Thrust Highlights

Introduction

Ferrimagnetic insulators offer a promising route toward low-loss magnonic devices, including magnon interconnects, as pure spin currents can propagate without accompanying charge current. We have identified low loss epitaxial spinel structure ferrite thin films as a promising  medium for magnon generation and propagation. However, we need to understand the magnon modes that can exist in these materials. 

Highlight

We report the first direct spectroscopic observation of magnons in low-damping epitaxial ferrimagnetic insulating Li_0.5(Al,Fe)_2.5O₄ thin films at room temperature. Using inelastic Brillouin light scattering (BLS), the study reveals multiple thermal magnon modes that correspond to quantized standing spin-wave modes across the film thickness. The significance of this result lies in moving beyond the field’s traditional reliance on Y₃Fe₅O₁₂ (YIG). Although YIG has been the benchmark material for low-damping magnonics, its high-temperature growth requirements limit compatibility with CMOS fabrication. In contrast, the demonstrated Li-based ferrimagnetic insulating films provide a new, CMOS-compatible platform for magnonic devices, expanding the material landscape for integrated spin-wave technologies.

From a materials and device perspective, the magnon frequencies in these films are highly tunable through both film thickness and external magnetic field, offering a wider tuning range than YIG. Analysis of the magnetic-field-dependent magnon spectra enables extraction of key magnetic parameters, including the magnetic anisotropy and exchange constant. Complementary first-principles calculations further elucidate the ferrimagnetic ordering mechanism and quantitatively support the experimentally extracted exchange interactions.

Together, these results establish Li0.5(Al,Fe)2.5O4  thin films as a promising, CMOS-compatible ferrimagnetic magnon platform with strong tunability and well-understood magnetic properties.

Introduction

Antiferromagnet insulators (AFMI), such as NiO, are promising candidates for magnon media. In AFMI, pure spin currents can propagate without accompanying charge current. They can support magnon excitations at and above THz frequencies and are robust to external field perturbations due to the net zero magnetization. NiO is an excellent candidate for tunable magnon media because of its low magnetic losses. 

Highlight

CEEMag researchers have discovered a way to grow high-quality crystalline films of the antiferromagnet NiO and transfer them to different substrates without damaging the films. This technique allows ultrathin NiO films to be placed on a membrane support for electron transmission experiments. This setup enabled scientists to study the NiO lattice and its structural response when excited, which is crucial for understanding its magnetic properties.

Using high-energy electrons from the Megaelectronvolt Ultrafast Electron Diffraction instrument at the SLAC National Accelerator Laboratory, CEEMag researchers imaged the atomic structure of the NiO film as it was energized by an ultrashort laser pulse. The results showed that not only did atomic vibrations increase (a thermal response), but the film also became ‘flatter’ due to laser-induced strain, which reduced the film's mosaicity. This insight into the structural response of NiO will help improve our understanding and engineering of its dynamic magnetic properties.

Introduction

Magnetic tunnel junctions (MTJs) based on ferromagnets are canonical devices in spintronics, with wide-ranging applications in data storage, computing, and sensing. They simultaneously exhibit mechanisms for electrical detection of magnetic order through the TMR effect, and reciprocally, for controlling ferromagnetic order by electric currents through spin-transfer torque (STT). It was long assumed that neither of these effects could be sizeable in tunnel junctions made from antiferromagnetic materials, since they exhibit no net magnetization. Recently, we showed that all-antiferromagnetic tunnel junctions (AATJs) based on chiral antiferromagnets do exhibit TMR due to their non-relativistic momentum-dependent spin polarization and cluster magnetic octupole moment (CMO), which are manifestations of their spin-split band structure. However, the reciprocal effect, i.e., the antiferromagnetic counterpart of STT driven by currents through the AATJ, had been assumed non-existent due to the total electric current being spin-neutral. 

Highlight

We have discovered a new current-induced torque in all-antiferromagnetic tunnel junctions (AATJs) made of noncollinear chiral antiferromagnets. This torque, which we term octupole-driven spin-transfer torque (OTT), and the associated large tunneling magnetoresistance (TMR) ratio in AATJs provide a mechanism for efficient electrical detection and generation of magnons in antiferromagnets. We observed current-induced OTT switching in nanoscale PtMn₃|MgO|PtMn₃ AATJs with diameters of 50 to 200 nm, fabricated on a thermally oxidized silicon substrate (Figure a and b). These AATJ structures exhibited a record-high TMR value of 363% at room temperature and switching current densities of the order of 10 MA/cm². An example of this switching data is shown in Fig. c. We performed theoretical modeling that explains the origin of OTT in terms of the imbalance between intra- and inter-sublattice spin currents across the ATJ, and equivalently, in terms of the non-zero net cluster octupole polarization of each PtMn₃ layer.