Rotatable Magnetic Anisotropy Research Articles

Viscous magnetization decrease in first-order reversal curves induced by rotatable magnetic anisotropy in polycrystalline exchange-biased bilayers

A prototypical polycrystalline in-plane exchange bias system exhibited a viscous decrease of the ferromagnetic magnetization upon increasing external magnetic field when measuring first-order reversal curves. The phenomenon is investigated by means of angular-resolved vectorial magneto-optical Kerr magnetometry complemented by fits of model calculations and by Kerr microscopy. It is found that the viscous magnetization decrease is mediated by a rotatable magnetic anisotropy arising from thermally unstable antiferromagnetic grains coupled to the probed ferromagnet. This additionally manifests itself by a creeping domain wall motion in the ferromagnet due to thermally activated processes in the antiferromagnet. The investigations are in agreement with a generalized description of polycrystalline exchange bias systems and emphasize the relevance of understanding minor loop behavior addressing nonsaturated magnetic states for systems susceptible to dynamic changes on the hysteresis loop timescale.

Modulation of interfacial magnetic relaxation timeframes by partially uncoupled exchange bias

A set of partially uncoupled NiFe/Cu/IrMn exchange biased thin films with variable thickness of non-magnetic Cu spacer is characterized by ferromagnetic resonance (FMR) and Brillouin light scattering (BLS) techniques applied complementary to reveal time-scale dependent effects of uncoupling between ferromagnetic and antiferromagnetic layers on high-frequency magnetization dynamics. The results correlate with interfacial grain texture variations and static magnetization behavior. Two types of crystalline phases with correlated microwave response are revealed at the ferro–antiferromagnet interface in NiFe/Cu/IrMn thin films. The 1st phase forms well-textured NiFe/IrMn grains with NiFe (111)/IrMn (111) interface. The 2nd phase consists of amorphous NiFe/IrMn grains. Intercalation of NiFe/IrMn by Cu clusters results in relaxation of tensile strains at the NiFe/IrMn interface leading to larger size of grains in both the NiFe and IrMn layers. The contributions of well-textured and amorphous grains to the high-frequency magnetization reversal behavior are distinguished by FMR and BLS techniques. Generation of a spin-wave mode is revealed in the well-textured phase, whereas microwave response of the amorphous phase is found to originate from magnetization rotation dominated by a rotatable magnetic anisotropy term. Under fixed FMR frequency, the increase of Cu thickness results in higher magnetization rotation frequencies in the amorphous grains.

Non-volatile voltage control of in-plane and out-of-plane magnetization in polycrystalline Ni films on ferroelectric PMN–PT (001)pc substrates

We identify room-temperature converse magnetoelectric effects (CMEs) that are non-volatile by using a single-crystal substrate of PMN–PT (001)pc (pc denotes pseudocubic) to impart voltage-driven strain to a polycrystalline film of Ni. An appropriate magnetic-field history enhances the magnetoelectric coefficient to a near-record peak of ∼10−6 s m−1 and permits electrically driven magnetization reversal of substantial net magnetization. In zero magnetic field, electrically driven ferroelectric domain switching produces large changes of in-plane magnetization that are non-volatile. Microscopically, these changes are accompanied by the creation and destruction of magnetic stripe domains, implying the electrical control of perpendicular magnetic anisotropy. Moreover, the stripe direction can be rotated by a magnetic field or an electric field, the latter yielding the first example of electrically driven rotatable magnetic anisotropy. The observed CMEs are associated with repeatable ferroelectric domain switching that yields a memory effect. This memory effect is well known for PMN–PT (110)pc but not PMN–PT (001)pc. Given that close control of the applied field is not required as for PMN–PT (110)pc, this memory effect could lead the way to magnetoelectric memories based on PMN–PT (001)pc membranes that switch at low voltage.

Solid-state synthesis, rotatable magnetic anisotropy and characterization of Co1-xPtx phases in 50Pt/50fccCo(001) and 32Pt/68fccCo(001) thin films

We reported the phase formation sequences in 50Pt/50fcc-Co(001) and 32Pt/68fcc-Co(001) thin films after annealing up to 850 °C. In both cases, the ordered L10 phase formed first on the Pt/Co interface at ∼400 °C and as the annealing temperature increased the L10 phase transformed into the chemically disordered fcc A1 phase in 50Pt/50fcc-Co(001) at 750 °C and in 32Pt/68fcc-Co(001) films at 550 °C. Based on the analysis of solid-state reactions in thin films, a phase transition at ∼ 400 °C is predicted in Co-Pt systems with a 32–72% Pt composition. Torque measurements of the 50Pt/50fcc-Co(001) samples showed that the rotatable magnetic anisotropy coexisted with the three variants of L10 in a temperature range of 400–750 °C. An analysis of the torque curves revealed that the L10 films consist of a soft magnetic layer epitaxially intergrown to the substrate MgO(001) and a top layer having rotatable magnetic anisotropy. It showed that the magnetically hard properties of L10 films are associated with a rotatable magnetic anisotropy layer. A model of rotatable magnetic anisotropy is reasoned, which is founded on some identical mechanisms of rotatable magnetic anisotropy and magnetic-field-induced strains, explaining the ferromagnetic shape-memory effect in Heusler alloys. Our results suggested that the rotatable magnetic anisotropy phenomena may have an important role in the origin of perpendicular anisotropy in hard magnetic L10 structures.

Tailoring magnetism in self-intercalated Cr1+δTe2 epitaxial films

Magnetic transition metal dichalcogenide (TMD) films have recently emerged as promising candidates to host novel magnetic phases relevant to next-generation spintronic devices. However, systematic control of the magnetization orientation, or anisotropy, and its thermal stability, characterized by Curie temperature (Tc) remains to be achieved in such films. Here we present self-intercalated epitaxial Cr1+{\delta}Te2 films as a platform for achieving systematic/smooth magnetic tailoring in TMD films. Using a molecular beam epitaxy (MBE) based technique, we have realized epitaxial Cr1+{\delta}Te2 films with smoothly tunable over a wide range (0.33-0.82), while maintaining NiAs-type crystal structure. With increasing {\delta}, we found monotonic enhancement of Tc from 160 to 350 K, and the rotation of magnetic anisotropy from out-of-plane to in-plane easy axis configuration for fixed film thickness. Contributions from conventional dipolar and orbital moment terms are insufficient to explain the observed evolution of magnetic behavior with {\delta}. Instead, ab initio calculations suggest that the emergence of antiferromagnetic interactions with {\delta}, and its interplay with conventional ferromagnetism, may play a key role in the observed trends. To our knowledge, this constitutes the first demonstration of tunable Tc and magnetic anisotropy across room temperature in TMD films, and paves the way for engineering novel magnetic phases for spintronic applications.

Tunable high-frequency properties of flexible FeGa films with rotatable stripe domain

A comprehensive investigation of stress-induced anisotropy in 300 nm Fe81Ga19 (FeGa) thin films with stripe domain structures has been performed by comparing static and dynamic measurements. The formation and rotation of stripe domains under a compressive stress have been imaged by magnetic force microscopy and understood by micromagnetic simulations. A double magnetic anisotropy system, combing the stress-induced uniaxial magnetic anisotropy and the rotatable magnetic anisotropy caused by the stripe domain structure was achieved in FeGa thin films. The ferromagnetic resonance frequency of 300 nm FeGa thin films with stripe domains can be enhanced from 2.7 to 3.0 GHz without sacrificing the initial permeability. This found provides us a new approach via utilizing the double magnetic anisotropies in magnetic films with stripe domains to enhance the resonance frequency beyond the Snoek’s limit and extends the application for FeGa films to flexible microwave devices.

CoPt–Al2O3 Nanocomposite Films: Synthesis, Structure, and Magnetic Properties

The structure and magnetic properties of CoPt–Al2O3 nanocomposite films synthesized by the annealing of Al/(Co3O4 + Pt) bilayers on a MgO(001) substrate at 650°C in vacuum are investigated. The synthesized composite films contain ferromagnetic CoPt grains with an average size of 25–45 nm enclosed in a nonconducting Al2O3 matrix. The saturation magnetization (Ms ~ 330 G) and coercivity (Hc ≈ 6 kOe) of the films are measured in the film plane and perpendicular to it. The obtained films are characterized by a spatial rotational magnetic anisotropy, which makes it possible to arbitrarily set the easy magnetization axis in the film plane or perpendicular to it using a magnetic field stronger than the coercivity (H > Hc).

Preferential weakening of rotational magnetic anisotropy by keV-He ion bombardment in polycrystalline exchange bias layer systems

The consequences of keV light-ion bombardment of polycrystalline exchange bias layer systems on individual magnetic anisotropies composing the effective unidirectional magnetic anisotropy are investigated by vectorial magneto-optic Kerr effect measurements. Preferential reduction of rotatable magnetic anisotropy as compared to the other magnetic anisotropies is observable, disproving the intuitive assumption of an equable weakening of all magnetic anisotropies due to the bombardment. It is concluded that light-ion bombardment has a stronger impact on the magnetic characteristics of smaller grains in a polycrystalline antiferromagnet as compared to those of larger ones.

Rotational magnetic anisotropy in polycrystalline FePt films fabricated by solid-state synthesis

It is shown that annealing 550°C of the Fe/Pt bilayer films fabricated by layer-by-layer deposition onto polycrystalline Al2O3 substrates in an atomic ratio of Fe: Pt = 71: 29 leads to the formation of the inplane rotational magnetic anisotropy with the easy axis that can be aligned by magnetic field in any direction in the film plane. The increase in the Pt content in the investigated film system to 50 at % leads to an increase in the anisotropy constant to 6.5 × 106 erg/cm3, which exceeds the value in the Fe71Pt29 sample by a factor of 6. In addition to the rotational anisotropy, the Fe50Pt50 films are characterized by the perpendicular rotational magnetic anisotropy, which is higher than the film shape anisotropy. Therefore, the easy axis in these samples can be aligned by magnetic field in any spatial direction. According to the X-ray and magnetic measurement data, the synthesized samples have a polycrystalline structure consisting of two ordered phases: magnetically hard L10–FePt and magnetically soft L12–Fe3Pt. This gives us grounds to suggest that the rotational magnetic anisotropy originates from the epitaxial intergrowth and exchange coupling of these two phases.

Магнитная вращающаяся анизотропия в поликристаллических пленках FePt, полученных методом твердофазного синтеза

AbstractIt is shown that annealing 550°C of the Fe/Pt bilayer films fabricated by layer-by-layer deposition onto polycrystalline Al_2O_3 substrates in an atomic ratio of Fe: Pt = 71: 29 leads to the formation of the inplane rotational magnetic anisotropy with the easy axis that can be aligned by magnetic field in any direction in the film plane. The increase in the Pt content in the investigated film system to 50 at % leads to an increase in the anisotropy constant to 6.5 × 10^6 erg/cm^3, which exceeds the value in the Fe_71Pt_29 sample by a factor of 6. In addition to the rotational anisotropy, the Fe50Pt50 films are characterized by the perpendicular rotational magnetic anisotropy, which is higher than the film shape anisotropy. Therefore, the easy axis in these samples can be aligned by magnetic field in any spatial direction. According to the X-ray and magnetic measurement data, the synthesized samples have a polycrystalline structure consisting of two ordered phases: magnetically hard L1_0–FePt and magnetically soft L1_2–Fe_3Pt. This gives us grounds to suggest that the rotational magnetic anisotropy originates from the epitaxial intergrowth and exchange coupling of these two phases.

High rotatable magnetic anisotropy in MnBi thin films

The variations of the structural and magnetic properties of Bi/Mn/Bi and Mn/Bi/Mn trilayer film systems of equiatomic composition in the process of vacuum annealing are studied. The annealing of Bi/Mn/Bi films at a temperature of 270°C for an hour results in the synthesis of the well-studied highly oriented low-temperature LT-MnBi(001) phase with the perpendicular magnetic anisotropy K u ∼ 1.1 × 107 erg/cm3 and coercivity H C ∼ 1.5 kOe. In contrast to Bi/Mn/Bi, polycrystalline LT-MnBi nanoclusters are formed in Mn/Bi/Mn films under the same annealing conditions. A high rotatable magnetic anisotropy exceeding the shape anisotropy is detected in the films under consideration: the easy axis of anisotropy with the inclusion of the delay angle in magnetic fields above the coercivity H > H C = 9.0 kOe can be oriented in any spatial direction. It is shown that the nature of rotatable magnetic anisotropy is due to the structural coexistence of epitaxially coupled LT-MnBi and QHTP-Mn1.08Bi phases. The reported experimental results indicate the existence of a new class of ferromagnetic film media with the spatially tunable easy axis.

Phase formation sequence, magnetic and structural development during solid-state reactions in 72Pt/28fcc-Co (001) thin films

Abstract The phase formation sequence during the thermally induced solid-state reaction between polycrystalline Pt and epitaxial fcc-Co (001) films in the Pt/fcc-Co(001) bilayers are systematically examined using X-ray diffraction and magnetic measurements. The films have nominal atomic ratio Co:Pt = 28:72 and total thickness 300–400 nm. Annealing to the temperature of 375 °C does not change the structural and magnetic properties of the films; this is indicative of the absence of considerable mixing at the Co/Pt interface. With the subsequent increase of the annealing temperature, the phase formation in the Pt/fcc-Co(001) bilayers has been found to have two temperature (375 °C–575 °C and 575 °C–825 °C) intervals. Solid-state reaction between Pt and Co starts above 375 °C, and nanoclusters containing the ordered L10 phase epitaxially intergrow with the disordered A1 phase of the composition CoPt3 form and exist in the first temperature interval. The distinctive feature of the first interval is the formation of in-plane rotatable magnetic anisotropy. In the second temperature interval, the (L10 + A1) two-phase mixture grows into the ordered L12-CoPt3 phase leading to the disappearance of rotatable anisotropy. Possible origin of the rotatable magnetic anisotropy is discussed. The first magnetocrystalline anisotropy constant of L12-CoPt3 has the maximum value of −5.0·105egr/cm3 and order parameter 0.55 at 675 °C. A careful analysis of thin film solid-state reactions implies the existence of low-temperature transformation (∼375 °C) on the Pt-rich side of the Co-Pt system.

Magnetic and structural properties of Fe87Pt13–Al2O3 composite thin films synthesized by solid-state reactions

The structural and magnetic properties of Fe87Pt13 films synthesized by solid-state reactions and Fe87Pt13–Al2O3 composite films fabricated by aluminothermy are investigated. It is shown that the synthesized samples of both types are characterized by the rotational magnetic anisotropy, when the easy magnetization axis in the film plane can be set by a magnetic field. It is established that the value of rotational magnetic anisotropy in the Fe87Pt13–Al2O3 composite films is higher than in the Fe87Pt13 samples by an order of magnitude. The rotational magnetic anisotropy is assumed to be caused by the exchange coupling of the L10–FePt phase with the L12–Fe3Pt phase in the Fe87Pt13 films and magnetic iron oxides in the Fe87Pt13–Al2O3 samples.

Time-dependent rotatable magnetic anisotropy in polycrystalline exchange-bias systems: Dependence on grain-size distribution

Angular resolved measurements of the exchange bias field and the coercive field are a powerful tool to distinguish between different competing magnetic anisotropies in polycrystalline exchange bias layer systems. No simple analytical model is as yet available, which considers time dependent effects like enhanced coercivity arising from the grain size distribution of the antiferromagnet. In this work we expand an existing model class describing polycrystalline exchange bias systems by a rotatable magnetic anisotropy term to describe grain size correlated effects. Additionally, we performed angular resolved magnetization curve measurements using vectorial magnetooptic Kerr magnetometry. Comparison of the experimental data with the proposed model shows excellent agreement and reveals the ferromagnetic anisotropy and properties connected to the grain size distribution of the antiferromagnet. Therefore, a distinction between the different influences on coercivity and anisotropy becomes available.

High rotatable magnetic anisotropy in epitaxial L10CoPt(111) thin films

The evolution of the structural and magnetic properties in epitaxial film systems Co/Pt(111) of equiatomic composition during vacuum annealing has been presented. Annealing to the temperature of 400°C does not lead to the variation of the structural and magnetic properties of the films, which indicates the absence of considerable mixing of the Co/Pt interface. With the increase in the annealing temperature from 400 to 750°C, nanoclusters containing the main magnetically hard L10CoPt(111) phase epitaxially intergrown with the CoPt3 phase are formed. High rotatable magnetic anisotropy has been found in the prepared films. In magnetic fields above the coercive force (H > HC = 8 kOe), the easy anisotropy axis with the angle of lag taken into account can be oriented in any spatial direction. Possible mechanisms of the formation of the rotatable magnetic anisotropy have been discussed. It has been assumed that the high rotatable magnetic anisotropy makes the main contribution to the magnetic perpendicular anisotropy in CoxPt1–x films.

High-frequency magnetic properties and their thermal stability in diluted IrMn–Al2O3/FeCo exchange-biased multilayers

In this work, we prepare and characterize FeCo/IrMn–Al2O3 multilayers using magnetron sputtering method with various doping concentrations in order to study the doping effect of the antiferromagnetic layer in exchange-biased films on their high frequency magnetic properties and their thermal stability for fundamental research as well as for application. The influences of the nonmagnetic Al2O3 doping in the antiferromagnetic layers on the high frequency magnetic properties from 1GHz up to 8GHz and their thermal stability of the FeCo/IrMn exchange-biased multilayer have been investigated in the temperature range from room temperature up to 420K. The temperature behavior of exchange bias, static magnetic anisotropy field, dynamic magnetic anisotropy field, rotatable magnetic anisotropy field, resonant frequency and damping of the samples are presented and interpreted by classifying the antiferromagnet grains into random spin grains, frozen spin grains and rotatable spin grains. In particular, the roles and changes of each grain type with the doping amount and with temperature are discussed in conjunction with the variations of the magnetic and microwave parameters.

Rotatable magnetic anisotropy in Si/SiO2/(Co2Fe)xGe1−x Heusler alloy films

Polycrystalline (Co2Fe)xGe1−x Heusler alloy films are fabricated by sputtering on amorphous substrates and shown to possess three types of magnetic anisotropy. The nearly stoichiometric composition of x = 50 m.f.% shows a rectangular hysteresis loop and isotropic coercive and ferromagnetic resonance fields when the film is field-magnetized along any in-plane direction, thus predominantly possessing rotatable in-plane magnetic anisotropy. Higher-x compositions show evidence of two- and fourfold in-plane anisotropy superposed on the rotatable one. A qualitative model of the observed anisotropic magnetic properties is proposed. The model explains the rotatable anisotropy by taking into account dry friction for the in-plane rotation of the magnetization direction in a fine-grained polycrystalline film with the magnetic grain size smaller than the correlation length of the inter-grain exchange interaction. The observed two- and fourfold magnetic anisotropy contributions are attributed to partial texturing of the fine-grained films, even though the films are grown on amorphous SiO2 substrates. These results should be valuable for understanding and controlling the magnetic behaviour of highly spin-polarized Heusler alloy films used in various magnetic nanodevices.

Fabrication and magnetic properties of nanostructured amorphous Nd–Co films with lateral modulation of magnetic stripe period

Amorphous Nd–Co films with perpendicular magnetic anisotropy have been nanostructured in a lateral magnetic multilayer geometry in order to analyse and modify in a controlled way the configuration of its characteristic stripe domains as well as their rotation processes. Magnetic force microscopy measurements reveal that, actually, the artificial thickness modulation results in size effects on the magnetic stripes, so that the stripe domains configuration can be tuned with the lateral multilayer periodicity, and, due to the consequent reduction of the rotatable magnetic anisotropy, it is possible to modify the stripe rotation processes for in-plane magnetization reversal.

High thermal stability of zero-field ferromagnetic resonance above 5 GHz in ferrite-doped CoFe thin films

Ferrite doped CoFe films with stripe domains are demonstrated to possess zero-field ferromagnetic resonance frequency above 5 GHz. The high resonance frequency is driven by the rotatable magnetic anisotropy propagated from the stripe domain structure and exchange coupling between rotatable ferrimagnetic spins and the ferromagnetic grains. The high temperature results show that these films have excellent thermal stability, which may have a great implication for microwave applications.

Thermal stability of exchange-biased NiFe/FeMn multilayered thin films

A systematic study of the effect of ferromagnetic thickness on magnetic and microwave properties of exchange-biased NiFe/FeMn multilayered thin films was carried out with regards to thermal stability. The temperature-dependent microwave characteristics of the films were obtained from the near-field microwave microscopy technique and analysed based on Landau-Lifshitz-Gilbert equation. The complex microwave permeability spectra of the magnetic thin films up to 5 GHz in the temperature range from room temperature to 420 K were measured. It was found that thicker ferromagnetic layers helped to reduce the dependence of the magnetic properties on temperature, leading to better thermal stability. The saturation magnetization MS, dynamic magnetic anisotropy field HKdyn, and ferromagnetic resonance frequency fFMR were found to decrease with temperature, while the effective damping coefficient αeff was increased with temperature. We also investigate the rotational magnetic anisotropy field HKrot with temperature which gives a measure of the rotatable magnetization of the antiferromagnetic layers and its thermal stability.