Ryoichi Nakatani

We investigated a suitable growth condition of the antiferromagnetic Cr₂O₃(0001) thin film on the ferroelectric LiNbO₃ substrate. Crystallinity of Cr₂O₃, quantified using the X-ray diffraction, gets better with increasing growth temperature. Accompanying improved crystallinity, the Néel temperature, detected by the anomalous Hall effect, increased. Meanwhile, the high temperature growth degrades LiNbO₃. To solve this competition, we investigated a post annealing effect. For all the films studied, the X-ray diffraction intensity increased in maintaining the epitaxial relationship. On the contrary, the Néel temperature did not follow this simple trend. The initially amorphous film showed no appreciable anomalous Hall effect even after the crystallization, whereas in other films, the Néel temperature increased by the post annealing. The results suggest that the formation of epitaxial crystallites at low growth temperature and the subsequent growth by post annealing are suitable to prepare the well-ordered Cr₂O₃ on LiNbO₃.

To develop voltage‐controlled magnetization switching technologies for spintronics applications, a highly (422)‐oriented Co₂FeSi layer on top of the piezoelectric PMN‐PT(011) is experimentally demonstrated by inserting a vanadium (V) ultra‐thin layer. The strength of the growth‐induced magnetic anisotropy of the (422)‐oriented Co₂FeSi layers can be artificially controlled by tuning the thicknesses of the inserted V and the grown Co₂FeSi layers. As a result, a giant converse magnetoelectric effect (over 10⁻⁵ s m⁻¹) and a non‐volatile binary state at zero electric field are simultaneously achieved in the (422)‐oriented Co₂FeSi/V/PMN‐PT(011) multiferroic heterostructure. This study leads to a way toward magnetoresistive random‐access‐memory (MRAM) with a low power writing technology.

Using a molecular beam epitaxy technique, we experimentally demonstrate a multiferroic heterostructure consisting of metastable ferromagnetic Co3Mn on piezoelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT). Inserting a 2-nm-thick Fe layer between Co3Mn and PMN-PT(001) allows the formation of bcc Co3Mn layers even at an extremely low growth temperature of ∼80°C. Upon increasing this temperature to 200 °C, a bcc Co3Mn/Fe/PMN-PT(001) multiferroic heterostructure with a relatively large saturation magnetization of ∼1680 kA/m and an atomically flat interface is obtained, resulting in an obvious converse magnetoelectric (CME) effect. The large CME effect originates mainly from the strain-induced modulation of the magnetic anisotropy energy, supported by the first-principles calculations.

Antiferromagnet does not show the net magnetization, whereas the finite uncompensated moment can residue at the surface. On the surface of the magnetoelectric antiferromagnet, the finite boundary magnetization can acquire the magnetic response. In this paper, we address the magnetic response of the boundary magnetization in the Pt/magnetoelectric Cr2O3/Pt epitaxial trilayer based on the anomalous Hall effect (AHE) and the soft x-ray magnetic circular dichroism (XMCD). Decreasing the Cr2O3 thickness down to 15 nm, the film acquired the magnetic responsiveness, which manifested as the rectangular hysteresis in the magnetic field dependence of the AHE. The sizable XMCD intensity and the rectangular magnetic field dependence of the XMCD intensity revealed that the magnetic response was attributed to the interfacial Cr moment. The detailed investigation of AHE and XMCD revealed that the domain wall motion dominated the reversal process of the boundary magnetization, which was directly visualized by the scanning XMCD microscope.

We synthesized Co-Pt nanoparticles in a crystal composed of proteins with a cage structure (i.e., Pyrococcus furiosus virus-like particle (PfV)) and investigated their magnetic interactions. When the soaking concentrations of metal, Co, and Pt ions in the PfV crystal were below 5.5 mM, isolated Co-Pt nanoparticles were formed. The size of the primary particles was 2-3 nm, which was smaller than the inner core size of the cage structure. When the soaking concentrations increased above 11 mM, the Co-Pt nanoparticles formed aggregates exceeding 30 nm in size. The synthesized nanoparticles showed superparamagnetic behavior at 300 K, independent of the soaking concentration. The temperature dependence of the AC magnetic susceptibility and remanent magnetization curve verified the occurrence of magnetic interactions between the Co-Pt nanoparticles. These measurements revealed that the dipolar-dipolar interaction was significant in the closely packed isolated nanoparticles, whereas it was weaker in the aggregated Co-Pt nanoparticles, probably due to the increased distance between the particles.

We investigate magnetoelectric switching of perpendicular exchange bias with very low coercivity in a Pt/Co/Ir/Cr₂O₃/Pt epitaxial film. We also optimize the suitable Ir spacer thickness so that the film exhibits the perpendicular exchange bias greater than the coercivity up to the vicinity of the Néel temperature. Main impact of the Ir spacer layer is the significant reduction of coercivity less than 0.5 mT in maintaining both the perpendicular magnetic anisotropy and the perpendicular exchange bias. For the suitable structure, the perpendicular exchange bias was isothermally switched by the gate voltage in combination with the magnetic field. Analysis of the hysteresis of the exchange bias field as a function of the gate voltage suggested that the magnetoelectric coefficient was comparable to the reported value. This implies that the Ir layer does not degrade the efficiency to transfer the magnetoelectrically controlled antiferromagnetic order parameter to the ferromagnetic Pt/Co/Ir trilayer.

To investigate the magnetization process in the intermediate frequency region, the frequency dependences of coercivities in Ni and Co–Fe–B thin films were determined by anisotropic magnetoresistance measurements up to ∼160 kHz. In the low-frequency region (<5 kHz) the coercivity of Co–Fe–B was lower than that of Ni. However, the increasing rate of the coercivity in Ni was lower than that in Co–Fe–B. Consequently, above 19 kHz, the coercivity of Ni was lower than one of Co–Fe–B. Considering the basic material properties, better soft magnetic properties of Ni compared with Co–Fe–B should arise from the higher Walker breakdown field. This difference was mainly due to the Gilbert damping constant.

We investigate the magneto-optic Kerr effect in perpendicularly magnetized Pt/Co/Ir/Cr₂O₃/Pt thin films, associated with the antiferromagnetic–paramagnetic transition of the Cr₂O₃ layer. The magneto-optic Kerr rotation angle (θ _K) shows oscillatory behavior as a function of the photon energy of incident light owing to interference in the Cr₂O₃ layer. The temperature dependence of θ _K at 2.67 eV (λ = 465 nm), at which the largest θ _K is obtained, shows a sharp dip at 287.0 K. The dip temperature is similar to the reported Néel temperature for Cr₂O₃ thin films. Although the θ _K spectra measured at several temperatures are generally explained by the classical interference model, θ _K is enhanced at 2.36–2.79 eV (λ = 525–445 nm) close to the dip temperature. This peculiar enhancement in θ _K is discussed on the basis of the anomaly in the optical parameters of the Cr₂O₃ layer associated with the antiferromagnetic–paramagnetic transition.

Increase in the Néel temperature (T_N) of the 10-nm thick Cr₂O₃(0001) thin films by the lattice strain was experimentally investigated. T_N was determined based on the zero-offset anomalous Hall measurements for the Pt/Cr₂O₃/Pt epitaxial trilayer. The lattice strain was altered by the Pt buffer layer thickness and was evaluated by the lattice parameters. T_N was increased from 241.5 to 260.0 K by varying the Pt buffer layer thickness from 0 to 20 nm. For the film without the Pt buffer layer, the apparent critical exponent near T_N increased suggesting the distribution of T_N due to the inhomogeneous strain. The T_N value was weakly correlated with the axial ratio c/a and the lattice volume of the Cr₂O₃ layer V; T_N increases with decreasing c/a or increasing V. The increase in T_N by the reduction in c/a (or by increasing V) implies that the exchange coupling between the nearest neighbor Cr³⁺ spins has the significant role in the strain effect.

The magnetoelectric (ME) effect is one of the methods for electrically controlling the magnetization direction. In this study, we investigated the ME-driven domain wall creep and depinning using a Pt/Co/Au/ME-Cr₂O₃/Pt thin film. The domain switching process is governed by domain wall propagation rather than the nucleation of reversed domains, similar to a pure ferromagnet. The domain wall velocity v increases exponentially with the ME pressure, that is, the simultaneous application of magnetic H and electric E fields. The v–E curve under a constant H can be scaled by the ME pressure with the assistance of the exchange bias. We determined the depinning threshold, pinning energy scale, and depinning velocity based on the model for the magnetic domain wall creep. Compared with the depinning velocity in various other systems, it was suggested that the ME-driven mechanism could yield a fast domain wall velocity utilizing the low pressure.