Magnetic Reorientation Research Articles

First TPymT-Ln complexes (TPymT = 2,4,6-Tris(2-pyrimidyl)-1,3,5-triazine; Ln = Eu, Gd, Tb, Dy): Solvothermal synthesis, structure, magnetic and photoluminescent properties

Four novel mononuclear lanthanide complexes, [Ln(TPymT)(NO3)3(H2O)2] (Ln: Eu3+ (1), Gd3+ (2), Tb3+ (3) and Dy3+ (4)), have been solvothermally synthesized using multidentate 2,4,6-tris(2-pyrimidyl)-1,3,5-triazine (TPymT). Compounds 1–3 have been characterized by means of elemental analysis, FT-IR, and single-crystal/powder X-ray diffraction analysis. Compound 4 was structurally characterized. The Ln3+ ion in 1–4 is ten-coordinated, where TPymT serves as an N3 donor in an isostructural series. The alternating-current magnetic studies showed frequency dependence below ca. 10 K for 3, as an indication of a single-ion magnet. The activation energy for the magnetization reorientation was estimated as Ueff/kB = 95(9) K after applying a dc bias field 2000 Oe for 3. The photoluminescent studies clarified that 1 and 3 behaved as red and green light emitters with quantum yields as high as 17 and 39 %, respectively. The TD-DFT calculation supports the energy level scheme of ground and excited states of TPymT together with the reference compound 4′-phenyl-2,2’:6′,2″-terpyridine. The present work suggests a broad chance and motivation to apply TPymT to the field of 4f coordination chemistry.

Mn doping-induced twofold spin reorientation and multi-step spin switching in NdFeO3 single crystal

Magnetization reversal and magnetic state regulation are the basis of application for magnetic materials in information processing. Here, twofold spin reorientation transitions (SRT) and field-induced multi-step spin switching phenomena were observed in NdMn0.2Fe0.8O3 (NMFO) single crystals grown via the optical floating zone method. Magnetic anisotropy was systematically investigated in both Zero-Field-Cooled (ZFC) and Field-Cooled (FC) modes. The NMFO single crystal exhibited twofold SRTs: from Γ4 (Gx, Ay, Fz) to Γ1 (Ax, Gy, Cz) and from Γ1 (Ax, Gy, Cz) to Γ2 (Fx, Cy, Gz). Furthermore, a magnetic field-induced spin reorientation transition was identified along the c-axis. Additionally, closer proximity to the SRT temperature corresponded to a reduced magnetic field requirement for inducing the spin reorientation transition. These findings contribute to our comprehension of spin dynamics in NMFO, presenting possibilities for future device applications.

Role of Dy 4f electrons on magnetic coupling and reorientation in DyFeO3

Spin reorientation transition is an ubiquitous phenomenon observed in magnetic rare earth orthferrites RFeO3, which has garnered significant attention in recent years due to its potential applications in spintronics or magnetoelectric devices. Although a plenty of experimental works suggest that the magnetic interaction between R3+ and Fe3+ spins is at the heart of the spin reorientation, but a direct and conclusive theoretical support has been lacking thus far, primarily due to the challenging nature of handling R 4f electrons. In this paper, we explored DyFeO3 as an example by means of comprehensive first principles calculations, and compared two different approaches, where the Dy 4f electrons were treated separately as core or valence states, aiming to elucidate the role of Dy 4f electrons, particularly in the context of the spin reorientation transition. The comparison provides a solid piece of evidence for the experimental argument that the Dy3+−Fe3+ magnetic interactions play a vital role in triggering spin reorientation of Fe3+ moments at low temperatures. The findings revealed here not only extend our understanding on the underlying mechanism for spin reorientation transition in RFeO3, but also highlight the importance of explicit description of R 4f electrons in rationally reproducing their structural, electronic and magnetic properties.

Magnetocaloric effect at the reorientation of the magnetization in ferromagnetic multilayers with perpendicular anisotropy

We investigate the magnetocaloric effect obtained by the rotation of a magnetic field applied to an exchange-coupled multilayer system composed of two different ferromagnetic (FM) materials. We specifically consider a system in which the two FMs have perpendicular uniaxial anisotropy axes and utilize conditions which yield a reorientation of the total magnetization when compensation between the anisotropies of the two layers occurs. We calculate the consequent entropy change associated with the “artificial” reorientation. By using known parameters from MnBi and Co we predict an entropy change of Δs=0.34 J kg−1 K−1 for perfect coupling. Lastly, we study the behavior of the multilayer under a rotating magnetic field via a micromagnetic model. When the layer thicknesses are of the order of the local domain wall width, the magnetic field-induced entropy change can be obtained with magnetic fields one order of magnitude lower than in the uncoupled case.

Evolution of magnetic anisotropy in La0.67Sr0.33MnO3 thin films due to the appearance of out-of-plane magnetization at low temperature

We have observed quadrupolar angle-dependent anisotropic magnetoresistance (AMR) at low temperature in La0.67Sr0.33MnO3 (LSMO) films grown on SrTiO3 substrate, where the magnetic field was applied while rotating the direction of the applied magnetic field from out-of-plane (OP) to in-plane. At room temperature, the easy axis is in-plane and has a two-fold magnetic anisotropy. As the temperature decreases, it changes to a quadrupolar anisotropy. Along with the AMR, we have measured the magnetoresistance (MR) in the OP direction at various temperatures. Like the AMR, we found the appearance of a new magnetic configuration in the OP direction at low temperatures. We suggest that one of the important origins of the new OP magnetism, which appears at temperature much lower than the Curie temperature (Tc), is the d3z2−r2 orbital at the interface between the LSMO films and SrTiO3 substrate. It has been reported that despite the tensile strain imposed by the SrTiO3 substrate, the d3z2−r2 orbital exists at the interface and causes the OP magnetism. We demonstrate by a polarized x-ray resonant scattering that there is a higher density of d3z2−r2 orbitals at the interface than in the region away from the interface. As other references have proposed, we suggest the orbital reconstructions and the orbital reconstruction-driven magnetic reorientation at the interface as the origin of the appearance of the OP magnetism present in the AMR and MR results.

Imaging of voltage-controlled switching of magnetization in highly magnetostrictive epitaxial Fe-Ga microstructures.

The magnetoelectric behavior of epitaxial Fe-Ga microstructures on top of a (001)-oriented PMN-PT piezoelectric substrate is imaged with magnetic X-ray microscopy. Additionally, the micron-scale strain distribution in PMN-PT is characterized by X-ray microdiffraction and examined with respect to the results of the Fe-Ga magnetoelectric switching. The magnetic reorientation of Fe-Ga is found to be strongly correlated with size, shape, and crystallographic orientation of the microstructures. In the case of square-shaped structures, size dictates the influence of the strain distribution on both the initialization of the ground state and on the magnetic reorientation during application of voltage. On the other hand, elliptical microstructures demonstrate completely different magnetic responses depending on the relative orientation of their long axis with respect to the crystallographic directions of the PMN-PT. This study demonstrates that engineering the behavior of highly magnetostrictive epitaxial microdevices is possible. It further elucidates that voltage-induced actuation can be largely tuned to achieve the desired type of magnetic switching ranging from vortex circulation reversal, domain wall motion, to a large rotation of magnetization. Because of the outstanding properties of the investigated material system, the reported findings are expected to be of great interest for the realization of next-generation energy-efficient magnetic memory and logic devices.

Size and density control of skyrmions with picometer CoFeB thickness variations—observation of zero-field skyrmions and skyrmion merging

The controlled formation and adjustment of size and density of magnetic skyrmions in Ta/CoFeB/MgO trilayers with low Dzyaloshinskii–Moriya interaction is demonstrated. Close to the out-of-plane to in-plane magnetic spin reorientation transition, we find that small energy contributions enable skyrmion formation in a narrow window of 20 pm in CoFeB thickness. Zero-field stable skyrmions are established with proper magnetic field initialization within a 10 pm CoFeB thickness range. Using magneto-optical imaging with quantitative image processing, variations in skyrmion distribution and diameter are analyzed quantitatively, the latter for sizes well below the optical resolution limit. We demonstrate the controlled merging of individual skyrmions. The overall demonstrated degree of comprehension of skyrmion control aids to the development of envisioned skyrmion based magnetic memory devices.

Electrochemical manipulation of spin reorientation in FePO[formula omitted] by Li-ion intercalation

Density functional theory plus U and Monte Carlo simulations reveal that the electrochemical lithiation/delithiation process has a dramatic effect on magnetic structure and magnetocrystalline anisotropy (MCA) of an olivine LixFePO4 (0 ≤ x ≤ 1) structure. We demonstrate that MCA undergoes a transition from the magnetic easy axis along the [100] direction for FePO4 (fully delithiated phase) to the [010] direction for the partially and fully lithiated phases of LixFePO4 with x ≥ 0.25. This magnetization reorientation is the result of the interplay between the lithiation/delithiation induced changes in the spin–orbit coupled 3d-electron orbital states near the Fermi level and the high-spin crystal field split of the Fe3＋ and Fe2＋ ionic states. Furthermore, the lithium intercalation substantially reduces the energy deviation of an antiferromagnetic state from a ferromagnetic phase, reflecting the experimental observation of lowering the Néel temperature from 125 K for FePO4 to 52 K for LiFePO4, which are fairly reproduced by the Monte Carlo simulation. These findings open interesting prospects for exploiting electrochemical lithiation/delithiation process to manipulate magnetization direction in antiferromagnet spintronics.

In-plane reorientation induced single laser pulse magnetization reversal

Single Pulse All Optical Switching represents the ability to reverse the magnetization of a nanostructure using a femtosecond single laser pulse without any applied field. Since the first switching experiments carried out on GdFeCo ferrimagnets, this phenomena has been only recently extended to a few other materials, MnRuGa alloys and Tb/Co multilayers with a very specific range of thickness and composition. Here, we demonstrate that single pulse switching can be obtained for a large range of rare earth–transition metal multilayers, making this phenomenon much more general. Surprisingly, the threshold fluence for switching is observed to be independent of the laser pulse duration. Moreover, at high laser intensities, concentric ring domain structures are induced. These striking features contrast to those observed in Gd based materials pointing towards a different reversal mechanism. Concomitant with the demonstration of an in-plane magnetization reorientation, a precessional reversal mechanism explains all the observed features.

Investigations of Shape Deformation Behaviors of the Ferromagnetic Ni-Mn-Ga Alloy/Porous Silicone Rubber Composite towards Actuator Applications.

Ferromagnetic shape memory alloys (FSMAs), which are potential candidates for future technologies (i.e., actuators in robots), have been paid much attention for their high work per volume and rapid response as external stimulation, such as a magnetic field, is imposed. Among all the FSMAs, the Ni-Mn-Ga-based alloys were considered promising materials due to their appropriate phase transformation temperatures and ferromagnetism. Nevertheless, their intrinsic embrittlement issue and sluggish twin motion due to the inhibition of grain boundaries restrict their practicability. This study took advantage of the single-crystal Ni-Mn-Ga cube/silicone rubber composite materials to solve the two aforementioned difficulties. The single-crystal Ni-Mn-Ga cube was prepared by using a high-temperature alloying procedure and a floating-zone (FZ) method, and the cubes were verified to be the near-{100}p Ni-Mn-Ga alloy. Various room temperature (RT) curing silicone rubbers were utilized as matrix materials. Furthermore, polystyrene foam particles (PFP) were used to provide pores, allowing a porous silicone rubber matrix. It was found that the elastic modulus of the silicone rubber was successfully reduced by introducing the PFP. Additionally, the magnetic field-induced martensite variant reorientation (MVR) was greatly enhanced by introducing a porous structure into the silicone rubber. The single-crystal Ni-Mn-Ga cube/porous silicone rubber composite materials are considered to be promising materials for applications in actuators.

Reversible magnetic domain reorientation induced by magnetic field pulses of fixed direction

Reversible magnetic domain reorientation induced by magnetic field pulses of fixed direction

Temperature dependence of optical fiber current sensor with external conversion

The paper presents the research of a optical fiber current sensor with external conversion (OFCS-EC) as a function of temperature. The sensor was developed in the Department of Optoelectronics. The sensor was tested in the temperature range from 23 °C – 80 °C. For each of the temperatures, the sensitivity was determined on the basis of a sinusoidal waveform. Thus, the linearity of the transfer characteristics was checked. The main purpose of the experiment was to check the limits of sensitivity changes in changing thermal conditions and whether the sensor meets the standards used in the power industry. Full Text: PDF References M.J. Weber, "CRC Handbook of Laser Science and Technology. Supplement 2: Optical Materials", CRC Press, Boca Raton, FL, USA (1995). CrossRef S. Ju, Y. Lee, Y.T. Ryu, S.G. Kang, J. Kim, P.R. Watekar, B.H. Kim, Y. Lee, Y.H. An, C.J. Kim, W.T. Han, "Temperature Dependence of Faraday Rotation of Glass Optical Fibers Doped with Quantum Dots of CdSe and CdMnTe", Phys. Status Solidi A, 216: 1800549 (2019). CrossRef E. Lage, L. Beran, A.U. Quindeau, L. Ohnoutek, M. Kucera, R. Antos, S. R. Sani, G.F. Dionne, M. Veis, C.A. Ross; Temperature-dependent Faraday rotation and magnetization reorientation in cerium-substituted yttrium iron garnet thin films. APL Mater. 5, 036104 (2017). CrossRef S. Ju, J. Kim, K. Linganna, P.R. Watekar, S.G. Kang, B.H. Kim, S. Boo, Y. Lee, Y.H. An, C.J. Kim, et al. "Temperature and Vibration Dependence of the Faraday Effect of Gd2O3 NPs-Doped Alumino-Silicate Glass Optical Fiber". Sensors, 18, 988 (2018). CrossRef A. Jeffrey, D. and R.M. Bunch, "Temperature dependence of the Faraday rotation of Hoya FR-5 glass," Appl. Opt. 23, 633 (1984). CrossRef K. Barczak, J. Juraszek, "Optoelectronic system for detecting short-circuits in low voltage networks", Photon. Lett. Pol, 14, 37 (2022). CrossRef K. Barczak, K. Mazniewski, "Optical fiber current sensor with external conversion for measurements of low AC electric currents", Proc. SPIE, 10455 (2017). CrossRef

Elucidating Individual Magnetic Contributions in Bi-Magnetic Fe3 O4 /Mn3 O4 Core/Shell Nanoparticles by Polarized Powder Neutron Diffraction.

Heterogeneous bi-magnetic nanostructured systems have had a sustained interest during the last decades owing to their unique magnetic properties and the wide range of derived potential applications. However, elucidating the details of their magnetic properties can be rather complex. Here, a comprehensive study of Fe3 O4 /Mn3 O4 core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented. The results show that while at low fields the Fe3 O4 and Mn3 O4 magnetic moments averaged over the unit cell are antiferromagnetically coupled, at high fields, they orient parallel to each other. This magnetic reorientation of the Mn3 O4 shell moments is associated with a gradual evolution with the applied field of the local magnetic susceptibility from anisotropic to isotropic. Additionally, the magnetic coherence length of the Fe3 O4 cores shows some unusual field dependence due to the competition between the antiferromagnetic interface interaction and the Zeeman energies. The results demonstrate the great potential of the quantitative analysis of polarized neutron powder diffraction for the study of complex multiphase magnetic materials.

Reprogrammable magnetization pattern and shape morphing of phase-change magnetic soft composites

Magnetic soft composites with programmable magnetization patterns are highly desirable for diverse applications due to their multimodal magnetic response and locomotion. Despite substantial progress achieved in magnetic programming approaches, reprogrammable magnetization using low magnetic fields is still in development and challenging. Here, we report a one-step multi-direction magnetic reorientation strategy capable of reprogramming magnetization patterns in soft composites consisting of an elastomer matrix and magnetized ferromagnetic microparticles encapsulated by a phase-change polymer. Our approach is first based on the overall heating of these soft composites to achieve polymer phase transition, and then use magnet arrays to locally regulate the magnetic field distribution to achieve magnetic reorientation and programmable magnetization. Importantly, this approach allows us to reconstruct the desired magnetization patterns by reheating and redesigning the applied magnetic field. Using this approach, we numerically and experimentally demonstrate that multimodal magnetization and deformation patterns of millimeter-level strip-shaped and multi-arm phase-change soft composites can be achieved without changing the composite structure and the externally applied magnetic field. Furthermore, we demonstrate its functional applications in developing a multistate circuit switching, as well as a flexible magnetic recording that can present a variety of information such as numbers, letters, and face-based patterns.

Magnetic domain-dependent ultrafast optical demagnetization in stripe domain films

We investigated femto- and picosecond-time magnetization dynamics in a ferromagnetic Ni80Fe20 film with varying thicknesses (wedge-shaped film). We observed that the thickness gradient strongly affects the magnetic moment distribution, causing a magnetization reorientation from in-plane to out-of-plane, and formatting a stripe domain (SD) at the thicker end of the wedge. The magnetization dynamics measurements reveal that the part of the film displaying SDs follows a substantially faster demagnetization and magnetization recovery and smaller magnetization quenching compared to the in-plane domain film. The experiments and micromagnetic simulations support that the decrease in relaxation time is caused by a magnetic anisotropy of the films introduced by SD formation. Our results point out that the micromagnetic structure plays an important role in the magnetization dynamics in ferromagnetic films after optically triggered demagnetization.

Field-free spin–orbit torque switching of an antiferromagnet with perpendicular Néel vector

The field-free spin–orbit torque induced 180° reorientation of magnetization is beneficial for the high performance magnetic memory. The antiferromagnetic material (AFM) can provide a higher operation speed than the ferromagnetic counterpart. In this paper, we propose a trilayer AFM/insulator/heavy metal structure as the AFM memory device. We show that the field-free switching of the AFM with a perpendicular Néel vector can be achieved by using two orthogonal currents, which provide a uniform damping-like torque and uniform field-like torque, respectively. The reversible switching can be obtained by reversing either current. A current density of 1.79 × 1011 A/m2 is sufficient to induce the switching. In addition, the two magnetic moments become noncollinear during switching. This enables an ultrafast switching within 40 ps. The device and switching mechanism proposed in this work offer a promising approach to deterministically switch the AFM with the perpendicular Néel vector. It can also stimulate the development of an ultrafast AFM-based MRAM.

Memory of frozen and rotatable antiferromagnetic spins in epitaxial CoO(1 1 1)/Fe and NiO(1 1 1)/Fe bilayers

We combined X-ray magnetic linear and circular dichroism together with magnetooptic Kerr effect measurements in order to follow the magnetic properties of epitaxial CoO(111)/Fe(110) and NiO(111)/Fe(110) bilayers. We find that in both studied cases ferromagnetic sublayer plays a dominant role and determines the magnetic state of the neighboring antiferromagnet, however different interaction scenarios are observed. In CoO/Fe bilayers the antiferromagnetic spins are frozen and their orientation is imprinted by magnetization of Fe layer as the system passes the Néel temperature of CoO. For NiO/Fe bilayers, the antiferromagnetic spins are rotatable and always follow the reorientation of Fe magnetization that can be controlled by external magnetic field or via the temperature and thickness driven spin reorientation transition in Fe(110).

Spin-wave nonreciprocity at the spin-flop transition region in synthetic antiferromagnets

We investigate the frequency nonreciprocity in CoFeB/Ru/CoFeB synthetic antiferromagnets near the spin-flop transition region, where the magnetic moments in the two ferromagnetic layers are noncollinear. Using conventional Brillouin light scattering, we perform systematic measurements of the frequency nonreciprocity as a function of an external magnetic field. At near zero magnetic field, where the antiparallel alignment of the magnetic moments in the two layers is established, we observe a significant frequency nonreciprocity of up to a few GHz, which vanishes when the relative magnetization orientation switches into the parallel configuration at a large magnetic field. For the intermediate values of the magnetic field, where the system transitions from the antiparallel to the parallel orientation, a nonmonotonous dependence of the frequency nonreciprocity is found, with a maximum frequency shift around the spin-flop critical point. This nontrivial dependence of the nonreciprocity versus field is attributed to the nonmonotonous dependence of the dynamic dipolar interaction, which is the main factor that causes asymmetry in the dispersion relation. Furthermore, we found that the sign of the frequency shift changes even without switching the polarity of the bias field. These results show that one can precisely control the nonreciprocal propagation of spin waves via field-driven magnetization reorientation.

Magnetic and elastic oscillations in manganese-zinc spinel crystal in the region of spin reorientation

The formula for the magnetic dissipation parameter of ferrimagnetic crystals depending on the constant magnetic field and the anisotropy field was obtained. It was shown that in the region of magnetic reorientation in a manganese-zinc spinel crystal, magnetic and elastic vibrations of infinitely small amplitude and low frequency are excited, which are characterized by giant damping in a wide temperature range.

Magnetic-field modulation of topological electronic state and emergent magneto-transport in a magnetic Weyl semimetal

The modulation of topological electronic state by an external magnetic field is highly desired for condensed-matter physics. Schemes to achieve this have been proposed theoretically, but few can be realized experimentally. Here, combining transverse transport, theoretical calculations, and scanning tunneling microscopy/spectroscopy (STM/S) investigations, we provide an observation that the topological electronic state, accompanied by an emergent magneto-transport phenomenon, was modulated by applying magnetic field through induced non-collinear magnetism in the magnetic Weyl semimetal EuB6. A giant unconventional anomalous Hall effect (UAHE) is found during the magnetization re-orientation from easy axes to hard ones in magnetic field, with a UAHE peak around the low field of 5 kOe. Under the reasonable spin-canting effect, the folding of the topological anti-crossing bands occurs, generating a strong Berry curvature that accounts for the observed UAHE. Field-dependent STM/S reveals a highly synchronous evolution of electronic density of states, with a dI/dV peak around the same field of 5 kOe, which provides evidence to the folded bands and excited UAHE by external magnetic fields. This finding elucidates the connection between the real-space non-collinear magnetism and the k-space topological electronic state and establishes a novel manner to engineer the magneto-transport behaviors of correlated electrons for future topological spintronics.