Magnetic Anisotropy Switching Research Articles

Laser-driven first-order spin reorientation and Verwey phase transitions in magnetite Fe3O4 beyond the range of thermodynamic equilibrium

Ultrafast photo-induced phase transitions occurring under the impact of femtosecond laser pulses provide versatile opportunities for switching solids between distinctly-different crystalline, electronic, and magnetic states and thus modify their functional properties in a significant way. In this paper, we report on the laser-induced spin reorientation and Verwey phase transitions in a single crystalline ferrimagnetic magnetite ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. Using femtosecond optical and magneto-optical pump-probe techniques, we define the range of the initial sample temperatures and laser fluences when partial or complete photo-induced phase transitions occur from a monoclinic insulating to a cubic metallic state with concomitant switching of magnetic anisotropy from the uniaxial to the cubic one. We thus reveal a connection between these phase transitions when driven by femtosecond laser pulses. Using transient linear and quadratic magneto-optical effects, we examine magnetization dynamics launched the switching of the magnetic anisotropy axis. We unveil the presence of the domains undergoing the laser-induced phase transitions even below the established threshold fluence for the transitions, as well as when the material is initially in the cubic phase. This is the manifestation of the first order of these both laser-induced phase transitions beyond the range of thermodynamic equilibrium.

Band shifting and magnetic anisotropy switching induced by electric field in CrI3/1T′-MX2 heterojunction

Manipulating magnetism by the electric field (EF) is a well-established technology that has been applied in two-dimensional materials systems. Here, based on first-principles calculations, we study the influence of the EF on the band structure and magnetic characteristics of the CrI3/1T′-MX2 heterojunction. The results show amounts of charge exchange at the interface accompanying an obvious band shifting at the Γ point around the Fermi level. The magnetic moment of CrI3/1T′-MX2 is visibly increased under an external EF relative to isolated CrI3. Particularly, the magnetic anisotropy of CrI3/1T′-MX2 switching from out-of-plane to in-plane can be modulated by the EF, while this change does not occur in isolated CrI3. In addition, the topological properties of the heterojunction are still preserved by a nontrivial topological invariant 2 = 1. Our findings provide theoretical guidance for the spintronic application of CrI3/1T′-MX2 materials.

Magnetic anisotropy manipulation and interfacial coupling in Sm3Fe5O12 films and CoFe/Sm3Fe5O12 heterostructures* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0303603 and 2016YFA0401803), the National Natural Science Foundation of China (Grant Nos. U2032218, 11574316, 11874120, 61805256, and 11904367), the Plan for Major Provincial Science & Technology Project (Grant No. 202003a05020018), and the Key Research

The magnetic anisotropy manipulation in the Sm3Fe5O12 (SmIG) films and its effect on the interfacial spin coupling in the CoFe/SmIG heterostructures were studied carefully. By switching the orientation of the Gd3Ga5O12 substrates from (111) to (001), the magnetic anisotropy of obtained SmIG films shifts from in-plane to out-of-plane. Similar results can also be obtained in the films on Gd3Sc2Ga3O12 substrates, which identifies the universality of such orientation-induced magnetic anisotropy switching. Additionally, the interfacial spin coupling and magnetic anisotropy switching effect on the spin wave in CoFe/SmIG magnetic heterojunctions have also been explored by utilizing the time-resolved magneto–optical Kerr effect technique. It is intriguing to find that both the frequency and effective damping factor of spin precession in CoFe/SmIG heterojunctions can be manipulated by the magnetic anisotropy switching of SmIG films. These findings not only provide a route for the perpendicular magnetic anisotropy acquisition but also give a further path for spin manipulation in magnetic films and heterojunctions.

Engineering new limits to magnetostriction through metastability in iron-gallium alloys

Magnetostrictive materials transduce magnetic and mechanical energies and when combined with piezoelectric elements, evoke magnetoelectric transduction for high-sensitivity magnetic field sensors and energy-efficient beyond-CMOS technologies. The dearth of ductile, rare-earth-free materials with high magnetostrictive coefficients motivates the discovery of superior materials. Fe1−xGax alloys are amongst the highest performing rare-earth-free magnetostrictive materials; however, magnetostriction becomes sharply suppressed beyond x = 19% due to the formation of a parasitic ordered intermetallic phase. Here, we harness epitaxy to extend the stability of the BCC Fe1−xGax alloy to gallium compositions as high as x = 30% and in so doing dramatically boost the magnetostriction by as much as 10x relative to the bulk and 2x larger than canonical rare-earth based magnetostrictors. A Fe1−xGax − [Pb(Mg1/3Nb2/3)O3]0.7−[PbTiO3]0.3 (PMN-PT) composite magnetoelectric shows robust 90° electrical switching of magnetic anisotropy and a converse magnetoelectric coefficient of 2.0 × 10−5 s m−1. When optimally scaled, this high coefficient implies stable switching at ~80 aJ per bit.

Bandgap opening and magnetic anisotropy switching by uniaxial strain in graphene/CrI3 heterojunction

Two-dimensional van der Waals heterojunctions are significant building blocks for optoelectronic and spintronic devices. By first-principle calculations, we study the influence of uniaxial strain on the bandgap and magnetism of graphene/CrI3 heterojunction. The results show that the bandgap size of graphene in heterojunction can reach up to 0.5 eV, and it increases more obvious at the compressive strain relative to the isolated graphene resulting from stronger interlayer proximity exchange. Interestingly, we find that the proximity exchange can induce charge accumulation like a dumbbell between the nearest neighbor C–C atoms along the armchair direction. Unexpectedly, the uniaxial strain induces the magnetic anisotropy switching from out-of-plane to in-plane, while this change does not occur in isolated CrI3. Our findings can provide theoretical guidance for the application of graphene/CrI3 materials.

Large magnetoelectric coupling in multiferroic oxide heterostructures assembled via epitaxial lift-off

Epitaxial films may be released from growth substrates and transferred to structurally and chemically incompatible substrates, but epitaxial films of transition metal perovskite oxides have not been transferred to electroactive substrates for voltage control of their myriad functional properties. Here we demonstrate good strain transmission at the incoherent interface between a strain-released film of epitaxially grown ferromagnetic La0.7Sr0.3MnO3 and an electroactive substrate of ferroelectric 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 in a different crystallographic orientation. Our strain-mediated magnetoelectric coupling compares well with respect to epitaxial heterostructures, where the epitaxy responsible for strong coupling can degrade film magnetization via strain and dislocations. Moreover, the electrical switching of magnetic anisotropy is repeatable and non-volatile. High-resolution magnetic vector maps reveal that micromagnetic behaviour is governed by electrically controlled strain and cracks in the film. Our demonstration should inspire others to control the physical/chemical properties in strain-released epitaxial oxide films by using electroactive substrates to impart strain via non-epitaxial interfaces.

Magnetic anisotropy switching induced by shape memory effect in NiTi/Ni bilayer

Strain modulation during a two-way shape memory effect (TWSME) in a sputtered nitinol NiTi is used to reliably induce and switch by 90° a uniaxial magnetic anisotropy of a 20 nm thick Ni film during the thermal cycle from 300 K to 400 K. NiTi strain and its distribution are carefully measured by digital image correlation during tensile prestrain and subsequent temperature cycles in order to compare with Ni strain extracted from the magnetometry measurement and from transmission electron microscopy. In a NiTi/Ni bilayer, a variation of 2.7% strain in NiTi during the TWSME generates 1.3% strain in Ni, which results in a transition from −2 × 104 J/m3 in-plane magnetic anisotropy to +1 × 105 J/m3. Such a composite system offers a way to timely ease writability while maintaining high thermal stability at rest in magnetic media.

In Operando Study of the Hydrogen-Induced Switching of Magnetic Anisotropy at the Co/Pd Interface for Magnetic Hydrogen Gas Sensing.

Heterostructures exhibiting perpendicular magnetic anisotropy (PMA) have traditionally served the magnetic recording industry. However, an opportunity exists to expand the applications of PMA heterostructures into the realm of hydrogen sensing using ferromagnetic resonance (FMR) by exploiting the hydrogen-induced modifications to PMA that occur at the interface between Pd and a ferromagnet. Here, we present the first in operando depth-resolved study of the in-plane interfacial magnetization of a Co/Pd film which features tailorable PMA in the presence of hydrogen gas. We combine polarized neutron reflectometry with in situ FMR to explore how the absorption of hydrogen at the Co/Pd interface affects the heterostructures spin-resonance condition during hydrogen cycling. Experimental data and modeling reveal that the Pd layer expands when exposed to hydrogen gas, while the in-plane magnetic moment of the Co/Pd film increases as the interfacial PMA is reduced to affect the FMR frequency. This work highlights a potential route for magnetic hydrogen gas sensing.

In-plane to perpendicular magnetic anisotropy switching in heavily-Fe-doped ferromagnetic semiconductor (Ga,Fe)Sb with high Curie temperature

We report switching of magnetic anisotropy (MA) from in-plane to perpendicular with increasing the thickness d of a (001)-oriented ferromagnetic-semiconductor (FMS) (Ga0.7,Fe0.3)Sb layer with a high Curie temperature (Tc > 320 K), using ferromagnetic resonance at room temperature. We show that the total MA energy (E) along the [001] direction changes its sign from positive (in-plane) to negative (perpendicular) with increasing d above an effective critical value \mathrm{d}_\mathrm{C}^\mathrm{*}\ ~ 42 nm. We reveal that (Ga,Fe)Sb has two-fold symmetry in the film plane. Meanwhile, in the plane perpendicular to the film including the in-plane [110] axis, the two-fold symmetry with the easy magnetization axis along [110] changes to four-fold symmetry with easy magnetization axis along <001> with increasing d. This peculiar behavior is different from that of (Ga,Mn)As, in which only the in-plane MA depends on the film thickness and has four-fold symmetry due to its dominant cubic anisotropy along the <100> axes. This work provides an important guide for controlling the easy magnetization axis of high-Tc FMS (Ga,Fe)Sb for room-temperature device applications.

The contribution of distinct response characteristics of Fe atoms to switching of magnetic anisotropy in Fe4N/MgO heterostructures

The modulation of magnetic anisotropy is very promising for the realization of energy-efficient memory devices. In this work, we investigate the effects of interfacial oxidation and electric field on the magnetic anisotropy of the Fe4N/MgO heterostructure using first-principles calculations. The Fe4N/MgO heterostructure exhibits in-plane magnetic anisotropy, while interfacial oxidation induces a perpendicular magnetic anisotropy. In addition, the electric field-controlled switching of magnetic anisotropy is achieved. The magnetic anisotropy of FeI and FeII atoms in Fe4N has distinct response characteristics to interfacial oxidation and electric field, where the FeII atom is more sensitive and its magnetic anisotropy is reversed from the in-plane to the out-of-plane direction. This suggests the tunability of magnetic anisotropy in the Fe4N/MgO heterostructure, which offers the possibility for further application of the Fe4N-based magnetic tunnel junction.

Low-Voltage Control of (Co/Pt) x Perpendicular Magnetic Anisotropy Heterostructure for Flexible Spintronics.

The trend of mobile Internet requires portable and wearable devices as bio-device interfaces. Electric field control of magnetism is a promising approach to achieve compact, light-weight, and energy-efficient wearable devices. Within a flexible sandwich heterostructure, perpendicular magnetic anisotropy switching was achieved via low-voltage gating control of an ionic gel in mica/Ta/(Pt/Co) x/Pt/ionic gel/Pt, where (Pt/Co) x acted as a functional layer. By conducting in situ VSM, EPR, and MOKE measurements, a 1098 Oe magnetic anisotropy field change was determined at the bending state with tensile strain, corresponding to a magnetic anisotropy energy change of 3.16 × 105 J/m3 and a giant voltage tunability coefficient of 0.79 × 105 J/m3·V. The low voltage and strain dual control of magnetism on mica substrates enables tunable flexible spintronic devices with an increased degree of manipulation.

Non-Volatile Ferroelectric Switching of Ferromagnetic Resonance in NiFe/PLZT Multiferroic Thin Film Heterostructures.

Magnetoelectric effect, arising from the interfacial coupling between magnetic and electrical order parameters, has recently emerged as a robust means to electrically manipulate the magnetic properties in multiferroic heterostructures. Challenge remains as finding an energy efficient way to modify the distinct magnetic states in a reliable, reversible, and non-volatile manner. Here we report ferroelectric switching of ferromagnetic resonance in multiferroic bilayers consisting of ultrathin ferromagnetic NiFe and ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT) films, where the magnetic anisotropy of NiFe can be electrically modified by low voltages. Ferromagnetic resonance measurements confirm that the interfacial charge-mediated magnetoelectric effect is dominant in NiFe/PLZT heterostructures. Non-volatile modification of ferromagnetic resonance field is demonstrated by applying voltage pulses. The ferroelectric switching of magnetic anisotropy exhibits extensive applications in energy-efficient electronic devices such as magnetoelectric random access memories, magnetic field sensors, and tunable radio frequency (RF)/microwave devices.

Switching of magnetic anisotropy in a fcc antiferromagnet with direction-dependent interactions

Direction-dependent anisotropic exchange is a common feature of magnetic systems with strong spin-orbit coupling. Here we study the effect of such exchange upon macroscopic magnetic anisotropy for a face-centered-cubic model. By several theoretical techniques, we show that, both in the paramagnetic and ordered phases, the magnetic anisotropy is induced by fluctuations. Moreover, the magnetic anisotropy differs in the paramagnetic and ordered phases: in the paramagnetic phase the susceptibility is maximum along the $\langle111\rangle$ directions, while the magnetic moments orient along $\langle110\rangle$ or $\langle100\rangle$ in the ordered phase. We suggest that such "anisotropy switching" can be a common feature of strongly spin-orbit coupled magnets.

Modification of structure and magnetic anisotropy of epitaxial CoFe2O4 films by hydrogen reduction

Heteroepitaxial CoFe2O4 (CFO) thin films with different thicknesses were deposited on MgO (001) substrates. The as-deposited CFO films show a clear switching of magnetic anisotropy with increasing film thickness. The thinner films (&amp;lt;100 nm) show a perpendicular magnetic anisotropy due to the out-of-plane compressive strain. The thicker films exhibit an in-plane easy axis owing to the dominating shape anisotropy effect. The magnetostriction coefficient of CFO films is estimated to be λ[001] = −188 × 10−6. Metallic CoFe2 films were obtained by annealing the as-deposited CFO films in forming gas (Ar 93% + H2 7%) at 450 °C. XRD shows that CoFe2 films are textured out-of-plane and aligned in-plane, owing to lattice matching between CoFe2 and MgO substrate. TEM results indicate that as-deposited films are continuous while the annealed films exhibit a nanopore mushroom structure. The magnetic anisotropy of CoFe2 films is dominated by the shape effect. The results demonstrate that hydrogen reduction can be effectively used to modify microstructures and physical properties of complex metal oxide materials.

Electric field controlled reversible magnetic anisotropy switching studied by spin rectification

In this letter, spin rectification was used to study the electric field controlled dynamic magnetic properties of the multiferroic composite which is a Co stripe with induced in-plane anisotropy deposited onto a Pb(Mg1∕3Nb2∕3)O3-PbTiO3 substrate. Due to the coupling between piezoelectric and magnetoelastic effects, a reversible in-plane anisotropy switching has been realized by varying the history of the applied electric field. This merit results from the electric hysteresis of the polarization in the nonlinear piezoelectric regime, which has been proved by a butterfly type electric field dependence of the in-plane anisotropy field. Moreover, the electric field dependent effective demagnetization field and linewidth have been observed at the same time.

Switching of Magnetic Anisotropy of a Single Magnetic Molecule Observed by Scanning Tunneling Microscopy(Research)

Switching of Magnetic Anisotropy of a Single Magnetic Molecule Observed by Scanning Tunneling Microscopy(Research)

Diode-MTJ Crossbar Memory Cell Using Voltage-Induced Unipolar Switching for High-Density MRAM

This letter presents a diode-magnetic tunnel junction (MTJ) magnetic random access memory cell in a 65-nm complimentary metal-oxide-semiconductor compatible process. A voltage-controlled magnetic anisotropy switching mechanism, in addition to STT, allows for a unipolar set/reset write scheme, where voltage pulses of the same polarity, but different amplitudes, are used to switch the MTJs. A small crossbar array is constructed from 65-nm MTJs fabricated on a silicon wafer, with switching voltages ~ 1 V and thermal stability greater than 10 years, with discrete germanium diodes as access devices to allow for read/write operations. The crossbar architecture can be extended to multiple layers to create a 3-D stackable, nonvolatile memory with a sub-1F <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> effective cell size.

Magnetism of Iron (II) Phthalocyanine at Surfaces -Spin, Magnetic Anisotropy and Kondo Effect-

Two topics are presented about the magnetic properties of iron (II) phthalocyanine (FePc) adsorbed on metal substrates. The spin state and switching of magnetic anisotropy of FePc on Cu(110) and Cu(110)(2×1)-O are discussed based on the spin excitation spectra and the magnetic field evolution in the first topic. In the second topic, the formation of Kondo resonance state is described for FePc on Au(111) and the spectral evolution from an isolated molecule to the cluster and the Kondo lattice is discussed by the competition of the Kondo effect and antiferromagnetic Rudermann-Kittel-Kasuya-Yosida coupling between the molecular spins.

Switching of magnetic anisotropy in epitaxial CoFe2O4 thin films induced by SrRuO3 buffer layer

The magnetic anisotropy of epitaxial spinel ferrite CoFe2O4 films grown on SrTiO3 substrates by pulsed laser deposition can be reoriented by inserting a thin SrRuO3 buffer layer. Without SrRuO3, the CoFe2O4 films show a uniaxial anisotropy with the easy axis perpendicular to the film plane, while inserting a SrRuO3 buffer layer results in the switching of the easy axis into the in-plane orientation. This is associated with a tensile and a compressive strain for the films without and with buffer layer, respectively, which is also correlated with the different density of interfacial misfit dislocations. As a result, the ferrite films can be effectively tailored by using SrRuO3.

Adsorption-Induced Switching of Magnetic Anisotropy in a Single Iron(II) Phthalocyanine Molecule on an Oxidized Cu(110) Surface

We examined the zero-field splitting of an iron(II) phthalocyanine (FePc) attached to clean and oxidized Cu(110) surfaces and the dependence on an applied magnetic field by inelastic electron tunneling spectroscopy with STM. The symmetry of the ligand field surrounding the Fe atom is lowered on the oxidized surface, switching the magnetic anisotropy from the easy plane of the bulk to the easy axis. The zero-field splitting was not observed for FePc on a clean Cu(110) surface, and the spin state converts from triplet to singlet due to the strong coupling of Fe d states with the Cu substrate, as is also confirmed by photoelectron spectroscopy. These findings demonstrate the importance of coupling at the molecule-substrate interface for manipulating the magnetic properties of adsorbates.