We have probed directly the temperature and magnetic field dependence of pinned uncompensated magnetization at the interface of antiferromagnetic FeF2 with Cu, using FeF2-Cu-Co spin valves. Electrons polarized by the Co layer are scattered by the pinned uncompensated moments at the FeF2-Cu interface giving rise to giant magnetoresistance. We determined the direction and magnitude of the pinned uncompensated magnetization at different magnetic fields and temperatures using the angular dependencies of resistance. The strong FeF2 anisotropy pins the uncompensated magnetization along the easy axis independent of the cooling field orientation. Most interestingly, magnetic fields as high as 90 kOe cannot break the pinning at the FeF2-Cu interface. This proves that the pinned interfacial magnetization is strongly coupled to the antiferromagnetic order inside the bulk FeF2 layer. Studies as a function of FeF2 crystalline orientation show that uncompensated spins are only detected in a spin valve with (110) crystal orientation, but not in valves containing FeF2 (100) and FeF2 (001). This observation is in agreement with symmetry-related considerations which predict the equilibrium boundary magnetization for the FeF2 (110) layer.

The martensitic transformation arrest phenomenon in Ni45Co5Mn36.6In13.4 meta-magnetic shape memory alloy (MMSMA) single crystals was investigated as a function of secondary annealing heat treatments, using thermo-magnetometry and transmission electron microscopy (TEM). Dark-field images of the austenite phase at room temperature revealed the long range L21 and B2 ordered microstructural landscape with different morphologies in the annealed single crystals. Their measured thermomagnetic responses demonstrated full transformation arrest after certain heat treatments and unique microstructural morphologies. Martensitic transformation hysteresis, range, and enthalpy were measured in the annealed partially- or non-arrested single crystals. With the data, herein, we found that the samples with long range L21 ordering exhibit martensitic transformation at higher temperatures than some of the crystals of the same composition showing predominantly B2 order. This finding opposes the previous reports on the effect of annealing on the martensitic transformation characteristics of MMSMAs. We provide evidence that long range order promoted with high temperature annealing is not the only microstructural feature capable of influencing the martensitic transition in NiCoMnIn MMSMAs. Data suggests that quenched in vacancies, in addition to long range order, also influence the transformation characteristics.

We observed and studied pinned uncompensated magnetization in an antiferromagnet using magnetoresistance measurements. For this, we developed antiferromagnet-ferromagnet spin valves (AFSVs) that consist of an antiferromagnetic layer and a ferromagnetic one, separated by a nonmagnetic conducting spacer. In an AFSV, the uncompensated magnetization in the antiferromagnet affects scattering of spin-polarized electrons giving rise to giant magnetoresitance (GMR). By measuring angular dependence of AFSVs' resistance, we detected pinned uncompensated magnetization responsible for the exchange bias effect in an antiferromagnet- only exchange bias system Cu/FeMn/Cu. The fact that GMR measured in this system persists up to 110 kOe indicates that the scattering occurs on strongly pinned uncompensated magnetic moments in FeMn. This strong pinning can be explained if this pinned uncompensated magnetization is a thermodynamically stable state and coupled to the antiferromagnetic order parameter. Using the AFSV technique, we confirmed that the two interfaces between FeMn and Cu are magnetically different: The uncompensated magnetization is pinned only at the interface with the bottom Cu layer.

The (110) surface of iron fluoride (FeF2) is especially relevant to the understanding of the exchange bias phenomenon, which has important applications in the sensor industry, and has been extensively explored, both theoretically and experimentally. Here we investigate this FeF2 surface by means of ab initio techniques. We compute the (110) surface reconstruction, energetics, magnetic moments, band structure, charge density and electron localization function, for the two possible terminations (Fe and F). The surface reconstruction modifies the atomic and electronic structure of the free surface, yielding magnetism of a magnitude of 0.1μB per surface unit cell. Moreover, the charge density also changes, which alters the bonding in the vicinity of the surface. All these changes are expected to be relevant for exchange bias, that is once a ferromagnetic layer is deposited on the FeF2 surface.

To study the effect of non-magnetic layer (Cu) on magnetic properties of antiferromagnetic FeMn, multilayers of Ta(5 nm)/[FeMn(t)/Cu(5 nm)]10/Ta(5 nm), where t is varied in the range of 5–15 nm, are fabricated by a combination of RF and DC magnetron sputter deposition. Magnetization curves for these samples exhibit magnetic hysteresis, and when the samples are cooled in an applied magnetic field, the hysteresis loops are shifted. This shift is attributed to an “intrinsic” exchange bias effect (i.e., it is observed without a separate ferromagnetic layer). Presented temperature and thickness dependences of the coercive field, magnetic moment, and exchange bias field provide insights into the origin and mechanism of the observed intrinsic exchange bias.

Ensemble-averaged exchange bias in arrays of Fe/FeF2 nanodots has been deconvoluted into local, microscopic, bias separately experienced by nanodots going through different reversal modes. The relative fraction of dots in each mode can be modified by exchange bias. Single domain dots exhibit a simple loop shift, while vortex state dots have asymmetric shifts in the vortex nucleation and annihilation fields, manifesting local incomplete domain walls in these nanodots as magnetic vortices with tilted cores.

S. López-Moreno,1,* A. H. Romero,2 J. Mejı́a-López,3 A. Muñoz,4 and Igor V. Roshchin5,6 1Facultad de Ciencias, Universidad Nacional Autónoma de México, Apdo. Post. 70-646, México D. F. 04510, México 2CINVESTAV-Queretaro Libramiento Norponiente No 2000 Real de Juriquilla 76230 Queretaro, Qro, México 3Facultad de Fı́sica, Pontificia Universidad Católica, Av. Vicuña Mackenna 4860, Santiago, Chile 4Departamento de Fı́sica Fundamental II, Instituto de Materiales y Nanotecnologı́a Universidad de La Laguna,

We report systematic ab initio calculations of the electronic band structure, phonon dispersion relation, and the structural characterization of FeF2 in the rutile (P4(2)/mnm) structure as well as in several high-pressure phases by means of the generalized gradient approximation (GGA) + U approximation. Using the phonon dispersion relations, we calculated the Gibbs free energy and evaluated the phase transitions at 300 K, at which most experimental measurements are performed. Calculated Raman and infrared vibrational modes, lattice parameters, and electronic structure for all considered crystalline structures are compared with available experimental data. Our calculations show that at 5.33 GPa, the FeF2 undergoes a second-order proper ferroelastic phase transition, rutile -> CaCl2-type structure. This result is supported by the softening of the elastic shear module C-s in the rutile phase, the softening (hardening) of the B-1g (A(g)) Raman active mode in the rutile (CaCl2-type) structure near the transition pressure, and the decrease of the square of the spontaneous strain e(ss) from the CaCl2-type structure. This demonstrates that the rutile -> CaCl2-type phase transition is driven by the coupling between the Raman active B-1g mode and shear modulus C-s. At 8.22 GPa, the CaCl2-type structure undergoes a first-order phase transition to the Pbca phase, a distorted fcc Pa (3) over bar phase with a volume reduction of Delta V approximate to 7%, as reported in experiments. Upon further increase of the pressure, the Pbca phase transforms to a Fmmm phase othorhombic center-type structure at similar to 20.38 GPa, with Delta V approximate to 2.5%. Finally, at 25.05 GPa, there is a phase transition to the orthorhombic cotunnite structure (Pnma space group), with Delta V approximate to 5.8%, which is stable up to 45 GPa, the largest considered pressure. The coordination number for the Fe ion in each phase is 6, 6, 6, 8, and 9 for rutile, CaCl2-type, Pbca, Fmmm, and cotunnite structures, respectively. The evolution of the band gap, phonon frequencies, and magnetic moment of Fe ion as a function of the applied pressure is reported for all studied phases. The exchange constants J(1), J(2), and J(3), calculated for rutile and the lowest Gibbs free-energy high-pressure phases, are reported.