arXiv subjects

T. Hajiri

Publications and source records attributed to T. Hajiri.

11 recordsLinked to original sources

arXiv2021

Spin-Orbit Torque Switching of Noncollinear Antiferromagnetic Antiperovskite Manganese Nitride Mn$_3$GaN

Noncollinear antiferromagnets have promising potential to replace ferromagnets in the field of spintronics as high-density devices with ultrafast operation. To take full advantage of noncollinear antiferromagnets in spintronics applications, it is important to achieve efficient manipulation of noncollinear antiferromagnetic spin. Here, using the anomalous Hall effect as an electrical signal of the triangular magnetic configuration, spin-orbit torque switching with no external magnetic field is demonstrated in noncollinear antiferromagnetic antiperovskite manganese nitride Mn$_3$GaN at room temperature. The pulse-width dependence and subsequent relaxation of Hall signal behavior indicate that the spin-orbit torque plays a more important role than the thermal contribution due to pulse injection. In addition, multistate memristive switching with respect to pulse current density was observed. The findings advance the effective control of noncollinear antiferromagnetic spin, facilitating the use of such materials in antiferromagnetic spintronics and neuromorphic computing applications.

cond-mat.mtrl-sci↗

arXiv2020

Scaling the electrical current switching of exchange bias in fully-epitaxial antiferromagnet/ferromagnet bilayers

While the electrical current manipulation of antiferromagnets (AFMs) has been demonstrated, the extent of the studied AFM materials has been limited with few systematic experiments and a poor understanding. We compare the electrical current switching of the exchange-bias field ($H_{ex}$) in AFM-Mn$_3A$N/ferromagnet-Co$_3$FeN bilayers. An applied pulse current can manipulate $H_{ex}$ with respect to the current density and FM layer magnetization, which shifts exponentially as a function of the current density. We found that the saturation current density and exponential decay constant $τ$ increase with the local moment of AFM Mn atoms. Our results highlight the effect of the AFM local moment to electrical current switching of $H_{ex}$, although it has a near-zero net magnetization, and may provide a facile way to explore the electrical current manipulation of AFM materials.

cond-mat.mes-hall↗

arXiv2020

Epitaxial growth and orientation-dependent anomalous Hall effect of noncollinear antiferromagnetic Mn$_3$Ni$_{0.35}$Cu$_{0.65}$N films

We report the growth of noncollinear antiferromagnetic (AFM) Mn$_3$Ni$_{0.35}$Cu$_{0.65}$N films and the orientation-dependent anomalous Hall effect (AHE) of (001) and (111) films due to nonzero Berry curvature. We found that post-annealing at 500$^\circ$C can significantly improve the AHE signals, though using the appropriate post-annealing conditions is important. The AHE and magnetization loops show sharp flipping at the coercive field in (111) films, while (001) films are hard to saturate by a magnetic field. The anomalous Hall conductivity of (111) films is an order of magnitude larger than that of (001) films. The present results provide not only a better understanding of the AHE in Mn$_3X$N systems but also further opportunities to study the unique phenomena related to noncollinear AFM.

cond-mat.mtrl-sci↗

arXiv2019

Electrical current switching of the noncollinear antiferromagnet Mn$_3$GaN

We report electrical current switching of noncollinear antiferromagnetic (AFM) Mn$_3$GaN/Pt bilayers at room temperature. The Hall resistance of these bilayers can be manipulated by applying a pulse current of $1.5\times10^6$~A/cm$^2$, whereas no significant change is observed up to $\sim10^8$~A/cm$^2$ in Mn$_3$GaN single films, indicating that the Pt layer plays an important role. In comparison with ferrimagnetic Mn$_3$GaN/Pt bilayers, a lower electrical current switching of noncollinear AFM Mn$_3$GaN is demonstrated, with a critical current density two orders of magnitude smaller. Our results highlight that a combination of a noncollinear AFM antiperovskite nitride and a spin-torque technique is a good platform of AFM spintronics.

cond-mat.mtrl-sci↗

arXiv2019

Anomalous Hall effect in non-collinear antiferromagnetic antiperovskite Mn$_{3}$Ni$_{1-x}$Cu$_{x}$N

We report the anomalous Hall effect (AHE) in antiperovskite Mn$_{3}$NiN with substantial doping of Cu on the Ni site (i.e. Mn$_{3}$Ni$_{1-x}$Cu$_{x}$N), which stabilizes a noncollinear antiferromagnetic (AFM) order compatible with the AHE. Observed on both sintered polycrystalline pieces and single crystalline films, the AHE does not scale with the net magnetization, contrary to the conventional ferromagnetic case. The existence of the AHE is explained through symmetry analysis based on the $Γ_{\rm 4g}$ AFM order in Cu doped Mn$_{3}$NiN. DFT calculations of the intrinsic contribution to the AHE reveal the non-vanishing Berry curvature in momentum space due to the noncollinear magnetic order. Combined with other attractive properties, antiperovskite Mn$_{3}$AN system offers great potential in AFM spintronics.

cond-mat.str-el↗

arXiv2018

Change of Fermi surface states related with two different $T_{\rm c}$-raising mechanisms in iron pnictide superconductor

Evolution of Fermi surface (FS) states of NdFeAs$_{1-x}$P$_x$O$_{0.9}$F$_{0.1}$ single crystals with As/P substitution has been investigated. The critical temperature $T_{\rm c}$ and the power law exponent ($n$) of temperature-dependent resistivity ($ρ(T) = ρ_0 + AT^n$) show a clear correlation above $x=$0.2, suggesting that $T_{\rm c}$ is enhanced with increasing bosonic fluctuation in the same type of FS state. Around $x=$0.2, all the transport properties show anomalies, indicating that $x$$\sim$0.2 is the critical composition of drastic FS change. The angle resolved photoemission spectroscopy has more directly revealed the distinct change of FS around $x=$0.2, that one hole FS disappears at Brillouin zone center and the other FS with propeller like shape appears at zone corner with decreasing $x$. These results are indicative of the existence of two types of FS state with different nesting conditions that are related with two $T_{\rm c}$-rising mechanisms in this system.

cond-mat.supr-con↗

arXiv2017

Impact of Anisotropy on Antiferromagnet Rotation in Heusler-type Ferromagnet/Antiferromagnet Epitaxial Bilayers

We report the magnetotransport properties of ferromagnet (FM)/antiferromagnet (AFM) Fe$_2$CrSi/Ru$_2$MnGe epitaxial bilayers using current-in-plane configurations. Above the critical thickness of the Ru$_2$MnGe layer to induce exchange bias, symmetric and asymmetric curves were observed in response to the direction of FM magnetocrystalline anisotropy. Because each magnetoresistance curve showed full and partial AFM rotation, the magnetoresistance curves imply the impact of the Fe$_2$CrSi magnetocrystalline anisotropy to govern the AFM rotation. The maximum magnitude of the angular-dependent resistance-change ratio of the bilayers is more than an order of magnitude larger than that of single-layer Fe$_2$CrSi films, resulting from the reorientation of AFM spins via the FM rotation. These results highlight the essential role of controlling the AFM rotation and reveal a facile approach to detect the AFM moment even in current-in-plane configurations in FM/AFM bilayers.

cond-mat.mtrl-sci↗

arXiv2014

Anomalous Superconducting-Gap Structure of Slightly Overdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$

We observed the anisotropic superconducting-gap (SC-gap) structure of a slightly overdoped superconductor, Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ ($x=0.1$), using three-dimensional (3D) angle-resolved photoemission spectroscopy. Two hole Fermi surfaces (FSs) observed at the Brillouin zone center and an inner electron FS at the zone corner showed a nearly isotropic SC gap in 3D momentum space. However, the outer electron FS showed an anisotropic SC gap with nodes or gap minima around the M and A points. The different anisotropies obtained the SC gap between the outer and inner electron FSs cannot be expected from all theoretical predictions with spin fluctuation, orbital fluctuation, and both competition. Our results provide a new insight into the SC mechanisms of iron pnictide superconductors.

cond-mat.supr-con↗

arXiv2013

Strong Correlation Effects of A-site Ordered Perovskite CaCu3Ti4O12 revealed by Angle-Resolved Photoemission Spectroscopy

We report angle-resolved photoemission spectroscopy (ARPES) results of A-site ordered perovskite CaCu$_3$Ti$_4$O$_{12}$. We have observed the clear band dispersions, which are shifted to the higher energy by 1.7 eV and show the band narrowing around 2 eV in comparison with the local density approximation calculations. In addition, the high energy multiplet structures of Cu 3$d^8$ final-states have been found around 8 - 13 eV. These results reveal that CaCu$_3$Ti$_4$O$_{12}$ is a Mott-type insulator caused by the strong correlation effects of the Cu 3$d$ electrons well hybridized with O 2$p$ states. Unexpectedly, there exist a very small spectral weight at the Fermi level in the insulator phase, indicating the existence of isolated metallic states.

cond-mat.str-el↗

arXiv2013

Strongly hybridized electronic structure of YbAl2: An angle-resolved photoemission study

We report the electronic structure of a prototypical valence fluctuation system, YbAl2, using angle-resolved photoemission spectroscopy. The observed band dispersions and Fermi surfaces are well described in terms of band structure calculations based on local density approximation. Strong hybridization between the conduction and 4f bands is identified on the basis of the periodic Anderson model. The evaluated small mass enhancement factor and the high Kondo temperature qualitatively agree with those obtained from thermodynamic measurements. Such findings suggest that the strong hybridization suppresses band renormalization and is responsible for the valence fluctuations in YbAl2.

cond-mat.str-el↗

arXiv2013

Magnetic origin of high-energy kink structure in heavily electron-doped Li$_{1+x}$FeAs

We report the origin of a high-energy kink structure of heavily electron-doped nonsuperconducting Li$_{1+x}$FeAs observed by three-dimensional angle-resolved photoemission spectroscopy. The $d_{xy}$ orbital at the center of the Brillouin zone is strongly renormalized, indicating stronger electron correlation, exceeding that of stoichiometric LiFeAs despite the fact that the orbital characters of all bands remain unchanged. Two types of kink structure on the $d_{yz}$ band were identified: an isotropic kink at the binding energy of 20 meV, and another kink located at 100 meV observed only in the (110) direction. The higher-energy kink is considered to originate from a magnetic interaction, because the peak energy is consistent with that of a spin excitation.

cond-mat.str-el↗