United States

The spin nematic order has attracted a lot of interest in the field of magnetism. It is a kind of multipole order of spins. The previous theoretical and numerical studies[1-3] predicted that the spin nematic order would be induced by the frustration of the ferromagnetic and antiferromagnetic exchange interactions, or the biquadratic interaction. The spin nematic order is characterized by the long-range four spin correlation and the two-magnon bound state. The previous numerical diagonalization and the finite-size scaling study had indicated that a two-magnon bound state can occur in the S=1 antiferromagnetic chain with the single-ion anisotropy under magnetic field[4].The recent analysis of the critical exponents of the spin correlation functions[5] suggested that this two-magnon bound state includes the spin nematic liquid phase, as well as the spin density wave (SDW) liquid one. The phase diagrams with respect to the anisotropy and the magnetization were obtained by the numerical diagonalization of finite size clusters. The same numerical analysis indicated that the spin nematic liquid phase appears in the magnetization process of the 1/2 spin ladder system with the anisotropic ferromagnetic rung interaction[6,7].These systems would be able to exhibit the spin nematic liquid even without the frustration or the biquadratic exchange interaction. &lt;br/&gt; In the present study, the S=1/2 distorted diamond spin chain is investigated using the numerical diagonalization of finite-size clusters. This system is a typical frustrated system. The recently discovered candidate material of this system, K3Cu3AlO2(SO4)4 called alumoklyuchevskite[8], includes the ferromagnetic interactions. Thus we investigated the distorted diamond chain with the ferromagnetic interactions, as well as the coupling anisotropy. As a result we found that a two-magnon bound state appears in the magnetization process. We also discus about the spin nematic liquid behaviors.&lt;br&gt;

**References**&lt;br&gt; [1] A. F. Andreev and A. Grishchuk, Sov. Phys. JETP 60, 267 (1984).&lt;br&gt;
[2] H. H. Chen and P. M. Levy, Phys. Rev. Lett. 27, 1383 (1971).&lt;br&gt;
[3] T. Hikihara et al., Phys. Rev. B 78, 144404 (2008).&lt;br&gt;
[4] T. Sakai, Phys. Rev. B 58, 6268 (1998).&lt;br&gt;
[5] T. Sakai, H. Nakano, K. Okamoto and R. Furuchi, J. Phys.: Conf. Ser. 2164, 012030 (2022).&lt;br&gt;
[6] T. Sakai, K. Okamoto and T. Tonegawa, phys. Status solidi B 247, 583 (2010)&lt;br&gt;
[7] T. Sakai et al., in preparation.&lt;br&gt;
[8] M. Fujihara et al., Sci. Rep. 7, 16785 (2017).&lt;br&gt;

MMM 2022

Spin Nematic Liquid of the S=1/2 Distorted Diamond Spin Chain in Magnetic Field

The spin nematic order has attracted a lot of interest in the field of magnetism. It is a kind of multipole order of spins. The previous theoretical and numerical studies[1-3] predicted that the spin nematic order would be induced by the frustration of the ferromagnetic and antiferromagnetic exchange interactions, or the biquadratic interaction. The spin nematic order is characterized by the long-range four spin correlation and the two-magnon bound state. The previous numerical diagonalization and the finite-size scaling study had indicated that a two-magnon bound state can occur in the S=1 antiferromagnetic chain with the single-ion anisotropy under magnetic field[4].The recent analysis of the critical exponents of the spin correlation functions[5] suggested that this two-magnon bound state includes the spin nematic liquid phase, as well as the spin density wave (SDW) liquid one. The phase diagrams with respect to the anisotropy and the magnetization were obtained by the numerical diagonalization of finite size clusters. The same numerical analysis indicated that the spin nematic liquid phase appears in the magnetization process of the 1/2 spin ladder system with the anisotropic ferromagnetic rung interaction[6,7].These systems would be able to exhibit the spin nematic liquid even without the frustration or the biquadratic exchange interaction. In the present study, the S=1/2 distorted diamond spin chain is investigated using the numerical diagonalization of finite-size clusters. This system is a typical frustrated system. The recently discovered candidate material of this system, K3Cu3AlO2(SO4)4 called alumoklyuchevskite[8], includes the ferromagnetic interactions. Thus we investigated the distorted diamond chain with the ferromagnetic interactions, as well as the coupling anisotropy. As a result we found that a two-magnon bound state appears in the magnetization process. We also discus about the spin nematic liquid behaviors. 

**References** [1] A. F. Andreev and A. Grishchuk, Sov. Phys. JETP 60, 267 (1984). 
[2] H. H. Chen and P. M. Levy, Phys. Rev. Lett. 27, 1383 (1971). 
[3] T. Hikihara et al., Phys. Rev. B 78, 144404 (2008). 
[4] T. Sakai, Phys. Rev. B 58, 6268 (1998). 
[5] T. Sakai, H. Nakano, K. Okamoto and R. Furuchi, J. Phys.: Conf. Ser. 2164, 012030 (2022). 
[6] T. Sakai, K. Okamoto and T. Tonegawa, phys. Status solidi B 247, 583 (2010) 
[7] T. Sakai et al., in preparation. 
[8] M. Fujihara et al., Sci. Rep. 7, 16785 (2017).

poster

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

The Conference was held from October 31 to November 4 and offers both in-person and on-demand content.

MMM 2022 is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a tutorial, and several special sessions. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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Metal-organic frameworks (MOFs) are a novel and the most prominent class of microporous materials for the applications such as gas storage and separation, catalysis, heat storage and liquid purification, owing to their unique structural diversity and tunability. [1] In the current scenario, understanding the magnetic properties of MOFs has started to be explored in the field of molecular magnetism, [2,3] and the electron paramagnetic resonance (EPR) spectroscopy is one of the inevitable tools to investigate the magnetic exchange interactions between spin centers to understand the local structure of the MOF materials [1,4,5]. Herein, EPR measurements at X- (9.4 GHz), Q- (34 GHz) and W-band (94 GHz) on paddlewheel (PW) type post-synthetic metal exchanged DUT-49(M, M): M-Zn, Mn, Cu MOFs were investigated. Temperature-dependent X-band measurements were recorded from T = 7 K to T = 170 K on DUT-49(Cu), DUT-49(Mn) and bimetallic DUT-49(CuZn) and DUT-49(CuMn) MOFs. Moreover, an EPR signal intensity of the magnetically coupled metal ion pairs in the PW units, which is proportional to the magnetic susceptibility, was extracted from the temperature-dependent X-band EPR data for all MOFs. The sign of the isotropic coupling constant (2J = -239 cm-1) determined via Bleaney Blowers susceptibility fit confirms the excited S= 1 state antiferromagnetic interaction above T = 60 K of DUT-49(Cu) within the paddlewheel. Also, other DUT-49(M, M) MOFs are expected to have antiferromagnetic interaction within the paddle wheel (see Table). X-band measurements at T =160 K confirm the excited spin states (SCuCu = 1, SCuMn = 3, and SMnMn = 5) of the antiferromagnetically (AFM) coupled ions of the PW units for the DUT-49(Cu), DUT-49(CuMn) and DUT-49(Mn) MOFs, respectively. While signals observed from DUT-49(CuMn) and DUT-49(CuZn) MOFs at T = 7 K could be due to the low spin states of the coupled paramagnetic ions within the PW and corresponds to the SCuMn = 2 and SCuZn = 1/2 spin states, respectively.

Magnetic Coupling of Divalent Metal Centers in Post Synthetic Metal Exchanged Bimetallic DUT 49 MOFs by EPR interrogations

The Spin-MOSFET is expected to show a high performance in integrated circuits (1). Efficient spin injection into semiconductors from ferromagnet is important for the realization of the Spin-MOSFET. However, it is difficult to realize efficient spin injection due to the conduction mismatch problem (2). To enhance the efficiency of spin injection, ferromagnets with high spin polarization and semiconductor like resistivity were proposed. We focused on a new class material with both these properties: Spin gapless semiconductor CoFeMnSi (CFMS). CFMS have a spin gapless structure and semiconductor behavior which have been proofed in theories and experiments (3, 4). 
In this study, we aim to investigate the crystal structure, magnetic and electrical properties of CMFS thin films grown on MgO substrates.
A structure of CFMS (50nm)/Ta (5nm) was deposited by magnetron sputtering on MgO (100) substrates. The annealing temperature (Ta) of CFMS layer was between 300 and 600oC. The composition of CFMS thin films was confirmed by ICP-MS (Co : Fe : Mn : Si = 25.5 : 24.2 : 26.3 : 23.9). X-ray diffraction (XRD) was performed for characterization of crystal structure. Fig. 1 shows the annealing temperature dependence of (111) peaks of CFMS thin films. The (111) peaks were obtained above Ta = 400oC, indicating that the epitaxial growth of highly ordered (L21 or Y-type) CMFS thin films was successful. Fig. 2 shows the temperature dependence of resistivity and conductivity of CFMS thin film at Ta = 500oC. Although the electrical conductivity σxx value of our sample is around 5.3×103 S/cm, about two orders of magnitude lower than the half metallic ferromagnetic alloy Co2MnSi (~105 S/cm), non-semiconductor behaviour of resistivity was observed above 50K. Composition dependence of the CFMS thin films properties and guideline for realization of spingapless semiconductor behaviour will be presented in the conferenece.
This research was conducted by participating in the GP-Spin and JST’s SPRING program, Tohoku University, and supported by CSIS Organization for Advanced Studies and the CSRN. 

**References:** 
(1) S. Datta et al., Appl. Phys. Lett. 56, 665 (1998). 
(2) G. Schmidt et al., Phy. Rev. B 62, R4790 (2000). 
(3) X. Dai et al., J. Appl. Phys. 105, 07E901 (2009). 
(4) L. Bainsla et al., Phy. Rev. B 91, 104408 (2015). 

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Investigation of Spin Gapless Semiconductor CoFeMnSi Thin Films on MgO Substrate

Materials showing half-metallic band structure with a high Curie temperature and moderate magnetization are attractive for spin-transport-based devices. Some Heusler compounds exhibit these features and have attracted much attention. We have synthesized one such compound, Co1.5Mo0.5FeAl, using arc melting and high vacuum annealing. The arc-melted ingot was annealed at 600 oC for 48 hours. The room temperature x-ray diffraction suggests that the sample has cubic crystal structure with A2 type disorder. The temperature dependence of magnetization measured at 1 kOe shows a very high Curie temperature of about 950 K. There is an anomaly in the M(T) curve near 750 K, Fig.1. The sample exhibits a moderate saturation magnetization of 68 emu/g at room temperature. These results show that Co1.5Mo0.5FeAl has potential for high temperature magnetic and spintronic applications. Here, we will also discuss the electronic and magnetic properties of cubic Co1.5Mo0.5FeAl predicted by our first principles calculations.
This research is supported by the National Science Foundation (NSF) under Grant Numbers 2003828 and 2003856 via DMR and EPSCoR. 

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Structural and Magnetic Properties of Co1.5Mo0.5FeAl Alloy

The promising properties of Co-based (i.e. Co2BC) Heusler alloys (HA) such as half-metallicity, magneto-caloric effect, shape-memory effect, spin-valve like behavior, etc. make them important from technological as well as fundamental point of view.1-5 In the present work, we have probed the role of substitution of 3d, 4d and 5d elements (i.e, with electronic configuration (EC): ndx where n = 3-5 ; x = number of electrons in d level) at the B site on structural, magnetic and electronic properties of a set of Co-based full HAs using DFT based first principle calculations.
We have optimized all the 30 combinations studied here in Fm-3m (A2BC conventional HA structure) and F-43m (AABC inverse HA structure) space groups using VASP.6 Thereafter, electronic structure (ES) calculations have been carried out using FPLAPW method as implemented in WIEN2k7 and results obtained using PBE and mBJ exchange-correlation terms have been compared. Further, XMCD calculations have been performed using single particle approximation and Fermi-golden rule as implemented in WIEN2k7.
It is observed that the HAs with EC: nd1-6,10 (n = 3, 4, 5) at B site have Fm-3m as the lowest energy structure. Calculated magnetic moment of these alloys with EC: nd1-6 are found to be in agreement with the Slater-Pauling (SP) rule with an integer or close to integer value, except 4d1, 4d6 and 5d6 as shown in Figure 1. Furthermore, alloys with EC: nd2-6 show half-metallic or half-metallic nature. However, alloys having EC: nd7-9 (along with 4d1 and 4d6) at B site are found to be in F-43m space group. Further, magnetic moment of EC: nd7-10 show deviation from the SP rule. In order to understand the impact of nd1-10 electrons on the local environment of various atomic sites, detailed analyses of the XMCD spectra have been carried out on the basis of changes in the orbital projected unoccupied density of states. 

**References:** 
1. T. Roy, et al., Phys. Rev. B., 93, 184102 (2016) and references therein 
2. M. Baral, et al., Phys. Rev. B., 99, 205136 (2019) and references therein 
3. K. Inomata, et al., J. Appl. Phys., 95, 7234 (2004) and references therein 
4. R. Dutt et al., J. Phys. Condens. Matter , 33, 045402 (2020) and references therein 
5. I. Galankis, et al., Phys. Rev. B., 66, 134428 (2002) and references therein 
6. G. Kresse et al., Phys. Rev. B., 54, 11169 (1996) and references therein 
7. P. Blaha, et al., J. Chem. Phys. 152, 074101 (2020) and references therein 

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Effect of substitution of 3d, 4d and 5d elements on structural, electronic, magnetic properties and XMCD spectra of Co based Full Heusler alloys: A DFT Study

Recently, Co2TiSi Heusler alloy is theoretically predicted that is one of the ferromagnetic Weyl semimetal (WSM) candidates, and also the half-metal (1, 2, 3), which has potential for both a large anomalous Hall effect (AHE) and a large spin polarization. It is also expected that the anomalous Hall effect can be enhanced by customizing Co2(Ti1-xVx)Si due to the Fermi level shift (4, 5). In this research, we optimized the fabrication conditions for producing Co2(Ti1-xVx)Si thin films, and investigated their AHE and anisotropic magnetoresistance (AMR) effect to discuss the electronic structure in Co2(Ti1-xVx)Si.
The UHV magnetron co-sputtering method was used for the preparation of thin films using Co2TiSi and Co2VSi targets. The structure of the sample was MgO (001) sub. / Co2(Ti1-xVx)Si (50 nm) / Ta (5 nm), and the doping amount of x was changed. We characterized the crystal structure, magnetic properties, AHE and AMR effect by XRD, SQUID, and PPMS, respectively.
High quality L21 ordered single crystal films (SL21 ≈ 70%) were successfully fabricated by applying adequate annealing process. Fig. 1 shows doping value x dependence of anomalous Hall angle (AHA), measured at 10 K. AHA of Co2(Ti1-xVx)Si sample films enhanced by V doping, and the highest AHA was observed near 3% at x = 0.31. Fig. 2 shows measurement temperature dependence of AMR radio when the electric current flowed in the Co2(Ti1-xVx)Si [110] and [100] directions. The magnitude of AMR ratio increases with decreasing temperature for both directions, but the sign is positive for [110] and negative for [100] direction, respectively. The s-d scattering theory of AMR (6) suggests that d-orbitals of majority spin electron are split by crystal field and the density of states of ε- and γ-orbitals are well different at the Fermi level. The change of magnitude of AMR for [100] direction implies that the Fermi level tuning was realized by V doping into the Co2TiSi Heusler alloy thin films.
This research was conducted by participating in the GP-Spin and JST's SPRING program, Tohoku University, and supported by CSIS Organization for Advanced Studies and the CSRN. 

**References:** 
(1) A. Bernevig, H. M. Weng, Z. Fang and X. Dai, J. Phys. Soc. Jpn., 87, 041001 (2018). 
(2) G. Q. Chang, S. Y. Xu, H. Zheng et al., Sci. Rep. 6. 38839; doi: 10.1038/srep38839 (2016). 
(3) J. Barth, G. H. Fecher, B. Balke et al., Phys. Rev. B 81, 064404 (2010). 
(4) J. Zou, Z. He, and G. Xu, Npj Computational Materials, 5(1) (2019). 
(5) I. Galanakis, P. H. Dederichs, and N. Papanikolaou, Phys. Rev. B 66, 174429 (2002). 
(6) S. Kokado and M. Tsunoda, J. Phys. Soc. Jpn. 88, 034706 (2019). 

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Anomalous Hall Effect and Anisotropic Magnetoresistance Effect in Co2(Ti1 xVx)Si Heusler Alloy Single Crystal Films

In order to improve the multiferroic properties of BiFeO3 (BFO), partial substitution of the A site for Bi has been reported to effective method. Last decade, two classes of dopants used include rare earth elements (R), such as Y, La, Nd, Pr, and Sm (1), and alkali earth elements, e.g. Ca, Sr, and Ba (2). These dopants could reduce the leakage current density and improve the ferroelectric as well as magnetic properties of BFO. Although the investigations on multiferroic properties of R-doped BFO are extensive, BFO doped with heavy rare earth (H) is less discussed. To substitute H, such as Gd, Tb, Dy, Ho, and Er, for Bi in BFO is expected to improve the magnetic properties because the larger magnetic moment (7.8-10.6 μB) for H ions may introduce an additional magnetic interactions and ordering in BFO (3). In this work, we adopted pulsed laser deposition (PLD) to develop Bi0.9H0.1FeO3 (BHFO) films on glass substrates. The microstructure, ferroelectric, and magnetic properties of BHFO films are studied. BHFO films were confirmed to mainly consist of the perovskite phase. The desired multiferroic properties with the suppressed leakage is reached. The magnetic properties with the enhanced magnetization are found for BHFO films: their Ms are 18.8, 18.9, 21.3, 25.6, 18.5 emu/cm3, for the films with H = Gd, Tb, Dy, Ho, and Er, respectively. The higher magnetic moment of H ion is, and larger Ms of the films is. The above behavior reveals magnetization of BFO films with H substitution is mainly dominated by the magnetic moment of H ion. Besides, good ferroelectric properties are also attained for BHFO films, where the largest remanent polarization (2Pr) of 120.1 µC/cm2 is achieved for H = Gd, related to low leakage and high BHFO(110) texture. It might be related to the larger ionic radius for Gd than other H. Relationship between leakage mechanisms and microstructure are also discussed in those BHFO films. 

**References:** 
(1) D. Kan et al., J. Appl. Phys. 110, 014106 (2011). 
(2) H.W. Chang et al., J. Alloys Compd. 683, 427 (2016). 
(3) N. Jeon et al., Appl. Phys. Lett. 98, 072901 (2011).

Enhanced multiferroic properties in Bi0.9H0.1FeO3 polycrystalline films (H = heavy rare earth elements)

Holmium substituted Samarium iron garnet i.e., Sm3-xHoxFe5O12 (x = 0 and 0.6) were synthesized using solid-state reaction route by sintering at 1400°C for 8 hours. Samples are found to be in single phase form as per XRD patterns and refined by Rietveld refinement technique using Ia-3d space group. The lattice constant is found to be decrease from 12.53Å to 12.43Å with Ho doping. Magnetic hysteresis loops (M-H) are measured at room temperature shows that saturation magnetization is decreases from 21.05 emu/g for x = 0 to 12.06 emu/g for x = 0.6 sample. The anisotropy constant is decreasing from 2.42×105 erg/cc for x = 0 to 1.7×105 erg/cc for x = 0.6 sample. Negative magnetization is observed for x = 0.6 sample which can be explained by using the equation i.e., Mnet = 3MFe(d) – [2MFe(a) + (3-x) MSm + xMHo], where Fe(d) and Fe(a) are the Fe atoms at tetrahedral and octahedral site respectively. The frequency dependence of real part of impedance Z' show a step like behavior which indicates the presence of dielectric relaxation in the samples. The merger of all Z' values at higher frequency is due to release of space charges. - Z'' vs. frequency is also plotted and all the plots show relaxation peaks. With the increase in temperature, the peaks are shifting towards higher frequency which suggests that the relaxation is due to thermally activated charge carriers.
Keywords: Ferrimagnetic, negative magnetization, relaxation behavior. 

**References:** 
1. Xiaobo Wu, Study on dielectric and magnetodielectric properties of Lu3Fe5O12 ceramics, Applied Physics Letters 87, 042901 (2005). 
2. Canglong Li, Spin reorientation, normal and inverse magnetocaloric effects in heavy rare-earth iron garnets, Ceramics International 46 (2020) 18758–18762. 
3. Tingsong Zhang, Dielectric and ferroelectric properties of Nd3Fe5O12, Solid State Communications 305 (2020) 113766. 
4. Yu-Jhan Siao, Journal of Applied Physics 109, 07A508 (2011). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/1322cc1f1e92d4d3c0f56be11b5f9c47.png)

Tuning of Magnetic and Electrical properties of Samarium Iron Garnet by Holmium Substitution

Transition metal oxides exhibit rich electrical and magnetic properties. The epitaxial strain plays a vital role to manipulate their magnetic ground state. In addition, Oxygen vacancies (Vo) are one of the most common point defects to play a key role in regulating the physicochemical properties of oxides. (1-4) 
In this context, LCO exhibits interesting phenomena in terms of the origin of magnetism.
In this work, Perovskite LaCoO3 (PV-LCO) films were grown at STO and LAO substrate. Then brownmillerite LaCoO2.5 (BM-LCO) films are formed by annealing in a vacuum at 450 °C for 4h and 500 °C for 4h, respectively. The XRD spectra show that there is no impurity phase during phase transitions, indicating that the films can be fully transformed. (Fig.1a) The magnetic properties were confirmed by SQUID-VSM. The 50-nm-thick PV-LCO film on STO exhibits an obvious ferromagnetism phase. (Fig.1b-c) The tensile strain leads to a decrease of ΔCF due to distortion of CoO6 octahedron, promotes the spin state transition. (5,6) 
However, the same thickness coherently strain film in compression on LAO exhibits extremely weak ferromagnetism. We anticipate the reason for this is because the eg orbitals are split into two separate energy levels due to existence of the JTD distortion, it is always accompanied by an increase of the bond distance rCo–O and suppression of CoO6 octahedral rotation and distortion. Interestingly, the BM-LCO shows no hysteresis loop. We believe the reason for this novel phenomenon is that the increase in ordered Vo leads to the expansion of the lattice, thus inhibiting the rotation and distortion of the CoO6 octahedron and causing the generation of anti-ferromagnetic behavior. (7,8)
In sum, we find that BM-LCO films in epitaxial strain and Vo ordering show no ferromagnetism. This phenomenon provided an effective way to regulate the physical properties of cobalt oxides. It would have a good application value in the fields of ion electrochemical sensors. 

**References:** 
(1) J. H. Ngai, F. J. Walker, and C. H. Ahn, Annual Review of Materials Research 44 (1), 1 (2014). 
(2) A. La Spina, P. Paolicchi, and A. Kryszczynska, Nature 428 (6981), 400 (2004). 
(3) J. Jeong, N. Aetukuri, and T. Graf, Science 339 (6126), 1402 (2013) 
(4) G.E. Sterbinsky, R. Nanguneri, and J.X. Ma, Ryan, Phys. Rev. Lett. 120 197201(2018). 
(5) Er-Jia Guo, Ryan D. Desautels, and David Keavney, Physical Rev B 75 (14) (2007). 
(6) Fuchs, D. C. Pinta, and T. Schwarz, Physic Rev. B 75, 144402 (2007). 
(7) D. Fuchs, E. Arac, and C. Pinta, Phys. Rev. B 77, 014434, (2008). 
(8) Q. Zhang, A. Gao, and F. Meng, Nat Commun 12(1), 1853 (2021). 

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Strain independent Regulation of Magnetic Properties by Topotactic Phase Transformation in LaCoO3 Thin Films

Bimagnetic nanoparticles show promise for applications in energy efficient magnetic storage media and magnetic device applications. The magnetic properties, including the exchange bias of nanostructured materials can be tuned by variation of the size, composition, and morphology of the core vs overlayer of the nanoparticles (NPs). The purpose of this study is to investigate the optimal synthesis routes, structure and magnetic properties of novel CoO/NiFe2O4 heterostructured nanocrystals (HNCs). In this work, we aim to examine how the size impacts the exchange bias, coercivity and other magnetic properties of the CoO/NiFe2O4 HNCs. The nanoparticles with sizes ranging from 10 nm to 24 nm were formed by the synthesis of an antiferromagnetic (AFM) CoO core and deposition of a ferrimagnetic (FiM) NiFe2O4 overlayer. A highly crystalline magnetic phase is more likely to occur when the morphology of the core-overgrowth is present, which enhances the coupling at the AFM-FiM interface. The CoO core NPs are prepared using thermal decomposition of Co(OH)2 at 600 °C for 2 hours in a pure argon atmosphere, whereas the HNCs are obtained first using thermal evaporation followed by hydrothermal synthesis. The structural and morphological characterization made using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) techniques verify that the HNCs are comprised of a CoO core and a NiFe2O4 overgrowth phase. Rietveld refinement of the XRD data shows that the CoO core has the rocksalt (Fd3(bar)m) crystal structure and the NiFe2O4 overgrowth has the spinel (C12/m1) crystal structure. SEM-EDS data indicates the presence and uniform distribution of Co, Ni, and Fe in the HNCs. XPS analysis of the O 1s, Co 2p, Ni 2p, and Fe 2p regions shows that both the core and overgrowth phases are well-structured. The results from PPMS magnetization measurements and high-resolution transmission electron microscopy (HR-TEM) of the CoO/NiFe2O4 HNCs will be discussed.

On the synthesis and characterization of bimagnetic CoO/NiFe2O4 heterostructured nanoparticles

In this work, we investigate the synthesis, along with the structural, magnetic, and surface chemical environment properties, of novel Mn-Co-NiO-based heterostructured nanocrystals (HNCs). The objective is to develop novel, well structurally ordered inverted antiferromagnetic (AFM) NiO – ferrimagnetic (FiM) spinel phase overgrowth HNCs. Inverted HNCs are particularly promising for magnetic device applications because their magnetic properties are more easily controlled by having well-ordered AFM cores, which can result in magnetic structures having large coercivities, tunable blocking temperatures, and other enhanced magnetic effects. The synthesis of the HNCs is accomplished using a two-step process: In the first step, NiO nanoparticles are synthesized using a thermal decomposition method. Subsequently, Mn-Co overgrowth phases are grown on the NiO nanoparticles via hydrothermal nanophase epitaxy, using a fixed pH level (~5.3) of the aqueous medium. This pH level was selected based on previous work in our laboratory showing that NiO/Mn3O4 HNCs of constant size have optimal coercivity and exchange bias when synthesized at a pH of 5.0.1 The crystalline structure, surface/interface chemical environment and gross morphology of the Mn-Co-NiO-based HNCs have been analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Scanning Electron Microscopy (SEM) techniques, respectively. Analysis using these techniques shows that the HNCs are composed of a NiO core and a CoMn2O4 overgrowth phase. Rietveld refinement of XRD data shows that the NiO core has the rocksalt (Fm-3m) cubic crystal structure and the CoMn2O4 overgrowth has the spinel (I41/amd) crystal structure. Moreover, an increased relative amount of the CoMn2O4 overgrowth phase is deposited with decreasing NiO core particle size during the synthesis of the HNCs. Analysis of the O 1s, Ni 2p, Co 2p, and Mn 2p regions of the XPS spectra measured from our samples is consistent with well-structured core/overgrowth phases having M-OH (M: Ni, Co, Mn) bonding at their surface regions. The results from PPMS magnetization and high-resolution transmission electron microscopy (HR-TEM) characterization of the Mn-Co-NiO-based HNCs will be discussed. 

**References:** 
(1) Shafe et al., ACS Applied Materials & Interfaces, 13, 24013−24023 (2021)

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