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Research Highlights

Magnetization reversal, damping properties and magnetic anisotropy of L10 ordered FeNi thin films

V. Thiruvengadam, B. B. Singh, T. Kojima, K. Takanashi, M. Mizuguchi, and S. Bedanta

Appl. Phys. Letters 115, 202402 (2019)

L10L10-ordered magnetic alloys such as FePt, FePd, CoPt, and FeNi are well known for their large magnetocrystalline anisotropy. Among these, the L10L10-FeNi alloy is an economically viable material for magnetic recording media because it does not contain rare earth and noble elements. In this work, L10L10-FeNi films with three different strengths of anisotropy were fabricated by varying the deposition process in a molecular beam epitaxy system. We have investigated magnetization reversal along with domain imaging via a magneto-optic Kerr effect based microscope. It is found that in all three samples, the magnetization reversal happens via domain wall motion. Furthermore, ferromagnetic resonance spectroscopy was performed to evaluate the damping constant (α) and magnetic anisotropy. It was observed that the FeNi sample with a moderate strength of anisotropy exhibits a low value of α∼4.9×10−3α∼4.9×10−3. In addition to this, it was found that the films possess a mixture of cubic and uniaxial anisotropies.

Skyrmion Racetrack memory with an antidot

Aroop Kumar Behera1, Chandrasekhar Murapaka, Sougata Mallick, Braj Bhusan Singh and Subhankar Bedanta

J. Phys. D: Appl. Phys. 54, 025001 (2020)

Skyrmion racetrack memory has a lot of potential in future non-volatile solid state devices. By application of current in such devices, both spin-orbit torque and spin-transfer torques are proven to be useful to nucleate and propagate skyrmions. However, the current applied during nucleation of successive skyrmions may have unwanted perturbation viz. Joule heating and the skyrmion Hall effect, on the propagation of previously generated skyrmions. Therefore, new methodology is desired to decouple the generation and propagation of skyrmions. Here, we present a novel route via micromagnetic simulations for generation of skyrmions from triangular antidot structure in a ferromagnetic nanotrack using local Oersted field. Antidots are holes in a magnetic nanoelement. Multiple skyrmions can be simultaneously generated by incorporating a greater number of antidots. Controlled skyrmion injection can be achieved by tuning the separation between the antidots that are placed at either end of the nanotrack. Here, we propose a novel design to realise skyrmionic racetrcak memory, where one can individually generate and manipulate the skyrmions within the nanotrack.

Effect of random anisotropy in stabilization of topological chiral textures

Gajanan Pradhan, Subhankar Bedanta

JMMM,528, 167805,2021

Ever increasing demand of skyrmion manipulation in nanodevices has brought up interesting research to 8 understand the stabilization of these topologically protected chiral structures. To understand the actual shape 9 and size of skyrmion observed experimentally, we have performed micromagnetic simulations to investigate 10 skyrmion stabilization in presence of random anisotropy in magnetic thin film system. Previous experimental 11 reports of skyrmion imaging in thin films depicts that the skyrmion shape is not perfectly circular. Here we 12 show via simulations that the shape of a skyrmion can get distorted due to the presence of different local 13 anisotropy energy. The values of uniaxial anisotropy constant (Ku) and random aniostropy constant (Kr) 14 are varied to understand the change in shape and size of a skyrmion and an antiskyrmion stabilized in a 15 square magnetic nanoelement. The skyrmion shape gets distorted and the size gets constant for high random 16 anisotropy energy in the system.

Magnetism at the interface of non-magnetic Cu and C60

Purbasha Sharangi, Pierluigi Gargiani, Manuel Valvidares and Subhankar Bedanta

PHYSICAL CHEMISTRY CHEMICAL PHYSICS, doi.org/10.1039/D0CP06326F(2020)

The signature of magnetism without a ferromagnet in a non-magnetic heterostructure is novel as well as fascinating from a fundamental research point of view. It has been shown by Al’Mari et al. that magnetism can be induced at the interface of Cu/C60 due to a change in the density of states. However, the quantification of such an interfacial magnetic moment has not been performed yet. In order to quantify the induced magnetic moment in Cu, we have performed X-ray magnetic circular dichroism (XMCD) measurements on Cu/C60 multilayers. We have observed room temperature ferromagnetism in the Cu/C60 stack. Further XMCD measurements show that a ∼0.01 μB per atom magnetic moment has been induced in Cu at the Cu/C60 interface.

Effect of spin glass frustration on exchange bias in NiMn/CoFeB bilayers

Sagarika Nayak, Palash Kumar Manna, Braj Bhusan Singha and Subhankar Bedanta

PHYSICAL CHEMISTRY CHEMICAL PHYSICS, doi.org/10.1039/D0CP05726F (2020)

Exchange bias in ferromagnetic/antiferromagnetic systems can be explained in terms of various interfacial phenomena. Among these, spin glass frustration can affect the magnetic properties in exchange bias systems. Here we have studied a NiMn/CoFeB exchange bias system in which spin glass frustration seems to play a crucial role. In order to account for the effect of spin glass frustration on magnetic properties, we have investigated the temperature and cooling field dependence of exchange bias. We have observed the decrease of exchange bias field (μ0HEB) with cooling field (μ0HFC) whereas there is a negligible effect on coercive field (μ0HC). Exponential decay of μ0HEB and μ0HC is found in this exchange bias system. Furthermore, training effect measurements have been performed to study the spin relaxation mechanism. We have fitted the training effect data with a frozen and rotatable spin relaxation model. We have determined the ratio of relaxation rates of interfacial rotatable and frozen spins in this study. The training effect data are also fitted with various other models. Furthermore, we have observed the shifting of the peak temperature towards higher temperature with frequency from the ac susceptibility data. The peak temperature vs. frequency data can be described by the Vogel–Fulcher law, which indicates the spin glass like state in the bilayer system.

Tunability of Domain Structure and Magnonic Spectra in Antidot Arrays of Heusler Alloy

Sougata Mallick, Sucheta Mondal, Takeshi Seki, Sourav Sahoo, Thomas Forrest, Francesco Maccherozzi, Zhenchao Wen, Saswati Barman, Anjan Barman, Koki Takanashi and Subhankar Bedanta

PHYSICAL REVIEW APPLIED 12, 014043 (2019)

Materials suitable for magnonic crystals demand low magnetic damping and long spin-wave propagation distance. In this context Co-based Heusler compounds are ideal candidates for magnonic based applications. In this work, antidot arrays (with different shapes) of epitaxial Co2Fe0.4Mn0.6Si Heusleralloy thin films are prepared using e-beam lithography and sputtering technique. Magneto-optic Kerr effect (MOKE) and ferromagnetic resonance analysis confirm the presence of dominant cubic and moderate uniaxial magnetic anisotropies in the thin film. Domain imaging via x-ray photoemission electron microscopy on the antidot arrays reveals chainlike switching or correlated bigger domains for different antidot shapes. Time-resolved MOKE microscopy is performed to study the precessional dynamics and magnonic modes of the antidots with different shapes. We show that the optically induced spin-wave spectra in such antidot arrays can be tuned by changing the shape of the holes. The variation in internal-field profiles, pinning energy barrier, and anisotropy modifies the spin-wave spectra dramatically within the antidot arrays with different shapes. We further show that by combining the magnetocrystalline anisotropy with the shape anisotropy, an extra degree of freedom can be achieved to control the magnonic modes in such antidot lattices.

Simultaneous observation of anti-damping and the inverse spin Hall effect in the La0.67Sr0.33MnO3/Pt bilayer system†
Pushpendra Gupta, Braj Bhusan Singh, Koustuv Roy, Anirban Sarkar, Markus Waschk, Thomas Brueckel and Subhankar Bedanta

Nanoscale, 2021, 13, 2714

Manganites have shown potential in spintronics because they exhibit high spin polarization. Here, by ferromagnetic resonance we have studied the damping properties of La0.67Sr0.33MnO3/Pt bilayers which are prepared by oxide molecular beam epitaxy. The damping coefficient (α) of a La0.67Sr0.33MnO3 (LSMO) single layer is found to be 0.0104. However the LSMO/Pt bilayers exhibit a decrease in α with an increase in Pt thickness. This decrease in the value of α is probably due to high anti-damping like torque. Furthermore, we have investigated the angle dependent inverse spin Hall effect (ISHE) to quantify the spin pumping voltage from other spin rectification effects such as the anomalous Hall effect and anisotropic magnetoresistance. We have observed a high spin pumping voltage (∼20 μV). The results indicate that both anti-damping and spin pumping phenomena occur simultaneously.

High Spin to Charge Conversion Efficiency in Electron Beam-Evaporated Topological Insulator Bi2Se3
Braj Bhusan Singh, Sukanta Kumar Jena, Manisha Samanta, Kanishka Biswas, and Subhankar Bedanta

ACS Appl. Mater. Interfaces 2020, 12, 47

Bi2Se3 is a well-established topological insulator (TI) having spin momentum locked Dirac surface states at room temperature and predicted to exhibit high spin to charge conversion efficiency (SCCE) for spintronics applications. The SCCE in TIs is characterized by an inverse Edelstein effect length (λIREE). We report an λIREE of ∼0.36 nm, which is the highest ever observed in Bi2Se3. Here, we performed spin pumping and inverse spin Hall effect (ISHE) in an electron beam-evaporated Bi2Se3/CoFeB bilayer. The Bi2Se3 thickness dependence of λIREE, perpendicular surface anisotropy (KS), spin mixing conductance, and spin Hall angle confirmed that spin to charge conversion is due to spin momentum locked Dirac surface states. We propose that the role of surface states in SCCE can be understood by the evaluation of KS. The SCCE is found to be high when the value of KS is small.

Large Spin Hall Angle and Spin-Mixing Conductance in the Highly Resistive Antiferromagnet Mn2Au
Braj Bhusan Singh and Subhankar Bedanta

PHYSICAL REVIEW APPLIED 13, 044020 (2020)

Antiferromagnetic materials (AFMs) have recently been attracting interest in research on spintronics due to their zero stray magnetic field, high anisotropy, and spin-orbit coupling. In this context, the bimetallic AFM Mn2Au has drawn attention because it exhibits unique properties and its Néel temperature is very high (TN = 1500 K). Here, we report investigations of spin pumping and the inverse spin Hall effect (ISHE) in a Mn2Au/Co-Fe-B bilayer system using ferromagnetic resonance. We find a large spin Hall angle θSH = 0.22, with a spin Hall conductivity σSH = 1.46 × 105(/2e) −1 m−1, comparable to that of Pt. Further, we evaluate the effective spin-mixing conductance g↑↓ eff = 3.27 × 1018 m−2 and intrinsic spin-mixing conductance g↑↓ r = 8.83 × 1018 m−2, which are higher than the previously reported value (1.40 × 1018 m−2 for Mn2Au/Y3Fe5O12).

Inverse spin Hall effect and spin pumping in the polycrystalline noncollinear
antiferromagnetic Mn3Ga
Braj Bhusan Singh, Koustuv Roy, J. Arout Chelvane, and Subhankar Bedanta

Phys. Rev. B 102, 174444(2020)

Noncollinear antiferromagnetic (AFM) materials have drawn research interest because they exhibit large anomalous Hall effect at room temperature (RT) due to large Berry curvature. Mn3Ga is a noncollinear AFM in which the order of Mn magnetic moments is arranged in inverse triangular configuration on a kagome lattice. It makes Mn3Ga a promising candidate for inverse spin Hall effect (ISHE) study which has not been studied before. In this work, investigation of ISHE and spin pumping in polycrystalline Mn3Ga/CoFeB heterostructures at RT has been performed. Angle-dependent measurements of ISHE have been performed in order to disentangle various spin-rectification effects. Spin-mixing conductance (g ↑↓ eff ), spin Hall angle (θSH ), and spin Hall conductivity (σSH ) are evaluated to be (5.0 ± 1.8) × 1018 m−2, 0.31 ± 0.01, and 7.5 × 105 (¯h/2e) −1 m−1, respectively. The observed value of θSH is higher than Mn3Sn and comparable to the IrMn3, which is also a noncollinear AFM. Large spin Hall angle makes Mn3Ga a promising candidate for future spintronics devices.

High spin mixing conductance and spin interface transparency at the interface of a Co2Fe0.4Mn0.6Si
Heusler alloy and Pt

Braj Bhusan Singh, Koustuv Roy, Pushpendra Gupta, Takeshi Seki, Koki Takanashi and Subhankar Bedanta

Singh et al. NPG Asia Materials (2021) 13:9

Ferromagnetic materials exhibiting low magnetic damping (α) and moderately high-saturation magnetization are required from the viewpoints of generation, transmission, and detection of spin waves. Since spin-to-charge conversion efficiency is another important parameter, high spin mixing conductance ðg"# r Þ is the key for efficient spinto-charge conversion. Full Heusler alloys, e.g., Co2Fe0.4Mn0.6Si (CFMS), which are predicted to be 100% spin-polarized, exhibit low α. However, g"# r at the interface between CFMS and a paramagnet is not fully understood. Here, we report investigations of spin pumping and the inverse spin Hall effect in CFMS/Pt bilayers. Damping analysis indicates the presence of significant spin pumping at the interface of CFMS and Pt, which is also confirmed by the detection of an inverse spin Hall voltage. We show that in CFMS/Pt, g"# r (1.70 × 1020 m−2 ) and the interface transparency (83%) are higher than the values reported for other ferromagnetic/heavy metal systems. We observed a spin Hall angle of ~0.026 for the CFMS/Pt bilayer system.

Magnetization reversal, damping properties and magnetic anisotropy of L10 -ordered FeNi thin films

V. Thiruvengadam, B. B. Singh, T. Kojima, K. Takanashi, M. Mizuguchi, and S. Bedanta

Appl. Phys. Lett. 115, 202402 (2019)

L10-ordered magnetic alloys such as FePt, FePd, CoPt, and FeNi are well known for their large magnetocrystalline anisotropy. Among these, the L10-FeNi alloy is an economically viable material for magnetic recording media because it does not contain rare earth and noble elements. In this work, L10-FeNi films with three different strengths of anisotropy were fabricated by varying the deposition process in a molecular beam epitaxy system. We have investigated magnetization reversal along with domain imaging via a magneto-optic Kerr effect based microscope. It is found that in all three samples, the magnetization reversal happens via domain wall motion. Furthermore, ferromagnetic resonance spectroscopy was performed to evaluate the damping constant (a) and magnetic anisotropy. It was observed that the FeNi sample with a moderate strength of anisotropy exhibits a low value of a 4:9 103. In addition to this, it was found that the films possess a mixture of cubic and uniaxial anisotropies.

Tuning spinterface properties in iron/fullerene thin films

Srijani Mallik1 , Amir Syed Mohd , Alexandros Koutsioubas, Stefan Mattauch , Biswarup Satpati , Thomas Brückel, and Subhankar Bedanta

Nanotechnology 30 (2019) 435705 (8pp)

In ferromagnetic (FM) metal/organic semiconductor (OSC) heterostructures charge transfer can occur which leads to induction of magnetism in the non-magnetic OSC. This phenomenon has been described by the change in the density of states in the OSC which leads to a finite magnetic moment at the OSC interface and it is called the ‘spinterface’. One of the main motivations in this field of organic spintronics is how to control the magnetic moment in the spinterface. In this regard, there are several open questions such as (i) which combination of FM and OSC can lead to more moment at the spinterface? (ii) Is the thickness of OSC also important? (iii) How does the spinterface moment vary with the FM thickness? (iv) Does the crystalline quality of the FM matter? (v) What is the effect of spinterface on magnetization reversal, domain structure and anisotropy? In this context, we have tried to answer the last four issues in this paper by studying Fe/C60 bilayers of variable Fe thickness deposited on Si substrates. We find that both the induced moment and thickness of the spinterface vary proportionally with the Fe thickness. Such behavior is explained in terms of the growth quality of the Fe layer on the native oxide of the Si (100) substrate. The magnetization reversal, domain structure and anisotropy of these bilayer samples were studied and compared with their respective reference samples without the C60 layer. It is observed that the formation of spinterface leads to a reduction in uniaxial anisotropy in Fe/C60 on Si (100) in comparison to their reference samples.

Inverse Spin Hall Effect in Electron Beam Evaporated Topological Insulator Bi2Se3 Thin Film

Braj B Singh et al.

physica status solidi (RRL)–Rapid Research Letters 13 (3), 2018, 1800492

Spintronics exploiting pure spin current in ferromagnetic (FM)/heavy metals (HM) is a subject of intense research. Topological insulators having spin momentum locked surface states exhibit high spin–orbit coupling and thus possess a huge potential to replace the HM like Pt, Ta, W, etc. In this context, the spin pumping phenomenon in Bi2Se3/CoFeB bilayers has been investigated. Bi2Se3 thin films are fabricated by electron beam evaporation method on Si (100) substrate. In order to confirm the topological nature of Bi2Se3, low temperature magnetotransport measurement on a 30 nm thick Bi2Se3 film which shows 10% magnetoresistance (MR) at 1.5 K has been performed. A linear increase in MR with applied magnetic field indicates the presence of spin momentum‐locked surface states. A voltage has been measured at room temperature to quantify the spin pumping which is generated via inverse spin Hall effect (ISHE). For the separation of spin rectification effects mainly produced by the FM CoFeB layer, in plane angular dependence of the dc voltage with respect to applied magnetic field has been measured. Our analysis reveals that spin pumping induced ISHE is the dominant contribution in the measured voltage.

Tuning of magnetic properties by alternating the order of hard/soft bilayers with various thicknesses

Sagarika Nayak, Braj Bhusan Singh, Sougata Mallick and Subhankar Bedanta

J. Phys. D: Appl. Phys. 52 (2019) 305301 (6pp)

Controlled tuning of magnetic anisotropy and damping is essential in ferromagnetic systems for various spintronic applications. In this context, magnetic bilayers with different thicknesses of soft (Co) and hard (Co40Fe40B20) layers have been prepared using magnetron sputtering technique by alternating the order of the constituent magnetic layers. We have observed an increase in the coercive field (HC) by increasing the thickness of the Co layer. Further, for the same constituent layers, HC increases for the case when CoFeB is at the bottom of Co. We further show that by this deposition methodology, one can systematically tune the domain structure. The line width and hence the Gilbert damping constant (α) of the bilayers can also be modified by alternating the order and thickness of the individual magnetic layers.

Effect of magnetic fullerene on magnetization reversal created at the Fe/C 60 interface

Srijani Mallik, Stefan Mattauch, Manas Kumar Dalai, Thomas Brückel, Subhankar Bedanta

Scientific Reports, vol 8, 5515 (2018)

Probing the hybridized magnetic interface between organic semiconductor (OSC) and ferromagnetic (FM) layers has drawn significant attention in recent years because of their potential in spintronic applications. Recent studies demonstrate various aspects of organic spintronics such as magnetoresistance, induced interface moment etc. However, not much work has been performed to investigate the implications of such OSC/FM interfaces on the magnetization reversal and domain structure which are the utmost requirements for any applications. Here, we show that non-magnetic Fullerene can obtain non-negligible magnetic moment at the interface of Fe(15 nm)/C60(40 nm) bilayer. This leads to substantial effect on both the magnetic domain structure as well as the magnetization reversal when compared to a single layer of Fe(15 nm). This is corroborated by the polarized neutron reflectivity (PNR) data which indicates presence of hybridization at the interface by the reduction of magnetic moment in Fe. Afterwards, upto 1.9 nm of C60 near the interface exhibits magnetic moment. From the PNR measurements it was found that the magnetic C60 layer prefers to be aligned anti-parallel with the Fe layer at the remanant state. The later observation has been confirmed by domain imaging via magneto-optic Kerr microscopy.

Relaxation dynamics in magnetic antidot lattice arrays of Co/Pt with perpendicular anisotropy

Sougata Mallick, Swapna Sindhu Mishra, Subhankar Bedanta

Scientific Reports, vol 8, 11648 (2018)

The topic of magnetic antidot lattice (MAL) arrays has drawn attention from both fundamental research as well as from application point of view. MAL arrays are promising candidates for making domain engineering in thin films. For various applications it is necessary to understand the magnetization reversal mechanism as well as the relaxation dynamics. In this context we have studied magnetic antidot lattice (MAL) arrays of Co/Pt with perpendicular anisotropy fabricated by combination of photolithography and sputtering deposition. Kerr microscopy domain imaging for the continuous thin film reveals the formation of typical bubble domains of perpendicular media with high anisotropy. However, presence of periodic holes in the MAL arrays lead to nucleation of localised smaller bubbles. We have performed simulations using object oriented micromagnetic framework (OOMMF) which reproduced the experimental results even considering antidot arrays in nano dimension. In literature it has been reported that in MAL arrays with in-plane anisotropy the domain propagation gets significantly hindered by the presence of the holes. However here we show that in perpendicularly magnetized Co/Pt the propagation of the domain walls is not restricted by the presence of the antidots. Further we have performed magnetic relaxation study and found that the global relaxation time for the MAL arrays of Co/Pt is faster as compared to it’s parent thin film. This behavior is opposite to what has been observed in literature for in-plane magnetized MAL arrays.

Effect of magnetic fullerene on magnetization reversal created at the Fe/C 60 interface

Srijani Mallik, Stefan Mattauch, Manas Kumar Dalai, Thomas Brückel, Subhankar Bedanta

Scientific Reports, vol 8, 5515 (2018)

360° domain walls in magnetic thin films with uniaxial and random anisotropy

N Chowdhury, W Kleemann, O Petracic, F Kronast, A Doran, A Scholl, S Cardoso, P Freitas, S Bedanta

Physical Review B 98 (13), 2018, 134440

X-ray photoemission electron microscopy (XPEEM) and magneto-optic Kerr effect (MOKE) microscopy have been performed on a metal-insulator multilayer of [Co80Fe20 (t = 1.8 nm)/Al2O3(3 nm)]9 to image 360° domain walls (DWs) along easy and hard axes, respectively. Their creation and annihilation can be directly visualized under application of a magnetic field. XPEEM experiments and micromagnetic simulations show that 360◦ DWs occur through the merger of 180◦ DWs of opposite chiralities along the easy axis. They are stable even under application of large magnetic fields. Formation of 360◦ DWs observed along the hard axis is attributed to symmetry breaking of the coherent spin rotation. Their formation in metal-insulator multilayers is explained as being due to the presence of an orientational dispersion of anisotropy axes in the film grains that is comparable to an overall uniaxial anisotropy term. Our results are confirmed numerically using micromagnetic simulations.

Superferromagnetism in dipolarly coupled L10 FePt nanodots with perpendicular magnetization

S. Bedanta et al.

Applied Physics Letters 107, 152410 (2015)

The magnetization reversal for perpendicularly magnetized L10-FePt (001) nanodots with different interdot distances was studied by magnetic domain observation. We show the results for two kinds of dot arrays: (i) the dots with physical percolation leading to direct exchange coupling, and (ii) the dots which are fully isolated experiencing only dipolar interaction. For the physically-percolated dot array, ferromagnetic domains were observed in which domain expanded with magnetic field. On the other hand, the array with the isolated FePt dots also exhibited domain like features resembling to the percolated dots, indicating the existence of superferromagnetism in the array of FePt nanodots.

Study of magnetization reversal processes in a thin Co film

N Chowdhury, S Bedanta and G S Babu

J. Magn. Magn. Mater. 336, 20 (2013)

The magnetization reversal has been studied both along the easy- and hard- axes for an in plane magnetized thin Cobalt film using magneto-optical Kerr effect (MOKE) microscope. We observe that magnetization reversal is governed by domain wall motion accompanied by nucleation when measured along the easy axis. However coherent rotation is observed during magnetization reversal when measured along the hard axis. The relaxation of magnetization in constant dc magnetic field measured along the easy axis show exponential behaviour which according to Fatuzzo - Labrune model indicates domain nucleated dominant process. Domain wall velocity plotted as a function of constant dc magnetic field shows creep and slide regime from which the depinning transition was extracted.

Figure: Domain wall velocity vs. magnetic field applied along the easy axis measured at room temperature. The solid line is the linear fit for slide region. The depinning field is marked as Hdep.

Controlling the anisotropy and domain structure with oblique deposition and substrate rotation

N Chowdhury and S Bedanta

AIP Advances 4, 027104 (2014)

Figure: Domain images for sample A, B and C are shown for θ = 0° (EA), 30°, 60° and 90° (HA), respectively. The images are taken near the coercive fields for the respective angles and the samples. The inset in figure (i) shows the zoomed image (60 μm x 60 μm) for the marked region.

Effect of substrate rotation on anisotropy and domain structure for a thin ferromagnetic film has been investigated in this work. For this purpose Co films with 10 nm thickness have been prepared by sputtering with oblique angle of incidence for various substrate rotations. This method of preparation induces a uniaxial anisotropy due to shadow deposition effect. The magnetization reversal is studied by magneto-optic Kerr effect (MOKE) based microscope in the longitudinal geometry. The Co films prepared by rotating the substrate with 10 and 20 rpm weakens the anisotropy but does not completely give isotropic films. But this leads to high dispersion in local grain anisotropy resulting in ripple and labyrinth domains. It is observed that the substrate rotation has moderate effect on uniaxial anisotropy but has significant effect on the magnetization reversal process and the domain structure.

Evidence of the spin glass state in (Bi1.88Fe0.12)(Fe1.42Te0.58)O6.87 Pyrochlore

G. S. Babu, S. Bedanta and M. Valant

Solid State Communications 158, 51 (2013)

The (Bi1.88Fe0.12)(Fe1.42Te0.58)O6.87 pyrochlore with very high Fe content undergoes a transition to the spin glass state at its glass temperature Tg ~ 21 K. The spin glass state in this compound has been evidenced by performing the so-called "memory effect" in the zero-field-cooled magnetization. The memory effect is clearly observed at temperatures below Tg and no such effect can be observed above Tg. Further, the "rejuvenation" effect, also has clearly been observed, which evidences the chaotic nature of the spin glass state. A possible origin of such glassy state is discussed.

Figure: Memory effect experiment in zero field cooled magnetization evidencing the spin glass state

Domain images of sample A taken by longitudinal Kerr microscope for different = 0° (a), 45° (b), 90° (c), -25°(d). The numeric names of the images in (a-d) basically indicate to the points in hysteresis loops as shown in Figure 2. The red arrows in the images represent the net magnetization in the particular area of the sample. The left column shows the direction of applied magnetic field (H) for the domain images to its corresponding rows.

Interplay of uniaxial and cubic anisotropy in epitaxial Fe thin films on MgO (001) substrate

Srijani Mallik, N. Chowdhury, and S. Bedanta

AIP Advances 4, (2014)

Epitaxial Fe thin films were grown on annealed MgO(001) substrates at oblique incidence by DC magnetron sputtering. Due to the oblique growth configuration, uniaxial anisotropy was found to be superimposed on the expected four-fold cubic anisotropy. A detailed study of in-plane magnetic hysteresis for Fe on MgO thin films has been performed by Magneto Optic Kerr Effect (MOKE) magnetometer. Both single step and double step loops have been observed depending on the angle between the applied field and easy axis i.e. along <100> direction. Domain images during magnetization reversal were captured by Kerr microscope. Domain images clearly evidence two successive and separate 90° domain wall (DW) nucleation and motion along cubic easy cum uniaxial easy axis and cubic easy cum uniaxial hard axis, respectively. However, along cubic hard axis two 180° domain wall motion dominate the magnetization reversal process. In spite of having four-fold anisotropy it is essential to explain magnetization reversal mechanism in 0°< <90° span as uniaxial anisotropy plays a major role in this system. Also it is shown that substrate rotation can suppress the effect of uniaxial anisotropy superimposed on four-fold anisotropy.

Size and Shape dependence study of Magnetization reversal in Magnetic Antidot Lattice arrays

Sougata Mallick and S. Bedanta*

J. Magn. Magn. Mater., 382, 158 (2015)

Magnetic Antidot Lattice (MAL) arrays of Co have been prepared in micron range using ultraviolet (UV) lithography technique with different shapes and sizes. Magnetization reversal in such MAL systems has been studied by magneto-optic Kerr effect (MOKE) based microscopy by varying the angle between the easy axis and the external magnetic field. The domain images evidence that the magnetization reversal along easy axis is highly dominated by nucleation of domains which is subsequently accompanied by domain wall motion. We have observed that with increase in active magnetic area domain size increases but on the contrary coercivity decreases. The presence of periodic holes turns the MALs magnetically hard when compared to similar thickness of continuous thin film. The magnetization relaxation along easy axis for the Co MAL at constant dc field fits very well with the exponential law of Fatuzzo-Labrune indicating domain nucleated dominant process.

Figure: Hysteresis measured by Longitudinal Kerr Microscope for triangular antidot along easy axis. Figs.(b) to (f) are domain images taken at fields marked by point 1 to 5 in (a) at 40 mT, + 14.35 mT, + 15.49 mT, 40 mT and 15.43 mT, respectively. The white arrow in image (b) represents the direction of applied magnetic field (H).

Magnetic domain imaging in L10 ordered FePt thin films with in-plane uniaxial magnetic anisotropy

Sougata Mallick, S. Bedanta, T. Seki and K. Takanashi

J. Appl. Phys. 116, 133904 (2014)

Epitaxial FePt thin films with various thicknesses, having in-plane uniaxial magnetic anisotropy, have been prepared at different deposition temperatures using sputtering technique. L10 ordering in the FePt thin film leads to very large uniaxial magnetocrystalline anisotropy. The magnetocrystalline anisotropy energy increases with the increase in film thickness and deposition temperature. Domain imaging performed by Magneto-optic Kerr microscopy with a longitudinal geometry suggests that the domain structure and magnetization reversal process strongly depend on the film thickness and the deposition temperature. For the films deposited at a certain temperature, there exists a critical thickness where the coercive and saturation fields show maxima.

Figure: Hysteresis loop measured by Longitudinal Kerr Microscope along easy axis for a FePt film with 20 nm thickness, Tsub = 400ºC). (b) to (f) are domain images taken at fields marked by point 1 to 5 in (a) respectively.

Controlling the size and relaxation dynamics of superferromagnetic domains

N. Chowdhury, S. Bedanta, S. Sing, and W. Kleemann

J. Appl. Phys. 117, 153907 (2015)

The magnetization reversal process has been studied on a discontinuous metal-insulator multilayer sample [Co80Fe20(1.3nm)/Al2O3(3nm)]10 showing superferromagnetic (SFM) behavior. Size and shape of SFM domains can be changed by varying the angle between the magnetic field and the easy axis. The magnetization reversal process along the easy axis (EA) is governed by domain wall motion. However, upon approaching the hard axis (HA) at increasing coherent rotation of superspins becomes dominant in the magnetization reversal process. This leads to the reduction of domain sizes for larger values of . The relaxation of magnetization has also been studied for various values of . We find fast relaxation along the easy axis, while the relaxation time increases for increasing ?. In general, the angular dependence of the magnetization reversal of SFM domains shows properties similar to continuous thin films.

Figure: Domain images for [Co80Fe20 (1.3nm)/ Al2O3 (3nm)]10 recorded with LMOKE micro-scopy with H directed along θ = 0° (EA), 30°, 45°, 60°, and 90° (HA).