Exchange Bias Field Research Articles

Temperature dependence of exchange bias in NiFe2O4/BiFeO3 bilayers

In BiFeO3-based ferromagnet/anitferromagnet (FM/AFM) bilayers, field cooling is required through the Néel temperature to achieve significant exchange bias (EB) at room temperature (RT). In this work, NiFe2O4/BiFeO3 (NFO/BFO) all-oxide bilayer films were selected and deposited on SrTiO3 substrates by pulsed laser deposition to avoid the possible interfacial oxidization during the field cooling process. In combination with X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS) and transmission electron microscopy (TEM) characterizations, the NFO/BFO bilayers are shown to have good crystalline layered structure. After field cooling at various temperatures, the samples show sizable EB effect at low temperature region, which is attributed to spin-glass-pinned effect. More importantly, significant EB can be obtained at RT after the sample was field cooled from temperatures higher than 400 K. The maximal values of the exchange bias field (HE) and the ratio between HE and coercivity (i.e. HE/HC) are 48 Oe and 0.6 at RT, respectively. The EB effect at RT is considered to result from the pinned and uncompensated spins near the NFO-BFO interface.

Observation of room temperature ferromagnetism and exchange bias in a 55Mn+ ion-implanted unintentionally doped [formula omitted]-[formula omitted] single crystal

In this manuscript, the structural and magnetic properties of a 55Mn+ ion-implanted unintentionally doped β-Ga2O3 single crystal are investigated. The implanted Mn+ ions cause crystalline degeneration due to the formation of defects in the surface of the substrate. Room temperature ferromagnetism (FM) is observed in the samples with implantation doses of 5 × 1014cm-2, 1 × 1015cm-2 and 5 × 1015cm-2. The zero-field-cooled (ZFC)/field-cooled (FC) measurement of magnetization reveals that the ion implantation process induces an antiferromagnetic (AFM) phase. The temperature evolution of the remanence asymmetry (ME) demonstrates the typical behavior of the exchange bias effect due to the coexistence of FM and AFM phases. Based on the distribution of Mn+ ions and defects and the relationship between the remanence asymmetry (ME) and the exchange bias field (HE), the single domain and core-shell models are used to investigate the exchange bias system. The formation mechanism of the ferromagnetic order is briefly discussed, which is rather different from the one corresponding to the bound magnetic polaron (BMP) model.

Investigation of 2π isolated skyrmion pinning using exchange bias

The 2π isolated chiral skyrmion is a magnetic configuration. Since the total topological charge is zero, the 2π isolated skyrmion driven by a spin-polarized current propagates strictly along the racetrack. The manipulation of 2π-skyrmion, e.g., pinning/depinning at a specific position of the racetrack, is significant. Here, we investigated the 2π-skyrmion pinning in a racetrack using exchange bias. A series of transversal AFM wires were set above the ferromagnetic (FM) racetrack. Spin waves were employed to induce 2π-skyrmion motion to study the dynamics of the 2π-skyrmion pinning. The AFM wires induce exchange bias at the AFM/FM crossing points, which can act as pinning sites. The working window for a 2π-skyrmion in a racetrack was investigated as a function of the exchange bias field, the frequency and amplitude of the oscillating magnetic field for exciting spin waves. The interaction mechanism between the 2π-skyrmion and the exchange bias was also studied. This work may provide guidance for the design of next-generation spintronics.

Giant Strain Control of Antiferromagnetic Moment in Metallic FeMn by Tuning Exchange Spring Structure

AbstractManipulation of the antiferromagnetic moment in antiferromagnets (AFMs) is a crucial issue for developing AFM‐based spintronic devices. Lattice strain is an effective strategy to modulate the antiferromagnetic moment and is traditionally based on a direct crystalline tailoring of AFMs. A novel method for strain tuning the antiferromagnetic moment by controlling the exchange spring in the AFM, which is applicable to other conventional AFM materials, is reported. Specifically, a TiNi(Nb) shape memory alloy (SMA) is used as the substrate of Ta/NiFe/FeMn multilayers. By thermally driven inverse martensitic phase transformation in the SMA, a significant strain of 1.3% is transferred into the film, which toggles a noticeable magnetic moment rotation of NiFe by nearly 90° in the film plane, resulting in a consequent twirling of the Néel vector of FeMn due to interfacial exchange interaction. In turn, the antiferromagnetic moment of FeMn is tailorable by tuning the exchange spring. Simultaneously, the exchange bias field is tuned significantly with a maximal variation of 350% due to the twist of the antiferromagnetic moment, which facilitates strain‐assisted magnetization reversal for developing a logic memory device. These findings provide an alternative strategy to advance the development of an AFM‐based memorizer by temperature‐driven strain engineering.

Martensitic transformation and large exchange bias in Mn-rich Ni–Mn–Sn thin films on mica substrates

The fabrication of high-quality Heusler alloy films with clear martensitic transformation is a challenging issue in the field of magnetic shape memory alloys. Here, flexible Mn-rich Ni–Mn–Sn (Mn49.9Ni40.4Sn9.7) thin films are successfully synthesized by pulsed laser deposition on mica substrates using Ti as buffer layer. XRD analysis reveals that films exhibit austenitic phase with L21 structure at room temperature. The surface morphology indicates polycrystalline structure of fine grains with size of about 60 nm and smooth surface with roughness of less than 1 nm. The temperature dependent magnetization measurement clearly confirms the existence of first-order martensitic transformation. Exchange bias was observed below 70 K with maximum exchange bias field of 148 Oe at 20 K, which is attributed to the exchange coupling between the coexisting antiferromagnetic and ferromagnetic phases due to the various occupations of Mn atoms.

Exchange bias and anomalous Hall effect in a wide temperature range of 5–300 K in non-collinear antiferromagnetic Mn–Cr–Sn alloy

In this paper, noncollinear antiferromagnets Mn3.1Sn0.9 and Mn2.5Cr0.6Sn0.9 with an Ni3Sn-type hexagonal structure has been synthesized. An exchange bias (EB) from 5 K to room temperature has been observed in Mn2.5Cr0.6Sn0.9 with a maximum EB field of 3166 Oe at 5 K, which decreases gradually to 60 Oe at 300 K. It is proposed that the EB is ascribed to the exchange anisotropy between the ferromagnetic component and the antiferromagnetic host due to the magnetic inhomogeneity of the materials. Large anomalous Hall effect ranging from 3.5 to 1 µΩ · cm persists from 5 K to 300 K, which is attributed to the non-vanishing Berry phase arising from the non-collinear spin structure. Both the exchange bias and anomalous Hall effect in a wide temperature range of 5–300 K suggest that the Mn–Cr–Sn alloy has a promising application prospect in antiferromagnetic based spintronics devices.

Spin glass and exchange bias behavior in magnetically frustrated Ni1−xMgxCr2O4 (x = 0.0–0.50)

Single-phase polycrystalline samples of Ni1−xMgxCr2O4 (x = 0.0–0.50) were prepared by sol gel route and their structural and magnetic properties were studied. Structural transformation from tetragonal (I41/amd) to cubic (Fd3-m) phase is observed at room temperature due to Mg substitution. Mg substitution gives rise to reduction in ferrimagnetic and antiferromagnetic transition temperatures along with a signature of spin glass like phase in samples of intermediate compositions, i.e., for x = 0.10, 0.20 and 0.30. The observed stretched exponential type relaxation of thermoremanent magnetization and the highly frustrated magnetic behavior confirm the glassy magnetic phase. Significant increase in exchange bias field under field cooled condition for the Mg substituted samples is observed. The origin of exchange bias in theses samples is explained by considering exchange anisotropy between the ferrimagnetic and antiferromagnetic components of canted spin. The training effect of exchange bias field is also observed.

Correlation between phase composition and exchange bias in CoFe/MnN and MnN/CoFe polycrystalline films

Magnetic properties and phase composition of both MnN/CoFe (MnN at top of bilayer) and CoFe/MnN films (MnN at bottom of bilayer) bilayers through annealing at various temperatures (Ta = 300-450 °C) and then cooling to room temperature under the application of an external magnetic field of 1.5 kOe are compared. The exchange bias field (HE), the magnitude of magnetic hysteresis loop shift, of the studied films is highly related to phase composition of antiferromagnetic (AF) layer. The increase of HE with increasing Ta in the range of 300-375 °C possibly results from the improvement of magnetocrystalline anisotropy of AF related to the promoted crystallinity and stress relaxation of tetragonal face-centered θ-MnN phase. The reduction of HE at higher Ta is due to the decreased volume fraction or disappearance of θ-MnN phase and the formation of impurity phases, such as Mn4N and Mn. The induction of impurity phases is possibly related to the diffusion of part of N out of MnN phase at higher Ta. Higher HE for CoFe/MnN than MnN/CoFe at Ta = 300-375 °C might be attributed to larger amount and higher degree of stress relaxation for θ-MnN phase. For CoFe/MnN film annealed at 375 °C, the highest HE = 562 Oe is attained, and the corresponding interfacial exchange energy of 0.47 mJ/m2 in this study is comparable to that reported by Meinert et al. [Phys. Rev. B 92, 144408 (2015)].

Chiral symmetry and scale invariance breaking in spin chains

The effects of the Dzyaloshinsky-Moriya interaction on finite size one-dimensional (1D) magnetic chains are investigated as a function of their length. The magnetic configuration the system adopts for varying boundary conditions are explored analytically, which leads to the appearance of chiral configurations that play a crucial role. The coercive and exchange bias fields show an unexpected chain length dependence, caused by the boundary conditions and by chiral symmetry breaking, which in turn leads to the breakdown of scale-invariance. Our treatment yields results in agreement with experimental evidence and ongoing research on phthalocyanine iron chains bonded to hydrogen.

Regeneration of perpendicular exchange-biased state in high temperature regime in Pt/Co/Au/Cr2O3/Pt stacked films

Exchange bias (EB) has been a hot research topic since its discovery in the 1950s. In this paper, we present the peculiar temperature dependence of perpendicular EB in Pt/Co/Au/Cr2O3/Pt stacked films: the regeneration of the EB in the high temperature regime. Three temperature-dependent states are identified: (1) low temperature exchange-biased state, (2) middle temperature coercivity-enhanced state, and (3) high temperature regenerated exchange-biased state. At the boundary of these states, both EB field (HEB) and coercivity (HC) abruptly switch with the relationship HEB = ΔHC. The low temperature disappearance of EB can be interpreted based on the classical Meiklejohn–Bean exchange anisotropy model. A more complex temperature dependence of the interfacial spin alignment and magnetic domain wall width in the Cr2O3 layer is involved in the high temperature regeneration of EB.

Electric field modulation of exchange bias at the Co/CoOx interface

We demonstrated the electric field (EF) effect on exchange bias (EB) in a perpendicularly magnetized $\mathrm{Co}/\mathrm{Co}{\mathrm{O}}_{x}$ layered structure. The antiferromagnetic $\mathrm{Co}{\mathrm{O}}_{x}$ layer was formed by naturally oxidizing the Co surface. The modulation of the EB field and the coercivity by gate voltage application through a dielectric layer was clearly observed below the blocking temperature ${T}_{\mathrm{B}}$. The modulation ratio of the EB field exhibited strong temperature dependence, and it increased as the temperature approached ${T}_{\mathrm{B}}$. One possible cause of the EB modulation detailed herein is the modulation of the electronic state at the $\mathrm{Co}/\mathrm{Co}{\mathrm{O}}_{x}$ interface, which is fundamentally different from the case in which multiferroic antiferromagnets are used.

Influence of atomic roughness at the uncompensated Fe/CoO(111) interface on the exchange-bias effect

The effect of interface roughness of ferromagnetic and antiferromagnetic layers on exchange bias is still not well understood. In this report we have investigated the effect of surface roughness in (111)-oriented antiferromagnetic CoO films on exchange bias with ferromagnetic Fe grown on top. The surface roughness is controlled at the atomic scale, over a range below ~ 0.35 nm, by varying layer thickness of the CoO films. It is observed that both exchange bias field ($H_{E}$) and coercivity ($H_{C}$) extensively depend on the atomic scale roughness of the CoO (111) at the interface with Fe film. An opposite dependence of $H_{E}$ and $H_{C}$ on interface roughness was found, which was ascribed to partially compensated spin states induced by the atomic roughness at the fully uncompensated CoO (111) surfaces and was corroborated using the Monte Carlo simulations. Moreover, the onset temperature for $H_{C}$ is found to be up to ~ 80 K below the blocking temperature ($T_{B}$) and the temperature dependence of $H_{C}$ follows the power law with a critical exponent equal to one, which indicates that, in this system, $H_{C}$ is more of an interface-related property than $H_{E}$.

Exchange-biased nanocomposite ferromagnetic insulator

Magnetic insulating and exchange bias (EB) are two basic magnetic states with actual and potential applications in spintronic and magnonic devices. However, ferromagnetic insulators (FMIs) are rare, and it is even more challenging to obtain a FMI with large EB. In this paper, high performance of FMIs and EB in $\mathrm{LaMn}{\mathrm{O}}_{3}$:NiO nanocomposite epitaxial thin films is reported. The antiferromagnetic NiO nanoparticles, randomly embedded in a ferromagnetic $\mathrm{LaMn}{\mathrm{O}}_{3}$ matrix, enhance the robustness of the insulating state and result in an EB field as large as 1100 Oe. The simultaneously enhanced resistivity and EB are attributed to localized oxygen vacancy, preserved strain state, and increased ferromagnetic-antiferromagnetic interface ratio. This paper provides not only a material candidate for spintronics but also a referential strategy to design artificial materials with multifunctionality.

Giant exchange bias effect in all-3d-metal Ni38.8Co2.9Mn37.9Ti20.4 thin film

In this work, a giant exchange bias field of over 1 kOe, which is uncommon in film systems, is obtained in all-3d-metal Heusler-type Ni38.8Co2.9Mn37.9Ti20.4 thin films. The thermomagnetic curves show an unambiguous noncoincidence between zero-field-cooled and field-cooled protocols, indicating the presence of magnetically inhomogeneous phases in the film in the low temperature range. Magnetic measurements, including magnetic relaxation and magnetic memory effects, demonstrate that the exchange bias should be attributed to the exchange interaction between ferromagnetic clusters and the antiferromagnetic host in the Ni38.8Co2.9Mn37.9Ti20.4 thin film at low temperature. The local Mn-Co-Mn configuration provides the ferromagnetic contribution, which is formed in the antiferromagnetic Heusler main phase due to the small amount of Co-substitution for Ni. The achievement of the large exchange bias effect in the Ni38.8Co2.9Mn37.9Ti20.4 thin film is a significant addition to the existing multiple magneto-responsive effects in all-3d-metal Ni-Mn-based Heusler systems.

Edge Domain Walls in Ultrathin Exchange-Biased Films

We present an analysis of edge domain walls in exchange-biased ferromagnetic films appearing as a result of a competition between the stray field at the film edges and the exchange bias field in the bulk. We introduce an effective two-dimensional micromagnetic energy that governs the magnetization behavior in exchange-biased materials and investigate its energy minimizers in the strip geometry. In a periodic setting, we provide a complete characterization of global energy minimizers corresponding to edge domain walls. In particular, we show that energy minimizers are one-dimensional and do not exhibit winding. We then consider a particular thin-film regime for large samples and relatively strong exchange bias and derive a simple and comprehensive algebraic model describing the limiting magnetization behavior in the interior and at the boundary of the sample. Finally, we demonstrate that the asymptotic results obtained in the periodic setting remain true in the case of finite rectangular samples.

Positive exchange bias effect in LaCr0.5Fe0.5O3 perovskite

This work reports the synthesis and characterization of perovskites with LaCr0.5Fe0.5O3 stoichiometry. The perovskite was synthesized by an alternative route to traditional sol-gel method, the precursor was prepared using gelatin as a chelating agent and further calcined 2 h at 1200 °C. The sample is investigated by combining X-ray diffraction, Mössbauer spectroscopy and scanning electron microscopy with chemical microanalysis. The Rietveld refinement showed the formation of the La(OH)3 secondary phase in addition to the main perovskite phase. The M-T measurements showed a negative field cooling magnetization with a compensation temperature depending on the cooling field. The findings in the M-T measurements are discussed in terms of competition between the single ion magnetic anisotropy and the antisymmetric Dzyaloshinsky-Moriya interactions. The M-H measurement at 300 K revealed a hysteretic behavior ascribed to ferromagnetic clusters. At low temperature the field cooling M-H curves showed a positive exchange bias field and a downward shift of the hysteresis. The shifting of the hysteresis and the exchange bias field began at 250 K, a bit below the Néel transition. The hysteresis shift is ascribed to an antiferromagnetic coupling between the ferromagnetic clusters and the antiferromagnetic LaCr0.5Fe0.5O3 phase.

Exchange bias effect in hierarchical NiO/NiFe2O4 sub-microcubes fabricated via a self-sacrifice template process

In this study, we report a facile in situ synthesis of hierarchical NiO/NiFe2O4 sub-microcubes by annealing Ni3[Fe(CN)6]2 sub-microcubes fabricated via a one-step solution-precipitation method. The temperature-dependent magnetization curves show that there exists a collective freezing of the moments of interface spin clusters at TB≈296K, which further indicates the appearance a coupling interaction between the ferrimagnetic NiFe2O4 and antiferromagnetic NiO nanoparticles. Furthermore, hysteresis loops measurements reveal the existence of an exchange bias effect in the NiO/NiFe2O4 composite system. Interestingly, no training effect of exchange bias is observed, which indicates that the magnetic pinning interaction between NiFe2O4 and NiO is stable and robust. Exchange bias field shows an exponential dependence on temperature and vanishes at about 150 K, which reveals the strong coupling interaction does not improve effectively the thermal stability of exchange bias effect in the current system.

Chemical pressure induced substantial negative exchange bias on zero field cooled nanocrystalline LaFeO3

We report a distinctly different zero feld cooled negative exchange bias(EB) effect in Ni doped LaFeO3 nanoparticle where the EB field is both temperature and concentration dependent. Nanoparticles of Ni substituted LaFeO3 is prepared by a sol-gel process. The broadening of diffraction peaks with Ni is due to the combined effect of octahedral distortion and reduction in particle size while rulling out any structural phase change. A substantial EB is acquired below spin reorientation transitions which further reduces with higher chemical pressure of Ni. EB field is found to be ∼5.71 kOe at 5 K, revealing a large value compared to similar zero field cooled EB systems. A simple model is proposed based on tunable shell-core magnetic phases to explain these unique EB effects. Present study also aimed to explore the spin and oxidation state of Fe vis a vis magnetic ordering in LaFeO3 together with consequence of Ni doping.

Phase transformation, magnetic properties, and exchange bias of Heusler alloy Mn<sub>50–x</sub>Cr<sub>x</sub>Ni<sub>42</sub>Sn<sub>8</sub>

In this paper, phase transformations, magnetic properties and exchange bias of Mn<sub>50–<i>x</i></sub>Cr<sub><i>x</i></sub>Ni<sub>42</sub>Sn<sub>8</sub> (<i>x</i> = 0, 0.4, 0.6, 0.8) polycrystalline samples are investigated. It is found that each of all the alloys has a tetragonal martensite structure at room temperature. The transformation temperature decreases with the increase of Cr content. The maximum magnetization difference between martensite and austenite phase is ∆<i>M</i> = 7.61 emu/g. The change of magnetic properties is mainly related to the change of Mn-Mn distance and the hybridization strength between Ni(A)-Mn(D). The ferromagnetism of martensite can be enhanced by Cr doping. The exchange bias field is observed to reach up to as high as 2624 Oe in Mn<sub>50</sub>Ni<sub>42</sub>Sn<sub>8</sub> alloy after cooling from room temperature to 5 K in 500 Oe magnetic field, which decreases gradually with the increase of Cr content. Furthermore, the exchange bias field increases first and then followed by a decrease with the increase of the cooling field in Mn<sub>49.2</sub>Cr<sub>0.8</sub>Ni<sub>42</sub>Sn<sub>8</sub>. This is mainly attributed to the change of the interface exchange coupling between the spin glass state and antiferromagnetic region.

Electric control of exchange bias in Co/FeOx bilayer by resistive switching

Two types of electric control of exchange bias (EB) by resistive switching (RS), i.e. conductive-filament-RS (type I) and interface-barrier-RS (type II) were observed in the Si/SiO2Ti/Pt/FeOx/Co/ITO multilayer devices, which were fabricated by magnetron sputtering. It is difficult for the type I device to control EB, which may be due to that the quantity of conductive filaments is not enough to modify the antiferromagnetic structure of FeOx near the Co/FeOx interface. However, the electric control of EB can be accomplished in the type II device. Compared with low-resistance-state (LRS), the exchange bias field (HE) increases a little but the coercivity (HC) increases significantly at high-resistance-state (HRS). We consider that the migration of the oxygen vacancies under different voltages is able to mediate the interfacial barrier height, leading to the bipolar RS effect and the change of EB as well. This provides a way for designing new types of spintronic devices based on electric control.