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Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators.

Chung S, Eklund A, Iacocca E, Mohseni SM, Sani SR, Bookman L, Hoefer MA, Dumas RK, Åkerman J - Nat Commun (2016)

Bottom Line: Static and dynamic magnetic solitons play a critical role in applied nanomagnetism.Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers.Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden.

ABSTRACT
Static and dynamic magnetic solitons play a critical role in applied nanomagnetism. Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers. Here, we perform a detailed experimental determination of the full droplet nucleation boundary in the current-field plane for a wide range of nanocontact sizes and demonstrate its excellent agreement with an analytical expression originating from a stability analysis. Our results reconcile recent contradicting reports of the field dependence of the droplet nucleation. Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

No MeSH data available.


Related in: MedlinePlus

Orthogonal NC-STO characterization.(a) Schematic of the orthogonal NC-STOs showing a pseudo-spin-valve composed of a Co fixed layer, a Cu spacer and a Co/Ni free layer with perpendicular magnetic anisotropy. The current enters the stack through a nanocontact (NC). The field H is applied perpendicularly to the plane. (b,c) Representative power density spectra as a function of field magnitude and current are shown in b and c, respectively. Droplet nucleation is observed as a frequency drop accompanied by a dramatic increase in power. The nucleation can also be monitored by a jump in resistance, as shown by the yellow data in b and c. (d,e) Show corresponding microwave power integrated around the main high-frequency peak (red) as well as in a low-frequency region from 0.1 to 1 GHz (black). Both the main signal power and the onset of low-frequency dynamics are clear signatures of droplet nucleation.
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f1: Orthogonal NC-STO characterization.(a) Schematic of the orthogonal NC-STOs showing a pseudo-spin-valve composed of a Co fixed layer, a Cu spacer and a Co/Ni free layer with perpendicular magnetic anisotropy. The current enters the stack through a nanocontact (NC). The field H is applied perpendicularly to the plane. (b,c) Representative power density spectra as a function of field magnitude and current are shown in b and c, respectively. Droplet nucleation is observed as a frequency drop accompanied by a dramatic increase in power. The nucleation can also be monitored by a jump in resistance, as shown by the yellow data in b and c. (d,e) Show corresponding microwave power integrated around the main high-frequency peak (red) as well as in a low-frequency region from 0.1 to 1 GHz (black). Both the main signal power and the onset of low-frequency dynamics are clear signatures of droplet nucleation.

Mentions: The investigated devices are orthogonal NC-STOs (Fig. 1a) with a perpendicularly magnetized Co/Ni free layer, and an in-plane magnetized Co fixed layer (see ‘Methods' section). The Co/Ni free layer has a saturation magnetization Ms of μ0Ms=0.9 T and a PMA field of μ0Hk=1.35 T, where μ0 is the permeability of free space; the Co fixed layer has a saturation magnetization of about μ0Ms,p=1.6 T. Figure 1b shows the field-dependent NC-STO magnetodynamics for a NC radius of 40 nm, where the perpendicular field μ0H was swept from 0.05 to 0.85 T at a constant device current of Idc=−7.7 mA, where the negative current polarity indicates electrons flowing from the free to the fixed layer. For low magnetic fields, a faint signal is observed, corresponding to moderate-angle precession close to the ferromagnetic resonance frequency3334. Above a critical field of about 0.48 T, a sudden frequency drop and a dramatic increase in microwave power (red data points in Fig. 1d) are observed, marking the nucleation of a droplet. A similar droplet nucleation transition is observed in Fig. 1c,e as a function of current in a constant perpendicular field of μ0H=0.625 T. The yellow line in Fig. 1b,c shows how the NC-STO magnetoresistance (MR) exhibits a step-like increase at the droplet nucleation stemming from the partial reversal of the free layer magnetization underneath the NC. The nucleation transition can also be detected by measuring the integrated microwave noise power between 0.1 and 1 GHz, black data points in Fig. 1d,e. This technique is based on the fact that broad, low-frequency features in the spectrum can be related to random transitions between different dynamical regimes. In our experiments, we hypothesize that only thermally driven spin waves and droplets are supported (corroborated by the theory given in ref. 17), suggesting that the appearance of low-frequency noise is related to the nucleation and drift instabilities of droplets. This conclusion is supported by the good agreement between the MR and microwave noise power techniques to detect a droplet, as shown in Fig. 1d,e. These are particularly useful at high fields, where the excited microwave frequencies surpass the bandwidth of our low noise amplifier (30 GHz) and spectrum analyzer (40 GHz).


Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators.

Chung S, Eklund A, Iacocca E, Mohseni SM, Sani SR, Bookman L, Hoefer MA, Dumas RK, Åkerman J - Nat Commun (2016)

Orthogonal NC-STO characterization.(a) Schematic of the orthogonal NC-STOs showing a pseudo-spin-valve composed of a Co fixed layer, a Cu spacer and a Co/Ni free layer with perpendicular magnetic anisotropy. The current enters the stack through a nanocontact (NC). The field H is applied perpendicularly to the plane. (b,c) Representative power density spectra as a function of field magnitude and current are shown in b and c, respectively. Droplet nucleation is observed as a frequency drop accompanied by a dramatic increase in power. The nucleation can also be monitored by a jump in resistance, as shown by the yellow data in b and c. (d,e) Show corresponding microwave power integrated around the main high-frequency peak (red) as well as in a low-frequency region from 0.1 to 1 GHz (black). Both the main signal power and the onset of low-frequency dynamics are clear signatures of droplet nucleation.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4837446&req=5

f1: Orthogonal NC-STO characterization.(a) Schematic of the orthogonal NC-STOs showing a pseudo-spin-valve composed of a Co fixed layer, a Cu spacer and a Co/Ni free layer with perpendicular magnetic anisotropy. The current enters the stack through a nanocontact (NC). The field H is applied perpendicularly to the plane. (b,c) Representative power density spectra as a function of field magnitude and current are shown in b and c, respectively. Droplet nucleation is observed as a frequency drop accompanied by a dramatic increase in power. The nucleation can also be monitored by a jump in resistance, as shown by the yellow data in b and c. (d,e) Show corresponding microwave power integrated around the main high-frequency peak (red) as well as in a low-frequency region from 0.1 to 1 GHz (black). Both the main signal power and the onset of low-frequency dynamics are clear signatures of droplet nucleation.
Mentions: The investigated devices are orthogonal NC-STOs (Fig. 1a) with a perpendicularly magnetized Co/Ni free layer, and an in-plane magnetized Co fixed layer (see ‘Methods' section). The Co/Ni free layer has a saturation magnetization Ms of μ0Ms=0.9 T and a PMA field of μ0Hk=1.35 T, where μ0 is the permeability of free space; the Co fixed layer has a saturation magnetization of about μ0Ms,p=1.6 T. Figure 1b shows the field-dependent NC-STO magnetodynamics for a NC radius of 40 nm, where the perpendicular field μ0H was swept from 0.05 to 0.85 T at a constant device current of Idc=−7.7 mA, where the negative current polarity indicates electrons flowing from the free to the fixed layer. For low magnetic fields, a faint signal is observed, corresponding to moderate-angle precession close to the ferromagnetic resonance frequency3334. Above a critical field of about 0.48 T, a sudden frequency drop and a dramatic increase in microwave power (red data points in Fig. 1d) are observed, marking the nucleation of a droplet. A similar droplet nucleation transition is observed in Fig. 1c,e as a function of current in a constant perpendicular field of μ0H=0.625 T. The yellow line in Fig. 1b,c shows how the NC-STO magnetoresistance (MR) exhibits a step-like increase at the droplet nucleation stemming from the partial reversal of the free layer magnetization underneath the NC. The nucleation transition can also be detected by measuring the integrated microwave noise power between 0.1 and 1 GHz, black data points in Fig. 1d,e. This technique is based on the fact that broad, low-frequency features in the spectrum can be related to random transitions between different dynamical regimes. In our experiments, we hypothesize that only thermally driven spin waves and droplets are supported (corroborated by the theory given in ref. 17), suggesting that the appearance of low-frequency noise is related to the nucleation and drift instabilities of droplets. This conclusion is supported by the good agreement between the MR and microwave noise power techniques to detect a droplet, as shown in Fig. 1d,e. These are particularly useful at high fields, where the excited microwave frequencies surpass the bandwidth of our low noise amplifier (30 GHz) and spectrum analyzer (40 GHz).

Bottom Line: Static and dynamic magnetic solitons play a critical role in applied nanomagnetism.Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers.Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden.

ABSTRACT
Static and dynamic magnetic solitons play a critical role in applied nanomagnetism. Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers. Here, we perform a detailed experimental determination of the full droplet nucleation boundary in the current-field plane for a wide range of nanocontact sizes and demonstrate its excellent agreement with an analytical expression originating from a stability analysis. Our results reconcile recent contradicting reports of the field dependence of the droplet nucleation. Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

No MeSH data available.


Related in: MedlinePlus