Best Nikon D3100 lens for portraits?

Are there any inexpensive lenses that have a low f stop number such as in the 1.4 to 2 range that will work for the Nikon D3100?

Ideal lenses would be ones at a focal range of around 50mm.

I would like to take pictures with a shallow depth of field, subjects being either objects or humans.

examples of the kinds of pictures I would like to achieve are as follows:

The limit you will face on your D3100 is that the least expensive Nikon lenses available in the 50mm range will not autofocus on your camera. In my judgement, autofocus is not critical for portraits, unless you're talking about action portraits where you want to concentrate on framing and following the subject and don't want to worry about focusing while you're doing it.

If you really want autofocus with your D3100, here are some good portrait lens choices:
AF-S DX 35mm f/1.8 G Nikkor, about $US 200.
AF-S 50mm f/1.4 G, about $US 500.
AF-S 85mm f/1.4 G, about $US 1800

If you can live without autofocus, there are some less expensive choices at each focal length and f-stop combination.
AF 50mm f/1.8 D, about $US 135
AF 50mm f/1.4 D, about $US 350
AF 85mm f/1.8 D, about $US 460

You can also find non-D AF lenses that will work fine in manual focus and provide full automatic metering on your D3100. Here's a compatibility discussion from Ken Rockwell that talks about these Nikon autofocus and metering issues with entry-level Nikon DSLRs. You can use manual focus Nikon lenses as well, but the camera's metering system won't work - you will need to manually determine and set your exposure, as you probably don't remember doing back the 1960s with your Nikon F and Leica M cameras.

http://www.kenrockwell.com/...y-lens.htm

You should also be aware that you can't visually focus precisely or judge exact depth of field on the finder screen at faster than f/2.8 or so with current Nikon DSLRs - you will need to rely on the focus indicator light in the viewfinder for precise point of focus. The finder image will show you the depth of focus at about f/2.8, so the extra brightness and small focus depth of the faster lenses won't help in manual focusing. This is true even of the pro Nikon DSLR cameras unless fitted with special 3rd-party finder screens - the finder optics seem to be optimized for the best possible image with the f/2.8 pro zooms.