Transient ferromagnetic-like state mediating ultrafast reversal of antiferromagnetically coupled spins

Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism. Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. Usually the dynamics are studied by observing the collective response of exchange-coupled spins, that is, spin resonances, after an external perturbation by a pulse of magnetic field, current or light. The periods of the corresponding resonances range from one nanosecond for ferromagnets down to one picosecond for antiferromagnets. However, virtually nothing is known about the behaviour of spins in a magnetic material after being excited on a timescale faster than that corresponding to the exchange interaction (10-100?fs), that is, in a non-adiabatic way. Here we use the element-specific technique X-ray magnetic circular dichroism to study spin reversal in GdFeCo that is optically excited on a timescale pertinent to the characteristic time of the exchange interaction between Gd and Fe spins. We unexpectedly find that the ultrafast spin reversal in this material, where spins are coupled antiferromagnetically, occurs by way of a transient ferromagnetic-like state. Following the optical excitation, the net magnetizations of the Gd and Fe sublattices rapidly collapse, switch their direction and rebuild their net magnetic moments at substantially different timescales; the net magnetic moment of the Gd sublattice is found to reverse within 1.5 picoseconds, which is substantially slower than the Fe reversal time of 300 femtoseconds. Consequently, a transient state characterized by a temporary parallel alignment of the net Gd and Fe moments emerges, despite their ground-state antiferromagnetic coupling. These surprising observations, supported by atomistic simulations, provide a concept for the possibility of manipulating magnetic order on the timescale of the exchange interaction.     

Magnetic exchange force microscopy with atomic resolution

The ordering of neighbouring atomic magnetic moments (spins) leads to important collective phenomena such as ferromagnetism and antiferromagnetism. A full understanding of magnetism on the nanometre scale therefore calls for information on the arrangement of spins in real space and with atomic resolution. Spin-polarized scanning tunnelling microscopy accomplishes this but can probe only conducting materials. Force microscopy can be used on any sample independent of its conductivity. In particular, magnetic force microscopy is well suited to exploring ferromagnetic domain structures. However, atomic resolution cannot be achieved because data acquisition involves the sensing of long-range magnetostatic forces between tip and sample. Magnetic exchange force microscopy has been proposed for overcoming this limitation: by using an atomic force microscope with a magnetic tip, it should be possible to detect the short-range magnetic exchange force between tip and sample spins. Here we show for a prototypical antiferromagnetic insulator, the (001) surface of nickel oxide, that magnetic exchange force microscopy can indeed reveal the arrangement of both surface atoms and their spins simultaneously. In contrast with previous attempts to implement this method, we use an external magnetic field to align the magnetic polarization at the tip apex so as to optimize the interaction between tip and sample spins. This allows us to observe the direct magnetic exchange coupling between the spins of the tip atom and sample atom that are closest to each other, and thereby demonstrate the potential of magnetic exchange force microscopy for investigations of inter-spin interactions at the atomic level.     

铁磁180°畴层结构中的畴层振动模

本文探讨了具有反平行磁化矢量的铁磁层状结构的磁畴振动谱,给出了普遍的色散方程.当外加磁场为零,整个分布成180°畴层状态时,给出了一般的色散曲钱.理论预言可能同时存在一种具有二维铁磁性和一维反铁磁性的新型磁有序材料.     

应力对铁磁/反铁磁双层体系交换偏置场的影响

研究在铁磁/反铁磁层体系中,当铁磁层中存在应力时对体系交换偏置场的影响.分别计算了改变应力大小和方向两种情况下,磁滞回线的变化以及偏置场随外磁场角度依赖关系的改变.结果表明:应力场沿着易轴方向时,偏置场随外磁场角度依赖关系表现为随着应力场的增大,偏置场的最大值变大,其最大值所对应的位置逐渐远离易轴.在外磁场与易轴成一定角度时,交换偏置场向左移动,并且阻碍沿磁场方向的磁化随着应力场的增加;当应力场旋转90°时,偏置场随外磁场角度依赖关系表现为随着应力场的增大,偏置场的最大值减小,其最大值所对应的位置逐渐靠近易轴.在外磁场与易轴成一定角度时,交换偏置场向右移动,并且促进沿磁场方向的磁化随着应力场的增加.     

Laser-induced ultrafast spin reorientation in the antiferromagnet TmFeO3

All magnetically ordered materials can be divided into two primary classes: ferromagnets and antiferromagnets. Since ancient times, ferromagnetic materials have found vast application areas, from the compass to computer storage and more recently to magnetic random access memory and spintronics. In contrast, antiferromagnetic (AFM) materials, though representing the overwhelming majority of magnetically ordered materials, for a long time were of academic interest only. The fundamental difference between the two types of magnetic materials manifests itself in their reaction to an external magnetic field-in an antiferromagnet, the exchange interaction leads to zero net magnetization. The related absence of a net angular momentum should result in orders of magnitude faster AFM spin dynamics. Here we show that, using a short laser pulse, the spins of the antiferromagnet TmFeO3 can indeed be manipulated on a timescale of a few picoseconds, in contrast to the hundreds of picoseconds in a ferromagnet. Because the ultrafast dynamics of spins in antiferromagnets is a key issue for exchange-biased devices, this finding can expand the now limited set of applications for AFM materials.     

Beating the superparamagnetic limit with exchange bias

Interest in magnetic nanoparticles has increased in the past few years by virtue of their potential for applications in fields such as ultrahigh-density recording and medicine. Most applications rely on the magnetic order of the nanoparticles being stable with time. However, with decreasing particle size the magnetic anisotropy energy per particle responsible for holding the magnetic moment along certain directions becomes comparable to the thermal energy. When this happens, the thermal fluctuations induce random flipping of the magnetic moment with time, and the nanoparticles lose their stable magnetic order and become superparamagnetic. Thus, the demand for further miniaturization comes into conflict with the superparamagnetism caused by the reduction of the anisotropy energy per particle: this constitutes the so-called 'superparamagnetic limit' in recording media. Here we show that magnetic exchange coupling induced at the interface between ferromagnetic and antiferromagnetic systems can provide an extra source of anisotropy, leading to magnetization stability. We demonstrate this principle for ferromagnetic cobalt nanoparticles of about 4 nm in diameter that are embedded in either a paramagnetic or an antiferromagnetic matrix. Whereas the cobalt cores lose their magnetic moment at 10 K in the first system, they remain ferromagnetic up to about 290 K in the second. This behaviour is ascribed to the specific way ferromagnetic nanoparticles couple to an antiferromagnetic matrix.     

Quantum simulation of antiferromagnetic spin chains in an optical lattice

Understanding exotic forms of magnetism in quantum mechanical systems is a central goal of modern condensed matter physics, with implications for systems ranging from high-temperature superconductors to spintronic devices. Simulating magnetic materials in the vicinity of a quantum phase transition is computationally intractable on classical computers, owing to the extreme complexity arising from quantum entanglement between the constituent magnetic spins. Here we use a degenerate Bose gas of rubidium atoms confined in an optical lattice to simulate a chain of interacting quantum Ising spins as they undergo a phase transition. Strong spin interactions are achieved through a site-occupation to pseudo-spin mapping. As we vary a magnetic field, quantum fluctuations drive a phase transition from a paramagnetic phase into an antiferromagnetic phase. In the paramagnetic phase, the interaction between the spins is overwhelmed by the applied field, which aligns the spins. In the antiferromagnetic phase, the interaction dominates and produces staggered magnetic ordering. Magnetic domain formation is observed through both in situ site-resolved imaging and noise correlation measurements. By demonstrating a route to quantum magnetism in an optical lattice, this work should facilitate further investigations of magnetic models using ultracold atoms, thereby improving our understanding of real magnetic materials.     

双二次耦合场对铁磁/反铁磁双层膜中交换偏置场的影响

研究了界面双线性耦合和双二次耦合对铁磁/反铁磁双层膜中交换偏置场的影响.结果表明:随着双线性耦合场或双二次耦合场的增加,交换偏置场的最大值位置向易轴靠近.另外,交换偏置场随外磁场角度的变化曲线随着双二次耦合场的增加,逐渐趋于正弦图像.其根源在于双二次耦合场与铁磁层单轴各向异场二者作用相互抵消,使曲线趋于光滑.铁磁层的磁滞回线对双二次耦合场也有一定的依赖.     

Observation of coupled magnetic and electric domains

Ferroelectromagnets are an interesting group of compounds that complement purely (anti-)ferroelectric or (anti-)ferromagnetic materials--they display simultaneous electric and magnetic order. With this coexistence they supplement materials in which magnetization can be induced by an electric field and electrical polarization by a magnetic field, a property which is termed the magnetoelectric effect. Aside from its fundamental importance, the mutual control of electric and magnetic properties is of significant interest for applications in magnetic storage media and 'spintronics'. The coupled electric and magnetic ordering in ferroelectromagnets is accompanied by the formation of domains and domain walls. However, such a cross-correlation between magnetic and electric domains has so far not been observed. Here we report spatial maps of coupled antiferromagnetic and ferroelectric domains in YMnO3, obtained by imaging with optical second harmonic generation. The coupling originates from an interaction between magnetic and electric domain walls, which leads to a configuration that is dominated by the ferroelectromagnetic product of the order parameters.     

Origin of ferromagnetic exchange interactions in a fullerene-organic compound

Organic ferromagnets, which exhibit exchange interactions between unpaired electrons in pi-orbitals, are rare, and the origin of ferromagnetism in these compounds has so far remained unexplained. Tetrakis(dimethylamino)ethylene-fullerene[60] (TDAE-C60) shows a transition to a ferromagnetic state with fully saturated s = 1/2 molecular spins at the relatively high Curie temperature (for organic materials) of 16 K (ref. 4). It has been suggested that the orientations of the C60 molecules may be important for ferromagnetism in this material, but in the absence of structural data at low temperatures there has been little progress towards understanding these microscopic interactions. Here we report the results of a comparative structural study of two different magnetic forms of TDAE-C60 crystals at low temperatures, correlating the structural properties--in particular, the intermolecular orientations--with the magnetic properties. We find that both ferromagnetism and spin-glass-like ordering are possible in this material, and depend on the orientational state of C60 molecules. This resolves the apparent contradictions posed by different macroscopic measurements, and opens the way to a microscopic understanding of pi-electron ferromagnetic exchange interactions in organic materials.     

Magnetic and non-magnetic phases of a quantum spin liquid

A quantum spin-liquid phase is an intriguing possibility for a system of strongly interacting magnetic units in which the usual magnetically ordered ground state is avoided owing to strong quantum fluctuations. It was first predicted theoretically for a triangular-lattice model with antiferromagnetically coupled S = 1/2 spins. Recently, materials have become available showing persuasive experimental evidence for such a state. Although many studies show that the ideal triangular lattice of S = 1/2 Heisenberg spins actually orders magnetically into a three-sublattice, non-collinear 120° arrangement, quantum fluctuations significantly reduce the size of the ordered moment. This residual ordering can be completely suppressed when higher-order ring-exchange magnetic interactions are significant, as found in nearly metallic Mott insulators. The layered molecular system κ-(BEDT-TTF)(2)Cu(2)(CN)(3) is a Mott insulator with an almost isotropic, triangular magnetic lattice of spin-1/2 BEDT-TTF dimers that provides a prime example of a spin liquid formed in this way. Despite a high-temperature exchange coupling, J, of 250 K (ref. 6), no obvious signature of conventional magnetic ordering is seen down to 20 mK (refs 7, 8). Here we show, using muon spin rotation, that applying a small magnetic field to this system produces a quantum phase transition between the spin-liquid phase and an antiferromagnetic phase with a strongly suppressed moment. This can be described as Bose-Einstein condensation of spin excitations with an extremely small spin gap. At higher fields, a second transition is found that suggests a threshold for deconfinement of the spin excitations. Our studies reveal the low-temperature magnetic phase diagram and enable us to measure characteristic critical properties. We compare our results closely with current theoretical models, and this gives some further insight into the nature of the spin-liquid phase.     

1/5掺杂反铁磁海森堡自旋链的基态

利用密度矩阵重整化群方法研究1/5掺杂对自旋1/2海森堡反铁磁链的影响.研究表明,掺人侧自旋能减弱近邻自旋关联但增强长程自旋关联,同时侧自旋的掺入破坏了子格对称性,导致系统基态反铁磁和铁磁长程序共存.侧自旋对量子涨落的抑制作用随自旋关联距离的增加而减弱,且其自身受到量子涨落的影响最小.对于1/5掺杂反铁磁海森堡自旋链,量子涨落的影响明显,引起了交错磁化率47%的减弱.     

具有掺杂界面的磁超晶格的磁相变研究

采用蒙特卡罗模拟方法研究具有无序掺杂界面的复杂磁性系统的磁特性,重点计算了系统掺杂界面层(ApB1-p)上A类原子的掺杂浓度p及界面交换耦合常数对磁相变的影响.通过模拟,获得了系统的相图以及磁化强度、磁化率等随温度的变化规律.模拟结果表明,纳米磁性薄膜的无序掺杂界面极大地决定了系统的相图.     

Direct measurement of antiferromagnetic domain fluctuations

Measurements of magnetic noise emanating from ferromagnets owing to domain motion were first carried out nearly 100 years ago, and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise. However, this must be sampled at spatial wavelengths of the order of several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present a direct measurement of the fluctuations in the nanometre-scale superstructure of spin- and charge-density waves associated with antiferromagnetism in elemental chromium. The technique used is X-ray photon correlation spectroscopy, where coherent X-ray diffraction produces a speckle pattern that serves as a 'fingerprint' of a particular magnetic domain configuration. The temporal evolution of the patterns corresponds to domain walls advancing and retreating over micrometre distances. This work demonstrates a useful measurement tool for antiferromagnetic domain wall engineering, but also reveals a fundamental finding about spin dynamics in the simplest antiferromagnet: although the domain wall motion is thermally activated at temperatures above 100 K, it is not so at lower temperatures, and indeed has a rate that saturates at a finite value-consistent with quantum fluctuations-on cooling below 40 K.     

光学界面自旋波与界面各向异性

计入外磁场、体和界面单轴各向异性后，详细研究了铁磁或反铁磁层间耦合的自旋双链系统中的光学界面自旋波及其存在的充要条件．结果发现：对于不同的界面各向异性，该系统可以存在０、１和２个光学界面自旋波，其能量随铁磁层间耦合强度的增大而升高，却随反铁磁层间耦合强度的增大而降低．     

Effect of the Interface Anisotropy on Optical Interface Spin Waves in an A-B-A System

计入外磁场、体和界面单轴各向异性后，研究了铁磁和反铁磁层间耦合的Ａ－Ｂ－Ａ三链系统中的光学界面自旋波及其存在的充要条件．结果发现，在三链系统中，有两种不同类型的光学界面自旋波，它们在不同界面各向异性条件下，均可存在０～２个自旋波解     

纳米级自旋分离的铁磁共振成像研究

自旋间的相互作用力对原子级别纳米构造体磁性质的理解是极为重要的.磁交换力显微镜(MExFM)是测量原子磁矩相互作用力的一种创新手段,但是这种手段不能分离表面形貌和自旋信息.我们提出的铁磁共振磁交换力显微镜(FMR-MExFM)实现了磁性材料表面的磁交换力信息和表面形貌信息的分离.为了充分利用铁磁共振效应,高效率的微波照射机构是极为重要的一项.在本研究中,通过照射机构的仿真和优化设计得到了微波在同轴电缆间的衰减、照射机构直径以及同轴电缆-探针距离之间的关系,同时得到了最优化的实验条件,在此基础上提高了铁磁共振检测的灵敏度.运用改进后的FMR-MExFM,成功地完全分离了磁性信息和表面结构信息,实现了磁性信息N极和S极的180°的相位差.本研究对FMR-MExFM的开发、磁信息检测具有重要的作用.     

A-B-A系统界面各相异性与声学型界面自旋波存在条件

计入外磁场、铁磁和反铁磁层间耦合、体和界面各向异性后,研究了Ａ－Ｂ－Ａ三链系统中物声学型界面自旋波及其存在条件,结果发现,在此系统中,存在奇、偶两种不同宇称的声学型界面自旋波,在没的界面各向异性条件,这两种声学型界面自旋波解的个数分别在０￣２之间变化。     

Ni／NiO交换偏置系统的制备与测试

将具有60keV能量的O离子注入通过分子外延束方法生长的Ni铁磁性薄膜,再将注入离子与基质原子发生化学反应所生成的反铁磁性NiO颗粒镶嵌于铁磁性薄膜中,在Ni／NiO界面处通过交换相互作用构建起交换偏置系统,在对系统进行场冷后,成功测量到磁滞回线的平移,在此基础上研究了系统的微观结构和磁特性,并对其中交换偏置的来源做出初步解释．通过调制注入离子的剂量,系统研究了不同因素对交换偏置的影响,优化了交换偏置系统的制备方法．