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.     

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.     

多铁材料BiNi_xFe_(1-x)O_3(x=0～0.5)磁性的第一性原理计算

采用基于密度泛函理论的第一性原理计算多铁材料BiNi_xFe_(1-x)O_3(x=0,0.125,0.167,0.25,0.5)各晶体结构的电子性质.能带结构、Mulliken电荷以及自旋磁矩等计算结果表明:由Ni离子部分替代掺杂Fe离子可使体系由反铁磁有序转变为局部亚铁磁有序,晶体总自旋磁矩随Ni离子浓度的增大而增大;Ni离子掺杂明显抑制了特定位置Fe离子的磁矩,这是由于Ni-eg轨道少数自旋方向的电子态被50%占据以及Ni离子与Fe离子之间的超交换作用所致.     

Pressure-induced crystallization of a spin liquid

Liquids are expected to crystallize at low temperature. The only exception is helium, which can remain liquid at 0 K, owing to quantum fluctuations. Similarly, the atomic magnetic moments (spins) in a magnet are expected to order at a temperature scale set by the Curie-Weiss temperature theta(CW) (ref. 3). Geometrically frustrated magnets represent an exception. In these systems, the pairwise spin interactions cannot be simultaneously minimized because of the lattice symmetry. This can stabilize a liquid-like state of short-range-ordered fluctuating moments well below theta(CW) (refs 5-7). Here we use neutron scattering to observe the spin liquid state in a geometrically frustrated system, Tb(2)Ti(2)O(7), under conditions of high pressure (approximately 9 GPa) and low temperature (approximately 1 K). This compound is a three-dimensional magnet with theta(CW) = -19 K, where the negative value indicates antiferromagnetic interactions. At ambient pressure Tb(2)Ti(2)O(7) remains in a spin liquid state down to at least 70 mK (ref. 8). But we find that, under high pressure, the spins start to order or 'crystallize' below 2.1 K, with antiferromagnetic order coexisting with liquid-like fluctuations. These results indicate that a spin liquid/solid mixture can be induced by pressure in geometrically frustrated systems.     

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.     

Magnetoresistive behavior and magnetization reversal of NiFe/Cu/CoFe/IrMn spin valve GMRs in nanoscale

The magnetoresistance behavior and the magnetization reversal mode of NiFe/Cu/CoFe/IrMn spin valve giant magnetoresistance (SV-GMR) in nanoscale were investigated experimentally and theoretically by nanosized magnetic simulation methods. Based on the Landau-Lifshitz-Gilbert equation, a model with a special gridding was proposed to calculate the giant magnetoresistance ratio (MR) and investigate the magnetization reversal mode. The relationship between MR and the external magnetic field was obtained and analyzed. Studies into the variation of the magnetization distribution reveal that the magnetization reversal mode, that is, the jump variation mode for NiFe/Cu/CoFe/IrMn, depends greatly on the antiferromagnetic coupling behavior between the pinned layer and the antiferromagnetic layer. It is also found that the switching field is almost linear with the exchange coefficient.     

在分子自旋量子系统中的可控多体纠缠态

在由N＋1个相互作用的反铁磁分子环构成的量子自旋系统中,可以调控1种多体纠缠态。N个周边分子环的电子自旋和1个中心分子环的电子存在相互交换,从而在分子间形成可调的相互作用。通过整个系统的有效自旋哈密顿量解析得出系统的量子动力学行为。研究发现在量子涨落的条件下,1种高精度的形纠缠态可以被制备出来。通过控制分子间的相互作用,这种多体纠缠态也可以从一些分子环传输到其他分子环上。     

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.     

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.     

Spectroscopy of spontaneous spin noise as a probe of spin dynamics and magnetic resonance

Not all noise in experimental measurements is unwelcome. Certain fundamental noise sources contain valuable information about the system itself-a notable example being the inherent voltage fluctuations (Johnson noise) that exist across any resistor, which allow the temperature to be determined. In magnetic systems, fundamental noise can exist in the form of random spin fluctuations. For example, statistical fluctuations of N paramagnetic spins should generate measurable noise of order N spins, even in zero magnetic field. Here we exploit this effect to perform perturbation-free magnetic resonance. We use off-resonant Faraday rotation to passively detect the magnetization noise in an equilibrium ensemble of paramagnetic alkali atoms; the random fluctuations generate spontaneous spin coherences that precess and decay with the same characteristic energy and timescales as the macroscopic magnetization of an intentionally polarized or driven ensemble. Correlation spectra of the measured spin noise reveal g-factors, nuclear spin, isotope abundance ratios, hyperfine splittings, nuclear moments and spin coherence lifetimes-without having to excite, optically pump or otherwise drive the system away from thermal equilibrium. These noise signatures scale inversely with interaction volume, suggesting a possible route towards non-perturbative, sourceless magnetic resonance of small systems.     

Chiral magnetic order at surfaces driven by inversion asymmetry

Chirality is a fascinating phenomenon that can manifest itself in subtle ways, for example in biochemistry (in the observed single-handedness of biomolecules) and in particle physics (in the charge-parity violation of electroweak interactions). In condensed matter, magnetic materials can also display single-handed, or homochiral, spin structures. This may be caused by the Dzyaloshinskii-Moriya interaction, which arises from spin-orbit scattering of electrons in an inversion-asymmetric crystal field. This effect is typically irrelevant in bulk metals as their crystals are inversion symmetric. However, low-dimensional systems lack structural inversion symmetry, so that homochiral spin structures may occur. Here we report the observation of magnetic order of a specific chirality in a single atomic layer of manganese on a tungsten (110) substrate. Spin-polarized scanning tunnelling microscopy reveals that adjacent spins are not perfectly antiferromagnetic but slightly canted, resulting in a spin spiral structure with a period of about 12 nm. We show by quantitative theory that this chiral order is caused by the Dzyaloshinskii-Moriya interaction and leads to a left-rotating spin cycloid. Our findings confirm the significance of this interaction for magnets in reduced dimensions. Chirality in nanoscale magnets may play a crucial role in spintronic devices, where the spin rather than the charge of an electron is used for data transmission and manipulation. For instance, a spin-polarized current flowing through chiral magnetic structures will exert a spin-torque on the magnetic structure, causing a variety of excitations or manipulations of the magnetization and giving rise to microwave emission, magnetization switching, or magnetic motors.     

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 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.     

Coherent spin manipulation without magnetic fields in strained semiconductors

A consequence of relativity is that in the presence of an electric field, the spin and momentum states of an electron can be coupled; this is known as spin-orbit coupling. Such an interaction opens a pathway to the manipulation of electron spins within non-magnetic semiconductors, in the absence of applied magnetic fields. This interaction has implications for spin-based quantum information processing and spintronics, forming the basis of various device proposals. For example, the concept of spin field-effect transistors is based on spin precession due to the spin-orbit coupling. Most studies, however, focus on non-spin-selective electrical measurements in quantum structures. Here we report the direct measurement of coherent electron spin precession in zero magnetic field as the electrons drift in response to an applied electric field. We use ultrafast optical techniques to spatiotemporally resolve spin dynamics in strained gallium arsenide and indium gallium arsenide epitaxial layers. Unexpectedly, we observe spin splitting in these simple structures arising from strain in the semiconductor films. The observed effect provides a flexible approach for enabling electrical control over electron spins using strain engineering. Moreover, we exploit this strain-induced field to electrically drive spin resonance with Rabi frequencies of up to approximately 30 MHz.     

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.     

“1111”型稀磁半导体的研究进展

我们成功地制备了载流子和自旋分离的"1111"型块材稀磁半导体(La,AE)(Zn,TM)AsO(AE=Ba,Sr;TM=Mn,Fe)居里温度TC可以达到40K.我们研究了载流子和局域磁矩对铁磁有序的调制作用,在(La1-xSrx)(Zn0.9Mn0.1)AsO(x=0.10,0.20,0.30)中,控制Mn的浓度为10%,改变Sr的掺杂浓度,当Sr的掺杂量为10%时,我们可以观测到～30 K的铁磁转变温度;而当Sr的掺杂量达到30%时,铁磁转变温度和有效磁矩都大幅度地降低.我们运用缪子自旋共振和中子散射等微观测量手段研究了该系列材料的自旋动力学,缪子自旋共振的测量表明铁磁有序转变发生在整个样品内,即样品是块材稀磁半导体;缪子自旋共振测量得到的"1111"型稀磁半导体静态局域场振幅as与居里温度TC的关系和(Ga,Mn)As,"111"型Li(Zn,Mn)As,以及"122"型(Ba,K)(Zn,Mn)2As2一致,表明这些体系拥有相同的磁性起源机制.我们对该系列稀磁半导体的研究有利于进一步揭示包括(Ga,Mn)As在内的稀磁半导体的磁性起源机制.     

液态甲胺中双稳态溶剂化双电子:神秘的自旋交叉动力学及双电子交换

本文利用从头算分子动力学模拟研究了一类普适液体介质(液态甲胺)中溶剂化双电子的结构及动力学行为,揭示了其独特的结构特征及时间演化动力学, 特别是发现了溶剂化双电子具有迷人的自旋交叉动力学特征,是一类双稳态的双极化子。模拟结果表明,液态甲胺中2个电子优先定域于2个独立的溶剂笼,且进行非协同时间演化。单态时具有稳态动力学演化特征,而三态时则表现为电子交换动力学。非常有趣的是单态及三态轨迹均表明此类动态双极化子展现了双稳态,在时间演化上呈现铁磁态与反铁磁态之间频繁的自旋转换。本工作首次详细报道了一类广泛应用液态介质中双稳态溶剂化双电子的动态自旋交叉现象及动力学规律,为探索此类磁性液体的实际应用提供了新的见解。     

Universal quantum computation with the exchange interaction

Various physical implementations of quantum computers are being investigated, although the requirements that must be met to make such devices a reality in the laboratory at present involve capabilities well beyond the state of the art. Recent solid-state approaches have used quantum dots, donor-atom nuclear spins or electron spins; in these architectures, the basic two-qubit quantum gate is generated by a tunable exchange interaction between spins (a Heisenberg interaction), whereas the one-qubit gates require control over a local magnetic field. Compared to the Heisenberg operation, the one-qubit operations are significantly slower, requiring substantially greater materials and device complexity--potentially contributing to a detrimental increase in the decoherence rate. Here we introduced an explicit scheme in which the Heisenberg interaction alone suffices to implement exactly any quantum computer circuit. This capability comes at a price of a factor of three in additional qubits, and about a factor of ten in additional two-qubit operations. Even at this cost, the ability to eliminate the complexity of one-qubit operations should accelerate progress towards solid-state implementations of quantum computation.     

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.