Antiferromagnetic Mn Research Articles

Tailoring interlayer coupling and coercivity in Co/Mn/Co trilayers by controlling the interface roughness

Epitaxial Co/Mn/Co trilayers with a wedged Mn layer were grown on Cu(001) and studied by magneto-optical Kerr effect measurements. The bottom Co film as well as the Mn film exhibits a layer-by-layer growth mode, which allows to modify both interface roughnesses on the atomic scale by tuning the thicknesses of the films to achieve a certain filling of their topmost atomic layers. The onset of antiferromagnetic order in the Mn layer at room temperature was found at thicknesses of 4.1 (4.8) and 3.4 (4.0) atomic monolayers (ML) for a filled (half-filled) topmost atomic layer of the bottom Co film in Mn/Co bilayers and Co/Mn/Co trilayers, respectively. Magnetization loops with only one step were found for a trilayer with half-filled topmost atomic layer of the bottom Co film, while loops with two separate steps have been observed in trilayers with an integer number of atomic layers in the bottom Co film. The coercivity of the top Co film shows an oscillation with 1 ML period as a function of the Mn thickness above 10 ML, which is interpreted as the influence of the atomic-scale control of the interface roughness on the interface exchange coupling between the antiferromagnetic Mn and the top ferromagnetic (FM) Co layer. The strength of the magnetic interlayer coupling between the top and bottom Co layers through the Mn layer for an integer number of atomic layers in the bottom Co layer, deduced from minor-loop measurements, exhibits an oscillation with a period of 2 ML Mn thickness, indicative of direct exchange coupling through the antiferromagnetic Mn layer. In addition, a long-period interlayer coupling of the two FM layers with antiparallel coupling maxima at Mn thicknesses of 2.5, 8.2, and 13.7 ML is observed and attributed to indirect exchange coupling of the Rudermann-Kittel-Kasuya-Yosida type.

Chemical and phase composition of GaMnAs/GaAs/InGaAs spin light-emitting diodes

The technique of X-ray photoelectron spectroscopy combined with ion profiling is used for the quantitative chemical analysis of the structure of spin light-emitting diodes with a GaMnAs spin-injection layer and an InGaAs quantum well. The depth distribution of phases is determined, and the causes of redistribution of the phases are clarified. In the spin-injection layer, the fractions of antiferromagnetic Mn and ferromagnetic MnAs are at the same level. The procedure for phase separation and verification of the correctness of determination of the content of components is improved.

Growth and magnetic property of antiperovskite manganese nitride films doped with Cu by molecular beam epitaxy

Manganese nitrides thin films on MgO (100) substrates with and without Cu-doping have been fabricated by plasma assisted molecular beam epitaxy. Antiperovskite compounds Mn3.6Cu0.4N have been grown in the case of Cu-doping, and the pure Mn3N2 single crystal has been obtained without Cu-doping. The Mn3.6Cu0.4N exhibits ferrimagnetism, and the magnetization of Mn3.6Cu0.4N increases upon the temperature decreasing from 300 K to 5 K, similar to Mn4N. The exchange bias (EB) effects emerge in the Mn3.6Cu0.4N films. The EB behavior is originated from the interfaces between ferrimagnetic Mn3.6Cu0.4N and antiferromagnetic metal Mn, which is verified to be formed by the data of x-ray photoelectron spectroscopy. The present results not only provide a strategy for producing functional antiperovskite manganese nitrides, but also shed promising light on fabricating the exchange bias part of spintronic devices.

Mechanically Driven Alloying and Magnetic Properties in Immiscible Mn<sub>80</sub>Bi<sub>20</sub> Nanocrystalline Powders

The paper has described the formation of nanocrystalline Mn80Bi20 powders by mechanical alloying and studied the changes of structure and magnetic properties of the powders during the process of ball milling by using X-ray diffraction and saturation magnetization σs measurements. The solid solubility of bismuth in manganese increases with milling time and tends to a stable value after 80h milling. The σs of Mn80Bi20 increases abruptly with milling time at the early stage and begins to decrease after 15h. At the time of 15h, the σs reaches a maximum, which is about 7Am2/kg. The result shows an interesting information that the antiferromagnetic Mn and the diamagnetic Bi produce ferromagnetic Mn80Bi20 in process of mechanical alloying.

Magnetic ordering of implanted Mn in HOPG substrates

Currently, significant research efforts are geared towards using carbon-based materials for electronic applications. Here we report the observation of magnetic ordering of implanted Mn in HOPG substrates. Superconducting quantum interference device measurements show higher moments for Mn-doped samples and also the existence of double and inverted hysteresis in both undoped and doped samples. High-resolution transmission electron microscopy shows the presence of nanocrystals in implanted samples. Grazing incidence synchrotron x-ray diffraction studies show the presence of three stable manganese carbides, including antiferromagnetic Mn${}_{23}$C${}_{6}$. X-ray magnetic circular dichroism (XMCD) measurements show ferromagnetic ordering of Mn moments at temperatures below \ensuremath{\sim}100 K. However, a very weak XMCD signal indicates that only about 1$%$ of Mn atoms are ferromagnetically ordered. We conclude that the observed Mn ferromagnetic ordering is caused by uncompensated Mn moments on the surface of antiferromagnetic Mn${}_{23}$C${}_{6}$ nanocrystals that are aligned by the local magnetic field.

Magnetic interactions in the martensitic phase of Mn rich Ni-Mn-In shape memory alloys

The magnetic properties of Mn2Ni(1+x)In(1−x) (x = 0.5, 0.6, 0.7) and Mn(2−y)Ni(1.6+y)In0.4 (y = −0.08, −0.04, 0.04, 0.08) shape memory alloys have been studied. Magnetic interactions in the martensitic phase of these alloys are found to be quite similar to those in Ni2Mn(1+x)In(1−x) type alloys. Doping of Ni for In not only induces martensitic instability in Mn2NiIn type alloys but also affects magnetic properties due to a site occupancy disorder. Excess Ni preferentially occupies X sites forcing Mn to the Z sites of X2YZ Heusler composition resulting in a transition from ferromagnetic ground state to a state dominated by ferromagnetic Mn(Y)-Mn(Y) and antiferromagnetic Mn(Y)-Mn(Z) interactions. These changes in magnetic ground state manifest themselves in observation of exchange bias effect even in zero field cooled condition and virgin magnetization curve lying outside the hysteresis loop.

Rational Design of a Photomagnetic Chain: Bridging Single-Molecule Magnets with a Spin-Crossover Complex

The spin-crossover complex [Fe(LN5)(CN)2]·H2O (1, LN5 = 2,13-dimethyl-3,6,9-12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), reported previously by Nelson et al. in 1986, was reinvestigated, and its structure determined by single crystal X-ray diffraction for the first time. The reaction between [Mn(III)(saltmen)(H2O)](+) and this photomagnetic linker yielded the trinuclear molecular complex [{Mn(saltmen)}2FeHS(LN5)(CN)2](ClO4)2·0.5CH3OH (2) and the one-dimensional compound [{Mn(saltmen)}2FeLS(LN5)(CN)2](ClO4)2·0.5C4H10O·0.5H2O (3) depending on the addition order of the reagents (HS: High-Spin; LS: Low-Spin). Compound 3 exhibits a wave-shaped chain structure built from the assembly of the trinuclear [Mn(III)-NC-Fe(II)] motif found in 2. Static magnetic measurements revealed the existence of antiferromagnetic Mn(III)···Fe(II) (Fe(II) HS, S = 2) interactions in the trinuclear entity of 2 via the cyanido bridge leading to an ST = 2 ground state. In the case of 3, concomitant ferromagnetic and antiferromagnetic exchange interactions are found along the chain due to the presence of two crystallographically independent {Mn2(saltmen)2} units, which behave differently as shown by the magnetic susceptibility analysis, while the Fe(II) (LS, S = 0) cyanido-bridging moiety is isolating these dinuclear Mn(III) units. ac susceptibility experiments indicated slow relaxation of the magnetization arising from the ferromagnetically coupled [Mn2] units (τ0 = 1.1 × 10(-7) s and Δ(eff)/k(B) = 13.9 K). Optical reflectivity and photomagnetic properties of 1 and 3 have been investigated in detail. These studies reveal that the photomagnetic properties of 1 are kept after its coordination to the acceptor Mn(III)/saltmen complexes, allowing in 3 to switch "on" and "off" the magnetic interaction between the photoinduced Fe(II) HS unit (S = 2) and the Mn(III) ions. To the best of our knowledge, the compound 3 represents the first example of a coordination network of single-molecule magnets linked by spin-crossover units inducing thermally and photoreversible magnetic and optical properties.

Weak ferromagnetism and domain effects in multiferroic LiNbO3-type MnTiO3-II

Magnetic order consistent with multiferroism has been observed in the acentric LiNbO${}_{3}$-type, high pressure form II of MnTiO${}_{3}$ using neutron diffraction and magnetization measurements. Spin order below the 28 K magnetic transition has propagation vector (0 0 0), and spins lie in the $ab$ plane. Representation symmetry analysis shows that the antiferromagnetic Mn${}^{2+}$ spin component observed by neutron scattering, of magnitude 3.9(1) \ensuremath{\mu}${}_{B}$ at 2 K, coexists with a weak ferromagnetic component of magnitude 0.0014 \ensuremath{\mu}${}_{B}$. This magnetization is perpendicular to the electrical polarization resulting from cation displacements in this acentric structure, permitting coupled switches of the two ferroic orders. The spin order is stable to fields of at least 5 T; however, facile domain reorientation within the $ab$ plane enhances the antiferromagnetic susceptibility and a constant magnetization/field ($M$/$H$) is observed in field strengths greater than \ensuremath{\sim}1.5 T. Suppression of the {101} neutron intensity with field follows the same Brillouin-dependence as $M$/$H$ and enables the antiferromagnetic easy axes to be identified as parallel to the $ab$ plane hexagonal axes.

Anomalous surface segregation behaviour of some 3d elements in ferromagnetic iron

The segregation of Cr in Fe is known to be anomalous since the barrier for surface segregation of Cr is not determined by the topmost surface layer, as one would expect, but rather by the subsurface layer where the energy of segregation is much larger and endothermic. This has been attributed to a complex interaction involving the antiferromagnetism of Cr and the ferromagnetism of Fe. We report in this paper the results of our ab initio electronic structure calculations on the segregation behaviour of all the 3d elements on the (1 0 0) surface of ferromagnetic iron in the hope of better understanding this phenomenon. We find a similar behaviour for the segregation of the next antiferromagnetic 3d element Mn in Fe, where the subsurface layer is also found to block the segregation of Mn to the surface. On the other hand, ferromagnetic Co exhibits a normal segregation behaviour. The elements Sc, Cu and Ni do not form solid solutions with ferromagnetic iron. The early elements Ti and V are non-magnetic in their metallic states, but are strongly polarized by Fe, and develop magnetic moments which are aligned antiferromagnetically to those of Fe atoms. While the subsurface layer blocks the segregation of Ti to the surface, no blocking behaviour is found for the segregation of V. The segregation behaviour of all these elements is strongly correlated with the displacement of the solute atoms on the surface of Fe. The elements showing anomalous segregation behaviour are all displaced upwards on the surface, while those showing normal segregation are pulled inwards. These results indicate that the antiferromagnetism of the segregating element plays the key role in the anomalous segregation behaviour in Fe.

Physical properties of antiferromagnetic Mn doped ZnO samples: Role of impurity phase

Structural, morphological, optical, and magnetic properties of nanocrystalline Zn1−xMnxO samples (x=0.01, 0.02, 0.04, 0.06, 0.08 and 0.10) prepared by the sol–gel route are studied by X-ray diffraction (XRD), Scanning electron microscopy (SEM), UV–visible absorption spectroscopy, Superconducting quantum interference device (SQUID) magnetometry and positron annihilation lifetime spectroscopy (PALS). XRD confirms formation of wurzite structure in all the Mn-substituted samples. A systematic increase in lattice constants and decrease in grain size have been observed with increase in manganese doping concentration up to 6at% in the ZnO structure. An impurity phase (ZnMnO3) has been detected when percentage of Mn concentration is 6at% or higher. The optical band gap of the Mn-substituted ZnO samples decrease with increase in doping concentration of manganese whereas the width of the localized states increases. The antiferromagnetic exchange interaction is strong in the samples for 2 and 4at% of Mn doping but it reduces when the doping level increases from 6at% and further. Positron life time components τ1 and τ2 are found to decrease when concentration of the dopant exceeds 6at%. The changes in magnetic properties as well as positron annihilation parameters at higher manganese concentration have been assigned as due to the formation of impurity phase.

Parity effect in the ground state localization of antiferromagnetic chains coupled to a ferromagnet.

We investigate the ground states of antiferromagnetic Mn nanochains on Ni(110) by spin-polarized scanning tunneling microscopy in combination with theory. While the ferrimagnetic linear trimer experimentally shows the predicted collinear classical ground state, no magnetic contrast was observed for dimers and tetramers where noncollinear structures were expected based on abinitio theory. This striking observation can be explained by zero-point energy motion for even-numbered chains derived within a classical equation of motion leading to nonclassical ground states. Thus, depending on the parity of the chain length, the system shows a classical or a quantum behavior.

Structural and magnetic properties of (Fe/Mn) exchange-biased multilayers

Exchange-biasing of ferromagnetic (F) Fe layers by adjacent antiferromagnetic (AF) Mn layers has been investigated in (Fe/Mn)10 multilayered films. This study has been focused on the relationship between the evolution of the exchange-bias field and the evolution of the film microstructure as a function of the deposition temperature. The increase of the deposition temperature results in the formation of an Fe–Mn alloy at the interfaces and columnar features whose size increases with the deposition temperature. In parallel, the exchange-bias field decreases significantly, due to interface roughness.

Anisotropic thermal expansion and cooperative Invar and anti-Invar effects in mn alloys.

We have investigated thermal expansion of a tetragonal Mn(88)Ni(12) alloy by x-ray diffraction, Mn and Ni K-edge extended x-ray-absorption fine-structure spectroscopy, and the computational simulations based on the path-integral effective-classical-potential theory. It is found from the x-ray diffraction that the tetragonal lattice constant c exhibits almost no thermal expansion like an Invar alloy, while the lattice constant a shows even larger thermal expansion than usually expected from anharmonicity, implying significant anisotropy in thermal expansion. The extended x-ray-absorption fine-structure reveals that the Mn local environment is actually tetragonally distorted, while the Ni one retains its inherent cubiclike symmetry. Combined with the computational simulations, it is concluded that large thermal expansion along the a axis originates from the anti-Invar effect, while negligibly small thermal expansion along the c axis originates from the cooperative Invar effect. Namely, the tetragonally distorted more stable antiferromagnetic Mn state gives a significantly smaller (slightly longer) atomic radius along the a (c) axis than the radius of the spherical paramagnetic state.

Carbon-Induced Ferromagnetism in the Antiferromagnetic Metallic Host Material Mn3ZnN

Carbon-for-nitrogen substitution (51 at% at most) was achieved in the antiferromagnetic metallic host material Mn(3)ZnN. The various carbon-doped compounds were studied using synchrotron X-ray diffraction, and their electrical resistivities, specific heats, and degrees of magnetization were measured for temperatures of 2-400 K. The sharp antiferromagnetic-to-paramagnetic transition of the host material at 185 K broadened markedly as the carbon content was increased, and a significant ferromagnetic character was found to coexist with the antiferromagnetism when the carbon concentration exceeded 27 at%. This critical magnetic behavior is likely in part due to the increase in the density of states at the Fermi level and the increase in the distance between neighboring Mn atoms. The exact mechanism responsible for the induction of the complicated magnetic state could not be determined. However, the results demonstrate clearly that the chemical tuning of the X site in antiperovskite Mn(3)AX materials is as useful as that of the A and Mn sites and can be used to develop the properties of these materials for practical applications.

Spin lattice relaxation of proton NMR in Mn formate di-urea single crystal at low temperatures

We have studied the spin-lattice relaxation of proton-NMR on the antiferromagnetic Mn formate di-urea below the Néel temperature. Two kinds of relaxation time, T′1 and T1″ are observed. When temperature decreases, T′1 increases slightly and T″1 shows a minimum. This suggests that two kinds of temperature dependence of Mn2+ spontaneous magnetizations coexist.

Exchange Biasing Single Molecule Magnets: Coupling of TbPc2 to Antiferromagnetic Layers

We investigate the possibility to induce exchange bias between single molecule magnets (SMM) and metallic or oxide antiferromagnetic substrates. Element-resolved X-ray magnetic circular dichroism measurements reveal, respectively, the presence and absence of unidirectional exchange anisotropy for TbPc(2) SMM deposited on antiferromagnetic Mn and CoO layers. TbPc(2) deposited on Mn thin films present magnetic hysteresis and a negative horizontal shift of the Tb magnetization loop after field cooling, consistent with the observation of pinned spins in the Mn layer coupled parallel to the Tb magnetic moment. Conversely, molecules deposited on CoO substrates present paramagnetic magnetization loops with no indication of exchange bias. These experiments demonstrate the ability of SMM to polarize the pinned uncompensated spins of an antiferromagnet during field-cooling and realize metal-organic exchange-biased heterostructures using antiferromagnetic pinning layers.

Single Molecule Magnetoresistance with Combined Antiferromagnetic and Ferromagnetic Electrodes

The magnetoresistance of a hydrogen-phthalocyanine molecule placed on an antiferromagnetic Mn(001) surface and contacted by a ferromagnetic Fe electrode is investigated using density functional theory based transport calculations and low-temperature scanning tunneling microscopy. A large and negative magnetoresistance ratio of ~50% is observed in combination with a high conductance. The effect originates from a lowest unoccupied molecular orbital (LUMO) doublet placed almost in resonance with the Fermi energy. As a consequence, irrespective of the mutual alignment of magnetizations, electron transport is always dominated by resonant transmission of Mn-majority charge carries going through LUMO levels.

Magnetic instability in Mn2NiSb

Mn 2 NiSb can exist in both ferromagnetic and antiferromagnetic states because there is only a small total energy difference between the two states. In its electronic structure, two different types of energy gaps exist and are close to the Fermi level. The relative position of the Fermi level to the gap determines the ferromagnetic or antiferromagnetic state. That is the origin of the magnetic instability in Mn 2 NiSb. A 70% decrease of the total moment is predicted between the ferromagnetic and antiferromagnetic Mn 2 NiSb.

First-principles DFT + U studies of the atomic, electronic, and magnetic structure of α-MnO2 (cryptomelane)

Density functional theory DFT+U calculations are used to investigate α-MnO2, a structure containing a framework of corner and edge sharing MnO6 octahedra with tunnels in between. Placing K+ ions into the tunnels stabilizes α-MnO2 with respect to the rutile-structure β-MnO2 phase, in agreement with experiment. The computed magnetic structure has antiferromagnetic (ferromagnetic) Mn–Mn interactions between corner-sharing (edge-sharing) octahedra. Pure α-MnO2 is found to be a semiconductor with an indirect band gap of 1.3eV. Water and related hydrides (OH−; H3O+) can also be accommodated in the tunnels; the equilibrium K–O distance increases with increasing oxygen hydride charge.

Cyanido-Bridged Fe(III)–Mn(III) Heterobimetallic Materials Built From Mn(III) Schiff Base Complexes and Di- or Tri-Cyanido Fe(III) Precursors

The reaction of [Fe(III)L(CN)(3)](-) (L being bpca = bis(2-pyridylcarbonyl)amidate, pcq = 8-(pyridine-2-carboxamido)quinoline) or [Fe(III)(bpb)(CN)(2)](-) (bpb = 1,2-bis(pyridine-2-carboxamido)benzenate) ferric complexes with Mn(III) salen type complexes afforded seven new bimetallic cyanido-bridged Mn(III)-Fe(III) systems: [Fe(pcq)(CN)(3)Mn(saltmen)(CH(3)OH)]·CH(3)OH (1), [Fe(bpca)(CN)(3)Mn(3-MeO-salen)(OH(2))]·CH(3)OH·H(2)O (2), [Fe(bpca)(CN)(3)Mn(salpen)] (3), [Fe(bpca)(CN)(3)Mn(saltmen)] (4), [Fe(bpca)(CN)(3)Mn(5-Me-saltmen)]·2CHCl(3) (5), [Fe(pcq)(CN)(3)Mn(5-Me-saltmen)]·2CH(3)OH·0.75H(2)O (6), and [Fe(bpb)(CN)(2)Mn(saltmen)]·2CH(3)OH (7) (with saltmen(2-) = N,N'-(1,1,2,2-tetramethylethylene)bis(salicylideneiminato) dianion, salpen(2-) = N,N'-propylenebis(salicylideneiminato) dianion, salen(2-) = N,N'-ethylenebis(salicylideneiminato) dianion). Single crystal X-ray diffraction studies were carried out for all these compounds indicating that compounds 1 and 2 are discrete dinuclear [Fe(III)-CN-Mn(III)] complexes while systems 3-7 are heterometallic chains with {-NC-Fe(III)-CN-Mn(III)} repeating units. These chains are connected through π-π and short contact interactions to form extended supramolecular networks. Investigation of the magnetic properties revealed the occurrence of antiferromagnetic Mn(III)···Fe(III) interactions in 1-4 while ferromagnetic Mn(III)···Fe(III) interactions were detected in 5-7. The nature of these Mn(III)···Fe(III) magnetic interactions mediated by a CN bridge appeared to be dependent on the Schiff base substituent. The packing is also strongly affected by the nature of the substituent and the presence of solvent molecules, resulting in additional antiferromagnetic interdinuclear/interchain interactions. Thus the crystal packing and the supramolecular interactions induce different magnetic properties for these systems. The dinuclear complexes 1 and 2, which possess a paramagnetic S(T) = 3/2 ground state, interact antiferromagnetically in their crystal packing. At high temperature, the complexes 3-7 exhibit a one-dimensional magnetic behavior, but at low temperature their magnetic properties are modulated by the supramolecular arrangement: a three-dimensional antiferromagnetic order with a metamagnetic behavior is observed for 3, 4, and 7, and Single-Chain Magnet properties are detected for 5 and 6.