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In recent years multiferroic materials have great application potential in the ferroelectric random access memory and emerging spintronics development setting off the boom of the multiferroic materials research. It was assume that the ferroelectric and magnetic properties cannot coexist at the same temperature range or their coupling is weak making the searching for multiferroics in dream. Multiferroic materials have been discovered in recent years. Multiferroics may even be induced due to the strong coupling between artificial layers by growing multilayer technique. [1] [2] Among of which the BiFeO3 compound contains of antiferromagnetic and ferroelectric properties at room temperature. The BiFeO3 thin films manifests a larger spontaneous porlarization than that of the bulk and is highly capable of industrial application. In this study, Pb doped BFO (BPFO) this films were grown on top the conductive SrRuO3 layers. By varying BPFO¡¦s thickness, the strain effect the relative physical properties were studied. It is found that the lattice constants of SRO is totally different to that of STO substrate indicating strain relaxation. Similar phenomena are also discovered for BPFO thin film which is believed due to the tilting effect on the SRO/STO interface.

<p> Magnetic susceptibility data indicate that Tb2Mo2O7 undergoes a spin glass transition at 25 K. The crystal structure is consistent with the fully ordered pyrochlore model. Short range order, involving ferromagnetic and antiferromagnetic correlations, is observed down to 8 K by neutron scattering. </p> <p> Sr2Cr03F and SrLaCr04 have the same magnetic structure. The neutron diffraction data are consistent with two different models. In both cases the magnetic cell is √2a and c and the magnetic moments lie out of the plane. However, in one model the in-plane projection is canted and in the other it is colinear. Sr2Cr03F has a Tc value of 132(2) K and a susceptibility maximum at 280 K. SrLaCr04 has a Tc value of approximately 200 K and a susceptibility maximum near 400 K. The critical· exponent β for Sr2Cr03F is 0.26(4). The (100) magnetic reflection of both SrLaCr04 and Sr2Cr03F shows short range order correlations above Tc. </p> / Thesis / Master of Science (MSc)

The present rapid increase in the number of space missions demands a decrease in the cost of satellite equipment, but also requires the development of instruments that have low power consumption, low weight, and small size.Anisotropic magnetoresistance (AMR) sensors can answer these needs on account of their small size, weight, and power consumption. AMR sensors also produce lower noise than either giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) devices and are thus more suitable for space applications.The type of AMR sensor developed in this study was a Planar Hall EffectBridge (PHEB) sensor. The FM layer was also coupled with an AFM layer in order to fix the internal magnetization of the FM layer.One technique that was employed in order to meet the low-noise requirement was to make the FM layer thicker than has previously been attempted.In doing so, the exchange bias field between the AFM layer and the FMlayer is no longer high enough to bias the thicker FM layer, so in order to correct this unwanted effect, the material stack was upgraded to two AFM–FM interfaces. With this configuration, it became possible to increase the exchange field by up to 60%. Stronger exchange bias leads to a thicker FMlayer and so to lower noise in the device performance. Another strategy that was used to lower the resistance of the device was to implement an NiFeX alloy instead of the standard NiFe. NiFeX consists of an alloy of NiFe andCu, Ag, or Au; the last of these is known to have very low resistivity.This solution leads to a significant lowering of the device’s resistance. A recent technological advance used to fabricate devices with lower resistance is to deposit a multilayer of AFM–FM.

Thesis advisor: Michael Naughton / A helium gas-pressure system has been tested and then used to investigate the nature of a phase transition from the normal metallic state to an antiferromagnetic spin density wave state in the quasi-one dimensional molecular organic conductor (TMTSF)2PF6. This metallic state superconducts at low temperature and high pressure, in such a way that the insulating antiferromagnetic state competes with the superconducting state. The physics motivation was to examine the possibility of a ¿quantum critical point¿ near the critical pressure of the sample where these latter two states may coexist. The technical motivation was to make the first tests of the pressure system at cryogenic temperatures to ensure that it is the appropriate tool for the planned investigations. Using temperature sweeps at various fixed pressures on a single crystal sample, we were able to obtain several points on the pressure versus temperature phase boundary separating the metallic and SDW states. We have thus verified that the helium gas-pressure system is indeed capable of facilitating these types of experiments, and future measurements will be done at lower temperatures accessing the superconducting state. / Thesis (BS) — Boston College, 2003. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Physics. / Discipline: College Honors Program.

An intriguing magnetodielectric (MD) behavior is observed in geometrically frustrated spin-tetrahedral systems Cu2Te2O5X2 (X = Cl and Br). While the phase transition observed in the Cl-system at TN=18.5 K is consistent with 3D antiferromagnetic ordering, the phase transition at To=11.5 K in the Br-system has several unusual features. Concomitantly, a pronounced ferroelectric ordering is observed coinciding with TN of Cl-system and To of Br-system. At the highest applied magnetic field 90 kOe, the temperature dependent dielectric behavior with almost frequency independent well defined at TN~18.5 K are decrease compared with that at zero field for Cl-system, and at the highest applied magnetic field 90 kOe, the temperature dependent dielectric behavior with almost frequency independent well defined at Tm~30 K are decrease compared with that at zero field for Br-system. The ferroelectricity is ascribed to the polarization of the Te4+ lone-pair electrons, while the MD effect is argued to be due to exchange interaction involving frustrated tetramer clusters and intercluster exchange bridges.

The [ReCl6]2-, [ReBr6]2- and [ReCl4(ox)]2- anions are crystallised with the organic 4,4’- bipyridinium dication (4,4-H2bipy). Magnetometry reveals exotic behaviour of the [4,4’- H2bipy][ReCl6] and [4,4’-H2bipy][ReBr6] salts which demonstrate spin-canting, antiferromagnetic exchange interactions and metamagnetism. Single crystal X-ray structures at T = 3, 14 and 20 K of the [4,4’-H2bipy][ReBr6] salt reveal the behaviour to be purely of magnetic origin as no structural changes are observed. For the [4,4’-H2bipy][ReCl4(ox)] compound an antiferromagnetic exchange interaction of 10.2 cm-1 between the anions is observed (Chapter 2). The complexes (NBu4)2[(ReCl5)2(μ-pyrazine)], (NBu4)2[(ReBr5)2(μ-pyrazine)], (NBu4)2[(ReBr5)2(μ-pyrimidine)] and (NBu4)2[(ReBr5)2(μ-triazine)] are structurally and magnetically characterised in Chapter 3. Magnetic measurements reveal the ReIV ions bridged by a 1,4-heterocyclic amine to exhibit strong antiferromagnetic coupling induced by the linearity of the bridging ligand. The two dimers bridged by a 1,3-heterocyclic amine exhibit intramolecular ferromagnetic exchange and at low temperature an intermolecular antiferromagnetic coupling is observed for the (NBu4)2[(ReBr5)2(μ-triazine)] complex due to the presence of short intermolecular Br···Br distances. Six molecular ReIVCuII chains of formula {[Cu(L)4][ReCl6]}n (L = imidazole, 1- methylimidazole, 1-vinylimidazole, 1-butylimidazole, 1-vinyl-1,2,4-triazole or dimethylformamide) are characterised structurally and magnetically in Chapter 4. SQUID magnetometry and theoretical calculations reveal the chains to exhibit ferromagnetic exchange interactions, which increase as the Re–Cl–Cu bond angle decreases. The {[Cu(vinylimidazole)4][ReCl6]}n chain exhibit magnetic order at TC = 2.4 K, and the {[Cu(imidazole)4][ReCl6]}n network exhibits ferrimagnetic behaviour. Eight complexes of the [ReCl6]2- and [ReBr6]2- anions crystallised with the [MII(L•)2]2+ (M = Fe, Co or Cu) or [Ni(L•)(CH3CN)3]2+ cations (L• = 4-dimethyl-2,2-di(2-pyridyl)oxazolidine N-oxide) are characterised structurally and magnetically in Chapter 5. The [Co(L•)2]2+ cation shows evidence of a gradual, thermally induced spin-crossover transition in variable-temperature magnetic and structural experiments. The [Ni(L•)(CH3CN)3]2+ cation show exchange of the coordinated acetonitrile molecules for atmospheric water upon drying. The nickel-radical magnetic coupling is ferromagnetic in all cases, demonstrating spin-canting behaviour with an ordering temperature of T = 2.7 K for the [ReCl6]2- based compound, and intermolecular antiferromagnetic exchange interactions for the [ReBr6]2- based complex.

This thesis is focused on two open problems of antiferromagnetic (AFM) spintronics: manipulation of AFM coupled moments and development of new materials combining AFM and semiconductor properties. We present three particular methods enabling AFM moments manipulation. The rst method, based on the exchange spring effect in an AFM/FM double layer, strongly de- pends on the AFM layer thickness and temperature. We systematically vary these two parameters and identify the conditions when AFM moments can be manip- ulated. By the second method, cooling an AFM in a magnetic eld through the critical temperature, we prove the concept of a fully AFM-based (containing no FM) spintronic device. The last studied method is based on current induced effects in nanostructures containing an AFM. By systematic study of samples with and without AFM we demonstrate the ability of AFM moments to absorb a current induced torque. Relying neither on a FM nor on cooling in magnetic eld, this method represents an elegant way of AFM moments manipulation. In the second experimental part new materials for AFM spintronics are discussed, and one representative example, CuMnAs, is studied in detail. Characterization of bulk and epitaxial CuMnAs is presented and rst spintronic functionality is shown. Powered by TCPDF (www.tcpdf.org)

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A combinação das funções de Green para operadores de criação e destruição de bosons com as transformações de Holstein e Primakoff é utilizada na análise de sistemas antiferromagnéticos, isolantes e anisotrópicos, na região de baixas temperaturas. Os limites de estabilidade das fases antiferromagnética e \"spin-flop\", assim como a transição \"spin-flop\"-paramagnética são determinados em função da temperatura. A Hamiltoniana modelo considerada leva em conta as interações de intercâmbio anisotrópicas entre primeiros e segundos dos vizinhos numa rede cúbica simples e anisotropias de íon único, dos tipos uniaxial e ortorrômbico. Em particular, as fronteiras de fase \"spin-flop\"-paramagnéticas dos antiferromagnetos NiCl2.6H2O, CoCl2. 6H2O, NiCl2.4H2O e MnCl2.4H2O são determinadas e comparadas com os dados experimentais. As interações dipolares são consideradas explicitamente no circulo da fronteira de fase \"spin-flop\"-paramagnética do EuTe, obtendo-se boa concordância com a experiência. / The combination of Greens functions for boson creation and destruction operators with the Holstein - Primakoff transformations is employed in the analysis of insulating and anisotropic antiferromagnetic systems at very low temperatures. The stability limits of antiferromagnetic and spin- flop phases, as well as the spin-flop-paramagnetic transition are determined as a function of temperature. The model Hamiltonian considered takes account of the anisotropic exchange interactions between first and second neighbors for a simple cubic lattice and of single ion anisotropies, of the uniaxial and orthorhombic types. Particularly, the spin-flop-paramagnetic phase boundaries of the antiferromagnetics NiCl2.6H2O, CoCl2. 6H2O, NiCl2.4H2O and MnCl2.4H2O are determined and compared with experimental values. The dipolar interactions are explicitly considered in the calculation of the spin-flop-paramagnetic phase boundary of antiferromagnetic EuTe, and we obtain good agreement with the experiments.