Ferromagnetic Interfacial Coupling Research Articles

Mean-Field and Monte Carlo calculations of phase transitions in a core–shell Ising nanotube

A cylindrical Ising nanotube that consists of 3/2 core spins surrounded by 5/2 shell spins is introduced and studied with Mean-Field approximation and Monte Carlo simulations in the presence of crystal and external magnetic fields. The effects of positive and negative interfacial coupling constants on the magnetic properties have been examined. The thermal behaviors of the order parameters and different types of macroscopic instabilities are presented. The model exhibits compensation points, first- and second-order phase transitions. Richer magnetic properties are obtained in the antiferromagnetic core–shell coupling case. In the presence of ferromagnetic interfacial coupling, the simulations sometimes generated long-lived metastable states. These states jumped to stable thermodynamic ones under thermal fluctuations via first-order phase transitions.

Emergence of electric-field-tunable interfacial ferromagnetism in 2D antiferromagnet heterostructures

Van der Waals (vdW) magnet heterostructures have emerged as new platforms to explore exotic magnetic orders and quantum phenomena. Here, we study heterostructures of layered antiferromagnets, CrI3 and CrCl3, with perpendicular and in-plane magnetic anisotropy, respectively. Using magneto-optical Kerr effect microscopy, we demonstrate out-of-plane magnetic order in the CrCl3 layer proximal to CrI3, with ferromagnetic interfacial coupling between the two. Such an interlayer exchange field leads to higher critical temperature than that of either CrI3 or CrCl3 alone. We further demonstrate significant electric-field control of the coercivity, attributed to the naturally broken structural inversion symmetry of the heterostructure allowing unprecedented direct coupling between electric field and interfacial magnetism. These findings illustrate the opportunity to explore exotic magnetic phases and engineer spintronic devices in vdW heterostructures.

Effect of antiferromagnetic interfacial coupling on spin-wave resonance frequency of multi-layer film

We investigate the spin-wave resonance (SWR) frequency in a bicomponent bilayer and triple-layer films with antiferromagnetic or ferromagnetic interfacial couplings, as function of interfacial coupling, surface anisotropy, interface anisotropy, thickness and external magnetic field, using the linear spin-wave approximation and Green’s function technique. The microwave properties for multi-layer magnetic film with antiferromagnetic interfacial coupling is different from those for multi-layer magnetic film with ferromagnetic interfacial coupling. For the bilayer film with antiferromagnetic interfacial couplings, as the lower (upper) surface anisotropy increases, only the SWR frequencies of the odd (even) number modes increase. The lower (upper) surface anisotropy does not affect the SWR frequencies of the even (odd) number modes. For the multi-layer film with antiferromagnetic interfacial coupling, the SWR frequency of modes m=1, 3 and 4 decreases while that of mode m=2 increases with increasing thickness of the film within a proper parameter region. The present results could be useful in enhancing our fundamental understanding and show the method to enhance and adjust the SWR frequency of bicomponent multi-layer magnetic films with antiferromagnetic or ferromagnetic interfacial coupling.

Constructing a magnetic handle for antiferromagnetic manganites

An intrinsic property of antiferromagnetic materials is the compensation of the magnetic moments from the individual atoms that prohibits the direct interaction of the spin lattice with an external magnetic field. To overcome this limitation we have created artificial spin structures by heteroepitaxy between two bulk antiferromagnets ${\mathrm{SrMnO}}_{3}$ and ${\mathrm{NdMnO}}_{3}$. Here, we demonstrate that charge transfer at the interface results in the creation of thin ferromagnetic layers adjacent to $A$-type antiferromagnetism in thick ${\mathrm{NdMnO}}_{3}$ layers. A novel interference based neutron diffraction technique and polarized neutron reflectometry are used to confirm the presence of ferromagnetism in the ${\mathrm{SrMnO}}_{3}$ layers and to probe the relative alignment of antiferromagnetic spins induced by the coupling at the ferro- to antiferromagnet interface. A density functional theory analysis of the driving forces for the exchange reveals strong ferromagnetic interfacial coupling through quantifiable short range charge transfer. These results confirm a layer-by-layer control of magnetic arrangements that constitutes a promising step on a path towards isothermal magnetic control of antiferromagnetic arrangements as would be necessary in spin-based heterostructures like multiferroic devices.

A breakthrough in the chemical imaging of single molecule: sub-nm tip-enhanced Raman spectroscopy

CO2FeAl/(Ga,Mn)As bilayer shifted with a small ferromagnetic interaction field of +24 Oe and –23 Oe at 15 K. The observed ferromagnetic interfacial coupling is supported by ab initio density functional calculations. For details of this study, please refer the paper cited in [4]. These findings may provide a viable pathway to design room temperature semiconductor spintronic devices through a magnetic proximity effect. Professor Zhao’s team plans to further enhance the magnetic proximity effect and then increase the thickness of the (Ga,Mn)As layer in which Mn ions remain spin polarized up to room temperature. Their final goal is to design and process (Ga,Mn)As-based spintronic devices that operate effectively at room temperature.

Ferromagnetic Interfacial Interaction and the Proximity Effect in aCo2FeAl/(Ga,Mn)AsBilayer

The magnetic properties of a Co2FeAl/(Ga,Mn)As bilayer epitaxied on GaAs (001) are studied both experimentally and theoretically. Unlike the common antiferromagnetic interfacial interaction existing in most ferromagnet-magnetic semiconductor bilayers, a ferromagnetic interfacial interaction in the Co2FeAl/(Ga,Mn)As bilayer is observed from measurements of magnetic hysteresis and x-ray magnetic circular dichroism. The Mn ions in a 1.36 nm thick (Ga,Mn)As layer remain spin polarized up to 400 K due to the magnetic proximity effect. The minor loops of the Co2FeAl/(Ga,Mn)As bilayer shift with a small ferromagnetic interaction field of +24 Oe and -23 Oe at 15 K. The observed ferromagnetic interfacial coupling is supported by ab initio density functional calculations. These findings may provide a viable pathway for designing room-temperature semiconductor spintronic devices through magnetic proximity effect.

Modification of exchange bias by cooling field without changing the ferromagnetic magnetization

In this work, cooling fields (HFC) with different signs or magnitudes were applied on ferromagnetic (FM)/antiferromagnetic (AF) films [Pt(10Å)/Co(4Å)]4/NiO(tNiOÅ) when FM magnetization of Pt/Co multilayers was kept in positive saturation state at room temperature. Compared to results at HFC=+5kOe, it is seen that HFC=−1kOe suppressed the exchange bias field (HE) and enhanced the coercivity (HC) at the same time. The phenomenon indicates that AF spins can be modified by cooling field without changing the FM magnetization. The experimental results are understood by the competition between the FM/AF interfacial exchange coupling and Zeeman energy in FM/AF systems with ferromagnetic interfacial coupling.

Exchange Bias in a Granular System of Antiferromagnetic Nanoparticles with Strong Magnetic Anisotropy Embedded in a Ferromagnetic Matrix

Recently, magnetic properties of nanoparticles have gained high interest from both technological and fundamental research, stemming partially from the applications in high-density magnetic storage media and biomedicine. In order to stabilize the recording units in nanometer scales at finite temperatures, the most challenge is to beat the superparamagnetism. Exchange bias, which represents a shift in the hysteresis loop induced by intimate contact between heterogeneous components, may be used to solve this issue. We present a nanogranular system with an antiferromagnetic-ferromagnetic cores-matrix morphology and study the interfacial-coupling and field-cooling dependence of exchange bias by conducting a modified Monte Carlo Metropolis method. When the interfacial coupling is antiferromagnetic, the competition between Zeeman and interfacial-coupling energies determines the strength and sign of exchange bias. Positive exchange bias appears when the cooling field overcomes the interfacial coupling. Whereas in the system with such a special morphology, the ferromagnetic anisotropy and exchange coupling may also determine the exchange bias behaviors to some extent when the interfacial coupling is antiferromagnetic and the strength of cooling field is intermediate. On the other hand, in the system with ferromagnetic interfacial coupling, exchange bias is negative constantly and linear roughly with not large interfacial coupling but independent of cooling field. The phenomena are clarified on the basis of the theories of surplus magnetization and pinning effect as well as by means of the microscopic spin configurations of system. [DOI: 10.1380/ejssnt.2011.126]

The effect of field-cooling strength and interfacial coupling on exchange bias in a granular system of ferromagnetic nanoparticles embedded in an antiferromagnetic matrix

A modified Monte Carlo Metropolis method is performed to simulate the effect of field-cooling strength and interfacial coupling on exchange bias and coercivity of a system with ferromagnetic cores embedded in an antiferromagnetic matrix, based on three-dimensional classical Heisenberg model. The results show that the exchange bias changes from negative value to positive value with increasing cooling field as the interfacial coupling is antiferromagnetic, whereas coercivity is almost unchanged due to the small size of ferromagnetic cores. After applying a weak cooling field, the exchange bias of system with antiferromagnetic interfacial coupling may be positive just when the value of interfacial coupling is not large and exhibit a positive maximum. However, the exchange bias is constantly positive and increasing with increasing values of interfacial coupling after applying a strong cooling field. The exchange bias of system with ferromagnetic interfacial coupling is independent of cooling field and increases for larger interfacial coupling, but the sign is negative due to the constantly positive net magnetization of antiferromagnetic surface. It is obvious that the positive exchange bias depends on the field-cooling strength to a large extent. However, the interfacial coupling may change the configuration of antiferromagnetic matrix to influence the exchange bias.

Quantum study of the spin wave spectrum of a superlattice with ferromagnetic interfacial coupling

In this paper the authors have carried out a quantum study of the features of the spin wave spectrum of a superlattice composed of alternating layers of two different ferromagnetic materials or a ferromagnetic material and a non-magnetic material, with a ferromagnetic coupling at the interfaces. Depending on the intensity of the interactions in the interfaces and on the layer thickness very different types of thermal behaviour of the magnetization can be obtained. In the case of the ferromagnetic-non-magnetic system a clear tendency to a bidimensional type of behaviour is shown when the exchange coupling at the interfaces is very small, and a clear dependence of the thermal demagnetization on the layer thickness is observed. In the case of the system composed of two different ferromagnetic materials, in the weak-coupling limit at the interfaces, the authors observe a more rapid decrease of the magnetization with temperature for thinner unit cells. The opposite type of behaviour is observed in the strong-coupling limit.