Thermal creation of stronger spin-transfer torque in oscillators and memories

Oscillators and magnetic random-access memories (MRAMs) investigated today rely on spin-transfer torque (STT) carried by an electric current flowing through a magnetic tunnel junction (MTJ) having barrier composition MgO.\footnote{See the STT review by D. Ralph and M. Stiles, J. Magn. Magn. Mater. \textbf{320, }1190 (2008).} Experiments confirm the theoretical upper bound \textit{$\tau $}$_{e}$=1/2 on the torque yield (defined as dimensionless torque per unit supplied electric current). This bound limits the performance potential of STT-MRAM in which current supplied by one transistor within each cell switches the information bit. Replacement of electric current with heat flow (supplied by a Joule heater) carried by magnons may provide a greater torque yield \textit{$\tau $}$_{h}$.\footnote{J. Slonczewski, Phys. Rev. B \textbf{82}, 054403 (2010).} The essential structure for this \textit{thermagnonic} spin transfer (TMST) comprises a stack of three nano layers: a spontaneously magnetized insulator (\textit{ferrite} for brevity), a non-magnetic metallic spacer, and the free metallic magnet responding to the transferred torque. Phonons carry most of the heat flowing through the ferrite. But spin-1 magnons also carry a portion of it and deposit pure spin polarization into the spacer whose free electrons transport it to the free magnet. Ferrite-metal interfaces also occur in a spin-Seebeck effect.\footnote{The talk by E. Saitoh in this Symposium} Principles of spin relaxation provide estimates of \textit{$\tau $}$_{h}$ based on existing data for sd-exchange, superexchange, and non-magnetic interfacial thermal resistance; \textit{$\tau $}$_{h}$ may exceed \textit{$\tau $}$_{e }$by one order of magnitude.\footnote{J. Slonczewski, Phys. Rev. B \textbf{82}, 054403 (2010).} Related results of an FMR spin-pumping experiment\footnote{B. Heinrich et al, Phys. Rev. Letts. \textbf{107}, 066604 (2011).} and DFT computations\footnote{The talk by K. Xia in this Symposium.} support the potential of TMST-MRAM. In the case of an oscillator, TMST could increase its efficiency and enable largely independent controls of frequency and output voltage.