Perpendicular Exchange Bias Field Research Articles

In-plane and perpendicular exchange bias effect induced by an antiferromagnetic D019 Mn2FeGa thin film

Abstract Fe-doped D019 Mn3Ga films were studied in terms of both their in-plane and perpendicular exchange bias field, Hex, induced in the attached ferromagnetic layer. The (Mn,Fe)3Ga films were deposited on a Si substrate with a Ru buffer layer with a (0 0 0 1)-oriented single D019 phase at room temperature. Consequently, in-plane and perpendicular Hex were measured in bilayers with a CoFe layer and a [Co/Pt] multilayer, respectively. In-plane Hex was found to be dependent on the Fe compositions, showing the largest value of 446 Oe at 120 K for 10-nm-thick Mn1.99Fe0.41Ga. Perpendicular Hex was dependent on not only the Fe compositions but also the thickness of (Mn,Fe)3Ga, exhibiting the maximum of 163 Oe at 120 K for 5-nm-thick Mn1.96Fe0.67Ga. The median blocking temperature of both in-plane and perpendicular Hex systems for 10-nm-thick (Mn,Fe)3Ga were measured to be 235 and 240 K, respectively. The measured in-plane and perpendicular Hex induced by (Mn,Fe)3Ga are generated by a spin structural change from noncollinear to noncoplaner in ab-plane as expected from the previous theoretical study [A. Kundu and S. Ghosh, Intermetallics 93, 209 (2018)].

All-Oxide Nanocomposites to Yield Large, Tunable Perpendicular Exchange Bias above Room Temperature.

In all-oxide-based spintronic devices, large exchange bias effect with robustness against temperature fluctuation and compatibility with perpendicular magnetic recording is highly desired. In this work, rock-salt antiferromagnetic NiO with a Néel temperature ( TN) of ∼525 K and spinel ferrimagnetic NiFe2O4 with a high Curie temperature, TC, ≈ 790 K and TC > TN were chosen as compatible materials to form a well-phase-separated, vertically aligned nanocomposite thin film. In this nanoengineered thin film, an exchange bias effect with a blocking temperature far above room temperature has been achieved. A large perpendicular exchange bias field of up to 0.91 kOe with an interfacial exchange energy density of 0.11-0.34 erg/cm2 was obtained at room temperature. It was also demonstrated that the exchange bias effect can be easily tuned by changing the alignment of the magnetic moments in the NiO phase using substrates of different crystalline orientations and by changing the microstructure of the film with substrates of different lattice parameters. The results demonstrate that proper choice of the phases (including use of nonperovskite compositions) and careful strain engineering and nanostructure engineering makes oxide nanocomposites strong potential candidate systems for next generation spintronic devices.

Perpendicular exchange coupling effects in ferrimagnetic TbFeCo/GdFeCo hard/soft structures

Bilayers consisting of magnetically hard TbFeCo and soft GdFeCo alloy were fabricated. Exchange-spring and sharp switching in a step-by-step fashion were observed in the TbFeCo/GdFeCo hard/soft bilayers with increasing GdFeCo thickness. A perpendicular exchange bias field of several hundred Oersteds is observed from the shift of minor loops pinned by TbFeCo layer. The perpendicular exchange energy is derived to be in the range of 0.18–0.30 erg/cm2. The exchange energy is shown to increase with the thickness of GdFeCo layer in the bilayers, which can be attributed to the enhanced perpendicular anisotropy of GdFeCo layer in our experimental range.

Simultaneous achievement of high perpendicular exchange bias and low coercivity by controlling ferromagnetic/antiferromagnetic interfacial magnetic anisotropy

This study investigates the influence of Pt and Au spacer layers on the perpendicular exchange bias field and coercivity of Pt/Co/(Pt or Au)/Cr2O3/Pt films. When using a Pt-spacer, the perpendicular exchange bias was highly degraded to less than 0.1 erg/cm2, which was about half that of the Au-spacer system. The Au spacer also suppressed the enhancement in coercivity that usually occurs at around room temperature when using Pt. It is suggested that this difference in exchange bias field is due to in-plane interfacial magnetic anisotropy at the Pt/Cr2O3 interface, which cants the interfacial Cr spin from the surface normal and results in degradation in the perpendicular exchange bias.

Enhancement of perpendicular magnetic anisotropy of Co layer in exchange-biased Au/Co/NiO/Au polycrystalline system

The perpendicular exchange bias in NiO(antiferromagnet)/Co(ferromagnet) polycrystalline layer films is studied. It is found that the NiO layer forces the Co layer magnetization to be oriented perpendicular to the film plane in a greater thickness range than is found in the Au/Co/Au system. Simultaneously, a large coercivity and a significant perpendicular exchange bias field were observed that are owing to the interlayer exchange bias coupling between NiO and Co, which supports the perpendicular magnetic anisotropy of the Co layer. These findings are confirmed by magnetometry and magnetoresistance measurements.

Exchange Bias Driven by Dzyaloshinskii-Moriya Interactions

The exchange bias effect in a compensated IrMn3/Co(111) system is studied using multiscale modeling from ab initio to atomistic spin model calculations. We evaluate numerically the out-of-plane hysteresis loops of the bilayer for different thicknesses of the ferromagnetic layer. The results show the existence of a perpendicular exchange bias and an enhancement of the coercivity of the system. To identify the origin of the exchange bias, we analyze the hysteresis loops of a selected bilayer by tuning the different contributions to the exchange interaction across the interface. Our results indicate that the exchange bias is primarily induced by Dzyaloshinskii-Moriya interactions, while the coercivity is increased mainly due to a spin-flop mechanism.

Pt / Co ] 4 / NiO thin film perpendicular magnetic anisotropy dependence on Co layer thickness

We have examined the dependence of the magnetism from perpendicular magnetic anisotropy with Co thickness in [Pt/Co] multilayers coupled to a NiO layer. We measure a perpendicular exchange bias field that changes polarity with Co thickness and find that with 2 nm of Co the perpendicular magnetic anisotropy is no longer present. Further evidence of the change in the Co layers preferred orientation is offered by zero field cooled susceptometry, where the low temperature susceptibility is negative for thin Co layers (0.9–1.2 nm) and becomes positive with increasing Co thickness (1.5–2.0 nm). Thin Co layered films also exhibit a compensation point around 100 K. These results indicate that the Co and NiO interface moments are coupled strongly and that there is competition between Pt/Co and Co/Ni at the film layer interfaces that sets the overall anisotropy from perpendicular and parallel configurations.

Influence of Interface Roughness, Film Texture, and Magnetic Anisotropy on Exchange Bias in <formula formulatype="inline"> <tex>$[{\hbox{Pt/Co}}]_{3}/{\hbox{IrMn}}$</tex> </formula> and <formula formulatype="inline"> <tex>${\hbox{IrMn}}/[{\hbox{Co/Pt}}]_{3}$</tex> </formula> Multilayers

Co/Pt multilayers can exhibit considerable exchange bias along the film normal when grown onto or covered by an antiferromagnetic IrMn layer. The magnitude of the bias effect depends strongly on the selection of the buffer layers and is largest for Co/Pt multilayers with small perpendicular anisotropy. This counterintuitive result is explained by opposite dependencies of exchange bias and magnetic anisotropy on the degree of film texture and grain size. Interface roughness also influences the perpendicular exchange bias field. The largest biases are measured on smooth films. Interlayer mixing during post-deposition annealing procedures decreases the exchange bias field of IrMn-capped Co/Pt multilayers.

In‐plane and perpendicular exchange bias in [Pt/Co]/NiO multilayers

AbstractExchange bias in [Pt/Co]/NiO multilayers were studied as a function of film thickness and [Pt/Co] layer repetition. A strong temperature dependence of the coercivity, Hc, and exchange bias field, Hex, was observed for the thick and thinnest [Pt/Co]/NiO multilayers. While the thinnest [Pt(3 nm)/Co(1.25 nm)]4/NiO multilayers exhibits no in‐plane exchange bias field, a perpendicular Hex⊥ ∼ ‐150 Oe at 80 K was measured. By contrast, the thickest [Pt(12 nm)/Co(10 nm)]1/NiO multilayers exhibited an in‐plane Hex// ∼ ‐600 Oe (with Hex// ∼ ‐1300 Oe at 5 K) with no measurable perpendicular exchange bias field. The estimated interfacial exchange coupling energy implies the effective Co layer thickness contributing to the exchange bias is effective only in Co layer in contact with NiO bottom layer. AC susceptibility and the temperature dependence of Hex show that the a 1.25 nm thick Co component enables perpendicular exchange bias with a reduced blocking temperature TB∼200 K, compared to that (TB∼250 K) for the thick [Pt/Co]/NiO multilayers. This is attributed to disordered CoPt phases that formed due to intermixing between Co and Pt during deposition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Co80Fe20 insertion layer on perpendicular exchange bias characteristics in [Pd∕Co]5∕FeMn bilayered thin films

Effects of Co80Fe20 insertion layer on the perpendicular exchange bias characteristics in [Pd∕Co]5∕FeMn bilayered thin films have been investigated by changing the Co80Fe20 insertion layer thickness. Hysteresis loops for both in-plane and perpendicular to the film plane were measured by using a vibrating sample magnetometer and an extraordinary Hall effect. It was found that the thin Co80Fe20 insertion layer was very effective in improving the perpendicular exchange bias characteristics in [Pd∕Co]5∕FeMn bilayered thin films. By increasing Co80Fe20 insertion layer thickness from 0to1.24nm, the exchange bias field was significantly increased up to 332Oe and then decreased down to 60Oe, while the pinned coercivity was monotonically decreased. The effective magnetic anisotropy was kept at positive up to 0.6nm thick of Co80Fe20 insertion layer. It was experimentally confirmed that the perpendicular exchange bias field was promisingly larger than the pinned coercivity when the Co80Fe20 insertion layer had the thickness in the range between 0.37 and 0.56nm. The significant increase of the perpendicular exchange bias field and the reduction of pinned coercivity after inserting Co80Fe20 thin films are thought to be attributed to the modification of interfacial roughness between the FeMn and adjacent top Co layers and the stress-induced change of perpendicular magnetization direction of the top Co layer, which is adjacent to FeMn layer.

Application of the magnetoelectric effect to exchange bias

Owing to its magnetoelectric properties the antiferromagnetic (AF) domain structure of Cr 2 O 3 can be controlled when cooling through the Néel-point in simultaneously applied magnetic and electric fields. Recently we achieved complete switching of the perpendicular exchange bias field from positive to negative values and vice versa by variation of the applied electric freezing field at constant external magnetic field in the system Cr 2 O 3 / [ Co / Pt ] n [P. Borisov, et al., Phys. Rev. Lett. 94 (2005) 117203]. In order to image uncompensated spins at the ferromagnetic–AF interface, X-ray photoemission electron microscopy (X-PEEM) was performed on Cr 2 O 3 (1 1 1)/Pt/Co/Pt heterolayers after cooling in magnetic and electric freezing fields. The orientation of the uncompensated spins and the magnetic structure at the interface measured at the Cr and the Co L 3 , 2 absorption edges are discussed.

Simulating a perpendicular exchange-biased structure with a partially covering nonmagnetic insertion at the FM/AFM interface

Abstract The existence of minority spins, opposite to the perpendicular exchange bias, and majority spins aligned with this bias, and the dependence of the perpendicular exchange-bias field on the imprint effect, caused by the partially covering spacer at the ferromagnetic/antiferromagnetic (FM/AFM) interface, have been studied using Ising-type simulations. The present investigation suggests that the main factors influencing this phenomenon were dependent on the FM/AFM interface morphology, the balance between FM/AFM coupling and the FM-spins coupling, and the numerical balance between minority and majority spins. It was also determined that the imprint phenomenon can be used to enhance the perpendicular exchange-bias for small partial insertions at the FM/AFM interface.

Magnetization reversal in perpendicular exchange-biased multilayers

Co/Pt multilayers with perpendicular magnetic anisotropy exhibit an exchange bias when covered with an IrMn layer. The exchange bias field, which is about 7 mT for 3 Co/Pt bilayer repetitions and a Co layer thickness of 5 A, can be increased up to 16.5 mT by the insertion of a thin Pt layer at the Co/IrMn interface. The interfacial magnetic anisotropy of the Co/IrMn interface (KSCo/IrMn =-0.09 mJ/m2) favours in-plane magnetization and tends to tilt the Co spins away from the film normal. Dynamical measurements of the magnetization reversal process reveal that both thermally activated spin reversal in the IrMn layer and domain wall nucleation in the Co/Pt multilayer influence the interfacial spin structure and therefore the strength of the perpendicular exchange bias field.

Magnetoelectric Switching of Exchange Bias

The perpendicular exchange bias field, H(EB), of the magnetoelectric heterostructure Cr2O3(111)/(Co/Pt)(3) changes sign after field cooling to below the Néel temperature of Cr2O3 in either parallel or antiparallel axial magnetic and electric freezing fields. The switching of H(EB) is explained by magnetoelectrically induced antiferromagnetic single domains which extend to the interface, where the direction of their end spins controls the sign of H(EB). Novel applications in magnetoelectronic devices seem possible.

Correlation between perpendicular exchange bias and magnetic anisotropy in IrMn∕[Co∕Pt]n and [Pt∕Co]n∕IrMn multilayers

Perpendicular exchange bias has been observed for IrMn∕[Co∕Pt]n and [Pt∕Co]n∕IrMn multilayers in the as-deposited state. The exchange bias field is largest when the IrMn film is grown on magnetically saturated Co∕Pt multilayers (8.12mT for n=3), whereas it is considerably smaller when domain formation in the IrMn film occurs before Co∕Pt deposition (3.30mT for n=3). After annealing at 220°C in an out-of-plane magnetic field the perpendicular exchange bias field and magnetic anisotropy are considerably larger for the Co∕Pt multilayers with an IrMn film at the bottom. The apparent correlation between bias and anisotropy is explained by the dependence of the perpendicular exchange bias field on the orientation of the Co spins near the Co∕IrMn interface.