance of magnetism and conductivity at the interface between a polar and a nonpolar insulator.
Contact Karsten Rode
Low-magnetization alloys for in-plane spin transfer torque devices, K. Oguz, M. Ozdemir, O. Dur and J. M. D. Coey, J. Appl. Phys. (2012) 111 113904 (2012)
Site-specific order and magnetism in tetragonal Mn3Ga thin films, K. Rode, N. Baadji, D Betto, Y. C. Lau, H. Kurt, M. Venkatesan, P. Stamenov, S. Sanvito, J. M. D. Coey, E. Fonda, E. Otero, F. Choueikani, P. Ohresser, F. Porcher and G. Andrè, Physical Review B 87 184427 (2013)
Introduction to magnetic oxides, J. M. D. Coey, M. Venkatesan and H J Xu, in S. B Ogale, T Venkatesanm editors Functional Oxides: New Science and Novel Applications Wiley,Weinheim 2013 Ch 1.