Spin Transfer Torque Magnetoresistive Random Access Memory
77
presence of air gap between the sample and GCPW [186]. Nonetheless, the magnitude of h ac is appropriately small to excite the magnetization within the linear regime
[187].
One drawback of such frequency sweeping mode is the challenges associated with
achieving matching RF impedance over the entire bandwidth. The design of GCPW or
microstrip follows the microwave transmission theory and a dielectric material with
high permittivity ε r is desirable so as to minimize dielectric loss. Impedance matching
can be achieved by tuning the gap distances between s and w. In addition, tapering s
may also help to tune the inductance and capacitance of the coplanar waveguide, and
the introduction of via holes can help to reduce radiative losses. Most importantly,
the GCPW should not be plated using electroless nickel/immersion gold (ENIG),
a method that is commonly deployed in PCB finishing, due to the presence of the
ferromagnetic nickel. Instead, immersion silver is deployed to ensure low insertion
loss, even at higher frequencies.
The end launch connectors and coaxial cables should be free of any magnetic
impurities to avoid the influence of external magnetic fields [179]. The VNA should
also be placed at a sufficient distance from the electromagnet, as the microwave source
is a YIG-based oscillator that might be influenced by external magnetic fields. If airspaced coaxial cables are used, over bending of the cables may lead to irreversible
damage. The dielectric within the cable is air for minimal dielectric loss, and the
inner conductor is supported by spacers placed at regular intervals to prevent the
inner conductor from touching the outer ground shield.
The importance of FMR spectroscopy in the development of MRAM arises from
its ability to decouple the anisotropy contributions from the various ferromagnetic
sections in the MTJ. Since the effective fields generated from intrinsic contributions
are different for the soft layer and hard layers, the ferromagnetic resonance conditions
will differ for each layer within the MTJ, as shown in Fig. 12. For example, the
material choice for the soft layer (CoFeB) typically has a lower PMA in contrast to
the hard layers, and therefore resonates at a lower microwave frequency of ~5 GHz.
The free layer is also shown to have resonance points that are symmetric about zero
external magnetic field. Due to the antiferromagnetic coupling, the FMR of HL1
and HL2 will switch at the coercivity points of the SAF structure. In contrast, the
M-H loop measurements of the hard axis (e.g. VSM, MOKE, SQUID) will be the
total contribution from the entire MTJ stack as the all magnetic moments will rotate
coherently towards its hard axis. Since crystallinity, interlayer diffusion and surface
roughness plays an important role in K eff , truncating out the free layer section is not
a good representative.
6.2 Current In-Plane Tunneling (CIPT)
Four-point probe measurements (also known as the Kelvin technique) have been
commonly used to accurately monitor the sheet resistance of materials. It has been
77
presence of air gap between the sample and GCPW [186]. Nonetheless, the magnitude of h ac is appropriately small to excite the magnetization within the linear regime
[187].
One drawback of such frequency sweeping mode is the challenges associated with
achieving matching RF impedance over the entire bandwidth. The design of GCPW or
microstrip follows the microwave transmission theory and a dielectric material with
high permittivity ε r is desirable so as to minimize dielectric loss. Impedance matching
can be achieved by tuning the gap distances between s and w. In addition, tapering s
may also help to tune the inductance and capacitance of the coplanar waveguide, and
the introduction of via holes can help to reduce radiative losses. Most importantly,
the GCPW should not be plated using electroless nickel/immersion gold (ENIG),
a method that is commonly deployed in PCB finishing, due to the presence of the
ferromagnetic nickel. Instead, immersion silver is deployed to ensure low insertion
loss, even at higher frequencies.
The end launch connectors and coaxial cables should be free of any magnetic
impurities to avoid the influence of external magnetic fields [179]. The VNA should
also be placed at a sufficient distance from the electromagnet, as the microwave source
is a YIG-based oscillator that might be influenced by external magnetic fields. If airspaced coaxial cables are used, over bending of the cables may lead to irreversible
damage. The dielectric within the cable is air for minimal dielectric loss, and the
inner conductor is supported by spacers placed at regular intervals to prevent the
inner conductor from touching the outer ground shield.
The importance of FMR spectroscopy in the development of MRAM arises from
its ability to decouple the anisotropy contributions from the various ferromagnetic
sections in the MTJ. Since the effective fields generated from intrinsic contributions
are different for the soft layer and hard layers, the ferromagnetic resonance conditions
will differ for each layer within the MTJ, as shown in Fig. 12. For example, the
material choice for the soft layer (CoFeB) typically has a lower PMA in contrast to
the hard layers, and therefore resonates at a lower microwave frequency of ~5 GHz.
The free layer is also shown to have resonance points that are symmetric about zero
external magnetic field. Due to the antiferromagnetic coupling, the FMR of HL1
and HL2 will switch at the coercivity points of the SAF structure. In contrast, the
M-H loop measurements of the hard axis (e.g. VSM, MOKE, SQUID) will be the
total contribution from the entire MTJ stack as the all magnetic moments will rotate
coherently towards its hard axis. Since crystallinity, interlayer diffusion and surface
roughness plays an important role in K eff , truncating out the free layer section is not
a good representative.
6.2 Current In-Plane Tunneling (CIPT)
Four-point probe measurements (also known as the Kelvin technique) have been
commonly used to accurately monitor the sheet resistance of materials. It has been