Showing papers on "Antiferromagnetism published in 2007"

Defect-induced magnetism in graphene

Abstract: We study from first principles the magnetism in graphene induced by single carbon atom defects For two types of defects considered in our study, the hydrogen chemisorption defect and the vacancy defect, the itinerant magnetism due to the defect-induced extended states has been observed Calculated magnetic moments are equal to 1µB per hydrogen chemisorption defect and 112–153µB per vacancy defect depending on the defect concentration The coupling between the magnetic moments is either ferromagnetic or antiferromagnetic, depending on whether the defects correspond to the same or to different hexagonal sublattices of the graphene lattice, respectively The relevance of itinerant magnetism in graphene to the high-TC magnetic ordering is discussed

Chiral magnetic order at surfaces driven by inversion asymmetry

Abstract: 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.

Spin dynamics of the spin-1/2 kagome lattice antiferromagnet ZnCu3(OH)6Cl2.

Abstract: We have performed thermodynamic and neutron scattering measurements on the $S=1/2$ kagom\'e lattice antiferromagnet ${\mathrm{ZnCu}}_{3}(\mathrm{OH}{)}_{6}{\mathrm{Cl}}_{2}$. The susceptibility indicates a Curie-Weiss temperature of ${\ensuremath{\theta}}_{\mathrm{CW}}\ensuremath{\simeq}\ensuremath{-}300\text{ }\text{ }\mathrm{K}$; however, no magnetic order is observed down to 50 mK. Inelastic neutron scattering reveals a spectrum of low energy spin excitations with no observable gap down to 0.1 meV. The specific heat at low-$T$ follows a power law temperature dependence. These results suggest that an unusual spin liquid state with essentially gapless excitations is realized in this kagom\'e lattice system.

Raman scattering in nanosized nickel oxide NiO

Abstract: Magnetic ordering in nanosized (100 and 1500 nm) nickel oxide NiO powders, prepared by the plasma synthesis method, was studied using Raman scattering spectroscopy in a wide range of temperatures from 10 to 300 K. It was observed that the intensity of two- magnon band decreases rapidly for smaller crystallites size. This effect is attributed to a decrease of antiferromagnetic spin correlations and leads to the antiferromagnetic-to- paramagnetic phase transition .

Exchange bias using a spin glass

Abstract: Exchange bias is commonly manifested as the hysteresis-loop shift observed when a ferromagnet is in contact with an antiferromagnet. Here, we report observations of exchange bias with unusual features of a ferromagnet in contact with a spin glass, demonstrating that this is a phenomenon of greater generality. The easily measured properties of the ferromagnet allow access to the internal magnetic degrees of freedom of the glass to which they are coupled. Our results show that a Co/CuMn bilayer system exhibits all the rich phenomena of coercivity enhancement, bias-field shifts and training effects associated with a conventional ferromagnet/antiferromagnet system. Nevertheless, striking differences arise, such as an orientation reversal of the bias field in a small temperature range just below the blocking temperature. We argue that all features can be understood within the context of a random-field model for long-ranged oscillatory Ruderman–Kittel–Kasuya–Yosida (RKKY) coupled spins.

Spin-liquid state in the S=1/2 hyperkagome antiferromagnet Na4Ir3O8.

Abstract: A spinel related oxide, Na(4)Ir(3)O(8), was found to have a three dimensional network of corner shared Ir(4+) (t(2g)(5)) triangles. This gives rise to an antiferromagnetically coupled S = 1/2 spin system formed on a geometrically frustrated hyperkagome lattice. Magnetization M and magnetic specific heat C(m) data showed the absence of long range magnetic ordering at least down to 2 K. The large C(m) at low temperatures is independent of applied magnetic field up to 12 T, in striking parallel to the behavior seen in triangular and kagome antiferromagnets reported to have a spin-liquid ground state. These results strongly suggest that the ground state of Na(4)Ir(3)O(8) is a three dimensional manifestation of a spin liquid.

Ferromagnetism in nanoscale BiFeO3

Abstract: A remarkably high saturation magnetization of ~0.4mu_B/Fe along with room temperature ferromagnetic hysteresis loop has been observed in nanoscale (4-40 nm) multiferroic BiFeO_3 which in bulk form exhibits weak magnetization (~0.02mu_B/Fe) and an antiferromagnetic order. The magnetic hysteresis loops, however, exhibit exchange bias as well as vertical asymmetry which could be because of spin pinning at the boundaries between ferromagnetic and antiferromagnetic domains. Interestingly, like in bulk BiFeO_3, both the calorimetric and dielectric permittivity data in nanoscale BiFeO_3 exhibit characteristic features at the magnetic transition point. These features establish formation of a true ferromagnetic-ferroelectric system with a coupling between the respective order parameters in nanoscale BiFeO_3.

Quantum magnetism in the paratacamite family: towards an ideal kagomé lattice.

Abstract: We report muon spin rotation measurements on the $S=1/2$ (${\mathrm{Cu}}^{2+}$) paratacamite ${\mathrm{Zn}}_{x}{\mathrm{Cu}}_{4\ensuremath{-}x}(\mathrm{OH}{)}_{6}{\mathrm{Cl}}_{2}$ family. Despite a Weiss temperature of $\ensuremath{\sim}\ensuremath{-}300\text{ }\text{ }\mathrm{K}$, the $x=1$ compound is found to have no transition to a magnetic frozen state down to 50 mK as theoretically expected for the kagom\'e Heisenberg antiferromagnet. We find that the limit between a dynamical and a partly frozen ground state occurs around $x=0.5$. For $x=1$, we discuss the relevance to a singlet picture.

Ferromagnetism in nanoscale BiFeO3

Abstract: A remarkably high saturation magnetization of ∼0.4μB∕Fe along with room temperature ferromagnetic hysteresis loop has been observed in nanoscale (4–40nm) multiferroic BiFeO3 which in bulk form exhibits weak magnetization (∼0.02μB∕Fe) and an antiferromagnetic order. The magnetic hysteresis loops exhibit exchange bias and vertical asymmetry which could be because of spin pinning at the boundaries between ferromagnetic and antiferromagnetic domains. Interestingly, both the calorimetric and dielectric permittivity data in nanoscale BiFeO3 exhibit characteristic features at the magnetic transition point. These features establish the formation of a true ferromagnetic-ferroelectric system with a coupling between the respective order parameters in nanoscale BiFeO3.

Synthesis and size-dependent exchange bias in inverted core-shell MnO|Mn3O4 nanoparticles.

Abstract: Core−shell nanoparticles of MnO|Mn3O4 with average particle sizes of 5−60 nm, composed of an antiferromagnetic (AFM) core and a ferrimagnetic (FiM) shell, have been synthesized and their magnetic properties investigated. The core−shell structure has been generated by the passivation of the MnO cores, yielding an inverted AFM-core|FiM-shell system, as opposed to the typical FM-core|AFM-shell. The exchange-coupling between AFM and FiM gives rise to an enhanced coercivity of ∼8 kOe and a loop shift of ∼2 kOe at 10 K, i.e., exchange bias. The coercivity and loop shift show a non-monotonic variation with the core diameter. The large coercivity and the loop shift are ascribed to the highly anisotropic Mn3O4 and size effects of the AFM (i.e., uncompensated spins, AFM domains, and size-dependent transition temperature).

Exchange bias behavior in Ni-Mn-Sb Heusler alloys

Abstract: The authors report the observation of exchange bias in bulk polycrystalline Ni50Mn25+xSb25−x Heusler alloys. Shifts in hysteresis loops of up to 248Oe were observed in the 5T field cooled samples. The observed exchange bias behavior in Ni50Mn25+xSb25−x is attributed to the coexistence of antiferromagnetic and ferromagnetic exchange interactions in the system. Such behavior is an addition to the multifunctional properties of the Ni50Mn25+xSb25−x Heusler alloy system.

Single crystal-to-single crystal transformation from ferromagnetic discrete molecules to a spin-canting antiferromagnetic layer.

Abstract: A unique single crystal-to-single crystal transformation from 0D discrete molecules to a 2D coordination polymer exhibits magnetic property changes from a short-range ferromagnetic coupling to a long-range canting antiferromagnetic ordering.

Enhancement of ferromagnetic properties in BiFeO3 polycrystalline ceramic by La doping

Abstract: The authors present the structure transformation and magnetic properties of Bi1−xLaxFeO3 (x=0.0–0.15) ceramics prepared by a conventional solid-state reaction processing. Magnetic measurements reveal that remnant magnetization of 15% La-doped BiFeO3 has enhanced about 20 times as compared to pure BiFeO3. It is the structural phase transition (R3c–C222) near x=0.15 that destructs the spin cycloid, and thus enhances the ferromagnetic properties significantly. In these Bi1−xLaxFeO3 ceramic samples, besides the known antiferromagnetic Neel temperature TN1∼615K, another Neel temperature TN2∼260K can be observed due to the trace impurity phase of Bi2Fe4O9 in these ceramic samples.

Order-by-disorder and spiral spin-liquid in frustrated diamond-lattice antiferromagnets

Abstract: Frustration refers to competition between different interactions that cannot be simultaneously satisfied—a familiar feature in many magnetic solids. Strong frustration leads to highly degenerate ground states and a large suppression of ordering by fluctuations. Key challenges in frustrated magnetism include the characterization of the fluctuating spin-liquid regime and determination of the mechanism of eventual order at lower temperature. Here, we study a model of a diamond-lattice antiferromagnet appropriate for numerous spinel materials. With sufficiently strong frustration, a massive ground-state degeneracy develops amongst spirals whose propagation wavevectors reside on a continuous two-dimensional ‘spiral surface’ in momentum space. We argue that an important ordering mechanism is entropic splitting of the degenerate ground states, an elusive phenomenon called ‘order by disorder’. A broad spiral spin-liquid regime emerges at higher temperatures, where the underlying spiral surface can be directly revealed through spin correlations. We discuss the agreement between these predictions and the well-characterized spinel MnSc2S4.

Diluted graphene antiferromagnet.

Abstract: We study RKKY interactions between local magnetic moments for both doped and undoped graphene. In the former case interactions for moments located on definite sublattices fall off as 1/R2, whereas for those placed at interstitial sites they decay as 1/R3. The interactions are primarily (anti)ferromagnetic for moments on (opposite) equivalent sublattices, suggesting that at low temperature dilute magnetic moments embedded in graphene can order into a state analogous to that of a dilute antiferromagnet. In the undoped case we find no net magnetic moment in the ground state, and demonstrate numerically this effect for ribbons, suggesting the possibility of an unusual spin-transfer device.

Spin correlations in the electron-doped high-transition-temperature superconductor Nd2-xCexCuO4+/-delta.

Abstract: High-transition-temperature (high-T(c)) superconductivity develops near antiferromagnetic phases, and it is possible that magnetic excitations contribute to the superconducting pairing mechanism. To assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional antiferromagnetic spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping and appears to overlap with the superconducting phase. The archetypal electron-doped compound Nd2-xCexCuO4+/-delta (NCCO) shows bulk superconductivity above x approximately 0.13 (refs 3, 4), while evidence for antiferromagnetic order has been found up to x approximately 0.17 (refs 2, 5, 6). Here we report inelastic magnetic neutron-scattering measurements that point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not coexist. The data reveal a magnetic quantum critical point where superconductivity first appears, consistent with an exotic quantum phase transition between the two phases. We also demonstrate that the pseudogap phenomenon in the electron-doped materials, which is associated with pronounced charge anomalies, arises from a build-up of spin correlations, in agreement with recent theoretical proposals.

Giant magnetocaloric effect in antiferromagnetic ErRu2Si2 compound

Abstract: Giant magnetocaloric effect has been observed in ErRu2Si2, which is associated with field-induced metamagnetic transition from antiferromagnetic to ferromagnetic state. The maximum values of magnetic entropy change (−ΔSMmax) and adiabatic temperature change (ΔTadmax) for a field change of 7T are evaluated to be 19.3J∕kgK and 15.9K, respectively, around 5.5K within the temperature range of 4–25K. The value of ΔTadmax is even larger than other potential magnetic refrigerant materials reported in the same temperature range and also comparable to room temperature giant magnetocaloric materials exhibiting first-order magnetic transition from paramagnetic to ferromagnetic state.

A “Star” Antiferromagnet: A Polymeric Iron(III) Acetate That Exhibits Both Spin Frustration and Long‐Range Magnetic Ordering

Abstract: The preparation of new geometrically spin-frustrated magnetic materials that approximate theoretical models is a challenge. Although theMermin–Wagner theorem indicates that long-range magnetic order can exist in two dimensions at zero Kelvin, order can be destroyed either by quantum fluctuations or geometric frustration even at this temperature. Theoretical studies indicate that the ground state of a spin-1/2 Heisenberg antiferromagnet is most likely to be semiclassically ordered. However, the interplay of geometric frustration and quantum fluctuations has been found to give rise to a paramagnetic ground state without semi-classical long-range order in two types of lattice. The first of these lattices is the famous Kagom+ lattice (T8) and the second is the so-called “star” lattice (T9; Scheme 1), which may serve as a new example of a quantum paramagnet. 5] The triangles are corner-sharing in the Kagom+ lattice whereas they are separated by a bridge in the star lattice, which means that their next-nearest-neighbor exchange interactions are different. 5] The magnetic J exchange pathways in the Kagom+ lattice are all equivalent, whereas the intra-triangular JT pathway in the star lattice is weaker than the inter-triangular JD pathway. In contrast to the rapid development of Kagom+-type antiferromagetic lattices 7] and related, geometrically spin-frustrated lattices, there appears to date to be no report of a compound with a genuine star lattice. Triangular clusters with superexchange pathways, such as the widely employedM3(m3-O) clusters, whereMmay be Fe , Fe, Co, Ni, Cu, V, or Cr, can be used to generate frustrated lattices, including the desired magnetically frustrated star lattice. This star lattice can be described in vertex notation as 3.12 (see Scheme S1 in the Supporting Information), a lattice that is a uniform, three-connected twodimensional net with large voids. Three-connected node subunits that prefer to bond in a planar fashion, such as the basic cationic iron(III) carboxylate cluster [Fe3(m3-O)(mO2CR)6L3] , where L may be water, methanol, or pyridine, must be used to avoid three-dimensional connections. These carboxylate clusters are good potential building blocks because they are easily prepared, prefer planar bonding, and the R groups and L ligands can easily be varied. The cationic [Fe3(m3-O)(m-O2CR)6L3] + moiety has previously served as a sixor three-connected node (see Scheme S2 in the Supporting Information) to form either threeor zero-dimensional porous frameworks depending upon the nature of the carboxylate, which may be either fully or partially substituted by dicarboxylates; the L ligands are usually retained as terminal ligands. Although no example is known to date, it should be possible to substitute the L ligands located in the triangular [Fe3(m3-O)(m-O2CR)6L3] + cation plane with other bridging bidentate ligands that are better at both mediating antiferromagnetic interactions and producing a two-dimensional star lattice. Herein, we report the use of bidentate acetate bridging ligands to link [Fe3(m3-O)(mOAc)6] + cations together to form [Fe3(m3-O)(m-OAc)6(H2O)3][Fe3(m3-O)(m-OAc)7.5]2·7H2O (1), a new compound with the desired star lattice. Single-crystal X-ray diffraction studies of 1 at 293 and 90 K revealed that isolated [Fe3(m3-O)(m-OAc)6(H2O)3] + cations (Figure 1) occupy the dodecagonal channels formed by the stacking of acetate-bridged [Fe3(m3-O)(m-OAc)7.5] 1/2 anionic layers; the dihedral angle between the triangular [Fe3(m3-O)(m-OAc)6(H2O)3] + cations and the [Fe3(m3-O)(mScheme 1. A comparison of the Kagom (T8, left) and star (T9, right) lattices with indication of the magnetic J exchange pathways.

Observation of exchange bias in the martensitic state of Ni50Mn36Sn14 Heusler alloy

Abstract: Exchange bias was observed in the Ni50Mn36Sn14 Heusler alloy after field cooling by means of hysteresis loop measurement. The hysteresis loops shift along the axis of an applied field and its magnitude significantly increased with decreasing temperature below 70K. This effect could be understood as a result of exchange anisotropy created at the interface between an antiferromagnet and a ferromagnet in the phase separated of martensitic state. Above 70K, however, the exchange bias field disappeared and the coercivity significantly reduced owing to the fact that the pinning between an antiferromagnet and a ferromagnet becomes weaker with increasing temperature.

Exchange bias in bulk Mn rich Ni–Mn–Sn Heusler alloys

Abstract: An experimental study on the exchange bias properties of bulk polycrystalline Ni50Mn50−xSnx Heusler alloys has been performed. Martensitic transformations have been observed in the alloys for some critical Sn concentrations. The alloys, while in their respective martensitic phases, are found to exhibit exchange bias effect. Shifts in hysteresis loops of up to 225Oe were observed in the 50kOe field cooled samples. The observed exchange bias behavior in Ni50Mn50−xSnx is attributed to the coexistence of antiferromagnetic and ferromagnetic exchange interactions in the system.

Occurrence of a rare 49·66 structural topology, chirality, and weak ferromagnetism in the [NH4][MII(HCOO)3] (M = Mn, Co, Ni) frameworks

Abstract: We report the synthesis, crystal structures, thermal, IR, UV-vis, and magnetic properties of a series of divalent transition metal formates, [NH4][M(HCOO)3], where M = divalent Mn, Co, or Ni. They crystallize in the hexagonal chiral space group P6(3)22. The structure consists of octahedral metal centers connected by the anti-anti formate ligands, and the ammonium cations sit in the channels. The chiral structure is a framework with the rarely observed 49.66 topology, and the chirality is derived from the handedness imposed by the formate ligands around the metals and the presence of units with only one handedness. The thermal properties are characterized by a decomposition at ca. 200 degrees C. The three compounds exhibit an antiferromagnetic ground state at 8.4, 9.8, and 29.5 K for Mn, Co, and Ni, respectively. The last two display a weak spontaneous magnetization due to a small canting of the moments below the critical temperature, and the Co compound shows a further transition at lower temperatures. The isothermal magnetizations at 2 K show spin-flop fields of 600 Oe (Mn), 14 kOe (Co), and above 50 kOe (Ni) and a small hysteresis with a remnant magnetization of 25 cm3 G mol(-1) (Co) and 50 cm3 G mol(-1) (Ni) and coercive field of 400 Oe (Co) and 830 Oe (Ni).

Realization of magnetic field-induced reversible martensitic transformation in NiCoMnGa alloys

Abstract: Effect of a magnetic field on martensitic transformation in the NiCoMnGa alloys was investigated. A field-induced reversible martensitic transformation from the martensitic phase of low magnetization to the parent phase of high magnetization has been realized. The substitution of Co for Ni atoms has turned the magnetic ordering of the parent phase from partially antiferromagnetic to ferromagnetic, resulting in a large magnetization change across the transformation, which dramatically enhances the magnetic field driving force. The transformation temperature can be downshifted by magnetic field at a rate up to 14K∕T in Ni37Co13Mn32Ga18. Other mechanism details were also discussed.

Molecular Thin Films: A New Type of Magnetic Switch

Abstract: The magnetic coupling of flexible metal phthalocyanine (MPc) thin films can be modified depending on the polymorphic form adopted by the crystals. A simple annealing procedure can switch the couplings from antiferromagnetic to ferromagnetic (MnPc) or paramagnetic (CuPc), opening up avenues for spintronic applications. Density functional and perturbation theories rationalize these trends and attribute the coupling mechanism to indirect exchange.

1,2,3-Triazolate-bridged tetradecametallic transition metal clusters [M14(L)6O6(OMe)18X6] (M = FeIII, CrIII and VIII/IV) and related compounds: Ground-state spins ranging from S = 0 to S = 25 and spin-enhanced magnetocaloric effect

Abstract: We report the synthesis, by solvothermal methods, of the tetradecametallic cluster complexes [M14(L)6O6(OMe)18Cl6] (M=FeIII, CrIII) and [V14(L)6O6(OMe)18Cl6-xOx] (L=anion of 1,2,3-triazole or derivative). Crystal structure data are reported for the {M14} complexes [Fe14(C2H2N3)6O6(OMe)18Cl6], [Cr14(bta)6O6(OMe)18Cl6] (btaH=benzotriazole), [V14O6(Me2bta)6(OMe)18Cl6-xOx] [Me2btaH=5,6-Me2-benzotriazole; eight metal sites are VIII, the remainder are disordered between {VIII-Cl}2+ and {VIV=O}2+] and for the distorted [FeIII14O9(OH)(OMe)8(bta)7(MeOH)5(H2O)Cl8] structure that results from non-solvothermal synthetic methods, highlighting the importance of temperature regime in cluster synthesis. Magnetic studies reveal the {Fe14} complexes to have ground state electronic spins of S

Multiferroic phases of Eu 1-x Y x MnO 3

Abstract: We report on structural, magnetic, dielectric, and thermodynamic properties of (Eu:Y)MnO3 for Y doping levels 0 <= x < 1. This system resembles the multiferroic perovskite manganites RMnO3 (with R= Gd, Dy, Tb) but without the interference of magnetic contributions of the 4f-ions. In addition, it offers the possibility to continuously tune the influence of the A-site ionic radii. For small concentrations x <= 0.1 we find a canted antiferromagnetic and paraelectric groundstate. For higher concentrations x <= 0.3 ferroelectric polarization coexists with the features of a long wavelength incommensurate spiral magnetic phase analogous to the observations in TbMnO3. In the intermediate concentration range around x = 0.2 a multiferroic scenario is realized combining weak ferroelectricity and weak ferromagnetism, presumably due to a canted spiral magnetic structure.

Origin of ferromagnetic response in diluted magnetic semiconductors and oxides

Abstract: This paper reviews the present understanding of the origin of ferromagnetic response that has been detected in a number of diluted magnetic semiconductors (DMSs) and diluted magnetic oxides (DMOs) as well as in some nominally magnetically undoped materials. It is argued that these systems can be grouped into four classes. To the first belong composite materials in which precipitations of a known ferromagnetic, ferrimagnetic or antiferromagnetic compound account for magnetic characteristics at high temperatures. The second class forms alloys showing chemical nanoscale phase separation into the regions with small and large concentrations of the magnetic constituent. Here, high-temperature magnetic properties are determined by the regions with high magnetic ion concentrations, whose crystal structure is imposed by the host. Novel methods enabling a control of this spinodal decomposition and possible functionalities of these systems are described. To the third class belong (Ga, Mn)As, heavily doped p-(Zn, Mn)Te, and related semiconductors. In these solid solutions the theory built on the p–d Zener model of hole-mediated ferromagnetism and on either the Kohn–Luttinger kp theory or the multi-orbital tight-binding approach describes qualitatively, and often quantitatively, thermodynamic, micromagnetic, optical, and transport properties. Moreover, the understanding of these materials has provided a basis for the development of novel methods, enabling magnetization manipulation and switching. Finally, in a number of carrier-doped DMSs and DMOs a competition between long-range ferromagnetic and short-range antiferromagnetic interactions and/or the proximity of the localization boundary lead to an electronic nanoscale phase separation. These materials exhibit characteristics similar to colossal magnetoresistance oxides.

Direct measurement of antiferromagnetic domain fluctuations

Abstract: 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.

Large magnetostriction and negative thermal expansion in the frustrated antiferromagnet ZnCr2Se4.

Abstract: A detailed investigation of ZnCr2Se4 is presented which is dominated by strong ferromagnetic exchange but orders antiferromagnetically at TN=21 K. Specific heat and thermal expansion exhibit sharp first-order anomalies at the antiferromagnetic transition. TN is shifted to lower temperatures by external magnetic fields and finally is fully suppressed by a field of 65 kOe. The relative length change DeltaL/L(T) is unusually large and exhibits negative thermal expansion alpha below 75 K down to TN indicating strong frustration of the lattice. Magnetostriction DeltaL/L(H) reveals large values comparable to giant magnetostrictive materials. These results point to a spin-driven origin of the structural instability at TN explained in terms of competing ferromagnetic and antiferromagnetic exchange interactions.

Lattice contraction and magnetic and electronic transport properties of Mn3Zn1−xGexN

Abstract: The lattice and electronic and magnetic transport properties of the antiperovskite structure Mn3Zn1−xGexN compounds were investigated. For Mn3ZnN, there is a magnetic transition from antiferromagnetic to paramagnetic near 185K. Correspondingly, the resistivity shows an abrupt drop, but any sudden change of lattice parameters is not found. However, it is interesting that the partial substitution of Ge for Zn induces a lattice contraction near the magnetic transition temperature, where a drop of the resistivity remain, and the transition temperature point increases and the temperature range is broadened with increasing doped Ge contents. The thermodynamics properties were also investigated.