Magnetic
2021/03
Ashish Chhaganlal Gandhi, Srikrishna Tummala, Hsin-Hao Chiu, Ming-Kang Ho, Tai-Yue Li, Chung-Kai Chang, Chia-Liang Cheng, Yen-Peng Ho and Sheng Yun Wu
ACS Appl. Nano. Mater. 2021, XXXX, XXX, XXX-XXX
The search for diluted magnetic semiconductors (DMSs) has gained immense research interest because of the coexistence of the charge and spin degree of freedom in a single substance to realize a particular class of spintronic devices, and a rare earth (RE)-doped transition-metal oxide (TMO) is one of the choices. This study intends to understand the effect of RE Sm3+ ion substitution in antiferromagnetic (AF) NiO nanoparticles (NPs) using modern cutting-edge techniques (synchrotron powder X-ray diffraction and soft X-ray absorption, Raman scattering, and superconducting quantum interference device magnetometry). A percolation threshold limit of about 1% incorporation of Sm3+ ions at the Ni2+ site was evident. An enhanced magnetic moment observed for an intermediate composition has been attributed to the interacting bound magnetic polaron. A core–shell model has been proposed such that the multivalent point defects reside at the surface of NPs, whereas the core of the particles retains AF properties. The exchange coupling mediated by interfacial frozen spins is the leading mechanism behind the magnetic memory effect at room temperature. The outcome of this study is vital for the future development of RE-functionalized TMO DMS spintronic devices and the understanding of their fundamental physics and chemistry.
Full Paper: https://pubs.acs.org/doi/abs/10.1021/acsanm.1c01535
2021/03
Room Temperature Magnetic Memory Effect in Nanodiamond/γ-Fe2O3 Composites
Ashish Chhaganlal Gandhi,Rajakar Selvam,Chia-Liang Cheng andSheng Yun Wu
Nanomaterials 11(3), 648, 2021
We report a room temperature magnetic memory effect (RT-MME) from magnetic nanodiamond (MND) (ND)/γ-Fe2O3 nanocomposites. The detailed crystal structural analysis of the diluted MND was performed by synchrotron radiation X-ray diffraction, revealing the composite nature of MND having 99 and 1% weight fraction ND and γ-Fe2O3 phases, respectively. The magnetic measurements carried out using a DC SQUID magnetometer show the non-interacting superparamagnetic nature of γ-Fe2O3 nanoparticles in MND have a wide distribution in the blocking temperature. Using different temperature, field, and time relaxation protocols, the memory phenomenon in the DC magnetization has been observed at room temperature (RT). These findings suggest that the dynamics of MND are governed by a wide distribution of particle relaxation times, which arise from the distribution of γ-Fe2O3 nanoparticle size. The observed RT ferromagnetism coupled with MME in MND will find potential applications in ND-based spintronics.
Full Paper: https://www.mdpi.com/2079-4991/11/3/648
2020/09
Ashish Chhaganlal Gandhi, Hsin-Hao Chiu, Kuan-Ting Wu, Chia-Liang Cheng, and Sheng Yun Wu
Appl. Surf. Sci. ,536, 147856, 2020
We report a possible route to achieve room temperature (RT) magnetic memory effect (MME) simply by thermal treatment from barely 1 % Fe-substituted NiO nanoparticles. The post-annealing of Ni0.99Fe0.01O nanoparticles leads to the formation of a Fe-rich NiO surface, resulting in an enhanced collective spin behavior because of the increase in interparticle interaction. This surface-spin driven collective spin behavior provided additional anisotropic energy leading to the occurrence of RT MME. This study demonstrates an approach to tune the magnitude of MME simply by controlling the post-annealing temperature for 1 % Fe-dopant concentration. The proposed method opens a route for implementing RT spintronic devices and a deeper understanding of their fundamental physics.
Full Paper: https://doi.org/10.1016/j.apsusc.2020.147856
2020/07
Ashish Chhaganlal Gandhi, Tai-Yue Li, B Vijaya Kumar, P Muralidhar Reddy, Jen-Chih Peng, Chun-Ming Wu, Sheng Yun Wu
Nanomaterial 10(7), 1318, 2020
The Fe-doped NiO nanoparticles that were synthesized using a co-precipitation method are characterized by enhanced room-temperature ferromagnetic property evident from magnetic measurements. Neutron powder diffraction experiments suggested an increment of the magnetic moment of 3 d ions in the nanoparticles as a function of Fe-concentration. The temperature, time, and field-dependent magnetization measurements show that the effect of Fe-doping in NiO has enhanced the intraparticle interactions due to formed defect clusters. The intraparticle interactions are proposed to bring additional magnetic anisotropy energy barriers that affect the overall magnetic moment relaxation process and emerging as room temperature magnetic memory. The outcome of this study is attractive for the future development of the room temperature ferromagnetic oxide system to facilitate the integration of spintronic devices and understanding of their fundamental physics.
Full Paper: https://www.mdpi.com/2079-4991/10/7/1318
2020/01
Ashish Chhaganlal Gandhi and Sheng Yun Wu
AIP Advances 10(1), 015211
We report room temperature (RT) ferromagnetism and magnetic memory effect in Ni0.95Fe0.05O nanoparticles (NPs) synthesize by hydrothermal method followed by post-annealing in an ambient atmosphere. The temperature and time-dependent magnetization measurements show that the effect of post-annealing at higher temperatures leads to enhancement in the intraparticle interactions. The enhanced intraparticle interaction has provided additional magnetic anisotropy energy resulting in RT ferromagnetic (FM) properties and enhanced magnetic memory effect. The findings from this study will be useful for the development and understanding of RT FM materials to facilitate the integration of spintronic devices.
Full Paper: https://aip.scitation.org/doi/full/10.1063/1.5129785
2019/11
Ashish Chhaganlal Gandhi, Tai-Yue Li, Jen-Chih Peng, Chin-Wei Wang, Ting Shan Chan, Jauyn Grace Lin, Sheng Yun Wu
ACS Applied Nano Materials 2(12), 8027-8042, 2019
The origin of the concomitant memory effect is still a controversial issue and poor evidence in the observation of nanocrystal systems. We report on a type of concomitant memory effect driven by first-field-induced unidirectional magnetic anisotropy at the interface of ferromagnetic CrO2-core and antiferromagnetic Cr2O3-shell nanorods, with the effect becoming less significant in pure CrO2 nanorods. To corroborate the results, a core–shell anisotropic energy model was used to determine the coherent rotation of magnetization and exchange coupling constant, giving direct access to the anisotropic interfacial properties of the core–shell nanoscale system without spatially resolved magnetic measurement. The outcome of this study will be useful for the future development of thermal memory devices.
Full Paper: https://pubs.acs.org/doi/abs/10.1021/acsanm.9b02084
2018/12
Ashish Chhaganlal Gandhi, Pradeep Raja Anandan, Yu-Chen Yeh, Tai-Yue Li, Chi-Yuan Wang, Yasuhiro Hayakawa, Sheng Yun Wu
ACS Applied Nano Materials 2(1), 278-290, 2019
Uniform hexagonal single phase Ni1–xFexO (x = 0, 0.01, 0.05, and 0.1) nanoparticles synthesized by a standard hydrothermal method are characterized with an enhanced lattice expansion along with a decrease in the microstrain, crystal size, and Ni occupancy as a function of the Fe concentration. The observed anomalous temperature and field dependent magnetic properties as a function of the Fe content were explained using a core–shell type structure of Ni1–xFexO nanoparticle such that the effect of Fe-doping has led to a decrease of disordered surface spins and an increase of uncompensated-core spins. Perfect incorporation of Fe3+ ions at the octahedral site of NiO was observed from the low Fe concentration; however, at a higher Fe content, 4:1 defect clusters (four octahedral Ni2+ vacancies surrounding an Fe3+ tetrahedral interstitial) are formed in the core of the nanoparticles, resulting in the transition of spin-glassy to the cluster-glassy system. An enhanced thermal magnetic memory effect is noted from the cluster-glassy system possibly because of increased intraparticle interactions. The outcome of this study is important for the future development of diluted magnetic semiconductor spintronic devices and the understanding of their fundamental physics.
Full Paper: https://pubs.acs.org/doi/full/10.1021/acsanm.8b01898
2018/05
Ashish Chhaganlal Gandhi, Tai-Yue Li, Ting Shan Chan and Sheng Yun Wu
Nanomaterials 8(5), 312, 2018
With the evolution of synthesis and the critical characterization of core-shell nanostructures, short-range magnetic correlation is of prime interest in employing their properties to develop novel devices and widespread applications. In this regard, a novel approach of the magnetic core-shell saturated magnetization (CSSM) cylinder model solely based on the contribution of saturated magnetization in one-dimensional CrO2/Cr2O3 core-shell nanorods (NRs) has been developed and applied for the determination of core-diameter and shell-thickness. The nanosized effect leads to a short-range magnetic correlation of ferromagnetic core-CrO2 extracted from CSSM, which can be explained using finite size scaling method. The outcome of this study is important in terms of utilizing magnetic properties for the critical characterization of core-shell nanomagnetic materials
Full Paper: https://www.mdpi.com/2079-4991/8/5/312
2018/04
W. C. Lin, C. L. Tsai, K. Ogawa, S. Yamada, A. C. Gandhi, J. G. Lin
Applied Physics Letters 112(16), 162403, 2018
The interfacial magnetic coupling between metallic Fe and the double-perovskite NdBaMn2O6 single crystal was investigated in the heterostructure of 4-nm Pd/10-nm Fe/NdBaMn2O6. A considerable magnetic coupling effect was observed in the temperature range coincident with the magnetic phase transition of NdBaMn2O6. When the temperature was elevated above 270 K, NdBaMn2O6 transformed from a state of antiferromagnetic fluctuating domains to a superparamagnetism-like (ferromagnetic fluctuation) state with high magnetic susceptibility. Concurrently, the interfacial magnetic coupling between the Fe layer and the NdBaMn2O6 crystal was observed, as indicated by the considerable squareness reduction and coercivity enhancement in the Fe layer. Moreover, the presence of the Fe layer changed the magnetic structure of NdBaMn2O6 from conventional 4-fold symmetry to 2-fold symmetry. These observations offer applicable insights into the mutual magnetic interaction in the heterostructures of metallic ferromagnetism/perovskite materials.
Full Paper: https://aip.scitation.org/doi/full/10.1063/1.5023119
2018/01
Swati R. Gawali, Ashish Chhaganlal Gandhi, Shrikrushna Shivaji Gaikwad, Jayashree Pant, Ting-Shan Chan, Chia-Liang Cheng, Yuan-Ron Ma & Sheng Yun Wu*
Scientific Reports 8(1), 249, 2018
We report the phonon and magnetic properties of various well-stabilized Co3O4 nanoparticles. The net valence in cobalt (II)/(III) cation can be obtained by subtracting the Co2+ ions in tetrahedral interstices and Co3+ ions in the octahedral interstices, respectively, which will possess spatial inhomogeneity of its magnetic moment via Co2+ in tetrahedra and Co3+ in octahedral configurations in the normal spinel structure. Furthermore, the distribution of Co2+/Co3+ governed by various external (magnetic field and temperature) and internal (particle size and slightly distorted CoO6 octahedra) sources, have led to phenomena such as a large redshift of phonon-phonon interaction and short-range magnetic correlation in the inverse spinel structure. The outcome of our study is important in terms of the future development of magnetic semiconductor spintronic devices of Co3O4.
Full Paper: https://www.nature.com/articles/s41598-017-18563-9
2017/12
Ashish Chhaganlal Gandhi, Ting Shan Chan, Jayashree Pant, Sheng Yun Wu
Nanoscale Research Letters 12(1), 1-8, 2017
After a decade of effort, a large number of magnetic memory nanoparticles with different sizes and core/shell compositions have been developed. While the field-cooling memory effect is often attributed to particle size and distribution effects, other magnetic coupling parameters such as inter- and intra-coupling strength, exchange bias, interfacial pinned spins, and the crystallinity of the nanoparticles also have a significant influence on magnetization properties and mechanisms. In this study, we used the analysis of static- and dynamic-magnetization measurements to investigate NiO nanoparticles with different sizes and discussed how these field-cooling strengths affect their memory properties. We conclude that the observed field-cooling memory effect from bare, small size NiO nanoparticles arises because of the unidirectional anisotropy which is mediated by the interfacial strongly pinned spins.
Full Paper: https://nanoscalereslett.springeropen.com/articles/10.1186/s11671-017-1988-x
2017/09
R Pradeep, A. C. Gandhi, Y. Tejabhiram, I. K. Md Mathar Sahib, Y. Shimura, L. Karmakar, D. Das, S. Y. Wu and Y. Hayakawa
Materials Research Express 4, 096103, 2017
Undoped and iron-doped NiO nanoparticle were synthesized by standard hydrothermal method. A detailed study is carried out on the effect of dopant concentration on morphology, structural, resonance and magnetic properties of NiO nanoparticle by varying the Fe concentration from 0.01 to 0.10 M. The synchrotron-x-ray diffraction confirmed that no secondary phase was observed other than NiO. The x-ray photoelectron spectroscopy studies revealed that, Fe was primarily in the trivalent state, replacing the Ni2+ ion inside the octahedral crystal site of NiO. The Electron paramagnetic studies revealed the ferromagnetic cluster formation at high doping concentration (5 and 10%). The ZFC–FC curves displayed an average blocking temperature around 180 K due to particle size distribution. The anomalous behaviour of spontaneous exchange bias (H SEB) and magnetic remanence (M r) for all Fe-doped samples observed at 5 K showed an increase (0.1316–0.1384 emu g−1) in the moment of frozen spin (M p) as the dopant concentration increased. The role of frozen spin moment in spontaneous exchange bias behaviour was discussed.
Full Paper: https://iopscience.iop.org/article/10.1088/2053-1591/aa7f9b/meta
2017/05
Ashish Chhaganlal Gandhi and Sheng Yun Wu
RSC Advances 7, 25512-25518, 2017
Understanding the magnetic anisotropy of antiferromagnetic (AF) transition metal oxides is a matter of importance for its future application in spintronics. In this study we have carried out temperature and field-dependent magnetization measurements to investigate the magnetic anisotropy of AF Cr2O3 nanorods (NRs) with XRD = 24 nm diameter. Our experimental findings suggest that the observed field-cooled memory effect from exchange-coupled Cr2O3 NRs is mediated by unidirectional anisotropy. This finding opens an opportunity for devices utilizing the anisotropy field of bare AF Cr2O3 nano-crystalline structures.
Full Paper: https://pubs.rsc.org/en/content/articlehtml/2017/ra/c7ra03934d
2017/04
Ashish Chhaganlal Gandhi, Y. Weng, and Jauyn Grace Lin
2017 IEEE International Magnetics Conference (INTERMAG)
Inverse spin Hall effect (ISHE) has been intensively investigated by injecting a spin current from ferromagnet (FM) to normal metal (NM) with large spin-orbit-coupling (SOC) and simultaneously detecting the induced electrical current.
Full Paper: https://ieeexplore.ieee.org/abstract/document/8007563
2017/04
Ashish Chhaganlal Gandhi and Jauyn Grace Lin
Journal of Physics: Condensed Matter 29(21), 215802, 2017
Finite sized Ni/NiO nanoparticles (NPs) are prepared by oxidizing pure Ni-NPs in an ambient atmosphere with varying annealing time (t A). A synchrotron radiation x-ray diffraction technique was used to estimate the grain size, weight fraction and lattice parameters of Ni and NiO. The temperature (T) dependencies of effective g-factor and line-width of ferromagnetic resonance (FMR) spectra for a series of Ni/NiO NPs are determined. Three T-regions with different FMR behaviors T> 200 K, 200> T> 130 K and T< 130 K are identified. In particular, for T< 200 K, the T-dependency of the g-factor reveals an evolution of exchange coupling between Ni and NiO due to the gradual oxidation of Ni NPs.
Full Paper: https://iopscience.iop.org/article/10.1088/1361-648X/aa6a7c/meta
2017/03
Ashish Chhaganlal Gandhi, Rajasree Das, Fang-Cheng Chou, Jauyn Grace Lin.
Journal of Physics: Condensed Matter 29(17), 175802, 2017
Understanding of magnetocrystalline anisotropy in CaFe 2 O 4 is a matter of importance for its future applications. A high quality single crystal CaFe 2 O 4 sample is studied by using synchrotron x-ray diffraction, a magnetometer and the electron spin resonance (ESR) technique. A broad feature of the susceptibility curve around room temperature is observed, indicating the development of 1D spin interactions above the on-set of antiferromagnetic transition. The angular dependency of ESR reveals an in-plane two-fold symmetry, suggesting a strong correlation between the room temperature spin structure and magnetocrystalline anisotropy. This finding opens an opportunity for the device utilizing the anisotropy field of CaFe 2 O 4.
Full Paper: https://iopscience.iop.org/article/10.1088/1361-648X/aa61f2/meta
2016/10
Ashish Chhaganlal Gandhi and Jauyn Grace Lin
Journal of Magnetism and Magnetic Materials 424, 221-225, 2016
Structural and magnetic properties of finite sized NiO nanoparticles are investigated with synchrotron X-ray diffraction (XRD), transmission electron microscopy, magnetometer and ferromagnetic resonance (FMR) spectroscopy. A minor Ni phase is detected with synchrotron XRD, attributed to the oxygen defects in the NiO core. A considerable exchange bias of ~100 Oe is observed at 50 K and it drops abruptly and vanishes above 150 K, in association with the reduction of frozen spins. FMR data indicate a strong interaction between ferromagnetic (FM) and antiferromagnetic (AFM) phases below 150 K, consistent with the picture of isolated FM clusters in AFM matrix.
Full Paper: https://www.sciencedirect.com/science/article/abs/pii/S0304885316319898
2016/03
Ashish Chhaganlal Gandhi, Hui-Yu Cheng, Yu-Ming Chang, Jauyn Grace Lin
Materials Research Express 3(3), 035017, 2016
The question of whether NiO nanoparticles contain metallic ferromagnetic Ni clusters is still a matter of debate, and it is very important from an application point of view. Resolving this problem relies on proper detection probes with high sensitivity and a systematic analysis that would be demonstrated in this study. NiO nanoparticles with mean size ranging from ~4 to 80 nm are synthesized by sol-gel method. Synchrotron x-ray diffraction, transmission electron microscopy, Raman and ferromagnetic resonance (FMR) spectroscopy are used to study the size effects on the structures and magnetic properties of nanoparticles. It is found that a minor Ni phase below 1% in NiO nanoparticles is traceable with synchrotron XRD, selective area electron diffraction and static/dynamic magnetic measurements. The Ni phase only exists in NiO nanoparticles with the size ranging from 8 to 20 nm, attributed to the oxygen vacancies in core structure. Our findings provide important information for controlling the magnetic properties of NiO nanoparticles.
Full Paper: https://iopscience.iop.org/article/10.1088/2053-1591/3/3/035017/meta
2015/12
Ashish Chhaganlal Gandhi, Jayashree Pant, Sheng Yun Wu
RSC advances 6(3), 2079-2086, 2016
We report the finite size effect and nickel vacancy defects in NiO nanoparticles that result in the formation of magnetic phase separation in uncompensated antiferromagnetic NiO-cores with frustrated and disordered spins at the surface NiO-shell. The inter-particle interaction is probed by analyzing the relaxation dynamics measurements. A significant index for the interaction of n derived from the dynamic magnetization is proposed, which paves the way for the examination of the spontaneous exchange bias mechanism and offers insight into the influence of the particle size and defects.
Full Paper: https://pubs.rsc.org/en/content/articlelanding/2015/ra/c5ra24673c/unauth#!divAbstract
2015/09
Ashish Chhaganal Gandhi, P Muralidhar Reddy, Ting-Shan Chan, Yen-Peng Ho, Sheng Yun Wu
RSC advances 5(103), 84782-84789, 2015
We report the influence of the nanosized effect on the superconducting properties of bimetallic In2Bi nanoparticles. In this study, the temperature- and applied magnetic field-dependence of the magnetization were utilized to investigate the electron-phonon coupling effect while controlling particle sizes 〈d〉 from 21(2) to 42(5) nm. As the particle size decreases, the electron-phonon constant λEP decreases rapidly, signaling the short-range electron-phonon coupling effect which acts to confine the electrons within a smaller volume, thereby giving rise to a higher superconducting transition temperature TC. An enhanced superconducting transition was observed from the temperature dependence of magnetization, revealing a main diamagnetic Meissner state below TC ∼ 5.72(5) K for 〈d〉 = 31(1) nm In2Bi nanoparticles. The variation of the TC is very sensitive to the particle size, which might be due to crystallinity and size uniformity of the samples. The electron-phonon coupling to low lying phonons is found to be the leading mechanism for the observed strong-coupling superconductivity in the In2Bi system.
Full Paper: https://pubs.rsc.org/en/content/articlelanding/2015/ra/c5ra14332b/unauth#!divAbstract
2014/11
S Y Wu, J Y Ji, P H Shih, A. C. Gandhi, T S Chan
Journal of Applied Physics 116, 193906, 2014
A study of “proteresis (inverted hysteresis)” in core-shell, Cu2O/CuO nanoparticles, is presented. Crystal and characteristic sizes are determined using the x-ray absorption near-edge structure method for the weak ferromagnetic core (Cu2O) and antiferromagnetic shell (CuO) nanoparticles. A core-shell anisotropic energy model is established to describe the observed proteretic behavior in Cu2O/CuO core-shell nanocrystals. The proteresis loop triggered by the applied magnetic field can be tentatively attributed to core-shell exchange coupling induced by the surface of the Cu2O core, hinting at a possible way to tune the strength of the exchange inter-coupling energy that can control the dynamic of proteresis and hysteresis in a core-shell system.
Full Paper:https://aip.scitation.org/doi/full/10.1063/1.4902524
