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How to manipulate magnetic states of antiferromagnets

Antiferromagnetic materials, which have drawn considerable attention recently, have fascinating features: they are robust against perturbation, produce no stray fields, and exhibit ultrafast dynamics. Discerning how to efficiently manipulate the magnetic state of an antiferromagnet is key to the development of antiferromagnetic spintronics. In this review, we introduce four main methods (magnetic, strain, electrical, and optical) to mediate the magnetic states and elaborate on intrinsic origins of different antiferromagnetic materials. Magnetic control includes a strong magnetic field, exchange bias, and field cooling, which are traditional and basic. Strain control involves the magnetic anisotropy effect or metamagnetic transition. Electrical control can be divided into two parts, electric field and electric current, both of which are convenient for practical applications. Optical control includes thermal and electronic excitation, an inertia-driven mechanism, and terahertz laser control, with the potential for ultrafast antiferromagnetic manipulation. This review sheds light on effective usage of antiferromagnets and provides a new perspective on antiferromagnetic spintronics.


Introduction
Spintronics, also called spin electronics, studies the effective control and manipulation of spin degrees of freedom in solid-state systems [1]. During the past three decades, spintronics has been widely researched worldwide, and its potential usage has drawn significant attention. Conventional spintronic devices depend on the manipulation of magnetic moments in ferromagnets [2]. However, as antiferromagnets exhibit myriad intriguing features, devices whose key parts are made of antiferromagnets have garnered much interest. They have inspired a new field called antiferromagnetic spintronics with the aim of complementing or replacing ferromagnets in the active components of spintronic devices [3]. Compared to ferromagnets, antiferromagnetic materials resist perturbation well, produce no stray fields, demonstrate ultrafast dynamics, and generate large magneto-transport effects [1, [3][4][5][6][7][8][9]. Accordingly, antiferromagnetic materials have become increasingly important and exhibit various promising applications including non-volatile memory [2, 10,11] and magnetic field probes [12].
In 2011, a spin-valve-like magnetoresistance of an antiferromagnet-based tunnel junction marked the formal emergence of antiferromagnetic spintronics [13].
Thereafter, extensive researches were conducted in this area, focused on the important roles antiferromagnets play in spintronics. Barely a year later, tunneling anisotropic magnetoresistance of antiferromagnetic tunnel junctions was realized at room temperature [14]. In 2014, a room-temperature antiferromagnetic memory resistor [11] as well as a strain-and electric field-controlled metamagnetic transition emerged [ With antiferromagnets, field-free switching of magnetization through spin-orbit torque has also been realized [34][35][36][37]. Therefore, determining how to manipulate the magnetic states of antiferromagnets efficiently is key to further development [5].
Research efforts have aimed to solve this problem via magnetic, strain, electrical, and optical manipulation. Magnetic control includes a strong magnetic field, exchange bias and field cooling, which is fundamental. Strain control involves the magnetic anisotropy effect or metamagnetic transition. Electrical control can be divided into two parts: electric field and electric current, which are convenient for practical applications. Optical control includes thermal and electronic excitation, an inertia-driven mechanism, and terahertz laser control, which has the potential for ultrafast antiferromagnetic manipulation. Some reviews of antiferromagnetic spintronics over the last decade have explicated its intrinsic theoretics and introduced various experimental methods [3-5, 12, 38-42]. For instance, a comprehensive review in 2016 illustrated the physical roots and spintronic devices related to different types of antiferromagnetic materials [5]. Other reviews pertained to information reading, writing, and storage in antiferromagnetic materials [38]; elaborated spin-transfer torques, 5 it was widely researched for over half a century. We can observe exchange bias in the magnetic hysteresis loop as its center shifts along the horizontal axis, which arises from the interfacial coupling of ferromagnets and antiferromagnets when heterostructures are field cooled below their respective Curie (TC) and Né el (TN) temperatures [46,47], as presented in figure 2. Exchange spring is a vital assumption of models describing exchange bias, having been proven experimentally in ferromagnets [94,95] and antiferromagnets [96]. For an antiferromagnet, if magnetic moments of the ferromagnet beside it are switched or rotated, a planar domain wall is wound up in the antiferromagnet [97], forming a spring-like structure due to exchange bias [96]. Magnetic control of antiferromagnets using the mechanism above was initially realized in the NiFe/IrMn/MgO/Pt system, where an IrMn antiferromagnet was laid next to the tunnel barrier, controlling the transport signal as shown in figure 3. The change of exchange-spring action and the rotation of antiferromagnets' magnetic moments is reflected in antiferromagnetic tunnelling anisotropic magnetoresistance (TAMR) and superconducting quantum interference device measurements [98]. In addition, the influence of temperature, amplitude of the field that triggers magnetic moment rotation, and thickness of antiferromagnetic films are all essential for altering magnetic moments in

Strain control of antiferromagnets
To avoid perturbations in the external magnetic field and to control magnetic moments in antiferromagnets without the influence of ferromagnets, strain is proposed as an intriguing means of manipulation via either the magnetic anisotropy effect or metamagnetic transition. For the former, strain exerts an influence on the lattice parameter and spin-orbit coupling, which has a strong relationship with magnetic anisotropies related to the ground-state energy, chemical potential, and density of states, being the origin of antiferromagnets' magnetic moment orientation [110]. It is worth noting that this method has been predicted and observed primarily in bimetallic antiferromagnets such as Mn2Au [44,110]  The other mechanism relies on the metamagnetic transition from the ferromagnetic to antiferromagnetic phase [111]. Recent works in this field have focused mostly on strained FeRh thin films [112,113]. The metamagnetic transition temperature from antiferromagnetic order to ferromagnetic order is approximately 350 K, meaning that FeRh may exhibit an antiferromagnetic state at room temperature [114][115][116][117][118][119], which is suitable for information storage [120]. A lattice expansion of 1% is noticeable, accompanied by the phase transition from antiferromagnet to ferromagnet in which point strain produced during the epitaxial growth process can also control the metamagnetic transition by modifying the electronic structure under tetragonal lattice distortion, resulting in magnetocrystalline anisotropy in thin films. indicated that a low electric field, such as E = 0.4 kV cm −1 , can cause the metamagnetic transition temperature to increase by about 25 K, which is adequate to isothermally convert the FeRh from the antiferromagnetic to ferromagnetic order. If the electric field is turned off, FeRh could restore the initial antiferromagnetic state.
As we mentioned above, the underlying cause for this phenomenon lies in electric field-induced strain, which can alter the lattice parameter and electronic structure of  due to the larger magnetic anisotropy of the top FeMn layer [147]. The progress in the electrical control of antiferromagnets through an ionic liquid provides a novel method for manipulation combining the electric field and exchange spring [148].
To manipulate the antiferromagnetic moments directly without ferromagnets, magnetic fields, or heating, an electric current becomes the priority with ultrafast spin dynamics [149,150]. Spin-orbit torque plays an important role in this mechanism to achieve all-electrical isothermal manipulation of antiferromagnetic moments at room be determined by optical measurements [153]. Therefore, more materials suitable for electrical control of their magnetic moments should be explored and utilized to enhance the range of materials considered appropriate for practical usage. Recently, current-induced switching is also realized in ferrimagnets with high thermal stability, such as Pt/CoGd [157] and Ta/Co1−xTbx system [158], as well as synthetic antiferromagnets [159,160]. Besides, the electric current density required for switching is 4.510 6 A cm −2 for CuMnAs [6] and 2×10 7 A cm −2 for Mn2Au [156], which should be further decreased to create energy-saving devices. can be triggered by laser [163,[173][174][175][176]. Similarly, through a phase transition, the spin configuration of antiferromagnet DyFeO3 would be changed into a noncollinear 21 spin state with a net magnetization from the collinear compensated state [177].

Optical control of antiferromagnets
In addition to these ultrafast dynamics, a fundamentally different scenario of spin switching in antiferromagnets has been discovered that has a slow response to an optical stimulus. Inertial behavior is exhibited due to the exchange interaction between spins, the process of which is as follows and is depicted in figure 10(a): a strong magnetic-field pulse generated by laser transfers sufficient momentum and kinetic energy to the spin system despite that spin orientation hardly changes during the action; however, the momentum is strong enough that spins overcome the potential barrier and reorient into a new metastable state long after the pulse, thus realizing switching in antiferromagnets. Inertia-driven spin switching has been observed in antiferromagnets like HoFeO3 [178,179] and YFeO3 [180], providing new insight into ultrafast recording and information storage.
We can employ new developments in terahertz technology to control the spin degree of freedom in antiferromagnets apart from manipulating charge motion.
Terahertz radiation is located between infrared light and microwave radiation, the frequency of which is approximately 0.1 to 10 THz, providing a generic ultrafast method to dominate previously inaccessible magnetic excitations in the electronic ground state. Earlier studies show that low-energy elementary excitations in solids like lattice vibrations and plasma oscillations can be manipulated by the terahertz field, which controls charge carriers through electric-dipole coupling. Magnetic terahertz interaction with magnetic-dipole coupling is much harder to harness due to the weak magnetic dipoles; even so, it is possible to realize ultrafast spin control in antiferromagnets because the spin of an electron is associated with a magnetic moment, and the time-dependent magnetic field exerts a Zeeman torque that can be used to control the spin [181][182][183]. High-intensity terahertz pulses have been applied 22 to investigate the spin reorientation, specifically the interplay between antiferromagnets and the magnetic component of intense terahertz transients, as shown in figure 10(b) [182].
The magnetic component couples to electron spins selectively decided via Zeeman interaction. The Zeeman torque enables the spins to turn out of the easy plane, and the anisotropy field forces them into precession motion with respect to their equilibrium direction. The out-of-plane spin component is precisely antiparallel in the two sublattices, whereas the in-plane coordinate oscillates in the same direction. Note that in terahertz systems, whose observed dynamics are solely motivated by the magnetic rather than the electric field, the terahertz pulse is aimed exclusively at the spins and does not deposit excessive heat in other degrees of freedom, unlike the mechanism of optical pulses heating the electron system. For example, the temperature of the excited NiO volume by terahertz pump pulse increases less than 10 μK even with the effects of complete relaxation of the magnon wave, thus qualifying the ultrafast coherent control of spins [182]. Besides using the magnetic field component of terahertz pulses to coherently control magnons in the electronic ground state by direct Zeeman interaction, nonlinearity of the terahertz electric field can also be used to control antiferromagnetic orders [181], which is much stronger than linear Zeeman coupling to the terahertz magnetic field. This is because the coupling of electronic orbital states to ordered spins in nearly all materials is highly correlated with the strength and direction of the magnetic anisotropy, at which point an ultrafast electric field pulse induced by intense terahertz pulses can change the orbital state of electrons abruptly and suddenly alter the magnetic anisotropy. Finally, nonlinear spin control is realized by triggering coherent magnon oscillations. This mechanism has successfully been demonstrated in antiferromagnetic TmFeO3 [184]. Antiferromagnetic HoFeO3 [185] 23 and NiO [186] also exhibit nonlinear magnetization dynamics but are still related to the terahertz magnetic field. Therefore, there are two complementary concepts of fully coherent coupling of nonlinearities depending on terahertz pulses: the magnetic field and electric field component of terahertz pulses [187]. The observation and control of the spin waves and resonance [188] in the time domain via terahertz technology have expanded to other antiferromagnetic materials such as MnO, NdFeO3, YFeO3, TbMnO3, HoMnO3, and so on. The spin wave of MnO is excited only by linearly polarized laser pulses ascribed to angular momentum conservation [189], whereas for NdFeO3 and YFeO3, the spin wave is driven by temperature-dependent in-plane anisotropy [190] and dielectric anisotropy of the crystal [191][192][193], respectively. As for TbMnO3 [194], HoMnO3 [193], and YMnO3 [196], the spin dynamics is driven by Optical methods to control antiferromagnetism uncover ultrafast information recording and processing for purely optical memory devices in the future.

Conclusion
In summary, the purpose of this review is to walk the readers through recent works on