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 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.     

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.     

Magnetic shape-memory effects in a crystal

Magnetic fields affect the motion of electrons and the orientation of spins in solids, but are thought to have little impact on crystal structure, particularly in compounds with low magnetic susceptibility, such as antiferromagnets. Here we describe an unexpected magnetic effect on crystal shape, in which the direction of the crystal's axes are swapped and the shape changes when a magnetic field is applied; this in turn induces curious memory effects in resistivity and magnetic susceptibility. Ironically, this phenomenon occurs in one of the most well-studied two-dimensional antiferromagnets, La(2-x)Sr(x)CuO(4).     

Electric polarization reversal and memory in a multiferroic material induced by magnetic fields

Ferroelectric and magnetic materials are a time-honoured subject of study and have led to some of the most important technological advances to date. Magnetism and ferroelectricity are involved with local spins and off-centre structural distortions, respectively. These two seemingly unrelated phenomena can coexist in certain unusual materials, termed multiferroics. Despite the possible coexistence of ferroelectricity and magnetism, a pronounced interplay between these properties has rarely been observed. This has prevented the realization of multiferroic devices offering such functionality. Here, we report a striking interplay between ferroelectricity and magnetism in the multiferroic TbMn2O5, demonstrated by a highly reproducible electric polarization reversal and permanent polarization imprint that are both actuated by an applied magnetic field. Our results point to new device applications such as magnetically recorded ferroelectric memory.     

An hour-glass magnetic spectrum in an insulating, hole-doped antiferromagnet

Superconductivity in layered copper oxide compounds emerges when charge carriers are added to antiferromagnetically ordered CuO(2) layers. The carriers destroy the antiferromagnetic order, but strong spin fluctuations persist throughout the superconducting phase and are intimately linked to superconductivity. Neutron scattering measurements of spin fluctuations in hole-doped copper oxides have revealed an unusual 'hour-glass' feature in the momentum-resolved magnetic spectrum that is present in a wide range of superconducting and non-superconducting materials. There is no widely accepted explanation for this feature. One possibility is that it derives from a pattern of alternating spin and charge stripes, and this idea is supported by measurements on stripe-ordered La(1.875)Ba(0.125)CuO(4) (ref. 15). Many copper oxides without stripe order, however, also exhibit an hour-glass spectrum. Here we report the observation of an hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the family of superconducting copper oxides. Our system has stripe correlations and is an insulator, which means that its magnetic dynamics can conclusively be ascribed to stripes. The results provide compelling evidence that the hour-glass spectrum in the copper oxide superconductors arises from fluctuating stripes.     

Spintronic materials and devices based on antiferromagnetic metals

In this paper, we review our recent experimental developments on antiferromagnet (AFM) spintronics mainly comprising Mn-based noncollinear AFM metals. IrMn-based tunnel junctions and Hall devices have been investigated to explore the manipulation of AFM moments by magnetic fields, ferromagnetic materials and electric fields. Room-temperature tunneling anisotropic magnetoresistance based on IrMn as well as FeMn has been successfully achieved, and electrical control of the AFM exchange spring is realized by adopting ionic liquid. In addition, promising spin-orbit effects in AFM as well as spin transfer via AFM spin waves reported by different groups have also been reviewed, indicating that the AFM can serve as an efficient spin current source. To explore the crucial role of AFM acting as efficient generators, transmitters, and detectors of spin currents is an emerging topic in the field of magnetism today. AFM metals are now ready to join the rapidly developing fields of basic and applied spintronics, enriching this area of solid-state physics and microelectronics.     

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.     

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.     

Direct observation of the alignment of ferromagnetic spins by antiferromagnetic spins

The arrangement of spins at interfaces in a layered magnetic material often has an important effect on the properties of the material. One example of this is the directional coupling between the spins in an antiferromagnet and those in an adjacent ferromagnet, an effect first discovered in 1956 and referred to as exchange bias. Because of its technological importance for the development of advanced devices such as magnetic read heads and magnetic memory cells, this phenomenon has received much attention. Despite extensive studies, however, exchange bias is still poorly understood, largely due to the lack of techniques capable of providing detailed information about the arrangement of magnetic moments near interfaces. Here we present polarization-dependent X-ray magnetic dichroism spectro-microscopy that reveals the micromagnetic structure on both sides of a ferromagnetic-antiferromagnetic interface. Images of thin ferromagnetic Co films grown on antiferromagnetic LaFeO3 show a direct link between the arrangement of spins in each material. Remanent hysteresis loops, recorded for individual ferromagnetic domains, show a local exchange bias. Our results imply that the alignment of the ferromagnetic spins is determined, domain by domain, by the spin directions in the underlying antiferromagnetic layer.     

双层三角晶格反铁磁材料c-轴电阻性质的研究

由t-J模型出发,在feimion-spin理论框架下研究了空穴型掺杂双层三角晶格反铁磁材料c-轴的反常电阻行为.与面内电荷动力学不同的是,c-轴电荷输运与材料的种类有关.材料中CuO2平面之间是否有Cu-O链,对于材料的物理性质有很大的影响.对于层间存在Cu-O链的材料,c-轴方向的电荷动力学主要是由CuO2平面内holon的散射决定的,并且其c-轴电荷动力学与ab平面的电荷动力学在定性上是相似的;而对于层间不存在Cu-O链的材料,c-轴方向的电荷动力学是由CuO2平面内holon的散射和层间的无序效应共同决定的.此外,c-轴方向的电阻率比平面内的电阻率大3～4个数量级,这就意味着大部分的载流子被限制在了ab平面内.因此,双层三角晶格反铁磁材料正常态电荷动力学存在着很强的各向异性行为.由三角晶格反铁磁材料得到的结果与四方晶格相类似,但是由于三角晶格反铁磁材料自身存在的几何阻挫效应,使得二者的结果稍有差异.     

Minority spin condensate in the spin-polarized superfluid 3He A1 phase

The magnetic properties of (3)He in its various phases originate from the interactions among the nuclear spins. The spin-polarized 'ferromagnetic' superfluid (3)He A(1) phase (which forms below 3 mK between two transition temperatures, T(c1) and T(c2), in an external magnetic field) serves as a material in which theories of fundamental magnetic processes and macroscopic quantum spin phenomena may be tested. Conventionally, the superfluid component of the A(1) phase is understood to contain only the majority spin condensate, having energetically favoured paired spins directed along the external field and no minority spin condensate having paired spins in the opposite direction. Because of difficulties in satisfying both the ultralow temperature and high magnetic field required to produce a substantial phase space, there exist few studies of spin dynamics phenomena that could be used to test the conventional view of the A(1) phase. Here we develop a mechanical spin density detector that operates in the required regime, enabling us to perform measurements of spin relaxation in the A(1) phase as a function of temperature, pressure and magnetic field. Our mechanical spin detector is based in principle on the magnetic fountain effect; spin-polarized superfluid motion can be induced both magnetically and mechanically, and we demonstrate the feasibility of increasing spin polarization by a mechanical spin filtering process. In the high temperature range of the A(1) phase near T(c1), the measured spin relaxation time is long, as expected. Unexpectedly, the spin relaxation rate increases rapidly as the temperature is decreased towards T(c2). Our measurements, together with Leggett-Takagi theory, demonstrate that a minute presence of minority spin pairs is responsible for this unexpected spin relaxation behaviour. Thus, the long-held conventional view that the A(1) phase contains only the majority spin condensate is inadequate.     

双层三角晶格反铁磁体c-轴电荷动力学研究

由t-J模型出发,在Feimion-spin理论框架下研究了有Cu-O链的双层三角晶格反铁磁体的c-轴电荷动力学.与面内电荷动力学不同的是,c-轴电荷输运与材料的种类有关,也就是说,它依赖于CuO2平面间的化学组成成分.材料中CuO2平面之间是否有Cu-O链,对于材料的物理性质有很大的影响.对于层间存在Cu-O链的材料,c-轴方向的电荷动力学主要是由CuO2平面内holon的散射决定的.双层三角晶格反铁磁体正常态电荷动力学与单层材料一样,也存在着很强的各向异性行为.由三角晶格得到的结果与四方晶格相类似,但是由于三角晶格自身存在的几何阻挫效应,使得二者的结果稍有差异.     

空穴型掺杂双层三角晶格反铁磁体积分自旋动力学研究

从t-J模型出发,在feimion-spin理论框架下研究了空穴型掺杂双层三角晶格反铁磁体积分自旋动力学.结果表明,与四方晶格材料不同,空穴型掺杂双层三角晶格反铁磁体积分自旋动力学不具有普适行为.结果还表明对于三角晶格反铁磁体,2个CuO2平面之间的耦合对自旋动力学的影响不是很大,双层三角晶格反铁磁体的自旋动力学与单层三角晶格大体相同.     

Electrical effects of spin density wave quantization and magnetic domain walls in chromium

The role of magnetic domains (and the walls between domains) in determining the electrical properties of ferromagnetic materials has been investigated in great detail for many years, not least because control over domains offers a means of manipulating electron spin to control charge transport in 'spintronic' devices. In contrast, much less attention has been paid to the effects of domains and domain walls on the electrical properties of antiferromagnets: antiferromagnetic domains show no net external magnetic moment, and so are difficult to manipulate or probe. Here we describe electrical measurements on chromium--a simple metal and quintessential spin density wave antiferromagnet--that show behaviour directly related to spin density wave formation and the presence of antiferromagnetic domains. Two types of thermal hysteresis are seen in both longitudinal and Hall resistivity: the first can be explained by the quantization of spin density waves due to the finite film thickness (confirmed by X-ray diffraction measurements) and the second by domain-wall scattering of electrons. We also observe the striking influence of the electrical lead configuration (a mesoscopic effect) on the resistivity of macroscopic samples in the spin density wave state. Our results are potentially of practical importance, in that they reveal tunable electrical effects of film thickness and domain walls that are as large as the highest seen for ferromagnets.     

Single-shot read-out of an individual electron spin in a quantum dot

Spin is a fundamental property of all elementary particles. Classically it can be viewed as a tiny magnetic moment, but a measurement of an electron spin along the direction of an external magnetic field can have only two outcomes: parallel or anti-parallel to the field. This discreteness reflects the quantum mechanical nature of spin. Ensembles of many spins have found diverse applications ranging from magnetic resonance imaging to magneto-electronic devices, while individual spins are considered as carriers for quantum information. Read-out of single spin states has been achieved using optical techniques, and is within reach of magnetic resonance force microscopy. However, electrical read-out of single spins has so far remained elusive. Here we demonstrate electrical single-shot measurement of the state of an individual electron spin in a semiconductor quantum dot. We use spin-to-charge conversion of a single electron confined in the dot, and detect the single-electron charge using a quantum point contact; the spin measurement visibility is approximately 65%. Furthermore, we observe very long single-spin energy relaxation times (up to approximately 0.85 ms at a magnetic field of 8 T), which are encouraging for the use of electron spins as carriers of quantum information.     

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.     

Emergent excitations in a geometrically frustrated magnet

Frustrated systems are ubiquitous, and they are interesting because their behaviour is difficult to predict; frustration can lead to macroscopic degeneracies and qualitatively new states of matter. Magnetic systems offer good examples in the form of spin lattices, where all interactions between spins cannot be simultaneously satisfied. Here we report how unusual composite spin degrees of freedom can emerge from frustrated magnetic interactions in the cubic spinel ZnCr(2)O(4). Upon cooling, groups of six spins self-organize into weakly interacting antiferromagnetic loops, whose directors -- the unique direction along which the spins are aligned, parallel or antiparallel -- govern all low-temperature dynamics. The experimental evidence comes from a measurement of the magnetic form factor by inelastic neutron scattering; the data show that neutrons scatter from hexagonal spin clusters rather than individual spins. The hexagon directors are, to a first approximation, decoupled from each other, and hence their reorientations embody the long-sought local zero energy modes for the pyrochlore lattice.     

Detecting excitation and magnetization of individual dopants in a semiconductor

An individual magnetic atom doped into a semiconductor is a promising building block for bottom-up spintronic devices and quantum logic gates. Moreover, it provides a perfect model system for the atomic-scale investigation of fundamental effects such as magnetism in dilute magnetic semiconductors. However, dopants in semiconductors so far have not been studied by magnetically sensitive techniques with atomic resolution that correlate the atomic structure with the dopant's magnetism. Here we show electrical excitation and read-out of a spin associated with a single magnetic dopant in a semiconductor host. We use spin-resolved scanning tunnelling spectroscopy to measure the spin excitations and the magnetization curve of individual iron surface-dopants embedded within a two-dimensional electron gas confined to an indium antimonide (110) surface. The dopants act like isolated quantum spins the states of which are governed by a substantial magnetic anisotropy that forces the spin to lie in the surface plane. This result is corroborated by our first principles calculations. The demonstrated methodology opens new routes for the investigation of sample systems that are more widely studied in the field of spintronics-that is, Mn in GaAs (ref. 5), magnetic ions in semiconductor quantum dots, nitrogen-vacancy centres in diamond and phosphorus spins in silicon.     

(B2-FeRh)-(L1_0-FePt)复合双层薄膜的热致反铁磁-铁磁转变

用磁控溅射法在加热的MgO(001)基片上生长FeRh-FePt双层取向薄膜,试图使两层都有序化并分析其反铁磁-铁磁转变性质.结果表明,覆盖于FePt层之上的FeRh层可以在不发生层间混合的前提下,于450°C依靠长时间热处理出现有序化,从而获得较为理想的(B2-FeRh)-(L10-FePt)双层复合薄膜.改变FeRh层的成分和控制FePt层的有序化程度能够裁剪其反铁磁-铁磁转变行为.在彻底有序化的FePt层上生长富Fe的FeRh层会导致热滞温度为负.而FePt层适当欠有序化则可以将反铁磁-铁磁转变温度由100°C提高到200°C,使其进一步远离室温.这有利于用来制作采用热辅助技术的垂直磁记录介质.从Pt扩散阈值的角度对反铁磁-铁磁转变举动变化的可能原因进行了讨论.