Shift Of Hysteresis Loop Research Articles

Non-exchange bias hysteresis loop shifts in dense composites of soft-hard magnetic nanoparticles: New possibilities for simple reference layers in magnetic devices

Exchange bias has been extensively studied in both exchange-coupled thin films and nanoparticle composite systems. However, the role of non-exchange mechanisms in the overall hysteresis loop bias is far from being understood. Here, dense soft-hard binary nanoparticle composites are used not only as a novel tool to unravel the effect of dipolar interactions on the hysteresis loop shift but also as a new strategy to enhance the bias of any magnet exhibiting an asymmetric magnetization reversal. Mixtures of equally sized, 6.8 nm, soft maghemite (γ-Fe2O3) nanoparticles (no bias—symmetric reversal) and hard cobalt doped γ-Fe2O3 nanoparticles (large exchange bias—asymmetric reversal) reveal that, for certain fractions of soft particles, the loop shift of the composite can be significantly larger than the exchange-bias field of the hard particles in the mixture. Simple calculations indicate how this emerging phenomenon can be further enhanced by optimizing the parameters of the hard particles (coercivity and loop asymmetry). In addition, the existence of a dipolar-induced loop shift (“dipolar bias”) is demonstrated both experimentally and theoretically, where, for example, a bias is induced in the initially unbiased γ-Fe2O3 nanoparticles due to the dipolar interaction with the exchange-biased hard nanoparticles. These results open a new paradigm in the large field of hysteresis bias and pave the way for novel approaches to tune loop shifts in magnetic hybrid systems beyond interface exchange coupling.

Asymmetric magnetization switching and programmable complete Boolean logic enabled by long-range intralayer Dzyaloshinskii-Moriya interaction

After decades of efforts, some fundamental physics for electrical switching of magnetization is still missing. Here, we report the discovery of the long-range intralayer Dzyaloshinskii-Moriya interaction (DMI) effect, which is the chiral coupling of orthogonal magnetic domains within the same magnetic layer via the mediation of an adjacent heavy metal layer. The effective magnetic field of the long-range intralayer DMI on the perpendicular magnetization is out-of-plane and varies with the interfacial DMI constant, the applied in-plane magnetic fields, and the magnetic anisotropy distribution. Striking consequences of the effect include asymmetric current/field switching of perpendicular magnetization, hysteresis loop shift of perpendicular magnetization in the absence of in-plane direct current, and sharp in-plane magnetic field switching of perpendicular magnetization. Utilizing the intralayer DMI, we demonstrate programable, complete Boolean logic operations within a single spin-orbit torque device. These results will stimulate investigation of the long-range intralayer DMI effect in a variety of spintronic devices.

Proximity effects induced exchange bias and perpendicular net moment in SrIrO3

The magnetic proximity effect enables the incorporation of magnetic properties into non-magnetic materials, thereby achieving high-performance electronic devices. The 5d perovskite-oxide iridates provide a versatile platform for investigating competing electronic states due to their strong spin–orbit coupling. By proximity to conventional 3d transition-metal oxide ferromagnets, originally paramagnetic SrIrO3 is expected to exhibit exotic magnetic phenomena. In this study, we report the emergence of perpendicular net moment and an unprecedented exchange bias effect in SrIrO3 sandwiched between two ferromagnetic LaMnO3 layers. The magnetization measurements clearly demonstrate antiferromagnetic coupling in the heterostructures, resulting in a shift of the magnetic hysteresis loops. Furthermore, magneto-transport measurements provide evidence for the induction of a net moment along the perpendicular direction of the SrIrO3 film plane due to proximity effects. Both the magnetic pinning effect and perpendicular magnetization can be controlled by an external magnetic field. These findings expand the potential applications of iridate-related heterostructures in modern information storage fields such as skyrmions, spin–orbit torques, and antiferromagnet-based spintronics.

Tuneable Vertical Hysteresis Loop Shift in Exchange Coupled La0.67Sr0.33MnO3‐SrRuO3 Bilayer

AbstractHarnessing extra degrees of freedom at the heterostructure interface is of crucial importance to bring additional functionalities in modern spintronic devices. Here, a vertical hysteresis loop shift (vertical bias) is demonstrated in an exchange biased system of ferromagnetic thin film heterostructure of La0.67Sr0.33MnO3 (10 nm)‐SrRuO3 (SRO) (20 nm), after field cooling with ±1 T below 100 K close to the Curie temperature (TC) ≈125 K of SRO and loop sweeping under ±1 T field. Besides, a positive exchange bias (HEB) is also observed below TC ≈125 K showing a maximum ≈11 mT at 2 K. The vertical shift is modeled closely using micromagnetic simulations and the layers’ thickness dependency is demonstrated. The reason for the shift is attributed to the simultaneous role of the interfacial antiferromagnetic interaction and the hard anisotropy of SRO against the Zeeman energy. Finally, from the experimental and simulation results, a generalized model of controllable and tunable vertical shift is proposed applicable for other material systems possessing glassy phases, uncompensated/canted spins, absent interfacial exchange coupling, etc., and hence can be informative for the use of vertical shift in future spintronic devices.

Second Harmonic Generation Versus Linear Magneto-Optical Response Studies of Laser-Induced Switching of Pinning Effects in Antiferromagnetic/Ferromagnetic Films

Composite magnetic nanostructures are a subject of high research interest, as they provide a number of exciting effects absent in live nature. Among others, much attention has been paid to the studies of exchange coupling in antiferromagnetic/ferromagnetic (AFM/FM) films, which leads to the pinning effect. It manifests itself as a widening and shift of the magnetic hysteresis loop with respect to zero value of the external magnetic field oriented along the pinning direction. In this work, we report on comparative studies of linear and nonlinear magneto-optical effects under the laser-induced switching of the pinning effect in IrMn/CoFe films of various thickness of the ferromagnetic CoFe layer. We found that the magneto-optical response of the pinned AFM/FM nanofilms appears with different hysteresis loop parameters in the transverse magneto-optical Kerr effect (MOKE) and interface-sensitive magnetization-induced second harmonic generation (SHG), indicating the diversity of the magnetic effects at interfaces compared to the bulk of the films.

The Role of Stress States on the Hysteric Behavior of Expansive Soil under Multiple Drying-Wetting Cycles

Expansive soils in the field are typically exposed to cyclic wetting and drying due to climatic fluctuations and subjected to a variety of stress conditions in nature or when used as compacted layers for the construction of hydraulic barriers or waste disposal facilities. The hysteric behavior of the soil-water characteristic curve (SWCC) is a key parameter for understanding, modeling, and interpreting the unsaturated behavior of these soils under such conditions. This study investigates the effect of stress states on the hysteresis behavior of soil-water characteristic curves (SWCCs) for compacted highly expansive clay over a range of matric suction between 0 and 1500 kPa. Two test series were performed, the first test series investigated the effect of stress states on the hysteresis of SWCCs during a single drying-wetting (DW) cycle. The second test series studied the combined effect of stress applied and multiple drying-wetting cycles on the SWCC hysteresis. For the sake of comparison, the overall SWCC hysteresis due to drying-wetting cycles was quantified using the average degree of hysteresis in terms of volumetric water content (ADHθ). Furthermore, contributors to the observed hysteresis were defined using two newly proposed measures; namely, average degree hysteresis in terms of gravimetric water content (ADHw) and in terms of volume change (ADHe*). The outcomes of this study indicate that consideration of stress states on the hysteresis of SWCC for expansive clay is of great importance. The results show a dual trend for the variation of ADHθ with applied vertical stress. Furthermore, multiple DW cycles induced a significant reduction in the hysteresis (ADHθ) under low- and high-stress states up to a certain level of DW cycles, then, no further changes in the hysteresis trend were detected. It was also found that hysteresis loops under a low-stress state were concentric in shape while hysteresis loops for specimens under a high-stress state were non-concentric, with a downward shift in hysteresis loops with the increase in DW cycles.

Engineering Pt/Co/AlO x heterostructures to enhance the Dzyaloshinskii–Moriya interaction

The study of interfacial Dzyaloshinskii–Moriya interaction (DMI) in perpendicularly magnetized structurally asymmetric heavy metal/ferromagnet multilayer systems is of high importance due to the formation of chiral magnetic textures in the presence of DMI. Here, we report the impact of cobalt oxidation at the Co/AlO x interface in Pt/Co/AlO x trilayer structures on the DMI by varying the post-growth annealing time, Al thickness and substrate. To quantify DMI we employed magneto-optical imaging of the asymmetric domain wall expansion, hysteresis loop shift, and spin-wave spectroscopy techniques. We further correlated the Co oxidation with low-temperature Hall effect measurements and x-ray photoelectron spectroscopy. Our results emphasize the importance of full characterization of the magnetic films that could be used for magnetic random access memory technologies when subjected to the semiconductor temperature processing conditions, as the magnetic interactions are critical for device performance and can be highly sensitive to oxidation and other effects.

Facile integration of giant exchange bias in Fe5GeTe2/oxide heterostructures by atomic layer deposition

Exchange bias arises from the interfacial exchange coupling in ferromagnet-antiferromagnet bilayers and manifests as a horizontal shift of the magnetic hysteresis loop, constituting a critical component underpinning a broad range of magnetoresistive logic and memory devices. The facile implementation of exchange bias in van der Waals (vdW) magnets would be a key step towards practical devices for emerging vdW spintronics. Here, we report an easy approach to establishing strong exchange bias in the vdW magnet ${\mathrm{Fe}}_{5}\mathrm{Ge}{\mathrm{Te}}_{2}$ by a single-step process---atomic layer deposition (ALD) of oxides on ${\mathrm{Fe}}_{5}{\mathrm{GeTe}}_{2}$. We successfully created exchange bias of 300--1500 Oe in ${\mathrm{Fe}}_{5}\mathrm{Ge}{\mathrm{Te}}_{2}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}, {\mathrm{Fe}}_{5}\mathrm{Ge}{\mathrm{Te}}_{2}/\mathrm{ZnO}$, and ${\mathrm{Fe}}_{5}\mathrm{Ge}{\mathrm{Te}}_{2}/{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ heterostructures, at 130 K. Control experiments showed that increasing the oxidant pulse duration in each ALD cycle or utilizing the stronger oxidant ${\mathrm{O}}_{3}$ can enhance the exchange bias strength, revealing the key role of the ALD oxidants. Our systematic work elucidates the essential role of ALD-enabled oxidization of ${\mathrm{Fe}}_{5}\mathrm{Ge}{\mathrm{Te}}_{2}$ in the formation of exchange bias, and establishes ALD of oxides as a facile, controllable, and generally effective approach to creating giant exchange bias in vdW magnets, representing an integral advance towards practical vdW spintronic devices.

Origin of spin-glass-like magnetic anomaly in the superconducting and multiferroic spin ladder BaFe2Se3

The spin ladder system ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{3}$ presents puzzling magnetic behaviors, such as a spin-glass-like phase below $\ensuremath{\sim}50$ K. In this paper, an exchange bias effect with a large vertical shift in the field-cooled hysteresis loop is observed below 50 K. We also evidence the existence of uncompensated spins by susceptibility, magnetic remanence, and hysteresis loop measurements. The thermoremanent and isothermoremanent magnetization curves evidence a two-dimensional diluted antiferromagnet (DAFF) nature. Moreover, a nanometer-sized layered structure is observed by scanning electron microscope (SEM) and confirmed by scanning transmission electron microscope (STEM) coupled with electron energy loss spectroscopy (EELS). The discovery of exchange bias in antiferromagnetic ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{3}$ crystals adds a new dimension to the research of its superconducting and multiferroic properties.

Atomistic study of Positive Exchange Bias in Hard/Soft ferromagnetic bilayer nanosystem

We present calculations for the Exchange spring nano-bilayer with intermixing of high and low anisotropic moments at the interface, where we study the unusual phenomenon of positive exchange bias. The results are well reproducible at different simulation temperatures. However, the amount of shift as well as the width of M-H loop is greatly influenced by the temperature itself. We illustrate interface roughness as an important cause for the shift of Hysteresis loop in Ferromagnetic Hard/Soft nano bilayer systems.

Coexistence of positive and negative exchange bias effect in Pr-doped NdCrO3 samples

Positive and negative exchange bias effects are observed in Nd1−xPrxCrO3 (0.00 ≤ x ≤ 1.00) polycrystalline samples. The shift of hysteresis loops, either opposing or following the direction of the field cooling, depends on the concentration of praseodymium. The temperature dependence of magnetization reveals a magnetic transition at T ≈ 220 K likely ascribed to the N é el temperature of Cr3+ magnetic moments. The non-Kramer Pr3+ ion seems to play an important role in the magnetic properties of this compound. In the range 0.00 ≤ x ≤ 0.60, all samples presented a decrease in the magnetization below TN and a clear compensation point. Pr-rich samples (x ≥ 0.70) show a paramagnetic-like behavior down to low temperatures. The most impressive result is the relation between reversal magnetization and the signal of exchange bias effect which seems be tuned by the Pr-doping.

Exchange bias and interface-related effects in two-dimensional van der Waals magnetic heterostructures: Open questions and perspectives

The exchange bias (EB) effect is known as a fundamentally and technologically important magnetic property of a magnetic bilayer film. It is manifested as a horizontal shift in a magnetic hysteresis loop of a film subject to cooling in the presence of a magnetic field. The EB effect in van der Waals (vdW) heterostructures offers a novel approach for tuning the magnetic properties of the newly discovered single-layer magnets, as well as adds a new impetus to magnetic vdW heterostructures. Indeed, intriguing EB effects have recently been reported in a variety of low-dimensional vdW magnetic systems ranging from a weakly interlayer-coupled vdW magnet (e.g., Fe3GeTe2) to a bilayer composed of two different magnetic vdW materials (e.g., Fe3GeTe2/CrCl3, Fe3GeTe2/FePS3, Fe3GeTe2/MnPS3, Fe3GeTe2/CrSe, Fe3GeTe2/CrOCl, Fe3GeTe2/CoPc, Fe5GeTe2/FePS3), to bilayers of two different vdW defective magnets (e.g., VSe2/MoS2), or to metallic ferromagnet/vdW defective magnet interfaces (e.g., Fe/MoS2). Despite their huge potential in spintronic device applications, the physical origins of the observed EB effects have remained elusive to researchers. We present here a critical review of the EB effect and associated phenomena such as magnetic proximity (MP) in various vdW heterostructure systems and propose approaches to addressing some of the emerging fundamental questions.

Coercivity and Exchange Bias in Ti-Doped Maghemite Nanoparticles

Ti-doped maghemite nanoparticles of average crystallite size 12.9 nm were synthesized using the sol–gel method. The XRD profile mainly showed the presence of maghemite phase with very small phases of TiO2 (rutile and anatase). Magnetization hysteresis loops of the nanoparticles were obtained between −4 T to +4 T at temperatures of 2, 10, 30, 50, 70, 100, 150, 200, and 300 K under field cooling (FC) of 1, 2, 3, and 4 T and zero-field cooling conditions (ZFC). The coercivity displayed nonmonotonic field dependence while it decreased sharply with temperature and vanished at 150 K at all fields. Horizontal hysteresis loop shifts were observed in the 2–150 K temperature range in both the ZFC and FC conditions. The exchange bias effect became negligible in both ZFC and FC states above 50 K. Magnetization vs. applied field measurements were conducted in both ZFC and FC cooled conditions at several temperatures in the range of 2–400 K, with spin freezing being observed below 50 K. The exchange bias effect obtained below 50 K is suggested to be attributed to the competing roles of the long-range dipolar and short-range exchange coupled interactions.

Deep learning for spin-orbit torque characterizations with a projected vector field magnet

Spin-orbit torque characterizations on magnetic heterostructures with perpendicular anisotropy are demonstrated on a projected vector field magnet via hysteresis loop shift measurement and harmonic Hall measurement with planar Hall correction. Accurate magnetic field calibration of the vector magnet is realized with the help of deep learning models, which are able to capture the nonlinear behavior between the generated magnetic field and the currents applied to the magnet. The trained models can successfully predict the applied current combinations under the circumstances of magnetic field scans, angle scans, and hysteresis loop shift measurements. The validity of the models is further verified, complemented by the comparison of the spin-orbit torque characterization results obtained from the deep-learning-trained vector magnet system with those obtained from a conventional setup comprised of two separated electromagnets. The damping-like spin-orbit torque (DL-SOT) efficiencies (|$\xi_{DL}$|) extracted from the vector magnet and the traditional measurement configuration are consistent, where |$\xi_{DL}$| $\approx$ 0.22 for amorphous W and |$\xi_{DL}$| $\approx$ 0.02 for $\alpha$-W. Our work provides an advanced method to meticulously control a vector magnet and to conveniently perform various spin-orbit torque characterizations.

Temperature and frequency dependent defect dipole kinematics in "hard" piezoelectric ceramics

“Hard” piezoelectric ceramics exhibit typical shifted hysteresis loops due to the presence of oxygen vacancies and defect dipoles. The kinetic characteristics of oxygen vacancies and defect dipoles significantly affect the hardening effect and application scenario of “hard” piezoelectric ceramics. The scaling behavior of the internal bias field is an effective method to investigate the kinetic characteristics of oxygen vacancies and defect dipoles. This work systematically studied the temperature and frequency-dependent relation of the internal bias field in “hard” lead zirconate titanate (PZT-8) ceramics with tetragonal phase. The linear relation of the internal bias field on the temperature and electric field amplitude is given. Simultaneously, the internal bias field exhibits distinct relaxation characteristics with the frequency variation. The stretching relaxation relation between internal bias field and frequency was also determined. In addition, the kinetic characteristics of oxygen vacancy and defect dipole dependent on frequency and temperature were studied and analyzed. Our works provide the scientific theory of defect kinetics for hard-doping perovskite ferroelectrics and guide the device application of “hard” PZT ceramics.

Temperature-driven spin switching and exchange bias in the ErFeO3 ferrimagnet

The compensated ferrimagnet orthoferrite $\mathrm{ErFe}{\mathrm{O}}_{3}$ exhibits the exchange bias (EB) effect, which was previously detected as the traditional shift of the magnetization hysteresis loops $M$ vs $H$ near the compensation temperature ${T}_{\mathrm{comp}}=45\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This paper shows that another specific phenomenon, temperature-driven spin switching and exchange bias, occurs in this ferrimagnet. Namely, the EB manifests itself as the temperature shift of the hysteresis loops $M$ vs $T$, which occurs upon successive cooling and heating in a weak magnetic field. The $M$ vs $T$ loops limiting the region of coexistence of negative and positive magnetization are shifted towards lower or higher temperatures, depending on the sign of the applied magnetic field, which causes the unidirectional EB anisotropy. The EB anisotropy energy, which contributes to the energy barrier for switching spins to an equilibrium state, determines the shift in the switching temperature ${T}_{\mathrm{sw}}$. Exchange-biased spin switching in $\mathrm{ErFe}{\mathrm{O}}_{3}$ is discussed within a model, which explains the magnetic compensation of the canted ferromagnetic moment of the ${\mathrm{Fe}}^{3+}$ spins and the opposite moment of paramagnetic ${\mathrm{Er}}^{3+}$ spins induced by the antiferromagnetic exchange interaction between the Er and Fe ions.

Role of an in-plane ferromagnet in a T-type structure for field-free magnetization switching

Deterministic magnetization switching driven by current-induced spin–orbit torque (SOT) without an external magnetic field has potential applications in magnetic random access memory. Here, we realized the field-free magnetization switching in a T-type structure (CoFeB/W/CoFeB), where the two CoFeB layers have perpendicular magnetic anisotropy and in-plane magnetic anisotropy (IMA), respectively. We discovered that the direction of symmetry-breaking field is parallel to the magnetization of the bottom CoFeB (IMA), which cannot be explained by a stray field of this layer. In addition, by placing a 2.5-nm thick insulating layer of MgO between the bottom CoFeB and W layer (CoFeB/MgO/W/CoFeB) to block the interlayer exchange coupling and the spin current from the bottom CoFeB, the field-free SOT switching was still achieved, primarily due to the Néel orange-peel effect in our devices. By using micromagnetic simulations, the roughness of angstrom magnitude was introduced into the model to calculate the symmetry-breaking field, finding a qualitative agreement with experiments. Moreover, we obtained the spin Hall angle of CoFeB (θSH = −0.024) by the current-induced hysteresis loop shift method, and the contribution of the effective efficiency χ from the bottom CoFeB was accounted for about 26% of the total in the current-induced SOT switching process. These results indicated that an in-plane ferromagnet layer in the T-type structure provides not only the symmetry-breaking field but also spin current for the field-free SOT magnetization switching.

Генерация магнитоиндуцированной второй гармоники в тонких пленках с интерфейсами ферромагнетик/антиферромагнетик

We experimentally studied the generation of optical second harmonic (SH) and magneto-optical effects at the SH frequency in thin films consisting of ferromagnetic and antiferromagnetic materials, including exchange-bonded layers, one of which is pinned with an antiferromagnetic layer. As expected, the loops of the linear magneto-optical Kerr effect are shifted relative to the zero magnetic field value, which reflects the presence of a pronounced exchange interaction in such structures. It is shown that in the case of the nonlinear Kerr magneto-optical effect this effect is also present, but it is significantly smaller than in the nonlinear case, and leads to the observation of only one hysteresis loop instead of two. The studied dependence of the magneto-optical response at the TH frequency on the probing laser radiation power demonstrates that its growth leads to a decrease in the magnetic hysteresis loop shift of the TH.

Magnetization induced second harmonic generation in thin films with ferromagnet/antiferromagnet interfaces

We experimentally studied the generation of optical second harmonic (SH) and magneto-optical effects at the SH frequency in thin films consisting of ferromagnetic and antiferromagnetic materials, including exchange-bonded layers, one of which is pinned with an antiferromagnetic layer. As expected, the loops of the linear magneto-optical Kerr effect are shifted relative to the zero magnetic field value, which reflects the presence of a pronounced exchange interaction in such structures. It is shown that in the case of the nonlinear Kerr magneto-optical effect this effect is also present, but it is significantly smaller than in the nonlinear case, and leads to the observation of only one hysteresis loop instead of two. The studied dependence of the magneto-optical response at the TH frequency on the probing laser radiation power demonstrates that its growth leads to a decrease in the magnetic hysteresis loop shift of the TH. Keywords: magnetization-induced SHG, ferromagnet/antiferromagnet interface, pinned structure, thin films

Additive Dzyaloshinskii–Moriya interaction in Pt/Co/Re films

We report the Dzyaloshinskii–Moriya interaction (DMI) in Pt/Co/Re films. By measuring the current-induced hysteresis loop shift, we find that the interfacial DMI in the Pt/Co/Re structure is 2.1 pJ/m, which is stronger than that in Ir/Co/Pt asymmetric multilayers. The large DMI in this system can be attributed to an additive DMI at the Pt/Co and Co/Re interfaces; the Co/Re interface hosts a large DMI whose sign is the same as that at the Pt/Co interface. The additive DMI due to the large DMI at the Co/Re interface is consistent with theoretical predictions. The result provides a way to control the formation of chiral spin textures, such as magnetic skyrmions and chiral domain walls.