Lattice Softening in Metastable bcc <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mi>Co</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi>Mn</mml:mi><mml:mrow><mml:mn>100</mml:mn><mml:mstyle displaystyle="false" scriptlevel="0"><mml:mtext>−</mml:mtext></mml:mstyle><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math> (001) Ferromagnetic Layers for a Strain-Free Magnetic Tunnel Junction
Kevin Elphick, Kenta Yoshida, Tufan Roy, Tomohiro Ichinose, Kazuma Kunimatsu, Tomoki Tsuchiya, Kazuya Suzuki, Masahito Tsujikawa, Yasuyoshi Nagai, Shigemi Mizukami, Masafumi Shirai, Atsufumi Hirohata
Abstract
In spintronics, one of the long-standing questions is why the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mrow><a:mi>Mg</a:mi><a:mi mathvariant="normal">O</a:mi></a:mrow></a:math>-based magnetic tunnel junction (MTJ) is almost the only option for achieving a large tunneling magnetoresistance (TMR) ratio at room temperature, although this is not as large as the theoretical prediction. This study focuses on the development of an almost strain-free MTJ using metastable bcc <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><e:msub><e:mi>Co</e:mi><e:mi>x</e:mi></e:msub><e:msub><e:mi>Mn</e:mi><e:mrow><e:mn>100</e:mn><e:mstyle displaystyle="false" scriptlevel="0"><e:mtext>−</e:mtext></e:mstyle><e:mi>x</e:mi></e:mrow></e:msub></e:math> (<j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><j:mi>Co</j:mi><j:mtext>−</j:mtext><j:mi>Mn</j:mi></j:math>) ferromagnetic films. We investigate the degree of crystallization in MTJs consisting of <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><m:mi>Co</m:mi><m:mstyle displaystyle="false" scriptlevel="0"><m:mtext>−</m:mtext></m:mstyle><m:mi>Mn</m:mi><m:mo>/</m:mo><m:mrow><m:mi>Mg</m:mi><m:mi mathvariant="normal">O</m:mi></m:mrow><m:mo>/</m:mo><m:mi>Co</m:mi><m:mstyle displaystyle="false" scriptlevel="0"><m:mtext>−</m:mtext></m:mstyle><m:mi>Mn</m:mi></m:math> in relation to their TMR ratios. Cross-section high resolution transmission electron microscopy reveals that almost consistent lattice constants of these layers for 66 ≤ ≤ 83 with large TMR ratios of 229% at room temperature, confirming the soft nature of the <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><u:mi>Co</u:mi><u:mstyle displaystyle="false" scriptlevel="0"><u:mtext>−</u:mtext></u:mstyle><u:mi>Mn</u:mi></u:math> layer with some dislocations at the <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><z:mrow><z:mi>Mg</z:mi><z:mi mathvariant="normal">O</z:mi></z:mrow><z:mo>/</z:mo><z:msub><z:mi>Co</z:mi><z:mn>75</z:mn></z:msub><z:msub><z:mi>Mn</z:mi><z:mn>25</z:mn></z:msub></z:math> interfaces. calculations confirm the crystalline deformation stability across a broad compositional range in <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><db:mi>Co</db:mi><db:mstyle displaystyle="false" scriptlevel="0"><db:mtext>−</db:mtext></db:mstyle><db:mi>Mn</db:mi></db:math>, proving the advantage of a strain-free interface for much larger TMR ratios.