Chemical Substitutions in Tl 2 Mn 2 O 7 Pyrochlore,Synthesized Under High Pressure: Effects on the Colossal Magnetoresistance

Tl 2 Mn 2 O 7 pyrochlore has recently been described as a half-metallic ferromagnet showing colossal magnetoresistance (CMR) properties. New series of Tl 2 Mn 2 O 7 derivatives have been prepared under high pressure conditions. We have replaced Tl 3+ cations by Bi 3+ and Cd 2+ , leading to different modifications of the physical properties, with dramatic improvements of CMR. The Mn 4+ cations have also been partially replaced by Sb 5+ and Te 6+ . In particular, moderate Sb substitution leads to significant increments of T C . In this work we discuss on the effects of the different chemical substitutions on the structural, magnetic and transport properties.     

Magnetic excitations in an itinerant ferromagnet near quantum criticality

We determine the initial temperature dependence of the exchange splitting Delta(T) in the weak itinerant ferromagnet ZrZn2 (T{C}=28 K) using the de Haas-van Alphen effect. There is a large decrease in Delta with temperature in the range 0.5< or =T< or =4 K. A comparison of Delta(T) with the magnetization M(T) shows that the dominant process responsible for the reduction of M is not the thermal excitation of spin waves, but a repopulation of the spin- upward arrow and spin- downward arrow Fermi surfaces. This contrasts with the behavior in Fe where there is no observable change in Delta and the thermal excitation of spin waves is the only observable spin-flipping process at low temperatures.     

Anomalous Hall effect in highly Mn-Doped silicon films

The transport and magnetic properties of Mn _x Si_{1 ? x} films with a high (x ≈ 0.35) content of Mn produced by laser deposition at growth temperatures of 300–350°C have been studied in a temperature range of 5–300 K in magnetic fields of up to 2.5 T. The films exhibit a hole-type metallic conductivity and a relatively weak change of magnetization in a temperature range of 50–200 K. An anomalous Hall effect with an essentially hysteretic behavior from 50 K up to ≈230 K has been discovered. The properties of the films are explained by the two-phase model, in which ferromagnetic clusters containing interstitial Mn ions with a localized magnetic moment are embedded in the matrix of a weak band MnSi_{2 ? x} (x ≈ 0.3) type ferromagnet with delocalized spin density.     

Randomness-induced XY ordering in a graphene quantum hall ferromagnet

Valley-polarized quantum Hall states in graphene are described by a Heisenberg O(3) ferromagnet model, with the ordering type controlled by the strength and the sign of the valley anisotropy. A mechanism resulting from electron coupling to the strain-induced gauge field, giving a leading contribution to the anisotropy, is described in terms of an effective random magnetic field aligned with the ferromagnet z axis. We argue that such a random field stabilizes the XY ferromagnet state, which is a coherent equal-weight mixture of the K and K' valley states. The implications such as the Berezinskii-Kosterlitz-Thouless ordering transition and topological defects with half-integer charge are discussed.     

Study of CeNi4Mn by neutron diffraction

We report neutron diffraction measurements on CeNi₄Mn, which has recently been identified as a soft ferromagnet with a sizeable spin-transport polarization. Our data show conclusively that the Mn atoms occupy a unique site (4c) in the unit cell, which has the symmetry of the cubic MgCu₄Sn-type structure. We infer a moment of 4.6 μ_B on Mn at 17 K, which is oriented ferromagnetically along the {101} plane. The amplitude of the Mn vibrational motion is found to be larger than that of Ce and Ni atoms at all temperatures, thereby lending support to theoretical prediction of rattling phonon modes in this compound.     

Spin-wave relaxation in a quantum hall ferromagnet

We study spin wave relaxation in quantum Hall ferromagnet regimes. Spin-orbit coupling is considered as a factor determining spin nonconservation, and external random potential as a cause of energy dissipation making spin-flip processes irreversible. We compare this relaxation mechanism with other relaxation channels existing in a quantum Hall ferromagnet. The article is published in the original.     

Stabilization of metastable expanded face-centered-tetragonal manganese

The structural and magnetic properties of Mn prepared on single crystalline face-centered-tetragonal (fct) Co(001) were investigated. Mn grows coherently up to at least 50 monolayers (ML) and adopts a metastable expanded fct(001) phase [c/a = 1.055(5)]. This new fct-Mn phase was recently predicted theoretically by Hafner and Spisák. Studies of magnetic Mn/Co interface exchange interactions prove the room temperature antiferromagnetic state for thicknesses above 2.5 ML. The magnetic anisotropy of the thin Mn is high enough to induce a significant exchange anisotropy for Mn thicknesses as low as 6 ML. The potential of fct-Mn to become a novel model system for systematic studies on the exchange interactions at antiferromagnet/ferromagnet interfaces is discussed.     

Anomalous Hall effect in ferromagnetic semiconductors

We present a theory of the anomalous Hall effect in ferromagnetic (III, Mn)V semiconductors. Our theory relates the anomalous Hall conductance of a homogeneous ferromagnet to the Berry phase acquired by a quasiparticle wave function upon traversing closed paths on the spin-split Fermi surface. The quantitative agreement between our theory and experimental data in both (In, Mn)As and (Ga, Mn)As systems suggests that this disorder independent contribution to the anomalous Hall conductivity dominates in diluted magnetic semiconductors. The success of this model for (III, Mn)V materials is unprecedented in the longstanding effort to understand origins of the anomalous Hall effect in itinerant ferromagnets.     

Criticality in inhomogeneous magnetic systems: application to quantum ferromagnets

We consider a phi4 theory with a position-dependent distance from the critical point. One realization of this model is a classical ferromagnet subject to nonuniform mechanical stress. We find a sharp phase transition where the envelope of the local magnetization vanishes uniformly. The first-order transition in a quantum ferromagnet also remains sharp. The universal mechanism leading to a tricritical point in an itinerant quantum ferromagnet is suppressed, and in principle, one can recover a quantum critical point with mean-field exponents. Observable consequences of these results are discussed.     

NMRON studies of the quasi-2-dimensional ferromagnet 54Mn-Mn(COOCH3)2·4H2O

Pulsed and continuous wave NMRON, and NMR thermally detected by Nuclear Orientation have been used to investigate the magnetic properties of the quasi-2-dimensional ferromagnet ⁵⁴Mn-Mn(COOCH₃)·4H₂O. The NMR frequencies of both the ⁵⁴Mn spins and the abundant ⁵⁵Mn spins have been determined for the two crystalline lattice sites Mn1 and Mn2. The line widths of the ⁵⁴Mn resonances are homogeneously broadened to about 300 kHz, and the ⁵⁵Mn resonances are significantly pulled down in frequency. The spin-lattice relaxation times in low applied magnetic fields are short with T₁ for the Mn2 site being much less than the value for the Mn1 site. Level crossing is observed between the Mn1 and Mn2 resonances in an applied field of 2.6 T. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of spin-polarized current on superconductivity and the realization of large magnetoresistance

The superconducting state can be influenced by injecting spin-polarized current in a controlled manner by properly tailoring the interfacial transmittivity between a ferromagnet (F) and a superconductor (S), resulting in a large magnetoresistance of over 1100% for a F/I/S/I/F multilayer system (I insulator). Because of the competition between ferromagnetism and superconductivity, the superconducting transition temperature (T(C)) in the spin-parallel configuration is shifted below that in the spin antiparallel configuration. The T(C) shift is attributed to ferromagnet-induced nonequilibrium spin carriers in the superconductors.     

Magnetic Resonance Study of the Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Doped with Manganese

Crystals of 3D topological insulators, bismuth telluride Bi₂Te₃, doped with manganese were studied using electron spin resonance (ESR) spectroscopy together with the SQUID magnetometry, transport measurements, and X-ray characterization. The obtained ESR data, such as the temperature and the angular dependence of the resonance field, reveal the specific critical behavior and confirm the ferromagnetic ordering of Mn spins even at modest doping. In addition to the studies of the critical behavior of diluted ferromagnet Bi_2?x Mn _x Te_3, we also discuss the effects of the limited solubility of Mn ions giving rise to microscopic inclusions of the spurious magnetic phases which were revealed using ESR technique.     

Observation of quantum coherence in mesoscopic molecular magnets

By applying a transverse magnetic field B( perpendicular) of sufficient strength to the uniaxial molecular magnets Fe8 and Mn12, the tunneling splitting Delta(t) of their S = +/-10 magnetic ground states can be made large compared to perturbations such as hyperfine and dipolar interactions. We present evidence for such a Delta(t) from magnetic specific heat data below 1 K that is consistent with coherent quantum mechanical tunneling in a "mesoscopic" system under such conditions.     

Change in the magnetic ground state of LaFe11.4Al1.6 compound by the substitution of Mn for Fe

The structure and magnetic properties of La(Fe_{1-x}Mn_x)_{11.4}Al_{1.6} (0≤x≤0.25)compounds have been studied. The NaZn_{13}-type structure is preserved and the lattice parameter increases linearly with increasing the Mn concentration. The magnetic ground state changes from the antiferromagnetic to the spin-glass or the cluster-glass state by the substitution of Mn for Fe. Furthermore, a field-induced transition from cluster glass to ferromagnet is found for the samples with x=0.05 and 0.10.     

Quantum Hall ferromagnetism in a two-valley strained Si quantum well

Tilted field magnetotransport study was performed in a two-valley strained Si quantum well and hysteretic diagonal resistance spikes were observed near the coincidence angles. The spike around filling factor ν=3 develops into a giant feature when it moves to the high-field edge of the quantum Hall (QH) state and quenches for higher tilt angles. When the spike is most prominent, its peak resistance is temperature independent from T20 mK up to 0.3 K, which is different from the critical behavior previously reported near the Curie temperature of the QH ferromagnet in AlAs quantum wells. Our data suggest a strong interplay between spins and valleys near the coincidence.     

Negatively charged excitons in semimagnetic CdSe/ZnSe/ZnMnSe quantum dots

Low-temperature (T = 1.6 K) photoluminescence (PL) of individual CdSe/ZnSe/ZnMnSe quantum dots (QDs) with different magnitudes of the sp-d exchange interaction between the magnetic impurity ions and charge carriers has been studied in a magnetic field up to 12 T applied in the Faraday and Voigt geometry. The magnitude of the interaction was controlled by changing the fraction (η_{e, h}) of the squared wave function of charge carriers in the semimagnetic barrier by means of variation of the nonmagnetic (ZnSe) layer thickness. It is established that the sp-d exchange interaction leads to a change in the sign of the effective hole g factor even for η_{e, h} ～ 5%, while further increase in the interaction magnitude is accompanied by a rapid growth in the magnitude of spin splitting for both electrons and holes. The quantum yield of PL exhibits a significant decrease due to nonradiative Auger recombination with the excitation of Mn ions only for η_{e, h} ～ 12%, while the rate of the holes spin relaxation starts growing only for still higher η_{e, h} values. In a strong magnetic field perpendicular to the sample plane, the alignment of Mn spins leads to suppression of the Auger recombination only in the excited spin state. For a small rate of the hole spin relaxation, this leads to a rather unusual result: the emission from an excited trion state predominates in strong magnetic fields.     

Hydrogen induced changes in the crystal structure and magnetic properties of UCoGe

Hydrogen pressure of 0.5-140 bar has been applied to synthesize hydrides of UCoGe. Besides an α hydride crystallizing in the structure type of the parent compound, which loses the weak ferromagnetism found in pure UCoGe, two distinctly different β hydrides were identified. The almost pure β hydride (UCoGeH(1.7)) is a ferromagnet below T(C) = 50 K. The highest H(2) pressures (> 130 bar) produce admixture of another hydride called β' hydride, with less H/f.u. and T(C) = 8 K, obtained presumably as a decay product of a full hydride UCoGeH(2.0) unstable at ambient conditions. The value of the Sommerfeld coefficient of electronic specific heat γ increases over 100 mJ mol(-1) K(-2) for the magnetic hydrides.     

Criteria for the absence of quantum fluctuations after spontaneous symmetry breaking

The lowest-energy state of a macroscopic system in which symmetry is spontaneously broken, is a very stable wavepacket centered around a spontaneously chosen, classical direction in symmetry space. However, for a Heisenberg ferromagnet the quantum groundstate is exactly the classical groundstate, there are no quantum fluctuations. This coincides with seven exceptional properties of the ferromagnet, including spontaneous time-reversal symmetry breaking, a reduced number of Nambu–Goldstone modes and the absence of a thin spectrum (Anderson tower of states). Recent discoveries of other non-relativistic systems with fewer Nambu–Goldstone modes suggest these specialties apply there as well. I establish precise criteria for the absence of quantum fluctuations and all the other features. In particular, it is not sufficient that the order parameter operator commutes with the Hamiltonian. It leads to a measurably larger coherence time of superpositions in small but macroscopic systems.     

Quantum transport study of canted antiferromagnetic phase in the bilayer quantum Hall state

We examine the ν=2 bilayer quantum Hall (QH) state in clean two-dimensional electron systems (2DESs) to study effects due to not only the layer degree of freedom called pseudospin but also the real spin degree of freedom. The novel canted antiferromagnetic phase (CAF phase) has been predicted to emerge from subtle many-body electron interactions between the singlet (S) and ferromagnet (F) phases. Though several experiments indicate an onset of the CAF phase, a systematic transport study is not yet to be demonstrated. We have carried out magnetotransport measurements of the ν=2 bilayer QH state using a sample with tunneling energy . Activation energy was precisely measured as a function of the total density of the 2DES and the density difference between the two layers. Results support an appearance of the CAF phase between the S and F phases.