Research: New Magnetic Materials

Spin valves

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.

An exchange-biased magnetic tunnel junction (MTJ) with an MgO barrier. The resistance changes greatly as the free layer switches in a low applied field. We have been making MTJs with >200% magnetoresistance since 2006. The exchange bias and TMR depend critically on annealing temperature Ta.

Thermal stability is determined by the ten-year data retention condition KuV > 1.5 eV, where V is the magnetic volume and K_u the uniaxial anisotropy constant. We need K_u > 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, J_c, required for spin torque switching is proportional to α M_s, where M_s 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 J_c. 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.

Heusler Alloys

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: Mn₂Ru_0.5Ga.

Fig.1 The structure of a cubic X2YZ Heusler compound (a), and its tetragonally-distorted counterpart (b). The magnetic hysteresis loop is of a c-axis oriented thin film of Mn3Ga with Ku = 2.35 MJm3.

Oxides

Another very versatile family of materials are oxides, such as those with the perovskite structure. Examples include Pauli paramagnets [CaRuO₃], metallic [SrRuO₃] and half-metallic ferromagnets [(La_0.7Sr_0.3)MnO₃], weak ferromagnets [LaMnO₃] and antiferromagnets [LaNiO₃]. Helical magnets [BiFeO₃] ferroelectrics [BaTiO₃] and insulators [LaAlO₃, LaAlO₃] also belong to this family. Oxides can often exhibit the spinel structure exemplified by MgAl₂O₄, of which CoFe₂O₄ and NiFe₂O₄ are examples of ferromagnetic materials, the former being a useful magnetostrictive material and both having potential applications as spin filters. 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 and functionalities. Of particular interest is the combination of ferroelectricity and ferromagnetism via strain or electric field mediated magnetoelectric interactions. The understanding and control of these and related phenomena are the subject of the field of multiferroics.

Fig.1 BaTiO₃ in ferroelectric polarisation states corresponding to the Ti⁴⁺ ions being displaced relative to the O^2- sublattice. At room temperature the positively charged Ti ion can sit either above or below the TiO₂ planes, and can be displaced relative to the O ions with an external electric field. This leads to both ferroelectric polarisation and piezoelectricity in this interesting perovskite oxide.

Fig.2 Magnetisation curves of CoFe₂O₄ thin films, with the applied field both in the plane of the film and out, on (a) La_0.26Sr_0.76Al_0.61Ta_0.37O₃ and (b) MgAl₂O₄ substrates. The deviation from the nominally cubic anisotropy of bulk CoFe₂O₄ under the constraint of epitaxial strain illustrates the scale of the magnetostriction in this spinel ferrite.

Contact Karsten Rode

rodek@tcd.ie

Further Reading

[1] Cubic Mn2Ga thin films: Crossing the spin gap with ruthenium, H. Kurt, K. Rode, P. Stamenov, M. Venkatesan, Y. . Lau, E. Fonda, and J. M. D. Coey, Phys. Rev. Lett. , 112, no. 2, 2014.

[2] Magnetic dead layers in La_0.7Sr_0.3MnO₃ revisited, S.B. Porter, M. Venkatesan, P. Dunne, B. Doudin, K. Rode, J.M.D. Coey, IEEE Trans.Magn., (2017) 53 1-4 (2017)

[3] Site-specific order and magnetism in tetragonal Mn3Ga thin films, F. Eskandari, S.B. Porter, M. Venkatesan, P. Kameli, K. Rode, J.M.D. Coey, Physical Review Materials 1 074413 (2017)