High magnetic sensitivity via large-diameter-vortex stability in magnetic tunnel junctions through controlled anisotropy

Magnetic-annealing-induced uniaxial anisotropy is shown to influence vortex formation in magnetic tunnel junction (MTJ) free layers of various diameters, with micromagnetic simulations demonstrating a threshold for single-vortex formation that exhibits an inverse diameter dependence. Angular <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>M</a:mi></a:math>-<c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>H</c:mi></c:math> curves were measured for MTJ multilayers under different annealing conditions, and reveal that a two-step orthogonal magnetic annealing process reduces the uniaxial anisotropy in the free layer from <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mn>1040</e:mn><e:mspace width="0.2em"/><e:mi mathvariant="normal">J</e:mi><e:mo>/</e:mo><e:msup><e:mi mathvariant="normal">m</e:mi><e:mn>3</e:mn></e:msup></e:math> to <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mn>130</j:mn><j:mspace width="0.2em"/><j:mi mathvariant="normal">J</j:mi><j:mo>/</j:mo><j:msup><j:mi mathvariant="normal">m</j:mi><j:mn>3</j:mn></j:msup></j:math>. This reduction in anisotropy allows for vortex formation in MTJ sensors as large as <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mn>40</o:mn><o:mspace width="0.2em"/><o:mtext fontfamily="times">μ</o:mtext><o:mi mathvariant="normal">m</o:mi></o:math>. Increasing the MTJ diameter leads to an enhancement of magnetic sensitivity up to <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"><t:mn>2.80</t:mn><t:mi mathvariant="normal">%</t:mi><t:mo>/</t:mo><t:mi>Oe</t:mi></t:math> with the conventional definition, or <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mn>1.78</w:mn><w:mi mathvariant="normal">%</w:mi><w:mo>/</w:mo><w:mi>Oe</w:mi></w:math> via an ac sensitivity measurement that verifies the nonhysteretic response. This result demonstrates an eightfold increase in vortex-MTJ sensitivity, compared with standard MTJs that have diameters limited to 5 <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"><z:mtext fontfamily="times">μ</z:mtext><z:mi mathvariant="normal">m</z:mi></z:math>, and this work provides the foundation for highly sensitive vortex-MTJ-based sensors for applications that require a nonhysteretic, ultrasensitive response.