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Magnetic Yoking and Tunable Interactions in FePt-Based Hard/Soft Bilayers

View Article: PubMed Central - PubMed

ABSTRACT

Magnetic interactions in magnetic nanostructures are critical to nanomagnetic and spintronic explorations. Here we demonstrate an extremely sensitive magnetic yoking effect and tunable interactions in FePt based hard/soft bilayers mediated by the soft layer. Below the exchange length, a thin soft layer strongly exchange couples to the perpendicular moments of the hard layer; above the exchange length, just a few nanometers thicker, the soft layer moments turn in-plane and act to yoke the dipolar fields from the adjacent hard layer perpendicular domains. The evolution from exchange to dipolar-dominated interactions is experimentally captured by first-order reversal curves, the ΔM method, and polarized neutron reflectometry, and confirmed by micromagnetic simulations. These findings demonstrate an effective yoking approach to design and control magnetic interactions in wide varieties of magnetic nanostructures and devices.

No MeSH data available.


Micromagnetic simulations.(a) Cross-section through a domain-wall at remanence after in-plane saturation for a block with dimensions x = 100 nm, y = 100 nm, z = (tL10 + tA1), where tL10 = 4 nm and tA1 = 2 nm. Top and bottom panels show out-of-plane and in-plane magnetization, respectively, across a domain wall for selected layers (z = 1 nm and 4 nm, i.e., base and top of the L10-FePt, and 5 nm and 6 nm, i.e., base and top of the A1- FePt). Dashed curves show theoretical magnetization curves, vertical dashed lines represent the theoretical domain-wall width . (b) as in (a), but now for tA1 = 9 nm. (c) Contributions to free energy density F (top panel) along the magnetization curve shown in the bottom panel. All energy terms are normalized to the stray field energy constant. (d) Side-view example magnetization configuration in a long narrow strip (x = 1000 nm, y = 10 nm, z = (tL10 + tA1), with tL10 = 4 nm and tA1 = 5 nm) hosting domain walls in both the L10 and A1 layer, under in-plane field (µ0Hx = −0.1 T). Color coding indicates x-component of the magnetization. (e,f) Example magnetic configuration from large model (x = 1500 nm, y = 1500 nm, z = 10 nm) after domain nucleation, with color contrast in (e) indicating the out-of-plane magnetization, and in (f) indicating the into-the-page magnetization.
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f5: Micromagnetic simulations.(a) Cross-section through a domain-wall at remanence after in-plane saturation for a block with dimensions x = 100 nm, y = 100 nm, z = (tL10 + tA1), where tL10 = 4 nm and tA1 = 2 nm. Top and bottom panels show out-of-plane and in-plane magnetization, respectively, across a domain wall for selected layers (z = 1 nm and 4 nm, i.e., base and top of the L10-FePt, and 5 nm and 6 nm, i.e., base and top of the A1- FePt). Dashed curves show theoretical magnetization curves, vertical dashed lines represent the theoretical domain-wall width . (b) as in (a), but now for tA1 = 9 nm. (c) Contributions to free energy density F (top panel) along the magnetization curve shown in the bottom panel. All energy terms are normalized to the stray field energy constant. (d) Side-view example magnetization configuration in a long narrow strip (x = 1000 nm, y = 10 nm, z = (tL10 + tA1), with tL10 = 4 nm and tA1 = 5 nm) hosting domain walls in both the L10 and A1 layer, under in-plane field (µ0Hx = −0.1 T). Color coding indicates x-component of the magnetization. (e,f) Example magnetic configuration from large model (x = 1500 nm, y = 1500 nm, z = 10 nm) after domain nucleation, with color contrast in (e) indicating the out-of-plane magnetization, and in (f) indicating the into-the-page magnetization.

Mentions: The results from our micromagnetic simulations underpin the yoking mechanism inferred from the experimental data. As long as tA1 < lex (e.g., 2 nm in Fig. 5a), yoking is practically absent; magnetization in the soft phase is strongly exchange coupled to that in the hard phase, which results in a narrow Néel-type domain wall (nearly 180 degrees) with only a minor change in domain-wall width when going from the hard to the soft layer. The out-of-plane and in-plane component of the wall magnetization closely follow the theoretical curve given by and , respectively (dashed curves). For a tA1 ≈ 2lex (Fig. 5b), the domain wall in the upper part of the soft phase widens considerably and has a smaller amplitude variations due to the now predominant in-plane magnetization component in the soft layer. From the energy diagram (Fig. 5c), it can be seen that the demagnetizing energy for tA1 = 9 nm is greatly reduced compared to tA1 = 2 nm. This gain is a direct consequence of flux closure due to yoking and more than offsets the cost in exchange energy needed to deflect the soft-layer magnetization in-plane. For the case that the soft-layer domain is orthogonal to the hard-layer domain wall, shown in Fig. 5d for the modeled strip (1000 nm long, 10 nm wide) and Fig. 5e for the large block, the soft layer follows the dipolar fields forming a yoke structure, as described above. Figure 5d also demonstrates a multi-domain reversal in the thick A1 layer which is not commensurate with the domain structure in the L10 underlayer. Interestingly, despite their incommensurate domain structures, for the case that the soft-layer domain is oriented parallel to the domain wall, Fig. 5f, the yoking effect is still present and re-orients the soft-layer near the hard-layer domain wall to follow the dipolar fields. Thus, independent of the domain structure in the soft-layer, yoking is expected to occur and facilitates the dipolar interactions.


Magnetic Yoking and Tunable Interactions in FePt-Based Hard/Soft Bilayers
Micromagnetic simulations.(a) Cross-section through a domain-wall at remanence after in-plane saturation for a block with dimensions x = 100 nm, y = 100 nm, z = (tL10 + tA1), where tL10 = 4 nm and tA1 = 2 nm. Top and bottom panels show out-of-plane and in-plane magnetization, respectively, across a domain wall for selected layers (z = 1 nm and 4 nm, i.e., base and top of the L10-FePt, and 5 nm and 6 nm, i.e., base and top of the A1- FePt). Dashed curves show theoretical magnetization curves, vertical dashed lines represent the theoretical domain-wall width . (b) as in (a), but now for tA1 = 9 nm. (c) Contributions to free energy density F (top panel) along the magnetization curve shown in the bottom panel. All energy terms are normalized to the stray field energy constant. (d) Side-view example magnetization configuration in a long narrow strip (x = 1000 nm, y = 10 nm, z = (tL10 + tA1), with tL10 = 4 nm and tA1 = 5 nm) hosting domain walls in both the L10 and A1 layer, under in-plane field (µ0Hx = −0.1 T). Color coding indicates x-component of the magnetization. (e,f) Example magnetic configuration from large model (x = 1500 nm, y = 1500 nm, z = 10 nm) after domain nucleation, with color contrast in (e) indicating the out-of-plane magnetization, and in (f) indicating the into-the-page magnetization.
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f5: Micromagnetic simulations.(a) Cross-section through a domain-wall at remanence after in-plane saturation for a block with dimensions x = 100 nm, y = 100 nm, z = (tL10 + tA1), where tL10 = 4 nm and tA1 = 2 nm. Top and bottom panels show out-of-plane and in-plane magnetization, respectively, across a domain wall for selected layers (z = 1 nm and 4 nm, i.e., base and top of the L10-FePt, and 5 nm and 6 nm, i.e., base and top of the A1- FePt). Dashed curves show theoretical magnetization curves, vertical dashed lines represent the theoretical domain-wall width . (b) as in (a), but now for tA1 = 9 nm. (c) Contributions to free energy density F (top panel) along the magnetization curve shown in the bottom panel. All energy terms are normalized to the stray field energy constant. (d) Side-view example magnetization configuration in a long narrow strip (x = 1000 nm, y = 10 nm, z = (tL10 + tA1), with tL10 = 4 nm and tA1 = 5 nm) hosting domain walls in both the L10 and A1 layer, under in-plane field (µ0Hx = −0.1 T). Color coding indicates x-component of the magnetization. (e,f) Example magnetic configuration from large model (x = 1500 nm, y = 1500 nm, z = 10 nm) after domain nucleation, with color contrast in (e) indicating the out-of-plane magnetization, and in (f) indicating the into-the-page magnetization.
Mentions: The results from our micromagnetic simulations underpin the yoking mechanism inferred from the experimental data. As long as tA1 < lex (e.g., 2 nm in Fig. 5a), yoking is practically absent; magnetization in the soft phase is strongly exchange coupled to that in the hard phase, which results in a narrow Néel-type domain wall (nearly 180 degrees) with only a minor change in domain-wall width when going from the hard to the soft layer. The out-of-plane and in-plane component of the wall magnetization closely follow the theoretical curve given by and , respectively (dashed curves). For a tA1 ≈ 2lex (Fig. 5b), the domain wall in the upper part of the soft phase widens considerably and has a smaller amplitude variations due to the now predominant in-plane magnetization component in the soft layer. From the energy diagram (Fig. 5c), it can be seen that the demagnetizing energy for tA1 = 9 nm is greatly reduced compared to tA1 = 2 nm. This gain is a direct consequence of flux closure due to yoking and more than offsets the cost in exchange energy needed to deflect the soft-layer magnetization in-plane. For the case that the soft-layer domain is orthogonal to the hard-layer domain wall, shown in Fig. 5d for the modeled strip (1000 nm long, 10 nm wide) and Fig. 5e for the large block, the soft layer follows the dipolar fields forming a yoke structure, as described above. Figure 5d also demonstrates a multi-domain reversal in the thick A1 layer which is not commensurate with the domain structure in the L10 underlayer. Interestingly, despite their incommensurate domain structures, for the case that the soft-layer domain is oriented parallel to the domain wall, Fig. 5f, the yoking effect is still present and re-orients the soft-layer near the hard-layer domain wall to follow the dipolar fields. Thus, independent of the domain structure in the soft-layer, yoking is expected to occur and facilitates the dipolar interactions.

View Article: PubMed Central - PubMed

ABSTRACT

Magnetic interactions in magnetic nanostructures are critical to nanomagnetic and spintronic explorations. Here we demonstrate an extremely sensitive magnetic yoking effect and tunable interactions in FePt based hard/soft bilayers mediated by the soft layer. Below the exchange length, a thin soft layer strongly exchange couples to the perpendicular moments of the hard layer; above the exchange length, just a few nanometers thicker, the soft layer moments turn in-plane and act to yoke the dipolar fields from the adjacent hard layer perpendicular domains. The evolution from exchange to dipolar-dominated interactions is experimentally captured by first-order reversal curves, the &Delta;M method, and polarized neutron reflectometry, and confirmed by micromagnetic simulations. These findings demonstrate an effective yoking approach to design and control magnetic interactions in wide varieties of magnetic nanostructures and devices.

No MeSH data available.