Magnetic powder includes: an epsilon-phase iron oxide-based compound selected from ε-Fe.sub.2O.sub.3 or a compound represented by Formula (1). The magnetic powder has an average particle diameter of 8 nm to 25 nm, a ratio of Hc to Hc′ of from 0.6 to 1.0, and Hc′ satisfying Expression (II). Hc′ represents a magnetic field at which a value of Expression (I) becomes zero in a magnetic field-magnetization curve obtained by performing measurement at a maximum applied magnetic field of 359 kA/m, a temperature of 296 K, and a magnetic field sweeping speed of 1.994 kA/m/s. M represents magnetization and H represents applied magnetic field. Hc represents a magnetic field at which magnetization becomes zero in the magnetic field-magnetization curve. In Formula (1), A represents at least one metal element other than Fe, and a represents a number that satisfies a relationship of 0<a<2.
d.sup.2M/dH.sup.2  Expression (I)
119 kA/m<Hc′<2380 kA/m  Expression (II)
ε-A.sub.aFe.sub.2-aO.sub.3  (1)

Magnetic powder includes: an epsilon-phase iron oxide-based compound selected from ε-Fe.sub.2O.sub.3 or a compound represented by Formula (1). The magnetic powder has an average particle diameter of 8 nm to 25 nm, a ratio of Hc to Hc′ of from 0.6 to 1.0, and Hc′ satisfying Expression (II). Hc′ represents a magnetic field at which a value of Expression (I) becomes zero in a magnetic field-magnetization curve obtained by performing measurement at a maximum applied magnetic field of 359 kA/m, a temperature of 296 K, and a magnetic field sweeping speed of 1.994 kA/m/s. M represents magnetization and H represents applied magnetic field. Hc represents a magnetic field at which magnetization becomes zero in the magnetic field-magnetization curve. In Formula (1), A represents at least one metal element other than Fe, and a represents a number that satisfies a relationship of 0<a<2.
d.sup.2M/dH.sup.2  Expression (I)
119 kA/m<Hc′<2380 kA/m  Expression (II)
ε-A.sub.aFe.sub.2-aO.sub.3  (1)

An electronic device includes an electromagnetic interference shield having a layer of conductive material covering at least a portion of the electronic device and having a skin depth of less than 2 μm for electromagnetic signals having frequencies in a kilohertz range.

An electronic device includes an electromagnetic interference shield having a layer of conductive material covering at least a portion of the electronic device and having a skin depth of less than 2 μm for electromagnetic signals having frequencies in a kilohertz range.

In some variations, the invention provides a method of depositing nanoparticles on a substrate, comprising: providing a substrate having a positive or negative surface charge; optionally depositing a polymer on the substrate, wherein the polymer has opposite charge polarity compared to the substrate; and simultaneously depositing first nanoparticles and second nanoparticles onto the substrate, wherein the first nanoparticles and the second nanoparticles have opposite charge polarities during depositing. Other variations provide a method of depositing a layer of nanoparticles on a substrate, the method comprising: providing a substrate having a positive or negative surface charge; providing faceted nanoparticles; preparing a nanoparticle solution containing the nanoparticles; and adjusting surface charge of the nanoparticles by changing the solution pH to reduce the magnitude of average zeta potential of the nanoparticles, thereby causing aggregation of the nanoparticles onto the substrate surface. Very high packing densities may be achieved with these methods.

In some variations, the invention provides a method of depositing nanoparticles on a substrate, comprising: providing a substrate having a positive or negative surface charge; optionally depositing a polymer on the substrate, wherein the polymer has opposite charge polarity compared to the substrate; and simultaneously depositing first nanoparticles and second nanoparticles onto the substrate, wherein the first nanoparticles and the second nanoparticles have opposite charge polarities during depositing. Other variations provide a method of depositing a layer of nanoparticles on a substrate, the method comprising: providing a substrate having a positive or negative surface charge; providing faceted nanoparticles; preparing a nanoparticle solution containing the nanoparticles; and adjusting surface charge of the nanoparticles by changing the solution pH to reduce the magnitude of average zeta potential of the nanoparticles, thereby causing aggregation of the nanoparticles onto the substrate surface. Very high packing densities may be achieved with these methods.

Magnetic powder includes: an epsilon-phase iron oxide-based compound selected from ε-Fe.sub.2O.sub.3 or a compound represented by Formula (1). The magnetic powder has an average particle diameter of 8 nm to 25 nm, a ratio of Hc to Hc′ of from 0.6 to 1.0, and Hc′ satisfying Expression (II). Hc′ represents a magnetic field at which a value of Expression (I) becomes zeroin a magnetic field-magnetization curve obtained by performing measurement at a maximum applied magnetic field of 359 kA/m, a temperature of 296 K, and a magnetic field sweeping speed of 1.994 kA/m/s. M represents magnetization and H represents applied magnetic field. Hc represents a magnetic field at which magnetization becomes zero in the magnetic field-magnetization curve. In Formula (1), A represents at least one metal element other than Fe, and a represents a number that satisfies a relationship of 0<a<2.

d.sup.2M/dH.sup.2  Expression (I)

119 kA/m<Hc′<2380 kA/m  Expression (II)

ε-A.sub.xFe.sub.2-xO.sub.3  (1)

Magnetic powder includes: an epsilon-phase iron oxide-based compound selected from ε-Fe.sub.2O.sub.3 or a compound represented by Formula (1). The magnetic powder has an average particle diameter of 8 nm to 25 nm, a ratio of Hc to Hc′ of from 0.6 to 1.0, and Hc′ satisfying Expression (II). Hc′ represents a magnetic field at which a value of Expression (I) becomes zeroin a magnetic field-magnetization curve obtained by performing measurement at a maximum applied magnetic field of 359 kA/m, a temperature of 296 K, and a magnetic field sweeping speed of 1.994 kA/m/s. M represents magnetization and H represents applied magnetic field. Hc represents a magnetic field at which magnetization becomes zero in the magnetic field-magnetization curve. In Formula (1), A represents at least one metal element other than Fe, and a represents a number that satisfies a relationship of 0<a<2.

d.sup.2M/dH.sup.2  Expression (I)

119 kA/m<Hc′<2380 kA/m  Expression (II)

ε-A.sub.xFe.sub.2-xO.sub.3  (1)

A method of producing a magnetic powder includes performing heat treatment on first particles that contain ferrous oxide to prepare 5 second particles that contain ε-iron oxide.

A method of producing a magnetic powder includes performing heat treatment on first particles that contain ferrous oxide to prepare 5 second particles that contain ε-iron oxide.