Chapter Three - Magnetic Exchange Phenomena Probed by Neutron Scattering

Frontier molecular orbitals play a crucial role in determining the magnetic behavior and exchange interactions in organic radicals. In this study, we investigate the underlying mechanism influencing the need for orbital planarity and the role of frontier orbital overlap in magnetic exchange interactions. To study this, we designed a series of 12 polyacene-coupled triarylmethyl diradicals, systematically increasing in length of polyacene. We have used nine different DFT functionals for the calculation of the magnetic exchange coupling constant (J). The calculation of magnetic exchange coupling reveals that the GGA functionals define a more accurate spin state, hence more correct magnetic behavior than the meta-GGA and hybrid functionals. We have studied the effect of orbital orientation and their energy gap to understand the high magnetic exchange coupling in the higher polyacene-coupled diradicals. Our calculations revealed that the planarity and overlap of the frontier molecular orbitals are one of the key factors in influencing the strength and behavior of the magnetic exchange interactions in diradicals. Specifically, the overlap between SOMOs and LUMO influences the strength of the magnetic exchange interaction.

We study how a bias voltage changes magnetic exchange interactions. We derive a general expression for magnetic exchange interactions for systems coupled to reservoirs under a bias potential and apply it to spin valves. We find that for metallic systems, the interlayer exchange coupling shows a weak oscillatory dependence on the bias potential. For tunneling systems, we find a quadratic dependence on the bias potential and derive an approximate expression for this bias dependence for a toy model. We give general conditions for when the interlayer exchange coupling is a quadratic function of bias potential.

We investigate the far-from-equilibrium nature of magnetic anisotropy and exchange interactions between molecular magnets embedded in a tunnel junction. By mapping to an effective spin model, these magnetic interactions can be divided into three types: isotropic Heisenberg, anisotropic Ising, and anisotropic Dzyaloshinski-Moriya contributions, which are attributed to the background nonequilibrium electronic structures. We further demonstrate that both the magnetic self- and exchange interactions can be controlled either electrically by gating and tuning the voltage bias, or thermally by adjusting the temperature bias. We show that the Heisenberg and Ising interactions scale linearly, while the Dzyaloshinski-Moriya interaction scales quadratically, with the molecule-lead coupling strength. The interactions scale linearly with the effective spin polarizations of the leads and the molecular coherence. Our results pave a way for smart control of magnetic exchange interactions at atomic and molecular levels.

The interlayer exchange coupling (IEC) in magnetic multilayers determines the spin orientation of individual magnetic layers comprising the system. The dependence of the resistance on the resulting spin configuration in turn leads to the phenomenon known as giant magnetoresistance (GMR). [1] The GMR effect is of key importance to the area of spintronics, in which the spin degree of freedom is utilized in the operation of electronic devices. The behavior of IEC in metallic ferromagnetic multilayers has been extensively investigated, and the ability to control the IEC by structural parameters to be either ferromagnetic (FM) or antiferromagnetic (AFM) is now well established. In contrast, in multilayers consisting of FM semiconductors such as GaMnAs, the ability to change the IEC from FM to AFM is not well understood. Recently, however, AMF IEC was observed in GaMnAs/GaAs:Be multilayers [1]. These systems, however, show somewhat different behavior from their metallic counterparts. Specifically, the IEC in the GaMnAs/GaAs:Be multilayer system is observed to be long range, typically of over 10 nm, so that not only interactions between nearest neighbor (NN) layers of the multilayer, but also between next-nearest-neighbor (NNN) layers are involved during the magnetization reversal process. [2].

Exchange bias and interlayer exchange coupling are interface driven phenomena. Since an ideal interface is very challenging to achieve, a clear understanding of the chemical and magnetic natures of interfaces is pivotal to identify their influence on the magnetism. We have chosen Ni80Fe20/CoO(tCoO)/Co trilayers as a model system, and identified non-stoichiometric Ni-ferrite and Co-ferrite at the surface and interface, respectively. These ferrites, being ferrimagnets typically, should influence the exchange coupling. However, in our trilayers the interface ferrites were found not to be ferro- or ferri-magnetic; thus having no observable influence on the exchange coupling. Our analysis also revealed that (i) interlayer exchange coupling was present between Ni80Fe20 and Co even though the interlayer thickness was significantly larger than expected for this phenomenon to happen, and (ii) the majority of the CoO layer (except some portion near the interface) did not contribute to the observed exchange bias. We also identified that the interlayer exchange coupling and the exchange bias properties were not interdependent.

To overcome the detrimental effect of charge transfer from a transition metal to 2D substrates like graphene, we have grown ultrathin antiferromagnetic α-Fe2O3 layers on both sides of the graphene surface. Anomalous magnetic behavior, viz., coercivity and exchange bias, increases with increasing temperature with strong ferromagnetic ordering. The highest values of coercivity and large exchange bias are obtained as 3335 Oe and 2361 Oe, respectively. Large enhancement (646%) in exchange bias is observed with an increase in temperature from 2 K to 70 K. Interlayer exchange coupling between the ferromagnetic layers becomes strongest at 300 K to achieve an ultralow coercivity of 22 Oe by growing an α-Fe2O3 phase on both sides of the graphene surface. A 32% negative magnetoresistance is observed as a result of exchange bias which changes with temperature. All these results are explained on the basis of the charge transfer effect at the interface of the graphene/α-Fe2O3 nanostructure at the low temperature region and the spin canting effect of surface states at the higher temperature region. Theoretical Density Functional Theory calculation is also done to understand the interface interaction, quantitative evaluation of charge transfer, and density of states.

Van der Waals (vdW) magnetic heterostructures offer a versatile platform for engineering interfacial spin interactions with atomic precision, enabling nontrivial spin textures and dynamics behavior. In this work, robust asymmetric magnetization reversal and exchange bias are reported in Fe3GeTe2 (FGT), driven by interlayer exchange coupling with the A‐type antiferromagnet CrSBr. Despite the orthogonal magnetic anisotropies–out‐of‐plane easy axis in FGT and in‐plane in CrSBr–a strong interfacial exchange interaction that gives rise to pronounced and switchable exchange bias and asymmetric switching in FGT is observed, persisting up to the Néel temperature of CrSBr (∼132 K) as revealed by anomalous Hall effect measurements. The microscopic origin of this behavior is uncovered through cross‐sectional magnetic imaging of the domain structure using off‐axis electron holography. The results reveal that the asymmetric switching and exchange bias arise from the influence of CrSBr on the domain configuration of FGT, where the in‐plane antiferromagnetic state of CrSBr promotes the formation of stripe‐like domain structures in FGT with circular rotation of magnetization in the cross‐sectional bc plane defined by the easy axes of both FGT and CrSBr. These findings elucidate the mechanism of exchange bias in orthogonally coupled vdW systems and demonstrate a pathway for stabilizing 3D domain structures in ferromagnets through interfacial exchange interactions.

The interlayer exchange coupling (IEC) in magnetic multilayers determines the spin orientation of individual magnetic layers comprising the system. The dependence of the resistance on the resulting spin configuration in turn leads to the phenomenon known as giant magnetoresistance (GMR). [1] The GMR effect is of key importance to the area of spintronics, in which the spin degree of freedom is utilized in the operation of electronic devices. The behavior of IEC in metallic ferromagnetic multilayers has been extensively investigated, and the ability to control the IEC by structural parameters to be either ferromagnetic (FM) or antiferromagnetic (AFM) is now well established. In contrast, in multilayers consisting of FM semiconductors such as GaMnAs, the ability to change the IEC from FM to AFM is not well understood. Recently, however, AMF IEC was observed in GaMnAs/GaAs:Be multilayers [1]. These systems, however, show somewhat different behavior from their metallic counterparts. Specifically, the IEC in the GaMnAs/GaAs:Be multilayer system is observed to be long range, typically of over 10 nm, so that not only interactions between nearest neighbor (NN) layers of the multilayer, but also between next-nearest-neighbor (NNN) layers are involved during the magnetization reversal process. [2].

Maghemite nanoparticles functionalised with Co(II) coordination complexes at their surface show a significant increase of their magnetic anisotropy, leading to a doubling of the blocking temperature and a sixfold increase of the coercive field. Magnetometric studies suggest an enhancement that is not related to surface disordering, and point to a molecular effect involving magnetic exchange interactions mediated by the oxygen atoms at the interface as its source. Field- and temperature-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) studies show that the magnetic anisotropy enhancement is not limited to surface atoms and involves the core of the nanoparticle. These studies also point to a mechanism driven by anisotropic exchange and confirm the strength of the magnetic exchange interactions. The coupling between the complex and the nanoparticle persists at room temperature. Simulations based on the XMCD data give an effective exchange field value through the oxido coordination bridge between the Co(II) complex and the nanoparticle that is comparable to the exchange field between iron ions in bulk maghemite. Further evidence of the effectiveness of the oxido coordination bridge in mediating the magnetic interaction at the interface is given with the Ni(II) analog to the Co(II) surface-functionalised nanoparticles. A substrate-induced magnetic response is observed for the Ni(II) complexes, up to room temperature.

Magnetic exchange interactions within the asymmetric dimetallic compounds [hqH2][Ln2(hq)4(NO3)3]·MeOH, (Ln = Er(III) and Yb(III), hqH = 8-hydroxyquinoline) have been directly probed with EPR spectroscopy and accurately modeled by spin Hamiltonian techniques. Exploitation of site selectivity via doping experiments in Y(III) and Lu(III) matrices yields simple EPR spectra corresponding to isolated Kramers doublets, allowing determination of the local magnetic properties of the individual sites within the dimetallic compounds. CASSCF-SO calculations and INS and far-IR measurements are all employed to further support the identification and modeling of the local electronic structure for each site. EPR spectra of the pure dimetallic compounds are highly featured and correspond to transitions within the lowest-lying exchange-coupled manifold, permitting determination of the highly anisotropic magnetic exchange between the lanthanide ions. We find a unique orientation for the exchange interaction, corresponding to a common elongated oxygen bridge for both isostructural analogs. This suggests a microscopic physical connection to the magnetic superexchange. These results are of fundamental importance for building and validating model microscopic Hamiltonians to understand the origins of magnetic interactions between lanthanides and how they may be controlled with chemistry.

Ion beam irradiation has been shown to be an interesting tool for tailoring the magneticproperties of thin films and multilayers. The modified properties include magneticanisotropy, interlayer exchange coupling, exchange bias, magnetic domain structureand magnetization reversal. In this work, new results are shown concerning theenhancement, by one order of magnitude, of the antiferromagnetic coupling strengthin amorphous CoSi/Si multilayers by irradiating Si(100) substrates with 1 keVAr+ ions. The ion beam exposure induces an increase of the substrate roughness, from 0.07 to0.88 nm, which enhances antiferromagnetic coupling in the magnetic multilayers grown ontop. One possible mechanism governing this enhancement is discussed, related to theformation of magnetic/non-magnetic regions where dipolar interactions could stabilize theantiferromagnetic alignment. The presence of non-magnetic regions is suggestedby the observed trend to superparamagnetism, and is expected since the Curietemperature of the amorphous CoSi alloy used is slightly above but very closeto room temperature. Accordingly, small fluctuations in the local composition,leading to an enrichment of Si, would produce non-magnetic regions enablingdipolar interactions to take place. Furthermore, the ion beam induced increase ofroughness makes surface diffusion of the atoms arriving at the sample difficult,favoring the formation of local non-magnetic inhomogeneities. Finally, the role ofother possible mechanisms to enhance antiferromagnetic coupling is also brieflydiscussed.

Magnetic exchange coupling (J) is one of the important spin Hamiltonian parameters that control the magnetic characteristics of single-molecule magnets (SMMs). While numerous chemical methodologies have been proposed to modify ligands and control the J value, and magneto-structural correlations have been developed accordingly, altering this parameter through non-chemical means remains a challenging task. This study explores the impact of an Oriented-External Electric Field (OEEF) on over twenty lanthanide-radical complexes using Density Functional Theory (DFT) and ab initio Complete Active Space Self-Consistent Field (CASSCF) methods. Five complexes - [{(Me3Si)2N]2Gd(THF)}2(μ-η2:η2-N2)] (1), [Gd(Hbpz3)2(dtbsq)] (2), [Gd(hfac)3(IM-2py)] (3), [Gd(hfac)3(NITBzImH)] (4), and [Gd(hfac)3{2Py-NO}(H2O)] (5) - were selected for detailed analysis, revealing significant OEEF effects on magnetic exchange interactions and structural parameters. Various parameters such as bond distances, bond angles, and torsional angles were examined as a function of OEEF to establish guiding principles for molecule selection. In complexes 1, 2, and 3, OEEF influenced torsional angles and altered exchange interactions. Complex 4 demonstrated enhanced ferromagnetic coupling under OEEF, reaching a maximum J value of +5.3 cm-1. Complex 5 reveals switching the sign of JGd-rad exchange interaction from antiferromagnetic to ferromagnetic under OEEF, highlighting the potential of electric fields in designing materials with tuneable magnetic properties. These findings offer valuable insights for future research and applications in advanced materials and molecular electronics.

SmCo/FeCo/SmCo trilayer was deposited with two different thickness configurations for soft phase (FeCo); 50 nm/10 nm/50 nm and 50 nm/25 nm/50 nm were deposited on Si (111) substrate and Ta (50 nm) seed layer by RF magnetron sputtering in a pressure, p, of 30 - 35 m Torr. After deposition the films were annealed under Ar atmosphere at temperature T equal to 923 and 973 for different times followed by quenching in water. X-ray diffraction patterns were obtained to identified phase presents and calculate average crystallite size. To study the effect of configuration thickness in soft phases, DC magnetic measurements were carried out; the measurements were done in the temperature interval of 300 - 50 K. Hysteresis loops collected at low temperatures exposed an increment in coercivity with the decrease of T and at same time, presented a “knee” in the second quadrant of the demagnetization curve, which suggests that the inter-layer exchange coupling becomes less effective, being more evident for sample with 50 nm/25 nm/50 nm thickness. Moreover, δM (H) plots were calculated from magnetic measurements at three different temperatures, T, equal to 300, 150 and 50 K, which corroborates that the dipolar interactions became stronger when thickness of soft phases increases. Finally, the thickness effect is attributed to the SmCo5 phase magnetocrystalline anisotropy constant, which is responsible for the exchange coupling length.

Individual molecular spins are promising quantum states for emerging computation technologies. The "on surface" configuration of molecules in proximity to a magnetic film allows control over the orientations of molecular spins and coupling between them. The stacking of planar molecular spins could favor antiferromagnetic interlayer couplings and lead to pinning of the magnetic underlayer via the exchange bias, which is extensively utilized in ultrafast and high-density spintronics. However, fundamental understanding of the molecular exchange bias and its operating features on a device has not been unveiled. Here, we showed tunable molecular exchange bias and its asymmetrical magnetotransport characteristics on a device by using the metalloporphyrin/cobalt hybrid films. A series of the distinctive molecular layers showcased a wide range of the interfacial exchange coupling and bias. The transport behaviors of the hybrid bilayer films revealed the molecular exchange bias effect on a fabricated device, representing asymmetric characteristics on anisotropic and angle-dependent magnetoresistances. Theoretical simulations demonstrated close correlations among the interfacial distance, magnetic interaction, and exchange bias. This study of the hybrid interfacial coupling and its impact on magnetic and magnetotransport behaviors will extend functionalities of molecular spinterfaces for emerging information technologies.

We study the influence of collective magnetic excitations on the interlayer exchange coupling (IEC) in metallic multilayers. The results are compared to other models that explain the temperature dependence of the IEC by mechanisms within the spacer or at the interfaces of the multilayers. As a main result we find that the reduction of the IEC with temperature shows practically the same functional dendence in all models. On the other hand the influence of the spacer thickness, the magnetic material, and an external field are quite different. Based on these considerations we propose experiments, that are able to determine the dominating mechanism that reduces the IEC at finite temperatures.