Magnetic Easy Axis Changes Research Articles

Spin-reorientation driven emergent phases and unconventional magnetotransport in quasi-2D vdW ferromagnet Fe4GeTe2

Non-trivial spin textures driven by strong exchange interaction, magneto-crystalline anisotropy, and electron correlation in a low-dimensional magnetic material often lead to unusual electronic transitions. Through a combination of transport experiments in exfoliated nanoflakes down to 16 layers and first principle calculations, we unravel emergent electronic phases in quasi-2D van der Waals ferromagnet, Fe4GeTe2, possessing ferromagnetic TC ~ 270 K, along with a spin-reorientation transition (TSR ~ 120 K) with the change of magnetic easy axis. Two electronic transitions are identified. The first transition near TSR exhibits a sharp fall in resistivity, followed by a sign change in the ordinary Hall coefficient (R0), together with, maximum negative magnetoresistance (MR) and anomalous Hall conductivity. Another unusual electronic transition, hitherto unknown, is observed near ~ 40–50 K (TQ), where R0 again changes sign and below which, the resistivity shows a quadratic temperature dependence, and MR becomes positive. An analysis of the experimental data further uncovers the role of competing inelastic scattering processes in anomalous magnetotransport behavior. The density-functional theory based first-principle calculations unveil two possible magnetic phases, followed by a low-energy model Hamiltonian which captures the essence of these phases as well as explains the observed magnetotransport behavior. Thus, we demonstrate an interplay between magnetism and band topology and its consequence on electron transport in Fe4GeTe2, important for spintronic applications.

Strain and voltage control of the magnetic anisotropy in Co2MnSi thin film

The strain effect on the magnetic anisotropy (MA) in free-standing Co2MnSi(001) thin film and the voltage control MA in Au-capped MgO/Co2MnSi thin film are investigated within the framework of density functional theory. We find that the MA of free-standing film with MnSi-termination is sensitive upon strain, while the magnetic easy axis changes from in-plane to out-of-plane under the applied in-plane 1% extension strain. The MA of Au-capped Co2MnSi/MgO shows a ∧-shaped electric-field dependence and a large voltage-controlled MA value (VCMA) of 225 fJ (Vm)−1. The VCMA is caused by the change of the spin-flip term, which is related to the energy shift of the Co dz 2 minority-spin band near the Fermi level. Based on this study, it is expected that Co2MnSi can potentially be used in magnetoelectric spintronic devices.

Scaling of the Thermally Induced Sign Inversion of Longitudinal Spin Seebeck Effect in a Compensated Ferrimagnet: Role of Magnetic Anisotropy

AbstractThe authors report on a systematic investigation of the longitudinal spin Seebeck effect (LSSE) in a GGG(Gd3Ga5O12)/GdIG(Gd3Fe5O12)/Pt film series exhibiting an in‐plane magnetic easy axis with a compensation temperature (TComp) that decreases from 270 to 220 K when decreasing GdIG film thickness from 272 to 31 nm, respectively. For all the films, the LSSE signal flips its sign below TComp. The authors demonstrate a universal scaling behavior of the temperature dependence of LSSE signal for the GdIG films around their respective TComp. Additionally, the authors demonstrate LSSE in a 31 nm GdIG film grown on a lattice‐mismatched GSGG (Gd3Sc2Ga3O12) substrate that exhibits an out‐of‐plane magnetic easy axis at room temperature. However, this sample reveals a spin reorientation transition where the magnetic easy axis changes its orientation to in‐plane at low temperatures. A clear distinction is observed in the LSSE signal for the GSGG/GdIG(31 nm)/Pt heterostructure, relative to GGG/GdIG(31 nm)/Pt showing an in‐plane magnetic easy axis. The findings underscore a strong correlation between the LSSE signal and the orientation of magnetic easy axis in compensated ferrimagnets and opens the possibility to tune LSSE through effective anisotropy.

Transition from Ferromagnetic Semiconductor to Ferromagnetic Metal with Enhanced Curie Temperature in Cr2Ge2Te6 via Organic Ion Intercalation.

Magnetism in the two-dimensional limit has become an intriguing topic for exploring new physical phenomena and potential applications. Especially, the two-dimensional magnetism is often associated with novel intrinsic spin fluctuations and versatile electronic structures, which provides vast opportunities in 2D material research. However, it is still challenging to verify candidate materials hosting two-dimensional magnetism, since the prototype systems have to be realized by using mechanical exfoliation or atomic layer deposition. Here, an alternative manipulation of two-dimensional magnetic properties via electrochemical intercalation of organic molecules is reported. Using tetrabutyl ammonium (TBA+), we synthesized a (TBA)Cr2Ge2Te6 hybrid superlattice with metallic behavior, and the Curie temperature is significantly increased from 67 K in pristine Cr2Ge2Te6 to 208 K in (TBA)Cr2Ge2Te6. Moreover, the magnetic easy axis changes from the ⟨001⟩ direction in Cr2Ge2Te6 to the ab-plane in (TBA)Cr2Ge2Te6. Theoretical calculations indicate that the drastic increase of the Curie temperature can be attributed to the change of magnetic coupling from a weak superexchange interaction in pristine Cr2Ge2Te6 to a strong double-exchange interaction in (TBA)Cr2Ge2Te6. These findings are the first demonstration of manipulation of magnetism in magnetic van der Waals materials by means of intercalating organic ions, which can serve as a convenient and efficient approach to explore versatile magnetic and electronic properties in van der Waals crystals.

Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction

The electric field-induced ∼180° magnetization reversal is realized for a sputtered CoFeB/MgO-based magnetic tunnel junction with perpendicular magnetic easy axis in a static external magnetic field. Application of bias voltage with nanoseconds duration results in a temporal change of magnetic easy axis in the free layer CoFeB to in-plane, which induces precessional motion of magnetization in the free layer. The magnetization reversal takes place when the bias voltage pulse duration is adjusted to a half period of the precession. We show that the back and forth magnetization reversal can be observed by using successive application of half-period voltage pulses.

Directional change of magnetic easy axis of arrays of cobalt nanowires: Role of non-dipolar magnetostatic interaction

Abstract Room temperature magnetization of two dimensional (2D) arrays of cobalt nanowires (NWs) having diameter 50 and 150 nm prepared by electrodeposition are studied in details. Diffraction patterns of the NWs reveal that the crystallites of the NWs become more textured on decreasing their diameter. Magnetic hysteresis loop measurements show the magnetic easy axis changes its direction from axial to perpendicular direction of NWs on increasing the length of the NWs. The magnetostatic interaction among the NWs, known as the key factor in defining the easy direction is found not to be dipolar at all the circumstances. An aspect ratio (length/diameter of NWs) dependence of the non-dipolar interaction in 150 nm NWs is evident from the static magnetization as well as from ferromagnetic resonance (FMR) measurements.

マグネタイトウィスカーからの電界放出電子のスピン偏極度

We observed the Verwey transition, metal-insulator transition, in a spin-polarization of field-emitted electrons from a topmost surface of a 〈110〉-oriented magnetite whisker by means of field-emitted electron polarimetry. Raising temperature of the whisker to about 120 K, the polarization of field-emitted electrons increased from 4 to 11% with a rotation of its direction. The rotation angle corresponds to a change of magnetic easy axis from 〈001〉 to 〈111〉 direction caused by the Verwey transition. At magnetite temperature of 40 K, the spin polarization was sensitive to increase of the applied field. Eventually, the direction of spin polarization reversed, which can not be explained taking in to account conventional free electron model. These results reveal that the Verwey transition occurs even at a surface as well as in a bulk and that the energy width and level of t2g band become narrower and lower than that of bulk.

Observation of Rotation of Spin-Polarization with the Verwey Transition at a Surface of a <110>-Oriented Magnetite Whisker by Field-Emitted Electron Polarimetry

We observed the Verwey transition, i. e., metal-insulator transition, in a spin-polarization of field-emitted electrons from a topmost surface of a -oriented magnetite whisker by means of field-emitted electron polarimetry. With a rise in temperature of the whisker to about 120 K, the polarization of field-emitted electrons increased from 4 to 11% with a rotation of its direction. The rotation angle corresponds to a change of magnetic easy axis from to direction caused by the Verwey transition. This result reveals that the Verwey transition occurs even at a surface as well as in a bulk. [DOI: 10.1380/ejssnt.2011.322]

Magnetic Anisotropy Modulation in Ta/ CoFeB/ MgO Structure by Electric Fields

We investigate the magnetic anisotropy in as-deposited and annealed Ta/ CoFeB/ MgO samples prepared by sputtering and its CoFeB thickness dependence. The magnetic easy axis changes from in-plane to perpendicular with decreasing CoFeB thickness. The thickness, at which magnetic easy axis direction changes, is increased by annealing. It is also shown that the magnetic anisotropy can be modulated by electric field and its modulation ratio is larger for the annealed samples.

Fabrication and Magnetization Reversal Processes for Co/Cu Multilayer Nanowires

Co/Cu multilayer nanowires fabricated in an array using anodized aluminium oxide (AAO) template has been investigated. Experimental conditions are optimized to fabricate Co/Cu multilayer systems with fixed Cu and variable Co layer thicknesses. Magnetization reversal mode is found to depend on the Co layer thickness. A transition occurs from coherent rotation to a combination of coherent and curling rotation at around t(Co) = 60 nm with increasing t(Co). The reversal modes have been investigated using the magnetic hysteresis loops measured at room temperature for Co/Cu nanowires placed at various angles between the directions of the nanowire axis and external fields using a vibrating sample magnetometer. The magnetic easy axis changes from the direction perpendicular to nanowires to that parallel to the nanowires at around t(Co) = 60 nm, indicating a change in the magnetization reversal mode. The reversal mode for the nanowires with thin disk-shaped Co layers is of a coherent rotation type, while that for long rod-shaped Co layers can be explained by a combination of coherent rotation and a curling mode.

Magnetic properties of Sr3-xCaxRu2O7

We report the magnetization data of single crystalline Sr3-xCaxRu2O7 (x = 1.5, 2.0, 3.0) grown by a floating-zone method. In the magnetic phase diagram of this system, a phase with a ferromagnetic correlation (0.5 ≤x ≤ 1.2) adjoins a two-dimensional (2D) metallic and antiferromagnetic phase (1.2 ≤ x ≤ 3). In the antiferromagnetic and 2D-metallic phase, the magnetic susceptibilities indicate that the magnetic easy axis changes continuously from the ab-plane to c-axis with increasing Sr content. Our results exhibit that the metamagnetic transition of Ca3Ru2O7, which is on the basis of a tunneling magnetoresistance, shifts to lower field and the transition becomes broad with increasing Sr content. Moreover, the magnetization data at T = 5 K for x = 1.5, 2.0 show a hysteresis, which suggests that the magnetic ground state is a canted antiferromagnetic one in this phase.

Structural Distortion and Physical Properties in Double-Layered Ruthenate Sr3-xCaxRu2O7

We have investigated the structural, magnetic, and electronic properties of single-crystalline Sr 3- x Ca x Ru 2 O 7 (0 ≤ x ≤3) grown by a floating-zone method. The results of X-ray diffraction, magnetic susceptibility, specific heat, and electrical resistivity measurements reveal that this system shows a novel phase diagram. With increasing Ca content, the enhanced paramagnetism of Sr 3 Ru 2 O 7 is suppressed, and a phase with a ferromagnetic correlation (\(0.5 \lesssim x \lesssim 1.2\)) emerges. This phase adjoins an antiferromagnetic two-dimensional metallic phase (\(1.2 \lesssim x \leq 3.0\)). At the border between these two phases, discontinuous changes in the lattice parameters are observed. In the ferromagnetically correlated phase, the density of states is significantly enhanced. In the antiferromagnetic phase, the magnetic easy axis changes continuously from the c -axis to the a b -plane with increasing Ca content. These magnetic and electronic properties are strongly coupled with structural dist...

Studies of magnetic axis switching phenomenon in magnetite

The results of magnetic and structural studies of magnetic easy axis switching in magnetite are presented. This phenomenon was first observed after the easy axis was established by the in-field cooling across the Verwey transition. Although the subsequent application of magnetic field at low temperatures along certain <1 0 0> is almost as effective in establishing the easy axis as field cooling, the repeated easy axis redirection below T V causes the field required to switch the axis to become lower. We have also shown that the change of magnetic easy axis is reflected in the change of crystallographic c direction. Finally, the energy required to switch the axis was calculated and was found comparable to the characteristic energy of the Verwey transition.

Magnetic torque of λ-(BETS) 2FeCl 4

Magnetic torque of λ-(BETS) 2 FeCl 4 is measured up to 15 T and down to 0.4 K. Clear first-order spin flop transition between the antiferromagnetic (AF) and the canted antiferromagnetic (CAF) phases is observed in the magnetic field of about 1.2 T and below 9.5 K. The AF easy axis is found to be in the direction tilted by about 30° from the c-axis to the b-axis, which is different from the previous reports on the easy axis (the c-axis). At the step-like transition with hysteresis around H = 10 T between CAF and the field induced ferromagnetic (F) (paramagnetic) phases, the direction of the magnetic easy axis changes. The two transitions on the H - T phase diagram are considered on the basis of the coupled two spin system; Fe 3+ (S = 5/2) and (BETS) 2 (S = 1/2).

Composition-driven spin-reorientation transition in ferromagnetic alloy films

A spin-reorientation transition upon a variation of the Fe concentration $x$ is observed in ultrathin fcc ${\mathrm{Fe}}_{x}{\mathrm{Co}}_{1\ensuremath{-}x}$ alloy films on Cu(001) for $x$g77%. For film thicknesses below 4 monolayers the magnetic easy axis changes from in-plane on the Co-rich side to out-of-plane on the Fe-rich side. Since no structural transitions accompany this spin-reorientation transition, varying the alloy composition can be interpreted as varying the number of $3d$ electrons. Upon changing the average $d$-band filling by as little as 0.1 electron strong changes of the anisotropy energy are observed.

Nonequilibrium magnetization near the reorientation phase transition of Fe/Ag(100) films.

The static and dynamic magnetic properties of ultrathin Fe/Ag(100) films are investigated in the vicinity of the reorientation transition where the magnetic easy axis changes from out-of-plane to in-plane orientation. In a certain temperature range below the reorientation transition temperature ${T}_{r}$ a time dependent decay of the remanent out-of-plane magnetization is observed. The origin of this relaxation behavior is discussed, and implications for a thermodynamic description of the reorientation phase transition are outlined.

Stress effects of water sorption in cure baked photoresist underlayers on the magnetic easy axis of Permalloy film overlayers

Variations in the uniaxial mechanical stress of Permalloy films can change the magnetic easy axis of the Permalloy films through the magnetostrictive effect. Conventional inductive film heads are fabricated with the use of cure baked photoresist (CBP). We show that water absorption in the CBP can produce significant changes in the magnetic properties of Permalloy films similar to those used in recording head yokes. The diffusion coefficient of water in 3 /spl mu/m thick CBP films was measured with a quartz crystal microbalance and found to be 2.2/spl times/10/sup -9/ cm/sup 2//s at 25 degrees C. A large and reversible change in the magnetic easy axis is observed in 1 /spl mu/m thick Permalloy films that overlay a mound of CBP when it is exposed to water vapor. We infer from the observed magnetic easy axis changes that uniaxial stresses of greater than 5/spl times/10/sup 8/ dynes/cm/sup 2/ are produced due to the swelling and contraction of the underlaying CBP. >

Simultaneous observation of the smoke signals during the growth of bcc Fe on the Ag(100) surface

Simultaneous observation of the surface magneto-optic Kerr effect (SMOKE) and reflection high-energy electron diffraction (RHEED) intensity oscillation was done during the epitaxial growth of bcc Fe on fcc Ag(100) surface. The SMOKE system using the high-frequency (100 kHz) ac method has a high sensitivity (10−4 deg) and is able to obtain an absolute value of the Kerr rotation angle (φK ). φK of Fe/Ag(100) was measured at room temperature under magnetic fields of 0 and 100 Oe. When film thickness (d) is thicker than five monatomic layers (ML’s), φK of the longitudinal Kerr effect at 100 Oe increases with increasing d. No longitudinal SMOKE signal, however, was observed below 5 ML, while a polar Kerr signal was observed only in the range of 4 ML&amp;lt;d&amp;lt;5 ML. This suggests that the magnetic easy axis changes from perpendicular to in-plane at 5 ML, and below 4 ML the film is hardly magnetized by 100 Oe because of its high coercivity. These properties are discussed in relation to the crystal growth mechanism.